MAGIC-ANGLE-SPINNING NMR-STUDIES OF ACID SITES IN ZEOLITE H-ZSM-5

1H, 11C, 17Al, and 29Si magic-angle-spinning (MAS) NMR was used to elucidate the nature of the catalytic activity of zeolite H-ZSM-5. 1H MAS NMR of sealed samples after mild hydrothermal dealumination shows that the enhanced activity for n-hexane cracking is not due to an enhanced Brønsted acidity. The concentrations of the various OH groups and aluminous species suggest that the reason for the enhanced catalytic activity is the interaction of the n-hexane molecule with a bridging hydroxyl group and with extra-framework aluminium species. Loading the samples with HCOOH or HCl shows that those extra-framework aluminium species, which give rise to the enhanced activity, cannot be easily removed from their positions, and are therefore immobilized by the zeolitic framework. © 1991.