The acidity of a series of commercial dealuminated H−Y zeolites was studied by means of MAS NMR and XPS spectroscopy. 27Al MAS NMR has shown the presence of three types of Al in the dealuminated samples:  framework tetrahedral Al, nonframework octahedral Al, and a third type of Al. As inferred from the framework Si/Al ratio obtained by 29Si MAS NMR spectra and from the evolution of the unit cell parameter determined by XRD, this third type of Al may be localized into the zeolite framework. XPS characterization of the samples in their acid form has allowed us to establish the relative concentration of Al at the outer surface of the particles. Upon adsorption of ammonia on the dealuminated zeolites, three types of acid sites have been evidenced by XPS. The analysis of the XPS data indicates that the strength of the acid sites associated with tetrahedral framework Al atoms is not influenced by the other types of Al. The catalytic results of heptane and decane hydroisomerization have evidenced that the acid sites of certain dealuminated H−Y zeolites may be very efficient. The information provided by MAS NMR, combined with the XPS results and literature data, has allowed us to partially elucidate the origin of the high activity of some dealuminated H−Y zeolites.

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Mg Ka X-ray photoelectron spectroscopy (XPS) was used to establish the relative concentration and the chemical state of aluminum at the surface of dealuminated mordenites. A method for the binding energy calibration is proposed. Modified Auger parameters of AI have been determined for different zeolites and clay minerals as well as for dealuminated mordenites. The data are compared with those available in the literature. Ranges of values for modified Auger parameter of octahedral and tetrahedral aluminum are reported. The results obtained on highly dealuminated mordenites are consistent with the presence of surface tricoordinated AI. IntroductionMordenite is used in several industrial catalytic processes.'v2 Dealumination of mordenite is nowadays a common way to improve the catalytic performances as well as the catalyst life.3-4