Simultaneous hydrogenation and acid-catalyzed conversion of the biomass-derived furans in solvents with distinct polarities

Article abstract of DOI:10.1039/c5ra22414d

Furfural and 5-hydroxymethylfurfural (HMF), the two typical biomass-derived furans, can be converted into biofuels and value-added chemicals via hydrogenation or acid catalysis or both. The potential competition between the hydrogenation and the catalyzed-conversion of HMF and furfural has been investigated with Pd/C and Amberlyst 70 as the catalysts at 170°C in various solvents. In water, the hydrogenation of HMF or the derivatives of HMF could take place, but the acid-catalyzed conversion of HMF to the diketones (2,5-hexanedione) was the dominant reaction pathway. On the contrary, with ethanol as the solvent, the full hydrogenation of HMF to 2,5-tetrahydrofurandimethanol was the dominant route, and the acid-catalyzed routes became insignificant. The efficiency for hydrogenation of HMF was much higher in ethanol than in water. As for furfural, its hydrogenation proceeded more efficiently in the polar solvents (i.e. ethanol, diethyl ether) than in non-polar solvents (i.e. toluene): a polar solvent tended to favor the hydrogenation of the furan ring in furfural over that of the carbonyl group in the same furfural.

Full text of DOI:10.1039/c5ra22414d

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Simultaneous hydrogenation and acid-catalyzed
conversion of the biomass-derived furans in
solvents with distinct polarities†
Cite this: RSC Adv., 2016, 6, 4647
*
Xun Hu, Sri Kadarwati, Yao Song and Chun-Zhu Li
Furfural and 5-hydroxymethylfurfural (HMF), the two typical biomass-derived furans, can be converted into
biofuels and value-added chemicals via hydrogenation or acid catalysis or both. The potential competition
between the hydrogenation and the catalyzed-conversion of HMF and furfural has been investigated with
Pd/C and Amberlyst 70 as the catalysts at 170 ꢀC in various solvents. In water, the hydrogenation of HMF or
the derivatives of HMF could take place, but the acid-catalyzed conversion of HMF to the diketones (2,5-
hexanedione) was the dominant reaction pathway. On the contrary, with ethanol as the solvent, the full
hydrogenation of HMF to 2,5-tetrahydrofurandimethanol was the dominant route, and the acid-
catalyzed routes became insignificant. The efficiency for hydrogenation of HMF was much higher in
ethanol than in water. As for furfural, its hydrogenation proceeded more efficiently in the polar solvents
(i.e. ethanol, diethyl ether) than in non-polar solvents (i.e. toluene): a polar solvent tended to favor the
hydrogenation of the furan ring in furfural over that of the carbonyl group in the same furfural.
Received 26th October 2015
Accepted 23rd December 2015
DOI: 10.1039/c5ra22414d
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previous studies have shown that the sugars and the biomass-
derived furans had drastically different tendencies towards
the polymerization in water and in methanol.22,23 Upgrading of
Introduction
Furfural and 5-hydroxymethylfurfural (HMF) are two important
furans derived from hydrolysis/pyrolysis of lignocellulosic
biomass. They are the platform chemicals for manufacturing
value-added chemicals or biofuels.1,2 The transformation of
HMF and furfural usually proceeded via either acid catalysis or
hydrogenation.3–7 However, hydrogenation and acid catalysis
are coupled for conversion of the furans in some circum-
stances.8–11 One specic example is the conversion of furfural
into methyltetrahydrofuran (2-MTHF), a fuel additive,12–14 which
involves both acid-catalyzed dehydration reactions and hydro-
genation reactions.15,16 From HMF to 2-MTHF also involves the
acid-catalyzed degradation of HMF to levulinic acid (LA) and the
subsequent dehydration/hydrogenation of LA.17,18 The acid-
catalyzed reactions and the hydrogenation reactions may
occur simultaneously, together determining the product
distribution.
furans via hydrogenation coupled with acid catalysis have been
investigated in a number of studies.24–28 In the co-presence of
a hydrogenation catalyst and an acid catalyst, hydrogenation
and acid catalysis may compete and drastically affect reaction
network. Furthermore, the competition may be varied in
different solvents, which, however, has not been fully under-
stood or been purposely investigated. This is a critical knowl-
edge gap. Understanding how acid catalysis and hydrogenation
competes will provide the essential information for process
optimization in biorenery.
In this study, the hydrogenation of HMF and furfural with
Pd/C only or coupled with Amberlyst 70 (A70, a commercial
solid acid catalyst) were preformed in water, ethanol, methyl
formate, diethyl ether, THF and toluene, respectively. The
selection of Pd/C and A70 was based on the considerations that
they are commercially available catalysts and were commonly
employed in hydrogenation reactions or acid-catalyzed reac-
tions. The selection of the solvents was based on the consider-
ation of their distinct polarities. The competition between the
acid catalysis and hydrogenation in the varied reaction envi-
ronments was explored.
Both acid catalysis and hydrogenation are affected remark-
ably by the reaction environment (i.e. solvent and etc.).19–23
Mediating solvent is an important strategy to control the
product selectivity during the upgrading of the furans and
sugars via either hydrogenation or acid catalysis.19–23 Solvents
play the roles far more than providing a reaction medium. Our
Our results demonstrated that (1): in water, the acid-
catalyzed conversion of HMF was dominate, while, in ethanol,
the hydrogenation of HMF was dominant; (2): the hydrogena-
tion of furfural proceeded more efficiently in polar solvents
than in non-polar solvents; (3) polarities of the solvents strongly
affect the reactivities of the furan ring and the carbonyl group in
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/c5ra22414d
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furfural towards hydrogenation; (4): in the co-presence of Pd/C
GC-MS, UV uorescence spectrometer and FT-IR were used
and A70 catalysts, the dominance of the hydrogenation reaction to analyze the products. Details for the procedures can refer to
could suppress, but not terminate the polymerization of HMF or our previous work.20,29 Not all products can be identied with
furfural; (5) synergistic effects between Pd/C and A70 was the GC-MS spectral library (NIST MS Search 2005). Thus, some
observed. A70 catalyzed the dehydration of hydroxyl groups to products were identied by analyzing their mass spectrum. The
form carbon–carbon double bonds or p-conjugated structures, proposed reaction pathways for dissociation of these products
while Pd/C catalyzed the hydrogenation of these unsaturated in the mass spectrometer were presented in the ESI.† It is
functionalities, which worked together to eliminate the oxygen difficult to obtain standards for all the products for quanti-
in the furans or their derivatives.
cation. Thus, relative proportion of the compound, which was
calculated by the peak area of this compound divided by total
peak area of all the products in the spectrum, was used to
understand their relative abundance in the products.
Experimental
Hydrogenation of the furans was carried out in a Hastalloy auto-
clave reactor (Autoclave Engineers, Division of Snap-Tite Inc.
volume; 100 ml). Experimental procedures were similar to that
published elsewhere.29 Typically, 2 g of HMF or furfural together
Results and discussion
Conversion of HMF in water
with 40 ml of solvent as well as 0.5 g of Pd/C catalyst and/or 0.5 g HMF has an unsaturated furan ring and a carbonyl group,
of A70 catalyst were used. Forty bar hydrogen was fed into the which was expected to be fully hydrogenated to form 2,5-tetra-
reactor before heating to 170 ꢀC in ca. 10 min with a stirring rate hydrofurandimethanol (TFDM) with Pd/C as the catalyst.
of 350 rpm. Additional hydrogen was fed at 170 ꢀC to maintain 70 However, the relative proportion of TFDM was quite small
bar throughout the experiment. Aer holding at 170 ꢀC for 60 (Scheme 1). The main products were 2,5-hexanedione and 5-
min, the products were quenched with a cooling coil inside the hydroxy-2-hexanone. 2,5-Hexanedione was probably formed via
reactor vessel. The liquid products and the catalysts were collected the reverse of the Paal–Knorr synthesis.30,31 The Paal–Knorr
together and were separated aer the catalyst was settled down in synthesis is a reaction to synthesize furans or pyrroles from 1,4-
the bottom of the sample container. The selection of 170 ꢀC as the diketones, which is catalyzed by acids.30,31 2,5-Hexanedione is
reaction temperature was based on the consideration that the a 1,4-diketones and opening of the furan ring via the reverse
maximum operating temperature for A70 is 190 ꢀC.
Paal–Knorr synthesis was a possible route for its formation.
Scheme 1 Distribution of the typical products in hydrogenation of HMF in water in the absence and presence of A70 catalyst. Reaction
conditions: HMF: 2 g; Pd/C: 0.5 g; A70: 0.5 g; solvent: 40 ml; T ¼ 170 ꢀC; PH at 170 ꢀC ¼ 70 bar; reaction time: 60 min; stirring rate: 350 rpm.
2
Reaction conditions for those in other schemes were similar. “(x%)” is relative proportion of the product in all products. “(Yield: x%)” is the yield of
the product. Conversions of HMF were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S1 in
ESI.† 5-Hydroxy-2-hexanone was identified by manually analysing its mass spectrum (Fig. S2 and S3†). It needs to note here that the mass
balances in the above experiments is less than 100%. This is because a number of unknown products could not be identified by the library of the
GC-MS, as shown in Fig. S1.†
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In the absence of A70, the formation of the signicant solvents or the mixture of organic solvents and water are more
amount of 5-hexanedione and 5-hydroxy-2-hexanone in water efficient than in water.33 The organic solvents such as g-valer-
indicates that the hydrogenation of the furan ring was not effi- olactone could prevent the re-precipitation of by-products
cient. HMF has the chance to be converted via the reverse Paal– derived from lignin on the surface of cellulose, which helps to
Knorr synthesis before the hydrogenation of the furan ring. maintain accessibility to the reactive cellulose surface. In
Water might act as an acid catalyst to promote the opening of the comparison, water itself could not effectively suppress the re-
furan ring under the subcritical conditions. The presence of the precipitation. This may be the reason for the relatively low
strong acid catalyst, A70, further promoted the ring opening efficiency for the hydrolysis of biomass with water as the sole
reactions, leading to the formation of more 2,5-hexanedione and solvent. However, in the conversion of the simple molecules like
more 5-hydroxy-2-hexanone (Scheme 1). In addition, small HMF, which does not have a complex macro polymeric struc-
amount of LA (yield: 4%) was also produced in the presence of ture, no such a problem (the re-precipitation) is existed. HMF
A70. From the conversion of HMF to LA several intermediates could be converted efficiently in water via the acid-catalyzed
with unsaturated ring structures or doubles bonds were routes.
invoved.32 These unsaturated species had the chance to make
their ways to LA.
Although water was found not to be the good solvents for the
hydrogenation of HMF in this study, it was regarded as the good
Both LA and 2,5-hexanedione were formed via the acid- one for the hydrogenolysis of lignin derived phenolics34,35 or the
catalyzed reactions. These results conrm that the hydrogena- oxidation of HMF.36 The lignin derived phenolics have the
tion of HMF in water was not taken place effectively, and the either bonds and the benzene ring structures. Under sub-
acid-catalyzed reactions were dominant, even in the absence of critical conditions water behaves as an acid, which might help
A70. The presence of A70 further promoted the acid-catalyzed to break down the ether bond in lignin.
reactions. The characterization of the liquid products with FT-
The furan ring in HMF, however, could not be effectively
IR also proved that the hydrogenation was not efficient in hydrogenated in water, while the benzene ring structures could.
water as the unsaturated carbon–carbon double bonds could Although both the furan ring and the aromatic ring in the lignin
still be detected (Fig. 1a).
derived compounds have aromaticity, their reactivities towards
Water has a relatively higher acidity than ethanol at the sub- hydrogenation or acid-catalysis are probably different. The
critical or supercritical conditions. This might be the reason for oxygen in the furan rings could facilitate the ring-opening
the high efficiency for the acid-catalyzed conversion of HMF in reactions of the furans via acid catalysis while in water
water. It was reported that the hydrolysis of biomass in organic opening the aromatic ring via acid-catalyzed reactions is rela-
tively difficult.
Conversion of HMF in ethanol
In ethanol, the acid-catalyzed reactions were no match with the
hydrogenation reactions (Scheme 2). HMF had virtually no
chance to form LA or ethyl levulinate. The fully hydrogenated
products (i.e. TFDM) were the dominated ones. Characterization
of the liquid products with FT-IR showed that the abundance of
the carbon double bonds was negligible in ethanol while that in
water was signicant (Fig. 1). Solubility of hydrogen is much
higher in ethanol than in water,37 which might be a reason for the
high efficiency of hydrogenation in ethanol.
Ethanol also affected the distribution of the products in
a way different from water. TFDM and tetrahydro-5-methyl-2-
furanmethanol (TMFE) were dominant. Ethanol is less acidic
than water, which might negatively affect its capability and
involvement in the ring-opening reactions. However, in the
presence of the strong acid catalyst, A70, the amounts of 2,5-
hexanedione formed were still small (0.6% versus 0.9% in the
absence of A70). Thus, the low acidity of ethanol was not the
main reason for the formation of the signicantly less amount
of 2,5-hexanedione in ethanol. The hydrogenation of the furan
ring in HMF probably proceeded very quickly in ethanol, and
thus the conversion of HMF to 2,5-hexanedione via the reverse
Paal–Knorr synthesis was signicantly suppressed. This was
also conrmed by the result that HMF virtually had no chance
to form levulinic acid/ethyl levulinate aer the hydrogenation of
the furan ring.
Fig. 1 Characterisation of the liquid products in hydrogenation of
HMF in water and ethanol with FT-IR. The liquid samples were diluted
with methanol/chloroform (volume/volume: 1/4) to 2 wt% before the
characterisation.
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Scheme 2 Distribution of the typical products in hydrogenation of HMF in ethanol in the absence and presence of A70 catalyst. Conversions of
HMF were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S4 in ESI.† ETHM was identified by
analyzing its mass spectrum manually (Fig. S5 and S6†).
In ethanol, A70 promoted the formation of TMFE (relative polymer was centred at 319 nm.39 The polymerization of HMF
proportion: 18.1% without A70 versus 27.0% with A70). This was clearly suppressed under the hydrogenation conditions,
indicated that the dehydration of the hydroxyl group in TFDM even in the presence of A70.
followed by hydrogenation was promoted by the acid catalyst.
The above results showed that water and ethanol affected the
A70 also promoted the formation of tetrahydropyran-2-carbinol conversion of HMF via the hydrogenation/acid catalysis in very
(THPC) (1.5% versus 9.7%). These ring-containing products different ways. This encouraged us to explore the conversion of
were preserved in ethanol.
furfural, another important biomass-derived furans, in the
In presence of an acid-catalyst, the decomposition of HMF is various solvents with Pd/C and/or A70 as the catalysts.
always followed by polymerization.38 In the absence of A70, the
soluble polymer was not detected with a UV orescence spec-
Conversion of furfural in ethanol
trometer in the hydrogenation of HMF with either water or
ethanol as the solvent (Fig. 2). The hydrogenation reactions
helped to eliminate the polymerization reactions. However, in
the presence of A70, the soluble polymers were still detected.
The soluble polymer could grow bigger (longer wavelength) in
water than in ethanol. However, the size of the conjugated p-
bond structures in the soluble polymer was smaller than that in
acid-catalyzed conversion of HMF, where peak of the soluble
In ethanol, tetrahydrofurfuryl methanol (TFM) and 2-
ethoxymethyl-tetrahydrofuran (EMTF), the ring-containing
products, were the major ones in the hydrogenation of
furfural (Scheme 3). No signicant amounts of ring-opening
products were detected. The ring-containing products could
be preserved in hydrogenation of furfural in ethanol, which was
similar to the results in hydrogenation of HMF in ethanol. TFM
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formation of ethyl levulinate from furfural here. In addition to
FA, trace amount of tetrahydrofuran-2-carbaldehyde (THFC)
was also detected, indicating that the hydrogenation of the
furan ring and the carbonyl group in furfural might not proceed
via a specic sequence, which will be discussed further below.
Conversion of furfural in methyl formate
In methyl formate, the yield of TFM (Scheme 4) was remark-
ably lower than that in ethanol. This was due to the formation
of tetrahydrofurfuryl acetate via the esterication between
TFM and the formic acid produced by the dissociation of
methyl formate or the transesterication. The formation of
tetrahydrofurfuryl acetate was further promoted over A70.
More interestingly, methyl formate affected the hydrogenation
of the furan ring and the carbonyl group of furfural in
a different way.
Both the furan ring and the carbonyl group in furfural could
be hydrogenated, but it was more difficult to hydrogenate the
carbonyl group than the furan ring of furfural in methyl
formate, as evidenced by the higher relative proportion of THFC
than that of FA (Scheme 4). Methyl formate has an ester group
with a relatively polar C–H bond (H–COOCH3) as the carbon is
connected to the oxygen with a high electron-withdrawing
capability. Thus, the carbonyl group in furfural might form
hydrogen bonds with methyl formate, increasing the difficulty
for its hydrogenation. The conjugated p bonds in furfural,
however, could not form the hydrogen bonds, which might be
the reason for its higher reactivity towards hydrogenation.
Fig. 2 Constant energy (ꢁ2800 cmꢁ1) synchronous spectra for the
soluble polymer formed in hydrogenation of HMF in water and
ethanol. The products were diluted in methanol to 3200 ppm before
the determination.
was the main product but it was further converted into EMTF
via the etherication even without A70, while the presence of
A70 remarkably promoted the formation of EMTF.
Conversion of furfural in diethyl ether
A small amount of ethyl levulinate was also formed (yield:
2.5%) in the presence of A70 (Scheme 3). Furfuryl alcohol (FA)
was the intermediate from furfural to levulinates.23 It was
produced from the partial hydrogenation of furfural. Appar-
ently, in the presence of A70, FA had a certain chances to be
further converted into ethyl levulinate before its full hydroge-
nation to TFM. The conversion of FA into LA proceeded very
quickly even at 130 ꢀC,23 which was probably the reason for the
The solvents with ether group (diethyl ether) and hydroxyl group
(ethanol) had very different impacts on the hydrogenation
reactions. As can be seen from Scheme 5, the yields of TFM were
signicantly higher in diethyl ether than in ethanol. Diethyl
ether did not react with TFM to form EMTF in the absence of
A70. More importantly, the acid-catalyzed reactions could not
compete with the hydrogenation reactions in diethyl ether. No
FA was detected while the amount of LA formed was quite small.
Scheme 3 Distribution of the typical products in hydrogenation of furfural in ethanol in the absence and presence of A70 catalyst. Conversions
of furfural were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S7 in ESI.† EMTF (Fig. S8 and
S9†) and THFC (Fig. S10 and S11†) were identified by analyzing its mass spectrum manually.
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Scheme 4 Distribution of the typical products in hydrogenation of furfural in methyl formate in the absence and presence of A70 catalyst.
Conversions of furfural were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S12 in ESI.†
Scheme 5 Distribution of the typical products in hydrogenation of furfural in diethyl ether in the absence and presence of A70 catalyst.
Conversions of furfural were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S13 in ESI.†
Hydrogenation of the furan ring possibly proceeded more effi- THF as the solvent. For example, gamma-valerolactone (GVL)
ciently in diethyl ether than in ethanol or in methyl formate. is a product from the hydrogenation/dehydration of LA.40,41 It
Consequently, furfural had little chance to go to FA or LA. was detected only when THF was used as solvent. Clearly, LA
Similar to that in methyl formate, the formation of THFC was could be hydrogenated and then dehydrated to form GVL
favoured in diethyl ether. The hydrogenation of the furan ring in THF.
was very quick, leaving little chance for the formation of LA
from FA.
Di-tetrahydrofuran methyl ether formed from the intermo-
lecular etherication of TFM was also detected. Some even
much bigger compounds, the soluble polymers, were formed in
not only THF but also in other solvents (Fig. 3). Furfural has
a very high tendency towards polymerization at the elevated
temperatures.42 It polymerizes in the presence of Pd/C catalyst
to a small extent, but to a more pronounced extent with the co-
presence of A70 (Fig. 3). The size of the p-conjugated structures
were smaller than that in acid-treatment of furfural with only
A70 as the catalyst, where a intensied peak centered at 379 nm
Conversion of furfural in THF
THF is a cyclic ether, and it behaved quite differently from
diethyl ether as the solvent for the hydrogenation of furfural
(Scheme 6). Hydrogenation of the furan ring was not efficient
in THF, and furfural had the chance to be converted into FA
and then into LA, especially in the presence of A70. In addi-
tion, there were also some interesting products formed with
Scheme 6 Distribution of the typical products in hydrogenation of furfural in THF in the absence and presence of A70 catalyst. Conversions of
furfural were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S14 in ESI.† Di-tetrahydrofuran
methyl ether was identified by analyzing its mass spectrum manually (Fig. S15 and S16†).
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Fig. 3 Constant energy (ꢁ2800 cmꢁ1) synchronous spectra for the soluble polymer formed in hydrogenation of furfural in the various solvents.
The products were diluted in methanol to 3200 ppm before the determination.
were observed.39 The presence of hydrogen and Pd/C catalyst toluene and hence majority of the acidic sites on the inner
could limit the growth of the soluble polymer, but could not surface are not accessible.43–45 Thus, TFM formation in
completely prevent the polymerization of furfural.
toluene was not affected much by A70 catalyst. The methyl-
cyclohexane formed from the hydrogenation of toluene was
also not affected much by the presence of A70 (2.3% without
A70 versus 2.4% with A70). This result further conrmed the
negligible effect of A70 on the hydrogenation reactions in
toluene.
The relative proportion of THFC and the yields of FA were
remarkably higher in toluene than in other solvents (Scheme 7).
Further to this, even in the presence of A70, the abundance of
THFC did not decrease signicantly. Both THFC and FA have
the unsaturated double bonds, but they could not be effectively
hydrogenated in toluene. In addition, trace amount of furfural
still could be detected at the end of the test in toluene but not in
other solvents. This indicated the relatively low hydrogenation
Conversion of furfural in toluene
The product distribution during the hydrogenation of furfural
in toluene (Scheme 7) was very different from that in other
solvents. In the presence of A70, TFM formation was decreased
in ethanol, methyl formate, diethyl ether and THF, due to the
competition between hydrogenation and acid catalysis.
However, in toluene, TFM formation was even increased to
a small extent in the co-presence of Pd/C catalyst and A70
catalyst, which was opposite to that in other solvents.
A70 is a solid acidic resin catalyst bearing the polar acidic
sites (–SO3H) on its local surface. It cannot swell effectively in
Scheme 7 Distribution of the typical products in hydrogenation of furfural in toluene in the absence and presence of A70 catalyst. Conversions
of furfural were >99% under the conditions specified above. GC-MS spectra for the products are presented in Fig. S17 in ESI.†
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efficiency in toluene. Both toluene and furfural have the
As can be seen in Fig. 4a, in toluene, the ratio between the
conjugated p bonds and toluene is a very good solvent to extract TFM yield in the presence of A70 and in the absence of A70 was
furfural. It was possible that the interaction between furfural above 1. The effect of A70 on the formation of TFM was not
and toluene had a shielding effect, negatively affecting the remarkable. In comparison, the yields of TFM varied signi-
hydrogenation of the furan ring and the carbonyl group in cantly in ethanol in the presence and the absence of A70 with
furfural.
the ratio of ca. 0.5. One important difference between toluene
In the relatively polar solvents such as methyl formate, and ethanol is their distinct polarities. This inspired us to
diethyl ether and THF, the hydrogenation of the furan ring in correlate dielectric constants of the solvents with their effects
furfural proceeded more efficiently than the hydrogenation of on TFM formation. It was found that the ratio of TFM yields
the carbonyl group, as evidenced by the presence of the rela- without A70 to that with A70 (Fig. 4a) followed an opposite trend
tively higher abundance of THFC. In toluene, no such effect to the dielectric constants from ethanol to toluene (Fig. 4b).
was observed. Hydrogenation of the furan ring and the Clearly, the effect of A70 on the hydrogenation reactions was
carbonyl group in furfural proceeded in parallel, producing more pronounced in the polar solvent than in the non-polar
THFC and FA with the similar relative proportions (9.8% solvent.
versus 8.7%). This result, from an opposite angel, conrmed
In addition, in toluene, the ratio of the THFC abundance in
that the hydrogenation of the furan ring in furfural was more the absence and the presence of A70 were also much higher in
favorable in the polar solvent.
toluene (0.8) than in methyl formate (0.3) or THF (0.4). This
further conrmed the insignicant effect of A70 on the hydro-
genation reactions in toluene. Solvent polarity has been known
to signicantly affect behaviors of the solid acid catalysts.44–46
The solid acidic resin catalyst cannot swell effectively in the
non-polar solvent such as toluene. Therefore, the effect of the
acid catalyst on the hydrogenation reactions was compromised
in toluene.
Solvent dielectric constant versus TFM formation
In the absence or presence of A70, the difference in terms of
TFM yields was signicant in the solvents investigated but
except that in toluene. The solvents affected the competition
between the hydrogenation and the acid catalysis in different
ways. TFM was the main product from furfural in the various
solvents via either hydrogenation or hydrogenation coupled
with acid catalysis. The ratio of its yields in the absence and the
presence of A70 could roughly indicate the effect of A70 on the
hydrogenation of furfural.
Roles of the solvents in the conversion of the furans
According to the above results, it was found that the solvents
used for the acid-catalyzed conversion and hydrogenation of the
furans was not just a reaction medium, but affected the
conversion of the furans in many ways. Firstly, solvent
remarkably affected the reaction pathways of the furans. For
example, in water the acid-catalyzed conversion of HMF to 1,4-
diketones was dominant while in ethanol the hydrogenation of
HMF was dominant (Scheme 1 and 2). Secondly, solvents can be
the reactants and could modify the reaction network. For
example, ethanol or methyl formate reacts with the furans or
the reaction intermediates (Scheme 2 and 4), which signi-
cantly changed the product distributions. Thirdly, solvents can
affect the catalytic behaviors of the acid catalysts. A70 catalyst
used in this study could swell effectively in the polar solvents
like alcohols but not in the less polar solvents like toluene,44
which affected the competition of the acid-catalyzed reactions
and the hydrogenation reactions. Fourthly, solvents affected the
solubility of hydrogen, which might further affect the hydro-
genation reactions. The solubility of hydrogen is much higher
in ethanol than in water,33 which might be the reason for the
relatively high efficiency of hydrogenation of HMF in ethanol.
Fihly, the properties of the solvents also affected the conver-
sion of the furans. For example, the polar solvent tended to
favor the hydrogenation of the furan ring over the carbonyl
groups of furfural (Fig. 4). In the less polar solvents like toluene,
the furan ring and the carbonyl groups had almost the equal
chance to be hydrogenated. The competitive absorption
between the solvents and hydrogen is also one factor that
might affect the conversion of furans, which needs further
investigation.
Fig. 4 (a) Ratio of TFM yields in the presence of Pd/C and A70 to the
TFM yields in the presence of Pd/C in the various solvents. (b) Dielectric
constants of the solvents. aThe ratio calculated in methyl formate also
included that of tetrahydrofurfuryl acetate. The ratio calculated based
on only TFM yields was presented in Fig. S18.†
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9 H. Heeres, R. Handana, D. Chunai, C. B. Rasrendra,
B. Girisuta and H. J. Heeres, Green Chem., 2009, 11, 1247–
1255.
10 M. Balakrishnan, E. R. Sacia and A. T. Bell, Green Chem.,
2012, 14, 1626–1634.
11 T. Thananatthanachon and T. B. Rauchfuss, Angew. Chem.,
2010, 122, 6766–6768.
12 V. Pace, P. Hoyos, L. Castoldi, P. D. Mara and A. R. Alcantara,
ChemSusChem, 2012, 5, 1369–1379.
13 X.-L. Du, Q.-Y. Bi, Y.-M. Liu, Y. Cao, H.-Y. He and K.-N. Fan,
Green Chem., 2012, 14, 935–939.
14 W. Yang and A. Sen, ChemSusChem, 2010, 3, 597–603.
15 T. Stein, P. M. Grande, W. Leitner and P. D. Mara,
ChemSusChem, 2011, 4, 1592–1594.
16 S. Dutta, S. De, B. Saha and M. I. Alam, Catal. Sci. Technol.,
2012, 2, 2025–2036.
17 P. P. Upare, J.-M. Lee, Y. K. Hwang, D. W. Hwang, J.-H. Lee,
S. B. Halligudi, J.-S. Hwang and J.-S. Chang, ChemSusChem,
2011, 4, 1749–1752.
18 J. Zhang, S. Wu, B. Li and H. Zhang, ChemCatChem, 2012, 4,
1230–1237.
19 D. J. Liu and E. Y.-X. Chen, Green Chem., 2014, 16, 964–981.
20 X. Hu, C. Lievens, A. Larcher and C.-Z. Li, Bioresour. Technol.,
2011, 102, 10104–10113.
21 H. Kobayashi and A. Fukuoka, Green Chem., 2013, 15, 1740–
1763.
22 X. Hu and C.-Z. Li, Green Chem., 2011, 13, 1676–1679.
23 X. Hu, R. J. M. Westerhof, L. Wu, D. Dong and C.-Z. Li, Green
Chem., 2015, 17, 219–224.
Conclusions
Potential competition of the acid-catalyzed conversion and the
hydrogenation of HMF and furfural were investigated in the
solvents with distinct polarities with A70 and Pd/C as the cata-
lysts. Our results showed that the solvents signicantly affected
the reaction pathways of the furans and the dominance of the
hydrogenation or the acid-catalyzed reactions in the solvents. In
the co-presence of A70 catalyst and Pd/C catalyst, the acid-
catalyzed conversion of HMF was dominant in water while the
hydrogenation of HMF was dominant in ethanol. The hydro-
genation reactions were the preferred routes in ethanol while
the acid-catalyzed reactions were effectively suppressed even in
the presence of A70. In water, it is vice versa. The distinct
reactivities of HMF towards hydrogenation and acid catalysis in
water and in alcohols need to be considered during the
upgrading of HMF to other value-added chemicals.
The competitive conversions of furfural via the acid-
catalyzed routes or the hydrogenation routes were affected
signicantly by polarities of the solvents. The hydrogenation of
furfural proceeded more effectively in the polar solvents than in
the non-polar solvents. In addition, in polar solvents the furan
ring in furfural was more reactive towards hydrogenation than
the carbonyl group in furfural. In the non-polar solvents, their
reactivities towards hydrogenation were similar. During
upgrading of the furans, the competition between acid catalysis
and hydrogenation in the varied solvents should be taken into
consideration and be controlled to facilitate the production of
the targeted products.
24 V. V. Ordomsky, J. C. Schouten, J. Schaaf and T. A. Nijhuis,
Appl. Catal., A, 2013, 451, 6–13.
25 A. D. Sutton, F. D. Waldie, R. Wu, M. Schlaf, L. A. P. Silks III
and J. C. Gordon, Nat. Chem., 2013, 5, 428–432.
26 P. Panagiotopoulou, N. Martin and D. G. Vlachos, J. Mol.
Catal. A: Chem., 2014, 392, 223–228.
27 Y. Nakagawa, K. Takada, M. Tamura and K. Tomishige, ACS
Catal., 2014, 4, 2718–2726.
28 M. Chidambaram and A. T. Bell, Green Chem., 2010, 12,
1253–1262.
Acknowledgements
This Project is 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. This project received funding from ARENA
as part of ARENA's Emerging Renewables Program.
29 X. Hu, Y. Wang, D. Mourant, R. Gunawan, C. Lievens,
W. Chaiwat, M. Gholizadeh, L. Wu, X. Li and C.-Z. Li,
AIChE J., 2013, 59, 888–900.
30 V. Amarnath and K. Amarnath, J. Org. Chem., 1995, 60, 301–
307.
References
1 V. Schiavo, G. Descotes and J. Montech, Bull. Soc. Chim. Fr.,
1991, 128, 704–711.
2 J.-P. Lange, E. Heide, J. Buijtenen and R. Price,
ChemSusChem, 2012, 5, 150–166.
31 G. Wang, Z. Guan, R. Tang and Y. He, Synth. Commun., 2010,
40, 370–377.
´
3 R. Alamillo, M. Tucker, M. Chia, Y. Pagan-Torres and
J. Dumesic, Green Chem., 2012, 14, 1413–1419.
4 Y. Nakagawa, M. Tamura and K. Tomishige, ACS Catal.,
2013, 3, 2655–2668.
5 S. Wang, V. Vorotnikov and D. G. Vlachos, ACS Catal., 2015,
5, 104–112.
32 A. Kruse and A. Gawlik, Ind. Eng. Chem. Res., 2003, 42, 267–
279.
33 J. S. Luterbacher, J. M. Rand, D. M. Alonso, J. Han,
J. T. Youngquist, C. T. Maravelias, B. F. Peger and
J. A. Dumesic, Science, 2014, 343, 277–280.
6 B. M. Nagaraja, A. H. Padmasri, B. D. Raju and K. S. R. Rao, J. 34 J. Zhang, J. Teo, X. Chen, H. Asakura, T. Tanaka, K. Teramura
Mol. Catal. A: Chem., 2007, 265, 90–97. and N. Yan, ACS Catal., 2014, 4, 1574–1583.
7 Y. Yang, C.-W. Hu and M. M. Abu-Omar, ChemSusChem, 35 J. Zhang, H. Asakur, J. Rijn, J. Yang, P. Duchesne, B. Zhang,
2012, 5, 405–410.
X. Chen, P. Zhang, M. Saeys and N. Yan, Green Chem., 2014,
16, 2432–2437.
´
8 Y. Roman-Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic,
Nature, 2007, 447, 982–985.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 4647–4656 | 4655

View Article Online
RSC Advances
Paper
36 S. Siankevich, G. Savoglidis, Z. Fei, G. Laurenczy, 41 Z.-P. Yan, L. Lin and S. Liu, Energy Fuels, 2009, 23, 3853–
D. T. L. Alexander, N. Yan and P. J. Dyson, J. Catal., 2014,
315, 67–74.
3858.
42 X. Hu, C. Lievens and C.-Z. Li, ChemSusChem, 2012, 5, 1427–
1434.
43 X. Hu, Y. Song, L. Wu, M. Gholizadeh and C.-Z. Li, ACS
Sustainable Chem. Eng., 2013, 1, 1593–1599.
44 B. Corain, M. Zecca and K. Jerabek, J. Mol. Catal. A: Chem.,
2001, 177, 3–20.
45 X. Hu, R. Westerhof, D. Dong, L. Wu and C.-Z. Li, ACS
Sustainable Chem. Eng., 2014, 2, 2562–2575.
46 X. Hu, S. Wang, R. J. M. Westerhof, L. Wu, Y. Song, D. Dong
and C.-Z. Li, Fuel, 2015, 141, 56–63.
37 P. Purwanto, R. M. Deshpande, R. V. Chaudhari and
H. Delmas, J. Chem. Eng. Data, 1996, 41, 1414–1417.
38 X. Hu, L. Wu, Y. Wang, D. Mourant, C. Lievens, R. Gunawan
and C.-Z. Li, Green Chem., 2012, 14, 3087–3098.
39 X. Hu, S. Kadarwati, S. Wang, Y. Song, M. D. M. Hasan and
C.-Z. Li, Fuel Process. Technol., 2015, 137, 212–219.
40 L. Deng, Y. Zhao, J. Li, Y. Fu, B. Liao and Q.-X. Guo,
ChemSusChem, 2010, 3, 1172–1175.
4656 | RSC Adv., 2016, 6, 4647–4656
This journal is © The Royal Society of Chemistry 2016