Optimum Yield of the Purely Heterogeneous Oxidative Dimerization of Methane

Abstract

The optimal C_2-yield per pass for the purely heterogeneous catalytic oxidation of methane by O_2 on Sm_2O_3 under continuous cofeed operation: η_(opt) ≤ 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 CH_4 and O_2 flow rates, catalyst mass, residence time, and temperature variations on the objective function η = methane conversion × C_2-selectivity for the scheme: CH_4 (+O_2) ⇒ CH_3(→) 1/2 C_2H_6⇒ 1/2 C_2H_4⇒ CO. The fact that the derived η_(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 solely 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%.