Mechanism of chemical vapour deposition of silicon alkoxide on mordenites

Abstract

The mechanism of deposition of silicon tetramethoxide on mordenites has been investigated from the viewpoints of the saturated surface concentration of deposited silicon, the products evolved, and i.r. spectroscopy of the deposited layer. The saturated concentration of silicon on the mordenites exceeded that of a monolayer covering by silica, and this was influenced by the degree of drying. It was found from desorption profiles that methanol was formed primarily upon deposition on H-mordenite at 291 K, but was adsorbed in zeolites. On Na-mordenite, however, the formation of methanol, accompanied by a small amount of dimethylether, was observed. Infrared spectra of the deposited layers revealed the presence of the methoxide groups. By assuming a simple model for deposition on NaM, it was found that 2.4–2.8 methoxide molecules were converted following deposition on the external surface. Based on these observations and on comparisons with SiO₂ and SiO₂–Al₂O₃, the following mechanism of deposition was proposed. First, the alkoxide reacts with the hydroxide on the external surface to yield the anchored trimethoxide and methanol; secondly, the trimethoxide is hydrolysed by the remaining water to form hydroxide groups, which react further with the gaseous alkoxide or the vicinal trimethoxide. A polymeric silica layer consisting of siloxane bonds is thus obtained which controls the pore-opening size of the zeolites.