Dehydrogenation and aromatization of methane in the absence of oxygen on Mo/HZSM-5 catalysts before and after NH_4OH extraction

摘要

Mo/HZSM-5 catalysts show good catalytic reactivity in the absence of oxygen for the dehydrogenation and aromatization of methane at 973 K. The active Mo species were investigated by combining catalytic studies on Mo/HZSM-5 catalysts before and after NH_4OH extraction with XRD, BET, NH_3-TPD and TPR analysis. The XRD patterns show that Mo species are well dispersed on the zeolite surface. The specific surface areas decrease with increasing Mo loading but they can be restored to a large extent by NH_4OH extraction. NH_3-TPD results suggest that the Mo species prefer to deposit on the strong acid sites of HZSM-5 zeolite. TPR profiles show that there is a kind of Mo species which is easily reduced. No TPR peaks could be obviously observed if the Mo/ HZSM-5 catalysts were extracted by NH_4OH solution. The results of NH_4OH extraction experiment and other relevant characterization studies suggest that there are several kinds of Mo species deposited on the surface. By referring to the Mo species on Al_2O_3 supported MoO_3 samples, we propose that the dissolvable Mo species in NH_4OH solution are MoO_3 crystallites and their aggregates in octahedral coordination, while the unsollable Mo species mainly are Al_2(MoO_4)_3 and MoO_4~(2-) in tetrahedrally coordinated form. The catalytic performance of Mo/HZSM-5 catalysts before and after NH_4OH extraction illustrates that Mo species in small MoO_3 crystallites with octahedral coordination form are active for methane activation in the absence of oxygen on Mo/HZSM-5 catalysts, while Mo species in tetrahedrally coordinated form is less active for the reaction.