Alkoxysilane production from silica and dimethylcarbonate catalyzed by alkali bases: A quantum chemical investigation of the reaction mechanism

Abstract

Density functional theory (DFT) calculations were carried out to investigate mechanistic details of the reaction between silica and dimethylcarbonate (DMC) catalyzed by an alkali base. Various experimental studies have reported that several silicon dioxide sources can react with DMC in the presence of alkali base catalysts to produce tetramethoxysilane (TMOS), but details of the reaction mechanism are still elusive. Our DFT calculations suggest that the reaction can be characterized by four mechanistic steps. The first two steps include the activation of Si-O bonds by the alkali base catalyst, and the cleavage of the Si-O bond forming -Si+ and -O-. In the third step, the O- moiety reacts with the methyl group of DMC to form the -O-CH3 moiety; this is the rate-determining step of the overall reaction. Finally, transfer of a methoxy group from DMC to the silicon occurs to produce a compound in which two Si-O bonds in silica are replaced by two Si-OCH3: i.e., dimethoxylsilyloxide. DFT calculations reveal that the rate-determining step of the reaction depends strongly on the nature of the cationic part of the alkali base catalysts. The order of barrier height of the rate-determining step was computed to be LiOH > KOH > CsOH, and this trend is in agreement with previous experimental studies. The Li cation was found to interact with DMC to form a very stable intermediate compound that causes the barrier height of the reaction between the O- moiety of the activated SiO2 and DMC to be higher than that of the reaction catalyzed by other cations.