Under the current energy and environmental challenges, the efficient utilization of methane has become a focus of research. In the previous article, we explored in detail the basic principles of photocatalytic methane conversion.
Building on this, this article will delve into the specific processes of photocatalytic methane conversion, including partial oxidation of methane to produce high-value-added chemicals, reforming reactions to generate syngas, coupling reactions to form more complex hydrocarbon compounds, as well as achieving selective combustion and functionalization under mild conditions. These conversion pathways not only demonstrate the diversity and flexibility of photocatalytic technology but also offer multiple possibilities for the efficient utilization of methane.
Depending on the different reaction products, methane conversion reactions can be divided into partial oxidation of methane, methane reforming, methane coupling, and methane combustion.[1] Among them, partial oxidation of methane is a promising route for producing valuable oxides such as methanol, formaldehyde, and formic acid.[2-4] Methanol is considered an ideal methane conversion product because it is a transportable liquid that can be used as both fuel and a basic chemical feedstock.[5-7] Traditional methane conversion to methanol follows an energy-intensive multi-step route, where methane undergoes steam reforming at high temperatures to produce syngas (CO and H₂). Syngas is then used to synthesize methanol under high pressure.[8-10] A single-step conversion of methane to methanol is an alternative route with lower energy consumption.[11,12] However, methanol is prone to over-oxidation, leading to the production of other byproducts. Therefore, increasing the selectivity for methanol in the partial oxidation of methane is crucial.[13]
With the involvement of oxidants such as O₂, H₂O₂, and N₂O, methane tends to undergo thermodynamically favored oxidation. Among these oxidants, O₂ is the most commonly chosen for photocatalytic partial oxidation of methane to methanol due to its cost-effectiveness and availability (Equation 1).
2CH₄ + O₂ → 2CH₃OH, ΔG⁰₂₉₈ₖ = −223 kJ·mol⁻¹ (1)
Methane reforming for hydrogen production mainly includes two processes: Steam Reforming of Methane (SRM) and Dry Reforming of Methane (DRM), among which SRM is one of the most important methods for large-scale hydrogen production in today’s chemical industry (Equation 2).[14,15] SRM is an endothermic reaction between methane and steam conducted at high temperatures (750-950°C) and high pressures (14-20 atm). This process often results in the production of large amounts of carbon dioxide, a byproduct of the water-gas shift reaction (CO + H₂O → CO₂ + H₂).[16,17] DRM allows for the co-conversion of two greenhouse gases, CH₄ and CO₂ (Equation 3), to produce syngas, which serves as a raw material for producing methanol, low-carbon olefins, and other useful chemicals (Figure 1).[18-20] Precious metal catalysts (e.g., Pt, Pd, and Ru) and non-precious metal catalysts (primarily Ni) exhibit excellent performance in methane reforming.[21-23] However, carbon deposition can occur on the catalyst surface through two processes: pyrolysis (CH₄ → 2H₂ + C) and the Boudouard reaction (2CO → CO₂ + C), both of which lead to catalyst deactivation.[24-27] Notably, below 700°C, the Boudouard reaction becomes thermodynamically favorable, while pyrolysis is more favorable at higher temperatures.[28-30]
CH₄ + H₂O → CO + 3H₂, ΔG⁰₂₉₈ₖ = 142.1 kJ·mol⁻¹ (2)
CH₄ + CO₂ → 2CO + 2H₂, ΔG⁰₂₉₈ₖ = 171 kJ·mol⁻¹ (3)
The high energy consumption of methane reforming leads to the re-release of greenhouse gases, which contradicts the goal of sustainable development. Photocatalytic methane reforming under mild conditions is an anticipated alternative route that utilizes renewable clean light energy to drive the reaction.[31] Currently, much of the research on photocatalytic methane reforming focuses on developing advanced photocatalysts with improved efficiency for hydrogen or syngas production.
While steam methane reforming is a mature industrial production method, it still relies on high temperatures (800-1000°C) to promote the endothermic methane reforming process. This not only results in high energy consumption but also shortens the catalyst’s lifespan. In the field of catalyst research, pursuing advanced catalysts with higher reactivity and stability remains a primary goal in steam methane reforming research. These advancements have the potential to lower reaction temperatures and reduce catalyst costs. On the other hand, traditional steam methane reforming relies on thermal catalysis, requiring large fossil fuel consumption and releasing significant amounts of greenhouse gases into the atmosphere. Transitioning to steam methane reforming technology driven by clean and sustainable solar energy has the potential to catalyze an industrial revolution within the existing industrial framework.
Figure 1. a) Photocatalytic SRM reaction on Rh/TiO₂ under visible light irradiation. b) Relationship between H₂ production rate and reaction temperature on Rh/TiO₂ photocatalyst under light or dark conditions.[32] c) Diagram of single H₂ separation and sequential separation of H₂ and CO₂. d) H₂/CO₂ yield and ratio over 6000 SRM reaction cycles with an average CH₄ conversion rate of 95.41%.[33]
Methane coupling with C₂₊ hydrocarbons (mainly ethane and ethylene) provides an alternative route to directly convert methane into useful chemicals[34,35]. The process can be divided into two categories depending on the presence of an oxidant (Equations 4~7): Methane Oxidative Coupling (OCM) and Methane Non-Oxidative Coupling (NOCM):
2CH₄ → C₂H₆ + H₂, ΔG⁰₂₉₈ₖ= 68.8 kJ·mol⁻¹ (4)
2CH₄ → C₂H₄ + 2H₂, ΔG⁰₂₉₈ₖ= 169.6 kJ·mol⁻¹ (5)
4CH₄ + O₂ → 2C₂H₆ + 2H₂O, ΔG⁰₂₉₈ₖ= −320 kJ·mol⁻¹ (6)
2CH₄ + O₂ → C₂H₄ + 2H₂O, ΔG⁰₂₉₈ₖ= −288 kJ·mol⁻¹ (7)
Due to thermodynamic limitations, NOCM often requires high reaction temperatures to achieve acceptable methane conversion efficiencies (Figure 2). However, operating at such high temperatures often leads to carbon deposition on the catalyst surface, resulting in catalyst deactivation and shortened lifespan. This severely hinders the practical industrial application of the catalytic system.[36,37] With the introduction of oxidants (mainly O₂), the OCM process can couple methane into light olefins under favorable thermodynamic conditions. However, the presence of oxygen in the reaction system inevitably leads to the generation of by-products such as CO and CO₂.[38-40]
It is generally believed that thermal catalytic OCM follows a heterogeneous-homogeneous catalytic reaction mechanism, where the activation of methane to methyl radicals occurs on the catalyst surface, followed by the coupling of methyl radicals as a spontaneous homogeneous reaction in the gas phase. This means that the control of selectivity for OCM may be limited by catalyst engineering itself.[41-43] Adjusting the interaction between methyl species and catalysts via photocatalytic pathways could be one of the important methods to enhance product selectivity.
Figure 2. a) Photocatalytic OCM activity on ZnO/TiO₂ with different metal co-catalysts. b) Proposed reaction process on ZnO with Au co-catalyst.[44] c) Photocatalytic OCM activity on ZnO and different metal/ZnO samples. d) Proposed scheme of the photocatalytic OCM mechanism on metal-loaded ZnO. e) Linear relationship between calculated d-* centers and selectivity for C₂₊ products on metal/ZnO photocatalysts.[45]
Methane Combustion
Due to the high hydrogen/carbon ratio of methane, it is a clean fuel alternative to coal and oil, with lower carbon dioxide emissions. However, residual methane in the exhaust that does not react fully can lead to unpleasant greenhouse effects, with an intensity approximately 20 times that of carbon dioxide.[46,47]
To address this issue, the Methane Combustion Reaction (Equation 8) represents the complete oxidation process of methane and has become a research focus for removing residual methane.[48] In the direct combustion process, once the ignition temperature is reached, the entire reaction follows a free radical chain reaction mechanism, with the reaction temperature rapidly soaring to 1200℃.[49] However, higher temperatures increase the likelihood of incomplete combustion of methane, leading to emissions of carbon monoxide, soot particles, and other hydrocarbon by-products.
During combustion, nitrogen in the air can be oxidized into toxic nitrogen oxides. Compared to direct combustion, catalytic combustion can better control the reaction process and allows for lower reaction temperatures (typically below 1000℃), thus reducing emissions of carbon pollutants and nitrogen oxides (Figure 6).[50,51] Given that thermal catalytic methane combustion still requires relatively high temperatures (>400℃), photocatalytic methane combustion, as a branch of environmental photocatalysis, has emerged as a promising alternative operating at low temperatures.[52,53]
CH₄ + 2O₂ → CO₂ + 2H₂O, ΔG⁰₂₉₈ₖ = −801 kJ·mol⁻¹ (8)
Active oxygen generated by transition metal oxides under light irradiation plays a crucial role in various photocatalytic environmental applications.[54]
Figure 3. a) UV-visible diffuse reflectance spectrum and wavelength-dependent apparent quantum yield of Ag/ZnO. b) Schematic diagram of the photocatalytic CH₄ combustion process on Ag/ZnO.[55] c) CH₄ conversion curves on HPMC and control samples with/without irradiation. HPC: PdO/CeO₂ coated on halloysite nanotubes, HMC: Mn₃O₄/CeO₂ coated on halloysite nanotubes. d) Proposed photocatalytic CH₄ combustion mechanism on HPMC.[56]
Photocatalytic Methane Functionalization
Selective catalytic functional group reactions of C–H bonds are an effective method for constructing C–C and C–X bonds, especially in the synthesis of pharmaceutical molecules[57-59]. The introduction of methyl groups can significantly enhance the biological activity of molecules, a phenomenon known biologically as the "methyl effect", providing a simple and effective approach for developing active pharmaceutical molecules.[60] Additionally, methane is considered a promising methylating agent, potentially replacing currently used toxic dimethyl sulfate and iodomethane.[61,62]
Furthermore, another major component of natural gas, ethane, can also be selectively converted to amines, with TONs reaching up to 9700. This photocatalytic system can efficiently and selectively carry out a series of methane functionalization reactions, such as the alkylation and arylation of methane (Figure 4). Notably, photocatalysts can be modified to adjust the electrophilicity of alkoxy radicals through the structure of alcohol compounds as hydrogen transfer catalysts, thereby allowing for regioselective modulation of various types of C–H bonds, including two different C–H bond substitution types in propane and butane molecules. Moreover, gas-liquid two-phase flow photochemical reactions of methane, ethane, and other gases have been successfully realized through flow chemistry and glass microreactor technology, achieving considerable conversion efficiencies and laying the foundation for scaled applications.
Figure 4 a) Photocatalytic amination of alkanes in continuous flow photoreactor for scaled applications. b) Photocatalytic alkylation and arylation reactions of methane. a,b) Reproduced with permission.[63] © 2018 American Association for the Advancement of Science. c) Proposed mechanism of methane functionalization with aryl and alkyl radicals.
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