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HDO of HMF under 6.8 bar H2.[5] Raines et al. reported the pro-
duction of DMF from untreated corn stover, giving 9% DMF
yield based on the cellulose content of the corn stover.[6]
Raines’s two-step process of DMF preparation involved the
CrCl3–HCl catalyzed transformation of corn stover into HMF,
followed by HDO of HMF to DMF using Cu–Ru/C catalyst in
the presence of H2. In this process, toxic chromium salt along
with mineral acid was used as a catalyst for the degradation of
corn stover into HMF. Sen et al. reported the conversion of car-
bohydrates to 2,5-dimethyltetrahydrofuran (DMTHF)[13] in 81%
yield using homogeneous RhCl3/HI catalyst. In the same year,
Bell et al. attempted HMF HDO with activated carbon (AC) sup-
ported palladium, ruthenium, and rhodium catalysts in 1-ethyl-
3-methylimidazolium chloride ([EMIM]Cl) solvent under 62 bar
H2 pressure.[14] Among several reactions, the Pd/C-catalyzed
HDO reported maximum 19% HMF conversion with 13% DMF
selectivity. Although both HMF conversion and DMF selectivity
improved to 47% and 32%, respectively, when acetonitrile was
mixed with [EMIM]Cl solvent, total DMF yield (15%) was not
high enough. An additional drawback of this method was that
ionic liquid decreased the solubility of H2. Hence, high pres-
sures of H2 (62 bar) were required, making the process energy-
intensive. Under similar reaction conditions, the authors
showed that Ru/C catalyst failed to produce DMF from HMF.
Most recently, Ru/Co3O4 has been reported as an effective cata-
lyst for HMF HDO in THF.[15] The only disadvantage of this pro-
cess is that the reaction requires high catalyst loading (40 wt%
based on substrate loading).
of the combined ZnCl2–Pd/C catalyst for a simple reaction
system, we investigated HMF HDO under mild reaction condi-
tions.
Catalyst screening
A preliminary reaction for HMF HDO with combined Pd/C and
ZnCl2 catalytic species was carried out in MeOH solvent at
1508C and 20 bar H2 pressure by using 0.04 g Pd/C, 0.04 g
ZnCl2, and 0.2 g HMF (1.58 mmol). The yield of DMF was 39%
after 2 h with about 75% conversion of HMF. The yield of DMF
remained unchanged when the same reaction was repeated
for 4 and 8 h. Based on previous reports,[18] it is hypothesized
that etherification of HMF with MeOH in the presence of the
Lewis acid ZnCl2 results in the formation of 5-methoxymethyl-
furfural (MMF), and hence blocks the hydroxymethyl group of
HMF from the desired HDO reaction. Therefore, we used THF
as a solvent for HMF conversion by adopting the following ex-
perimental conditions: HMF=3.96 mmol (0.50 g), Pd/C=
0.05 g, ZnCl2 =0.05 g, and THF=15 mL at 1508C and 22 bar of
H2. The later reaction achieved 85% DMF yield with complete
HMF conversion in 8 h (Supporting Information, Figure S3).
This DMF yield is about 82% higher than that observed by Bell
and co-workers[14] using Pd/C catalyst alone at 62 bar H2 pres-
sure. The GC–MS analysis of the product solution showed the
peaks for MTHFA and HD, accounting for their respective
yields of 2.6% and 1.6%. Other products (OP) accounting for
9–10% of total carbon mass balance corresponds to the un-
identified peaks between retention times 9.6 to 12 min as
shown in the GC chromatogram (Figure S3). Another reaction
at lower H2 pressure (8 bar) gave similar DMF yield (84%). A
control experiment without ZnCl2 produced only 27% DMF
under comparable reaction conditions between HMF and Pd/C
catalyst (Supporting Information, Figure S4). As evident by the
comparison in Figure 1, a small amount of ZnCl2 improved
DMF yield as much as 60%. ZnCl2 alone did not catalyze the
reaction as HMF remained unconverted without Pd/C.
Recently, we have demonstrated that Pd/C is very effective
catalyst in the presence of a small amount of Lewis-acidic ZnCl2
for HDO of monomeric lignin surrogate molecules.[16] Herein,
we demonstrate the benefit of a Pd/Zn/C catalytic system for
the conversion of HMF to DMF under mild reaction conditions
and low H2 pressures. From the two solvent systems studied,
tetrahydrofuran (THF) is a more effective solvent than metha-
nol. The synergistic effect of ZnCl2 with Ru/C and Ni/C catalysts
has also been investigated for HMF HDO under comparable re-
action conditions. The results show that the Pd/Zn/C catalyst is
most effective for quantitative conversion of HMF to DMF with
very high selectivity. The effectiveness of the later catalyst is
also examined for one-pot conversion of fructose to DMF.
Assuming that the acidity of ZnCl2 could be the reason for
the enhanced activity of the Pd/Zn system, we tested the activ-
ity of the Pd/C catalyst in the presence of a small amount of
Amberlyst-15 as co-catalyst. Under comparable conditions, a re-
action using 0.025 g Amberlyst-15 in place of ZnCl2 resulted in
the formation of the furan ring-hydrogenation product of DMF,
2,5-dimethyl tertrahydrofuran (DMTHF), as a major product
(13%), along with small amount of DMF (6%) and other ring-
hydrogenation intermediate products such as 2-hexanone
(1.6%), tetrahydrofurfural alcohol (THFA) (3%), MTHFA (6%), 2-
methyl-tetrahydrofuran-5-aldehyde (4.5%), 2,5-bis(dihydroxy)
tetrahydromethyfuran (13%), and some unidentified peaks
(Supporting Information, Figure S5). The formation of multiple
ring-hydrogenation products has also been observed for HMF
Results and Discussion
AC-supported palladium, rhodium, and ruthenium catalysts are
effective for hydrogenation of organic substrates at low tem-
perature and pressure. However, these catalysts have proved
ineffective for hydrogenation of HMF even at high pressures of
H2 (62 bar).[14] A recent study has demonstrated a synergistic
effect between zinc and palladium, and reported a significant
improvement in yields and selectivity of the corresponding hy-
drodeoxygenation products from their respective phenolic al-
cohol and aldehyde precursors.[16] Mechanistic studies have
suggested Zn2+ adsorption onto AC, and the resulting bimetal-
lic catalyst activated phenolic substrates via binding to ÀOH
groups and inducing reactivity with PdÀH sites on the surface
via hydrogen spillover. To examine the catalytic effectiveness
[19]
HDO using HI/RhI3 and ruthenium–porous metal oxide (Ru–
PMO)[20] catalysts. This result demonstrates that the acidity of
ZnCl2 is not the only reason for the enhanced activity of the
catalyst, resulting in high DMF yield, as the reaction in the
presence of Amberlyst-15 favored furan ring hydrogenation
with the formation of multiple products.
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