Chemistry of the silica surface: reaction with phosphorus pentachloride

Solid-state 31P and 29Si NMR experiments, with Magic-Angle Spinning (MAS), were used to elucidate the chemistry that occurs when silica gel is treated with phosphorus pentachloride. A low-loading regime (in which the molar ratio of initial PCl5 to surface silanols sites is <<1) and a high-loading regime (in which this ratio is approximately 1) were examined. For each regime, the results for limited and intentional exposure to moisture are presented. The occurrence of phosphorus bridging between two adjacent silanols sites is observed. Bridging structures based on Si-O-P-O-P-O-Si linkages are also indicated.