Oxygen Activation and Pathways in High-Temperature Reactions of Light Alkane Oxidation: A Seeming Simplicity of Kinetic Description

The kinetic regularities of the catalytic oxidation processes that proceed via a stepwise redox mechanism are considered. The main assumptions of the Mars–van-Krevelen model widely used to describe such processes are analyzed, and their correspondence to actual catalytic systems—typical catalysts for the partial oxidation of light alkanes (oxidative coupling of methane, oxidative dehydrogenation of C2+ alkanes, oxidative cracking of C3+ alkanes)—is examined. Certain features of the composition and operating conditions of these catalysts require some revision of existing notions about the activation of molecular oxygen, including factors that determine its interaction with active sites and participation in the catalytic cycle. Particular attention is paid to: the thermodynamic and kinetic conditions for the realization of the redox mechanism (thermochemistry of oxide systems, the lifetime of oxygen in a bound state); suitability of the redox-type mechanisms in the case of catalysts that do not contain cations with a variable oxidation state; chemical and phase transformations, which are part of the catalytic redox cycle; interpretation of the results of kinetic experiments in the study of heterogeneous-homogeneous processes and the influence of free-radical reactions on the values of experimentally determined kinetic parameters.