Fine control of the pore-opening size of the zeolite mordenite by chemical vapour deposition of silicon alkoxide

Abstract

Chemical vapour deposition (c.v.d.) of Si(OCH₃)₄ on the H form of mordenite has been carried out in order to control the pore-opening size without affecting its acidic properties. It has been shown that Si(OCH₃)₄ is deposited irreversibly on the zeolite. Because the molecular size of the alkoxide is larger than the pore size, the alkoxide does not enter the pore and the silicon compound is deposited on the external surface. The alkoxide may be deposited by reaction with hydroxide, thus covering the external surface of zeolite crystal after subsequent reactions. Calcination with oxygen removes the hydrocarbon residue and produces silica-coated H-mordenite (SiHM). The SiHM thus obtained has been characterized by temperature-programmed desorption (t.p.d.) of NH₃, adsorption experiments and X-ray photoelectron spectroscopy. The deposition of the alkoxide does not change the acidity but reduces the size of pore opening. Enrichment of Si on the external surface of the zeolite is confirmed. One can therefore conclude that SiO₂ covers the external surface of the zeolite, thus reducing the effective size of the pore opening. The pore size is effectively reduced by ca. 0.1 and 0.2 nm upon formation of 1–2 and 3 molecular layers of silicon oxide, respectively.