Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Study on the Synergistic Effect of SiO2 and H2O on Oxidative Coupling of Methane over Mn-Na2WO4/SiO2 Catalyst

Mn-Na2WO4-based catalysts with different supports were prepared using the incipient wetness impregnation method and evaluated for their oxidative coupling of methane (OCM) reaction performance. The results demonstrated that the SiO2- supported catalyst exhibited the best catalytic performance, and the introduction of H2O further enhanced its activity. Under the conditions of a feed gas mixture of CH4/O2/H2O = 6:1:24 at 800 °C and atmospheric pressure, the CH4 conversion and C2+ selectivity over the Mn-Na2WO4/SiO2 catalyst increased from 28.4% and 77.4% (without H2O) to 33.2% and 84.9%, respectively. In contrast, the catalytic activity using TiO2 and MgO supports drastically declined. Characterizations using X-ray diffraction (XRD), in situ infrared spectroscopy (In-situ IR), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), and oxygen temperature-programmed desorption (O2-TPD) revealed that the introduction of steam not only served as a diluent to decrease the partial pressures of the reactants CH4 and O2, thereby reducing deep oxidation reactions in the gas phase, but also interacted with surface oxygen species (O2 2-) and silica to form adjacent surface-bonded disilanol free radicals Si-OIH. This interaction facilitated the dehydrogenation and coupling of two methane molecules on the catalyst surface, resulting in the generation of C2+ products, significantly enhancing the catalytic activity and selectivity of OCM.

Degradation mechanisms of organic compounds in molten hydroxide salts: a radical reaction yielding H2 and graphite

Molten salts are used in various waste treatments, such as recycling, recovery or making inert. Here, we present a study of the degradation mechanisms of organic compounds in molten hydroxide salts. Molten salt oxidation (MSO) using carbonates, hydroxides and chlorides is known for the treatment of hazardous waste, organic material or metal recovery. This process is described as an oxidation reaction due to the consumption of O₂ and formation of H₂O and CO₂. We have treated various organic products, carboxylic acids, polyethylene and neoprene with molten hydroxides at 400 °C. However, the reaction products obtained in these salts, especially carbon graphite and H₂ without CO₂ emission, challenges the previous mechanisms described for the MSO process. Combining several analyses of the solid residues and the gas produced during the reaction of organic compounds in molten hydroxides (NaOH-KOH), we demonstrate that these mechanisms are radical-based instead of oxidative. We also show that the obtained end products are highly recoverable graphite and H₂, which opens a new way of recycling plastic residues.

Analysis of measured high-resolution doublet rovibronic spectra and related line lists of 12CH and 16OH

Detailed understanding of the energy-level structure of the quantum states as well as of the rovibronic spectra of the ethylidyne (CH) and the hydroxyl (OH) radicals is mandatory for a multitude of modelling efforts within multiple chemical, combustion, astrophysical, and atmospheric environments. Accurate empirical rovibronic energy levels, with associated uncertainties, are reported for the low-lying doublet electronic states of 12CH and 16OH, using the Measured Active Rotational-Vibrational Energy Levels (MARVEL) algorithm. For 12CH, a total of 1521 empirical energy levels are determined in the primary spectroscopic network (SN) of the radical, corresponding to the following seven electronic states: X 2Π, A 2Δ, B 2Σ-, C2 Σ+, D 2Π, E 2Σ+, and F 2Σ+. The energy levels are derived from 6348 experimentally measured and validated transitions, collected from 29 sources. For 16OH, the lowest four doublet electronic states, X 2Π, A 2Σ+, B 2Σ+, and C 2Σ+, are considered, and a careful analysis and validation of 15 938 rovibronic transitions, collected from 45 sources, results in 1624 empirical rovibronic energy levels. The large set of spectroscopic data presented should facilitate the refinement of line lists for the 12CH and 16OH radicals. For both molecules hyperfine-resolved experimental transitions have also been considered, forming SNs independent from the primary SNs.

Alkali metal halide-coated perovskite redox catalysts for anaerobic oxidative dehydrogenation of n-butane

Oxidative dehydrogenation (ODH) of n-butane has the potential to efficiently produce butadiene without equilibrium limitation or coke formation. Despite extensive research efforts, single-pass butadiene yields are limited to <23% in conventional catalytic ODH with gaseous O₂. This article reports molten LiBr as an effective promoter to modify a redox-active perovskite oxide, i.e., La_0.8Sr_0.2FeO₃ (LSF), for chemical looping-oxidative dehydrogenation of n-butane (CL-ODHB). Under the working state, the redox catalyst is composed of a molten LiBr layer covering the solid LSF substrate. Characterizations and ab initio molecular dynamics (AIMD) simulations indicate that peroxide species formed on LSF react with molten LiBr to form active atomic Br, which act as reaction intermediates for C─H bond activation. Meanwhile, molten LiBr layer inhibits unselective CO₂ formation, leading to 42.5% butadiene yield. The redox catalyst design strategy can be extended to CL-ODH of other light alkanes such as iso-butane conversion to iso-butylene, providing a generalized approach for olefin production.

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.

Activation and catalytic transformation of methane under mild conditions

In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH₄) under mild conditions for both chemical synthesis with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH₄ to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH₄ to value-added intermediate compounds including but not limited to CH₃OH, HCHO, HCOOH, and CH₃COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH₄ under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH₄ to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH₄ or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH₄ to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH₄ under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.

Noncatalytic Oxidative Coupling of Methane (OCM): Gas-Phase Reactions in a Jet Stirred Reactor (JSR)

Oxidative coupling of methane (OCM) is a promising technique for converting methane to higher hydrocarbons in a single reactor. Catalytic OCM is known to proceed via both gas-phase and surface chemical reactions. It is essential to first implement an accurate gas-phase model and then to further develop comprehensive homogeneous-heterogeneous OCM reaction networks. In this work, OCM gas-phase kinetics using a jet-stirred reactor are studied in the absence of a catalyst and simulated using a 0-D reactor model. Experiments were conducted in OCM-relevant operating conditions under various temperatures, residence times, and inlet CH₄/O₂ ratios. Simulations of different gas-phase models related to methane oxidation were implemented and compared against the experimental data. Quantities of interest (QoI) and rate of production analyses on hydrocarbon products were also performed to evaluate the models. The gas-phase models taken from catalytic reaction networks could not adequately describe the experimental gas-phase performances. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics; it is recommended for further use as the gas-phase model for constructing homogeneous-heterogeneous reaction networks.

Elucidating the effects of individual components in KxMnOy/SiO2 and water on selectivity enhancement in the oxidative coupling of methane

To date, a great number of various materials have been tested for the oxidative coupling of methane (OCM).

Status and prospects of the decentralised valorisation of natural gas into energy and energy carriers

We critically review the recent advances in process, reactor, and catalyst design that enable process miniaturisation for decentralised natural gas upgrading into electricity, liquefied natural gas, fuels and chemicals.

Fluctuating Storage of the Active Phase in a Mn-Na2 WO4 /SiO2 Catalyst for the Oxidative Coupling of Methane

Structural dynamics of a Mn-Na2 WO4 /SiO2 catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na2 WO4 with Mn7 SiO12 . The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.

Pyrolysis of mixtures of methane and ethane: activation of methane with the aid of radicals generated from ethane

Inert CH₄ molecules can be activated and incorporated into pyrolysis products with the aid of radicals generated by pyrolysis of C₂H₆.

Transition of surface phase of cobalt oxide during CO oxidation

Transition of surface phase of cobalt oxide nanoparticle catalyst during CO oxidation in 60–200 °C tracked through AP-XPS.

Integrated In Situ Characterization of a Molten Salt Catalyst Surface: Evidence of Sodium Peroxide and Hydroxyl Radical Formation

Sodium-based catalysts (such as Na₂ WO₄ ) were proposed to selectively catalyze OH radical formation from H₂ O and O₂ at high temperatures. This reaction may proceed on molten salt state surfaces owing to the lower melting point of the used Na salts compared to the reaction temperature. This study provides direct evidence of the molten salt state of Na₂ WO₄ , which can form OH radicals, using in situ techniques including X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), laser induced fluorescence (LIF) spectrometry, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). As a result, Na₂ O₂ species, which were hypothesized to be responsible for the formation of OH radicals, have been identified on the outer surfaces at temperatures of ≥800 °C, and these species are useful for various gas-phase hydrocarbon reactions, including the selective transformation of methane to ethane.