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Formaldehyde is an important organic chemical raw material and one of the most significant derivatives of methanol. It is widely used in the production of resins such as urea-formaldehyde, phenol-formaldehyde, and melamine, as well as in the manufacture of urotropin, pentaerythritol, and dyes. In agriculture, it can be used as a pesticide and disinfectant. With the rapid development of the automotive, construction, and decoration industries, formaldehyde, as a traditional bulk chemical raw material, has become a product with high growth potential; there are great prospects for developing high-tech downstream products based on it. The production methods of formaldehyde include: the non-catalytic oxidation method using liquefied petroleum gas as a raw material, methane oxidation, methanol air oxidation, methanol dehydrogenation, and dimethyl ether oxidation, among others. This paper provides a brief introduction to methods for producing formaldehyde, such as the methane oxidation method, the methanol air oxidation method (using Ag and Fe-Mo catalysts), the methanol dehydrogenation method, and the dimethyl ether oxidation method. It also reviews the research progress on the catalysts used in these various production methods. 1 Direct oxidation of methane: Currently, the industrial synthesis of formaldehyde from methane involves 3 steps: (1) Steam reforming of methane to produce syngas ； (2) Production of methanol from syngas ； (3) Methanol is further oxidized to formaldehyde. The three-step process for producing formaldehyde requires high-pressure equipment, has a complex production workflow, high technical requirements, high energy consumption with low efficiency, and thus results in relatively high costs. In comparison, the direct oxidation of methane can simplify process equipment and make efficient use of energy, making it an ideal approach for the development and utilization of natural gas. In the reaction of direct oxidation of methane to formaldehyde, formaldehyde acts merely as an intermediate in the oxidation process of methane; it is highly unstable and tends to be further oxidized to CO and CO2, resulting in a very low yield of formaldehyde, around 2% only. In recent years, various catalysts for the direct oxidation of methane to formaldehyde have been developed, among which those supported with Mo or V exhibit good performance. 1.1 Mo-based catalysts: Wang Chengxue et al. used MoO3 and V2O5 as active components, and investigated the catalytic performance when SiO2, SnO2, and Al2O3 were used as carriers, among which SiO2 proved to be the best carrier ； The effects of a series of additives on the performance of the Mo/SiO2 catalyst system were also studied. It was found that the addition of Fe2O3, V2O5, CuO, B2O3, La2O3, CeO2, MgO, and SnO2 all helped to increase the methane conversion rate, while the addition of P2O5, La2O3, Cr2O3, and CeO2 improved the selectivity for formaldehyde. Zhang Xin et al. used ZrO2 as a carrier to prepare a series of Mo/ZrO2 catalysts for the selective oxidation of methane to formaldehyde. At a Mo mass fraction of 12%, under the conditions of n(CH4):n(O2):n(N2) = 10:1:3, gas space velocity of 12 L/(g·h), pressure of 5.0 MPa, and temperature of 400°C, the methane conversion rate was 8.3%, the formaldehyde selectivity was 47.8%, the formaldehyde yield was 4.0%, and the space-time yield was 396 g/(kg·h). Studies have shown that Mo/ZrO2 catalysts mainly consist of ZrO2 and Zr(MoO4)2, and the performance of the catalyst is closely related to the properties of Zr(MoO4)2. 1.2 V-based catalysts: Literature reports indicate that the performance of V2O5/SiO2 catalysts is similar to that of MoO3/SiO2 catalysts. Nguyen et al. used a new preparation method to load V oxides on SiO2. At a V mass fraction of 2.2%, under reaction conditions of 581°C and atmospheric pressure, the methane conversion rate was 3.5%, the selectivity for formaldehyde was 74.2%, and the space-time yield of formaldehyde was 2.435 kg/(kg·h). The high activity of the catalyst can be attributed to the high dispersion of V-active species on the carrier. These mononuclear V-active species consist of 1 V=O bond and 3 bridging V-O-Si bonds, or 1 -OH group and 2 bridging V-O-Si bonds; the -OH group and V-O-Si bonds contribute to the regeneration of active sites or the activation of methane. The literature reports mesoporous molecular sieves (such as MCM-41, MCM-48, SBA-15, etc.) as catalysts for the selective oxidation of methane. Bemdt et al. loaded VOx on MCM-41 and MCM-48 molecular sieves, and prepared a V/MCM-41 catalyst with a V mass fraction of 2.8% using the impregnation method. Using this catalyst, the space-time yield of formaldehyde is 2.255 kg/(kg·h), but the selectivity for formaldehyde is only 26.3%. They believe that VVO(OH)x(OSi)3-x or the corresponding reducible VⅣO(OH)x(OSi)2-x are the actual active sites, which is why the space-time yield of formaldehyde over VOx/MCM catalysts is high. Du et al. incorporated V into the framework of MCM-41 molecular sieves. Due to the large specific surface area of the V-MCM-41 catalyst, the active V species are highly dispersed, resulting in high activity and selectivity for formaldehyde. Compared with MCM series molecular sieves, SBA-15 molecular sieves have larger pore sizes, thicker pore walls, and higher thermal stability, which facilitates the desorption of partial oxidation products and prevents deep oxidation. Fornes et al. found that when SBA-15 molecular sieve is used as a carrier, VOx exhibits superior catalytic performance for the selective oxidation of methane. Based on various characterizations, it is speculated that when the V mass fraction is below 3%, V-O species may primarily be dispersed in a mononuclear form on the pore walls of SBA-15 molecular sieves, and can be reduced at lower temperatures; this may be one of the reasons for the good performance of the VO/SBA-15 catalyst. However, the structure-directing agents used to synthesize SBA-15 molecular sieves are relatively expensive. 1.3 Fe-containing catalysts Literature reports on the use of certain methane selective oxidation catalysts in which redox couples serve as active sites; for example, one active site (such as Cu1+/2+) is used to activate oxygen, while another active site (such as Fe3+/2+ or Sn4+/2+) is used to activate methane. On a Cu1+/Fe3+/ZnO catalyst, using air as the oxidant, the space-time yield of formaldehyde is 76 g/(kg·h), with a selectivity of 10% for formaldehyde. Parmaliana et al. studied the FeOx/SiO2 catalyst system and found that a Fe2O3 mass fraction of less than 0.3% is highly favorable for the formation of formaldehyde, but the selectivity for formaldehyde decreases when the Fe2O3 mass fraction exceeds 0.3%. They believe that on the SiO2 surface there are isolated Fe3+ ions, small Fe2O3 clusters, and large Fe2O3 particles; these different Fe3+ species can alter the surface activity of SiO2, thereby enhancing the catalyst’s activity. Arena et al. also believe that the preparation method of the FeO/SiO2 catalyst and the loading amount of Fe greatly affect the catalyst’s activity. Beata synthesized H-type or Na-type Fe/ZSM-5 catalysts with different Fe/Si ratios; increasing the Fe content in the ZSM-5 molecular sieve enhanced the catalyst’s activity and raised the methane conversion rate, but it also led to easier formation of CO. The Fe3+ ions in the molecular sieve framework exist in a mutually isolated tetra-coordinated structure; these highly isolated Fe3+ ions can increase the conversion rate of methane and the selectivity for formaldehyde. However, only a very limited amount of Fe3+ can be incorporated into the mesoporous molecular sieve framework. The Fe3+ ions in FePO4 also exist in a tetra-coordinated form, with the iron oxide tetrahedra being separated by phosphorus oxide tetrahedra. Wang et al. used the co-impregnation method to introduce FePO4 into the pores of mesoporous MCM-41 or SBA-15 molecular sieves, resulting in highly dispersed nanoparticles or clusters; under the same reaction conditions, the conversion rate of methane and the selectivity for formaldehyde were improved. 1.4 Other catalysts: Zhang Ting and others prepared a series of Mo/La-Co-O catalysts. Experimental results showed that when Mo is loaded on the La-Co-O support, Mo-O species exist in an amorphous state on the surface of this support and interact with it; such interactions can modify the properties of the catalyst, thereby affecting its performance. The research team also prepared Mo-Cr-V-Bi-Si multi-component composite oxide catalysts using the coprecipitation method. Under reaction conditions of n(CH4):n(O2) = 10:1, gas space velocity of 6500 h-1, pressure of 5.0 MPa, and temperature of 460–490°C, the methane conversion rate was 8%–10%, while the selectivity for formaldehyde was 60%–80%. Matsumura et al. studied supported Sb oxide catalysts. Catalysts with oxidized diamond as the support exhibited high activity, whereas those using SiO2 as a support had almost no activity. X-ray diffraction and UV-Vis spectroscopy analyses showed that α-Sb2O4 species were formed on diamond, which exhibited some activity in the selective oxidation of methane. In contrast, the Sb6O13 species formed on the SiO2 surface were more stable and showed little activity in the oxidation of methane to formaldehyde. Zhang et al. studied the SbOx/SiO2 catalyst system and concluded that, in the reaction of methane oxidation to formaldehyde, highly dispersed SbOx species are more active than aggregated SbOx species. The research team also prepared an MSU-2-supported Sb-V mixed oxide catalyst, namely the Sb5V1.12O/MSU-2 catalyst with a 5% mass fraction of Sb2O5 and a 1.12% mass fraction of V2O5. Under conditions of a reaction gas ratio of V(CH4):V(O2):V(He) = 2:1:7, a gas flow rate of 0.14 mol/h, and a temperature of 638°C, the conversion rate of methane was 9.6%, and the yield of formaldehyde was 2.9%. 2 Methanol air oxidation method: Depending on the type of catalyst used, the production of formaldehyde via methanol air oxidation can be divided into two methods: the Ag catalyst method and the Fe-Mo catalyst method. 2.1 Ag catalyst method The Ag catalyst method operates above the explosion limit of methanol-air, with an excess of methanol. Methanol undergoes oxidation and dehydrogenation reactions at atmospheric pressure and temperatures of 580–740°C. Approximately 50%-60% of the formaldehyde is produced through oxidation, while the remaining formaldehyde is generated via dehydrogenation. The reaction equation is: CH3OH + 1/2O2 → CH2O + H2O. CH3OH → CH2O + H2. Ag catalysts are classified into electrolytic Ag catalysts and pumice Ag catalysts based on their structural forms. The electrolytic Ag catalyst achieves higher methanol conversion rates and formaldehyde yields compared to the pumice Ag catalyst, with lower raw material consumption and a simpler production process ； However, it has a short service life and is prone to poisoning; the raw materials need to be purified in advance to ensure high purity, and significant electricity consumption is required during catalyst preparation. The pumice Ag catalyst has low activity, poor selectivity, and high methanol consumption. Therefore, there is a need to find a catalyst with high activity, high selectivity, and a long service life. Ag and its alloy catalysts exhibit high activity. Deng Jingfa et al. used electrolytic Ag as a catalyst, achieving an alcohol conversion rate of 97.6%, a formaldehyde yield of 87.1%, and a formaldehyde selectivity of 89.2%. Pestryakov studied the catalytic activity of As catalysts modified with oxides of metals such as Zr, Ce, La, Rh, and CS in the oxidation of methanol to formaldehyde. When the mass fraction of the modifier was 1%-10%, it was possible to alter the effective charge of the supported Ag, its reducing properties, the dispersion of the metal, surface acidity, and the degree of carbon deposition on the catalyst during the reaction. Among these factors, the modification of the electronic state of the catalytic active sites (Ag+, Agnδ+) had the greatest impact on the catalytic performance of the Ag catalyst. By using catalysts containing 1%-5% by mass of Zr oxide and Ce oxide, a yield of over 80% for formaldehyde can be achieved under conditions of n(O2):/2(CH3OH) = 0.35 and a temperature of 600-700°C. Li et al. prepared a ceramic catalyst with 20% Ag by weight using kaolin; the interaction between Ag and the ceramic was strong, and the Ag species on the surface were in an ionic state with a high positive charge, which increased the yield of formaldehyde. Dai et al. prepared an Ag-SiO2-Al2O3 catalyst; when the molar ratio of silicon to aluminum was (8.5:1.5)–(9:1) and the mass fraction of Ag was 20%, the methanol conversion rate was 97.8%, the formaldehyde selectivity was 92.8%, and the formaldehyde yield was 90.8%. Under similar reaction conditions, the conversion rate of methanol, the selectivity for formaldehyde, and the yield on the electrolytic Ag catalyst were 93.6%, 90.2%, and 84.4%, respectively. Dai et al. found that the addition of a small amount of the organic halide CH3I could enhance the catalytic activity of electrolytic Ag catalysts. Characterization using scanning electron microscopy and X-ray photoelectron spectroscopy showed that a small amount of CH3I interacted with the Ag surface to form AgI microcrystals with a specific structure, thereby increasing the specific surface area of the catalyst. Qian et al. also investigated the effect of the CH3I additive on the performance of Ag catalysts in the oxidation of methanol to formaldehyde. They found that after treatment with CH3I, the conversion rates of methanol and oxygen decreased at longer reaction times; the selectivity for formaldehyde increased, while the yield of formaldehyde did not increase. However, the formation of the by-product formic acid was suppressed. 2.2 Fe-Mo catalyst method: The Fe-Mo catalyst method operates above the lower explosion limit of methanol-air mixtures, with an excess of air, and the reaction takes place at atmospheric pressure and temperatures between 280–350°C. The reaction equation is: CH3OH + 1/2O2 → CH2O + H2O. Compared to the traditional Ag catalyst method, the Fe-Mo catalyst method requires lower reaction pressures and temperatures; it achieves a high conversion rate for methanol, approaching 100%, and also exhibits high selectivity for formaldehyde ； It has a high production capacity, low methanol consumption per unit of product, and a long service life for the catalyst ； Without the need for a distillation unit, high-concentration formaldehyde can be produced, with a low alcohol content in the resulting formaldehyde product. However, the Fe-Mo catalyst method has a slightly higher power consumption due to the large amount of air circulation during synthesis, as well as higher initial investment costs. 2.2.1 Carrier-free Fe-Mo catalysts Soares et al. prepared Fe-Mo catalysts via coprecipitation and investigated the effect of Mo content on catalyst activity. An excess of Mo can increase the specific surface area of the catalyst and alter its acidity, but the conversion rate of methanol per unit surface area of the catalyst remains unchanged. Fe2(MoO4)3 is the active phase; an excess of Mo (n(Mo):n(Fe)=3) can improve the selectivity for formaldehyde. Therefore, in Fe-Mo catalysts, an excess of Mo is necessary to ensure the catalyst’s stability, high activity, and selectivity. Adding the additives Ce, La, and V all helps to improve the performance of Fe-Mo catalysts; adding La