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, t he metal ion loading, and the zeolite acidity. Nonoxidative methane ac tivation is characterized by an induction period prior to the formatio n of benzene, toluene, and naphthalene. During this induction period t he supported metal oxide is reduced and CO, CO2, and H2O are formed. O ptimum catalytic performance was obtained for CO-prereduced 2-4 wt% TM I/H-ZSM-5 zeolites, and their activities decreased in the order: Mo (1 8.3)> W (10.8)> Fe (5.7)> V (3.9)> Cr (1.5). The numbers in parenthese s are the rates of methane reaction (molec. reacted/metal atom/h) obta ined after 3 h on-stream. Impregnated materials always had a higher ac tivity and shorter initial induction period than solid state ion-excha nged materials, and the induction period decreased with increasing tim e at moderate reduction temperature and with increasing metal oxide lo ading. Based on a detailed comparison of the different systems, it is evident that the catalytic properties depend on the presence of Bronst ed acid sites and the distribution of the TMI's in the zeolite materia l. (C) 1998 Academic Press.