Effects of zinc and tin promoters on the copper-catalyzed conversion of silicon to methylchlorosilanes

The direct reaction of methyl chloride with silicon to produce methylchlorosilanes was studied using CuSi and CuZnSnSi contact masses. In particular, their activation, transient and steady-state reaction kinetics, and deactivation were examined. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to study these processes. High purity starting materials were employed so that the effect of the zinc and tin promoters could be unambiguously determined.;The formation of active sites on CuSi and CuZnSnSi occurred by an autocatalytic (in terms of metallic copper) surface reaction between CuCl and silicon which primarily produced tetrachlorosilane. The rate of silane production during steady-state reaction kinetics, however, was increased by over an order of magnitude when zinc and tin were added to CuSi and the selectivity for dimethyldichlorosilane (DMDC) was greatly enhanced. These changes are attributed to the formation of new sites on the promoted mass which incorporated both zinc and tin. The increased reactivity of these sites are proposed to result from their complementary effects, that is, zinc accelerated the formation of Si-CH;Although the amount of carbon (i.e., coke) deposited on the CuSi surface during the direct reaction was greater than that deposited on CuZnSnSi, the promoted sample experienced more extensive deactivation. This apparent anomaly was explained in terms of zinc's ability to direct methyl groups to surface Si atoms active for silane formation and away from copper rich areas where coke was more readily formed. This resulted in less carbon deposition on CuZnSnSi; however, if coke was formed, it was more likely to block active sites and result in loss of activity.