Deciphering the Mechanistic Role of Individual Oxide Phases and Their Combinations in Supported Mn–Na2WO4 Catalysts for Oxidative Coupling of Methane

Oxidative Coupling of Methane: A Review Study on the Catalytic Performance

Extensive data on the characteristics and performance of the catalysts synthesized and tested for methane oxidative coupling (OCM) is available in thousands of reports published during the last four decades. Revisiting and analyzing the general trends recognizable in those data could improve the current understanding of the catalyst functionality under different reaction conditions. This is instrumental in determining the direction of future research aiming for more efficient OCM catalysts and reactors. These are the subjects of the comprehensive analysis reported in this paper, which covers the main aspects associated with the analysis of the OCM catalytic performance, including the catalyst characteristics, reaction mechanism, and reactor operation. Special attention was devoted to analyzing these aspects in the framework of thermal-reaction engineering and, accordingly, critically reviewing the reported catalytic performances in the literature.

Selective Catalytic Combustion of Hydrogen under Aerobic Conditions on Na2WO4/SiO2

Na2WO4/SiO2, a material known to catalyze alkane selective oxidation including the oxidative coupling of methane (OCM), is demonstrated to catalyze selective hydrogen combustion (SHC) with >97 % selectivity in mixtures with several hydrocarbons (CH4, C2H6, C2H4, C3H6, C6H6) in the presence of gas-phase dioxygen at 883-983 K. Hydrogen combustion rates exhibit a near-first-order dependence on H2 partial pressure and are zero-order in H2O and O2 partial pressures. Mechanistic studies at 923 K using isotopically-labeled reagents demonstrate the kinetic relevance of H-H dissociation and absence of O-atom recombination. In situ X-ray diffraction (XRD) and W LIII-edge X-ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na2WO4 crystallinity and lack of second-shell coordination with respect to W6+ cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase at temperatures below the bulk melting point of Na2WO4 (971 K) during SHC catalysis. The Na2WO4/SiO2 SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox-active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes.