W/HZSM-5 catalyst for methane dehydroaromatization: a multinuclear MAS NMR study

Abstract

Multinuclear solid-state NMR techniques were employed to investigate the interaction between tungsten species and zeolite in W/HZSM-5 catalysts prepared by controlled impregnation. Al-27, Si-29 NMR experiments suggest that introduction of W species leads to the formation of distorted four-coordinate framework and six-coordinate extra-framework aluminum species due to the interaction of the W species and framework aluminum, and the former can be further transformed into the latter, especially at high W loadings. H-1 MAS NMR indicates that some of the W species disperse on the external surface and some of them diffuse into the internal channels of the zeolite, which causes a decrease of the Bronsted acid sites and silanols as well as an increase of the extra-framework AI-OH species in the W/HZSM-5 catalysts. Cooperation of the W species and the remaining Bronsted acid sites is responsible for the activity of the W/HZAM-5 catalysts for methane dehydroaromatization. As revealed by C-13 NMR experiments, an induction period exists for methane dehydroaromatization reaction and W6+ oxide on/in the zeolite is likely to be converted into W4+ oxide during the reaction. It is W4+ oxide rather than carbide tungsten that acts as active metal sites for initial activation of methane on the W/HZSM-5 catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.