Effect of reaction conditions on the oxidative coupling of methane over doped MnOx-Na2WO4/SiO2 catalyst

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

Light olefins are important feedstocks and platform molecules for the chemical industry. Their synthesis has been a research priority in both academia and industry. There are many different approaches to the synthesis of these compounds, which differ by the choice of raw materials, catalysts and reaction conditions. The goals of this review are to highlight the most recent trends in light olefin synthesis and to perform a comparative analysis of different synthetic routes using several quantitative characteristics: selectivity, productivity, severity of operating conditions, stability, technological maturity and sustainability. Traditionally, on an industrial scale, the cracking of oil fractions has been used to produce light olefins. Methanol-to-olefins, alkane direct or oxidative dehydrogenation technologies have great potential in the short term and have already reached scientific and technological maturities. Major progress should be made in the field of methanol-mediated CO and CO₂ direct hydrogenation to light olefins. The electrocatalytic reduction of CO₂ to light olefins is a very attractive process in the long run due to the low reaction temperature and possible use of sustainable electricity. The application of modern concepts such as electricity-driven process intensification, looping, CO₂ management and nanoscale catalyst design should lead in the near future to more environmentally friendly, energy efficient and selective large-scale technologies for light olefin synthesis.

Oxidative Coupling of Methane: Examining the Inactivity of the MnOx -Na2 WO4 /SiO2 Catalyst at Low Temperature

Oxidative coupling of methane (OCM) catalyzed by MnO_x -Na₂ WO₄ /SiO₂ has great industrial promise to convert methane directly to C_2-3 products, but its high light-off temperature is the most challenging obstacle to commercialization and its working mechanism is still a mystery. We report the discovery of a low-temperature active and selective MnO_x -Na₂ WO₄ /SiO₂ catalyst enriched with Q² units in the SiO₂ carrier, being capable of converting 23 % CH₄ with 72 % C_2-3 selectivity at 660 °C. From experiments and theoretical calculations, a large number of Q² units in the MnO_x -Na₂ WO₄ /SiO₂ catalyst is a trigger for markedly lowering the light-off temperature of the Mn³⁺ ↔Mn²⁺ redox cycle involved in the OCM reaction because of the easy formation of MnSiO₃ . Notably, the MnSiO₃ formation proceeds merely through the SiO₂ -involved reaction in the presence of Na₂ WO₄ : Mn₇ SiO₁₂ +6 SiO₂ ↔7 MnSiO₃ +1.5 O₂ . The Na₂ WO₄ not only drives the light-off of this cycle but also gets it working with substantial selectivity toward C_2-3 products. Our findings shine a light on the rational design of more advanced MnO_x -Na₂ WO₄ based OCM catalysts through establishing new Mn³⁺ ↔Mn²⁺ redox cycles with lowered light-off temperature.

Oxidative coupling of methane-comparisons of MnTiO3-Na2WO4 and MnOx-TiO2-Na2WO4 catalysts on different silica supports

The oxidative coupling of methane (OCM) converts CH₄ to value-added chemicals (C₂₊), such as olefins and paraffin. For a series of MnTiO₃-Na₂WO₄ (MnTiO₃-NW) and MnO_x-TiO₂-Na₂WO₄ (Mn-Ti-NW), the effect of loading of MnTiO₃ or MnO_x-TiO₂, respectively, on two different supports (sol-gel SiO₂ (SG) and commercial fumed SiO₂ (CS)) was examined. The catalyst with the highest C₂₊ yield (21.6% with 60.8% C₂₊ selectivity and 35.6% CH₄ conversion) was 10 wt% MnTiO₃-NW/SG with an olefins/paraffin ratio of 2.2. The catalyst surfaces with low oxygen-binding energies were associated with high CH₄ conversion. Stability tests conducted for over 24 h revealed that SG-supported catalysts were more durable than those on CS because the active phase (especially Na₂WO₄) was more stable in SG than in CS. With the use of SG, the activity of MnTiO₃-NW was not substantially different from that of Mn-Ti-NW, especially at high metal loading.

Oxidative Coupling of Methane for Ethylene Production: Reviewing Kinetic Modelling Approaches, Thermodynamics and Catalysts

Ethylene production via oxidative coupling of methane (OCM) represents an interesting route for natural gas upscaling, being the focus of intensive research worldwide. Here, OCM developments are analysed in terms of kinetic mechanisms and respective applications in chemical reactor models, discussing current challenges and directions for further developments. Furthermore, some thermodynamic aspects of the OCM reactions are also revised, providing achievable olefins yields in a wide range of operational reaction conditions. Finally, OCM catalysts are reviewed in terms of respective catalytic performances and thermal stability, providing an executive summary for future studies on OCM economic feasibility.

Na2WO4/Mn/SiO2 Catalyst Pellets for Upgrading H2S-Containing Biogas via the Oxidative Coupling of Methane

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.

Oxidative Coupling of Methane: Perspective for High-Value C2 Chemicals

The oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H4 and C2H6) has aroused worldwide interest over the past decade due to the rise of vast new shale gas resources. However, obtaining higher C2 selectivity can be very challenging in a typical OCM process in the presence of easily oxidized products such as C2H4 and C2H6. Regarding this, different types of catalysts have been studied to achieve desirable C2 yields. In this review, we briefly presented three typical types of catalysts such as alkali/alkaline earth metal doped/supported on metal oxide catalysts (mainly for Li doped/supported catalysts), modified transition metal oxide catalysts, and pyrochlore catalysts for OCM and highlighted the features that play key roles in the OCM reactions such as active oxygen species, the mobility of the lattice oxygen and surface alkalinity of the catalysts. In particular, we focused on the pyrochlore (A2B2O7) materials because of their promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding for OCM reaction.

SiO2@MnO x @Na2WO4@SiO2 core-shell-derived catalyst for oxidative coupling of methane

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared and their fabrication was confirmed using transmission electron microscopy. The formation of Mn-based nanosheets on the silica spheres is important for the deposition of nanoscopic Na₂WO₄. The SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were used for the oxidative coupling of methane at a temperature of 700–800 °C at which the nanostructures were completely destroyed. Although the core–shell structures did not survive the high-temperature oxidative coupling of methane, the selective production of olefins and paraffins can be attributed to highly dispersed Na₂WO₄ derived from confined core–shell structures.

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared for the oxidative coupling of methane.

Study on the unsteady state oxidative coupling of methane: effects of oxygen species from O2, surface lattice oxygen, and CO2 on the C2+ selectivity

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na₂WO₄/Mn/SiO₂. The reaction conditions, including the methane-to-oxygen ratio, ratio of feed gas dilution by N₂, quantity of catalyst, and feed flow rate were adjusted for the continuous flow fixed bed reaction system. While the O₂ gas initiated methyl radical formation from methane, the surface lattice oxygen atoms improved the dehydrogenation of paraffins to olefins without significant activation of methane. The addition of CO₂ as a mild oxidizing agent was also tested and slightly improved OCM selectivity with slightly lower methane conversion were observed.

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na₂WO₄/Mn/SiO₂.

Synthesis of Value-Added Chemicals via Oxidative Coupling of Methanes over Na2WO4-TiO2-MnO x /SiO2 Catalysts with Alkali or Alkali Earth Oxide Additives

Na₂WO₄-TiO₂-MnO _x /SiO₂ (SM) catalysts with alkali (Li, K, Rb, Cs) or alkali earth (Mg, Ca, Sr, Ba) oxide additives, which were prepared using incipient wetness impregnation, were investigated for oxidative coupling of methane (OCM) to value-added hydrocarbons (C₂₊). A screening test that was performed on the catalysts revealed that SM with Sr (SM-Sr) had the highest yield of C₂₊. X-ray photoelectron spectroscopy analyses indicated that the catalysts with a relatively low binding energy of W 4f_7/2 facilitated a high CH₄ conversion. A combination of crystalline MnTiO₃, Mn₂O₃, α-cristobalite, Na₂WO₄, and TiO₂ phases was identified as an essential component for a remarkable improvement in the activity of the catalysts in the OCM reaction. In attempts to optimize the C₂₊ yield, 0.25 wt % Sr onto SM-Sr achieved the highest C₂₊ yield at 22.9% with a 62.5% C₂₊ selectivity and a 36.6% CH₄ conversion. A stability test of the optimal catalyst showed that after 24 h of testing, its activity decreased by 18.7%.

Catalyst screening for the oxidative coupling of methane: from isothermal to adiabatic operation via microkinetic simulations

OCM catalysts underperforming in typical isothermal conditions could result in above average performances in adiabatically-relevant operating conditions.