Effect of oxide layers on the reaction of silicon with methanol into trimethoxysilane using copper (I) chloride catalyst

When the SiO₂-overlayer of silicon surface was removed by treating silicon grains with aqueous HF solution, the reactivity of silicon towards methanol increased. Thus, trimethoxysilane was formed with almost complete conversion of silicon in three hours and > 98% of selectivity, when the mixture of the silicon grains and a catalyst (copper (I) chloride) was preheated at 513 K and brought in contact with methanol at 513 K. When the silicon with the SiO₂-overlayer was used for the reaction after its mixture with copper (I) chloride was heated at 513 K, the reaction started after a long induction period and the rate of the silicon conversion was very low. However, when the preheating was done at 723 K, the reaction proceeded without induction period, irrespective of the presence of the SiO₂-overlayer. The selectivity for trimethoxysilane was only about 60%. X-ray photoelectron spectroscopy was used to evaluate the thickness of the oxide layers of silicon surface. The morphology of the reacting silicon grains was also examined by SEM.