Selective conversion of cellulose to hydroxymethylfurfural in polar aprotic solvents

Article abstract of DOI:10.1002/cctc.201402299

Herein, we report a new reaction pathway to produce hydroxymethylfurfural (HMF) from cellulose under mild reaction conditions (140-190°C; 5 mM H ₂SO₄) in polar aprotic solvents (i.e. THF) without the presence of water. In this system, levoglucosan is the major decomposition product of cellulose, followed by dehydration to produce HMF. Glucose, levulinic acid, and formic acid are also produced as a result of side reactions with water, which is a by-product of dehydration. The turnover frequency for cellulose conversion increases as the water content in the solvent decreases, with conversion rates in THF being more than twenty times higher than those in water. The highest HMF yield from cellulose was 44% and the highest combined yield of HMF and levulinic from cellulose was 53%, which are nearly comparable to yields obtained in ionic liquids or biphasic systems. Moreover, the use of a low boiling point solvent, such as THF, facilitates recovery of HMF in downstream processes.

Full text of DOI:10.1002/cctc.201402299

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DOI: 10.1002/cctc.201402299
Selective Conversion of Cellulose to
Hydroxymethylfurfural in Polar Aprotic Solvents
Ronen Weingarten, Alexandra Rodriguez-Beuerman, Fei Cao, Jeremy S. Luterbacher, David
Martin Alonso, James A. Dumesic, and George W. Huber*[a]
Herein, we report a new reaction pathway to produce hydroxy-
methylfurfural (HMF) from cellulose under mild reaction condi-
tions (140–1908C; 5 mm H2SO4) in polar aprotic solvents (i.e.
THF) without the presence of water. In this system, levogluco-
san is the major decomposition product of cellulose, followed
by dehydration to produce HMF. Glucose, levulinic acid, and
formic acid are also produced as a result of side reactions with
water, which is a by-product of dehydration. The turnover fre-
quency for cellulose conversion increases as the water content
in the solvent decreases, with conversion rates in THF being
more than twenty times higher than those in water. The high-
est HMF yield from cellulose was 44% and the highest com-
bined yield of HMF and levulinic from cellulose was 53%,
which are nearly comparable to yields obtained in ionic liquids
or biphasic systems. Moreover, the use of a low boiling point
solvent, such as THF, facilitates recovery of HMF in downstream
processes.
desired formation of humins.[6] In aqueous systems HMF pro-
duction is maximized at relatively high temperatures (200–
3008C) and short reaction times (order of seconds or minutes),
and is readily converted to formic acid and levulinic acid. The
latter compound is also
chemical.[7]
a versatile biobased platform
The use of ionic liquids (ILs) as solvents for HMF production
has been proposed due to the solvation capabilities of the ILs.
A HMF yield of 51% from fructose was obtained by Li et al.
when a high concentration of feed (67 wt%) was used in 1-
butyl-3-methylimidazolium chloride ([C4mim]Cl).[8] Binder and
Raines developed a process to convert lignocellulosic biomass
to HMF using N,N-dimethylacetamide (DMA) containing lithium
chloride (LiCl) as a solvent.[9] HMF yields of up to 54% were ob-
tained with 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) as
an additive and a mixture of CrCl2/HCl as the catalyst. Rinaldi
et al. showed that solid acid catalysts can be used in 1-butyl-3-
methylimidazolium chloride ([BMIM]Cl) to selectively depoly-
merize cellulose to produce glucose and HMF.[10] Zhang and
co-workers have reported HMF yields of 55% from cellulose
with a mixture of CuCl2 and CrCl2 dissolved in [EMIM]Cl at rela-
tively low temperatures.[11] A comprehensive review covering
the process chemistry of HMF production from various feed-
stocks is given by van Putten et al.[12]
5-Hydroxymethylfurfural (HMF) is an alternative nonpetroleum
precursor which can be used as a building block chemical for
the production of various high-volume organic chemicals with
numerous potential industrial applications. These chemicals in-
clude 2,5-furandicarboxylic acid (FDCA), which can serve as
a precursor in the polymer industry,[1] and 2,5-dimethylfuran
(DMF), which can be used as a liquid transportation fuel.[2]
DMF can also be used to produce p-xylene via cycloaddition
with ethylene combined with dehydration over acidic zeolites
and acidic oxides.[3] Alamillo et al. have shown quantitative
yields of 2,5-di-hydroxymethyl-tetrahydrofuran (DHMTHF) from
HMF with ruthenium-supported oxide catalysts.[4]
Significant challenges hinder the industrial use of ILs for pro-
duction of HMF. Owing to their high costs, quantitative recov-
ery and reuse of ILs (at least 98%) is necessary to make the
process economically attractive.[13] Relative low cellulose solu-
bility (10–15 wt%) in ILs,[14] high viscosity and high toxicity of
ILs are also impeding factors.[15] Thermal and chemical stability
of ILs are also in question, as new compounds have been de-
tected derived from side reactions between HMF and imidazo-
lium-based ILs.[16] Extensive work has been reported by Jꢀrꢁme
and co-workers to produce HMF from biomass derived feed-
stock in alternative solvent systems that are comparable with
imidazolium-based ILs.[17] Alternative approaches have also
been investigated using biphasic reaction systems with organic
solvents that can extract the HMF from the aqueous phase
before it undergoes further degradation reactions.[18] Phase
modifiers (i.e. NaCl) can be added to the aqueous phase to
help enhance HMF partitioning into the immiscible organic
phase and consequently impede further HMF degradation.[19]
Herein we introduce a new approach to produce HMF from
cellulose in polar aprotic solvents without using water as a co-
solvent. We show that this reaction system is able to produce
HMF from cellulose in yields that approach those obtained in
ILs or biphasic systems.[20] Moreover, HMF and other reaction
HMF is produced conventionally from glucose (in low yields)
or fructose (in high yields) by a triple dehydration step with
mineral acids in water.[5] It would be highly desirable to be
able to produce HMF from cellulose, which is a more abundant
and lower value feedstock than fructose. However, in aqueous
systems, HMF is only produced in low yields (between 8 to
21%) from cellulose because of miscibility limitations and un-
[a] R. Weingarten, A. Rodriguez-Beuerman, Dr. F. Cao, Dr. J. S. Luterbacher,
Dr. D. M. Alonso, Prof. J. A. Dumesic, Prof. G. W. Huber
Department of Chemical and Biological Engineering
University of Wisconsin-Madison
3018 Engineering Hall, 1415 Engineering Drive
Madison, WI 53706-1691
E-mail: huber@engr.wisc.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402299.
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Scheme 1. Proposed reaction scheme for HMF production from cellulose in polar aprotic solvents under acidic conditions.
byproducts can be separated from the polar aprotic solvent
using conventional separation technologies like distillation and
evaporation. This approach is in contrast to previous percep-
tions of liquid phase cellulose decomposition wherein the
presence of water has been thought to be necessary to con-
vert cellulose into HMF via hydrolysis route.[21]
alyst to produce levulinic acid and formic acid. Once glucose is
formed, it can also undergo dehydration to produce HMF, as
well as undergo degradation to produce humic species. Furfu-
ral was also detected in carbon yields lower than 7%. It has
been reported that furfural is a by-product of levoglucosan[25]
and/or glucose[26] decomposition. Separate decomposition
studies with HMF in THF and sulfuric acid showed that HMF is
relatively stable in the non-aqueous environment with conver-
sions below 8% at 1908C after 180 min. Other studies have
also mentioned that THF prevents further degradation of furfu-
ral and HMF.[27]
Shown in Scheme 1 is our proposed reaction pathway to
produce HMF from cellulose in polar aprotic solvents. Cellulose
initially undergoes reaction to produce levoglucosan under
dilute acidic conditions at temperatures in the range of 140–
1908C. It has been reported that levoglucosan can be pro-
duced from cellulose in the organic solvent sulfolane (36%
yield after 2 min at 3308C), but relatively higher temperatures
(200–3308C) were required to perform the reaction and a cata-
lyst was not used.[22] The same authors showed relatively high
yields of levoglucosenone (38%) and furfural (20%) under
acidic conditions.[23] Our experiments show that acid is neces-
sary to produce levoglucosan as confirmed by blank studies
(without acid catalyst) with cellulose in THF, which resulted in
negligible cellulose conversion (no detectable products) at
1708C after 6 h. The levoglucosan then undergoes a double
dehydration step to produce HMF. The water produced in this
reaction can react with levoglucosan to produce glucose.[24]
HMF can also undergo rehydration with water over an acid cat-
The polar aprotic solvents including g-valerolactone (GVL),
acetone and THF showed considerable higher yields of HMF
from cellulose compared to ethyl acetate, water and ethanol,
as shown in Figure 1. The HMF yield increased in the following
order: ethanol<water<ethyl acetate !GVL<acetoneꢀTHF.
Reactions in water and ethanol, both protic solvents, resulted
in the lowest HMF yields. Despite being a polar aprotic solvent,
the yields obtained in ethyl acetate were only somewhat
higher compared to the protic solvents. We believe that this
behavior can be attributed to the instability of ethyl acetate
under the applied reaction conditions where water (from the
dehydration reaction) reacts with ethyl acetate to form ethanol
and acetic acid. Both of these by-products were detected with
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Figure 1. Cellulose decomposition in polar protic and aprotic solvents under
acidic conditions. HMF production as a function of reaction time at 1708C.
Cellulose loading was 5 wt% and solvent volume was 60 mL. Catalyst con-
~
&
*
centration was 5 mm sulfuric acid. Water ( ), THF ( ), GVL ( ), ethyl ace-
!
^
tate ( ), acetone (N), ethanol ( ).
HPLC when ethyl acetate was used as a solvent. Acetone has
been shown to be a valuable solvent for HMF production from
carbohydrates;[28] however, acetone is not stable under acidic
conditions, as it undergoes aldol-type reactions to form dimers
and trimers.[28a]
Figure 2. Cellulose decomposition in THF/water mixtures under acidic condi-
tions. Carbon yield of major products as a function of reaction time at
1708C. a) levoglucosan, b) glucose, c) HMF, d) levulinic acid. Cellulose load-
ing was 5 wt% and reaction volume was 60 mL. Catalyst concentration was
Previous work has shown GVL to be a promising solvent for
biomass processing.[29] Alonso et al. used a monophase system
comprised of a solution of 90 wt% GVL and 10 wt% water as
the solvent with Amberlyst 70 to selectively produce levulinic
acid from cellulose with yields close to 70%.[30] Increasing the
amount of water in the solvent decreased the reaction rate.
GVL/water solutions were also used to convert the hemicellu-
lose and cellulose fractions of lignocellulosic biomass to furfu-
ral and levulinic acid respectively.[26,31] However, it has also
been reported that GVL reacts with water under acidic condi-
tions[32] and also undergoes oxidation to form degradation
products in the presence of molecular oxygen.[33] In this re-
spect, we performed stability studies with GVL and sulfuric
acid under inert atmosphere, and we detected traces of levu-
linic acid and pentenoic acid[34] after 60 min at 1708C. Identical
studies with THF resulted in no identifiable degradation
products.
~
&
*
5 mm sulfuric acid. Water ( ), water/THF 1:1 v/v ( ), water/THF 1:9 v/v ( ),
^
water/THF 1:40 v/v ( ), THF (N).
major products detected were levoglucosan, glucose, HMF and
levulinic acid. Higher carbon yields of levoglucosan were ob-
served in pure THF and in the 40:1 THF/water mixture (7 and
8% respectively after 60 min) compared to the other reaction
mixtures. Levoglucosan is most likely the primary decomposi-
tion product of cellulose in THF. Stoichiometry requires that
water be a reactant for the production of glucose from cellu-
lose.[36] We also identified trace amounts of other anhydrosu-
gars including levoglucosenone, 1,4:3,6-dianhydro-a-d-gluco-
pyranose and 1,6-anhydro-b-d-glucofuranose, which are by-
products of levoglucosan dehydration and isomerization reac-
tions.[23] These byproducts are identical to those detected from
gas-phase cellulose pyrolysis.[37] Glucose is also observed when
pure THF is the reaction solvent.
THF was selected as the reaction solvent for more detailed
studies. Biphasic mixtures of THF and water have been used in
a wide array of biomass conversion processes, including furfu-
ral production from hemicellulose, HMF production from glu-
cose and the use of co-solvent systems (with and without
metal halides) to produce the aforementioned products from
maple wood.[27a,35] Cellulose decomposition with dilute sulfuric
acid was performed in five different solvent systems (pure THF,
pure water, a 1:1 THF/water mixture, a 9:1 THF/water mixture,
and a 40:1 THF/water mixture), as shown in Figure 2. The
Separate experiments with levoglucosan in THF under acidic
conditions confirmed that HMF and levulinic acid can be pro-
duced directly from levoglucosan (Figure S1 in the Supporting
Information). Hu et al. also reported that levoglucosan under-
goes dehydration to form HMF in the presence of Amberlyst
70 in organic solvents, however it is important to note that
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their reaction systems were not completely free of water due
to the ion resin which was not dried prior to the reaction (ca.
57% water content).[25] The HMF yield was higher when glu-
cose was used as a feedstock compared to levoglucosan, but
by no more than 3%. Levoglucosan (21% after 30 min) was
also observed as a product from glucose dehydration when
THF was used as a solvent (Figure S2). In contrast, only trace
levels of levoglucosan were observed when water was used as
a solvent for glucose dehydration. Similar high yields of levo-
glucosan have been observed for glucose dehydration with
Amberlyst 15 in the polar aprotic solvent N,N-dimethylforma-
mide.[38] The rate of glucose conversion in THF was higher than
that in water (above 90% conversion after 5 min in THF), im-
plying the formation of unknown reaction intermediates when
THF was used as a solvent. This shows the significance of the
solvent effect on the reaction chemistry.
dation reactions, and 4) opportunity to use solid catalysts with
improved stability in the reaction media.
The product selectivity can be modified by adjusting the re-
action temperature. The levoglucosan yield increased with in-
creasing temperature as shown in Figure S6a (39% yield after
1 min at 2108C). The HMF yield went through a maximum at
1908C (Figure S6c). Additional studies performed to depict the
effect of cellulose loading demonstrated that the levoglucosan
yield increased with a decrease in the initial cellulose loading,
as shown in Figure S7b, whereas an increase in the HMF yield
was observed with increased cellulose loading (Figure S7c).
The carbon yield of all the detectable products and their rela-
tive carbon selectivity at 1908C in THF as the solvent are
shown in Figure 3. The HMF yield increased steadily with time
reaching 44% after 120 min at 1908C (Figure 3a). The HMF
yield then remained constant, even after 4 h, and the relative
HMF selectivity increased with time to 65% (Figure 3b). At
1908C levoglucosan went through a maximum yield of 25%
after 30 min. The glucose yield (18% maximum) decreased
with reaction time at 190 and 2108C. The cumulative detecta-
ble products carbon yield at 1908C went through a maximum
of 88% after 60 min, decreasing to 67% after 4 h, suggesting
that undesired humins form in this reaction. The levulinic acid
yield also decreased with reaction time at 190 and 2108C (23%
In general, the carbon yields of HMF and levulinic acid in-
creased as the water content in the solvent decreased (Fig-
ure 2c). On the other hand, increased yields of all four prod-
ucts were observed when relatively small amounts of water
(water/THF 1:40 v/v) were added to pure THF. The effect of
water on HMF yield at relatively low water concentrations is
shown in Figure S3. These studies show that adding relatively
small amounts of water (<2.5 vol% water) is in fact beneficial
for HMF production. The initial turnover frequencies (TOF) for
cellulose conversion and HMF formation per sulfuric acid site
were calculated from the data in Figure S4 and Figure 2 re-
spectively for four different solvent systems (pure THF, pure
water, a 1:1 THF/water mixture, and a 9:1 THF/water mixture).
The TOF for cellulose conversion in THF was more than twenty
fold higher than the TOF in pure water and the TOF for HMF
formation in THF was more than forty times higher than in
water (Figure S5). It is notable to mention that a fraction of the
cellulose (up to 50%) can also produce insoluble humins in
pure water and this was not accounted for in the TOF calcula-
tions. This would only alter the results by a factor of two at
the most. Nevertheless, it is clear that water (above 2–3 vol%)
has a significant inhibition effect on cellulose decomposition,
as well as the dehydration reaction. We believe that the acid
sites are less reactive in the presence of water, owing to solva-
tion of the proton by water molecules. For example, the Gibbs
free energy for solvation of a proton changes from À265.9 kcal
molÀ1 in liquid water to À260.2 kcalmolÀ1 in an aprotic solvent
such as acetonitrile.[39] Thus, the proton catalyst is stabilized in
an aprotic solvent to a less extent than in water (by 5.7 kcal
molÀ1), leading to higher reactivity of the proton, provided
that the solvent has a more moderate effect on the transition
state for the acid-catalyzed reaction relative to the reactant.
This higher reactivity of the Brønsted acid catalyst in an aprotic
solvent allows for the use of low acid concentrations to per-
form the reaction, as mentioned in previous studies with
GVL.[40] The TOF for cellulose conversion decreases from 190 to
52 hrÀ1 as the solvent changes from 0% H2O to 10% H2O. It
has previously been shown that minimizing the water concen-
tration in the reactor leads to a number of advantages:[26,40]
1) enhanced rate of furfural/HMF production from monosac-
charides; 2) facilitated product recovery; 3) mitigation of degra-
Figure 3. Cellulose decomposition in THF under acidic conditions at 1908C.
a) Carbon yield of detectable products; b) Relative carbon selectivity of de-
tectable products. Cellulose loading was 1 wt% and reaction volume was
60 mL. Catalyst concentration was 5 mm sulfuric acid. Levoglucosan (&),
glucose (&), HMF (&), furfural (&), levulinic acid (&), formic acid (&).
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We have presented a new method to convert cellulose into
HMF in polar aprotic solvents (i.e. THF) under dilute acid condi-
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major decomposition product of cellulose, followed by dehy-
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This material is based upon work supported as part of the Insti-
tute for Atom-efficient Chemical Transformations (IACT), an
Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences. This
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Keywords: biomass · hydroxymethylfurfural · homogeneous
catalysis · polar aprotic solvent · tetrahydrofuran
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