A Multitechnique Characterization of the Acidity of Dealuminated Mazzite

Abstract

The acidity of steam-dealuminated mazzite has been studied using heat flow calorimetry, temperature programmed desorption, break X-ray photoelectron, and infrared spectroscopy. Particular emphasis has been given on evaluating the influence of framework and nonframework aluminum species on the nature, strength, and accessibility of the acid sites by preparing mazzite samples covering a broad range of global and framework compositions. It is shown that the strong acid sites present in dealuminated mazzite are associated with the framework aluminum atoms, whereas nonframework species contribute essentially to the weaker acidity. The presence of very strong sites has been revealed by the various techniques. Initial and intermediate heats of adsorption of ammonia were higher (by about 10 to 20 kJ/mol) than those usually reported for dealuminated zeolites. Three families of sites have been identified: (i) very strong Lewis sites, whose assignment to framework species is supported by the photoelectron and infrared data, (ii) structural Br/nsted sites, associated with bridging hydroxyl groups, which exhibit an acid strength higher than those of mordenite and faujasite, and (iii) Lewis and Br/nsted sites with weak or medium strength, originating from nonframework aluminum-containing species formed by high-temperature hydrolysis of structural Al–OH–Si groups. Evaluation of the performance of platinum/mazzite catalyst forn-paraffin isomerization under industrial-like conditions reveals an activity and selectivity higher than that of commercial platinum/mordenite, with increased production of the high-octane isomer 2,2-dimethyl-butane.