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PRODUCTION OF FURFURAL: OVERVIEW AND CHALLENGES

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 PRODUCTION OF FURFURAL:
OVERVIEW AND CHALLENGES
MEHDI DASHTBAN, ALLAN GILBERT, PEDRAM FATEHI*

The renewability and abundance of lignocellulosic biomass make it a viable resource for the production of platform chemicals such as fur-
ABSTRACT

fural. Currently, furfural is industrially produced through the acid hydrolysis of agro-based biomass. Alternatively, it can be produced from
woody biomass in an integrated forest biorefinery. In this study, various processes developed for producing furfural at industrial and labora-
tory scales will be reviewed. Generally, furfural production yield was higher in bi-phasic systems than in aqueous systems. However, furfural
production in bi-phasic systems encounters certain technical challenges, including solvent recovery, process complexity, and environmental
issues, which prevent its practical implementation at industrial scales. This study also reviews the advantages and disadvantages of various
furfural production processes proposed in the literature.

INTRODUCTION
The majority of synthetic products are will increase its overall revenues [6–8]. biomass could be used in the production
currently produced from oil-based ma- Canada has enormous forest and of various value-added chemicals in dif-
terials. Emerging biorefinery technolo- agro-based resources. According to the ferent biorefinery scenarios.
gies could help produce these products BIOCAP Canada Foundation, 42% of to- Generally, altered value-added chem-
from biomass. Plant biomass is com- tal land area in Canada was covered with icals such as dissolving pulp [2,3], nano-
posed of three major components: cel- forest in 2003, and approximately 6.8% of crystalline cellulose (NCC), [14], and bio-
lulose, hemicelluloses, and lignin. As the land was used as farms for agriculture [9]. fuels such as ethanol [15] can be produced
consumption of lignocellulosic biomass Moreover, a part of the harvested forest from the cellulose in lignocellulosic mate-
increases globally, the production of lig- and agro-based biomass has not yet been rials . Lignin can also be used in the pro-
nocellulosic residues (wastes) will also used (i.e., it is wasted) [9]. Table 1 lists duction of value-added chemicals, includ-
increase. Recently, the accommodation some of the wastes produced in Canada ing phenols and adhesives [16]. Although
of lignocellulosic residues from differ- from 2001 to 2008. It was reported that hemicelluloses can be used in the produc-
ent pulping processes in the production mill residues accounted for approximately tion of ethanol or xylitol, their industrial
of various value-added products was dis- 2.7 million metric tonnes (million MT) in application through bioconversion routes
cussed [1–5]. The biorefinery will also as- 2005, while agro-based residues were ap- is challenging. On the other hand, hemi-
sist the Canadian pulp and paper industry proximately 17.8 million MT in 2001 [9– celluloses can be chemically converted to
to produce various value-added chemi- 11]. Highly abundant municipal solid and furfural. This conversion is significantly
cals (in addition to traditional pulp prod- animal wastes could also be considered as faster and more industrially attractive than
ucts) from its renewable feedstock, which potential biomass resources [11–13]. This the bioconversion of cellulose or hemicel-
luloses to ethanol [7].
Attempts at chemical conversion
of mono-sugars derived from biomass
hemicelluloses to fuels and chemicals have
been extensively reported in the literature.
For example, 5-hydroxymethylfurfural
(HMF) can be produced from glucose and
be further converted to 2,5-dimethylfuran
PEDRAM FATEHI MEHDI DASHTBAN ALLAN GILBERT (DMF). DMF has 40% higher energy den-
Chemical Engineering Chemical Engineering Chemical Engineering sity than ethanol [17]. Levulinic acid (LA)
Department, Department, Department,
Lakehead University Lakehead University Lakehead University and furfural can also be produced from
Thunder Bay, On Thunder Bay, On Thunder Bay, On 5- and 6-carbon mono-sugars in catalytic
Canada Canada Canada reactions [7]. LA can be further converted
*Contact: pfatehi@lakeheadu.ca
to levulinate (EL), which is considered as

44 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012
 SPECIAL BIOREFINERY ISSUE

fuel [18], or to succinic acid by oxidation. used as a monomer or a solvent in the pentoses were then converted (i.e., cyclo-
Formic acid, which is a value-added chem- pharmaceutical and chemical industries dehydrated) into furfural in a subsequent
ical, is also formed as a by-product in the [7]. LA, which is produced from furfural stage, and then furfural was recovered by
LA production process [7]. via furfuryl alcohol routes, can function steam stripping from solution. The draw-
This review focuses on various pro- as a platform chemical with applications backs of this process were low yield (less
cesses which have been developed for in the pharmaceutical, agricultural, and than 50% based on mono-sugars), sub-
producing furfural from woody biomass, petroleum industries as a fuel or fuel ad- stantial steam requirement, high effluent
agricultural residues, and commercially ditive [22]. production, (i.e., very acidic wastes), and
available sugars. It also addresses the chal- high operating cost, which led to the clo-
lenges and technical problems associated CURRENT FURFURAL PRODUC- sure of plants in developed countries in
with laboratory- and commercial-scale ERS the 1990s [21,23]. The rather low yield
furfural production processes. of this process was attributed to the fact
Currently, China, South Africa, and the that the first step (hydrolysis) was 50 times
FURFURAL Dominican Republic are the major pro- faster than the second step (dehydration).
ducers of furfural and its derivatives Consequently, a significant number of side
Furfural, an organic compound (hetero- [6,21]. All these major furfural produc- reactions occurred because of the high
cyclic aldehyde) with a boiling point of ers use agricultural residues as feedstock: availability of mono-sugars in the process,
161.7°C (at 1 atm) and a density of 1160 corncobs in China and bagasse in South which ultimately reduced the quantity of
kg/m3, was first isolated as a by-product Africa and the Dominican Republic. Cur- mono-sugars available for furfural pro-
of formic acid production in 1821 and rent furfural production is estimated to duction [21,24].
later produced through hydrolysis and de- be 300,000 tons/year globally, with the Recently, Westpro has modified the
hydration of various crops. A large-scale largest furfural production plant in the Quaker Oats Technology process in China
furfural production plant was started by Dominican Republic having a capacity of (Huaxia Furfural Technology) into a con-
the Quaker Oats Company in 1922 using 35,000 tons/year [21,23]. tinuous process. This method uses fixed-
agricultural by-products such as sugarcane bed reactors and a continuous dynamic
bagasse and corn cobs. The main reason INDUSTRIAL- AND PILOT-SCALE azeotropic distillation refining process,
for furfural production was the fact that FURFURAL PRODUCTION which led to 4%–12% production yield
massive quantities of oat hulls remained with respect to the initial weight of dry
unused after production of rolled oats In principle, any lignocellulosic residues biomass used (i.e., corn cobs, rice hulls,
[19]. Traditionally, furfural was produced containing pentosans can be used as a flax dregs, cotton hulls, sugarcane bagasse,
using a two-step process: 1) hydrolysis of feedstock for furfural production. The and wood) [21,23]. However, no detailed
lignocellulosic biomass using heat and acid first furfural production plant was a batch information is available with regard to this
(mainly sulphuric acid) to release pentoses process originally developed by Quaker technology.
(mostly xylose) from biomass and 2) cy- Oats Technology in the 1920s in the SupraYield is another modification
clodehydration of the pentoses using acid United States. In this process, biomass of the Quaker Oats Technology process
and steam to produce furfural [19,20]. was treated with acid (2.2 wt.% (OD of introduced in the late 1990s [19,21]. In
Furfural was traditionally used for biomass) aqueous sulphuric or phosphoric this technology, lignocelluloses (sugarcane
resin production (in foundry technologies) acid) and steam at 153°C in a hydrolysis bagasse) are hydrolysed in one stage, and
and as a solvent for lubricant production step which could convert the pentosans then pentoses are converted into furfural
[21]. Recently, it was proposed as a plat- in the biomass to pentoses. The generated in aqueous solution at its boiling point
form chemical for the production of oth-
er furan-based chemicals such as furfuryl TABLE 1 Lignocellulosic wastes from different sources.
alcohol (FFA), tetrahydrofurfuryl alcohol Annual production
Lignocellulosic Wastes References
(THFA), methyltetrahydrofuran (MTHF), (million MT)
furoic acid, furfurylamine, and methylfu- Forest-based residues
ran [22]. These furfural derivatives have Forest floor residues (roadside, annually) 12.8 [11]
high potential for industrial applications. Mill residues (2005) 2.7 [10,11]
For example, furfuryl alcohol produced Agricultural-based residues
by hydrogenation of furfural can replace Across Canada (2001) 17.79 [9]
oil-based binders in the foundry industry Municipal solid waste (MSW)
[7]. Tetrahydrofuran (THF) can be pro- Across Canada (2008) 25.8 [13]
duced from furfural (by decarbonylation Animal wastes
and then catalytic hydrogenation) and be Across Canada (2006) 180 [11-13]

J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012 45
(with or without phosphoric acid). The so- nia, Hungary, and Finland [24]. Although salts were used to improve furfural pro-
lution containing furfural is then adiabati- this process had a higher furfural yield, the duction, and the results showed that the
cally flash-distilled, which facilitates the significant quantity of acidic wastes pro- concentration of acid (HCl or H2SO4) and
transfer of the furfural formed from the duced was an environmental concern. NaCl affected furfural production selectiv-
aqueous phase to the vapour phase. This In 2008, CIMV (Compagnie Indus- ity, yields, and separation. Furfural yields
process has a production yield of 50%– trielle de la Matière Végétale) launched its of over 83% with a purity of over 99%
70% and is less expensive than the tradi- first pilot-scale lignocellulosic biorefinery were obtained under optimal conditions
tional process described above. The high facility in Pomacle (Marne, France). This (0.18 wt.% HCl, 1.7 M NaCl at 200°C)
temperature of this process (240°C) pro- process is based on a continuous fraction- [27]. One advantage of this method was
motes the conversion of mono-sugars to ation of biomass (wheat straw) under acid- the minimal formation of by-products
furfural. This process was initially investi- ic conditions (e.g., acetic acid and formic such as HMF, acetic acid, and formic acid
gated at a pilot scale in South Africa and acid at 185°C–210°C). The fractionated [25]. Because this process is a continuous
then commercialized by the Proserpine components (lignin, cellulose, and hemi- reactor and distillation tower, it also fea-
Cooperative Sugar Mill in Queensland, celluloses) would follow different routes: tures low energy consumption, and there-
Australia in 2009 [21]. hemicelluloses would be converted to xy- fore this technology seems to be a more
Alternatively, a single-step furfural litol, furfural, and furfuryl alcohol [25], appropriate alternative to the traditional
production process based on simultane- cellulose to bleached pulp (with properties batch process. However, the optimal yield
ous hydrolysis and dehydration of lig- similar to those of hardwood pulps), and is achieved only when dilute solutions (5
nocellulosic biomass to furfural in dilute lignin to resins and adhesives [19,26]. wt.%) of pentosans are used. Therefore,
sulphuric acid solution was developed by In another process, furfural was pro- lignocellulosic biomass cannot be directly
Vedernikov. Because the pentoses pro- duced using a novel reactor known as the used in this process. In other words, bio-
duced would spontaneously be converted multi-turbine column (MTC) developed at mass hemicelluloses must be initially sepa-
to furfural, the presence of surplus pen- TU Delft (Delft University of Technol- rated from the biomass and then diluted.
toses in the solution would be limited, ogy) in The Netherlands. In this process, This technology is at the early stage of its
and therefore the occurrence of side re- both acid hydrolysis of hemicellulosic development, implying that more in-depth
actions and hence by-products would pentosans (obtained from pre-hydrolysis analyses are required for commercializa-
be marginal. As a result, more pentoses of straw) to xylose and conversion to fur- tion of this process [28].
would be converted to furfural (furfural fural occur in one continuous reactor (i.e.,
yield was 75% in this case). In addition, it is a one-step process). In this technol- LABORATORY-SCALE FURFURAL
biomass cellulose appears to be preserved ogy, 5% (wt.) aqueous solution contain- PRDUCTION
in this process, which promotes the down- ing pentosans was used as the feedstock,
stream application of cellulose in the pro- which was directly added to the column Because current furfural production plants
duction of other value-added chemicals, [25]. Furfural was produced in water in are still practicing original or modified
such as ethanol. This technology was as- the reactor and simultaneously transferred versions of the Quaker Oats Technology
sessed in the former Soviet Union, Slove- into the vapour phase. Various acids and process at industrial scales, the production

TABLE 2 Current industrial furfural production processes and their specifications.

Method Year of operation Raw materials Process conditions Furfural yields Scale References
Quaker Oats 1922 Sugarcane Acid catalysts, 50% Industrial [21,23]
Technology bagasse/corncobs steam (153 °C)
SupraYield 2009 Sugarcane Acid catalysts, 50-70% Industrial [19,21]
bagasse high temperatures
(180-240 °C)
Single-step 2000 Sugarcane Concentrated 75% Industrial [24]
continuous bagasse/corncobs sulphuric acid,
process steam
CIMV 2008 Wheat straw High temperature N/A Pilot plant [25]
(230 °C) and
pressure
MTC 2012 Straw Acid hydrolysis, 83% Pilot plant [25,28]
high temperature
(200 °C)

23
46 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012
 SPECIAL BIOREFINERY ISSUE

yield and cost are not satisfactory, and en- of reaction [30]. aqueous-acid catalytic and autocatalytic
vironmental concerns with acidic process In another study (Table 3), the con- systems is that acid is added to the aque-
wastes still exist. Many studies have been version of commercial xylose to furfural ous-acid catalytic systems. Moreover, the
carried out to improve furfural produc- was assessed under aqueous subcritical production of by-products is usually high-
tion technology so that the process has conditions using hot compressed water er in acid-catalytic than in autocatalytic
a higher yield and lower cost and is more (HCW) at a temperature range of 200°C– systems, which is due to the more exten-
environmentally friendly. In this regard, 300°C and 200 bar (Table 3) [31]. In this sive occurrence of side reactions in acidic
different catalytic systems using mono- or process, formic and lactic acids (i.e., by- systems. Therefore, the process condi-
bi-phasic processes have been developed, products) were formed, which reduced the tions, including temperature, time, acid
as discussed in the following sections. pH of the solution from 6.5 to 3.2. The and pentose concentrations, should be
high pressure, temperature, and formed carefully monitored in furfural production
Furfural production in autocatalytic acids promoted xylose conversion into processes using this technique. In this re-
systems furfural, which would ultimately provide gard, furfural formation was studied using
Table 3 lists various autocatalytic systems a more environmentally friendly process. commercial xylose solutions (0.067–0.20
for furfural production that have been de- In addition, the xylose conversion rate was mol/l) as a feedstock and formic acid as
scribed in the literature. Furfural can be increased from 70% to 100% by increas- the catalyst in a batch process under dif-
produced by treating biomass at high tem- ing reaction temperature from 260°C to ferent experimental conditions [29]. The
perature (200°C or higher) and pressure 300°C at 200 bar [31]. A similar result was results showed that the xylose conversion
in batch systems. In this process, acetic obtained in another study using commer- rate was increased from 6.2% to 98.2%
acid is liberated from hydrolysis of acetyl cially available pentoses (arabinose, xylose, by increasing temperature from 120°C to
groups associated with biomass hemicel- galacturonic acid, and glucuronic acid) at 200°C. The furfural yield was increased
luloses and acts as a catalyst for the re- 240°C and 10 MPa under aqueous sub- (from 3.0% to 65.4%) by increasing tem-
action. Furthermore, hemicelluloses are critical water conditions. The pH of the perature (from 140°C to 200°C), whereas
isolated from biomass, depolymerized to solution was also reported to decrease to it decreased with increasing time (from 20
pentoses and hexoses, and simultaneously approximately 3.0 in this study [32]. min to 40 min). The selectivity of furfural
the pentoses and hexoses are converted to Although these processes can be production varied from 42.3% to 72.7%
furfural and HMF respectively [29]. The practically implemented in industry, fur- under different conditions [29]. The high-
absence of mineral acid implies that the fural production yield is still low, and a est furfural yield of 65.4% was obtained
aqueous solutions (and therefore aqueous considerable quantity of by-products (e.g., using 0.067 mol/l xylose concentration
wastes) of this system are not very corro- formic acid) is formed. Furthermore, op- and 30 wt.% formic acid concentration at
sive or toxic (i.e., environmental concerns erating a very highly pressurized furfural 200°C after 20 min of reaction. These re-
are reduced). production reactor may be technically sults demonstrate that the process condi-
In one study, furfural production challenging. tions should be monitored to reduce the
was practiced from commercial xylose in a occurrence of side reactions. Note that
batch reactor at high temperature (180°C Furfural production in aqueous-acid formic acid is formed during the decom-
to 220°C) and 10 MPa pressure in water catalytic systems position reactions of hemicelluloses and
[30]. As a result, a maximum of 50% fur- In these systems, various acids such as hy- furfural. Therefore, the concentration of
fural (based on xylose) and formic acid (an drochloric, phosphoric, acetic, or formic formic acid as a catalyst will increase in a
undisclosed concentration) were formed acids are added to the reactor, in which process using this technique.
as the main product and by-products of biomass is converted to furfural (as ap- Overall, it can be inferred that fur-
this process respectively, while xylose con- plied at industrial scales) in aqueous solu- fural production yield is generally higher
version was 95.8% at 220°C after 50 min tions. Therefore, the difference between in homogeneous catalytic systems than

TABLE 3 Laboratory-scale furfural production processes using autocatalytic systems.

Method Raw materials Process conditions Furfural yields By-products References
High temperature liquid water Xylose High temperature 50% Formic acid [30]
(HTLW) (180-220 °C)
Hot compressed water Xylose High temperature 0.3 molar Formic acid [31]
(HCW) (200-300 °C), 200 bar yield1
Subcritical water Pentose High temperature 0.3 molar Formic acid [32]
(240 °C), 10 MPa yield1
1
The yield was determined based on the molar conversion of biomass to furfural.

J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012 47
in autocatalytic systems. The application biorefinery producing furfural from a hydrolysis stage at 130°C for 30 min us-
of strong mineral-acid catalysts, such as model spent liquor from FORMACELL ing sulphuric acid at pH 1, and then the
H2SO4 or HCl, produced a higher furfu- pulping (i.e., formic acid in solution) was monomers were converted to furfural in
ral yield than organic acids such as formic investigated [35]. The reaction was per- the second stage at 200°C. The furfural
acid in the homogeneous systems. The formed using two different solvent sys- produced in the second stage was ab-
main drawback of these systems is the tems including a mixture of acetic acid sorbed by an organic phase containing
environmental concerns of dealing with and water (ACETOSOLV) with or with- tetrahydrofuran (THF), which resulted in
acidic process wastes. The separation, pu- out adding 0.1% HCl as a catalyst. Furfu- 90% furfural yield (based on the xylose in
rification, and recovery of acids are costly ral production was analyzed using these the aqueous phase) [36].
and technically challenging. acid solutions containing 20 mmol of xy- In another study, various raw materi-
lose, glucose, mannose, or xylan at 165°C. als (e.g., sucrose, inulin, starch, cellobiose,
Furfural production in bi-phasic Maximum furfural production of 55% xylan, fructose, and xylose) were used as a
systems was obtained using FORMACELL liquor feedstock at 10–30 (wt.%) concentrations
Table 4 lists some laboratory-scale bi-pha- at 165°C after 240 min. In the case of at 413°K–443°K. The feedstocks were
sic systems for furfural production. Furfu- ACETOCELL, 40.8% furfural yield was converted into HMF and furfural in an
ral can be produced in a batch process in obtained in the absence of HCl and 46.4% aqueous phase (water and DMSO at ratios
a bi-phasic system. In this system, furfural in the presence of 0.1% HCl. Moreover, of 5:5, 4:6, and 3:7), and then the HMF
is produced from pentose (mainly xylose) HMF and acetoxymethylfurfural (AMF) and furfural were transferred to an or-
in an aqueous phase, and the formed fur- were produced from the hexoses in the ganic phase containing MIBK-2-butanol
fural will simultaneously be transferred mixtures, with maximum yields of 18% mixture (7:3 ratio) or DCM [37]. Furfural
from the aqueous phase to an organic and 22% respectively in ACETOSOLV selectivity was 91% and 76% for the de-
phase in which the furfural is extracted. In (without HCl) [35]. hydration of xylose and xylan respectively,
this system, the reaction medium (aque- In another attempt (Table 4), pro- implying that furfural production is de-
ous phase) containing acid catalysts has a duction of furfural from mixed northern pendent on the molecular weight of hemi-
limited concentration of furfural because hardwoods was investigated (Table 4). In celluloses, in that the larger the molecule,
the furfural is spontaneously transferred this work, wood chips consisting of ma- the lower the furfural selectivity will be.
as it is formed, which improves the over- ple, beech, birch, poplar, and aspen were Generally, the presence of acid, salt,
all furfural product yield. The interest of a initially hydrolyzed in a pilot-scale reactor and two phases in one process is opera-
bi-phasic reaction is to minimize side reac- (160°C with H-factor of 360 h) with hot tionally challenging. Moreover, the sol-
tions and to avoid the azeotropic point of water or green liquor (a mixture of so- vent, acid, and salt should be recovered
the furfural-water mixture, which makes dium carbonate, sodium sulphide, and so- to obtain an economically viable process.
separation difficult [33]. The aqueous dium hydroxide) to extract hemicelluloses The recovery process might be operable
phase usually consists of water or a wa- [36]. The extracted hemicelluloses (10.7% at laboratory scales, but would be complex
ter-dimethyl sulphoxide (DMSO) mixture wt. concentration, which contained 2% and expensive at industrial scales. Conse-
and an acid (HCl or H2SO4). The organic hemicelluloses in wood chips) were then quently, the commercialization of these
phase, however, consists of different or- used as a feedstock for furfural produc- bi-phasic systems is challenging and may
ganic solvents such as methylisobutylk- tion in a continuous bi-phasic reactor [36]. not be technically possible or economical
etone (MIBK), MIBK-2-butanol mixture, The aqueous phase was a dilute mineral- with available technology.
tetrahydrofuran (THF), or dichlorometh- acid solution (0.44 M HCl or H2SO4) and
ane (DCM), which have great affinity for NaCl (5–20 g/100 g hemicelluloses). Xy- Furfural production in solid catalyt-
absorbing furfural [34]. lan from the extracted hemicelluloses was ic/aqueous systems
In one study, the possibility of a converted into monomers in a separate In industry, 80%–85% of catalytic pro-

TABLE 4 Laboratory-scale furfural production processes using bi-phasic systems.

Method Raw materials Process conditions Furfural yields By-products References
ACETOCELL FORMACELL pulping Acetic acid/formic 46.4% HMF and [35]
acid, 165°C AMF
Continuous two zone Waste aqueous Sulphuric acid, 90% Humins [36]
biphasic reactor hemicellulose solutions 110-200°C
Biphasic reactor Biomass-derived mono-, Sulfuric acid, 91%1 HMF [37]
di- and polysaccharides 413-443 K
1
xylan yield

23
48 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012
 SPECIAL BIOREFINERY ISSUE

esses use solid catalysts instead of liquid two-stage process, however, controls the 125°C and 15% H2SO4 in the second stage
ones [34]. Similarly, a furfural production depolymerization and conversion reac- at 125°C in the presence of TiO2 as the
process can be implemented using solid tions in two separate stages, meaning that catalyst [38]. This process yielded 14.9%
catalysts in aqueous solutions. If used in each can be optimized separately, which furfural based on dry mass of rice hulls.
the furfural production process, the solid may improve overall furfural production,
catalysts can be readily filtered from the but at the expense of a more complex Furfural production in solid catalyt-
product suspension, which implies that process [38]. ic/bi-phasic systems
the separation (and hence the recovery) of In one study, one- and two-stage Recently, the use of solid catalysts in bi-
solid catalysts is easily feasible at industrial processes were used to produce furfural phasic systems has attracted attention.
scales. This fairly simple recovery system using rice hulls as feedstock [38]. The re- Table 5 lists some laboratory-scale furfu-
significantly reduces the operational cost sults showed that furfural production was ral production processes in solid catalytic/
of furfural production at industrial scales very limited (0.7%–3.34% on a dry basis) bi-phasic systems. This interest can be
[34]. For this reason, over the past few when the liquid/solid range was 13–50 ascribed mainly to the significantly higher
years, several studies have been carried ml/g and the sulphuric acid concentration furfural yield of these processes compared
out to discover stable and recyclable solid was 3 to 35 wt.% in a one-stage process at to other catalytic processes. In this respect,
catalysts for the conversion of saccharides 125°C. However, furfural production was a combination of water and MIBK or tol-
into furfural. enhanced in the presence of metallic ox- uene as a two-phase system (1:3 v:v) was
Similarly to other catalytic systems ides (i.e., AIC13, ZnC12, ZnO, and TiO2) studied for converting xylose (commer-
for furfural production, lignocellulosic as the solid catalysts in the reaction sys- cial xylose) to furfural at 170°C [39]. The
biomass can be used as the feedstock for tem. Among the solid catalysts used, TiO2 use of H-mordenite (0.5%) in the system
furfural production in solid catalytic sys- demonstrated the most efficient catalytic resulted in furfural selectivity and xylose
tems in one- or two-stage processes. A affinity for furfural production (4.3% fur- conversion rate of 95% and 66% respec-
one-stage process enables simultaneous fural based on dry mass of rice hulls). Al- tively (with a furfural yield of approximate-
depolymerization of pentosans to xylose ternatively, a two-stage process was inves- ly 63%) [39]. This furfural selectivity was
and conversion of pentose to furfural. A tigated using 3% H2SO4 in the first stage at higher than the selectivity (approximately

TABLE 5 Laboratory-scale furfural production in solid catalytic/aqueous systems.

Method Raw materials Process conditions Furfural yields By-products References
Sulphuric acid, metallic oxide
One- and two-stage
Rice hulls (AIC13, ZnC12, ZnO and TiO2) 14.9% N/A [38]
processes
catalysts, 125 °C
Batch mode two- H-mordenite catalyst, water-
Xylose 63% N/A [39]
phase process MIBK/toluene solvent, 170 °C
Bi-phasic process, DMSO, water,
HPAs-catalytic
Xylose water/toluene or water/MIBK 58-67% N/A [40]
process
solvents, 140-200 °C
Microporous and mesoporous
Micro-mesoporous
niobium silicates (Na,H-AM-11
niobium (Nb) silicate- Xylose 50% N/A [41]
and Nb-MCM-41), water-toluene
catalytic process
solvent, 140-180 °C
Microporous SAPO- SAPO-5, 11 and 40 catalysts,
Xylose 38% N/A [42]
catalytic process water-toluene solvent, 170 °C
Mesoporous TUD-1- Mono-, di- and AL-TUD-1 catalyst, water-toluene
60% HMF [43]
catalytic process polysaccharides solvent, 170 °C
Micro-mesoporous Micro-mesoporous silicas with
silica-sulphonic acid Xylose sulphonic acid groups, water- 75% N/A [44]
group catalytic process toluene solvent, 140 °C
Sulphated zirconia Mesoporous SZ-MCM-41, water-
Xylose 46% N/A [45]
(SZ) catalytic toluene solvent, 160 °C
SBA-15-SH(C), SBA-15-SO3H(C)
Mesoporous SBA-15
Xylose and SBA-15-SO3H(G), water- 70% N/A [46]
catalytic process
toluene solvent, 130-180 °C

J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012 49
70%) of a process using a mineral-acid other by-products. In this case, the larger system for four consecutive runs, with no
(HCl or H2SO4) solution under otherwise pores of the mesoporous catalyst facilitat- significant furfural yield loss.
similar conditions [39]. However, this ed the diffusion of formed furfural from As is well known, the surface prop-
method suffers from a lower xylose con- the pores to the reaction medium and erties of solid catalysts play an important
version rate and from high solvent usage, eventually to the organic phase. role in their performance. In this regard,
causing safety and environmental issues Alternatively, microporous silicoalu- the influence of surface modification of
for large-scale applications. minophosphates (SAPO) have been used solid catalysts in increasing furfural pro-
Heteropolyacids (HPA) have low as microporous solid zeolite SAPOs duction yield was investigated. In one
volatility, low corrosivity, and high flex- (SAPO-5, -11, and -40) in bi-phasic sys- study, surface-modified silica (0.7 meq/g
ibility. Literature results show that the use tems at 170°C for furfural production [42]. sulphonic acid groups grafted onto the
of HPA as a catalyst produced a smaller In this study, the batch system was oper- surface) was used as a solid catalyst (2%)
quantity of by-products compared to min- ated using 3% initial xylose concentration in a one-phase DMSO system at 110°C–
eral acids (in furfural production) [40]. In and a water-toluene mixture (3:7). In the 170°C. In this system, the furfural selec-
one study, H3PW12O40 (PW), H4SiW12O40 absence of a solid catalyst, a small furfural tivity and yield were 82% and 75% re-
(SiW), and H3PMo12O40 (PMo) were used yield (5%) was obtained. The maximum spectively. Adding unmodified catalyst
for furfural production from commercial furfural yield (38%) and selectivity (55%) (MCM-41) resulted in 30% xylose conver-
xylose (3 wt.% solution) in a homoge- were obtained using SAPO-11 (2%) under sion and 4% furfural selectivity under the
neous liquid phase in water. The results the same conditions. The SAPO-5 cata- same reaction conditions. Alternatively,
showed a low xylose conversion rate (34%) lysts retained their catalytic activity after the silica was used in a bi-phasic system
and furfural selectivity (2%) in the absence three consecutive recyclings, suggesting of DMSO or water and toluene under the
of a catalyst. Adding 2% PW catalyst in that these materials can be reused in the same experimental conditions. In this case,
DMSO (as the solvent) at 140°C improved reaction [42]. the maximum yield of 75% was obtained
the xylose conversion rate and furfural se- Although the analyses described using a mesoporous catalyst (MCM-41-
lectivity to 100% and 67% respectively. above have demonstrated various char- SO3Hs) and a water-toluene solvent after
Alternatively, the solid catalysts were used acteristics of furfural production in solid 24 h at 140°C [44]. The modified meso-
in water/toluene or water/MIBK (3:7 ra- catalytic/ bi-phasic systems, the main raw porous catalyst (i.e., MCM-41-SO3Hs)
tio) in a bi-phasic system [40]. The furfural material for these analyses was commer- used in the study showed a higher furfu-
yield (58%–67%) obtained in this system cially available xylose (in solution). How- ral selectivity and yield as well as a higher
was comparable to the furfural yield of ever, the hydrolysis of wood and agro- conversion rate compared to unmodified
the H2SO4 catalytic system under other- based biomass results in hemicelluloses mesoporous catalyst (i.e., MCM-41) in bi-
wise the same conditions [40]. Although with different molecular weights, implying phasic systems. Note that solid catalysts
the HPAs used in the study are promis- that the earlier investigations may not be may lose their surface activity after being
ing candidates for furfural production, the truly representative of furfural produc- used in the reaction process. Therefore,
possibility of recycling catalysts remains tion in a biorefinery. Interestingly, in one it is essential to investigate whether solid
to be assessed. study, furfural production was performed catalysts are reusable. In the current study
Niobium-containing materials such using mono- and polysaccharide (1%) so- using surface-modified silica, the stability
as hydrated niobium oxide and niobium lutions at 170°C in the presence of 2% of the catalyst was tested through regen-
phosphate could also be considered as aluminum-containing mesoporous TUD- erating (washing) with methanol and treat-
catalysts for furfural production. In one 1 (AL-TUD1) [43]. In the case of mono- ment with H2SO4. The use of the recov-
study, microporous and mesoporous nio- saccharides, approximately 60% furfural ered solid catalyst in furfural production
bium (Nb) silicates (H-AM-11 and Nb- and 17%–20% HMF yields were obtained resulted in 37% lower xylose conversion
MCM-41) were used as solid catalysts in using xylose and hexose respectively. In and 34% furfural selectivity under optimal
a water-toluene mixture (3:7 ratio) in a the case of polysaccharides, simultaneous reaction conditions [44].
bi-phasic system. The reaction was con- hydrolysis and dehydration occurred un- In another bi-phasic system with a
ducted using commercial xylose (3 wt.%) der these conditions. The conversion of water-toluene mixture (7:1 ratio), sulphat-
with micro- and mesoporous catalysts xylan or inulin to furfural or HMF yielded ed zirconia materials (SZ), including sul-
(3%) at 160°C [41]. The results showed approximately 18% and 20% respectively. phated zirconia (MSZ), alumina-modified
xylose conversions of 85% and 90% and These results demonstrate that furfural MSZ (MSAZ), and sulphated and persul-
furfural yields of 46% and 50% for mi- production is dependent on the proper- phated zirconia supported on MCM-41
cro- and mesoporous catalysts respectively ties of hemicelluloses and the content of mesoporous silica (SZ-MCM-41, SAZ-
[41]. The longer the retention of furfural available pentosans in hemicelluloses. The MCM-41, and PSZ-MCM-41 or PSZA-
in the pores of the catalyst, the greater is AL-TUD-1 catalyst used in this process MCM-41 respectively) were used as the
the possibility of conversion of furfural to seems to be stable after recirculating in the solid catalysts in a batch system [45]. In

23
50 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012
 SPECIAL BIOREFINERY ISSUE

the absence of any catalyst, the bi-phasic this will address a part of the environmen- backs of these processes, which have led
system yielded only 12% xylose conver- tal concerns associated with the use of to the closure of many furfural produc-
sion and less than 1% furfural production mineral acids in conventional furfural pro- tion plants. Due to its wide range of ap-
at 160°C after 4 h. Adding the unmodified cesses. Mesoporous solid catalysts such as plications, furfural has been considered as
catalyst (SZ, 2%) to the system increased SBA-15-SO3H(C) or AL-TUD-1 showed one of the most promising value-added
the furfural yield to 9% and the xylose a higher furfural yield than microporous chemicals that can be produced from lig-
conversion rate to 48%. However, apply- solid catalysts such as SAPOs or AM-11. nocellulosic biomass. As the need for fur-
ing different modified zirconia catalysts The main disadvantages of these sol- fural increases globally, various processes
(2%) enhanced the catalytic reaction and id-catalyst processes are that they do not and raw materials are being considered for
increased the furfural yield (22%–46%) have a sufficiently high xylose conversion furfural production. Large quantities of
and the xylose conversion rate (50%–96%) and furfural yield compared to conven- lignocellulosic residues (wastes) from dif-
under the same reaction conditions. The tional mineral-acid processes; that solid ferent agro- and forestry-based industries
maximum furfural yield of 46% with over catalysts might be expensive or difficult to are available globally. These materials have
90% xylose conversion was obtained using produce at industrial scales; that lignocel- been considered as feedstocks for furfural
mesoporous modified (PSZA-MCM-41 lulosic biomass has seldom been used as a production. The results for autocatalytic
and SAZ-MCM-41) zirconia catalysts feedstock to a furfural production process processes are not convincing in terms of
(2%) [45]. Among the catalysts used in the in these systems (i.e., commercial xylose yield and selectivity. The use of bi-phasic
study, SAZ-MCM-41 showed the highest has mainly been used as a feedstock), im- systems has shown more promising results
stability, with almost no significant loss of plying that the results of the present stud- in terms of furfural yields and selectivity.
selectivity for furfural production even af- ies may not be representative of furfural However, these systems may not be com-
ter three recycling runs [45]. production from lignocellulosic feedstock mercialized soon because the recovery of
In another study using surface- in solid catalytic systems; and that large solvents is not an easy task. Moreover, lit-
functionalized (with sulphonated group) quantities of solvents and solid catalysts in tle in-depth analysis has been carried out
mesoporous silica (SBA-15) [46], a batch bi-phasic systems make their commercial- on producing furfural from lignocellulosic
catalytic reaction was carried out using ization extremely challenging due to the biomass. Therefore, research on solvent
3% commercial xylose concentration and complexity of the process and the need to recovery design and on the use of ligno-
toluene-water (3:1 ratios) as a bi-phasic recycle the materials. Moreover, some of cellulosic feedstock as raw material for
system at different temperatures (130°C– the solid catalysts seem to become deacti- these systems is urgently needed. Solid-
180°C). In the absence of catalyst, a low vated after a few runs, which makes their catalytic systems have fewer environmen-
xylose conversion rate and furfural selec- recyclability unfavourable. tal impacts and are technically feasible.
tivity of 37.2% and 12.9% respectively Production of wood chips is exten- However, furfural yield and selectivity are
were obtained at 160°C using toluene- sive in Canada. However, the Canadian not as high as in mineral-acid processes.
water (3:1 ratio) as the solvent. Adding un- pulp and paper sector has been faced with In addition, little progress has been made
modified mesoporous catalyst (SBA-15) a significant decline in production in re- in using lignocellulosic biomass as raw
under the same reaction conditions slight- cent years, which implies that the availabil- material for solid-catalyst systems. Exten-
ly increased the furfural selectivity and ity of wood chips for the production of sive research is ongoing to discover a solid
yield to 13.5% and 5% respectively. The other value-added chemicals (such as fur- catalyst with high reaction performance
maximum furfural selectivity and yield of fural) is high [46]. As stated above, bi-pha- and recyclability. In this context, surface
74% and 70% respectively were obtained sic systems have shown promising results modification of catalysts has also been
using SBA-15-SO3H(C) as the catalyst at in terms of furfural production, but need attempted. Although the results may be
160°C. The furfural yields (70%) obtained technical breakthroughs to facilitate their promising, the effect of recycling on sur-
in this study using the solid acid catalyst commercialization. Moreover, advance- face modification is still unknown, which
SBA-15-SO3H(C) were comparable to the ments in the treatment and recovery of presents a challenge for this approach. The
yield (approximately 70%) of the conven- acidic wastes would help promote furfural use of solid catalysts in bi-phasic systems
tional method using liquid H2SO4 acid un- production in aqueous systems. is unlikely to be industrially implemented
der otherwise the same conditions [46]. in the near future due to the complexity
Regardless of the reaction media CONCLUSIONS of these systems.
used for furfural production, the main
advantage of using solid catalysts is their Furfural has been industrially produced in ACKNOWLEDGEMENTS
technically viable separation from the re- mineral-acid solutions (aqueous-acid cata-
action media (and from products). This lytic solutions). Environmental concerns, The authors would like to acknowledge
will decrease the recovery cost of solid low yield, and poor furfural selectivity NSERC for supporting this research
catalysts at industrial scales. In addition, (from hemicelluloses) are the main draw- through an NSERC Discovery Grant.

J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.2, NO.4, 2012 51
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