A SupramolecularView on the Cooperative Role of Brønstedand Lewis Acid Sites in Zeolites for Methanol Conversion

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Bronsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Bronsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]+ and double protonated binuclear metal clusters [M(μ-OH)2M]2+ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The i...