Resolving the Types and Origin of Active Oxygen Species Present in Supported Mn-Na2WO4/SiO2 Catalysts for Oxidative Coupling of Methane

Photodriven Methane Conversion on Transition Metal Oxide Catalyst: Recent Progress and Prospects

Methane as the main component in natural gas is a promising chemical raw material for synthesizing value-added chemicals, but its harsh chemical conversion process often causes severe energy and environment concerns. Photocatalysis provides an attractive path to active and convert methane into various products under mild conditions with clean and sustainable solar energy, although many challenges remain at present. In this review, recent advances in photocatalytic methane conversion are systematically summarized. As the basis of methane conversion, the activation of methane is first elucidated from the structural basis and activation path of methane molecules. The study is committed to categorizing and elucidating the research progress and the laws of the intricate methane conversion reactions according to the target products, including photocatalytic methane partial oxidation, reforming, coupling, combustion, and functionalization. Advanced photocatalytic reactor designs are also designed to enrich the options and reliability of photocatalytic methane conversion performance evaluation. The challenges and prospects of photocatalytic methane conversion are also discussed, which in turn offers guidelines for methane-conversion-related photocatalyst exploration, reaction mechanism investigation, and advanced photoreactor design.

Low-Temperature Light-off MnOx-Na2WO4-Based Catalysts: A Step Forward to OCM Process Industrialization

Oxidative coupling of methane (OCM) catalyzed by MnOx-Na2WO4-based catalysts has great industrial potential to convert CH4 directly to C2-3 products, but the high light-off temperature is a big challenge to OCM commercialization. The reaction mechanism studies disclosed that O2/CH4-activation relevant "Mn2+↔Mn3+" redox cycle is tightly linked with the catalyst light-off. One concept is thus put forward that the OCM light-off temperature could be lowered once a "Mn2+↔Mn3+" redox cycle was established to be triggered at low temperature over MnOx-Na2WO4-based catalysts. The relevant studies in recent years are reviewed, showing that the establishment of low-temperature light-off "Mn2+↔Mn3+" redox cycle over the MnOx-Na2WO4-based catalysts indeed works effectively toward a low-temperature light-off OCM process. Moreover, three perspectives for the OCM industrialization are discussed based on this concept, including monolithic catalyst, fluidized-bed method and chemical-looping process.

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.

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO4 catalytic active sites resulting in surface WO5 sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn2O5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH4 than the poisoned surface WO5 sites or the original WO4 sites. However, the surface reaction intermediates formed from CH4 activation over the Mn2O5 surface sites are more stable than those formed over the Na2WO4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na2WO4 surface sites towards CH4 activation but can yield MnO x surface sites that activate CH4 faster than Na2WO4 surface sites, but unselectively.

New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2 WO4 /SiO2 Catalysts

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na₂ WO₄ /SiO₂ catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H₂ -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na₂ WO₄ active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na-WO_x sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that (i) surface Na-WO_x sites are responsible for selectively activating CH₄ to C₂ H_x and over-oxidizing CH_y to CO and (ii) molten Na₂ WO₄ phase is mainly responsible for over-oxidation of CH₄ to CO₂ and also assists in oxidative dehydrogenation of C₂ H₆ to C₂ H₄ . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na₂ WO₄ /SiO₂ catalysts.