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.