Rates of alkoxylation and hydrolysis of alkoxyl groups on silica surfaces

The rate of reactions on silica surfaces was investigatedfor the alkoxylation and hydrolysis of alkoxyl groups. Silica gel particles were contacted with 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 apparent rate constants could be calculated by applying the progressivereaction model. The apparent rates based on the total number of surface groups were determined for alkoxylation reactions. Among the alcohols investigated, alkoxylation with methanol showed the smallest activation energy and the largest rate constant. The rates of hydrolysis of alkoxyl groups were also measured on samples which had been alkoxylatedto about 10% of the initial surface groups. The activation energy of hydrolysis was almost constant for all alkoxyl groups. However, the hydrolysis of methoxy groups showed the largest rate constant. The influence of preheating treatment on the rate of alkoxylation was examined for the reaction with 1-butanol. Even when the reaction temperature was the same, the rate constant increased with increasing preheating temperature. The increase in the rate constant could be attributed to the increase in the frequency factor, since the preheating treatment increased the activation energy.