Upgrading biomass-derived furans via acid-catalysis/hydrogenation: The remarkable difference between water and methanol as the solvent

Article abstract of DOI:10.1039/c4gc01826e

In methanol 5-hydroxymethylfurfural (HMF) and furfuryl alcohol (FA) can be selectively converted into methyl levulinate via acidcatalysis, whereas in water polymerization dominates. The hydrogenation of HMF, furan and furfural with the exception of FA is

Full text of DOI:10.1039/c4gc01826e

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Upgrading biomass-derived furans via
acid-catalysis/hydrogenation: the remarkable
difference between water and methanol as
the solvent†
Cite this: DOI: 10.1039/c4gc01826e
Received 20th September 2014,
Accepted 7th October 2014
DOI: 10.1039/c4gc01826e
www.rsc.org/greenchem
Xun Hu, Roel J. M. Westerhof, Liping Wu, Dehua Dong and Chun-Zhu Li*
1
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In methanol 5-hydroxymethylfurfural (HMF) and furfuryl alcohol ester produced in methanol than in water. Numerous efforts
(
FA) can be selectively converted into methyl levulinate via acid- have been devoted to the development of acid catalysts or
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catalysis, whereas in water polymerization dominates. The hydro- hydrogenation catalysts for the production of these furans.
genation of HMF, furan and furfural with the exception of FA is However, much less attention was given to roles of solvent,
much more selective in methanol than in water.
which, in many cases, is almost equally important to a catalyst
in both acid-catalyzed reactions and hydrogenation reactions.
In this study, the acid-treatment (catalyst: Amberlyst 70)
and hydrogenation (catalyst: Pd/C) of the biomass-derived
furans (FA, HMF, furan and furfural) were performed in water
and methanol to demonstrate the importance of methanol in
the catalytic reactions. Our results indicate that: (1) the furans
with distinct molecular structures demonstrate very different
levels of interaction with methanol and water; (2) in methanol
both FA and HMF can be selectively converted into methyl
levulinate. In water both FA and HMF polymerized but to very
different extents (FA ≫ HMF); (3) the hydrogenation efficiency
and selectivity of HMF, furan and furfural with the exception
of FA is much more efficient in methanol than in water.
Introduction
Furans (e.g. furfural and 5-hydroxylmethylfurfural) can be pro-
duced from abundantly available biomass via hydrolysis. They
are the platform chemicals which can be upgraded to value-
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added chemicals such as pharmaceutical intermediates and
3
fuel additives via acid-catalysis or hydrogenation or both. For
example, 5-hydroxylmethylfurfural (HMF) and furfuryl alcohol
(FA, the hydrogenation product of furfural) can be converted
4
into levulinic acid/ester via acid-catalyzed conversion or to
5
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2
-methyltetrahydrofuran (MTHF) and valerate esters via
hydrogenation. Furfural can also be converted to alkyl levuli-
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nates (fuel additives) or to MTHF via acid-catalysis coupled
with hydrogenation.
Results and discussion
During either acid-catalysis or hydrogenation, the solvent
and the catalyst are the critical factors for determining overall
efficiency of the processes. The solvent not only provides a
reaction environment but also may react directly with reac-
tants, modifying their behaviors. For example, during the acid-
catalyzed conversion of levoglucosan and glucose, water as a
solvent promotes polymerization and diminishes production
of levulinic acid, while methanol reacts with the sugars, stabil-
izes the reactive intermediates and promotes methyl levulinate
Acid-catalyzed conversion of the furans in water and methanol
Furfuryl alcohol. FA can be produced via partial hydrogen-
ation of furfural, which is an important intermediate for the
production of levulinic acid/ester from furfural or xylose. Via
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acid-catalysis in water FA is converted into levulinic acid,
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while in alcohols levulinates are the main products. Water
and the alcohols make a small difference in the conversion of
FA (Fig. 1a), but make huge difference in terms of the yield of
levulinic acid and the alkyl levulinates (Fig. 1b and Scheme 1).
FA was selectively converted into methyl levulinate in
methanol or ethyl levulinate in ethanol, but in water the oppo-
site was observed. Polymerization of FA in the acidic environ-
ment is believed to be the main reason for low selectivity of
levulinic acid in water. In the acid-treatment of FA in water,
the used catalyst became very sticky, which was difficult to be
scrubbed from the reactor wall and stirrer. In methanol, the
polymerization is suppressed and the production of methyl
9
production. During the one-step conversion of xylose to levuli-
nic acid, using water and methanol as solvents also makes
remarkable difference: much higher yields of levulinic acid/
Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box
U1987, Perth, WA 6845, Australia. E-mail: chun-zhu.li@curtin.edu.au;
Fax: (+) 61 8 9266 1138; Tel: (+) 61 8 9266 1131
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc01826e
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Fig. 1 Acid-catalyzed conversion of furfuryl alcohol in water, methanol
and ethanol. Reaction conditions: furfuryl alcohol: 3 g, Amberlyst 70:
Fig. 2 Acid-catalyzed conversion of HMF in water and methanol. Reac-
tion conditions: HMF loaded: 3 g, Amberlyst 70: 3 g, solvent: 100 ml, T =
3
g, solvent: 100 ml, T = 130 °C, residence time: 90 min.
170 °C, residence time: 90 min.
HMF. HMF, the dehydration product from the C6 sugars
such as glucose and fructose, can be further converted into
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levulinic acid in the presence of an acid catalyst. The conver-
sion of HMF in methanol is much faster than in water
(
Fig. 2a), which is due to the acetalisation and etherification
reactions. Similar to that of FA, in methanol selective conver-
sion of HMF to methyl levulinate and in water the non-selec-
tive conversion were expected. However, unexpectedly, the
yield of levulinic acid from HMF in water reaches ca. 81%
(
Fig. 2b). The following experiments without sampling con-
firmed the result that the yield of levulinic acid from HMF in
water is not significantly lower than that of methyl levulinate
in methanol.
In our previous study about the conversion of levoglucosan
in methanol, we found that HMF reacted with methanol,
forming 5-(methoxymethyl)-2-furancarboxadhyde and 2-(di-
9
methoxymethyl)-5-(methoxymethyl)furan. It was believed that
the conversion of the hydroxyl group to the ether functionality
and the carbonyl group to the acetal functionality suppressed
the polymerization of HMF. This leads to selective production
of methyl levulinate from HMF in methanol. However, the
polymerization of HMF in both methanol and water did occur,
as evidenced by the formation of a soluble polymer with the
progress of the reaction (Fig. S2†), but to a very limited level.
In addition, the result here also proves that in water the
Scheme 1 Acid-catalyzed conversion of FA, HMF and furfural in water
and methanol.
levulinate from FA is the preferred reaction pathway. The for- decomposition of HMF to levulinic acid is the preferred reac-
mation of acetal in the alcohols possibly suppresses the aldol tion pathway, but not the polymerization.
condensation which leads to polymer formation.
The transformation of FA to levulinic acid initiates from the from glucose in water is typically ca. 50% (mol basis). The
It has been well documented that the yield of levulinic acid
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hydration and opening of the furan rings. Many reactive loss of levulinic acid production is due to the polymerization
16
compounds with the carbonyl group and carbon–carbon during the dehydration of glucose or HMF or both. Here
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double bonds were produced as intermediates. In the acidic based on the yields of levulinic acid from HMF (ca. 81%) and
environment they can easily polymerize via aldol conden- from glucose (ca. 50%, another experiment under similar con-
sation. The UV florescence characterization clearly showed the ditions), we concluded that: (1) the polymerization of HMF
formation of the extensive conjugated carbon–carbon double itself contributes to ca. 20% of the loss of levulinic acid pro-
bonds (Fig. S1†). In methanol the polymerization of these reac- duction; (2) the polymerization incurred from glucose contrib-
tive intermediates were suppressed while the formation of utes another ca. 30% to the loss of levulinic acid production,
methyl levulinate were promoted. This result is in line with which was calculated by the loss of levulinic acid production
our previous study that methanol plays a crucial role in sup- directly from glucose (ca. 50%) minus that directly from HMF
pression of polymerization during the acid-catalyzed conver- (ca. 20%); (3) polymerization takes place in both the de-
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sion of C6 sugars like levoglucosan and glucose.
hydration of glucose to HMF and the degradation of HMF to
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levulinic acid, but more significant in the initial dehydration alcohol (TFA) as the main product. Yields of TFA are similar in
of glucose. Effort should be made to suppress both polymeriz- water and methanol. The polymerization of FA in water during
ation in the up-stream dehydration of the sugars and the the hydrogenation is negligible, as confirmed by the UV fluore-
down-stream degradation of HMF, which are the keys for scence result that no soluble polymer was observed (Fig. S4†).
enhancing levulinic acid production from sugars in water.
The polymerization of FA is accompanied by the formation of
Both HMF and FA in water can be converted into levulinic the reactive intermediates with carbon–carbon double bonds
acid, but their tendencies towards polymerization are very or carbonyl groups. During hydrogenation, once these reactive
different. Although the molecular structures of HMF and FA intermediates were formed, they could be readily hydrogenated
are different, the intermediates involved during their trans- and consequently the polymerization is terminated. In
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formation to levulinic acid have similar structures. It has not addition some acid-catalyzed reactions like etherification
been understood why FA has a much higher tendency towards occurred as well, producing tetrahydrofurfuryl methyl ether
polymerization than HMF in water, while methanol or ethanol (TFME) from TFA. Opening of the furan ring via hydrogenation
can stabilize FA and promote the selective conversion of FA to also happened but in the minor route.
methyl levulinate or ethyl levulinate.
HMF. HMF has an additional carbonyl group when com-
Furan/furfural. Furan industrially is manufactured by the pared with FA, and the hydrogenation of HMF is much more
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palladium-catalyzed decarbonylation of furfural. It, unlike complex (Scheme 3). HMF was mainly hydrogenated to cis-2,5-
that of FA and HMF, does not have a hydroxyl group or a carbo- tetrahydrofurandimethanol (TFM, its identification was based
nyl group. Its reactivity represents the reactivity of the furan on the analysis of its mass spectrum, see Fig. S5 and S6†).
ring itself. As is shown in Fig. S3,† furan is less reactive than There no trans-2,5-tetrahydrofurandimethanol is observed.
both FA and HMF due to its slower conversion than FA and This indicates that the carbonyl group on the furan ring was
HMF (Fig. 1a and 2a) under similar conditions. Unfortunately, hydrogenated first and then the furan ring. This is because if
the products from furan degradation were not able to be the furan ring was hydrogenated first, the formed intermediate
identified.
can access the metallic sites from two opposite directions,
Furfural is the main product from the dehydration of which will inevitably create hydrogenated isomers for TFM.
xylose. It is so reactive towards polymerization that methanol Another possibility is that the hydrogenation of the carbonyl
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and water make almost no difference. Polymer is the only group and the furan ring occurred subsequently when both
main product in either methanol or water (Scheme 1). Furfural the carbonyl group and the furan ring have access to the met-
itself contains the reactive carbonyl group and the furan ring. allic sites.
Opening of the furan ring of furfural under acidic conditions
TFM has at least four ways to go under the conditions
would produce more reactive intermediates. These intermedi- employed (Scheme 3), which includes cracking of the hydroxy-
ates, unlike those from the degradation of HMF, cannot be methyl group to TFA and methyl formate, hydrodeoxygenation
further converted into the relatively stable levulinic acid. With of the hydroxyl group to tetrahydro-5-methyl-2-furanmethanol,
the prolonged residence time, they have no other way to go but opening of the furan ring and then dehydrate to tetrahydro-
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polymerizes as their main destiny.
Hydrogenation of the furans in water and methanol
Furfuryl alcohol. In the acidic environment the conversion
of FA in water and methanol makes big difference. In compari-
son, in the almost neutral environment with Pd/C as the cata-
lyst, the hydrogenation of FA in water and methanol makes no
marked difference (Scheme 2). Hydrogenation of the furan
ring is the main reaction pathway and the conjugated π bonds
were hydrogenated in one-step, producing tetrahydrofurfuryl
Scheme 2 Hydrogenation of furfuryl alcohol in water and methanol.
Reaction conditions: furfuryl alcohol loaded: 2 g, Pd/C loaded: 2 g,
solvent: 40 ml, T = 170 °C, residence time: 120 min, P = 70 bar. TFA:
tetrahydrofurfuryl alcohol; TFME: tetrahydrofurfuryl methyl ether.
Scheme 3 Hydrogenation of HMF in water and methanol. Reaction con-
ditions are the same as that in Scheme 2. THMF: tetrahydro-5-(methoxy-
methyl)furan-2-yl)methanol; TFA: tetrahydrofurfuryl alcohol.
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Scheme 4 Hydrogenation of furfural in water and methanol. Reaction
conditions are the same as that in Scheme 2.
pyran-2-carbinol, and etherification to tetrahydro-5-(methoxy-
methyl)furan-2-yl)methanol (THFM, its identification was
based on the analysis of its mass spectrum, see Fig. S7 and
Scheme 5 Acid-catalyzed conversion and hydrogenation of furfural in
water and methanol. Reaction conditions: furfural loaded: 2 g, Pd/C
S8†). Interestingly, THFM has both the trans and cis isomers. loaded: 2 g, Amberlyst 70 loaded: 2 g; solvent: 40 ml, T = 170 °C, resi-
During the etherification of cis-TFM, methanol can attack dence time: 120 min, P = 70 bar. TFA: tetrahydrofurfuryl alcohol. MMHF:
2-(methoxymethyl)tetrahydrofuran.
from both sides of the furan ring, forming the isomers. This is
different from the hydrogenation of HMF that the molecule
accesses the metallic sites from only one side of the furan
ring.
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In water, TFM is also the main product, but its abundance 70, furfural mainly polymerized. With only Pd/C, furfural
is much lower than that in methanol (Scheme 3 and Fig. S9†). was mainly converted into TFA. With both Amberlyst 70 and
The selectivity for hydrogenation of HMF is much lower in Pd/C, the acid-catalysis and hydrogenation were expected to
water than in methanol. This is also evidenced by the occur in parallel.
formation of 2,5-hexanedione in water, but was not detected
in methanol. Clearly, hydrogenation of HMF in methanol is lysts it was found that the hydrogenation reactions dominated.
much more efficient in methanol than in water. TFA is the main product (yield: 44.3%) from the hydrogenation
In methanol, with both Amberlyst 70 and Pd/C as the cata-
Furfural. Hydrogenation of furfural, like that of HMF, (Scheme 5). Only a very small amount of furfural was partially
creates a clear difference in water and in methanol (Scheme 4). hydrogenated to 2-(methoxymethyl)furan and further forming
The furan ring and the carbonyl group can be hydrogenated trace amounts of methyl levulinate (yield: 0.2%) via acid-cata-
in either water or methanol, producing TFA as the main lysis (Scheme 5). The presence of Amberlyst 70 in methanol
product. TFA yields are much higher in methanol than in mainly catalyzes the etherification to form the ethers like 1,5-
water. This is not even counted in the ether, TFME, formed via dimethoxy-pentane.
etherification of TFA. The polymerization of furfural and
In water, the hydrogenation and acid-catalysis occurred in
hydrogenation of furfural at the elevated temperatures parallel. TFA is one of the main products, but the yield (10.6%)
are competitive reactions. Furfural is very reactive towards is much lower than that in methanol. Levulinic acid yield
polymerization due to its reactive carbonyl group and furan (8.4%) in water is much higher than the yield of its counter-
ring, while saturation of the carbonyl group and the furan ring part (methyl levulinate) in methanol (Scheme 5). This clearly
eliminates the reactive functional groups and prevent further indicates that in water furfural has much more chance to be
polymerization. The soluble polymer was not detected in water partially hydrogenated to furfuryl alcohol. In methanol the
(Fig. S10†). The low selectivity towards hydrogenation of the furan ring and the carbonyl group were hydrogenated very
furan ring in furfural in water is responsible for the low yields quickly. Furfural has little chance to go to FA and methyl levu-
of TFA production. For example, cyclopentanone was formed linate. Obviously, hydrogenation proceeds very efficiently in
in water but not in methanol. Its formation in water was methanol even in the presence of a strong solid acid catalyst.
probably promoted by the synergistic effect of metal and the In water the hydrogenation is much less efficient. The hydro-
protons generated by water at high temperature. In addition, genation and the acid-catalysis occur in parallel.
furfural also cracked to butyrolactone in water, while in metha-
Furan. The structure of furan is very simple. It does not
nol the products are mainly from the hydrogenation reactions. have the carbonyl group and the hydroxyl group attached to
The selective hydrogenation in methanol favors only the the furan ring. However, furan also behaves very differently
production of the direct hydrogenation product, TFA.
during the hydrogenation in water and methanol (Scheme 6).
Catalytic conversions of furfural in the presence of both the Furan cracked to acetic acid in water while formic acid in
hydrogenation catalyst (Pd/C) and the acid catalyst (Amberlyst methanol (in the form of methyl formate). Tetrahydrofuran
7
0) were also performed to understand their competitions (THF) is the main hydrogenation product and its yield is
(
Scheme 5). This is because acid-catalysis and hydrogenation also much higher in methanol than in water (Scheme 6). Simi-
are always coupled during the conversion of furans, sugars larly, this is believed to be the selective hydrogenation in
and cellulose to the fuel or fuel additives. With only Amberlyst methanol.
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HP-INNOWax capillary column. Acetone was used as the
solvent to dissolve the samples. The temperature of column
was maintained at 40 °C for 1.3 min after injection of the
sample, and then was increased to 260 °C at a heating rate of
0 °C min− and was maintained at this temperature for
1
1
1
0 min. The detailed procedures and parameters of the instru-
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0
ment are described in the literature. The soluble polymers
were characterized with a Perkin-Elmer LS50B spectrometer by
measuring the size and abundance of the conjugated π bonds
in the polymer. Details about the parameters are described in
Scheme 6 Hydrogenation of furan in water and alcohol. Reaction con-
ditions are the same as that in Scheme 2.
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the literature. Yield of products is defined by the formula:
Conclusions
Yields ðmol%Þ ¼ ðamount of the product produced=
theoretical amount of the product from
xylose loadedÞ ꢀ 100%
The biomass derived furans behave very differently during
their acid-catalyzed conversion in water and in methanol. FA
and HMF can be selectively converted into methyl levulinate in
methanol. In water they both polymerize but to different
extents. Furfuryl alcohol mostly (>80%) polymerizes in water
while a much smaller portion of HMF (ca. 20%) polymerizes. Acknowledgements
Furfural, in comparison, polymerizes either in water or metha-
nol almost completely.
This Project was supported by the Commonwealth of Australia
under the Australia–China Science and Research Fund as
well as Curtin University of Technology through the Curtin
Research Fellowship Scheme.
For furan, HMF and furfural the hydrogenation is much
more efficient in methanol than in water. The acid-catalyzed
reactions can even compete with the hydrogenation of furfural
in water but not in methanol. The efficiency for the acid-cata-
lysis and hydrogenation of the furans in methanol is much
higher or at least equal (for hydrogenation of furfuryl alcohol)
to that in water. Hence, methanol or other alcohols are the pre-
ferred solvents for the upgrading of furans via both acid-cata-
lysis and hydrogenation.
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Experimental section
All the chemicals used in this study are of analytical grade.
Commercially available solid acid catalysts, Amberlyst 70
(obtained from Dow Chemical), and Pd/C (obtained from
Sigma-Aldrich) were used directly without any further treat-
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1
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is 40 bar and was maintained at 70 bar at 170 °C by feeding
additional hydrogen, if needed.
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