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aluminium sec-butoxide in the presence of the metal. This mode of
preparation was developed by the group of Park who reported that
the AlO(OH)-entrapped metal nanoparticles form highly active cat-
alysts [30–33]. Optimum reaction conditions were established with
benzaldehyde as a model compound (Eq. (2)) before investigating
the scope of the reaction for other aldehydes and ketones.
Parr autoclave at 100 ◦C. Due to their lower reactivity, the catalytic
transfer hydrogenation of ketones was carried out using 200 mg of
2 wt.% Ru/AlO(OH). For recycling tests, the used catalyst was recov-
ered by centrifugation, washed with water followed by ethanol and
dried at room temperature before use.
Ru/AlO(OH)
OH
O
+ KHCO3
+ HCOOK + H2O
(2)
2. Experimental
3. Results and discussion
2.1. Catalyst preparation
3.1. Catalyst characterization
The Ru/AlO(OH) catalysts were synthesized following Ref. [30],
with a slight modification where ethanol was replaced with
2-butanol. In a typical preparation for 1 g of 1 wt.% Ru/AlO(OH) cata-
lyst, RuCl3·xH2O (20.6 mg, 0.1 mmol), (sec-BuO)3Al (4 g, 16.5 mmol)
and 2-butanol (2.4 mL, 26.2 mmol) were added into 50 mL round
bottom flask equipped with condenser. After stirring the solution
at 100 ◦C for 1 h, water (4 mL, 22.2 mmol) was added and the stir-
ring was continued for another 30 min. The resulting black solid
was filtered, washed with acetone, and dried in the air at room
temperature. Samples with ruthenium loadings of 0.5, 2, 5, 8 and
10 wt.% were prepared by the same procedure as well as one con-
taining only AlO(OH). Similarly, 1 wt.% of Ag, Cu, and Ni entrapped
in AlO(OH) were prepared using AgNO3, Cu(NO3)2 and Ni(NO3)2 as
precursors, respectively.
For comparison, a 1 wt.% Ru(OH)x/Al2O3 catalyst (wt.% refers to
Ru) was also prepared by the wet impregnation method. To prepare
1 g of catalyst, ␥-Al2O3 (0.99 g) was added to 20 ml aqueous solution
of RuCl3·xH2O (20.6 mg, 0.1 mmol). The suspension was stirred at
room temperature for 1 h before adjusting the pH to 13 by adding
1 M NaOH dropwise. After stirring for another 5 h, the solid was
filtered off, washed with deionized water and dried at room tem-
perature.
2ꢀ ∼ 28.2◦, 38.3◦, 48.9◦, 54.9◦ and 64.2◦, which are characteristic of
AlO(OH) (JCPDS Card No. 21-1307). No peaks related to ruthenium
were detected for the Ru/AlO(OH) samples even when the loading
increased to 10 wt.% (Fig. 1). This may be due to the small ruthenium
decreased for ruthenium loadings of 8 and 10 wt.%.
The 0.5 wt.% Ru/AlO(OH) sample had a high surface area of
476 m2 g−1 and a large pore volume of 1.36 cm3 g−1, similar to the
textural properties of the support (Table 1). The nitrogen isotherms
of the sample showed a Type 4 hysteresis indicating the presence
of mesopores (Fig. S1a). The sample contained pores that were
bimodally distributed with a narrow distribution of smaller pores
from 2 to 5 nm and a broader distribution of larger pores from 5 to
18 nm (Fig. S1b). With higher ruthenium loading, both the surface
area and pore volume decreased such that for 10 wt.% Ru, the sur-
face area was only 152 m2 g−1 and the pore volume 0.18 cm3 g−1
.
Furthermore, the larger pores of 5–18 nm decreased with load-
ing and samples with 5–10 wt.% Ru only had pores of 2–5 nm. The
absence of larger pores above 5 nm in these higher loaded samples
might at first suggest pore blockage by large ruthenium particles.
However, TEM measurements showed that the ruthenium parti-
cles for 10 wt.% Ru/AlO(OH) were only slightly larger than in 1 wt.%
Ru/AlO(OH) (Fig. 2). The mean particle size was 1.5 nm and 1.8 nm
for 1 and 10 wt.% Ru loading, respectively. Taking this and the
2.2. Catalyst characterization
The surface area and pore volume of the catalysts were deter-
mined by nitrogen adsorption (Micromeritics Tristar 3000). Prior
to the measurement, the sample was degassed under nitrogen at
100 ◦C for 5 h. Powder X-ray diffraction (XRD) measurements were
carried out using a Siemens D5005 diffractometer equipped with a
Cu anode and variable slits. The diffractograms were measured for
the 2 range of 20◦ to 80◦ with a step size of 0.02◦ and a dwell time of
1 s/step. Transmission electron microscopy (TEM) was performed
using a JEOL JEM 3010 HRTEM. X-ray photoelectron spectroscopy
(XPS) measurements were made using a VG-Scientific ESCALAB
Mark 2 spectrometer equipped with a hemispherical electron ana-
lyzer and a Mg K␣ X-ray source (1253.6 eV, 300 W). All binding
energies were calibrated using the C 1s peak at 284.6 eV as the
reference.
(g)
(f)
(e)
(d)
(c)
2.3. Catalytic testing
(b)
(a)
For a typical reduction, 2 mmol of the aldehyde substrate,
0.504 g (6 mmol) potassium formate, 0.54 mL (30 mmol) water and
5 mL (65 mmol) dimethylformamide (DMF) were added to a 25 mL
round-bottom flask. After heating the reaction mixture to 100 ◦C
under a flow of nitrogen, 100 mg of 1 wt.% Ru/AlO(OH) (0.5 mol %
of Ru) was added. Samples were taken at regular intervals and ana-
lyzed by gas chromatography (GC) and gas chromatography mass
spectrometry (GC–MS). For comparison, the direct hydrogenation
of benzaldehyde using molecular H2 at 0.5 MPa was carried out in a
20
30
40
50
60
70
80
2-Theta (o)
Fig. 1. XRD diffractograms of (a) AlO(OH) and Ru/AlO(OH) with (b) 0.5, (c) 1, (d) 2,
(e) 5, (f) 8, and (g) 10 wt.% Ru.