Effects of crystal size and Si/Al ratio on the surface properties of H-ZSM-5 zeolites

Highly crystalline H-ZSM-5 zeolite samples with crystal sizes from 0.5 to 5 μm and Si/Al ratios ranging from 20 to 70 have been prepared and characterized by IR spectroscopy, SEM and XRD techniques. The surface properties of these materials have been compared with those of conventional small crystal size materials by using FT-IR spectroscopy of the surface hydroxyl groups and of adsorbed carbon monoxide. The external surface has been investigated using pivalonitrile (2,2-dimethyl-propionitrile) as an adsorption probe. Data show that the basic structure of the zeolite active sites do not depend significantly on the crystal size. Brønsted acid sites of high acid strength are constituted by bridging OH groups located inside the zeolite channels, while Lewis acid sites and weakly acidic silanol groups are found on the external surface in all cases.