MAS MMR studies on the dealumination of Zeolite MCM-22

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 Al-27 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 T-3, T-4, T-7, T-10, and T-12 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.