Accessibility of the acid sites in dealuminated small-port mordenites studied by FTIR of co-adsorbed alkylpyridines and CO

A novel methodology for characterization and quantification the acidic sites of different accessibility has been developed for dealuminated zeolite catalysts. The method is based on probing the accessibility of acid sites by IR spectroscopy of adsorbed alkylpyridines, followed by the characterization of the non-accessible sites by IR spectroscopy of subsequently adsorbed CO. The gradual increase of the size of alkylpyridine probe molecules in a series of experiments allows a step-by-step characterization of the nature and strength of sites with different accessibility. The approach proposed was tested on the series of small-port mordenites dealuminated up to Si/Al ratios of 11, 12, 14 and 18 and characterized by various techniques including XRD, TPD-NH3, nitrogen adsorption-desorption, H-1 MAS NMR and IR spectroscopy. It has been demonstrated that upon dealumination of the small-port MOR, two phenomena related to accessibility occur: (i) the acid sites in the side pockets become accessible for such probe molecules as pyridine due to the partial destruction of side pockets upon dealumination (ii) subsequently, the formation of secondary mesopore system due to further dealumination makes the zeolite crystals completely accessible to relatively bulky molecules such as lutidine, collidine, 2,6-di-tert-butylpyridine. (C) 2004 Published by Elsevier Inc.