Reactions of silicon intermediates relevant to the dechlorination of chlorofluorocarbons.

The threat to the Earth's ozone layer posed by chlorofluorocarbons (CFCs) arises because of the photolytic generation of chlorine atoms in the stratosphere. New methods of rendering CFCs less harmful by dechlorinating them, are therefore of environmental importance. Preliminary investigations have suggested that silylene chemistry may be useful in this respect. Silylenes have also been shown to be formed as gas-phase intermediates during the Direct Synthesis of methylchlorosilanes. This work reports on a study of the possibility of using these silylenes for the dechlorination of CFCs using the technology of the Direct Synthesis, and the effect that CFCs would have upon methylchlorosilane product distribution. The gas-phase reactions of silylenes with chlorofluorocarbons were studied. Silylenes (:SiMe2, :SiMeCl and :SiCl2), were generated thermally from appropriate disilane precursors in the presence of halomethanes (CF2Cl2 and CFCl3). The reactions proceeded via radical mechanisms, initiated by the decomposition of an adduct formed between the CFC and silylene, resulting in chlorine abstraction to yield chlorosilyl and halomethyl radicals. Abstraction proceeded most efficiently with :SiMe2. The mechanisms were propagated by the resultant halomethyl radicals which acted as chain carriers. Although silylene insertion reactions were observed, radical reactions were dominant. The reactions of silylenes with methyl chloride were also studied but were found to be less extensive, with die greater strength of the C-Cl bond allowing insertion reactions to become competitive. All reactions were found to proceed much more efficiently in the presence of a relatively weak Si-H bond. To understand further the role of radical reactions in the dehalogenation of CFCs, silyl radicals were generated directly using mercury photosensitisation. Similar mechanisms were indicated, although reaction was more extensive as a result of the increased number of gas-phase radical species. A laboratory scale reactor was used to simulate the addition of CFCs to the Direct Synthesis, and to study their effect on product distribution. Surface reactions dominated, with the adsorption of CFCs leading to greater chlorination of the methylchlorosilane products.