Catalytic activity of H-forms of zeolites in the isomerization of supercritical n-pentane and their physicochemical properties

The acidic properties of the H-forms of zeolites ZSM-5, Beta, Y, and mordenite are studied by diffuse reflectance IR spectroscopy using n-pentane as a probe molecule. The decreasing order of Brønsted acid site strengths is constructed. The isopentane selectivity in n-pentane isomerization under supercritical conditions (260°C, 130 atm) increases in the order H-ZSM-5 < H-Beta < H-mordenite(11) ≈ H-Y with decreasing strength of Brønsted sites. Catalytic data are analyzed jointly with the results of physicochemical studies of H-mordenite (temperature-programmed ammonia desorption, benzene adsorption, and IR spectroscopy). Under the supercritical conditions, the conversion of n-pentane on mordenite is determined by the total acidity of the zeolite and also by the accessibility of the acid sites inside the zeolite channels to the reactant.