Core-shell Fe / molecular sieve catalyst and application of core-shell Fe / molecular sieve catalyst in preparation of formic acid by catalyzing methane oxidation

Abstract

The invention discloses an iron / molecular sieve catalyst with a core-shell structure, a preparation method of the iron / molecular sieve catalyst and application of the iron / molecular sieve catalyst in preparation of formic acid by catalyzing methane oxidation, and belongs to the technical field of natural gas conversion and energy. The catalyst is formed by taking a Fe-ZSM-5 molecular sieve with catalytic activity as an inner core and taking a porous and inert oxide shell as a shell layer. In the core-shell structure, the internal Fe-ZSM-5 provides a high-dispersion iron active site and an acid site for activating methane; the external porous oxide shell promotes mass transfer of reaction active intermediates, meanwhile, the catalytic inertia of the oxide shell effectively inhibits excessive reaction and loss of active species, and retention of the target product formic acid is remarkably promoted. Therefore, the synergistic effect of the structure provided by the invention can greatly improve the activity and stability of the catalyst in the reaction of preparing formic acid by methane oxidation, especially the selectivity to formic acid.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of natural gas conversion, energy science and technology, and particularly relates to a core-shell type iron / molecular sieve catalyst, a preparation method thereof and application of the catalyst in catalyzing mild oxidation of methane to formic acid. BACKGROUND

[0002] With the global oil resources becoming increasingly exhausted, methane (CH4) as a rich and cheap clean energy has attracted widespread attention. However, the methane molecule has high stability, with a C-H bond energy of 439 kJ / mol, high ionization energy and weak acidity, which leads to extremely difficult activation and conversion, and is referred to as a "holy grail" challenge in the catalysis field. Efficient conversion of methane into high-value-added chemicals (such as formic acid) can not only alleviate its greenhouse effect, but also realize high-value utilization of resources, which has great strategic significance.

[0003] Metallic molecular sieve catalysts (such as ZSM-5) have shown potential in methane conversion due to their regular pores and adjustable acidity. However, the existing technologies generally have the following limitations: (1) active species are prone to agglomeration and deactivation; (2) mass transfer of reaction intermediates is limited; (3) target product formic acid is prone to excessive oxidation and decomposition; and (4) traditional synthesis gas routes are not economical and environmentally friendly.

[0004] Core-shell molecular sieves can theoretically optimize the mass transfer path and protect the active center through the synergistic effect of the core active site and the shell limited space. However, the design of core-shell catalysts for selective oxidation of methane to formic acid, especially the use of inert shells to inhibit over-reaction and improve product retention, is still in the blank. The present application optimizes the design of methane oxidation catalysts, making it easier for reaction active intermediates to diffuse and for products to be retained, and the reaction system is more environmentally friendly. SUMMARY

[0005] To solve the core problems of low selectivity of catalysts, excessive oxidation of products, high energy consumption of synthesis gas in reaction path dependence and poor process economy in the existing technology for methane oxidation to formic acid, the present application proposes an innovative iron-based molecular sieve catalyst with a core-shell structure and a preparation method thereof. The catalyst uses Fe-ZSM-5 molecular sieve as an active core, and a porous inert oxide shell is constructed on the outer surface thereof to form a unique core-shell limited system.

[0006] To achieve the above-mentioned purposes, the present application adopts the following technical solutions: One of the purposes of the present application is to protect a core-shell type iron / molecular sieve catalyst, which is composed of Fe-ZSM-5 as an inner core and a porous and inert oxide shell wrapped outside. In the inner core, Fe is anchored in the microporous channels of the ZSM-5 molecular sieve in a highly dispersed cationic state.

[0007] Further, the loading of Fe in the catalyst is 0.03-4 wt%.

[0008] Further, the oxide shell is S-1 molecular sieve, TS-1 molecular sieve or silicon oxide, and the thickness is 20-100 nm, and the pore size is about 0.55 nm.

[0009] Further, the ratio of the shell to the core in the catalyst can be converted into the mass ratio of SiO2 contained therein as 1:0.2-1:8.

[0010] The second object of the present application is to protect the preparation method of the core-shell type iron / molecular sieve catalyst, which comprises the following steps: 1) tetraethyl orthosilicate, aluminum nitrate, tetrapropylammonium hydroxide (TPAOH) and water are mixed, then ammonia water is added, and hydrothermal crystallization is carried out after stirring at room temperature for 1 h; 2) the solid product after crystallization in step 1) is washed with deionized water until neutral, then centrifuged, filtered, dried, and calcined in an air environment to obtain ZSM-5 molecular sieve; 3) the ZSM-5 molecular sieve obtained in step 2) is slowly added with a solution of iron nitrate, stirred at room temperature for 1 h, and then hydrothermal crystallization is carried out; 4) the solid product after crystallization in step 3) is washed with deionized water until neutral, then centrifuged, filtered, dried, and calcined to obtain Fe-ZSM-5; 5) the Fe-ZSM-5 obtained in step 4) is added to an aqueous solution containing a silicon source, and after alcohol removal, the obtained slurry is moved to a hydrothermal reaction kettle for hydrothermal crystallization; 6) the solid product after crystallization in step 5) is washed with deionized water until neutral, then centrifuged, filtered, dried, and calcined to obtain the core-shell type iron / molecular sieve catalyst.

[0011] Further, the mass ratio of tetraethyl orthosilicate, aluminum nitrate, TPAOH, ammonia water and water used in step 1) is 13.8748:0.8324:13.5038:10.3542:22.7505.

[0012] Further, the concentration of the iron nitrate solution in step 3) is 0.5 mol / L.

[0013] Further, the amount of the iron nitrate solution in step 3) is converted into the mass ratio of iron nitrate contained therein to ZSM-5 molecular sieve as 0.02-0.5:0.5-5.

[0014] Further, the speed of adding the iron nitrate solution to the ZSM-5 molecular sieve in step 3) is 20-100 μL / min.

[0015] Further, the silicon source in step 5) includes any one of SiO2 and tetraethyl orthosilicate.

[0016] Further, the amount of the silicon source and water in step 5) is calculated according to the molar ratio of SiO2 to H2O contained in the silicon source, which is 1:50.

[0017] Further, the alcohol removal in step 5) is specifically stirring in a 60 ℃ water bath for 2 h, and then increasing the temperature of the water bath to 80 ℃ and keeping for 1 h.

[0018] Further, in the operation, the temperature of the hydrothermal crystallization is 100-250 ℃, and the time is 4-72 h.

[0019] Further, in the operation, the temperature of the drying is 30-120 ℃, and the time is 1-24 h.

[0020] Further, in the operation, the temperature of the calcination is 200-600 ℃, and the time is 4-24 h.

[0021] The preparation method of the catalyst of the application comprises two key stages. The first stage is the construction of an active core: the ZSM-5 molecular sieve is immersed in a soluble iron salt (such as ferric nitrate) solution, and the iron species is uniformly loaded by the immersion method; then the solvent is removed by drying at 30-120 ℃, and the iron ions are firmly anchored in the form of highly dispersed isolated Fe 3+ on the channels and surface of the ZSM-5 molecular sieve by calcination in an air atmosphere at 200-600 ℃ for 4-24 h, forming a high-activity Fe-ZSM-5 core; The second stage is the construction of an inert shell: an in-situ hydrothermal crystallization method is used, a silicon source (such as SiO2 or tetraethyl orthosilicate) is used as a precursor, and by adjusting the crystallization temperature (100-250 ℃) and time (4-72 h), a layer of oxide shell with a thickness of 20-100 nm and a pore size of about 0.55 nm is grown on the surface of the Fe-ZSM-5 core, so as to completely cover the core and not block the active sites of the core layer.

[0022] The third object of the application is to protect the application of the core-shell type iron / molecular sieve catalyst in the oxidation of methane to formic acid.

[0023] Further, the reaction conditions of the core-shell type iron / molecular sieve catalyst in the oxidation of methane to formic acid are a temperature of 30-150 ℃ and a pressure of 1-5 MPa.

[0024] The application has the following beneficial effects: The present application uses iron salt and microporous molecular sieve as raw materials, and prepares a core-shell Fe / molecular sieve catalyst through hydrothermal treatment, drying and calcination. The unique structure of the catalyst enables the modification of the outer surface while retaining the catalytic performance of the core phase molecular sieve. Through the synergistic effect of the core phase metal active site and the multi-level pore structure of the core-shell, the catalytic activity and stability and the selectivity of the target product can be effectively improved. Compared with the traditional Fe-ZSM-5 catalyst, the core-shell Fe / molecular sieve catalyst prepared by the present application utilizes the framework structure of ZSM-5 molecular sieve and the inertness of the molecular sieve to restrict the product or active species in the pore, greatly improving the activity of the catalyst. Compared with ordinary molecular sieve catalysts, the core-shell molecular sieve catalyst has superior performance and certain universality, and the inertness of the shell layer is beneficial to the retention of the product.

[0025] In the core-shell Fe / molecular sieve catalyst obtained by the present application, the active metal is in a monodispersed cationic state, which can effectively reduce the amount of metal used. When applied to the reaction of selective oxidation of energy molecules such as methane to prepare formic acid, the catalyst can exhibit high yield and good catalytic effect, and the by-products are few. No additional additives or organic solvents are needed in the reaction, which is green and environmentally friendly, suitable for large-scale production, and has good application prospect in the field of small molecule catalytic conversion.

[0026] The present application can also change different inert carriers as the shell layer of the core-shell molecular sieve. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 XRD patterns of Fe-ZSM-5 and Fe-ZSM-5@xS-1 prepared for the examples.

[0028] Figure 2 Scanning electron microscope images and pore size distribution patterns of Fe-ZSM-5 (a, b) and Fe / ZSM-5@4S-1 (c, d) prepared for the examples.

[0029] Figure 3 Transmission electron microscope images of Fe-ZSM-5 (a) and Fe-ZSM-5@4S-1 (b) prepared for Example 2.

[0030] Figure 4 Comparison of the catalytic performance of Fe-ZSM-5@4S-1 prepared for Example 2 at different reaction temperatures.

[0031] Figure 5 Cycle performance test chart of Fe-ZSM-5@4S-1 prepared for Example 2. DETAILED DESCRIPTION

[0032] A core-shell Fe / molecular sieve catalyst, the preparation of which comprises the following steps: 1) Mix tetraethyl orthosilicate, aluminum nitrate, and TPAOH with water, then add ammonia water. Stir at room temperature for 1 hour, and then hydrothermally crystallize at 100-250°C for 4-72 hours. 2) washing the solid product crystallized in step 1) with deionized water until neutral, centrifuging, filtering, drying at 30-120° C. for 1-24 h, placing in a muffle furnace, and calcining at 200-600° C. for 4-24 h in an air environment to obtain a ZSM-5 molecular sieve; 3) Dissolve ferric nitrate in water to prepare a 0.5 mol / L ferric nitrate solution, then slowly add the solution dropwise to the ZSM-5 molecular sieve obtained in step 2) at a rate of 20-100 μL / min. After stirring at room temperature for 1 hour, transfer the resulting sample to a polytetrafluoroethylene-lined container and place it in a hydrothermal reactor for hydrothermal crystallization in a constant temperature oven at 100-250°C for 4-72 hours. 4) washing the solid product crystallized in step 3) with deionized water until neutral, centrifuging, filtering, drying at 30-120° C. for 1-24 h, and calcining at 200-600° C. in an air environment in a muffle furnace for 4-24 h to obtain Fe-ZSM-5; 5) Adding the Fe-ZSM-5 obtained in step 4) to the aqueous solution containing the silicon source, stirring in a 60°C water bath for 2 h, then heating the water bath to 80°C and maintaining for 1 h. The resulting slurry is then transferred to a polytetrafluoroethylene-lined container and placed in a hydrothermal reactor for hydrothermal crystallization in a constant temperature oven at 100-250°C for 4-72 h. 6) The solid product crystallized in step 5) is washed with deionized water until neutral, centrifuged, filtered, dried at 30-120° C. for 1-24 h, and then placed in a muffle furnace and calcined at 200-600° C. in air for 4-24 h to obtain a core-shell iron / molecular sieve catalyst having an Fe loading of 0.1-4 wt%.

[0033] Wherein, the mass ratio of tetraethyl orthosilicate, aluminum nitrate, TPAOH, ammonia water and water used in step 1) is 13.8748:0.8324:13.5038:10.3542:22.7505.

[0034] The amount of the ferric nitrate solution in step 3) is converted based on a mass ratio of ferric nitrate to ZSM-5 molecular sieve of 0.02-0.5:0.5-5.

[0035] The silicon source in step 5) includes any one of SiO2 and tetraethyl orthosilicate; the amount of the silicon source and water is converted based on a molar ratio of SiO2 to H2O in the silicon source of 1:50.

[0036] In order to make the content of the present application more convenient to understand, the technical solutions of the present application are further described below in combination with specific embodiments, but the present application is not limited thereto.

[0037] Example 1 Synthesis of Fe-ZSM-5 molecular sieve: 1) 13.8748 g of tetraethyl orthosilicate (TEOS), 13.5038 g of tetrapropylammonium hydroxide (TPAOH), and 12.7505 g of H2O were sequentially added into a beaker, and the mixture was continuously stirred at a speed of 450 r / min at room temperature for 30 min. 10.3542 g of NH3·H2O was slowly added into the beaker by using a disposable dropper, and the mixture was continuously stirred for 30 min. 0.8324 g of Al(NO3)3·9H2O was dissolved in 10 g of deionized water, and the solution was slowly dropped into the beaker by using a syringe pump at a rate of 200 μL / min. After the dropping was completed, the mixture was continuously stirred for 1 h to fully mix the components. Then, the sample was transferred into a 100 mL polytetrafluoroethylene liner, and was loaded into a hydrothermal reactor. The hydrothermal reaction was performed in an oven at 170 ℃ for 48 h. After the reactor was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, and was then centrifuged, filtered, and dried in an oven at 80 ℃ overnight. Then, the sample was ground and placed in a muffle furnace, and was calcined at 550 ℃ for 6 h to obtain H-type ZSM-5, which was labeled as ZSM-5.

[0038] 2) 4 mL of a solution containing 0.1158 g of Fe(NO3)3·9H2O was slowly dropped into a beaker containing 1 g of ZSM-5 by using a syringe pump at a rate of 50 μL / min. After the dropping was completed, the mixture was continuously stirred for 1 h to fully mix the components. Then, the sample was transferred into a 100 mL polytetrafluoroethylene liner, and was loaded into a hydrothermal reactor. The hydrothermal reaction was performed in an oven at 170 ℃ for 48 h. After the reactor was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, and was then centrifuged, filtered, and dried in an oven at 80 ℃ overnight. Then, the sample was ground and placed in a muffle furnace, and was calcined at 550 ℃ for 6 h to obtain H-type ZSM-5 containing 0.4 wt% of Fe, which was labeled as Fe-ZSM-5.

[0039] Example 2 TEOS, TPAOH, H2O were mixed in the molar ratio of n(SiO2):n(TPAOH):n(H2O) = 1:0.5:50, then a certain amount of Fe-ZSM-5 powder prepared in Example 1 was added, and after stirring in a 60 ℃ water bath for 2 h, the water bath was heated to 80 ℃ and kept for 1 h. Then the slurry was moved to a 100 mL polytetrafluoroethylene liner and loaded into a hydrothermal reactor in a 170 ℃ constant temperature oven for hydrothermal reaction for 24 h. After the reactor was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, then centrifuged, filtered, and the sample was dried in an 80 ℃ oven overnight. Then the sample was ground and placed in a muffle furnace and calcined at 550 ℃ for 6 h to obtain the catalyst, labeled as Fe-ZSM-5@xS-1, where x represents the mass ratio of SiO2 in S-1 to SiO2 in Fe-ZSM-5.

[0040] Table 1 is the texture properties of Fe-ZSM-5 and Fe-ZSM-5@xS-1 prepared in the examples.

[0041] Table 1 Texture properties of catalysts prepared in the examples

[0042] As can be seen from the results in Table 1, the specific surface area of the core-shell structure catalysts is slightly larger than that of the parent Fe-ZSM-5, and with the increase of the coating amount, the specific surface area decreases, and the pore volume is uniformly distributed around 0.2 cm 3 / g, which shows that the shell coated on the outside of the parent is a porous structure, which may be due to the addition of the template TPAOH in the secondary hydrothermal epitaxial growth process, which makes part of Si in the parent Fe-ZSM-5 dissolved to form pores. Compared with the parent, the core-shell catalyst after coating has a more abundant pore system, which is more advantageous for the catalyst to limit the free radical intermediates produced in the reaction process, thereby promoting the reaction.

[0043] The Fe-ZSM-5 and Fe-ZSM-5@xS-1 prepared in the examples were characterized by XRD, and the results are shown in Figure 1 . In the figure, only the diffraction peaks of the molecular sieve are presented, and the diffraction peaks of Fe and Fe2O3 particles are not observed, which shows that no metal particles are formed in the prepared Fe-ZSM-5@xS-1, and the metal species is in a highly dispersed state.

[0044] The catalysts prepared in the examples were characterized by scanning electron microscopy, and the results are shown in Figure 2From the morphology analysis, it can be seen that the prepared Fe-ZSM-5 is in the shape of regular round cake, with the particle size ranging from 330 nm to 450 nm, and the average particle size concentrated at 386 nm. The Fe-ZSM-5@4S-1 catalyst after coating becomes in the shape of hexagonal plate, with the particle size ranging from 475 nm to 775 nm, and the average particle size concentrated at 631 nm. At the same time, it can be seen from the figure that the particle size of the core-shell catalyst increases after the S-1 shell layer is epitaxially grown on the matrix Fe-ZSM-5 by the direct hydrothermal method.

[0045] The Fe-ZSM-5 and Fe-ZSM-5@4S-1 catalysts prepared in the examples were subjected to transmission electron microscopy analysis, and the results are shown in Figure 3 .

[0046] From the morphology analysis, it can be seen that the Fe-ZSM-5 is in the shape of round cake, and the Fe-ZSM-5@4S-1 coated with S-1 becomes in the shape of hexagonal plate, and it is obvious that the round cake-shaped core and the hexagonal shell, indicating that the core-shell catalyst with Fe-ZSM-5 as the core and S-1 as the shell is successfully prepared.

[0047] Comparative Example 1 A solution of 4 mL containing 0.1158 g Fe(NO3)3·9H2O was slowly dropped into a beaker containing 1 g of S-1 at a rate of 50 μL / min by a syringe pump, and after the dropping was completed, the stirring was continued for 1 h to fully mix the components, and then the sample was transferred to a 100 mL polytetrafluoroethylene liner, and was loaded into a hydrothermal reaction kettle, and was hydrothermally treated in a constant temperature oven at 170 ℃ for 48 h. After the reaction kettle was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, and then was centrifuged, filtered, and then the sample was dried in an oven at 80 ℃ overnight. After that, the sample was ground and placed in a muffle furnace, and was calcined at 550 ℃ for 6 h to obtain H-type S-1 containing Fe0.4wt%, which was marked as Fe-S-1.

[0048] Comparative Example 2 In a beaker, 13.8748 g of tetraethyl orthosilicate (TEOS), 13.5038 g of tetrapropylammonium hydroxide (TPAOH), and 12.7505 g of H2O were sequentially added, and the mixture was continuously stirred at a speed of 450 r / min at room temperature for 30 min. Then 10.3542 g of NH3·H2O was slowly added using a disposable dropper, and the stirring was continued for 30 min to mix uniformly. 0.8324 g of Al(NO3)3·9H2O was dissolved in 10 g of deionized water, and the solution was slowly dropped into the beaker at a rate of 200 μL / min using a syringe pump. After the dropping was completed, 1 g of Fe-S-1 prepared in Comparative Example 1 was added, and the stirring was continued for 1 h to fully mix the components. Then the sample was transferred to a 100 mL polytetrafluoroethylene liner, and was loaded into a hydrothermal reactor, which was placed in a constant-temperature oven at 170 ℃ for hydrothermal reaction for 48 h. After the reactor was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, and then was centrifuged, filtered, and dried in an oven at 80 ℃ overnight. Then the sample was ground and placed in a muffle furnace, and was calcined at 550 ℃ for 6 h to finally obtain Fe-S-1@ZSM-5.

[0049] Application Example 1 The catalysts prepared in the examples and comparative examples were applied to the selective hydrogenation reaction of methane. Specifically, 20 mg of the catalyst was placed in a reactor, and 15 ml of 30% hydrogen peroxide aqueous solution was added as a solvent. After the reactor was sealed, 2 MPa of CH4 was charged, and the temperature was set to 90 ℃, and the reaction time was 1 h. After the reaction was completed, the reactor was cooled to room temperature, and the gas after the reaction was collected with a gas bag and was detected by gas chromatography. At the same time, the catalyst after the reaction was washed with water, and the supernatant was taken after centrifugation and was subjected to liquid nuclear magnetic resonance test (0.01 wt% of D2O-DSS as an internal standard). The product selectivity and yield were measured and calculated, and the results are shown in Table 2.

[0050] Table 2 Catalytic performance table

[0051] As can be seen from Table 2, compared with other catalysts, the catalyst prepared by taking Fe-ZSM-5 as the core and S-1 as the shell showed the best catalytic performance in terms of both the formic acid yield and the selectivity.

[0052] Example 3 TEOS, TBOT, TPAOH, H2O were mixed in the molar ratio of n(SiO2):n(TPAOH):n(H2O) = 1:0.0167:0.5:50 (wherein the molar ratio of TEOS and TBOT used was 1:0.0167); followed by the addition of a certain amount of Fe-ZSM-5 powder prepared in Example 1, after stirring in a 60 °C water bath for 2 h, the water bath was heated to 80 °C and kept for 1 h. Then the slurry was moved to a 100 mL polytetrafluoroethylene liner and loaded into a hydrothermal reactor, which was placed in a constant temperature oven at 170 °C for 24 h. After the reactor was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, then centrifuged, filtered, and the sample was dried in an 80 °C oven overnight. Then the sample was ground and placed in a muffle furnace, calcined at 550 °C for 6 h to obtain the catalyst, labeled as Fe-ZSM-5@xTS-1, wherein x represents the mass ratio of SiO2 in TS-1 to SiO2 in Fe-ZSM-5 powder.

[0053] Application Example 2 The Fe-ZSM-5@xTS-1 prepared according to Example 3 was used to carry out selective hydrogenation of methane under the conditions given in Application Example 1, and the results obtained are shown in Table 3.

[0054] Table 3 Comparison of catalytic activity of core-shell molecular sieve catalysts with TS-1 as shell

[0055] As can be seen from Table 3, the catalyst prepared with Fe-ZSM-5 as core and TS-1 as shell also has good formic acid yield and selectivity.

[0056] Example 4 SiO2 and H2O were mixed in the molar ratio of n(SiO2):n(H2O) = 1:50, followed by the addition of a certain amount of Fe-ZSM-5 powder prepared in Example 1, after stirring in a 60 °C water bath for 2 h, the water bath was heated to 80 °C and kept for 1 h. Then the slurry was moved to a 100 mL polytetrafluoroethylene liner and loaded into a hydrothermal reactor, which was placed in a constant temperature oven at 170 °C for 24 h. After the reactor was naturally cooled to room temperature, the obtained solid product was washed with deionized water until neutral, then centrifuged, filtered, and the sample was dried in an 80 °C oven overnight. Then the sample was ground and placed in a muffle furnace, calcined at 550 °C for 6 h to obtain the catalyst, labeled as Fe-ZSM-5@xSiO2, wherein x represents the mass ratio of SiO2 in the system to SiO2 in Fe-ZSM-5 powder.

[0057] Application Example 3 The Fe-ZSM-5@xSiO2 prepared in Example 4 was subjected to selective hydrogenation of methane under the conditions given in Application Example 1. The results are shown in Table 4.

[0058] Table 4 Comparison of catalytic activity of core-shell molecular sieve catalysts with SiO2 as shell

[0059] As can be seen from Table 4, the catalyst prepared with Fe-ZSM-5 as the core and SiO2 as the shell also has good formic acid yield and selectivity.

[0060] Application Example 4 The effect of different reaction temperatures on the performance of Fe-ZSM-5@4S-1 was investigated. The specific operation was as follows: 0.02g of Fe-ZSM-5@4S-1 catalyst was weighed and placed in a reactor, and 15ml of 30% hydrogen peroxide solution was added as solvent. After sealing the reactor, 2 MPa of CH4 was filled in, and the temperature was raised to 50℃, 70℃ and 90℃ respectively and the reaction was carried out for 1 hour. After the reaction was completed, the reactor was cooled to room temperature, and the gas after the reaction was collected with an air bag for gas chromatography detection. The product selectivity and yield were measured and calculated. The results are shown in Fig. Figure 4 .

[0061] Depend on Figure 4 It can be seen that with the increase of reaction temperature, the yield and selectivity of formic acid are improved.

[0062] Application Example 5 Catalytic cycle stability test, collect the Fe-ZSM-5@4S-1 catalyst after the reaction of Example 1, wash and remove the reaction product, wash the catalyst with water until no product is attached, centrifuge and place in an oven at 80 ° C for 4 hours. The dried catalyst is placed in a muffle furnace and calcined in static air at 550 ° C for 4 hours. After cooling to room temperature, weigh 0.02 g of the regenerated Fe-ZSM-5@4S-1 catalyst and place it in a reactor, and add 15 ml of 30% hydrogen peroxide aqueous solution as a solvent. After sealing the reactor, fill it with 2 MPa of CH4, heat it to 80 ° C and react for 1 hour. After the reaction is completed, cool the reactor to room temperature, collect the gas after the reaction with an air bag for gas chromatography detection, measure and calculate the product selectivity and yield, the results are shown in the figure. Figure 5 .

[0063] Figure 5 The results showed that the regenerated Fe-ZSM-5@4S-1 catalyst still had good reaction activity.

[0064] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A core-shell iron / molecular sieve catalyst, characterized in that: The catalyst is composed of Fe-ZSM-5 as a core and a porous, inert oxide shell wrapped around the core; in the core, Fe is anchored in the micropores of the ZSM-5 molecular sieve in a highly dispersed cation state.

2. The core-shell iron / molecular sieve catalyst according to claim 1, characterized in that: The Fe loading in the catalyst is 0.03-4 wt %.

3. The core-shell iron / molecular sieve catalyst according to claim 1, characterized in that: The oxide shell is S-1 molecular sieve, TS-1 molecular sieve or silicon oxide; the ratio of the shell to the core in the catalyst is converted according to the mass ratio of SiO2 contained therein of 1:0.2~1:

8.

4. A method for preparing the core-shell iron / molecular sieve catalyst according to claim 1, characterized in that: The following steps are involved: 1) Tetraethyl orthosilicate, aluminum nitrate, and tetrapropylammonium hydroxide were mixed with water, and then ammonia water was added. The mixture was stirred at room temperature for 1 hour and then hydrothermal crystallization was performed. 2) washing the solid product obtained from the crystallization in step 1) with deionized water until neutral, centrifuging, filtering, drying, and then calcining in air to obtain a ZSM-5 molecular sieve; 3) slowly adding the ferric nitrate solution dropwise to the ZSM-5 molecular sieve obtained in step 2), stirring at room temperature for 1 h and then performing hydrothermal crystallization; 4) washing the solid product obtained from the crystallization in step 3) with deionized water until neutral, and then centrifuging, filtering, drying, and calcining to obtain Fe-ZSM-5; 5) adding the Fe-ZSM-5 obtained in step 4) to the aqueous solution containing the silicon source, removing the alcohol, and transferring the resulting slurry to a hydrothermal reactor for hydrothermal crystallization; 6) The solid product after crystallization in step 5) is washed with deionized water until neutral, and then centrifuged, filtered, dried, and calcined to obtain the core-shell iron / molecular sieve catalyst.

5. The preparation method according to claim 4, characterized in that: The mass ratio of tetraethyl orthosilicate, aluminum nitrate, tetrapropylammonium hydroxide, ammonia water and water used in step 1) is 13.8748:0.8324:13.5038:10.3542:22.7505.

6. The preparation method according to claim 4, wherein: The concentration of the ferric nitrate solution in step 3) is 0.5 mol / L, and its amount is converted according to the mass ratio of ferric nitrate to ZSM-5 molecular sieve of 0.02~0.5:0.5~5; its dropwise addition rate is 20~100 μL / min.

7. The preparation method according to claim 4, characterized in that: The silicon source in step 5) includes any one of SiO2 and tetraethyl orthosilicate; the amount of the silicon source and water is converted based on a molar ratio of SiO2 to water in the silicon source of 1:

50.

8. The preparation method according to claim 4, wherein: In step 5), the alcohol is removed by stirring in a 60°C water bath for 2 h, and then the water bath is heated to 80°C and maintained for 1 h.

9. The preparation method according to claim 4, wherein: During the operation, the hydrothermal crystallization temperature is 100-250°C, and the time is 4-72 h; the calcination temperature is 200-600°C, and the time is 4-24 h.

10. Use of the core-shell iron / molecular sieve catalyst according to claim 1 in the production of formic acid by oxidation of methane.

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