Solid-State 29Si NMR Study of Polycondensation During Heat Treatment of Sol-Gel-Derived Silicas

摘要

Solid state magic angle spinning (MAS) and cross polarization Si NMR spectroscopy was used to monitor polycondensation in alkoxide-derived silica gels during thermal treatment from 25 to 800 C. Gels prepared from tetramethylorthosilicate under both neutral and basic conditions were studied, and differences in the evolution of chemical structure during thermal treatment are readily observed by NMR. As thermal treatment proceeds the extent of condensation parallels the BET surface area. For uncatalyzed gels, the amount of cross linking increases, and the BET surface area decreases on each step of heating through 800 C. The base catalyzed gel depolymerizes slightly on heating from 150 to 450 C, accompanied by a slight increase in BET area, then undergoes extensive cross linking on heating from 450 to 800 C, with a large decrease in surface area. After heating to 800 C both gels have similar degrees of cross linking and surface area.