Methane dehydroaromatization-A study on hydrogen use for catalyst reduction,role of molybdenum,the nature of catalyst support and significance of Bronsted acid sites

摘要

Methane dehydroaromatization was studied over Mo/SiO2 and Mo/HZSM-5 with different Mo loadings(2, 5, 10 wt%) at 973 K and 1023 K in a recirculating batch reactor. H2 pretreatment at 1023 K prior to methane activation has significantly improved the catalyst activity with increase in Mo loading and reduced the induction time on benzene formation in both Mo/SiO2 and Mo/HZSM-5. 10 wt% Mo/HZSM-5 gave a maximum methane conversion of 19% and ~67% benzene selectivity at 1023 K. The XRD analysis of used catalysts revealed that the Mo O3 species were converted to β-Mo2C phase. Studies on Mo/SiO2 catalysts showed that benzene was formed even in the absence of acidic zeolite sites. Reactions of ethylene in the presence of pure silica, HZSM-5 and in a blank reactor revealed that conversion of ethylene to aromatics was similar in case of the blank reactor and silica. Thus, it is believed that molybdenum carbide sites act as active sites only for C–H bond activation of methane and ethylene formation. Even though, ethylene can undergo subsequent oligomerization without any catalytic aid to form benzene at973 K and above addition of acidic zeolites improved the selectivity of benzene.