Oxidative dimerization of methane over sodium-promoted calcium oxide

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% C 2 (sum of ethane and ethylene) selectivity was achieved at a 33% methane conversion over 2.0 g of 15 wt% Na CaO catalyst at 725 °C with a gas mixture of CH 4 O 2 = 2 . The other products were CO, CO 2, and H 2. EPR results indicated that [Na +O −] centers in Na CaO are responsible for the catalytic production of CH 3· from methane via hydrogen atom abstraction. These CH 3· radicals dimerize, primarily in the gas phase, to form C 2H 6, which further oxidizes to C 2H 4. Increasing temperatures reverse the gas-phase equilibrium CH 3· + O 2 ⇄ CH 3O 2· to produce more CH 3· and increase the C 2 selectivity. The CH 3O 2· eventually is converted to carbon oxides under the reaction conditions employed; therefore, increasing O 2 pressures decrease the C 2 selectivity. There is evidence that CH 3O 2· in the presence of C 2H 6 initiates a chain reaction that 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.