A Fe-based catalyst for the synthesis of formic acid under mild conditions and its preparation method

Abstract

This invention discloses a Fe-based catalyst for the selective oxidation of methane to formic acid under mild conditions and its preparation method. Fe is introduced into the framework and channels of a pure silicon molecular sieve by high-temperature dissolution and secondary recrystallization. After calcination in an oxidizing atmosphere, two Fe species with different functions are formed on the molecular sieve: one providing methane adsorption sites and the other providing methane activation sites. The catalyst prepared by this invention successfully loads Fe species onto an Al-free molecular sieve, thereby achieving highly selective synthesis of formic acid, providing another approach for the preparation of methane oxidation catalysts.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to an Fe-based catalyst for the synthesis of formic acid under mild conditions and its preparation method. Background Technology

[0002] Methane is the simplest hydrocarbon compound, widely distributed in mineral and biomass resources such as natural gas, shale gas, coalbed methane, biogas, oil refinery gas, and methane hydrate (combustible ice). Natural gas refers to a naturally occurring, combustible mixture of hydrocarbons and non-hydrocarbon gases found underground, primarily composed of methane (approximately 80-90%), with smaller amounts of ethane, propane, butane, carbon monoxide, and trace amounts of rare gases. Natural gas is a clean energy source; its development and utilization can reduce the use of coal and oil, thereby improving air quality and securing its important place in the global energy structure. The methane molecule has a stable tetrahedral structure, with the central carbon atom interacting with four hydrogen atoms via sp(s). 3 Hybridization results in four identical carbon and hydrogen atoms. Under standard conditions, the bond cleavage energy of the first CH bond in methane reaches 439.3 kJ·mol⁻¹. -1 Among all alkanes, methane has the highest bond energy of the CH bond. Simultaneously, methane possesses high ionization energy, low proton affinity, acidity, and a large HOMO / LUMO energy level difference. This means that methane is a relatively stable molecule, requiring highly volatile reaction conditions such as high temperatures and strong acids to be activated. Therefore, achieving the activation and selective conversion of CH4 under mild conditions remains a global challenge.

[0003] Currently, the main pathways for converting methane into high-value-added chemicals and liquid fuels are divided into two categories: indirect conversion and direct conversion. Indirect processes rely on obtaining syngas (CO and H2) through reforming reactions or partial oxidation, followed by the production of other high-value-added chemicals via Fischer-Tropsch synthesis or alcohol synthesis. Direct methane conversion involves various reactions, such as oxidative coupling to ethylene (OCM), formaldehyde, and anaerobic aromatization of methane. Direct methane conversion offers advantages such as a short process flow, low energy consumption, and low cost. However, designing highly active catalysts for the oxidation of CH4 to organic oxygen-containing compounds under mild conditions remains a challenge. This is because the active surfaces of catalysts used for CH4 oxidation at low temperatures are easily poisoned by oxygen or hydrogen peroxide. Even at low temperatures, O2 and H2O can easily deactivate the catalytic center through the immediate oxidation of supported metal clusters. From this perspective, encapsulating the active center in a microporous environment is a better option for CH4 oxidation conversion under mild conditions. In fact, most catalysts for the catalytic conversion of CH4 under mild conditions have their catalytic centers encapsulated in the micropores of silica-alumina molecular sieves, such as ZSM-5 (J. Am. Chem. Soc., 2023, 145, 5888), SSZ-13 (Fuel, 2022, 309, 122178), and MOR (J. Am. Chem. Soc., 2023, 145(23): 12928), containing metal atoms or in metal-oxygen clusters in MOFs (ACS Catal., 2022, 12, 11159). However, the introduction of Al limits the subsequent Fe content and may promote side reactions such as ethane / propane dehydrogenation and dry reforming of methane. Therefore, pure silica zeolite is an ideal support for the catalyst in the methane oxidation reaction. This invention improves the Fe content in the molecular sieve through dissolution and recrystallization and successfully applies it to the methane oxidation reaction. Summary of the Invention

[0004] The purpose of this invention is to provide an Fe-based catalyst for the synthesis of formic acid under mild conditions and its preparation method. Fe is introduced into the framework and channels of pure silicon molecular sieves to obtain an Fe-based catalyst for the oxidation of methane to formic acid under mild (70 °C) conditions.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing an Fe-based catalyst for the synthesis of formic acid under mild conditions includes the following steps:

[0007] (1) Pure Si molecular sieve, template agent and water are mixed and subjected to high temperature hydrothermal treatment to obtain precursor sol, and then cooled to room temperature;

[0008] (2) The aqueous solution containing Fe source is slowly introduced into the precursor sol, stirred evenly, and then redetermined by high-temperature hydrothermal treatment;

[0009] (3) The solution obtained in step (2) is centrifuged, washed and dried, ground into powder and then calcined in a high-temperature atmosphere to obtain the Fe-based catalyst.

[0010] Furthermore, the topological structure of the pure Si molecular sieve in step (1) is any one of MFI, BEA, and CHA.

[0011] Further, the template agent mentioned in step (1) is one of tetrapropylammonium hydroxide, tetraethylammonium hydroxide and N,N,N-trimethyladamantaneammonium hydroxide.

[0012] Further, in step (1), the molar ratio of pure Si molecular sieve, template agent and water is 1:(0.2-2.0):(10-50).

[0013] Furthermore, the hydrothermal treatment temperature in step (1) is 100-230°C. o C, the hydrothermal treatment time is 3-6 hours.

[0014] Furthermore, the hydrothermal treatment temperature in step (2) is 140-230°C. o C, the high-temperature hydrothermal treatment time is 24-72 hours.

[0015] Furthermore, the Fe source mentioned in step (2) is one of Fe(NO3)3·9H2O, FeCl3·6H2O or EDTA-Fe.

[0016] Furthermore, the roasting temperature in step (3) is 450-900 °C and the time is 6-12 h.

[0017] The above preparation method yields an Fe-based catalyst for the synthesis of formic acid under mild conditions.

[0018] The Fe content in the Fe-based catalyst is 0.1-1.5 wt%.

[0019] Application of the above-mentioned Fe-based catalyst in the selective oxidation of methane to formic acid.

[0020] The beneficial effects of this invention are:

[0021] (1) The present invention successfully implants highly dispersed Fe into the molecular sieve framework and channels through a synthesis method of dissolution and secondary crystallization, and exhibits excellent formic acid generation activity in the selective oxidation of methane to formic acid.

[0022] (2) The pure Si-type Fe-based catalyst prepared by the present invention has high activity under mild conditions, providing an efficient catalyst for the selective oxidation of methane to formic acid, thereby promoting the further development of the selective oxidation of methane technology. Attached Figure Description

[0023] Figure 1 The images show the XRD patterns of the catalysts obtained in Example 1 and Comparative Example 1.

[0024] Figure 2 The images show the UV-Vis spectra of the catalysts obtained in Example 1 and Comparative Example 1.

[0025] Figure 3 The images show TEM images of the catalysts obtained in Example 1 (A) and Comparative Example 1 (B); where red represents Fe, cyan represents O, and green represents Si.

[0026] Figure 4 The CH4-TPD diagrams are of the catalysts obtained in Example 1 and Comparative Example 2.

[0027] Figure 5 The images show the XRD patterns of the catalyst obtained in Example 1 before and after the reaction.

[0028] Figure 6 The images show the UV-Vis spectra of the catalyst obtained in Example 1 before and after the reaction.

[0029] Figure 7 This is a graph showing the cycle stability evaluation of the catalyst obtained in Example 1. Detailed Implementation

[0030] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto.

[0031] Example 1 0.5Fe-MFI

[0032] 4.48 g of tetrapropylammonium hydroxide and 39.63 g of water were mixed and stirred until homogeneous. Then, 25.51 g of tetraethyl orthosilicate was slowly added dropwise, and stirring continued until the solution became clear and transparent. The solution was then placed in a 50 °C water bath and stirred for 30 min. The temperature was then increased to 80 °C, and the solution was stirred continuously to remove the ethanol generated from the hydrolysis of tetraethyl orthosilicate. Water was added continuously during the process to maintain a constant total mass. The solution was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated for 48 h to crystallize the solution. After crystallization, the solution was filtered and washed with deionized water until neutral. It was then dried at 80 °C overnight, ground into powder, and calcined in air at 550 °C for 6 h at a heating rate of 2 °C / min to obtain Silicalite-1 molecular sieve.

[0033] 3 g of Silicalite-1 molecular sieve, 10.17 g of tetrapropylammonium hydroxide, and 30.50 mL of water were mixed thoroughly and stirred at room temperature for 2 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.0720 g of Fe(NO3)3·9H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and dynamically hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). After the reaction, the product was washed with water and centrifuged until neutral. It was then dried at 80 °C overnight and ground into powder. The powder was calcined at 550 °C in air at a heating rate of 2 °C / min for 6 h to obtain a 0.5Fe-MFI catalyst (with a Fe content of 0.5 wt%).

[0034] Example 2 0.3Fe-MFI

[0035] The synthesis steps of Silicalite-1 molecular sieve were the same as in Example 1. 3 g of Silicalite-1 molecular sieve, 10.17 g of tetrapropylammonium hydroxide, and 30.50 mL of water were mixed thoroughly and stirred at room temperature for 2 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.0432 g of Fe(NO3)3·9H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). Finally, after repeated washing with water and centrifugation to neutrality, the mixture was dried at 80 °C overnight and ground into powder. The powder was then calcined at 550 °C in air atmosphere for 6 h at a heating rate of 2 °C / min to obtain a 0.3Fe-MFI catalyst (with a Fe content of 0.3 wt%).

[0036] Example 3 1.2Fe-MFI

[0037] The synthesis steps of Silicalite-1 molecular sieve were the same as in Example 1. 3 g of Silicalite-1 molecular sieve, 10.17 g of tetrapropylammonium hydroxide, and 30.50 mL of water were mixed thoroughly and stirred at room temperature for 2 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.1728 g of Fe(NO3)3·9H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). Finally, after repeated washing with water and centrifugation to neutrality, the mixture was dried at 80 °C overnight and ground into powder. The powder was then calcined at 550 °C in air atmosphere for 6 h at a heating rate of 2 °C / min to obtain a 1.2Fe-MFI catalyst (with a Fe content of 1.2 wt%).

[0038] Example 4 1.5Fe-MFI

[0039] The synthesis steps of Silicalite-1 molecular sieve were the same as in Example 1. 3 g of Silicalite-1 molecular sieve, 10.17 g of tetrapropylammonium hydroxide, and 30.50 mL of water were mixed thoroughly and stirred at room temperature for 2 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.2160 g of Fe(NO3)3·9H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). Finally, after repeated washing with water and centrifugation to neutrality, the mixture was dried at 80 °C overnight and ground into powder. The powder was then calcined at 550 °C in air atmosphere for 6 h at a heating rate of 2 °C / min to obtain a 1.5Fe-MFI catalyst (with a Fe content of 1.5 wt%).

[0040] Example 5 0.5Fe-MFI-FeCl3

[0041] The synthesis steps of Silicalite-1 molecular sieve were the same as in Example 1. 3 g of Silicalite-1 molecular sieve, 10.17 g of tetrapropylammonium hydroxide, and 30.50 mL of water were mixed thoroughly and stirred at room temperature for 2 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.048 g of FeCl3·6H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). Finally, after repeated washing with water and centrifugation to neutrality, the mixture was dried at 80 °C overnight and ground into powder. The powder was then calcined at 550 °C in air atmosphere for 6 h at a heating rate of 2 °C / min to obtain a 0.5Fe-MFI-FeCl3 catalyst (where the Fe content was 0.5 wt%).

[0042] Example 6 0.5Fe-BEA

[0043] 1.5 g sodium hydroxide, 0.075 g silicon dioxide seed crystals, 5.9 g tetraethylammonium hydroxide, and 7.5 g fumed silica were sequentially added to a mortar, mixed thoroughly, and ground for 30 min. The resulting powder was then transferred to a polytetrafluoroethylene-lined high-pressure reactor and hydrothermally heated at 145 °C for 16 h. Afterward, the powder was washed with deionized water until neutral, dried overnight at 110 °C, and then ground to obtain the final powder. 5 g of the powder was then ion-exchanged in 100 g of a 1.0 mol / L ammonium nitrate solution at 90 °C for 3 h, centrifuged, washed three times with deionized water, dried overnight at 80 °C, and ground into powder. The powder was then calcined in air at 550 °C for 6 h at a heating rate of 2 °C / min to obtain pure Si BEA molecular sieve.

[0044] 3 g of pure Si BEA molecular sieve, 7.36 g of tetraethylammonium hydroxide, and 30.60 mL of water were mixed uniformly and stirred at room temperature for 2 h. The mixture was then transferred to a PTFE-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.072 g of Fe(NO3)3·9H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a PTFE-lined autoclave and hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). Finally, after repeated washing with water and centrifugation to neutrality, the mixture was dried at 80 °C overnight and ground into powder. The powder was then calcined at 550 °C in air atmosphere for 6 h at a heating rate of 2 °C / min to obtain a 0.5Fe-BEA catalyst (with a Fe content of 0.5 wt%).

[0045] Example 7 0.5Fe-CHA

[0046] 1.06 g of N,N,N-trimethyladamantane ammonium hydroxide and 5.4 g of water were mixed and stirred until homogeneous. 6 g of fumed silica was slowly added and stirred for 30 min, then aged at room temperature for 24 h. 0.1 g of hydrofluoric acid was then slowly added dropwise while stirring. The resulting gel was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 150 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). After hydrothermal treatment, the gel was repeatedly washed with water and centrifuged until neutral. It was then dried at 100 °C overnight, ground into powder, and calcined in air at 700 °C for 8 h at a heating rate of 1 °C / min to obtain pure Si CHA molecular sieves.

[0047] 3 g of pure Si CHA molecular sieve, 10.57 g of N,N,N-trimethyladamantane ammonium hydroxide, and 31.70 mL of water were mixed uniformly and stirred at room temperature for 2 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 170 °C for 3 h to obtain a precursor sol. After cooling to room temperature, an aqueous solution containing 0.0720 g of Fe(NO3)3·9H2O was slowly introduced into the precursor sol and stirred at room temperature for 3 h. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated at 200 °C for 48 h (equipment: Yamato Constant Temperature Oven DKN312C, Japan). Finally, after repeated washing with water and centrifugation to neutrality, the mixture was dried at 80 °C overnight and ground into powder. The powder was then calcined at 550 °C in air atmosphere for 6 h at a heating rate of 2 °C / min to obtain a 0.5Fe-CHA catalyst (with a Fe content of 0.5 wt%).

[0048] Comparative Example 1 0.5Fe / MFI-IWI

[0049] The synthesis steps of Silicalite-1 molecular sieve were the same as in Example 1. 0.036 g of Fe(NO3)3·9H2O was dissolved in 600 µL of water and sonicated for 15 min. This solution was then impregnated into 1 g of Silicalite-1 molecular sieve by an equal volume, aged at room temperature for 3 h, and then dried overnight at 80°C. After thorough grinding, the solution was calcined in air at 550°C for 6 h at a heating rate of 2 °C / min to obtain the 0.5Fe / MFI-IWI catalyst (where the Fe content was 0.5 wt%).

[0050] Comparative Example 2 0.5Fe / ZSM-5-IWI

[0051] 6.23 g of tetrapropylammonium hydroxide and 83.75 g of water were mixed and stirred until homogeneous. Then, 25.51 g of tetraethyl orthosilicate was slowly added dropwise while stirring until clear and transparent. Next, 0.34 g of sodium aluminate was added and stirring continued for 30 min. The solution was then placed in a 50 °C water bath and stirred for 30 min. The temperature was then increased to 80 °C, and the solution was continuously stirred to remove the ethanol generated from the hydrolysis of tetraethyl orthosilicate. Water was added continuously during this process to maintain a constant total mass. The solution was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally heated for 72 h to crystallize the solution. After crystallization, the solution was filtered and washed with deionized water until neutral. It was then dried at 80 °C overnight and ground into powder. The powder was calcined at 550 °C in air at a heating rate of 2 °C / min for 6 h. 5 g of the prepared powder was weighed and added to 100 mL of 1 mol / L ammonium nitrate solution. The mixture was stirred in an 80 °C water bath for 2 h and then centrifuged. This process was repeated three times. After centrifugation, the solid was dried at 80 °C overnight and then ground into powder. The powder was then calcined in air at 550 °C for 6 h at a heating rate of 2 °C / min to obtain ZSM-5 (Si:Al=30) molecular sieve.

[0052] 0.036 g of Fe(NO3)3·9H2O was dissolved in 600 µL of water and sonicated for 15 min. The solution was then impregnated with an equal volume of 1 g of ZSM-5 (Si:Al=30) molecular sieve, aged at room temperature for 3 h, and then dried overnight at 80 °C. After thorough grinding, the solution was calcined in air at 550 °C for 6 h at a heating rate of 2 °C / min to obtain the 0.5Fe / ZSM-5-IWI catalyst (where the Fe content was 0.5 wt%).

[0053] Figure 1The figures show the XRD patterns of the catalysts obtained in Example 1 and Comparative Example 1. As can be seen from the figures, both Example 1 and Comparative Example 1 conform to the MFI configuration standard card, indicating that the introduction of Fe does not change the molecular sieve crystal structure. Furthermore, no Fe-related diffraction characteristic peaks were detected in any of the catalysts, which may be due to the Fe species being below the XRD detection limit.

[0054] Figure 2 The images show the UV-Vis spectra of the catalysts obtained in Example 1 and Comparative Example 1. As can be seen from the images, a large amount of oligomeric Fe can be observed in 0.5Fe / MFI-IWI (Comparative Example 1). x O y Species (350-500 nm) and aggregated Fe2O3 particles (>500 nm); 0.5Fe-MFI (Example 1) showed no obvious aggregation, with most of the Fe particles existing as isolated Fe on the framework. 3+ Species (<250 nm) and Fe isolated outside the skeleton 3+ The species (250-350 nm) are present. This indicates that the Fe species in Example 1 enter the molecular sieve framework and channels, are well dispersed, and do not aggregate.

[0055] Figure 3 The images show TEM images of the catalysts obtained in Example 1 (A) and Comparative Example 1 (B). As can be seen from the images, the 0.5Fe-MFI (Example 1) catalyst has a smooth surface with a distinct square outline, and no Fe nanoparticles were observed, indicating that Fe has good dispersion on the catalyst. Further analysis of the mapping shows that Si, O, and Fe are uniformly distributed, with no Fe species aggregation and most of them located inside the molecular sieve. In contrast, the 0.5Fe / MFI-IWI catalyst exhibits a square shape but a rough surface with a slightly elliptical morphology, and Fe nanoparticles can be observed on its surface. x O y Nanoparticles. Further analysis of the mapping shows that Si and O are uniformly distributed, but Fe species clearly aggregate on the catalyst surface, indicating that 0.5Fe / MFI forms Fe species oligomers or clusters.

[0056] Figure 4 The figures show the CH4-TPD spectra of the catalysts obtained in Example 1 and Comparative Example 1. As can be seen from the figures, the characteristic absorption peak of the 0.5Fe / MFI-IWI (Comparative Example 1) catalyst is located at 92°C. o C, with a smaller peak area, indicates that 0.5Fe / MFI has difficulty adsorbing large amounts of methane. In contrast, the characteristic absorption peak of the 0.5Fe-MFI (Example 1) catalyst is located at 122. oC, with a peak area much larger than that of 0.5Fe / MFI-IWI, indicates that 0.5Fe-MFI exhibits good adsorption performance for methane and can provide more methane adsorption sites.

[0057] The catalysts synthesized in the above examples and comparative examples were used to synthesize formic acid.

[0058] Application Example 1

[0059] 20 mg of 0.5Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 1 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0060] Application Example 2

[0061] 20 mg of 0.5Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0062] Application Example 3

[0063] 20 mg of 0.5Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 3 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0064] Application Example 4

[0065] 20 mg of 0.5Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 4 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0066] Application Example 5

[0067] 20 mg of 0.3Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0068] Application Example 6

[0069] 20 mg of 1.2Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0070] Application Example 7

[0071] 20 mg of 1.5Fe-MFI catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0072] Application Example 8

[0073] 20 mg of 0.5Fe-MFI-FeCl3 catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0074] Application Example 9

[0075] 20 mg of 0.5Fe-BEA catalyst, 15 mL of water and 0.5667 g of hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0076] Application Example 10

[0077] 20 mg of 0.5Fe-CHA catalyst, 15 mL of water and hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0078] Application Comparative Example 1

[0079] 20 mg of 0.5Fe / MFI-IWI catalyst, 15 mL of water and hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0080] Application Comparative Example 2

[0081] 20 mg of 0.5Fe / ZSM-5-IWI catalyst, 15 mL of water and hydrogen peroxide (30% aqueous solution) were added to a 50 mL batch reactor, and methane gas was introduced at 2 MPa. The reaction was carried out at 70 °C for 30 min. The reaction results are shown in Table 1.

[0082] Table 1. Experimental results of methane oxidation to formic acid in application examples and comparative examples.

[0083]

[0084] As can be seen from the table, the space-time yield and selectivity for formic acid preparation using the catalyst synthesized by the equal-volume impregnation method (using Comparative Examples 1 and 2) are both lower than those in Examples 1-10, indicating that the catalyst synthesized by this method has high activity in the selective oxidation of methane to formic acid. Furthermore, Fe metal itself can simultaneously enhance both methane adsorption sites and methane activation sites without introducing a second auxiliary metal.

[0085] Figure 5 The images show the XRD patterns of the catalyst obtained in Example 1 before and after the reaction. The images show that the catalyst retains the MFI configuration after the reaction, and no diffraction characteristic peaks related to the Fe species were observed.

[0086] Figure 6 The images show the UV-Vis spectra of the catalyst obtained in Example 1 before and after the reaction. No oligomeric iron oxides or iron oxide agglomerates were observed in the catalyst after the reaction, and the peak distribution of the original catalyst remained basically unchanged, indicating that the Fe species in the catalyst after the reaction can maintain their original state well.

[0087] Figure 7 The graph shows the cyclic stability evaluation of the catalyst obtained in Example 1. As can be seen from the graph, the catalytic activity remains at a certain level in each cycle, and the formic acid selectivity is greater than 90%, indicating that the catalyst has good cyclic stability.

[0088] The specific embodiments described above are further explanations of the technical solutions and beneficial effects of the present invention, and are not intended to limit the implementation methods. All equivalent changes and modifications made within the scope of the claims of this invention should be considered within the scope of this invention.

Claims

1. A method for preparing an Fe-based catalyst for the synthesis of formic acid under mild conditions, characterized in that: Includes the following steps: (1) Pure Si molecular sieve, template agent and water are mixed and subjected to high temperature hydrothermal treatment to obtain precursor sol, and then cooled to room temperature; (2) The aqueous solution containing Fe source is slowly introduced into the precursor sol, stirred evenly, and then redetermined by high-temperature dynamic hydrothermal treatment; (3) The precipitate obtained in step (2) is centrifuged, washed and dried, ground into powder and then calcined in a high-temperature atmosphere to obtain the Fe-based catalyst; When the topology of the pure Si molecular sieve is MFI, the template agent is tetrapropylammonium hydroxide; when the topology of the pure Si molecular sieve is BEA, the template agent is tetraethylammonium hydroxide; when the topology of the pure Si molecular sieve is CHA, the template agent is N,N,N-trimethyladamantaneammonium hydroxide.

2. The preparation method according to claim 1, characterized in that: The molar ratio of pure Si molecular sieve, template agent and water in step (1) is 1:(0.2-2.0):(10-50).

3. The preparation method according to claim 1, characterized in that: The hydrothermal treatment temperature in step (1) is 100-230 ℃, and the hydrothermal treatment time is 3-6 h.

4. The preparation method according to claim 1, characterized in that: The dynamic hydrothermal treatment temperature in step (2) is 140-230 ℃, and the hydrothermal treatment time is 24-72 h.

5. The preparation method according to claim 1, characterized in that: The Fe source mentioned in step (2) is one of Fe(NO3)3·9H2O, FeCl3·6H2O or EDTA-Fe.

6. The preparation method according to claim 1, characterized in that: The roasting temperature in step (3) is 450-900 ℃ and the time is 6-12 h.

7. The Fe-based catalyst obtained by the preparation method according to any one of claims 1 to 6, characterized in that: The Fe content in the Fe-based catalyst is 0.1-1.5 wt%.

8. The application of the Fe-based catalyst as described in claim 7 in the selective oxidation of methane to formic acid.

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