Effects of phosphorus and silicon purity on coke formation and reaction kinetics in the direct synthesis of methylchlorosilanes

The direct reaction of methyl chloride with silicon to produce methylchlorosilanes was studied using CuZnSnSi(hp), CuZnSnPSi(hp), CuZnSnSi(tech) and CuZnSnPSi(tech). The effects of phosphorus and impurities present in technical grade silicon on the coke formation and the reaction kinetics were investigated using FTIR spectroscopy and temperature programmed oxidation (TPO) technique.; CO₂ profiles obtained by performing temperature programmed oxidation (TPO) on post-reaction contact masses revealed two major peaks, referred to as the α-peak (550–590 K) and the β-peak (620–660 K). TPO experiments conducted on carbon black suggested that the combined presence of copper and chlorine was required to catalyze the oxidation of coke. It also suggested that different sites could be responsible for the different peaks that were observed. Nevertheless, the contribution of different types of coke to the observed peaks cannot be conclusively ruled out. The α-peak is proposed to be due to oxidation of coke near sites that are rich in copper and chlorine while the β-peak results from oxidation of coke near copper silicide sites (lean in chlorine). Thus, an increase in the peak size could be due either to an increase in the amount of coke deposited or to an increase in the number of the respective sites.; The contribution of impurities present in technical grade silicon to coke formation appeared to be very small and could be detected only when the total amount of coke formed was low. Phosphorus suppressed coke formation at high temperatures and pressures. Phosphorus may have a role in adsorbing and dissociating methyl chloride. It is proposed that phosphorus may also stabilize the dissociated methyl radical.; Reaction orders and activation energies were calculated using a simplified power law expression. The reaction orders were independent of temperature for all the contact masses. Rapid deactivation was observed in CuZnSnSi(tech), CuZnSnPSi(tech) and CuZnSnPSi(hp) which required the use of a deactivation profile to extract reaction orders and activation energies. The existence of strong synergism when zinc and tin are present in the contact mass appears to have masked the effect of phosphorus and impurities present in the technical grade silicon.