Identification of Mo active species for methane dehydro-aromatization over Mo/HZSM-5 catalysts in the absence of oxygen:: 1H MAS NMR and EPR investigations

The effect of pre-reduction with H-2 at a low temperature, i.e. 623 K, on the structure of the Mo/HZSM-5 catalyst prepared by impregnation has been studied and characterized by means of H-1 MAS NMR and EPR techniques. It has been revealed that there are three kinds of Mo species on the Mo/HZSM-5 catalyst, namely, the Mo-6c(6+) oxide species with distorted octahedral coordination, the Mo-5c(6+) oxide species with square pyramidal coordination, and the Mo6+ oxide species associated with the Bronsted acid sites. The first two Mo oxide species exist mainly on the external surface of the HZSM-5 zeolite, and can be readily reduced to form the beta-Mo2C species, while the last one resides primarily in the channels, and is difficult to be fully reduced to form Mo carbide species. The Mo oxide species associated with the Bronsted acid sites can be partially reduced to the MoOxCy species during the induction period, which play a crucial role in the methane dehydro-aromatization (MDA) reaction. The MoOxCy species with face centered cubic (fcc) structure are more active and more selective towards benzene, yet more stable than those Mo carbide species with hexagonally close packed (hcp) structure. It is likely that pre-reduction with H-2 at 623 K can enhance the topotactic transformation of the Mo specie from the hcp structure into the fcc structure, thus can improve greatly the catalytic activity and selectivity towards mono-aromatics, as well as the stability of the Mo/HZSM-5 catalyst. (c) 2005 Elsevier B.V. All rights reserved.