Selective synthesis of trialkoxysilanes by the reaction of metallic silicon with alcohols using copper(I) chloride as the catalyst

The reactions of silicon with ethanol, propanols and butanols were studied using copper(I) chloride as the catalyst. Primary alcohols reacted with silicon, while secondary and tertiary alcohols did not. The reactions of silicon with ethanol, 1-propanol, and 1-butanol gave high silicon conversions of 95, 85, and 77%, respectively, at 513 K without heating treatment of the silicon-catalyst mixture. Under these conditions, the selectivities for trialkoxysilanes were very high (>99%). The pretreatment of the silicon-catalyst mixtures at high temperatures (i.e., 723 K) to facilitate the formation of Cu₃Si Phase is not favorable for the reactions: Both the silicon conversion and the selectivity for trialkoxysilane are sharply depressed. Metallic copper is easily formed on the silicon surface during the reactions after high temperature pretreatment. The metallic copper catalyzes the dehydrogenation of alcohols to aldehydes, which inhibit the silicon-alcohol reactions. It also catalyzes the reaction of trialkoxysilane with alcohol to give tetraalkoxysilane. This brings about the decrease in the selectivity for the trialkoxysilane. However, the high silicon conversion together with high selectivity was obtained by adding thiophene or propyl chloride, which is a poison for copper catalysts, to the alcohol feed. Though 2-propanol does not react with silicon, dimethoxyisopropoxysilane and methoxydiisopropoxysilane were formed appreciably when 2-propanol was fed to the silicon together with methanol. This phenomenon was discussed in relevance of the mechanism of the silicon-alcohol reactions.