Methane Dehydro-aromatization under Nonoxidative Conditions over Mo/HZSM-5 Catalysts:EPR Study of the Mo Species on/in the HZSM-5 Zeolite

摘要

EPR characterization of the Mo species on/in the HZSM-5 zeolite is described. Four different EPR signals, denoted as signals A,B,D,and E, respectively, have been recorded on the Mo/HZSM-5 sample during its stepwise reduction by methane. Different Mo species were identified on the basis of their reducibility and the nature of the signal concerned. There are two kinds of Mo species on the Mo/HZSM-5 sample, which are located at different positions on/in the　 HZSM-5 zeolite. The first kind of the Mo species is polynuclear and located on the external surfadce. They are either in the octahedral-coordinated MoO_3 crystallite form (denoted as MoO_3oct) or in the MoO_x form with a square-pyramidal coordination (denoted as MoO_xsqu).The second kind of the Mo species is associated with the Al atom in the lattice channels of the zeolite. The Mo species associated with Al are mononuclear species, which migrate or diffuse into the channels of the　 HZSM-5 during calcination. The EPR signals of migrating Mo ion (signals D and E)have hyperfine structures which are caused by the interaction between the Mo species and the lattice Al atom. And the corresponding Mo species are located at two different positions close to the Bronsted Al atoms: Al(I)...MoO_x and Al(II)...MoO_x. The order of reducibility of these four kinds of species is as follows:MoO_3octapprox=MoO_xsqu>Al(I)...MoO_x>Al(II)...MoO_x.It was found that the O_2~- species might exist on the surface of the Mo/HZSM-5 catalyst during the induction period of the reaction.The variation of the different Mo species in a 6% Mo/HZSM-5 sample during the reaction course is illustrated, and the relationship between the catalytic performance and the intensity of the ESR spectra is discussed. It is proposed that Mo_2C is located at the external surface, while partial reduced Mo species associated with the Al atom is inside the channel during the reaction. Both of these Mo species play a key role in methane dehydro-aromatization.