Methane dehydrogenation and aromatization in the absence of oxygen on Mo/HZSM-5: A study on the interaction between Mo species and HZSM-5 by using Al-27 and Si-29 MAS NMR

By using a high-resolution solid state nuclear magnetic resonance spectrometer with Al-27 and Si-29 probes, the interaction between Mo species and HZSM-5 of Mo/HZSM-5 catalysts has been studied. The results show that there is a strong interaction between Mo species and HZSM-5 zeolite. The framework aluminum in the zeolite can be easily extracted by the introduction of Mo species. The extractability of framework aluminum by Mo species increases with increasing Mo loading and the calcination temperature, The extraction process leads to the formation of non-framework Al at first and then a new crystalline phase of Al-2(MoO4)(3). The dealumination of the catalyst having a Mo loading of 15% and had been calcined at 973 K is so severe that all the aluminum in the framework are extracted and no framework Al could be detected by Al-27 MAS NMR. The catalyst, therefore, lost its catalytic activity for methane dehydrogenation and aromatization in the absence of oxygen, The Si/Al ratio measured from Si-29 MAS NMR further confirms the dealumination process observed by Al-27 MAS NMR. The MAS NMR results give us an evidence that Al-2(MoO4)(3) crystallites are much less active for the reaction.