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Recent advances in hydrothermal carbonisation:

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Cite this: Green Chem., 2020, 22,

from tailored carbon materials and biochemicals
to applications and bioenergy
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4747

Sabina A. Nicolae,*a Heather Au, b Pierpaolo Modugno,a Hui Luo,b

Anthony E. Szego,c Mo Qiao, b Liang Li,d Wang Yin,e Hero J. Heeres, e

Nicole Berged and Maria-Magdalena Titirici *a,b

Introduced in the literature in 1913 by Bergius, who at the time was studying biomass coalification, hydro-
thermal carbonisation, as many other technologies based on renewables, was forgotten during the
“industrial revolution”. It was rediscovered back in 2005, on the one hand, to follow the trend set by
Bergius of biomass to coal conversion for decentralised energy generation, and on the other hand as a
novel green method to prepare advanced carbon materials and chemicals from biomass in water, at mild
temperature, for energy storage and conversion and environmental protection. In this review, we will
present an overview on the latest trends in hydrothermal carbonisation including biomass to bioenergy
Received 20th March 2020, conversion, upgrading of hydrothermal carbons to fuels over heterogeneous catalysts, advanced carbon
Accepted 27th June 2020
materials and their applications in batteries, electrocatalysis and heterogeneous catalysis and finally an
DOI: 10.1039/d0gc00998a analysis of the chemicals in the liquid phase as well as a new family of fluorescent nanomaterials formed
rsc.li/greenchem at the interface between the liquid and solid phases, known as hydrothermal carbon nanodots.

1. Introduction formation,1–5 in times when there was only little concern

about the temporary shortage of fossil fuel supplies. The last
Our world is facing hard and unprecedented challenges due to two decades have seen renewed interest in this topic.6–13 The
the constant increase of population and technology develop- main advantage of HTC is that, as the process takes place in
ment which implies a growing demand for energy and water, biomass does not require a preliminary drying step,
advanced materials that can no longer rely on fossil fuel exploi- thus saving great amounts of energy and time, and further-
tation. There is a strong need for a new productive system more allowing processing of wet mixtures, including aqueous
where clean energy generation and storage go hand in hand waste and sewage sludge.14
with sustainable materials production and studies on Fig. 1 illustrates a schematic of the HTC process and poss-
Hydrothermal Carbonisation (HTC) of biomass are aimed at ible products, which will also be covered in this review. In
developing a clever technology to balance them both. The term
hydrothermal carbonisation describes a thermal treatment
that takes place in aqueous medium heated at subcritical
temperatures (180–250 °C), under self-generated pressure.
HTC has been known to scientists over a century ago and it
has been used as a way to mimic the natural process of coal

a
School of Engineering and Materials Science, Queen Mary University of London,
Mile End Road, London E1 4NS, UK
b
Department of Chemical Engineering, Imperial College London, South Kensington
Campus, London, SW7 2AZ, UK. E-mail: m.titirici@imperial.ac.uk
c
Materials and Environmental Chemistry Department, Stockholm University, SE-106
91 Stockholm, Sweden
d
Civil and Environmental Engineering, University of South Carolina, Columbia, SC
29201, USA
e
Chemical Engineering Department, ENTEG, University of Groningen, Nijenborgh 4, Fig. 1 Schematic representation of the HTC process with the main
9747 AG Gronigen, Netherlands topics represented here.

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aqueous media, at subcritical temperatures and pressures, on the fluorescence of the CDs) and other chemicals,29–31 as
lignocellulosic materials undergo a vast series of transform- catalysts29 or for imaging of cells.32
ations such as hydrolysis, dehydration, bond cleavage, for- The first section of this review will be dedicated to the pro-
mation of new bonds, and condensation. The ultimate pro- duction of advanced carbon materials from biomass or
ducts of these reactions can be grouped into liquid species (or biomass derivatives with focus on applications in energy
water-soluble compounds), solid species ( primary and second- storage and conversion and environmental production.
ary chars and carbon dots) and gases (water vapour, CO, CO2, Following this, we will discuss the composition of the liquid
and CH4).15 Water-soluble compounds include furan deriva- phase depending on the precursors used and the various HTC
tives (furfural and 5-hydroxymethylfurfural) and carboxylic conditions as well as methods for the separation of these com-
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acids. Many of the soluble chemicals found in the liquid ponents from the liquid phase and their transformation in
phase after HTC are commercially valuable, and a few of them other viable chemicals using heterogeneous catalysis. Next, we
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have been recognized as platform chemicals for a transition to will address the production of fluorescent nanometre-sized
a greener chemistry.16 In some cases, the liquid phase derived carbons. We will then focus on HTC as a viable route to
from HTC treatment of biowaste feedstocks can also be used convert various raw biowaste materials into hydrothermal
in agriculture for irrigation or recycled for a new batch of carbons and provide a thorough characterisation of each
biomass conversion. Primary chars are the direct product of product depending on the raw precursors and hydrothermal
hydrothermal dehydration and pyrolysis of biomass. They are conditions. Finally, we will present the possibility of upgrading
referred to as biochars or hydrochars to highlight their biologi- some of the hydrothermal carbons to fuels via reforming over
cal origin, their charred appearance (that corresponds at a heterogeneous catalysts. For each section we will provide an
physical level to a higher energy density value in terms of the overview based on recently published papers in the literature
higher heating value compared to that of the original and our own perspectives on where scientific knowledge and
biomass17,18) and the hydrothermal process employed to understanding the fundamentals are still needed for HTC to
prepare them. The higher energy density value makes them fit become a viable technique for both the production of
for combustion to generate decentralised energy with lower advanced materials and chemicals as well as decentralised
CO2 emissions when compared with coal power plants, since bioenergy using biowaste.
they are produced from biomass which can, in principle, be
regrown to adsorb back the generated CO2. Secondary chars
are the result of further dehydration, condensation and 2. Advanced hydrothermal carbon
polymerization of water dissolved compounds. The resulting materials
particles present a spherical shape and small sizes in the
domain of microns. They have long been known to form 2.1. Fundamentals and gaps in hydrothermal carbonisation
during HTC of sugars or lignocellulosic biomass. They are also for advanced carbon production
often, confusingly, called humins or hydrochars. The former The three main components of lignocellulosic biomass are
name has a long documented history of misuse19 and orig- cellulose, hemicellulose and lignin. Cellulose is a linear poly-
inally referred to the alkali-insoluble fraction of humus. saccharide of glucose units bonded together by β-(1,4)-glucosi-
Throughout the times its meaning has shifted to denote the dic bonds. Hemicellulose is a polysaccharide too, but it differs
dark-brownish, amorphous products obtained by acid hydro- from cellulose in that it is made of a number of different sugar
lysis of proteins or sugars. As for the word hydrochars, we monomers, such as xylose, glucose, mannose and galactose.
already saw that it describes hydrothermally charred biomass. This variety of monomers makes the overall structure of hemi-
Therefore, in order to avoid any further confusion, we will use cellulose less stronger than that of cellulose, with shorter
the expression hydrothermal carbon (HT carbon) to denote the branched chains. Lignin is a highly randomly cross-branched
carbonaceous microspheres obtained via hydrothermal car- polymer of phenylpropane derivatives. Among the three afore-
bonization of cellulosic biomass. Their porosity can be mentioned biopolymers, lignin is the most stable and least
changed using natural templates or activation procedures; prone33 to hydrolysis under hydrothermal conditions, requir-
their chemical and physical properties can be easily modified ing temperatures as high as 250 °C to start decomposing.34
by their combination with different components, such as in- Therefore, polysaccharides, their monomers and their
organic compounds, and they can be easily functionalized decomposition products will only be taken into account in the
leading to a large number of oxygenated groups present on the following description of HTC reaction pathways. However, it is
surface. Carbon dots are quasi-spherical fluorescent carbon important to stress that real biomass always contains lignin to
nanoparticles,20,21 with diameters around or below 10 nm,22 some extent, and its presence is known to hinder the hydro-
an amorphous or nanocrystalline structure with sp2 carbon lysis of long saccharide chains under hydrothermal con-
clusters23 and a high amount of oxygen atoms and carboxylic ditions, lowering conversion yields of both soluble and in-
groups on their surface.24 Although their mechanism of for- soluble products.35 On the other hand, some experiments on a
mation is still not very well understood, these nanoparticles mixture of components including saccharides, proteins, lipids
have been successfully employed as probes for the detection of and lignin, although performed in a range of temperatures
metal ions25–28 (due to the quenching effect of these species beyond those considered in this review, showed a clear syner-

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gistic effect of the different components, with an increased hyde from which lactic acid and acetic acid are derived. The
yield of biocrude in the case of the mixture, compared to the presence of lactic acid (LacA) in the hydrothermal conversion
average yield obtained by treatment of single precursors.36 of sugars was explained with a mechanism involving an initial
Decomposition of cellulosic materials under hydrothermal retro-aldol condensation of fructose, yielding glyceraldehyde
conditions can be summarized in a few basic steps: hydrolysis and dihydroxyacetone. Both of these compounds can be de-
of cellulose and hemicellulose chains into their constituting hydrated to pyruvaldehyde, which in turn is converted to LacA
monomers (mainly glucose and other hexose and pentose with a benzylic acid rearrangement (Fig. 6). The benzylic acid
sugars); dehydration of C6 sugars to 5-hydroxymethylfurfural rearrangement requires basic catalysis, and this explains
(5-HMF) and C5 sugars to furfural; decomposition to lower why lactic acid is found in relevant amounts when basic
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molecular weight compounds or, alternatively, condensation conditions are employed for hydrothermal conversion of cellu-
and aromatization reaction, to produce hydrothermal losic biomass.40,41 However, it was demonstrated that water
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carbon.37 A quite exhaustive scheme of the complex network of under subcritical conditions could provide sufficient hydroxide
reaction pathways of decomposition of cellulose under hydro- ions to catalyse the reaction in the absence of any other
thermal conditions has recently been proposed by Buendia- sources of OH−. In fact, lactic acid can also be found among
Kandia et al.38 (Fig. 2). The scheme was elaborated by studying the HTC products of sugars when an acid is added as a
the decomposition of microcrystalline cellulose under hydro- catalyst.42
thermal conditions, testing three temperatures (180 °C– Decarbonylation and decarboxylation reactions account for
220 °C–260 °C), sampling the liquid phase every 20 minutes the production of CO and CO2. Finally, due to limited access
for an overall reaction time of 120 minutes. Their results con- of water molecules on the inner cellulose fibres, pyrolysis of
firmed the pathway proposed by Matsumura:39 long chains of cellulose occurs, leading to the direct formation of the primary
cellulose are firstly hydrolysed to smaller oligomers or mono- hydrothermal carbon. As for hydrothermal carbon, this
mers of glucose. Glucose can undergo isomerization to fruc- material is structured as spherical microspheres6 whose
tose via the Lobry de Bruyn–Alberda–van Ekenstein transform- chemical structure consists of condensed furan rings linked
ation or epimerization to mannose. either via the α-carbon or via sp2- or sp3-type carbon.45 This
As said before, furfural and 5-HMF derive from dehydration suggests that 5-HMF and furfural are the main building blocks
of pentoses (Fig. 3) and hexoses (Fig. 4), respectively. Levulinic of hydrothermal carbon. 5-HMF is a rather unstable and reac-
acid (LA), also named 4-oxopentanoic acid, is derived from tive molecule under hydrothermal conditions. A closer look at
5-HMF, which, under hydrothermal conditions, is rehydrated its chemical structure is useful in understanding its reactivity
to form LA along with formic acid (Fig. 5). Glucose oligomers and thus to propose hypotheses about its role in the formation
can undergo dehydration before complete hydrolysis, produ- of HTC spheres. 5-HMF possesses one aldehyde group and one
cing cellobiosan and subsequently levoglucosan. Retro-aldol hydroxymethyl group at positions 2 and 5, respectively, on the
condensation of glucose produces erythrose and glycolalde- furan ring.

Fig. 2 Scheme of the reaction pathways of cellulose decomposition as proposed by Buendia-Kandia et al.; reproduced with permission from ref.
38.

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Fig. 3 Mechanism of dehydration of xylose to furfural via an acyclic pathway; adapted from ref. 43.

aromaticity when compared to benzene, can behave as a diene,

undergoing nucleophilic addition, ring-opening reaction or
Diels–Alder cycloaddition.
A recent investigation by van Zandvoort et al.48 has provided
very important insights into the chemical structure of hydro-
thermal carbon. In fact, by means of 1D and 2D solid-state
Nuclear Magnetic Resonance (NMR) spectra of 13C-labeled
hydrothermal carbon, various different linkages have been
identified in the 2D NMR spectra, ranging from the most
abundant Cα–Caliphatic and Cα–Cα linkages to minor ones such
Fig. 4 Mechanism of dehydration of fructose to 5-HMF; adapted from as Cβ–Cβ and Cβ–Caliphatic cross-links. Furan rings have been
ref. 44. found to be linked by short aliphatic chains; levulinic acid is
also included in the structure through covalent bonds. A
chemical structure (Fig. 8) was proposed.48
Brown et al.49 have simulated Raman spectra of model con-
stituents of hydrothermal carbon by means of Density
Functional Theory (DFT). By fitting the experimental Raman
hydrothermal carbon spectrum with a 12-peak fit, they have
found that two model structures could reproduce the main fea-
tures of the hydrothermal carbon: its Raman spectrum and its
elemental analysis: (1) a structure consisting of arene domains
comprising 6–8 rings connected via aliphatic chains; and (2) a
furan/arene structure consisting primarily of single furans and
2 or 3 ring arenes. However, subsequent NMR and infrared
(IR) analyses suggested that the latter model is more consist-
ent with the experimental observations. A nucleation pathway
was proposed by Sevilla and Fuertes50 which derived obser-
vations from the dissolution behaviour of cellulose under
Fig. 5 Mechanism of rehydration of 5-HMF to levulinic acid with loss of
a carbon atom in the form of formic acid; adapted from ref. 46.
hydrothermal conditions, hence known as “the soluble
pathway”. This pathway includes: (i) hydrolysis of cellulose
chains, (ii) dehydration and fragmentation into soluble pro-
ducts of the monomers that come from the hydrolysis of cell-
The aldehyde group can undergo aldol condensation with ulose, (iii) polymerization or condensation of the soluble pro-
α-ketones or aldehydes (Fig. 7) or react with alcohols or diols ducts, (iv) aromatization of the polymers thus formed, (v)
to give (hemi)acetals. The acetalisation of the aldehyde group appearance of a short burst of nucleation and (vi) growth of
activates the furan ring towards electrophilic aromatic substi- the nuclei so formed by diffusion and linkage of species from
tution. The hydroxymethyl group, on the other hand, can be the solution to the surface of the nuclei.50 More recently,
subject to nucleophilic substitution, as the mildly acidic con- Tsilomelekis et al.51 have studied acid catalysed degradation of
ditions can protonate the hydroxyl group making it a better 5-HMF by means of ATR-FTIR spectroscopy, Scanning Electron
leaving group. Finally, the whole furan ring, due to its lower Microscopy (SEM) and Dynamic Light Scattering (DLS) to

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Fig. 6 Mechanism of formation of lactic acid through retro aldol condensation and a benzylic rearrangement; adapted from ref. 42.
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Fig. 7 Tautomeric forms of HMF and the reaction to a dimer via aldol condensation; adapted from ref. 47.

addition lead to the formation of bigger hydrothermal carbon

particles. However, Cheng et al.53 have noted that hydro-
thermal carbon can be partially dissolved by multistage dis-
solution in an organic solvent to oligomers that have mass
numbers ranging from 200 to 600 Da, as detected by liquid
chromatography coupled with mass spectrometry (LC-MS).
Although this observation has led the authors to speculate that
hydrothermal carbon is actually an aggregate of oligomeric
species rather than a disordered polymer,53 there might be a
different interpretation. In fact, a very recent paper by Higgins
et al. reported striking evidence of a core/shell structure in
glucose-derived hydrothermal carbon. This evidence, obtained
by combined STXM and NEXAFS observations, points to a
“harder”, more condensed and hydrophobic core, surrounded
Fig. 8 Chemical structure of hydrothermal carbon proposed by van by a “softer” shell, rich in aldehydic/carboxylic groups. These
Zandvoort; adapted from ref. 48. reactive functionalities serve as binding sites for the growth of
the carbon particles.54 Formic acid and levulinic acid play a
key role in this scenario: the former, due to its rather high pKa,
understand the growth mechanism of 5-HMF derived hydro- significantly increases the rate of conversion of C6 sugar to
thermal carbon. They have proposed a mechanism based on 5-HMF in an autocatalytic fashion, therefore speeding up the
an initial nucleophilic attack of a 5-HMF carbonyl group to the growth of spherical particles. Levulinic acid, on the other
α- or β-position of the furanic ring, along with aldolic conden- hand, has a lower pKa and therefore does not have a strong
sation and etherification. Small, soluble oligomers grow catalytic effect, but it does affect the growth of the spherical
heavier until they form small solid nuclei through phase separ- particles taking part in the process as building units and
ation. Small particle aggregation and continuous 5-HMF slowing the growth by reducing the surface density.52

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Moreover, Jung et al. have stated that significant growth of phology.56 Moreover, it has been demonstrated that a second
carbon particles occurs when the 5-HMF concentration has subsequent HTC of the carbon spheres results in the uniform
already dramatically declined. This points to an alternative growth of the pre-existing particles without any relevant for-
model for growth of particles based on coalescence, which is mation of new ones.58 Simsir et al.59 have studied the effects of
also supported by the further observation that addition of elec- different reaction times on HTC of different kinds of feed-
trolytes in the reaction medium increases particle sizes by stocks (glucose, cellulose, chitin, chitosan, and wood chips) at
reducing repulsive forces between particles.47 This debate is a a fixed temperature of 200 °C. In this study, glucose-derived
proof that a thorough comprehension of the mechanisms carbon spheres are not observed before a 12 hour long treat-
underlying HTC has not been achieved yet and there is con- ment, with an average diameter of around 800 nm for a resi-
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siderable space for further research. dence time between 12 h and 36 h, and a slightly lower average
2.1.1. Influence of feedstock, reaction time and tempera- diameter of 500–600 nm with the increase in residence time to
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ture on the material morphology 48 h. This has been explained with the plausible existence of
2.1.1.1. Feedstock. As highlighted in the previous section, an equilibrium between the growth and decomposition of
furfural and 5-HMF, originating from the dehydration of pen- spherical carbon particles for longer residence times. Chitin is
toses and hexoses, respectively, play a key role in the formation insensitive to HTC, while cellulose and wood chips produce
of hydrothermal carbon spheres, due to their reactivity. hard carbon spheres, too. Finally, chitosan derived hydro-
Consequently, it is reasonable to expect chars with a similar thermal carbon appears in the form of densely aggregated
morphology and structure, regardless of the carbon precursor, structures.
as long as any of the aforementioned compounds are present 2.1.2. Influence of feedstock, reaction time and tempera-
in the reaction medium. Titirici et al.55 have demonstrated ture on hydrothermal carbon yields and chemical character-
that all hexose (and 5-HMF)-derived carbon spheres have the istics. Process conditions (e.g., reaction time and reaction
same morphology. Similarly, carbon spheres obtained from temperature) and feedstock properties have been shown to
the HTC of xylose and furfural are indistinguishable in shape. influence hydrothermal carbon yields and chemical
Further confirmations come from Falco et al.,8 who have com- composition.60–63 Reaction temperature has been reported to
pared the morphology of the HTC secondary char derived from be quite influential on these gross hydrothermal carbon pro-
glucose, cellulose and rye straw. While glucose and cellulose- perties. The influence of reaction time, however, varies and
derived carbon spheres appear similar in shape, it is argued appears to be somewhat dependent on carbonisation kinetics.
that long cellulose fibers are disrupted under hydrothermal The combined influence of reaction time and temperature is
conditions, leading to the formation of shorter chains that often modelled by using a severity factor approach,37,64,65
adopt a spherical shape to minimize contact with water. Rye which provides a means for comparing results from experi-
straw behaviour is similar to that of cellulose, with micro- ments conducted at different times and temperatures.65,66
spheres appearing on the surfaces of the fibers that suffer only Severity factor ( f ) is defined as:37
minor disruption.
2.1.1.2. Reaction time and temperature. Reaction time and
f ¼ 50t 0:2  e ð3500=TÞ
temperature have been shown to influence the hydrothermal
carbon morphology. Sevilla et al.,13 studying hydrothermal
carbon yields from the HTC of glucose, sucrose and starch, where t is time (s), and T is the final reaction temperature (K).
noted that in any case, a rise of the precursor concentration, Increases in reaction severity correlate with an increase of reac-
reaction time or reaction temperature resulted in an increased tion temperature and/or reaction time and provide a relative
yield of hydrothermal carbon and in the diameter of the micro- measure of reaction severity among carbonisation studies facil-
spheres. Romero-Anaya56 have substantially confirmed the itating the comparison of multiple studies. To understand how
observation, noting also that carbon sphere growth reaches a hydrothermal carbon yields and chemical compositions differ
maximum (at fixed reaction time and precursor concentration) for different feedstocks, the yields and chemical characteristics
at 200 °C. Temperature has a big impact on hydrothermal of hydrochars reported in the literature following the carbonis-
decomposition of cellulose. Depolymerization of microcrystal- ation of different organic feedstocks were collected and com-
line cellulose in water-soluble compounds is known to start at pared. Results from this comparison are presented in
180 °C.57 However, it has been shown that depolymerisation of Fig. 9–12. These box plots illustrate the distribution of the col-
cellulose to sugar oligomers becomes predominant at 220 °C, lected data, as well as median and average values. These col-
while temperatures as high as 260 °C cause a decline of sugar lected data represent the carbonisation of a variety of feed-
oligomers in favour of decomposition products such as 5-HMF stocks over a large range of process conditions. To facilitate
or carboxylic acids.38 Reaction time seems to have an impact comparison between these studies, severity factors associated
on the morphology and size of carbon spheres only up to a with each set of experimental data collected were calculated. In
certain point. In fact, it has been noted that, over treatment each figure, the feedstocks are listed in order of the lowest
times of 12, 24 and 48 h, sugar derived carbon spheres achieve average severity factor to the greatest average severity factor
bigger and more uniform sizes from 12 h to 24 h, but longer and a plot of the average severity factors is included above
reaction times do not produce any change in the size or mor- each box plot.

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Fig. 9 Distributions associated with the hydrothermal carbon yields of different feedstocks based on literature-collected data. The line in each box
represents the median value. The ends of each box represent the 25th and 75th percentiles associated with the data. The red diamonds represent
the average values. The lines and data points represent the scatter of data beyond the 10th and 90th percentiles. The numbers in parentheses follow-
ing each feedstock category represent the number of data points represented on the plot.

Fig. 11 Distributions associated with the percentage of hydrogen in the

hydrothermal carbon. The line in each box represents the median value.
The ends of each box represent the 25th and 75th percentiles associated
Fig. 10 Distributions associated with the (left) percentage of carbon in with the data. The red diamonds represent the average values. The lines
the hydrothermal carbon and (right) the percentage of initially present and data points represent the scatter of data beyond the 10th and 90th
carbon that remains in the hydrothermal carbon following carbonis- percentiles. The numbers in parentheses following each feedstock cat-
ation. The line in each box represents the median value. The ends of egory represent the number of data points represented on the plot.
each box represent the 25th and 75th percentiles associated with the
data. The red diamonds represent the average values. The lines and data
points represent the scatter of data beyond the 10th and 90th percen- This observation is not surprising, as lignin has been shown
tiles. The numbers in parentheses following each feedstock category
to be only mildly influenced when exposed to HTC.8,35 The car-
represent the number of data points represented on the plot.
bonisation of municipal solid waste (MSW), digestate, wood,
and yard waste also results in average and median yields
greater than 60%. Carbonisation of oils (e.g., bio-oil and pyrol-
Fig. 9 presents a comparison of hydrothermal carbon yields ysis oil)67,68 results in significantly lower hydrothermal carbon
associated with literature-collected data. Results from this ana- yields. Similarly, the HTC of paper and algae also results in
lysis indicate that the average and median yields obtained relatively lower yields than those associated with the majority
when carbonizing the majority of feedstocks range from 40 to of other feedstocks. In general, as the reaction severity
60%. The carbonisation of lignin results in the greatest increases, compound volatilisation and solubilisation increase,
average and median yields (average: 66% and median: 68%). resulting in decreased solid yields.8,69–71 This general trend is

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yard waste (Fig. 10 right), two biomass materials that are ideal
candidates for conversion via HTC because of their high initial
moisture contents.
The majority of the generated hydrothermal carbon, regard-
less of feedstock, contains hydrogen concentrations ranging
from approximately 4 to 6% (Fig. 11). An exception to this is
the hydrothermal carbon generated when carbonising food
waste, which contains an average hydrogen percentage of
7.2%. The feedstocks that result in the greatest average loss of
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hydrogen to either the gas or liquid-phase are algae and paper,

while greater than 60% of the initially present hydrogen
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remains in the hydrothermal carbon when carbonising food

waste, lignin, MSW, and plant materials. At relatively low and
high severity factors, slight trends exist, suggesting that at
these reaction severities, process conditions are potentially
more influential than feedstock properties on the fraction of
initially present hydrogen that remains within the hydro-
Fig. 12 Distributions associated with the percentage of oxygen in the
thermal carbon. Not surprisingly, a small amount of the
hydrothermal carbon. The line in each box represents the median value. oxygen present in the initial feedstocks remains integrated
The ends of each box represent the 25th and 75th percentiles associated within the hydrothermal carbon following carbonisation
with the data. The red diamonds represent the average values. The lines (Fig. 12), which is beneficial when considering the use of
and data points represent the scatter of data beyond the 10th and 90th hydrothermal carbon in different applications, such as an
percentiles. The numbers in parentheses following each feedstock cat-
egory represent the number of data points presented on the plot.
energy source. This observation is consistent with that
reported in the literature; the decrease in the solid-phase
oxygen content has been reported as being the most influen-
tial factor contributing to the decrease in solid recoveries.76
observed for individual feedstocks. However, when collectively Food waste, lignin, and plant materials, on average, retain the
investigating trends of all feedstocks, no trend with an increas- largest fraction of oxygen embedded within the initial feed-
ing severity factor (from left to right on the figure) is observed, stocks, when compared to other carbonised feedstocks.
suggesting that the feedstock properties significantly influence Generally, as the reaction severity increases, the hydrothermal
hydrothermal carbon yields (more than the reaction con- carbon oxygen content decreases.76,78,79 Interestingly, trends
ditions), consistent with that which has been previously in the average hydrothermal carbon oxygen content are not
reported.72–75 Lu and Berge72 reported that changes in feed- observed with increasing severity factors (from left to right on
stock properties/complexity influence hydrothermal carbon the figure), suggesting that feedstock properties also influence
yield. the hydrothermal carbon oxygen content.
Fig. 9–12 illustrate how changes in feedstock and process
conditions influence the chemical properties of hydrothermal 2.2. Synthesis of porous carbon materials
carbon. Based on the compiled data, the majority of the Hydrothermal carbon materials have been of great interest,
carbon content of the hydrothermal carbon generated from over the years, due to their wide range of applications. Used in
the carbonisation of different organics generally ranges from many ways, such as for cleaning the dyes present in water and
40 to 65%, with the hydrothermal carbon generated from sugar, or for the removal of unpleasant odours,80 present-day
starch, xylose, and glucose containing the largest average applications mention them in industry as adsorbents for gas
hydrochar carbon content (Fig. 10 left). Generally, as the reac- purification81–83 and water treatment84,85 or a catalyst and cata-
tion severity increases, the carbon content of the hydrothermal lyst-supports.86,87 They are, also, often used in the fields of
carbon increases.8,71,76,77 However, trends in the average energy storage, fuel cells and chromatography
hydrothermal carbon content are not observed with increasing technologies.88,89 In order to improve their features, a great
severity factor (from left to right on the figure), suggesting number of studies are focused on developing new synthetic
that, like that associated with hydrothermal carbon yield, feed- approaches for creating porous structures and enhancing the
stock properties significantly influence the hydrothermal surface area, along with gaining a fundamental understanding
carbon content. Fig. 10 right provides a summary of the frac- of their properties.90,91 As mentioned before, the pristine
tion of initially present carbon that remains integrated within hydrothermal carbons are nonporous and sometimes not
the hydrothermal carbon following carbonisation. The great- applicable for further usage.92–95 To overcome this, different
est, on average, loss of carbon from the feedstock to the gas strategies, including templating methods (soft and hard
and/or liquid-phases occurs when carbonising paper, straw, templating),93,94,96 or chemical activation, using alkaline com-
and algae. The greatest retention of initially present carbon in pounds, KOH,95,97 NaOH,85 and ZnCl298 or acids, such as
the hydrothermal carbon occurs when carbonising food and H3PO4,99 in combination with HTC, have been proposed.

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Fig. 13 Graphical representation of the templating methods: a – soft templating and b – hard templating; reproduced with permission from ref.
104.

2.2.1. Templating methods. One of the first attempts at organic acids,93 and natural polymers (lignin, chitin, and
using templating methods for the development of porous cellulose).109,110 For example, Liang et al.111 synthesised
carbon materials has been made by Gilbert et al.100 in 1982. porous carbon spheres via soft templating assisted HTC, using
They synthesised porous glassy carbons by impregnating a Pluronic F108 as a template and phenol-formaldehyde as a
silica template with a phenol-formaldehyde resin mixture. This precursor. The mixture was subjected to HTC at 170 °C for 6 h,
methodology reflects the “hard (exo) templating” approach followed by carbonisation at 600 °C under a N2 atmosphere.
and is based on a mixture of a carbon precursor (usually phe- Further characterization revealed the formation of spherical
nolic resins) with a hard template. In this way, the carbon pre- carbon particles with a microporous structure. As a result of
cursor will infiltrate into the structure of the template and it the high number of micropores, the carbon spheres possessed
will be carbonized within the pores (at high temperatures, high specific surface area and high pore volume, with values
>700 °C). The template is ultimately removed, leaving behind a of about 1481 m2 g−1 and 0.9 cm3 g−1 respectively. Pluronic
well-defined structure (see Fig. 13b).101,102 Later on, Liang F127, in combination with xylose, has been reported by Liu
et al.103 developed a new strategy based on the self-assembly et al.112 for the production of carbonaceous long-range
properties of block copolymers and aromatic resins, such as ordered mesostructures. The same approach has been used by
phloroglucinol, or resorcinol/formaldehyde. This method Xie et al.113 for the synthesis of hierarchically porous materials
known as “soft (endo) templating” is a classical way to produce with tunable properties. In both cases, the obtained materials
inorganic porous materials (see Fig. 13a). As HTC is a versatile have been characterized as a mesoporous structure with a
process, it can be easily combined with the above-mentioned surface area of around 450 m2 g−1. Zhou et al.105 reported the
methods, in order to synthesise porous carbons derived from synthesis of core-mesoporous shelled carbon spheres via HTC
sustainable resources. and soft templating. Starting from 2,4-dihydroxybenzoic acid,
2.2.1.1. Soft templating in HTC. Nanostructured HTC– hexamethylenetetramine and Pluronic P123, they produced
derived materials can be synthesised using polymeric tem- uniform carbon spheres with an average diameter of ∼141 nm
plates of defined size and shape. In soft-templating, an amphi- and shell thicknesses of ∼30 nm (see Fig. 14a and b).
philic molecule such as a surfactant or block-copolymer self- Characterisation techniques revealed the porous structure with
assembles with a carbon precursor into an organized meso- a surface area of up to 648 m2 g−1 and a large pore volume
phase, which is stabilised by thermal treatment. The process is (1.06 cm3 g−1). In a similar way, Xiao et al.96 reported the syn-
controlled by several parameters, such as the concentration, thesis of porous carbon materials starting from glucosamine.
temperature, hydrophilic or hydrophobic reaction, pH, etc. During the study, they investigated the influence of pH and
Generally, the templates are polymers such as poly(ethylene the amount of template. By moving from neutral to acidic con-
oxide)-b-poly( propylene oxide)-b-poly(ethylene oxide)triblock- ditions, the specific surface area significantly increased, from
coplymers (PEO-b-PPO-b-PEO) from the Pluronic 550 m2 g−1 up to 980 m2 g−1, as a result of P123 micelle for-
family,96,105,106 polystyrene-b-poly(ethylene oxide) (PS-b- mation being favoured at pH 2. Also, an improvement in tex-
PEO)107 or polystyrene-b-poly(4-vinlpiridine)(PS-b-P4VP),108 tural properties was observed on increasing the amount of
trioctylamine@ferrocene.93 Usually, the carbon precursors polymer. SEM micrographs show the formation of agglomer-
consist of small clusters of phenol-formaldehyde, “resol” or ated small particles, confirming the transformation of glucos-
phloroglucinol-formaldehyde resins, but many researchers amine in carbon spheres (Fig. 14c), and TEM reveals the
adopted it by using different carbon precursors, including worm-like structure of the sample, related to the formation of
carbohydrates,90,94 nitrogen and carbon-rich compounds,94,96 micro- or mesopores (Fig. 14d). P123 together with sodium

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Fig. 15 (a) SEM, (b) TEM, and (c) HRTEM micrographs of ordered meso-
porous carbons derived from fructose (reproduced with permission
from ref. 90). (d and e) SEM images of ordered mesoporous carbons and
(f and g) TEM images of the same samples with an Im3m mesoporous
Fig. 14 Typical (a) SEM and (b) TEM images of hollow core-meso- structure; reproduced with permission from ref. 115.
porous shelled carbon spheres; reproduced with permission from ref.
105. (c) SEM and (d) TEM images of hydrothermal porous carbons
(reproduced with permission from ref. 96). (e) and (f ) SEM images with
different magnifications of BHCS. (g–i) TEM images of BHCS under meter of 240 nm and high surface area, in the range of
different magnifications (reproduced with permission from ref. 93). 446–566 m2 g−1. SEM and TEM results are illustrated in
Fig. 15d–g.
2.2.1.2. Hard templating in HTC. The hard-templating
oleate (SO) have been reported by Chen et al.114 in the syn- method is based on the structure replication approach using
thesis of asymmetric flask-like hollow carbon materials. Zhang sacrificial templates. The advantage of hard templating com-
et al.93 reported the synthesis of hollow carbon spheres start- bined with HTC is that polar functionalities are directly
ing from ascorbic acid and trioctylamine @ferrocene-in-water. present on the surface of the synthesised material. Different
The authors obtained bowl-like hollow carbon spheres (BHCS templates can and have been employed, such as silica,116
– see Fig. 14e–i) with a microporous structure, a diameter of zeolite,117,118 and metal organic frameworks.119 An advantage
approximately 0.53 nm and a specific surface area of about of hard templating over soft templating is a better control of
200 m2 g−1. the carbon structure, and a higher possibility of obtaining
The synthesis of ordered porous carbon materials derived hierarchical structures.120 Xiao et al.94 reported the synthesis
from D-fructose has been reported by Kubo et al.90 In this way, of mesoporous carbon materials by combining soft and hard
the HTC of D-fructose was performed at 130 °C for a long time, templating strategies. In this way, they used different sacchar-
in order to ensure the stability of the template. The final com- ides (D-glucose_1, D-fructose_2, D-glucosamine hydro-
posite was characterised by ordered porosity with pore dia- chloride_3, D-maltose_4, sucrose_5, and β-cyclodextrin_6), tri-
meters being around 10 nm and wall thicknesses of 6 nm, block copolymers and tetraethyl orthosilicate (TEOS). During
indicating that the self-assembly of the polymer with fructose HTC, Pluronic P123 forms micelles that are covered by a silica
was successful (Fig. 15a–c). The material was characterised by layer, coming from the hydrolysis of TEOS and the subsequent
a 257 m2 g−1 surface area and a 0.14 cm3 g−1 pore volume. condensation reaction of orthosilicic acid. The silica interacts
Chen et al.115 synthesised carbon nanospheres starting from a with the micelles via hydrogen bonds and acts as a protector
urea-phenol-formaldehyde (UPF) resin and Pluronic F127. The for the micelles during the HTC. The carbon precursor reacts
resulting materials had a well-ordered cubic Im3m mesostruc- on the silica surface, and forms hydrothermal carbon, without
ture in large domains with spherical shapes with a mean dia- harming the micelles. In this way, after the template removal,

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Fig. 16 TEM images of (a) OHTC_1, (b) OHTSiC_1 (before etching the silica template) and (c) OHTSi_1 (after burning the carbon at 600 °C), repro-
duced with permission from ref. 94; TEM pictures of (d) SBA-15/C composites after calcination at 900 °C, (e) pristine carbon materials after template
removal and (f ) S-doped carbon materials after template removal (reproduced with permission from ref. 121).

the carbon presents an ordered structure. The obtained 2.2.2. Activation methods. Aside from the templating
materials were denoted as OHTC-X (where X refers to the methods, activation strategies have been highly implemented
carbon source) and were characterised by type IV isotherms, in the generation of porosity in the carbon
specific for mesoporous structures, with a H2 hysteresis loop, structure.97,99,122–124 Generally, there are two types of acti-
indicating a multilayer adsorption of nitrogen. The surface vations that could be applied on hydrothermal carbons: physi-
areas ( pore volume) are in between 300 m2 g−1 (0.55 cm3 g−1) cal and chemical. In the case of physical activation, the hydro-
and 520 m2 g−1 (0.40 cm3 g−1). TEM images illustrate the thermal carbon is treated at temperatures in the range of
ordered structure of the materials (Fig. 16). To assess the 600–900 °C, under an inert atmosphere (carbonisation) or in
ordered character of the pores the authors viewed the samples an oxygen or steam flow (activation/oxidation). For the chemi-
with and without a silica template (Fig. 16a and b). It was cal activation, the hydrothermal carbon is mixed with an acid,
observed that the carbonaceous powders preserve the ordered a strong base or a salt before carbonisation.122 The most
orientation after template removal. To confirm that the silica common chemical activators are KOH,97,125 NaOH,85 Li2C2O4,
formed in situ during the HTC is the structure directing agent, Na2C2O4, K2C2O4,126 ZnCl2 98 and H3PO4.99 Zhu et al.126 syn-
the carbon was burned by calcination, leaving behind ordered thesised porous carbon materials via the HTC of pineapple
silica (Fig. 16c). Hard templated carbons have been prepared waste and chemical activation. In this way, the biomass was
by Wang et al.121 using SBA-15 as a sacrificial template for the washed, pulverized, and sieved, followed by mixing with alkali
synthesis of ordered mesoporous doped carbons. Starting metal oxalate (Li2C2O4, Na2C2O4 and K2C2O4) in a mass ratio
from glucose and using HTC, the mixtures are connected of 1 : 2 (biomass : alkali metal) and HTC for 10 h at 210 °C.
through carbon bridges and cover the template uniformly. The Porosity measurements revealed the formation of nonporous
final material is highly populated with oxygenated groups, and macroporous structures for the Li2C2O4 and Na2C2O4 acti-
coming from the dehydration compounds of glucose during vated carbons (ACs) and microporous structures for the ACs
the HTC. Part of these are removed during further carbonis- derived from the activation with K2C2O4. When comparing the
ation, at 900 °C, and the template is removed by NH4HF2 porosity data such as the pore volume (V0) and narrow micro-
leaching. TEM pictures of the obtained materials show the pore volume (Vn), Vn is bigger for the K2C2O4 ACs, and the
presence of a well-ordered structure of the composites C/ Li2C2O4 and Na2C2O4 ACs present opposite results. The
SBA-15 (Fig. 16d) which is stable also after the template authors explained these textural differences among the AC
removal (Fig. 16e and f ). The porosity was confirmed also by series with Li2C2O4, Na2C2O4 and K2C2O4 by the different reac-
N2 sorption isotherms, both doped and undoped carbons tion pathways of the oxalates during the activation process. In
showing type IV isotherms characteristic of mesoporous this way, when K2C2O4 at high temperature is used, a larger
materials with pores ranging from 2 to 8 nm and surface areas amount of CO is released in the third stage, which causes the
of 253 m2 g−1 and 341 m2 g−1, respectively. micropore formation. Sevilla et al.127 reported the synthesis of

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microporous carbon via the HTC of microalga and glucose dispersed in a certain amount of H3PO4 and deionized water,
mixtures. To ensure the microporosity, the hydrothermal and HTC was performed at 200 °C for 24 h. Also, in the pres-
carbon was chemically activated using KOH at elevated temp- ence of an activation agent, the carbon yield was higher, prob-
eratures, 650–750 °C, in a mass ratio of 2 KOH/hydrothermal ably because H3PO4 acts as a catalyst for the hydrothermal
carbon. The obtained materials were characterised by their carbon formation.
microporous structure with an apparent surface area of 2.2.3. Summary. To date, HTC has been intensively reported
1800 m2 g−1 to 2200 m2 g−1 at the maximum activation temp- as one of the best strategies to produce carbon materials with
erature (750 °C). Li et al.128 synthesised a composite structure tunable physical and chemical properties. By combining the HTC
of a graphene-like nanosheet (GN)/porous carbon framework with templating approaches, the HTC carbon structure can be
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using waste-peanut skin as a biomass precursor. The pro- highly improved in terms of surface area and pore volume. Also,
cedure was based on a H2SO4 assisted HTC followed by KOH chemical and physical activation helps to enhance the specific
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activation, and during the study the concentration of H2SO4 surface area and create porosity. Table 1 summarises a few
was varied. TEM and SEM results showed that the graphitisa- examples of materials prepared via HTC in combination with
tion level was very much influenced by the acid concentration. templating strategies or activation processes as well as their main
In this way, the sample without H2SO4 presented a typical porosity features (surface area and total pore volume).
amorphous carbon structure while the one with the highest
H2SO4 concentration possessed three-layer structures with a
clear interlayer spacing of 0.38 nm, indicating the graphene- 3. Heteroatom doped carbon
like morphology. The samples were characterized by a micro– materials
mesoporous structure, with a high number of micropores. Shi
et al.129 prepared ACs starting from glucose and glucosamine, As described in the previous section, HTC can be successfully
in parallel, and activation with KOH. During the synthesis, used in the production of porous carbon materials using
they produced the hydrothermal carbon via HTC followed by renewable resources. Furthermore, due to the versatile charac-
chemical activation, as a subsequent step. The resulting ter of this technique, different strategies can be used in
materials possessed a porous structure, regardless of the tandem in order to boost the properties of the final materials,
carbon precursor, and the microporosity was observed only as presented in section 2.2. Although the textural properties
after KOH activation. The ACs were characterized by large promote this type of material as a good candidate for appli-
surface areas, between 778 m2 g−1 and 1513 m2 g−1, depending cation in gas storage fields or water treatment experiments,
on the activation parameters. ACs with micro–mesoporous sometimes its chemical and physical properties are limited,
structures have been prepared by Fuertes et al.,95 starting from making it a second option candidate for applications such as
potato starch and eucalyptus sawdust, via HTC at 250 °C, and electrocatalysis or energy storage. A very common method to
activation with KOH and melamine. The powders had a overcome this consists of functionalization of the carbon struc-
micro–mesoporous bimodal structure. It was observed that ture with various heteroatoms, such as nitrogen,105,136
melamine has the ability to extend the pore size distribution boron,137,138 sulphur121,139 and phosphorus.140 This brings
range of the activated carbons, generating mesopores.95 Again, some advantages including improved catalytic performance
independent of the biomass source, the surface areas were and better selectivity for different reactions, such as the
improved by the melamine incorporation, reaching a oxygen reduction reaction, and higher adsorption capacity
maximum of 3420 m2 g−1, and 2.37 cm3 g−1 for pore volume. when it comes to gas storage and water treatment.
Boyjoo et al.98 reported the synthesis of ACs starting from
Coca-Cola via HTC, followed by chemical activation with 3.1. Nitrogen-doped carbon materials
ZnCl2, denoted as CMC_1 and CMC_2 (mass ratio of ZnCl2/ Nitrogen-doped carbon materials (NCs) have received reco-
HTC carbon = 1 and 3, respectively), and with KOH, denoted gnition since the earlier studies, due to their remarkable
as CMC_3 (KOH/HTC carbon = 4). Surface area measurements chemical and physical properties. The hydrothermal carbon is
revealed the formation of microporous structures. The nonacti- highly improved by N incorporation due to the following
vated sample presents a moderate surface area, 405 m2 g−1, reasons: the nitrogen atoms are more electronegative than
and pore sizes in the micropore domain, maybe due to H3PO4 carbon, due to the two lone pair electrons, and this provides a
present in the biomass. The samples activated with ZnCl2 higher electrochemical activity for the NCs, which makes them
achieve surface areas of about 1082 m2 g−1 and 80% of the potential catalysts for the oxygen reduction reaction and CO2
pore volume consists of micropores. On increasing the amount reduction;87 N-doping effectively increases the electrical con-
of activator, the surface area almost doubles, but the pore size ductivity and creates defects that can provide enough space for
distribution shifts towards mesopores, with the percentage of ion, electrolyte or gas diffusion that make them viable for gas
narrow micropores (<0.8 nm) decreasing down to 29%. Plata storage,141 fuel cells and energy storage.139,142 NCs can have
et al.130 reported the synthesis of ACs prepared through different structures, such as 1-D N-doped carbon nanotubes
H3PO4-assisted HTC, from different biomass sources (sawdust, (NCNTs),143 N-doped nanofibers (NCNFs),144 2-D N-doped gra-
almond shells, hemp residues and coconut shells) and H3PO4 phene (NG)145 or 3-D hierarchical nanostructures with
for pore development. In a typical synthesis the biomass was different dimensions.146 Depending on the N-bonding, four

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Table 1 Surface area and pore volume of various carbonaceous materials prepared via HTC coupled with soft/hard templating or chemical/physical
activation

SBET PV
Sample name Carbon precursor Synthesis method Template/activating agent (m2 g−1) (cm3 g−1) Ref.

NPCSs Phenol- Soft templating-HTC Pluronic F108 1480 0.9 111

formaldehyde
BHCSs Ascorbic acid Trioctylamine@ferrocene-in-water 199 NA 93
emulsion
OHTC-1 Glucose Soft and hard templating- Pluronic P123, TEOS 520 0.40 94
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HTC
HPNC-1 Glucosamine Soft templating-HTC Pluronic P123, 0.5 mmol, pH = 7 550 0.43 96
HPNC-2 hydrochloride Pluronic P123, 0.25 mmol, pH = 2 650 0.54 96
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HPNC-3 Pluronic P123, 0.75 mmol, pH = 2 710 0.54 96

HPNC-4 Pluronic P123, 1 mmol, pH = 2 980 0.78 96
HPC-3 Glucose Pluronic P123, 0.75 mmol, pH = 2 490 0.29 96
Assai-MW-CO2 Assai stone Microwave-assisted HTC CO2 (80 ml min−1) 1100 0.45 131
and physical activation
CNSA_6 Peanut skins H2SO4 assisted HTC and KOH (1 : 1 mass ratio) 1886 1.02 128
chemical activation
HPCS_1.2 Starch HTC-chemical activation (NH4)2Fe(SO4)2 973 0.27 132
AC hydrochar – dry Giant bamboo KOH (4 : 1 mass ratio) 2117 1.14 133
impregnated KOH
Hydrochar – wet 2262 1.30 133
impregnated KOH
BCF Bamboo sticks KOH (6 : 1 mass ratio) 12 — 134
CMC-1 Coca Cola waste ZnCl2 (1 : 1 mass ratio) 1082 0.43 98
CMC-2 ZnCl2 (3 : 1 mass ratio) 1994 0.87 98
CMC-3 KOH (4 : 1 mass ratio) 1405 0.80 98
COSHTC_3 Coconut shell NaOH (3 : 1 mass ratio) 876 0.44 85
AA-0 Potato starch KOH (4 : 1 mass ratio) 3000 1.41 135
AA-2M KOH (4 : 1 mass ratio) and melamine 3280 2.37 135
(2 : 1 mass ratio)
AA-3M Potato starch KOH (4 : 1 mass ratio) and melamine 3220 2.27 135
(3 : 1 mass ratio)
AC-0 Cellulose KOH (4 : 1 mass ratio) 3100 1.46 135
AC-2M KOH (4 : 1 mass ratio) and melamine 3540 2.22 135
(2 : 1 mass ratio)
AS-0 Sawdust KOH (4 : 1 mass ratio) 2690 1.28 135
AS-2M KOH (4 : 1 mass ratio) and melamine 3420 2.30 135
(2 : 1 mass ratio)
AS-3M KOH (4 : 1 mass ratio) and melamine 2990 2.35 135
(3 : 1 mass ratio)

main types of N can be distinguished: pyrrolic N, pyridinic N, Usually, the carbon is functionalized during HTC followed
quaternary N/graphitic N and N oxides of pyridinic N (illus- by high-temperature annealing.148 A wide range of nitrogen
trated in Fig. 17). precursors can be used, including ammonia,149 urea,148,150
melamine,151 nitric acid152 etc. The chemical and physical pro-
perties, pore structure, content and types of nitrogen function-
alities are all influenced by the nitrogen precursors and heat
treatment conditions. Liu et al.153 reported the synthesis of
NCs using natural biowaste as a carbon precursor and mela-
mine as a N source, succeeding in incorporating about 1.75
at% N, according to XPS data. Starting from chitosan and
gaseous NH3, Wu et al.154 proposed the synthesis of NCs via
HTC for energy storage applications and CO2 capture. In this
way, they synthesised two sets of materials: NCs derived from
chitosan and NCs derived from glucose and gaseous NH3. The
nitrogen incorporation was much more successful for the
carbons obtained via the HTC of chitosan, about 4.61 wt% N
was incorporated, and half of it was attached in the pyrrolic
form. When NH3 was used as a N donor, more pyridinic func-
tionalities were reported, with a N content of about 3.58 wt%.
Fig. 17 Schematic structure of N doped carbons; reproduced with per- Using aqueous NH3, Schipper et al.155 reported the synthesis
mission from ref. 147. of NCs via the HTC of glucose, containing about 9 wt%

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N. Zhang et al.156 synthesised NCs with 5.5 at% N, starting 5-HMF, the main intermediate compound formed during
from the HTC of glucose, followed by dry impregnation of the HTC.89
resulting hydrothermal carbon with melamine. Ren et al.157
reported the HTC of NCs derived from microalgae, containing 3.2. Sulphur doped carbon materials
up to 2.89 at% N, coming exclusively from the biomass source. Together with N, sulphur has gained a lot of attention as a
Preuss et al.136 reported the usage of lyophilised ovalbumin dopant for the carbon structure. Sulphur-doped carbons (SCs)
from chicken egg white as a N source for the synthesis of NCs, are widely used as cathodes for Li–S batteries, due to the high
starting from glucose and using HTC. The highest N content theoretical capacity of S (around 1675 mA h g−1).165 Other
obtained in this study was 3.3 at%, which reduces to 2.9 at% applications of SCs include electrodes for supercapacitors,166
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after further activation is applied. Qiao et al.158 reported that energy storage as hydrogen storage media,166 photoactive
the NCs improve the electrocatalytic activity of multiwall materials for light-harvesting and electrocatalysis. SCs can be
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carbon nanotubes towards oxygen reduction reactions. The N usually prepared using two strategies: (i) incorporation of S
was incorporated mostly as graphitic N (57.16%) and pyridinic into the structure of synthetic or commercial carbon using a
N (18.26%), the rest being coordinated with Fe or in an oxi- sulphur precursor by sulphurisation,167 and (ii) heating of mix-
dized form. Alatalo et al.159 performed the synthesis of NCs tures of carbon and sulphur precursors.168,169 Generally, the
starting from glucose and cellulose in the presence of a soy sulphur donor can be any sulphur-containing compound,
bean additive as a structure directing agent and a nitrogen pre- including elemental sulphur, H2S, SO2, CS2, sulphur-contain-
cursor. The resulting materials showed good nitrogen incor- ing organic compounds and polymers.121,139,166,170 Emran
poration: between 1.0 and 3.7 wt% for pristine materials and et al.171 proposed the synthesis of sulphur-doped carbon
0.2–1.9 wt% for calcined carbons. Table 2 shows a few more microspheres (S-MCMS) via a one-pot HTC for the motorisa-
examples of NCs synthesised in the literature. tion of ascorbic acid and lemon juice in food and pharmaceu-
The examples mentioned above are proof that incorporation ticals. S-MCMS were prepared from D-glucose and thiourea as
of nitrogen atoms within the carbon structure using HTC is the S source, followed by annealing at different temperatures
possible, but how nitrogen incorporates into the carbon struc- (700 °C, 800 °C, and 900 °C). The obtained materials had an
ture is still under debate. So far, the Maillard mechanism164 average size of 0.5–5 μM and a highly microporous structure.
has been accepted as playing an important role in the process. The annealing temperature had a high influence on the struc-
The mechanism consists of a group of reactions between redu- ture, and S-MCMS-900 possessed the highest surface area,
cing sugars and amino acids. It is impossible to provide a with the smallest sphere size and more sp2 carbon chain dis-
clear pathway for the formation of NC materials during HTC tortion. Roldán et al.172 synthesised SCs via the HTC of
because throughout these reactions, hundreds of compounds D-glucose activated with ZnCl2 and thiophenecarboxaldehyde
can be formed and react again themselves. The main steps of as the S source. For comparison, NCs and N, S-doped carbon
the Maillard mechanism are described in short in Fig. 18.89 materials have been prepared using the same approach. The
The first products formed during the Maillard reaction are SCs possess C–S–C bonding derived from the thiophenic
glucosamines via a nucleophilic attack of an amine on the group and a small amount of S was linked to Zn, forming ZnS
aldehyde of the sugar (Fig. 18, I). The glucosamines are further sphalerite, inhibiting, in this way, the complete removal of the
transformed to a Schiff base, by dehydration (Fig. 18, II) which activating agent. In terms of textural properties, the SCs exhibi-
can, in turn, suffer rearrangements to aminoketones (Fig. 18, ted surface areas around 550 m2 g−1 with a 0.9 cm3 g−1 pore
III and IV). Compound III can form an α-dicarbonyl species volume, and according to XPS results 6.7 wt% of the sulphur
(Fig. 18, V), which after successive dehydration can form was incorporated. In an earlier study, the same author pre-
pared SCs for water treatment applications.168 Using the same
approach, they prepared mesoporous SCs. For comparison,
Table 2 Example of NCs synthesised in the literature
NCs were also prepared. SCs possessed a mesoporous struc-
ture, with 89% mesoporosity and a pore size of approximately
Nitrogen 34 nm, containing a high amount of S atoms incorporated
Sample Nitrogen content onto the carbon structure, 8.3 wt%, which is double that of the
name Carbon precursor source (at%) Ref.
N atoms incorporated, 3.9 wt%.
AG650 Glucose and Microalgae 2.65 127
AG700 microalgal mixtures 1.43 3.3. Boron doped carbon materials
AG750 0.68
N-GCS-2 Peach extract NH4OH 9.33 160 Besides nitrogen and sulphur, boron represents a promising
AMBC-600 Bamboo shoot Melamine 12.9 161 dopant for carbon materials. The advantage of boron incorpor-
AMBC-700 6.2 ation lies in the fact that it creates defects, by introducing an
AMBC-800 2.7
HPCS-1.2 Starch (NH4)2Fe 2.3 132 uneven charge distribution which enhances the electro-
(SO4)2 chemical performance by improving the charge transfer
H-N400 Sucrose (NH4)2SO4 6.57 162 between neighbouring carbon atoms.173 Also, the incorpor-
H-N600 7.33
H-N800 6.54 ation of boron could improve the conductivity of carbon
GN-700-4 Glucose Urea 6.20 163 materials by increasing the density of hole-type charge car-

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Fig. 18 Some examples of the Maillard reaction; reproduced with permission from ref. 89.

riers.174 All these improvements make them good candidates ture, with surface areas between 73 m2 g−1 and 362 m2 g−1 and
for different applications, such as energy storage175 and boron content between 0.05 and 0.19 wt%.
electrocatalysis.144,173,174 Sun et al.176 reported the synthesis of
boron-doped porous graphitic carbon materials (BCs) derived 3.4. Phosphorus doped carbon materials
from chitosan (BNGC-2-900). The resulting hydrothermal Phosphorus (P) is another important heteroatom that gained
carbon was chemically activated using ZnCl2 followed by car- its place on the list of dopants for the carbon structure. P
bonisation at elevated temperatures (900 °C). The produced doping can bring a lot of advantages to the carbon structure,
BCs had high surface areas, up to 1567 m2 g−1, and a pore such as improving electrochemical stability by suppressing the
volume of about 0.48 cm3 g−1. XPS results revealed the pres- formation of electrophilic oxygen species and inhibiting the
ence of carbon, nitrogen (4.99%), boron (3.47%) and oxygen combustion of the oxygen species, which consequently also
atoms. The successful doping of the materials with boron was contributes to enhancing the cycling stability, capacity reten-
confirmed by the presence of peaks from the C 1s spectrum tion ratio and operating voltage window for capacitors.140
corresponding to C–B (283.4 eV), C–C (284.7 eV), C–N (285.6 Regarding their synthesis, P doped carbon materials (PCs) can
eV), C–O (286.1 eV), CvO (286.4 eV), and C–O–B (288.7 eV). be prepared either in a two-step synthesis of mesoporous
Incorporation of boron atoms in the hydrothermal carbon can carbon followed by P incorporation,177 or through a one-pot
affect both the morphology and particle size, according to a HTC.178 Li et al.177 reported the synthesis of P and N co-doped
study conducted by Kalijadis et al.138 For their synthesis, carbon microspheres following a two-step strategy using
different concentrations of boric acid (0.1, 0.2, 0.6 and 1 wt%) D-glucose and (NH4)HPO4. Both doped and undoped materials
were mixed with D-glucose and HTC was performed at 180 °C present a spherical morphology with a smooth surface, as
for 24 h. For the undoped sample, the particles present a shown by SEM (see Fig. 19a and b). The heteroatom incorpor-
uniform diameter of about 3 µm and a few particles were ation was confirmed by XPS measurements, with P–O, P–C and
about 7 µm. By incorporation of boron atoms, the particle size C–N bonds being identified. Quantitative XPS analysis revealed
is increased, up to 20 µm, and the degree of homogeneity is that P was incorporated at ∼1.19 at% and N at 1.00 at% in
decreased. This enlargement phenomenon is attributed to the NPCM.
catalytic effect that boric acid could have during the HTC, by Wu et al.178 synthesised PCs starting from glucose and tet-
effective conversion of D-glucose to produce 5-HMF. Based on raphenylphosphonium bromide (C24H20P(Br)) via HTC com-
textural characterisation, BCs consist of a microporous struc- bined with a soft templating method. The mixture was treated

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Fig. 19 SEM images of (A) CM and (B) NPCM; reproduced with permission from ref. 177.
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at 180 °C for 4 h during HTC, followed by template removal at 4. Applications

800 °C for 2 h, under an inert atmosphere. During the syn-
thesis, the amount of P donor was varied, and the resulting Porous carbon materials have been used in multiple appli-
samples were denoted as P-CHS-1, 2 and 3, with the increasing cations since long ago. There is clear evidence of the usage of
number referring the increment of the P donor. For compari- ACs along the years, for example in ancient times charcoal was
son, undoped samples were also prepared. P incorporation used to adsorb unpleasant odours or to clean water. Later, in
was examined by FTIR: when comparing CHS and P-CHS-2, 1773, AC was mentioned as an adsorptive material in a gas
similar peaks are identified, at 3060 cm−1 for aromatic C–H experiment by Scheele. In 1862, Frederick Lipscombe rinsed
stretching vibrations and aromatic C–H out of plane defor- activated carbon for commercial applications, by using the
mation vibrations below 900 cm−1, aliphatic C–H stretching material to purify potable water, and in 1881 Heinrich Kayser
vibrations at 2857 and 2924 cm−1 and C–H scissoring mentioned the ability of charcoal to take up gases. ACs were
vibrations at 1453 cm−1. This is clear evidence that the HTC of implemented at the industrial scale at the beginning of the
D-glucose is not influenced by P doping. The CvC band is twentieth century, by Chemische Werke, and during the First
present in both samples, with vibrations at 1588 cm−1, and World War it was used in gas masks by American soldiers to
both contain O–H (3440 cm−1), CvO (1705 cm−1) and C–O protect them from poison gas.180–183 Due to a series of advan-
(1278 and 1026 cm−1) bands. Among all these peaks, P-CHS-2 tages, such as the low cost of the raw materials, the high
shows a few more, at 1109 cm−1 assigned to P–O combination, degree of microporosity,127 high surface areas (1000 m2
at 1085 cm−1 assigned to the ionized linkage P+–O− or P–O–C, g−1),184 and their good chemical and thermal stability, porous
and two vibration bands at 725 and 682 cm−1, due to P–C, carbon materials are used in a wide range of applications, as
which confirms the doping of P in the carbon. Guo et al.179 described below.
prepared phosphorus-doped carbon nanotubes (PCNTs). For
this, the mixture was heated at 170 °C for 12 h. Compared to
4.1. Energy storage
raw CNTs, PCNTs show a lower surface area and a smaller pore
volume of pores, due to the removal of highly disordered Hydrothermal carbons represent a useful and sustainable
amorphous carbon regions. The functional groups on the class of materials for use in energy storage. They have been
surface of PCNTs were investigated by XPS. The amount of P extensively explored as graphite replacements in lithium-ion
was about 1.66 at% and O was about 6.98 at%. Overall, the –P– batteries and have enabled the development of sodium-ion
O groups were dominant, and they formed via the acid reac- technologies which have been previously inaccessible, as well
tion with the –OH groups from the edge of the sp2 nanotube as further the development of supercapacitor electrodes and
layer. The main P containing groups were –PvO and –P–C. fuel cells.
4.1.1. Batteries. Li-ion batteries are a well-established and
widely available commercial technology, used extensively as
3.5. Summary power sources for portable devices such as mobile phones and
Heteroatom doped carbon materials showed their potential in laptops, power tools, electric vehicles, and many other consu-
a wide range of applications, but they seem a “go to” option in mer products, since they are lightweight and have a high
fields such as electrocatalysis for the ORR or CO2 reduction energy density. Carbon is the most commonly used anode
and supercapacitors. As presented above, HTC represents a material, traditionally in the form of graphite, which is natu-
useful method for the synthesis of NCs, SCs, BCs or PCs either rally occurring, abundant and inexpensive. However, recent
in one pot or two step reactions. The advantage of HTC over developments have been made in the use of hard carbons for
other synthesis methods lies in the possibility of being able to electrode materials, which are able to achieve significantly
use sustainable carbon precursors and any source to dope the higher capacities, and show an improved rate capability,
heteroatom. Also, HTC implies a small energy consumption cyclability and efficiency. Furthermore, the field of sodium-ion
and the process itself is easy and eco-friendly. batteries is growing rapidly and suitable anodes are also

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necessary, since graphite cannot be used in an analogous way. Hard carbons exhibit good capacity retention, but often
Hard carbons have also been found to be effective anodes for poor rate capability; conversely, pure soft carbons have
sodium-ion batteries, with a rapidly growing body of literature. superior rate capability, but suffer rapid capacity fade.
HTC is an appealing route for obtaining useful carbon Combining the two can result in a composite anode with
materials from cheap and abundant biomass; for example, improved electrochemical properties. Hard–soft composites
holly185 and ginkgo leaves,186 sugarcane,187 sweet potato,188 prepared for K-ion batteries exhibited a higher rate capability
pollen,189 peanut skin128,190 apricot shell,191 and corn stalks192 than the pure hard carbon (81 mA h g−1 vs. 45 mA h g−1 at 10
have all been converted hydrothermally for use in energy C) and superior cycling stability compared to pure soft carbon
storage applications. Several studies have compared hydrother- (93% vs. 55% after 200 cycles at 1 C).196 The hard carbon pro-
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mally treated precursors against directly pyrolysed biomass vided a porous and robust structure to accommodate the inter-
and found that those materials which have first undergone calation of potassium ions, whilst the conductivity offered by
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HTC function significantly better as electrodes in lithium- or the soft carbon regions facilitated fast ion transport to main-
sodium-ion batteries. Wang et al.190 attributed the larger tain performance at high current rates. Similarly, hard carbon
initial capacity achieved in their hydrothermally treated spheres with a soft carbon coating displayed improved initial
peanut skin to the presence of significant micropores and coulombic efficiency (54% to 83%) and a capacity retention of
mesopores, and showed that a higher fraction of mesopores 93% after 100 cycles, when tested as anodes in Na-ion bat-
was desirable for fast ion diffusion channels. In contrast, two teries.197 In addition, they found that increasing the carbonis-
reports by Zheng et al. comparing HTC and direct pyrolysis of ation temperature from 1000 °C to 1600 °C could significantly
holly leaves185 and sweet potato188 found that the greater increase the plateau capacity, hypothesising that this effect
microporosity introduced by HTC played an important role in was due to the larger number of nanopores at higher tempera-
the improved electrochemical performances. The discrepan- ture. The direct addition of graphene derivatives is another
cies between these studies likely arise from the intrinsic differ- common approach for synthesising hard/soft composites;
ences in the nanostructure of the starting biomass. The often graphene/graphite oxide is combined with the hard
storage mechanism of sodium in hard carbons is still poorly carbon precursor prior to HTC, which serves to reduce the gra-
understood, and hydrothermally treated biomass was used as phene sheets in situ.187,198 In these studies, the enhanced
the precursor for tailored structures in these fundamental sodium storage capacity and cycling stability of the composite
studies. Hard carbons derived from reed straw and carbonised were ascribed to the complementary effect of the different
at 900–1500 °C exhibited variations in graphitic domains and morphological characteristics of the carbons which provided
microporosity, resulting in different contributions to both the ample sodium storage in the micropores and large interlayer
sloping and plateau capacity regions.193 Sodium storage in the spaces, minimised volume variation during cycling and
sloping region was attributed to adsorption in the micropores ensured sufficient contact area with the electrolyte for efficient
and defect sites, and Na+ intercalation into the graphite-like diffusion and charge transport.198 Aside from pure carbon
layers in the plateau region (Fig. 20 and Table 3). materials, various carbon/inorganic composites have been
Hydrothermally prepared shaddock peel was used to inves- explored as electrode materials.195,202,203 For Li-ion batteries,
tigate the relationship between capacity and disordered turbos- elements such as Si and Sn are promising since lithium can
tratic nanodomains, with the resulting carbons exhibiting form alloys with either in a ratio of 4.4 : 1, resulting in a signifi-
mainly sloping capacity.199 The main mechanism of sodium- cantly higher theoretical capacity (e.g. Li4.4Si 4200 mA h g−1 vs.
ion storage was attributed to the balance of charge conduc- 372 mA h g−1 for LiC6 in graphite). Unfortunately, formation
tivity, the sodium-ion diffusion rate, and the number of of these alloys is generally accompanied by severe volume
adsorption sites. Well-defined precursors, such as glucose194 changes, leading to rapid capacity fade with cycling.
or phenolic resin,200 are also often used for systematic studies. Furthermore, some of the alloy species are soluble in the elec-
For phenolic resin carbonised at 800–1500 °C, it was found trolyte solution and thus the anode gradually decomposes. In
that increasing carbonisation temperatures decreased the addition, such materials on their own have low intrinsic con-
d-spacing between graphitic layers, and promoted the shift ductivity which can result in overall poor performance. In
from microporosity to mesoporosity in the materials, resulting several studies, the combination of these inorganic particles
in a decrease in surface area and total pore volume.200 The with hard or soft carbons serves to stabilise volume changes
material carbonised at intermediate temperature (1250 °C) and improve conductivity, resulting in superior cycling per-
showed the best reversible capacity which was attributed to the formance. A comparison between hard carbon-coated SnO2
balance between optimised d-spacing and the pore structure. and a mechanically mixed composite demonstrated that the
Interestingly, the material carbonised at 1500 °C had the carbon coating could significantly stabilise capacity fading.191
highest first cycle coulombic efficiency (62.5%), due to the Jeong et al.201 produced silicon coated hard carbon attached to
lower surface area resulting in less significant SEI formation a graphite scaffold; the overall reduced contact area with the
(Fig. 21a), whilst the material carbonised at a lower tempera- electrolyte served to suppress SEI formation, whilst the graph-
ture (1000 °C), despite being less electrically conductive, ite facilitated electron transport. The report further showed
exhibited a better rate capability at higher C rates, arising from that the electrode thickness and amount of binder strongly
the wider d-spacing (Fig. 21b). affected the lithiation capacity; particularly in the case of alloy

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Fig. 20 LHS: (a) Plateau and sloping curves of RS-x electrodes. (b) Amount and contribution rate of capacity from the plateau and sloping region in
RS-x electrodes; capacity contributed at 0.1–4 C by (c) sloping regions and (d) plateau regions. RHS: Schematic illustration of the sodium storage
mechanism of RS-x electrodes. Reproduced with permission from ref. 193.

materials, the binder content can additionally stabilise volume cursor served to stabilise the electrode during long cycles and
expansion and contraction on cycling, an important factor at high current densities.192 Hydrothermally prepared MoS2
which is not often discussed in the literature. Similarly, Zhang was combined with PVP and SnO2 and it also showed excellent
et al.204 supported SnO2 particles on rGO and used the growth cycling stability at high current densities for both Na and K
of hard carbon on the surface to limit the particle size, result- storage206 attributed to the sandwich structure formed by the
ing in a uniform size distribution. Good contact between SnO2 MoS2 and carbon coating. Well-aligned nanosheets of SnS2
particles and the hard carbon-rGO scaffold was achieved by C– have been grown hydrothermally on a carbon cloth (Fig. 22),
O–Sn covalent bonds which enhanced conductivity, whilst the and when tested as an anode for sodium-ion batteries they
rGO/hard carbon matrix prevented agglomeration of SnO2 and exhibited enhanced electrochemical properties in comparison
limited the volume expansion to improve cycling stability. with pure SnS2.205 The enhanced behaviour was attributed to
Carbon/transition metal oxide and dichalcogenide electrodes the introduction of the carbon cloth substrate, which provided
are also being developed for energy storage.207,209 An electrode the contact area for both the electrolyte and active material
material derived from avocado and MnO, coated with alumina and enabled rapid Na-ion transport, improved electrical con-
for stability, was tested in a Li-ion cell, and the high capacity ductivity, and relieved aggregation and pulverisation of the
and good rate capability (Table 2) were explained by the struc- SnS2 nanosheets.
ture inherited from the avocado precursor which provided a Cathode materials have also been developed using similar
good buffer for volume expansion, and the Al2O3 coating with methodology, for example, the hydrothermally assisted syn-
greater ionic conductivity.210 In another study with MnO-gra- thesis of Na7V4(P2O7)4(PO4)/C nanorods, which are formed by
phene nanopeapods, structural stability was introduced with a simultaneous crystallisation of Na7V4(P2O7)4(PO4) and in situ
graphene scaffold, which also enhanced electrical conduc- carbonisation of surfactants on the surface.212 The carbon
tivity, resulting in high Li storage capacity.211 For Na-ion bat- formed as a thin layer around the nanorods, and limited par-
teries, analogous carbon-supported MoS2 materials showed ticle growth, resulting in a uniform particle size. With an
promising performance, in which MoS2 nanosheets were verti- aqueous electrolyte, a capacity of 51.2 mA h g−1 was achieved
cally grown on cotton-derived carbon fibres which act as a at a current density of 80 mA g−1, and 72% of the initial
robust conductive support and result in greatly enhanced capacity was maintained even at 1000 mA h g−1. The hydro-
cycling stability. Even at high current densities, much higher thermal production of a Na3V2O2x(PO4)2F3−2x/carbon compo-
capacities could be maintained, compared to the capacity site further demonstrated the importance of the conductive
retention of bare MoS2 nanosheets. In a similar approach, carbon for overall cell performance.213 In a Li–S system, a com-
MoS2 nanosheets were grown on the surface of carbonised posite electrode of nickel, sulfur and carbon derived from
corn stalks, whereby the intrinsic structure of the biomass pre- bamboo exhibited a high initial capacity (1198 mA h g−1 at 0.2

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Table 3 Electrochemical performance of carbon and composite battery anodes obtained by HTC. FCCE: first cycle coulombic efficiency

Reversible
Hydrothermal Carbonisation capacity FCCE
Precursor conditions temperature Activation Template (mA h g−1) (%) Application Ref.

Glucose 180 °C, 24 h 1100 °C (Ar), 800 °C — — 160 at 50 mA — Na-ion 194

(H2) g−1
Glucose 180 °C, 6 h 800 °C (N2), 300 °C — Na2MoO4 250 at 50 mA 50 Na-ion 195
(air) template g−1
Sugar/ 195 °C, 5 h 1100 °C (HTC — — 261 at 28 mA 67 K-ion 196
perylenetetracarboxylic glucose), 900 °C (HTC g−1
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dianhydride (PTCDA) glucose + PTCDA)

Sucrose, toluene 190 °C, 5 h (sucrose) 1600 °C (HTC sucrose — — 312 at 30 mA 83 Na-ion 197
+ toluene) g−1
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Sucrose, graphite oxide 190 °C, 12 h 700 °C — — 296 at 50 mA 44 Na ion 198

g−1
Sugarcane bagasse, 150 °C, 4 h 750 °C (treatment KOH — 617 at 200 mA 56 Li-ion 187
graphene oxide and activation of g−1
sugarcane)
Holly leaf 180 °C, 10 h 800 °C — — 318 at 20 mA 50 Na-ion 185
g−1
Ginkgo leaf 250 °C, 12 h 700 °C KOH — 200 at 200 mA 31 Na-ion 186
g−1
Sweet potato 160 °C, 10 h 800 °C — — 320 at 100 mA 33 Li-ion 188
g−1
Rape pollen grains 180 °C, 24 h 600 °C — — 145 at 100 mA — Na-ion 189
g−1
Peanut skin 180 °C, 12 h 900 °C KOH — 681 at 1000 mA 45 (Li), Li-ion, Na- 128
g−1 (Li), 302 at 27 ion
100 mA g−1 (Na)
(Na)
Peanut skin 180 °C, 24 h 800 °C KOH — 431 at 100 mA 34 Na-ion 190
g−1
Apricot shell 200 °C, 40 h 1000 °C — — 184 at 0.1 C 72 Na-ion 191
Reed straw 200 °C, 24 h 1300 °C — — 372 at 25 mA 77 Na-ion 193
g−1
Shaddock peel 180 °C, 24 h 600 °C — — 287 at 50 mA 57 Na-ion 199
g−1
Phenolic resin 500 °C, 12 h 1250 °C — — 311 at 20 mA 60 Na-ion 200
g−1
Nano-Si, sucrose, graphite 190 °C, 6 h 1000 °C — — 879 at 30 mA 81 Li-ion 201
g−1
Sn, glucose 180 °C, 5 h 500 °C — — 452 at 0.5 C — Li-ion, Na- 202
(Li), 487 at 0.5 ion
C (Na)
Polyethylene glycol (PEG), 100 °C, 6 h 600 °C (carbon — Crab-shell 298 at 100 mA 40 Li-ion 203
SnCl2 template calcination, template g−1
H2/Ar)
Graphite oxide, glucose, 180 °C, 12 h — — — 1468 at 80 mA 64 Li-ion 204
SnCl4 g−1
Carbon cloth, SnCl4, 160 °C, 16 h — — — 1040 at 200 mA 33 Na-ion 205
thioacetamide g−1
SnCl4, apricot shell 180 °C, 20 h (SnO2 500 °C — — 288 at 0.1 C 50 Na-ion 191
synthesis), 200 °C,
20 h (SnO2-
hydrocarbon)
Na2MoO4, CH3CSNH2, poly- 180 °C, 12 h (MoS2), 500 °C (MoS2 + SnO2 — — 396 at 100 mA 68 Na-ion, 206
vinylpyrrolidone (PVP), 120 °C, 10 h (MoS2 + + carbon) g−1 (Na), 312 at (Na), K-ion
SnCl4 SnO2 + carbon) 50 mA g−1 (K) 73 (K)
Corn stalks, Na2MoO4, 180 °C, 24 h (MoS2 − 600 °C (corn stalks), — — 1231 at 100 mA 72 Li-ion 192
thiourea carbon) 650 °C (carbon + g−1
MoS2)
Loofah sponges, graphene 160 °C, 6 h (loofah), 400 °C (HTC loofah + — — 838 at 200 mA 55 Li-ion 207
oxide, (NH4)6Mo7O24, 200 °C, 24 h (HTC GO), 600 °C 2 h (HTC g−1
thiourea loofah + GO + MoS2) loofah + GO + MoS2)
Cotton, (NH4)6Mo7O24, 220 °C, 24 h 80 °C (H2/Ar) — — 444 at 100 mA 82 Na-ion 208
thiourea g−1
Cotton, (NH4)6Mo7O24, 220 °C, 24 h 80 °C (H2/Ar) — — 444 at 100 mA 82 Na-ion 208
thiourea g−1
Citric acid, (NH4)6Mo7O24, 180 °C, 5 h (MoO2 + 600 °C (carbon — NaCl 367 at 100 mA 47 Na-ion, 209
ascorbic acid carbon) template, H2/Ar), g−1 (Na), 213 at (Na), K-ion
400 °C (MoO2 + 50 mA g−1 (K) 45 (K)
carbon)
Avocado, KMnO4, 180 °C, 24 h 600 °C (carbon + — — 1165 at 150 mA 67 Li-ion 210
trimethylaluminium (avocado) MnO) g−1
KMnO4, benzoic acid 160 °C, 10 h 1200 °C — — 1168 at 50 mA 86 Li-ion 211
g−1

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Fig. 21 (a) Cycle performances of phenolic resin-based hard carbon sphere electrodes at 100 mA g−1. (b) Rate performances of phenolic resin-
based hard carbon sphere electrodes. Reproduced with permission from ref. 200.

C), good cycling stability (1030 mA h g−1 after 200 cycles) and where specific surface areas up to 3000 m2 g−1 were
an improved rate capability.134 These improvements were achieved222 (Table 4). Typical capacitances for these materials
attributed to the structural stability and high conductivity are in the range 200 to 350 F g−1, at moderate current densities
arising from the Ni particles embedded in the carbon frame- of 0.1–1 A g−1. The introduction of mesoporosity using ZnCl2
work. Lastly, peanut shell has been used as both a cathode is also beneficial for ion diffusion with an activated hydrother-
and anode for a Na-ion hybrid capacitor;214 hydrothermal mally carbonised granulated sugar achieving a capacity of 140
treatment of the peanut shell for the cathode resulted in a F g−1.223 Another study found that templated starch, using
hierarchically porous material with a sheet-like morphology ammonium ferrous sulfate as the porogen,132 outperformed
and a surface area comparable to that of graphene materials its unmodified counterparts because the hierarchically porous
(2396 m2 g−1) and high levels of oxygen doping (∼13 wt%). The structure and large SSA contribute a large part to the capaci-
assembled Na-ion capacitor yielded specific energies of 201, tance mechanism. Yeast cells have also been used as a biologi-
76 and 50 W h kg−1 at specific powers of 285, 8500 and 16 500 cal template to synthesise nitrogen-doped porous hollow
W kg−1, respectively, and achieved 88% capacity retention after carbon spheres, followed by in situ deposition of MnO2.224 The
100 000 cycles at 51.2 A g−1. resulting composite exhibited an ultrahigh specific capaci-
4.1.2. Supercapacitors. While batteries can deliver high tance (Table 4) and when assembled in an asymmetric capaci-
specific energy, supercapacitors are a high-power density tor the maximum energy density operating at a 2.0 V voltage
means of electrochemical energy storage with a long cycle life window was 41.4 W h kg−1 at a power density of 500 W kg−1
and a wide operational temperature range, currently unattain- and still maintained 23.0 W h kg−1 at a power density of 7901
able with Li-ion batteries. However, their comparatively low W kg−1 (Fig. 23). Porosity may also be influenced by the
energy density (<10 W h kg−1)215 restricts their use to a few solvent; a comparison of ionothermal (ICT) and hydrothermal
seconds of charge/discharge, thus limiting their more wide- carbonisation on jujun grass218 showed that the material
spread use for energy harvesting applications. There are two derived from ITC exhibited a hierarchical pore structure of
main classes of supercapacitors: electric double layer capaci- micro- and mesopores, not found in the HTC product.
tors (EDLCs) and pseudocapacitors. EDLCs store charge via Accordingly, the ITC carbon had a high specific capacitance of
electrostatic adsorption of electrolyte ions on the surface of 336 F g−1 at 1 A g−1, whilst the HTC product only reached 220
conductive porous electrodes, whereas pseudocapacitors store F g−1.
energy through redox reactions at the interface between the In addition, the introduction of inorganic nanoparticles to
electrode and electrolyte. In general, pseudocapacitors exhibit carbon225,226 or doping with heteroatoms can introduce comp-
a higher energy density than EDLCs, but carbon-based lementary pseudocapacitive storage mechanisms, thereby
materials for EDLCs have superior cycling stability and rate improving the overall performance of the material.
capability, and so can be operated at higher charge and dis- Nitrogen,216,218,222,227 oxygen,219,227 sulfur,186 and iron222 have
charge rates and have much longer lifetimes.215 Hierarchical all been incorporated into HTC materials, with the aim of
pore structures enable ion mass transport, and therefore improving capacitance (Table 4). Nitrogen is a common
porous carbon materials are an important material for super- dopant, often arising as a natural component of the precursor.
capacitor electrodes. HTC materials generally show low poro- Interestingly, a comparison of hydrothermally carbonised
sity and require further activation after treatment. To increase glucose and glucosamine216 showed that after activation with
the surface area, KOH activation was applied to HTC carbons KOH, the surface area of HTC glucose was higher than that of
from many different biomass sources,216–221 as well as basic HTC glucosamine (1766 vs. 1238 m2 g−1), but that there was no
biomass building blocks such as glucose and glucosamine significant difference in their capacitances (280 and 287 F g−1,

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Fig. 22 (a) Larger-magnification SEM images of SnS2 with a flower-like feature. (b) Typical low-magnification SEM image of SnS2/CC. (c) and (d)
Larger-magnification SEM images of SnS2 on CC showing the well-aligned nanosheet architecture. (e) EDS exploration of SnS2/CC. (f ) SEM image of
the marked region of SnS2/CC and relevant to EDS plots of (g) Sn and (h) S. (i) TEM and ( j and k) HRTEM images, with inset showing the FFT pattern,
and (l) the corresponding SAED pattern of the SnS2 sample. Reproduced with permission from ref. 205.

respectively, at 0.2 A g−1). The discrepancy was ascribed to the 0.5 A g−1, and excellent cycling stability.227 The enhanced per-
presence of pyridone and pyridine groups which contribute formance compared to that of commercial activated carbons
pseudocapacitance, and of pyrrole groups which improve elec- was attributed to the presence of heteroatoms which provide
tron mobility and electron transfer reactions in the carbon additional pseudocapacitance, as well as improving electrical
matrix. Nitrogen- and oxygen-doped hydrothermally carbo- conductivity and wettability, enhancing electron and ion trans-
nised tobacco rods showed a high capacitance of 286 F g−1 at portation. Similarly, N-doping in hydrothermal lignin

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Table 4 Electrochemical performance of carbon and composite supercapacitor electrodes obtained by HTC

Carbonisation Activation/ Capacitance Capacitance

Precursor Hydrothermal conditions temperature template (F g−1) retention Ref.

Ginkgo leaf 250 °C, 12 h 700 °C (activation) KOH 350 at 1 A 98% after 186
g−1 30 000 cycles at
5 A g−1
Rye straw 240 °C, 24 h 750 °C KOH 229 at 1 A — 216
g−1
Hemp stems 160 °C, 12 h 600 °C (pre- KOH 318 at 0.1 A 96% after 217
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activation), 800 °C g−1 10 000 cycles at

(activation) 4 A g−1
Jujun grass 180 °C, 10 h (ionothermal) 900 °C (activation) KOH 336 at 1 A 87% after 2000 218
g−1 cycles at 1 A g−1
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Stem bark 130 °C, 2 h, KOH 700 °C KOH 320 at 0.5 A 94% after 219
g−1 10 000 cycles at
2 A g−1
Rice husk 180 °C, 10 h, NaOH 500 °C (pre- KOH 302 at 1 A 89% after 5000 221
activation), 800 °C g−1 cycles at 10 A
(activation) g−1
Tobacco rods 200 °C, 12 h 800 °C KOH 287 at 0.5 A 96% after 227
g−1 10 000 cycles at
5 A g−1
Starch 200 °C, 8 h 800 °C Ammonium 248 at 0.5 A 93% after 132
ferrous sulfate g−1 10 000 cycles at
(NH4)2Fe(SO4)2 2 A g−1
Lignin 200 °C, 24 h 800 °C KOH 312 at 1 A 98% after 220
g−1 20 000 cycles at
10 A g−1
Glucose 200 °C, 24 h 700 °C KOH 247 at 0.2 A — 222
g−1
Glucosamine 200 °C, 24 h 700 °C KOH 250 at 0.2 A — 222
g−1
Granulated sugar 200 °C, 24 h 700 °C (activation), ZnCl2 136 at 0.1 A — 223
800 °C (H2 reduction) g−1
Lignin, graphene oxide 240 °C, 16 h — — 190 at 0.5 A 87% after 228
g−1 10 000 cycles at
10 A g−1
Glucose, graphene oxide 200 °C, 16 h 800 °C KOH 223 at 0.1 A 87% after 229
g−1 10 000 cycles at
1 A g−1
Graphene foam, polyvinyl 190 °C, 12 h 800 °C KOH 188 at 0.5 A 100% after 230
alcohol (PVA), g−1 10 000 cycles at
polyvinylpyrrolidone (PVP) 2 A g−1
Distillers dried grains with 200 °C, 18 h 600 °C (activation), KOH 102 at 1 A 92% after 5000 231
solubles, graphene oxide then microwave for g−1 cycles at 5 A g−1
30 s at 1200 W
Glutaraldehyde, KMnO4 180 °C, 5 h (yeast + 850 °C (hollow Yeast cells 255 at 1 A 89% after 5000 224
glutaraldehyde), 160 °C, carbon) g−1 cycles at 10 A
3 h (hollow carbon + g−1
MnO2)
Carbon cloth, Co(NO3)2, 150 °C, 6 h (CoMoO4 + 350 °C (CoMoO4 + — 2024 at 1 A 91% after 5000 225
Na2MoO2, Ni(NO3)2, C6H12N4 carbon cloth), 90 °C, 12 h carbon cloth) g−1 cycles at 1 A g−1
(NiCo-LDH)
NiCl2, Se, graphene oxide, 120 °C, 12 h — — 1280 at 1 A 98% after 2500 226
hydrazine g−1 cycles at 5 A g−1

improved conductivity and surface wettability with electrolyte ric acid-assisted HTC, and KOH activation, a carbon material
ions, whilst the hierarchical bowl-like pore structure and large with a S-content up to ∼8 wt% was obtained, with a hierarchi-
specific surface area (2218 m2 g−1) resulted in a high specific cal porosity, and a specific surface area of 1132 m2 g−1, higher
capacitance of 312 F g−1 at 1 A g−1 and excellent cycling stabi- than those of materials produced without activation or treat-
lity (98% after 20 000 cycles at 10 A g−1, Fig. 24).220 Even at 80 ment with sulfuric acid. Further heating of the sample in air
A g−1, 81% capacitance was retained, likely due to the high to remove sulphur resulted in an even higher surface area of
conductivity and efficient ion transport pathways in the hier- 1757 m2 g−1. In spite of the lower surface area, the S-doped
archical pore structure. material showed the best performance, achieving a specific
Ginkgo leaves have been used as a precursor to sulphur- capacitance of 364 F g−1 at 0.5 A g−1, with an aqueous electro-
doped carbons, and highly porous carbons.186 Following sulfu- lyte. At a high rate of 50 A g−1, the specific capacitance was

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Fig. 23 The electrochemical performances of samples in a 1 M Na2SO4 electrolyte. (a) CV curves at a scan rate of 50 mV s−1. (b) GCD curves at a
current density of 1 A g−1. (c) CV curves at different scan rates. (d) GCD curves at different current densities of MnO2/HCS-30. (e) Nyquist plots of
samples. (f ) Cycle life and coulombic efficiency of MnO2/HCS-30 at a current density of 10 A g−1. Reproduced with permission from ref. 224.

still 230 F g−1 suggesting that sulphur-doping can increase the vs. 153 F g−1 at 0.1 A g−1) and retained 87% capacity after
pseudocapacitance in the presence of sulfone and sulfoxide 10 000 cycles at 1 A g−1. The performance was attributed to
moieties and creates a more polarised surface which facilitates good conductivity and the hybrid cellular structure. In
charge transfer. The addition of graphene derivatives to another work, graphene foam produced by CVD was used as
biomass precursors results in hybrid xerogel structures with a support for a hybrid hydrogel of polyvinyl alcohol (PVA) and
varied morphologies and porosities, depending on the precur- polyvinylpyrrolidone (PVP).230 KOH activation increased the
sor, component ratios, and HTC or activation conditions. In surface area to 2994 m2 g−1, and a gravimetric capacitance of
one study, the addition of GO to lignin during HTC was found 188 F g−1 at a current density of 0.5 A g−1 was achieved. The
to modify the pore structure and increase the specific surface hybrid also showed excellent long-term stability at a current
area, whilst lignin could effectively reduce the GO sheets density of 2 A g−1, with no capacitance loss after 10 000
under hydrothermal conditions, and prevent aggregation of cycles. The performance can be attributed to the highly
rGO sheets by interrupting the π–π stacking.228 rGO improved porous structure and large accessible surface area, which are
electrical conductivity, and provided structural stability, stabilised by the graphene foam, and enable rapid and revers-
enabling long cycling. Similarly, xerogels of hydrothermally ible ion transport. The presence of GO in the HTC of distillers
carbonised glucose-GO followed by KOH activation resulted in dried grains with water-soluble chemicals changed the mor-
cellular structures exhibiting well-connected, continuous and phology of the particles from spheres to flakes.231 Additional
very thin carbon walls (∼5–15 nm) which favoured ionic KOH activation increased the surface area, although it
diffusion and electronic conduction.229 The thickness and con- remained low compared to those of other activated carbons
nectivity of the walls could be altered by varying the amount of (479 m2 g−1). Short microwave irradiation reduced the GO
KOH, and it was found that the carbon xerogel with very thin, sheets to increase the degree of carbonisation and graphitisa-
connected walls and a relatively high graphitic character (from tion. The symmetric cell showed a specific capacitance of 102
a lower amount of KOH) delivered a much higher specific F g−1 at 1 A g−1, whilst 92% capacitance was retained after
capacitance than that of a commercial activated carbon (223 5000 cycles at 5 A g−1. The presence of GO is thought to

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Fig. 24 (a) SEM, TEM and HR-TEM images of hierarchical porous nitrogen-doped carbon showing bowl-like pore structures. (b) CV curves, (c)
charge/discharge profiles, and (d) cycling stability and coulombic efficiency at 10 A g−1 in a 6 M KOH electrolyte. Reproduced with permission from
ref. 220.

promote biomass carbonisation and graphitisation, as well as ducts. It is worth noting that due to the limited electric con-
increasing electrical conductivity. ductivity of the HTC carbon, a follow-up carbonisation process
is usually applied at high temperature under an inert atmo-
4.2. Electrocatalysis sphere (usually under nitrogen or argon at above 700 °C) to
Electrocatalysis plays a crucial role in clean energy conversion further improve the electric conductivity of the carbon matrix
to increase the rate, efficiency and selectivity of the chemical and facilitate the assembly of heteroatom-derived active
transformation.232 This process involves the electrochemical sites.237,238 For example, nitrogen-doped carbon materials were
conversion of water, CO2, and N2 into added-value products produced using natural halloysite as a template and urea as
such as hydrogen, oxygenates and ammonia. Besides, the the nitrogen source.235 A flaky morphology was obtained with
oxygen reduction reaction (ORR) plays an important role in glucose as a precursor, whereas using furfural resulted in rod-
energy conversion devices such as fuel cells and metal–air bat- like structures (Fig. 25a and b). The metal-free electrocatalysts
teries. Its sluggish kinetics is a main barrier for the improve- were tested for the ORR in alkaline aqueous electrolytes, and
ment of the overall efficiency of fuel cell devices. Currently the rod-like catalyst demonstrated a better performance than
commercial catalysts for the ORR are Pt group metals. These the flaky material. Considering similar amounts of N species
metals demonstrated high catalytic activity towards the ORR; and graphitization degrees in the as-produced samples, the
nevertheless they are scarce and easy to degrade especially in superior performance of the rod-like catalysts was attributed to
an acidic electrolyte. Precious metal free nanocarbon with a the higher surface area and larger pore volume which provided
heteroatom dopant are considered as promising electrocata- more active sites, a greater complexity in pore size distribution,
lysts to replace the current commercial Pt group metals. The and a rod-like morphology which facilitated electron transport.
heteroatom dopants include but are not limited to non-metal Compared to a commercial Pt/C (20 wt%) catalyst, the carbon
atoms such as N, P, S, B, and transition metals such as Fe, Co, catalysts demonstrated higher retention in diffusion limiting
Ni, etc.233,234 These heteroatom dopants within the carbon current density (after 3000) cycles and enhanced methanol tol-
matrix induce heteroatom–carbon bonds and the polarization erances (Fig. 25c). When tested as cathodes in a single cell H2/
of these bonds generates active sites on the adjacent carbon O2 anion exchange membrane fuel cell, the rod-like catalyst
matrix and results in a reduced energy barrier towards electro- delivered a peak power density as high as 703 mW cm−2 (vs.
catalysis, while simultaneously increasing the electrical con- 1106 mW cm−2 with the commercial Pt/C cathode catalyst)
ductivity due to the higher concentration of delocalised elec- (Fig. 25d). Efforts have also been made to investigate doping
trons within the π-system.136,159,235–237 with other heteroatoms such as boron, sulfur and
Heteroatom doped carbon materials can be obtained from phosphorus.121,136,239 Carbogels derived from glucose and
direct hydrothermal carbonisation of carbon and heteroatom ovalbumin were produced to study the synergistic effect of
precursors. A template can also be involved during the HTC boron and nitrogen and were proved to generate higher elec-
process to help tune the morphology of the as-obtained pro- tron transfer numbers and lower hydrogen peroxide yields

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Fig. 25 STEM images of (a) FU and (b) GU in transmission electron mode. (c) The LSV curves of GU, FU and the benchmark Pt/C (20 wt%) catalysts
(scan rate = 10 mV s−1, rotation rate = 1600 rpm) obtained in O2 saturated aqueous 0.1 M KOH recorded both before and after 3000 potential cycles
(between 0.6 and 1.2 V). (d) Beginning-of-life H2/O2 anion-exchange membrane fuel cell (AEMFC) performance curves recorded at 60 °C for the GU
and FU cathode catalysts (along with PtRu/C anodes): inlet gas flows were supplied with flow rates of 1 SLPM (83% relative humidity and without gas
back-pressurization); reproduced with permission from ref. 235.

than those observed in purely N-doped systems. The presence with a very low overpotential of 0.35 V to achieve 10 mA cm−2
of sulfur decreased the surface area and nitrogen content in alkaline medium. In the other half of the water splitting
resulting in diminished ORR performance.136 In contrast, a reaction, suitable catalysts are also required for the oxygen
sulfur doping of 5.5 wt% in SiO2-templated mesoporous evolution reaction; in the past, the development of fuel cells
ordered carbons was found to enhance the electrocatalytic was held back due to the slow kinetics of the OER. Non-noble
activity in the ORR in alkaline solution, likely due to the fact metal alternatives for the OER are often based on transition
that the mesoporous structure was retained from the templat- metal oxides, while carbon-based materials have generally
ing method.121 Further study on the N/B/S/P doped carbon been underexplored due to their relatively poor performance.
matrix from the same research group. Nonetheless, hydrothermally prepared transition metal dichal-
SBA-15 templated carbon was doped with different hetero- cogenides have also been prepared for the HER, for instance,
atom precursors at low and high concentrations (Fig. 26a), as the in situ hydrothermal synthesis of MoS2/guar gum carbon
well as a combination of nitrogen/boron, nitrogen/sulfur and hybrid nanoflowers possessed an extremely low onset potential
nitrogen/phosphorus (Fig. 26b).240 The correlation of material of approximately 20 mV, a low overpotential of 125 mV at
properties (surface area and surface chemistry) with the 10 mA cm−2 and a small Tafel slope of 34 mV dec−1, nearly
electrocatalytic activity in an alkaline electrolyte showed that identical to those of the bulk platinum standard.243 The per-
the doping of N via the HTC method is easier than that of B or formance was ascribed to the nanoflower architecture which
P (when comparing the yield of the dopant). N doping into the provided ample active sites; furthermore, strong interactions
carbon matrix resulted in improved catalytic activity and elec- between the MoS2 nanoflakes and guar gum enabled long-
tron transfer numbers, whilst the other dopants resulted in term stability and microstructural integrity, with nearly 100%
similar slightly improved performance possibly by creating activity retention after 2000 cycles. In another study, combin-
defects in the carbon matrix. Despite tremendous efforts in ing MoSe2 with NiSe2 in composite nanowires on a carbon
this area, most metal-free electrocatalysts demonstrate limited fibre paper skeleton resulted in better performance than either
catalytic activity. It is also proved that the influence of trace MoSe2 or NiSe2 alone;244 this observation was explained by the
amounts of metal impurities on the high performance “metal- abundance of active sites resulting from the suppression of
free” electrocatalysts cannot be ignored.238 Non-precious metal restacking of MoSe2 nanosheets, and the high conductivity of
dopants such as Fe, Co, and Ni, in co-existence with non-metal NiSe2 which facilitated the transfer of electrons from the elec-
dopants, was demonstrated to boost the catalytic activity of the trode to the active sites. In addition to the heteroatom dopant
carbon electrocatalyst. These metal–N–C electrocatalysts have in the carbon matrix, the HTC approach also activates carbon
shown comparable catalytic activity, higher stability and toler- cloths by creating oxygen-containing functional groups on its
ance toward MeOH when compared to commercial Pt–C.241 surface using peroxovanadium complexes, which results in a
Bifunctional nanocarbon composites can also be obtained via higher specific surface area and faster electron transfer rate,
HTC by careful design of the nanocarbon matrix and active when compared to pristine carbon cloths.245 The overpotential
sites and tested for the ORR. 2D crystalline carbons were (310 mV) at 10 mA cm−2 of the activated carbon cloth is much
obtained from the HTC of glucose, fructose or cellulose with lower than that of the pristine material and comparable to that
guanine, which played an important role in producing the 2D- of RuO2/C (280 mV), making the carbon cloth a competitive
morphology of the resultant carbon materials.242 The porous non-metal catalyst for the OER. The HTC process is also a
N-doped carbons were not only found to be highly active powerful strategy in the assembly of carbon nanostructures of
towards the ORR but they also showed efficiency for the HER various dimensions (D), with 0D, 1D and 2D nanostructures

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Fig. 26 Combination graph correlating the materials surface area and dopant species with the electrocatalytic activity (alkaline media) for SBA-15
templated carbons (a) doped with different heteroatom precursors in low and high concentrations. (b) Doped with nitrogen/boron, nitrogen/sulfur
and nitrogen/phosphorus. Reproduced with permission from ref. 240.

being able to be assembled into higher 3D structures which activity limited the performance of active sites. However, when
possess a well developed hierarchical morphology to both the electrical activity reached a certain level, active sites play
facilitate mass transfer and accommodate the active sites to an important role.237 Based on this hybrid structure, we
provide superior electrocatalytic conductivity. Highly porous further reported a FeNC electrocatalyst using carbon nano-
and granular Ni–Co nanowires were grown hydrothermally on tubes to provide electrical conductivity. The Fe residues in the
a woven carbon fibre fabric and then coated with a conductive carbon nanotubes coordinate with N in the hybridized HTC
shell by glucose carbonisation.246 The structure of the nano- carbon layer to form highly efficient Fe-N based active sites
wires greatly increased the catalytic surface area delivering an while the HTC carbon layer protects the Fe–N active sites from
overpotential of 302 mV at a current density of 10 mA cm−2. being poisoned by the by-product and therefore highly
The conductive carbon layer not only facilitated electron trans- improve the long-term stability.158 HTC is an environmentally
port throughout the entire electrode but also prevented frag- benign and highly efficient synthesis strategy for electrocatalyst
mentation of the nanowires during the reaction, which trans- production. However, due to the complicated mechanism
lated to an overall increase of the structural integrity and during the process, the structural regulation of the well-
hence a more reliable performance. HTC was also used to defined active site configuration remains to be explored.
prepare 3D hierarchical structures of mesoporous SnO2 Meanwhile, the HTC synthesis of single-atom catalysts would
nanosheets supported on a flexible carbon cloth, which could also be an interesting research direction to further improve the
efficiently and selectively electrochemically reduce CO2 to catalytic performance of the catalyst.
formate under aqueous conditions.247 The electrode exhibited
a partial current density of 45 mA cm−2 at a moderate overpo- 4.3. Heterogeneous catalysis
tential (0.88 V) with high faradaic efficiency (87%), even larger HTC derived carbon materials have been used in catalysis in
than those of most gas diffusion electrodes. The presence of many ways. On their own, they can be used as catalysts, mostly
the SnO2 particles is thought to be the reason why high per- as solid acid catalysts. This is done by introducing strong
formances are achieved due to the fact that they show high Brønsted acidity mainly by the presence of sulfonated groups
selectivity in the reduction of CO2, they present a highly on the surfaces of the materials. Another widespread use of
porous structure which provides a large surface area increasing these materials is as a catalyst support. The tunability of their
the contact surface between the electrode and electrolyte and surface polarity and area facilitates the anchoring of metal
facilitating mass and charge transfer, and the robustness of nanoparticles, which can then be used in different reactions.
their structure allows them to maintain the high stability of Hydrothermal carbon is particularly interesting in this respect
the electrocatalyst during long-term operation. Our group because its properties lead to a decrease in leaching of the
reported a series of graphene–HTC carbon hybrids, in which active species when compared to classic ACs. A last and least
we applied graphene oxide as an electrically conductive sub- studied use of these materials is as templates. Since hydro-
strate, and N-containing biomass derived HTC carbon was thermal carbon can be eliminated by combustion in air at
used to provide active sites (Fig. 27). temperatures that are not too drastic, they can be used as
The sandwich structure managed to decouple the influence structure directing agents.
of electroactivity and active sites by changing the ratio of GO 4.3.1. Sulfonated hydrothermal carbon catalysts. Addition
and HTC carbon. It has been reported that a low electrical of sulfonated groups to HTC synthesised carbon materials give

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Fig. 27 (a) Schematic illustration of the fabrication procedure of 3D catalytic electrodes. (b) SEM images of CCS Ni-Co Nws on carbon fibre fabric.
(c) Polarisation curves of different 3D catalytic electrodes for the OER in 1 m KOH solution; reproduced with permission from ref. 248.

values, and taking into account the amount of sulphur added,

the obvious conclusion is that additional oxygen other than
that of the sulfonated groups was added during the treatment
with sulphuric acid. This may occur by water addition to
double bonds, hydrolysis of furan groups, or addition of ether
Fig. 28 Schematic view of sulfonation of porous carbon. functionalities. The sulfonated material was tested in the ester-
ification of glycerol with acetic, butyric, and caprylic acids and
the catalytic performance was compared to those of commer-
rise to introduction of sulfonic acid groups (Fig. 28) leading to cial sulfonated resins.252 Turnover numbers of glycerol and
the formation of a solid acid catalyst that can be used in cata- acetic acid were in the same range for both the commercial
lytic reactions. These materials can be filtered and easily recov- and hydrothermal carbon materials. Regeneration of these
ered. Such materials are generally made by treatment of acid catalysts usually involves the treatment of spent catalysts
porous carbon with concentrated sulphuric acid at high temp- with acid, cleaving esterified sulphonic groups.253 Pileidis
eratures. Many sulfonated hydrothermal carbon catalysts have et al.254 also prepared such materials (HTC conditions: 230 °C,
been applied in esterification reactions,249–251 mainly in the 24 h) and used them as catalysts for the esterification of levuli-
synthesis of biodiesel. nic acid. In this case, not only glucose but also cellulose and
One such case is the one prepared by Roldán et al.249 After rice straw were used as carbon sources. These sources led to
the catalyst was tested in the esterification reaction of palmitic the formation of materials with 80, 76, and 70% of carbon (for
acid with methanol, a difference in the deactivation of the glucose, cellulose and rice straw, respectively) and were then
catalyst was observed and this depended on the activation sulfonated (80 °C, 4 h) introducing 5–6% of sulphur.
temperature employed for each catalyst. When temperatures Esterification was carried out at 60 °C and after 3 h almost full
lower than 500 °C were used, the deactivation of the catalyst conversion was achieved with the glucose-derived material
was attributed to the formation of sulfonate esters on the with a 97% selectivity toward the ester. The second best per-
surface while for those treated at higher temperatures it is formance was recorded for the material prepared from rice
speculated that accumulation of reactants and products in the straw with 92% of both conversion and selectivity. With carbo-
pores is the main cause of deactivation. Similar catalysts were nised and sulfonated cellulose, 89% conversion and selectivity
employed for the esterification of other molecules such as gly- were observed. It is worth noting that it has been reported that
cerol.252 Glucose was used as a carbon source and the glucose sulfonation at high temperature (150 °C) induces changes in
solution was treated for 19 h at 195 °C leading to the prepa- the structure of hydrothermal carbon. This is due to a decrease
ration of a carbonaceous material which contained 67.9% C in the abundance of furanic groups and an increase in the
and 27.5% of oxygen. It was observed that after sulfonation presence of benzenic rings.255 Alternative methods of sulfona-
(150 °C, 15 h) the carbon content decreased to 55.8% and the tion have been shown such as the direct HTC in the presence
oxygen content increased to 40.5%. From this change in of sulfonic precursors (mainly hydroxyethylsulfonic acid).256

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The materials prepared using this method presented very high or without further modification have been employed as carbon
stability and reusability, enabling future applications. supports for metals.259–263 The process in most cases is the
4.3.2. Pristine hydrothermal carbon as catalysts. same: a carbon source is HTC yielding a material that is acti-
Sulfonation is a simple method to introduce strongly acidic vated (thermally or chemically) and is then used to support the
sites into hydrothermal carbon. However, also pristine surfaces metal precursor which is then reduced by the addition of a
possess catalytic properties due to the high amount of reducing agent (NaBH4 for example), as elaborated in section
hydroxyl and carboxylic groups. This has been demonstrated 2.2.2. In some cases, the reduction step can be skipped when
in the application of such catalysts for the 5-HMF production using pristine carbon surfaces.264,265 Glucose-derived hydro-
from fructose in ionic liquids.257 Hydrothermal carbon is pro- thermal carbon supports and stabilizes the metal nano-
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duced from glucose at 180 °C in 10 h and employed after oven- particles and keeps them active for prolonged time under reac-
drying without any further treatment. The results show that tion conditions. Palladium nanoparticles supported onto
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fructose was converted into 5-HMF with a maximum yield of hydrothermal carbon were employed for the Suzuki–Miyaura
89% after a 90 min reaction time. The stability of these cata- coupling.259 The material demonstrated high catalytic activity
lysts was not properly evaluated, and hence clarification would for the reaction of many aryl halides and boronic acids. It
be needed; however this shows that even sulphur-free surfaces could be recycled for up to five times through simple centrifu-
of carbon present catalytic activity in some reactions such as gation. In liquid state reactions, leaching of the supported
the dehydration of fructose. Hydrothermal carbon also metal is usually a problem but the properties of hydrothermal
permits alkaline functionalisation of surfaces and their use in carbon favour the redeposition of leached particles during
catalysis.258 As such, carbon spheres were synthesized from cooling down of the reaction. More elaborate supports can be
glucose at 160 °C maintaining the temperature for 12 h. designed by combining HTC with a porous polymer as the
Thereafter, acidic functionalities such as carboxylic and template as done by Cheng at al.266 In this case, a polymer was
hydroxyl groups were neutralized with sodium hydroxide at introduced during the HTC process serving as a template
room temperature. The sodium-hydroxide-treated hydrochar which was then removed at 700 °C under a reducing atmo-
was active for the base-catalysed aldol condensation.258 sphere. This thermal treatment increased the BET surface area
Benzaldehyde was reacted with acetaldehyde to form cinna- significantly. This material was then loaded with gold nano-
maldehyde. High selectivity (94%) was achieved at 34% conver- particles and tested for the hydrogenation of 4-nitrophenol to
sion based on benzaldehyde. At this conversion, the cinnamal- 4-aminophenol with sodium borohydride, resulting in high
dehyde also started to react with acetaldehyde to produce a catalytic activity. As mentioned before, hydrochar can be
higher weight homolog. In comparison with sodium hydroxide treated under basic conditions neutralising any acid surface
solutions, the modified hydrothermal carbon is less active but functionality. This can be interesting when using them as sup-
more selective and can be used in three catalytic runs with the ports such as when loading palladium NPs and using them in
same activity. As a summary, it can be stated that hydro- oxidation reactions.258 The absence of acid sites lowers the
thermal carbon has promising potential as a metal-free cata- number of side reactions that can occur augmenting selectivity
lyst for industrial applications. Introduction of strong and the high dispersion of the metal allows high activities to
Brønsted acid sites is achieved in a straightforward manner by be achieved. This high dispersion is aided by this basic pre-
sulfonation, for example, by treatment with sulphuric acid. treatment of the material as evidenced by the smaller palla-
However, the oxygen functionalities of pristine hydrothermal dium NPs observed for those samples treated with a higher
carbon can also be used for catalytic transformation. concentration of basic solution (2.7 nm versus 7.5 nm). HTC
4.3.3. Hydrothermal carbon as a catalyst support. ACs are can also be performed in the presence of metal oxide
classical supports for numerous catalysts found in commercial particles.267–269 Hence, by utilising magnetic metal oxide cores
processes. This is due to their high stability and surface area. a magnetically active material can be obtained. In this way,
In general, activated carbons possess a high surface area of active catalysts for the Suzuki–Miyaura cross-coupling reaction
1000–1500 m2 g−1, and they are highly apolar and hydrophobic have been prepared with palladium and platinum nano-
due to a low oxygen content. They, however, present a problem particles as active sites.267 In this work, Fe3O4 particles (mag-
when used as a support for metals such as palladium and that netite) were introduced during the carbonisation of glucose at
is leaching, which greatly decreases the activity and reusability 180 °C for 4 h. After this, palladium or platinum nanoparticles
of the catalyst. In contrast, hydrothermal carbon has a polar were deposited on the carbon shell and the whole material was
surface and a much lower surface area. By reduction of the protected by a further layer of approx. 35 nm thickness of
oxygen content and increasing that of carbon, the properties mesoporous silica. Silica was added to prevent sintering of the
of hydrothermal carbon can become closer to those of AC.249 metal nanoparticles while its porosity allowed the organic
By tuning these parameters (surface area and polarity), the molecules tested to pass through it. The magnetite particles
deposition of metal precursors can be enhanced, indicating had a uniform diameter of approx. 180 nm composed of nano-
that not only can the textural properties be similar to those of crystals of approx. 8 nm size. The carbon shell thickness was
ACs but they have also been reported to enhance the reuseabil- found to be approx. 15 nm. The size of the supported palla-
ity of the catalysts, most probably by diminishing the aggrega- dium and platinum particles was determined by HRTEM and
tion and leaching of the metals.259 Hydrothermal carbons with mean diameters of 14 and 25 nm were obtained, respectively.

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The hydrophilic surface of the hydrothermal carbon 4.4. Gas storage

facilitated dispersion of the particles in water during the meso-
porous silica synthesis and allowed a regular coverage of the The porous carbon materials, obtained via HTC combined with
particles. templating/activation techniques, have been intensively used for
4.3.4. Hydrothermal carbon as a sacrificial component. gas capture and separation. Different gases such as CO2,83,98,273–275
The defined structure and geometry of the spheres synthesised H2,135 SO2,82 H2S and CH4,276,277 have a good affinity for the
from HTC can be used as a template since it can be easily carbon structure, which make the ACs an obvious choice, over the
removed with thermal treatment in air at about 500 °C.270–272 classical gas separation methodologies.278
With this in mind, very effective catalysts for the low tempera- 4.4.1. CO2 adsorption. CO2 is commonly discussed as a
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ture oxidation of CO were produced.270 In his work, Zhao greenhouse gas and the main cause of global warming. As
et al.270 produced gold nanoparticles by bringing in contact such, many research studies have been focused on the syn-
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the gold precursor and glucose in water (Fig. 29). Glucose thesis of porous carbon materials for CO2 adsorption. In a
plays two main roles in this synthesis: it acts as a reducing recent work, Rao et al.279 reported high CO2 uptake for carbon
agent and it is also a carbon source for generating the hydro- adsorbents derived from glucose and decorated with nitrogen.
thermal carbon. Once the gold nanoparticles were formed, the The authors described different variations of the adsorption
cerium precursor was added, and the mixture was heated to capacity depending on several parameters. In this way, they
180 °C for different periods of time (1, 6, 10, and 20 h). observed that the CO2 uptake increases on raising the carbon-
Afterwards, the solid was collected and calcined at different isation temperature from 600 °C to 650 °C, but a further
temperatures in a range of 300–600 °C for 6 h, eliminating all increase affects negatively the adsorption process. During the
hydrothermal carbon. study, they reported the maximum CO2 adsorption capability
TEM images (of the calcined samples) showed gold nano- of 4.26 mmol g−1 and 6.70 mmol g−1 at 25 °C and 1 bar (RTP),
particles of an average size of 11 nm after 1 h of HTC and and in contrast to other studies which correlate the adsorption
the whole diameter of the core–shell structure was about properties with the volume of narrow micropores, the authors
40 nm. When the HTC was prolonged to 10 h, the gold reported that the best performing samples in their own work
nanoparticles grew to 16 nm and the shells to 103 nm. The possessed medium values of the narrow micropore volume,
best catalytic performance in carbon monoxide oxidation was and their good performance is mainly correlated with the syn-
shown by the material that was subjected to HTC for 10 h thesis procedure. Zhu et al.126 synthesised porous carbon
and was calcined subsequently at 600 °C. This catalyst adsorbents via the HTC of pineapple waste. The resulting
allowed the reduction of the reaction temperature from 300 hydrothermal carbon was further activated, with Li2C2O4,
to 155 °C for full conversion and it was tested on stream for Na2C2O4 and K2C2O4, in order to develop the porosity and
70 h without any deactivation being evident. In a similar enhance the CO2 uptake. The maximum CO2 adsorption
way, cobalt nanoparticles protected within hollow meso- capacity values were 5.32 mmol g−1 at 0 °C and 1 bar (STP) and
porous silica spheres were synthesized271 starting from 4.25 mmol g−1 in RTP, and it was observed for the carbon acti-
hydrothermal carbon spheres with an approximate diameter vated with K2C2O4, being correlated with the high volume of
of 100–150 nm synthesised from glucose carbonised at narrow micropores, about 0.92 cm3 g−1. Sevilla et al.135
180 °C for 4.5 h. After impregnation with cobalt nitrate pro- reported important adsorption capacities for CO2 using porous
viding nanoparticles of approx. 4 nm and the synthesis of a carbon materials derived from three different biomass
mesoporous silica shell (thickness: approx. 20 nm) with sources: starch, cellulose and sawdust, by means of HTC and
CTAB as a soft template, all the organic material was chemical activation. The high surface areas (2690–3540 m2
removed by calcination at 430 °C. This synthesis procedure g−1) combined with bimodal porosity in the micromesopore
provided a catalytic material with interesting performance in range led to high CO2 adsorption capacities, such as
the epoxidation of alkenes. When the cobalt/silica hollow 20–21 mmol g−1, at 20 bar and 25 °C. By extending the
spheres were employed in the epoxidation of styrene with pressure range, up to 40 bars, the CO2 uptake was increased to
oxygen, a 94% selectivity toward the epoxide was achieved at 30–31 mmol g−1. Xiao et al.96 reported adsorption capacities
almost complete conversion (Table 5). for CO2 of about 4.7 mmol g−1 in STP, using nitrogen-doped
carbon materials obtained from glucosamine via the HTC–soft
templating approach. CO2 adsorption was performed also by
Boyjoo et al.,98 who reported an adsorption capacity of about
5.22 mmol g−1 in RTP, using activated hydrothermal carbon
produced from Coca-Cola waste as adsorbents. The value was
even higher in STP, reaching 6.27 mmol g−1. In their study,
they prepared three activated carbons, starting from the same
biomass precursor and changing either the mass ratio between
the activator and hydrothermal carbon or the activation agent
Fig. 29 Schematic representation of the synthesis of core–shell distri- (activation with ZnCl2, denoted as CMC_1 and CMC_2; mass
bution of gold particles and cerium oxide. ratio of ZnCl2/HTC carbon = 1 and 3, respectively, and KOH,

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Table 5 Catalytic materials prepared through HTC compiled from the literature

Carbon Carbonisation Role of carbon Reaction Conversion Selectivity

Sample name precursor conditions material Reaction tested conditions Product obtained (%) (%) Ref.

HTC_673_SO3H Glucose 673 K, N2 atmosphere, Heterogeneous Esterification of Reflux, 85 °C, 4 h Methyl palmitate 90 — 249
H2SO4, 473 K, Ar catalyst palmitic acid with
Critical Review

atmosphere methanol
SHTC Glucose HTC 195 °C, 19 h, Heterogeneous Esterification of 115 °C, 10 h, 9 : 1 Monoacetin 98 5 252
H2SO4, 150 °C, 15 h catalyst glycerol with acetic AcOH/gly Diacetin 38
acid Triacetin 57
SHTC Glucose HTC 195 °C, 19 h, Heterogeneous Esterification of 115 °C, 10 h, 9 : 1 Monoacetin 99 1 252
H2SO4, 150 °C, 15 h catalyst glycerol with Butyric OH/gly Diacetin 24
butyric acid Triacetin 75
SHTC Glucose HTC 195 °C, 19 h, Heterogeneous Esterification of 115 °C, 10 h, 9 : 1 Monoacetin 98 2 252
H2SO4, 150 °C, 15 h catalyst glycerol with Caprylic OH/gly Diacetin 42

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caprylic acid Triacetin 56
HTC_Glu_S Glucose HTC 240 °C, 24 h, Heterogeneous Esterification of 60 °C, 3 h Ethyl levulinate 97 97 254
H2SO4, 80 °C, 4 h catalyst levulinic acid with
ethanol
HTC_Cell_S Cellulose HTC 240 °C, 24 h, Heterogeneous Esterification of 60 °C, 3 h Ethyl levulinate 89 88.9 254
H2SO4, 80 °C, 4 h catalyst levulinic acid with
ethanol
HTC_RS_S Rye straw HTC 240 °C, 24 h, Heterogeneous Esterification of 60 °C, 3 h Ethyl levulinate 92.2 92.2 254
H2SO4, 80 °C, 4 h catalyst levulinic acid with
ethanol
Carbon microspheres Glucose HTC 180 °C, 10 h Heterogeneous Dehydration of 100 °C, 90 min, 5-HMF 89 100 257
catalyst fructose [BMIM][Cl] ionic
liquid,
CNS_3OH Glucose 160 °C, 12 h, 3 M NaOH Heterogeneous Aldol condensation 40 °C, 5h Cis- and trans- 33.9 94 258
solution, RT, 1h catalyst of benzaldehyde cinnamaldehyde
and acetaldehyde Cis- and trans-5-phenyl- 6
2,4-pentadienal
Cis- and trans-7-phenyl- 0
2,4, 6-heptatrienal
Pd@UCS Sucrose 180 °C, 8 h, kayexalate, Catalyst support Suzuki coupling 50 °C, 1 h Coupling product of 99 — 259
activated in air, 800 °C, for Pd NPs reaction bromobenzene and
1h phenylboronic acid
Au-5/C Glucose 180 °C, 24 h, modified Catalyst support Hydrogenation of RT, 180s 4-Aminophenol 100 — 266
polymer template, for Au NPs 4-nitrophenol
carbonised in 5% H2
with a heat ramp to
700 °C for 5 h
Fe3O4@C-Pd@mSiO2 Glucose 180 °C, 4 h Layer over Fe3O4 Suzuki–Miyaura 70 °C, 6 h Coupling product of 99 — 267
and support for crosscoupling phenylboronic acid and
Pd NPs reaction p-iodoacetophenone
20 h-600 °C-Au@CeO2 Glucose 180 °C, 20 h, HAuCl4, Sacrificial Low temperature From RT to 300 °C — 100 — 270
CeCl3 and CO(NH2)2, component CO oxidation at 1 °C min−1
calcined at 600 °C, 6 h
Co/HSM Glucose 180 °C, 4.5 h, calcined Sacrificial Epoxidation of 100 °C, 3 h, Styrene oxide 98 94 271
at 400 °C in air for 7 h component styrene with bubbling with O2
molecular oxygen at atmospheric
pressure in DMF
Co/HSM Glucose 180 °C, 4.5 h, calcined Sacrificial Epoxidation of dec- 110 °C, 3 h, Dec-1-en oxide 93 93 271
at 400 °C in air for 7 h component 1-en with molecular bubbling with O2
oxygen at atmospheric
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denoted as CMC_3 KOH/HTC carbon = 4). The adsorption 4.5. Water treatment
capacities were approximately 4.8 mmol g−1 for ZnCl2 activated
samples and 6.27 mmol g−1 for the KOH activated sample, Water contamination with organic pollutants or heavy metals
with values reported in STP. More examples of carbonaceous attracted significant attention because of their high toxicity
materials derived from biomass resources and used in CO2 and non-degradability.281 Among their applications, porous
adsorption applications are listed in Table 6. carbon materials derived from sustainable resources represent
4.4.2. H2, CH4 and H2S adsorption. Correa et al.133 con- a green and effective method to adsorb water pollutants.
ducted a comparative study of the adsorption properties of Several studies reported their ability to take up both
activated carbons produced from giant bamboo via HTC and organic85,168,282–284 and inorganic molecules.285–290
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pyrolysis, followed by chemical activation with KOH. The 4.5.1. Removal of organic pollutants. Roldán et al.168 used
materials were tested for H2 adsorption at −196 °C and 1 mesoporous carbon doped with N and S, prepared via a one-
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

bar. The results showed that the best H2 uptake was regis- pot HTC, for water treatment, for the adsorption of two dyes:
tered for the carbon material synthesised via HTC, followed methylene blue (1.43 nm × 0.61 nm × 0.4 nm) and Rhodamine
by KOH activation, with 3.2 wt% corresponding to 16 mmol B (1.59 nm × 1.18 nm × 0.56 nm). They observed that as a
g−1. Sevilla et al.135 reported high adsorption capacities for general trend the adsorption capacity was larger for methylene
H2. At −196 °C and 20 bar the total H2 uptake of biomass blue, in contrast to the other dye, which may be mainly due to
derived carbons was in between 6.4 and 6.8 wt%, equivalent size-based reasons, the second one being slightly larger. Both
to 32–34 mmol g−1. H2 adsorption has been studied by Xiao molecules are cationic; so the interaction with the support
et al.96 using soft templating-HTC carbon adsorbents. The does not significantly vary. Also, both materials perform better
maximum H2 uptake was 9 mmol g−1 under the same before pyrolysis, explained by the higher oxygen functional
adsorption conditions as those used by Correa et al.133 The group content and lower graphitization degree which offer a
difference may be assigned to the textural properties, as the more hydrophilic surface enhancing the electrostatic inter-
materials synthesised by Correa133 present a higher surface actions with the dyes. In this case, the presence of the dopant
area, pore volume and micropore volume (SBET = 2117 m2 greatly enhances the adsorption properties by creating
g−1; Vtotal = 1.14 cm3 g−1; and Vn = 0.49 cm3 g−1) when com- different pore sizes in the material. The adsorption capacities
pared to the carbons reported by Xiao96 (SBET = 980 m2 g−1; Vtotal were about 106 mg g−1 for rhodamine B and 123 mg g−1 for
= 0.78 cm3 g−1; and Vn = 0.46 cm3 g−1). It is worth noting that methylene blue, on S-doped porous carbon activated with
the synthesis methods are different; while Correa133 used HTC ZnCl2, with a surface area of 288 m2 g−1 and a pore volume of
and chemical activation, Xiao96 reported HTC-soft templating. 0.56 cm3 g−1. Alatalo et al.169 proposed methylene blue
CH4 adsorption, at high pressure and ambient temperature, for removal from aqueous media using meso-microporous soft
activated nanocarbon produced by microwave-assisted HTC, was templated carbons prepared via a HTC–salt templating
measured by Cruz et al.131 They found that CH4 uptake method. The materials were derived from fructose and acti-
increases with pressure, up to a maximum of 8 wt% at 40 bars. vated with mixtures of LiCl/ZnCl2 in a one-pot HTC at 180 °C.
Shang et al.280 studied the adsorption of H2S on biochars Two types of materials were prepared, one from pure fructose
derived from agricultural and forestry waste, such as camphor (FruLi) containing surface polar oxygenated functionalities
tree, rice hull and bamboo. The maximum H2S uptake was and a second one from fructose and 2-thiophenecarboxyalde-
about 11.25 mmol g−1 for the biochar derived from rice hull hyde (TCA), resulting in thiophenic sulphur doped within the
having the highest specific surface area (115 m2 g−1) and the final carbon network (FruLi+TCA). The adsorption experiments
highest carbon content. Sethupathi et al.276 used biochars as of methylene blue for both carbon materials were performed
potential absorbents of methane, carbon dioxide and hydrogen in the pH range of 3–8, but a minor effect was observed. The
sulphide. They investigated four types of optimized biochars influence of temperature was also studied, in the range
derived from perilla leaf, soybean Stover, Korean oak and 20–60 °C. During these measurements it was observed that the
Japanese oak. When adsorbed in tandem, CO2 and H2S adsorption efficiency increased slightly when the temperature
compete for the adsorption sites, resulting in low adsorption was increased from 20 to 40 °C, maybe due to decreased solu-
capacity of the material. To confirm the competition phenom- tion viscosity leading to an enhanced diffusion rate of adsorp-
enon, the authors conducted a single-gas study of CO2, H2S and tive molecules across the external boundary layer and in the
CH4. During the measurement it was observed that all the bio- internal pores.282 At equilibrium ( pH 6 at 20 °C with a 24 h
chars showed a longer H2S adsorption breakthrough time when contact time) the maximum adsorption capacity was approxi-
compared to a simultaneous study, and a single-gas adsorption mately 96 mg g−1 for FruLi and 64 mg g−1 for FruLi+TCA. In a
measurement for CO2 revealed about 6.6 mmol g−1 adsorption recent study, Correa et al.133 reported very high adsorption
capacity, significantly higher than the adsorption capacity capacity for methylene blue, about 735 mg g−1 using activated
obtained during the simultaneous tests. It was also observed carbon prepared from different chars. Xiong et al.284 used a
that all biochars showed a higher preference for H2S over CO2. carbon-silicate composite, synthesised by HTC, for methylene
This is because CO2 is generally captured via physisorption, blue removal. The maximum adsorption capacity was 418 mg
while H2S adsorption depends on both physisorption and local g−1, considerably higher than that obtained for the unmodi-
pH within the pores. fied hydrothermal carbon.

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Table 6 CO2 uptake on porous carbons obtained via HTC

CO2 uptake
(mmol g−1)

Sample Carbon Carbonisation Activation SBET Vtotal Vmicro Vnarrow microb 0 °C, 25 °C,
name precursor conditions method (m2 g−1) (cm3 g−1) (cm3 g−1) (cm3 g−1) 1 bar 1 bar Ref.
a
GN-600- Glucose and 600 °C, N2 KOH 821 0.42 0.29 0.46 5.33 3.99 279
1 urea atmosphere
GN-650- Glucose and 650 °C, N2 KOH 1734 0.78 0.62a 0.77 6.70 4.26 279
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1 urea atmosphere
GN-700- Glucose and 700 °C, N2 KOH 2394 1.13 0.93a 0.87 6.46 3.92 279
1 urea atmosphere
—
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C-K-500 Pineapple 500 °C, N2 K2C2O4 422 0.04 0.82 2.71 2.22 126
waste atmosphere
C-K-600 Pineapple 600 °C, N2 K2C2O4 644 0.05 — 0.91 3.82 3.16 126
waste atmosphere
C-K-700 Pineapple 700 °C, N2 K2C2O4 1076 0.08 — 0.92 5.32 4.25 126
waste atmosphere
AA-0 Potato starch 800 °C, N2 KOH 3000 1.41 1.09c — — 2.80 135
atmosphere
AA-3M Potato starch 800 °C, N2 KOH and 3220 2.37 1.07c — — 2.50 135
atmosphere melamine
AC-0 Cellulose 800 °C, N2 KOH 3100 1.46 1.05c — — 2.80 135
atmosphere
AC-2M Cellulose 800 °C, N2 KOH and 3540 2.22 1.28c — — 2.30 135
atmosphere melamine
AS-2M Sawdust 800 °C, N2 KOH and 3420 2.30 1.16c — — 2.20 135
atmosphere melamine
CMC-3 Coca Cola High T°C, inert KOH 1405 0.80 0.05d 0.45e 6.27 5.20 98
waste atmosphere
CMC-2 Coca Cola High T°C, inert ZnCl2 1994 0.87 0.51d 0.26e 4.84 3.00 98
waste atmosphere
HPNC-3 Glucosamine 600 °C, N2 Pluronic P123 710 0.54 0.37d 0.32e 4.20 — 96
atmosphere templated
HPNC-4 Glucosamine 600 °C, N2 Pluronic P123 980 0.78 0.46d 0.40e 4.70 — 96
atmosphere templated
a
Calculated from the t-plot method. b Pore volume of narrow micropores (<1 nm) obtained from CO2 adsorption in STP. c Micropore volume
determined by the DR method. d Micropore volume calculated from the pores <2 nm based on the NLDFT method. e Ultramicropore volume cal-
culated for the pores <0.7 nm based on the NLDFT method.

4.5.2. Metal recycling. Porous carbon materials prepared their hydrated ionic radius. Overall, the KOH ACs were much
via HTC received recognition also for their ability to adsorb better adsorbents for the heavy metals, compared to the pure
and recycle heavy metals from aqueous solutions.281,285,286,291 ones, probably due to the increased binding sites associated
Sun et al.286 reported the synthesis of KOH ACs, starting from with oxygen-containing functional groups. When tested in the
different feedstocks (sawdust, wheat straw, and corn stalk), for multi-metal experiment, Cd(II) sorption capacities on mHCs
the removal of Cd(II) and multiple metals (Pb(II), Cu(II) and Zn were only 4.12–4.73 mg g−1, much lower than that in a single-
(II)) from water. The materials have been prepared via HTC at metal system (30.40–40.78 mg g−1), which is a clear indication
200 °C for 20 h and have been denoted as H_SD (sawdust), of the competition for surface adsorption. Han et al.288 investi-
H_WS (wheat straw) and H_CS (corn stalk). After HTC, a part gated the properties of hydrothermal carbon as sorbents for
of the obtained amount was modified with KOH, being Cd(II), as well. The adsorbents have been prepared via HTC
denoted as mH_SD, mH_WS and mH_CS. The characterisation from swine solids and poultry litter, characterized and used
results showed that the hydrothermal carbon had a high for Cd(II) and Sb(III). The maximum adsorption capacities for
content of C, together with N, O and H. In terms of porosity, Cd(II) were 19.80 mg g−1 for the poultry litter hydrothermal
the powders possessed surface areas between 4.4 and 9 m2 carbon and 27.18 mg g−1 for the swine solids. The uptake for
g−1. Both hydrothermal carbon samples, KOH modified and Sb(III) was considerably lower, with a maximum of 3.98 mg g−1
unmodified, were tested for Cd(II) adsorption. The samples for the swine solid derived hydrothermal carbon. A recent
needed about 2 h to reach the apparent equilibrium. Cd(II) was study regarding Cd(II) removal from aqueous solutions was
more readily removed by the KOH ACs, about 80%, compared conducted by Xiong et al.284 Here, the authors proposed the
to less than 10% for the pure hydrothermal carbons. When HTC synthesis of a silicate-carbon composite as an efficient
used to separate multiple heavy metals, hydrothermal carbons adsorbent for Cd(II) and methylene blue. The maximum Cd(II)
and ACs adsorb the metals in the following order: Pb(II) > Cu uptake, after a 60 minute equilibration time was 108 mg g−1
(II) > Cd(II) > Zn(II), which coincides with the reverse order of for the composite materials, which was higher than that

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obtained for pure hydrothermal carbon (34.62 mg g−1) or for sities (up to 35 MJ kg−1 dry hydrothermal carbon) and struc-
magnesium silicate (70.42 mg g−1), the precursor used during tures similar to coal, making them candidates for replacing
the synthesis. Cai et al.287 reported the removal of uranium(VI) and/or supplementing fossil fuel-derived energy.37,295–297
from aqueous solutions using carbonaceous spheres derived Several studies have reported that hydrothermal carbonisation
from glucose via HTC. In a typical adsorption experiment improves the fuel characteristics (e.g., aromaticity) of biowaste,
10 mg of carbon powder was dispersed in 20 mL of different making them more comparable to lignite and bituminous
concentrations of U(VI) solutions (20–140 mg L−1), and the mix- coals.298 Hydrothermal carbon energy content data collected
tures were stirred at room temperature at different pH values from the literature are summarized in Fig. 30. The results indi-
and different times. The U(VI) uptake was determined by the cate that, on average, hydrothermal carbons generated from
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difference in the solution concentrations obtained from the carbonisation of food waste and animal feeds result in the
UV-VIS measurements. During the study, it was observed that largest energy contents, while the hydrothermal carbon gener-
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the pH plays a crucial role in U(VI) adsorption. In this way, the ated from the carbonisation of sludge results in the lowest
amount of uranium adsorbed on the carbon spheres registered energy content. Several studies reported that the hydrothermal
a sharp increase from pH 1.5 to 3.5, then becoming constant carbon energy content increases with increasing reaction
at this value. The maximum uranium uptake was 163 mg g−1, severity.18,71,76,299–301 It is possible that the high average
obtained at pH 4.5. The process is influenced by the pH energy content data associated with hydrothermal carbon gen-
because at low pH values the functional groups from the erated from animal feed is skewed by process conditions, since
microsphere’s surface were protonated, creating positive the conditions associated with the carbonisation of this feed-
charges, followed by electrostatic repulsion between the posi- stock resulted in the highest average severity factor. However,
tive charge existing on the surface and the uranyl ions, no defined trends associated with the severity factor appear to
decreasing the adsorption capacity. Also, the competition be present in these data.
between H+ and uranyl ions for the active sites at low pH It is important to note that both the hydrothermal carbon
results in poor U(VI) uptake. The contact time was tested as energy content and hydrothermal carbon yield (e.g., hydro-
well. In the first 9 h, the uranium uptake increased rapidly, fol- thermal carbon mass) are important when determining the
lowed by a small variation reaching the maximum adsorption total energy that can be recovered from hydrothermal carbon.
capacity of 163 mg g−1 at a contact time of 22 h. When the Results from previously conducted studies detailed in the lit-
uranium concentration increases from 20 to 80 mg L−1, the erature indicate that the hydrothermal carbon energy content
adsorption capacity registers a sharp increase, followed by a
slow increment beyond 80 mg L−1, due to the saturation of
active sites onto the surface of the HTC microspheres. A
similar study has been reported by Zheng et al.291 who used
glucose HTC carbon spheres functionalized with 4-aminoace-
tophenone oxime groups for the adsorption of uranyl ions.
Following a similar strategy, they obtained the maximum
adsorption capacity at pH 6, followed by a decrease, due to the
formation of hydroxide products of UO22+, such as UO2(OH)3−
and (UO2)3 (OH)7−, which generate electrostatic repulsion
between these anions and the adsorbents. Also, at an
optimum pH = 6, the amount of uranium adsorbed was sig-
nificantly improved from 55.7 ± 1.5 to 366.8 ± 16.0 mg g−1,
after functionalization with 4-aminoacetophenone oxime
groups. The maximum adsorption capacity obtained in this
study for uranyl ions was 588.2 mg g−1 at pH 6 and 60-minute
contact time, for the functionalized carbon spheres. Other
studies dealing with the adsorptive removal of uranium using
hydrothermal carbon have been reported by Yu et al.,285 Deng
et al.,292 and Han et al.290

4.6 Bioenergy source–bioenergy potential in hydrothermal

carbon
Fig. 30 Distributions associated with the hydrothermal carbon energy
The solid (hydrothermal carbon) and liquid products resulting content of different feedstocks based on literature-collected data. The
from the carbonisation of biomass have significant bioenergy line in each box represents the median value. The ends of each box rep-
resent the 25th and 75th percentiles associated with the data. The red
potential, which has resulted in numerous studies investi-
diamonds represent the average values. The lines and data points rep-
gating the use of hydrothermal carbon and/or the process resent the scatter of data beyond the 10th and 90th percentiles. The
water as fuel sources.72,293,294 Hydrothermal carbons generated numbers in parentheses following each feedstock category represent
from biomass have been reported to have high energy den- the number of data points represented on the plot.

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generally increases with increasing reaction temperature and, based chemicals such as furfural, 5-hydroxymethylfurfural and
in a way, reaction time.17,61,69,71,302–305 Conversely, an increase levulinic acid. Valorisation of these highly condensed poly-
in temperature and to some degree time generally result in a meric hydrothermal carbons48,314,315 by catalytic conversions
reduction in hydrothermal carbon generation.8,68,69,71,76,306,307 is of high interest but has received limited attention to date.
Therefore, understanding the combined effect of these hydro- This chapter gives an overview of the research carried out on
thermal carbon properties is needed when determining the the catalytic conversion of hydrothermal carbons to liquid pro-
value of using hydrothermal carbon as a bioenergy source. ducts to be used as biofuels or, after fractionation, as a source
Combining statistical models for predicting hydrothermal of valuable biobased chemicals such as alkylphenolics and
carbon yields and energy contents based on literature-collected aromatics. The emphasis will be on four technologies, viz. (i)
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data,308 the total energy that may be recovered from different catalytic hydrotreatments with molecular hydrogen or hydro-
organic waste materials (at a constant reaction temperature, gen donors such as isoporopanol or formic acid, (ii) catalytic
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time, and initial solid concentration) can be calculated. pyrolysis, (iii) aqueous phase reforming and (iv) gasification.
Results from this analysis are shown in Fig. 31. In this plot, 4.7.1. Catalytic hydrotreatment. Catalytic hydrotreatment
the feedstocks are listed in order of the lowest carbon content involves bringing in contact the hydrothermal carbons with
to the greatest carbon content, indicating that there is a molecular hydrogen (or a hydrogen donor such as formic acid
relationship between the feedstock carbon content and the (FA)) in the presence of a catalyst. This methodology has been
total energy that is available for use in the generated hydro- explored in detail for the depolymerisation of lignin to low
thermal carbon. Blending of hydrothermal carbon with coal molecular weight chemicals.316 Typical catalysts for lignin con-
has been reported to improve the combustion efficiency of low versions are heterogeneous metal supported catalysts such as
rank coals for energy generation.309–312 Gao et al.313 reported NiMo and CoMo on various supports.317,318 Relatively harsh
that the presence of a secondary char deposited on the hydro- conditions (temperatures up to 450 °C, pressures up to 200
thermal carbon surface is responsible for the high oxidative bar) are required for substantial depolymerisation activity. The
reactivity of the hydrothermal carbon. The generation of this use of hydrogen donor solvents in both the presence and
secondary char (tar-like deposits on the hydrothermal carbon absence of metal catalysts has been reported. A well-known
surface) has been reported to increase with the reaction sever- hydrogen donor is FA, which is in situ, either thermally or cata-
ity (e.g., increase in reaction time and temperature). Gao lytically, converted to hydrogen and CO/CO2. Other solvents,
et al.313 indicate that blending of hydrothermal carbons up to either for dilution or to act as a hydrogen donor, are alcohols
10% with bituminous coal is possible and does not lead to (ethanol, methanol, and isopropanol (IPA)), and water.
fuel segregation (e.g., uneven burning) during combustion. Temperatures between 300 °C and 450 °C have been explored
for lignin, with reaction times ranging from 2 to 17 h.
However, in contrast to lignin, only a limited number of lique-
4.7. Catalytic conversions of hydrothermal carbon
faction studies on hydrothermal carbon using a catalytic
Hydrothermal carbons are inevitable byproducts from acid- hydrotreatment strategy have been reported. Trautmann
catalyzed conversions of C5 and C6 sugars to important bio- et al.319 performed research on a catalytic treatment of hydro-
thermal carbon, referred to as biocoals in their research,
obtained by HTC of various biomass sources (green waste,
straw, bark, pine wood meal and wood) and horse or swine
manure with the objective to obtain a liquid product with
potential to be used as a biofuel. Various biocoals were tested
using Ni on titania as the catalyst at a temperature of 400 °C
using molecular hydrogen (100 bar) and tetralin as the hydro-
gen donors (batch, 2 h reaction time). Oil yields of about
32–36 wt% have been reported, the major by-product being
gas-phase components (45–58 wt%). The product oils were
shown to contain limited amounts of bound oxygen (<5 wt%),
and a H/C molar ratio of about 1.3. The higher heating value
(HHV) of the oils was > 40 MJ kg−1 oil. Subsequent studies320
were aimed at minimizing the formation of gas-phase com-
ponents by lowering the liquefaction temperature from 400 to
350 °C and reducing the batch time from 2 to 1 h. Indeed, the
gas yield dramatically decreased from 45–58 wt% to 14 wt%,
Fig. 31 Calculated total energy recoverable from different organic while the product-oil yield was nearly similar. These findings
waste materials. Note that the waste materials are listed in order of their
suggest that liquefaction of hydrothermal carbon to oils by a
carbon content, ranging from the lowest value (left) to the greatest
value (right). Each value represents the average value obtained from
catalytic hydrotreatment is feasible. Wang et al.321 reported an
different model combinations. The error bars represent the standard extensive catalyst screening study using various (noble) metals
deviations associated with these data. (Pt, Ru, Ni, and Rh) on a range of supports (C, Al2O3, TiO2,

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ZrO2, and CeO2) for the reactive liquefaction of hydrothermal gomeric compounds (GPC). Main GC detectable species
carbon in isopropanol (IPA) in the absence of hydrogen gas. arising from the hydrochar were substituted alkylphenolics,
Experiments were carried out in a batch reactor using an artifi- naphthalenes and cyclic alkanes (GC-MS-FID, GC × GC).
cial model hydrothermal carbon derived from glucose with iso- Recently Saha and co-workers325 reported the catalytic hydro-
propanol as the solvent at 400 °C for a 3 h batch time. Initial treatment of hydrothermal carbons in methanol with
studies using noble metal catalysts (Rh, Pt, Pd, and Ru) on a additional molecular H2 as the hydrogen source using various
carbon support revealed that Pt was the best catalyst in terms noble metals supported on carbon. Experiments were per-
of hydrothermal carbon conversion (77%) and amounts of formed in a batch autoclave at 350 °C under 30 bar H2 with a
alkylphenolics and aromatics in the product oil (GC × GC-FID). 3 h batch time. The highest yield of the product oil was
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Subsequent support screening studies (TiO2, ZrO2, and CeO2) 43 wt% at 55% conversion of the hydrothermal carbon using
were performed using Pt as the active metal and the results Pt/C. All other catalysts were less effective (Ru, Pd and Rh).
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were compared with those of Pt/C. The best results were However, the amounts of GC detectable in the oil (10%) were
obtained with Pt/C when considering hydrothermal carbon relatively low. Reaction parameters, including the reaction
conversion. However, Pt/CeO2 was shown to be more attractive time, temperature, H2 pressure and catalyst-to-hydrothermal
when considering the amounts of alkylphenolics in the carbon mass ratio were optimized. Higher reaction tempera-
product oils (20.4 wt% based on hydrothermal carbon intake). tures were shown to have a positive effect on the hydrothermal
Detailed liquid product analysis (GPC, GC-MS, and GC × GC) carbon conversion. However, the product oil yield does not
including blank reactions in the absence of hydrothermal progressively increase with temperature and shows a
carbons revealed that the hydrochars are mainly converted to maximum at about 400 °C due to excessive gasification at high
monomeric and oligomeric alkylphenolics and aromatics (GPC temperatures. Isotopic-labelling experiments were conducted
and GC). IPA is not inert and is converted to acetone and using 13C-labeled methanol to investigate whether methanol
hydrogen, and the latter is the hydrogen source for the various only serves as a hydrogen source or also reacts with the inter-
metal catalysed hydrogenolysis and hydro(deoxy)genation reac- mediates or products of the liquefaction reaction. The results
tions. In addition, acetone is converted to aldol condensation imply that methanol is incorporated as an –OCH3 group in the
products (such as methylisobutylketone, MIBK) and hydrogen- products, formed by for instance esterification reactions of
ation products derived thereof. An overview of the various reac- hydrothermal carbon-derived fragments and as methyl groups
tions occurring during the reactive liquefaction process is on aromatic units, likely by alkylation reactions.
given in Fig. 32. Though a complex reaction mixture is 4.7.2. (Catalytic) pyrolysis. Liquefaction of hydrothermal
obtained with products from both hydrothermal carbon and carbon obtained from the HTC of glucose and fructose using
the solvent, efficient separation technology may allow the sep- pyrolysis technology with the objective to obtain high liquid
aration of the aldol condensation products (such as MIBK) yields was investigated by Rasrendra et al.326 Pyrolysis GC-MS
and alkylphenolics from the mixture. Both components are (300–600 °C, 10 s, He atmosphere) showed the presence of
commercially available bulk chemicals with a high application furanics and organic acids in the vapour phase, though the
range. For instance, MIBK is used as a solvent, whereas mix- individual components were present in only minor amounts
tures of alkylphenolics may be used as a replacement of (<1 wt%). Micro-pyrolysis (500 °C, 12 s, N2 atmosphere) yielded
phenol in phenol based adhesive formulations.322 In addition, 30 wt% of gaseous and liquid products, the remainder being a
mixtures of alkylphenolics may also be used as biofuel blend- solid char. Liquid yields were by far lower than that obtained
ing agents to improve diesel engine performance.323 for a typical lignin sample (Kraft lignin) under similar con-
Further systematic studies to obtain high amounts of alkyl- ditions. As such the authors concluded that pyrolysis of hydro-
phenolics were performed using a Pt/C catalyst in IPA.321 The thermal carbon is cumbersome and gives relatively low liquid
highest hydrothermal carbon conversion achieved in this yields. The catalytic pyrolysis of hydrothermal carbons, includ-
study was 72%. The main hydrothermal carbon derived pro- ing those from the HTC of model sugars and industrial hydro-
ducts were aromatics, alkylphenolics and aliphatic hydro- thermal carbons, for BTXNE synthesis (benzene, toluene,
carbons, which was confirmed by performing blank reactions xylenes, naphthalene, and ethylbenzene) has been reported by
with IPA and Pt/C in the absence of hydrothermal carbon. The Agarwal et al.327 Initially, the catalytic pyrolysis of a synthetic
highest amount of alkylphenolics was 14% based on GC hydrothermal carbon from glucose using various zeolite cata-
detectable products in the liquid phase after the reaction. The lysts was investigated in a small-scale catalytic pyrolysis set-up
catalytic hydrotreatment of hydrothermal carbon in the pres- (PTV-GC/MS) at 550 °C to identify the best catalyst for liquid
ence of mixtures of FA and IPA using supported Ru catalysts products enriched in valuable bulk chemicals such as BTXNE.
has also been investigated.324 Best results were obtained using Remarkable differences in the yields of aromatics (BTXNE)
Ru/C and hydrothermal carbon conversions up to 69% were were observed for the various catalysts. Lowest BTXNE yields
achieved using a combination of FA/IPA as the hydrogen were found for neutral and basic zeolites, indicating that
donor. Elemental analysis showed that the oils have a con- acidic zeolites are desirable for the aromatization of the pyroly-
siderably reduced oxygen content compared to the hydro- tic vapours from hydrothermal carbon. For such acidic zeo-
thermal carbon feed, with HHVs up to 38 MJ kg−1. The lites, the BTXNE yield increased in the order: ferrierite 20 <
product oils were shown to consist of both monomeric and oli- HY–80 < HY 5.1 < H-mordenite 20 < H-ZSM-5. The catalytic

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Fig. 32 Overview of reaction pathways occurring during the catalytic hydrotreatment of hydrothermal carbon in IPA; reproduced with permission
from ref. 324.

pyrolysis of synthetic and industrial hydrothermal carbons conditions (225–250 °C, pH 9–11, 20 h, batch) using various
using HZSM-5 with different SiO2/Al2O3 ratios (23, 50 and 80) supported Pt-based catalysts. Best results were obtained using
and as such with different acidity levels (highest for 23) was Pt–Re on a ZrO2 support giving up to 8 wt% of a product-oil
investigated. The results revealed that HZSM-5 with a SiO2/ which was shown to mainly contain phenolics and aromatic
Al2O3 ratio of 50 gives the highest yield of aromatics for both ketones (GC-MS). Insoluble hydrothermal carbons, without a
types of hydrothermal carbons. The BTXNE yield is signifi- prior alkaline pre-treatment, were also investigated. In this
cantly higher for the industrial hydrothermal carbon (10 wt%) case, only 3 wt% of a product oil was formed together with
than that of the synthetic one (1.5 wt%). In addition, catalytic comparable gas yields as for the alkali-treated hydrothermal
pyrolysis experiments were performed with an industrial and carbon. The product oil was shown to contain more furanic
synthetic hydrothermal carbon at an in house-built reactor structures and less phenolics. This difference in product distri-
setup consisting of two reactors (the first reactor for the pyrol- bution is related to the difference in structures between the
ysis of the hydrothermal carbon and the second for the zeolite soluble, alkali treated hydrothermal carbon and the original
catalyst) to obtain a liquid product and to determine the mass hydrothermal carbon. The latter was shown to be rich in fura-
balance. The total amount of detectable aromatic species in nics, whereas the solubilisation in an alkaline medium
the product oil for industrial and synthetic hydrothermal resulted in the formation of a structure with more ( polycyclic)
carbons were at max. 9 and 2 wt% on hydrothermal carbon aromatics present. As such, this study shows catalytic conver-
intake, respectively, which confirms the results from the sions of (solubilised) hydrothermal carbon under APR con-
PTV-GC/MS experiments. These studies reveal that a thermo- ditions resulting in the formation of a gas phase with some
chemical pyrolysis approach to depolymerise hydrothermal H2, CH4 and CO2 and a liquid product with phenolics and/or
carbon suffers from low liquid yields and the formation of sub- furanics.
stantial amounts of solid char. Catalytic pyrolysis approaches 4.7.4. Gasification. Gasification and particularly steam
aiming for aromatics such as BTXNE seem more promising. reforming of hydrothermal carbon (900–1200 °C) with the
4.7.3. Hydrothermal carbon liquefaction using catalysts objective to obtain syngas (mixture of COx and hydrogen) was
and conditions typical for aqueous phase reforming. van studied by Hoang et al.328 Various alkali–metal-based catalysts
Zandvoort et al.314 recently reported liquefaction studies using (Na2CO3, K2CO3, Cs2CO3 and CaCO3) were screened for the
solubilised glucose hydrothermal carbons. Obtained by an reactions and Na2CO3 showed the highest activity. Gas phase
alkali treatment, under typical aqueous phase reforming (APR) analyses showed that the syngas typically has a H2/CO ratio of

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Table 7 Products of decomposition of lignocellulosic biomass under hydrothermal conditions335–338

Aldehydes, ketones and monosaccharides Furan derivatives Carboxylic acids Phenols

Glucose Furfural Pyruvic acid Guaiacol

Fructose 5-HMF Glycolic acid Catechol
Xylose Furfuryl alcohol Acetic acid Cresol
Erythrose 2-Methylbenzofuran Lactic acid
Dihydroxyacetone Soluble polymers Formic acid
Levoglucosan Levulinic acid
Pyruvaldehyde Propionic acid
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Glyceraldehyde
Formaldehyde
Acetaldehyde
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2,5-Dioxo-6-hydroxyhexanal

2. However, substantial loss of carbon to vapours was observed alcohol are derived from the hemicellulosic fraction of
during the heating up stage in the gasifier (up to 45 wt% on biomass.331–334 Table 7 shows an extended list of the main car-
intake). To improve carbon yields, a second catalytic reactor bonisation products grouped in the four above-mentioned
was proposed to gasify the volatiles formed during the heating classes.
phase to increase the solid to gas carbon efficiency. Dry/CO2 5-Hydroxy-methylfurfural (5-HMF) is probably the most
reforming of hydrothermal carbons with Na2CO3 was also renowned and sought-after of all hydrothermal carbonisation
studied.329 Without a catalyst, only 7 wt% of the hydrothermal products. It is derived from dehydration of
carbon was converted at 750 °C, which increased to 20 wt% at C6 monosaccharides. The huge interest around this com-
900 °C. The addition of Na2CO3 substantially increased the pound arises from the reactivity of its two functional groups,
hydrothermal carbon conversion rate, and a near quantitative methoxy- and aldehydic, and the possibilities of derivatisation
conversion was achieved at 725 °C. that they offer. In fact, 5-HMF has been recognized as a valu-
4.7.5. Conclusions. Considerable progress has been made able bio-based chemical building block16 which can play a key
in the liquefaction of hydrothermal carbons obtained by HTC role not only as an intermediate for the production of the
using a number of catalytic approaches. All findings reveal biofuel dimethylfuran (DMF), but also for other biomass-
that the oligomeric/polymeric and recalcitrant structures of derived intermediates, such as 2,5-furan-dicarboxylic acid,339
hydrothermal carbons may be ( partly) depolymerised to liquid 2,5-dimethylfuran,340,341 adipic acid and levulinic acid (LA).342
and gas phase products, though the product yields are gener- As a consequence, a few processes for industrial scale syn-
ally rather low due to the formation of substantial amounts of thesis of 5-HMF from monosaccharides have already been
solid residues. The liquid phase obtained by a catalytic hydro- patented. AVA Biochem, a Swiss based company, has been
treatment has potential to be used as a source for interesting involved in the industrial scale production of 5-HMF derived
bulk chemicals after fractionation. This opens new avenues for from biomass since 2013, with an annual production of 20
the development of added-value product outlets for hydro- tonnes. AVA Biochem’s approach is based on carbonisation of
thermal carbons beyond the use as a solid fuel. As such, this fructose in water at high pressure. Fructose is in turn obtained
will have a positive effect on the techno-economic viability of from previous treatment of biomass waste. The traditional dis-
biorefinery schemes involving the conversion of C5 and C6 advantage of the production of a remarkable amount of hydro-
sugars to biobased chemicals such as levulinic acid and HMF thermal carbon as a byproduct of condensation and polymeriz-
with the inevitable formation of hydrothermal carbons. ation of 5-HMF is avoided by extracting 5-HMF from the
system before the condensation and polymerization steps take
place. The final product is either a 99.9% pure crystalline solid
5. Soluble carbonisation products or is available in solution for further processing. Moreover, as
the conversion of fructose in a single step is not complete, the
The complexity of transformations and reaction pathways process has been designed to incorporate a recycling step of
occurring during hydrothermal carbonisation of ligno- the unreacted sugar, to enhance overall conversion. These
cellulosic biomass is reflected in the large number of soluble series of improvements have allowed the process to be scaled
products found in the aqueous phase. However, all chemicals to the industrial level.343
can be grouped into four classes: aldehydes/ketones (including Levulinic acid (LA) is considered as one of the most promis-
saccharides), furan derivatives, carboxylic acids and ing platform chemicals produced from lignocellulosic biomass
phenols.76,303,330–334 First three classes of compounds can for fuels and chemicals.344,345 It is regarded as a speciality
always be found in a relatively high amount or in traces after chemical that finds applications for several purposes, such as
hydrothermal carbonisation of mono- and polysaccharides, a source of polymer resins, animal feed and food as well as
whereas phenols are only found in real biomass HTC, as they components of the flavouring and fragrance industry, textile
arise from hydrolysis of lignin. Xylose, furfural and furfuryl dyes, additives, extenders for fuels, antimicrobial agents, her-

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bicides and plasticizers.346,347 The biorefinery process employs summary of the fraction of initially present carbon transferred
cellulosic biomass for the synthesis of levulinic acid and to the gas and liquid-phases following carbonisation. As
formic acid. The conversion takes place in two steps: the shown, a small fraction of initially present carbon is trans-
ground biomass is injected into the first reactor, where it is ferred to the gas-phase, which is consistent with that reported
then mixed with dilute sulfuric acid at a temperature of in the literature. Generally, as the reaction severity increases,
210–220 °C and a pressure of 25 bar. A residence time of 12 the fraction of carbon in the gas-phase increases.71,76,303,353
seconds is sufficient for the hydrolysis and dehydration reac- The carbonisation of paper results in the greatest transfer of
tion to occur, leading to the synthesis of a 5-HMF intermedi- carbon to the gas, which is not surprising because the con-
ate. Subsequently, the mixture is moved to a second, larger ditions associated with the carbonisation of paper also rep-
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reactor. Here the acidic catalyst concentration is the same, resent the largest average severity factor. Conversely, a signifi-
while temperature and pressure are milder. A longer residence cant fraction of carbon is transferred to the liquid-phase as a
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time of 20 minutes is necessary to complete the reaction of result of carbonisation, which may have significant impli-
rehydration of 5-HMF to levulinic acid and formic acid. cations associated with using the liquid-phase as an energy
Furfural and other by-products are removed at this stage. source via anaerobic digestion.354,355 The distribution of data
Water is subsequently boiled off, along with the remaining describing this fraction of carbon is quite large, likely because
volatiles, and levulinic acid is extracted from the resulting of the significant influence process conditions have on liquid-
mixture by distillation. In this process, approximately 50% of phase carbon contents.68,71,72,76,353,356 Unlike that associated
the initial mass of 6-carbon sugars is converted to LA, 20% to with other carbonisation products, there is a trend associated
formic acid and 30% to tar.348 with the severity factor, which is consistent with that reported
Furfural is largely produced by dehydration of pentose in the literature. At low average severity factors, large fractions
sugars rich in hemicellulose. In fact, currently, commercial of carbon are transferred to the liquid-phase. As average sever-
production of furfural exclusively relies on dehydration of ity factors increase, the transfer of carbon to the liquid-phase
lignocellulosic biomass. The typical process involves the break- decreases. These trends are consistent with those reported in
ing down of the hemicellulose fraction of a lignocellulosic the literature.71,76
biomass in a 3% sulfuric acid solution for 3 hours, in a range
of temperatures between 170 and 185 °C. Under these con- 5.2. Influence of feedwater and catalysts on soluble
ditions, 40–50% of the potential furfural is obtained. The carbonisation products
undissolved residual solid, composed of lignin and cellulose, HTC of biomass is usually performed under acidic conditions,
is recovered and employed for energy recovery, while the acid because hydrolysis and dehydration reactions are catalyzed by
catalyst is recycled.349 Several mechanisms have been specu- hydronium ions. As mentioned before, 5-HMf is the pivotal
lated to explain pentose dehydration to furfural including both compound in the whole process of transformation, as it is the
acyclic and cyclic intermediates. However, it is reasonable to starting point for both conversions to dissolved products (levu-
think that the most plausible mechanism under acidic con- linic acid) and solid products (hydrothermal carbon).
ditions involves acyclic xylose isomerization to xylulose fol- Therefore, any consideration of the influence of reaction para-
lowed by further dehydration and cyclization to furfural.43 The meters on the final products of hydrothermal carbonisation
interest in bio-derived furfural arises from the many possibili- must take into account the reactivity of 5-HMF. The rate-limit-
ties that this molecule offers in terms of catalytic upgrading to ing step in the dehydration of glucose to 5-HMF in acidic
fuel additives.350 medium is the isomerization of glucose to fructose.44,357 This
The abovementioned compounds have received a great deal of explains the reluctance of glucose, compared to fructose, in
attention because of their primary role in the transition to sustain- dehydrating to 5-HMF: in fact, isomerization of glucose to fruc-
able, bio-based fuels and materials. However, among the numer- tose is base catalyzed and therefore it is slower under typical
ous other chemical compounds, some organic acids such as lactic acidic conditions for the synthesis of 5-HMF.44
acid, formic acid and acetic acid are worth mentioning. Despite The nature of the acid used to catalyse the dehydration of
their limited strategic relevance, they still have commercial value saccharides plays a major role in the final yield and distri-
due to their abundance as by-products in the hydrothermal conver- bution of products. Lu et al. noted faster kinetics in HCl cata-
sion of biomass and levulinic acid synthesis. Lactic acid (LacA) is a lysed hydrolysis of cellulose compared to that of H2SO4 in the
promising platform chemical for the synthesis of solvents (ethyl same concentration.356 Reiche et al. described a different
lactate), biodegradable plastics (polylactic acid, PLA) and other behaviour in HTC of 20 wt% glucose solutions treated with
polymer precursors (acrylic acid).16,351 Formic acid can serve as a HCl or HNO3 with the same synthesis pH. Nitric acid, in fact,
starting compound for the synthesis of green solvents,348 whereas due to the oxidizing properties of NO3−, drives the conversion
acetic acid could replace inorganic acids for the synthesis of towards higher yields of hydrothermal carbon in spite of the
roadway deicer salts.352 formation of 5-HMF, levulinic acid and formic acid.
Hydrochloric acid, conversely, catalyses more efficient conver-
5.1. Element distribution in the liquid and gas-phases sion of glucose to HMF and levulinic acid subsequently, with
Significantly less composition data have been reported for the much lower HTC carbon yields. It is also worth pointing out a
gas and liquid carbonisation products. Fig. 33 provides a strong imbalance between levulinic acid and formic acid, in

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Fig. 33 Distributions associated with the percentage of initially present carbon in the (a) gas and (b) liquid-phases following the carbonisation of
different feedstocks. The line in each box represents the median value. The ends of each box represent the 25th and 75th percentiles associated
with the data. The red diamonds represent the average values. The lines and data points represent the scatter of data beyond the 10th and 90th per-
centiles. The numbers in parentheses following each feedstock category represent the number of data points represented on the plot.

favour of the former, for both HCl and HNO3 acidified of 88% in biphasic medium (water : MeTHF) at 200 °C in
samples, despite a theoretical ratio of 1 : 1. This effect is par- 60 min ( pH 1) with FeCl3 and a yield of 56% for 5-HMF in a
ticularly strong at a synthesis pH of 0. Under these conditions, biphasic solution at higher pH ( pH = 2) in 180 min with
the strong acidity makes formic acid and other carboxylic FeSO4. Weiqi et al.364 have studied the synergistic effect of
acids break down into gaseous species (CO and CO2), also coupling CrCl3 and H3PO4 on glucose conversion to LA com-
causing a slight rise in the final pH.358 pared to a single use of CrCl3 or H3PO4. The highest LA
Hydrothermal treatment of poly- and monosaccharides yield of 54.24% was obtained from 100% glucose conversion
under basic conditions leads to a quite significant change in at 170 °C for 240 min. CO2 in hydrothermal applications can
the scenario. In fact, although bases such as NaOH, KOH and be considered as both a Lewis acid in gaseous form or
Ca(OH)2 are all able to accelerate the hydrolysis of cellulose to Brønsted acid in its hydrated form. In any case, it has been
glucose, they somehow change the pathway of carbonisation, proved to be effective at catalysing carbohydrates to 5-HMF.
slowing down its dehydration to 5-HMF, with a consequent Lin et al.365 have obtained a 5-HMF yield of 60.33% from
buildup of the glucose concentration and lower yields of levuli- fructose conversion at 190 °C for 20 min with a CO2 pressure
nic acid.338,356 On the other hand, a rise in formic acid and of 2 MPa. There is proof that compressed CO2 is also
lactic acid concentrations is observed with higher synthesis effective at hindering the formation of undesired 5-HMF oli-
pH,338 pointing out the preferred pathway of degradation of gomers, thus improving the conversion yield of sugar to
the hexoses, involving retro aldol condensation, as shown in 5-HMF.366 Using heterogeneous catalysis has the significant
Fig. 6. advantage of avoiding tedious recovery steps that are necess-
Salt anions too can affect the conversion of sugars under ary for homogeneous catalysts. The most recent hetero-
hydrothermal conditions. In fact, anions with good leaving geneous catalysts proposed include transition metal
group qualities can strongly accelerate the rate of dehydra- oxides,367 phosphates,368,369 zeolites,370 organic polymers,371
tion of fructose to 5-HMF, provided that they are also small and carbon sulfates.372–374
and good nucleophiles. This is due to their intervention in
the first step of dehydration of fructose, which involves sub- 5.3. Influence of reaction time and temperature on soluble
stitution and elimination reactions on the C2 carbon.359 carbonisation products
Lewis acids are also of interest in the homogeneous catalysis Several studies have investigated the effect of reaction time
of dehydration reaction as substitutes of traditional Brønsted and temperature on product yields in the liquid phase after
acids, due to their lower corrosivity. Various Lewis acids have hydrothermal carbonisation of carbohydrates and ligno-
been studied for carbohydrate conversion. Dehydration of cellulosic feedstock. Table 8 provides a summary of the reac-
glucose has been achieved using CrCl2 in an ionic liquid tion conditions used in the papers taken into account. Seen
with 70% yield of HMF;360 CrCl3 and HCl with 59% yield of from the perspective of reaction time, in the early stages of
HMF in water (NaCl)/THF biphasic medium;361 and SnCl4 in polysaccharide conversion, the HTC liquid phase is character-
an ionic liquid to afford a 64% yield of HMF.362 More ised by a relatively high concentration of sugars (glucose) and
recently, Jiang et al.363 have achieved a yield of levulinic acid aromatics (5-HMF). As a general trend, it appears that hydro-

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Table 8 Summary of carbon precursors and reaction conditions used to study the impact of reaction parameters on product yields in the HTC
liquid phase

Carbon precursor Biomass/water ratio (wt%) Reaction temperature (°C) Reaction time Ref.

Starch 10% 180, 200, 220, 240 2 to 40 min 330

Biomass (Jeffrey pine and white fir) 12.5% 215, 235, 255 275, 295 30 min 303
255 5 to 60 min
Cellulose 20% 225, 250, 275 0 to 96 h 76
Cellulose, wheat straw, poplar 12.5% 200, 230, 260 (fixed) 0 to 8 h 331
160–260 (dynamic) —
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Coconut husk, rice husk 10% 140, 160, 180, 200 4h 332
200 1, 2, 3 h
Cassava rhizome 20%, 10%, 6.6% 160, 180, 200 1, 2, 3 h 333
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Corncomb 20%, 10%, 6.6% 160, 180, 200 1, 2, 3 h 334

Sucrose, glucose, fructose 2% (sucrose) 1.05% (glu., fru.) 180 2 to 24 min 357
200, 220 2 to 18 min

lysis of polysaccharides to simple sugars consistently increases direct distillation unfavourable. Therefore, every process
with temperature, with the maximum yield shifting to shorter designed to convert cellulosic biomass into platform chemi-
reaction time as temperature increases.303,330 The highest cals must deal with a liquid–liquid extraction step. A good
sugar yields are generally found between 180 °C and 230 °C solvent for liquid–liquid extraction must fulfil some basic
and between 1 hour and 2 hours.331,333,334 Fructose yields are requirements: it must extract effectively and selectively the
generally lower than glucose yields330,331 and this may serve as compound of interest; it must be recoverable by distillation, in
proof that the reaction of fructose dehydration to 5-HMF is order to be recycled, thus lowering the production costs; it
kinetically faster than the isomerization of glucose to fructose. must be poorly soluble in water, in order to minimize losses
Therefore, in the early stages of hydrolysis of cellulose, the during extraction; it must have a fairly different density from
concentration of glucose builds up, whereas fructose is readily water, to allow quick separation. In addition to these funda-
consumed. 5-HMF yield is very dependent on both the reaction mental features, from the perspective of a greener process, it
time and temperature and its production is strictly connected should also be inexpensive and non-toxic.375 Several organic
to the sugar content in the liquid phase. Although the time solvents have been evaluated for the liquid–liquid extraction of
scale of the 5-HMF concentration peak may vary from a few 5-HMF, levulinic acid and furfural. Methyl isobutyl ketone
minutes to hours, depending on the type and size of the (MIBK),376,377 2-butanol,340,376 THF,378 and 2-MTHF378 are
reactor,330,331,333,334 it always follows the maximum in mono- most frequently used for the extraction of HMF, although
saccharide concentration. 5-HMF prefers moderate tempera- some other alternative solvents such as dimethylcarbonate,369
ture for optimal yields (180–220 °C),331,333,334 while higher o-propylphenol379 or hexafluoroisopropanol380 have been pro-
temperatures cause faster conversion but lower yields, due to posed due to their remarkably high partitioning coefficients.
the increase of secondary reactions. Experiments on the con- MIBK in particular has been proven to be the best solvent for
version of simple sugar monomers (glucose and fructose) and the extraction of 5-HMF in a counter current, due to the com-
dimers (sucrose), although being conducted in the presence of bined effect of its high partition coefficient and its lower solu-
an acid catalyst and therefore being kinetically faster, lead to a bility in water, compared to other solvents. Addition of sodium
similar conclusion.357 Xylose and furfural are only minor pro- chloride further improves the partition coefficient.381
ducts in the hydrothermal carbonisation of hexoses.357 In con- Exploiting the salting-out effect of some common salts is an
trast, furfural becomes much more relevant in hemicellulose- effective method to maximize the liquid–liquid extraction.
rich biomass and its decline throughout the process is slower Salting-out efficiency has been demonstrated to be roughly
compared to that of 5-HMF, which indicates a slightly higher independent of the nature of the cations; in fact, it only
stability.331 Finally, organic acids tend to prevail as the reac- depends on the anion used. Moreover, the partitioning behav-
tion time proceeds and higher temperatures are used, with iour of the salt is roughly independent of the extraction
acetic acid being particularly high in concentration in HTC of solvent used.382 Pentasodium phytate, a green and cheap
lignin-rich biomass.303,331–334 salting-out agent derived from cereals and currently con-
sidered waste, has also been considered as an alternative to in-
5.4. Chemical isolation organic salts. In a comparative study with other inorganic salts
HTC typically occurs in water or aqueous medium, where the in a water/1-butanol/HMF system, the most pronounced separ-
chemical products of interest are found at the end of the reac- ation of HMF than the other tested sulfates has been
tion. Typical compounds found in the water phase are car- shown.383 Salts can also increase the separation efficiency
boxylic acids, aldehydes, ketones and furfural derivatives. when a system of two mixed solvents is used instead of one
Although the yields of conversion may vary, the concentrations single pure solvent. However, subsequent recovery of the sol-
of these products are quite diluted, making separation by vents can be reduced upon addition of salts due to the for-

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mation of complexes.384 When ionic liquids are used as reac- ation, nucleation (aromatization) and growth. Wang et al.
tion media for dehydration of sugars to HMF and levulinic reported a simple model of the different stages in 2012.393
acid, extraction with the aforementioned solvent leads to very Some people also infer the final step as surface passivation by
poor yields. This has been explained with the strong H-bond the residue precursor.394,395 Liu and co-workers prepared nitro-
interaction between the hydroxyl group of HMF and the anion gen and sulfur co-doped CDs via hydrothermal treatment of
of the IL.385 In fact, it has been observed that the addition of L-cys and NH3·H2O.
394
They have claimed that L-cys molecules
small chain alcohols, by interfering with the aforementioned went through a few steps from decomposition to passivation,
H-bond, results in a significant improvement of the extraction as shown in Fig. 34A. The HRTEM images of CDs being impec-
efficiency of HMF by MIBK.386 Compressed CO2 as the extrac- cable gradually support the possible formation mechanism. In
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tion solvent of HMF from ILs has also been considered, one of the earlier reports, Sahu et al. have proposed a simpli-
proving to be a promising approach due to the combination of fied 3-step explanation for the formation mechanism of hydro-
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

both the extraction efficiency and enhanced solubility of thermally synthesized CDs.20 The authors have concluded that
glucose in the IL medium.387 Brouwer et al.388 have recently the dehydration and decomposition of fructose/glucose gave
proposed a method to separate furfural, levulinic and formic rise to different soluble chemical compounds, as mentioned
acid in different stages from the same acidic aqueous stream. before in the liquid phase. Hydronium ions formed from these
Furfural is extracted in the first stage with toluene. acids acted as a catalyst in subsequent decomposition reaction
Subsequently, levulinic acid and formic acid are extracted with stages. The polymerisation and condensation of these soluble
a mixture of 30% TOPO (trioctylphosphine oxide) in MIBK, chemicals gave rise to soluble polymers where aromatization
with a high partition coefficient and selectivity over sulfuric took place via aldol condensation, cycloaddition and hydroxy-
acid. A double temperature swing back extraction has been methyl-mediated furan resin condensation.50 When the con-
performed to concentrate LA and FA, therefore lowering the centration of aromatic clusters reached a critical supersatura-
energy required for the final distillation step to obtain levuli- tion point, burst nucleation took place, and carbon dots were
nic acid. grown. Similarly, the particle formation of CDs reported from
banana juice396 and sweet potato397 also follows the three
steps of carbonisation of carbohydrates. Later Qu et al. syn-
6. Carbon dots thesized N-doped graphene quantum dots (GQDs) with a
hydrothermal method with citric acid and urea.398 They con-
Apart from the formation of new chemical compounds in the cluded that the formation of N-doped GQDs involved two
liquid phase, there is an interface grown between the solid steps. In the first step, citric acid molecules self-assembled
carbon microsphere and liquid chemicals, which is considered into sheet structures and dehydrolyzed to form a graphene
as a third product. This is where the nucleation process takes framework, and the reaction between citric acid and amines
place as a new phase is formed, leading to thermodynamic occurred. Through the second step of intramolecular dehydro-
instability. The small nuclei formed at this interface represent lysis, amides reacted with carboxylic groups and formed pyrro-
the fluorescent carbon dots (CDs) with a size generally below lic N. Similar methods have also been reported by Bai et al. for
10 nm, which were previously reported via the HTC process.389 N-doped CDs produced from citric acid and ethylenedia-
Because of some unique properties, such as low toxicity, high mine,407 and Atchudan et al. with Chionanthus retusus fruit
photoluminescence (PL) quantum yield, facile modification extract.400 Our group has been pioneering in the field of HTC
and excellent electron donating/accepting abilities, CDs have for the last decade and based on the structural investigation of
found applications in many fields, ranging from bioimaging to CDs, we propose an evolutional mechanism, which involves
sensors, LEDs, photocatalysis, solar cells and more.24,390,391 the influence of the gas phase during the reaction.401,402 We
The electronic states and optical properties that make CDs believe that some of the polymeric nuclei formed initially
suitable for the above-mentioned applications are strictly con- during the hydrothermal process act as hot spots (due to temp-
nected to their structural features, which can be controlled erature gradients existing in the autoclave during the HTC
during synthesis. On that note, a detailed mechanism for the process). At the same time, CO/CO2 gas along with H2 are
formation of CDs from molecular precursors is of vital impor- formed in situ in the autoclave from partial gasification of
tance to better understand the unique properties of these glucose under subcritical conditions. These gases deposit onto
materials and also facilitate control of their key characteristics. the hot spot nuclei in a CVD-like process via CO2 reduction
under autogenic pressures (30 bars) leading to the formation
6.1. Formation mechanism of crystalline carbons. With the observation from the HRTEM
So far, two different mechanisms have been proposed in the images in Fig. 34B, we hypothesize that the first crystalline
literature. One suggests a growth according to the classical La form of CDs is sp3-bonded carbon obtained from an amor-
Mer model,392 where the resulting nuclei grow uniformly and phous/carbon black type which then can further convert into
isotropically by diffusion of solutes towards the particle carbon anions which subsequently form graphitic struc-
surface until the final size is attained.392 It is usually divided tures.403 During the incipient stage of glucose transformation
into several stages, which depending on the classification can via the “hot spot CVD” mechanism mentioned above, a
be summarized as decomposition, condensation, polymeris- mixture of sp2/sp3 carbon nanostructures are initially formed

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Fig. 34 (A) Schematic illustration of the possible mechanism for the formation of N,S-CDs using L-cys as the carbon source, HR-TEM of CDs at
different formation stages, and the PL behaviour under different pH values; reproduced with permission from ref. 394. (B) HR-TEM images showing
the evolution of glucose derived CDs and the influence of solvent polarity on the excitation-dependent PL properties; reproduced with permission
from ref. 401. (C) Mechanism of the reaction from NTA to N-doped GQDs; reproduced with permission from ref. 404. (D) (a) PL QY of CDs as a func-
tion of reaction time, (b) evolution of the relative amount of C–C, CvC, and CvN bonded C atoms inside the CDs during the synthesis, and (c) the
scheme of different structures contributing to PL with respect to time; reproduced with permission from ref. 405.

which are later converted into predominately sp2 carbon mation of large-sized polymeric nanoparticles through inter-
nanostructures.403 molecular dehydration occurred upon high temperature and
Jiang and co-workers modelled the polymerization process pressure exposure of the precursor. As the reaction proceeded,
of nitrilotriacetic acid (NTA) to form N-doped GQDs under nucleation started as the aforementioned polymeric nano-
hydrothermal conditions (250 °C, 4 h), and concluded that the particles partitioned, resulting in the coexistence of polymers
mechanism involved dehydration and nucleophilic addition and CDs. Upon further increase in heating time, CDs con-
between different NTA molecules to form a conjugated system, stantly surpassed the polymer nanoparticles until the latter
as shown in Fig. 34C, which grows into larger CQDs as the disappeared. Similar processes have also been reported by
reaction continues.404 The other formation mechanism Yang et al. with amino acid with respect to the time
involves local nucleation on some high energy areas. Dai et al. increase.407 Unfortunately, in all the above cases, the for-
synthesized hydrothermal CDs and proposed a formation mation mechanism appears as a hypothetical suggestion and
mechanism based on the temperature increase.406 At first, for- there is no clear evidence via in situ characterisation to

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support it. Therefore, an in-depth insight involving in situ tech- first 30 min and the size remained constant for the rest of the
niques is crucial for the survival of these systems initially and synthesis. Instead of growing in size, they undergo a substan-
later to accomplish their commercialization. tial transformation of the internal structure, with larger aro-
matic domains and a higher level of graphitization, as seen in
6.2. Reaction parameters Fig. 34D. The PL QY decreases accordingly, with a shorter PL
At this stage, it is crucial to explore the parameters affecting lifetime, which implies an increase in the nonradiative recom-
the properties of the resulting CDs during their synthesis. It is bination process.405
well-known that surface groups in particular are highly depen-
dent on the conditions of the synthesis, such as the solvent, 6.3. Photo-related applications
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temperature and pH. The solvents can affect the excitation By understanding the formation mechanism and controlling
dependent PL properties, as shown in the maps in Fig. 34B, the reaction parameters to manipulate the optical properties
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and this is generally attributed to the presence or absence of of CDs, researchers have been pushing forward to utilize this
oxygen-containing functional groups. Such groups are formed material for practical applications. Due to their easy and cheap
easily in solvents with stronger hydrophilicity and higher synthesis, low toxicity, high (aqueous) solubility, good optical
polarity (such as water and ethanol), enhancing blue emission. properties with high fluorescence quantum yield, facile modi-
In contrast, low polarity solvents such as cyclohexane and fication and stability against photobleaching, CDs made from
toluene show a negligible amount of oxygen-related groups, HTC are superior to the traditional semiconductor quantum
decreasing blue emission, while increasing the red one.408,409 dots. Interesting facts about this material have been discovered
Temperature is another key factor in the preparation of CDs. from research over the past few years, which makes it a good
Several physical, chemical and optical properties of the result- candidate for various applications, such as bio-imaging, optro-
ing products can be affected by the reaction temperature such nics, photocatalysis and photovoltaics. Our group has been
as the particle size, crystallization and surface functionality working on applying CDs as sustainable alternatives for some
and as a consequence the PL intensity and peak hazardous materials in solar cells and photocatalysis. In an
position.410,411 The different pH environment during HTC can early report, biomass derived CDs have been used to sensitize
largely affect the PL properties of CDs by affecting the revers- ZnO nanorods to visible light, allowing their use in solid-state
ible protonation and deprotonation of the abundant func- nanostructured solar cells.389 The performance of the devices
tional groups.412 Each functional group contributes to the depended on the functional groups on the CD surface, and it
overall emission in a different way in different pH ranges, is likely that the functionalisation determines the binding of
therefore leading to intensity variation or even shifting the CDs to the ZnO surface and the nature of the charge transpor-
peak position. Compared to the other parameters, the reaction tation and recombination. Later we further investigated the
time can influence the CD formation stage, which changes the influence of functionalisation on solar cell performance.415 Six
structural and optical properties accordingly. Bai and co- types of CDs were produced from different precursors and
workers changed the reaction time of metronidazole derived used as sensitizers for TiO2-based solar cells. By investigating
CDs and found that the formation of CDs starts after 3 h. They the structural and optical properties of different CDs we have
monitored the surface functional group changes using FTIR shown that the combination of amine and carboxylic acid
techniques and learned that the surface groups of CDs groups is more beneficial for enhancing the solar cell perform-
changed gradually, with –NO2 reduced to N–H, while C–OH ance. In a more recent work, chitosan derived CDs with nitro-
and C–H oxidized to C–O–C and COOH. The abundant surface gen doping were studied as potential photocatalysts.414 The
groups enabled further carbonisation of the CDs, as well as CDs were covalently bonded on a conductive substrate and
granted good solubility in water. After 8 h reaction, the PL tested as a photoanode. As shown in Fig. 35, the CD electrode
intensity became much stronger than that at 3 h, while the is capable of producing a photocurrent under visible light,
chemical groups changed a little.399 The effect of reaction time owing to the visible light absorption, easily accessed energy
on the QY is also investigated, which showed a volcano-shaped states and good charge transfer ability, which can be used to
trend with the highest QY obtained at a 6 h reaction time.413 sensitize other semiconductor photocatalysts or it can act as a
Jiang and co-workers combined experimental and theoretical photocatalyst itself to drive chemical reactions.
calculations on nitrogen-doped GQDs to study the formation
mechanism. As the reaction proceeded, the size of the planar
structure increased and a size change occurred from non- 7. Summary and perspective
uniform to uniform. This phenomenon was explained by the
stability of the different products: the less stable intermediate HTC represents a useful strategy for a variety of present-day
small fractions would decompose into the original precursor applications. The high flexibility in reaction parameters,
while the relatively stable N-doped graphene layers would grow including the reaction time and temperature, plus the easiness
at high temperature and pressure.404 A different formation in combining the feedstock with different catalysts, on the one
behaviour of CDs has been reported by Feldmann and co- hand, in order to enhance the valuable chemicals that can be
workers using citric acid and ethylenediamine as the precur- found in the liquid phase and/or the hydrothermal carbon
sors. They have found that the CDs grew rapidly during the yield, or various templates, activators and/or heteroatom

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Fig. 35 (a) TEM images of as-prepared CDs; (b) fabrication of the CD electrode; (c) chopped photocurrent generation under visible light illumina-
tion and (d) the proposed reaction mechanism of CDs as the photocatalyst; reproduced with permission from ref. 414.

doping, on the other hand, in order to produce porous carbon starting material. The advantage of combining HTC with side
structures decorated with functionalities, make the HTC one strategies, such as templating methods (soft or hard templat-
of the most used approaches, around the research world, when ing) or chemical activation, results in the production of highly
it comes to transforming waste into valuable products via a porous carbon structures characterised by their large specific
sustainable route. surface area and large pore volume, as detailed in section 2.
As is summarised here, HTC can be used in degradation of This is a great achievement for further involvement of these
biomass and biomass derivatives in three different products, materials in environmental applications such as air depollu-
such as liquid phase containing chemicals (5-HMF, levulinic tion and water treatment or in energy storage applications,
acid, formic acid, and acetic acid), solid carbon materials and including Li-ion batteries and supercapacitors, as electrode
fluorescent carbons. Regarding the biomass decomposition materials. Carbon-based materials have superior cycling stabi-
and hydrothermal carbon formation, several mechanisms have lity and rate capability, and so can be operated at higher
been proposed and the main steps are hydrolysis, dehydration, charge and discharge rates and have much longer lifetimes.
polymerization, aromatization and nucleation. Many studies Moreover, the hierarchical pore structures enable ion mass
have been focused on the influence of the feedstock, reaction transport, and therefore porous carbon is an important
time and temperature on the materials properties and carbon material for supercapacitor electrodes. Other applications of
yield, summarizing that as the reaction severity increases, the HTC carbon include their usage as electrocatalysts in the
compound solubilisation and volatilisation increase, decreas- ORR. Due to the easiness of combining the carbon precursor
ing the solid yields. Also, when the trends of many feedstocks (biomass) with different chemicals, a wide range of hetero-
were collectively investigated, it was observed that feedstock atom doped carbons can be synthesised via HTC, as discussed
properties have a significant influence on the hydrothermal in section 3. The heteroatoms doped within the carbon matrix
carbon yield. When it comes to the solid composition, it was induce heteroatom–carbon bonds, and the polarisation of
observed that regardless of the biomass precursor, the result- these bonds generates active sites on the adjacent carbon
ing hydrothermal carbons contain between 4 and 6% hydro- matrix resulting in a reduced energy barrier towards electroca-
gen; meanwhile the oxygen content seems to be highly depen- talysis. Simultaneously, the electrical conductivity is increased
dent on the precursor. Regarding their structural properties, due to the higher concentration of delocalised electrons within
the resulting powders consist mainly of nonporous spherical the π-system. In addition, the HTC carbon powders can be suc-
particles, with variation in sizes and shape depending on the cessfully employed as a catalyst support as the tunability of

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their surface polarity and area facilitates the anchoring of mechanism of these challenging materials. Understanding the
metal nanoparticles, which can then be used in different fundamentals will allow better control over the synthesis and
reactions. upscaling of these materials. This is valid not only for the
Chemicals in the liquid phase such as 5-HMF, levulinic carbon dots, but also for the HTC materials, where still certain
acid, formic acid, lactic acid and acetic acid, represent a huge fundamental aspects such as nucleation, growth, aggregation,
opportunity as a source of bio-based materials and fuel addi- and the role of water remain to be elucidated and where in
tives. Their synthesis is strictly connected to that of hydro- operando characterisation techniques during the HTC
thermal carbons and therefore their joint production rep- materials formation will play an important role in overcoming
resents an added value. Of course, this opportunity comes these major challenges.
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with several challenges. Yields of conversion are still relatively Of crucial importance in the future, an area where not
poor, separation from reaction medium is difficult and cor- many scientific breakthroughs have been seen, is processing
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

rosion risk due to highly acidic process water is high. All of HTC materials. This could involve techniques such as electro-
these issues must be addressed to allow the successful transit spinning, electrowriting or 3D printing to create 3D architec-
of these processes to industrial scale production. tures with tunable properties for a wide range of applications
The newly discovered third product, carbon dots, holds from energy storage to wearable electronics. Finally, new appli-
great potential in many applications, especially in photocataly- cations of these materials must be explored along with already
sis and photovoltaics. In section 6, the formation mechanism emerging applications, with focus on energy and the
and the reaction parameters were discussed, giving guidance environment.
to future investigations on this intriguing material. Given the New applications will emerge especially in the case of
fact that HTC is an easy and low-cost process, and the CDs carbon dots, in particular in photocatalysis, biosensors and
formed from this method show excellent bio-compatible and optoelectronic applications. Emerging applications of HTC in
optical properties, producing high quality CDs with HTC to batteries and electrocatalysis will continue to rise. In particu-
replace the traditional hazardous semiconductor quantum lar, the development of multi-ion batteries based on dual
dots could be one of the future directions towards carbon electrodes designed with the right configuration for
sustainability. both alkali metals and anion storage as well as other hybrid
Overall, publications from the past few years show that HTC supercapacitor-battery systems will be crucial in the near
has been intensively researched and upgraded to be used in future. These batteries could become compostable, whereby
novel and up-to-date applications. Despite the fact that the after the end of life, a battery containing no current collector
process has been known since 100 years ago, this review but only carbon electrodes could be mixed with soil using the
reveals that the research community has high interest in HTC, concept of Terra Pretta. In terms of electrocatalysis using HTC
and there are still gaps to be fulfilled and debates to be solved, supports to anchor single atom sites or dimer sites to control
in terms of the mechanism of carbon sphere formation and the selectivity to various products, especially for oxygen, CO2
carbon quantum dot formation. Moreover, considerable atten- and nitrogen, reaction will be crucial. Particularly promising
tion should be devoted to upscaling the HTC process, will be the use of the resulting modified HTC materials as elec-
especially related to the advanced materials production requir- trocatalysts to further convert the liquid chemicals from the
ing advanced reactor design and process engineering. This aqueous phase into high end products and/or fuels. If we
includes also the energetic and techno-economic analysis could create a circular process where the HTC products could
along with their environmental impact at each life stage. be tailor made with the right electrochemical properties to be
A great deal of fundamentals in HTC have already been able to further catalyze under the application of an external
understood but there is still a lack of details. For example, the bias the conversion of liquid phase chemicals such as HMF
mechanism of nucleation, the rate-limiting step, the kinetics and LA into other valuable components, the economic value of
of the reaction, and the mechanism of the reaction still the HTC process would increase.
remain ambiguous. Something that is not well understood so Other applications of increasing importance will be based
far is the role and influence of pressure during the HTC on using HTC adsorbents for critical metal recovery to recover
process and how the reaction mechanism/nucleation change metals of crucial economic importance from batteries, fuel
in the presence of external gases, i.e. when the HTC process is cells, electrolysers, solar panels or wind turbines and facilitate
run under an inert gas or CO2. In addition, it is still not clear the movement towards a circular economy.
what dictates the formation of the amorphous HTC spheres Finally, clearer correlations, especially when using real
and what dictates the formation of the carbon dots in solution. biomass, must be made between the (bio)chemistry of the
We assume that a gas phase reaction is involved in the for- biomass precursor and the resulting carbon materials given
mation of the carbon dots which is yet to be proved. To under- that biomass is variable. For the production of advanced
stand the missing fundamentals, in situ/operando characteris- materials, it is desired that raw biomass is separated into pure
ation using synchrotron facilities (Small Angle X-Ray/Neutron components before being transformed by means of hydro-
Diffraction and Pair Distribution Function) is crucial. In thermal processes. For bioenergy, the biomass-final product
addition, on-line monitoring using NMR or HPLC-MS of the characteristics must be understood and optimised to ensure
reaction products will be able to shed light on the formation reproducibility.

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Life cycle assessment and life cycle costing along with a 19 J. A. Rice and P. MacCarthy, Geoderma, 1988, 43, 65–73.
careful estimation of energy balances must become an integral 20 S. Sahu, B. Behera, T. K. Maiti and S. Mohapatra, Chem.
part of future studies reporting either advanced materials pro- Commun., 2012, 48, 8835.
duction or biomass to bioenergy processes. Only then, we can 21 Z. C. Yang, M. Wang, A. M. Yong, S. Y. Wong, X. H. Zhang,
ensure truly sustainable, environmentally benign and econ- H. Tan, A. Y. Chang, X. Li and J. Wang, Chem. Commun.,
omically viable processes. So far almost none of the reports on 2011, 47, 11615–11617.
HTC and products are accompanied by LCA; however this 22 X. T. Zheng, A. Ananthanarayanan, K. Q. Luo and P. Chen,
must change and become the new norm. Small, 2015, 11, 1620–1636.
23 S. C. Ray, A. Saha, N. R. Jana and R. Sarkar, J. Phys. Chem.
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C, 2009, 113, 18546–18551.

Conflicts of interest 24 S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010,
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

49, 6726–6744.
There are no conflicts to declare. 25 Y. Chen, Y. Wu, B. Weng, B. Wang and C. Li, Sens.
Actuators, B, 2016, 223, 689–696.
26 N. Wang, Y. Wang, T. Guo, T. Yang, M. Chen and J. Wang,
References Biosens. Bioelectron., 2016, 85, 68–75.
27 T. N. J. I. Edison, R. Atchudan, J. J. Shim, S. Kalimuthu,
1 F. Bergius and H. Specht, Verlag Wilhelm Knapp, Halle der B. C. Ahn and Y. R. Lee, J. Photochem. Photobiol., B, 2016,
Saale, Ger, 1913, p. 58. 158, 235–242.
2 E. Berl and A. Schmidt, Justus Liebigs Ann. Chem., 1932, 28 J. Liao, Z. Cheng and L. Zhou, ACS Sustainable Chem. Eng.,
493, 97–123. 2016, 4, 3053–3061.
3 E. Berl and A. Schmidt, Justus Liebigs Ann. Chem., 1932, 29 M. Jahanbakhshi and B. Habibi, Biosens. Bioelectron.,
493, 124–135. 2016, 81, 143–150.
4 E. Berl and A. Schmidt, Justus Liebigs Ann. Chem., 1932, 30 J. Li, L. Zhang, P. Li, Y. Zhang and C. Dong, Sens.
493, 135–152. Actuators, B, 2018, 258, 580–588.
5 E. Berl and A. Schmidt, Justus Liebigs Ann. Chem., 1932, 31 Y. Guo, L. Yang, W. Li, X. Wang, Y. Shang and B. Li,
496, 283–303. Microchim. Acta, 2016, 183, 1409–1416.
6 Q. Wang, H. Li, L. Chen and X. Huang, Carbon, 2001, 39, 32 R. Atchudan, T. N. J. I. Edison, M. G. Sethuraman and
2211–2214. Y. R. Lee, Appl. Surf. Sci., 2016, 384, 432–441.
7 C. Falco, F. Perez Caballero, F. Babonneau, C. Gervais, 33 H. Wikberg, T. Ohra-aho, F. Pileidis and M.-M. Titirici,
G. Laurent, M.-M. Titirici and N. Baccile, Langmuir, 2011, ACS Sustainable Chem. Eng., 2015, 3, 2737–2745.
27, 14460–14471. 34 D. Kim, K. Lee and K. Y. Park, J. Ind. Eng. Chem., 2016, 42,
8 C. Falco, N. Baccile and M.-M. Titirici, Green Chem., 2011, 95–100.
13, 3273–3281. 35 E. Dinjus, A. Kruse and N. Tröger, Chem. Eng. Technol.,
9 C. Falco, J. P. Marco-Lozar, D. Salinas-Torres, E. Morallon, 2011, 34, 2037–2043.
D. Cazorla-Amorós, M.-M. Titirici and D. Lozano-Castelló, 36 J. Lu, Z. Liu, Y. Zhang and P. E. Savage, ACS Sustainable
Carbon, 2013, 62, 346–355. Chem. Eng., 2018, 6, 14501–14509.
10 C. Falco, J. M. Sieben, N. Brun, M. Sevilla, T. der Mauelen, 37 A. Funke and F. Ziegler, Biofuels, Bioprod. Biorefin., 2010,
E. Morallón, D. Cazorla-Amorós and M.-M. Titirici, 4, 160–177.
ChemSusChem, 2013, 6, 374–382. 38 F. Buendia-Kandia, G. Mauviel, E. Guedon, E. Rondags,
11 M.-M. Titirici, A. Thomas and M. Antonietti, New J. Chem., D. Petitjean and A. Dufour, Energy Fuels, 2018, 32(4),
2007, 31, 787–789. 4127–4138.
12 M.-M. Titirici, A. Thomas and M. Antonietti, J. Mater. 39 Y. Matsumura, M. Sasaki, K. Okuda, S. Takami, S. Ohara,
Chem., 2007, 17, 3412–3418. M. Umetsu and T. Adschiri, Combust. Sci. Technol., 2006,
13 M. Sevilla and A. B. Fuertes, Chem. – Eur. J., 2009, 15, 178, 509–536.
4195–4203. 40 X. Yan, F. Jin, K. Tohji, T. Moriya and H. Enomoto,
14 L. Wang, Y. Chang and A. Li, Renewable Sustainable Energy J. Mater. Sci., 2007, 42, 9995–9999.
Rev., 2019, 108, 423–440. 41 X. Yan, F. Jin and K. Tohji, AIChE J., 2010, 56, 2727–
15 M.-M. M. Titirici and M. Antonietti, Chem. Soc. Rev., 2010, 2733.
39, 103–116. 42 F. Jin, Z. Zhou, H. Enomoto, T. Moriya and
16 T. Werpy and G. Petersen, Other Inf. PBD 1 Aug 2004, H. Higashijima, Chem. Lett., 2004, 33, 126–127.
2004, Medium: ED; Size: 76 pp. 43 B. Danon, G. Marcotullio and W. De Jong, Green Chem.,
17 S. Román, J. M. V. Nabais, C. Laginhas, B. Ledesma and 2014, 16, 39–54.
J. F. González, Fuel Process. Technol., 2012, 103, 78–83. 44 R. J. Van Putten, J. N. M. Soetedjo, E. A. Pidko, J. C. Van
18 A. Alvarez-Murillo, S. Roman, B. Ledesma and E. Sabio, Der Waal, E. J. M. Hensen, E. De Jong and H. J. Heeres,
J. Anal. Appl. Pyrolysis, 2015, 113, 307–314. ChemSusChem, 2013, 6, 1681–1687.

4792 | Green Chem., 2020, 22, 4747–4800 This journal is © The Royal Society of Chemistry 2020
 View Article Online

Green Chemistry Critical Review

45 N. Baccile, G. Laurent, F. Babonneau, F. Fayon, 71 L. Li, R. Diederick, J. R. V. Flora and N. D. Berge, Waste
M.-M. Titirici and M. Antonietti, J. Phys. Chem. C, 2009, Manage., 2013, 33, 2478–2492.
113, 9644–9654. 72 X. Lu and N. D. Berge, Bioresour. Technol., 2014, 166, 120–
46 G. Yang, E. A. Pidko and E. J. M. Hensen, J. Catal., 2012, 131.
295, 122–132. 73 K. Aydıncak, T. Yumak, A. Sınag and B. Esen, Ind. Eng.
47 D. Jung, M. Zimmermann and A. Kruse, ACS Sustainable Chem. Res., 2012, 51, 9145–9152.
Chem. Eng., 2018, 6, 13877–13887. 74 N. D. Berge, K. S. Ro, J. Mao and J. R. V. Flora, Environ.
48 I. Van Zandvoort, E. J. Koers, M. Weingarth, Sci. Technol., 2011, 45, 5696–5703.
P. C. A. Bruijnincx, M. Baldus and B. M. Weckhuysen, 75 S. Kang, X. Li, J. Fan and J. Chang, Ind. Eng. Chem. Res.,
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Green Chem., 2015, 17, 4383–4392. 2012, 51, 9023–9031.

49 A. B. Brown, B. J. McKeogh, G. A. Tompsett, R. Lewis, 76 X. Lu, P. Pellechia, J. R. V. Flora and N. Berge, Bioresour.
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

N. A. Deskins and M. T. Timko, Carbon, 2017, 125, 614– Technol., 2013, 138, 180–190.
629. 77 S. S. Jamari and J. R. Howse, Biomass Bioenergy, 2012, 47,
50 M. Sevilla and A. B. Fuertes, Carbon, 2009, 47, 2281–2289. 82–90.
51 G. Tsilomelekis, M. J. Orella, Z. Lin, Z. Cheng, W. Zheng, 78 X. Cao, K. S. Ro, M. Chappell, Y. Li and J. Mao, Energy
V. Nikolakis and D. G. Vlachos, Green Chem., 2016, 18, Fuels, 2011, 25, 388–397.
1983–1993. 79 C. Areeprasert, P. Zhao, D. Ma, Y. Shen and K. Yoshikawa,
52 Y. Qi, B. Song and Y. Qi, RSC Adv., 2016, 6, 102428– Energy Fuels, 2014, 28, 1198–1206.
102435. 80 M. Sevilla, G. A. Ferrero and A. B. Fuertes, Carbon, 2017,
53 Z. Cheng, J. Everhart, G. Tsilomelekis, V. Nikolakis, 114, 50–58.
B. Saha and D. Vlachos, Green Chem., 2018, 20, 997–1006. 81 R. Azargohar, Production of Activated Carbon and its
54 L. J. R. Higgins, A. P. Brown, J. P. Harrington, A. B. Ross, Catalytic Application for Oxidation of Hydrogen Sulphide,
B. Kaulich and B. Mishra, Carbon, 2020, 161, 423–431. 2009.
55 M. Titirici, M. Antonietti and N. Baccile, Green Chem., 82 E. Atanes, A. Nieto-Márquez, A. Cambra, M. C. Ruiz-Pérez
2008, 10, 1204–1212. and F. Fernández-Martínez, Chem. Eng. J., 2012, 211–212,
56 A. J. Romero-Anaya, M. Ouzzine, M. A. Lillo-Ródenas and 60–67.
A. Linares-Solano, Carbon, 2014, 68, 296–307. 83 K. V. Kumar, K. Preuss, L. Lu, Z. X. Guo and M. M. Titirici,
57 Y. Yu and H. Wu, Ind. Eng. Chem. Res., 2010, 49, 3902– J. Phys. Chem. C, 2015, 119, 22310–22321.
3909. 84 M. A. Islam, A. Benhouria, M. Asif and B. H. Hameed,
58 M. Li, W. Li and S. Liu, Carbohydr. Res., 2011, 346, 999– J. Taiwan Inst. Chem. Eng., 2015, 52, 57–64.
1004. 85 M. A. Islam, M. J. Ahmed, W. A. Khanday, M. Asif and
59 H. Simsir, N. Eltugral and S. Karagoz, Bioresour. Technol., B. H. Hameed, J. Environ. Manage., 2017, 203, 237–244.
2017, 246, 82–87. 86 P. P. Sharma, J. Wu, R. M. Yadav, M. Liu, C. J. Wright,
60 S.-M. Alatalo, E. Repo, E. Mäkilä, J. Salonen, C. S. Tiwary, B. I. Yakobson, J. Lou, P. M. Ajayan and X.-D.
E. Vakkilainen and M. Sillanpää, Bioresour. Technol., 2013, D. Zhou, Angew. Chem., Int. Ed., 2015, 54, 13701–13705.
147, 71–76. 87 H. Wang, J. Jia, P. Song, Q. Wang, D. Li, S. Min, C. Qian,
61 L. Li, J. R. V. Flora, J. M. Caicedo and N. D. Berge, L. Wang, Y. F. Li, C. Ma, T. Wu, J. Yuan, M. Antonietti and
Bioresour. Technol., 2015, 187, 263–274. G. A. Ozin, Angew. Chem., Int. Ed., 2017, 56, 7847–7852.
62 S. Nizamuddin, H. A. Baloch, G. J. Griffin, N. M. Mubarak, 88 G. W. Liu, T. Y. Chen, C. H. Chung, H. P. Lin and
A. W. Bhutto, R. Abro, S. A. Mazari and B. S. Ali, C. H. Hsu, ACS Omega, 2017, 2, 2106–2113.
Renewable Sustainable Energy Rev., 2017, 73, 1289–1299. 89 M.-M. M. Titirici, R. J. White, N. Brun, V. L. Budarin,
63 I. Oliveira, D. Blöhse and H.-G. Ramke, Bioresour. D. S. Su, F. del Monte, J. H. Clark and M. J. MacLachlan,
Technol., 2013, 142, 138–146. Chem. Soc. Rev., 2015, 44, 250–290.
64 K. U. Suwelack, D. Wüst, P. Fleischmann and A. Kruse, 90 S. Kubo, R. J. White, N. Yoshizawa, M. Antonietti and
Biomass Convers. Biorefin., 2016, 6, 151–160. M. M. Titirici, Chem. Mater., 2011, 23, 4882–4885.
65 K. Suwelack, D. Wust, M. Zeller, A. Kruse and J. Krumpel, 91 K. V. Kumar, S. Gadipelli, K. Preuss, H. Porwal, T. Zhao,
Biomass Convers. Biorefin., 2016, 6, 347–354. Z. X. Guo and M. M. Titirici, ChemSusChem, 2017, 10,
66 H. P. Ruyter, Fuel, 1982, 61, 1182–1187. 199–209.
67 Y. Fu, J. Ye, J. Chang, H. Lou and X. Zheng, Fuel, 2016, 92 M.-M. M. Titirici, R. J. White, C. Falco and M. Sevilla,
180, 591–596. Energy Environ. Sci., 2012, 5, 6796–6822.
68 D. Knežević, W. van Swaaij and S. Kersten, Ind. Eng. Chem. 93 Z. Zhang, M. Qin, B. Jia, H. Zhang, H. Wu and X. Qu,
Res., 2010, 49, 104–112. Chem. Commun., 2017, 53, 2922–2925.
69 M. K. Akalın, K. Tekin and S. Karagoz, Bioresour. Technol., 94 P. W. Xiao, L. Zhao, Z. Y. Sui, M. Y. Xu and
2012, 110, 682–687. B. H. Han, Microporous Mesoporous Mater., 2017, 253,
70 D. Knežević, W. P. M. van Swaaij and S. R. A. Kersten, Ind. 215–222.
Eng. Chem. Res., 2009, 48, 4731–4743. 95 A. B. Fuertes and M. Sevilla, Carbon, 2015, 94, 41–52.

This journal is © The Royal Society of Chemistry 2020 Green Chem., 2020, 22, 4747–4800 | 4793
 View Article Online

Critical Review Green Chemistry

96 P. W. Xiao, D. Guo, L. Zhao and B. H. Han, Microporous 122 F. Rodríguez-Reinoso and M. Molina-Sabio, Carbon, 1992,
Mesoporous Mater., 2016, 220, 129–135. 30, 1111–1118.
97 L. Zhu, F. Shen, R. L. Smith and X. Qi, Energy Technol., 123 J. Guo, Y. Luo, A. C. Lua, R.-A. Chi, Y.-L. Chen, X.-T. Bao
2017, 5, 452–460. and S.-X. Xiang, Carbon, 2007, 45, 330–336.
98 Y. Boyjoo, Y. Cheng, H. Zhong, H. Tian, J. Pan, 124 J. Jin, S. Tanaka, Y. Egashira and N. Nishiyama, Carbon,
V. K. Pareek, S. P. Jiang, J. F. Lamonier, M. Jaroniec and 2010, 48, 1985–1989.
J. Liu, Carbon, 2017, 116, 490–499. 125 X. Zhou, Z. Jia, A. Feng, X. Wang, J. Liu, M. Zhang, H. Cao
99 Q.-P. Fabian, R.-R. Ramiro, M. Emilia and C.-A. Diego, and G. Wu, Carbon, 2019, 152, 827–836.
ChemPlusChem, 2016, 81, 1349–1359. 126 M. Zhu, W. Cai, F. Verpoort and J. Zhou, Chem. Eng. Res.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

100 M. T. Gilbert, J. H. Knox and B. Kaur, Chromatographia, Des., 2019, 146, 130–140.
1982, 16, 138–146. 127 M. Sevilla, W. Gu, C. Falco, M. M. Titirici, A. B. Fuertes
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

101 S. Kubo, R. Demir-Cakan, L. Zhao, R. J. White and and G. Yushin, J. Power Sources, 2014, 267, 26–32.
M. M. Titirici, ChemSusChem, 2010, 3, 188–194. 128 J. Li, H. Qi, Q. Wang, Z. Xu, Y. Liu, Q. Li, X. Kong and
102 S. Z. Niu, S. Da Wu, W. Lu, Q. H. Yang and F. Y. Kang, J. Huang, Electrochim. Acta, 2018, 271, 92–102.
Xinxing Tan Cailiao/New Carbon Mater., 2017, 32, 289–296. 129 J. Shi, N. Yan, H. Cui, Y. Liu, Y. Weng, D. Li and X. Ji,
103 C. Liang, K. Hong, G. A. Guiochon, J. W. Mays and S. Dai, J. CO2 Util., 2017, 21, 444–449.
Angew. Chem., Int. Ed., 2004, 43, 5785–5789. 130 F. Quesada-Plata, R. Ruiz-Rosas, E. Morallón and
104 Y. Zhang, L. Liu, B. Van Der Bruggen and F. Yang, D. Cazorla-Amorós, ChemPlusChem, 2016, 81, 1349–
J. Mater. Chem. A, 2017, 5, 12673–12698. 1359.
105 T. Zhou, R. Ma, Y. Zhou, R. Xing, Q. Liu, Y. Zhu and 131 O. F. Cruz, J. Silvestre-albero, M. E. Casco, D. Hotza and
J. Wang, Microporous Mesoporous Mater., 2018, 261, 88–97. C. R. Rambo, Mater. Chem. Phys., 2018, 216, 42–46.
106 S. Wang, C. Han, J. Wang, J. Deng, M. Zhu, J. Yao, H. Li 132 X. Lu, C. Jiang, Y. Hu, H. Zhong, Y. Zhao, X. Xu and
and Y. Wang, Chem. Mater., 2014, 26, 6872–6877. H. Liu, J. Appl. Electrochem., 2018, 48, 233–241.
107 S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, S. A. Bagshaw, 133 C. Rodríguez Correa, C. Ngamying, D. Klank and A. Kruse,
E. Prouzet and T. J. Pinnavaia, Science, 1995, 269, 1242– Biomass Convers. Biorefin., 2018, 8, 317–328.
1244. 134 X. Zhang, Y. Zhong, X. Xia, Y. Xia, D. Wang, C. Zhou,
108 J. Texter, L. Zhao, P. W. Xiao, F. P. Caballero, B. H. Han W. Tang, X. Wang, J. B. Wu and J. Tu, ACS Appl. Mater.
and M. M. Titirici, Carbon, 2017, 112, 117–129. Interfaces, 2018, 10, 13598–13605.
109 Suhas, V. K. Gupta, P. J. M. Carrott, R. Singh, 135 M. Sevilla, W. Sangchoom, N. Balahmar, A. B. Fuertes and
M. Chaudhary and S. Kushwaha, Bioresour. Technol., 2016, R. Mokaya, ACS Sustainable Chem. Eng., 2016, 4, 4710–
216, 1066–1076. 4716.
110 J. Deng, M. Li and Y. Wang, Green Chem., 2016, 18, 4824– 136 K. Preuss, L. C. Tǎnase, C. M. Teodorescu, I. Abrahams
4854. and M. M. Titirici, J. Mater. Chem. A, 2017, 5, 16336–
111 Z. Liang, L. Zhang, H. Liu, J. Zeng, J. Zhou, H. Li and 16343.
H. Xia, Results Phys., 2019, 12, 1984–1990. 137 L. S. Panchakarla, A. Govindaraj and C. N. R. Rao, Inorg.
112 J. Liu, L. Xie, Z. Wang, S. Mao, Y. Gong and Y. Wang, Chim. Acta, 2010, 363, 4163–4174.
Chem. Commun., 2019, 56, 229–232. 138 A. Kalijadis, J. DorCevidć, T. Trtić-Petrović, M. Vukčević,
113 L. Xie, Z. Wang, J. Liu, Y. Gong, S. Mao, G. Lü, X. Ma, M. Popović, V. Maksimović, Z. Rakočević and Z. Laušević,
L. Yu and Y. Wang, Carbon, 2019, 155, 611–617. Carbon, 2015, 95, 42–50.
114 C. Chen, H. Wang, C. Han, J. Deng, J. Wang, M. Li, 139 J. Wu, J. Hu, K. Song, J. Xu and H. Gao, J. Alloys Compd.,
M. Tang, H. Jin and Y. Wang, J. Am. Chem. Soc., 2017, 139, 2017, 704, 1–6.
2657–2663. 140 W. Ma, L. Xie, L. Dai, G. Sun, J. Chen, F. Su, Y. Cao,
115 A. Chen, Y. Yu, Y. Zhang, W. Zang, Y. Yu, Y. Zhang, H. Lei, Q. Kong and C. M. Chen, Electrochim. Acta, 2018,
S. Shen and J. Zhang, Carbon, 2014, 80, 19–27. 266, 420–430.
116 Q. Kong, L. Zhang, M. Wang, M. Li, H. Yao and J. Shi, Sci. 141 B. Ashourirad, A. K. Sekizkardes, S. Altarawneh and
Bull., 2016, 61, 1195–1201. H. M. El-Kaderi, Chem. Mater., 2015, 27, 1349–1358.
117 N. P. Stadie, S. Wang, K. V. Kravchyk and M. V. Kovalenko, 142 E. Antolini, Renewable Sustainable Energy Rev., 2016, 58,
ACS Nano, 2017, 11, 1911–1919. 34–51.
118 H. Nishihara and T. Kyotani, Chem. Commun., 2018, 54, 143 O. S. G. P. Soares, R. P. Rocha, A. G. Gonçalves,
5648–5673. J. L. Figueiredo, J. J. M. Órfão and M. F. R. Pereira, Appl.
119 B. Liu, H. Shioyama, T. Akita and Q. Xu, J. Am. Chem. Soc., Catal., B, 2016, 192, 296–303.
2008, 130, 5390–5391. 144 L. Wang, Y. Wang, M. Wu, Z. Wei, C. Cui, M. Mao,
120 S. Dutta, A. Bhaumik and K. C.-W. Wu, Energy Environ. J. Zhang, X. Han, Q. Liu and J. Ma, Small, 2018, 1800737,
Sci., 2014, 7, 3574–3592. 1800737.
121 L. P. Wang, W. S. Jia, X. F. Liu, J. Z. Li and M. M. Titirici, 145 K. Sharma, D. Hui, N. H. Kim and J. H. Lee, Nanoscale,
J. Energy Chem., 2016, 25, 566–570. 2019, 11, 1205–1216.

4794 | Green Chem., 2020, 22, 4747–4800 This journal is © The Royal Society of Chemistry 2020
 View Article Online

Green Chemistry Critical Review

146 A. Zhang, S. Cao, Y. Zhao, C. Zhang and A. Chen, New J. 169 S. M. Alatalo, E. Mäkilä, E. Repo, M. Heinonen,
Chem., 2018, 42, 6903–6909. J. Salonen, E. Kukk, M. Sillanpää and M. M. Titirici, Green
147 A. Sarapuu, E. Kibena-Põldsepp, M. Borghei and Chem., 2016, 18, 1137–1146.
K. Tammeveski, J. Mater. Chem. A, 2018, 6, 776–804. 170 S. S. Zhang, Inorg. Chem. Front., 2015, 2, 1059–1069.
148 M. Borghei, N. Laocharoen, E. Kibena-Põldsepp, 171 M. Y. Emran, M. A. Shenashen, A. A. Abdelwahab,
L. S. Johansson, J. Campbell, E. Kauppinen, M. Abdelmottaleb and S. A. El-Safty, New J. Chem., 2018,
K. Tammeveski and O. J. Rojas, Appl. Catal., B, 2017, 204, 42, 5037–5044.
394–402. 172 L. Roldán, L. Truong-Phuoc, A. Ansón-Casaos, C. Pham-
149 Y. Song, G. Liu and Z.-Y. Y. Yuan, RSC Adv., 2016, 6, Huu and E. García-Bordejé, Catal. Today, 2018, 301, 2–10.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

94636–94642. 173 M. H. Yeh, Y. S. Li, G. L. Chen, L. Y. Lin, T. J. Li,

150 T. Berthold, C. R. Castro, M. Winter, G. Hoerpel, H. M. Chuang, C. Y. Hsieh, S. C. Lo, W. H. Chiang and
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

M. Kurttepeli, S. Bals, M. Antonietti and N. Fechler, K. C. Ho, Electrochim. Acta, 2015, 172, 52–60.
ChemNanoMat, 2017, 3, 311–318. 174 M. H. Yeh, Y. A. Leu, W. H. Chiang, Y. S. Li, G. L. Chen,
151 A. L. Yaumi, M. Z. A. Bakar and B. H. Hameed, Energy, T. J. Li, L. Y. Chang, L. Y. Lin, J. J. Lin and K. C. Ho,
2018, 155, 46–55. J. Power Sources, 2018, 375, 29–36.
152 W. He, P. Xue, H. Du, L. Xu, M. Pang, X. Gao, J. Yu, 175 J. Jiang, P. Nie, S. Fang, Y. Zhang, Y. An, R. Fu,
Z. Zhang and T. Huang, Int. J. Hydrogen Energy, 2017, 42, H. Dou and X. Zhang, Chin. Chem. Lett., 2018, 29, 624–
4123–4132. 628.
153 F. Liu, Y. Gao, C. Zhang, H. Huang, C. Yan, X. Chu, Z. Xu, 176 L. Sun, Y. Fu, C. Tian, Y. Yang, L. Wang, J. Yin, J. Ma,
Z. Wang, H. Zhang, X. Xiao and W. Yang, J. Colloid R. Wang and H. Fu, ChemSusChem, 2014, 7, 1637–1646.
Interface Sci., 2019, 548, 322–332. 177 Y. Li, Z. Wang, L. Li, S. Peng, L. Zhang, M. Srinivasan and
154 Q. Wu, G. Zhang, M. Gao, L. Huang, L. Li, S. Liu, C. Xie, S. Ramakrishna, Carbon, 2016, 99, 556–563.
Y. Zhang and S. Yu, J. Alloys Compd., 2019, 786, 826–838. 178 J. Wu, C. Jin, Z. Yang, J. Tian and R. Yang, Carbon, 2015,
155 F. Schipper, S. Kubo and T. P. Fellinger, J. Sol-Gel Sci. 82, 562–571.
Technol., 2019, 89, 101–110. 179 M. Q. Guo, J. Q. Huang, X. Y. Kong, H. J. Peng, H. Shui,
156 W. Zhang, Z. Ren, Z. Ying, X. Liu and H. Wan, J. Alloys F. Y. Qian, L. Zhu, W. C. Zhu and Q. Zhang, Xinxing Tan
Compd., 2018, 743, 44–51. Cailiao/New Carbon Mater., 2016, 31, 352–362.
157 M. Ren, Z. Jia, Z. Tian, D. Lopez, J. Cai, M. M. Titirici and 180 R. C. Bansai and M. Goyal, Activated Carbon Adsorption,
A. B. Jorge, ChemElectroChem, 2018, 5, 2686–2693. CRC Press, Boca Raton, 2005.
158 M. Qiao, S. S. Meysami, G. A. Ferrero, F. Xie, H. Meng, 181 F. Y. Chang, K. J. Chao, H. H. Cheng and C. S. Tan, Sep.
N. Grobert and M. M. Titirici, Adv. Funct. Mater., 2017, Purif. Technol., 2009, 70, 87–95.
1707284, 1–7. 182 H. Marsh and F. Rodríguez-Reinoso, Characterization of
159 S. M. Alatalo, K. Qiu, K. Preuss, A. Marinovic, M. Sevilla, Activated Carbon, 2006.
M. Sillanpää, X. Guo and M. M. Titirici, Carbon, 2016, 96, 183 T. Gupta, Carbon Black, Gray Transparent, 2017, pp. 1–319.
622–630. 184 C. Laginhas, J. M. V. Nabais and M. M. Titirici,
160 R. Atchudan, T. N. J. I. Edison, S. Perumal and Y. R. Lee, Microporous Mesoporous Mater., 2016, 226, 125–
Appl. Surf. Sci., 2017, 393, 276–286. 132.
161 G. Huang, Y. Wang, T. Zhang, X. Wu and J. Cai, J. Taiwan 185 P. Zheng, T. Liu, X. Yuan, L. Zhang, Y. Liu, J. Huang and
Inst. Chem. Eng., 2019, 96, 672–680. S. Guo, Sci. Rep., 2016, 6, 1–9.
162 K. G. Latham, W. M. Dose, J. A. Allen and S. W. Donne, 186 E. Hao, W. Liu, S. Liu, Y. Zhang, H. Wang, S. Chen,
Carbon, 2018, 128, 179–190. F. Cheng, S. Zhao and H. Yang, J. Mater. Chem. A, 2017, 5,
163 L. Rao, R. Ma, S. Liu, L. Wang, Z. Wu, J. Yang and X. Hu, 2204–2214.
Chem. Eng. J., 2019, 362, 794–801. 187 M. Imtiaz, C. Zhu, Y. Li, M. S. Pak, I. Zada, S. W. Bokhari,
164 F. C. de Oliveira, J. S. dos R. Coimbra, E. B. de Oliveira, Z. Chen, D. Zhang and S. Zhu, J. Alloys Compd., 2017, 724,
A. D. G. Zuñiga and E. E. G. Rojas, Crit. Rev. Food Sci. 296–305.
Nutr., 2016, 56, 1108–1125. 188 P. Zheng, T. Liu, J. Zhang, L. Zhang, Y. Liu, J. Huang and
165 X. Ma, G. Ning, Y. Wang, X. Song, Z. Xiao, L. Hou, S. Guo, RSC Adv., 2015, 5, 40737–40741.
W. Yang, J. Gao and Y. Li, Electrochim. Acta, 2018, 269, 83– 189 W. Li, J. Huang, L. Feng, L. Cao, Y. Ren, R. Li, Z. Xu, J. Li
92. and C. Yao, J. Alloys Compd., 2017, 716, 210–219.
166 A. Elmouwahidi, J. Castelo-Quibén, J. F. Vivo-Vilches, 190 H. Wang, W. Yu, J. Shi, N. Mao, S. Chen and W. Liu,
A. F. Pérez-Cadenas, F. J. Maldonado-Hódar and Electrochim. Acta, 2016, 188, 103–110.
F. Carrasco-Marín, Chem. Eng. J., 2018, 334, 1835–1841. 191 E. Demir, M. Aydin, A. A. Arie and R. Demir-Cakan,
167 K. Anoop Krishnan, K. G. Sreejalekshmi, V. Vimexen and J. Alloys Compd., 2019, 788, 1093–1102.
V. V. Dev, Ecotoxicol. Environ. Saf., 2016, 124, 418–425. 192 L. Ma, B. Zhao, X. Wang, J. Yang, X. Zhang, Y. Zhou and
168 L. Roldán, Y. Marco and E. García-Bordejé, Microporous J. Chen, ACS Appl. Mater. Interfaces, 2018, 10, 22067–
Mesoporous Mater., 2016, 222, 55–62. 22073.

This journal is © The Royal Society of Chemistry 2020 Green Chem., 2020, 22, 4747–4800 | 4795
 View Article Online

Critical Review Green Chemistry

193 J. Wang, L. Yan, Q. Ren, L. Fan, F. Zhang and Z. Shi, 217 Y. Wang, R. Yang, M. Li and Z. Zhao, Ind. Crops Prod.,
Electrochim. Acta, 2018, 291, 188–196. 2015, 65, 216–226.
194 R. Väli, A. Jänes, T. Thomberg and E. Lust, J. Electrochem. 218 Y. Liu, B. Huang, X. Lin and Z. Xie, J. Mater. Chem. A,
Soc., 2016, 163, A1619–A1626. 2017, 5, 13009–13018.
195 H. Tang, M. Wang, T. Lu and L. Pan, Ceram. Int., 2017, 43, 219 T. Wei, X. Wei, Y. Gao and H. Li, Electrochim. Acta, 2015,
4475–4482. 169, 186–194.
196 Z. Jian, S. Hwang, Z. Li, A. S. Hernandez, X. Wang, 220 L. Zhang, T. You, T. Zhou, X. Zhou and F. Xu, ACS Appl.
Z. Xing, D. Su and X. Ji, Adv. Funct. Mater., 2017, 27, 1–6. Mater. Interfaces, 2016, 8, 13918–13925.
197 Y. Li, S. Xu, X. Wu, J. Yu, Y. Wang, Y. S. Hu, H. Li, L. Chen 221 C. Li, D. He, Z.-H. Huang and M.-X. Wang, J. Electrochem.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

and X. Huang, J. Mater. Chem. A, 2015, 3, 71–77. Soc., 2018, 165, A3334–A3341.
198 L. Yin, Y. Wang, C. Han, Y. M. Kang, X. Ma, H. Xie and 222 K. K. Lee, W. Hao, M. Gustafsson, C. W. Tai, D. Morin,
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

M. Wu, J. Power Sources, 2016, 305, 156–160. E. Björkman, M. Lilliestråle, F. Björefors, A. M. Andersson
199 R. Li, J. Huang, Z. Xu, H. Qi, L. Cao, Y. Liu, W. Li and J. Li, and N. Hedin, RSC Adv., 2016, 6, 110629–110641.
Energy Technol., 2018, 6, 1080–1087. 223 M. Härmas, T. Thomberg, T. Romann, A. Jänes and
200 H.-L. Wang, Z.-Q. Shi, J. Jin, C.-B. Chong and C.-Y. Wang, E. Lust, J. Electrochem. Soc., 2017, 164, A1866–A1872.
J. Electroanal. Chem., 2015, 755, 87–91. 224 W. Du, X. Wang, J. Zhan, X. Sun, L. Kang, F. Jiang,
201 S. Jeong, X. Li, J. Zheng, P. Yan, R. Cao, H. J. Jung, X. Zhang, Q. Shao, M. Dong, H. Liu, V. Murugadoss and
C. Wang, J. Liu and J. G. Zhang, J. Power Sources, 2016, Z. Guo, Electrochim. Acta, 2019, 296, 907–915.
329, 323–329. 225 Y. Zhao, X. He, R. Chen, Q. Liu, J. Liu, J. Yu, J. Li,
202 J.-S. Choi, H.-J. Lee, J.-K. Ha and K.-K. Cho, J. Nanosci. H. Zhang, H. Dong, M. Zhang and J. Wang, Chem. Eng. J.,
Nanotechnol., 2018, 18, 6459–6462. 2018, 352, 29–38.
203 S. Y. Son, S.-A. Hong, S. Y. Oh, Y.-C. Lee, G.-W. Lee, 226 B. Kirubasankar, V. Murugadoss, J. Lin, T. Ding, M. Dong,
J. W. Kang, Y. S. Huh and I. T. Kim, J. Nanosci. H. Liu, J. Zhang, T. Li, N. Wang, Z. Guo and S. Angaiah,
Nanotechnol., 2018, 18, 6463–6468. Nanoscale, 2018, 10, 20414–20425.
204 Q. Zhang, Q. Gao, W. Qian, H. Zhang, Y. Tan, W. Tian, 227 Y. Q. Zhao, M. Lu, P. Y. Tao, Y. J. Zhang, X. T. Gong,
Z. Li and H. Xiao, J. Mater. Chem. A, 2017, 5, 19136– Z. Yang, G. Q. Zhang and H. L. Li, J. Power Sources, 2016,
19142. 307, 391–400.
205 L. Wang, J. Yuan, Q. Zhao, Z. Wang, Y. Zhu, X. Ma and 228 W. Ye, X. Li, J. Luo, X. Wang and R. Sun, Ind. Crops Prod.,
C. Cao, Electrochim. Acta, 2019, 308, 174–184. 2017, 109, 410–419.
206 Z. Chen, D. Yin and M. Zhang, Small, 2018, 14, 1–9. 229 M. Enterría, F. J. Martín-Jimeno, F. Suárez-García,
207 Y. Guo, Y. Zhang, Y. Wang, D. Zhang, Y. Lu, R. Luo, J. I. Paredes, M. F. R. Pereira, J. I. Martins, A. Martínez-
Y. Wang, X. Liu, J. K. Kim and Y. Luo, Electrochim. Acta, Alonso, J. M. D. Tascón and J. L. Figueiredo, Carbon,
2019, 296, 989–998. 2016, 105, 474–483.
208 X. Li, Y. Yang, J. Liu, L. Ouyang, J. Liu, R. Hu, L. Yang and 230 F. Barzegar, A. Bello, O. O. Fashedemi, J. K. Dangbegnon,
M. Zhu, Appl. Surf. Sci., 2017, 413, 169–174. D. Y. Momodu, F. Taghizadeh and N. Manyala,
209 S. Bao, S.-H. Luo, S.-X. Yan, Z.-Y. Wang, Q. Wang, J. Feng, Electrochim. Acta, 2015, 180, 442–450.
Y.-L. Wang and T.-F. Yi, Electrochim. Acta, 2019, 307, 293– 231 Y. Wang and L. Jiang, ACS Sustainable Chem. Eng., 2017, 5,
301. 5588–5597.
210 W. ur Rehman, Y. Xu, X. Du, X. Sun, I. Ullah, Y. Zhang, 232 Z. W. She, J. Kibsgaard, C. F. Dickens, I. Chorkendorff,
Y. Jin, B. Zhang and X. Li, Appl. Surf. Sci., 2018, 445, 359– J. K. Nørskov and T. F. Jaramillo, Science, 2017, 355(6321),
367. eaad4998.
211 Z. Xiao, G. Ning, Z. Yu, C. Qi, L. Zhao, Y. Li, X. Ma and 233 C. Tang, M. M. Titirici and Q. Zhang, J. Energy Chem.,
Y. Li, Nanoscale, 2019, 11, 8270–8280. 2017, 26, 1077–1093.
212 C. Deng, S. Zhang and Y. Wu, Nanoscale, 2015, 7, 487–491. 234 F. Cao, M. Zhao, Y. Yu, B. Chen, Y. Huang, J. Yang, X. Cao,
213 V. Palomares, M. Blas, P. Serras, A. Iturrondobeitia, Q. Lu, X. Zhang, Z. Zhang, C. Tan and H. Zhang, J. Am.
A. Peña, A. Lopez-Urionabarrenechea, L. Lezama and Chem. Soc., 2016, 138, 6924–6927.
T. Rojo, ACS Sustainable Chem. Eng., 2018, 6, 16386– 235 Y. Lu, L. Wang, K. Preuß, M. Qiao, M. M. Titirici, J. Varcoe
16398. and Q. Cai, J. Power Sources, 2017, 372, 82–90.
214 J. Ding, H. Wang, Z. Li, K. Cui, D. Karpuzov, X. Tan, 236 H. Yu, L. Shang, T. Bian, R. Shi, G. I. N. Waterhouse,
A. Kohandehghan and D. Mitlin, Energy Environ. Sci., Y. Zhao, C. Zhou, L.-Z. Wu, C.-H. Tung and T. Zhang, Adv.
2015, 8, 941–955. Mater., 2016, 28, 5140–5140.
215 H. Lu and X. S. Zhao, Sustainable Energy Fuels, 2017, 1, 237 M. Qiao, C. Tang, G. He, K. Qiu, R. Binions, I. P. Parkin,
1265–1281. Q. Zhang, Z. Guo and M. M. Titirici, J. Mater. Chem. A,
216 D. Salinas-Torres, D. Lozano-Castelló, M. M. Titirici, 2016, 4, 12658–12666.
L. Zhao, L. Yu, E. Morallón and D. Cazorla-Amoros, 238 K. Preuss, M. Qiao and M. Titrici, Hydrothermal
J. Mater. Chem. A, 2015, 3, 15558–15567. Carbon Materials for the Oxygen Reduction Reaction,

4796 | Green Chem., 2020, 22, 4747–4800 This journal is © The Royal Society of Chemistry 2020
 View Article Online

Green Chemistry Critical Review

in Carbon-Based Metal-Free Catalysts, ed. L. Dai, Wiley, 264 Y. M. Lu, H. Z. Zhu, W. G. Li, B. Hu and S. H. Yu, J. Mater.
2018, vol. 2, pp. 369–401. Chem. A, 2013, 1, 3783–3788.
239 W. Wan, Q. Wang, L. Zhang, H. W. Liang, P. Chen and 265 S. P. Dubey, A. D. Dwivedi, I.-C. Kim, M. Sillanpaa,
S. H. Yu, J. Mater. Chem. A, 2016, 4, 8602–8609. Y.-N. Kwon and C. Lee, Chem. Eng. J., 2014, 244, 160–167.
240 K. Preuss, A. M. Siwoniku, C. I. Bucur and M.-M. Titirici, 266 J. Cheng, Y. Wang, C. Teng, Y. Shang, L. Ren and B. Jiang,
ChemPlusChem, 2019, 84, 457–464. Chem. Eng. J., 2014, 242, 285–293.
241 G. He, M. Qiao, W. Li, Y. Lu, T. Zhao, R. Zou, B. Li, 267 Z. Sun, J. Yang, J. Wang, W. Li, S. Kaliaguine, X. Hou,
J. A. Darr, J. Hu, M. M. Titirici and I. P. Parkin, Adv. Sci., Y. Deng and D. Zhao, J. Mater. Chem. A, 2014, 2, 6071–
2017, 4, 1600214. 6074.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

242 B. Huang, Y. Liu and Z. Xie, J. Mater. Chem. A, 2017, 5, 268 J. Yu, L. Yan, G. Tu, C. Xu, X. Ye, Y. Zhong, W. Zhu and
23481–23488. Q. Xiao, Catal. Lett., 2014, 144, 2065–2070.
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

243 Y. Cheng, K. Pang, X. Wu, Z. Zhang, X. Xu, J. Ren, 269 F. Zhang, H. Hu, H. Zhong, N. Yan and Q. Chen, Dalton
W. Huang and R. Song, ACS Sustainable Chem. Eng., 2018, Trans., 2014, 43, 6041.
6, 8688–8696. 270 B. He, Q. Zhao, Z. Zeng, X. Wang and S. Han, J. Mater.
244 L. Zhang, T. Wang, L. Sun, Y. Sun, T. Hu, K. Xu and F. Ma, Sci., 2015, 50, 6339–6348.
J. Mater. Chem. A, 2017, 5, 19752–19759. 271 Z. Q. Shi, L. X. Jiao, J. Sun, Z. B. Chen, Y. Z. Chen,
245 D. Huang, S. Li, X. Zhang, Y. Luo, J. Xiao and H. Chen, X. H. Zhu, J. H. Zhou, X. C. Zhou, X. Z. Li and R. Li, RSC
Carbon, 2018, 129, 468–475. Adv., 2014, 4, 47–53.
246 S. H. Bae, J. E. Kim, H. Randriamahazaka, S. Y. Moon, 272 K. Huang, J. Liu, L. Wang, G. Chang, R. Wang,
J. Y. Park and I. K. Oh, Adv. Energy Mater., 2017, 7, 1–11. M. Lei, Y. Wang and Y. He, Appl. Surf. Sci., 2019, 487,
247 F. Li, L. Chen, G. P. Knowles, D. R. MacFarlane and 1145–1151.
J. Zhang, Angew. Chem., Int. Ed., 2017, 56, 505–509. 273 G. Singh, K. S. Lakhi, I. Y. Kim, S. Kim, P. Srivastava,
248 S.-H. H. Bae, J.-E. E. Kim, H. Randriamahazaka, S.-Y. R. Naidu and A. Vinu, ACS Appl. Mater. Interfaces, 2017, 9,
Y. Moon, J.-Y. Y. Park and I.-K. K. Oh, Adv. Energy Mater., 29782–29793.
2017, 7, 1601492. 274 M. Liu, Y. Pang, B. Zhang, P. De Luna, O. Voznyy, J. Xu,
249 L. Roldán, E. Pires, J. M. Fraile and E. García-Bordejé, X. Zheng, C. T. Dinh, F. Fan, C. Cao, F. P. G. De Arquer,
Catal. Today, 2015, 249, 153–160. T. S. Safaei, A. Mepham, A. Klinkova, E. Kumacheva,
250 D. R. Lathiya, D. V. Bhatt and K. C. Maheria, Bioresour. T. Filleter, D. Sinton, S. O. Kelley and E. H. Sargent,
Technol. Rep., 2018, 2, 69–76. Nature, 2016, 537, 382–386.
251 Z.-E. Tang, S. Lim, Y.-L. Pang, H.-C. Ong and K.-T. Lee, 275 N. Querejeta, M. V. Gil, C. Pevida and T. A. Centeno,
Renewable Sustainable Energy Rev., 2018, 92, 235–253. J. CO2 Util., 2018, 26, 1–7.
252 C. De La Calle, J. M. Fraile, E. García-Bordejé, E. Pires and 276 S. Sethupathi, M. Zhang, A. U. Rajapaksha, S. S. R. Lee,
L. Roldán, Catal. Sci. Technol., 2015, 1–7. N. M. Nor, A. R. Mohamed, M. Al-Wabel, S. S. R. Lee and
253 S. Kang and J. Yu, Sugar Technol., 2018, 20, 182–193. Y. S. Ok, Sustainability, 2017, 9, 1–10.
254 F. D. Pileidis, M. Tabassum, S. Coutts, M. M. Ttitirici and 277 M. C. Castrillon, K. O. Moura, C. A. Alves, M. Bastos-Neto,
M.-M. Titirici, Cuihua Xuebao/Chin. J. Catal., 2014, 35, D. C. S. Azevedo, J. Hofmann, J. Möllmer, W. D. Einicke
929–936. and R. Gläser, Energy Fuels, 2016, 30, 9596–9604.
255 J. M. Fraile, E. García-Bordejé, E. Pires and L. Roldán, 278 D. Y. C. Leung, G. Caramanna and M. M. Maroto-Valer,
Carbon, 2014, 77, 1157–1167. Renewable Sustainable Energy Rev., 2014, 39, 426–
256 I. F. Nata, M. D. Putra, C. Irawan and C.-K. Lee, J. Environ. 443.
Chem. Eng., 2017, 5, 2171–2175. 279 L. Rao, R. Ma, S. Liu, L. Wang, Z. Wu, J. Yang and X. Hu,
257 X. Qi, N. Liu and Y. Lian, RSC Adv., 2015, 5, 17526–17531. Nitrogen enriched porous carbons from D-glucose with excel-
258 Y. Yan, Y. Dai, S. Wang, X. Jia, H. Yu and Y. Yang, Appl. lent CO 2 capture performance, Elsevier B.V., 2019.
Catal., B, 2016, 184, 104–118. 280 G. Shang, Q. Li, L. Liu, P. Chen and X. Huang, J. Air Waste
259 J. Chen, J. Zhang, W. Sun, K. Song, D. Zhu and T. Li, Appl. Manage. Assoc., 2016, 66, 8–16.
Organomet. Chem., 2017, 32, 3. 281 M. I. Inyang, B. Gao, Y. Yao, Y. Xue, A. Zimmerman,
260 A. Eiad-ua, B. Jomhataikool, W. Gunpum, N. Viriya- A. Mosa, P. Pullammanappallil, Y. S. Ok and X. Cao, Crit.
empikul and K. Faungnawakij, Mater. Today: Proc., 2017, Rev. Environ. Sci. Technol., 2016, 46, 406–433.
4, 6153–6158. 282 X. He, K. B. Male, P. N. Nesterenko, D. Brabazon, B. Paull
261 J. Liu, N. Wickramaratne, S. Qiao, et al., Nat. Mater., 2015, and J. H. T. Luong, ACS Appl. Mater. Interfaces, 2013, 5,
14, 763–774. 8796–8804.
262 C. Gai, F. Zhang, T. Yang, Z. Liu, W. Jiao, N. Peng, T. Liu, 283 M. A. Islam, I. A. W. Tan, A. Benhouria, M. Asif and
Q. Lang and Y. Xia, Green Chem., 2018, 20, 2788–2800. B. H. Hameed, Chem. Eng. J., 2015, 270, 187–195.
263 P. Wataniyakul, P. Boonnoun, A. T. Quitain, M. Sasaki, 284 T. Xiong, X. Yuan, X. Chen, Z. Wu, H. Wang, L. Leng,
T. Kida, N. Laosiripojana and A. Shotipruk, Catal. H. Wang, L. Jiang and G. Zeng, Appl. Surf. Sci., 2018, 427,
Commun., 2018, 104, 41–47. 1107–1117.

This journal is © The Royal Society of Chemistry 2020 Green Chem., 2020, 22, 4747–4800 | 4797
 View Article Online

Critical Review Green Chemistry

285 X. F. Yu, Y. H. Liu, Z. W. Zhou, G. X. Xiong, X. H. Cao, 310 C. Liu, J. Xu, J. Hu, H. Zhang and R. Xiao, Chem. Eng.
M. Li and Z.-B. Zhang, J. Radioanal. Nucl. Chem., 2014, Technol., 2017, 40, 1092–1100.
300, 1235–1244. 311 G. K. Parshetti, S. K. Hoekman and R. Balasubramanian,
286 K. Sun, J. Tang, Y. Gong and H. Zhang, Environ. Sci. Pollut. Bioresour. Technol., 2013, 135, 683–689.
Res., 2015, 22, 16640–16651. 312 A. M. Smith, S. Singh and A. B. Ross, Fuel, 2016, 169, 135–
287 H. Cai, X. Lin, L. Tian and X. Luo, Ind. Eng. Chem. Res., 145.
2016, 55, 9648–9656. 313 L. Gao, M. Volpe, M. Lucian, L. Fiori and J. L. Goldfarb,
288 L. Han, H. Sun, K. S. Ro, K. Sun, J. A. Libra and B. Xing, Energy Convers. Manage., 2019, 181, 93–104.
Bioresour. Technol., 2017, 234, 77–85. 314 I. Van Zandvoort, E. R. H. Van Eck, P. De Peinder,
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

289 J. Lu, F. Fu, Z. Ding, N. Li and B. Tang, J. Hazard. Mater., H. J. Heeres, P. C. A. Bruijnincx and B. M. Weckhuysen,
2017, 330, 93–104. ACS Sustainable Chem. Eng., 2015, 3, 533–543.
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

290 B. Han, E. Zhang, G. Cheng, L. Zhang, D. Wang and 315 I. Van Zandvoort, Y. Wang, C. B. Rasrendra, E. R. H. Van
X. Wang, Chem. Eng. J., 2018, 338, 734–744. Eck, P. C. A. Bruijnincx, H. J. Heeres and
291 Z. Zheng, Y. Wang, W. Zhao, G. Xiong, X. Cao, Y. Dai, B. M. Weckhuysen, ChemSusChem, 2013, 6, 1745–1758.
Z. Le, S. Yu, Z. Zhang and Y. Liu, J. Radioanal. Nucl. 316 M. Kleinert and T. Barth, Energy Fuels, 2008, 22, 1371–
Chem., 2017, 312, 187–198. 1379.
292 G. Deng, Y. Zhang, X. Luo and J. Yang, Appl. Organomet. 317 A. Kloekhorst and H. J. Heeres, ACS Sustainable Chem.
Chem., 2018, e4473. Eng., 2015, 3, 1905–1914.
293 P. Biller and A. B. Ross, Biofuels, 2012, 3, 603–623. 318 A. L. Jongerius, R. Jastrzebski, P. C. A. Bruijnincx and
294 S. M. Heilmann, L. R. Jader, L. A. Harned, M. J. Sadowsky, B. M. Weckhuysen, J. Catal., 2012, 285, 315–323.
F. J. Schendel, P. A. Lefebvre, M. G. von Keitz and 319 M. Trautmann, A. Löwe and Y. Traa, Green Chem., 2014,
K. J. Valentas, Appl. Energy, 2011, 88, 3286–3290. 16, 3710–3714.
295 M. Escala, T. Zumbühl, C. Koller, R. Junge and R. Krebs, 320 M. Trautmann, S. Lang and Y. Traa, Fuel, 2015, 151, 102–
Energy Fuels, 2013, 27, 454–460. 109.
296 D. Kim, K. Yoshikawa and K. Park, Energies, 2015, 8, 321 Y. Wang, S. Agarwal, Z. Tang and H. J. Heeres, RSC Adv.,
12412. 2017, 7, 5136–5147.
297 E. Steurer and G. Ardissone, Energy Procedia, 2015, 79, 47– 322 A. Effendi, H. Gerhauser and A. V. Bridgwater, Renewable
54. Sustainable Energy Rev., 2008, 12, 2092–2116.
298 X. Zhuang, H. Zhan, Y. Huang, Y. Song, X. Yin and C. Wu, 323 M. E. Boot-Handford, J. C. Abanades, E. J. Anthony,
Bioresour. Technol., 2018, 267, 17–29. M. J. Blunt, S. Brandani, N. Mac Dowell, J. R. Fernández,
299 D. Basso, F. Patuzzi, D. Castello, M. Baratieri, E. C. Rada, M. C. Ferrari, R. Gross, J. P. Hallett, R. S. Haszeldine,
E. Weiss-Hortala and L. Fiori, Waste Manage., 2016, 47, P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano,
114–121. R. T. J. Porter, M. Pourkashanian, G. T. Rochelle, N. Shah,
300 V. Benavente, E. Calabuig and A. Fullana, J. Anal. Appl. J. G. Yao and P. S. Fennell, Energy Environ. Sci., 2014, 7,
Pyrolysis, 2015, 113, 89–98. 130–189.
301 M. T. Reza, W. Yan, M. H. Uddin, J. G. Lynam, 324 Y. Wang, S. Agarwal, A. Kloekhorst and H. J. Heeres,
S. K. Hoekman, C. J. Coronella and V. R. Vásquez, ChemSusChem, 2016, 9, 951–961.
Bioresour. Technol., 2013, 139, 161–169. 325 Z. Cheng, B. Saha and D. G. Vlachos, ChemSusChem, 2018,
302 J. Chen, Z. Chen, C. Wang and X. Li, Mater. Lett., 2012, 67, 11, 3545.
365–368. 326 C. B. Rasrendra, M. Windt, Y. Wang, S. Adisasmito,
303 S. K. Hoekman, A. Broch and C. Robbins, Energy Fuels, I. G. B. N. Makertihartha, E. R. H. Van Eck, D. Meier and
2011, 25, 1802–1810. H. J. Heeres, J. Anal. Appl. Pyrolysis, 2013, 104, 299–307.
304 I. H. Hwang, H. Aoyama, T. Matsuto, T. Nakagishi and 327 S. Agarwal, D. van Es and H. J. Heeres, J. Anal. Appl.
T. Matsuo, Waste Manage., 2012, 32, 410–416. Pyrolysis, 2017, 123, 134–143.
305 S. Kieseler, Y. Neubauer and N. Zobel, Energy Fuels, 2013, 328 T. M. C. Hoang, L. Lefferts and K. Seshan, ChemSusChem,
27, 908–918. 2013, 6, 1651–1658.
306 L. Garcia Alba, C. Torri, C. Samorì, J. Van Der Spek, 329 T. M. C. Hoang, E. R. H. Van Eck, W. P. Bula,
D. Fabbri, S. R. A. Kersten and D. W. F. Brilman, Energy J. G. E. Gardeniers, L. Lefferts and K. Seshan, Green
Fuels, 2012, 26, 642–657. Chem., 2015, 17, 959–972.
307 S. M. Heilmann, H. T. Davis, L. R. Jader, P. A. Lefebvre, 330 M. Nagamori and T. Funazukuri, J. Chem. Technol.
M. J. Sadowsky, F. J. Schendel, M. G. von Keitz and Biotechnol., 2004, 79, 229–233.
K. J. Valentas, Biomass Bioenergy, 2010, 34, 875–882. 331 M. T. Reza, B. Wirth, U. Lüder and M. Werner, Bioresour.
308 L. Li, Y. Wang, J. Xu, J. R. V. Flora, S. Hoque and Technol., 2014, 169, 352–361.
N. D. Berge, Bioresour. Technol., 2018, 262, 284–293. 332 K. Nakason, B. Panyapinyopol, V. Kanokkantapong,
309 C. He, K. Wang, Y. Yang and J.-Y. Wang, Energy Fuels, N. Viriya-empikul, W. Kraithong and P. Pavasant, J. Energy
2014, 28, 6140–6150. Inst., 2017, 1–11.

4798 | Green Chem., 2020, 22, 4747–4800 This journal is © The Royal Society of Chemistry 2020
 View Article Online

Green Chemistry Critical Review

333 K. Nakason, B. Panyapinyopol, V. Kanokkantapong, 355 E. Erdogan, B. Atila, J. Mumme, M. T. Reza, A. Toptas,
N. Viriya-empikul, W. Kraithong and P. Pavasant, J. Energy M. Elibol and J. Yanik, Bioresour. Technol., 2015, 196, 35–
Inst., 2018, 91, 184–193. 42.
334 K. Nakason, B. Panyapinyopol, V. Kanokkantapong, 356 X. Lu, J. R. V. Flora and N. D. Berge, Bioresour. Technol.,
N. Viriya-empikul, W. Kraithong and P. Pavasant, Biomass 2014, 154, 229–239.
Convers. Biorefin., 2018, 8, 199–210. 357 D. Steinbach, A. Kruse, J. Sauer and P. Vetter, Energies,
335 F. S. Asghari and H. Yoshida, Ind. Eng. Chem. Res., 2006, 2018, 11, 645.
45, 2163–2173. 358 S. Reiche, N. Kowalew and R. Schlögl, ChemPhysChem,
336 S. K. R. Patil and C. R. F. Lund, Energy Fuels, 2011, 25, 2014, 16, 579–587.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

4745–4755. 359 P. Körner, S. Beil and A. Kruse, React. Chem. Eng., 2019, 4,
337 R. Becker, U. Dorgerloh, E. Paulke, J. Mumme and 747–762.
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

I. Nehls, Chem. Eng. Technol., 2014, 37, 511–518. 360 H. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang,
338 M. T. Reza, E. Rottler, L. Herklotz and B. Wirth, Bioresour. Science, 2007, 316, 1597 LP – 1600.
Technol., 2015, 182, 336–344. 361 V. Choudhary, S. H. Mushrif, C. Ho, A. Anderko,
339 J. J. Pacheco and M. E. Davis, Proc. Natl. Acad. Sci. U. S. A., V. Nikolakis, N. S. Marinkovic, A. I. Frenkel, S. I. Sandler
2014, 111, 8363–8367. and D. G. Vlachos, J. Am. Chem. Soc., 2013, 135, 3997–
340 Y. Román-Leshkov, C. J. Barrett, Z. Y. Liu and 4006.
J. A. Dumesic, Nature, 2007, 447, 982–985. 362 S. Hu, Z. Zhang, J. Song, Y. Zhou and B. Han, Green
341 J. Jae, W. Zheng, R. F. Lobo and D. G. Vlachos, Chem., 2009, 11, 1746–1749.
ChemSusChem, 2013, 6, 1158–1162. 363 Y. Jiang, L. Yang, C. M. Bohn, G. Li, D. Han, N. S. Mosier,
342 G. M. G. Maldonado, R. S. Assary, J. Dumesic and J. T. Miller, H. I. Kenttämaa and M. M. Abu-Omar, Org.
L. A. Curtiss, Energy Environ. Sci., 2012, 5, 6981– Chem. Front., 2015, 2, 1388–1396.
6989. 364 W. Weiqi and W. Shubin, Chem. Eng. J., 2017, 307, 389–
343 S. Krawielitzki and T. M. Kläusli, Ind. Biotechnol., 2015, 398.
11, 6–8. 365 H. Lin, Q. Xiong, Y. Zhao, J. Chen and S. Wang, AIChE J.,
344 T. Werpy, G. Petersen, A. Aden, J. Bozell, J. Holladay, 2016, 63, 257–265.
J. White, A. Manheim, D. Eliot, L. Lasure and S. Jones, 366 X. Fu, J. Dai, X. Guo, J. Tang, L. Zhu and C. Hu, Green
Top value added chemicals from biomass. Volume 1-Results Chem., 2017, 19, 3334–3343.
of screening for potential candidates from sugars and syn- 367 L. Yang, J. Su, S. Carl, J. G. Lynam, X. Yang and H. Lin,
thesis gas, 2004. Appl. Catal., B, 2015, 162, 149–157.
345 J. J. Bozell and G. R. Petersen, Green Chem., 2010, 12, 539– 368 Q. Hou, M. Zhen, L. Liu, Y. Chen, F. Huang,
554. S. Zhang, W. Li and M. Ju, Appl. Catal., B, 2018, 224, 183–
346 Q. Fang and M. A. Hanna, Bioresour. Technol., 2002, 81, 193.
187–192. 369 A. Dibenedetto, M. Aresta, L. Di Bitonto and C. Pastore,
347 S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, ChemSusChem, 2016, 9, 118–125.
H. Kato and S. Hayashi, J. Am. Chem. Soc., 2008, 130, 370 H. Li, S. Saravanamurugan, S. Yang and A. Riisager, Green
12787–12793. Chem., 2016, 18, 726–734.
348 D. J. Hayes, S. Fitzpatrick, M. H. B. Hayes and 371 K. Dong, J. Zhang, W. Luo, L. Su and Z. Huang, Chem.
J. R. H. Ross, in Biorefineries-Industrial Processes and Eng. J., 2018, 334, 1055–1064.
Products: Status Quo and Future Directions, ed. B. Kamm, 372 X. Qi, H. Guo, L. Li and R. L. Smith, ChemSusChem, 2012,
P. R. Gruber and M. Kamm, Wiley, 2006, pp. 139– 5, 2215–2220.
164. 373 J. Wang, L. Zhu, Y. Wang, H. Cui, Y. Y. Zhang and
349 A. S. Mamman, J. Lee, Y. Kim, I. T. Hwang, N. Park, Y. Y. Zhang, J. Chem. Technol. Biotechnol., 2017, 92, 1454–
Y. K. Hwang and J. Chang, Biofuels, Bioprod. Biorefin., 1463.
2008, 438–454. 374 J. Howard, D. W. Rackemann, J. P. Bartley, C. Samori and
350 J. P. Lange, E. Van Der Heide, J. Van Buijtenen and W. O. S. Doherty, ACS Sustainable Chem. Eng., 2018, 6,
R. Price, ChemSusChem, 2012, 5, 150–166. 4531–4538.
351 M. Dusselier, P. Van Wouwe, A. Dewaele, E. Makshina and 375 A. G. Demesa, A. Laari, E. Tirronen and I. Turunen, Chem.
B. F. Sels, Energy Environ. Sci., 2013, 6, 1415– Eng. Res. Des., 2015, 93, 531–540.
1442. 376 Y. Román-Leshkov, J. N. Chheda and J. A. Dumesic,
352 F. Jin, Z. Zhou, A. Kishita and H. Enomoto, J. Mater. Sci., Science, 2006, 312, 1933–1938.
2006, 41, 1495–1500. 377 T. Shimanouchi, Y. Kataoka, M. Yasukawa, T. Ono and
353 S. K. Hoekman, A. Broch, C. Robbins, B. Zielinska and Y. Kimura, Solvent Extr. Res. Dev., 2013, 20, 205–
L. Felix, Biomass Convers. Biorefin., 2013, 3, 113– 212.
126. 378 J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and
354 B. Wirth and J. Mumme, Appl. Bioenergy, 2013, 1–10. J. A. Dumesic, Green Chem., 2013, 15, 85–90.

This journal is © The Royal Society of Chemistry 2020 Green Chem., 2020, 22, 4747–4800 | 4799
 View Article Online

Critical Review Green Chemistry

379 L. C. Blumenthal, C. M. Jens, J. Ulbrich, F. Schwering, 400 R. Atchudan, T. N. J. I. Edison, D. Chakradhar,

V. Langrehr, T. Turek, U. Kunz, K. Leonhard and S. Perumal, J. J. Shim and Y. R. Lee, Sens. Actuators, B,
R. Palkovits, ACS Sustainable Chem. Eng., 2016, 4, 228– 2017, 246, 497–509.
235. 401 N. Papaioannou, A. Marinovic, N. Yoshizawa, A. E. Goode,
380 E. Weingart, L. Teevs, R. Krieg and U. Prüße, Energy M. Fay, A. Khlobystov, M. Titirici and A. Sapelkin, Sci.
Technol., 2018, 6, 432–440. Rep., 2018, 8, 6559.
381 E. C. Sindermann, A. Holbach, A. de Haan and 402 H. Luo, N. Papaioannou, E. Salvadori, M. M. Roessler,
N. Kockmann, Chem. Eng. J., 2016, 283, 251–259. G. Ploenes, E. R. H. van Eck, L. C. Tanase, J. Feng, Y. Sun,
382 S. Mohammad, G. Grundl, R. Müller, W. Kunz, Y. Yang, M. Danaie, A. Belen Jorge, A. Sapelkin,
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

G. Sadowski and C. Held, Fluid Phase Equilib., 2016, 428, J. Durrant, S. D. Dimitrov and M. M. Titirici,
102–111. ChemSusChem, 2019, 12, 4432–4441.
Open Access Article. Published on 01 July 2020. Downloaded on 3/6/2023 6:41:55 AM.

383 G. Grundl, M. Müller, D. Touraud and W. Kunz, J. Mol. 403 H. Shengliang, T. Fei, B. Peikang, C. Shirui,
Liq., 2017, 236, 368–375. S. Jing and Y. Jing, Mater. Sci. Eng., B, 2009, 157(1–3), 11–
384 F. Liu, S. Sivoththaman and Z. Tan, Sustainable Environ. 14.
Res., 2014, 24, 149–157. 404 R. Tian, S. Zhong, J. Wu, Y. Geng, B. Zhou, Q. Wang and
385 H. Wang, S. Liu, Y. Zhao, H. Zhang and J. Wang, ACS W. Jiang, J. Mater. Chem. C, 2017, 5, 9174–9180.
Sustainable Chem. Eng., 2016, 4, 6712–6721. 405 F. Ehrat, S. Bhattacharyya, J. Schneider, A. Löf,
386 J. Zhou, T. Huang, Y. Zhao, Z. Xia, Z. Xu, S. Jia, J. Wang R. Wyrwich, A. L. Rogach, J. K. Stolarczyk, A. S. Urban and
and Z. C. Zhang, Ind. Eng. Chem. Res., 2015, 54, 7977– J. Feldmann, Nano Lett., 2017, 17, 7710–7716.
7983. 406 B. Dai, C. Wu, Y. Lu, D. Deng and S. Xu, J. Lumin., 2017,
387 X. Sun, Z. Liu, Z. Xue, Y. Zhang and T. Mu, Green Chem., 190, 108–114.
2015, 17, 2719–2722. 407 J. Yang, W. Chen, X. Liu, Y. Zhang and Y. Bai, Mater. Res.
388 T. Brouwer, M. Blahusiak, K. Babic and B. Schuur, Sep. Bull., 2017, 89, 26–32.
Purif. Technol., 2017, 185, 186–195. 408 Z. Tian, X. Zhang, D. Li, D. Zhou, P. Jing, D. Shen, S. Qu,
389 J. Briscoe, A. Marinovic, M. Sevilla, S. Dunn and R. Zboril and A. L. Rogach, Adv. Opt. Mater., 2017, 5,
M. Titirici, Angew. Chem., Int. Ed., 2015, 54, 4463–4468. 1700416.
390 R. Wang, K. Q. Lu, Z. R. Tang and Y. J. Xu, J. Mater. Chem. 409 T. Zhang, J. Zhu, Y. Zhai, H. Wang, X. Bai, B. Dong,
A, 2017, 5, 3717–3734. H. Wang and H. Song, Nanoscale, 2017, 9, 13042–
391 G. A. M. Hutton, B. C. M. Martindale and E. Reisner, 13051.
Chem. Soc. Rev., 2017, 46, 6111–6123. 410 S. K. Bhunia, A. Saha, A. R. Maity, S. C. Ray and
392 X. Sun and Y. Li, Angew. Chem., Int. Ed., 2004, 43, 597– N. R. Jana, Sci. Rep., 2013, 3, 1473.
601. 411 R. Purbia and S. Paria, Biosens. Bioelectron., 2016, 79, 467–
393 X. Jia, J. Li and E. Wang, Nanoscale, 2012, 4, 5572–5575. 475.
394 Z. Song, F. Quan, Y. Xu, M. Liu, L. Cui and J. Liu, Carbon, 412 P. Yang, J. Zhao, L. Zhang, L. Li and Z. Zhu, Chem. – Eur.
2016, 104, 169–178. J., 2015, 21, 8561–8568.
395 M. L. Liu, B. Bin Chen, C. M. Li and C. Z. Huang, Green 413 Q. Xu, Y. Liu, C. Gao, J. Wei, H. Zhou, Y. Chen, C. Dong,
Chem., 2019, 21, 449–471. T. S. Sreeprasad, N. Li and Z. Xia, J. Mater. Chem. C, 2015,
396 B. De and N. Karak, RSC Adv., 2013, 3, 8286–8290. 3, 9885–9893.
397 J. Shen, S. Shang, X. Chen, D. Wang and Y. Cai, Mater. Sci. 414 D.-W. Zhang, N. Papaioannou, N. M. David, H. Luo,
Eng., C, 2017, 76, 856–864. H. Gao, L. C. Tanase, T. Degousée, P. Samorì, A. Sapelkin,
398 D. Qu, M. Zheng, L. Zhang, H. Zhao, Z. Xie, X. Jing, O. Fenwick, M.-M. Titirici and S. Krause, Mater. Horiz.,
R. E. Haddad, H. Fan and Z. Sun, Sci. Rep., 2015, 4, 5294. 2018, 5, 423–428.
399 J. Liu, S. Lu, Q. Tang, K. Zhang, W. Yu, H. Sun and 415 A. Marinovic, L. S. Kiat, S. Dunn, M. M. Titirici and
B. Yang, Nanoscale, 2017, 9, 7135–7142. J. Briscoe, ChemSusChem, 2017, 10, 1004–1013.

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