Highly active and selective Li/MgO catalysts for methane transformation to C2 hydrocarbons: experimental and DFT study

Abstract

The development of energy-efficient and economically attractive ways of utilization of methane for the production of chemicals/liquid fuels remains one of the long-standing challenges in catalysis. Li/MgO catalysts synthesized by a successive deposition–sublimation method show high activity and stability during the oxidative conversion of methane to olefins. A surfactant-assisted solvothermal method was used to synthesize the MgO support and then Li was deposited onto the support MgO by applying a successive deposition–sublimation method, where the initially deposited Li was sublimated and stabilized at 900 °C and this process was done for two consecutive times to prepare the final Li-doped MgO catalyst. For comparison purposes, several Li loaded catalysts were also prepared by the deposition method only. Different analytical techniques were used to characterise the prepared catalysts in order to understand and relate their physico-chemical properties to their catalytic activity. The catalyst prepared by the deposition–sublimation method (3.6Li/MgO^Dep–Sub) showed high methane conversion and C₂ selectivity compared to the catalysts prepared by only deposition method. The 3.6Li/MgO^Dep–Sub catalyst showed ∼38% CH₄ conversion with 55% ethylene selectivity and 23% ethane selectivity with a total C₂ selectivity of 78% at 700 °C, and the catalyst was highly stable without losing its activity even after 24 h time on stream. It was found that the catalyst prepared by the deposition method deactivates rapidly with time and the activity is also low. DFT studies showed that Li doping to MgO has a significant promotional effect on the methane C–H activation. The C–H activation barrier was reduced by nearly 85 kcal mol⁻¹ over the Li-doped MgO surface, Li₂–MgO(100), compared to the undoped MgO(100) surface.