The influence of molybdenum loading on activity of ZSM-5 zeolite in dehydroaromatization of methane

Abstract

Mo/ZSM-5 type catalysts prepared by different methods of loading the same quantity of molybdenum were tested by methane dehydroaromatization. The catalyst samples were impregnated with ammonium molybdate in different ways: by standard wet impregnation method (WIM), by wet impregnation combined by the treatment in rotavapour (RV), by wet impregnation assisted by ultrasound (US) or by microwave (MW) irradiations. The fifth catalyst was prepared by mechanical mixing of zeolite with ammonium molybdate (MM). Prepared catalysts were characterized by X-ray diffraction (XRD), Fourier-Transform Infra-Red Spectroscopy (FTIR), Temperature Programmed Desorption of Ammonia (TPDA), Scanning Electron Microscopy (SEM), Field-Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and by physical adsorption of nitrogen. The strength of the acid sites was compared in the n-hexane cracking. It has been revealed that the sample impregnated with assistance of microwaves had the highest activity in methane conversion. For this sample the finest dispersion of molybdenum species was observed in TEM-pictures, and other characteristics showed the highest intrusion of molybdenum into zeolite pores and their association with Brønsted acid sites. The lowest activity showed the catalyst prepared by mechanical mixing (MM), followed by the catalyst prepared by simple wet impregnation method (WIM). Samples RV and US had higher activities as sample WIM but lower than MW.