Yoshimi Kawashima

A shape-selective platinum-loaded mordenite catalyst for the hydrocracking of paraffins by the chemical vapour deposition of silicon alkoxide

A platinum-loaded H-mordenite (PtHM) catalyst has been modified by the chemical vapour deposition of silicon alkoxide in order to improve its shape-selectivity for the hydrocracking of paraffins. The paraffins reacted on the silica-coated PtHM according to their molecular size, and reactant shape-selectivity was achieved by choosing the extent of modification. Silica was only deposited on the external surface of the zeolite to form mono-, di- or tri-layers of SiO2, with the internal acid and metal sites unpoisoned, thus effectively narrowing the pore exit size only. Platinum metal on the external surface was not completely covered by the silicon oxide.

Mechanism of chemical vapour deposition of silicon alkoxide on mordenites

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.

Growth of Silica and its Controlling of Pore-opening Size on CVD Zeolites

This paper describes how silica grows on the external surface of zeolite, and how it controls the pore-opening size. Silica growth and control of pore-opening size is not affected by the kind of cation but only by the composition of zeolite. The larger the silica content, the more similar the silica layer to the basal plane grows. Due to the subtle difference between overlayer and zeolite, the pore-opening size can be narrowed. Because of the similarity, thicker layers are required for control in the highly siliceous zeolites.