The formation of active sites on the catalyst is crucial for the selective oxidation of methane to formaldehyde^[78]. For Mo-based catalysts, the structure of molybdenum species is one of the factors influencing the selective oxidation of methane. Meanwhile, researchers have found that amorphous SiO₂can enhance both the stability and activity of the catalyst. It also provides a larger surface area, improving the contact between reactants and catalyst, thereby increasing catalytic activity^[79-80]. Therefore, Mo-based catalysts supported on SiO₂are widely used in the selective oxidation of methane. Ohler et al.^[81]found a direct relationship between the selective oxidation activity of methane and the concentration of reduced sites on the catalyst surface. The peroxide generated from the reaction of O₂with low-concentration reductive molybdate species is the active species responsible for formaldehyde production in the selective oxidation of methane. According to the oxidation mechanism of CH₄on MoO _x/SiO₂(Figure 4a), the five-coordinated species with isolated Mo \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O bonds can be readily reduced by H₂into molybdate species containing Mo^Ⅳcations. In the gas phase, O₂reacts with low-concentration reductive molybdate species present on the catalyst surface to form peroxides. Subsequently, these molybdenum peroxide species react with CH₄to produce CH₂O and H₂O, or regenerate stable molybdate species upon reaction with H₂O. Isotope experiments indicate that oxygen from the silica support does not directly participate in the catalytic cycle for oxidizing CH₄^[61]. However, some studies also suggest that SiO₂itself can serve as an active site for the oxidation of methane to formaldehyde. Miceli et al.^[82]observed that the activity on SiO₂is due to specific surface sites with donor properties that activate molecular oxygen (i.e., Si³⁺deposited at structural defects in SiO₂). Simulation studies of the SiO₂surface suggest that reducing sites and siloxane bridges on the SiO₂surface can effectively activate O₂and CH₄molecules, respectively.

In recent years, studies have found that changes in Mo loading can affect the structure of molybdenum species on the catalyst surface, thereby influencing methane conversion and formaldehyde selectivity. Catalysts with highly dispersed molybdenum species exhibit higher activity in the selective oxidation of methane^[83].Smith et al. investigated the effect of MoO₃/SiO₂ catalyst loading on the activity and selectivity of the selective oxidation of methane to formaldehyde. Characterization results confirmed that at lower Mo loadings, highly dispersed silicon-molybdenum species with terminal Mo \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O sites exist, leading to the maximum formaldehyde production rate. As the loading increases, the highly dispersed Mo \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O bonds gradually transform into Mo—O—Mo bonds, increasing the polymerization degree of molybdenum species and resulting in non-selective oxidation of methane. At high loadings, crystalline MoO₃ forms on the catalyst surface, accelerating the deep oxidation of the reaction^[84].Spencer and Miceli^[80,82] also found that the formation of MoO₃ crystals at high Mo loadings may mask the methane activation sites, which is unfavorable for the generation of the target product, formaldehyde. Increasing the MoO₃ loading reduces the number of active sites on the catalyst surface, thus the activity of MoO₃/SiO₂ catalyst decreases as the metal oxide loading increases. In addition to amorphous SiO₂ supports, catalysts with highly dispersed Mo species prepared by loading Mo onto other silica-based materials also achieve higher formaldehyde selectivity. Studies have shown that Mo loaded on SBA-1^[85] exhibits better activity and selectivity for the selective oxidation of methane to formaldehyde compared to loading on amorphous SiO₂. This is because, under strongly acidic conditions, the cubic mesoporous material SBA-1 can incorporate transition metal ions Mo into its framework, forming highly dispersed active centers. Valverde et al.^[86] prepared Mo/HZSM-5 catalysts using the impregnation method and applied them to the selective oxidation of methane. The results showed that at low Mo loadings, molybdenum species exist in a highly dispersed tetrahedral form and may be associated with Brönsted acid sites of HZSM-5 zeolite (Figure 4b). As the Mo loading increases, molybdenum species are predominantly polyoxomolybdates, and the carrier surface gradually becomes covered by molybdenum, reducing the active sites for methane activation and consequently lowering formaldehyde selectivity. Therefore, research suggests that monomeric Mo \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O sites on the catalyst are the active sites for formaldehyde formation in the selective oxidation of methane. Miao et al.^[87], by comparing the physicochemical properties of fresh and used catalysts, found that the number of tetrahedral Mo species chemically bonded to the surface of the MCM-22 support hardly changed after the reaction. However, at higher Mo loadings, octahedral polyoxomolybdates with weaker interactions with MCM-22 leached out during the reaction, suggesting that isolated tetrahedral molybdenum oxides are more stable during the reaction. Stability evaluation results also confirmed that the HCHO yield remained unchanged on catalysts containing isolated tetrahedral molybdenum oxides, while the study found that octahedral polyoxomolybdate species were associated with the formation of deep oxidation products (CO _x). Currently, most catalysts for the selective oxidation of methane are prepared by impregnation, but this method may result in fewer active sites and sometimes lead to pore blockage of the support. Studies have shown that other catalyst preparation methods can achieve higher formaldehyde selectivity. Chen et al.^[88] applied catalysts prepared by hydrothermal synthesis and impregnation to the selective oxidation of methane to formaldehyde. The results indicated that in the Mo-KIT-6 catalyst prepared by hydrothermal synthesis, Mo species existed in a highly dispersed form on the catalyst, thus achieving higher formaldehyde selectivity. In contrast, the Mo/KIT-6 catalyst prepared by impregnation had highly polymerized Mo species, resulting in lower formaldehyde selectivity. Furthermore, on the 8-Mo-KIT-6 catalyst, Mo \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O sites were more easily activated, thus exhibiting stronger anti-coking ability (Figures 4c, d). Aoki et al.^[89], comparing the activity of MoO₃/SiO₂ catalysts prepared by impregnation and sol-gel methods for the selective oxidation of methane, found that the sol-gel method-prepared catalyst exhibited superior activity and selectivity in selectively oxidizing methane to oxygenated compounds under excess water vapor conditions. Characterization results showed that, compared to the impregnation method, the polymerization degree of molybdenum species in the catalyst prepared by impregnation was higher. However, Lou et al.^[90], studying the selective oxidation of methane to formaldehyde on MoO _x/SBA-15 catalysts under atmospheric pressure and its kinetics, found that at high MoO _xloadings, the aggregated MoO _xspecies exhibited significantly higher activity than when existing as isolated MoO _xspecies at low loadings. This may be due to the fact that aggregated MoO _xformed more active centers and packed more tightly, thereby enhancing methane activation capability.

In recent years, studies have shown that Mo-based catalysts modified with elements such as P, Fe, and V exhibit improved performance in the selective oxidation of methane. Yang et al.^[91]investigated the catalytic performance of MoO₃/SBA-15 and P-modified P-MoO₃/SBA-15 catalysts in the selective oxidation of methane. The results revealed that phosphorus-modified molybdenum oxide oligomeric clusters encapsulated within the SBA-15 channels demonstrated excellent catalytic activity for the selective oxidation of methane to formaldehyde. Wang et al.^[92]found that the addition of phosphorus enabled the formation of a structure similar to phosphomolybdic heteropolyacid on the surface of MoO _xsupported on SBA-15, thereby enhancing methane conversion and formaldehyde selectivity. Yang et al.^[93]compared the activities of Mo/SiO₂and Fe-Mo/SiO₂catalysts in the partial oxidation of methane and found that the Fe-Mo/SiO₂catalyst could enhance the low-temperature activity for the selective oxidation of methane to formaldehyde. The study suggested that Fe serves as the site for activating C―H bond intermediates, while Mo is the site for converting these intermediates into formaldehyde. Ortiz et al.^[94]compared the formaldehyde selectivity of FeMoO _x/SiO₂and MoO _x/SiO₂catalysts, and the results indicated that the FeMoO _x/SiO₂catalyst exhibited the best catalytic performance. This was attributed to the iron oxide in the FeMoO _x/SiO₂catalyst acting as an oxygen supplier, providing oxygen to the selective molybdenum sites. This oxygen-supplying capability helped improve the selectivity of the methane oxidation reaction over the catalyst. Pei et al.^[95]demonstrated that P-Mo-V mixed oxide supported on SiO₂catalysts exhibited good methane conversion and formaldehyde selectivity. The P-Mo-V mixed oxides were highly dispersed within the mesoporous silica catalyst, and the presence of these active species enhanced both methane conversion and formaldehyde selectivity.

Although mesoporous silica has attracted considerable attention due to its unique mesoporous structure, large surface area, and high thermal stability, and has become the preferred support for the selective oxidation of methane to formaldehyde^[96-97], studies have also shown that catalysts prepared by loading molybdenum species onto other metal oxide supports can achieve similarly high performance in the selective oxidation of methane. Fodor et al.^[70] investigated the effect of K₂MoO₄deposited on different supports on the selective oxidation of methane to formaldehyde, and found that the K₂MoO₄/SiO₂catalyst exhibited the highest formaldehyde selectivity, whereas the formaldehyde selectivity was relatively lower on catalysts supported by Al₂O₃, TiO₂, and MgO. This is because, for other metal oxides as supports, complete oxidation of methane is the dominant process, resulting in poor formaldehyde selectivity^[98]. Numerous studies have demonstrated that ZrO₂possesses complex physicochemical properties (reducibility, oxidizability, acidity, and basicity), making it an ideal support for various reactions^[99]. Research on the selective oxidation of methane has also confirmed that using ZrO₂as a support can influence the crystallite size, reducibility, oxidation state of Mo, and surface composition of the catalyst, thereby determining its catalytic performance. Zhang et al.^[100] examined the catalytic performance of Mo/ZrO₂catalysts in the selective oxidation of methane to formaldehyde, and the results indicated that there was a certain interaction between Mo and the ZrO₂support, forming a new phase Zr(MoO₄)₂. The Mo \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O group selectively oxidizes methane to formaldehyde, while crystalline MoO₃accelerates the deep oxidation of formaldehyde. Composite oxides play an important role in olefin oxidation, but their application in alkane catalytic oxidation has been less studied. Studies have shown that different composite oxide catalysts can also achieve high formaldehyde selectivity in the selective oxidation of methane. Cui Xianghao et al.^[101] investigated the relationship between the structure of molybdate catalysts containing Mg, La, Ce, and Eu and their activity in the selective oxidation of methane. Characterization results revealed that in the CeMoO₄series catalysts, the Ce⁴⁺oxidation state remained unchanged before and after the reaction, while Mo⁶⁺was reduced to Mo⁵⁺, resulting in high formaldehyde selectivity. When Ce/Mo=0.68, the formaldehyde selectivity increased to 90%. Carlsson et al.^[102] studied the catalytic performance of iron molybdate powder catalysts for the selective oxidation of methane under oxygen pulse conditions, and the results showed that a higher methane/oxygen ratio could improve formaldehyde selectivity. It is evident that transient operation with mixed gases is also a feasible method for controlling product selectivity. Akiyama et al.^[103] found that in the presence of water vapor, composite oxides of Mo and Cu (Cu-MoO _x) achieved high formaldehyde selectivity in the selective oxidation of methane. Among them, the Cu₃Mo₂O₉catalyst, which exhibits strong redox properties, achieved the highest formaldehyde yield.

In summary, the performance of supported MoO _xcatalysts for the selective oxidation of methane is significantly influenced by the support, promoters, and preparation methods. The formation of MoO _xspecies in the catalyst serves as the active site for the selective oxidation of methane. Introducing other components into the catalyst can alter the structure of surface-active species, thereby enhancing the catalyst's performance in the selective oxidation of methane.

Studies have shown that three types of VO _xspecies exist on the surface of supported vanadium oxide catalysts: isolated tetrahedral monomeric vanadium species, oligomeric VO _x, and crystalline V₂O₅(Figure 5a) ^[109]. Launay et al.^[110-111]reported that under aqueous conditions, highly active, hydroxylated monomeric vanadium species appeared in significant proportions in the VO _x/SiO₂catalyst, resulting in improved catalytic activity for the selective oxidation of methane to formaldehyde (Figure 5b). Berndt et al. applied the VO _x/MCM catalyst to the selective oxidation of methane to formaldehyde and found that highly dispersed VO _xspecies could achieve a higher formaldehyde space-time yield. They discovered that during steam co-feeding, the VO _x/MCM catalyst formed surface species such as V^ⅤO(OH) _x(OSi≡)_{3- x}, which contributed to the catalyst's enhanced formaldehyde space-time yield. The corresponding reduced site, V^ⅣO(OH) _x(OSi≡)_{2- x}, was identified as the true active site of the catalyst^[112]. Based on these findings, researchers proposed a mechanism for the formation of active sites on the VO _x/MCM catalyst (Figure 5c). Earlier studies suggested that V^Ⅴsites might serve as the active centers for the selective oxidation of alkanes^[113]. Cormick et al.^[114]compared the performance of pyrophosphate vanadium catalysts with and without promoters in the selective oxidation of methane, finding that pure pyrophosphate vanadium, containing only V^Ⅳsites, was unfavorable for partial methane oxidation. However, modification with Fe or Cr could stabilize or enhance the formation of isolated V^Ⅴsites on the catalyst surface, thereby improving formaldehyde selectivity. Sun et al.^[115]also confirmed through in-situ Raman characterization that isolated V⁵⁺species were the active sites for methane selective oxidation on the V₂O₅/SiO₂catalyst. Makowski et al.^[116]found that vanadium-based catalysts supported on gas-phase silica carriers containing highly dispersed tetrahedrally coordinated vanadium species exhibited high methane oxidation activity and formaldehyde selectivity. Kartheuser et al. investigated the relationship between the dispersion of vanadium oxides on silica supports and the selective oxidation of methane to formaldehyde, demonstrating that when the silica support surface had highly dispersed vanadium species, the catalyst achieved higher formaldehyde selectivity, with the smallest vanadium oxide particles at this point. Conversely, when the vanadium oxide particles were larger, the likelihood of formaldehyde colliding repeatedly with the surface increased, leading to a decrease in formaldehyde selectivity^[117].

The above studies indicate that highly dispersed vanadium species are one of the factors influencing the activity of methane selective oxidation. Additionally, different supports significantly affect the catalytic activity of the catalyst. The surface structure of the support (surface area, pore structure, and morphology) influences the dispersion of vanadium oxides, thereby affecting the catalytic performance. Earlier researchers confirmed that mesoporous pure silica SBA-15 material with a high surface area is an excellent support for dispersing vanadium oxide species^[105].Wallis et al.^[118]demonstrated that high-surface-area mesoporous silica materials (MCM-41 and MCM-48) can form highly stable and well-dispersed VO _xactive sites, significantly enhancing formaldehyde yield. Building on this, the research group investigated how the properties of SBA-15 supports at different aging temperatures affect the catalytic performance of VO _x/SBA-15 catalysts in the selective oxidation of methane to formaldehyde. The results showed that the V/SBA₇₀catalyst exhibited the best formaldehyde selectivity. At this point, the SBA-15 support had the highest specific surface area and the smallest micropore size, which facilitated the formation of more monomeric and oligomeric VO _xspecies, thus achieving optimal catalytic performance. Pirovano et al.^[119]applied supported VO _xcatalysts prepared using nanoporous glass and mesoporous silica as supports in the selective oxidation of methane. The results indicated that catalysts prepared on nanoporous glass supports with low surface area and large pore sizes formed larger vanadium oxide particles on the surface, reducing formaldehyde selectivity. In contrast, mesoporous silica supports (with high surface area and small pore sizes) significantly improved the dispersion of VO _xspecies, leading to higher formaldehyde selectivity. However, Yang et al.^[120]prepared VO _xsupported catalysts using MCF-1 and SBA-15 as supports, respectively, and applied them to the selective oxidation of methane. They found that the MCF-17 support, with its large specific surface area and pore size, favored the dispersion of vanadium species, resulting in better formaldehyde selectivity.

Different catalyst preparation methods can adjust the dispersion of catalytic active sites, thereby enhancing the catalytic performance of the catalyst. However, systematic studies on the impact of catalyst preparation methods on the catalytic performance of mesoporous silica-supported vanadium oxide catalysts and their application in the selective oxidation of methane are still lacking. Currently, the most commonly used method is impregnation; Pirovano et al.^[119]synthesized VO _x/SBA-15 catalysts using two different methods—wet impregnation and initial wet impregnation—and applied them to the selective oxidation of methane. The study showed that the VO _xspecies in the catalyst prepared by initial wet impregnation exhibited higher dispersion on the support surface, resulting in greater formaldehyde selectivity. Yang et al.^[120]used solvent-free impregnation (where the precursor and support are physically mixed in mortar) and wet impregnation to prepare VO _x/SBA-15 catalysts for the selective oxidation of methane to formaldehyde. The results indicated that the former method could produce a catalyst with more tetrahedral monovanadate species compared to the latter, thus achieving a higher net formaldehyde yield. However, traditional impregnation methods do not always effectively disperse the active species, and the active sites may shift during the reaction, reducing catalytic activity and leading to less-than-ideal formaldehyde yields^[121-122]. Therefore, to improve the dispersion of active components on the support, Song et al. prepared V/MSN catalysts with highly dispersed active sites using a grafting method and applied them to the selective oxidation of methane. The results showed that the V/MSN catalyst prepared by grafting, featuring highly dispersed VO₄tetrahedral active centers, could stably convert methane into formaldehyde with high formaldehyde selectivity (Figure 5d)^[123]. Ruddy et al. prepared catalysts using thermal decomposition of molecular precursors (TMP) and traditional wet impregnation methods, comparing their performances in the selective oxidation of methane. They found that using the TMP method to graft OVOSi(O^tBu)₃₃and OV(O^tBu)₃precursors onto the SBA-15 support selectively yielded uniform and isolated monovanadate species, resulting in higher formaldehyde selectivity (Figure 5e)^[124-125]. Nguyen et al. synthesized mesoporous SiO₂-supported vanadium oxide catalysts using co-condensation and impregnation methods, respectively, and examined their performance in the selective oxidation of methane. The results indicated that the catalyst synthesized by co-condensation contained more highly dispersed vanadium monomer species, leading to higher activity and formaldehyde selectivity^[126]. Yang et al.^[127]prepared efficient SiO₂@V₂O₅@Al₂O₃core-shell nanostructures using hydrothermal synthesis and atomic layer deposition (ALD), applying them to the selective oxidation of methane (Figure 5f). The results showed that, compared to traditional impregnation methods, the Al₂O₃shell facilitated interactions between Al₂O₃and V₂O₅nanoparticles, providing a new interface for generating highly dispersed T_dmonomers with V―O―Al bonds (Figure 5g), thereby enhancing catalytic performance. The sol-gel method is a simple catalyst preparation technique that also allows better control over the particle size and morphology of the active phase. Loricera et al.^[128]found that the VO _x/SiO₂catalyst prepared by the sol-gel method contained highly dispersed isolated or oligomeric surface vanadium species, resulting in higher formaldehyde yields. Zhang et al.^[129]synthesized V-MCM-41 catalysts using direct hydrothermal synthesis (DHT) and template ion exchange (TIE) methods, examining their performance in the selective oxidation of methane. The results showed that the vanadium-based tetrahedra in the catalyst prepared by TIE were predominantly dispersed on the MCM-41 support walls, with a higher concentration of V \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O, thus exhibiting superior catalytic activity. In contrast, the vanadium-based tetrahedra produced by the DHT method were mainly located within the MCM-41 support framework, weakening the double-bond characteristics between vanadium and oxygen and resulting in lower catalytic activity.

In the selective oxidation of methane, mesoporous SiO₂with a large surface area and an ordered pore structure (such as MCM-41 and SBA-15 mentioned above) is often used instead of amorphous SiO₂as a support, and improved catalyst preparation methods (such as the sol-gel method, ion exchange method, and grafting method mentioned above) are commonly employed to achieve highly dispersed vanadium species and thus prepare highly active catalysts. Additionally, researchers have found that adding promoters to VO _x/SiO₂catalysts can also significantly enhance methane conversion and formaldehyde yield. Shimura et al.^[130]investigated the effect of Ga₂O₃as a promoter on the performance of VO _x/SiO₂catalysts in the selective oxidation of methane, and found that Ga₂O₃enhanced the reducibility of the active VO _xspecies without reducing their dispersion, resulting in higher catalytic activity for the VO _x/Ga₂O₃/SiO₂catalyst. Wallis et al.^[131]demonstrated that Ti as a promoter could induce the formation of more monomeric and readily reducible VO _xspecies in V/Ti-SBA-15 catalysts, thereby improving formaldehyde selectivity. Zhang et al.^[132]compared the performance of Sb-V-O/SiO₂and VO _x/SiO₂catalysts in the selective oxidation of methane to formaldehyde. The results indicated that the Sb-V mixed oxide formed on the Sb-V-O/SiO₂catalyst could provide more active sites, enhancing methane adsorption and activation and leading to higher formaldehyde yields. However, some studies have also confirmed that metal modification does not necessarily improve the methane selective oxidation performance of catalysts. Irusta et al.^[133]prepared sodium-modified vanadium-based catalysts by pre-impregnating SiO₂support with sodium and investigated their performance in the selective oxidation of methane. The results showed that compared to unmodified vanadium-based catalysts, the sodium-modified catalyst exhibited poorer formaldehyde selectivity. This was because the addition of sodium led to the formation of vanadate-like compounds, reducing the concentration of V \stackrel{\mathrm{ }\mathrm{ }\mathrm{ }\mathrm{ }}{̿}O, which prevented methane molecules from being activated through hydrogen extraction and promoted formaldehyde decomposition, resulting in lower catalyst activity and selectivity. In addition to silica as a support, certain metal oxides (TiO₂and Al₂O₃) are also common supports for supported catalysts; however, the performance of catalysts supported on metal oxides for the partial oxidation of methane is not ideal. Arena et al.^[51]compared the performance of V₂O₅/SiO₂and V₂O₅/TiO₂catalysts in the selective oxidation of methane to formaldehyde, and found that the main product of the reaction catalyzed by the V₂O₅/TiO₂catalyst was CO, and it also exhibited weaker methane activation ability. Parmaliana et al.^[81]also studied the selective oxidation of methane over catalysts supported on metal oxides, and found that the products of this reaction catalyzed by vanadium-based catalysts supported on TiO₂and Al₂O₃were primarily CO _x. Koranne et al.^[107]confirmed in their study that formaldehyde produced by the selective oxidation of methane could be further oxidized by acid sites on alumina, and therefore no formaldehyde was detected on the V₂O₅/Al₂O₃catalyst.

In summary, it can be seen that vanadium-based catalysts exhibit excellent catalytic performance in the selective oxidation of methane. The formation of monomeric and oligomeric vanadium species within the catalyst structure serves as the active site for methane activation to produce formaldehyde. Modifying the support type and catalyst preparation method, or introducing other metal components into the catalyst structure, can help increase the number of active species and maintain their dispersion, thereby enabling the catalyst to demonstrate superior catalytic performance.

Studies have shown that modifying iron-based catalysts with P results in an appropriate P/Fe ratio, forming FePO₄nanoparticles that can further enhance formaldehyde selectivity. He et al.^[139]pointed out that the FePO₄nanoclusters formed by the interaction between iron and phosphorus in the P-FeO _x-SiO₂catalyst exhibit high formaldehyde selectivity. On the P-FeO _x-SiO₂(P/Fe=0.5) catalyst, the single-pass yield of formaldehyde reaches as high as 2.4%, which is consistent with the conclusion that FePO₄supported on SiO₂promotes methane conversion^[140]. McCormick et al.^[141]investigated the activity of methane selective oxidation over silica-supported iron phosphate catalysts with varying iron phosphate contents. The results indicated that as the iron phosphate content increased, the number of FePO₄crystals also increased, leading to poorer formaldehyde selectivity. In contrast, at low loading levels, the pentacoordinate Fe³⁺species isolated by phosphate groups exhibited higher formaldehyde selectivity than Fe₂O₃crystals. Wang et al.^[142]studied the catalytic performance of iron phosphate catalysts supported on a silicon-based carrier for the selective oxidation of methane. They found that when the loading was below 40 wt%, the FePO₄species were highly dispersed on the surface of the silicon-based carrier or formed small nanoscale clusters, resulting in high catalytic activity and formaldehyde selectivity. However, as the loading increased, the clusters began to aggregate outside the mesopores, forming small FePO₄crystal particles, thereby reducing the catalyst's activity.

The choice of support significantly affects the catalytic activity and selectivity of iron species in the selective oxidation of methane, with different supports directly influencing the dispersion of Fe³⁺ species. Most of the catalysts mentioned in the above studies use mesoporous silica as the support. Silica, as a high-surface-area support, ensures high dispersion of metal oxides and prevents secondary reactions between formaldehyde and CO _x. Wachi et al.^[143]compared the performance of methane selective oxidation over iron dinuclear polyoxometalate catalysts supported on various metal oxides (Al₂O₃, ZrO₂, and CeO₂) and SiO₂. The results indicated that the highly dispersed state of the iron dinuclear polyoxometalate precursor on SiO₂led to the formation of more dispersed iron oxide nanoclusters, thereby enhancing methane conversion and formaldehyde yield. They also proposed a surface reaction mechanism for formaldehyde on Al₂O₃and SiO₂(Figure 6d), showing that Al₂O₃promotes the over-oxidation of formaldehyde. This study is consistent with the findings reported by McCormick et al.^[144]regarding the methane selective oxidation performance of FePO₄supported on Al₂O₃and SiO₂, where the superior catalytic performance is attributed to the highly coordinated and easily reducible Fe species isolated by phosphate groups on the SiO₂support. In mesoporous materials, due to the confinement effect of the ordered mesopores, the growth of highly dispersed Fe species and FeO _xnanoclusters into Fe₂O₃particles is restricted^[57]. Therefore, SBA-15, with its large surface area, holds promise as a support for preparing highly efficient iron-based catalysts. Wang et al.^[145]found that iron phosphate catalysts supported on SBA-15, which has larger pores and higher inertness, achieved higher formaldehyde selectivity, whereas the limited acidic sites within the pores of the MCM-41 support led to continuous conversion of formaldehyde, thus reducing the formaldehyde selectivity of the FePO₄/MCM-41 catalyst.

In addition, better catalytic activity for the selective oxidation of methane has also been observed on certain metal composite oxide catalysts. Matsuda et al. found that increasing the P/Fe ratio in triangular FePO₄nanoparticle catalysts led to an increase in acidic sites on the catalyst surface, thereby enhancing catalytic activity and improving formaldehyde selectivity. The researchers also proposed a reaction mechanism: first, CH₄is oxidized on the FePO₄surface to produce HCHO and partially reduced FePO_{4- δ}; subsequently, FePO_{4- δ}is rapidly oxidized back to FePO₄by O₂, thus establishing a catalytic cycle. FePO₄with uniform Lewis acid sites and weak basic sites facilitates the activation of C—H bonds, while its weak interaction with oxygen-containing compounds (such as HCHO) results in the selective direct oxidation of CH₄to HCHO (Figure 6e)^[146]. Nedyalkova et al.^[147]found that inserting iron into the crystal lattice of Ce₂Zr_{2- x}Fe _xO_8-∂catalysts can prevent the formation of aggregated Fe₂O₃species, and as a result, the formaldehyde selectivity of this catalyst increases with the increase in iron content.

The dispersion of iron species is also related to the preparation method of the catalyst. In the traditional impregnation method, the use of water as a solvent inevitably leads to the aggregation of elemental iron in the precursor solution during hydrolysis and drying, forming bulk iron oxides and thus deteriorating the catalyst performance^[148]. Therefore, alternative methods need to be considered for preparing highly efficient catalysts for the selective oxidation of methane. Studies by Arena et al.^[67,137]have shown that compared with the initial wet impregnation method, the adsorption-precipitation method can further enhance the dispersion of iron species on the catalyst surface, significantly improving the methane selective oxidation performance of FeO _x/SiO₂catalysts. Research has confirmed that the sol-gel method can produce supported catalysts with uniformly distributed active phases. Fajardo et al.^[149]found that controlling the pH during the sol-gel preparation process can prevent iron precipitation and ensure high dispersion of Fe³⁺species within the silica support, thereby enabling the catalyst to achieve superior methane oxidation performance. He et al.^[139]observed that the sol-gel method yields catalysts with higher dispersion of iron species, resulting in higher formaldehyde selectivity compared to FeO _x-SiO₂catalysts prepared by impregnation. Yu et al.^[148]confirmed that compared to the initial wet impregnation and liquid ion exchange methods, the solid-state ion exchange method can effectively inhibit the initial aggregation of iron species and the formation of bulk iron oxides, maximizing the proportion of isolated iron species in the Fe/ZSM5 catalyst and thus achieving excellent catalytic performance for methane oxidation. Zhang et al.^[129]found that the direct hydrothermal synthesis (DHT) method can form isolated tetrahedrally coordinated Fe—O species within the support framework, whereas the template ion exchange (TIE) method primarily produces octahedrally coordinated polymeric iron oxide clusters. Consequently, Fe-MCM-41 catalysts prepared by the DHT method exhibit higher methane conversion and formaldehyde selectivity.

In summary, Fe³⁺species in the structure of iron-based catalysts serve as the active sites for the selective oxidation of methane. Mesoporous silica materials, with their large surface area and pore size, are often used as supports for methane selective oxidation reactions. By modifying the preparation methods, iron-based catalysts with highly dispersed active sites can be synthesized. Additionally, adjusting the content and distribution of active components in iron-based catalysts can also facilitate the development of highly efficient catalysts for methane selective oxidation reactions.