Ammonolysis of silica gels containing methyl groups

Abstract

Silica gels containing a constant concentration of methyl groups directly bonded to silicon atoms have been prepared from methyltriethoxysilane (MTES) and mixtures of tetraethylorthosilicate (TEOS) with dimethyldiethoxysilane (DMDES) and hexamethyldisiloxane (HMDS). These gels were subsequently ammonolyzed. The temperature range in which significant nitrogen is incorporated into a gel lies between 600 ° C and 800 ° C for the gel derived from MTES, between 800 ° C and 1000 ° C in the gel derived from DMDES with TEOS and at 1000 ° C in the gel derived from HMDS with TEOS. Three different nitridation processes operate depending on the temperature. The processes are reaction of ammonia with (i) methyl groups bonded to silicon atoms, (ii) SiOH groups and (iii) SiOSi bonds. The low-temperature process is dominant in MTES-derived gel and the middle- and high-temperature processes are dominant in DMDES- and HMDS-derived gels. The nitridation processes depend on the amount of SiOH groups in the gel, the thermal stability of methyl groups present and the reactivity of the gel with SiOH or SiNH 2 groups to form SiOSi, SiNHSi or nitride bonds.