ACIDITY IN ZEOLITES AND THEIR CHARACTERIZATION BY DIFFERENT SPECTROSCOPIC METHODS

An overview of the different acidic sites in zeolites is presented her e. It is followed by a discussion of the methods for identification of the various acidic sites and their distribution in some widely used z eolites such as faujasite, ZSM-5 and mordenite. Emphasis is laid on th e discussion of various techniques, their relative advantages and disa dvantages and several example studies, where these techniques are appl ied. The techniques discussed include titration in aprotic solvents, t emperature-programmed desorption, microcalorimetric measurement of the heats of adsorption of probe molecules, IR spectroscopy with and with out probe molecules, MAS NMR, photoelectron spectroscopy and position annihilation spectroscopy. From this study it appears that a single te chnique is not sufficient for the characterization and interpretation of zeolite acid sites and for the prediction of catalytic activity. Co mbination of different techniques such as NMR with LR can provide more information about the structure of acid sites of zeolites.