A novel catalyst containing a platinum complex in polyethylene glycol medium supported on silica gel for vapor-phase hydrosilylation of acetylene with trichlorosilane or trimethoxysilane

Article abstract of DOI:10.1039/b203524c

Hydrosilylation of acetylene with trichlorosilane or trimethoxysilane was carried out using a vapor-phase flow reactor with use of tetraammineplatinum(II) chloride in polyethylene glycol medium supported on silica gel as a catalyst, which is an active and thermally stable supported liquid-phase catalyst prepared readily from easily available materials, tetraammineplatinum(II) chloride, polyethylene glycol and silica gel.

Full text of DOI:10.1039/b203524c

A novel catalyst containing a platinum complex in polyethylene glycol
medium supported on silica gel for vapor-phase hydrosilylation of
acetylene with trichlorosilane or trimethoxysilane
a
b
c
b
Masaki Okamoto,* Hironari Kiya, Hiromi Yamashita and Eiichi Suzuki*
a
Department of Applied Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552,
Japan
b
c
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku,
Tokyo 152-8550, Japan
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
Gakuencho, Sakai, Osaka 599-8531, Japan
Received (in Cambridge, UK) 12th April 2002, Accepted 21st June 2002
First published as an Advance Article on the web 2nd July 2002
Hydrosilylation of acetylene with trichlorosilane or trime-
thoxysilane was carried out using a vapor-phase flow reactor
with use of tetraammineplatinum(II) chloride in polyethyl-
ene glycol medium supported on silica gel as a catalyst,
which is an active and thermally stable supported liquid-
phase catalyst prepared readily from easily available materi-
als, tetraammineplatinum(II) chloride, polyethylene glycol
and silica gel.
stream, prior to the reaction of acetylene (76 kPa) with
trichlorosilane (19 kPa) at 150 °C. The products were identified
by GC-MS and analyzed by gas chromatography.
Fig. 1 shows time courses of vinyltrichlorosilane yield in the
hydrosilylation of acetylene with trichlorosilane over various
catalysts. In all runs, a main product was vinyltrichlorosilane,
the by-reactions being disproportionation of trichlorosilane and
double hydrosilylation of acetylene to form 1,2-bis(trichlor-
osilyl)ethane. Alumina-supported platinum metal catalyst was
active for the hydrosilylation. However, an initial yield of
vinyltrichlorosilane was below 40%, and deactivation was
serious, probably due to strong adsorption of acetylene on the
platinum metal surface. The silica-supported tetraammineplati-
num(II) chloride catalyst showed a very high activity up to 2 h
of time on stream, and then the yield quickly decreased to 35%.
Using the catalyst containing tetraammineplatinum(II) chloride
in PEG medium supported on silica gel resulted in a high and
stable activity, the yield of vinyltrichlorosilane and the
trichlorosilane conversion being 72% and 84%, respectively.
When the reaction was continued to 36 h, the activity did not
decrease.
Hydrosilylation is one of the most important reactions for
1
formation of carbon–silicon bonds. For example, acetylene
hydrosilylation with trichlorosilane and trialkoxysilane gives
vinyltrichlorosilane and vinyltrialkoxysilane, respectively,
which are useful chemicals as silane-coupling reagents. Gen-
erally, hydrosilylation is performed in the liquid phase using a
1
homogeneous catalyst. However, separation of the catalyst
from the reaction mixture is troublesome. A vapor-phase flow
reaction using a solid catalyst is one of the solutions for this
problem. A supported platinum-metal catalyst, such as platinum
on activated carbon, is a well-known solid catalyst for
1
hydrosilylation. However, acetylene is strongly adsorbed on
2
the platinum metal surface, probably leading to deactivation of
Some platinum complexes were tested for the hydrosilylation
instead of tetraammineplatinum(II) chloride. Hexachloropla-
tinic acid, which is the most typical homogeneous catalyst for
the hydrosilylation, was used. However, the reaction did not
proceed. The EXAFS experiments of the used catalyst revealed
that platinum is completely reduced to the metal during the
reaction. Potassium hexachloroplatinate also showed a high
the catalyst in vapor-phase hydrosilylation of acetylene. A non-
metal catalyst is suitable for vapor-phase hydrosilylation of
acetylene.
A supported liquid-phase catalyst (SLPC) consists of a
catalyst component dissolved in a liquid coated on the surface of
a porous material, and has both properties of homogeneous and
3–5
heterogeneous catalysts. SLPCs have been used for hydro-
3
4
5
formylation, selective oxidation and hydrochlorination, and
have shown good performances. In these reports, the liquid
phase was a molecule with a high boiling temperature or a
molten salt.
Here, we report a new type of SLPC and its application to
hydrosilylation. A polymer, polyethylene glycol whose vapor
6
pressure is very low at a high temperature, was used as a liquid
phase in the SLPC. The catalyst containing tetraammineplatinu-
m(II) chloride in polyethylene glycol medium supported on
silica gel showed a high activity for acetylene hydrosilylation as
well as a high stability under the reaction conditions.
A typical catalyst was prepared as follows: silica gel, Micro
2
21
21
Bead 300A (surface area 128 m g , pore volume 1.17 mL g
,
average pore diameter 27.5 nm) from Fuji Silysia Chemical
Ltd., was dried at 170 °C for 24 h in an oven. Tetra-
ammineplatinum(II) chloride (Pt(NH ) Cl ·H O, 19.4 mg) was
3 4 2 2
dissolved in a mixture of 1.15 g of polyethylene glycol (PEG,
average molecular weight: 1000) and 3 mL of water. After
adding 2.15 g of the silica gel to the solution, water was further
poured into the mixture until the solution had a total volume of
Fig. 1 Time courses of vinyltrichlorosilane yield in hydrosilylation of
acetylene with trichlorosilane over platinum metal supported on alumina
8
mL. The mixture was settled for 24 h and dried using a rotary
(a), tetraammineplatinum(II) chloride supported on silica gel (b), and
evaporator to remove water. The catalyst (300 mg) was placed
in a reactor tube (quartz, i.d. 10 mm) of a fixed-bed flow reactor
system. The catalyst was pretreated at 150 °C for 1 h in a helium
tetraammineplatinum(II) chloride in PEG medium supported on silica gel
(c). The reaction was carried out at 150 °C at 19 kPa of trichlorosilane and
76 kPa of acetylene using 300 mg of the catalyst.
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activity, the yield of vinyltrichlorosilane being 67% at 6 h.
Platinum(IV) acetylacetonate and potassium tetrachloroplatinate
gave only 21% and 30% yield, respectively, at 6 h.
at 1.96 Å is not the same as those due to Pt–Pt and Pt–O bonds
in platinum metal and platinum oxide. Dichlorodiammineplati-
num(II), which is well-known as the thermal decomposition
product of tetraammineplatinum(II) chloride, gave the Pt–Cl
peak at 1.96 Å (two chlorine ligands are coordinated to
platinum). The intensity of dichlorodiammineplatinum(II) was,
however, lower than those at 1.96 Å in Fig. 2(d–f). The
coordination numbers of chlorine resulting from EXAFS
analysis were 4.7, 5.6 and 5.3, respectively, and were much
higher than 2 in tetraammineplatinum(II) chloride. This sug-
gests that the Pt–Cl peak at 1.96 Å in Fig. 2(d–f) is not due to
dichlorodiammineplatinum(II). Thus, it is plausible that the
chloride ion is coordinated to platinum with elimination of a
Silica-supported tetraammineplatinum(II) chloride dissolved
in PEG catalyst also had a high activity for hydrosilylation with
ethylene and propylene. The reaction of trichlorosilane (19 kPa)
with ethylene (76 kPa) at 150 °C gave an 86% yield of
ethyltrichlorosilane. A by-reaction was only disproportionation
of trichlorosilane, tetrachlorosilane yield being 3%. When
propylene (76 kPa) was used instead of ethylene, the n-
propyltrichlorosilane yield was 62%, iso-propyltrichlorosilane
not being formed.
Using trimethoxysilane (11 kPa) in place of trichlorosilane,
hydrosilylation of acetylene (44 kPa) was carried out at 150 °C.
Vinyltrimethoxysilane was selectively formed with a high
yield, 67%.
part of the ammine ligands from tetraammineplatinum(IV
)
chloride during the pretreatment, while during the reaction the
chloride ion originating from trichlorosilane is coordinated to
platinum in place of the ammine ligands. Actually, we have
observed that, in the EXAFS transform of potassium hexa-
chloroplatinate in which six chlorines are bonded to platinum
with octahedral coordination, the peak due to the platinum–
chlorine bond was at the same distance as the Pt–Cl peaks in
Fig. 2(e, f), and the intensity and the coordination number of
chlorine were also almost the same. Hexachloroplatinic acid,
the most typical homogeneous catalyst, is very active for the
liquid-phase hydrosilylation, and potassium hexachloroplati-
nate in polyethylene glycol medium supported on silica was
also a very active catalyst for vapor-phase hydrosilylation as
mentioned above. These active catalysts have six chlorines and
no other ligands. A high coordination number of chlorine
present as a sole ligand is consistent with the EXAFS results of
tetraammineplatinum(II) chloride dissolved in PEG on silica gel
after the reaction.
Fig. 2 shows Pt LIII-edge EXAFS results of the catalyst
before and after the reaction. In the EXAFS Fourier transform of
tetraammineplatinum(II) chloride (Fig. 2(a)), there was one
strong peak at 1.78 Å (without phase-shift correction), which
can be assigned to a platinum–nitrogen bond (Pt–N) in square-
7
planar coordination. This Pt–N peak was also observed in the
Fourier transform of tetraammineplatinum(II) chloride dis-
solved in PEG on silica gel (Fig. 2(b)), and its peak intensity did
not change, indicating that square-planar coordination still
remained. After the pretreatment, a shoulder peak appeared at
1.96 Å, which is attributed to the platinum–chloride bond (Pt–
Cl) as mentioned below. Thus, the environment of platinum in
the catalyst changed at the pretreatment step. After 1 h of
reaction, the Pt–N peak at 1.78 Å disappeared, while the
intensity of the Pt–Cl peak at 1.96 Å increased with increasing
time on stream till 3 h and then did not change. At 6 h, this Pt–Cl
peak intensity was the same as that at 3 h. As mentioned above,
the vinyltrichlorosilane yield increased with increasing time on
stream till 2 h and then remained at 72%. This change of the
yield with time on stream is very similar to that of the intensity
change of the Pt–Cl peak at 1.96 Å. This strongly suggests that
the platinum species during the reaction is a complex giving the
transforms (e) and (f) in Fig. 2. The position of the Pt–Cl peak
The SLPC reported here can be readily prepared from easily
available raw materials, i.e. tetraammineplatinum(II) chloride,
PEG and silica gel. Furthermore, this catalyst can be handled in
an air atmosphere, and its thermal stability is higher than
platinum complexes. The SLPC, containing a polymer as a
liquid phase, is expected to have applications in various vapor-
flow reactions.
The EXAFS experiments were performed at the SPring-8
with the approval of the Japan Synchrotron Radiation Research
Institute (Proposal No. 2001B0023-NX-np). We thank Dr.
Yuichi Ichihashi in the National Institute of Advanced In-
dustrial Science and Technology (AIST) for his help in EXAFS
experiments.
Notes and references
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Fig. 2 Pt LIII-edge EXAFS Fourier transforms of tetraammineplatinum(II
)
chloride (a), tetraammineplatinum(II) chloride dissolved in PEG supported
on silica gel (b) after the pretreatment (c) and 1 h (d), 3 h (e) and 6 h (f)
reaction. The reaction was carried out at 150 °C at 19 kPa of trichlorosilane
and 76 kPa of acetylene using 300 mg of the catalyst.
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