CN1159098C - A kind of iron-based catalyst for Fischer-Tropsch synthesis and preparation method thereof - Google Patents

Abstract

A method for preparing iron-based catalysts for Fischer-Tropsch synthesis, using cheap ferrous sulfate raw materials, oxidizing with oxidant _H2O2 to obtain ferric sulfate solution, then mixing with lanthanum nitrate and copper salt _mixed solution, and using basic compounds for rapid Co-precipitation, after the precipitation slurry is washed, the slurry is further added to the potassium silicate water glass solution of different modulus, and then spray-dried to obtain an iron-based catalyst, or dried and roasted and then pressed into tablets to obtain an iron-based catalyst suitable for a fixed-bed reactor. base catalyst. The preparation method has low cost and is suitable for industrial continuous large-scale production, and the prepared catalyst can be used for both fixed-bed Fischer-Tropsch synthesis reaction and slurry-state Fischer-Tropsch synthesis reaction.

Description

A kind of iron-based catalyst for Fischer-Tropsch synthesis and preparation method thereof

Technical field:

The invention belongs to a preparation method of a catalyst, in particular to an iron-based catalyst containing lanthanum additives for Fischer-Tropsch synthesis and a preparation method thereof.

Background technique:

Fischer-Tropsch synthesis is a method for synthesizing liquid fuels from syngas (CO+H ₂ ) discovered in the 1920s, and since the 1950s, large-scale industrialization of coal-based synthetic liquid fuels has been realized in South Africa (see Dry ME Paper "The Fischer-Tropsch Process-Commercial Aspects" published in Catalysis Today, 1990, 6(3) 183-206). Iron-based catalysts are commonly used in Fischer-Tropsch synthesis. The synthesis gas is reacted in a fixed-bed, fluidized-bed or slurry-bed reactor equipped with an iron-based catalyst to produce the required hydrocarbon products, including gasoline, diesel, Wax, naphtha, low-carbon olefins, etc. Syngas can be produced from carbon-containing materials such as coal, natural gas, coal bed methane, coke, and biomass. In order to improve the conversion efficiency of synthesis gas, the selectivity of hydrocarbon products and adapt to the type of reactor used, the development of new iron-based catalysts has been ongoing.

U.S. Mobil Company discloses a kind of method that adopts continuous co-precipitation method to prepare the Fe-Cu-K Fischer-Tropsch synthesis catalyst of low nitrogen content in U.S. Patent USP4617288, specifically: mix the iron nitrate and copper nitrate solution of measuring ratio and appropriate The concentrated ammonia solution is pumped into a high-level tank with stirring water vapor to keep the temperature at 80-90 °C and the pH between 6.6-6.8. The sedimentation slurry is continuously introduced into a settling tank, filtered with suction, and washed with deionized water until it is free. Nitrate, then add a certain amount of K ₂ CO ₃ solution, beating, drying, and roasting at 300°C to prepare a catalyst with a nitrogen content of less than 100ppm. This method is suitable for industrial continuous production.

U.S. Rentech Company discloses a kind of preparation method of Fischer-Tropsch synthesis iron-based catalyst for slurry bed reactor in U.S. Patent USP5504118 and Chinese invention patent CN1113905A, specifically: dissolving metal iron and copper with nitric acid to obtain ferrous nitrate , ferric nitrate and copper nitrate mixed solution, oxygen must be passed into the solution when dissolving to reduce the nitric oxide content, and a small amount of overflowing NO ₂ is absorbed and removed by KOH solution. Add ammonia water to the hot nitrate mixture, and control the pH at 7.4 to obtain a precipitated slurry, then wash it with chlorine-free high-quality water until there is no ammonium nitrate, then add potassium carbonate solution, make a slurry, the weight percentage of the catalyst in the slurry The content is about 8-12%, and then spray-dried with a spray dryer to remove most of the water to obtain roughly spherical catalyst particles with a diameter ranging from 5-50 μm. Finally, the catalyst is heated to about 315°C in the air to remove residual water , to stabilize the catalyst.

U.S.A. Texas A&M University Burkur et al. disclose a kind of Fe/Cu/K/ _SiO in magazine Ind.Eng.Chem.Res., 1990,29,1588-1599 Preparation method of catalyst, specifically: in suitable ratio Add ammonia solution to the mixed solution of ferric nitrate and copper nitrate kept at 82°C for continuous co-precipitation, wash the precipitate thoroughly with deionized water and suction filter, add a certain amount of 26wt% K ₂ SiO ₃ solution, re-pulp and make the pH ≤ 6 , after vacuum drying, impregnated with a certain amount of KHCO ₃ solution, then dried and baked at 300°C for 5 hours, and then formulated in Ind.Eng.Chem.Res., 1999, 38, 3270-3275 as A catalyst of 100Fe/3Cu/4K/16SiO ₂ (by weight) was used in a slurry bed reaction and was found to have high activity and high selectivity of C ₅ ⁺ and light olefins for syngas feedstocks with low H ₂ /CO ratio sex.

But above-mentioned several kinds of catalyzers are because the raw material price of using is more expensive, cause the cost of catalyzer high, the price is on the high side.

Invention content:

The purpose of the present invention is to provide a Fischer-Tropsch synthesis iron-based catalyst with low price and high activity and high selectivity of hydrocarbons above _C5 and low-carbon olefins and a preparation method thereof.

The object of the present invention is achieved in this way: adopt cheap ferrous sulfate raw material, obtain ferric sulfate solution by oxidant _H2O2 oxidation, then mix with lanthanum nitrate and copper salt mixed solution, carry out quick co _- precipitation with basic compound, precipitate After the slurry is washed, the slurry is further added with potassium silicate water glass solutions of different moduli, and then spray-dried to obtain an iron-based catalyst, or dried and calcined, and pressed into tablets to obtain an iron-based catalyst suitable for a fixed-bed reactor.

The weight ratio composition of the catalyst of the present invention is: Fe:La:Cu:K:SiO ₂ =100:0.02-2:0.01-5:0.5-10:5-30.

The weight ratio of K as described above is preferably 1-7.

The above SiO ₂ weight ratio is preferably 8-27.

The preparation method of the present invention comprises the steps:

(1) molar concentration is that 0.5～10mol/L ferrous sulfate solution, concentrated sulfuric acid and weight percent concentration are 3～30wt% hydrogen peroxide solution is ferrous sulfate by molar ratio: sulfuric acid: hydrogen peroxide=1: 1.6～2.0: 3.7 Proportional mixing, under the condition of oxidation temperature of 20-80°C, oxidation reaction time of 1-10 hours, to prepare ferric sulfate solution;

(2) According to the weight ratio of the catalyst, it is composed of Fe: La: Cu: K: SiO ₂ =100: 0.02-2: 0.01-5: 0.5-10: 5-30, the total molar concentration of copper salt and lanthanum nitrate is 1 ～4mol/L copper lanthanum aqueous solution;

(3) the alkaline precipitating agent is formulated into a molar concentration of 1 to 6mol/L precipitating agent aqueous solution;

(4) According to the composition of the catalyst, after mixing the ferric sulfate solution and the copper lanthanum aqueous solution, under the condition that the precipitation temperature is 20-95°C, add the precipitant aqueous solution to make the solution pH=5-10, and the rapid precipitation time is 5-60 minutes ;

(5) Stand for aging for 1 to 2 hours, filter with suction, and wash the precipitate repeatedly with deionized water until there is no sulfate ion;

(6) Deionized water is added to the precipitate for slurrying, and the molar ratio is SiO ₂ : K ₂ O=1～10 Potassium silicate water glass, the amount of potassium silicate water glass is added according to the composition of the catalyst, and stirred Uniformly, obtain catalyst slurry;

(7) a. The catalyst slurry is dried, roasted, tableted and broken, and 20-40 meshes are selected as the fixed-bed Fischer-Tropsch synthesis catalyst;

b. Spray-dry and roast the catalyst slurry, and select 50-100 μm as the slurry-bed Fischer-Tropsch synthesis catalyst.

The molar concentration of the above-mentioned ferrous sulfate solution is preferably 1-5 mol/L.

The above-mentioned ferrous sulfate can be a by-product of titanium dioxide factory and steel factory.

The weight percent concentration of the above-mentioned hydrogen peroxide solution is preferably 10-20%.

The above-mentioned oxidation temperature is preferably 25 to 60°C.

The above-mentioned oxidation reaction time is preferably 2 to 6 hours.

The molar ratio of the above-mentioned ferrous sulfate, sulfuric acid and hydrogen peroxide is preferably 1:1.8～1.95:4～6.

The copper salts mentioned above are copper nitrate, copper sulfate, copper acetate.

The total molar concentration of the above-mentioned lanthanum nitrate and copper salt is preferably 1.5-3.0 mol/L.

The above-mentioned precipitating agent is Na ₂ CO ₃ , ammonia water, NaOH, KOH, K ₂ CO ₃ , etc., preferably Na ₂ CO ₃ , ammonia water, NaOH, preferably at a concentration of 1.5-4.5 mol/L.

The molar concentration of the above-mentioned precipitant aqueous solution is preferably 1.5-4.5 mol/L.

The above-mentioned precipitation temperature is preferably 60-90°C.

The pH value as mentioned above is preferably 7-8.5.

The above-mentioned settling time is preferably 10 to 30 minutes.

The molar ratio of SiO ₂ to K ₂ O as described above is preferably 2-5.

Compared with the prior art, the present invention has the following advantages:

(1) described iron source raw material in the present invention prepares iron-based catalyst process is ferrous sulfate, and price is cheap, and source is wide, can be the by-product ferrous sulfate of titanium dioxide factory, steelworks, compare with preparation method in the past, can Greatly reduce the cost of catalyst production.

(2) The oxidizing agent of the ferrous sulfate in the process of preparing the iron-based catalyst in the present invention is H ₂ O ₂ , which has the characteristics of small addition amount, complete oxidation, and difficulty in bringing in impurities.

(3) The lanthanum promoter is realized by adding lanthanum nitrate solution in the process of preparing the iron-based catalyst in the present invention, and the proportion of the lanthanum promoter in the catalyst is 0.01% to 5%, usually 0.02% to 2% compared with iron , which accounts for a relatively small proportion in the cost of the catalyst, and will not significantly increase the manufacturing cost of the catalyst.

(4) The iron-based catalyst prepared by the present invention has high reactivity and high selectivity to hydrocarbons above C5 and low-carbon olefins, and is especially suitable for synthesizing diesel oil, gasoline and gasoline from coal-based synthesis gas in a slurry bed reactor. wax products.

Detailed ways:

The following are the detailed content of the present invention and the effect examples of Fischer-Tropsch synthesis reaction, and the protection scope of the present invention is not limited by these examples.

Example 1

Dissolve 100kg of FeSO ₄ ·7H ₂ O in 360L of deionized water, and then add 6.5L of 3mol/L sulfuric acid solution. Add 45 L of 15% H ₂ O ₂ aqueous solution to the mixed solution, and oxidize it under vigorous stirring at 35° C. for 4 hours, and then stop gas flow. 2.3kg of Cu(NO ₃ ) ₂ ·3H ₂ O and 12.6g of La(NO ₃ ) ₃ ·6H ₂ O were dissolved in 10L of deionized water to obtain a mixed solution of nitric acid and lanthanum nitrate. Add the mixed solution of copper nitrate and lanthanum nitrate into the ferric sulfate solution prepared above to obtain a mixed salt solution containing iron, lanthanum and copper. 120kg _{of Na2CO3} was dissolved into 400L of deionized water to obtain an aqueous sodium carbonate solution. Heat the mixed salt solution and sodium carbonate solution with water vapor through the jacket and keep them at 85°C, and use acid-resistant pumps and alkali-resistant pumps to continuously pump the mixed salt solution and sodium carbonate solution into a vigorously stirred tank at the same flow rate In the process, keep the temperature at 85°C, the pH of the mixed solution=8-8.5, and mix evenly

The continuous co-precipitation process was completed within 20 minutes, and the pH value was 8.2. Leave it to age for 2 hours, filter it with suction, and wash it repeatedly with deionized water until there is no sulfate group detected by _BaCl2 aqueous solution. Add 50L of water to the filter cake for re-pulping, add 7.3L of potassium silicate water glass solution containing 20wt% _SiO2 with a modulus of 3.6 into the slurry, stir vigorously, and then dry the slurry in an oven at 110°C. Then calcined at 350° C. for 5 hours to obtain 32 kg of iron-based catalyst, the catalyst weight ratio is Fe/La/Cu/K/SiO ₂ =100:0.02:3:3:10, and the catalyst is marked as C. After the catalyst is sieved, the catalyst with 20-40 mesh particles is taken to evaluate the Fischer-Tropsch synthesis reaction performance in a fixed-bed laboratory device.

Example 2

According to the steps of Example 1, after the obtained filter cake is beaten, add 7.3L of potassium silicate water glass solution containing 20wt% _SiO2 with a modulus of 3.6 into the slurry, and stir vigorously evenly. Then add 180L of deionized water, beat again, and use the slurry to spray dry to obtain a powder of approximately spherical particles. The powder is then roasted at 350°C for 5 hours to obtain 32.5kg of iron-based catalyst. The weight ratio of the catalyst is It is Fe/La/Cu/K/SiO ₂ =100:0.02:3:3:10, and the catalyst is marked as D. After the catalyst is sieved, the 50-100 μm segment particles are taken to evaluate the Fischer-Tropsch synthesis reaction performance in a slurry bed laboratory device.

Example 3

Dissolve 60kg of FeSO ₄ ·7H ₂ O in 150L of deionized water, and then add 4L of 3mol/L sulfuric acid solution. 75 L of 5% H ₂ O ₂ aqueous solution was added to the mixed solution, and oxidized at 50° C. for 5.5 hours under vigorous stirring. 2.3kg of Cu(NO ₃ ) ₂ ·3H ₂ O and 18.8g of La(NO ₃ ) ₃ .6H ₂ O were dissolved in 10L of deionized water to obtain a mixed salt solution of copper nitrate and lanthanum nitrate. Add the mixed salt solution of copper nitrate and lanthanum nitrate into the ferric sulfate solution prepared above to obtain the mixed salt solution containing iron, lanthanum and copper. 240L of 9.5wt% ammonia solution was placed in another storage tank. The mixed salt solution and ammonia solution are heated to 65°C with water vapor through the jacket, respectively, and the acid-resistant pump and alkali-resistant pump are used to continuously pump the mixed salt solution and ammonia solution into a vigorously stirred tank at the same flow rate. Keeping the temperature at 65°C, the mixing and continuous coprecipitation process was completed within 30 minutes, and the pH value was 7.5. Leave it to age for 1 hour, filter it with suction, and wash it repeatedly with deionized water until there is no sulfate group detected by BaCl ₂ aqueous solution. Add 35L of water to the filter cake for re-pulping, add 3.6L of potassium silicate water glass solution containing 25wt% _SiO2 with a modulus of 2.5 into the slurry, stir vigorously, and then dry the slurry in an oven at 110°C. Then calcined at 400° C. for 3 hours to obtain 20 kg of iron-based catalyst with a weight ratio of Fe/La/Cu/K/SiO ₂ =100:0.5:5:5:10. The catalyst is marked as E. After the catalyst was sieved, it was then used for fixed-bed reaction evaluation according to Example 1.

Example 4

According to the steps of Example 1, the filter cake was beaten and potassium silicate water glass solution was added and stirred evenly. Then add 120L of deionized water, beat again, and spray dry the slurry to obtain a powder of approximately spherical particles, and then roast at 400 ° C for 3 hours to obtain 21 kg of iron-based catalysts, with a weight ratio of Fe/La/ Cu/K/SiO ₂ =100:0.5:5:5:10, the catalyst is designated as F. After the catalyst is sieved, the 50-100 μm segment particles are taken to evaluate the Fischer-Tropsch synthesis reaction performance in a slurry bed laboratory device.

Example 5

Dissolve 150kg of FeSO ₄ ·7H ₂ O in 450L of deionized water, and then add 14L of 2mol/L sulfuric acid solution. 50 L of 20% H ₂ O ₂ aqueous solution was added to the mixed solution, and oxidized at 40° C. for 2 hours with vigorous stirring. 4.6kg of Cu(NO ₃ ) ₂ ·3H ₂ O and 75.3g of La(NO ₃ ) ₃ .6H ₂ O were dissolved in 20L of deionized water to obtain a mixed salt solution of copper nitrate and lanthanum nitrate. Mix with the prepared ferric sulfate solution to obtain a mixed salt solution containing iron, lanthanum and copper. The NaOH of 57kg is dissolved in 500L deionized water to obtain sodium hydroxide aqueous solution. Heat the mixed salt solution and sodium hydroxide solution separately through the jacket with water vapor and keep them at 85°C, and use the acid-resistant pump and alkali-resistant pump to continuously inject the mixed salt solution and sodium hydroxide solution at the same flow rate for a vigorous stirring In the tank, the temperature was kept at 80°C, the mixing and continuous coprecipitation process was completed within 60 minutes, and the pH value was 7.8. Stand for aging for 3 hours, filter with suction, and wash repeatedly with deionized water until no sulfate radicals are detected with BaCl ₂ aqueous solution. Add 80L of water to the filter cake for re-pulping, add _13L of potassium silicate water glass solution containing 25wt% SiO2 with a modulus of 3.0 into the slurry, stir vigorously, then dry the slurry at 100°C in an oven, and then Calcined at 350° C. for 4 hours to obtain 48 kg of iron-based catalyst, the weight ratio is Fe/La/Cu/K/SiO ₂ =100:0.8:4:6:15, and the catalyst is marked as G. After the catalyst was sieved, it was then used for fixed-bed reaction evaluation according to Example 1.

Example 6

According to the steps in Example 3, the filter cake was beaten and potassium silicate water glass solution was added and stirred evenly. Then add 320L of deionized water, beat again, and spray dry the slurry to obtain a powder of approximately spherical particles, and then roast at 350C for 4 hours to obtain 49kg of iron-based catalyst, the weight ratio is Fe/La/Cu /K/SiO ₂ =100:0.8:4:6:15, the catalyst is denoted as H. After the catalyst is sieved, the 50-100 μm segment particles are taken to evaluate the Fischer-Tropsch synthesis reaction performance in a slurry bed laboratory device.

Catalyst Fischer-Tropsch synthesis reaction evaluation experiment was carried out on laboratory equipment, in which C, E, and G catalysts were selected in fixed-bed reactors, the raw material gas H ₂ /CO=2, space velocity 1500NL/Lcat.h were used, The reaction temperature is 250°C and the reaction pressure is 2.0MPa; D, F, and H catalysts are evaluated in a mechanically stirred slurry bed reactor, the catalyst particles are 50-100 μm, and liquid paraffin is installed inside, and the content of the catalyst in the liquid paraffin is 15% , the raw material gas used is H ₂ /CO=0.67, the space velocity is 2000NL/kgFe.h, the reaction temperature is 260°C, and the reaction pressure is 2.0MPa. The selection results are listed in Table 1.

Table 1 Catalyst evaluation results

Catalyst No. C D E E F F G G H

CO conversion rate, % 87.0 78.0 86.0 81.0 87.36 75.35

CO conversion to _CO2

32.0 48.3 28.3 47.5 27.89 48.5

selectivity, %

Hydrocarbon selectivity, wt%

_CH4 6.32 2.60 5.85 3.21 7.53 3.00

C ₂ -C ₄ 26.40 10.35 24.56 12.35 24.36 13.80

C ₅ -C ₁₁ 30.12 12.56 32.47 13.81 31.45 11.90

C ₁₂ ⁺ 37.16 74.49 37.12 70.63 36.66 71.30

C ₂ ⁼ -C ₄ ⁼ the C ₂ -C ₄ hydrocarbon

                                                                                                                     

Medium ratio, %

It can be seen from Table 1 that the catalyst has high reactivity in a fixed-bed reactor, methane selectivity is lower than 8%, hydrocarbons above _C5 are above 67%, and low-carbon olefins account for _C2 - _C4 hydrocarbons. More than 66%, so the catalyst is a fixed-bed catalyst with excellent performance. The catalyst is used in a slurry bed reactor. Under low H ₂ /CO ratio feed gas and high space velocity, the catalyst has high reactivity, the CO conversion rate is above 75%, and the methane selectivity is 3.5%. In the distribution of hydrocarbons, hydrocarbons above _C12 account for a large proportion, and low-carbon olefins account for more than 80% of _C2 - _C4 hydrocarbons, so this catalyst is especially suitable for coal-based synthesis in slurry bed reactors Gas synthesis of diesel, gasoline, wax and other products.

Claims (18)

1. An iron-based catalyst for Fischer-Tropsch synthesis, characterized in that the weight ratio composition of the catalyst is: Fe:La:Cu:K:SiO ₂ =100:0.02～2:0.01～5:0.5～10:5～30. 2 . The iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 1 , wherein the weight ratio of K is 1-7. 3 . 3. a kind of iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 1, is characterized in that described _SiO The weight ratio is 8～27. 4. the preparation method of a kind of Fischer-Tropsch synthesis iron-based catalyst as claimed in claim 1, is characterized in that described preparation method comprises the steps: (1) molar concentration is that 0.5～10mol/L ferrous sulfate solution, concentrated sulfuric acid and weight percent concentration are 3～30wt% hydrogen peroxide solution is ferrous sulfate by molar ratio: sulfuric acid: hydrogen peroxide=1: 1.6～2.0: 3.7 Proportional mixing, under the condition of oxidation temperature of 20-80°C, oxidation reaction time of 1-10 hours, to prepare ferric sulfate solution; (2) According to the weight ratio of the catalyst, it is composed of Fe: La: Cu: K: SiO ₂ =100: 0.02-2: 0.01-5: 0.5-10: 5-30, the total molar concentration of copper salt and lanthanum nitrate is 1 ～4mol/L copper lanthanum aqueous solution; (3) the alkaline precipitating agent is formulated into a molar concentration of 1 to 6mol/L precipitating agent aqueous solution; (4) According to the composition of the catalyst, after mixing the ferric sulfate solution and the copper lanthanum aqueous solution, under the condition that the precipitation temperature is 20-95°C, add the precipitant aqueous solution to make the solution pH=5-10, and the rapid precipitation time is 5-60 minutes ; (5) Stand for aging for 1 to 2 hours, filter with suction, and wash the precipitate repeatedly with deionized water until there is no sulfate ion; (6) Deionized water is added to the precipitate for slurrying, and the molar ratio is SiO ₂ : K ₂ O=1～10 Potassium silicate water glass, the amount of potassium silicate water glass is added according to the composition of the catalyst, and stirred Uniformly, obtain catalyst slurry; (7) a. The catalyst slurry is dried, roasted, tableted and broken, and 20-40 meshes are selected as the fixed-bed Fischer-Tropsch synthesis catalyst; b. Spray-dry and roast the catalyst slurry, and select 50-100 μm as the slurry-bed Fischer-Tropsch synthesis catalyst. 5. the preparation method of a kind of iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, is characterized in that described ferrous sulfate solution molar concentration is 1～5mol/L. 6. the preparation method of a kind of Fischer-Tropsch synthesis iron-based catalyst as claimed in claim 4 is characterized in that described ferrous sulfate is the by-product of titanium dioxide factory, steelworks. 7. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the weight percent concentration of the hydrogen peroxide solution is 10-20%. 8 . The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4 , characterized in that the oxidation temperature is 25-60° C. 9. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the oxidation reaction time is 2 to 6 hours. 10. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the molar ratio of the ferrous sulfate, sulfuric acid and hydrogen peroxide is 1: 1.8～1.95: 4～6. 11. the preparation method of a kind of Fischer-Tropsch synthesis iron-based catalyst as claimed in claim 4 is characterized in that described copper salt is copper nitrate, copper sulfate or copper acetate. 12. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the total molar concentration of said lanthanum nitrate and copper salt is 1.5-3.0 mol/L. 13. A method for preparing an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, wherein the precipitating agent is Na ₂ CO ₃ , ammonia, NaOH, KOH or K ₂ CO ₃ , preferably Na ₂ CO ₃ , ammonia or NaOH. 14. The method for preparing an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the molar concentration of the aqueous precipitant solution is 1.5-4.5 mol/L. 15. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the precipitation temperature is 60-90°C. 16. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the pH value is 7-8.5. 17. The preparation method of an iron-based catalyst for Fischer-Tropsch synthesis as claimed in claim 4, characterized in that the precipitation time is 10 to 30 minutes. 18. The method for preparing an iron-based catalyst for Fischer-Tropsch synthesis according to claim 4, characterized in that the molar ratio of SiO ₂ to K ₂ O is 2-5.