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 at a 33% methane conversion over 2.0 g of 15 wt% Na CaO catalyst at 725 °C with a gas mixture of CH4 O2 = 2. The other products were CO, CO2, and H2. EPR results indicated that [Na+O-] centers in Na CaO are responsible for the catalytic production of CH3· from methane via 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 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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry