A method for improving the performance of Fe-ZSM-5 in catalyzing selective oxidation of methane to formic acid by pretreatment in a reducing atmosphere

Abstract

This invention discloses a method for improving the performance of Fe-ZSM-5 catalyzed selective oxidation of methane to formic acid through reducing atmosphere pretreatment, belonging to the field of catalyst preparation and methane catalytic conversion technology. The preparation method of the Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid includes the following steps: activating the Fe-ZSM-5 molecular sieve to be activated under a reducing atmosphere to complete the preparation. This invention, through reducing atmosphere activation treatment, significantly improves the performance of the Fe-ZSM-5 catalyst in the selective oxidation of methane to formic acid reaction. The dispersion of iron species in the catalyst is significantly improved, the original large-size Fe2O3 agglomerates are greatly reduced, and the iron species mainly take the form of highly active isolated Fe. 3+ It exists in ionic or small-sized Fe-O cluster form. After testing the selective oxidation of methane to formic acid under mild reaction conditions, it was found that the catalyst's methane conversion activity and selectivity for the target product, formic acid, were simultaneously improved.

Description

Technical Field

[0001] This invention relates to the fields of catalyst preparation and methane catalytic conversion technology, and in particular to a method for improving the selective oxidation performance of methane to formic acid by Fe-ZSM-5 catalyst through reducing atmosphere pretreatment. Background Technology

[0002] Methane is a major component of resources such as natural gas and shale gas, and is not only abundant but also a clean and high-quality energy option. However, the application value of natural resources like natural gas and shale gas is limited due to difficulties in transportation and high operating costs. Furthermore, the methane emitted from coal mines contains a certain amount of gas, which is often directly burned, resulting in resource waste and exacerbating the greenhouse effect. In traditional industrial utilization pathways, methane typically needs to be first converted into syngas (H2+CO) through steam reforming, followed by Fischer-Tropsch synthesis to prepare the target product. However, this process requires high temperature and pressure, is cumbersome, complex, and costly. Therefore, developing a method to directly convert methane into high-value-added products is of great significance.

[0003] The selective oxidation of methane to prepare oxygen-containing compounds faces the following challenges: First, the activation of the C-H bonds in methane is difficult. Due to the highly symmetrical tetrahedral configuration of the methane molecule, its C-H bonds are difficult to polarize. Furthermore, the high HOMO-LUMO band gap (9.91 eV) and ionization energy (12.61 eV) hinder electron transfer during catalysis. Second, the selectivity of the reaction products is poor. The dissociation energy of the methane C-H bonds is high (440 kJ / mol), while the resulting oxygen-containing compounds (such as methanol with a C-H bond dissociation energy of 388.4 kJ / mol) are typically more reactive, easily undergoing over-oxidation to generate CO2, leading to low selectivity of the target product. Therefore, achieving efficient activation and directional transformation of the methane C-H bonds is crucial for the selective oxidation of methane to prepare oxygen-containing compounds.

[0004] Methanol, formaldehyde, formic acid, and acetic acid, among other low-carbon oxygen-containing compounds, are important basic chemical raw materials, widely used in olefins, aromatics, polymer materials, resins, pesticides, pharmaceuticals, fuels, and food. Therefore, the on-site selective conversion of methane into these high-value-added liquid oxygen-containing chemical products can reduce natural gas transportation costs and help reduce greenhouse gas emissions, thus having a positive impact on improving natural gas utilization and optimizing the energy structure.

[0005] In nature, methane monooxygenases (MMOs) can efficiently catalyze the selective oxidation of methane to methanol under aerobic conditions. MMOs mainly include two types: soluble methane monooxygenases (sMMOs) and particulate methane monooxygenases (pMMOs), with their active centers being binuclear iron structures and copper-based structures, respectively. However, limitations in temperature, pressure, and solvent tolerance of the enzyme catalytic system restrict their practical application in industrial processes. Therefore, researchers in this field urgently need to develop heterogeneous catalysts with both high activity and high selectivity to mimic the catalytic function of MMOs. Among them, the Fe-ZSM-5 catalyst shows good potential in this reaction due to its suitable acidity, regular pore structure, and tunable active iron species.

[0006] Currently, Fe-ZSM-5 is mainly prepared using impregnation, ion exchange, or hydrothermal synthesis methods. However, these methods still have several problems: First, the iron species are unevenly dispersed, easily agglomerating on the surface of the support (ZSM-5) to form low-activity Fe2O3 clusters or particles, resulting in a reduction in the number of highly active mononuclear or binuclear iron species. Second, the ability to regulate the structure of active sites is limited, making it difficult to precisely control the coordination environment, redox properties, and interaction with the support of iron, thus restricting the simultaneous improvement of catalyst activity and selectivity. Although some studies have attempted to modify Fe-ZSM-5 by adjusting the iron species introduction method, optimizing distribution, changing synthesis methods, or adding additives, the preparation process is often complex and lacks specificity for the methane oxidation to formic acid reaction, resulting in limited performance improvement. Therefore, developing a simple, efficient, and precisely controllable pretreatment method for the active site structure of Fe-ZSM-5 is of great significance for promoting the development of the selective methane oxidation to formic acid technology. Summary of the Invention

[0007] The purpose of this invention is to provide a method for improving the performance of Fe-ZSM-5 catalyst in the selective oxidation of methane to formic acid through reducing atmosphere pretreatment, thereby solving the aforementioned problems in the background art. This invention significantly improves the performance of the Fe-ZSM-5 catalyst in the selective oxidation of methane to formic acid through reducing atmosphere activation treatment, and the dispersion of iron species in the catalyst is significantly improved.

[0008] To achieve the above objectives, the present invention provides the following technical solution: One of the technical solutions of this invention is to provide a method for preparing a Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid, comprising the following steps: The Fe-ZSM-5 molecular sieve to be activated was activated under a reducing atmosphere to obtain the Fe-ZSM-5 molecular sieve catalyst.

[0009] Preferably, the reducing atmosphere is ethylene, carbon monoxide, ammonia, methane, or a hydrogen / argon mixture, and the gas flow rate is 30–100 mL / min.

[0010] Preferably, the activation treatment temperature is 300–700°C, and the activation time is 1–12 hours.

[0011] Preferably, the preparation method of the Fe-ZSM-5 molecular sieve to be activated includes the following steps: A mixture of silicon source, aluminum source, iron source, template agent and water was prepared and stirred to obtain Fe-ZSM-5 precursor gel. The gel was then crystallized and calcined to obtain the calcined product. The calcination product was subjected to ammonia exchange treatment to obtain the Fe-ZSM-5 molecular sieve to be activated.

[0012] Preferably, the silicon source is one or more selected from tetraethyl orthosilicate, silica sol, and silica. The aluminum source is one or more of sodium aluminate, aluminum nitrate and aluminum sulfate; The iron source is one or more of ferric nitrate, ferric chloride, ferric sulfate, ferric oxalate, and ferric acetylacetone. The template agent is tetrapropylammonium hydroxide or tetrapropylammonium bromide; The molar ratio of Si in the silicon source to Al in the aluminum source is 20–200; the molar ratio of Fe in the iron source to Si in the silicon source is 0.001–0.01; and the molar ratio of the template agent to Si in the silicon source is 0.1–0.3.

[0013] Preferably, the crystallization reaction temperature is 150–180°C, and the crystallization time is 24–72 hours; The calcination conditions are as follows: in an air atmosphere, the temperature is increased to 450-550°C at a heating rate of 2-5°C / min, and held for 3-8 hours.

[0014] Preferably, the ammonia exchange treatment includes the following steps: (1) The roasted product is mixed with NH4NO3 solution and subjected to ammonia exchange at 50-80°C for 1-3 hours. After solid-liquid separation and washing with water, the ammonia exchange product is obtained. (2) Repeat step (1) 2 to 4 times to obtain the Fe-ZSM-5 molecular sieve to be activated; The mass-to-volume ratio (g / mL) of the calcined product to the NH4NO3 solution is 0.1–1; the concentration of NH4NO3 in the NH4NO3 solution is 0.5–2 mol / L.

[0015] The second technical solution of the present invention provides a Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid, obtained according to the above preparation method.

[0016] The third technical solution of the present invention provides an application of the above-mentioned Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid in the field of selective oxidation of methane to formic acid.

[0017] The fourth technical solution of the present invention provides a method for improving the performance of Fe-ZSM-5 catalytic selective oxidation of methane to formic acid through reducing atmosphere pretreatment, using the above-mentioned Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid as the catalytic component, comprising the following steps: Methane, an aqueous hydrogen peroxide solution, and the Fe-ZSM-5 molecular sieve catalyst were mixed and reacted to complete the preparation of formic acid. The concentration of hydrogen peroxide in the hydrogen peroxide aqueous solution is 0.2–2 mol / L, and the partial pressure of methane during the reaction process is 3 MPa; The reaction is carried out at a temperature of 50–80°C, a pressure of 0.5–3 MPa, and a time of 15–60 minutes.

[0018] The technical principle of this invention is as follows: 1. The crystallization reaction of this invention can introduce iron species into the ZSM-5 framework, followed by ammonium exchange and activation in a reducing atmosphere, thereby achieving precise control over the structure of the Fe active center and the surface properties of the catalyst: 2. Differential activation of Fe species by reducing atmosphere: The effects of different reducing gases on Fe 3+ The reducing power of each element differs. The stronger the reducing power (e.g., H2), the more effectively it promotes the deep reduction and redispersion of Fe species, forming more highly active isolated Fe. 3+ or partially reduced Fe 2+ Species, optimizing their electronic structure and coordination environment. The stronger the reducing power, the stronger the selective oxidation performance of methane to formic acid.

[0019] 3. Acidity control and surface cleaning: The reducing atmosphere can partially reduce the skeletal aluminum species, reduce the strong acid sites, and at the same time remove residual organic matter and adsorbed water on the surface, purify the pores, and improve mass transfer.

[0020] 4. Synergistic enhancement of catalytic performance: The preparation process defined in this invention optimizes the accessibility of Fe active centers, electron density, and redox cycle efficiency of the product, thereby significantly improving the activation ability of methane and the selectivity for formic acid under low-temperature liquid phase conditions.

[0021] The beneficial technical effects of the present invention are as follows: This invention significantly improves the performance of the Fe-ZSM-5 catalyst in the selective oxidation of methane to formic acid through reducing atmosphere pretreatment. Characterization results from transmission electron microscopy (TEM), ultraviolet-visible absorption spectroscopy (UV-Vis), and X-ray absorption fine structure (XAFS) show that after appropriate reducing atmosphere treatment, the dispersion of iron species in the catalyst is significantly improved, the original large-sized Fe2O3 agglomerates are greatly reduced, and the iron species mainly exist as highly active isolated Fe. 3+ It exists in the form of ions or small-sized Fe-O clusters.

[0022] After testing the selective oxidation of methane to formic acid under mild reaction conditions, it was found that the catalyst's methane conversion activity and selectivity for the target product, formic acid, were simultaneously improved. Example data show that activation treatment with a reducing gas significantly increased the formic acid yield of the catalyst compared to untreated samples or samples treated in an air atmosphere.

[0023] Ammonia-programmed temperature desorption (NH3-TPD) analysis confirmed that this activation process can moderately modulate the acid strength distribution of the molecular sieve support, reducing the number of strong acid sites, thereby effectively inhibiting the excessive decomposition or deep oxidation of formic acid products during the reaction. This method is simple to operate, operates under mild conditions, and is easy to scale up, making it highly valuable for practical applications. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the preparation and performance evaluation process of the Fe-ZSM-5 molecular sieve catalyst of the present invention.

[0026] Figure 2 The FT-EXAFS spectra of Fe-ZSM-5(H2 / Ar) and Fe-ZSM-5(Air) are shown.

[0027] Figure 3 The UV-Vis spectra of Fe-ZSM-5(H2 / Ar) and Fe-ZSM-5(Air) are shown.

[0028] Figure 4 The TEM mapping spectrum of Fe-ZSM-5(H2 / Ar).

[0029] Figure 5 This is the TEM mapping spectrum of Fe-ZSM-5(Air). Detailed Implementation

[0030] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0031] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, are also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0032] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. It should be noted that any aspects of this invention not described in detail are conventional practices in the art and are not the focus of this invention.

[0033] The terms “comprising,” “including,” “having,” “containing,” etc., used in this invention are all open-ended terms, meaning that they include but are not limited to.

[0034] This invention discloses a method for preparing a Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid, comprising the following steps: S1: Preparation of Fe-ZSM-5 precursor gel A mixture of silicon source, aluminum source, iron source, template agent and water was prepared and stirred at room temperature for 2 to 12 hours to ensure thorough mixing and preliminary hydrolysis, thereby obtaining Fe-ZSM-5 precursor gel. The silicon source is one or more of tetraethyl orthosilicate, silica sol, and silica. The aluminum source is one or more of sodium aluminate, aluminum nitrate and aluminum sulfate; The iron source is one or more of ferric nitrate (Fe(NO3)3), ferric chloride (FeCl3), ferric sulfate (Fe2(SO4)3), ferric oxalate (Fe2(C2O4)3), and ferric acetylacetone (Fe(C5H7O2)3), preferably ferric oxalate; The template agent is tetrapropylammonium hydroxide or tetrapropylammonium bromide; The molar ratio (Si / Al) of Si in the silicon source to Al in the aluminum source is 20-200, preferably 30-80; the molar ratio of Fe in the iron source to Si in the silicon source is 0.001-0.01, preferably 0.002-0.005; and the molar ratio of the template agent to Si in the silicon source is 0.1-0.3. S2: Crystallization reaction The Fe-ZSM-5 precursor gel described in step S1 was transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene for crystallization. After crystallization, the product was naturally cooled to room temperature, centrifuged or filtered, washed with water until the filtrate was neutral, and ground to obtain white precursor I. The crystallization reaction is carried out at a temperature of 150–180°C for 24–72 hours. S3: Catalyst calcination and ammonia exchange (1) The precursor I is calcined to obtain the calcined product; (2) The calcined product is placed in NH4NO3 solution and subjected to ammonia exchange in a water bath at 50-80°C for 1-3 hours. The product is then separated by centrifugation or filtration and washed with water until the filtrate is neutral to obtain the ammonia exchange product. (3) Repeat step (2) 2 to 4 times, and then dry at 80 to 120°C for 6 to 12 hours to obtain the white Fe-ZSM-5 molecular sieve to be activated; The calcination conditions are as follows: in an air atmosphere, the temperature is raised to 450-550°C at a heating rate of 2-5°C / min, and held for 3-8 hours. The mass-to-volume ratio (g / mL) of the calcined product to the NH4NO3 solution is 0.1–1; the concentration of NH4NO3 in the NH4NO3 solution is 0.5–2 mol / L. S4: Atmospheric activation of the catalyst The Fe-ZSM-5 molecular sieve to be activated was placed in a tube furnace and activated under a reducing atmosphere. After activation, it was cooled to room temperature under an inert atmosphere to obtain the Fe-ZSM-5 molecular sieve catalyst. The reducing atmosphere is ethylene (C2H4), carbon monoxide (CO), ammonia (NH3), methane (CH4) or a hydrogen / argon mixture (H2 / Ar), and the gas flow rate is 30-100 mL / min. The activation treatment temperature is 300-700℃, preferably 550℃, and the activation time is 1-12 hours.

[0035] The present invention also discloses the application of the above-mentioned Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid in the field of selective oxidation of methane to formic acid.

[0036] This invention also discloses a method for improving the performance of Fe-ZSM-5 catalytic selective oxidation of methane to formic acid through reducing atmosphere pretreatment, using the above-mentioned Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid as the catalytic component, comprising the following steps: Methane, an aqueous hydrogen peroxide solution, and the Fe-ZSM-5 molecular sieve catalyst were mixed and reacted to complete the preparation of formic acid. The concentration of hydrogen peroxide in the hydrogen peroxide aqueous solution is 0.2–2 mol / L, and the partial pressure of methane during the reaction process is 3 MPa; The reaction is carried out at a temperature of 50–80°C, a pressure of 0.5–3 MPa, and a time of 15–60 minutes.

[0037] Unless otherwise specified, "room temperature" in this invention refers to 10-30°C.

[0038] All raw materials used in the following embodiments and comparative examples of the present invention are commercially available products.

[0039] Example 1 A method for preparing a Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid includes the following steps: S1: Preparation of Fe-ZSM-5 precursor gel A mixture of silicon source, aluminum source, iron source, template agent and water was prepared and stirred at room temperature for 12 hours to fully mix and preliminarily hydrolyze the mixture, thus obtaining Fe-ZSM-5 precursor gel. The silicon source is tetraethyl orthosilicate; The aluminum source is aluminum nitrate; The iron source is ferric oxalate; The template agent is tetrapropylammonium hydroxide. The molar ratio of Si in the silicon source to Al in the aluminum source is 30; the molar ratio of Fe in the iron source to SiO2 in the silicon source is 0.01; and the molar ratio of the template agent to SiO2 in the silicon source is 0.2. S2: Crystallization reaction The Fe-ZSM-5 precursor gel from step S1 was transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene for crystallization. After crystallization, the product was naturally cooled to room temperature, centrifuged or filtered, washed with water until the filtrate was neutral, and ground to obtain white precursor I. The crystallization reaction temperature was 180℃, and the crystallization time was 72 hours. S3: Catalyst calcination and ammonia exchange (1) Precursor I was calcined to obtain the calcined product; (2) The calcined product was placed in NH4NO3 solution and subjected to ammonia exchange in a water bath at 80°C for 3 hours. The product was then centrifuged and washed with water until the filtrate was neutral to obtain the ammonia exchange product. (3) Repeat step (2) 4 times, and then dry at 80°C for 12 hours to obtain white Fe-ZSM-5 molecular sieve to be activated (the Fe content is 0.2 wt%). The calcination conditions were as follows: in an air atmosphere, the temperature was increased to 550°C at a heating rate of 2°C / min and held for 6 hours. The mass-to-volume ratio (g / mL) of the calcination product to the NH4NO3 solution was 1:10; the concentration of NH4NO3 in the NH4NO3 solution was 1 mol / L. S4: Atmospheric activation of the catalyst Five portions of Fe-ZSM-5 molecular sieve to be activated (1 g) were placed in tube furnaces and activated under a reducing atmosphere (C2H4, CO, NH3, CH4 and a hydrogen / argon mixture, respectively; the volume percentage of H2 in the hydrogen / argon mixture was 10%) (550℃ for 3 hours, with a flow rate of 40 mL / min for the reducing atmosphere); after activation, the catalysts were cooled to room temperature under an inert atmosphere to obtain Fe-ZSM-5 molecular sieve catalysts (denoted as Fe-ZSM-5(C2H4), Fe-ZSM-5(CO), Fe-ZSM-5(NH3), Fe-ZSM-5(CH4), and Fe-ZSM-5(H2 / Ar) respectively).

[0040] Comparative Example 1 The only difference from Example 1 is that step S4 is modified to the following form: Take 1 g of the Fe-ZSM-5 molecular sieve to be activated in Example 1, place it in a tube furnace, and activate it in an air atmosphere (550℃ for 4 hours, air flow rate of 40 mL / min); after activation, cool it to room temperature in an inert atmosphere to obtain the Fe-ZSM-5 molecular sieve catalyst.

[0041] Example 1 A method for improving the selective oxidation performance of methane to formic acid catalyzed by Fe-ZSM-5 catalyst through reducing atmosphere pretreatment was tested using the products of Example 1 and Comparative Example 1 as catalytic components. The steps are as follows: Methane, an aqueous solution of hydrogen peroxide (hydrogen peroxide concentration of 0.5 mol / L, specifically 15 ml H2O + 0.77 ml H2O2), and a catalyst sample (addition amount of 30 mg) were added to a batch autoclave and reacted (reaction temperature of 70℃, pressure of 3 MPa, time of 30 minutes; the partial pressure of methane during the reaction was 3 MPa). After the reaction was completed, the product was collected by ice-water condensation, thus completing the preparation.

[0042] use 1 Quantitative analysis of the product was performed by H NMR (AVANCE NEO 400 MHz): 450 μL of the product collected by ice water condensation was mixed with 50 μL of deuterated water containing dimethyl sulfoxide (DMSO) as an internal standard and then detected by nuclear magnetic resonance. 1 The chemical shifts of H are assigned as follows: DMSO methyl proton δ = 2.7 ppm, methanol methyl proton δ = 3.4 ppm, methylperoxymethylene proton δ = 3.8 ppm, hydroxymethylperoxymethylene proton δ = 5.1 ppm, formic acid proton δ = 8.3 ppm; The yield and selectivity of formic acid were calculated using the internal standard method. The test results are shown in Table 1.

[0043] Table 1 As shown in Table 1, all reducing atmosphere pretreatments (Example 1) improved the catalytic performance of Fe-ZSM-5, and the improvement was closely related to the reducing power of the reducing gas. The H2 / Ar mixture, due to its strongest reducing power, showed the best activation and dispersion effect on Fe species, increasing formic acid yield by approximately 116% with a selectivity of 92.5%. CO was the next best, while C2H4, NH3, and CH4 showed relatively limited improvement.

[0044] Example 2 (Effect of Activation Temperature) In a reducing atmosphere of hydrogen / argon mixture, catalysts were prepared by modifying only the activation temperature in S4 of Example 1 (from 550℃ to 350℃, 450℃, and 700℃, respectively) (denoted as Fe-ZSM-5(H2 / Ar)-350℃, Fe-ZSM-5(H2 / Ar)-450℃, and Fe-ZSM-5(H2 / Ar)-700℃, respectively), and the effect of activation temperature on the selective oxidation of methane to formic acid was evaluated. The test results are shown in Table 2.

[0045] Table 2 As shown in Table 2, the formic acid yield and selectivity of the catalyst reached their optimal values ​​when pretreated at 550℃. This is mainly because excessively low temperatures lead to insufficient activation, while excessively high temperatures induce side reactions, resulting in more formic acid being converted into CO2.

[0046] Example 3 (Effect of Activation Time) Catalysts (denoted as Fe-ZSM-5(H2 / Ar)-1h, Fe-ZSM-5(H2 / Ar)-6h, and Fe-ZSM-5(H2 / Ar)-12h, respectively) were prepared by modifying only the activation time in S4 of Example 1 (from 3h to 1h, 6h, and 12h). The effect of activation time on the selective oxidation of methane to formic acid was evaluated. The test results are shown in Table 3.

[0047] Table 3 Table 3 shows that the formic acid yield is optimal when the treatment time is 3 hours. Further extending the treatment time has little effect on increasing the formic acid yield. This is mainly because the Fe species in Fe-ZSM-5 are fully activated when the treatment time reaches 3 hours. If the treatment time is too short, the Fe species in Fe-ZSM-5 will not be fully activated, leading to a decrease in formic acid yield.

[0048] Comparative Example 2 (ZSM-5 as catalyst only) The only difference from Example 1 is that step S4 is modified to the following form: ZSM-5 molecular sieve was prepared according to steps S1-S3 of Example 1, omitting only the iron source in step S1 of Example 1. One g of ZSM-5 molecular sieve was placed in a tube furnace and activated under a hydrogen / argon mixed atmosphere (550℃ for 3 hours, hydrogen / argon mixed atmosphere flow rate of 40 mL / min). After activation, it was cooled to room temperature under an inert atmosphere to obtain the ZSM-5 molecular sieve catalyst. Then, the catalytic performance of selective oxidation of methane to formic acid was tested according to the conditions of Example 1.

[0049] Test results show that when the ZSM-5 molecular sieve catalyst prepared in Comparative Example 2 is used as the catalytic component, almost no formic acid is generated (the formic acid yield is 20 μmol, which is extremely low and selectivity is not considered), proving that Fe is an essential active center. The role of pretreatment is to optimize the position, structure and valence state of Fe species.

[0050] The above data fully demonstrate the necessity, significant effect, and structure-activity relationship of the preparation process defined in this invention for improving the performance of the catalyst in the selective oxidation of methane to formic acid.

[0051] Figure 1 This is a schematic diagram of the preparation and performance evaluation process of the Fe-ZSM-5 molecular sieve catalyst of the present invention.

[0052] The coordination environment, speciation, and spatial distribution of iron species in Fe-ZSM-5 catalysts treated with hydrogen / argon (H2 / Ar) and air atmospheres were compared and studied using techniques such as Fourier transform X-ray absorption fine structure spectroscopy (FT-EXAFS), ultraviolet-visible absorption spectroscopy (UV-Vis), and transmission electron microscopy elemental surface distribution (TEM mapping).

[0053] Figure 2 The FT-EXAFS spectra of Fe-ZSM-5(H2 / Ar) and Fe-ZSM-5(Air) are shown.

[0054] Depend on Figure 2 It can be seen that, compared with the air-treated sample (the product of Comparative Example 1), in Figure 2 Compared to Fe-ZSM-5 (referred to as Fe-ZSM5-Air) after H2 reduction treatment (Fe-ZSM-5(H2 / Ar in Example 1), in... Figure 2 In the form of Fe-ZSM5-H2, iron species tend to exist in a mononuclear form that is dispersed at the atomic level.

[0055] Figure 3 The UV-Vis spectra of Fe-ZSM-5(H2 / Ar) and Fe-ZSM-5(Air) are shown.

[0056] Figure 3 UV-Vis spectral analysis further confirmed that the reducing atmosphere activation treatment can effectively regulate the deposition and polymerization state of iron species. Compared with the sample treated with Air in Comparative Example 1 (in... Figure 3 (Referring to Fe-ZSM-5-Air), Fe-ZSM-5 activated in a reducing atmosphere (Fe-ZSM-5(H2 / Ar in Example 1), in...) Figure 3 In Fe-ZSM-5-H2, iron species tend to be distributed outside the molecular sieve framework and mainly exist in the form of highly active isolated mononuclear or binuclear ferrite clusters, while the characteristic signals of oligomeric iron species and large-size Fe2O3 oxides are significantly weakened.

[0057] Figure 4 The TEM mapping spectrum of Fe-ZSM-5(H2 / Ar).

[0058] Figure 5 This is the TEM mapping spectrum of Fe-ZSM-5(Air).

[0059] Depend on Figure 4-5 It can be seen that the distribution of iron on the ZSM-5 molecular sieve support is significantly more uniform on the Fe-ZSM-5 catalyst after activation in a reducing atmosphere, which further confirms that pretreatment in a reducing atmosphere can significantly improve the dispersion of iron species and inhibit their aggregation.

[0060] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid, characterized in that, Includes the following steps: The Fe-ZSM-5 molecular sieve to be activated was activated under a reducing atmosphere to obtain the Fe-ZSM-5 molecular sieve catalyst.

2. The preparation method according to claim 1, characterized in that, The reducing atmosphere is ethylene, carbon monoxide, ammonia, methane, or a hydrogen / argon mixture, and the gas flow rate is 30–100 mL / min.

3. The preparation method according to claim 1, characterized in that, The activation treatment is performed at a temperature of 300–700°C for 1–12 hours.

4. The preparation method according to claim 1, characterized in that, The preparation method of the Fe-ZSM-5 molecular sieve to be activated includes the following steps: A mixture of silicon source, aluminum source, iron source, template agent and water was prepared and stirred to obtain Fe-ZSM-5 precursor gel. The gel was then crystallized and calcined to obtain the calcined product. The calcination product was subjected to ammonia exchange treatment to obtain the Fe-ZSM-5 molecular sieve to be activated.

5. The preparation method according to claim 4, characterized in that, The silicon source is one or more of tetraethyl orthosilicate, silica sol, and silica. The aluminum source is one or more of sodium aluminate, aluminum nitrate and aluminum sulfate; The iron source is one or more of ferric nitrate, ferric chloride, ferric sulfate, ferric oxalate, and ferric acetylacetone. The template agent is tetrapropylammonium hydroxide or tetrapropylammonium bromide; The molar ratio of Si in the silicon source to Al in the aluminum source is 20–200; the molar ratio of Fe in the iron source to Si in the silicon source is 0.001–0.01; and the molar ratio of the template agent to Si in the silicon source is 0.1–0.

3.

6. The preparation method according to claim 4, characterized in that, The crystallization reaction is carried out at a temperature of 150–180°C for 24–72 hours. The calcination conditions are as follows: in an air atmosphere, the temperature is increased to 450-550°C at a heating rate of 2-5°C / min, and held for 3-8 hours.

7. The preparation method according to claim 4, characterized in that, The ammonia exchange process includes the following steps: (1) The roasted product is mixed with NH4NO3 solution and subjected to ammonia exchange at 50-80°C for 1-3 hours. After solid-liquid separation and washing with water, the ammonia exchange product is obtained. (2) Repeat step (1) 2 to 4 times to obtain the Fe-ZSM-5 molecular sieve to be activated; The mass-to-volume ratio (g / mL) of the calcined product to the NH4NO3 solution is 0.1–1; the concentration of NH4NO3 in the NH4NO3 solution is 0.5–2 mol / L.

8. A Fe-ZSM-5 molecular sieve catalyst for the selective oxidation of methane to formic acid, obtained by the preparation method according to any one of claims 1-7.

9. The application of the Fe-ZSM-5 molecular sieve catalyst of claim 8 for the selective oxidation of methane to formic acid in the field of selective oxidation of methane to formic acid.

10. A method for improving the performance of Fe-ZSM-5 catalyst for selective oxidation of methane to formic acid by reducing atmosphere pretreatment, wherein the Fe-ZSM-5 molecular sieve catalyst of claim 8 for selective oxidation of methane to formic acid is used as the catalytic component, characterized in that, Includes the following steps: Methane, an aqueous hydrogen peroxide solution, and the Fe-ZSM-5 molecular sieve catalyst were mixed and reacted to complete the preparation of formic acid. The concentration of hydrogen peroxide in the hydrogen peroxide aqueous solution is 0.2–2 mol / L, and the partial pressure of methane during the reaction process is 3 MPa; The reaction is carried out at a temperature of 50–80°C, a pressure of 0.5–3 MPa, and a time of 15–60 minutes.

Images