The Selective Oxidation of Methane to C2-hydrocarbons over Li+-Doped TiO2: Catalytic and Mechanistic Studies

Abstract

This chapter presents the performance of Li+-doped TiO2 catalysts and mechanistic conclusions derived from various transient experiments related to the reactivity of C2–hydrocarbons with the lattice oxygen of the catalyst. The influence of dopants on the catalytic performance of TiO2 and other rare-earth oxides for the oxidative coupling of methane (OCM) has recently gained significant interest among many researchers. Doping of a metal oxide with a metal cation of valence lower or higher than that of the parent oxide results in changes in the electronic structure of the crystal (Fermi energy level), in acidity and basicity characteristics, and in lattice oxygen ion mobility. These parameters have been found to influence catalytic activity and selectivity under OCM conditions, and, in certain catalyst formulations, correlations between the aforementioned physicochemical and catalytic parameters has been determined. Although a large number of papers related to the mechanism of the OCM reaction have been published, many questions still remain concerning the actual mechanistic carbon and oxygen pathways over a given catalytic system. In particular, the exact role of lattice versus adsorbed oxygen species towards the formation of the undesired COx products, and the relative contribution of gas-phase and surface reaction steps towards methane coupling are of critical concern. Steady-state tracing techniques would certainly be of valuable importance in clarifying many mechanistic aspects of this complicated reaction system. However, these techniques have only found a limited application thus far. © 1994, Elsevier Science & Technology. All rights reserved.