Catalytic and multinuclear MAS NMR studies of a thermally treated zeolite ZSM-5

Abstract

Catalysts of ZSM-5 type were obtained by synthesis with tetrapropyl-ammonium-bromide (TPABr) as a template and subsequent hydrothermal treatment up to 1173 K. 1H-, 27Al-, and 29Si-NMR spectroscopy and the direct oxidation of benzene with N 2O as a catalytic test were applied to characterize the products. 27Al MAS NMR and 27Al 3QMAS NMR studies of the samples give evidence that fivefold-coordinated extra-framework aluminum species exist if the product is treated at 1173 K and then rehydrated. The values of all characteristic catalytic key data are highest when the catalysts are precalcined at 1173 K. This is a clue that species giving rise to fivefold-coordinated aluminum species in the hydrated products are responsible for the high catalytic key data. The nature of these species is not yet clear. Two models for the species are in agreement with our experimental findings.