1
256
Chemistry Letters Vol.37, No.12 (2008)
Remarkable Support Effects of Gallium Compounds on the Activity and Selectivity
of Ru Metal Catalyst for Liquid-phase Citral Hydrogenation
ꢀ
Hiroshi Nishiyama, Junichi Takeuchi, Hiroshi Hayase, Nobuo Saito, and Yasunobu Inoue
Department of Chemistry, Nagaoka University of Technology, Nagaoka 940-2137
(Received August 4, 2008; CL-080752; E-mail: inoue@analysis.nagaokaut.ac.jp)
Various types of p-block metal oxides and nitrides were
employed as catalyst supports for Ru metal in liquid-phase citral
solution as a reducing agent was added to the solution. After
reduction, the catalysts were recovered by filtration, washed with
water and acetone, and dried at room temperature. The liquid-
phase hydrogenation of citral was carried out at a hydrogen pres-
sure of 1.3 MPa and at 393 K under constant stirring conditions
in an autoclave equipped with a thermocouple, heater and a
rotator. About 2.0 g of catalyst and 130 mL of citral (98.0% pure)
were loaded without solvent into the autoclave. The reaction
products were analyzed by a gas chromatograph with an FID de-
tector. The conversion was taken as a measure of catalytic activ-
ity and determined by the consumption of citral at a given reac-
tion time, whereas the selectivity was defined as the percentage
of nerol/geraniol to the total of the other products.
3þ
hydrogenation. Compounds containing Ga ions, in particular
GaOOH, were found to exhibit remarkable support effects on
the enhancement of catalytic activity and reaction selectivity
for the production of ꢀ,ꢁ-unsaturated alcohols (nerol and gera-
niol).
The selective hydrogenation of ꢀ,ꢁ-unsaturated aldehydes
to unsaturated alcohols on heterogeneous catalysts is of great
synthetic importance for the production of various fine chemi-
cals in the flavor, fragrance, and pharmaceutical industries. In
particular, the ꢀ,ꢁ-unsaturated alcohols nerol and geraniol are
important in the perfume industry and are obtained in the hydro-
genation of the ꢀ,ꢁ-unsaturated aldehyde citral (3,7-dimethyl-
Under the present reaction conditions, the products identi-
fied were nerol/geraniol (N/G), citronellal (CNL), isogeraniol
(n), and citronellol (CNA). Figure 1 shows the activity and selec-
tivity of citral hydrogenation on Ru metals loaded on various
p-block metal oxides. Both the activity of the catalysts and the
selectivity for N/G production were larger in the order
ꢁ-Ga O > In O > SnO > SiO > GeO . Previously report-
2,6-octadienal), which contains three unsaturated bonds; i.e.,
conjugated C=O and C=C bonds and an isolated C=C bond.
Difficulty in selective hydrogenation to nerol/geraniol is due
to the fact that the hydrogenation of a C=C bond is thermody-
namically more favorable than C=O hydrogenation.
2
3
2
3
2
2
2
ed high selectivities for N/G production on supported Ru metal
catalysts were 87.8% at a conversion of 21% for Ru–Sn/TiO2
Ruthenium is a potentially useful metal as a catalyst for
selective C=O bond hydrogenation because of its wide d band
character which favors interactions between the metal surface
6
(TiO was subjected to high-temperature reduction at 673 K)
2
3
and 82% at 70% conversion for Ru–Ce/active carbon in which
heptane and 2-propanol were used as solvents, respectively. The
1
and the C=O bond. It has been established that the supports
for metal-supporting catalysts play an important role in the im-
provement of catalytic performance. The citral hydrogenation
has been examined with Ru metals dispersed on various supports
result that Ru/ꢁ-Ga O catalyst provided a selectivity of 80% at
35% conversion under solvent-free reaction conditions was indi-
2
3
cative of compatible support effects of ꢁ-Ga O on the selectiv-
2
3
2–4
2–4
1
5
such as Al2O3, carbon, SiO2, KL zeolite (K9Al9Si27O72),
ity for Ru-catalyzed C=O hydrogenation. The transition-metal
6
and TiO2; however, significant support effects on nerol/gera-
niol production have not been reported. The conventional sup-
ports lead to poor performance with low yields of the desired
products. Therefore, the discovery of supports that have strong
enhancing effects on Ru metal for the citral hydrogenation is
an important issue. In the present study, we have investigated
various p-block metal oxides and nitrides for use as supports
3
þ
and found that compounds containing Ga ions, in particular
GaOOH, make excellent supports that significantly enhance
the ability of Ru to promote the selective hydrogenation of citral
for nerol/geraniol production.
The p-block metal oxides employed were commercially
available Ga2O3 (99.99% pure, Soekawa Chem.), In2O3
(
(
99.999%, Nacalai Tesque), SnO2 (ER, Nacalai Tesque), SiO2
GR, Nacalai Tesque), GeO2 (99.99%, Nacalai Tesque), and
the metal nitrides were BN (>99%, Kojundo Chem. Lab.),
GaN (>99:99%, Aldrich), InN (99.9%, Aldrich), Si3N4
(>99:9%, Kojundo Chem. Lab.), AlN (98%, Aldrich), and
Ge3N4 (>99:99%, Aldrich). Ruthenium metal (2.5 wt %) was
loaded on the supports using a liquid-phase reduction method.
Figure 1. Conversion and selectivity of liquid phase citral hy-
drogenation over Ru metal loaded on various p-block metal ox-
ide supports. Reaction time: 6 h.
.
Ruthenium chloride (RuCl3 H2O) as a starting material and
the supports were dispersed in water, and then NaBH4 aqueous
Copyright Ó 2008 The Chemical Society of Japan
Nishiyama, Hiroshi
