Angewandte
Chemie
topology and nanosheet morphology (Al-MFI-ns) as
a Brønsted acid catalyst. Both TH steps result in the oxidation
of the hydrogen donor, which can be readily separated from
the products and regenerated under mild gas-phase condi-
tions by the use of inexpensive catalysts (e.g., Ni or Cu) that
are not active, selective, or stable for the production of
GVL.[19] Importantly, the catalysts, solvents, and mild reaction
conditions used in this study are compatible with those
required to convert hemicellulose into Fur in high yields.[18a]
Therefore, this catalytic system offers an attractive stream-
lined strategy for the production of GVL from lignocellulosic
biomass without the use of precious metals or high pressures
of molecular H2.
The conversion of methyl levulinate (ML) into GVL via
methyl/butyl 4-HPs was used to screen the TH activity of
various solid Lewis acids in the presence of 2-butanol as the
hydrogen donor. Of the two TH steps proposed in Scheme 1,
the conversion of ML into methyl 4-HP features a less
favorable thermodynamic equilibrium because both methyl 4-
HP and 2-butanol are secondary alcohols. However, the
spontaneous lactonization of methyl 4-HP to give GVL drives
the reaction to completion. Zr-Beta showed the highest
activity of all solid Lewis acid zeolites studied, with the
formation of GVL in 97% yield at 393 K and an initial GVL
production rate of 0.026 mmolGVL(mmolZr)À1 sÀ1 (Table 1,
of magnitude slower than the conversion of ML (see Table 1
entries 2 and 10), and the low initial selectivity for the
formation of GVL (28%) is attributable to the esterification
of LA with 2-butanol. Quantitative conversion of LA/BL into
GVL occurred during longer reaction times (Table 1,
entry 11). Importantly, the catalyst appears to be very
stable. It showed no signs of deactivation after 48 h on
stream in a packed-bed flow reactor (see Figure S10 in the
Supporting Information). Zr-Beta also converted Fur into FA
and
FE
quantitatively
with
rates
exceeding
0.33 mmolGVL(mmolZr)À1 sÀ1 (Table 1, entry 1).
In a comparative study of several hydrogen donors, Zr-
Beta maintained its TH activity and high GVL selectivity for
all systems investigated (see Table S1 in the Supporting
Information). Such versatility is important because Fur is
produced in many different solvents and, to minimize unit
operations, the subsequent conversion of Fur into GVL
should be performed in the same solvent system. When
primary alcohols, secondary alcohols with alkyl chains longer
than C4, or solvent mixtures with lower hydrogen-donor-to-
substrate ratios were used (e.g., the use of THF with
4 equivalents of 2-butanol), the reaction rate was slower
than when pure 2-butanol or 2-propanol was used. Notably,
selectivity for the formation of GVL always remained above
95%, regardless of the solvent used. Recently, it was shown
that GVL itself is an excellent solvent for the production of
Fur from hemicellulose.[20] Zr-Beta generated quantitative
amounts of GVL from ML and LA in a 1:1 (w/w) GVL/2-
butanol solvent mixture (see Table S1); thus, the reaction
product may also be used as an effective reaction solvent.
Next, the ring opening of FA/FE to LA/BL was coupled
with the two TH steps to complete the “one-pot” conversion
of Fur into GVL. Ring opening by hydrolytic cleavage of the
entry 2). In contrast, Sn-Beta showed
a reactivity of
0.0014 mmolGVL(mmolSn)À1 sÀ1; with this catalyst, 26%
conversion was observed after 5 h with a selectivity of 95%
(Table 1, entry 5). Al-Beta, Ti-Beta, and Al-MFI-ns showed
drastically lower reaction rates (Table 1, entries 6–8). The
detectable TH activity of Al-based zeolites is attributed to the
Lewis acidity of extraframework Al species. ZrO2 was
unreactive under the present reaction conditions (Table 1,
entry 9). The conversion of LA into GVL was almost an order
À
furanic C O bond is typically promoted with Brønsted acids,
such as aluminosilicates and ion-exchange resins. These
catalysts, however, may also promote side reactions that
result in the formation of soluble high-molecular-weight
species and insoluble humins. Of all acids tested in the initial
screening, Al-Beta, Amberlyst-70, and Al-MFI-ns generated
GVL in the highest yield (44, 66, and 62%, respectively;
Table 2, entries 1, 4, and 10). However, Amberlyst-70 and Al-
Beta also converted 8 and 16 wt%, respectively, of the 2-
butanol solvent into sec-butyl ether, whereas Al-MFI-ns
converted less than 2 wt% of the 2-butanol. The crystalline
nanosheet morphology of Al-MFI-ns provides larger surface
areas and shorter molecular-diffusion lengths as compared to
those of micron-sized zeolite crystals, as well as improved
hydrothermal stability as compared to that of amorphous
mesostructured silicates.[21] The use of Al-MCM-41 and Al-
MFI zeolite (crystal size ca. 300 nm) led to the formation of
GVL in moderate yields, whereas sulfuric acid produced
virtually no GVL. Sulfuric acid fully converted FA/FE
mixtures into levulinate derivatives, but GVL yields were
low owing to slow rates for the second TH step. Besides FA,
FE, LA, and BL, the only other intermediate detected in
significant concentrations was 5-methyl-2(5H)-furanone
(MF), which is the product of LA dehydration.[6] The 4-HPs
were never observed, which suggests that the lactonization is
very fast under the conditions investigated.
Table 1: Transfer-hydrogenation reactions with solid Lewis acids in 2-
butanol.[a]
Entry Catalyst
Feed[b] Product[c]
t
[h]
Conv.[d] Sel.[e] Prod.
[%]
[%]
rate[f]
1
2
3
4
5
6
7
8
9
Zr-Beta
Zr-Beta
Zr-Beta
Zr-Beta
Sn-Beta
Al-Beta
Ti-Beta
Fur
ML
ML
ML
ML
ML
ML
FA+FE
GVL
GVL
GVL
GVL
GVL
GVL
GVL
GVL
GVL
GVL
0.08
0.25
1
5
5
5
5
5
5
98
25
60
97
26
11
1
96 327
95 26
>99 17
>99 5.4
95 1.4
98 0.6
94 0.05
66 0.02
Al-MFI-ns[g] ML
5
[h]
ZrO2
ML
LA
LA
<1
12
98
–
–
10
11
Zr-Beta
Zr-Beta
0.25
11
28 3.7
>99 2.5
[a] Reaction conditions: Lewis acid catalyst: 1 mol%, reactant: 6.7 wt%
((molreactant)/(molmetal)=100), 393 K. [b] Fur, furfural; ML, methyl
levulinate; LA, levulinic acid. [c] FA, furfural alcohol; FE, furfuryl ether;
GVL, g-valerolactone. [d] Conversion: ([mmol(feed converted)]/[mmol
(feed initial)])ꢁ100. [e] Selectivity: ([mmolproduct]/[mmol(feed con-
verted)])ꢁ100. [f] Production rate: [(mmolproduct)(mmolme-
tal)À1 sÀ1]ꢁ10À3. [g] The reaction was carried out with 6.25 mol% Al (i.e.,
16:1 ML/Al(tetrahedral)). [h] The reaction was carried out with a 1:5
ZrO2/ML mass ratio; ZrO2 had a particle size <100 nm and a surface
area ꢀ25 m2 gÀ1
.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
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