Conversion of sugars to ethylene glycol with nickel tungsten carbide in a fed-batch reactor: High productivity and reaction network elucidation

Article abstract of DOI:10.1039/c3gc41431k

Bifunctional nickel tungsten carbide catalysis was used for the conversion of aqueous sugar solutions into short-chain polyols such as ethylene glycol. It is shown that very concentrated sugar solutions, viz. up to 0.2 kg L ^-1, can be converted without loss of ethylene glycol selectivity by gradually feeding the sugar solution. Detailed investigation of the reaction network shows that, under the applied reaction conditions, glucose is converted via a retro-aldol reaction into glycol aldehyde, which is further transformed into ethylene glycol by hydrogenation. The main byproducts are sorbitol, erythritol, glycerol and 1,2-propanediol. They are formed through a series of unwanted side reactions including hydrogenation, isomerisation, hydrogenolysis and dehydration. Hydrogenolysis of sorbitol is only a minor source of ethylene glycol. To assess the relevance of the fed-batch system in biomass conversions, both the influence of the catalyst composition and the reactor setup parameters like temperature, pressure and glucose addition rate were optimized, culminating in ethylene glycol yields up to 66% and separately, volume productivities of nearly 300 g_EG L^-1 h^-1.

Full text of DOI:10.1039/c3gc41431k

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Conversion of sugars to ethylene glycol with
nickel tungsten carbide in a fed-batch reactor:
high productivity and reaction network elucidation
Cite this: Green Chem., 2014, 16,
695
Roselinde Ooms,a Michiel Dusselier,a Jan A. Geboers,b Beau Op de Beeck,a
Rick Verhaeven,a Elena Gobechiya,a Johan A. Martens,a Andreas Redlc and
Bert F. Sels*a
Bifunctional nickel tungsten carbide catalysis was used for the conversion of aqueous sugar solutions into
short-chain polyols such as ethylene glycol. It is shown that very concentrated sugar solutions, viz. up to
0.2 kg L−1, can be converted without loss of ethylene glycol selectivity by gradually feeding the sugar
solution. Detailed investigation of the reaction network shows that, under the applied reaction conditions,
glucose is converted via a retro-aldol reaction into glycol aldehyde, which is further transformed into
ethylene glycol by hydrogenation. The main byproducts are sorbitol, erythritol, glycerol and 1,2-propane-
diol. They are formed through a series of unwanted side reactions including hydrogenation, isomerisation,
hydrogenolysis and dehydration. Hydrogenolysis of sorbitol is only a minor source of ethylene glycol. To
assess the relevance of the fed-batch system in biomass conversions, both the influence of the catalyst
composition and the reactor setup parameters like temperature, pressure and glucose addition rate were
optimized, culminating in ethylene glycol yields up to 66% and separately, volume productivities of nearly
Received 19th July 2013,
Accepted 15th October 2013
DOI: 10.1039/c3gc41431k
www.rsc.org/greenchem
300 gEG L−1 h−1
.
demand for polyesters.28,29 Nowadays, ethylene glycol is pro-
duced mainly from ethylene, a petrochemical or bioethanol-
Introduction
Biomass has great potential as a substitute for fossil feedstock derived product.30 An alternative route, namely the direct cata-
for the renewable production of transportation fuels and lytic conversion of cellulosic biomass to ethylene glycol, can
industrially important chemicals. It is estimated that more reduce the dependence of the polymer and antifreeze indus-
than 90% of the global annual amount of terrestrial plant tries on petroleum or multi-step processes, enabling the pro-
biomass produced through photosynthesis – corresponding to duction of packaging materials from renewable sources.
57 × 109 tonnes of elemental carbon – is not digestible by
The formation of ethylene glycol from (reduced) sugars has
humans and this renewable resource may thus be used for fuel been reported since 1935.31–41 Sorbitol, the hydrogenation
or chemical purposes without competition with the food- product of glucose, was often used as feed and catalytically
industry.1–4 In particular, cellulose – a β-1,4-homopolymer of converted into a mixture of glycerol, ethylene glycol and 1,2-
glucose – has been highlighted recently as a suitable and propanediol. Although product distributions varied with the
abundant carbon source for the production of chemicals.5–27
catalyst type, additives like CaO and reaction conditions
Ethylene glycol is a valuable product used in antifreeze (pressure, substrate concentration and temperature), 1,2-
liquids and as a precursor for polymers such as polyethylene propanediol always appeared to be one of the most dominant
terephthalate (PET). The global annual production of ethylene products. It was only since the recent discovery by Ji et al.,42,43
glycol is experiencing a tremendous increase due to the rising who reported the direct catalytic conversion of cellulose to
ethylene glycol, that the selective formation of ethylene glycol
was conceivable. In their first publication, they noted a
aCenter for Surface Science and Catalysis, KU Leuven, Kasteelpark Arenberg 23,
3001, Heverlee, Belgium. E-mail: bert.sels@biw.kuleuven.be; Fax: (+32) 1632 1998;
Tel: (+32) 1632 1593
remarkable 61% yield of ethylene glycol with a nickel-
promoted tungsten carbide catalyst in a one-pot aqueous
batch reaction of 30 min at 518 K. In later studies,44,45 the
yield was further increased up to 75% by using different
carbon supports with a better pore structure. Others recently
reported the use of near-stoichiometric amounts of WO3 in
bMax-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim
an der Ruhr, Germany
cTEREOS SYRAL SAS, Z.I et Portuaire B.P.32, F-67390 Marckolsheim, France.
E-mail: andreas.redl@tereos.com; Tel: +33388581615
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combination with Ru on carbon for the conversion of sugars other identified intermediates, viz. products of hydrogenolysis
like glucose to 50% ethylene glycol.46 Based on cellulose con- and dehydration like 1,2-propanediol, 5-HMF and sorbitan,
version with tungstic acid (H2WO4) and Ru on carbon, the sug- was evaluated as well. Through careful analysis of the product
gestion was made by Tai et al.47 that dissolved HxWO3 is the distributions of the different experiments and accurate deter-
active component responsible of catalyzing the C–C bond clea- mination of the carbon mass balance in liquid and gas, we
vage. Those findings opened the way for other combinations of here confirm that there are two reaction paths to ethylene
hydrogenation catalysts and tungsten-based catalysts.48,49 glycol: (i) in the major route, glucose undergoes a selective
Unfortunately, low substrate concentration (often less than retro-aldol reaction to glycol aldehyde and a tetrose, the latter
10 g L−1 in water) and sometimes complicated catalyst is then cleaved as well via retro-aldol into two additional glycol
synthesis, in casu nickel tungsten carbide, could hamper aldehyde units which lead to ethylene glycol via hydro-
industrial upscaling in its present form. The mechanism of genation; (ii) unselective hydrogenolysis of reduced sugars
C–C bond cleavage leading to ethylene glycol formation and (ranging from triols to hexitols) also produces some additional
the intermediates involved are not yet fully elucidated. While a ethylene glycol. Byproducts are mainly formed through hydro-
direct hydrogenolysis mechanism was mentioned in the first genation of glucose (and lower monosaccharides) to stable sor-
reports, recent research by Zhang and coworkers points to a bitol (and corresponding polyols) and their unselective C–C
selective retro-aldol mechanism at the origin of the C–C hydrogenolysis.
cleavage, followed by hydrogenation.45,48,50
With the acquired mechanistic understanding, reaction
In the frame of an industrial collaboration, we thoroughly conditions, reactor set-up and catalyst composition were
investigated the bifunctional Ni tungsten carbide system to adapted to stimulate retro-aldol, while suppressing glucose
convert renewable feedstock to ethylene glycol. Glucose hydrogenation and dehydration and polyol hydrogenolysis.
instead of cellulose was used, as highly concentrated cellulosic Ethylene glycol yields up to 66% were ultimately achieved from
suspensions are very difficult to handle in state-of-the-art highly concentrated glucose syrups, while at shorter reaction
industrial processing. A fed-batch system was selected as this times, a staggering ethylene glycol productivity of 293 g L−1
reactor type simulates the slow release of glucose from cellu- h−1 was reached, though somewhat at the expense of the ethy-
lose through hydrolysis in a batch reaction, ensuring low lene glycol yield. Carbon mass balances approached 90 mol%
glucose concentration. As such, degradation of thermolabile in most cases after analysis of C in both gas and liquid phases.
glucose is prevented, while still allowing the ability to pump in Preliminary reuse experiments indicated that the catalyst
highly concentrated glucose syrups, viz. up to 333 g L−1 in this system could be recycled with acceptable losses in ethylene
work. Moreover, such concentrated glucose streams of indus- glycol yield and limited W-leaching in the cold reaction filtrate.
trial grade are available today at a reasonable price, whereas in Useful byproducts are sorbitol, erythritol/threitol, 1,2-propane-
the future, they could become cheaper via the hydrolysis of cel- diol and 1,2-butanediol.
lulose in second generation biorefineries. Indeed, the release
of glucose from cellulose is becoming increasingly viable with
solid acid catalysts and enzymes, foreseeing glucose availa-
Materials and methods
Catalyst preparation
bility on a large scale.11,22,51,52 Therefore, processes that
convert glucose into valuable chemicals53 – e.g. via fed-batch
processing50 with solid catalysts, as reported in this contri- The preparation of the tungsten carbide catalysts, viz. 2% Ni–
bution for ethylene glycol – will be of importance to fuel the 30% W2C/AC-973 (W2C = tungsten carbide, AC = activated
economic feasibility of the future biorefineries. Very recently, carbon, 973 = reduction temperature in K during pretreatment)
Zhang and coworkers reported an engineering study using a was adapted from the procedure described by Ji et al.:42 2 g of
semi-continuous setup for producing ethylene glycol from activated carbon support (Darco KB-B, surface area 1500 m2
aqueous glucose (final concentration 50 g L−1) with the combi- g−1) was impregnated with 3 ml of an aqueous solution con-
nation of a homogeneous tungsten catalyst and heterogeneous taining 0.315 g Ni(NO3)2 hydrate and 1.177 g (NH4)6H2W12O40
Ru on carbon.50
and dried overnight at 493 K. The dried powder was reduced
Besides enabling the use of a highly concentrated glucose in a pure hydrogen flow (at 1600 ml g−1 h−1 contact time)
feed (final concentration 200 g L−1), the fed batch set-up in according to the reported procedure. However, after reduction
our study is also ideal to unambiguously clarify the reaction the catalyst was not passivated in a 1% O2/N2 flow for
network of ethylene glycol formation by performing a series of 12 hours, but directly transferred into the reactor under a
experiments with different substrates. In this way, the conver- nitrogen atmosphere. The synthesis of this catalyst is very
sion of any unstable intermediate is easily assessed because its subtle and the above procedure should be followed puncti-
degradation is avoided as a result of the limited contact time. liously to obtain reproducible data. Particular attention should
Three groups of intermediates were examined: aldoses be paid that the catalyst is not exposed to air after the
(ranging from trioses to hexoses), ketoses and the reduced reduction step. Powder XRD (X-ray diffraction) patterns were
forms of these sugars such as sorbitol, xylitol, erythritol/ recorded on a STOE STADI P Combi diffractometer with
threitol, glycerol and ethylene glycol. Using the latter provides an image plate position sensitive detector (IP PSD) in the
insight into its stability as it is the desired product. The fate of region 2θ = 10 to 60° (Δ2θ = 0.03°). The measurements were
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performed in transmission mode at room temperature using 7673 auto sampler, a 50 m Poraplot Q column and a FID were
CuKα1 radiation with λ = 1.54056 Å selected by means of a used for a better separation of the C1 to C3 compounds such
Ge(111) monochromator. The ICDD database PDF-2, release as methanol, ethanol, ethylene glycol and 1,2-propanediol.
2008 was used for identification of the phases present in the Long-chain polyols (C4, C5 and C6) were additionally quanti-
sample.54 The reflections at 2θ = 34.5°, 38.2°, 39.6° and 52.4° fied after derivatisation of the reaction products via silylation9
indicate the presence of the W2C-phase [01-079-0743]. This is and analysed with a Hewlett Packard 5890 Series II GC
in agreement with the W2C reflections measured by Ji et al.42 equipped with a 50 m CP-Sil-5CB column and FID. All reported
Occasionally an additional reflection at 2θ = 40.5° appeared as product yields are expressed as the molar fraction of carbon
well, which is tentatively assigned to metallic W [01-089-2767]. (mol% C) represented by that product, relative to the total
However, the presence of this reflection entailed no noticeable amount of carbon introduced in the reactor. ICP-OES measure-
effect on the activity of the catalyst. The presence of NiW ments for W content determination in the reaction filtrate
[00-047-1172] is evidenced by a few reflections with the stron- were measured at 224.88 nm. Samples were hereto filtered and
gest one at 2θ = 43.5°.
sodium hydroxide was added. Volumetric productivity values
were calculated by dividing the total weight of ethylene glycol
formed by the time of reaction and the total liquid reaction
Catalytic reaction
In a typical batch experiment, the 100 ml stainless steel reactor volume (in g L−1 h−1).
(Parr Instruments Co.) was loaded with 0.8 g catalyst (2% Ni–
30% W2C/AC-973) and 50 ml of the 200 g L−1 aqueous glucose
solution. After flushing with hydrogen, the reactor was pres-
Results and discussion
Performance of batch vs. fed-batch reactor set-ups
surized with 6.0 MPa hydrogen and heated to 518 K under con-
stant stirring at 750 ppm. The moment at which the reactor
reached a temperature of 518 K was taken as the starting point
of the reaction. After one hour, the reactor was quickly cooled
in an ice bed. Samples were taken from the cold reaction solu-
tion after opening the reactor.
In this work, we studied the possibility of adapting the cellu-
lose to ethylene glycol reaction system initiated by Zhang and
coworkers42,43,50 to a fed-batch reactor using concentrated
glucose syrups to achieve high ethylene glycol volume produc-
tivities. In an initial experiment we benchmarked the glucose
to ethylene glycol conversion in the fed-batch reactor against
that in the reference batch reactor. Table 1 shows that the
batch experiment starting from a 200 g L−1 glucose solution
results in less than 10% ethylene glycol yield. The dominant
In a typical fed-batch experiment, the reactor was loaded
with 0.8 g catalyst and 20 ml water. After flushing with hydro-
gen, the reactor was pressurized with 6.0 MPa hydrogen and
heated to 518 K. After reaching the reaction temperature, the
sugar solution was fed gradually into the reactor with a Waters
515 HPLC pump. During the course of the reaction, 30 ml of
an aqueous 333 g L−1 (D-)glucose syrup was added at a constant
flow rate of 0.167 ml min−1, leading to a total reaction time of
3 hours. In this way, a total of 10 g glucose and 50 ml water
were present in the reactor, corresponding to a 200 g L−1 feed
concentration in this work. Other addition rates were evaluated
as well and they lead to different reaction times, e.g.
15 minutes in the case of a feeding rate of 2 ml min−1. After
introducing the entire glucose solution, the reactor was cooled
and the samples were taken in the same way as in the batch
experiment. The fed-batch recycling experiments were per-
formed with 0.8 g 2% Ni–30% W2C/AC starting with 4.5 MPa
H2 pressure and a total reaction time of 1 hour. At the end of
the reaction, the vessel was cooled and the catalyst was recove-
red by centrifugation (50 min, 10 000 rpm) and subsequent
decantation. Then, the catalyst was washed with 50 ml of H2O,
followed by centrifugation/decantation. The wet catalyst was
transferred into the reaction vessel with 20 ml of water for the
2nd run under identical reaction conditions. The recycling pro-
cedure was repeated for the 3rd and 4th runs.
Table 1 Comparison of product distribution and yields for the conver-
sion of glucose in a batch versus fed-batch reactor
Reactor set-up
Batcha
100
Fed-batchb
100
Conversion
Yield of products
Sorbitol
28
1
10
3
3
0.2
0.2
6
2
3
3
4
0.7
47
0.3
0.4
83
Mannitol
Sorbitan isomers
Xylitol
Arabinitol
20
3
0.5
3
Erythritol
Threitol
1,2-Butanediol
Glycerol
0.5
2
3
1,2-Propanediol
n-Propanol
Ethylene glycol
Ethanol
Methanol
Carbon balance
5
2
8
0.1
0.3
76
a Conditions batch set-up: the reactor was loaded with 50 ml H2O, 10 g
glucose and 0.8 g 2% Ni–30% W2C/AC-973, pressurized with 60 bar H2
and kept at 518 K during 3 hours. b Conditions fed-batch set-up: the
reactor was initially loaded with 20 ml H2O and 0.8 g 2% Ni–30% W2C/
AC-973, pressurized with 60 bar H2 and heated to 518 K. In 3 hours,
30 ml of a 333 g L−1 aqueous glucose solution was pumped into the
reactor vessel bringing the overall feed concentration of glucose to
Analysis of the reaction products
The reaction products were analysed using an Agilent 1200
Series HPLC equipped with a Varian MetaCarb 67C column
and a RID. Millipore water was used as a mobile phase. In
addition, a Hewlett Packard 5890 GC, equipped with a HP
200 g L−1
.
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products are reduced C6 sugars such as sorbitol and their de- a glucose molecule into 3 ethylene glycol molecules formally
hydrated forms such as sorbitan. Besides the products listed requires six hydrogen atoms, the reaction in water needs to be
in Table 1, a significant amount of char was found in performed under hydrogen pressure. As such, one obvious
the reactor vessel after reaction. This char is likely formed reaction is the direct hydrogenation of glucose to sorbitol. Its
through the decomposition of glucose at high reaction subsequent hydrogenolysis could lead to ethylene glycol, as
temperatures.55–57 Knežević et al.55 also reported char for- was suggested earlier.31–40 If this is the main route to ethylene
mation from glucose in hot compressed water. A known route glycol, the Ni tungsten carbide catalyst should have a certain
to char is the condensation of sugars and their dehydration unique hydrogenolytic activity, mainly producing ethylene
products like 5-hydroxymethylfurfural (5-HMF) to oligomeric glycol instead of 1,2-propanediol and glycerol. Previous
and polymeric fractions, which eventually become insoluble. research on glucose decomposition in hot-compressed water
High glucose concentrations are more prone to such 5-HMF suggested the occurrence of three other chemical
and char formation.58 In this respect, it is important to note reactions:56,58–60 dehydration to 5-HMF (and others), cleavage
that no such amount of char is observed in a typical batch to smaller compounds through retro-aldol reaction and
reaction starting from cellulose instead of glucose.42,43,50 This aldose–ketose isomerisation. These reactions could all
observation is likely explained by the gradual release of somehow play a role in the formation of ethylene glycol.
glucose through rate determining hydrolysis of the recalcitrant
Of the four reaction types, dehydration of glucose will be
cellulose. In accordance with the literature, microcrystalline considered first. This reaction has been reported to produce
Avicel cellulose (1 wt% in water) was fully converted after mainly 1,6-anhydroglucose (AHG) and 5-HMF.56,58,60,61
24 hours at 518 K in our setup, yielding 59 wt% ethylene glycol However, no considerable amounts of AHG or 5-HMF were
at a carbon mass balance of 78 mol%. The issue of char for- detected in our batch nor fed-batch reaction, as shown in
mation emphasises the importance of a low glucose concen- Table 1. Their absence could indicate fast condensation/
tration and implies that a batch reactor setup is not ideal to polymerization under the applied reaction conditions, leading
achieve high volume productivities.
to char. Taking into account the mass balance, there will
The proposed benefits of the fed-batch reactor strategy are indeed be some char formed in the fed-batch set-up, but
clear from the data in Table 1: the ethylene glycol yield additional experiments using 5-HMF and AHG as a feed did
increases six-fold to 47% at the expense of reduced (and sub- exclude their role in ethylene glycol synthesis. Glucose dehy-
sequently dehydrated) sugars, while the mass balance dration was thus dismissed as a primary reaction pathway to
increases from 76% to 83%, when compared to the batch ethylene glycol. The reaction is thus considered as an undesir-
reactor experiment. To close the mass balance, gas phase and able side-reaction that needs to be avoided as much as pos-
solid residues were analyzed as well, but both pointed to less sible to improve the carbon efficiency. In other words, the
than 0.5 mol% C content of the input carbon. This indicates catalyst preferably contains no (strong) acidity so as to circum-
little char formation.
vent such catalyzed dehydration.
The fed-batch setup in Table 1 produces about 32 g L−1 h−1
The possible primary reaction pathways for converting
ethylene glycol. To explain the tremendous difference between glucose to ethylene glycol that should still be considered are
fed-batch and batch, a true understanding of the reaction hydrogenation (potentially followed by dehydration and/or
network was ambiated. Afterwards, the fed-batch system was hydrogenolysis), retro-aldol and aldose–ketose isomerisation.
further optimized with regard to temperature, pressure and These pathways, visualized in Scheme 1, were further studied
feed rate.
in more detail by feeding their corresponding products in the
fed-batch reactor.
Hydrogenation. As can be seen from Table 1, sorbitol, its
dehydration products (sorbitan and isosorbide) and shorter
Investigation of the reaction network
Table 1 shows that sorbitol and erythritol are the main bypro- sugar alcohols like erythritol, glycerol and ethylene glycol are
ducts in the fed-batch reaction. Low amounts of mannitol, the main products of the conversion of glucose in fed-batch
threitol, glycerol, sorbitan isomers, 1,2-butanediol and 1,2- mode. As sorbitol is by far the most dominant byproduct,
propanediol were also formed (<5% yield), as well as trace direct hydrogenation of glucose occurs during reaction. More-
amounts of xylitol, arabinitol, n-propanol, ethanol and metha- over, the higher ethylene glycol yield in the fed-batch was
nol (<1% yield). To elucidate the whole reaction network accompanied with a decrease in sorbitol yield, when compared
including these products and to track the main reaction path- to the batch reaction. This means that sorbitol or its dehy-
ways towards ethylene glycol formation, these identified dration products could be key intermediates in the formation
chemicals were fed into the reactor under the same reaction of ethylene glycol via (un)selective hydrogenolysis. If not, it
conditions of Table 1, albeit at somewhat lower concentration. means that sorbitol (and its dehydrates) is a dead-end product,
For the network study, 30 ml of a 17 g L−1 substrate solution is which renders the fast and selective glucose hydrogenation to
gradually fed in the 100 ml reactor over 3 hours with constant be an undesirable competitive pathway. To distinguish the two
feed rate.
possibilities, pure isosorbide and a sorbitol dehydration
Four different reaction pathways starting from glucose were mixture consisting mainly of sorbitan isomers were fed into
considered a priori in the study. Firstly, since the conversion of the reactor. The data are collected in Table 2. In addition, the
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Scheme 1 Proposed primary reaction routes for the conversion of glucose to ethylene glycol.
Table 2 Distribution and yield of products for the conversion of sorbi-
tol dehydrates
Table 3 Distribution and yield of products for the conversion of
reduced sugarsa
Substrate
Sorbitol dehydration mixturea
23b
Isosorbide
19
Ethylene
Substrate
Sorbitol Xylitol Erythritol Glycerol glycol
Conversion
Product distribution
Sorbitol
Sorbitan isomers
Isosorbide
Ethylene glycol
Carbon balance
Conversion
Product distribution
Sorbitol
Mannitol
Sorbitan
Xylitol
Arabinitol
Erythritol
Threitol
Glycerol
1,2-Propanediol
n-Propanol
Ethylene glycol
Ethanol
Methanol
Carbon balance
68
55
51
15
7
9
54
7
0
70
32
2
5
0.4
0.8
1
81
0
81
45
3
5
1
5
11
3
15
0.2
0.7
89
Reaction conditions: fed-batch reactor loaded with 20 ml H2O and
0.8 g 2% Ni–30% W2C/AC-973, pressurized with 60 bar H2 and heated
to 518 K. a In 3 hours, 30 ml of a 17 g L−1 of the sorbitol dehydration
mixture (>75% of sorbitan) was pumped into the reactor vessel.
Product distribution and carbon balance calculated on the total
amount of sorbitan isomers, sorbitol and isosorbide in the reactor.
b Conversion of sorbitan.
49
9
2
13
—
15
—
2
1
8
85
8
0.8
2
—
3
99
14
6
14
0.2
0.4
85
93
—
1
90
94
a Reaction conditions: fed-batch reaction; reactor loaded with 20 ml
H2O and 0.8 g 2% Ni–30% W2C/AC-973, pressurized with 60 bar H2
and heated to 518 K. In 3 hours, 30 ml of a 17 g L−1 substrate solution
was pumped into the reactor vessel.
product distribution from reactions with pure sorbitol and
shorter reduced sugars like xylitol, erythritol, glycerol and
ethylene glycol were examined as well and reported in Table 3.
The dehydrated sorbitol products are not very reactive
under the applied reaction conditions as reflected by the mod-
erate conversion. Since no ethylene glycol or other degradation
products are detected from the conversion of sorbitan isomers
(in the sorbitol dehydration mixture) and isosorbide, it is clear
that these compounds are not at play in the formation of
ethylene glycol. For both reactions, low mass balances were
measured, probably due to polymerisation reactions, while no
other products were detected. The latter is in agreement with
an earlier report on the conversion of cellulose and sorbitol
to isosorbide which reports the formation of insoluble
byproducts.25
much more stable under the applied reaction conditions. Such
high product stability is of course advantageous for the fed-
batch reaction with regard to ethylene glycol production. Con-
version of glycerol reveals 1,2-propanediol as the main
product. It was previously reported62,63 that glycerol can be
converted via dehydration to hydroxyacetone and subsequent
hydrogenation to 1,2-propanediol. The conversion of erythritol
and xylitol, reduced sugars with a C4 and C5 backbone
respectively, results in approximately equal amounts of C2 and
C3 compounds. This product distribution differs from that of
the reaction with sorbitol as C3 compounds were analysed as
the main products there, with a total selectivity of 53%.
Because ethylene glycol is not the main product from sorbitol
(16% selectivity at 14% yield), its direct formation from
The conversion of the set of reduced sugars (see Table 3)
reveals that their stability increases with decreasing carbon
chain length: while sorbitol is slowly converted into other pro-
ducts, ethylene glycol – the desired product in this study – is
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sorbitol is not considered as the major pathway. Indeed, if
In parallel, dehydration products like sorbitan isomers
ethylene glycol originated primarily from direct hydrogenolysis (mainly 1,4-sorbitan) are formed as well, further leading to iso-
of sorbitol, a reaction starting from sorbitol would be expected sorbide and polymerization products as shown in Table 2.
to yield more ethylene glycol than a reaction starting from Thus, despite some ethylene glycol formation from sorbitol,
glucose, which is not the case (compare data in Table 3 vs. the hydrogenation of glucose to sorbitol is clearly not a domi-
Tables 1 and 4). Moreover, the data from Table 3 show that sor- nant pathway to ethylene glycol in the catalytic system, which
bitol is converted through two different pathways (presented is in agreement with the report by Zhao et al.50 Moreover,
in Scheme 2). The dominant pathway is sorbitol C–C and C–O since sorbitol is significantly more stable than glucose under
hydrogenolysis, forming mainly C3 compounds such as gly- the applied reaction conditions, its formation through hydro-
cerol, 1,2-propanediol and n-propanol in addition to smaller genation should be avoided in the interest of maximizing the
amounts of ethylene glycol.
yield of ethylene glycol.
Table 4 Distribution and yield of products for the conversion of aldosesa
Substrate
Glucose
100
Mannose
100
Galactose
100
Xylose
100
Arabinose
Erythrose
100
Glyceraldehyde
100
Conversion
Yield of products
Sorbitol
Galactitol
Mannitol
100
9
—
3
4
—
8
—
15
—
2
Sorbitan
3
3
Xylitol
Arabinitol
Erythritol
Threitol
Glycerol
1,2-Propanediol
n-Propanol
Ethylene glycol
Ethanol
Methanol
Carbon balance
0.4
0.3
6
2
7
0.4
0.7
8
3
6
4
4
45
0.2
0.4
87
—
—
3
7
11
6
—
33
0.2
0.7
78
15
4
0.5
—
20
8
1
9
0.7
—
25
8
13
5
1
54
18
2
7
0.3
1
8
2
—
36
0.2
0.5
75
2
3
—
49
0.5
0.9
71
29
0.3
2
34
0.3
1
81
82
82
a Reaction conditions: fed-batch reaction; reactor loaded with 20 ml H2O and 0.8 g 2% Ni–30% W2C/AC-973, pressurized with 60 bar H2 and
heated to 518 K. In 3 hours, 30 ml of a 17 g L−1 substrate solution was pumped into the reactor vessel.
Scheme 2 Primary conversion routes for sorbitol.
700 | Green Chem., 2014, 16, 695–707
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Retro-aldol reaction. Retro-aldol reaction of glucose results aldol Cα–Cβ splitting (to threose and glycol aldehyde), followed
in glycol aldehyde and erythrose, which may be cleaved accord- by carbonyl hydrogenation.
ingly into two glycol aldehyde molecules. Subsequent hydro-
The conversion of the C5 aldoses – xylose and arabinose –
genation of glycol aldehyde leads to the formation of ethylene resulted in the formation of equal amounts of C2 and C3 pro-
glycol. Various aldoses such as galactose, xylose, arabinose and ducts. This is well in agreement with a retro-aldol pathway: the
glyceraldehyde, and ketoses such as fructose, erythrulose and aldoses are cleaved into glyceraldehyde and glycol aldehyde,
dihydroxyacetone were fed into the reactor to evaluate the retro- which yield glycerol and ethylene glycol upon hydrogenation.
aldol/hydrogenation reaction sequence as a possible route to Conversion of erythrose (a C4) through a sequential retro-aldol
ethylene glycol. The results of the aldoses and ketoses are col- and hydrogenation reaction is expected to yield mainly ethy-
lected in Tables 4 and 5, respectively. The high conversion of lene glycol. This is indeed supported by the data in Table 4,
these monosaccharides reflects their high reactivity in the reac- where ethylene glycol is the main product. Erythritol, formed
tion conditions. Aldoses for instance are quickly converted into via carbonyl hydrogenation of erythrose, is the main by-
more stable reaction products. Their high reactivity implies that product. Reaction with glyceraldehyde resulted in a fairly low
they will be absent in the final product mixture.
selectivity to ethylene glycol. Instead, glyceraldehyde was
Every aldose leads to detectable amounts of the corres- mainly hydrogenated to glycerol with some subsequent hydro-
ponding sugar alcohol originating from carbonyl hydroge- genolysis forming 1,2-propanediol and n-propanol.
nation. As discussed in the previous section, these reduced
Generally, the high selectivity for cleaving the α–β C–C bond
sugars can undergo unselective hydrogenolysis and dehy- with respect to the aldehyde group (e.g. see Scheme 1) strongly
dration reactions and their stability depends on their chain supports the retro-aldol cleavage mechanism, in agreement
length, with ethylene glycol as the most stable one. For these with some earlier studies.50,58,64,65
reactivity reasons, the amount of the corresponding sugar
alcohols that are retrieved from these sugars decreases with solutions undergo significant isomerisation under reaction
increasing chain length. conditions similar to ours, even in the absence of
Isomerisation. It has been previously observed that glucose
a
Carbonyl hydrogenation of galactose to galactitol seems catalyst.58,64,65 To assess the influence of aldose–ketose
slightly more competitive when compared to that of glucose isomerisation pathways such as that of glucose to fructose on
and mannose, while mannose led to the highest ethylene the product spectrum, the following ketoses were assessed as
glycol yield. This could indicate that selective C–C splitting is substrates in the fed-batch reactor: fructose, erythrulose and
sensitive to the conformation of the hydroxyls in the sugar dihydroxyacetone (DHA). The product distributions from
molecule, which are involved in the coordination with the cata- these reactions are shown in Table 5. Like the aldoses
lyst site. Interestingly, the product ratio of erythritol vs. threitol studied in Table 4, all ketoses are completely converted into
reverses when comparing the reaction outcome of mannose more stable reaction products including their respective
and glucose with galactose. The stereoselectivity of the higher sugar alcohols. Again, these sugar alcohols will undergo
threitol content with galactose agrees with the selective retro- further hydrogenolysis, forming C2 and C3 products.
However, closer inspection of the product distribution again
indicates that retro-aldol is the dominant C–C bond cleavage
mechanism.
Table 5 Distribution and yield of products for the conversion of
ketosesa
More specifically, the conversion of fructose results in
about 57% of C3 compounds such as glycerol, 1,2-propanediol
and n-propanol. This is consistent with the report of Zhang
and co-workers on the preferential conversion of inuline (= a
polyfructan) rich artichoke to 1,2-propanediol.66 In the conver-
sion of erythrulose, C3 compounds are also the main products,
but with a lower carbon selectivity of about 23%. Finally, the
conversion of dihydroxyacetone produces almost no smaller
fragments (>90% of all carbon remains contained in the
C3 fraction), glycerol and 1,2-propanediol being the main
products.
These observations are consistent with retro-aldol cleavage
between the α and β position relative to the carbonyl and lead
to a higher selectivity for C3 compounds in the conversion of
fructose compared to glucose: the C3/C2 product ratio
amounts to 8.8 for fructose, compared to 0.4 for glucose. This
difference in ratio unambiguously proves that retro-aldol Cα–Cβ
cleavage prevails over unselective hydrogenolysis, which
should otherwise yield similar ratios for both hexoses. Since
dihydroxyacetone does not have a reactive β position to the
Substrate
Fructose
100
Erythrulose
100
DHA
100
Conversion
Yield of products
Sorbitol
Mannitol
Sorbitan
Xylitol
Arabinitol
Erythritol
Threitol
1,2-Butanediol
Glycerol
1,2-Propanediol
n-Propanol
Ethylene glycol
Ethanol
Methanol
Carbon balance
5
5
2
0.2
0.2
0.6
0.4
—
24
28
5
10
9
13
2
21
0.4
10
0.4
5
66
25
1
6
1
0.5
0.6
78
0.1
0.4
94
71
a Reaction conditions: fed-batch reaction; reactor loaded with 20 ml
H2O and 0.8 g 2% Ni–30% W2C/AC-973, pressurized with 60 bar H2
and heated to 518 K. In 3 hours, 30 ml of a 17 g L−1 substrate solution
was pumped into the reactor vessel. DHA = dihydroxyacetone.
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Scheme 3 Main reaction pathways for the conversion of glucose. The blue box is the major and preferred route in this work.
carbonyl, no retro-aldol reaction can take place and any (summarized as well in Scheme 3) lead to C3 products, glucose
smaller (C1 and C2) compounds are expected to result from isomerisation will always result in an increase in C3 product
hydrogenolysis of glycerol. Since these compounds are almost selectivity.
absent, hydrogenolysis is not a preferred reaction pathway
Reaction network: summary. Scheme 3 summarizes the
under the applied reaction conditions. By comparing the data main reaction paths for the formation of ethylene glycol from
of erythrose (Table 4) and erythrulose (Table 5) conversion, glucose over the 2% Ni–30% W2C/AC-973 catalyst. Our fed-
differences in the product spectrum again support retro-aldol batch study includes the origin and faith of nearly 20 interest-
cleavage rather than random tetrol hydrogenolysis: up to 50% ing intermediates and side products and confirms the latest
of EG was obtained for erythrose compared to only 10% for mechanistic insights of earlier reports50 on this reaction. The
erythrulose. Interestingly, the previously reported (Lewis) acid- retro-aldol reaction is the dominating pathway, forming glycol
catalyzed conversion of tetroses to vinyl glyoxal, an interesting aldehyde and erythrose. Erythrose undergoes another retro-
precursor for novel polyester building blocks,26,67 was not aldol reaction forming two additional glycol aldehyde mole-
observed in the applied reaction conditions, possibly due to cules. Glycol aldehyde is subsequently hydrogenated very
the preferred retro-aldol reaction instead of the retro-Michael rapidly and selectively to ethylene glycol, which is fairly stable
dehydration68 in the presence of the Ni–W2C catalyst at high under the reaction conditions.
temperatures.
In light of previous studies of this catalytic system,45 we
In summary, isomerisation during the glucose to ethylene presume that the retro-aldol activity is attributed to the tung-
glycol reaction will have a profound influence on the final sten component of the catalyst and the hydrogenation activity
product distribution. Isomerisation of glucose to fructose will is associated with the metallic nickel component of the cata-
lead to byproducts according to two parallel pathways. Firstly, lyst. How the tungsten component of the catalyst stimulates
fructose is rapidly hydrogenated to sorbitol/mannitol, which the selective C–C bond cleavage is not yet fully understood,
undergoes further hydrogenolysis and dehydration reactions but there might be a link with the unique epimerization
as in Scheme 2. Alternatively, fructose undergoes a retro-aldol activity of tungsten and molybdenum oxides at slightly acidic
reaction, eventually leading to C3 products such as glycerol, pH, firstly reported by Bilik.69–73 Isotope labeling in glucose
1,2-propanediol and n-propanol. Since both of these pathways to mannose epimerization experiments demonstrated the
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involvement of a 1,2-carbon shift due to a catalyzed C–C clea- higher ethylene glycol yields and a reduced formation of sugar
vage between position α and β with respect to the aldehyde alcohols like sorbitol. The hypothesis is indeed in agreement
group.71 Probably, the high temperature applied in this work with the data shown in Fig. 1. Ethylene glycol yield increases
prohibits the reformation of the new C–C between the alde- with temperature up to 66 mol% C at 533 K, while lower reac-
hyde carbon and Cβ, that normally occurs in the epimerization tion temperatures favour the co-formation of sorbitol.
mechanism. However, more fundamental studies are required
to support this hypothesis.
Pressure. In the pressure range of 20–75 bar, hydrogenation
is observed to be first order with respect to hydrogen
Byproducts are mainly formed through direct hydrogen- pressure.74,76,77 This pressure dependency can be used to sup-
ation of glucose and the intermediate erythrose to sorbitol/ press the hydrogenation reaction, thereby limiting the sorbitol
mannitol and erythritol, respectively. These byproducts may yield. In addition, it has been observed that in supercritical
undergo further hydrogenolysis to smaller fragments. conditions, lower pressures favour retro-aldol reactions.78–80
Additionally, isomerisation of glucose to fructose also leads to Therefore, lowering the hydrogen pressure is expected to
the formation of byproducts, since the subsequent retro-aldol decrease the influence of the hydrogenation reaction and
reaction of fructose produces C3 sugars, dihydroxyacetone and increase the influence of the retro-aldol reaction, leading to
glyceraldehyde. These trioses are hydrogenated to glycerol, higher ethylene glycol yields. Fig. 2 shows that by lowering the
which leads to only small amounts of ethylene glycol as a hydrogen pressure from 60 bar to 45 bar, sorbitol yield
result of slow hydrogenolysis.
decreased and ethylene glycol yield slightly increased.
The mechanistic insights into the reaction network are However, a further decrease in hydrogen pressure to 30 bar
further used to rationalise the influence of reaction conditions
(temperature, pressure), fed-batch parameters (glucose
addition rate) and catalyst composition on the yield and
volume productivity of ethylene glycol.
Fine-tuning of reaction conditions
Temperature. As demonstrated in Table 1, sorbitol formed
by hydrogenation of glucose is the main side product. It is
therefore expected that promotion of the retro-aldol reaction
over the hydrogenation reaction will lead to an increase in
ethylene glycol yield. The expected difference in activation
energy between retro-aldol and hydrogenation reactions could
be exploited to achieve this promotion.
In an overview by Crezee et al.,74 the activation energy for
hydrogenation of glucose to sorbitol is reported to be highly
Fig. 1 Effect of reaction temperature on product yields and conversion.
Reaction conditions: fed-batch reaction; reactor loaded with 20 ml H2O
and 0.8 g 2% Ni–30% W2C/AC-973, pressurized with 60 bar H2 and
heated to desired temperature. In 3 hours, 30 ml of a 333 g L−1 glucose
dependent on the type of catalyst. Déschamp et al.75 reported
an apparent activation energy of 67 kJ mol−1 over a nickel cata-
lyst, while an apparent activation energy of 55 kJ mol−1 over a
■
▲
)
solution was pumped into the reactor vessel. ( ) Ethylene glycol, (
ruthenium catalyst was reported by Crezée et al.74 (both deter-
mined between 343 and 403 K). Based on these data, it can be
assumed that the apparent activation energy of hydrogenation
over the 2% Ni–30% W2C/AC-973 catalyst ranges somewhere
between 50 and 70 kJ mol−1. The activation energy of the retro-
aldol reaction was examined by Yoshida et al.65 They investi-
gated glucose decomposition in hot compressed liquid and
determined an activation energy of 126 kJ mol−1 for the trans-
formation of glucose into erythrose. Although this value may
be lowered somewhat by the presence of the 2% Ni–30% W2C/
AC-973 catalyst, the activation energy of the retro-aldol reaction
is still expected to be significantly higher than that of the
hydrogenation reaction since a C–C bond needs to be cleaved.
The recent paper by Zhao et al. confirms the differences in
energy barriers: they report 148 and 38 kJ mol−1 for the retro-
aldol and hydrogenation of glucose, respectively, under similar
conditions with a W based salt and Ru/carbon.50 A direct
consequence of the corroborated assumptions is that the
retro-aldol reaction should be favoured over the hydrogenation
◊
sorbitol, ( ) mass balance.
Fig. 2 Effect of initial pressure. Reaction conditions: fed-batch reac-
tion; reactor loaded with 20 ml H2O and 0.8 g 2% Ni–30% W2C/AC-973,
pressurized with an initial desired H2 pressure and heated to 518 K. In
3 hours, 30 ml of a 333 g L−1 glucose solution was pumped into the
■
▲
◊
reaction by increasing the reaction temperature, resulting in reactor vessel. ( ) Ethylene glycol, ( ) sorbitol, ( ) mass balance.
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resulted in a lower ethylene glycol yield and an unfavourable the great value of the fed-batch set-up for sugar and biomass
carbon mass balance. This can be attributed to parallel conver- conversions. For comparison, ethylene glycol productivities in
sion routes with the highly reactive glycol aldehyde,50,67 since glucose conversion reactions in a batch or fed-batch set-up
its hydrogenation to ethylene glycol is hampered due to the under the same reaction conditions (see Table 1) are 5 g L−1
lower hydrogenation capacity of the system under the con- h−1 and 32 g L−1 h−1, respectively. There, this six-fold diffe-
ditions. It can therefore be concluded that the optimal initial rence was caused merely by changing the reactor set-up,
hydrogen pressure is situated around 45 bar.
without further optimising the pressure, temperature and feed
Rate of glucose addition. A very important feature of a fed- addition rate. When glucose was gradually released from cellu-
batch set-up is the ability to vary the rate of substrate addition. lose by using a batch set-up, which Ji et al.42,43 achieved by
Fig. 3 shows the influence of the addition rate by adding 30 ml starting from 10 g L−1 cellulose in water, an ethylene glycol
of a 333 g L−1 glucose solution at varying flow rates, leading to yield of 61 wt% was obtained. This corresponds to an ethylene
different reaction times. A faster glucose addition rate logically glycol productivity of 12.2 g L−1 h−1. The fed-batch system not
increases the momentary glucose concentration in the reactor. only allows the use of a higher substrate final concentration,
Different glucose decomposition studies found a shift in the e.g. 200 gglucose L−1, additionally the substrate addition rate
reaction path by changing glucose concentration. The general can be readily adapted as well. By doing so, a maximum ethy-
findings are that dehydration, for instance ultimately to lene glycol volume productivity of approximately 25 times
5-HMF, is promoted at high glucose concentrations, while retro- higher than the productivity obtained with cellulose in a batch
aldol reactions are promoted at low glucose concentrations.
These trends are supported by the fed-batch data shown in
set-up, viz. 293 g L−1 h−1, was reached.
Continuous pressure. It was shown earlier that the optimal
Fig. 3 and from earlier kinetic studies:50 fast addition of hydrogen pressure of this system is situated around 45 bar at
glucose (high glucose concentration; right-hand side) leads to room temperature. After heating the reactor to 518 K, this
a moderate ethylene glycol yield and a relatively low carbon translates to a combined hydrogen and steam pressure of
mass balance. This is probably due to glucose dehydration and about 100 bar. In the interest of lowering the operating
decomposition reactions, forming insoluble char. Decreasing pressure of the set-up, we investigated the option of working
the rate of glucose addition leads to an increased ethylene under a continuous hydrogen supply. Fig. 4 shows that a con-
glycol yield and a higher carbon mass balance due to pro- stant reaction pressure at 60 bar is high enough to reach an
motion of the retro-aldol reaction over side reactions. However, ethylene glycol yield of approximately 55%, on par with the
at the lowest glucose addition rates, ethylene glycol yield stabi- ethylene glycol yield at 100 bar in the non-continuous reactor
lizes around 50% in the applied conditions.
system.
Catalyst composition. In the 2% Ni–30% W2C/AC-973 cata-
For industrial set-ups, the volumetric productivity (g L−1
h−1) of ethylene glycol is also an important factor, next to the lyst system, nickel is believed to catalyze the hydrogenation
classically reported molar yield. Increasing the rate of glucose reaction, while W2C catalyzes the retro-aldol reaction.45 Vari-
addition leads to ethylene glycol productivities near 300 g L−1 ation of the ratio of these two compounds is therefore a
h−1 (right-hand axis in Fig. 3) without drastically compromis- straightforward strategy to improve the interplay between the
ing the ethylene glycol yield (left-hand axis). Such high produc- hydrogenation and retro-aldol functionalities of the system,
tivities are unattainable for a batch process and typically prove provided that their close spatial proximity is not very crucial in
Fig. 3 Effect of glucose addition rate. Reaction conditions: fed-batch
reaction; reactor loaded with 20 ml H2O and 0.8 g 2% Ni–30% W2C/
AC-973, pressurized with 45 bar H2 and heated to 518 K. 30 ml of a
333 g L−1 glucose solution was pumped into the reactor vessel during a
Fig. 4 Effect of continuous pressure. Reaction conditions: fed-batch
reaction; reactor loaded with 20 ml H2O and 0.8 g 2% Ni–30% W2C/
AC-973. After reaching the reaction temperature of 518 K, the reactor
was pressurized with a continuous H2 pressure. In 1 hour, 30 ml of a
desired time. Left axis: ( ) ethylene glycol, ( ) sorbitol, ( ) mass balance.
Right axis: (×) ethylene glycol productivity.
333 g L−1 glucose solution was pumped into the reactor vessel. ( ) Yield
■
■
▲
◊
▲
◊
ethylene glycol, ( ) yield sorbitol, ( ) mass balance.
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Fig. 5 Effect of catalyst proportions. Reaction conditions: fed-batch
reaction; reactor loaded with 20 ml H2O, 0.8 g 30%W2C/AC-973 and the
desired amount of 2%Ni/AC-973, pressurized with 45 bar H2 and heated
to 518 K. In 1 hours, 30 ml of a 33 g L−1 glucose solution was pumped
■
▲
◊
into the reactor vessel. ( ) Yield ethylene glycol, ( ) yield sorbitol, ( )
mass balance.
Fig. 6 Yield of ethylene glycol and % of leached tungsten after each of
4 runs (relative to the W input in each run). Reaction conditions: fed-
batch reaction; reactor loaded with 20 ml H2O and 0.8 g 2% Ni–30%
W2C/AC-973. After reaching the reaction temperature of 518 K, the
reactor was pressurized with 45 bar H2. In 1 hour, 30 ml of a 333 g L−1
glucose solution was pumped into the reactor vessel.
the mechanism. In the reactions shown in Fig. 5, the catalytic
functions were present on two separate activated carbon sup-
ports and mixed physically in the reactor. In these experiments
the amount of nickel bearing catalyst is varied, while the tung-
sten catalyst content remains constant. The results confirm
the mode of catalyst operation proposed by Zheng et al.45
Hydrogenation activity decreased with decreasing nickel con-
tents, enabling high ethylene glycol yields up to 60%. On the
other hand, hydrogenation activity increased with nickel
content, resulting in increased sorbitol formation and a con-
comitant decrease of ethylene glycol yield. The clear dual cata-
lyst performance in these experiments is interesting, since it
demonstrates that the presence of closely associated active
sites on the same carrier is not a necessity. Spatial proximity of
the active species is not a determining factor for making ethy-
lene glycol.
system or continuous feed) and glucose addition rate. The
catalyst showed a gradual loss of activity.
The use of the fed-batch reactor allowed us to systematically
investigate the reaction network with identification of the
origin of nearly all byproducts. The importance of the retro-
aldol reaction, followed by hydrogenation, for the formation of
ethylene glycol was unambiguously confirmed in this reaction
network study. This mechanistic insight will open the way for
designing and developing new catalysts, which favour retro-
aldol reaction, while suppressing unwanted isomerisation,
dehydration and direct hydrogenation side reactions in the
reductive atmosphere.
The use of a fed-batch reactor entails many other advan-
tages for the conversion of glucose to ethylene glycol. While
simulating the release of glucose from cellulose upon hydro-
lysis during the reaction, gradual feeding of glucose in the fed-
batch reactor occurs in a more controlled way and allows
higher throughputs compared to real cellulosic biomass. For
the latter, it is known that hydrolysis rates are fluctuating due
to the heterogeneous nature of cellulose, with its crystalline
and amorphous parts, and the presence of impurities. Com-
pared to batch experiments, gradual addition of concentrated
aqueous glucose solutions results in high ethylene glycol selec-
tivities caused by a diminished hydrogenation to sorbitol and
a diminished isomerisation to fructose – both leading to
unwanted C3 compounds – and by bypassing the degradation
route to furanics (and ultimately to char) under the high reac-
tion temperature applied. The use of a fed-batch reactor is able
to provide a high volumetric productivity of ethylene glycol
and thus results in a highly concentrated ethylene glycol
product stream. The latter is important with regard to an
economically feasible separation of ethylene glycol from the
Catalyst reuse experiments. In order to assess whether the
heterogeneous catalyst can be recycled and reused for fed-batch
experiments, four successive runs were carried out. Between each
run, the catalyst was collected and washed with water before re-
use. As can be seen in Fig. 6, recycling of the 2% Ni–30% W2C/
AC-973 catalyst is possible with a gradual loss of ethylene glycol
yield in each successive run. No appreciable amount of W was
detected in cold reaction filtrates (see labels in Fig. 6).
Conclusions
Bifunctional nickel containing tungsten carbide catalysts, pre-
viously used for the conversion of cellulose to ethylene glycol,
are also very effective for the conversion of glucose in a fed-
batch set-up with an ethylene glycol yield ranging from 36 to
66% at full glucose conversion, corresponding to an ethylene
glycol volume productivity between 30 and 300 g L−1 h−1. The
final yield and volume productivity depend on the catalyst
composition, reaction temperature, hydrogen pressure (closed
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aqueous solution. However, as in the current ethylene oxide
based production, where glycols are already being separated
from water via distillation, this hurdle will be overcome.81
4 U. Roland, D. Sell and T. Hirth, Renewable Raw Materials:
New Feedstocks for the Chemical Industry, Wiley-VCH,
Weinheim, 2011.
Fine-tuning of the glucose addition rate resulted in
a
5 A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107,
2411–2502.
6 P. L. Dhepe and A. Fukuoka, ChemSusChem, 2008, 1, 969–975.
7 R. Rinaldi and F. Schüth, ChemSusChem, 2009, 2, 1096–
1107.
8 R. Rinaldi and F. Schüth, Energy Environ. Sci., 2009, 2, 610–626.
9 J. Geboers, S. Van de Vyver, K. Carpentier, K. de Blochouse,
P. A. Jacobs and B. F. Sels, Chem. Commun., 2010, 46, 3577–
3579.
maximum ethylene glycol productivity of 293 g L−1 h−1 at an
ethylene glycol yield of about 35% at 518 K and 45 bar partial
H2 pressure. Higher ethylene glycol yields around 66% were
obtained at higher temperature and lower flow rates. The influ-
ence of temperature, pressure and catalyst composition was
fully assessed. Interestingly, an optimal bifunctional catalyst
system was obtained by physically combining a balanced
amount of Ni on activated carbon with W2C on activated
carbon. Spatial proximity of both catalytic species seems there- 10 S. Van de Vyver, J. Geboers, M. Dusselier, H. Schepers,
fore of less importance.
T. Vosch, L. Zhang, G. Van Tendeloo, P. A. Jacobs and
In light of the use of glucose as a platform chemical in
B. F. Sels, ChemSusChem, 2010, 3, 698–701.
future second generation biorefineries, high productivity 11 S. Van de Vyver, L. Peng, J. Geboers, H. Schepers, C. F. de,
values could render the production of bio-derived ethylene
glycol from glucose more economically feasible. Fed-batch
C. J. Gommes, B. Goderis, P. A. Jacobs and B. F. Sels, Green
Chem., 2010, 12, 1560–1563.
reactor set-ups easily tolerate the use of various kinds of 12 R. Palkovits, K. Tajvidi, J. Procelewska, R. Rinaldi and
soluble biomass fractions such as sugar alcohols, ketoses and A. Ruppert, Green Chem., 2010, 12, 972–978.
aldoses. By replacing the initial feed composition with other 13 P. Gallezot, Top. catal., 2010, 53, 1209–1213.
available sugar blends like sucrose and hydrolysates of (hemi)- 14 J. Geboers, S. Van de Vyver, R. Ooms, B. Op de Beeck,
cellulose, alcohol product composition and proportion may be
changed readily and can be predicted based on this reaction
P. A. Jacobs and B. F. Sels, Catal. Sci. Technol., 2011, 1, 714–
726.
network study. Such flexibility foresees a fast anticipation on 15 J. Geboers, S. Van de Vyver, K. Carpentier, P. A. Jacobs and
varying market demands and prices of the produced polyols
such as ethylene glycol, 1,2-propanediol and glycerol.
B. F. Sels, Chem. Commun., 2011, 47, 5590–5592.
16 J. Geboers, S. Van de Vyver, K. Carpentier, P. A. Jacobs and
B. F. Sels, Green Chem., 2011, 13, 2167–2174.
Future work should concentrate on new catalyst designs
with focus on balancing the various active sites to further 17 S. Van de Vyver, J. Geboers, P. A. Jacobs and B. F. Sels,
improve the retro-aldol reaction route at the expense of the ChemCatChem, 2011, 3, 82–94.
other competitive but undesired reaction pathways. Long term 18 S. Van de Vyver, J. Thomas, J. Geboers, S. Keyzer, M. Smet,
catalyst stability in hot condensed water will be another chal-
lenge to tackle in the near future before ethylene glycol pro-
W. Dehaen, P. A. Jacobs and B. F. Sels, Energy Environ. Sci.,
2011, 4, 3601–3610.
duction from aqueous glucose can be conceived commercially. 19 R. Palkovits, K. Tajvidi, A. M. Ruppert and J. Procelewska,
Chem. Commun., 2011, 47, 576–578.
20 M. J. Climent, A. Corma and S. Iborra, Green Chem., 2011,
Acknowledgements
13, 520–540.
21 S. Van de Vyver, J. Geboers, W. Schutyser, M. Dusselier,
R.O. acknowledges the financial support of Syral. M.D.
P. Eloy, E. Dornez, J. W. Seo, C. M. Courtin, E. M. Gaigneaux,
acknowledges “FWO Vlaanderen” (grant 1.1.955.10N) for finan-
P. A. Jacobs and B. F. Sels, ChemSusChem, 2012, 5, 1549–1558.
cial support and his appointment as a postdoctoral researcher.
22 N. Meine, R. Rinaldi and F. Schüth, ChemSusChem, 2012, 5,
Methusalem CASAS is acknowledged for long term funding
1449–1454.
(E.G.) and the Belgian government is acknowledged for finan-
23 M. Benoit, A. Rodrigues, K. De Oliveira Vigier, E. Fourre,
cial support through IAP funding (Belspo). B.O. thanks the EU
J. Barrault, J.-M. Tatibouet and F. Jerome, Green Chem.,
FP7 project BIOCORE that is supported by the European Com-
2012, 14, 2212–2215.
mission through the Seventh Framework Programme for
24 D. M. Alonso, S. G. Wettstein and J. A. Dumesic, Green
Research and Technical development under contract no.
Chem., 2013, 15, 584–595.
FP7-241566, for financial support.
25 B. Op de Beeck, J. Geboers, S. Van de Vyver, J. Van Lishout,
J. Snelders, W. J. J. Huijgen, C. M. Courtin, P. A. Jacobs and
B. F. Sels, ChemSusChem, 2013, 6, 199–208.
26 M. Dusselier, P. Van Wouwe, A. Dewaele, E. Makshina and
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