Process for preparing organohalosilanes

Details Process for preparing organohalosilanes Process for preparing organohalosilanes

2001-02-14

2002-01-15

Shaver, Paul F. (Department: 1621)

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

Organic compounds

Silicon containing

Reexamination Certificate

active

06339167

ABSTRACT:

BACKGROUND OF THE INVENTION
A process for preparing organohalosilanes from metallic silicon and halogenated hydrocarbons using a copper or copper compound catalyst was first disclosed in U.S. Pat. No. 2,380,995, and is today called the Rochow reaction after the name of the inventor. The silicone industry has carried out organohalosilane synthesis using this direct synthesis process ever since it was invented. That is, organochlorosilanes such as methylchlorosilane are synthesized by the Rochow reaction in which an organic halide such as an alkyl halide (e.g., methyl chloride) or a halogenated aryl compound (e.g., halobenzene) is passed through metallic silicon and a catalyst component composed of a copper catalyst and a small amount of co-catalyst to induce a direct reaction in a vapor phase. In this reaction, because the cost of metallic silicon accounts for a large portion of the raw material costs, it is essential to increase the conversion of metallic silicon and also to maintain reaction conditions in such a way as to bring the formation ratio of the many by-products that generally form together with the main product into line with the demand-supply balance for organochlorosilanes. Industrially, the reaction is generally carried out in a reactor such as a fluidized bed reactor, vibrating fluidized bed reactor or stirred tank reactor while adding the catalyst component. Although activation to bring the reaction to a steady state takes a long time, the steady state is relatively shortlived. Accordingly, it is important to minimize the decline in activity (i.e., the rate of decline in the reaction rate and selectivity) due to the accumulation of deactivated catalyst component as the reaction proceeds in order to enable long-term operation, and thereby increase the conversion of metallic silicon to useful silanes.
The aluminum present as an impurity in industrial metallic silicon reportedly has a large impact on the reaction rate and selectivity. For example, according to Norwegian Patent No. 169831, the ternary phase FeAl
3
Si
2
in silicon provides enhanced reactivity, and the quaternary phase Fe
4
Si
6
Al
4
Ca provides enhanced selectivity. However, the reactivity and selectivity cannot both be increased. That is, it is well known that in the Rochow reaction the aluminum present as an impurity in the industrial metallic silicon serving as one of the starting materials is essential for increasing the catalytic activity, yet it lowers the selectivity for diorganodihalosilanes that are in high demand. The form and reactivity of aluminum present in metallic silicon has been the subject of considerable research and debate.
British Patent No. 2153697 teaches that both the reactivity and selectivity of a direct synthesis process are increased by the use of a copper catalyst comprising a mixture of copper, Cu
2
O and CuO, from 200 to 5,000 ppm of a tin-containing compound, and from 50 to 5,000 ppm of aluminum or an aluminum-containing compound. Unfortunately, this approach fails to provide significant increases in reactivity and selectivity.
H.M. Rong has proposed, in Norwegian Patent No. 950760, a process for the production of alkylhalosilanes by reacting elemental silicon with an alkyl halide at an elevated temperature in the presence of a copper-based catalyst and an optional promotor. However, such a process does not achieve significant increases in reactivity and selectivity. Although the examples of aluminum cited in this prior art include metallic aluminum, aluminum alloys, aluminum-containing silicon alloys and solid aluminum-containing compounds, none of these has sufficient activity by itself. Various active forms of these aluminum substances have been proposed, but no accepted view yet exists on their efficacy, nor have any specific procedures been described for activating such aluminum substances.
Thus, although aluminum has been the subject of considerable research, such efforts have involved merely the use of aluminum impurities in metallic silicon by default as the reaction promotor. No art disclosed to date has proposed activating the aluminum in metallic silicon, which is present primarily as aluminum silicide, to put it to effective use. Because the prior art uses as a starting material aluminum-containing metallic silicon, which has both advantages and drawbacks in the Rochow reaction, the concentration of aluminum within the reaction system is difficult to control, as is also the starting material itself.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a process for preparing organohalosilanes by the Rochow reaction that can shorten the time required for activation, increase the selectivity for desired diorganodihalosilanes, prolong the steady state of the reaction and improve conversion of the silicon.
The inventor has found it to be effective, when preparing organohalosilanes by the Rochow process, to add to the reaction system at least one promotor selected from among activated aluminum, activated aluminum alloys and activated aluminum carbide.
As noted above, there is little doubt that at least some of the aluminum present as aluminum alloy within metallic silicon reacts with halogenated hydrocarbons to form aluminum halides. The aluminum halides reportedly react with the oxide film present on the surface of the metallic silicon, inducing a surface-activating effect. Moreover, it has also been reported that the presence of aluminum halides increases the vapor pressure of copper halides that form from the copper catalyst, thus facilitating diffusion of the copper catalyst and ultimately promoting the catalytic effect of the copper. In any case, it appears to be indisputable that the presence of aluminum promotes the reaction.
Yet, at the same time, the aluminum halides that form as by-products are very strong Lewis acids, and are indeed familiar as catalysts for organohalosilane disproportionation reactions. Hence, the presence of excess aluminum halide gives rise to disproportionation within the reaction system of the diorganodihalosilanes which are primary constituents of the reaction product and are preferably obtained in the highest possible yield, resulting in an undesirable increase in silane by-products such as monoorganotrihalosilanes and triorganomonohalosilanes.
More specifically, in the activating reaction stage at the beginning of the Rochow reaction, the formation of much aluminum halide is advantageous because it is necessary to activate the catalyst component, but once a steady state has been reached, the formation of little aluminum halide is preferred. Achieving in this way the mutually conflicting goals of improved reactivity and improved selectivity requires very close and careful control of the reaction, yet the prior art carries out reactions which attempt to achieve this delicate effect using the aluminum impurities already present within the metallic silicon, such as alloys with silicon. Such aluminum impurities within metallic silicon originate from impurities within the silica starting materials used in the metallurgical production process, and so are not uniformly present throughout the metallic silicon. Rather, they form intermetallic compounds with silicon and other metals or compounds with nonmetallic elements, and are dispersed throughout the silicon as impurity zones. The slow rate of aluminum halide formation from reactions with organic halides has made it necessary to take one of two approaches: either use a higher aluminum content than would otherwise be warranted or use metallic silicon having a low aluminum content to achieve good selectivity in an activation period having a long initial stage. Hence, it is impossible in practice to control the active aluminum within the catalyst component to the required level, and such control by itself cannot increase both the reactivity and the selectivity.
Upon investigating instead reactions between what is referred to in the prior art as “active aluminum” and halogenated hydrocarbons, the inventor learned that within a temperature range of 250 to 400°

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