circles. The mean diameter of Ru particles was 9 nm, calculated
by the Scherrer’s equation, which was reported as the best
particle size for the selective hydrogenation of cellobiose
using H2 of 5 MPa.11 Therefore, we conclude that the highly
dispersed cationic Ru species is active for the transfer hydro-
genation reaction, whereas Ru metal nanoparticles are inactive
for this reaction.
In summary, Ru/AC(N), Ru/C-Q10 and Ru/CMK-3 catalysts
were active for the transfer hydrogenation of cellulose to sugar
alcohols. Ru/AC(N) also catalysed the hydrogenation of cellulose
at a H2 pressure as low as 0.8 MPa. The catalytic activity of
Ru/AC(N) is significantly different from those of the typical
catalysts for the cellulose hydrogenation, such as Ru/Al2O3,
which require high pressures of H2. It is proposed that the active
species for the transfer hydrogenation is a cationic Ru species.
We thank Prof. W. Ueda for the H2-TPR study. This work
was financially supported by JSPS KAKENHI (20226016).
Fig. 1 XRD patterns of (a) AC(N), (b) Ru/AC(N) and (c) the
differential pattern of (b)–(a).
AC(N) gave a broad scattering pattern of amorphous carbon
(2y = 231, 431 and 801) and sharp diffraction peaks of quartz,
which is an impurity of AC(N) [Fig. 1(a)]. After the impregna-
tion of Ru, no diffraction peaks of the Ru metal appeared in
the XRD [Fig. 1(b) and (c)], even when the Ru loading was
increased to 10 wt% (Fig. S2, ESIw). These data show that the
Ru species on AC(N) was highly dispersed and/or not fully
reduced to zero-valent particles during the catalyst preparation.
We performed H2-temperature programmed reduction (TPR)
of Ru/AC(N) (Fig. S3, ESIw), and 4 peaks appeared at 410,
530, 670 and 860 K with the H/Ru atomic ratios of 13, 3.5, 2.9
and 11, respectively. The second or the third peak might be
assigned to the reduction of the Ru species by correlating the
peak areas with the Ru concentration, suggesting tri- or tetra-
valent Ru. Other peaks are due to the reduction of the surface
functional groups on the carbon support such as quinones and
aromatic rings. In the XPS analysis, the electron binding
energy of Ru 3p3/2 for the Ru/AC(N) catalyst was 463.1 eV,
which is in the range of tri- to tetra-valent states and higher
than that for the Ru metal (461.9 eV, Fig. S4, ESIw). Fig. S5
(ESIw) shows the curve fitting of the XPS data for Ru/AC(N),
and the spectrum was fitted by those of RuO2 (99.7%) and Ru
metal (0.3%), giving a similar electronic state to that of RuO2.
It is thus indicated that the Ru species on AC(N) is not metal
but tetra- or tri-valent. Fig. 2 shows the XRD patterns of
Al2O3, Ru/Al2O3 and the differential pattern of Ru/Al2O3
minus Al2O3. The pattern of Al2O3 indicated the presence of
a g-Al2O3 phase [Fig. 2(a)]. After the loading of Ru, the
diffraction pattern of the Ru metal was observed with the
peaks of g-Al2O3 [Fig. 2(b)]. The differential pattern [Fig. 2(c)]
clearly showed the peaks of the Ru metal, marked with black
Notes and references
z The catalyst supports used in this study are as follows. CMK-1 and
CMK-3 were synthesised according to procedures in the literature.12
Carbon (C-Q10, BET surface area 840 m2 gꢀ1) was prepared using an
amorphous silica (Q-10, Fuji Silysia) in the same manner as for CMK-3.
Activated carbons were purchased from Wako (activated charcoal),
denoted as AC(W) and Aldrich (SX Ultra, Norit), denoted as AC(N).
Carbon blacks (VULCAN XC72 and BP2000) were supplied from
Cabot. Other supports are TiO2 (P-25, Degussa), ZrO2 (JRC-ZRO-2,
Catalysis Society of Japan) and Al2O3 (JRC-ALO-2, Catalysis Society
of Japan). Supported Ru catalysts (Ru metal loading 2 wt%) were
prepared by a conventional impregnation method as follows: RuCl3 aq.
(0.202 mmol in 5 mL of water) was dropped into a mixture of a catalyst
support (1.00 g) and water (20 mL), and the mixture was stirred for
16 h. After drying in vacuo, the solid was reduced in a fixed-bed flow
reactor with H2 (30 mL minꢀ1) at 673 K for 2 h. In the cases using oxide
supports, the precursors were calcined with O2 (30 mL minꢀ1) at
673 K before the H2 reduction. Transfer hydrogenation of cellulose
was carried out in a stainless steel (SUS316) high-pressure reactor
(OM Lab-Tech MMJ-100, 100 mL). The detail is described in the ESI.w
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Fig. 2 XRD patterns of (a) Al2O3, (b) Ru/Al2O3 and (c) the
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c
2368 Chem. Commun., 2011, 47, 2366–2368
This journal is The Royal Society of Chemistry 2011