Angewandte
Chemie
were 84% and 80%, respectively (Table 3, entries 9 and 10).
The observations indicate that it is possible to recycle the
RuCl /PPh catalyst and considerably reduce the cost of the
3
3
catalytic conversions.
Compared to PPh , other hydrophobic ligands such as
3
PCy3 and dppe provide slightly lower yields (Table 3,
entries 11 and 12). On the other hand, it is surprising to find
that water-soluble phosphine ligands including tppms and
tppts give significantly lower yields of the products (ca. 50%;
Table 3, entries 13 and 14). Furthermore, in the absence of
any phosphine ligand, the yield is even lower (21%, Table 3,
entry 15).
To demonstrate the practical potential of the process, we
applied the method to the conversion of glucose into GVL.
First, through acidic dehydration (catalyzed by 0.8m HCl at
2
208C in an autoclave) of glucose (400 mL, 15 wt% aqueous
solution), we obtained a mixture of LA and formic acid. After
pH neutralization and distillation (to remove some water) to
give a total volume of 50 mL, the aqueous mixture contained
4
2 wt% LA and 17 wt% formic acid. To this mixture we
added the RuCl /PPh3 catalyst (0.1 mol%) and pyridine;
3
catalytic hydrogenation at 1508C produced GVL in 83%
yield. The overall yield from glucose is 48%. Thus, we
demonstrate that this new technology can be used to convert
glucose into GVL with good efficiency.
Figure 1. Effect of CO on the hydrogenation of LA. Reaction condi-
tions: 1508C, 12 h, 200 mmol LA, 20 mmol pyridine, 0.2 mmol RuCl3,
2
Finally, intrigued by the surprisingly good performance of
0.6 mmol PPh , and 4 MPa H . a) Reaction conducted in 50 wt%
3
2
2
water; b) Reaction conducted with 4 MPa CO (black) and without CO2
the water-insoluble PPh ligand in the Ru-catalyzed hydro-
3
(gray).
genation in aqueous media, we also examined the RuCl3/
PPh -catalyzed hydrogenation of LA in 50 wt% aqueous
3
solution with 4 MPa H2 in the absence of formic acid.
Surprisingly the yield of this hydrogenation reaction is
actually low (45%, Figure 1a). In comparing the results
from direct hydrogenation and hydrogenation using formic
acid as the H source, we suspect that the CO produced in the
decomposition of formic acid may be an overlooked factor.
Indeed, when we intentionally added CO2 to the direct-
hydrogenation system, a steady increase of yield was observed
purification and without using external H supply. A striking
2
positive CO effect on the Ru-catalyzed hydrogenation was
2
observed, which may be used to explain the good results of
the aqueous hydrogenation using water-insoluble ligands. In a
model experiment with glucose as a biomass-derived carbo-
hydrate GVL was produced in 48% yield by an operationally
simple sequence (i.e. acidic dehydration followed by Ru-
catalyzed hydrogenation). Further optimization may even-
tually make the approach industrially viable for the trans-
formation of non-food biomass into GVL as a valuable
chemical.
2
2
(
Figure 1a). The maximum yield (ca. 100%) was obtained
when the at a CO pressure of 4 MPa (equal to the pressure of
2
H ).
2
Additional experiments showed that the concentration of
water also has an effect. In neat LA, the yield was 100% with
4
MPa CO and 78% without CO . Almost identical yields
2 2
were obtained with or without CO when 25 wt% water was
added. When 50 wt% water was added, the yield was 100%
2
Experimental Section
All catalysts and chemicals were obtained commercially and used
without further purification. All the catalytic experiments were
carried out in a 100 mL autoclave made of zirconium alloy. Before
with CO2 and 45% without CO . When 75% water was
2
added, the positive effect of adding CO vanished. All the
2
each run the autoclave was purged with N to exclude air. The mixture
2
above observations indicate that adding CO2 can greatly
improve the Ru-catalyzed hydrogenation. The rationale for of
the positive CO2 effect on hydrogenation remains to be
of substrates and catalyst was heated to the desired temperature in
less than half an hour with stirring at 1000 rpm. To recover the
catalyst, the reaction mixture was subjected to vacuum distillation to
remove water and GVL. The residue was added to the mixture of LA
and formic acid for the next run. For direct hydrogenation using H2,
examined future studies. Nonetheless, the positive CO effect
2
explains why simple the RuCl /PPh catalyst can promote the
3
3
aqueous hydrogenation of LA efficiently.
the autoclave was pressurized with H
2
to 2 MPa at room temperature
three times. Then H with or without CO was filled to 8 or 4 MPa.
In summary, we have demonstrated that an inexpensive,
recyclable RuCl /PPh /pyridine catalyst system can be used to
2
2
After the reaction, the mixture was analyzed by GC-MS (Thermal
Trace GC Ultra with a PolarisQ ion trap mass spectrometer)
equipped with a TR-35MS capillary column (30 m ꢁ 0.25 mm ꢁ
3
3
convert a 1:1 aqueous mixture of levulinic acid and formic
acid into g-valerolactone in high yields. This process may find
important applications for the efficient production of GVL
from biomass-derived carbohydrates without intermediate
0.25 mm).
The carbohydrates were hydrolyzed in 0.8m HCl at 2208C in a
500 mL stainless-steel autoclave with vigorous stirring for 1 h. The pH
Angew. Chem. Int. Ed. 2009, 48, 6529 –6532
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6531