(651f) Investigating the Role of Molybdenum during the Direct Conversion of Methane to Liquids Under Non-Oxidative Conditions

Methane dehydroaromatization (MDA) over Mo/HZSM-5 is still being researched due to the fact that the mechanism is still not well-deciphered. In an attempt to understand the contribution of molybdenum in the MDA process, this research has studied the catalytic conversion of methane over MoO₃/SiO₂ catalyst. Silica was chosen as the inert support so that the role of molybdenum could be better understood. A recirculating batch reactor was used for that purpose. Catalysts of different metal loadings were prepared by incipient wetness impregnation and sol gel method. The prepared catalysts were characterized by Brunauer-Emmett-Teller (BET) surface area, inductively coupled plasma-mass spectroscopy (ICP-MS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). XRD analysis showed that at low Mo loadings, molybdenum was uniformly dispersed on the silica support but at higher Mo loadings crystalline α-MoO₃ phase was present. Sintering and agglomeration of metal take place at higher loadings which is evident from SEM. The catalytic activity of both catalysts were studied at two different temperatures (973 K and 1023 K). It was found that with increase in temperature the conversion of methane increases. Selectivity of ethylene increases with increase in reaction time and temperature. Benzene was the major product formed with trace amounts of ethylene and ethane. A maximum of 1.2% conversion of methane was achieved at 1023 K using 2 wt % Mo/SiO₂ prepared by sol gel method. The role of Mo and a mechanism of the reaction was proposed. The XRD analysis of used catalysts revealed the presence of β-Mo₂C phase, obtained during the interaction of molybdenum with methane. These molybdenum carbide sites act as active sites for C-H bond activation and ethylene formation. The ethylene undergoes subsequent oligomerization to form aromatics like benzene. It is interesting to note that benzene was formed even in absence of acidic zeolite sites. Thus, acidic zeolite sites are not the primary sites for methane aromatization. It can be postulated that the free H⁺ radicals formed on C-H bond activation would help in ethylene oligomerization on active metal sites.