Elucidating the effects of individual components in KxMnOy/SiO2 and water on selectivity enhancement in the oxidative coupling of methane

Abstract

To date, a great number of various materials have been tested for the oxidative coupling of methane (OCM). On the basis of previous statistical analysis of OCM-related literature data, we developed novel catalysts based on K₂MnO₄ or KMnO₄ supported on SiO₂ showing selectivity to C₂-hydrocarbons (C₂H₄ and C₂H₆) up to 68 or 66%. The analysis of catalysts containing the single components (Mn/SiO₂, K₂O/SiO₂ or SiO₂) demonstrates that the co-existence of K and Mn is essential for achieving high selectivity to the desired products. The selectivity can be additionally improved when the OCM reaction is performed in the presence of water. The positive effect of water is attributed to the suppressed direct oxidation of CH₄ to CO₂. Water also enhances the rates of methane conversion into C₂-hydrocarbons and CO. Besides, the effects of water are more pronounced for samples based on crystalline SiO₂ rather than amorphous SiO₂. Transient experiments with isotopically labelled oxygen suggest that the accelerating effect should be of heterogenous nature. As the positive influence of water on the selective methane oxidation to C₂-hydrocarbons has been established over different catalysts, it may be advantageous to consider it when developing novel materials for the OCM reaction and even for other oxidation reactions.