Study of W/HZSM-5-based catalysts for dehydro-aromatization of CH4 in absence of O-2. II. Action of promoters Zn and Li

By correlating the results of the NH3-TPD characteristic study and the catalyst activity assay of the W/HZSM-5-based catalysts, we confirmed that the intensity and concentration of the surface B-acid sites have pronounced effects on the catalyst performance for dehydroaromatization of methane (DHAM). It was found experimentally that, by addition of a proper amount of Mg2+, the strong B-acid sites at the catalyst surface could be effectively eliminated, whereas the addition of a proper amount of Zn2+ or Li+ resulted not only in eliminating most of the strong surface B-acid sites but also in generating a kind of new medium-strong acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for dehydro-aromatization of methane; on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. By simultaneous addition of Mg2+ and Zn2+, optimized adjustment in surface acidity of the catalyst could be realized. On the other hand, the doping of the Zn2+ or Li+ component to the tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W component at the surface of the catalyst, thus leading to a pronounced decrease in the reduction temperature for the hard-to-be-reduced W6+ species and an increase in quantity of the reducible W6+ species at the reaction temperature for DHAM, as has been evidenced by the results of a H-2-TPR study on the reducibility of the Zn2+ (or La3+, Li+, Mn2+)-promoted W/HZSM-5 system. The above two roles that Zn2+ and Li+ as promoters played both contributed to the persistence of high methane conversion and benzene selectivity, and the alleviation of coke deposition, as well as the prolongation of the catalyst lifetime.