Preparation of hydrogenosilanes by catalytic hydrogenolysis...

Details Preparation of hydrogenosilanes by catalytic hydrogenolysis... Preparation of hydrogenosilanes by catalytic hydrogenolysis...

2001-01-25

2001-08-07

Shaver, Paul F. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Silicon containing

Reexamination Certificate

active

06271407

ABSTRACT:

The field of the invention is that of the production of hydroalkylsilanes from polysilanes and in particular disilanes.
More specifically, a subject-matter of the invention is a process for the preparation of hydrohaloalkylsilanes by catalytic hydrogenolysis of the Si—Si bond of haloalkylpolysilanes with gaseous hydrogen under pressure.
In the silicone industry, organohalosilanes (that is to say, in practice, methylchlorosilanes) are the fundamental units involved in the construction of silicone polymers (polyorganosiloxanes, POSs). Methylchlorosilanes are virtually always manufactured by direct synthesis (Rochow-Müller synthesis) with silicon and methyl chloride. This synthesis results in (CH
3
)
2
SiCl
2
(65-85%), CH
3
SiCl
3
(7-18%), (CH
3
)
3
SiCl (2-4%), CH
3
SiHCl
2
(0.5%) and polysilanes (6-8%).
These polysilanes are compounds of high boiling point which constitute the heavy fractions of the distillations carried out in order to separate the various monomers produced by direct synthesis. These polysilanes comprise disilanes which are: (CH
3
)Cl
2
Si—SiCl(CH
3
)
2
; (CH
3
)Cl
2
Si—SiCl
2
(CH
3
); (CH
3
)Cl
2
Si—Si(CH
3
)
3
; (CH
3
)
2
ClSiSiCl(CH
3
)
2
, denoted respectively by Me
12
, Me
11
, Me
13
and Me
22
, each figure in these indices corresponding to the number of methyl substituents carried by either one of the silicon atoms of the disilanes. Me
12
, Me
11
and Me
13
are also known as “cleavable” disilanes. They represent 80% of the heavy fraction, against 10% for Me
22
; approximately 10% of the mass being composed of products comprising Si—O—Si and Si—CH
2
—Si units.
One of the routes for enhancing in value these disilanes involves their conversion into hydrogenated organohalosilanes: dichloromethylhydrosilanes (MeH), monochlorodimethylhydrosilanes (Me
2
H) or chloromethyldihydrosilane (MeH
2
).
A particularly advantageous outlet for these hydrogenated haloorganosilanes, such as hydrogenated chloromethylsilanes, is the functionalization of polyorganosiloxanes and in particular those obtained from the dimethyldichlorosilane monomer. By hydrolysis and condensation, MeH, MeH
2
and Me
2
H can be integrated into silicone backbones to form monohydrogenated and dihydrogenated siloxyl units D (SiO
2/2
) and monohydrogenated siloxyl units M (SiO
1/2
).
Haloorganosilanes are also reaction intermediates which can be used in the preparation of polysilanes by the formation of an Si—Si bond via a dehydrogenation. Another application of hydrogenated haloorganosilanes can be the hydrosilylation of organic compounds.
The present invention relates to the problem of producing, in a simple and profitable way, hydrogenated haloorganosilanes by catalytic hydrogenolysis of polysilanes.
The catalytic hydrogenolysis of the Si—Si bonds of polysilanes of the heavy fraction from direct synthesis represents the most advantageous option of the prior art for enhancing in value. It makes it possible to convert by-products of low added value into reaction intermediates of high added value.
Two types of catalytic hydrogenolysis of polysilanes are distinguished according to the hydrogen source used, namely either hydrochloric acid in solution or gaseous hydrogen under pressure.
The process disclosed in Patent FR 1,447,304 relates to the first type of catalytic hydrogenolysis. The reaction which governs the process according to this patent is as follows:
Me
3−n
Cl
n
Si—SiCl
m
Me
3−m
+HCl→HMe
3−n
SiCl
n
+Me
3−m
SiCl
m+1
The catalyst employed is chosen from amides or organic bases. This process suffers from two major disadvantages. The first is related to the stoichiometry of the reaction, according to which, per mole of disilane, only one mole of hydrogenated chloromethylsilane is obtained. The second disadvantage is related to the specificity of the catalysts used in this type of process. This is because the said catalysts are preferably active with respect to disilanes comprising at least one monoalkylated fragment (MeCl
2
Si) at the expense of the disilanes which do not comprise it.
There exist several prior technical proposals in which the reaction chamber is fed with gaseous hydrogen under pressure. All these proposals have cumbersome and drastic operating conditions in common.
In this mood, French Patent No. 2,085,143 discloses the hydrogenolysis of disilanes Me
12
in the presence of an organometallic catalyst composed of a complex palladium salt, bis(tributylphosphine)dichloropalladium (PBu
3
)
2
PdCl
2
.
This hydrogenolysis catalyst is employed in a proportion of 1% by weight. The hydrogen pressure of 211×10
5
Pa employed is so high that it is out of the question on an industrial scale. The reaction temperature is relatively high: 120° C. The nevertheless lengthy reaction time does not make it possible to obtain high conversion ratios (74%). Finally, it should be emphasized that the starting materials are not composed of a mixture of polysilanes but of a single disilane.
U.S. Pat. No. 4,079,071 teaches, for its part, the hydrogenolysis of a mixture of disilanes Me
11
, Me
12
, Me
22
, Me
13
and Me
23
in the presence of 1% of hydrogenolysis catalysts formed by bis(tributylphosphine)dichloronickel (Pbu
3
)
2
NiCl
2
at a temperature of 150° C. under a hydrogen pressure of 50×10
5
Pa for 6 hours. Here again, it should be noted that, from an industrial viewpoint, these are not ideal operating conditions.
U.S. Pat. No. 5,175,329 discloses a process for the conversion of polysilanes, formed by a mixture of disilanes Me
13
+Me
22
+Me
12
+Me
11
with methyltrichlorosilanes added, into hydrogenated or nonhydrogenated monosilanes. This conversion is carried out using a hydrogenolysis catalyst chosen from nickel-, palladium- or platinum-based organometallic compounds or from complex salts of these same metals. One example given for this catalyst is bis(tributylphospine)dichloronickel: (PBu
3
)
2
NiCl
2
. According to one alternative form, the hydrogenolysis catalyst can be composed of inorganic forms of these metals, Ni, Pd and Pt, attached, for example, to a support of the alumina type. The redistribution catalyst is chosen from quaternary phosphonium or ammonium halides or aluminium or boron halides. It can be, for example, a tetrabutylphosphonium chloride. The hydrogenolysis is carried out at 135° C. under a hydrogen pressure of 36×10
5
Pa for very long periods of time, which can reach 17 hours, for a yield of hydrogenated monosilanes with respect to all the silicon involved of the order of 40%. In the light of such results, it is difficult to maintain that this prior process is economically viable on an industrial scale.
French Patent No. 2,342,981 is targeted at the hydrogenolysis of mixtures of disilanes of the type of those comprising 81% of tetrachlorodimethyldisilane (Me
11
), 17% of trichlorotrimethyldisilane (Me
21
) and 2% of dichlorodimethyldisilane (Me
22
) in the presence of a catalyst formed by an aprotic base of the hexamethylphosphoric triamide (HMPT) type and by divided nickel obtained by prior or in situ reduction of an inorganic or organic derivative (cyclopentadienylnickel or nickelocene) by a reducing agent or by hydrogen. The reaction is carried out under a hydrogen pressure of 25×10
5
Pa at 110° C. The reaction time is less than 1 hour for a conversion of the order of 70% and a selectivity for hydrogenated monosilanes of the order of 56%. In this instance, the operating conditions are milder and therefore more acceptable but the performance in terms of conversion and selectivity is very inadequate.
In this process, the examples given show that the catalyst is formed either by use of a sophisticated and expensive organometallic nickel compound or by reduction of nickel chloride by a further addition of a powerful and expensive reducing agent, such as triethylsilane.
In such a technical environment, one of the essential objectives of the present invention is to provide a process for the preparation of hydrogenated alkylmonosilanes, by catalytic hydrogenolysis under hydrogen pressure

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