Influence of Extraframework Aluminum on the Brønsted Acidity and Catalytic Reactivity of Faujasite Zeolite

A series of faujasite zeolites was modified by extraframework Al (Al_EF) with the goal to investigate the influence of such species on the intrinsic Brønsted acidity and catalytic activity towards paraffin cracking. The chemical state of Al_EF and zeolite acidity were investigated by ²⁷Al MAS NMR and CO_ads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect-free Y zeolites were obtained by substitution of framework Al by Si with (NH₄)₂SiF₆. In accordance with the next-nearest-neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in CO_ads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by Al_EF only resulted in a small fraction of cationic Al_EF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less Al_EF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Brønsted acid sites perturbed by cationic Al_EF species. The results of MQMAS ²⁷Al NMR spectroscopy confirmed the presence of sites perturbed by Al_EF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although CO_ads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from Al_EF, they were less suitable to predict the reactivity if the Brønsted acid sites were affected by cationic Al_EF species.