Methane dehydro-aromatization over Mo/HZSM-5 in the absence of oxygen: A multinuclear solid-state NMR study of the interaction between supported Mo species and HZSM-5 zeolite with different crystal sizes

The interaction between Mo species and a conventionally microsized and particularly nanosized HZSM-5 support was studied by high-resolution multinuclear solid-state NMR techniques. As proved by Al-27 and Si-29 MAS as well as CP/MAS NMR investigations, this interaction was so strong that the framework aluminum of both microsized and nanosized HZSM-5 zeolites could be extracted. With increasing Mo loading, more nonframework aluminum, resonanced at ca. 30 ppm, appeared in the Al-27 MAS NMR spectrum of the Mo-loaded nanosized HZSM-5 catalyst. Meanwhile, this strong interaction led to the formation of more new Al-2(MoO4)(3) crystallines on the nanosized HZSM-5 support than on the microsized HZSM-5 support. The appearance of Al-2(MoO4)(3) crystallines resulted in fewer active catalysts for the methane dehyro-aromatization. The results of H-1 MAS NMR using perfluorotributyl amine as a probe molecule demonstrated that Mo species preferentially reacted with the silanols and nonframework AlOH on the external surface of microsized and nanosized HZSM-5 zeolites. In addition, impregnated Mo species remained predominantly on the external surface of the nanosized HZSM-5 zeolite, although there was a possibility that they might migrate into the lattice channels of the microsized HZSM-5 zeolite. The migration of some Mo species into the zeolite channels might be beneficial for the conversion of methane to aromatics in the absence of oxygen. (C) 1999 Academic Press.