Iron-based catalyst for directly preparing olefin from synthesis gas as well as preparation method and application of iron-based catalyst

Abstract

The invention relates to an iron-based catalyst for directly preparing olefin from synthesis gas as well as a preparation method and application of the iron-based catalyst. The preparation method comprises the following steps: preparing a polymer solution and an iron salt solution, mixing to obtain a mixed solution a, mixing the mixed solution a and a nitrate solution b, and carrying out a co-precipitation reaction to obtain co-precipitation slurry; centrifuging, washing and drying the obtained coprecipitation slurry to obtain a catalyst precursor; and calcining the obtained catalyst precursor to obtain the iron-based catalyst, wherein the mass ratio of the polymer to the ferric salt in the mixed solution a is 1: (0.6-40). Compared with the prior art, the catalyst is prepared by innovatively adopting a method of introducing an auxiliary agent into a polymer in situ, and the synergistic effect between the auxiliary agent and the iron-based catalyst active component is remarkably enhanced, so that the CO2 and methane selectivity of the catalyst in a Fischer-Tropsch synthesis reaction is effectively reduced, and meanwhile, the selectivity of target olefin is also effectively improved.

Description

Technical Field

[0001] This invention relates to the field of catalyst preparation technology, specifically to an iron-based catalyst for the direct synthesis of olefins from syngas and its preparation method. Background Technology

[0002] Existing processes for converting syngas to olefins mainly fall into two categories: one involves converting olefins via oxygen-containing intermediates such as methanol or dimethyl ether, a process known as MTO (methanol-to-olefins) or DMTO (dimethyl ether-to-olefins), but this route has numerous operational steps and a complex process; the other involves directly producing olefins from syngas using a one-step Fischer-Tropsch synthesis reaction, which can be categorized into oxide molecular sieve tandem catalyst systems or metal catalyst systems such as iron, cobalt, and ruthenium. Among metal catalysts, iron-based catalysts are widely used in research on one-step syngas-to-olefins processes due to their high activity and low cost.

[0003] Existing patents have made some progress in improving olefin selectivity, for example, by introducing Na promoters into iron-based catalysts; however, research on the suppression and optimization of one-carbon byproducts such as methane and carbon dioxide in the syngas-to-olefins process is still limited. Conventional methods to suppress CO2 generation include increasing the carbon content of the CO2 in the feed gas, which reduces the carbon utilization rate of the catalyst; or constructing a hydrophobic layer such as a Si-based support in the catalyst, which inevitably reduces the intrinsic activity of the catalyst. Highly selective olefin products are crucial for industrial applications, and the generation of byproducts such as methane and carbon dioxide not only reduces carbon efficiency but also increases the cost of separation and treatment.

[0004] In summary, existing technologies face the problem of high selectivity for one-carbon byproducts (methane and carbon dioxide) and low selectivity for olefins in one-step synthesis of olefins from syngas. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defects of high selectivity of one-carbon byproducts and low selectivity of olefins in the one-step synthesis of olefins from syngas in the prior art, thereby providing an iron-based catalyst for the direct synthesis of olefins from syngas, its preparation method and application.

[0006] According to an embodiment of the present invention, in a first aspect, a method for preparing an iron-based catalyst for the direct synthesis of olefins from syngas is provided, comprising the following steps: A polymer solution and an iron salt solution are prepared and mixed to obtain a mixed solution a. Mixed solution a is then mixed with a nitrate solution b to undergo a coprecipitation reaction, resulting in a coprecipitation slurry. The obtained coprecipitation slurry is centrifuged, washed, and dried to obtain a catalyst precursor. The obtained catalyst precursor is then calcined to obtain the iron-based catalyst. In mixed solution a, the mass ratio of polymer to iron salt is 1:0.6-40.

[0007] In some embodiments of the present invention, the polymer solution includes one of polyvinylpyrrolidone solution, glucose solution, and polyethylene glycol solution; The polymer solution has a viscosity-average molecular weight of 8,000-80,000 and a concentration of 40-200 g / L.

[0008] In some embodiments of the present invention, the iron salt solution is a solution with one or more of the following as solutes: ammonium ferrohydride, potassium ferrocyanide, sodium ferrocyanide, potassium ferricyanide, and sodium ferricyanide, and the concentration of the iron salt solution is 0.015-0.5 mol / L.

[0009] In some embodiments of the present invention, the nitrate solution b is a solution with one or more of the following as solutes: ferric nitrate, calcium nitrate, zinc nitrate, manganese nitrate, copper nitrate, magnesium nitrate, and cerium nitrate, and the concentration of the nitrate solution b is 0.0175-0.225 mol / L.

[0010] In some embodiments of the present invention, the reaction conditions for the coprecipitation reaction include: a coprecipitation temperature of 20-80°C, a reaction time of 20-120 min, and the reaction being carried out under stirring conditions.

[0011] In some embodiments of the present invention, the centrifugation speed is 2000-10000 r / min, and the centrifugation time is 2-10 min; the washing is performed by washing with deionized water and / or ethanol 2-10 times. The drying temperature is 50-150℃, and the drying time is 8-20 hours.

[0012] In some embodiments of the present invention, the calcination temperature is 400-650°C and the calcination time is 1-10 hours.

[0013] In some embodiments of the present invention, the calcination temperature is 450-600°C, and the calcination is carried out under an inert atmosphere; The inert atmosphere is selected from argon, ammonia, nitrogen, and helium.

[0014] According to some embodiments of the present invention, in a second aspect, an iron-based catalyst for the direct synthesis of olefins from syngas is provided, which is prepared by the above-described preparation method.

[0015] According to some embodiments of the present invention, in a third aspect, the application of the above-described catalyst in the one-step synthesis of olefins from syngas is provided, wherein the specific conditions for the one-step process are: a temperature of 250-340°C and a space velocity of 5000-400000 h⁻¹. -1The pressure is 0.1-5.5 MPa, the ratio of H2 to CO in the synthesis gas is 0.5-3:1, and the reactor used in the one-step process is a fixed bed reactor, a slurry bed reactor, or a fluidized bed reactor.

[0016] The technical solution of this invention has the following advantages: 1. The present invention provides a method for preparing an iron-based catalyst for the direct synthesis of olefins from syngas, comprising the following steps: preparing a polymer solution and an iron salt solution, mixing them to obtain a mixed solution a, mixing the mixed solution a with a nitrate solution b to undergo a coprecipitation reaction, obtaining a coprecipitate slurry; centrifuging, washing, and drying the obtained coprecipitate slurry to obtain a catalyst precursor; calcining the obtained catalyst precursor to obtain the iron-based catalyst; wherein, the mass ratio of polymer to iron salt in the mixed solution a is 1:0.6-40.

[0017] This invention innovatively employs an in-situ polymer introduction method as a nitrate promoter to prepare the catalyst, significantly enhancing the synergistic effect between the nitrate promoter and the active component of the iron-based catalyst. This effectively reduces the selectivity of the catalyst for CO2 and methane in the Fischer-Tropsch synthesis reaction, while also effectively improving the selectivity for the target olefin. The mechanism by which this invention achieves the above effects may be that by using the polymer as a structure directing agent, through its interaction with the precursor during the preparation process, precise control and high dispersion of the catalyst particle structure are effectively realized, inhibiting particle aggregation and sintering during pyrolysis, and ensuring the structural stability of the final catalyst, the high dispersion of the active phase, and excellent catalytic performance.

[0018] 2. The iron-based catalyst for the direct synthesis of olefins from syngas provided by this invention is a stable single-phase catalyst, which effectively ensures the structural stability of the catalyst in the one-step synthesis of olefins from syngas, making it less prone to phase change or deactivation, thereby improving the service life and reaction performance of the catalyst.

[0019] 3. This invention provides an application of the catalyst prepared by the above method in the one-step synthesis of olefins from syngas. This invention reduces the selectivity of CO2 and methane in the one-step synthesis of olefins from syngas, while improving the catalytic activity and the selectivity of the target olefin. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1These are the XRD spectra of the catalyst precursors prepared in Examples 1-4 of this invention before calcination; Figure 2 This is the XRD pattern of the catalyst prepared in Example 3 of this invention after undergoing Fischer-Tropsch synthesis. Figure 3 This is a graph showing the trend of conversion rate of the catalyst prepared in Example 3 of the present invention in the Fischer-Tropsch synthesis reaction; Figure 4 The images are HAADF images of the catalysts; where a and b are HAADF images of the catalyst in Example 3, and c and d are HAADF images of the catalyst in Comparative Example 1. Detailed Implementation

[0022] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0023] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0024] Example 1 Add 8 g of polyvinylpyrrolidone (PVP) with a viscosity-average molecular weight of 20,000 to 200 mL of water and stir until completely dissolved. Then add 0.003 mol of ammonium ferrocyanide to prepare a mixed aqueous solution a of ammonium ferrocyanide and PPVP. In solution a, the concentration of ammonium ferrocyanide is 0.015 mol / L and the concentration of PPVP is 40 g / L.

[0025] Take another 200 mL of water and add 0.003 mol of ferric nitrate and 0.001 mol of 50% copper nitrate solution to prepare a mixed aqueous solution b of ferric nitrate and copper nitrate. In solution b, the concentration of ferric nitrate is 0.015 mol / L, the concentration of copper nitrate is 0.0025 mol / L, and the concentration of nitrate solution b is 0.0175 mol / L. Mix the mixed aqueous solution a and mixed aqueous solution b under stirring to undergo a coprecipitation reaction. The reaction temperature is controlled at 25℃, and the reaction time is 30 minutes to obtain a precipitate slurry. The obtained precipitate slurry is centrifuged at 10000 r / min for 10 minutes, then washed 8 times with deionized water, and finally dried at 120℃ for 12 hours, and granulated to obtain the catalyst precursor.

[0026] One g of catalyst was placed in a fixed-bed reactor and calcined in an ammonia atmosphere to obtain catalyst Fe3Cu[Fe(CN)6]3·10H2O. The calcination temperature was 550℃, the time was 5 hours, and the ammonia gas flow rate was 100 ml / min. After calcination, the reactor was switched to a syngas environment of 5 MPa (volume ratio H2:CO = 2:1), and the reaction space velocity was set to 80,000 h⁻¹. -1 The reaction temperature was controlled at 300℃. Data on the reaction products are shown in Table 1.

[0027] Example 2 Add 8g of polyethylene glycol with an average molecular weight of 8000 to 200 ml of water and stir until the polyethylene glycol is completely dissolved. Then, add 0.03 mol of potassium ferrocyanide to the solution to prepare a mixed aqueous solution a of potassium ferrocyanide and polyethylene glycol. In mixed aqueous solution a, the concentration of ammonium ferrocyanide is 0.15 mol / L and the concentration of polyethylene glycol is 40 g / L.

[0028] Add 0.02 mol of ferric nitrate and 0.005 mol of calcium nitrate to 200 ml of water to prepare a mixed aqueous solution b of ferric nitrate and calcium nitrate. In mixed aqueous solution b, the concentration of ferric nitrate solution is 0.1 mol / L, the concentration of calcium nitrate solution is 0.025 mol / L, and the concentration of nitrate solution b is 0.125 mol / L. Mix all the mixed aqueous solutions a and b together and allow a co-precipitation reaction to occur under stirring at 50 °C for 100 minutes. After the reaction, a precipitate is formed, and the resulting precipitate slurry is centrifuged at 5000 r / min for 3 min. After centrifugation, wash five times with deionized water and dry at 50 °C for 8 hours. Granulate to obtain the catalyst precursor.

[0029] 1 g of the catalyst was placed in a fixed-bed reactor and calcined in an ammonia atmosphere to obtain catalyst Fe3Ca[Fe(CN)6)]3·6H2O. The calcination temperature was 600℃ and the time was 3 hours, with an argon atmosphere flow rate of 50 ml / min. Then, 0.5 MPa of syngas (volume ratio H2:CO = 3:1) was introduced into the reactor, and the reaction space velocity was adjusted to 5000 h⁻¹. -1 The reaction temperature was 250℃, and the reaction products are shown in Table 1.

[0030] Example 3 Add 30g of polyvinylpyrrolidone (PVP) with a viscosity-average molecular weight of 40,000 to 200ml of water and stir until the PPVP is completely dissolved. Then, add 0.05mol of sodium ferrocyanide to this solution to prepare a mixed aqueous solution a of sodium ferrocyanide and PPVP. In mixed aqueous solution a, the concentration of ammonium ferrocyanide is 0.25mol / L, and the concentration of PPVP is 150g / L. Add 0.03mol of ferric nitrate and 0.03mol of 50%wt zinc nitrate solution to 200ml of water to prepare a mixed aqueous solution b of ferric nitrate and zinc nitrate. In mixed aqueous solution b, the concentration of ferric nitrate solution is 0.15mol / L, the concentration of zinc nitrate solution is 0.075mol / L, and the concentration of nitrate solution b is 0.225mol / L. Mix mixed aqueous solution a and mixed aqueous solution b together and, under stirring, a coprecipitation reaction occurs at 80℃ for 20 minutes. After the reaction, a precipitate was formed, and the resulting precipitate slurry was centrifuged at 2000 r / min for 10 minutes. After centrifugation, it was washed twice with ethanol and dried at 80℃ for 20 hours. Granulation was then performed to obtain the catalyst precursor.

[0031] 0.5 g of the catalyst precursor was placed in a fixed-bed reactor and calcined in an ammonia atmosphere to obtain the catalyst Fe2Zn2[Fe(CN)6)]·20H2O. The calcination temperature was 450℃ and the time was 1 hour, with the ammonia atmosphere flow rate at 50 ml / min. Then, 5.5 MPa synthesis gas (volume ratio H2:CO = 0.5:1) was introduced into the reactor, and the reaction space velocity was adjusted to 400,000 ml·h. -1 The reaction temperature was 340℃, and the reaction products are shown in Table 1.

[0032] Example 4 Add 40 g of polyvinylpyrrolidone (PVP) with a viscosity-average molecular weight of 80,000 to 200 mL of water and stir until completely dissolved. Then add 0.003 mol of potassium ferricyanide to prepare a mixed aqueous solution a of potassium ferricyanide and PVP. In mixed solution a, the concentration of potassium ferricyanide is 0.015 mol / L and the concentration of PVP is 200 g / L.

[0033] Add 0.003 mol of ferric nitrate and 0.006 mol of 50% cerium nitrate solution to another 200 mL of water to prepare a mixed aqueous solution b of ferric nitrate and cerium nitrate. In mixed solution b, the concentration of ferric nitrate is 0.015 mol / L, the concentration of cerium nitrate is 0.015 mol / L, and the concentration of nitrate solution b is 0.03 mol / L. Mixed aqueous solution a and mixed aqueous solution b are mixed under stirring and co-precipitated at 60 °C for 80 minutes to produce a precipitate. The resulting precipitate slurry is centrifuged at 8000 r / min for 5 minutes. After centrifugation, it is washed 10 times with deionized water, then dried at 150 °C for 10 hours, granulated, and the catalyst precursor is obtained.

[0034] 0.5 g of the catalyst precursor was placed in a fixed-bed reactor and calcined in a helium atmosphere to obtain the catalyst FeCe3[Fe(CN)6]3·10H2O. The calcination temperature was 550℃, the time was 10 hours, and the helium flow rate was 50 ml / min. After calcination, 0.1 MPa of syngas (H2:CO = 2.5:1) was introduced into the reactor, and the reaction space velocity was set to 15000 h⁻¹. -1 The reaction temperature was controlled at 340℃. Data on the reaction products are shown in Table 1.

[0035] Example 5 Add 8 g of glucose with a viscosity-average molecular weight of 20,000 to 200 mL of water and stir until completely dissolved. Then add 0.1 mol of ammonium ferrocyanide to prepare a mixed aqueous solution a of ammonium ferrocyanide and glucose. In solution a, the concentration of ammonium ferrocyanide is 0.5 mol / L and the concentration of glucose is 40 g / L.

[0036] Take another 200 mL of water and add 0.003 mol of ferric nitrate and 0.001 mol of 50% copper nitrate solution to prepare a mixed aqueous solution b of ferric nitrate and copper nitrate. In solution b, the concentration of ferric nitrate is 0.015 mol / L, the concentration of copper nitrate is 0.0025 mol / L, and the concentration of nitrate solution b is 0.0175 mol / L. Mix the mixed aqueous solution a and mixed aqueous solution b under stirring to undergo a coprecipitation reaction. The reaction temperature is controlled at 20℃, and the reaction time is 120 minutes to obtain a precipitate slurry. The obtained precipitate slurry is centrifuged at 10000 r / min for 2 minutes, then washed 8 times with deionized water, and finally dried at 120℃ for 12 hours, and granulated to obtain the catalyst precursor.

[0037] One g of catalyst was placed in a fixed-bed reactor and calcined under a nitrogen atmosphere to obtain catalyst Fe3Cu[Fe(CN)6]3·10H2O. The calcination temperature was 550℃, the time was 5 hours, and the nitrogen flow rate was 100 ml / min. After calcination, the reactor was switched to a syngas environment of 5 MPa (volume ratio H2:CO = 2:1), and the reaction space velocity was set to 80,000 h⁻¹. -1 The reaction temperature was controlled at 300℃. Data on the reaction products are shown in Table 1.

[0038] Comparative Example 1 A solution of 0.01 mol ferric nitrate was prepared in 200 mL of water, with a concentration of 0.05 mol / L. Under stirring, a 1 mol / L ammonia solution was slowly added dropwise to this solution until the pH reached 8, forming a ferric hydroxide precipitate. The coprecipitation reaction was carried out in a water bath (80 °C) for 60 minutes. After the reaction, the precipitate was centrifuged at 5000 r / min for 5 minutes, washed five times with deionized water, dried at 120 °C for 12 hours, and then calcined in a muffle furnace at 500 °C for 5 hours to obtain the catalyst precursor Fe₂O₃.

[0039] The obtained Fe2O3 precursor was placed in a fixed-bed reactor at a concentration of 1 g in a syngas atmosphere and activated by calcination at 280°C for 10 hours. Then, syngas at 5 MPa (H2:CO = 2:1 volume ratio) was introduced into the reactor, and the reaction space velocity was adjusted to 80,000 h⁻¹. -1 The reaction temperature was 300℃, and the reaction products are shown in Table 1.

[0040] Comparative Example 2 Weigh 5 grams of silicon dioxide (SiO2) and add it to 200 ml of deionized water. Disperse the silicon dioxide evenly under stirring to form a suspension. Add 0.01 mol of ferric nitrate (Fe(NO3)3·9H2O) solution to the silicon dioxide suspension to achieve a ferric nitrate concentration of 0.05 mol / L. Continue stirring and allow to stand at room temperature for 1 hour to allow the ferric nitrate to fully impregnate the silicon dioxide surface. Filter the impregnated silicon dioxide suspension and collect the iron-containing silicon dioxide precipitate. Dry the precipitate in a drying oven at 120°C for 12 hours to remove excess moisture, obtaining the Fe / SiO2 precursor. Place the dried Fe / SiO2 precursor in a muffle furnace and calcine at 450°C for 5 hours with an air flow rate of 50 ml / min to convert the ferric nitrate onto the silicon dioxide surface into active iron species.

[0041] 1 g of the catalyst precursor was placed in a fixed-bed reactor and activated in a hydrogen atmosphere at 350 °C for 5 hours, with a hydrogen flow rate of 100 ml / min. Then, 5 MPa of syngas (H2:CO = 2:1 volume ratio) was introduced into the reactor, and the reaction space velocity was adjusted to 80,000 h⁻¹. -1 The reaction temperature was 300℃, and the reaction products are shown in Table 1.

[0042] Comparative Example 3 Add 10g of polyvinylpyrrolidone to 200ml of water and stir until the polyvinylpyrrolidone is completely dissolved. Then add 0.003mol of sodium ferrocyanide to the solution to prepare a mixed aqueous solution a of sodium ferrocyanide and polyvinylpyrrolidone. In mixed aqueous solution a, the concentration of sodium ferrocyanide is 0.015mol / L and the concentration of polyvinylpyrrolidone is 50g / L.

[0043] 0.003 mol of ferric nitrate was added to 200 ml of water to prepare ferric nitrate solution b. The concentration of ferric nitrate solution in solution b was 0.015 mol / L. The mixed aqueous solution a and mixed aqueous solution b were completely mixed together, and a co-precipitation reaction occurred under stirring at 50℃ for 30 minutes. A precipitate was formed after the reaction, and the resulting precipitate slurry was centrifuged at 10000 r / min for 10 min. After centrifugation, it was washed three times with deionized water and dried at 120℃ for 12 hours. Granulation was then performed to obtain the catalyst precursor Fe4[Fe(CN)6]3·10H2O.

[0044] 1 g of the catalyst precursor was placed in a fixed-bed reactor and calcined in a CO atmosphere at 550 °C for 1 hour, with a CO flow rate of 50 ml / min, to obtain the catalyst. Then, 5 MPa of syngas (H2:CO = 2:1 volume ratio) was introduced into the reactor, and the reaction space velocity was adjusted to 80,000 h⁻¹. -1 The reaction temperature was 300℃, and the reaction products are shown in Table 1.

[0045] Table 1 Catalyst Evaluation Results

[0046] Among them, olefin selectivity refers to the selectivity of olefins in total hydrocarbons.

[0047] As shown in Table 1, compared with the iron-based catalyst in the comparative example, the single-phase iron carbide catalyst prepared by this method exhibits low CO2 selectivity and high olefin selectivity in the one-step synthesis of olefins from syngas. The main reason for this is that the catalyst prepared by this method is initially a single-phase iron carbide (Fe3C in Example 3, see...). Figure 2The minimal presence of Fe3O4 phase results in the lowest possible CO2 selectivity. Furthermore, the catalyst promoters prepared by this method are highly dispersed on the catalyst surface, preventing agglomeration and achieving better results. For example... Figure 4 As shown, the catalyst was characterized by transmission electron microscopy (FEI Talos F200A), and the distribution pattern of each element was obtained by energy dispersive X-ray spectroscopy (EDS). In Example 3, the auxiliary agent zinc was highly dispersed on the surface of iron carbide, while in Comparative Example 1, zinc agglomerated into lumps and separated from iron carbide.

[0048] The crystal structure of the catalyst was characterized using a Bruker D8 Advance X-ray diffractometer (Germany). A cobalt target was used as the X-ray source, and powder diffraction patterns of the samples were acquired at room temperature. Figure 1 The XRD patterns of the catalyst precursors prepared in Examples 1, 2, 3, and 4 illustrate that the catalyst precursors have similar structures. Figure 2 The image shows the XRD pattern of the catalyst prepared in Example 3 after undergoing Fischer-Tropsch synthesis. Figure 2 The results show that the catalyst prepared in Example 3, after Fischer-Tropsch synthesis, is a single-phase iron carbide, and its XRD pattern indicates a Fe3C structure. The performance evaluation of the catalyst in the Fischer-Tropsch synthesis reaction was carried out in a fixed-bed reactor. Figure 3 The figure shows the trend of conversion rate of the catalyst prepared in Example 3 during the Fischer-Tropsch synthesis reaction. Figure 3 The results show that the conversion rate of the catalyst prepared in Example 3 remained basically stable during the Fischer-Tropsch synthesis reaction over 100 hours.

[0049] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing an iron-based catalyst for the direct synthesis of olefins from syngas, characterized in that, Includes the following steps: A polymer solution and an iron salt solution are prepared and mixed to obtain a mixed solution a. Mixed solution a is then mixed with a nitrate solution b to undergo a coprecipitation reaction, resulting in a coprecipitation slurry. The obtained coprecipitation slurry is centrifuged, washed, and dried to obtain a catalyst precursor. The obtained catalyst precursor is then calcined to obtain the iron-based catalyst. In mixed solution a, the mass ratio of polymer to iron salt is 1:0.6-40.

2. The preparation method according to claim 1, characterized in that, The polymer solution includes one of polyvinylpyrrolidone solution, glucose solution, and polyethylene glycol solution; The polymer solution has a viscosity-average molecular weight of 8,000-80,000 and a concentration of 40-200 g / L.

3. The preparation method according to claim 1, characterized in that, The iron salt solution is a solution with one or more of the following as solutes: ammonium ferrohydride, potassium ferrocyanide, sodium ferrocyanide, potassium ferricyanide, and sodium ferricyanide, and the concentration of the iron salt solution is 0.015-0.5 mol / L.

4. The preparation method according to claim 1, characterized in that, The nitrate solution b is a solution with one or more of the following as solutes: ferric nitrate, calcium nitrate, zinc nitrate, manganese nitrate, copper nitrate, magnesium nitrate, and cerium nitrate. The concentration of the nitrate solution b is 0.0175-0.225 mol / L.

5. The preparation method according to claim 1, characterized in that, The reaction conditions for the coprecipitation reaction include: a coprecipitation temperature of 20-80℃, a reaction time of 20-120 min, and the reaction being carried out under stirring conditions.

6. The preparation method according to claim 1, characterized in that, The centrifugation speed is 2000-10000 r / min, and the centrifugation time is 2-10 min; the washing is performed by washing with deionized water and / or ethanol 2-10 times. The drying temperature is 50-150℃, and the drying time is 8-20 hours.

7. The preparation method according to claim 1, characterized in that, The calcination temperature is 400-650℃, and the calcination time is 1-10h.

8. The preparation method according to claim 7, characterized in that, The calcination temperature is 450-600℃, and the calcination is carried out under an inert atmosphere; The inert atmosphere is selected from argon, ammonia, nitrogen, and helium.

9. An iron-based catalyst for the direct synthesis of olefins from syngas, characterized in that, It is prepared by any of the preparation methods described in claims 1-8.

10. The application of the catalyst according to claim 9 in the one-step synthesis of olefins from syngas, characterized in that, The specific conditions for the one-step method are: temperature 250-340℃, and space velocity 5000-400000 h⁻¹. -1 The pressure is 0.1-5.5 MPa, the ratio of H2 to CO in the synthesis gas is 0.5-3:1, and the reactor used in the one-step process is a fixed bed reactor, a slurry bed reactor, or a fluidized bed reactor.

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