Mechanism of silation of silica with hexamethyldisilazane

The silation of silica with hexamethyldisilazane (HMDS) was examined over the temperature range 150-450 degrees C. The products and sequence of the reactions at the silica surface were determined. The major products are hexamethyldisiloxane (HMDSO) and ammonia, with lesser amounts of nitrogen and methane. 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 reaction of HMDS with silica is sequential. Reaction of pendant silyl groups with surface acidic hydroxyls to eliminate methane and the concentration of residual silyl moieties on the silica surface both increase with temperature of reaction.