1. Introduction
The industrial conversion of methane (CH4) to methanol (CH3OH) typically follows an indirect route, involving the initial step of CH4 steam reforming to generate syngas (CO and H2) at elevated temperature (above 800 °C). Subsequently, the synthesis of CH3OH takes place at high pressure (ca. 100 atm) using a Cu-Zn-Al catalyst. Although widely applied on a large scale, this commercial method is unsuitable for small-scale production due to its demanding reaction conditions, intricate operational processes, energy-intensive requirements and high equipment costs.1 Consequently, there is a growing interest in the direct oxidation of methane to methanol (DOMtM) under mild conditions, offering significant potential for implementation at distributed and small-scale plants.2 For over a century, researchers have explored DOMtM through both homogeneous and heterogeneous catalysis. Homogeneous catalysis typically involves the use of fuming sulfuric acid3 or trifluoroacetic acid4 as reaction solvents. Complex catalysts featuring Pt, Pd, Au or Hg noble metals as active centers have been employed. In the realm of heterogeneous catalysis, various catalytic materials, including metals5 and metal oxides6 have been intensely investigated.
Recently, inspired by the binuclear iron and copper active sites observed in natural methane monooxygenase (MMO), researchers have explored iron- and copper-based zeolite catalysts for DOMtM with high selectivity.7 Iron-based zeolites exhibit proficient N2O decomposition (N2O + (Fe2+)α → (Fe3+-O-)α + N2) at temperatures below 300 °C, with the α-O species identified as the active component for DOMtM.8 Copper-based zeolites show notable catalytic efficacy in DOMtM, especially when O2 or H2O is employed as oxidants, positioning Cu-MOR as a highly promising catalytic material for DOMtM.9 To overcome the high energy barrier (Ea) of CH4 oxidation and to inhibit excessive oxidation of CH3OH, a multi-step chemical looping approach has been proposed. This method involves catalyst pre-activation of the catalyst with O2 at high temperatures, followed by a low-temperature reaction with CH4 to generate adsorbed CH3OH species. Subsequently, extraction through either solvent or steam leads to the production of CH3OH. The primary objective of this approach is to safeguard the CH3OH formed on the catalyst surface from excessive oxidation, thereby achieving a superior CH3OH selectivity exceeding 90%. However, the intricate multi-step process involves frequent switches in feeding gases and adjustments in temperature, introduces discontinuities in CH3OH production, diminishes overall reaction efficiency, and entails substantial energy consumption.
Non-thermal plasma (NTP) stands out as a powerful method for activating and converting CH4 to CH3OH. Energetic electrons within the NTP effectively activate CH4 and O2 molecules, generating reactive radical species (CHx and O species).10,11Additionally, the low gas temperature in NTP plays a crucial thermodynamic role in CH3OH production, as excessively high temperatures can lead to CH3OH decomposition or its reforming with water vapor, producing CO and CO2. A dielectric barrier discharge (DBD) is one of the most common methods to produce NTP and is widely employed in plasma-assisted DOMtM. Nozaki demonstrated the conversion of CH4 into oxygenates in a microplasma reactor with a single-pass yield of 5-20% and a selectivity of 70-30%.12 Furthermore, when employing a Cu/ZnO/Al2O3 (CZA) catalyst for DOMtM, oxidized Cu species exhibited higher CH3OH selectivity compared to copper metal (Cu0) species, suggesting that Cu+ or Cu2+ may serve as active components in plasma-assisted DOMtM.13Subsequently, Chawdhury et al. found that a CuO/γ-Al2O3 catalyst enhanced the CH3OH selectivity, with high Cu loading facilitating formaldehyde (HCHO) generation.14 Recently, Li et al. reported a strategy to overcome the trade-off relationship between CH4 conversion and CH3OH selectivity through co-feeding H2O vapor with CH4 and O2 over a Pt2/BN-na catalyst.9
In summary, Cu-based zeolite catalysts exhibit notable CH3OH selectivity in thermal catalytic DOMtM, while NTP facilitates DOMtM with impressive CH4 conversion at low temperatures. Consequently, the synergistic utilization of NTP and Cu-based zeolite catalysts emerges as a promising strategy for DOMtM.