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Summary: Biogas, which contains methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), and other minor compounds, is a promising energy source and chemical feedstock (not least because it is generated from waste); however, its application as fuel is limited because of its low calorific value, which can be mitigated by upgrading biogas to high‐quality fuels via the oxidative coupling of methane (OCM), a catalytic process that increases its higher heating value from 39.9 (CH4, Wobbe index = 53.5 MJ/Nm3) to 41.7 MJ/Nm3 (OCM product mixture, Wobbe index = 54.4 MJ/Nm3), thus achieving compliance with the fuel standards in many countries (Wobbe index = 45.5‐55.0 MJ/Nm3). Among the compounds produced during OCM, H2S is formed in minor amounts but strongly suppresses biogas valorization, mainly because of potential catalyst poisoning. To understand the effects of sulfur compounds on OCM catalysts, three such catalysts, namely perovskites (SrTiO3 and LaAlO3) and Na2WO4/Mn/SiO2 (NWM), were used to convert biogas into olefins and paraffins. The oxidation of H2S in biogas produced sulfur dioxide, which did not strongly affect NWM but was strongly adsorbed by perovskites and, in turn, significantly reduced their OCM activity. Furthermore, long‐term (100 hours) stability testing demonstrated that the OCM activity of NWM was stable in the presence of H2S. Consequently, the influence of sulfur compounds on the catalysts during the OCM process was elucidated, including catalyst poisoning via sulfur species adsorption. The strategy to suppress the detrimental effects of sulfur poisoning was suggested, which can contribute to the development of feasible process for valorizing biogas to high quality fuels and chemicals. [ABSTRACT FROM AUTHOR]

• Perovskite-supported alkaline-earth metal oxide catalysts were prepared. • These catalysts were applied in the oxidative coupling of methane. • The selective production of C 2+ compounds was observed below 700 °C. • Low temperature reaction was achieved with the described oxide/support combination. • Barium oxide exhibited the highest C 2+ yield at the desired temperature. Perovskite-supported alkaline-earth metal oxide catalysts were prepared for application in the oxidative coupling of methane (OCM). The selective production of C 2+ compounds, including ethane, ethylene, acetylene, propane, and propylene, was observed at reaction temperatures < 700 °C. This lower reaction temperature was not achieved with pure perovskites but only with the combination of alkaline-earth metal oxides and perovskite supports. The formation of complex mixed oxides, such as Ba-Ca-Ti-O x , Ba-Sr-Ti-O x , and Ba 2 TiO 4 , was also observed for these catalysts. This contributed to the improved C 2+ yield at the lower reaction temperature. The strong basicity, which contributes to the improved OCM activity, was also observed for these catalysts. [ABSTRACT FROM AUTHOR]

• Descriptors for oxidative coupling of methane were investigated. • Density functional theory described the actual catalysis results. • Optimum methyl radical adsorption energy was found for good catalysts. • The roles of surface oxygen species were revealed by surface characterizations. Catalytic descriptors were studied to design optimum catalysts for the oxidative coupling of methane (OCM) by combining density functional theory (DFT) calculations and actual reaction experiments. SrTiO 3 perovskite catalysts, selected for OCM, were modified using metal dopants, and their electronic structures were calculated using the DFT method. The CH 3 adsorption energy E ads (CH 3) and the oxygen vacancy formation energy E f (vac) exhibited volcano-type correlations with the C 2+ selectivity and O 2 -consumption for the formation of CO x , respectively. The optimum catalytic activity, represented by the C 2+ selectivity, was obtained for E ads (CH 3) = −2.0 to −1.5 eV, indicating that overly strong adsorption of methyl radicals (or easily dissociated C H bonds of methane) and relatively insufficient oxygen supplementation to the catalyst surface improve deep oxidation to CO and CO 2. Praseodymium (Pr)- and neodymium (Nd)-doped SrTiO 3 catalysts confirm the DFT-predicted optimum electronic structure of the OCM catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Na-W-Mn-Ba-Ti composite catalyst exhibited 26.0 % C 2+ yield at 700 °C. • Optimum temperature, 700 °C, is 100 °C lower than that of conventional Mn-Na 2 WO 4. • Nanostructures of catalyst adjusted the redox of Mn and W. • H 2 -TPR, TEM, XRD, Raman, and others elucidated the catalytic activity. The oxidative coupling of methane (OCM) using hybrid catalysts containing BaTiO 3 perovskite and Mn-Na 2 WO 4 exhibited high activity and high selectivity at low temperature: 66.3 % C 2+ (olefins and paraffins) selectivity and 25.9 % C 2+ yield at 700 °C which is 100 °C lower than that used for Mn-Na 2 WO 4 catalysts (800 °C). Upon the preparation of complex catalysts, the insertion of Mn into BaTiO 3 (BaTi(Mn)O 3) and the formation of Na x Mn(Ti)O 2 improved the oxygen-supplying ability of the catalysts. Additionally, strong interactions between the WO 4 2− anions and BaTi(Mn)O 3 or Na x Mn(Ti)O 2 stabilized the WO 4 2− anions on the surface and improved the methane activation ability of the catalysts for the favorable production of hydrocarbons. The nanoscopic modification of catalysts was confirmed using H 2 –temperature-programmed reduction, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy results. [ABSTRACT FROM AUTHOR]

Biogas is a promising renewable energy source; however, it needs to be upgraded to increase its low calorific value. In this study, oxidative coupling of methane (OCM) was selected to convert it to a higher fuel standard. Prior to establishing the scaled-up OCM process, the effect of organic/inorganic binders on catalytic activity was examined. The selection of the binders and composition of the catalyst pellet influenced the pore structure, fracture strength, and catalytic activity of the catalyst pellets. It was also observed that the O2 supply from the inorganic binder is a key factor in determining catalytic activity, based on which the composition of the catalyst pellets was optimized. The higher heating value increased from 39.9 (CH4, Wobbe index = 53.5 MJ/Nm3) to 41.0 MJ/Nm3 (OCM product mixture, Wobbe index = 54.2 MJ/Nm3), achieving the fuel standard prescribed in many countries (Wobbe index = 45.5–55.0 MJ/Nm3). The reaction parameters (temperature, gas hourly space velocity, size of the reaction system, and the CH4/O2 ratio) were also optimized, followed by a sensitivity analysis. Furthermore, the catalyst was stable for a long-term (100 h) operation under the optimized conditions. [ABSTRACT FROM AUTHOR]

Abstract: The oxidative coupling of methane (OCM) to produce ethane and ethylene, valuable hydrocarbons, was scaled-up using differential, mid-scale, and bench-scale reactors with 0.030, 2.73, and 40g of packed catalysts, respectively. Organic and inorganic binders, such as methylcellulose, Al2O3, MgO, and TiO2, were mixed with Na2WO4/Mn/SiO2 catalyst powder to produce catalyst pellets, and TiO2-mixed catalyst pellets exhibited the highest C2 hydrocarbon yield despite the lowest active catalyst content. The formation of hot spots, which suppressed the selective conversion to C2 hydrocarbons by further oxidation to CO and CO2, was observed in the mid- and bench-scale reactors because of poor heat transfer. Inert-gas-diluted reactants and lower O2 flow may inhibit the formation of hot spots and allow the development of industry-feasible OCM processes. [Copyright &y& Elsevier]

[Display omitted] • SiO 2 -supported Na 2 WO 4 /Mn catalysts for oxidative coupling of methane are prepared. • Role of α-cristobalite formation in support is probed by assessing catalytic activity. • α-Cristobalite is shown to secure high activity by stabilizing WO 4 tetrahedrons. • Unprecedentedly high methane conversion and C 2+ selectivity are achieved. • Strategy for better catalyst design through zeolite support modification is provided. The oxidative coupling of methane (OCM) converts this abundant natural feedstock into value-added products and is typically performed in the presence of catalysts such as SiO 2 -supported Na 2 WO 4 /Mn to suppress the undesired deep oxidation affording CO and CO 2. Currently, the phase transformation of SiO 2 supports to α-cristobalite is known to be important for securing high OCM activity, although the underlying reasons of this influence remain debatable. Herein, Na 2 WO 4 /Mn catalysts supported on several SiO 2 -based materials including conventional amorphous SiO 2 and crystalline zeolites were prepared to close the above knowledge gap and thus promote the design of more efficient OCM catalysts. The best support was identified as calcined D -ITQ-1, which is a well-developed delaminated zeolite with a thin basal all-silica nanosheet structure facilitating the transition to the α-cristobalite phase. The corresponding catalyst retained high activity over 100 h and exhibited one of the highest yields (25.8%) of C 2+ hydrocarbons (paraffins and olefins) reported for dopant-free Na 2 WO 4 /Mn/SiO 2 catalysts to date. The results of XRD, Raman, and 29Si NMR indicated that this high performance can be ascribed to the stabilization of tetrahedral WO 4 units due to the transition from uncalcined silica to α-cristobalite. Thus, the present work advances our understanding of structural phase transitions and the concomitant formation of the desired active species in Na 2 WO 4 /Mn/SiO 2 catalysts and paves the way to the design of better catalysts through the modification of zeolite supports. [ABSTRACT FROM AUTHOR]

Abstract: The catalytic oxidative coupling of methane (OCM) on transition metal-doped TiO2 nanowire catalysts was performed and the effects of metal dopants were studied. With transition metal doping, the electric and optical properties of nanowires were adjusted, which seemed to improve the catalytic activity and selectivity of the OCM reaction. A Mn-doped TiO2 nanowire catalyst exhibited the highest C2 yield with the highest (ethylene)/(ethane) ratio because of its moderate oxidation activity, while a highly active Rh-doped TiO2 nanowire catalyst converted methane into fully oxidized CO and CO2. The electric conductivity assessed by UV–vis absorption represented the oxidation activity of the nanowire catalysts. [Copyright &y& Elsevier]

Abstract: A kinetic model of the oxidative coupling of methane was developed by considering ethane, ethylene, carbon monoxide, and carbon dioxide production in such a way that the reactor was designed for the reactions to take place in the catalyst-loading region as much as possible. The effectiveness of the reactor specification was validated by low conversion of methane in the blank test. A mechanism for catalytic reactions on the surface of Na2WO4/Mn/SiO2 gel and gas-phase radical reactions was suggested based on a literature survey. Reaction rates were developed by applying rapid equilibrium for adsorption and the quasi steady state approximation to the intermediate on the catalytic surface. Kinetic parameters were estimated by fitting experimental data with temperature, space velocity, and CH4/O2 ratio varied in full factorial manner. The validity of the model was corroborated by comparing its simulated results with experimental data. The effectiveness of the estimated parameters was discussed with respect to reported values. The effects of operating conditions were assessed using the developed model. [Copyright &y& Elsevier]