DISCRIMINATIVE CHARACTERIZATION OF SURFACE PROPERTIES IN THE INTERNAL EXTERNAL SURFACES OF MICROPOROUS CRYSTALS

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Bronsted acidic properties and surface areas of various zeolites have been characterized with discrimination of the internal and external surfaces by the pyridine- trialkylanine BDA (Base Dual-Adsorption) method, XPS, and the BET method using bulky molecules. The zeolites were inordenites, ZSM-5, ferrierite, L zeolite, and offretite/erionite in their hydrogen forms. The followings have been proved for all the zeolites:(1) The Bronsted (B) acid strength distribution is quite different between the internal and external surfaces. In the internal surface, both strong and weak B sites are located; whereas, in the external surface no strong B sites are present. External B sites are weak for L zeolite, ZSM-5, and offretitie/erionite or extremely weak for mordenites and ferrierite.(2) The internal Bronsted acid strength distribution shifts to higher acid strength with increasing Si/Al ratios in the framework. This shift is related to the concentration of Si(1Al) units, but not to the micropore structure or size.(3) External Bronsted acid strength has no relation to the Si/Al ratio in the external surface layer and depends on the kind of zeolites.(4) The surface density of framework Al atoms is smaller in extra-crystalline than in intracrystalline. Consequently the population of Bronsted acid sites in the external surface is lower than that expected from the external to internal surface area ratio.(5) Adsorption of bulky molecules such as perfluorotributylamine is useful for measuring the external surface area of zeolites with large channel openings.