compared to the other catalysts, Ru/C showed excellent perfor-
mance of hydrogenation giving a TOF of 5345 hÀ1. However,
the yield of GVL from the mixture of LA and formic acid cata-
lyzed by Ru/C was rather low (only 7% for 12 h), which could
be attributed to its inability to catalyze the decomposition of
formic acid for H2 generation and the low efficiency of transfer
hydrogenation at 1508C. The TOF of formic acid over Ru/C was
less than 10 hÀ1 and only 1% of formic acid was converted
after 12 h.
Scheme 2. Synthesis of immobilized RuII catalysts.
the reaction and 2) the following hydrogenation of LA in the
presence of high pressure H2 and CO2 as well as the catalyst.
Thus the three immobilized catalysts were tested in both re-
actions separately. The decomposition of formic acid was car-
ried out in 4m formic acid (HCOOH/HCOONa=9:1) aqueous
solution at 1208C. Table 1 (entries 1–3) shows that the immobi-
lized catalysts can decompose formic acid in water with high
Regarding the fact that CO2 derived from formic acid may
promote the Ru-catalyzed hydrogenation of LA, a mixture gas
of H2 and CO2 (1:1, 8 MPa) was employed for the hydrogena-
tion test with RuÀP/SiO2 or Ru/C. However, the TOFs of LA
(Table 1, entries 3 and 4) indicated no obvious positive effect
on both catalysts under the reaction condition in this study.
As we predicted, when using the RuII catalysts for transform-
ing the aqueous mixture of LA and formic acid, the overall
TOFs of the combined reactions decreased in the same order
as in LA hydrogenation. Compared to the other two catalysts,
Ru-P/SiO2 gave higher overall TOFs due to its better perfor-
mance for LA hydrogenation. By prolonging the reaction time
to 12 h, RuÀP/SiO2 gave a yield of 96% which is the highest
record in conversion of aqueous LA solution using equimolar
formic acid and demonstrated that RuÀP/SiO2 is a bifunctional
catalyst. Moreover, only 42 ugmlÀ1 Ru was detected in the re-
sulting mixture, corresponding to 1.4% Ru leaching of fresh
RuÀP/SiO2. According to a control experiment in which RuÀP/
SiO2 was separated from the mixture after 1 h and fresh formic
acid was added, less than 1% of GVL was generated in the
presence of the leached Ru species, indicating the reaction
was catalyzed by RuÀP/SiO2, not by leached Ru species.
Based on combined transmission electron microscopy (TEM)
microphotographs and energy-dispersive X-ray (EDX) spectros-
copy, ruthenium is presented atomically in Ru-P/SiO2 without
detectable clusters or nanoparticles (see the Supporting Infor-
mation, Figure S1). The X-ray photoelectron spectroscopy (XPS)
revealed that the Ru species in fresh Ru-P/SiO2 catalysts were
present in the RuII state, corresponding to the peaks with bind-
ing energies of 286.55 eV and 282.13 eV in 3d3/2 and 3d5/2
levels, respectively (Figure S2). The CP–MAS 29Si NMR (CP=
cross-polarization, MAS=magic angle spinning) measurements
clearly showed that Ru-P/SiO2 gave rise to a strong peak at
111.5 ppm and a weak peak at 65.2 ppm corresponding to the
NMR signals of siloxane [Qn =Si(OSi)n(OH)4Àn] and organosilox-
ane [Tm =RSi(OSi)m(OH)3Àm]. The Tm/(Qn+Tm) ratio was about
0.06. In addition, the CP–MAS 31P NMR spectra of Ru-P/SiO2 dis-
played two peaks at 38.7 ppm and 24.9 ppm corresponding to
the ÀPPh2 groups coordinated with Ru and free ÀPPh2 groups.
The relative integrated intensities of the two peaks were
0.82:1. Based on this value and total amount of ÀPPh2 groups
measured by inductively coupled plasma atomic emission
spectroscopy (ICP–AES) (0.39 mmolgÀ1), the amount of ÀPPh2
groups coordinated with Ru was about 0.17 mmolgÀ1. Com-
pared with the Ru content (0.14 mmolgÀ1), this suggested that
most Ru species were anchored by the ÀPPh2 groups. (see the
Supporting Information for more details of catalyst characteri-
zation).
Table 1. Transformation of LA and formic acid over heterogeneous cata-
lysts.
Entry Catalysts TOF of formic acid
TOF of LA
Overall TOF of LA
[a]
[b]
[c]
[hÀ1
]
[hÀ1
]
[hÀ1
]
1
2
3
RuÀN/
SiO2
RuÀS/
SiO2
RuÀP/
SiO2
Ru/C
Pd/C
Pt/C
1428
7357
3295
76
112
142
173
583 (453)[d]
447 (96%)
4
5
6
<10
<10
<10
5345 (5638)[d] 69 (7%)
68
16
Reaction conditions: [a] 4m formic acid (20 mL; HCOOH/HCOONa=9:1)
and catalyst (0.01 mmol, based on metal content) at 1208C for 1 h. [b] LA
(80 mmol), H2O (10 g), 4 MPa H2 and catalyst (0.06 mmol; 0.01 mmol Ru/
C) at 1508C for 1 h. [c] LA and formic acid (80 mmol), H2O (10 g), NaOH
(8 mmol), and catalyst (0.06 mmol) at 1508C for 1 h; the value in paren-
theses is the yield of GVL obtained under the same conditions for 12 h.
[d] The value in parenthesis is obtained under 4 MPa H2 and 4 MPa CO2.
efficiencies. The highest turnover frequency (TOF) of formic
acid was 7357 hÀ1 using RuÀS/SiO2 and even the worst one,
RuÀN/SiO2, gives a TOF of 1428 hÀ1 which is still much higher
than the TOFs of LA for hydrogenation. The decomposition of
formic acid in the presence of immobilized catalyst is carried
out selectively and no CO was detected by GC (see the Sup-
porting Information, Figure S7). Similar results were also report-
ed by Laurenczy et al. with an aim of developing H2 storage
system.[12] On the other hand, 4 MPa H2 was used for hydroge-
nation of 4m LA aqueous solution at 1508C. The hydrogena-
tion performances of the three catalysts decreased in the order
RuÀP/SiO2 >RuÀS/SiO2 >RuÀN/SiO2. The RuÀP/SiO2, which is a
simulate of RuCl3/PPh3 or [Ru(PPh3)3Cl2], showed the highest
activity (TOF=583 hÀ1) whereas the RuÀS/SiO2 and RuÀN/SiO2
gave TOFs of 142 and 76, respectively. Thus it is clear that the
hydrogenation of LA in water is the rate determining step for
the conversion of LA and formic acid in the presence of RuÀP/
SiO2 or RuÀS/SiO2 or RuÀN/SiO2. Furthermore, Ru/C, Pd/C and
Pt/C were also tested under 4 MPa H2 atmosphere. Remarkably,
ChemSusChem 2010, 3, 1172 – 1175
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