Mechanism of silation of silica with hexamethyldisilazane

The silation of silica with hexamethyldisilazane (HMDS) was examined over t he temperature range 150-450 degrees C. The products and sequence of the re actions at the silica surface were determined. The major products are hexam ethyldisiloxane (HMDSO) and ammonia, with lesser amounts of nitrogen and me thane. No methane is produced during reactions below 300 degrees C. Ammonia is the initial product detected, followed by HMDSO. The amount of nitrogen produced increases, and the amount of ammonia and HMDSO produced decreases , with increasing temperature of reaction. A mechanism is proposed. The rea ction of HMDS with silica is sequential. Reaction of pendant silyl groups w ith surface acidic hydroxyls to eliminate methane and the concentration of residual silyl moieties on the silica surface both increase with temperatur e of reaction.