NMR AND IR STUDIES OF ZEOLITE H-ZSM-5 MODIFIED WITH ORTHOPHOSPHORIC ACID

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 H3PO4 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. © 1990.