Fe / Fe3O4 composite material wrapped by iron carbide as well as preparation method and application of Fe / Fe3O4 composite material

Abstract

The invention discloses an iron carbide coated Fe / Fe3O4 composite material as well as a preparation method and application thereof, and belongs to the technical field of catalysts. The composite material takes an iron chain as a core, Fe3O4 particles are uniformly attached to the surface, and the outermost layer is wrapped by iron carbide. According to the Fe / Fe3O4 composite material coated with the iron carbide, the iron carbide on the outermost layer serves as a main active phase, CO dissociation and carbon chain growth can be efficiently catalyzed, and the utilization rate of active sites is maximized; the Fe3O4 particles in the middle layer dynamically adjust the proportion of H2 / CO in the synthesis gas through high water gas shift activity, a hydrogen-rich environment is provided for surface iron carbide, CO dissociation and carbon chain growth are promoted, and carbon deposition caused by insufficient H2 is avoided; the rigid framework of the iron chain core provides mechanical support for the composite material, sintering or crushing of the composite material under high-temperature and high-pressure conditions can be inhibited, and the service life of the composite material is prolonged. The three components cooperate to realize comprehensive improvement of activity, stability and inactivation resistance, so that the composite material still keeps high-efficiency long-period operation in a low H2 / CO ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and particularly relates to an iron carbide-coated Fe / Fe3O4 composite material, a preparation method thereof, and an application thereof. Background Art

[0002] Non-renewable energy sources such as coal, petroleum, and natural gas are important pillars of the modern industrial system and play an important role in society. They are widely used in aspects such as power generation, transportation, and manufacturing. To ensure energy security, it is necessary for people to continuously transform the energy structure from petroleum to coal, natural gas, and biomass. Among them, the Fischer-Tropsch synthesis (FTS) reaction is one of the important ways to achieve this transformation, and it can effectively convert coal into oil and gas.

[0003] It has been found through research that metals in Group VIII of the periodic table have certain catalytic activities for the FTS reaction. Among them, the catalytic effects of Ni, Ru, Co, and Fe are the strongest, and Fe-based catalysts and Co-based catalysts have been industrially applied. Compared with Co-based catalysts, Fe-based catalysts are inexpensive, suitable for syngas with a low hydrogen-carbon ratio, have a wide operating temperature range, and the product distribution can be adjusted. Therefore, the development of Fe-based catalysts is still a hot research issue at present.

[0004] However, due to the unsatisfactory activity, stability, and selectivity of pure Fe-based catalysts, it is difficult to achieve large-scale industrialization. To improve the performance of Fe-based catalysts, researchers need to modify the catalysts, such as optimizing the catalyst structure, modifying metal promoters, and regulating heteroatom doping. Summary of the Invention

[0005] The main object of the present invention is to propose an iron carbide-coated Fe / Fe3O4 composite material, a preparation method thereof, and an application thereof, aiming to solve the problems of low activity, complex product distribution, easy deactivation, and insufficient mechanical strength of existing iron-based catalysts, which cause catalyst particle breakage in the Fischer-Tropsch reaction.

[0006] To achieve the above object, the present invention proposes an iron carbide-coated Fe / Fe3O4 composite material, wherein the composite material has an iron chain as the core, Fe3O4 fine particles are uniformly attached to the surface, and the outermost layer is coated with iron carbide.

[0007] The present invention also proposes a preparation method of the iron carbide-coated Fe / Fe3O4 composite material, including the following steps:

[0008] S1. Add an iron salt to deionized water and stir until the iron salt is completely dissolved to obtain an iron salt solution;

[0009] S2. Add a borohydride and citric acid to the iron salt solution, continuously stir until it becomes a sol state, and dropwise add ammonia water to adjust the pH value;

[0010] S3. Place the sol in a constant temperature water bath, and the sol gradually transforms into a wet gel; place the wet gel in a vacuum drying oven for drying to obtain a porous dry gel.

[0011] S4. Place the porous dry gel in a tubular furnace, introduce a hydrogen / argon mixed gas, and carry out high-temperature calcination to obtain pure iron chains.

[0012] S5. After the temperature cools down to room temperature, purge with argon to remove residual hydrogen, then switch to an oxygen / argon mixed gas, and carry out high-temperature oxidation on the pure iron chains to obtain iron chains with uniformly attached magnetite particles on the surface.

[0013] S6. After the temperature cools down to room temperature, purge with argon to remove residual oxygen, then switch to a carbon monoxide / hydrogen mixed gas, and carry out high-temperature calcination on the iron chains with uniformly attached magnetite particles on the surface to finally obtain an Fe / Fe3O4 composite material wrapped with iron carbide.

[0014] Preferably, in step S1, the iron salt is at least one of ferric nitrate nonahydrate, ferrous nitrate, ferric chloride, and ferrous chloride; the concentration of the iron salt is 0.1 - 0.2 mol / L.

[0015] Preferably, in step S2, the borohydride is at least one of sodium borohydride, potassium borohydride, and lithium borohydride; the molar ratio of the iron salt, borohydride, and citric acid is 1:1.5 - 2:1 - 1.5; the stirring speed is 500 - 600 rpm, and the stirring time is 1.5 - 2 h; the pH value is 5 - 6.

[0016] Preferably, in step S3, the temperature of the constant temperature water bath is 60 - 80 °C, and the time of the constant temperature water bath is 8 - 12 h; the temperature of the vacuum drying is 60 - 80 °C, and the drying time is 12 - 24 h.

[0017] Preferably, in step S4, the volume ratio of hydrogen to argon in the hydrogen / argon mixed gas is 5:95; the calcination temperature is 400 - 500 °C, and the calcination time is 2 - 3 h.

[0018] Preferably, in step S5, the volume content of oxygen in the oxygen / argon mixed gas is 3 - 5%; the oxidation temperature is 250 - 300 °C, and the calcination time is 1 - 2 h.

[0019] Preferably, in step S6, the volume ratio of carbon monoxide to hydrogen in the carbon monoxide / hydrogen mixed gas is 1:1; the calcination temperature is 400 - 700 °C, and the calcination time is 1 - 3 h.

[0020] The present invention also provides an application of the iron carbide-coated Fe / Fe₃O₄ composite material, specifically applying the iron carbide-coated Fe / Fe₃O₄ composite material to the synthesis of long-chain olefins from syngas.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) For the iron carbide-coated Fe / Fe₃O₄ composite material provided by the present invention, the outermost layer of iron carbide can efficiently catalyze the dissociation of CO and carbon chain growth; iron carbide is the key active phase of Fischer-Tropsch synthesis. By stabilizing it on the surface of the composite material and directly exposing it to the reactants, the utilization rate of active sites is maximized.

[0023] (2) For the iron carbide-coated Fe / Fe₃O₄ composite material provided by the present invention, since the intermediate layer of Fe₃O₄ particles has high water-gas shift activity, it can convert CO and H₂O in the raw material gas into H₂ and CO₂. The Fe₃O₄ particles generate additional H₂ through the water-gas reaction, thereby increasing the H₂ / CO ratio, providing a hydrogen-rich environment for the outer layer of iron carbide, promoting CO dissociation and carbon chain growth, inhibiting excessive carbon deposition of iron carbide due to hydrogen deficiency, and maintaining the stability of the composite material.

[0024] (3) For the iron carbide-coated Fe / Fe₃O₄ composite material provided by the present invention, the rigid structure formed by the inner iron chain improves the mechanical strength of the composite material, reduces particle breakage caused by friction or pressure in the fixed-bed reactor, maintains the active surface area, and extends the service life of the composite material. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings.

[0026] Figure 1 It is a schematic cross-sectional structure diagram of the iron carbide-coated Fe / Fe₃O₄ composite material of the present invention;

[0027] Among them, 1. iron chain; 2. Fe₃O₄ particles; 3. iron carbide layer.

[0028] The realization, functional characteristics and advantages of the object of the present invention will be further described in conjunction with the embodiments with reference to the drawings. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] To avoid repetition, the items used in the following embodiments are all commercially available products without special instructions, and the methods used are all conventional methods without special instructions.

[0030] Example 1

[0031] A Fe / Fe3O4 composite material wrapped with iron carbide, the schematic cross-sectional structure diagram thereof is as Figure 1 shown, and its preparation method includes the following steps:

[0032] S1. Add 4.04 g of ferric nitrate nonahydrate to 50 mL of deionized water, stir until completely dissolved to obtain an iron salt solution;

[0033] S2. Add 0.76 g of sodium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the obtained solution at a rotation speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0034] S3. Place the sol in a constant temperature water bath at 60 °C for 12 h, and the sol turns into a wet gel; place the wet gel in a vacuum drying oven at 80 °C and dry for 24 h to obtain a porous dry gel;

[0035] S4. Place the porous dry gel in a tube furnace, and under a hydrogen / argon mixed atmosphere (the volume ratio of H2 and Ar2 is 5:95) with a flow rate of 30 sccm, heat it at 5 °C·min -1 to 500 °C and calcine for 3 h to obtain a pure iron chain;

[0036] S5. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual hydrogen, and then change to an oxygen / argon mixed gas, where the volume content of oxygen is 5%, the flow rate of the oxygen / argon mixed gas is 30 sccm, and heat it at 3 °C·min -1 to 300 °C and calcine the pure iron chain for 1.5 h to obtain an iron chain with uniformly attached magnetite particles on the surface;

[0037] S6. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual oxygen, and then change to a carbon monoxide / hydrogen mixed gas (the volume ratio of CO and H2 is 1:1), the flow rate of the carbon monoxide / hydrogen mixed gas is 30 sccm, and heat it at 3 °C·min -1 to 700 °C and calcine the iron chain with uniformly attached magnetite particles on the surface for 2 h to finally obtain a Fe / Fe3O4 composite material wrapped with iron carbide.

[0038] Example 2

[0039] A preparation method of a Fe / Fe3O4 composite material wrapped with iron carbide includes the following steps:

[0040] S1. Add 1.62 g of ferric chloride to 50 mL of deionized water and stir until completely dissolved to obtain an iron salt solution;

[0041] S2. Add 1.08 g of potassium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the resulting solution at a speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0042] S3. Place the sol in a constant temperature water bath at 60 °C for 12 h, and the sol turns into a wet gel; place the wet gel in a vacuum drying oven at 80 °C and dry for 24 h to obtain a porous dry gel;

[0043] S4. Place the porous dry gel in a tube furnace, and under a mixed atmosphere of hydrogen / argon with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat it at a rate of 5 °C·min -1 to 500 °C and calcine for 3 h to obtain a pure iron chain;

[0044] S5. After the temperature cools to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual hydrogen, and then switch to a mixed gas of oxygen / argon, where the volume content of oxygen is 5%, and the flow rate of the oxygen / argon mixed gas is 30 sccm. Heat it at a rate of 3 °C·min -1 to 300 °C and calcine the pure iron chain for 1.5 h to obtain an iron chain with uniformly attached Fe3O4 particles on the surface;

[0045] S6. After the temperature cools to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual oxygen, and then switch to a mixed gas of carbon monoxide / hydrogen (the volume ratio of CO and H2 is 1:1), and the flow rate of the carbon monoxide / hydrogen mixed gas is 30 sccm. Heat it at a rate of 3 °C·min -1 to 700 °C and calcine the iron chain with uniformly attached Fe3O4 particles on the surface for 2 h to finally obtain an Fe / Fe3O4 composite material wrapped with iron carbide.

[0046] Example 3

[0047] A preparation method of an Fe / Fe3O4 composite material wrapped with iron carbide, comprising the following steps:

[0048] S1. Add 4.04 g of ferric nitrate nonahydrate to 50 mL of deionized water and stir until completely dissolved to obtain an iron salt solution;

[0049] S2. Add 0.76 g of sodium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the resulting solution at a speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0050] S3. Place the sol in a constant temperature water bath at 60 °C for 12 h, and the sol is transformed into a wet gel; place the wet gel in a vacuum drying oven at 80 °C for drying for 24 h to obtain a porous dry gel;

[0051] S4. Place the porous dry gel in a tubular furnace, and under a hydrogen / argon mixed atmosphere with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat it at a rate of 5 °C·min -1 to 500 °C and calcine for 3 h to obtain a pure iron chain;

[0052] S5. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual hydrogen, and then change to an oxygen / argon mixed gas, where the volume content of oxygen is 5%, and the flow rate of the oxygen / argon mixed gas is 30 sccm. Heat it at a rate of 3 °C·min -1 to 250 °C and calcine the pure iron chain for 2 h to obtain an iron chain with uniformly attached Fe3O4 particles on the surface;

[0053] S6. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual oxygen, and then change to a carbon monoxide / hydrogen mixed gas (the volume ratio of CO and H2 is 1:1), and the flow rate of the carbon monoxide / hydrogen mixed gas is 30 sccm. Heat it at a rate of 3 °C·min -1 to 650 °C and calcine the iron chain with uniformly attached Fe3O4 particles on the surface for 3 h to finally obtain an Fe / Fe3O4 composite material wrapped with iron carbide.

[0054] Example 4

[0055] A preparation method of an Fe / Fe3O4 composite material wrapped with iron carbide, comprising the following steps:

[0056] S1. Add 4.04 g of ferric nitrate nonahydrate to 50 mL of deionized water, stir until completely dissolved to obtain an iron salt solution;

[0057] S2. Add 0.76 g of sodium borohydride and 2.88 g of citric acid to the iron salt solution, and then stir the obtained solution at a rotation speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0058] S3. Place the sol in a constant temperature water bath at 60 °C for 12 h, and the sol is transformed into a wet gel; place the wet gel in a vacuum drying oven at 80 °C for drying for 24 h to obtain a porous dry gel;

[0059] S4. Place the porous dry gel in a tubular furnace, and under a hydrogen / argon mixed atmosphere with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat it at a rate of 5 °C·min -1Heat to 450 °C and calcine for 3 h to obtain a pure iron chain;

[0060] S5. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual hydrogen, and then switch to an oxygen / argon mixture, where the volume content of oxygen is 5%, and the flow rate of the oxygen / argon mixture is 30 sccm. Heat at 3 °C·min -1 Heat to 300 °C and calcine the pure iron chain for 1.5 h to obtain an iron chain with magnetite particles uniformly attached to its surface;

[0061] S6. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual oxygen, and then switch to a carbon monoxide / hydrogen mixture (the volume ratio of CO and H2 is 1:1), and the flow rate of the carbon monoxide / hydrogen mixture is 30 sccm. Heat at 3 °C·min -1 Heat to 700 °C and calcine the iron chain with magnetite particles uniformly attached to its surface for 3 h to finally obtain an Fe / Fe3O4 composite material encapsulated with iron carbide.

[0062] Example 5

[0063] A method for preparing an Fe / Fe3O4 composite material encapsulated with iron carbide, comprising the following steps:

[0064] S1. Add 1.62 g of ferric chloride to 50 mL of deionized water and stir until completely dissolved to obtain an iron salt solution;

[0065] S2. Add 1.08 g of potassium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the resulting solution at a rotation speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0066] S3. Place the sol in a constant temperature water bath at 60 °C for 12 h, and the sol turns into a wet gel; place the wet gel in a vacuum drying oven at 80 °C and dry for 24 h to obtain a porous dry gel;

[0067] S4. Place the porous dry gel in a tubular furnace, and under a hydrogen / argon mixed atmosphere with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat at 5 °C·min -1 Heat to 450 °C and calcine for 3 h to obtain a pure iron chain;

[0068] S5. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual hydrogen, and then switch to an oxygen / argon mixture, where the volume content of oxygen is 5%, and the flow rate of the oxygen / argon mixture is 30 sccm. Heat at 3 °C·min -1Heat to 250 °C and calcine the pure iron chain for 2 h to obtain an iron chain with magnetite particles evenly attached to its surface;

[0069] S6. After the temperature is cooled to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual oxygen, and then switch to a carbon monoxide / hydrogen mixture (the volume ratio of CO and H2 is 1:1). The flow rate of the carbon monoxide / hydrogen mixture is 30 sccm. Heat at 3 °C·min -1 Heat to 650 °C and calcine the iron chain with magnetite particles evenly attached to its surface for 3 h to finally obtain an Fe / Fe3O4 composite material wrapped with iron carbide.

[0070] Comparative Example 1

[0071] A method for preparing a pure iron chain composite material, comprising the following steps:

[0072] S1. Add 4.04 g of iron(III) nitrate nonahydrate to 50 mL of deionized water and stir until completely dissolved to obtain an iron salt solution;

[0073] S2. Add 0.76 g of sodium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the resulting solution at a rotation speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0074] S3. Place the sol in a 60 °C constant temperature water bath for 12 h, and the sol turns into a wet gel; place the wet gel in an 80 °C vacuum drying oven and dry for 24 h to obtain a porous dry gel;

[0075] S4. Place the porous dry gel in a tubular furnace, and under a hydrogen / argon mixed atmosphere with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat at 5 °C·min -1 Heat to 500 °C and calcine for 3 h to obtain a pure iron chain composite material.

[0076] Comparative Example 2

[0077] A method for preparing an Fe / Fe3O4 composite material, comprising the following steps:

[0078] S1. Add 4.04 g of iron(III) nitrate nonahydrate to 50 mL of deionized water and stir until completely dissolved to obtain an iron salt solution;

[0079] S2. Add 0.76 g of sodium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the resulting solution at a rotation speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0080] S3. Place the sol in a 60°C constant temperature water bath for 12 h, and the sol transforms into a wet gel; place the wet gel in a vacuum drying oven at 80°C and dry for 24 h to obtain a porous dry gel;

[0081] S4. Place the porous dry gel in a tube furnace, and under a hydrogen / argon mixed atmosphere with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat it at a rate of 5°C·min -1 to 500°C and calcine for 3 h to obtain pure iron chains;

[0082] S5. After the temperature cools to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual hydrogen, and then switch to an oxygen / argon mixed gas, where the volume content of oxygen is 5%, the flow rate of the oxygen / argon mixed gas is 30 sccm, and heat it at a rate of 3°C·min -1 to 300°C and calcine the pure iron chains for 1.5 h to obtain an Fe / Fe3O4 composite material.

[0083] Comparative Example 3

[0084] A method for preparing an Fe composite material wrapped with iron carbide, comprising the following steps:

[0085] S1. Add 4.04 g of ferric nitrate nonahydrate to 50 mL of deionized water and stir until completely dissolved to obtain an iron salt solution;

[0086] S2. Add 0.76 g of sodium borohydride and 2.88 g of citric acid to the iron salt solution, then stir the resulting solution at a rotation speed of 600 rpm for 2 h until it becomes a sol state, and adjust the pH value to 6 with 25 wt% ammonia water;

[0087] S3. Place the sol in a 60°C constant temperature water bath for 12 h, and the sol transforms into a wet gel; place the wet gel in a vacuum drying oven at 80°C and dry for 24 h to obtain a porous dry gel;

[0088] S4. Place the porous dry gel in a tube furnace, and under a hydrogen / argon mixed atmosphere with a flow rate of 30 sccm (the volume ratio of H2 and Ar2 is 5:95), heat it at a rate of 5°C·min -1 to 500°C and calcine for 3 h to obtain pure iron chains;

[0089] S5. After the temperature cools to room temperature, purge with argon at a flow rate of 50 sccm for 30 min to remove residual oxygen, and then switch to a carbon monoxide / hydrogen mixed gas (the volume ratio of CO and H2 is 1:1), the flow rate of the carbon monoxide / hydrogen mixed gas is 30 sccm, and heat it at a rate of 3°C·min -1 to 700°C and calcine the pure iron chains for 2 h to obtain an Fe composite material wrapped with iron carbide.

[0090] Performance Test

[0091] The composite materials prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to performance tests for the synthesis of long-chain olefins from syngas. The specific test method was as follows: After the prepared composite materials were pressed into tablets, they were crushed into 20-40 mesh. 0.2 g of the composite material was loaded into a fixed-bed reactor, and the reaction conditions were 300 °C, a molar ratio of H2 to CO of 1, a pressure of 1 MPa, and a flow rate of 20 mL / min for the Fischer-Tropsch synthesis reaction. The specific test results are shown in Table 1.

[0092] Table 1 Performance Test of Composite Materials for the Synthesis of Long-chain Olefins from Syngas

[0093]

[0094]

[0095] As can be seen from Table 1, the CO conversion rates of the composite materials prepared in Comparative Examples 1-3 were low, all lower than 30%. The CO conversion rates of the composite materials provided in Examples 1-5 of the present invention were relatively high, all higher than 60%. The selectivity of the high-value product C5 in Examples 1-5 of the present invention + was higher than that of the comparative examples, and the selectivity of the by-product CH4 was lower than that of the comparative examples.

[0096] In summary, the iron carbide-coated Fe / Fe3O4 composite material provided by the present invention has a novel structure and excellent performance in the reaction for the preparation of long-chain olefins from syngas, with a high conversion rate and relatively good product selectivity.

[0097] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the patent protection scope of the present invention.

Claims

1. A Fe / Fe3O4 composite material wrapped with iron carbide, characterized in that: The composite material has an iron chain as the core, with Fe3O4 particles evenly attached to the surface, and the outermost layer is wrapped with iron carbide.

2. A method for preparing the composite material according to claim 1, characterized in that, It includes the following steps: S1. Add an iron salt to deionized water and stir until the iron salt is completely dissolved to obtain an iron salt solution. S2. Add a borohydride and citric acid to the iron salt solution, continuously stir until it becomes a sol state, and add ammonia water to adjust the pH value. S3. Place the sol in a constant temperature water bath, and the sol gradually turns into a wet gel; place the wet gel in a vacuum drying oven for drying to obtain a porous dry gel. S4. Place the porous dry gel in a tubular furnace, introduce a hydrogen / argon mixed gas, and carry out high-temperature calcination to obtain pure iron chains. S5. After the temperature cools down to room temperature, purge with argon to remove residual hydrogen, then switch to an oxygen / argon mixed gas, and carry out high-temperature oxidation on the pure iron chains to obtain iron chains with Fe3O4 particles evenly attached to the surface. S6. After the temperature cools down to room temperature, purge with argon to remove residual oxygen, then switch to a carbon monoxide / hydrogen mixed gas, and carry out high-temperature calcination on the iron chains with Fe3O4 particles evenly attached to the surface to finally obtain an Fe / Fe3O4 composite material wrapped with iron carbide.

3. The preparation method according to claim 2, characterized in that: In step S1, the iron salt is at least one of iron(III) nitrate nonahydrate, iron(II) nitrate, iron(III) chloride, and iron(II) chloride.

4. The preparation method according to claim 2, characterized in that: In step S2, the borohydride is at least one of sodium borohydride, potassium borohydride, and lithium borohydride.

5. The preparation method according to claim 2, characterized in that: In step S2, the molar ratio of the iron salt, borohydride, and citric acid is 1:1.5 - 2:1 - 1.

5.

6. The preparation method according to claim 2, characterized in that: In step S3, the temperature of the constant temperature water bath is 60 - 80°C, and the time of the constant temperature water bath is 8 - 12 h; the temperature of the vacuum drying is 60 - 80°C, and the drying time is 12 - 24 h.

7. The preparation method according to claim 2, characterized in that: In step S4, the volume ratio of hydrogen to argon in the hydrogen / argon mixed gas is 5:95; the calcination temperature is 400 - 500°C, and the calcination time is 2 - 3 h.

8. The preparation method according to claim 2, characterized in that: In step S5, the volume content of oxygen in the oxygen / argon mixed gas is 3 - 5%; the oxidation temperature is 250 - 300°C, and the calcination time is 1 - 2 h.

9. The preparation method according to claim 2, characterized in that: In step S6, the volume ratio of carbon monoxide to hydrogen in the carbon monoxide / hydrogen mixed gas is 1:1; the calcination temperature is 400 - 700°C, and the calcination time is 1 - 3 h.

10. Use of the composite material according to claim 1, characterized in that: Apply the Fe / Fe3O4 composite material wrapped with iron carbide to the synthesis of long-chain olefins from syngas.

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