Catalytic oxidative dimerization of methane

Abstract

The results of this research indicate that [M⁺O⁻] centers (M = Group IA ions) in Group IA/Group IIA oxides are responsible for the catalytic production of CH₃[superscript dot] radicals from CH₄ through hydrogen atom abstraction. These CH₃[superscript dot] radicals dimerize, primarily in the gas phase, to form C₂H₆, which further oxidizes to C₂H₄. The [M⁺O⁻] centers were formed by substituting cations of the promoters for those of the host metal oxides in an oxygen atmosphere at elevated temperatures. A higher concentration of such sites can be obtained if both cations have similar ionic radii, as in Li/MgO and Na/CaO. Most of these [M⁺O⁻] centers, however, are located in the sub-surface region and are not accessible for gases during catalysis. This observation is supported by the fact that the EPR spectrum of ([M⁺O⁻] was not broadened by O₂. Therefore, it is necessary to propose that over Group IA/IIA oxides containing [M⁺O⁻] there is a dynamic equilibrium in which hole transport via O²⁻ prevails between [M⁺O⁻ and oxide ions near the surface. Since the formation of the products is believed to proceed mainly in the gas phase, it is expected that the reactions are strongly influenced by the gas phase equilibrium CH₃[superscript dot] + O₂ = CH₃[superscript dot]O₂. Increasing the temperature yields more CH₃[superscript dot] radicals, which result in a higher C₂ selectivity. The ultimate fate of CH₃[superscript dot]O₂ is to CO and CO₂ under our reaction conditions, therefore increasing the O₂ pressure favors CH₃[superscript dot]O₂ and decreases the C₂ (C₂H₆ + C₂H₄) selectivity. However, there is evidence that CH₃[superscript dot]O₂, when in the presence of C₂H₆, can initiate a chain reaction to enhance the catalytic activity. The CH₃[superscript dot] also reacts with surface O²⁻ mainly to produce CO and CO₂ through a CH₃O⁻ intermediate. Thus, a high C₂ selectivity is usually obtained with catalysts of low surface area. The reduction of the surface area can be achieved either by promoting the catalysts with alkali metal ions or by heating the pure oxide at high temperatures in the presence of H₂O. Under optimum reaction conditions, promoted La₂O₃ is as active and selective as Group IA/IIA oxides in the formation of C₂ compounds. Moreover, pure La₂O₃ exhibits good C₂ selectivity (>70%), but only with high CH₄/O₂ ratios which prevent the further oxidation of C₂ compounds. Pure and promoted La₂O₃ are similar to Group IA/IIA oxides in that both types of catalysts are basic oxides, they have low surface areas, and they produce considerable amounts of CH₃[superscript dot] radicals during catalysis. However, there is no observable O⁻ species in the promoted and unpromoted lanthanum oxides.