Communications
transfer reduction of carbonyl compounds by formate salts to
obtained after a reaction time of 6 h are compared with those
obtained with other catalysts. Interestingly, when the Au/
ZrO2-VS catalyst was used, the 1:1 aqueous mixture of LA
and FA was converted quantitatively into GVL (Table 1,
entry 1).[18] This result is remarkable, and the system has the
added advantage that the catalyst can be reused at least five
times with minimal decrease in the yield (95%; Table 1,
entry 2). Of yet further interest is the fact that the total
turnover number (TON) based on five successive runs is as
high as 7600, and is thus the highest TON value ever reported
for the reduction of LA with FA (see Table S1). Importantly,
when we used gold supported on ZrO2 prepared by a routine
deposition–precipitation (DP) method as the catalyst, which
has a larger average gold-particle size (ca. 3.0 nm), a GVL
yield as high as 95% was still possible, although a longer
reaction time was required (Table 1, entry 9). The reaction
did not proceed at all with gold-free ZrO2; thus, the presence
of gold was essential for the high activity observed in the
reduction of LA.
In subsequent experiments performed at 908C under
otherwise identical reaction conditions, the Au/ZrO2-VS
catalyst exhibited very low activity (Table 1, entry 5). When
the reaction temperature was raised to 1208C, the yield of
GVL improved dramatically from 15 to 79% (Table 1,
entry 6). At an elevated temperature of 1808C, quantitative
formation of GVL was observed within three hours (Table 1,
entry 7). The catalytic reduction of LA to GVL has been
carried out previously with a variety of homogeneous or
heterogeneous Ru catalysts in high yield in the presence of a
large excess of molecular hydrogen at high pressures (3–
10 MPa) and low LA concentrations (7 wt%)[7b] or in organic
solvents rather than water.[4b,5] Strikingly, in our case, GVL
can also be produced in nearly quantitative yield from a
concentrated aqueous solution of LA (50 wt%) only in the
presence of FA produced in the original chemical hydrolysis
step (Table 1, entry 8).[19] The high activity of Au/ZrO2-VS
coupled with its excellent activity under highly acidic
conditions in the absence of molecular hydrogen significantly
improves the value of this gold-catalyzed LA reduction
process in terms of economic considerations and sustainabil-
ity.
A comparison with palladium, platinum, and ruthenium
NPs supported on zirconia as reference catalysts showed gold
to be far superior to other noble metals for the reduction of
LA with FA (Table 1, entries 13–17). Since it has been shown
that the choice of the support plays an important role in gold
catalysis, we also evaluated gold supported on TiO2 (Au/
TiO2). The use of this commercial catalyst resulted in the
formation of the desired product GVL in moderate yield
(Table 1, entry 10). In contrast, gold supported on silica (Au/
SiO2) and activated carbon (Au/C) were ineffective for the
reduction of LA under similar reaction conditions (Table 1,
entries 11 and 12). These results indicate that both the noble
metal and the nature of the support play a role in the catalytic
reduction of LA in the presence of FA as the sole hydrogen
source. Among the various catalysts examined in this study,
very small Au NPs deposited on ZrO2 (Au/ZrO2-VS)
exhibited by far the best catalytic performance.
give the corresponding alcohols under ambient conditions.[15]
The superior performance of Au catalysts for formate-
mediated transfer reduction led us to investigate the possi-
bilities offered by Au catalysts for LA reduction with FA as
the hydrogen source. Herein we show that gold NPs deposited
on acid-tolerant ZrO2 serve as a highly efficient and robust
heterogeneous catalyst for the generation of GVL from
biomass, and more specifically for the selective reduction of
LA with an equimolar amount of FA as the sole hydrogen
source. Moreover, we show that with a single gold catalyst, it
is possible to convert biomass-derived LA and ammonia or
primary amines into 5-methyl-2-pyrrolidone or its deriva-
tives[16] in a one-pot catalytic process; this reaction only
proceeds in the presence of FA as the reducing reagent.
We began our study by examining a diluted solution (pH
ꢀ 2.0) containing equimolar concentrations (0.43m) of LA
and FA in the presence of a catalyst (Table 1), as we expected
this setup to be the most straightforward for GVL production
from biomass-derived LA. We initially studied the reduction
of LAwith FA over a catalyst derived from very small Au NPs
(ca. 1.8 nm) supported on acid-tolerant ZrO2 (catalyst
denoted as Au/ZrO2-VS; see details in the Supporting
Information).[17] Transmission electron microscopy (TEM)
showed that a large fraction of the Au particles in this catalyst
had a diameter between 1.2 and 2.5 nm (see Figure S1 in the
Supporting Information). The time-course plot for the con-
version of LA with FA in the presence of Au/ZrO2-VS
(0.1 mol%) at 1508C is given in Figure S4, and the results
Table 1: Reductive transformation of LA into GVL with FA (1 equiv) in the
presence of various catalysts.[a]
Entry
Catalyst
t
[h]
T
[8C]
Yield of GVL
[mol%][b]
Selectivity
[mol%]
1
2[c]
3
4
5
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2-VS
Au/ZrO2
Au/TiO2
Au/SiO2
Au/C
Pd/ZrO2
Pt/ZrO2
Ru/ZrO2
Pd/C
6
6
1
3
6
6
3
6
10
6
6
6
6
6
6
6
6
6
150
150
150
150
90
99
95
68
86
15
79
99
99
95
55
1
trace
trace
trace
2
2
1
>99
>99
>99
>99
>99
>99
99
>99
99
99
99
–
–
–
99
99
99
–
6
7
120
180
150
150
150
150
150
150
150
150
150
150
150
8[d]
9[e]
10[f]
11
12[g]
13
14
15
16[h]
17[h]
18
Ru/C
ZrO2
n.r.
[a] Reaction conditions: LA (18 mmol), FA (18 mmol), metal
(0.1 mol%), water (40 mL), N2 (0.5 MPa); n.r.=no reaction. [b] The yield
was determined by GC with bis(2-methoxyethyl) ether as the internal
standard. [c] Results for the fifth run with a recycled catalyst. [d] LA
(60 mmol), FA (60 mmol), Au (0.1 mol%), water (4 mL), N2 (0.5 MPa).
[e] Au/ZrO2 was prepared by a routine DP method. [f] Au/TiO2 was
purchased from Mintek. [g] Au/C was purchased from the World Gold
Council (WGC). [h] Pd/C and Ru/C were purchased from Alfa Aesar.
7816
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7815 –7819