(573h) Co-Aromatization of Methane with Olefins

The recent discoveries of large reserves of natural gas (i.e., shale gas) in North America have motivated the development of viable methods to convert this cheap energy source into higher value products. The activation and conversion of methane has been a great challenge due to its symmetric molecular structure and high C-H bond energy. The aromatization of methane has drawn increasing attention, while its industrial application is limited by the high reaction temperature. It is reported that the co-fed higher hydrocarbons would assist the conversion of methane at low temperatures (400~600 ^oC). In the present work, methane, the main component of natural gas, is activated with novel zeolites supported catalysts loaded with active metals, and converted to aromatics when olefin formed from heavy oil cracked distillates is present and at 400 ^oC. Several representative model compounds including ethylene, propylene, 1-hexene, 1-octene, 1-decene and styrene are also employed to study the co-aromatization mechanism. The participation of methane in the aromatization is proven by the increased carbon number of product molecules, as well as the isotope labelling studies. The study of the reaction mechanism is executed from multiple aspects with several analytic instruments, including the GC-MS analysis of products, which provides the composition information, and Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy, which traces the evolution of surface reaction intermediates. The co-aromatization reaction undergoes a series of steps, involving catalytic cleavage of C-H bonds in methane molecules and addition of the formed CH_x and H moieties into the carbenium ion intermediates derived from olefin feedstock. ¹³C and ²D enriched methane are also employed to track the catalytic pathways. The NMR spectra of the isotope enriched products obtained from ¹³CH₄ and CD₄ runs reveal how the CH_x and H species are converted to aromatics. DRIFT spectra of the catalyst, which has absorbed higher hydrocarbon molecules (i.e., olefin model compounds), acquired under ¹³CH₄ and CD₄ environment also disclose the evolution of methane on the catalyst surface. This work clearly demonstrates the feasibility of the conversion of methane into more valuable chemicals under mild temperature. This mechanistic study would also benefit the rational design of catalyst for direct upgrading of heavy crude oil using natural gas as a promising further work.