Conversion of methane to benzene over transition metal ion ZSM-5 zeolites - I. Catalytic characterization

The conversion of methane to benzene in the absence of oxygen has been investigated over different TMI/H-ZSM-5 zeolites (with TMI = Mo, Fe, V, W, and Cr) as a function of the preparation and treatment method, the metal ion loading, and the zeolite acidity. Nonoxidative methane activation is characterized by an induction period prior to the formation of benzene, toluene, and naphthalene. During this induction period the supported metal oxide is reduced and CO, CO(2), and H(2)O are formed. Optimum catalytic performance was obtained for CO-prereduced 2-4 wt% TMI/H-ZSM-5 zeolites, and their activities decreased in the order: Mo (18.3)> W (10.8)> Fe (5.7)> V (3.9)> Cr (1.5). The numbers in parentheses are the rates of methane reaction (molec. reacted/metal atom/h) obtained after 3 h on-stream. Impregnated materials always had a higher activity and shorter initial induction period than solid state ion-exchanged materials, and the induction period decreased with increasing time at moderate reduction temperature and with increasing metal oxide loading. Based on a detailed comparison of the different systems, it is evident that the catalytic properties depend on the presence of Bronsted acid sites and the distribution of the TMI's in the zeolite material. (C) 1998 Academic Press.