Optimum yield of the purely heterogeneous oxidative dimerization of methane

The optimal C-2-yield per pass for the purely heterogeneous catalytic oxidation of methane by O-2 on Sm2O3 under continuous cofeed operation: eta(opt) less than or equal to 0.22 +/- 0.03, is computed from actual kinetic data for all the pseudoelementary chemical reactions involved. The optimization dispenses with adjustable kinetic parameters, and explores the combined effects of CH4 and O-2 flow rates, catalyst mass, residence time, and temperature variations on the objective function eta=methane conversion x C-2-selectivity for the scheme: CH4 (+O-2) double right arrow CH3 (-->) 1/2C(2)H(6) double right arrow 1/2C(2)H(4) double right arrow CO. The fact that the derived eta(opt) value essentially coincides with the largest C-2-yields reported on a myriad catalysts under vastly different conditions: (1) proves that the ceiling to selectivity is sorely determined by the competition among heterogeneous processes and (2) implies a substrate-independent selectivity pattern for the species formed in O-2 dissociative chemisorption. We briefly analyze the physical basis of these propositions and of the prospects of improving C-2-yields beyond 25%. (C) 1997 Academic Press.