Catalyst for disproportionating aryl- or alkyhalodisilanes into aryl- or alkylhalomonosilanes and aryl- or alkylhalopolysilanes

Catalysts composed of nitrogen-containing heterocyclic hydrocarbons, optionally fixed to a support material, for disproportionating aryl- and alkylhalodisilanes into the corresponding mono- and polysilanes, and a process of using these catalysts for disproportionating disilanes.

BACKGROUND OF THE INVENTION 
The invention relates to catalysts for the disproportionation of aryl- or 
alkylhalodisilane into aryl- or alkylhalomonosilane and aryl- or 
alkylhalopolysilane, and also to a process for the disproportionation of 
aryl- or alkylhalodisilanes using these catalysts. 
Aryl- and alkylhalosilanes, i.e. silanes substituted by aryl or alkyl 
groups and halogen atoms, in particular methylchlorosilanes, are valuable 
industrial starting materials for producing silicone products. 
Methylchlorosilanes such as dichlorodimethylsilane and 
trichloromethylsilane are prepared, inter alia, by the Mueller-Rochow 
synthesis. However, this produces, after distillation of the products, 
relatively high-boiling residues in amounts of up to 10% by weight. From 
these distillation residues there can be obtained a fraction which boils 
in the temperature range of about 150.degree. C. to 160.degree. C. and 
which essentially comprises alkylhalodisilanes, such as 
methylchlorodisilanes. From both economic and ecological points of view it 
is desirable to be able to work up the distillation residues and thus 
achieve complete utilization of the raw materials used. Previously known 
processes for catalytic disproportionation of, for example, 
methylchlorodisilanes into monomeric methylchlorosilanes and 
methylchloropolysilanes preferably use as catalytically active substances 
amines, for example NR.sub.3 where R.dbd.H, alkyl, aryl, and also the 
corresponding quaternary ammonium salts 
quaternary phosphonium salts 
hexamethylphosphoramide (HMPA) 
cyanides, in particular silver cyanide. 
Disadvantages of these catalytically active substances include, for 
example, a low activity or the fact that long reaction times are required, 
which results in polysilanes which, despite the long reaction times, often 
have a very syrupy consistency, which can be attributed to a high 
proportion of relatively low molecular-weight polysilanes and which makes 
their further processing difficult and leads to low yields on conversion 
into ceramic products. In addition, the proportion and the composition of 
the monosilane fraction obtained can only be varied within narrow limits. 
On the other hand, for example, the use of HMPA cannot be approved without 
question because of its physiologically questionable nature, despite its 
good catalytic properties. Despite the efforts of the prior art, there 
remains a need for improved catalysts for disproportionating aryl- and/or 
alkylhalodisilanes to monosilanes and polysilanes. 
SUMMARY OF THE INVENTION 
It is therefore the object of the invention to provide improved catalysts 
for disproportionating aryl- and/or alkylhalodisilanes and a process for 
using such catalysts to produce monosilanes and polysilanes. 
Another object of the invention is to provide a catalyst for 
disproportionating aryl- and/or alkylhalodisilanes which exhibits high 
activity and selectivity. 
A further object of the invention is to provide a catalyst for 
disproportionating aryl- and/or alkylhalodisilanes which is ecologically 
less objectionable. 
It is also an object of the invention to provide a catalyst and process for 
disproportionating aryl- and/or alkylhalodisilanes which facilitate 
control of the composition of the disproportionation products by adjusting 
the process parameters. 
These and other objects of the invention are achieved by providing a 
catalyst for disproportionating aryl- or alkylhalodisilanes, which 
catalyst comprises a nitrogen-containing heterocyclic hydrocarbon having 
at least one nitrogen atom in at least one 4- to 8-membered hydrocarbon 
ring, in which ring atoms adjacent the at least one nitrogen atom are 
selected from the group consisting of carbon atoms and nitrogen atoms. 
In accordance with further aspects of the invention, the objects are 
achieved by providing a process for catalytically disproportionating an 
aryl- or alkylhalodisilane to form monosilanes and polysilanes comprising 
contacting the aryl- or alkylhalodisilane at a temperature in the range 
from 50.degree. C. to 350.degree. C. with a catalyst comprising a 
nitrogen-containing heterocyclic hydrocarbon having at least one nitrogen 
atom in at least one 4- to 8-membered hydrocarbon ring, wherein ring atoms 
adjacent the at least one nitrogen atom are selected from the group 
consisting of carbon atoms and nitrogen atoms, whereby the aryl- or 
alkylhalodisilane reacts to form monosilanes and polysilanes, and 
recovering the resulting monosilanes and polysilanes. 
The invention thus relates to catalysts for the catalytic 
disproportionation of disilanes into mono- and polysilanes which catalysts 
are notable for high activity and selectivity and are ecologically less 
questionable, and with which the composition of the disproportionation 
products can be controlled by modification of the process parameters, and 
also to a process for carrying out the disproportionation using these 
catalysts. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The catalyst according to the invention comprises a heterocyclic 
hydrocarbon having at least one nitrogen atom in at least one hydrocarbon 
ring, in which the positioning of the nitrogen ensures sufficient polarity 
of the molecule. The heterocyclic hydrocarbons of the invention have at 
least one nitrogen atom in at least one 4- to 8-membered hydrocarbon ring, 
where the ring atoms adjacent to the nitrogen can be carbon or nitrogen 
and the hydrocarbon ring or rings are, independently of one another, 
aromatic or non-aromatic hydrocarbon rings. 
The nitrogen in the hydrocarbon ring can be substituted by 
H 
alkyl, branched or linear, preferably C.sub.1 to C.sub.6 -alkyl 
oxygen 
halogen 
trialkoxysilyl 
NR.sub.2, in which R.dbd.H, C.sub.1 to C.sub.6 -alkyl, linear or branched, 
or trialkoxysilyl or be bonded in the ring via 2 sigma and 1 .pi. bond or 
can form the bridgehead to a further ring. 
Depending on the bonding to the adjacent ring atoms, the carbon in the 
hydrocarbon ring can be mono- or disubstituted by 
H 
alkyl, branched or linear, preferably C.sub.1 to C.sub.6 -alkyl 
halogen 
oxygen 
NR.sub.2, in which R.dbd.H, C.sub.1 to C.sub.6 -alkyl, linear or branched, 
or trialkoxysilyl 
or carry a further hydrocarbon ring system or it forms the bridgehead to a 
further ring system. 
The substituents can, independently of one another, be identical or 
different. Nitrogen-containing heterocyclic hydrocarbons which are 
particularly suitable according to the invention as catalytically active 
substances for the disproportionation of aryl- and/or alkylhalodisilanes 
are, for example, 5-membered rings having from 1 to 3 nitrogen atoms in 
the hydrocarbon ring, preferably imidazole, 1-methylimidazole, 
2-methylimidazole, pyrazole, 3-methylpyrazole, pyrrolidone, 
N-methylpyrrolidone, 1,3-dimethyl-2-imidazolidone or triazole or 
6-membered rings having at least one nitrogen atom in the hydrocarbon 
ring, preferably2,2'-bipyridine, 
1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidone or N,N-dibutylpiperazine 
or polycyclic hydrocarbons having at least one nitrogen atom in the 
hydrocarbon ring, preferably benzimidazole, benzotriazole, Urotropin, 
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 
1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]non-5-ene, 
diazabicyclooctane (DABCO). 
These catalytically active substances can either be fixed to a support, 
preferably a siliceous support, or used directly as catalyst for the 
disproportionation. 
The catalytic disproportionation can be carried out either in the 
homogeneous phase or in the heterogeneous phase, the reaction on the 
catalyst being carried out at temperatures from 50.degree. to 350.degree. 
C. with formation of monosilanes and polysilanes. 
In a variant of the invention, the catalyst is a supported catalyst which 
is characterized by the catalytic component being fixed directly or via a 
spacer and/or via siloxy groups to the surface of a support material, 
preferably a siliceous support material. 
Suitable siliceous support materials within the scope of the invention 
include in general all inorganic materials composed of silicon dioxide. 
The siliceous support material can be a silicon dioxide material such as 
silica gel, porous glasses, silicates, silicalites, an aluminum silicate 
material or a zeolite material, where in the aluminum silicates and in 
zeolites the catalytic component can optionally in part also be covalently 
bonded directly (without spacer) or indirectly (with spacer) via Al--OH 
groups. Optionally, the siliceous support material on which the catalyst 
of the invention is based can also be a support material which is 
surface-modified in a known manner. 
These supported catalysts are very suitable for heterogeneous catalytic 
disproportionation since the catalytic component is firmly fixed to an 
inert support material, which enables easy separation of the products 
formed from the catalyst and thereby avoids the undesired contamination of 
the catalytically formed products by catalytically active component. 
Preferably, the disilanes to be disproportionated are fed to the catalyst 
in vapor or gaseous form. 
In this preferred procedure, the catalytic conversion is carried out at 
catalyst temperatures which are above the boiling points of the 
monosilanes formed as product. In particular, alkylhalodisilanes are 
reacted at a temperature of at least 100.degree. C., preferably at from 
120.degree. C. to 130.degree. C., under protective gas. 
If the temperature at the catalyst corresponds to at least the boiling 
point of the monosilanes formed, then these vaporize immediately after 
being formed on the catalyst and can be obtained directly by condensation 
alone without any further isolation measures. 
In a further process variant, therefore, the monosilanes formed as product 
are vaporized as early as during the reaction on the preferably heated 
catalyst and are thus isolated by distillative separation, while any 
higher-boiling polysilanes which may be formed in the reaction are fed 
back into and collected in the liquid phase from which the disilane 
starting materials for the gas-phase reaction on the catalyst are 
vaporized. 
The polysilanes collected in the liquid phase may, depending on the 
temperature of the liquid phase, already be partially converted to 
polycarbosilanes during the distillation and catalytic reaction. 
In a further process variant, therefore, the polysilanes contained in the 
liquid phase are subsequently thermally treated at temperatures up to 
400.degree. C. for further conversion into polycarbosilanes and the 
polycarbosilanes formed are isolated as product. This subsequent treatment 
can also be carried out in vacuo. 
The above-described process for the heterogeneous catalytic 
disproportionation of disilanes into mono- and polysilanes makes it 
possible to carry out the process continuously, for example for the workup 
of disilane-containing distillation residues from the Mueller-Rochow 
synthesis and for the catalyst-free isolation of the reaction products. 
A further advantage of this procedure is, besides the isolation of pure 
monosilanes, the formation of polymeric reaction products which can be 
crosslinked to a varying degree and are thus predominantly soluble or no 
longer soluble in organic solvents. The product can thereby be matched to 
the further processing desired. 
A further preferred procedure for the heterogeneous catalytic 
disproportionation of silanes comprises contacting the supported catalyst 
of the invention directly with the disilanes in the reaction vessel. The 
disproportionation is advantageously carried out under protective gas. The 
monosilanes formed are distilled off directly during the reaction. The 
polysilanes remaining in the liquid phase can be readily separated from 
the catalyst and subsequently thermally treated as described above. 
The catalysts of the invention are also directly suitable for the catalytic 
disproportionation of disilanes without being fixed to a support material. 
This procedure likewise enables the formation of pure monosilanes. The 
polysilanes, however, are contaminated by the catalyst component. The 
disproportionation is preferably carried out under protective gas at from 
100.degree. to 350.degree. C. 
A common feature of all these process variants is the ability to influence 
the composition of the disproportionation products in a targeted manner, 
for example by selection of the catalyst or by appropriate reaction 
conditions such as temperature, reaction time and amount of catalyst. 
The amount of catalyst used is preferably from 0.1 to 10%, the reaction 
time from 1 to 16 hours. 
It has been found that, for example when using imidazole or pyrazole as 
catalyst, reaction times of from 3 to 4 hours are necessary to form solid 
polysilane. 
Reaction temperatures of about 220.degree. C. are entirely sufficient for 
obtaining solid products if, for example, imidazole or pyrazole is used as 
catalyst. 
The composition and the consistency of the resulting polysilanes can be 
varied greatly via the reaction time and the temperature regime. A 
polysilane prepared at up to 300.degree. C. using imidazole is, for 
example, a golden yellow, foam-like and very brittle mass. Stopping the 
reaction at from 200.degree. to 220.degree. C. gives a glassy polysilane 
which can be remelted and which also has a higher chlorine content than 
the polysilanes treated at higher temperature. 
It is likewise possible to stop the reaction at a stage at which highly 
viscous polysilanes are present. 
The proportion and composition of the monomer fraction can be varied over a 
wide range by selection of the reaction conditions, the catalyst and the 
amount of catalyst.

The following examples are intended to illustrate the invention in further 
detail without restricting its scope. Unless otherwise specified, 
percentages are always % by weight. 
EXAMPLE 1 
100 ml of methylchlorodisilane mixture were admixed with 2% of catalyst in 
a 3-neck flask and heated under protective gas (N.sub.2) from room 
temperature to a maximum of 300.degree. C. The reaction was, if the 
polysilane formation had not already ended earlier, stopped after about 6 
hours. The cleavage products formed were distilled off during the reaction 
via a 30 cm packed column. 
The results of the experiment are shown in Table 1. 
EXAMPLE 2 
The experimental set-up was analogous to Example 1, but the packed column 
contained the catalyst fixed to a support and was wrapped with heating 
tape so that the catalyst was maintained at a temperature of from 
100.degree. to 140.degree. C. by means of the heating tape. 100 ml of 
methylchlorodisilane were likewise initially charged and slowly heated 
under protective gas from room temperature to a maximum of 350.degree. C. 
The reaction was ended when the temperature of the polysilane remained 
above 300.degree. C. for more than 30 minutes, or the temperature at the 
top of the column indicated that no monomers but only non-cleavable 
impurities from the disilane fraction, disilanes and low molecular-weight 
polysilanes were distilling over. 
The catalyst used was 3-(4,5-dihydroimidazol-1-yl)-propyltriethoxysilane 
fixed on silica gel. About 90 ml of monomers and 5 g of polysilane were 
obtained. 
EXAMPLE 3 
The experimental set-up was analogous to Example 1. 100 ml of disilane 
mixture were admixed with 10 g of imidazole fixed on silica gel. The 
reaction time was 80 minutes, the liquid phase temperature a maximum of 
330.degree. C. 77 ml of monomers and 11 g of polysilane/polycarbosilane 
mixture were obtained. 
TABLE 1 
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Yield 
Polysilane/poly- 
Heating rate 
Reaction time 
carbosilane 
Monosilane 
Catalyst .degree.C./min 
min g ml 
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TBD 0.7 315 20 70 
Imidazole 2.3 130 23 60 
1-Methylimidazole 
4.4 70 21 57 
2-Methylimidazole 
5.8 45 15 70 
Pyrazole 0.8 250 25 74 
3-Methylpyrazole 
5.6 55 11 80 
DABCO 4 75 22 62 
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The foregoing description and examples have been set forth merely to 
illustrate the invention and are not intended to be limiting. Since 
modifications of the described embodiments incorporating the spirit and 
substance of the invention may occur to persons skilled in the art, the 
scope of the invention should be construed to include all variations 
falling within the ambit of the appended claims and equivalents thereof.