Structural Evolution of Alkoxide Silica Gels to Glass: Effect of Catalyst pH

Abstract

Structural and energetic changes of alkoxide silica during densification were followed in situ using photoacoustic Fourier‐transform infrared spectroscopy (PAS‐FTIR), transposed temperature drop calorimetry, and drop solution calorimetry. Acid‐catalyzed silica gels contained higher concentrations of adsorbed water, silanol groups, and unreacted alkoxy groups than did the base‐catalyzed gels/precipitates. The surface species in acid‐catalyzed gels were removed easily, allowing this material to be densified at 800°C. The densified material was both structurally and energetically equivalent to fused silica glass. Base‐catalyzed gels had fewer silanol groups in the green state, but free silanol groups emerged during drying at 500°C. The base‐catalyzed gels did not densify below 1000°C, maintaining a high surface area of 100 m 2 /g. Base‐catalyzed alkoxide gels comprised strained ring structures; their IR spectra were similar to those of fumed silica, but they were energetically much less stable.