Synthesis & characterization of catalyst for methane conversion into higher hydrocarbons under non-oxidative conditions
Sourabh Mishra (Prof. K. K. Pant and Prof. M. A. Haider)
13th September 2017, Wednesday, 04:00‐04:30 pm (25+5 mins), ChE Seminar hall
Abundant availability of natural gas together with the improved technologies of its production makes it significant to find out efficient routes to convert methane (major content of natural gas) to value added chemicals. Direct non-oxidative methane conversion into higher hydrocarbons is a potential approach to exploit the natural gas. Zeolite supported molybdenum (Mo) is the well-known catalyst for this process with HZSM-5 and HMCM-22 as zeolite support. Porosity, framework structure and acidity of the said zeolites plays a crucial role in the catalytic performance. Also metal (Mo) loading to these zeolite supports tunes the acidity of the support and its dispersion which significantly affect the reaction. Thus the catalyst preparation and its characterization is the key factor for the process to achieve effective conversion with high product yield. In this regard, different factors, such as effect of metal (Mo) loading over zeolite support (HMCM-22), effect of SAR (SiO2/Al2O3) of the support varying the acidity of the catalyst, also reaction parameters such as space velocity (GHSV) have been studied for the direct non-oxidative methane conversion into higher hydrocarbons. Promoter (Cr & Ga) effect over the base catalyst (Mo/MCM-22) has also been tested for the catalytic performance.
Selection of molybdenum (Mo) as the base metal for the process was investigated by density functional theory. Molybdenum carbide clusters (MoxCy) formed during the initial carburization process of the reaction are suggested to be the active sites for C-H bond activation and C-C coupling to form ethylene which is subsequently aromatize at the neighbouring Brønsted acid sites of the zeolite. Theoretical calculations, utilising density functional theory simulations were applied to gain insights into the mechanism of C-H bond activation and C-C coupling reactions to form ethylene on the Mo4C2 cluster, which is proposed to be the most stable carbide cluster*. The methane dehydrogenation barrier was calculated to be 116 kJ/mol and for C-C coupling the activation energy was relatively higher (Ea=151kJ/mol). On increasing the cluster residual charge to +1, and thereby on less reduced carbide clusters the activation barriers for C-H activation (Ea=142kJ/mol) was significantly increased, and for C-C coupling significantly reduced (Ea=63kJ/mol). The theoretical results, therefore; suggested for a rational design and optimization of cluster charge, to obtain better catalytic activity, which is likely to be the case of improved activity for metal promoted Mo/MCM-22 catalyst as observed in the experiments.
*J. Gao, Y. Zheng, J. Jehng, Y. Tang, I. E. Wachs and S. G. Podkolzin, Science, 2015, 348, 686-690