Acidic property of Y- and mordenite-type zeolites with high aluminum concentration under dry conditions

The relationship between the acidic property and the composition of a zeoli te was precisely measured using Y- and mordenite-type samples with high alu minum contents. The ammonium form zeolite was evacuated and heated to yield the hydrogen form, and subsequently, the ammonia adsorption and temperatur e-programmed desorption were carried out in-situ without exposure of the hy drogen form sample to humid atmosphere. The number of the desorbed ammonia, showing the acid amount, was approximately same to the number of [Al] - [N a] atoms over the experimental range. This is quite different from the conv entionally observed relationship between the acidic propel-ty and compositi on, in which the acidity is absent at the high aluminum concentration where the secondary-neighboring aluminum species (Al-O-Si-O-Al) is formed. On th e basis of the X-ray diffraction, it is suggested that the lack of acidity on the hydrogen form zeolite with high aluminum concentration is due to the destruction of crystal structure, which is induced by exposure of the hydr ogen form sample to humidity after removal of ammonia. It is pointed out th at the isomorphously substituted aluminum in zeolite generates the acid sit e according to the stoichiometry even when the aluminum atoms locate the se condary-neighboring sites.