Angewandte
Chemie
ments of pyridine adsorption on g-Al O and Sn-SBA-15 and
2
3
Ref. [7] for Sn-beta). In contrast, high yields of furfural
> 70%) can be achieved in GVL for a variety of acid
(
catalysts containing Brønsted sites, such as sulfonic acid
functionalized catalysts (Amberlyst 70 (A70), Nafion SAC-
1
3, sulfonated carbon, and propylsulfonic acid functionalized
SBA-15), zeolites (H-ZSM-5, H-mordenite, and H-beta),
sulfated inorganic metal oxides (sulfated zirconia), and
homogenous mineral acid (0.02m H SO ).
2
4
These results suggest that the presence of Brønsted sites is
particularly important for the selective conversion of xylose
[
8]
into furfural, as suggested earlier by Weingarten et al. The
yields obtained using these catalysts are comparable to the
yields of furfural obtained when using sulfonic acid function-
alized materials in dimethyl sulfoxide (DMSO) as the solvent
[9]
(
i.e., 75% yield over sulfonic acid functionalized MCM-41).
However, the use of GVL, a biomass-derived solvent, and
zeolites having no functional groups prone to leaching, is
a substantial improvement. The use of zeolitic materials is of
particular interest because of their low cost, potential for
regeneration with a calcination treatment following deacti-
vation upon deposition of carbonaceous deposits (e.g.,
humins), and their high selectivity for furfural production
when GVL is used as the solvent. For example, the furfural
yield achieved using H-mordenite (H-M) in GVL is about
Figure 2. Furfural yield achieved over time when using H-M (^) and
A70 (&) as catalysts and a 2 wt% xylose feed. A) In GVL solvent
containing 10 wt% water. B) In water. Solid and dashed lines show the
xylose conversion over H-M and A70, respectively. All reactions were
carried out at 448 K. See Figure S3 for the furfural concentration
versus time for the cases presented in Figure 2.
8
0%, which is in contrast to the low yields of furfural reported
in the literature when using faujasite (40%) and mordenite
[10]
(
30%) in aqueous solution. Accordingly, H-M was chosen
in this work as the solid acid catalyst for further studies.
The production of xylose from hemicellulose is typically
carried out in aqueous solutions, and it is anticipated that
water will be present in xylose feed streams from biomass
resources. Therefore, we studied xylose dehydration over H-
M in GVL with various concentrations of water (0–20 wt%).
As the water concentration increased, the rate of furfural
production decreased, and this effect of water was particularly
significant at water concentrations higher than 10 wt% (see
Figure S2 in the Supporting Information). The presence of
[
11]
furfural yields. Several smaller molecules, such as formic
acid, formaldehyde, and acetaldehyde can be produced from
[12]
furfural fragmentation. In resinification reactions, furfural
reacts with another furfural molecule, while in condensation
reactions furfural can couple with pentose molecules or
intermediates. Therefore, the overall mass balance for this
reaction is similar to the furfural selectivity. Unidentified
products are common degradation products, such as solid
humins and soluble polymers. For example, in the best
reaction conditions for xylose conversion, that is, using H-M
as the catalyst and GVL with 10 wt% water as the solvent, the
products identified were 81% furfural and 4% formic acid
(Table 1, entry 1).
The reactant/catalyst mass ratios were adjusted for the
experiments in Figure 2 such that the initial rates for
degradation of furfural (through fragmentation and resin-
ification) would be similar in both solvent systems (see
Figure S4). At these conditions, as xylose conversion
increases, the yield of furfural decreases more significantly
over A70 compared to H-M, especially in the water. This
behavior suggests that as furfural is being formed from xylose,
it undergoes degradation not only by fragmentation or
resinification reactions, but also by condensation reactions
between furfural and reaction intermediates from xylose, and
such processes are more prominent over A70 in water.
The importance of furfural condensation reactions was
investigated further by conducting experiments using a feed
containing both xylose (2 wt%) and furfural (1 wt%) over
both catalysts in GVL (containing 10 wt% water; Figure 3A),
1
0 wt% H O was found to be acceptable as furfural could still
2
be obtained with high yields and rates. In addition, the
presence of water was found to facilitate the separation of
furfural from GVL due to the strong interactions between
water and furfural (see Table S1 in the Supporting Informa-
tion).
The effect of water on the rate of furfural formation
versus furfural degradation was studied by running experi-
ments in pure water as well as in GVL (containing 10 wt%
water) in the presence of H-M and A70 as the catalyst
(
Figure 2). A70 is a Brønsted acid catalyst commonly utilized
for sugar dehydration reactions. The data in Figure 2A show
the furfural yields and xylose conversions achieved over time
when starting from 2 wt% xylose in GVL; the data for both
H-M and A70 are shown. For comparison, the data in
Figure 2B show the results obtained from the same experi-
ments run in water. For both catalysts, the furfural yield
achieved is significantly lower when pure water is used as the
solvent. The presence of water has been reported to accel-
erate furfural degradation reactions (i.e., fragmentation,
resinification, and condensation), thereby decreasing the
Angew. Chem. Int. Ed. 2013, 52, 1270 –1274
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1271