NMR and IR studies of zeolite H-ZSM-5 modified with orthophosphoric acid

Abstract

By combined use of solid-state NMR, NMR self-diffusion techniques, and diffuse reflectance IR spectroscopy, important information on the nature of the interaction of orthophosphoric acid with zeolite H-ZSM-5 could be obtained. By 1H NMR self-diffusion experiments it was found that the phosphoric acid enters the ZSM-5 channel system. However, with increasing amounts of H3P04 deposited, there is an enrichment of phosphorus species near the external surface of the zeolite crystals, forming a transport barrier for diffusing molecules. 'Al MAS NMR shows that the decreased concentration of Bronsted acid sites in H-ZSM-5 after impregnation with orthophosphoric acid and a subsequent thermal treatment at 500°C is a direct result of a framework dealumination. Combined 27Al, 29Si, 31P, and 1H MAS NMR studies reveal that this dealumination process is promoted by the presence of increasing amounts of H 3PO 4 and results in the formation of occluded aluminum phosphate species. Furthermore, 1H MAS NMR shows that H3PO4 impregnation causes a drastic reduction in the concentration of nonacidic silanol groups. Both findings of 1H MAS NMR have been verified by diffuse reflectance IR spectroscopy. Consequently, treatment of ZSM-5 with orthophosphoric acid modifies acidity and diffusivity in the sample series under study. These effects can combine to give a useful catalyst of enhanced paraselectivity.