Enhanced catalytic performance of MnxOy–Na2WO4/SiO2 for the oxidative coupling of methane using an ordered mesoporous silica support

Catalysts informatics: paradigm shift towards data-driven catalyst design

Designing catalysts is a challenging matter as catalysts are involved with various factors that impact synthesis, catalysts, reactor and reaction. In order to overcome these difficulties, catalysts informatics is proposed as an alternative way to design and understand catalysts. The underlying concept of catalysts informatics is to design the catalysts from trends and patterns found in catalysts data. Here, three key concepts are introduced: experimental catalysts database, knowledge extraction from catalyst data via data science, and a catalysts informatics platform. Methane oxidation is chosen as a prototype reaction for demonstrating various aspects of catalysts informatics. This work summarizes how catalysts informatics plays a role in catalyst design. The work covers big data generation via high throughput experiments, machine learning, catalysts network method, catalyst design from small data, catalysts informatics platform, and the future of catalysts informatics via ontology. Thus, the proposed catalysts informatics would help innovate how catalysts can be designed and understood.

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.

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.

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.

Effect of thermal treatment on the stability of Na-Mn-W/SiO2 catalyst for the oxidative coupling of methane

In this study, we investigate the effect of thermal treatment/calcination on the stability and activity of a Na-Mn-W/SiO₂ catalyst for the oxidative coupling of methane. The catalyst performance and characterisation measurements suggest that the W species are directly involved in the catalyst active site responsible for CH₄ conversion. Under operating conditions, the active components, present in the form of a Na-W-O-Mn molten state, are highly mobile and volatile. By varying the parameters of the calcination protocol, it was shown that these molten components can be partially stabilised, resulting in a catalyst with lower activity (due to loss of surface area) but higher stability even for long duration OCM reaction experiments.

Multidimensional Classification of Catalysts in Oxidative Coupling of Methane through Machine Learning and High-Throughput Data

Understanding the unique features of catalysts is a complex matter as it requires quantitative analysis with a relatively large selection of catalyst data. Here, unique features of each catalyst within the oxidative methane of coupling (OCM) reaction are investigated by combining data science and high throughput experimental data. Visualization of high-throughput OCM data reveals that there are several groups of catalysts based on their response against experimental conditions. Unsupervised machine learning, in particular, the Gaussian mixture model, classifies the OCM catalysts into six groups based on similarity in catalytic activities. Data visualization and parallel coordinates unveil the unique catalytic features of each classified group. Each classified group is statistically analyzed where unique features of each group are defined in term of C₂ selectivity, CH₄ conversion, and their composition in each calssified group. Thus, systematic design of catalysts can be achieved in principle on the basis of the unique features of catalysts uncovered via data science.

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%.

Pd nanoparticles confined in mesoporous N-doped carbon silica supports: a synergistic effect between catalyst and support

Palladium nanoparticles of similar size were deposited on different supports, layers of carbon materials (with and without nitrogen doping) on the surface of a MCF (mesocellular foam) silica.

MnTiO3-driven low-temperature oxidative coupling of methane over TiO2-doped Mn2O3-Na2WO4/SiO2 catalyst

Oxidative coupling of methane (OCM) is a promising method for the direct conversion of methane to ethene and ethane (C₂ products). Among the catalysts reported previously, Mn₂O₃-Na₂WO₄/SiO₂ showed the highest conversion and selectivity, but only at 800° to 900°C, which represents a substantial challenge for commercialization. We report a TiO₂-doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst by using Ti-MWW zeolite as TiO₂ dopant as well as SiO₂ support, enabling OCM with 26% conversion and 76% C₂-C₃ selectivity at 720°C because of MnTiO₃ formation. MnTiO₃ triggers the low-temperature Mn²⁺↔Mn³⁺ cycle for O₂ activation while working synergistically with Na₂WO₄ to selectively convert methane to C₂-C₃. We also prepared a practical Mn₂O₃-TiO₂-Na₂WO₄/SiO₂ catalyst in a ball mill. This catalyst can be transformed in situ into MnTiO₃-Na₂WO₄/SiO₂, yielding 22% conversion and 62% selectivity at 650°C. Our results will stimulate attempts to understand more fully the chemistry of MnTiO₃-governed low-temperature activity, which might lead to commercial exploitation of a low-temperature OCM process.

Methane conversion into different hydrocarbons or oxygenates: current status and future perspectives in catalyst development and reactor operation

This Perspective highlights recent developments in methane conversion into different hydrocarbons and C₁-oxygenates. Our analysis identified possible directions for further research to bring the above approaches to a commercial level.