MAS NMR Studies on the Dealumination of Zeolite MCM-22

Abstract

The dealumination of Zeolite MCM-22 by calcination or hydrothermal treatment was investigated by conventional multinuclear solid-state NMR and checked by an ultrahigh-field NMR experiments (19.6 T) with a very fast spin rate (19.1 kHz). The presence or variation of different species (e.g. silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminum) during or after dealumination was detected, and their changes were followed. Four tetrahedral framework aluminum sites were identified upon the 27Al MAS NMR experiments showing chemical shifts at 61.5, 57.0, 53.5, and 50.0 ppm, respectively. After hydrothermal treatment, aluminum at 57.0 ppm which corresponded to T3, T4, T7, T10, and T12 sites in the orthorhombic model is preferentially expelled from the zeolitic framework. Moreover, a dehydroxylation process without the release of four-coordinated framework Al also happened, resulting in the presence of a tricoordinated framework aluminum species, which, together with the formation of tetrahedral framework aluminum species, may serve as the Lewis acid site of the zeolite.