Oxidative coupling of methane on Li/CeO2 based catalysts: Investigation of the effect of Mg- and La-doping of the CeO2 support

Abstract

The work presented herein reports on the oxidative coupling of methane (OCM) performance of a series of Li-free and Li-doped CeO2 and CeO2 modified with Mg2+ and La3+ catalysts. The supporting materials (Ce, Mg-Ce and La-Mg-Ce metal oxides) were synthesized using the microwave assisted sol-gel method, while lithium ions were added using the wet impregnation technique, to further affect the physicochemical properties, activity and selectivity of the materials, in terms of the desirable hydrocarbon products (C2H4 and C2H6). The materials were characterized towards their textural, structural and redox properties, surface basicity, and surface morphology using N2 adsorption/desorption, X-Ray Diffraction (XRD), Raman spectroscopy, CO2-Temperature Programmed Desorption (CO2-TPD), H2-Temperature Programmed Reduction (H2-TPR), Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS). Catalytic activity was assessed between 600 and 870 °C, at atmospheric pressure and different CH4:O2 molar ratios and Weigh-basis Gas Hourly Space Velocity (WGHSV). It is concluded that low specific surface area values, the existence of surface moderate basic sites, increased concentration of oxygen vacancies and the presence of electrophilic oxygen species (O2– and O22–) on the catalyst surface had a crucial role on the improvement of the catalytic performance in terms of the desirable products, mainly ethylene. It is noted that the addition of Li changed thoroughly the reaction pathway and the products’ distribution, with the C2 selectivity values exceeding 85%. The Li/Mg-Ce catalyst, presenting the higher population of intermediate basic sites and surface superoxide species, showed an improved catalytic activity in terms of XCH4 and production of ethylene, while the incorporation of La3+ into the crystal structure of CeO2 suppressed the production of ethylene. Finally, smaller CH4:O2 molar ratios suppressed the production of hydrocarbons, while enhanced residence times, favoured the dehydrogenation of C2H6 to C2H4.