Realumination of zeolite Y under acidic conditions

Abstract

Dealuminated zeolites Y were treated with aqueous solutions of various acids and ammonium salts to investigate the realumination behavior under acidic conditions. From the results of 27Al MAS NMR, 29Si MAS NMR and FT-IR measurements, it was found that a part of non-framework aluminum species in the dealuminated zeolite Y is effectively reinserted into the zeolite framework in CH3COONH4 and C6H5COONH4 aqueous solutions. Pyridine adsorption experiments also revealed that most of incorporated aluminum species generate tetrahedrally coordinated framework aluminum species, namely Brönsted acid sites. Although the realumination also proceeded in H2SO4 and CH3COOH aqueous solutions, large amounts of incorporated aluminum species were not necessarily responsible for generation of Brönsted acid sites. Framework connected aluminum species, presumably as 3-fold-coordinated Lewis acidic framework aluminum species, were mainly generated. In the TEM image of the realuminated zeolite Y, needle-like crystals with ca. 25–80 nm in length were observed, which are probably due to AlOOH generated from non-framework aluminum species.