Rates of alkoxylation and hydrolysis of alkoxyl groups on silica surfaces

The rate of reactions on silica surfaces was investigated for the alkoxylat ion and hydrolysis of alkoxyl groups. Silica gel particles were contacted w ith vapors of several alcohols and water in a flow-type reactor at various temperatures. The surface concentrations of alkoxyl groups were measured by thermogravimetric analyses to determine the rates of reactions. The appare nt rate constants could be calculated by applying the progressive reaction model. The apparent rates based on the total number of surface groups were determined for alkoxylation reactions. Among the alcohols investigated. alk oxylation with methanol showed the smallest activation energy and the large st rate constant. The rates of hydrolysis of alkoxyl groups were also measu red on samples which had been alkoxylated to about 10% of the initial surfa ce groups. The activation energy of hydrolysis was almost constant for all alkoxyl groups. However, the hydrolysis of methoxy groups showed the larges t rate constant. The influence of preheating treatment on the rate of alkox ylation was examined for the reaction with 1-butanol. Even when the reactio n temperature was the same. the rate constant increased with increasing pre heating temperature. The increase in the rate constant could be attributed to the increase in the frequency factor, since the preheating treatment inc reased the activation energy.