FTIR study of the relation, between extra-framework aluminum species and the adsorbed molecular water, and its effect on the acidity in ZSM-5 steamed zeolite

摘要

The infrared spectroscopy study of zeolite samples, obtained by steam treatment at 560‑960 °C of the ZSM‑5 catalyst (framework Si/Al ratio of 13), suggests an association between adsorbed molecular water and extra‑framework aluminum hydroxyls generated after treatment. Moreover, infrared spectroscopy of adsorbed pyridine shows the reduction of the densities of Br?nsted and Lewis sites, when treatment temperature rises, with contradicts the frequently accepted mechanism of the transformation of two bridged Si‑OH‑Al groups for each Lewis site generated. The gradual conversion of the octahedral extra-framework aluminum (Lewis‑associated) in polymeric species with low acidity is the most probable cause of this behavior. On the other hand, the apparent decline of the acid Br?nsted strength, with the increase in the temperature of the hydrothermal treatment, has two possible causes: a) the decreasing accessibility, of the pyridine molecular probe to bridged Si-OH-Al groups with the strongest Br?nsted acidity, inside the channels, and b) the gradual transformation of these groups into extra framework species of weak acidity.