EFFECT OF CARBON-DIOXIDE ON THE ACTIVATION-ENERGY FOR METHYL RADICAL GENERATION OVER LI/MGO CATALYSTS

The apparent activation energies for both the generation of CH3. radicals and the overall consumption of CH4 during the oxidative coupling reaction are strongly influenced by the presence of CO2. The intrinsic activation energy for the formation of CH3. radicals over Li/MgO catalysts is 26 +/- 2 kcal/mol, a value that increases to 37 +/- 3 kcal/mol upon the addition of only 0.04 Torr of CO2 to the reagents. The activation energy approaches a limiting value of ca. 50 kcal/mol at a CO2 partial pressure of greater-than-or-equal-to 1 Torr. Thus, under typical catalytic reaction conditions for the oxidative coupling of methane (e.g., 190 Torr of CH4, 95 Torr of O2), sufficient CO2 would be produced to attain this limiting value of E(a). However, by operating at sufficiently low partial pressures and conversion levels, an E(a) of 32 kcal/mol was obtained during the oxidative coupling reaction. Temperature-programmed desorption of CO2 from Li/MgO produces a peak at 630-degrees-C, which is consistent with the role of CO2 as a poison for the catalytic reactions.