Co-aromatization of olefin and methane over Ag-Ga/ZSM-5 catalyst at low temperature

Abstract

The massive exploitation of shale gas in the past decade has boosted the production of natural gas and reduced its price dramatically. The methane activation and following conversion into more valuable fuels and chemicals have thus become more and more attractive, while the introduction of hydrocarbons to enhance the methane activation at mild conditions represents a promising approach. In the present work, the co-aromatization of methane with propylene has been studied at 400°C. The presence of methane would increase the toluene to benzene ratio as well as the average carbon number of the formed liquid aromatic products compared to its propylene alone counterpart. Among the gas products, the formations of C3H8, C4H8 and C4H10 also get promoted when methane is present. The incorporation of methane into the product molecules is also directly evidenced by the 1H, 2D and 13C NMR spectroscopy of the liquid products obtained from the reaction between propylene (or styrene) and isotope labelled methane. Hydrogen from methane would contribute a large portion of the hydrogen in the product molecules, while the benzylic and aromatic hydrogen sites are favored compared with those on the alkyl side chains. The activation of methane is also observed in the DRIFT spectra when deuterium enriched methane is engaged as the methane source and evidenced by the escalated exothermic feature when olefin aromatization takes place under methane environment. The excellent catalytic performance of Ag-Ga/ZSM-5 might be because of the better dispersion of Ag and Ga on the ZSM-5 surface and moderate amount of strong Brösted and Lewis surface acid sites. All the observations suggest that methane might be activated nonoxidatively and converted into aromatics if suitable catalyst is charged under the assistance of co-existing olefin. The reported synergetic effect could potentially lead to the more economic utilization of abundant natural gas and petrochemical intermediates.