Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp 3 )–H Bond Activation

Open AccessCCS ChemistryMINI REVIEW14 Jun 2022Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp3)–H Bond Activation Yuheng Jiang†, Yingying Fan†, Siyang Li and Zhiyong Tang Yuheng Jiang† Chinese Academy of Science (CAS) Key Laboratory of Nanosystem and Hierarchy Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190 Center for Nanoscale Science and Technology, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871 University of Chinese Academy of Sciences, Beijing 100049 , Yingying Fan† Center for Advanced Analytical Science, School of Chemistry and Chemical Engineering, School of Civil Engineering, Analytical and Testing Center, Guangzhou University, Guangzhou 510006 , Siyang Li Chinese Academy of Science (CAS) Key Laboratory of Nanosystem and Hierarchy Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 and Zhiyong Tang *Corresponding author: E-mail Address: email protected Chinese Academy of Science (CAS) Key Laboratory of Nanosystem and Hierarchy Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202201991 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Mild and direct conversion of methane into high value-added products is a desired goal for chemistry, energy, and environment. The active species generated in the photocatalytic reaction system under mild conditions activate the inert C–H bond in methane and promote methane conversion. This review focuses on the developed mechanisms for C(sp3)–H bond activation in photocatalytic methane conversion, including radical and active site mechanisms. Particular emphasis is placed on the detailed summary of mechanism, characterization method, and corresponding application in photocatalytic methane conversion. We also discuss the challenges and prospects for future direction on C(sp3)–H bond activation mechanism in photocatalytic methane conversion. This review aims to promote the development of photocatalytic methane conversion and provides guidance for the design of high-efficiency photocatalysts. Download figure Download PowerPoint Introduction Methane is the main component of natural gas and has abundant reserves in the world.1 It is usually consumed as a fuel because of its highest mass heat (ca. 56 KJ g−1) compared with other hydrocarbons,2,3 but large amounts of CO2 produced by methane combustion exacerbate the greenhouse effect worldwide. Besides, methane is a good feedstock for C1 chemistry, which could potentially generate all the products now obtained from petroleum.3 However, owing to the high bond energy (439 kJ mol−1) and low polarization of methane C–H bonds, methane conversion usually requires harsh reaction conditions, typically causing carbon deposition, catalyst deactivation, and side reactions. Balancing the methane conversion ratio and product selectivity can be a challenge. In general, current methods for converting methane into high value-added products include direct and indirect pathways.4 As for the indirect way, methane is converted to syngas that is then evolved into high value-added products such as olefins and methanol via the Fischer–Tropsch reaction. Unfortunately, this reaction requires high temperature and pressure as well as high amounts of energy. Hence, direct conversion of methane under mild conditions to afford high value-added products is an ideal target. In using renewable solar energy as the energy source, photocatalysis serves as an ideal approach for the conversion of methane under mild conditions.5 The activation of the C–H bond is widely recognized as the rate-determining step (RDS) of methane conversion reaction, which lays the basis for its subsequent transformation into versatile products. According to Shilov and Shul'pin's definition, C–H bond activation refers to an increase in the activity of a molecule by certain actions, causing the inert C–H bond to be replaced by weaker and more easily functionalized bonds.6 Distinct from thermocatalytic systems, photocatalysts utilize light energy to overcome the activation barrier, which enables thermodynamically unfavorable reactions proceeding under mild conditions.7 With the help of a photocatalyst, many active intermediate species like free radicals and active sites with high spin density are formed, which can easily extract a hydrogen atom from methane with a very low activation barrier.4 Research on photocatalytic methane conversion has made many achievements. For example, Zuo et al. achieved visible light-catalyzed C–H bond amination, alkanization, and aromatization of methane at ambient temperature.8 Cerium salts were used as photocatalysts, and alkoxy radicals generated from simple alcohols through ligand-to-metal charge transfer (LMCT) excitation were employed as hydrogen atom transfer (HAT) catalysts. The turnover number (TON) reached 2900. This mixed-phase gas–liquid system could be realized in a continuous flow system for potential scale-up. Noël et al. used inexpensive decatungstate as a HAT photocatalyst to activate methane, and the produced carbon-center radicals were trapped by a variety of Michael acceptors, giving rise to hydroalkylated adducts with good yields and high selectivity.9 Khodakov et al. prepared silver-heteropolyacid-titania to stoichiometrically convert methane into ethane at room temperature.10 Through a photochemical loop strategy, the selectivity of ethane reached 90%, the yield was up to 9%, and the quantum efficiency was 3.5% at 362 nm. Tang et al. synthesized quantum-sized bismuth vanadate (q-BiVO4) nanoparticles with a template method. The q-BiVO4 catalyzed selective aerobic oxidation of methane to generate methanol (1.1 mmol g−1 productivity and 96.1% selectivity) or formaldehyde (13.1 mmol g−1 productivity and 86.7% selectivity).11 Wang et al. achieved photocatalytic anaerobic oxidation of methane to ethanol (productivity of 106 μmol gcat−1 h−1) with Cu-modified polymeric carbon nitride (PCN) through the methane–methanol–ethanol pathway.12 Nevertheless, the conversion rate of photocatalytic methane reaction is still quite low and the categories of reactions are limited. The partial oxidation of methane and anaerobic coupling of methane have been studied extensively, but there are only a few reports on the functionalization of methane in organic systems. We realize that the main reason is the lack of deep study on the activation mechanism of the C–H bonds of methane. The design of a photocatalyst for the activation of the inert C–H bond in methane is conducive to the efficient conversion of methane into high value-added products. We notice that there have been many reviews summarizing current research progresses of photocatalytic methane conversion,13–17 whereas the mechanism of C–H bond activation in photocatalytic methane conversion has not been discussed in detail. In this review, we will comprehensively summarize the C–H bond activation mechanism and the corresponding characterization methods in photocatalytic methane conversion. We classify the C–H bond activation mechanism into radical and active site mechanisms according to the category of active species. The radical mechanism refers to the activation of methane by the reaction with generated reactive radicals in solution. In contrast, when methane is adsorbed and activated directly on the photocatalyst surface, such a reaction is classified as an active site mechanism. Furthermore, the application of radical and active site mechanisms in photocatalytic methane conversion is elucidated. Finally, the challenges and prospects for future direction on C(sp3)–H bond activation mechanism in photocatalytic methane conversion are discussed. We hope that this review can deepen the mechanistic understanding of the photocatalytic methane conversion process and provide guidance for the design of high performance photocatalysts. Radical Mechanism General introduction Many active radicals are ubiquitous in the photocatalytic system, and they may abstract a hydrogen atom from methane (CH4 + R· → ·CH3 + RH) in a homolytic manner. The reaction barrier is low and the bond dissociation energy (BDE) of the newly formed R–H bond is larger than that of the broken CH3–H bond. Therefore, it is an exothermic reaction with a fast reaction rate.4 Figure 1 outlines the radical mechanism of photocatalytic methane activation involved with the hydroxyl radical (·OH), chlorine radical (·Cl), and alkyl radical (·OR).8 Figure 1 | Scheme of radical mechanism. Download figure Download PowerPoint Application of the radical mechanism ·OH mechanism In one photocatalytic system, ·OHs are the most common reactive oxygen species (ROS) in aqueous solution, which can be produced by the oxidation of water (·OH/H2O = 2.380 vs normal hydrogen electrode (NHE)), the decomposition of hydrogen peroxide (H2O2, H+/·OH = 1.14 vs NHE), or the reduction of O2 (O2 + 2H+ + 2e− → H2O2, O2/H2O2 = 0.695 V vs NHE, H2O2 + H+ + e− → ·OH, H2O2/·OH = 1.14 V vs NHE) (Figure 2a).18,19 ·OHs have strong oxidizing ability and are able to activate methane to produce ·CH3 in a homogeneous manner. This reaction is exothermic and may occur under mild conditions (ΔH = −60 kJ mol−1Ea = 15 kJ mol−1).13 Figure 2 | (a) ·OHs generated in photocatalytic reduction and oxidation steps of oxygen and water. (b) Conduction and valence bands positions for semiconductors used for photocatalytic methane oxidation. Download figure Download PowerPoint In 1998, Ogura et al. found that ·OH generated by 185 nm vacuum UV light photolysis of water vapor activated methane to produce methanol and formic acid.20,21 Subsequently, Noceti et al. used a mercury lamp as the light source in the solution system to catalyze the oxidation of methane to produce methanol, H2, O2, and CO.22 These pioneering works validated the feasibility of using ·OH to activate the C–H bond of methane. As shown in Table 1, using water, hydrogen peroxide, or oxygen as the oxidant, the photocatalytic conversion of methane to oxygen usually follows the ·OH mechanism. Evidently, the concentration and adsorption state of ·OH are the keys to determining the selectivity and conversion rate of the reaction. Table 1 | Photocatalytic Methane Conversion Involved with ·OH Mechanism in Aqueous Solution23 Entry Reactions Chemical equations ΔG0(298 K) kJmol−1 Methane, water, and oxygen/H2O2 1 Partial oxidation of methane (POM) 2CH4 + O2 → 2CH3OH −223 2 POM 2CH4 + O2 → 2HCHO + 2H2 −104 Only methane and water 3 Methane to methanol CH4 + H2O → CH3OH + H2 117 Introduction of photocatalysis system The realization of photocatalysis is mainly based on the energy band theory and the composition of reaction system. Typically, there are three reaction systems, that is, anaerobic oxidation, aerobic oxidation, and hydrogen peroxide oxidation. In the photocatalytic anaerobic methane oxidation system, water is the only source of ·OHs, which can be generated directly by water oxidation (·OH/H2O = 2.380 V vs NHE) or by in situ decomposition of hydrogen peroxide from water oxidation (H2O2, 2H+/2H2O = 1.76 V vs NHE, H2O2, H+/·OH = 1.14 V vs NHE). According to the energy band structure, TiO2,24 WO3,24–32 and BiVO433–36 have higher potential for water oxidation to the ·OH reaction and thus have the ability to activate the methane C–H bond through photocatalysis (Figure 2b). ·OHs may also be generated in indirect ways. For example, the holes of certain semiconductors only oxidize water to hydrogen peroxide through a two-electron process, then the additional active sites decompose the generated hydrogen peroxide to generate ·OHs in situ, thereby achieving methane C–H bond activation. In the anaerobic system, owing to the modest concentration of ·OHs and thermodynamic limitation, one most likely gets partial oxidation products such as methanol and formaldehyde, but the conversion rate of methane is very low. In comparison, the introduction of oxygen and hydrogen peroxide into the system helps to overcome the thermodynamic barrier of the reaction. As for the photocatalytic aerobic methane oxidation system, oxygen operates as an electron acceptor that significantly improves the conversion rate of methane. Note that oxygen is easily transformed into highly active radicals (e.g., ·O2− superoxide radical), resulting in the most stable thermodynamic overoxidation products (CO2 and CO).22,26,37 So, there are two requirements for choosing a photocatalyst. First, the conduction band position of catalyst should be lower than the potential of oxygen reduction to generate ·O2−. Second, the position of the valence band must be below the potential of water oxidation to generate ·OH. ZnO, WO3, and BiVO4 are all good photocatalysts for the selective oxidation of methane to methanol. The more negative conduction band position of NiO determines that methanol produced by methane oxidation is subsequently degraded by ·O2− to form CO2.24 With respect to the photocatalytic methane oxidation system using hydrogen peroxide as an oxidant, hydrogen peroxide may decompose to form ·OH, oxygen, ·O2−, singlet oxygen, and other ROS. In the absence of photocatalyst and using hydrogen peroxide as an oxidant, the methane conversion rate is rather low and the yield of partial oxidation products is very low. In a system using hydrogen peroxide as an oxidant, the conduction band position of photocatalyst should be higher than the potential of ·OH generated by the decomposition of hydrogen peroxide (H2O2 + H+ + e− → ·OH + H2O, H2O2, H+/·OH = 1.14 V vs NHE).19 Meanwhile, introducing a supported co-catalyst might promote the separation of photogenerated carriers and reduce the overpotential of hydrogen peroxide reduction; for instance, FeOx-supported TiO238 and FeOOH-supported mesoporous WO339 both exhibited high activity for the oxidation of methane to methanol with hydrogen peroxide. It is noted that the decomposition of hydrogen peroxide possibly generates a variety of products, but only ·OH can promote the methane oxidation reaction. To pursue a high hydrogen peroxide gain factor (gain factor = mol of H2O2 consumed for all products/mol of H2O2 consumed), it is very important to optimize the active sites in the photocatalyst. Wu et al. prepared a photocatalyst containing two types of active sites, Fe-Nx and Fe/FeC3 nanoparticles, by cracking the mixture of Fe-Zeolite imidazole frameworks (ZIF) and melamine.40 The iron in Fe-Nx with low spin state was discerned to be the active site for the decomposition of hydrogen peroxide to produce ·OH, whereas Fe/FeC3 nanoparticles contained higher electron density and tended to decompose hydrogen peroxide to produce oxygen. By finely tuning the active sites, the productivity of formic acid was up to 4659 μmol g−1 and the selectivity reached 90%. Control of ·OH concentration Since the ·OH mechanism is non-selective oxidation, the concentration of ·OH is the key to activating the C–H bond in methane and adjusting the product composition. ·OH concentration is affected by the efficiency of photogenerated carrier separation, the addition of oxygen, hydrogen peroxide, or sacrificial agents, and the choice of light source. Because ·OH results from the oxidation reaction between photogenerated holes and water, the separation efficiency of photogenerated carriers of photocatalytic materials determines the rate of ·OH generation. There are many ways to improve the separation efficiency of photogenerated carriers, such as supporting co-catalysts and forming heterojunctions, and the readers can refer to the published reviews.41,42 Introducing oxygen is one effective way to increase the concentration of ·OHs. For a photocatalyst with suitable conduction band positions, oxygen undergoes a two-step reduction process to generate ·OHs (O2 + 2H+ + 2e− → H2O2, O2/H2O2 = 0.695 V vs NHE, H2O2 + H+ + e− → ·OH, H2O2/·OH = 1.14 V vs NHE). In the selective conversion of methane to methanol using Au0.75/ZnO as photocatalyst, more ·OHs were produced after introducing oxygen and the methanol yield was five times that of the system in the absence of oxygen.18 Similarly, a 2.3-fold increase in ·OH concentration was observed when oxygen was introduced into the BiVO4-catalyzed methane oxidation system.11 Hydrogen peroxide is known to be ·OH generator, thus adding hydrogen peroxide to the reaction system was utilized as evidence to verify the ·OH mechanism.22 With the addition of hydrogen peroxide, more ·OHs were generated, which would promote the activation of methane and result in increased methane conversion and methanol yield. Importantly, an increased number of ·OHs often leads to a higher overoxidation product (CO, CO2) production and low methanol selectivity. As mentioned above, deep UV wavelength light might photolyze water to generate ·OH; therefore, choosing a suitable light source is an alternative method to control the ·OH concentration. Zhu et al. found that the use of a UV-filtered xenon light source was one of the reasons for the methanol selectivity of 85% in the bipyramidal BiVO4 catalyzed methane oxidation reaction.36 Murcia-Lopez et al. added nitrite ions in BiVO4-catalyzed oxidation of methane to methanol.34 The nitrite ions could inhibit water photolysis and act as ·OH sacrificial agents. As a result, the formation of CO2 was suppressed. Gondal et al. found that when a laser with a higher optical power density was used as a light source, WO3 photocatalyzed conversion of methane to methanol showed a faster reaction rate because of the higher rate of ·OH generation.26 In addition to the carrier separation efficiency, adding oxygen or hydrogen peroxide, and the light source, employing a photocatalyst with suitable band structure and active sites is another way to control the local ·OH concentration. Wang et al. used Cu-modified PCN (Cu/PCN) to catalyze the anaerobic oxidation of methane to ethanol under light irradiation (Figure 3a).12 The holes of PCN only oxidized water to hydrogen peroxide through a two-electron process, and as-generated hydrogen peroxide was decomposed to ·OH at the modified Cu sites, which guaranteed low local ·OH concentration (Figure 3b). Figure 3 | (a) Band structure alignment of PCN and Cu-0.5/PCN. (b) Hypothetical mechanism for photocatalytic anaerobic methane conversion over Cu/PCN. Reaction conditions: 20 mg of Cu-0.5/PCN suspended in 25 mL of H2O with continual stirring, 100 mL min−1 of CH4/N2, 500 W Xe lamp irradiation for 1 h. Reprinted with permission from ref 12. Copyright 2019 Springer Nature. Download figure Download PowerPoint ·OH not only activates C–H bonds in methane but also further oxidizes oxygenates like methanol. Therefore, controlling the concentration of ·OHs is a method to realize the product selectivity regulation in photocatalytic methane oxidation. Tang et al. achieved 96.6% methanol selectivity and 86.7% formaldehyde selectivity, respectively, in the photocatalytic methane oxidation system by optimizing the wavelength, intensity of the light source, the reaction time, and the volume of water (Figures 4a–4c).11 The reaction was performed at room temperature using q-BiVO4 nanoparticles as the catalyst and oxygen as the oxidant. Detailed characterization demonstrated that the reaction underwent a ·OH-dominated sequential oxidation step. Since formaldehyde was the oxidation product of methanol, increasing the oxidation capacity was an effective strategy to improve formaldehyde selectivity. The optimal selectivity to formaldehyde was achieved under ultraviolet irradiation (300–400 nm, 170 mW cm−2) for 7 h, while high methanol selectivity was obtained after 3 h of reaction under visible light (400–780 nm, 170 mW cm–2). Figure 4 | Selective oxidation of CH4 to (a) HCHO and (b) CH3OH. Reaction conditions: 10 mg q-BiVO4, 10 bar O2, 10 bar CH4, with either 10 mL H2O, 7 h reaction time, Hg lamp wavelength range 300–400 nm, intensity 170 mW cm–2 (a) or 10–80 mL H2O, 3 h reaction time, Xe lamp wavelength range 400–780 nm, intensity 170 mW cm–2 (b). Error bars denote the standard deviation of data from three tests. (c) Proposed photocatalytic methane oxidation reaction mechanism over q-BiVO4. Reprinted with permission from ref 11. Copyright 2021 Springer Nature. Download figure Download PowerPoint Adsorption state of ·OH Once the C–H bond in methane is activated, the CH3-surface transition state will form. If the CH3-surface interaction is energetically unfavorable or the geometrical configuration inaccessible, a radical-like ·CH3 will be observed.43 However, the surface of the photocatalyst interacts with free radicals in many cases, so the adsorption state of ·OH also affects the activation process of methane, and even determines the existing state of ·CH3. Surface oxygen vacancy, hydrophilicity or hydrophobicity, acidity or basicity, and photocatalyst crystal face all play an important role. La/WO3 is the earliest catalyst used for photocatalytic methane oxidation under gas–solid–liquid conditions. Villa et al. found that La doping was conducive to the formation of oxygen vacancies on the surface of WO3, which enhanced the adsorption capacity of water and promoted the formation of ·OHs. La/WO3 showed a higher methanol production than WO3.30 If the gas–solid–liquid ·OH followed a complete homogeneous reaction mechanism, the addition of ·OH sources such as hydrogen peroxide should promote methanol production. However, the opposite result was reported in the literature, so it was deduced that free-state ·OHs (·OHfre) and adsorbed-state ·OHs (·OHads) coexisted in the WO3-catalyzed system. Further fluorine modification on the surface of WO3 occupied the surface OH sites, which methanol production but a higher ethane production compared with the reaction mechanism the of of photogenerated oxidation of water or surface OH to generate ·OH, and activation of methane to generate ·CH3. only would with ·CH3 to produce methanol while free could Therefore, in the of on the surface, ·CH3 to form ethane rather than Murcia-Lopez et al. the acidity and of surface by doping and V into the which formed the doping the and acidity of surface, which was able to form a partial oxidation Through density theory et al. found that ·OH adsorption on and crystal of WO3, which the two ·OHs on the crystal exhibited the it to form hydrogen bonds and thus the highest activity (Figure WO3 with the highest ratio showed the activity and selectivity in the conversion of methane to (Figure Figure | (a) of ·OH adsorption on W of and The at the are corresponding adsorption energy. The and the hydrogen and oxygen (b) of WO3 WO3 and WO3 Scheme of WO3 with on Reprinted with permission from ref Copyright 2021 Download figure Download PowerPoint mechanism is an efficient HAT that can C(sp3)–H bonds including the one in methane. have been for the efficient of in photocatalytic methane conversion (Figure for example, (a) the photolysis of in situ generated under or (b) the direct electron transfer from to and (c) the Figure | (a) method in HAT reactions. (b) Scheme of of CH4 by is in an aqueous and CH4 is in The products, CH3OH and are in an aqueous (c) Proposed mechanism for functionalization of methane. (a) Reprinted with permission from ref Copyright 2021 Springer Nature. (b) Reprinted with permission from ref Copyright (c) Reprinted with permission from ref Copyright 2021 The for the of Download figure Download PowerPoint In Ogura et al. directly converted methane to methanol, and by at room conditions of V and nm the yield of was μmol and the current efficiency was The chlorine generated by the oxidation of was converted into under and then with methane to generate ·CH3. conditions of methanol was obtained from the of The of photochemical and the rate of chlorine and chlorine free thereby the selectivity of products. et al. a system of and water in ambient aerobic oxidation of methane to methanol and formic with yields of and respectively, and a methane conversion rate of As shown in Figure was produced in an aqueous solution by and The lower was the with methane and oxygen. light conditions, was decomposed into and oxygen, and the C–H bond in methane that with the singlet oxygen from oxygen to generate oxidation products. The bond in was more inert than the C–H bond in methane, while could methane and oxygen of methanol and formic As a result, this system the generated oxidation products into the water thereby the of et al. used produced from the process to activate methane and realized the of methane (Figure In with the mechanism that they = underwent an process irradiation to produce and then the a hydrogen atom from methane to generate The radical was further trapped by the and transformed into a This radical was by and by in a process, which generated the product and the yield of was under nm The two results reported are in a homogeneous system, and it is more to a photocatalytic system for methane conversion via chlorine for is because it is and abundant in Wang et al. reported a CH4 functionalization which was in with photocatalyst at room The generated active chlorine species converted methane to and Unfortunately, the mechanism was not The development of methane conversion in photocatalytic is very while the regulation of product selectivity a challenge. mechanism The of C–H bonds in methane not only to