ChemInform Abstract: Oxidative Dimerization of Methane Over Sodium‐Promoted Calcium Oxide.

Abstract

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 2 H 6 , which further oxidizes to C 2 H 4 . Increasing temperatures reverse the gas-phase equilibrium CH 3 · + O 2 ⇄ CH 3 O 2 · to produce more CH 3 · and increase the C 2 selectivity. The CH 3 O 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 3 O 2 · in the presence of C 2 H 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.