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Review of old chemistry and new catalytic

advances in the on-purpose synthesis
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Cite this: DOI: 10.1039/c4cs00105b

of butadiene
Ekaterina V. Makshina,*a Michiel Dusselier,a Wout Janssens,a Jan Degrève,b
Pierre A. Jacobsa and Bert F. Sels*a

Increasing demand for renewable feedstock-based chemicals is driving the interest of both academic and
industrial research to substitute petrochemicals with renewable chemicals from biomass-derived
resources. The search towards novel platform chemicals is challenging and rewarding, but the main
research activities are concentrated on finding efficient pathways to produce familiar drop-in chemicals
and polymer building blocks. A diversity of industrially important monomers like alkenes, conjugated
dienes, unsaturated carboxylic acids and aromatic compounds are thus targeted from renewable
feedstock. In this context, on-purpose production of 1,3-butadiene from biomass-derived feedstock is an
interesting example as its production is under pressure by uncertainty of the conventional fossil feedstock.
Ethanol, obtained via fermentation or (biomass-generated) syngas, can be converted to butadiene,
although there is no large commercial activity today. Though practised on a large scale in the beginning
of the 20th century, there is a growing worldwide renewed interest in the butadiene-from-ethanol route.
An alternative route to produce butadiene from biomass is through direct carbohydrate and gas
fermentation or indirectly via the dehydration of butanediols. This review starts with a brief discussion on
Received 6th March 2014 the different feedstock possibilities to produce butadiene, followed by a comprehensive summary of the
DOI: 10.1039/c4cs00105b current state of knowledge regarding advances and achievements in the field of the chemocatalytic
conversion of ethanol and butanediols to butadiene, including thermodynamics and kinetic aspects of the
www.rsc.org/csr reactions with discussions on the reaction pathways and the type of catalysts developed.

1. General introduction ‘‘Sinteticheskii Kauchuk’’ – synthetic caoutchouc) was made in

a one-stage process from grain ethanol, often referred to as the
1.1. Setting the scene Lebedev process. In addition to Russia, synthetic rubber from
1,3-Butadiene (C4H6 or BD) is the most important conjugated ethanol (KER: abbreviation of Polish – ‘‘kauczuk erytrenowy’’ –
diene, being the basis of a wide variety of synthetic rubbers, erythrene caoutchouc) was also produced in Poland in the early
elastomers and polymer resins upon polymerisation by itself or 1930s. Germany produced the rubber under the brand name
in conjunction with other polymerizable monomers. The global ‘‘Buna-S’’ and two routes starting from coal-derived acetylene were
BD demand in 2012 was approximately 10 million metric tons employed. The main route was the condensation of acetaldehyde
with an incremental growth of 1–2% per year. via 1,3-butanediol with an overall BD yield on acetylene of 60%;1
The synthetic BD industry started in 1920–1930; the synthetic the second route proceeded via 1,4-butynediol with an overall yield
rubber was proposed as an alternative for the manufacture of on acetylene of 70–75%.1 The first process involved the hydration
automobile tires. Sodium-butadiene rubber was the first syn- of acetylene to acetaldehyde, which is subsequently converted into
thetic rubber industrially produced worldwide. In Russia, the the aldol product, hydrogenated to 1,3-butanediol and dehydrated
S.K. synthetic rubber production (abbreviation from Russian to butadiene, while the second (Reppe) process used formaldehyde
and acetylene to produce 1,4-butynediol, which was hydrogenated
to 1,4-butanediol and dehydrated partly via tetrahydrofuran to BD.
a
Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, These routes are no longer practicable. In the US, BD routes
KU Leuven, Kasteelpark Arenberg 23, B-3001, Heverlee, Belgium.
employed were from oil sources and ethanol by a two-step
E-mail: bert.sels@biw.kuleuven.be, ekaterina.makshina@biw.kuleuven.be;
Fax: +32 16 321 998; Tel: +32 16 3 21 59
process using a mixture of ethanol and acetaldehyde.2
b
Department of Chemical Engineering, KU Leuven, Willem de Croylaan 46, In the US and most of Europe, the ethanol and acetylene
3001 Heverlee, Belgium routes have been abandoned after the world war because of the

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1
emerging production of BD from petroleum sources. The ethanol Table 1 Butadiene content (kg kg ethylene) for various feedstocks2,3
route only survived up to now in China and India. In the US,
Feedstock Butadiene/ethylene
a large percentage of BD production is obtained by dehydro-
genation of n-butane and n-butenes.2 Direct extraction from C4 Ethane 0.02
Propane 0.07
streams arising from naphtha crackers (ethylene plants) is Butane 0.07–0.11
substantially practised in Europe since the early seventies. Naphtha 0.13
The separation of BD from C4 streams though requires an Gas oil 0.26
expensive extractive distillation with a selectivity of only 4–5%
due to its azeotrope formation with butane. Recent trends in steam cracking show a lightened feedstock
The growth of ethylene production outpaced that of BD as a result of lower costs and larger availability of ethane from
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demand resulting in an oversupply of BD. Although this situa- natural gas like shale gas, this change in feedstock affecting the
tion has led to a shut-down of many on-purpose BD production supply and price of BD. Table 1 illustrates the lower BD yield
units, it also stimulated the development of chemical processes from lighter feedstock relative to naphtha and light atmospheric
using BD as the building block. BD is thus considered as a high gas oil in the steam cracker.2,3 Whereas an ethane-only cracker
value by-product of ethylene production from steam crackers with- has the lowest capital cost and the highest selectivity towards
out clear production economics. Since many years, the BD supply ethylene, it shows the lowest BD yield. As a consequence, restric-
and price have been influenced by the demand for ethylene. tions on the availability of BD and high BD prices are expected in

Dr Ekaterina V. Makshina Dr Michiel Dusselier obtained a

obtained her PhD degree in 2007 MSc in Engineering (Catalytic
in the frame of a cooperation Technology) at KU Leuven in
agreement (Co-tutelle) of Moscow 2009, studying in part at the TU
State University (Laboratory of München. He obtained his PhD in
Kinetics and Catalysis) and 2013 under the guidance of Prof.
Littoral University (Laboratory Sels and Jacobs on the topic of
of Catalysis and Environment), tailoring catalytic routes towards
Dunkerque. Her PhD thesis dealt and from lactic acid and
with the oxidation of methanol examining the synthesis and
and VOCs using supported oxide potential of bio-monomers. He
catalysts. Since 2010 she has was awarded the ACS Dr
Ekaterina V. Makshina been working as a post-doc Michiel Dusselier J. Breen memorial fellowship in
fellow in the Center for Surface 2013. As a postdoc, he currently
Chemistry and Catalysis (University of Leuven) in the group of Prof. focuses on zeolite catalysis and renewable conversion. He has
Sels on biomass transformation into commodity chemicals. She has authored 18 papers and 2 patents. Currently, he is staying in the
published 16 papers. group of Prof. Davis at Caltech.

Wout Janssens obtained his Jan Degrève obtained his PhD in

MSc in Engineering (Catalytic Chemical Engineering from SUNY
Technology) at KU Leuven in Buffalo, USA, in 1989 with a
2010. His master thesis was thesis on the numerical simula-
performed under the guidance of tion of combustion phenomena
Prof. Sels and dealt with the for the production of ceramic
synthesis and application of materials. He is presently
metal nanoparticles for catalytic professor of Chemical Engineering
purposes. Currently he is in the at KU Leuven, Belgium where he
process of finishing his PhD under teaches courses on the bachelor
the guidance of Prof. Sels, dealing and master level. His main fields
with the heterogeneously cata- of interest and expertise are
Wout Janssens lysed transformation of ethanol Jan Degrève design and analysis of chemical
into 1,3-butadiene. reactors, separation processes,
chemical process design and flowsheet simulation. He is also the
programme director for the advanced Master of Safety Engineering
at KU Leuven.

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the near future. Moreover, lightening of the feedstock for the

purpose of ethylene production leads to a deficit of C3, C4, C5
and pygas supply. Shortage of these chemicals, partially intro-
duced due to the shale gas revolution, may create new opportu-
nities for the production of biobased chemicals from renewable
feedstock like sugars, glycerin etc. One could argue about the
price of such chemicals, but their production cost will decrease as
the process technologies evolve and the sustainability benefits
should also not be overlooked.4,5 The worldwide change in feed-
stock usage asks for a revisit of on-purpose BD production.
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Future BD production will probably be diverse, highly depending

on the local availability and price of the different feedstocks such
as coal, oil, gas and biomass. Most relevant on-purpose BD
synthesis strategies are summarized in Scheme 1.
Biomass routes to butadiene. A return to the old BD produc-
tion routes from ethanol currently is gaining renewed interest,
since in a short time frame ethanol is one of the most relevant
sources of ‘‘bio’’-carbon for the chemical industry.6 As supplies
of ethanol increased rapidly in the last few years, mostly with
regard to its usage as a renewable fuel, its production is expected
to grow continuously. To illustrate its volume, the global ethanol
production amounts to more than 100 billion litres in 2012,7
which is an order of magnitude higher than the BD demand.
Usually, ethanol is made by fermentation of carbohydrates such
as starch (e.g. from corn) or sugar (e.g. from sugar cane and sugar
beet),8,9 though hydration of petroleum-based ethylene is still Scheme 1 Synthesis of 1,3-butadiene (BD) from various carbon-containing
practised. Chemical and fermentative routes via syngas to feedstocks. The focus of this review is shown in the red dashed line.
ethanol have also been investigated (see below). BDO = butanediols.
Obviously, the success of the ethanol-to-BD route will depend
on the cost of the upstream ethanol process. While the exploitation Europe and United States may well surpass the techno-economic
of the Brazilian sugarcane currently delivers the cheapest ethanol, performances. Many research efforts are presently undertaken
over time cellulosic-based second generation ethanol in Asia, developing more efficient ethanol production technologies from

Pierre A. Jacobs has been since Bert F. Sels obtained his PhD in
2008 professor emeritus with 2000 at KU Leuven in the field
special mandate at KU Leuven. of oxidation chemistry with
During his active career, he has heterogeneous catalysis. He was
been professor of Physical- awarded the DSM Chemistry
Chemistry and Catalysis at KU Award in 2000, and in 2005 the
Leuven and authored over 650 Incentive Award by the Societè
reviewed papers. The industrial Chimique Belge for his research
impact of his research is visible achievements. In 2005 he was
in numerous patents and culmi- appointed assistant professor at
nated in the recent nomination for KU Leuven, and full professor in
the 2013 Alwin Mittasch special 2008. He now teaches courses on
Pierre A. Jacobs prize marking the centennial com- Bert F. Sels spectroscopic tools in organic
missioning of the first ammonia chemistry, sustainability of
plant. Among the important awards received for his research are the chemical processes, spectroscopic tools in surface chemistry and
Paul Emmett Award in 1981, for his contribution to fundamental heterogeneous catalysis. He heads a research group in the Center
understanding of zeolite catalysts, the Donald Breck Award in 1998 for for Surface Chemistry and Catalysis (KU Leuven). His current field
his work on zeolite-based biomimetic catalysis. In recognition of his of research interest focuses on synthesis, characterization and use
contributions to bridge homogeneous, heterogeneous and bio-catalysis, of heterogeneous catalysis, mainly in transformation of renewable
he received the Synetix Award and the Kreitman Award in 2001. resources to chemicals. He was recently elected the co-chair of the
He was Karl Ziegler Guest Professor in 2004. IZA Catalysis Commission.

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low-cost and non-food/feed biomass feedstocks.10 Ethanol from under development. Syngas may originate from reforming of
cellulosic and algal sources, the so-called 2nd and 3rd genera- natural gas such as shale gas, although gasification of biomass
tion ethanol, is promising in this respect.11–17 or waste gases from the steel industry are valid alternatives.
While the industrial feasibility of the ethanol-to-BD route One route to BD runs via the production of 2,3-BDO followed by
has been proven long time ago, a number of improvements its dehydration. It was announced for technical implementation
both from an engineering and catalytic point of view is still vital by 2016 in the frame of new technology for adiponitrile (ADP) and
to attain high BD yield and high BD productivity. In order to nylon-6,6 production by LanzaTech and Invista.26 The butanediol
compete with existing oil- and gas-based technologies like C4 is produced from syngas via fermentation, for instance from the
dehydrogenation, issues related to carbon efficiency, catalyst waste gases of a steal producer or from natural gas steam
cost, toxicity, catalyst performance, feedstock tolerance, back- reforming.27 In the long term the direct gas fermentation of CO
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end optimization, catalyst lifetime and stability require further to BD is envisaged. In a partnership with Orochem Technologies,
optimization. So, it seems that extensive research is necessary both companies claim an economically viable thermocatalytic
to improve the viability of the ethanol route to BD vis-à-vis the 2,3-BDO dehydration process to BD with high selectivity. While
conventional routes. In 2013, Axens, IFPEN and Michelin have the CO gas fermentation lifts the process out of the sugar value
launched their joint research program to develop an economically chain (food – chemicals), the batch sugar production is replaced
competitive process to produce bio-sourced synthetic rubber from by continuous CO gas production.
bioethanol.18,19 Other routes to BD from natural gas were investigated. They
Next to ethanol, other bio-derived chemicals like butanols mainly focused on the direct and indirect production of chemical
(BuOH), viz. n-butanol and Gevo’s isobutanol, 2,3-butanediol and fermentation ethanol, which is converted to BD. Several
(2,3-BDO) and 1,4-butanediol (1,4-BDO) may also be useful to papers and reviews have been published on the catalytic conver-
produce BD. Most of the C4 alcohols are now available from sion of syngas to ethanol and higher alcohols using chemo-
renewable feedstocks via fermentation of biomass or syngas catalysis.28–31 Three chemocatalytic routes are known in the
(e.g. biomass derived).20,21 For instance, Genomatica – a company literature to convert syngas to ethanol: (i) the direct conversion
designing microorganisms for the conversion of sugars and via selective CO hydrogenation on a solid catalyst, (ii) methanol
cellulosic biomass to sustainable chemicals – is currently pro- homologation, which involves reductive carbonylation of methanol
ducing 1,4-BDO at a demonstration plant. The first commercial- over a redox catalyst, and (iii) a multistep process, wherein classical
scale plant is scheduled, with Novamont as a partner.22,23 methanol production from syngas, is followed by carbonylation to
Together with Versalis, Eni’s chemicals subsidiary leader in acetic acid, and subsequent hydrogenation to ethanol. None of
the production of elastomers, they recently announced the these routes have been commercially practised. The carbonylation
establishment of a strategic partnership to enable the produc- of methanol to ethanol is the most promising route, while the
tion of BD from renewable feedstocks. Main incentive of the direct formation of ethanol from syngas is probably commercially
collaboration is the successful direct production of million less interesting due to a low ethanol yield and selectivity.32 A recent
pound quantities of bio-based 1,4-BDO from biomass resources example of the methanol–acetic acid route (TCX technology) is
in 2011 by Genomatica.24 reported by Celanese.33
Acid catalysed dehydration of (bio-based) butanols leads to a The fermentative production of ethanol from syngas has also
mixture of n-butenes, requiring further dehydrogenation to gained considerable attention. The syngas could be an industrial
produce BD with existing technology.2 Next to dehydration of waste gas or it may be obtained by natural gas reforming or
monohydric alcohols direct butanediol (BDO) dehydration to BD gasification of biomass. The utilization of the whole biomass of
was also performed. This strategy has the advantage of avoiding low quality and the elimination of biomass fractionation are major
the expensive dehydrogenation step. The reaction involves a advantages here. The state-of-the art and the most important
double dehydration, preferably employing a catalytic process, challenges recently have been critically reviewed.34–38
the catalyst and the reaction conditions employed differing with Hydrocarbon and natural gas routes. Synthetic ethanol,
the position of the alcohol groups. Dehydration of 1,4-BDO is produced by the direct or indirect hydration of ethylene from
well-known from the acetylene-based Reppe route.2 It is carried the cracker or dehydrogenation of ethane in natural gas resources,
out at 280 1C by a rather complex method using an excess of may be converted to BD. However, the capacity of worldwide
steam and THF in the presence of a sodium phosphate catalyst. production of synthetic ethanol is rather limited and represents
The dehydration chemistry of 1,3-BDO is slightly different. High less than 10% of the global production.
BD yields above 80% were obtained at 400 1C using the same Dehydrogenation of butane from natural gas is a better
catalyst in the presence of steam. Dehydration of the other diols economical solution to BD production. This well-established
is less selective. So, 2,3-BDO easily dehydrates to methyl ethyl technology has been practiced until the 1990’s in Russia and
ketone, while its diacetate can be pyrolysed in high yield to BD the US. The main disadvantages of the process are the high
upon heating to 475–600 1C, the acetic acid co-product being endothermicity and the high process temperature (600–700 1C)
recycled.25 New catalytic advances in the dehydration of diols to achieve an economical equilibrium conversion. As the reac-
and polyols in detail will be described below. tion runs at incomplete conversion, separation of feed and
Gasification routes to butadiene. New chemical and bio- products is mandatory. Due to coke formation, catalysts usually
chemical technologies producing BD from syngas are currently run for only 5 to 15 minutes. As heat from catalyst regeneration

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fuels the endothermic dehydrogenation, the overall lifetime of is attempted in the first part. The contribution includes details
the catalysts is much longer. The Houdry-Catadiene process of the reaction network, mechanistic and thermodynamic con-
using a mixture of Al oxide and Cr oxide typically shows a siderations and compares catalytic data (in terms of BD yield,
reactor outlet with 15 vol% butadiene, and about 50% carbon ethanol conversion, carbon yield and volume productivity) of
yield.39 Somewhat higher yields (65%) are reported for the two- various catalyst types. Catalytic properties and structure/activity–
step Phillips Petroleum technology, involving butane dehydro- selectivity relationships were made as much as possible, if appro-
genation to n-butenes with Cr/Na/Al oxide, followed by their priate, in order to guide and inspire the reader.
extractive distillation and conversion to butadiene over Fe/K/Al Routes to BD from 4-carbon diols (BDO) have attracted
oxide.39 Application of vacuum or addition of water are used to much recent attention and this evolution is mainly explained
reduce the partial pressure (Le Chatelier) and to overcome coke by the availability of bio-derived BDOs from renewable and
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formation by stimulating the water gas shift reaction, respec- waste feedstocks. Though the chemistry seems less complicated
tively (see also technologies from Dow and Shell). The conver- and has already been described in part of the old BD processes
sion and selectivity of the process are also greatly improved by from acetylene and ethanol, the selective double dehydration of
hydrogen removal. In the presence of dioxygen, water is BDOs to BD under milder conditions remains challenging
formed, the exothermicity partly being used to compensate requiring unique catalysis. The second part of the review is
the energy required for the dehydrogenation. Whereas the therefore devoted to summarize and describe the most impor-
presence of water and O2 increases catalyst lifetime, O2 also tant catalytic achievements to dehydrate such diols including
helps the difficult abstraction of allylic hydrogen. With Phillips the mechanistic proposals and thermodynamics, the type of
O–X–D at 480–600 1C, 80% conversion and 90% BD selectivity catalyst developed and the current status.
were obtained. The Oxo-D process of PetroTex shows 93% BD
selectivity at 65% conversion.2 Both processes run only on
n-butenes since the addition of oxygen to butane would result 2. Ethanol-based butadiene
in products oxidation at the temperature required for butane 2.1. The technological options: one vs. two-step processes
dehydrogenation. Although use of iodine to remove H2 as HI In 1903 the catalytic synthesis of BD from ethanol was first
(Idas process, Shell France) has been proposed, serious corro- performed by Ipatiev in Russia.62 Low BD yield of about 1.5%
sion has been encountered. The yield of BD on butane was said was obtained by passing ethanol at 550–600 1C vapour over Al
to be 70%. Butenes could be also resourced from dehydrating powder. Filippov obtained a BD yield of 5% from diethyl ether
biobutanols to produce bio-based BD, though the price of the at 400–500 1C on the same catalyst.63 Ostromyslensky proposed
bioalcohols is currently too high for commercial use. a catalytic route from a mixture of ethanol and acetaldehyde
The dehydrogenation processes are competitive with the over alumina or clay catalysts at 440–460 1C, yielding 18% BD.64
cracking process when BD price is high and raw C4 is available. Maximoff claimed the formation of BD from a mixture of
Honeywell UOP in partnership with TPC group and BASF in ethanol with acetaldol (or crotonaldehyde) over aluminium
collaboration with Linde recently announced the development hydroxide.65 Later, Lebedev66–68 proposed his first process on
and licensing of such a process for the on-purpose BD produc- a mixture of zinc oxide and alumina at 400 1C. He achieved a BD
tion from butane.40–42 selectivity of 18%, directly from ethanol according to following
1.2. Scope of the review reaction (1):

The introductory section already partly reveals many feedstock (1)

possibilities for future on-purpose production of BD. Routes
from butane and n-butenes are probably the most developed Improvements by Lebedev’s group with an undisclosed
ones. Although they will not be described in this review, they catalyst composition showed higher yields with up to 70% BD
should be regarded as benchmark to alternative routes. The selectivity at 350 1C and atmospheric pressure.69 The process
most promising competitive routes are BD from ethanol and was examined both in Germany and in the US, but was not
BDOs. They form the main subject of this review. taken to the full scale. I.G. Farbenindustrie tested supported
Research on ethanol conversion to BD has been performed magnesia catalysts promoted by Co or Cr and reported 60%
intensively in the twenties to sixties of the 20th century. yield at 300 1C and atmospheric pressure.1 In the US a similar
A possible reaction mechanism and promising catalytic systems catalyst containing 59% magnesia, 2% chromic acid and 39%
have been presented in previous reviews.43–48 With the decreas- silica gel was also examined.1 This catalyst gave 38% BD yield
ing industrial interest for this reaction at that time, the scientific per pass with 56% selectivity at 400–425 1C.
interest also faded, while only a few publications analysing In addition to the direct ethanol conversion according to (1),
recent achievements appeared.49–52 Yet, there is now a steadily BD from a mixture of ethanol and acetaldehyde (or a so-called
growing interest of academia and industry in producing bio- two-steps process according to Ostromyslensky’s pathway) was
ethanol and using it as a platform molecule for various base studied. Union Carbide and Carbon Chemicals Corporation
chemicals,53–61 BD being an important chemical among them. performed the industrial manufacturing of BD from ethanol in
Therefore, a comprehensive review paper of old and recently the USA over tantalum oxide on silica using this two-step process
published reports on the catalytic conversion of ethanol to BD (Scheme 2). This process includes the partial dehydrogenation of

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Scheme 2 Two-step process of BD from ethanol.

ethanol to acetaldehyde, followed by the transformation of the

Scheme 4 BD formation mechanism according to Balandin.71
mixture of ethanol and acetaldehyde into BD. The yield of
ethanol was 18% at a conversion of 30% per pass.70
transformation of various reactants and mixtures thereof, the
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2.2. Routes and chemistry from ethanol to BD authors considered that the reactions, including the Lebedev
and Ostromyslensky synthesis, all proceeded through the
Ethanol as a building block can be converted into BD. The intermediate formation of crotonaldehyde.69,72,73,77,78 The
reaction network involves a series of reactions, leading via mechanism includes the formation of acetaldol via the aldol
multiple pathways to a variety of products that are controlled condensation of two acetaldehyde molecules, followed by
differently by kinetics and thermodynamic restrictions. Before dehydration of the aldol to crotonaldehyde, in accordance with
describing the thermodynamic and kinetic aspects, the different reaction (2):
reactions involved and the complex reaction network in the
synthesis of BD from ethanol will be clarified first.
Butadiene formation. Since the successful industrial pro- (2)
duction of BD from ethanol (or a mixture of ethanol and
acetaldehyde), the study of the reaction mechanism has been
performed extensively. Many reaction mechanisms proposed Further transformation of crotonaldehyde to BD has been
before 1945 have been discussed in depth in the review by the subject of many discussions for a long time. According to
Egloff and Hulla.44 This review briefly mentions the most Quattlebaum et al., crotonaldehyde is deoxygenated with ethanol,
important statements and conclusions of that period, but we as presented in Scheme 5(i).73 Moreover, the authors for the first
will focus more on the recently anticipated reaction schemes. time attempted a description of the mechanism taking into
Initially, Lebedev et al. proposed a radical mechanism to account the participation of surface atoms of the catalyst. They
describe the direct formation of BD from ethanol, in accordance assumed a synthesis route via hydrogen transfer from ethanol to
with the reactions in Scheme 3.68 A few years later, Balandin the enol-form of crotonaldehyde.73 It has been suggested that the
argued that this radical reaction mechanism was energetically reactive bridged siloxane of the silica-based catalyst participated in
possible.71 All possible combinations of molecules in the reaction the deoxygenation process by reacting with ethanol and the enol,
mixture were analysed based on the multiplet theory. Whereas the thereby supporting an intermolecular hydride shift (Scheme 5ii–iv),
formation of 49 different possible reaction products was predicted, while the oxide promoter (e.g., tantalum, zirconium or niobium
only 21 of them could be detected. Acetaldehyde was identified as oxides) performs the aldol condensation. The latter reaction
a primary product, while BD was assumed to be a third generation was identified as the rate-controlling step for the Ostromyslensky
product. Baladin concluded that the reaction proceeded via 1,3- process.73
BDO according to the following reaction sequence (Scheme 4).
This scheme was essentially similar to the mechanism
proposed earlier by Ostromyslensky for his two-step process
using a mixture of ethanol and acetaldehyde.64 Egloff and Hulla
supported the intermediate formation of 1,3-BDO.44
The above mechanistic proposals were all rejected a few
years later by different research groups.69,72–76 By studying the

Scheme 3 BD formation mechanism according to Lebedev.68 Scheme 5 BD formation mechanism according to Quattlebaum et al.73

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the conversion of crotonaldehyde into crotyl alcohol in the

presence of an excess of ethanol due to the repartition of
hydrogen, which occurs via the formation and decomposition
of an intermediate complex between ethanol and the aldehyde.
Later, Niiyama et al. studied the ethanol to BD conversion
over MgO–SiO2 catalysts. They assumed the ethanol dehydro-
genation as a rate-determining step on the basic catalyst.80 They
Scheme 6 Pathways of crotonaldehyde to BD.69,72 postulated for the first time that the intermolecular hydrogen
transfer between ethanol and crotonaldehyde proceeded via a
Meerwein–Pondorf–Verley mechanism (MPV), requiring the
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Jones et al.76 studied the reaction mechanism of the two-step participation of acid and basic sites. High BD yields necessitate
process over the same catalyst, viz. 2%Ta2O5–SiO2. They sug- a balanced ratio between the acidity and basicity.80
gested an opposite effect from the catalyst’s components, the Kvisle et al. also using the MgO–SiO2 catalyst81 concluded
unmodified silica being more effective for acetaldehyde conden- that the rate-limiting step consists in the condensation of
sation and the 2%Ta2O5–SiO2 material being more active for feed acetaldehyde occurring prior to the hydrogen transfer step,
mixtures requiring deoxygenation of the aldehyde. Thus, the rather than the dehydrogenation of ethanol to acetaldehyde.
proposed mechanism of the process (in Scheme 5i) was supported, In the absence of quantitative details, they also emphasized the
while reaction sequence ii–iv was rejected (Scheme 5), because synergic effect of the catalyst’s components for active and
participation of the Ta promoter was not included. selective catalysis.
At the same time, a slightly different pathway for crotonalde- Separately, the possible formation of 1,3-BDO as an active
hyde transformation was proposed (Scheme 6).69,72 Crotonalde- intermediate was studied in more detail.73,74,82 Based on the
hyde was first reduced to crotyl alcohol (Scheme 6i and ii), transformation of pure butanediol or its mixture with ethanol,
followed by its dehydration to BD (Scheme 6iii). Ethanol (or any it was concluded that the former did not participate in BD
other alcohol present in the system) served as a hydrogen donor. formation from ethanol.73,82 The analysis by Natta and Rigamonti
Kagan et al. over Lebedev type catalysts showed BD production by supported the thermodynamically impossible formation of 1,3-BDO
the direct reduction of crotonaldehyde with hydrogen.72 In view of from ethanol and acetaldehyde.74
the overall scheme of the reaction mechanism, it was concluded that Although the route of BD formation (Scheme 7) is generally
this hydrogenation process is less likely than the hydrogen transfer accepted, alternative schemes were proposed as well. In
from the alcohol to the aldehyde (Scheme 6ii). Natta and Scheme 8 it is suggested that BD formation results from an
Rigamonti74 and later Bhattacharyya and Sanyal,75 based on thermo- interaction between acetaldehyde and ethylene via the Prins
dynamic calculations of the proposed reaction scheme (Scheme 6), reaction.83,84 Thermodynamically, the Prins mechanism is
have confirmed that the formation of BD proceeded via the selective energetically possible but less favourable than the generally
reduction of crotonaldehyde by ethanol to crotyl alcohol, the direct accepted mechanism via aldol condensation.51,74 However,
reduction of crotonaldehyde by hydrogen being thermodynamically no experimental studies confirming the involvement of the
less favourable. In the overall reaction network (Scheme 7), croton- Prins reaction have been performed.
aldehyde formation from acetaldehyde, i.e. the aldol condensation In the literature, all mechanistic and reaction network studies
step, was indicated as the rate-controlling step.72,75 were based on the experimental results of catalytic tests using
The reaction network in Scheme 7 was also confirmed by various feed combinations, temperatures and contact times. There
Vinogradova et al. using C14 labelled acetaldehyde.79 They showed has been only one attempt studying BD formation from ethanol by
IR spectroscopy.86 Ethanol adsorption and its chemical transforma-
tion were studied over an alumina–zinc catalyst in the temperature
range of 20–400 1C. Above 300 1C a decrease of surface alkoxy-
species in the region 1080–1120 cm 1 with increasing reaction
temperature was observed at the expense of several new vibration
bands. The most intense signal was assigned to the characteristic
vibrations of CQO (1750 cm 1) and CQC–O (1035 and 1680 cm 1),
while the origin of other, less intense bands was not specified.
However, a conclusive mechanism was not proposed.

Scheme 7 Overall scheme of ethanol conversion to BD. Scheme 8 Prins reaction scheme for BD formation from ethanol.83–85

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Identification and origin of by-products. Reaction of ethanol product of deoxygenated acetaldol with subsequent rearrange-
to BD generates several by-products. Their formation has been ment (reaction (6))73 or it may result from dehydration of
studied intensively.73,87,88 Quattlebaum et al. proposed a classi- 1,3-BDO (see below, Section 3.2), or it could result from crotyl
fication of the by-products according to their formation mecha- alcohol isomerization.90 Butanal can be an isomerization pro-
nism,73 one group including the products formed by simple duct of crotyl alcohol (reaction (7)), while butenes and butanol
dehydration or ester-forming disproportionation, another one may be produced from butanal via deoxygenation (reaction (8))
containing the products formed similarly as in the reaction and reduction (reaction (9)), respectively.73 1,3-BDO was con-
cascade towards BD. The flow chart of the most important sidered as a product of acetaldol reduction (reaction (10)).
by-products is presented in Scheme 9. Furthermore, C6-hydrocarbons and C6-oxygenated products were
According to this classification, the first group includes formed by the condensation of C4-aldehydes such as butanal
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ethylene and diethyl ether, being products of the direct ethanol and crotonaldehyde with acetaldehyde via similar mechanisms
dehydration (reaction (1) and (2), Scheme 9). Ethyl acetal is a (reaction sequence, ii–vi).73
condensation product in the reaction between ethanol and The presence of molecules containing three and five carbon
acetaldehyde (reaction (3)). Ethyl acetate is a product of acet- atoms was rationalized by the cleavage of 1,3-BDO into
aldehyde disproportionation, also known as the Tishchenko propylene and formaldehyde (reaction (11), Scheme 9). The
reaction (reaction (4), Scheme 9). Finally, acetic acid is a occurrence of the last reaction explains the formation of
product of ethyl acetate hydrolysis (reaction (5), Scheme 9).73 C3-hydrocarbons and C3-oxygenated compounds as a result of
The second group of by-products consists of crotonaldehyde, an interaction of formaldehyde with ethanol and acetaldehyde,
a product of acetaldehyde condensation to acetaldol (reactions as well as C5-hydrocarbons and C5-oxygenated products as a
(iii), Scheme 9) and its subsequent dehydration (reaction (iv)); result of an interaction of formaldehyde with C4-oxygenated
crotyl alcohol, a product of the reduction of crotonaldehyde compounds.73 Also the reduction of formaldehyde could be a
(reaction (v)). Methyl ethyl ketone (butan-2-one) could be a source of methanol (reaction (12)), which further reacts with
other alcohols resulting in ether formation and ultimately in
aromatics according to the Methanol-to-Gasoline concept.91
An almost similar scheme of by-product formation was pro-
posed by Gorin et al.87,88 Despite the experimental evidence of
reaction (11) (Scheme 9),82 an alternative mechanism was
proposed to explain the formation of by-products with an odd
number of carbon atoms.88 The steps proposed are the keton-
ization of acetic acid, formed via reaction (5), resulting in
acetone with release of CO2 (reaction (13)). Acetone is further
reduced to isopropyl alcohol (reaction (14)), which is likely the
origin of propylene via dehydration (reaction (15)). Acetone
and isopropanol could participate in the formation of the
C5-hydrocarbon pool92,93 via mechanisms similar to those
proposed for BD formation (Scheme 9, reactions (ii)–(vi)).
Alternatively, it has been suggested that n-butanol is formed
directly via hydrogenation of crotonaldehyde (reaction (16),
Scheme 9)53,59,89 by surface hydrogen atoms resulting from
ethanol dehydrogenation (reaction (ii)), whereas the butenes can
be obtained by dehydration of n-butanol (reaction (17)).59 Further-
more, it has been also proposed that acetone could be produced
from acetaldol (reaction (18)).94–96 Reaction proceeds via formation
of surface carboxylate intermediate CH3CH(OH)CH2COO(s)
followed by dehydrogenation and decarboxylation.
All compounds identified as by-products in both Lebedev’s
and Ostromyslensky’s processes are summarized in Table 2.

2.3. Thermodynamic considerations

The thermodynamics for the conversion of ethanol to BD have
been described by several authors.43,51,74,75 The first attempt
was performed by Natta and Rigamonti74 in 1947, aiming at
unravelling the reaction mechanism. Seven different mecha-
Scheme 9 Reaction network of the main by-products.53,59,73,87–89 Reac-
nisms were discussed in this study. Based on the experimental
tions (ii)–(vi) and the products (in boxes) represent the generally accepted data and on the data for the Gibbs free energy of the different
reaction pathway towards BD. reaction steps in the temperature range 400–430 1C, the authors

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Table 2 By-products of ethanol conversion into butadiene48,73,78,87,88,97

Number of carbon atoms Hydrocarbons Oxygenated compounds

1 Methane Carbon monoxide
Carbon dioxide
Formaldehyde
Methanol

2 Ethylene Acetaldehyde
Ethane Acetic acid

3 Propylene Methyl ethyl ether

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Propane Acetone
Propanol and isopropanol

4 Butadiene Ethers: ethyl ether, ethyl vinyl ether

Butenes: 1-butenes, trans-2-butene, Esters: ethyl acetate
cis-2-butene; iso-butene Aldehydes: butanal, crotonaldehyde
Ketones: butanon
Alcohols: butanol, crotyl alcohol
Ethyl acetal

5 Pentenes: 2-pentene, 1-pentene, Amyl alcohol

2-methyl-2-butene, 2-methyl-1-
butene and 3-methyl-1-butene
Pentadienes: 1,3-pentadiene;
2-methyl-1,3-butadiene

6 Hexanes: both branched and Ethers: butyl acetate

strait-chain isomers Aldehydes: hexaldehyde and hexenal
Hexadienes: 1,3-cyclohexadiene, Alcohols: hexanol; 2-hexen-1-ol
2,4-hexadiene, 3-methyl-1,3-pentadiene
and 1,3-hexadiene benzene

7 Toluene

8 4-Vinylcyclohexene; p-xylene n-Octyl alcohol

concluded that the reaction sequence of reactions (ii) to (vi) in

Scheme 7 is the most probable pathway to form BD from
ethanol. Later, several groups carried out a more comprehensive
thermodynamic analysis as shown in Scheme 7.43,51,75
An Ellingham-type plot (Fig. 1) displays the change in Gibbs
free energy (DG, kJ mol 1) between 25 and 530 1C for the
different reactions shown in equation (i) to (vi). Calculations
were done in Aspen Pluss software for pure components in their
real, and not standard states at the given reaction temperature
and pressure. The ethanol to BD reaction is endothermic by
102–109 kJ mol 1 from 200 to 500 1C (not shown). The Gibbs free
energy change for the direct conversion of ethanol to BD shows
that the reaction (i) is favourable above 150 1C. Higher tempera-
ture shifts the equilibrium of the endothermic reaction to higher
conversions. Since the number of moles of products is higher
than that of the reagent, the reaction is pressure dependent, low
pressures being favourable for high equilibrium conversion.
Inspection of the individual reactions shows different thermo-
dynamic behaviour. The condensation of acetaldehyde to croton-
aldehyde ((iii) + (iv) in Fig. 1 and Schemes 7, 9) is characterized
by very low values of the Gibbs energy (DG). The reaction is
unfavourable above 200 1C, whereas the formation of acetalde-
Fig. 1 Calculated thermodynamic data for ethanol transformation into BD,
hyde from ethanol (ii) and the dehydration of crotyl alcohol to
DG values of the overall reaction (i) and the intermediate steps (ii–vi). The
BD (vi) are favourable above 250–300 1C. Thus, in the usual calculations were performed with Aspen Pluss software for mixtures of pure
temperature range applied for BD synthesis, viz. 350 to 430 1C, components at given temperature, p = 1 atm (DG is calculated in the real
the equilibrium of reaction (vi) (Fig. 1) is clearly shifted to state and thus different from DG0).

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BD formation. One reaction, viz. the bimolecular MPV ((v) in also stimulates reaction (iii) + (iv) to continuously produce
Fig. 1 and Schemes 7, 9), forming crotyl alcohol (CrOH), crotonaldehyde.
shows very low DG values over the entire temperature range. It may thus be concluded that the choice of the temperature
However, as long as CrOH is consumed by dehydration in the range for BD formation from ethanol will be defined by kinetic
consecutive reaction to BD (vi), the MPV reaction (v) will be factors, and thus by the ability of the catalyst components to
consuming crotonaldehyde (CrH) to form new CrOH. Further- accelerate the individual reactions.43
more, as the equilibrium of reaction (ii) is in favour of acet- Fig. 2 shows the equilibrium compositions (mole fractions)
aldehyde formation, an excess of acetaldehyde in the system in the direct conversion of ethanol to BD. These data were
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Fig. 2 Equilibrium compositions calculated for: (a) the overall reaction of ethanol to BD, reaction (i); (b) the reaction of ethanol to BD taking into account
the reaction sequence, reactions (ii) to (vi); (c) ethanol dehydrogenation to acetaldehyde, reaction (ii); (d) aldol condensation of acetaldehyde to
crotonaldehyde, reaction (iii) + (iv); (e) MPV reaction of ethanol with crotonaldehyde to synthesize crotyl alcohol, reaction (v), with a mixture of ethanol
and crotonaldehyde (molar ratio is 1 : 1); (f) dehydration of crotyl alcohol to BD, reaction (vi). The pressure in the calculations is assumed 1 atmosphere.
The simulation is performed using Equilibrium based reactor (REquil) in Aspen Pluss software. The reactions (i) to (vi) are in line with the reactions in Fig. 1
and in Scheme 7. (CrOH = crotyl alcohol; CrH = crotonaldehyde; AcH = acetaldehyde; BD = 1,3-butadiene).

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calculated at one atmosphere pressure using the Equilibrium

based reactor (REquil) in Aspen Pluss software. The results
indicate that an almost complete EtOH conversion could be
obtained above 227 1C (Fig. 2a). A very similar result is obtained
when the thermodynamic analysis was approached via the
reaction sequence (ii) to (vi) in Schemes 7 and 9 (Fig. 2b). Only
a slight decrease in BD concentration was found for tempera-
tures higher than 340 1C due to unconverted acetaldehyde and
crotonaldehyde (Fig. 2b).
The different steps of the reaction network (Schemes 7 and 9)
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were also individually analysed with respect to the equilibrium

compositions and conversion. Ethanol dehydrogenation is thermo-
dynamically favored by an increase in temperature,43,51,74,75 a
temperature higher than 430 1C being required to obtain full
ethanol conversion (Fig. 2c). In the commercially applied tem-
perature range of 325 to 430 1C, a 76 to 96% (on carbon basis)
thermodynamic yield of acetaldehyde (AcH) can be reached.
Further transformation of acetaldehyde into crotonaldehyde
proceeds via acetaldol (Scheme 7(iii)). The aldol reaction is
thermodynamically unfavourable (Fig. 1),51,75 but the subsequent
aldol dehydration into crotonaldehyde (CrH) (iv) is highly favor-
able (Fig. 1). The thermodynamics show full conversion of the
aldol over the entire temperature domain. The overall transforma-
tion of acetaldehyde into CrH (reactions (iii) + (iv)) inhibited by an
increase in temperature shows low DG values over the entire
temperature domain. Consequently, the equilibrium conversion
of acetaldehyde is 72% at 330 1C (on carbon basis) and decreases
with increasing temperature (Fig. 2d). Subtle acid–base catalysis is
required to assist this condensation reaction.
The subsequent step with crotonaldehyde is the MPV reaction
(v), which is also characterized by very low DG values.43,51,74 The
conversion of crotonaldehyde (CrH) into crotyl alcohol (CrOH) is
23–27% (from 325 to 430 1C, calculated on CrH basis) in the case
of an equimolar substrate feed (Fig. 2e). The equilibrium conver-
sion could be increased by increasing the molar ratio of EtOH to
CrH, with a value of 77%, when the molar ratio of EtOH to CrH is
20. CrOH formed as a product of the MPV reaction is then
dehydrated to BD (Fig. 2f). This reaction is thermodynamically
favorable irrespective of the temperature applied.
The thermodynamics were also analysed taking into account
the presence of side-products (Table 2). The effect of ethylene
and diethyl ether (DEE) on the thermodynamics was specified Fig. 3 Equilibrium composition calculated for the conversion of ethanol to
first. Fig. 3a shows the equilibrium composition of the reaction BD in case following by-products are allowed: (a) diethyl ether and ethylene;
mixture in the case where ethanol dehydration occurs together (b) 1-butanol; (c) butenes (1-butene, cis-2-butene, trans-2-butene). The
with ethanol-to-BD. Ethylene is clearly the most thermo- pressure in the system is 1 atmosphere. Simulation is performed using
the Equilibrium based reactor (REquil) in Aspen Pluss software. (BuOH =
dynamically stable product, whereas diethyl ether is formed
1-butanol; DEE = diethyl ether).
at temperatures below 150 1C. The BD yield is considerably
lower when ethanol dehydration is included in the thermo-
dynamic analysis (compare Fig. 3a with Fig. 2a and b). When the choice of the appropriate catalyst plays a significant role in
the temperature increases from 325 to 430 1C, in the presence the BD selectivity of the overall ethanol-to-BD process.
of ethylene in the product mixture, the thermodynamic yield of For reaction temperatures ranging from 350 to 430 1C,
BD is reduced to 24–27% (on carbon basis). When the reaction co-production of 1-BuOH in the ethanol-to-BD reaction is of
temperature is increased to 530 1C, the BD yield only slightly little importance, BD being formed preferentially according
increases (up to 30%). Thus dehydration of ethanol to ethylene to the thermodynamics (Fig. 3b). Conversely, all isomers of
should be avoided kinetically to attain high BD yields. As the butenes thermodynamically more favorable than BD (Fig. 3c)
reaction is catalysed by the acidity of the catalyst or the support, may be formed by acid catalysed dehydration of butanols.

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water and hydrogen simultaneously. Therefore, dehydration and

dehydrogenation were the prime targeted reactions for catalyst’s
components selection.68
In the first patents, Lebedev et al.66,67 have been referring
to a mixture of zinc oxide and alumina, yielding BD with a
selectivity of 18 wt%. Other compositions containing uranium
oxide98 and mixtures of aluminium hydrosilicate98,99 or floridin
(Fuller’s earth)98,100 with zinc or manganese oxide were suggested
as well. The dehydrogenation and dehydration components
should be present in an optimal ratio, viz. 25% dehydration
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and 75% dehydrogenation component, in order to attain a high

BD yield.68 However, this optimal ratio strongly depending on the
catalyst nature has been the subject of many discussions in the
Fig. 4 Equilibrium compositions calculated for the synthesis of BD from
an ethanol–acetaldehyde mixture with a molar ratio of EtOH to AcH of 3, more recent literature. Unfortunately, the elemental composition
via the reaction sequence (ii) to (vi) of Fig. 1 and Scheme 7. The pressure in the of the preferred catalyst, showing up to 42 wt% BD selectivity
system is 1 atmosphere. The simulation is performed using the Equilibrium (or close to 70% on carbon basis), remained undisclosed.68,69
based reactor (REquil) in Aspen Pluss software. The composition of a Lebedev industrial catalyst became avail-
able in 1941 after occupation of a factory for synthetic rubber
production in the Russian city of Yefremov by the German
Thus, optimization of catalyst properties is of key importance to army.43,74 Chemical analysis revealed the presence of 44.6 wt%
obtain high BD yields. of magnesia and 10 wt% of silica, next to a large number of
Thermodynamic simulation was also carried out for the two- small amounts of different elements.43,74 XRD analysis has shown
step Ostromyslensky process, co-feeding a mixture of ethanol the presence of magnesium oxide, silica and kaolin. Later on, in
and acetaldehyde to the catalyst (Fig. 4). Butadiene is the most the literature an abundant number of catalyst combinations has
favorable product in the reaction mixture. It seems that there is been described and tested. Note that the definition of a ‘‘Lebedev
no thermodynamic advantage when performing ethanol-to-BD catalyst’’ is ambiguous as in the literature two compositions, viz.
in two steps. The increase in BD yield and selectivity of the two- ZnO–Al2O3 and MgO–SiO2, have often been used by different
stage process can thus only be explained by kinetic factors, authors as a reference to Lebedev’s catalyst. The present review
related to the properties of the catalyst. categorises the catalysts into three groups, encompassing
Summarizing the thermodynamic aspects, reaction tem- (i) doped alumina catalysts (Table 3 and Fig. 5), (ii) pure and
peratures between 325 and 430 1C are preferred, since lower promoted magnesia–silica containing catalysts (see Table 4
temperatures lead to restricted acetaldehyde formation, whereas and 5; Fig. 6–11), and (iii) other catalysts (Table 6). Comparison
higher temperatures disfavor the aldol condensation. In the between the catalysts is made based on BD selectivity and yield,
preferred temperature range, BD yield and selectivity are total carbon conversion in the product stream (TC, %), activity
kinetically determined, the choice of the active catalyst being (gBD gcat 1 h 1) and volume productivity (gBD lcat 1 h 1).
of utmost importance.
Doped alumina catalysts. Early publications by Lebedev have
2.4 Catalyst development and reaction conditions claimed a BD selectivity of 31% with a ZnO–Al2O3 catalyst
The preceding analysis does not include the choice of the catalyst. (Table 3, entry 1).67 Several authors failed to reproduce this
The sequence of reactions (ii) to (vi) (Schemes 7 and 9) converting BD selectivity. Natta and Rigamonti reported a BD yield of 5.6%
ethanol to BD is favoured at high temperature, most side- with 40%ZnO–60%Al2O3, although a high selectivity to ethylene
reactions (hydrogenation and dehydration) being more affected. was observed (Table 3, entry 2).74 Reduction of the dehydration
Therefore the catalyst must be particularly active in reactions like activity of the alumina by treatment with sodium carbonate
dehydrogenation, aldol condensation and MPV, while avoiding increased the BD yield to 9%. Variations of Zn and Al ratios
side-reactions such as ethylene and butene formation. were investigated. The catalyst 25%ZnO–75%Al2O3 showed a
The data in the literature on agents capable of activating the BD yield of 10% (Table 3, entry 3),101 while with a 75%ZnO–
ethanol-to-BD conversion are abundant. They will be summar- 25%Al2O3 catalyst a BD yield of 15% with a selectivity of 20%
ized for both the one-step and two-step processes in the next has been reported (Table 3, entry 4).102 Despite the improve-
section. Further classification is based on the support type and ments, the selectivity was lower than commercially acceptable
the increasing number of modifiers in the catalyst composition. levels. Many authors therefore considered the unpromoted
Where possible, focus will be on structure–activity/selectivity ZnO–Al2O3 composition as ‘‘not very promising’’.
relationships. Natta and Rigamonti reported enhanced catalytic results
2.4.1 Catalysts in the Lebedev one-step process. Lebedev with MgO–Al2O3 catalysts rich in magnesia, yielding 10% BD
et al. were the first to comment on the compositional require- with a selectivity of 11% (Table 3, entry 5).74 This result was
ments of the catalyst. Based on inspection of the net reaction, explained by the higher condensation activity of magnesia,
it has been concluded that the catalyst should be able to remove compared to zinc oxide. Corson et al.101 have varied the

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Table 3 Catalytic activity of doped alumina materials in the direct butadiene synthesis from ethanol

WHSV/ LHSV/ TCa/ Yields/ Selectivity/ Activity/ Volume productivity/

Entry Catalyst T/1C h 1 h 1 % C% C% gBD gcat 1
h 1
gBD lcat 1 h 1 Ref.
1 ZnO–Al2O3 400 — — — — 31 — — 67
2 40%ZnO–60%Al2O3 360–380 — 0.3 — 5.6 — — 8 74
3 25%ZnO–75%Al2O3 425 0.25 0.4 67 10 15 0.01 17 101
4 75%ZnO–25%Al2O3 445 — 0.3 75 15 20 — 23 102
5 80%MgO–20%Al2O3 430 — 0.3 95 10 11 — 12 74
6 2%PbO–98%Al2O3 425 0.6 0.6 — 18 — 0.06 46 101
7 1%TiO2–9% ZrO2–90%Al2O3 425 0.5 0.6 — 17 — 0.05 43 101
8 30%Sb2O3–70%Al2O3 425 0.9 0.6 — 14 — 0.07 36 101
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9 10%Cr2O3–90%Al2O3 425 0.5 0.6 — 8 — 0.02 20 101

10 70% MgO–30%Al2O3 450 2.0 — 86 18 21 0.20 — 59
11 40%ZnO–60%Al2O3 425 1.5 1.6 94 56 59 0.50 421 104
12 20%MgO–80%Al2O3 425 1.5 1.5 — 48 — 0.40 322 104
13 40%Cr2O3–60%Al2O3 450 1.5 1.6 — 47 — 0.40 365 104
14 30%ZnO–1%K2O–4%SiO2–4%MgO–Al2O3 420 — 2.5 43 34 80 — 395 105
a
TC = Total conversion of ethanol.

was found, besides a low activity to ethylene and C4 side-

products. Conversely, catalysts with moderate basicity and high
acidity showed next to superior BD yields also high amounts of
other thermodynamically stable dehydration products like
ethylene and butenes (Section 2.3). Substitution of Fe for Al
in the hydrotalcite precursor, adjusting the acidity without
affecting the MgO-related basicity, yielded less acidic catalysts
showing at the same conversion high 1-butanol yields amount-
ing to 3 and 11% with Mg–Fe and Mg–Al catalysts, respectively,
as well as decreasing BD yield. Clearly, tuning of the acid–base
properties of the catalysts both in strength and number seems
to be the preferred way to attain high BD yields.
Extensive work on alumina-based catalysts containing mono-,
Fig. 5 Effect of MOx–Al2O3 catalyst composition on butadiene yield: (1)
binary and ternary oxides has been performed by Bhattacharyya
MgO–Al2O3 (400 1C); (2) SiO2–Al2O3 (425 1C); (3) ZrO2–Al2O3 (425 1C); (4) et al.75,104,106–108 None of the studied ternary oxide-based cata-
Fe2O3–Al2O3 (425 1C); (5) Cr2O3–Al2O3 (425 1C); (6) ZnO–Al2O3 (425 1C), lysts gave better results than the binary oxides. Among the binary
WHSV B 1.5 h 1.104 oxides, mixtures of alumina with magnesia, zirconia, chromium
oxide, calcium oxide, manganese oxide, silica, iron oxide, zinc
oxide and nickel oxide were investigated (Fig. 5).104 Most binary
MgO : Al2O3 ratios between 0.3 and 2.2 by co-precipitation of systems showed an optimum composition corresponding to a
magnesium and aluminium salts. However, such mixed oxides 40 : 60 weight ratio for MOx : Al2O3: for MgO–Al2O3 and SiO2–
showed poor activity, yielding less than 7% BD. Among other Al2O3 materials, catalysts containing 80% of alumina showed
alumina containing catalysts, the following oxide systems were enhanced catalytic performance (Fig. 5 and Table 3, entries 11–13).
found to be more active producing BD in yields higher than The best catalytic results were found with ZnO-containing alumina,
8%: PbO–Al2O3, TiO2–ZrO2–Al2O3, Sb2O3–Al2O3 and also Cr2O3– yielding at 425 1C and atmospheric pressure 56% BD with a
Al2O3 (Table 3, entries 6–9). Conversely, SiO2–Al2O3, ZrO2–Al2O3, selectivity of 59% to BD, corresponding to a volume productivity
Fe2O3–Al2O3 and NiO–Al2O3 yielded less than 7% BD. Thus, of 421 gBD L h 1 (entry 11). The catalyst showed a very high
none of the alumina-based materials were considered as a activity of 500 gBD kgcat h 1. Iron oxide on alumina was the
promising catalyst.101 second best catalyst (Fig. 5). In addition, the materials contain-
Hydrotalcite-derived mixed oxides are often used as basic ing nickel oxide and manganese oxide (not shown) were found to
catalysts, especially when a high dispersion of the elements in be active only in the dehydrogenation of ethanol, irrespective of
the composition is needed.103 León et al.59 using a MgAl hydro- the catalyst preparation method and the composition.
talcite precursor with a Mg : Al atomic ratio of 3 : 1 obtained at Binary oxide systems prepared by co-precipitation of the
450 1C using a WHSV of 2 h 1 a moderately high BD selectivity corresponding salts with aqueous ammonia showed higher
of about 21% at an ethanol conversion of 86%. Tuning the activity relative to mechanically blended samples and/or indivi-
acid–base properties of the catalyst by changing the synthesis dual oxides, likely due to synergetic effects of the various active
procedure by precipitation of hydrotalcites under low or high sites.104 Unfortunately, no physicochemical information of the
supersaturation conditions or under sonication, over catalysts different catalytic systems was available proving this statement.
with low acidity and basicity an accumulation of acetaldehyde Furthermore, metal nitrate precursors are preferred over chlorides

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and sulphates, because the anion could be completely removed the most acidic sites by K2O. Addition of hydrogen peroxide
by washing and calcination. In addition, substitution of aqueous (0.8–1.5 wt%) in the feed significantly improved the catalytic
ammonia as a precipitating agent by sodium or potassium results. A BD yield of 48% was found over the same catalyst
hydroxides resulted in adverse effects.107 Optimal contact times under similar conditions showing 90% selectivity for BD, corre-
attaining maximum BD yields were reported for the binary sponding to a productivity of 550 gBD lcat 1 h 1. The promoting
systems. A WHSV of 1.5 h 1 showed the highest BD yields for effect of peroxide was explained, on one hand, by hydroxylation
most catalysts, except for the SiO2–Al2O3 catalyst which showed of the catalyst surface during the reaction with peroxide and,
the highest BD yields at a WHSV of 1 h 1. Note that many authors on the other hand, by cleaning of the surface from coke by
were using much higher contact times.74,101,102 Bhattacharyya radicals resulting from peroxide decomposition.
et al. have clearly demonstrated the remarkable importance of In conclusion, numerous catalysts of the Zn–alumina type
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an optimal WHSV. In the case of ZnO–Al2O3, BD yield dropped were not retained because of low activity or selectivity. High
from 56 to 34% with a WHSV decreasing from 1.5 to 1 h 1, or to activity was reached after modification with promoters, facili-
12% with an increase of WHSV from 1.5 h 1 to 2 h 1.104 It is tating simultaneously the five elemental reactions for fast BD
worth mentioning that high BD selectivity and yields at a high formation, avoiding formation of the thermodynamically more
EtOH feed rate is beneficial because high space-time yields can be favourable side-products such as ethylene and butenes. High
reached (Table 3, compare entries 11–13 with entries 2–9). With temperature and low contact times were beneficial for the
regard to the temperature, the highest BD yield was observed at BD yield. Synthesis procedures and choice of precursors were
425 1C for most binary systems. Over the ZnO–Al2O3 catalyst a essential to reproduce the best performing catalysts.
decrease of the temperature from 425 to 375 1C showed a BD yield
decrease from 56 to 14%.104 Furthermore, it has been shown Magnesia–silica catalysts. Magnesia on silica has long been
that use of a fluidized bed reactor allowed us to significantly studied for converting ethanol in BD. Many catalytic studies
increase BD yield up to 73% compared to 56% obtained with a have been reported to reproduce the catalytic experiments and
fixed bed reactor.108 to better understand the active sites on the magnesia–silica
Tret’yakov et al. reported a similar zinc–alumina catalyst catalyst type. Table 4 compares the most important catalytic
promoted with a small percentage of potassium oxide, silica and data for un-promoted systems.
magnesia, viz. 30%ZnO–1%K2O–4%SiO2–4%MgO–Al2O3.105 They Szukiewicz109–111 has reported between 350 and 450 1C the
reported 34% BD yield with 80% selectivity at 420 1C and low production of BD from ethanol over a MgO–SiO2 catalyst with a
contact time, e.g. LHSV of 2.5 h 1 (Table 3, entry 14). Catalysts BD selectivity of 30–40%, using a LHSV of 0.3 h 1 (Table 4,
were synthesised by mixing of zinc, aluminium and potassium entry 1). Unfortunately, ethanol conversion data were absent.
oxides with suspension of magnesia and silica. The catalyst The catalyst had a MgO : SiO2 weight ratio of 60 : 40 and was
composition contained all the necessary active elements to prepared by wet-kneading of a physical mixture of the dry oxide
catalyse the sequence of transformations such as dehydrogena- powders in water, the amount of water added only specified as
tion (reaction (ii), Scheme 7) with Zn, the base-catalysed con- 1.5–4 parts of water to one part of powder mixture by weight.
densation (iii and iv) with MgO, the MPV reaction (v) with Zn or The aging time (2 to 12 h) was not essential. At 410 1C and with
MgO, and the dehydration (vi) with SiO2. The dehydration a LHSV of 0.4 h 1, Corson et al.,101 with a similar MgO–SiO2
of ethanol to ethylene is avoided possibly due to poisoning of catalyst, having a molar Mg/Si ratio of 2.2, were able to

Table 4 Catalytic activity of MgO–SiO2 materials in the direct butadiene synthesis from ethanol

WHSV/ LHSV/ TCa/ Yields/ Selectivity/ Activity/ Volume productivity/

Entry Mg/Si Preparation T/1C h 1 h 1 % C% C% gBD gcat 1
h 1
gBD lcat 1 h 1 Ref.
1 2.2 Wet-kneading, MgO + SiO2 350–400 — 0.3 — — 30–40 — — 109–111
2 2.3 Impregnation of SiO2 hydrogel 410 0.5 0.4 25 9 36 0.02 15 101
by Mg-salt
3 2.2 Wet-kneading, MgO + SiO2 430 — 0.3 84 35 41 — 48 112
4 2.2 Wet-kneading, MgO + SiO2 415 — 0.3 55 27 50 — 37 112
with NH4OH
5 2.2 Wet-kneading, MgO + SiO2 415 — 0.3 34 18 54 — 25 112
with H3COOH
6 5 Mg(OH)2 + SiO2 400 — 0.3 52 28 54 — 33 113
7 5 Same as entry 6, after 400 — 0.3 64 37 58 — 44 113
hydrothermal treatment
8 2 Commercial 440 0.30 0.2 70 37 48 0.06 31 114
9 3 Mg(OH)2 + colloidal SiO2 380 0.40 — 46 28 62 0.06 — 80
10 1 Wet-kneading, Mg(OH)2 + SiO2 350 0.15 — 50 42 84 0.04 — 115
11 0.83 Wet-kneading, Mg(OH)2 + SiO2 350 0.03 — 53 16 30 0.003 — 81
12 1 Wet-kneading, Mg(OH)2 + SiO2 350 0.20 — 76 15 20 0.02 — 116
13 6 Wet mixing MgO + SiO2 400 — — 66 32 48 — — 117
a
TC = Total conversion of ethanol.

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reproduce the data, viz. a BD yield of 9% and a BD selectivity of

36% (Table 4, entry 2).
A variety of procedures is known to synthesize magnesia–
silica catalysts. A comprehensive study of the preparation of
MgO–SiO2 catalysts (with constant Mg to Si molar ratio of
2.2 according to the method proposed by Szukiewicz109–111)
was carried out by Natta and Rigamonti.112 It was postulated
that the activity and selectivity of the magnesia–silica catalysts
strongly should be dependent on the synthesis procedure,
including the amount of water added, the time and temperature
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of aging, the drying and calcination procedure. XRD-analysis

demonstrated the requirement of a high dispersion of magnesia
on silica and the presence of a limited amount of amorphous
magnesia hydrosilicate phase (less than 10 wt% based on Mg), Fig. 6 Formation rates of BD and ethylene at 380 1C with a MgO content
resulting from the interaction of dissolved Mg2+ with the silanols of MgO–SiO2 catalysts (ethanol concentration of 10 vol%).80
of the silica surface. Wet kneading, followed by calcination at
450 1C, thus provided superior magnesia silica catalysts, at 430 1C
showing an ethanol to BD yield of 35% and a selectivity of 41% or The catalyst with 85 mol% (79 wt%) of MgO (Mg to Si molar ratio
volume productivity of 48 gBD l 1 h 1 (Table 4, entry 3). Mecha- of 5.7) possessed an optimal acid–base balance reaching a high
nical blending or mixing of the oxide powders in alcohol pre- BD formation rate of 42 mmolBD m 2 h 1 (Fig. 6). Unfortunately,
venting silicate formation showed poor catalytic performance. BD yield and selectivity were not reported.80
Stability is one of the key problems of magnesia–silica catalysts. Ohnishi et al. probably reported the highest BD yields ever
The formation of crystalline silicate from the amorphous magne- over magnesia–silica.115 At 350 1C after 10 min-on-stream the
sium hydrosilicate during the catalytic process was found to be butadiene yield and selectivity amounted to 42 and 84%,
responsible for the decrease in BD yield after regeneration.112 respectively (Table 4, entry 10). The catalysts were prepared by
During catalyst synthesis, prevention of extensive formation of wet-kneading of precipitated magnesium hydroxide and silica
magnesium hydrosilicate is possible by reduction of the Mg2+ gel derived from tetraethyl orthosilicate (TEOS). The nature of
solubility when cold water is used or acetic acid or aqueous the reagents was shown to be important, as substitution of
ammonia is added. Such catalysts showed a high stability upon magnesium nitrate and nitric acid for magnesium chloride and
regeneration. Though slightly less active, a high BD selectivity hydrochloric acid, respectively, showed reduced BD formation.
between 41 and 54% is reached (Table 4, entries 3–5). This was rationalised by incomplete removal of chloride ions
Hydrothermal treatment between 100 and 200 1C signifi- during washing of the precipitates, in agreement with earlier
cantly improved activity and selectivity of magnesia–silica results reported by Bhattacharyya et al.107 for alumina catalysts.
catalysts.113 Such treatment increased the specific surface area Maximum activity and BD selectivity were obtained for a Mg to
and pore volume of the catalyst. Hydrothermal treatment of a Si molar ratio of 1.
magnesia–silica catalyst, at 400 1C yielded 37% BD with a Many authors attempted reproducing such excellent data.
selectivity of 58% for a LHSV of 0.3 h 1 (Table 4, compare In the hands of Kvisle et al.81 samples prepared according to
entries 6 and 7). This mild hydrothermal treatment was also the Ohnishi method failed to show excellent performance. The
proposed for regeneration of deactivated catalysts. The catalytic better results at 350 1C only amounted to a BD yield of 16% with
result is close to the commercial MgO–SiO2 catalyst (with a molar a selectivity of 30% (Table 4, entry 11).
ratio of Mg to Si of 3) (Table 4, entry 8). A 2 h pretreatment From time-on-stream studies only showing steady-state
between 400 and 500 1C was defined as preferable. Higher behaviour and acceptable carbon mass balance after 20 to
temperatures resulted in a decreased BD yield.114 50 minutes on-stream, it was concluded that any catalytic result
Niiyama et al.80 studied the effect of the Mg/Si molar ratio in corresponding to shorter times-on-stream, viz. 10 min as used
the catalyst. Catalysts were prepared by kneading magnesium by Ohnishi et al., should be interpreted with caution. Recently,
hydroxide with colloidal silica, followed by drying and calcina- with a similar unmodified magnesia–silica catalyst, we only
tion at 600 1C for 3 h. Note that this pre-treatment temperature could reach at 350 1C and a WHSV of 0.2 h 1 a BD yield of 15%
is higher than the previously recommended one.112,114 At with a BD selectivity of 20% (Table 4, entry 12).116 This catalyst
380 1C a butadiene yield of 28% with a selectivity of 62% was performance is similar to that reported by Kvisle et al.81 Analysis
reported over the catalyst with a Mg to Si molar ratio of 3 with UV-vis and XRD showed that wet-kneading of the sample
(Table 4, entry 9). The acid–base properties of the magnesia– induces structural perturbations in the highly dispersed MgO,
silica catalysts in terms of the Mg to Si ratio have been investi- forming Mg–O–Si interactions like in amorphous magnesia
gated as well.80 Whereas the formation rate of ethylene was too hydrosilicate.81 Both criteria were already proposed by Natta112
high for samples with MgO content below 75 mol% (67 wt%), and Kovařı́k113 as a fingerprint of active and selective catalysts.
pure MgO was inactive for both dehydration and BD formation, Recently, Tong et al. reported the formation of BD from ethanol
the latter requiring a balance between acidic and basic sites. with 48% selectivity over magnesia–silica catalysts obtained by

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wet-mixing (Table 4, entry 13).117 An optimal calcination tempera- time keeping the remaining MgO highly dispersed. However, for
ture of 500 1C was suggested, which is in good agreement with the design of improved catalysts, more systematic characterization
data reported earlier. The best performance was obtained with a of the surface properties of the active catalysts is a prerequisite.
MgO content of 80 wt% (or a molar Mg to Si ratio of 6).
In addition to the catalyst composition and synthesis pro- Magnesia–silica materials doped with a dehydrogenation promoter.
cedure, reaction conditions also affect catalytic activity and BD The commercial magnesia–silica catalysts industrially used in USSR
selectivity. With the commercial catalyst (Table 4, entry 8) contained various dopants affording high and stable BD yields.
increasing BD yields are obtained at reaction temperatures up to These dopants were initially thought to improve the dehydrogena-
460 1C in agreement with thermodynamics, while the BD selectivity tion capacity of the commercial catalyst, resulting in faster
reaches a maximum of 48% at 430–440 1C and then decreases.114 conversion of ethanol to acetaldehyde. According to the patents
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Kvisle et al. have investigated the effect of the ethanol of Szukiewicz,109–111 modification with chromium oxide up to
concentration on its conversion to BD at 350 1C with their 10 wt% led to a significant increase in BD selectivity of about 5%.
magnesia–silica catalyst (entry 11) at a constant carrier gas flow The Cr modification reduced the formation of some by-products
rate of 18 ml min 1. The authors observed higher BD yields at like ethylene, while others, like butenes, were increasing. The
lower ethanol partial pressures (3000 ppm), in agreement with effect of different dopants on the catalytic performance has been
the increase in the number of product molecules in the studied in detail.
chemical reaction of ethanol to BD (eqn (1)).81 Lowering the Natta and Rigamonti74,112 have employed the addition of
partial pressure obviously leads to lower volume productivity, chromic acid as a promoter of a MgO–SiO2 catalyst with an
while an increase of the carrier gas flow rate (or decrease of the atomic Mg : Si ratio of 2.2. At 415 1C and with a LHSV of 0.2 h 1
contact time) at constant ethanol concentration in the feed (entry 1, Table 5) ethanol was converted to a high yield and
leads to a lower butadiene yield.80 selectivity of BD of 43 and 52%, respectively. With the Cr-free
Summarizing the results mentioned, it follows that high BD catalyst only the respective values of 35 and 41% were obtained.
yields (up to 30–40%) and selectivity (40 to 60%) are possible The positive effect of the Cr-promotion was connected to the
over the un-promoted binary magnesia–silica materials of formation of magnesium chromate preventing excessive forma-
Table 4 at temperatures ranging from 350 to 400 1C, using tion of magnesium silicate. It appears that modification with
space velocities from 0.3 to 0.5 h 1. For low partial pressures of chromium oxide has an effect similar to that of ammonia or
ethanol, a value of 10 vol% is a compromise between BD yield acetic acid (see before).
(selectivity) and volume productivity. Higher partial pressures Corson et al.101 showed that among the more than 500 tested
require higher reaction temperatures resulting in the formation of materials, the 2%Cr2O3–59%MgO–39%SiO2 catalyst was superior.
more side-products. Although contradictory data are reported on In a fixed-bed reactor at 400 1C with a WHSV of 0.4 h 1, it exhibited
the optimal Mg to Si molar ratio and the preferred synthesis a BD yield of 38% with 56% selectivity (Table 5, entry 2),
recipe, it appears that the catalytic performance of the MgO–SiO2 corresponding to a volume productivity of 65 gBD lcat 1 h 1.
catalysts is very sensitive to the acid–base properties of the Fig. 7 shows an overview of the promotion with 2 wt% Cr
synthesized material. There is agreement that the synthesis pro- of MgO–SiO2 catalysts with varying atomic ratios of Mg : Si,
cedure should promote formation of Mg–O–Si species, at the same as prepared and tested by different authors.101,102,112,116,118

Table 5 Catalytic activity of modified MgO–SiO2 materials and related clay minerals in the direct butadiene synthesis from ethanol

WHSV/ LHSV/ TCa/ Yield/ Selectivity/ Activity/ Volume productivity/

Entry Catalyst composition T/1C h 1 h 1 % C% C% g gcat 1 h 1
g lcat 1 h 1 Ref.
1 2%Cr2O3–59%MgO–39%SiO2 415 — 0.2 82 43 52 — 42 112
2 2%Cr2O3–59%MgO–39%SiO2 400 0.4 0.4 68 38 56 0.08 65 101
3 2%Cr2O3–79%MgO–19%SiO2 435 — 0.3 75 40 52 — 58 102
4 2%Cr2O3–79%MgO–19%SiO2 400 — 0.3 60 37 62 — 44 118
5 3%Cr2O3–54%MgO–43%SiO2 350 0.2 — 65 34 53 0.04 23 116
6 2%Cr2O3–76%MgO–11%SiO2–11% 435 — 0.3 76 48 63 — 82 102
kaolin
7 2%Cr2O3–76%MgO–11%SiO2–11% 400 — 0.3 75 47 63 — 56 118
kaolin
8 30%ZnO–30%MgO–40%SiO2 365 — 0.2 95 26 27 — 25 74
9 ZnO–MgO–SiO2–kaolin o410 — 0.3 67 46 69 — 64 121
10 3%ZnO–56%MgO–42%SiO2 400 0.7 — 98 45 46 0.19 117 116
11 2%Ta2O5–60%MgO–38%SiO2 420 0.4 0.4 68 34 50 0.08 58 101
12 10%NiO–28%MgO–62%SiO2 280 — — 59 53 90 — — 122
13 3%CuO–56%MgO–42%SiO2 400 0.7 — 86 44 53 0.20 123 116
14 4%Ag–55%MgO–41%SiO2 400 0.7 — 92 49 54 0.22 130 116
15 MgO/bentonite 440 — 0.4 — — 31 — — 123
16 Li+-containing fluorohectorite 375 0.3 — 32 16 48 0.03 — 124
17 6%Ag/Al–sepiolite 280 0.8 — 63 5.5 9 0.03 — 83
a
TC = Total conversion of ethanol.

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Fig. 9 Influence of the chromium loading on the BD selectivity for Cr2O3–

MgO–SiO2 materials with an atomic Mg : Si ratio of 3, carried out at 400 1C.119

yield of 40% with a selectivity of 52% over the magnesia–silica

catalyst with an atomic Mg : Si of 6 (Fig. 8; Table 5, entry 3).
Fig. 7 Catalytic performance of chromium oxide doped MgO–SiO2 materials, An increase of the chromium content of these catalysts resulted
containing 2 wt% of Cr2O3: cat. 1 – Mg : Si = 2.2;112 cat. 2 – Mg : Si = 2.3;101 in an increase of the BD yield.102 Silva et al.119 have been studying
cat. 3 – Mg : Si = 6;118 cat. 4 – Mg : Si = 6;102 cat. 5 – Mg : Si = 2116 (see the influence of the chromium loading on the catalytic perfor-
Table 5 for the reaction conditions). mance of MgO–SiO2 materials in the conversion of ethanol to BD
at 400 1C. The Cr2O3–MgO–SiO2 materials were prepared by
conventional impregnation on magnesia–silica catalysts. A steadily
Modification of magnesia–silica materials with a small amount
increasing activity was observed with increasing Cr content, the
of chromium oxide clearly results in higher BD yield and
effect on the BD selectivity being displayed in Fig. 9. Clearly, an
selectivity in comparison with the unmodified MgO–SiO2 samples
increase of Cr enhances acetaldehyde selectivity. Whereas BD
(compare Table 5, entries 1–5 and the corresponding data in
selectivity first increases until 50% upon addition of Cr up to
Table 4). Some authors reported a decrease in selectivity at
2 wt%, it drops with further enhanced Cr content.
temperatures higher than 400 1C, despite the higher BD
The opposite effect is seen for the ethylene selectivity. Cr was
yield.101,118 Comparison of the catalysts is difficult due to the
noted to promote ethanol dehydrogenation and to adjust the acid–
different reaction conditions.
base properties of the magnesia–silica material, by eliminating
László and co-workers102 have studied the Mg content for
strong acid sites with H0 r 3.3, thus avoiding the formation of
2 wt% Cr2O3 doped MgO–SiO2 materials at 435 1C with a LHSV
thermodynamically more favourable ethylene. Too high Cr con-
of 0.3 h 1. The catalytic data are summarized in Fig. 8.
tents should be avoided as well, since they introduce new acid sites
They observed that a lower silica content leads to a higher BD
related to the presence of Cr(III) species with distorted octahedral
yield and selectivity, while ethylene formation decreased. The
symmetry or to Cr(VI) species, as evidenced by UV-reflectance
observation again proofs the requirement of an optimal balance
bands at 27 000 and 37 000 cm 1, respectively.119
of the surface acid–base properties. They obtained the highest BD
Zinc oxide has also been used as a promoter for Lebedev’s
industrial magnesia–silica catalyst.120 Natta et al.,74 partially
substituting zinc oxide for magnesium oxide, reported lower BD
yield and selectivity in comparison to unmodified MgO–SiO2. At
365 1C and a LHSV of 0.2 h 1, with a 30%ZnO–30%MgO–40%SiO2
catalyst, a BD yield of about 26% at almost complete ethanol
conversion was obtained (compare entry 3 of Table 4 with entry 8
in Table 5). The ZnO modified materials were prepared either by
co-precipitation of the carbonates of magnesium and zinc or by
mechanical mixing of the oxides, followed by wet-kneading with
silica. The lower BD yield and selectivity were due to a reduced
dehydrogenation and condensation capacity as a result of the
formation of a solid solution between MgO and ZnO.
Berak et al., studying the influence of the zinc oxide promoter
loading on the performance of the magnesia–silica catalysts,121
reported an increase of the activity with increasing Zn content.
Fig. 8 Influence of the magnesium content on the activity and BD The reaction temperature to reach 66% ethanol conver-
selectivity of 2%Cr2O3–MgO–SiO2 at 435 1C and a WHSV of 0.3 h 1.102 sion decreased with an increase of the zinc oxide content.

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Kitayama et al.122 have shown that NiO promoted MgO–SiO2

is highly selective to BD. At 280 1C about 90% BD selectivity at
59% ethanol conversion (entry 12, Table 5) was found with a
10 wt% NiO supported on MgO–SiO2 with an atomic Mg : Si
ratio of 0.7. Catalysts were synthesized via impregnation of
Mg(OH)2–aerosil mixtures with nickel nitrate followed by calci-
nation at 400 1C for 2 h. They were shown to possess optimal
acid–base properties, although X-ray analysis still revealed the
presence of magnesium silicate. Whereas higher levels of Mg
resulted in the appearance of pure MgO phases, a decreased
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surface area was found, leading to lower activity. It was claimed

that substituted Ni is present as Ni2+ cations, dispersed into the
silicate and MgO lattice. However, no characterisation of acid–
base properties has been provided.
Fig. 10 Effect of the content of zinc promoter on the activity and Makshina et al.116 have performed a comparative study of
selectivity of the MgO–SiO2 catalyst at a total ethanol conversion of 67% magnesia–silica materials modified with various transition
with a WHSV of 0.3 h 1.121 metal oxides and metallic silver. Among the oxides, Ni, Cu and
Zn were promising with regard to the BD yield and selectivity,
while Fe and Mn showed insufficient performance. At 400 1C
Fig. 10 shows the effect of the Zn promoter loading on the BD and a WHSV of 0.7 h 1, Cu-loaded magnesia silica with an
yield and product selectivity. With increasing Zn content, the BD atomic Mg : Si ratio of 2 showed a BD yield of 47% at almost
yield first increased to 46% with a selectivity of 69% at a molar complete ethanol conversion, corresponding to a remarkable
ZnO : MgO ratio of 0.002 (entry 9, Table 5). Further increase of the volume productivity of 123 gBD lcat 1 h 1 (entry 13, Table 5).
Zn loading resulted in a decreased BD yield at the expense of Sequential impregnation is essential for keeping the specific
acetaldehyde, ethylene and other by-products formation. activity of the individual elements as high as possible. The
Makshina et al.116 recently confirmed that the modification atomic Mg : Si ratio influenced selectivity and activity, a value
of silica by co-impregnation with both zinc and magnesium of 2 being optimal. The presence of metallic Ag had a similar
salts results in a poor activity, yielding 17% of BD with a beneficial effect on the catalytic performance. under similar
selectivity of 31%, in comparison to similar materials obtained experimental conditions, the Ag promoted catalyst exhibited a
by consecutive modification of silica with MgO and ZnO.116 high selectivity of 54% for BD at full conversion, corresponding
At 400 1C and with a WHSV of 0.7 h 1, the latter catalyst showed to a formation rate of 220 g BD per kilogram catalyst per hour
a BD yield of 52% at almost full ethanol conversion, corre- (entry 14, Table 5).
sponding to a record volume productivity of more than Recently it has been reported that modification of the
100 gBD lcat 1 h 1 (entry 10, Table 5). Inspection of the side magnesia–silica catalyst with Zr and Zn oxides (1.5 and 0.5 wt%
products pointed to high levels of unconverted acetaldehyde respectively) leads to a significant increase in selectivity, e.g. at
with the co-impregnated catalyst, indicating that impregnation 325 1C and WHSV = 0.3 h 1 doped catalyst with a Mg : Si ratio of
leads to an un-balance between the dehydrogenation and aldol 85 : 15 showed 63 mol% selectivity towards BD at 40% of ethanol
condensation capacity. Possibly, the formation of new phases conversion while unpromoted material yielded butadiene with
in the co-impregnated sample blocks or prevents the creation of only 26 mol% selectivity at the same level of conversion.128
the active basic sites. XRD analysis showed the presence of Concluding this part, promotion of the magnesia silica catalyst
mixed Zn–Mg silicates in the co-impregnated sample. Further with elements showing dehydrogenation activity, generally
spectroscopic work is needed to confirm the rationalisation. improves both the catalyst activity and selectivity to BD. The
Furthermore, Ta, Ag, Ni, Mn, Cu, Mo, Fe, Co, and Zr oxides higher activity may be rationalised by faster acetaldehyde
were also used as promoters for MgO–SiO2 catalysts in the formation. However, the promoter loading is crucial since a
ethanol to BD reaction.101,113,116,122,125–128 balance with the basic sites, related to the highly dispersed
Among these redox dopants, tantalum oxide was found to be MgO, is important as unconverted acetaldehyde otherwise causes
an efficient promoter for magnesia–silica catalysts.101 These a decrease in BD yield. Some of the dopants may introduce strong
catalysts showed a butadiene yield of 34% with a selectivity of acidity or may block the mild acidity of the silica, e.g., by silicate
50% at 420 1C with a WHSV of 0.4 h 1 (Table 5, entry 11), formation. These effects need to be avoided to prevent diethyl
corresponding to a volume productivity of 58 gBD L 1 h 1. ether but mainly ethylene formation from ethanol or to facilitate
Similar to chromium oxide, an amount as low as 2 wt% of the dehydration of the aldol and crotyl alcohol to BD. Once
tantalum oxide was sufficient to boost the butadiene yield to used in the optimal content, the nature of the element seems
higher values relative to the unmodified catalyst. Variation of somewhat less crucial, since Cr, Zn, Ta, Cu, Ni, Zr oxides and
the magnesium content was also studied for the Ta promoted Ag show beneficial effects. However, a sequential loading of
samples. By changing the atomic Mg : Si ratio from 0.6 to 2, an the silica with the base, and the dopant using intermediate
increase of the BD yield from 7 to 34% has been reported. calcination or not, is crucial to attain high yield of and

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selectivity to BD. Co-impregnation often forms additional silica by other layered silicates such as talc and kaolinite showed
silicate phases or causes blockage or creation of acid sites. similarly reduced BD yields.129 Generally, both MgO and SiO2
There is no consensus on the optimal atomic ratio of Mg : Si in components being essential in the catalyst composition should
the doped catalysts, probably resulting from differences in the not be replaced for more than 20 wt% by the layered silica to
preparation methods and in the nature of the dopant. It is attain BD yields of 35 to 40%.129
therefore recommended to compare the different catalysts The situation is somewhat different when MgO
3SiO2
3H2O,
based on their surface properties like acid–base relationship an amorphous magnesium hydrosilicate, is used as a catalyst
(in terms of amounts, strength and location) rather than on the precursor.129 At 400 1C and with a LHSV of 0.3 h 1 it gives up to
basis of the elemental composition only. Today, systematic 80% yield of ethylene from ethanol. After precipitation of
studies with advanced surface characterization tools in combi- magnesium hydroxide with aqueous ammonia or sodium
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nation with catalytic measurements are missing for the ethanol- hydroxide (MgO 4 60 wt%) and subsequent calcination at 550 1C,
to-BD reaction. BD yields of over 50% were noticed. Based on X-ray diffraction, a
partially crystalline phase, viz. composition, 3MgO
4SiO2
2H2O,
Magnesia–silica catalysts promoted by alkaline metal cations could be identified next to MgO. Whereas the presence of free
and clay minerals. As ethylene is thermodynamically favoured in MgO has been suggested to be essential to ensure fast dehydro-
the reaction of ethanol to BD, promotion of magnesia-silica genation of ethanol to acetaldehyde, the precise catalytic role of
with alkaline metal oxides has been studied. Accordingly, the Mg silicate phase has not been clarified.
Butterbaugh and Spence125,126 have reported active MgO–SiO2 Aleixandre et al.123 have reported the effect of clay-modified
catalysts, yielding BD from ethanol with a selectivity up to 48% MgO on the BD selectivity. At 440 1C and using a LHSV of
at 435 1C, by contacting the precursor mixture of magnesium 0.4 h 1, four different clay minerals, viz. bentonite A and B,
hydroxide and silica with an alkaline solution at 90–100 1C for halloysite, and kaolin, were physically mixed with a variable
1–2 h. The alkaline treatment improved the BD selectivity at the amount of MgO (Fig. 11). Bentonite B was obtained from
expense of the ethylene selectivity. The most promising results bentonite A by extracting free silica. The MgO–clay catalyst
were obtained with alkali-digested MgO–SiO2 materials con- containing 57 wt% of bentonite A exhibited the highest BD
taining 20 to 50 wt% magnesia, corresponding to an atomic selectivity (31%). Unfortunately, no ethanol conversions were
Mg : Si ratio of 0.4 to 1.5. reported (entry 15, Table 5). Catalytic differences were rationa-
Later, Ohnishi et al. used post-impregnation with sodium or lised by taking into account the number of accessible active
potassium hydroxide followed by calcination at 500 1C, result- acid–base sites on the clay mineral. Compared to halloysite and
ing in the formation of highly active MgO–SiO2 catalysts. kaolin, montmorillonite – the main component of bentonites – is
At 350 1C and with a WHSV of 0.2 h 1, Na2O/MgO–SiO2 and known for its large interlamellar voids and high cation-exchange
K2O/MgO–SiO2 yielded 87 and 70% BD, respectively.115 Though capacity. However, the presence of different impurities in
the post-synthetic selective poisoning of acid sites by addition of bentonites like iron oxides which likely could promote ethanol
alkali is similar to that proposed by Butterbaugh and Spence,125,126 dehydrogenation was not taken into consideration.
the catalytic performance reported by Ohnishi et al. was almost In conclusion, the catalytic performance of magnesia–silica
twice as high. The different time-on-stream behaviour of the catalysts can be promoted with alkaline metal cations, by
catalyst possibly is at the origin of the different catalytic data. In suppressing (too) high acidity, thus reducing/blocking the
our experience, early sampling under non-steady state conditions ethanol to ethylene dehydration. Whereas modification of the
often leads to higher conversions. Within one to two hours BD magnesia–silica catalyst with layered silicates or clay minerals
yield and selectivity converge to lower steady state values, followed also results in an increase of BD selectivity, the catalytic role of
by slow deactivation at longer time-on-stream.
László et al.102 have studied ethanol to BD conversion with
magnesia–silica catalysts mixed with clays or clay-related
minerals. Replacement in a 2 wt% chromium oxide promoted
catalyst of half of the silica by kaolin, at 435 1C and with a LHSV
of 0.3 h 1 showed a BD yield and selectivity increase from 40 to
48% and from 54 to 63%, respectively, while the ethylene
selectivity decreased (compare entries 3 and 6, Table 5). Kovařı́k
et al.118 using a similar catalyst reported a similar increase
of the BD yield with partial substitution of silica for kaolin.
At 400 1C and with a LHSV of 0.3 h 1, 37 and 47% BD yields
were obtained with 2%Cr2O3–MgO–SiO2 and 2%Cr2O3–MgO–
SiO2–kaolin, respectively (compare entries 4 and 8, Table 5).
A complete substitution of silica for kaolin resulted in a reduced
BD yield and enhanced ethylene yield. Under the same condi-
tions, a MgO–kaolin catalyst yielded 22.5% BD, while the MgO– Fig. 11 Influence of the nature of various clay minerals on the selectivity
SiO2 catalyst produced 35% BD yield.74 Complete substitution of of the MgO/silicate catalysts (T = 440 1C, LHSV = 0.4 h 1).123

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the silicates is less clearly defined. Despite these improve- oxides132,134,136 or with mixtures of vanadium oxide with tung-
ments, addition of dehydrogenation modifiers is more effective sten or molybdenum oxides.137
to reach high BD yields. Gruver et al.83 have reported that a 6 wt% Ag promoted
sepiolite is able to produce BD from ethanol. At 280 1C and a
Magnesium silicate clay minerals. A series of unmodified Mg WHSV of 0.8 h 1, they obtained about 9% BD yield at 63%
and Si containing clay minerals have also been used in the ethanol conversion. As the major product was acetaldehyde,
ethanol to BD reaction. Dandy et al. demonstrated BD formation obtained at about 20% yield, the catalyst probably lacks appro-
over sepiolite for temperatures ranging from 200 to 300 1C.130 priate basicity needed to compensate for the fast dehydrogena-
Higher BD yields were generally obtained after modification of tion caused by the presence of metallic silver (entry 17, Table 5).
the clay with dopants, affecting the acid–base and dehydrogena- In summary, clay minerals like magnesia–silica catalysts
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tion properties of the catalyst. Suzuki et al.124 have employed require careful promotion of the dehydrogenation activity and
synthetic Li modified fluorohectorite as a catalyst in the ethanol fine-tuning of the acid–base properties in order to achieve high
to BD reaction at 375 1C using a LHSV of 0.3 h 1. The catalyst BD yields from ethanol. While the aforementioned modified
exhibited an excellent BD selectivity of 48% at 33% ethanol magnesia–silica catalysts generally show better BD yields and
conversion (Table 5, entry 16). An IR study with adsorbed pyridine selectivity than the promoted clays, individuals like ZnO pro-
and CO2 showed that the presence of Li+ and F in the catalyst moted sepiolite also exhibited promising catalytic properties.
modified the Lewis and Brønsted acid sites, thus optimising the
dehydration properties of the catalyst. Oxygen atoms bound to Other catalysts. Catalysts free of Mg2+ and Al3+ have been
Mg2+ at the edge of the layers were responsible for the dehydro- reported as well. Spence and Butterbaugh138 have reported the
genation activity. use of 20 wt% zinc oxide supported on silica (like diatomaceous
To further improve the dehydrogenation ability of the clay earth) for butadiene formation from ethanol, showing a selec-
catalysts, various metal oxide dopants have been explored. Cd, tivity of 58% and a yield of 29% at 450 1C using a LHSV of 1 h 1
Zn and Cu oxide promoters have been tested to improve the (Table 6, entry 1). The reaction temperature for BD formation
BD yields with smectite and bentonite clays.131 Smectite clay was lowered by modifying the Zn catalyst with phosphate or
modified with Cd showed better results than other catalysts tungstate. ZnO on silica and ZnO/Zn(II)phosphate on silica showed
yielding BD with 17% selectivity. Kitayama et al.132–137 used BD yields of 29 and 22% and a BD selectivity of 58 and 63%, at
sepiolites modified by impregnation with various transition 450 and 415 1C, respectively. Corson et al.101 have found a BD
metal oxides as ethanol conversion catalysts in a recirculation yield of 20% at 425 1C using 0.6 h 1 LHSV with a 10 wt% ZnO
reactor at 320 1C. For the unmodified sepiolite ethylene selec- on a silica catalyst (Table 6, entry 3).
tivity was very high, while after doping BD was one of the major Thorium and zirconium oxides on silica exhibit catalytic activity
products, the nature and amount of the precursor salt of the for BD formation from ethanol as well, at 445 1C amounting to BD
promoter determining the BD yield. With manganese content selectivities of 35 and 44%, respectively (Table 6, entries 4 and 5).
increasing from 39 to 79 mol% in the MnO2–sepiolite, the BD Unfortunately, no details about ethanol conversion were
yield increased spectacularly from 8.4 to 33.4%. The catalyst reported.127,139 Corson et al. reported 23% of BD yield using a
prepared from manganese acetate showed better activity than ZrO2–SiO2 catalyst (Table 6, entry 6).101
that from manganese chloride.135 Superior results were obtained Spence et al.127,139 have claimed copper and molybdenum
with ZnO modified sepiolite containing 4.4 wt% Zn, yielding oxides as potential dopants for the ZrO2–SiO2 catalyst. Unfortu-
63.5% of BD at full conversion after 2 h of reaction at 260 1C.133 nately, most of the catalytic data refer to catalytic compositions
High BD yields ranging from 33 to 58% were obtained over for which detailed information on the synthesis procedure and
sepiolites modified with copper, nickel, cobalt and vanadium the physicochemical properties of the catalyst surface is missing.

Table 6 Catalytic activity of various catalysts

WHSV/ LHSV/ TCa/ Yield/ Selectivity/ Activity/ Volume productivity/

Entry Catalyst composition T/1C h 1 h 1 % C% C% g gcat 1 h 1
g lcat 1 h 1 Ref.
1 20%ZnO/diatomaseous earth 450 — 1 50 29 58 — 127 138
2 ZnO + Zn–phosphate/diatomaseous earth 415 — 1.3 35 22 63 — 123 138
3 10%ZnO–90%SiO2 425 — 0.6 — 20 — 0.13 51 101
4 21% ZrO2/diatomaseous earth 445 — — — — 44 — — 127,139
5 21% ThO2/diatomaseous earth 445 — — — — 38 — — 127,139
6 9,5%ZrO2–90,5%SiO2 425 1.0 0.6 — 23 — 0.13 59 101
7 ZrO2/ZnO/SiO2–60Å 375 — 1.5 46 18b 39b — — 140
8 ZrO2/ZnO/SiO2–150Å 375 — 1.5 48 23b 48b — — 140
9 CuO/ZrO2/ZnO/SiO2–60Å 375 — 1.5 39 19 50b — — 140
10 Ag/ZrO2/SiO2 325 0.3 — 34 24 72 0.04 — 141
11 2%Ta2O5–98%SiO2 425 — 0.6 — 16 — — 41 101
12 1.1% CuO on 2%Ta2O5–98%SiO2 425 0.5 0.4 83 25 30 0.06 43 101
13 ZrO2–Fe2O3 (40 : 60) 425 1.5 — — 40 — 0.34 — 104
a b
TC = Total conversion of ethanol. Yield and selectivity are calculated in mol%.

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Conversion of ethanol to BD has recently been reported by catalyst (Table 6, entry 12). Although lead and cadmium oxide
Jones et al.140 over silica doubly promoted with metal oxides of silica promoters have been proposed, no systematic approach to
zinc, zirconium, copper, cobalt, manganese, cerium and hafnium. variations in catalyst composition was present nor was detailed
Emphasis was on the effect of the nature of the promoters and the information concerning the preparation methods available.
texture of the silica on the catalytic activity and BD selectivity. The Finally, although it has been postulated that modification of
best results were obtained over a ZnO–ZrO2/SiO2 catalyst contain- silica gel with 0.5–10 wt% titanium oxide should give suitable
ing 1.5 and 0.5 wt% ZrO2 and ZnO, respectively. Higher ethylene catalysts for ethanol transformation into BD, no catalytic data
selectivity occurred in parallel with higher promoter contents. were presented to support this.142
At 375 1C and a space velocity of 1.5 h 1, the catalyst showed Next to the reported silica supported oxide materials, silica-
18 mol% BD yield with a BD selectivity of 39 mol%, corre- free catalysts have been found to be active in the direct
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sponding to a high volume productivity of 124 gBD lcat 1 h 1 conversion of ethanol to BD. Bhattacharyya et al. studied binary
(entry 7, Table 6). Using this catalyst as reference, it could be systems containing zirconium or thorium oxides.104 Among the
shown that impregnation of the promoters rather than sol–gel tested materials, ZrO2–Fe2O3, having a weight ratio of 40 to 60,
synthesis is the preferred synthesis method to achieve high BD showed a high BD yield of 38% with 45% selectivity at 450 1C and
yields. High ethylene yields were observed as well. a space velocity of 1.5 h 1 (Table 6, entry 13). Arata et al.143 have
Selective poisoning of acid sites with alkali metal cations, as shown that promotion of zirconia with weakly basic TiO2 cata-
proposed by Ohnishi115 for the MgO–SiO2 system, was not very lysed the ethanol to BD reaction, yielding 12.4% BD at 330 1C.
successful for the Zn/Zr oxide silica system. In contrast, adding a Tsuchida et al.53 have employed hydroxyapatite to produce
third element like Cu oxide to the Zn–Zr doped silica significantly BD from ethanol at 350 1C and 1 h 1. The acid–base properties
improved the BD selectivity, yielding 19 mol% BD with a BD of the catalyst surface were controlled by changing the Ca to P
selectivity of 50 mol% (compare entries 7 and 9 in Table 6). Finally, ratio in the hydroxyapatite composition. Catalysts with a molar
an increase in BD yield and selectivity with pore size of the silica Ca : P ratio larger than 1.62 possessed the appropriate acid–base
support increasing from 40 to 150 Å was noticed for the Zn–Zr balance, yielding 24% BD at an ethanol conversion of 20%.
modified silica. Doped large pore silica yielded 23% BD with 48% 2.4.2 Two-step process. The two-step process of ethanol to
BD selectivity under the same reaction conditions (compare entries BD involves the dehydrogenation of ethanol to acetaldehyde in
7 and 8, Table 6). According to authors140 the large pore silica a separate step, followed by BD production in the second stage
catalyst contained lower proportion of acidic silanols, as measured by co-feeding ethanol and acetaldehyde. The latter reaction was
by the fractions of Q4 vs. Q2 and Q3 in 29Si MAS NMR that results in a discovery of Ostromyslensky at the beginning of the 20th
the decrease of ethylene yield. However diffusional effects and century.64 In this way, dehydrogenation of ethanol is decoupled
dispersion of supported metal oxides particles were not taken into from aldol condensation, MPV reduction and dehydration.
account to explain the changes in selectivity. Catalytic tests at Commercialized by Union Carbide, the process has been
different space velocities showed that a decrease of the space supplying the major amount of synthetic BD in the US during
velocity from 1.5 to 0.75 h 1 results in a slight increase in BD yield, 1943 to 1944.1 Although many studies on the two-step process
albeit with lower BD productivity. No explanation of the contact were not showing much pertinent information, most of the
time effect was advanced. It should be stressed that the preferred available bibliographic data refer to reactions of co-fed ethanol and
LHSV of 1.5 h 1 used is similar to that used by Bhattacharyya acetaldehyde in the presence of catalysts, previously developed for
et al.104,107,108 for the superior modified alumina catalysts. This the one-step process. It was attempted mostly to elucidate the
contact time is several times higher than that usually applied for reaction mechanism or to evaluate the effect of the presence of the
the magnesia–silica and other related catalytic systems (compare different components on the catalyst composition. In this respect,
Table 6, entries 7–9 with the entries in Tables 4 and 5). various authors noticed that addition of acetaldehyde results in an
Recently, Ordomskiy et al.141 have claimed that silver, cerium, tin increase of BD yield. However, it remains undecided whether the
and antimony oxide promoted ZrO2–SiO2 are highly selective for BD. two-step process should be preferred over the one-step process.
At 325 1C and with a WHSV of 0.3 h 1, 72% BD selectivity at an Corson et al. have reported that BD formation from ethanol
ethanol conversion of 34% was obtained over a 0.4 wt% Ag modified via the two-step process is preferable.101 This preference is
catalyst, corresponding to a productivity of 40 g kgcat 1 h 1 based on the enhanced flexibility for selection of appropriate
(entry 10, Table 6). A remarkably low temperature was used conditions for each reaction separately and on the decreased
compared to the usual temperature window for ethanol con- catalyst complexity with respect to composition and properties
version to BD. of the two catalyst types used in each stage. The two-step
Furthermore, a high BD yield has been observed over a process allowed a reduction of the reaction temperature from
tantalum oxide doped silica – the commercial catalyst developed 400–425 1C to 350 1C. The purity of BD extracted from the
for the two-step process.101 Good yields of BD up to 16% have 4-carbon cut amounting to 98% BD in the two-step process was
been observed over the catalysts with 2 wt% Ta2O5 on silica at only 80% in the one-step process.
425 1C and 0.6 h 1 (Table 6, entry 11).101 Under similar reaction Conversely, Bhattacharyya et al. have claimed that the one-
conditions, an increase in BD yield up to 25% with 30% BD step process is preferable.108 Under appropriate conditions,
selectivity was found upon doping the Ta oxide catalyst with Cu the amount of formed BD is high enough so that it is easily
oxide, likely due to an improved dehydrogenation capacity of the separated from other gases.108 Jones et al. showed that the

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addition of acetaldehyde next to ethanol does not result in any commercial silica gel, while the BD selectivity increased from
improvement of BD yield for catalysts that are able to produce 72–75 to 80%. Moreover, Ta2O5 supported on ordered silica
significant amounts of acetaldehyde (AcH) from ethanol.140 exhibited a better coke tolerance and stability with time-on-stream.
Ostromyslensky has reported that a higher yield of BD was The best catalytic results in terms of activity and selectivity were
obtained over aluminium oxide at 440–460 1C using an equi- obtained using silica support material with nano-sized morpho-
molar mixture of EtOH and AcH.64,144 Later, Maximoff and logy and large pore sizes, indicating the importance of efficient
Canonici145,146 have shown that aluminium sulphate, and basic pore diffusion. 2D or 3D organization of the mesopores like in
aluminium sulphate alone or deposited on pumice-stone or SBA-15 or KIT-6 was found to be less important. The advantage of
Kieselguhr, were capable of catalysing the formation of BD hierarchical mesoporous silicas is related to the ability of reaching
from EtOH–AcH mixtures at a reaction temperature ranging higher dispersion of tantalum making more active sites available
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from 280 to 450 1C yielding a BD selectivity of 33% with a molar and preventing agglomeration of Ta oxide during synthesis and
EtOH to AcH ratio of 2. catalytic reaction.
The two-step process of BD formation has been studied Romanovsky and Jordan148–150 have reported that the addition
intensively at Union Carbide and Carbon Chemical Corp. as of phosphate modifiers to magnesia–rich MgO–silica catalysts
early as 1947 with tantalum, zirconium or niobium oxide with an atomic Mg to Si ratio of 5 resulted in a higher conversion
promoted silica.70,78 The first step proceeded over a standard of the ethanol–acetaldehyde mixture and a higher BD selectivity.
copper dehydrogenation catalyst. The best results of the second Key promoters for silica–magnesia were 5 to 25 wt% calcium or
step were obtained over a catalyst containing 2 wt% tantalum calcium–nickel phosphate complexes having molar Ca to Ni ratio
oxide on silica. At 325–350 1C and a space velocity of 0.4–0.6 h 1, of 7.5–9.2. The best results were obtained at 400 1C using a
the catalyst produced 35% BD yield with a selectivity of 67% using LHSV of 0.4–1.0 h 1, yielding 21 and 23% BD with 56 and 58%
a mixture of 69% by weight ethanol, 24% acetaldehyde and 7% selectivity, respectively.
water. Higher Ta2O5 contents in the catalyst resulted in a further In addition to the catalyst composition, the reaction operat-
increase of the BD yield but at the expense of BD selectivity. It has ing conditions like temperature, contact time and feed molar
been stated that the activity of catalysts containing less than ratio of ethanol to AcH also played an important role in the
1 wt% of tantalum oxide was not reproducible.101 catalytic activity and BD selectivity. The old commercial process
The catalytic activity of zirconium and niobium oxides on operating between 325 and 350 1C used a mixture of 7 wt%
silica was lower than the Ta2O5–SiO2 material. 1.6 wt% ZrO2 on water and 93 wt% feed with a molar ethanol to AcH ratio of 2.7.
SiO2 showed a BD selectivity of 59% under comparable reaction Toussaint et al.78 investigated the influence of the reaction
conditions.78 Corson et al. have reported that hafnia promoted temperature, the molar ratio of EtOH to AcH and the space
silica gel was selective and active in the two-step production velocity on the catalyst activity and BD selectivity of a commer-
of BD. At 300 1C a BD yield of 30–40% and a BD selectivity of cial 2 wt% Ta2O5 on silica. High BD selectivity was obtained at
50–60% could made from an ethanol–acetaldehyde mixture.101 325 1C for a feed mixture with a EtOH to AcH ratio of 3, whereas
Ordomskiy et al. have claimed that zirconium, titanium, better results were found at 350 1C with a feed mixture ratio of
tantalum, niobium or magnesium oxides supported on silica EtOH to AcH of 2–2.5. Catalyst deactivation was less pronounced
promoted by silver, gold, copper, cerium, tin or antimony oxides at lower temperature. Though an increase in BD yield was
are highly active and selective catalysts for the production of BD observed at 350 1C with a EtOH to AcH feed molar ratio of 2, a
from a mixed acetaldehyde–ethanol feed at 325 1C.141 Among the considerable loss in activity was detected after 160 h on stream,
Ag promoted MOx–SiO2 catalysts, mildly basic titanium oxide when compared to a reaction at 325 1C using a feed molar ratio
was found to be the most selective catalyst showing a BD yield of of 3, the latter conditions being considered as optimal. Corson
25% and a BD selectivity of 72%. Ag promoted MgO-containing et al. have reported that for a 2%Ta2O5–SiO2 the optimal condi-
catalyst produced more BD, yielding 29%, but with a BD selec- tions for BD production were achieved at 350 1C, using a LHSV of
tivity of only of 64%. Among the zirconium containing materials, 0.6 h 1 and a molar EtOH to AcH molar feed ratio of 2.75.151 The
M–ZrO2–SiO2 doped with different metals, gold was claimed to amount of coke deposited on the catalyst was significantly higher
be the best choice producing BD with a yield of 25% and a when the EtOH to AcH molar ratio was decreased below 2.75,
selectivity of 82%. Promotion of a Ag modified ZrO2–SiO2 with resulting in faster catalyst deactivation.
cerium oxide further increased the BD yield when compared to The old commercial process with 2 wt% Ta2O5 on silica used
tin, antimony and sodium dopants, yielding 33% BD with 81% 7 wt% water in the feed. Romanovsky and Jordan employed 40
selectivity. Silica was the best support, and use of promoted to 60 wt% water with the phosphate supported magnesia–silica
alumina and aluminosilicate resulted in lower BD yields, 18 and catalyst to reach the highest BD yield of 23% at 410 1C using a
25%, respectively, and selectivity was 57 and 74%, respectively. LHSV of 0.4 h 1. However, the precise role of water has not
Chae et al.147 have studied the effect of the pore size, pore been investigated.148–150
volume and pore organisation on the catalytic conversion of an In conclusion, 2 wt% Ta2O5 on silica seems to be the pre-
ethanol–AcH mixture at 350 1C and a LHSV of 1 h 1 by using ferred catalyst for converting a mixture of ethanol and acet-
2 wt% Ta2O5 on several mesoporous silica like SBA-15, KIT-6 aldehyde to BD contains. High ethanol to acetaldehyde molar
and MCM-41 type materials. The BD yield increased from 22–24 ratios are required to obtain high BD selectivity and to avoid
till 37% when a SBA-15 silica support was used instead of a coke formation. Likely, water in the feed mixture has a

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beneficial though not further determined role in the catalysis. different BDOs is addressed first. 1,4-BDO is a versatile inter-
High catalyst stability and high BD yield are obtained for reaction mediate in the chemical industry with an annual production in
temperatures between 325 and 350 1C, the preferred contact times the range of 1 million metric tons.2 The main synthesis route
corresponding to a LHSV ranging from 0.3 to 1 h 1. Use of large is based on the Reppe process starting with the reaction of
pore size mesoporous silica is beneficial for the BD yield and acetylene with formaldehyde to form 2-butyne-1,4-diol, followed
selectivity due to high Ta oxide dispersion and efficient pore by hydrogenation to 1,4-BDO. The latter is not only a feedstock
diffusion. Best BD yield and selectivity were reported to be about for tetrahydrofuran (THF), but it is also used for polyurethanes
25 and 80% respectively, corresponding to a volume productivity and poly(butylene) terephthalate. Very recently, Genomatica
of 350 gBD l 1 h 1. unveiled an inventive bio-based route towards 1,4-BDO, using
a metabolically engineered Escherichia coli, which is capable of
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directly producing 1,4-BDO by fermenting different biomass-

3. Dehydration of C4 alcohols to BD derived sugars.152–154 Although this technology is immature,
3.1 Introduction and the feedstock origin it has been proven on a 2000 ton scale. Major players have
already licensed the technology for scale-up, paving the path for
Next to the ethanol-based route, alternative paths to on-purpose
replacing the non-renewable 1,4-BDO production.24
production of BD have been demonstrated. These routes are
Next to the 1,4-isomer, 2,3-BDO (Scheme 10) is also a potential
based on the (deoxy)dehydration of mono- and polyols with a
feedstock for BD. Though 2,3-BDO production by bacterial sugar
four-carbon (C4) backbone. In the literature the catalytic pro-
fermentation has been reported,155–157 its production was replaced
cessing of 1-butanol, several butanediols (BDOs) and even
by petrochemical routes based on sequential butene chlorohydri-
tetritols were reported for BD production via a direct or multi-
nation and chlorohydrin cyclization, followed by hydrolysis of the
step process. Scheme 10 provides an overview of the C4-based
resulting epoxides and their subsequent fractionation.155
alcohol routes and summarizes the major chemical trans-
A recently discovered approach to 2,3-BDO is gaining more
formations involved.
importance. The route is based on the fermentation of syngas
Double dehydration of several BDOs, viz. 1,3-, 2,3- and 1,4-BDO,
by acetogenic organisms.158 Feedstock syngas not only can be
leads to the formation of BD. In the search for novel on-purpose BD
derived from biomass gasification, but also from industrial gas
routes independent of the fossil oil feedstock, the origin of the
waste, e.g. from a steel mill. Several companies are actively
exploring gas fermentation to liquid fuels, mainly to ethanol,
Lanzatech being a pioneer in gas fermentation to 2,3-BDO.26,159
1,3-BDO is the third BDO isomer in Scheme 10. This diol is
the intermediate in the old synthesis of BD based on acetalde-
hyde from acetylene, followed by aldol condensation and
reduction of the intermediately formed 3-hydroxybutanal, a
route also included in Scheme 1.155 For this diol, no sustainable
production technology has been demonstrated at the com-
mercial level. However, numerous patents were filed in this
domain, using engineering of fermentative pathways.21,160–162
On the left-hand side of the lower part of Scheme 10,
another route to BD is shown, based on butanol dehydration.
n-Butanol and its isomers are dehydrated via a straightforward
acid-catalysed gas phase approach, leading to 1-butene (and
isomers).163–166 This could be an alternative way to biomass-
derived BD, as it plugs directly into the state-of-the-art industrial
dehydrogenation process of 1-butene.2,39 Though the latter process
has been practiced by Shell, Phillips and Petro-Tex, most plants
have been closed due to the high operational costs originating
from the elevated temperatures (up to 700 1C) needed to compen-
sate for the endothermicity. The catalyst is composed either of
oxides of Cr, Fe and/or Al (e.g., Fe2O3/bauxite) or of (Ca–Ni)
phosphates.2,39 Often, besides steam, oxygen is added to reduce
coking, oxidize hydrogen gas to water and generally to shift the
equilibrium away from the hydrocarbons. The oxidative variant
usually runs with mixed oxides based on Bi/Mo or Sn/Sb.2
The production of renewable butanol is experiencing a true
revival. The production of butanols relies on the fermentation of
carbohydrates in the so-called ABE fermentation,165,167 named
Scheme 10 Overview of 4-sugar alcohol based BD production routes. after its major products, viz. acetone, butanol and ethanol.

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The process was operated commercially in the first half of the

20th century, but abandoned again in favour of fossil alter-
natives.168 The focus of recent developments is aimed at the
synthesis of butanol, due to its high added-value and its proven
efficiency as a liquid transportation fuel with a high substitution
potential for both ethanol and biodiesel.168–170 Solventogenic
clostridia organisms are used to produce the acetone : butanol :
ethanol mixtures in a fixed 3 : 6 : 1 molar ratio. Large scale
economic breakthrough of this technology is hampered by the
high cost of the carbohydrate feedstock. Use of cheaper ligno-
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cellulosic biomass may present a great opportunity, though

considerable efforts are required to improve the performance
of the fermentation and to make the butanol recovery more
sustainable.168–170 As both ABE fermentation and the oxidative
dehydrogenation of butenes to BD have been applied commer-
cially in the past, a combined revival of these technologies could
potentially lead to bio-based BD, given the current drivers in this
market.171 One should however include the additional catalytic
dehydration to transform butanol to butenes. Although this
reaction is considered rather easy, butanol is produced in water
and its preceding recovery is a major issue.21,165,166 Due to its
biofuel potential and incentives therein, bio-butanol production
could likely increase dramatically on a short timescale,165 rapidly
followed by BD production from the butanols.
Scheme 11 BDO to BD reaction scheme with the major intermediate and
Scheme 10 also shows a one-step route based on the
side products.
deoxydehydration (DODH) reaction of 4-carbon sugar alcohols.
These tetritols are produced via hydrogenation of natural tetrose
sugars. As recently demonstrated in an efficient way by Shiramizu Dehydration of 2,3-BDO. Technically, the acid-catalysed
and Toste, the liquid phase one-pot double DODH approach dehydration of vicinal diols in the liquid phase covers a large
towards BD is based on homogeneous rhenium catalysts and field of research in the literature, mainly in the context of pinacol
involves the (transfer) hydrogenation with a sacrificial reductant. rearrangement, as first discovered by Fittig.176 Dehydration of
Unfortunately, tetrose sugars are not produced on a large scale pinacol or 2,3-dimethyl-2,3-butanediol, is the most studied sub-
today and remain an expensive and elusive feedstock.154,172–175 strate and leads to 3,3-dimethylbutan-2-one in the presence of
In conclusion, the introduction clearly points to the avail- acid catalysts. The reaction proceeds via dehydration according to
ability of on-purpose routes towards BD via 4-carbon mono-, a 1,2-rearrangement mechanism.177 The rearrangement occurs
di- and tetrols, which are derived from a renewable feedstock or by shifting a methyl group to the initially formed carbocation.
syngas. The main driver for the breakthrough is the success of Dehydration of 2,3-BDO follows the same chemistry, a hydride
engineering organisms to ferment carbohydrates or syngas to being shifted, leading to the formation of butan-2-one
available bio-butanols and BDOs. Selective dehydration is thus (Scheme 11). When a methyl shifts, 2-methyl propanal is formed
an essential step. The purpose of the next part is therefore to as a minor side product (Scheme 11). The classic pinacol chemistry
review and assess various catalytic dehydration processes for is described via the formation of an enol and thus hypothetically
the conversions of BDOs reported in the recent past with suggests the formation of 2-buten-2-ol, which then isomerizes into
an emphasis on BD production, including the chemistry and the ketone. However, the enol in the case of 2,3-BDO has never
thermodynamics of the reactions, the reaction network and the been observed and its existence is thought to be highly unlikely.
type of catalysts developed. In what follows, the DODH approach Rather, the dehydration mechanism more likely occurs either via
applied to tetritols will be assessed as well. OH abstraction with formation of a carbocation intermediate
(E1) or in a concerted fashion, via a protonated hydroxyl group
3.2. Mechanistic, catalytic and thermodynamic aspects of (oxonium, E2).178
BDO to BD dehydration chemistry Next to butan-2-one formation via the pinacol route, dehydra-
Double catalytic dehydration of 1,3-BDO, 1,4-BDO and 2,3-BDO tion of 2,3-BDO also follows another reaction mode, often denoted
leads to BD. The dehydration reaction with the different BDOs as 1,2-elimination, forming an alkene upon elimination of water.
leads to a mix of overlapping intermediates – originating from If one water molecule is removed, the unsaturated 3-buten-2-ol will
the first dehydration – and side-products, resulting from com- be formed, which may further dehydrate to BD. The ratio of the
peting reactions. The routes and chemistry of BDO double two dehydration types, presented in parallel in Scheme 11,
dehydration are schematically represented in Scheme 11, and depends on the chemical structure of the diol, the acidic
will be discussed for each type of BDO. property of the catalysts and the experimental conditions.178,179

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Most reports on the conversion of 2,3-BDO in the gas or liquid carbon backbone.189 Other zeolite topologies have also been
phase show a predominant pinacol type dehydration. The pinacol investigated. Lee et al. have investigated a series of zeolites with
reaction is accompanied by BD formation in the liquid phase only different Si/Al ratios within MFI, MOR, BEA and FAU topologies.
under non-classical conditions. For instance, Waldmann et al. They noted that the small pore zeolites favoured butan-2-one
have studied the dehydration of 2,3-BDO to BD in excess of pthalic formation,190 whereas the large pore zeolites mostly yielded
anhydride with benzenesulfonic acid.178,180 The pyrolysis of 2,3- acetals and ketals. However, no significant formation of BD was
acetoxy derivatives of 2,3-BDO into BD is also known.181 The use of reported. Very recently, Zhang et al. have studied the influence
amines also helps in guiding the dehydration to BD, while avoid- of boric acid modification of H-ZSM-5 zeolites191 in a study
ing butan-2-one formation from 2,3-BDO.182 Hereto, a vapour concerned with the effect of acid site density and strength. In
phase reaction over a catalyst, containing silica and tungsten oxide all cases however, BD yields were in the range of 1%.190–192
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on alumina, succeeded yielding 48% of BD from 2,3-BDO in a Olah et al. in a study on the use of H-Nafion, a perfluorinated
single pass at 330 1C. However, this high yield was only possible by resin with sulfonic acid groups, reported that this superacid
co-feeding butan-2-one, water and triethylamine. Later, a patent mainly catalysed the pinacol route.179 In a reactive distillation
was filed on a similar catalyst composition regarding the forma- setup, 1,3-dioxolane derivatives were predominantly formed via
tion of 2-methyl-butadiene (isoprene) in high yields (490%) from acetalisation of butan-2-one with 2,3-BDO, while butan-2-one
2-methyl-2,3-butanediol in the vapour phase without co-feeding was the main product in a continuous flow reactor setup. At
additional reagents. Unfortunately, the patent did not demonstrate 150 1C, the conversion of 2,3-BDO was about 78%, while the
the conversion of 2,3-BDO to BD.178,183 selectivity to BD only reached 4%. Butan-2-one and its acetals
The first real attempts on the single-step 2,3-BDO-to-BD accounted thus for about 94% of the recovered products. At
conversion without sacrificial or stoichiometric reagents date 175 1C, the yield of BD increased to 8% at full BDO conversion.
back to the 1940s. Bourns et al. have assessed a Morden Other strong acids like heteropolyacids examined by Török
bentonite clay as a catalyst and they noticed full conversion of et al. showed a dominant pinacol pathway.193 The preference
2,3-BDO into butan-2-one below 350 1C. Between 450 and 700 1C, for pinacol formation with acid catalysts is attributed to the
more gaseous products were formed containing small quantities strength of the acid sites. Strong acidity better stabilises the
of BD. Dilution of the feed stream with steam increased the BD oxonium ion transition state (thus the carbocation formed from
yield to 25%.184 Winfield was the first who truly aimed at making 2,3-BDO), hence favouring the 1,2-hydride (and also some 1,2-
BD form 2,3-BDO in as single step and thoroughly screened a methyl) shift. Ironically, when 2,3-dimethyl-2,3-butanediol
range of catalysts including several metal oxides and salts such (actual pinacol) is the substrate instead of 2,3-BDO, pinacol
as SiO2, B2O3, ThO2, BeO, CaHPO4, CaCO3 and CaSO4 in a rearrangement is less dominant and diene formation is more
vapour phase reactor setup in 1945.185 The author aimed at a pronounced. Ironically, when 2,3-dimethyl-2,3-butanediol (actual
reaction step, proceeding via 1,2-elimination towards 3-buten-2- pinacol) is the substrate instead of 2,3-BDO, pinacol rearrange-
ol, and further dehydration to BD (Scheme 11). While most ment is less dominant and diene formation is more pronounced.
catalysts promoted pinacol dehydration with almost exclusive This change in the reaction pathway is likely related to the relative
formation of butan-2-one, ThO2, prepared according to a specific ease of pinacol rearrangement with 2,3-BDO, since the migration
synthesis protocol, produced selectively BD. The best result was of the hydride as in 2,3-BDO is easier than the shift of a methyl
obtained over the ThO2 catalyst at 350 1C in a fixed-bed reactor group as in 2,3-dimethyl-2,3-butanediol.188,189
under reduced pressure, yielding 60% BD and 20% 3-buten-2-ol. Sato et al. have reported the use of basic CeO2 to dehydrate
According to the authors, the rate of the second dehydration 2,3-BDO at 425 1C, noting the formation of butan-2-one as the
step, converting 3-buten-2-ol to BD, appears inversely propor- major product, while no BD was formed (Table 7, entry 7B).
tional to the applied pressure. Winfield then thoroughly inves- Next to the unique ThO2 catalyst of Winfield, a recent patent
tigated the adsorption equilibria of his catalytic system to showed promising dehydration of 2,3-BDO to BD in the pre-
elucidate the profound effect of water adsorption and the con- sence of an hydroxyapatite–alumina catalyst at 320 1C, yielding
comitant retardation of the second dehydration.186 BD yields above 45%.194
The work of Winfield on ThO2 may be considered as the Finally, an alternative and original strategy was recently
breakthrough in converting 2,3-BDO to BD, since most other proposed, which tries to bridge the fermentative production
catalytic materials produced butan-2-one.178,187 Bartok et al. of 2,3-BDO with its conversion to BD. The strategy is composed
have reported that the pinacol route is the preferred dehydra- of (i) the esterification of the diol with formic or acetic acid –
tion pathway in the presence of zeolites like Na+ and Na+/H+ which are naturally present in glucose-to-BDO fermentation
exchanged X and Y type faujasites at 250 1C.188,189 The authors liquors-, followed by (ii) the pyrolysis of the di-esters formed
noticed that a lower acidity within X-type zeolites not only led to with production of BD at 500 1C. The yields of step 1 are in the
more pronounced 1,2-elimination and thus BD, but also to a range of 70–85% depending on the acid, whereas the pyrolysis
decreased conversion rate.188 For instance, at 250 1C, use of the of the pure di-esters yields BD in the range of 82–94%. This
NaH–X zeolite showed 59% conversion, distributed over 78% appears to be an effective way of avoiding pinacol type reactions,
butan-2-one, 4% 2-methyl-propanal and 16% BD.179,188 Zeolites but requires pyrolysis at elevated temperatures.195
generally require high reaction temperatures for dehydration In conclusion, direct dehydration of 2,3-BDO into BD in high
and therefore they also tend to catalyse fragmentation of the yields is very challenging due to the competitive butan-2-one

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Table 7 Exemplary catalytic conversions of BDOs in gas phase processing

WHSV/ TC/ SBD/ STHF/ S3-buten-1-ol/ S3-buten-2-ol/ S2-buten-1-ol/

Entry Feedstock Catalyst T/1C h 1 % mol% mol% mol% mol% mol% Ref.
1 1,4-BDO Al2O3 200 9.0 17 70 30 0 — — 202
2 1,4-BDO Al2O3 275 9.0 100 0 99 0 — — 202
3 1,4-BDO SiO2–Al2O3 200 9.0 100 5 92 0 — — 202
4 1,4-BDO ZrO2 275 9.0 15 49 37 0 — — 202
5 1,4-BDO CeO2 275 6.0 6 87 0 0 — — 202
6 1,4-BDO CeO2 425 6.0 73 5 7 56 — — 202
7A 1,4-BDO CeO2 425 1.8 91 24 — 36 — — 202
7Ba 2,3-BDO CeO2 425 13.2 51 0 — — 3 — 203
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8 1,4-BDO Yb2O3 375 20.3 40 — 3 82 — — 208

WHSV/ TC/ Sel.BD/ Sformaldehyde/ S3-buten-1-ol/ S3-buten-2-ol/ S2-buten-1-ol/

Entry Feedstock Catalyst T/1C h 1 % mol% mol% mol% mol% mol% Ref.
9 1,3-BDO SiO2–Al2O3 200 11.4 19 20 0 44 2 14 214
10 1,3-BDO SiO2–Al2O3 250 11.4 74 36 8 28 3 3 214
11 1,3-BDO Al2O3 250 11.4 49 0 28 15 0 4 214
12b 1,3-BDO TiO2 350 11.4 56 0 5 21 3 11 214
13c 1,3-BDO ZrO2 350 11.4 57 3 3 18 13 15 214
14 1,3-BDO CeO2 325 6.7 73 0 0 0 58 36 216
15 1,3-BDO Yb2O3 325 6.7 33 0 0 1 51 35 216
16 3-Buten-1-ol SiO2–Al2O3 250 5.7 42 13 58d — — — 214
17 2-Buten-1-ol SiO2–Al2O3 250 5.7 77 93 — — — — 214
18 3-Buten-2-ol SiO2–Al2O3 250 5.7 71 93 — — — 3 214
a
2,3-BDO as feed: major product is 64 mol% butan-4-one (thus pinacol rearrangement). b Other major product was 3-buten-2-one: 19 mol%/11 mol%.
c
Other major product was 3-buten-2-one: 19 mol%/11 mol%. d Major product is 58% propene instead of formaldehyde. TC presents the total
conversion of substrate (in %)

formation according to the pinacol rearrangement. While the high concentrations of unsaturated alcohols like 3-buten-1-ol
latter is catalysed preferably by most acid catalysts, Winfield’s and 2-buten-1-ol. Further increase of the temperature above
work of 1945 using ThO2 and recent work with hydroxyapatite 400 1C led to the predominant formation of BD.178 According to
prove that the competitive dehydration of 2,3-BDO to BD is Freidlin et al., with phosphate type catalysts THF was said not
possible, provided that the appropriate catalyst is used. There- to isomerise into the butenols, but to convert directly into BD at
fore, research urgently needs further and deeper exploration to temperatures above 360 1C.197
clarify the required catalytic properties and surface reactions. BD is thus formed both from THF and 3-buten-1-ol, as
Some companies159 very recently have announced development visually presented in Scheme 11. The primary pathway is said
of such new catalytic technology, in light of the recent break- to occur via 3-buten-1-ol, which is the product of dehydration
throughs in the production of 2,3-BDO from syngas and bio- via the catalysed 1,2-elimination mechanism.178 The work by
mass fermentation.158 Reppe et al. has confirmed that the 1,4-cyclodehydration is
Dehydration of 1,4-BDO. The acidic dehydration of 1,4-diols mildly exothermic and rather easy, while the conversion of THF
(g-diols) such as 1,4-BDO almost always leads to the formation to BD reaction is highly exothermic and is much more difficult.198
of cyclic ethers under medium temperature conditions. As sum- Nevertheless, they have described a process converting 1,4-BDO
marized in a comprehensive chapter by Bartok and Molnar,178 vapours mixed with steam, over a supported H3PO4 catalyst bed,
the 1,4-cyclodehydration reaction is nearly independent of the containing sodium phosphates. The residence time could be
chemical structure and the degree of substitution of the BDO. optimized to form almost quantitative yields of THF, including
It has been demonstrated with countless acidic catalysts,196 small percentages of BD. By separating BD and recycling THF
ranging from organic to mineral acids, metal salts like ZnCl2 over the catalyst bed, BD yields of over 90% were shown.198,199 The
and oxides like alumina and phosphates in both liquid and gas reverse process, viz. 1,4-BDO production from BD, is currently
phase conditions.178 Cyclodehydration of 1,4-BDO leads to the commercially practiced. This process proceeds via 1,4-diacetoxy-
formation of tetrahydrofuran (THF, in Scheme 11). As this butene formation from BD and acetic acid and subsequent
dehydration process is highly selective to the cyclic ether, it is hydrogenation followed by a final acid hydrolysis. Acetic acid can
obviously one of three commercially practiced processes to be recycled and, next to 1,4-BDO, THF is formed.2,200,201
produce THF. In the vapour phase, the dehydration of 1,4-BDO Sato et al. have studied the dehydration of 1,4-BDO to
over chromium oxide, alumina or calcium phosphate at 250–320 1C unsaturated alcohols. A wide range of oxides such as Al2O3,
almost quantitatively leads to THF.2 SiO2–Al2O3, ZrO2, MgO were screened as well as plethora of
Production of BD from 1,4-BDO demands a higher reaction lanthanide oxides, with a special focus on CeO2.202,203 Logically,
temperature. The formation of THF over Ca3(PO4)2 is selective the different catalytic behaviour of SiO2–Al2O3 and Al2O3
in the temperature window of 250 to 320 1C. At higher tem- showed that acidity influences the dehydration activity. The
peratures, the formation of some BD was observed along with dehydration reaction over alumina at 200 1C (entry 1 in Table 7)

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shows a conversion of 1,4-BDO of 17%, whereas full conversion long time has been an industrially practiced route.2,39,178 The
is obtained with SiO2–Al2O3 at the same temperature (entry 3, reaction, which proceeds mainly via the 1,2-elimination mecha-
Table 7). While at low conversions the selectivity to BD was nism, is catalysed by a plethora of acids, acid anhydrides and
high, viz. 70% for alumina, THF is the dominant product at full metal salts.
BDO conversion (entries 2 and 3), while no unsaturated alco- Different products such as BD, intermediately dehydrated
hols were formed. enol products, and other side-products have been encountered
Yamamoto et al.204,205 have investigated the use of ZrO2 for in the dehydration of 1,3-BDO. Their pathways are depicted in
the selective dehydration of 1,4-BDO to produce THF and Scheme 11. The intermediate products are the result of a
3-buten-1-ol at 275 1C and a WHSV of 9 h 1 (entry 4, Table 7). different regioselectivity in the first dehydration step. Accord-
The competitive formation of both products is likely due to the ing to the classic acid-catalysed 1,2-dehydration mechanism,
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presence of both acid and basic sites on ZrO2.204 This surface the diol should transform to 2-buten-1-ol, the most stable
composition is able to compete for 1,4-cyclodehydration to THF alkene in agreement with Zaitsev’s rules. The anti-Zaitsev
and 1,2-elimination to unsaturated alcohols. In order to avoid product, 3-buten-1-ol, is formed abundantly with 1,3-BDO. In
the formation of THF, the authors decided to investigate purely addition, while the elimination of water from a secondary alcohol
basic cerium oxides. Usage of CeO2 completely avoided the is usually easier than from a primary one due to the higher
formation of THF. At 275 1C and a WHSV of 6.0 h 1, only 6% stability of the more substituted carbocation, the terminal hydro-
1,4-BDO conversion was noticed with a high BD selectivity close xyl in the case of 1,3-BDO also undergoes serious dehydration,
to 90%, while a 73% conversion was obtained at 425 1C albeit rendering the formation of 3-buten-2-ol, as a third enol product
with a selectivity shift from BD to 3-buten-1-ol (compare entries in the mixture. BD formation via dehydration of the three enols
5 and 6 of Table 7). Upon increasing the contact time with a occurs with different reactivity for each enol type. Competitive
factor of three, BD selectivity increased from 5 to 24% at the isomerization with the formation of butan-2-one is an unwanted
expense of 3-buten-1-ol (entry 7A, Table 7), while a 91% con- side-reaction in view of BD production. Another mode of
version of 1,4-BDO was obtained. These results highlight the dehydration, namely 1,3-cyclodehydration resulting in the forma-
importance of tuning the contact time to promote single or tion of the cyclic 2-methyl oxirane, is usually not encountered for
double dehydration. The excellent redox properties of CeO2 is 1,3-BDO. Next to dehydration, fragmentations may occur via
responsible for a selective stimulation of dehydration to the retro-aldolisation if one of the hydroxyls is able to undergo
unsaturated alcohol.202,206 A similar behaviour was noticed with dehydrogenation to yield a carbonyl group.
In2O3207 and weakly basic, heavy rare earth oxides202,203,206,208–210 Patent literature on the synthesis of BD from 1,3-BDO is
such as Yb2O3 (entry 8, Table 7). The latter catalyst was very active ample. The patents describe the conditions and catalyst com-
and selective for dehydrating 1,4-BDO selectively to 3-buten-1-ol positions, which favour BD formation such as supported and
at 375 1C and a WHSV of 20.3 h 1 with a selectivity of 82% at a unsupported acidic and neutral phosphates like Na-(poly)-
conversion of 40%. The very high WHSV (and thus low contact phosphates (at 270 1C)2 and CePO4 (at 320 1C).212 Supported
time) reveals a high activity of Yb2O3. Although rather basic in H3PO4 and more complex compositions have also been used.213
nature, the catalytic action of the rare-earth oxides likely pro- BD yields over 90% are not uncommon in these patents.
ceeded via a concerted acid–base mechanism, as hinted from gas Sato et al.211 have thoroughly investigated the conversion of
poisoning experiments.211 Sato et al. recently reviewed their 1,3-BDO in the vapour phase with focus on the production of the
efforts in the dehydration of diols with rare-earth oxides.211 unsaturated alcohol, 3-buten-2-ol, rather than on BD (Scheme 11).
In conclusion, the most favourable dehydration route of Such enols are important raw materials for the synthesis of
1,4-BDO is cyclodehydration, rendering the formation of THF. custom and added-value specialty chemicals. Although not aimed
Although THF may be further dehydrated to BD, this reaction at producing BD, these studies are relevant for gaining mecha-
requires severe thermal conditions. The more straightforward nistic insight into the dehydration of 1,3-BDO over solid acid and
route from 1,4-BDO follows the double 1,2-elimination with base catalysts. A selection of the catalytic data is displayed in
intermediate formation of 3-buten-1-ol. This pathway is less Table 7 (entries 9 to 18). Strongly acidic catalysts such as
dominant over acidic catalysts. Neutral to basic catalysts like SiO2–Al2O3 were more active and yielded BD next to the enols
phosphates, ZrO2, CeO2 and Yb2O3 favour the formation of the (entries 9 to 13, Table 7). 19% of 1,3-BDO could be converted
unsaturated alcohol, BD being formed at high enough contact into 20% BD, 44% 3-buten-1-ol and 14% 2-buten-1-ol over SiO2–
time and increased reaction temperature. However, since the Al2O3 at 200 1C and WHSV of 11.4 h 1. Increasing the reaction
formation of BD was never the purpose of the aforementioned temperature to 250 1C, both the conversion and the BD selectivity
catalytic studies, we anticipate that much better catalytic results increased to 74% and 36%, respectively, (compare entries 9
are expected soon once the contact time, catalyst composition and 10, Table 7). Interestingly, and in contrast to dehydration
and other engineering parameters are fine-tuned with focus on of 1,4-BDO, dehydration of 1,3-BDO is more selective to BD in
1,4-BDO dehydration to BD. the presence of acid catalysts.
Dehydration of 1,3-BDO to BD. Dehydration of 1,3-BDO is Less acidic oxides like alumina tend to catalyze other reaction
the textbook example of 1,3-diols dehydration. As mentioned channels. In the presence of alumina, formation of formalde-
above, 1,3-BDO is the intermediate in the Reppe acetylene- hyde and 4-methyl-1,3-dioxane with 28 and 18% selectivity was
based route to BD. In fact, the double acidic dehydration for a noticed next to some 3-buten-1-ol (entry 11). Formaldehyde likely

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originated from 1,3-BDO dehydrogenation towards 4-hydroxy-

butan-2-one, followed by retro-aldol fragmentation. Formalde-
hyde further reacts with a new 1,3-diol molecule to form
1,3-dioxane.214
In the case of Zr and Ti oxides (entries 12 and 13), mainly
unsaturated alcohols were found, next to 3-buten-2-one,
indicative of the formation of 4-hydroxy-butan-2-one and its
dehydration, reaction temperatures as high as 350 1C being
required to achieve comparable 1,3-BDO conversions. CeO2
and Yb2O3 were very selective for the formation 3-buten-2-ol
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(from primary alcohol dehydration) and 2-buten-1-ol (Zaitsev’s

product from a secondary alcohol dehydration) (entries 14
and 15). Unfortunately no BD was formed over the mildly basic
oxides.206,211,214,215 At 325 1C and a WHSV of 6.7 h 1, 73% Fig. 12 Equilibrium composition calculated for 1,3-BDO to x-buten-y-ol,
1,3-BDO is converted into 3-buten-2-ol and 2-buten-1-ol with a performed using the Equilibrium based reactor (REquil) in Aspen Pluss
software.
selectivity of 58 and 36%, respectively.
The same authors have also investigated the dehydration of
the intermediate enols to BD using acidic SiO2–Al2O3. Interest- p = 1 atm (DH calculated in the real state and thus different
ingly, 3-buten-2-ol and 2-buten-1-ol are selectively converted to from DH0).
BD at 250 1C and a WHSV of 5.7 h 1, 2-buten-1-ol being slightly
C4H10O2 - C4H6 + 2H2O (3)
more reactive, while the anti-Zaitsev’s product 3-buten-1-ol is
less reactive, primarily forming propylene under the same The equilibrium composition shown in Fig. 12 for the
conditions (entries 16 to 18, Table 7). Clearly, the intermediate dehydration of 1,3-BDO to its three enol products shows that
formation of the latter enol should be limited as much as 2-buten-1-ol is favoured thermodynamically at low temperature,
possible, as almost no BD is formed from it. whereas the formation of 3-buten-1-ol gains importance at
In conclusion, dehydration of 1,3-BDO has been commer- higher temperature. 3-Buten-2-ol will be highly unfavourable
cially practiced at 400 1C in the presence of steam using a over the whole temperature range, if formation of the other two
mixture of NaPO3 and Na2P2O7, the pyrophosphate being the enols is possible. Clearly, the selective formation of BD from
dehydrating agent, yielding 80% of BD. Conversion of 1,3-BDO 1,3-BDO is a kinetic issue only.
in the presence of acid catalysts mainly follows three parallel As explained before, dehydration of 2,3-BDO shows the domi-
pathways in the first dehydration step, leading to a peculiar nant formation of butan-2-one rather than BD, due to the presence
product regioselectivity with three enol intermediates. While of an easy pinacol reaction channel. Fig. 13 shows the variation of
3-buten-2-ol and 2-buten-1-ol are selectively converted to BD, the equilibrium composition with respect to temperature. It should
intermediate formation of 3-buten-1-ol should be avoided since be noted that butan-2-one is the most favourable product up to
its transformation into BD is unselective. Here, the use of 500 1C. At higher temperatures, further dehydration according to
mildly basic rare earth metal oxides like CeO2 may be interest- the 1,2-elimination mechanism is favoured, resulting in the forma-
ing to apply in a two-stage approach with Brønsted acids like tion of BD. This means that tuning the catalytic properties towards
silica–alumina, since it converts 1,3-BDO exclusively into the 1,2-elimination is of utmost importance to attain BD selectively. If
two preferred enols, which are further dehydrated over the not, butan-2-one will be predominantly formed.
silica–alumina to attain high yields of BD. Despite the promis-
ing dehydration of 1,3-BDO to BD, production of 1,3-BDO from
sustainable resources is less developed when compared to the
other BDOs. Its limited availability via renewable routes pre-
sents a major bottleneck.
Thermodynamic considerations. The thermodynamics of
the BDO routes have been assessed by Aspen software based
on available Gibbs free energies for all three BDOs. In the
simulation of the overall reaction, viz. one molecule of BDO
converting to one molecule BD and two H2O molecules
(eqn (3)), the molar composition at steady state equilibrium
showed only the presence of BD in the range of 100 to 500 1C,
demonstrating that the reaction is favourable over a large
temperature window (data not shown). The reaction is slightly
endothermic by 62 kJ mol 1, 56 kJ mol 1 and 111 kJ mol 1 Fig. 13 Equilibrium composition calculated for 2,3-BDO to 3-buten-2-ol,
(at 250 1C) for 1,3-, 1,4-, and 2,3-BDO, respectively calculated in BD and butan-2-one, using the Equilibrium reactor (REquil) in Aspen Pluss
Aspen Pluss software for pure components at given temperature, software.

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In the thermodynamic simulations of 1,4-BDO dehydration Table 8 Catalytic results for the DODH approach applied to erythritol
to BD, the presence of THF in the equilibrium composition was
E Catalyst Solvent/red. Feed BDa 1a 2a 3a 4a 5a Ref.
unimportant (data not shown). Thus THF is not thermo-
dynamically favoured, while its abundant formation in the catalytic 1b Cp*ReO3 C6H5Cl/PPh3 ery 80 obs.b obs.b — — — 217
2c Re2(CO)10 3-Octanol f ery — — — — — 62 221
experiments demonstrates the fast kinetics of the 1,4-cyclo- 3d Bu4NReO4 benzene/SO3 ery 27 0 3 0 0 6 222
dehydration. In the long run, BD formation is favoured, albeit with 4e MeReO3 3-Octanol f ery 89 0 0 0 0 11 234
a high energy barrier. Lowering the energy barrier will be a great 5e MeReO3 3-Octanol f threig 81 0 0 0 13 0 234
6h HReO3 3-Octanol f ery 73 0 0 0 0 7 234
challenge for catalytic scientists if production of BD from 1,4-BDO is 7e MeReO3 3-Octanol f 2i 70 0 0 0 6 0 235
aimed at. 8e MeReO3 3-Octanol f 3i 70 0 0 0 0 0 235
9j XReO3k PhCl/PPh3 ery 18 0 6 2 — — 229
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3.3. Mechanistic and catalytic chemistry of deoxydehydration 10l XReO3k Pyridine/PPh3 ery 30 4 3 5 — — 229
11e XReO3k 3-Octanol f ery 67 0 0 0 — 7 229
Besides the dehydration route for converting BDOs into BD, there a
Yields of BD and compounds 1–5, as seen in Scheme 12i, are expressed in
also exists a catalytic deoxydehydration (DODH) methodology, mol%. b 0.044 M erythritol (ery), 1.5 mol% Re, 0.044 M PPh3, 135 1C, 28 h
involving dehydration with concomitant oxygen transfer to a obs = observed. c 1 M ery, 1.25 mol% Re, 1.7 mol% TsOH, 160 1C, 12 h, air.
d
reductant (Red.). This approach is generalized for vicinal diols in 0.2 M ery, 10 mol% Re, 0.3 M Na2SO3, 155 1C, 100 h. e 0.3 M ery,
2.5 mol% Re, 170 1C, 1.5 h, N2. f 3-Octanol plays the role of solvent and
eqn (4), L and M being ligand and (transition) metal, respectively. reductant. g Reaction with DL-threitol instead of erythritol, main side
product is a diastereoisomer of compound 4. h Conditions as e but at
155 1C for 5.5 h. i Resp. intermediates from Scheme 12i as substrate, 0.5 h of
(4) reaction. j 0.1 M erythritol, 2 mol% Re, 0.22 M PPh3, 180 1C, 24 h, N2. k X =
1,2,4-tri(tert-butyl)cyclopentadienyl. l Conditions as j but 15 h. E = Entry.

The metal-catalysed single stage DODH has first been reported

by Cook et al. based on a 1,2,3,4,5-pentamethylcyclopentadienyl–
ReO3 catalyst (Cp*ReO3) and triphenyl phosphine (PPh3) as the
sacrificial reductant.217 Although some homogeneous catalysts
based on Ru,218 Mo219 or V220 have since then been examined for
DODH potential, most reports focus on the use of Re221–231 as the
catalytic centre. Most DODH work was published in view of
deoxygenation of biomass derived oxygen-rich substrates to produce
new biofuels.232,233 Although erythritol was included as a substrate
in the original report by Cook and Andrews,217 the application of
DODH to sugar alcohols and real biomass substrates such as
glycerol and tetritols and not merely to aliphatic or aromatic
1,2-diols, recently received much interest.221,222,227,229,234,235 Cook
et al. have worked under biphasic conditions for DODH of erythritol
at 135 1C, using chlorobenzene as a solvent, yielding after 28 h
about 80% of BD (entry 1, Table 8). Next to BD as the major product,
3-butene-1,2-diol and cis-2-butene-1,4-diol were formed as well in a
molar ratio of 85 : 15. The latter enolic products are the result of a
single DODH reaction on either one of the 1,2 or 2,3-diol couple.
Scheme 12 i summarizes all possible enol products that can be
derived from erythritol via a single DODH reaction, viz. 3-butene-1,2-
diol (1), cis-2-butene-1,4-diol (2) and trans-2-butene-1,4-diol (3). Since
3-butene-1,2-diol is a vicinal diol, it may undergo a second DODH
resulting in BD formation. Cook and Andrews have verified the BD
formation route by feeding 3-butene-1,2-diol to the catalytic
system.217 They observed fast BD formation, whereas no direct
DODH was observed when feeding cis-2-butene-1,4-diol. They
have noticed some isomerisation of cis-2-butene-1,4-diol into
3-butene-1,2-diol under the catalytic conditions indicating Scheme 12 Tetritol to BD route: (i) main products: 3-butene-1,2-diol (1);
indirect formation of BD, albeit very slow. In the end, because cis-2-butene-1,4-diol (2), 3: trans-2-butene-1,4-diol (3); 1,4-
anhydroerythritol (4); 2,5-dihydrofuran (5). (ii) Proposed catalytic cycle,
the solubility of the three butenediols is much higher in
adapted from Shiramizu et al.235
chlorobenzene than the erythritol reagent, sequential DODH
is favoured promoting BD formation. Cook et al. also applied
the DODH protocol to xylitol.217 erythritol in the presence of p-toluenesulfonic acid. The reac-
Arceo et al.208 have studied the use of Re2(CO)10 in 3-octanol, tion (entry 2, Table 8) mainly showed 2,5-dihydrofuran for-
acting as solvent and the reductant for the conversion of mation. Erythritol underwent a sequential 1,4-cyclodehydration

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yielding 1,4-anhydroerythritol (4 in Scheme 12), followed by a vicinal main product. This points to the high preference of catalytic
DODH reaction leading to the formation of 2,5-dihydrofuran DODH rather than cyclodehydration under the conditions used.
(5 in Scheme 12). The presence of the acid additive was sug- Yi et al. also used MeReO3, but mainly reported on a solvent-
gested to be the driver for cyclisation to 1,4-anhydroerythritol. less DODH approach, practiced with glycerol.227 To remove and
Such a mechanism corroborates with the well-established collect the gaseous products, a continuous distillation setup was
substitution-independent acidic cyclisation of g-diols (see the used, mainly producing allyl alcohol from glycerol. The latter is
1,4-BDO section).178 not only the reactant, it also participates in transfer hydrogenation
In 2011, Ahmad et al. have investigated a sulphite driven with co-production of dihydroxyacetone. With 2 mol% of MeReO3
oxorhenium catalysed approach.222 Bu4NReO4 (perrhenate salts) and 1.5 molar equivalents of 1-heptanol as a transfer reagent,
was tested with a Na2SO3 reductant in benzene for the DODH of erythritol and threitol were converted mainly into gaseous
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erythritol (entry 3, Table 8) at 155 1C. BD, cis-2-butene-1,4-diol products.214 Using the continuous distillation procedure, no
and 2,5-dihydrofuran were encountered, with BD being the BD was condensed in the collector. Erythritol mainly yielded
major product (24%). MeReO3 was also used in combination 2,5-dihydrofuran and cis-but-2-enal, while meso-threitol was
with sulphites, but this Re catalyst performed less well. The converted into (3S,4S)-tetra-hydrofuran-3,4-diol, an isomer of
reaction products of these DODH reactions are fully in line with compound 4 in Scheme 12i.227 Differences with the results of
the proposed Re reaction system of Cook et al. with PPh3 (entry 1), Shiramizu and Toste234 (entries 4 and 5, Table 2) may originate
and helped corroborating the reaction path in Scheme 12i. either from the excess of sacrificial alcohol used or, more likely,
However, Nicholas et al.209 attributed the formation of from the differences in setup; viz. an open distillative system
2,5-dihydrofuran via fast 2,3-DODH, followed by a 1,4-cyclo- versus a closed approach. It appears that the open reactor
dehydration. Given the long reaction times, the sulphite225 promotes 1,4-cyclodehydration, next to DODH chemistry, pre-
system seemingly presents a less active DODH system for ferably leading to the cyclic species of Scheme 12 rather than to BD.
tetritols. Mechanistically, the first 1,2-diol DODH transforming The resemblance with the product spectrum of the Bergman
erythritol into 3-butene-1,2-diol is generally considered faster system is obvious.221 However, rather than being the effect of
than a 2,3-DODH (Scheme 12i). The second DODH, of 3-butene- Brønsted acidity (Bergman), here a plausible explanation could
1,2-diol into BD, is considered to be even faster, because this be a shift in reaction equilibria, caused by the open distillation,
diol was never noticed in the initial reaction stage. Although hereby promoting dehydration.
considerably slower, and thus not a major pathway, the initial To expand the scope of the oxorhenium catalyzed DODH,
2,3-diol DODH leading to the cis rather than to the trans form of Shiramizu and Toste have investigated different substrates.235
2-butene-1,4-diol, further confirms the preference of oxorhenium Oxorhenium compounds not only catalyse DODH chemistry,
for a syn-oriented elimination. but in parallel accelerate 1,3-OH shifts in allylic alcohols. The
Both Shiramizu and Toste234 and Yi et al.227 independently latter reaction thus formally presents 1,4-DODH and 1,6-DODH
have further explored the DODH reaction on erythritol, both with (eqn (5) and (6), respectively), rather than classic cis-vicinal or
a less complex and commercially available MeReO3 catalyst. The 1,2-DODH (eqn (4)).
former authors have provided a breakthrough in the frame of BD
production, by adopting a closed system in excess of a sacrificial
alcohol reductant – as in the work of Bergman221 – with the (5)
significantly simpler MeReO3 catalyst to assist the DODH. This
transfer hydrogenation, here demonstrated with 3-octanol to
3-octanone, is useful in view of the production of bio-alcoholic
solvents like isobutanol. Moreover, the formed ketone is easily
reduced and recycled, if necessary, or may be used in other (6)
reactions like aldol condensation. The authors have reported
very efficient double DODH on erythritol and DL-threitol at
170 1C, yielding BD in 89% yield at high conversion (entries 4 For intermediates like cis- and trans-2-butene-1,4-diol
and 5, Table 8). The major side-product from erythritol was (2 and 3, Scheme 12i) tested under conditions similar to those
2,5-dihydrofuran in line with the Bergman’s report, suggesting a of the original tetritol, very smooth progression of 1,4-DODH
dominant 1,4-cyclodehydration activity (see Scheme 12i). In the chemistry has been noticed for both isomers.222 The catalytic
case of DL-threitol, the major side-product was a trans isomer of results (entries 7 and 8, Table 8) indicate that the formation of a
1,4-anhydroerythritol, a cyclic internal trans-diol. Its formation 7-membered rhenium-diolate from the catalyst and the 1,4-diol
demonstrates the need for cis-diol substrates to get an efficient is excluded as both isomers showed identical reactivity. On this
vicinal-directed DODH. To check whether the introduction basis, the 1,3-OH shift causing isomerisation of allylic alcohols
of Brønsted acidity would result in an increase of the 2,5- may be included in Scheme 12i (dotted line). Note that Cook
dihydrofuran yield via the acidic 1,4-cyclodehydration path et al.217 already noticed the formation of 3-butene-1,2-diol from
(Scheme 12i), a reaction with HReO4 as a catalyst was performed cis-2-butene-1,4-diol, though at a much slower rate than the
under similar conditions (entry 6). However, no changes in DODH reaction. Shiramizu and Toste have proposed a catalytic
the product distribution were noted, while BD remained the cycle for their tandem isomerisation/DODH protocol, which is

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in fact the combination of a 1,4 and 1,2-DODH (Scheme 12ii). systems are worthy of exploring for BD production from tetri-
In the tentative catalytic cycle it is proposed that the 1,4-DODH tols, provided 2,5-dihydrofuran formation can be suppressed.
reaction proceeds through a 5-ring rheniumV-diolate before
expelling the isomerised olefin, i.e. 3-butene-1,2-diol (1) in the
case of erythritol. This intermediate is identical to the one in 4. Conclusions and perspectives
the last step of the final 1,2-DODH reaction, just before BD
expulsion (Scheme 12ii). This merged isomerisation-DODH On-purpose synthesis of BD has received a recent revival of
mechanism might explain the high overall DODH efficiency interest because its availability from naphtha and gas crackers
of the MeReO3–3-octanol system, in conjunction with the high is diminishing. Worldwide investment in BD extraction capa-
substrate solubility. Theoretical density functional studies city at the crackers and dehydrogenation of the butane fraction,
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by Qu et al. have demonstrated the possibility of a pathway either as a component of natural gas and shale gas or as a waste
involving a MeReO(OH)2 species formed from MeReO3 and fraction from petrorefinery, are two obvious ways to satisfy the
the reducing alcohol,231 coordinating diols through double needs of a firm BD market. Although the butane dehydrogena-
dehydration. Thus, the alcohol may act as a shuttle facilitating tion route to BD is an energy consuming process because of its
various hydrogen-transfer steps. Other mechanistic proposals, endothermicity, state-of-the-art technology has proven its relia-
elaborated for the synthesis of trans-stilbene, involve a bility in the past and new initiatives were recently announced.
rheniumIII-diolate, formed from 3-octanol reduction of methyl- However, driven by the shortage of light alkanes due to light-
dioxorheniumV.228 Finally, Raju et al. reported the conversion ening of cracker feeds and the increasing global demand for
of erythritol via DODH with a bulky 1,2,4-tri(tert-butyl)cyclo- bio-derived chemicals, biomass-based conversion might offer
pentadienyl-ReO3 catalyst.229 Entries 9 and 10 in Table 8 show new avenues for on-purpose BD production from renewable
these results, using PPh3 as a reductant in chlorobenzene or feedstock. The major technical challenge here is to produce BD
pyridine. Low BD yields were noticed and several intermediates, from biomass and/or biomass-derived syngas that will be cost
including trans-2-butene-1,4-diol, were obtained, pointing to an competitive with fossil oil-based BD. Several research and
inefficient DODH protocol. Under the conditions of Shiramizu development needs are required to answer critical questions
and Toste, 67% BD yield was noticed with the aforementioned regarding the technical, viz. process (operational) costs, and
bulky catalyst (entry 11), underlining the importance of solvent economic, viz. feedstock price and BD price, feasibility to
and the reductant in DODH reactions, as well as the appropri- integrate a bio-derived BD in the chemical industry.
ateness of using a sacrificial alcohol such as 3-octanol.221,223,234 Ethanol can be converted to BD. This route has been
As mentioned before, the four-carbon sugar-based feedstock practised on the industrial scale long before fossil oils and gases
is currently not produced on a large scale and certainly not on found general use. The industrial production of bio-ethanol is under
the scale requested for BD production. Therefore, it seems extensive development, enabling production of large amounts of
unlikely that it will become shortly a viable feedstock for BD ethanol in the near future. Other encouraging developments show
via DODH chemistry. Alternatively, the development has been ethanol production from syngas, either derived from natural gas,
reported for new routes to tetritols via decarbonylation,236 or to waste gas or biomass feedstock. Also methanol carbonylation,
tetroses via decarboxylation237 of larger carbohydrates such as followed by acetic acid hydrogenation might turn out promis-
pentoses and hexoses and via aldol condensation of glycolalde- ing for ethanol production. Meanwhile, fermentation became a
hyde.173 Pentose and hexoses are abundantly present in (hemi-)- very competitive process to ethanol.
cellulosic biomass,238–240 while glycol aldehyde is a dominant The conversion of ethanol to BD is thermodynamically
compound in bio-oils from biomass pyrolysis.173,241,242 Interest- feasible and proceeds via a sequence of reactions, receiving
ingly, the elegant DODH approach was also performed with general acceptance, viz. ethanol dehydrogenation, aldol con-
glucose-derived sugar alcohols such as mannitol and sorbitol, densation, MPV reduction, and dehydration. Clearly, a subtle
leading to a hexatriene as the major product.234 Such hexitol balance of the four reactions is required to achieve high
feedstocks can be directly derived from cellulosics.175,243,244 BD yield.
A second issue of current DODH next to feedstock avail- The literature overview indicates that a strong base like MgO
ability and cost lies in the use of the expensive complex Re or ZnO is essential to catalyse the aldol coupling, while Ag and
catalysts. Taking into account their turnover rate, their use on a CuO are good candidates to assist the dehydrogenation step.
large scale appears uneconomical for the production of BD. The catalytic system tolerates the presence of weak Brønsted
Note however that NH4ReO4 has a significantly lower cost.223 acidity, e.g. surface hydroxyls of silica, alumina or clays, cata-
For an industrial breakthrough of the DODH approach, lysing dehydration, though its amount and strength should be
research should be directed to the development of stable and limited, to avoid the thermodynamically favourable ethylene as
practical heterogeneous catalysts. Denning et al. have recently the side-product. The activity of the MPV hydrogen transfer
ventured into this area, with the synthesis of a carbon sup- reaction is likely most difficult to understand at the molecular
ported perrhenate DODH catalyst.224 In addition, the solvent- level, but it is related to the presence of Lewis acid sites,
less distillation of Ji et al.,214 which was mainly applied to on incompletely coordinated Mg2+ in MgO and some clays or on
convert glycerol, could be envisioned occurring in a full con- Al3+ in alumina and clays. The addition of ZrO2 is an interesting
tinuous mode. It seems that such open and continuous DODH option to assist aldol condensation and MPV reduction.

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Current BD selectivity with such catalysts ranges from 50 to (redox, acid–base) properties, is needed, ultimately helping
80%, the latter being obtained at modest conversion, thus improving overall catalytic performance.
showing room for improvement and development. Hydrocarbons Next to ethanol, there have been recent developments in
like ethylene and butenes and oxygenates like acetaldehyde are producing bio-derived 4-carbon alcohols, like n-butanol, iso-
produced, thereby reducing total BD selectivity. While ethylene butanol and various butanediols such as 1,3-BDO, 1,4-BDO and
may be valorised, the formation of butenes is unfortunate 2,3-BDO using fermentation of sugars. Also fermentation of
because of separation issues with BD. Another challenge of the syngas from natural gas or biomass has been demonstrated to
ethanol-to-BD reaction consists in its low volume productivity provide some of the diols. Dehydration of the monohydric
and often low BD concentration in the product stream. Current alcohols provides bio-derived butenes, which could be dehydro-
values range from 50 to 400 gBD h 1 per litre catalyst volume, with genated to BD, preferably in the presence of oxidants to
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values below 100 being reported most frequently. Thus one faces compensate for the endothermicity. Availability of cheap alco-
the challenging task to develop more active and selective catalysts hols will allow commercialization of the ODH technology.
at higher ethanol concentration. A low ethanol conversion should Dehydration of butanediols to BD has received increasing
not always be an inconvenience per se as efficient recycling of interest given the availability of bio- and gas-derived BDO’s.
unconverted ethanol is optional, as well as a product recycle since Each diol has its own unique features and challenges. Dehydra-
many by-products can be formed from the various intermediates tion of 2,3-BDO faces a facile pathway to butan-2-one, following
in the reaction sequence. Besides recycling reactors, reactor types a pinacol-type rearrangement. Whereas higher temperatures
like a fluidized bed could favour the cascade of consecutive will stimulate BD formation thermodynamically, kinetic control
reactions to BD due to better heat supply and back-mixing;108 is key to achieve high BD yields. This remains the key challenge
there has been no systematic study of the influence of the reactor for the catalytic chemist, especially with the renewable produc-
design and reactor type on the production of BD from ethanol. tion of 2,3-BDO just around the corner. For as such unknown
Most of the research and catalyst developments were devoted to reasons, a ThO2 catalyst currently delivers the best catalytic
experiments in plug flow reactors. performances.
There is also little information available about the importance Dehydration of 1,4-BDO suffers from the production of THF.
of the purity of the ethanol for BD production. Sporadically, a Again, use of higher temperatures is part of the thermodynamic
beneficial effect of water is reported, possibly as a dilute of ethanol solution, although kinetics to a major extent still determine the
or as a reactant that poisons strong acid–base sites. Others selectivity. THF can be further processed into BD, albeit at high
reported on the usage of diethyl ether in the feed stream as a temperatures. There is another challenging route proceeding
better reagent for BD production. The presence of impurities of according to a double dehydration with intermediate formation
larger alcohols like isoamyl alcohol in the bio-ethanol feed has not of 3-buten-1-ol. Suitable catalysts are neutral to basic like CeO2
been investigated in the context of on-purpose BD production. and Yb2O3. Combination of effective catalysis with the recent
Catalysts of particular interest for further improvement breakthrough in fermentative 1,4-BDO production is a route to
would include a combination of the abovementioned elements. BD that probably will be picked up in the very near future.
Catalyst preparation and pretreatment techniques are very Dehydration of 1,3-BDO is a classical route, as this diol was
important in order to maintain high dispersion and ideal the key intermediate in the acetylene-based industrial BD
mixtures/phases of the different functions. Structural modifiers production long time ago. Therein, cheap Na–polyphosphate
are likely required to stabilize the high dispersion against sinter- catalysis allowed good catalytic dehydration results.2 Alternatively, a
ing. Scalability and cost of catalyst preparation/modification are two-step reaction with CeO2 may be envisioned producing selec-
important factors to consider as well with these multi-component tively 2-buten-1-ol and 3-buten-2-ol, followed by their dehydration
catalysts. to BD using common Brønsted acid catalysts like silica–alumina.
Whereas whole series of catalyst compositions have been However, the bottleneck in this approach resides in the develop-
tested, many reports unfortunately lack thorough physicochemical ment of a renewable production of 1,3-BDO.
characterization and kinetic approaches defining the ideal Finally, the pioneering work of deoxydehydration of butane-
catalyst in terms of acid–base and redox properties. Studies of diols and tetritols is an elegant chemistry to produce BD.
individual reaction types in the conditions of BD formation are Although this route is probably far from commercialisation,
highly welcome,90,245,246 but this is not always possible due to it shows that creative thinking could lead to original catalytic
the thermodynamic constraints. They will form a first step in pathways towards useful drop-in chemicals from bio-derived
the deeper understanding of the correlation between the active chemicals. Development of cheap catalysts is essential though,
sites. A better understanding of the adsorption phenomena and and perhaps heterogeneous catalysis may offer the solution.
surface chemistry is crucial, especially of the critical reaction
steps determining selectivity and rate limiting events. Advanced
(in situ and operando) spectroscopic tools are available today to Acknowledgements
better monitor the reaction processes at the microscopic and
molecular level and the catalyst behaviour with nanoscale E.V.M. acknowledges the KU Leuven (Spiro fellowship) for
resolution under reaction conditions. Crucial information on financial support. M.D. thanks the Research Foundation
the overall catalyst behaviour, in terms of textural and chemical Flanders (FWO Vlaanderen) for a postdoctoral fellowship.

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