Development of polysilane-supported palladium/alumina hybrid catalysts and their application to hydrogenation

Article abstract of DOI:10.1039/b715220e

Novel immobilized Pd catalysts, polysilane-supported palladium/alumina hybrid catalysts, have been developed. The catalysts showed high catalytic activity for hydrogenation, and could be used in an organic solvent or under solvent-free conditions. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b715220e

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Development of polysilane-supported palladium/alumina hybrid catalysts and
their application to hydrogenation
Hidekazu Oyamada,a Takeshi Naito,a Shinpei Miyamoto,a Ryo Akiyama,a,b Hiroyuki Hagioa and
Shu¯ Kobayashi*a,b
Received 3rd October 2007, Accepted 13th November 2007
First published as an Advance Article on the web 23rd November 2007
DOI: 10.1039/b715220e
Novel immobilized Pd catalysts, polysilane-supported pal-
ladium/alumina hybrid catalysts, have been developed. The
catalysts showed high catalytic activity for hydrogenation,
and could be used in an organic solvent or under solvent-free
conditions.
confirmed by ICP analysis when nitrogen-containing compounds
such as nitrobenzene were used. Furthermore, these catalysts had a
somewhat limited applicability profile, since the carrier polysilane
dissolved in some organic solvents. In order to address these
issues, we planned to apply the PI method to polysilane-supported
catalysts. In this paper, we describe the synthesis of highly active PI
Pd/PSi on aluminium oxide and its application to hydrogenation.†
We first examined several metal oxides and the ratio to
Pd/PSi for the preparation of hybrid catalysts (Table 1). In a
typical procedure, poly(methylphenyl)silane (Mw = 3.2 × 104) and
Pd(OAc)2 were combined in THF at 0 ◦C for 1 h, and a metal oxide
was added to this mixture. After stirring for a further 1 h, MeOH
was added to form microencapsulated Pd (MC Pd/PSi) on the
Palladium is one of the most important hydrogenation catalysts
used in both laboratories and industry. While immobilized palla-
dium (Pd) catalysts such as Pd/C and Pd/SiO2, etc. havebeen often
employed, leaching of the palladium from the supports, moderate
yields of recovery, contamination of the reaction products with
palladium, and poisoning of the catalyst by heteroatoms such as
sulfur and nitrogen, etc. are sometimes serious problems especially
in the manufacture of pharmaceuticals.1 To address these issues,
new methods for the immobilization of homogeneous Pd catalysts
onto inorganic or organic supports have been widely investigated.2
However, tedious procedures are often needed for the preparation
of such heterogeneous catalysts, and lower activity/selectivity and
leaching of catalysts from the supports remain serious problems.
Previously, we have developed new methods for immobilizing
metal catalysts onto polymers, microencapsulation3 and polymer
incarcerated (PI) protocols,4 which are based on physical envel-
opment by the polymers, and electronic interaction between p
electrons of the benzene rings of the polystyrene-based polymers
and vacant orbitals of the metals. Next, we focused on polysilanes
as a support for heterogeneous catalysts. Polysilanes have been
widely studied due to their interesting electronic properties such
as high hole mobility, photoconductivity, and nonlinear optical
properties caused by r-conjugation of the silicon backbone.5–7 In
addition, polysilane is used as a starting material for ceramics
and synthetic methods for mass production are established.8
Recently, we have synthesized immobilized Pd and Pt catalysts
based on poly(methylphenyl)silane for the first time.9 Polysilane-
supported Pd catalysts (Pd/PSi) have been successfully used in
hydrogenation. The reactions proceeded in high yields, and the
catalysts could be recovered and reused almost quantitatively
by simple filtration. Polysilane-supported Pt (Pt/PSi) catalysts
were prepared by a similar method and showed high activity
in hydrosilylation. However, a small amount of Pd leaching was
◦
metal oxide. The resulting MC Pd/PSi (1) was heated at 140 C
for 2 h, washed sequentially with THF, CHCl3, and MeOH, and
dried under reduced pressure to afford PI Pd/PSi on the metal
oxide 2. We tested hybrid catalysts 2 on the hydrogenation of
ethyl cinnamate (Scheme 1). When SiO2 was used, recovery of
palladium was low (Table 1, entry 2). On the other hand, Al2O3,
TiO2, and ZrO2 gave promising results (entries 3, 6 and 9). It
was found that higher ratios of the metal oxides to Pd/PSi were
important for good recovery of palladium and high activity for
the hydrogenation (entries 4, 5, 8, 10, and 11).
Scheme 1 Hydrogenation of ethyl cinnamate
Among the metal oxides tested, alumina was revealed to be
superior to other metal oxides in terms of yield and quality of the
immobilized catalysts (entries 4 and 5). With these data in hand,
we further studied preparation conditions of the catalyst using
alumina. In a large-scale preparation of PI Pd/PSi, it was found
that the heating at 140 ◦C under neat conditions (cross-linking
step) gave low reproducibility, presumably due to irregularity of
heating and influence of oxygen. After further investigations, good
reproducibility was obtained when decane was used as a solvent
under an argon atmosphere. Next, several conditions for the cross-
linking step were examined (Table 2). It was found that whilst
palladium catalysts 4 that had been prepared at 120 ◦C and 140 ◦C
for 2 hours gave quantitative conversion in the hydrogenation
reaction, those prepared over a longer period of time gave lower
conversions in the subsequent hydrogenation (entries 2 and 4). On
aDepartment of Chemistry, School of Science and Graduate School of
Pharmaceutical Sciences, The University of Tokyo, The HFRE Division,
ERATO, Japan Science and Technology Agency (JST), Hongo, Bunkyo-
ku, Tokyo, 113-0033, Japan
bScience of Process Laboratories, ERATO, Japan Science and Technology
Agency (JST), Matoba, Kawagoe, Saitama, 350-1101, Japan. E-mail:
shu_kobayashi@chem.s.u-tokyo.ac.jp; Fax: +81 3 5684 0634; Tel: +81 3
5841 4790
◦
the other hand, using a preparation temperature of 160 C even
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Table 1 Preparation of PI Pd/PSi on metal oxide and hydrogenation reaction
Preparation of PI Pd/PSi on metal oxide
Hydrogenationa
Entry
Metal oxide (g)
Yield of 1 (g)
Yield of 2/1 (w/w)
Loading/lmol g−1
Recovery of Pd (%)b
Conversion (%)c
Leaching (%)d
1
2
3
4
5
6
7
8
9
None
0.72
1.81
1.63
5.62
10.57
1.63
5.61
10.56
1.60
5.61
10.48
—
13.4
49.8
72.3
15.1
8.3
60.1
11.7
7.3
96
51
91
81
85
78
65
74
81
90
79
94
88
55
99.8
99.8
74
76
>99.9
46
99.3
>99.9
NDf
0.27
0.23
<0.28
<0.28
<0.23
<0.28
<0.28
<0.23
<0.28
<0.30
SiO2 (1.0)
Al2O3 (1.0)
(5.0)
0.56
0.77
0.95
0.96
0.79
0.99
0.97
0.80
0.93
0.91
(10.0)
TiO2 (1.0)
(5.0)
(10.0)
ZrO2 (1.0)
(5.0)
63.7
16.2
8.2
10
11
(10.0)
a Reaction conditions: see Scheme 1. b Recovery over two steps. c Determined by GC analysis. d Determined by ICP analysis. e MC Pd/PSi was used. f Not
determined.
Table 2 Preparation of PI Pd/PSi on Al2O3 by the heat cross-linking method and hydrogenation reaction
Hydrogenationa
Preparation of PI Pd/PSi on Al2O3
Conversion (%)c
Entry
x/◦C
y/h
Yield of 4/3 (w/w)
Loading/lmol g−1
Recovery of Pd (%)b
1 h
2 h
Leaching (%)d
1
2
3
4
5
120
120
140
140
160
2
4
2
4
2
0.91
0.91
0.93
0.93
0.95
15.2
12.7
15.8
16.3
15.3
80
67
85
89
84
94
74
73
59
63
Quant.
93
Quant.
94
<0.74
1.06
<0.74
<0.74
<0.74
82
a Reaction conditions: ethyl cinnamate (1 mmol), Pd catalyst 4 (0.1 mol%), EtOH (1 mL) and H2 (1 atm), rt, 2 h. b Recovery of two steps. c Determined
by GC analysis. d Determined by ICP analysis.
over the shortened time of 2 hours only afforded a catalyst whose
activity was comparatively low (entry 5).
While N-methylmaleimide, maleic acid, and a diene were reduced
smoothly to afford N-methylsuccinimide, succinic acid, and alkane
in high yields, respectively, a small amount of Pd leaching was
observed (entries 4, 5 and 6). Furthermore, in the case of using
catalyst 4a, nitrobenzene was reduced smoothly to afford aniline
quantitatively. It is particularly noteworthy that the leaching of Pd
in this instance was lower than that observed when using the MC
Pd/PSi catalyst (entries 7 and 8).
The substrate scope of the hydrogenation was then examined
using 0.05 mol% PI Pd/PSi on alumina (4a) in organic solvents
such as ethanol and dichloromethane under hydrogen gas at at-
mospheric pressure and room temperature (Table 3). Gratifyingly
it was found that under these conditions ethyl cinnamate (entry 1)
and 2-cyclohexen-1-one (entry 2) were reduced smoothly in EtOH
to afford the desired products in high yields without leaching of
Pd. Chalcone also reacted smoothly to give the corresponding
ketone and alcohol in 91% and 6% yields, respectively (entry 3).
We also evaluated catalyst 4a in the hydrogenation of ethyl
cinnamate, N-methylmaleimide, and chalcone using 0.05 or
0.20 mol% of Pd under solvent-free conditions (Table 4).10 We
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Table 3 Substrate scope
Entry
1
Substrate
Solvent
EtOH
Conc./mol L−1
0.33
Time/h
1
Product
Yield (%)b
Leaching (%)c
>99.9d
<0.22
2
3
EtOH
EtOH
1.0
1.0
4
Quant.
91 (6)
<0.30
24
0.99
4
EtOH
0.50
1
97
0.48
5
6
EtOH
1.0
3.5
24
Quant.
98
0.94
3.6
CH2Cl2
0.50
7
EtOH
Hexane
1.0
0.33
8
1.5
>99.9d
>99.9d
0.34
2.28
8e
a Pd: 0.142 wt%, Al: 47.7 wt%, Si: 2.28 wt% (determined by ICP analysis). b Determined by 1H NMR analysis. c Determined by ICP analysis. d Conversion,
determined by GC analysis. e 0.50 mol% of MC Pd/PSi without Al2O3 was used. See ref. 9
Table 4 Hydrogenation without solvent
Entry
Substrate
4a/mol%
Temp.
RT
Time/h
Product
Yield (%)a
Leaching (%)b
1
0.05
0.05
8
16
>99.9c
<0.22
<0.10
2d
50 ◦
C
Quant.e
3
4
0.05
0.20
50 ◦
RT
C
1
1
58
79
—
—
5
6
0.05
0.20
50 ◦
50 ◦
C
C
24
24
80 (Trace)
95 (5)
—
0.12
a Determined by 1H NMR analysis. b Determined by ICP analysis. c Conversion, determined by GC analysis. d 25 mmol scale. e Determined by GC analysis
had previously found that when using MC Pd/PSi with a liquid
chemical such as ethyl cinnamate as substrate, the catalyst was
dissolved in the substrate and the palladium metal tended to
leach from the support. Pleasingly however, it was discovered
that in the case of 4a, the reaction could be conducted without
leaching of Pd (entries 1 and 2). Solid chemicals such as
N-methylmaleimide and chalcone could be used as substrates
directly and good chemical yields were obtained (entries 3, 4, 5
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and 6). Particularly, in the case of chalcone, it is noted that the
leaching of Pd was lower than that observed when using ethanol
as a solvent (entry 6, cf. Table 3, entry 3).
solvent or under solvent-free conditions. Further investigations
to prepare other immobilized metal catalysts as well as their
applications to other reactions are now in progress.11
In order to probe the structure of the catalyst, we analyzed PI
catalyst 4 (Table 2, entry 3) by 29Si CPMAS NMR (Fig. 1). In
the preparation of MC Pd/PSi on alumina (3), while a portion
of the Si–Si bonds were oxidized to form O–Si–O bonds, almost
the same signals (chart c) as for those of poly(methylphenylsilane)
(chart a) were observed. On the other hand, PI Pd/PSi on alumina
[catalyst 4 (Table 2, entry 3)] gave interesting spectra (chart d).
29Si NMR analysis revealed four signals at about −100, −40,
−33, and −25 ppm, corresponding to Si–O–Al, Si–Si, O–Si–O,
and Si–O–Si bonds, respectively. It was assumed that polysilane
was fixed on activated alumina by formation of linkages with
aluminium through oxygen. In addition, since alumina has a lot
of nanopores on the surface, it is expected that the polysilane-
supported palladium clusters are included in these nanopores and
stabilized. However, at the current time the precise structure of
catalyst 4 is still unclear, and further investigation and information
are needed.
Notes and references
† Synthesis of poly(methylphenylsilane): Methylphenyldichlorosilane
(382 g, 2.00 mol) was added dropwise to a suspension of sodium (96.6 g,
4.20 mol) in THF (1 L) over 30 min with vigorous stirring at reflux
temperature. After stirring for further 3.5 h, the mixture was cooled in
an ice bath and diluted with toluene (500 mL). 3 N HCl (500 mL) was
added dropwise to the mixture, the organic layer was separated, and water
was added to the aqueous layer to dissolve NaCl. The aqueous layer
was extracted with toluene and the combined organic layers were washed
with H2O, 5% aqueous solution of NaHCO3, H2O and brine. The organic
layer was dried over Na2SO4 and concentrated to ca. 600 mL in vacuo.
MeOH (750 mL) was added to the solution, the precipitate was collected
by filtration and washed with MeOH. The crude polysilane was dissolved
in toluene (370 mL) and precipitated by adding iPrOH. The precipitate
was collected, washed with toluene–iPrOH (1 : 5) and dried under reduced
pressure at 55 ◦C to afford poly(methylphenylsilane) (135 g, 56% yield).
Mw = 3.21 × 104, Mn = 1.12 × 104, Mw/Mn = 2.87.
General procedure for the preparation of PI Pd/PSi on Al2O3 (4) (Table 2):
Poly(methylphenylsilane) (100 g) was dissolved in THF (800 mL) and
cooled to 0 ◦C. To this solution, palladium(II) acetate (2.25 g, 10.0 mmol)
was added and the mixture was stirred for 1 h at this temperature. To the
dark brown polymer solution was added alumina (500 g), and the mixture
was allowed to warm to room temperature. After 2 h, MeOH (4 L) was
slowly added for coacervation and the resulting precipitate was collected
by filtration, washed with MeOH several times and dried under reduced
pressure at 55 ◦C to afford the microencapsulated palladium catalyst (MC
Pd/PSi on Al2O3 3, 581 g, Pd = 14.3 mmol g−1, 83% of Pd was loaded).
The Pd loading was determined by ICP analysis. MC catalyst 3 (20 g) was
suspended in decane (40 mL) at room temperature, and heated to 120–
160 ◦C for 2–4 h. The resulting solid was collected by filtration, washed
with hot THF and MeOH several times and dried under reduced pressure
at 55 ◦C. The polymer incarcerated palladium catalysts (PI Pd/PSi on
Al2O3 4) were obtained. The loading of Pd was determined by ICP analysis.
Hydrogenation catalyzed by PI Pd/PSi on Al2O3 (4a) (Table 3): A
typical experimental procedure is described for the hydrogenation of
ethyl cinnamate. Ethyl cinnamate (5 mmol) and PI Pd/PSi on Al2O3 (4a,
0.05 mol%) were combined in ethanol (15 mL). The mixture was stirred
for 1 h at room temperature under H2 atmosphere (1 atm). The yield and
conversion were determined by GC analysis with reference to naphthalene
or 1H NMR analysis with reference to 1,2,3,4-tetramethylbenzene (durene)
as an internal standard. The leaching of Pd from the support was
determined by ICP analysis.
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In summary, we have developed novel polysilane-supported
palladium/alumina hybrid catalysts. These catalysts showed high
catalytic activity in hydrogenation and could be used in an organic
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