Preparation of high-silica faujasites by treatment with silicon tetrachloride

Abstract

NaY has been dealuminated in highly controlled conditions by SiCl₄. The materials thus obtained, after subsequent washing and ammonium exchange, were characterized by chemical analysis, thermogravimetry, X-ray diffraction, infrared spectroscopy of the framework and surface hydroxyl groups and ²⁹Si nuclear magnetic resonance spectroscopy. In order to obtain highly crystalline materials in a reproducible way, a very specific dealumination and washing procedure has to be followed. The mechanism of the dealumination is complex and always combines direct isomorphic substitution with acid leaching of aluminium. The materials obtained have continuously varying properties as far as unit-cell dimensions and framework vibrations are concerned. In contrast, the amount of extra-lattice aluminium retained in the zeolite reaches a maximum at intermediate degrees of dealumination.