Studies on the Pyrolysis of Silica Gels Surface-treated with C₁-C₃ Alcohols

Abstract

The hyrolysis of silica gels treated with C₁-C₃ alcohols and the pyrolysis of the corrresponding alcohols in gaseous phase or over silica gels have been investigated from the analysis of their products with a quadrupole mass spectrometer. The pyrolysis of the surface-treated silica gels has taken place in the following three temperature ranges, (i) 30-200°C, (ii) 250-500°C and (iii) 350-550°C. In the pyrolysis of silica gel treated with methanol, the temperature ranges of (ii) and (iii) have shifted to higher temperature ranges, (ii) 250-650°C and (iii) 550-750°C, respectively. The analysis of mass spectra of the vapors evolved in the first temperature range has shown that the vapors evolved are originated from physically sorbed water or alcohol. The water vapor has been found to evolve even up to 800°C and to be due to the dehydration of residual surface silanols. The vapor evolved in the temperature range of (ii) comes mainly from the alcohol used. This may have been caused by the hydrolysis of the surface groups due to water produced in the condensation of surface silanols. The phyrolysis of the surface-treated silicagels in the temperature range of (iii) takes place at appreciably lower temperatures than those of pyrolysis of alcohol in gaseous phase or over silica gel as a catalyst. The main product of pyrolysis of silica gel treated with methanol is CO, whilst the main products of silica gels treated with C₂-C₃ alcohols are olefins corresponding to the alcohols used. These products differ from those produced in the pyrolysis of each alcohol in gaseous phase or over silica gel. Accordingly it is recognized that alcohols are not chemisorbed on silica gel, but they react with surface hydroxyl groups. It has also been found that the silica gel promotes the selective dehydration by other alcohols than methanol.