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.