Oxidative Dimerization of Methane Over Sodium-Promoted Calcium Oxides

Abstract

Sodium-promoted calcium oxides are active and selective catalysts for the partial oxidation of methane to ethane and ethylene using molecular oxygen as an oxidant. In a conventional fixed-bed flow reactor, operating at atmospheric pressure, a 45% C2 (sum of ethane and ethylene) selectivity was achieved to a 33% methane conversion over 2.0 g of the catalyst at 725°C with a gas mixture of CH4/O2 = 2. The other products were CO and CO2. EPR results indicate that [Na+O-] centers in Na/CaO are responsible for the catalytic production of CH3 from methane through hydrogen-atom abstraction. These CH3 radicals dimerize, primarily in the gas-phase, to form C2H6 which further oxidizes to C2H4. Increasing temperatures reverse the gas-phase equilibrium CH3+O2 ⇌ CH3O2 to produce more CH3 and increase the C2 selectivity. The CH3O2 eventually is converted to carbon oxides under the reaction conditions employed, therefore increasing O2 pressures decrease the C2 selectivity. There is evidence that CH3O2 in the presence of C2H6 initiates a chain reaction which enhances the methane conversion. The addition of Na to CaO also reduces the surface area of the catalysts, thus minimizing a nonselective oxidation pathway via surface methoxide intermediates.