Fe3C-containing composite catalyst as well as preparation method and application thereof

Abstract

The invention provides a preparation method of a composite catalyst containing Fe3C. The preparation method comprises the following steps: preparing a ferric salt solution and a precipitant solution; mixing the ferric salt solution and the precipitant solution in a parallel flow manner, and carrying out a co-precipitation reaction; carrying out hydrothermal aging on the obtained coprecipitation slurry; slurrying the obtained filter cake, carrying out spray drying, and roasting in a muffle furnace to obtain a catalyst precursor; and carrying out reduction carbonization on the catalyst precursor, and then carrying out decarbonization treatment to obtain the Fe3C-containing composite catalyst. The catalyst prepared by the invention is high in CO conversion rate and low in CO2 and CH4 selectivity in Fischer-Tropsch synthesis, and has excellent catalytic activity and stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a composite catalyst containing Fe3C and a preparation method thereof. Background Art

[0002] In the 1920s of the last century, German scientists F. Fischer and H. Tropsch discovered a method for synthesizing hydrocarbons or oxygen-containing compounds from syngas under the action of iron or cobalt catalysts. Since then, the method of producing various hydrocarbons and oxygen-containing organic compounds from syngas has been called the Fischer-Tropsch (F-T) synthesis method. This technology can not only effectively reduce air pollution, but also provide a new way for the development of alternative oil resources, and thus has attracted more and more attention.

[0003] The preparation methods of iron-based catalysts mainly include precipitation method and melting method. Among them, the catalyst prepared by the melting method has a lower specific surface area and lower activity, but higher mechanical strength, and is suitable for high-temperature Fischer-Tropsch synthesis processes using fluidized bed reactors. The catalyst prepared by the precipitation method has a higher specific surface area and higher activity, but slightly lower mechanical strength, and is suitable for low-temperature Fischer-Tropsch synthesis processes using fixed bed reactors and slurry bed reactors.

[0004] In the Fischer-Tropsch synthesis reaction system under the action of iron-based catalysts, in addition to the main reaction of generating hydrocarbons (CO + 2H2 = -CH2- + H2O), there are also water-gas shift reaction (CO + H2O = CO2 + H2) and methane formation reaction (CO + 3H2 = CH4 + H2O). Among them, the water-gas shift reaction can release hydrogen in water and supplement the hydrogen source to the reaction system. However, when the selectivity of the water-gas shift reaction is too high, it will lead to too high CO2 selectivity, resulting in too much inert CO2 gas in the system, which not only consumes CO, but also reduces the effective volume of the device and increases the operating cost. The methane formation reaction consumes CO and H2, but generates low-value methane and water. Therefore, the water-gas shift reaction and the methane formation reaction are side reactions that are desired to be suppressed as much as possible.

[0005] Patent CN107413362A uses organic sugars such as glucose as a carbon source, urea as a nitrogen source and pore-forming agent, mixes iron salts with these two substances and melts them, and calcines them at 700°C - 750°C in an inert atmosphere to obtain a graphene-coated Fe3C catalyst. For the catalyst prepared by the preparation method of patent CN107413362A, a large amount of carbon layers are coated on the surface of the active phase. Although some carbon layers with graphene structure are beneficial to the selectivity of the catalyst, the coverage of a large number of active sites makes the catalyst have to be evaluated at a higher temperature to achieve a certain activity.

[0006] Patent CN112569981 uses a mixed precipitation of an iron salt solution and an alkaline precipitant. The obtained precipitate is washed, dried, and calcined to obtain a precursor. The precursor is reduced in a hydrogen atmosphere at 470°C - 620°C, and then the reduced material is carbonized using syngas (hydrogen + CO) at 280°C - 420°C to obtain iron carbide Fe3C. Patent CN112569981 uses syngas to carbonize Fe at high temperature. This process is completed at high temperature, and carbon deposition is likely to form on the catalyst surface, which has an adverse effect on the catalyst activity.

[0007] Therefore, there is still a need in the art for a highly active iron-based catalyst with high CO conversion and low CO2 and CH4 selectivity for Fischer-Tropsch synthesis. Summary of the Invention

[0008] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a composite catalyst containing Fe3C and a preparation method thereof. This catalyst has high CO conversion and low CO2 and CH4 selectivity in Fischer-Tropsch synthesis.

[0009] The technical solution of the preparation method of the composite catalyst containing Fe3C of the present invention is as follows:

[0010] A preparation method of a composite catalyst containing Fe3C, comprising the following steps:

[0011] Step 1: Prepare an iron salt solution (aqueous iron salt solution);

[0012] Step 2: Prepare a precipitant solution;

[0013] Step 3: Mix the iron salt solution and the precipitant solution in a co-current manner for coprecipitation reaction;

[0014] Step 4: After the coprecipitation reaction is completed, put the coprecipitation slurry into a crystallization kettle for hydrothermal aging. After the slurry cools, wash and filter it repeatedly with deionized water to obtain a filter cake;

[0015] Or

[0016] After the coprecipitation reaction is completed, wash and filter the coprecipitation slurry repeatedly with deionized water to obtain a filter cake. Pulp the filter cake, put the obtained slurry into a crystallization kettle for hydrothermal aging, and centrifuge to obtain a filter cake after the reaction;

[0017] Step 5: After pulping the above filter cake, perform spray drying (to obtain microsphere catalyst), and then calcine it in a muffle furnace to obtain a catalyst precursor (Fe2O3 catalyst precursor);

[0018] Step 6: Perform reduction carbonization on the catalyst precursor, and then perform decarbonization treatment to obtain a composite catalyst containing Fe3C.

[0019] The preparation method according to the present invention, wherein, in step 1, the iron salt is Fe(NO3)3·9H2O, FeCl3·6H2O or Fe2(SO4)3·xH2O.

[0020] The preparation method according to the present invention, wherein, in step 1, the concentration of Fe in the iron salt solution 3+ is 0.5-3 wt%, for example, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt% or 3 wt%.

[0021] The preparation method according to the present invention, wherein, in step 2, water is added to the inorganic base, stirred evenly, and then potassium silicate solution or silica sol is added and mixed evenly to obtain a precipitant solution. Wherein, the inorganic base is concentrated ammonia water, sodium carbonate, sodium bicarbonate, potassium carbonate or potassium bicarbonate.

[0022] The preparation method according to the present invention, wherein, in step 2, the concentration of the inorganic base in the precipitant solution is 0.5-4 wt%, for example, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt% or 4 wt%.

[0023] The preparation method according to the present invention, wherein, in step 2, the potassium silicate solution or silica sol added contains 15 wt%-25 wt% of SiO2, for example, contains 15 wt%, 20 wt% or 25 wt% of SiO2.

[0024] The preparation method according to the present invention, wherein, in step 2, the molar ratio of the inorganic base in the precipitant solution to the silicon dioxide in the potassium silicate solution or silica sol is 3.7-40:1, for example, 3.7:1, 4:1, 7.4:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, preferably 3.7-30:1, more preferably 3.7-20:1.

[0025] The preparation method according to the present invention, wherein, in step 3, the flow rate ratio of the iron salt solution to the precipitant solution is 5-25:10 (the flow rate unit is g / min), for example, 5:10, 10:10, 15:10, 20:10, 25:10.

[0026] The preparation method according to the present invention, wherein, in step 3, the pH of the mixed solution of the iron salt solution and the precipitant solution is 5-9, for example, 5, 6, 7, 8 or 9.

[0027] According to the preparation method of the present invention, in step 3, the temperature of the coprecipitation reaction is: 20°C - 60°C, for example, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C or 60°C; the time is 30 - 120 min, for example, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min or 120 min.

[0028] According to the preparation method of the present invention, in step 4, after the coprecipitation reaction ends, the coprecipitation slurry is repeatedly washed and filtered with deionized water to obtain a filter cake, the filter cake is slurried to obtain a slurry with a solid content (calculated as Fe2O3) of 0.5 wt% - 4 wt%, and the slurry is put into a crystallization kettle for hydrothermal aging, and after the reaction ends, the filter cake is obtained by centrifugation.

[0029] According to the preparation method of the present invention, in step 4, the hydrothermal aging temperature is 160°C - 240°C, and the hydrothermal aging time is 2 h - 16 h.

[0030] According to the preparation method of the present invention, in step 4, it is repeatedly washed and filtered with deionized water until the conductivity of the filtrate is below 1 ms / cm.

[0031] According to the preparation method of the present invention, in step 5, the conditions for spray drying are: the inlet air temperature is 180 - 300°C, and the outlet air temperature is 100 - 150°C.

[0032] According to the preparation method of the present invention, in step 5, the conditions for roasting are: heating to 100 - 150°C (for example, 100°C, 110°C, 120°C, 130°C, 140°C or 150°C) at a rate of 5 - 15°C / min, holding at this temperature for 5 - 20 h, and then heating to 300°C - 600°C (for example, 300°C, 400°C, 500°C or 600°C) at a rate of 5 - 15°C / min, and holding at this temperature for 4 h - 10 h.

[0033] According to the preparation method of the present invention, in step 6, the conditions for reduction carbonization are: introducing a mixed gas of 5% - 100% CO (for example, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100% CO) and an inert gas, heating to 320°C - 370°C at a rate of 1°C / min - 15°C / min, and performing reduction carbonization at this temperature for 2 - 24 h; or introducing a reducing atmosphere of H2:CO = (20 - 0):1, heating to 320°C - 370°C at a rate of 1°C / min - 15°C / min, and performing reduction carbonization at this temperature for 2 - 24 h.

[0034] According to the preparation method of the present invention, in step 6, after reduction carbonization, the temperature is lowered to 200°C - 300°C (for example, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C or 300°C), and then decarbonization treatment is carried out.

[0035] According to the preparation method of the present invention, in step 6, the conditions for decarbonization treatment are as follows: a mixed gas of 5% - 100% hydrogen and an inert gas, heated at 1°C / min - 5°C / min to 420°C - 650°C, and decarbonization treatment is carried out for 1h - 12h.

[0036] According to the preparation method of the present invention, in step 6, the inert gas is nitrogen, Ar, or He.

[0037] Another object of the present invention is to provide a composite catalyst containing Fe₃C, which is prepared by the method described above.

[0038] According to the composite catalyst containing Fe₃C of the present invention, the composite catalyst containing Fe₃C is a Fe₃C and Fe₅C₂ composite catalyst; preferably, the composite catalyst containing Fe₃C is a θ-Fe₃C and χ-Fe₅C₂ composite catalyst.

[0039] Another object of the present invention is to provide the use of the above-mentioned composite catalyst containing Fe₃C in the Fischer-Tropsch synthesis reaction.

[0040] Beneficial effects

[0041] Through the preparation process combining co-precipitation and hydrothermal synthesis, the present invention prepares a Fe₂O₃ catalyst precursor with a special crystal plane orientation. After high-temperature reduction carbonization, an Fe₅C₂ phase iron carbide is formed, and then a composite catalyst with a Fe₃C crystal phase is prepared by decarbonization in a hydrogen atmosphere at 420°C - 650°C. Under the evaluation conditions, when switched to syngas, the catalyst transforms into a Fe₃C and Fe₅C₂ composite phase catalyst.

[0042] By using a Fe₂O₃ catalyst precursor with a special morphology and preparing a composite catalyst with a Fe₃C crystal phase by the method of carbonization first and then decarbonization, the present invention greatly reduces the carbon deposition on the active surface area of the catalyst, and well maintains the reaction activity and stability of the catalyst, and can have high activity at about 300°C. The present invention can be applied to a high-temperature Fischer-Tropsch system of 280°C - 350°C (preferably 280°C - 300°C) and has good Fischer-Tropsch reaction performance. Description of the drawings

[0043] Figure 1 The XRD spectrum of the Fe₂O₃ catalyst precursor with a (110) crystal plane dominant orientation prepared in Example 1 of the present application is shown.

[0044] Figure 2 XRD patterns of the composite catalysts with Fe3C crystal phase formed by hydrodecarbonylation after carbonization of the Fe2O3 catalyst precursors in Example 4 (decarbonization treatment temperature 420 °C) and Example 6 (decarbonization treatment temperature 600 °C) of this application are shown.

[0045] Figure 3 XRD pattern of the Fe5C2 catalyst prepared in Comparative Example 2 is shown.

[0046] Figure 4 XRD pattern of the Fe5C2 catalyst prepared in Comparative Example 3 is shown. Detailed implementation manners

[0047] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein.

[0048] The described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention belong to the scope of protection of the present invention.

[0049] Unless otherwise specified, the raw materials used in the following examples are all purchased commercially.

[0050] Examples

[0051] Example 1

[0052] Weigh 150 g of Fe(NO3)3·9H2O, dissolve it with deionized water and stir to prepare an iron salt solution with a concentration of 1.5 wt%. 3+ An iron salt solution with a concentration of 1.5 wt% is obtained.

[0053] Weigh 75 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with an NH3 concentration of 3 wt%. Then add 25 g of potassium silicate solution with a SiO2 content of 20%, and mix evenly.

[0054] Mix the iron salt solution and the precipitant solution in a co-current manner to carry out a co-precipitation reaction at a temperature of 40 °C. Control the pH value in the reaction kettle to 7 by adjusting the pump speeds of the two pumps respectively, and the reaction time is 40 min.

[0055] After the co-precipitation reaction is completed, put the obtained precipitation slurry into a crystallization kettle, hydrothermally age it at 170 °C for 5 h. After the slurry cools, wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake.

[0056] After slurrying the above-mentioned filter cake, spray drying is carried out under the conditions that the inlet air temperature is 200 °C and the outlet air temperature is 115 °C. The obtained microsphere catalyst is heated to 120 °C at a rate of 15 °C / min in a muffle furnace, held at this temperature for 8 h, then heated to 400 °C at a rate of 10 °C / min, and held at 400 °C for 5 h to obtain a catalyst precursor; this catalyst precursor is an Fe2O3 catalyst precursor with a preferential orientation of the (110) crystal plane, and its XRD pattern is as Figure 1 shown;

[0057] Put the catalyst precursor equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixed gas of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, then cool to 250 °C after carbonization, switch to hydrogen, heat to 440 °C at a rate of 1 °C / min, and carry out decarbonization treatment for 6 h to obtain an activated catalyst with a composite structure of θ-Fe3C and χ-Fe5C2. Its XRD pattern is the same as that of the activated catalyst with a composite structure of θ-Fe3C and χ-Fe5C2 prepared in Example 4 Figure 2 consistent ( Figure 2 XRD pattern A in).

[0058] Example 2

[0059] Weigh 150 g of Fe(NO3)3·9H2O, dissolve it with stirring in deionized water to prepare an iron salt solution with an Fe 3+ concentration of 1.5 wt%;

[0060] Weigh 75 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with NH3 of 3 wt%, and then add 35 g of potassium silicate solution with a SiO2 content of 20%, and mix evenly;

[0061] Mix the iron salt solution and the precipitant solution in a co-current manner for coprecipitation reaction at a temperature of 20 °C. Control the pH value in the reaction kettle to 7 by adjusting the pump speeds of the two pumps respectively, and the reaction time is 40 min;

[0062] After the coprecipitation reaction is completed, put the obtained precipitation slurry into a crystallization kettle, carry out hydrothermal aging at 160 °C for 16 h. After the slurry cools, wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake;

[0063] After slurrying the above-mentioned filter cake, spray drying is carried out under the conditions that the inlet air temperature is 250 °C and the outlet air temperature is 115 °C. The obtained microsphere catalyst is heated to 105 °C at a rate of 15 °C / min in a muffle furnace, held at this temperature for 8 h, then heated to 400 °C at a rate of 10 °C / min, and held at 400 °C for 5 h to obtain a catalyst precursor. Its XRD pattern is the same as that of the catalyst precursor prepared in Example 1 Figure 1Consistent;

[0064] Put the catalyst precursor equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixed gas of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, then cool down to 200 °C after carbonization, switch to a 50 vol% H2-N2 gas mixture, heat up to 450 °C at a rate of 1 °C / min, and perform decarburization treatment for 8 h to obtain a catalyst with an activated θ-Fe3C and χ-Fe5C2 composite structure. Its XRD pattern is the same as the XRD pattern of the activated θ-Fe3C and χ-Fe5C2 composite structure catalyst prepared in Example 4 Figure 2 Consistent ( Figure 2 XRD pattern A in).

[0065] Example 3

[0066] Weigh 150 g of Fe(NO3)3·9H2O and dissolve it with stirring in deionized water to prepare an iron salt solution with a concentration of 1.5 wt% 3+ ;

[0067] Weigh 100 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with 3 wt% NH3, then add 30 g of potassium silicate solution with 20% SiO2 content and mix evenly;

[0068] Mix the iron salt solution and the precipitant solution in a parallel flow for coprecipitation reaction at a temperature of 50 °C, and control the pH value in the reaction kettle to 9 by adjusting the pump speeds of the two pumps respectively, with a reaction time of 40 min;

[0069] After the coprecipitation reaction, obtain the precipitate slurry and wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm. After the obtained filter cake is slurried, put the precipitate slurry into a crystallization kettle and hydrothermally age it at 200 °C for 4 h. After the slurry cools, wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake;

[0070] After slurrying the above filter cake, perform spray drying under the conditions of an inlet air temperature of 220 °C and an outlet air temperature of 130 °C. The obtained microsphere catalyst is heated to 120 °C at a rate of 15 °C / min in a muffle furnace, kept at this temperature for 8 h, then heated to 400 °C at a rate of 10 °C / min and kept at 400 °C for 5 h to obtain a catalyst precursor. Its XRD pattern is the same as the XRD pattern of the catalyst precursor prepared in Example 1 Figure 1 Consistent;

[0071] Put the catalyst equivalent to 1 g of Fe in terms of the converted calculation into a fixed-bed device, use a mixed gas of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h. After carbonization, cool down to 250 °C, switch to a 30 vol% H2-Ar gas mixture, and heat up to 440 °C at a rate of 1 °C / min for decarburization treatment for 16 h to obtain the catalyst with an activated θ-Fe3C and χ-Fe5C2 composite structure. Its XRD pattern is the same as the XRD pattern of the catalyst with an activated θ-Fe3C and χ-Fe5C2 composite structure prepared in Example 4 Figure 2 consistent with( Figure 2 XRD pattern A in).

[0072] Example 4

[0073] Weigh 150 g of Fe(NO3)3·9H2O and dissolve it by stirring in deionized water to prepare an iron salt solution with a concentration of 1.5 wt% 3+ ;

[0074] Weigh 67 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with NH3 being 1 wt%, and then add 7.5 g of potassium silicate solution with 20% SiO2 content and mix evenly

[0075] Mix the iron salt solution and the precipitant solution in a parallel flow for coprecipitation reaction at a temperature of 60 °C. Control the pH value in the reaction kettle to 5 by adjusting the pump speeds of the two pumps respectively, and the reaction time is 40 min

[0076] After the coprecipitation reaction, obtain the precipitate slurry and wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm. After the obtained filter cake is slurried, put the precipitate slurry into a crystallization kettle and hydrothermally age it at 180 °C for 5 h. After the slurry cools down, wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake

[0077] After the above filter cake is slurried, carry out spray drying under the conditions that the inlet air temperature is 200 °C and the outlet air temperature is 115 °C. The obtained microsphere catalyst is heated to 120 °C at a rate of 15 °C / min in a muffle furnace, kept at this temperature for 8 h, and then heated to 400 °C at a rate of 10 °C / min and kept at 400 °C for 5 h to obtain the catalyst precursor. Its XRD pattern is the same as the XRD pattern of the catalyst precursor prepared in Example 1 Figure 1 consistent;

[0078] Put the catalyst equivalent to 1 g of Fe in mass after folding calculation into a fixed-bed device, use a mixed gas of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, then cool down to 250 °C after carbonization, switch to hydrogen, and heat up to 420 °C at a rate of 1 °C / min for decarbonization treatment for 12 h to obtain an activated θ-Fe3C and χ-Fe5C2 composite structure catalyst, and its XRD pattern is as Figure 2 shown ( Figure 2 XRD pattern A in

[0079] Example 5

[0080] Weigh 75 g of Fe2(SO4)3·xH2O and stir it with deionized water to dissolve and prepare an iron salt solution with a Fe 3+ concentration of 3 wt%.

[0081] Weigh 75 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with NH3 of 3.0 wt%, and then add 44.5 g of potassium silicate solution with a SiO2 content of 20%, and mix evenly;

[0082] Mix the iron salt solution and the precipitant solution in parallel for coprecipitation reaction at a temperature of 35 °C, and control the pH value in the reaction kettle to 7.5 by adjusting the pump speeds of the two pumps respectively, and the reaction time is 40 min;

[0083] After the coprecipitation reaction, obtain the precipitate slurry and wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm. After the obtained filter cake is slurried, put the precipitate slurry into a crystallization kettle and hydrothermally age it at 180 °C for 5 h. After the slurry cools, wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake;

[0084] After the above filter cake is slurried, perform spray drying under the conditions of an inlet air temperature of 300 °C and an outlet air temperature of 150 °C. The obtained microsphere catalyst is heated to 100 °C at a rate of 15 °C / min in a muffle furnace, held at this temperature for 8 h, and then heated to 400 °C at a rate of 10 °C / min and held at 400 °C for 5 h to obtain a catalyst precursor, and its XRD pattern is the same as that of the catalyst precursor prepared in Example 1 Figure 1 consistent;

[0085] Put the catalyst equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixed gas of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, then cool down to 250 °C after carbonization, switch to 5% vol H2-He, and heat up to 650 °C at a rate of 1 °C / min for decarbonization treatment for 24 h to obtain a catalyst with an activated θ-Fe3C and α-Fe composite structure. Its XRD pattern is the same as that of the catalyst with an activated θ-Fe3C and α-Fe composite structure prepared in Example 6 Figure 2 consistent with ( Figure 2 XRD pattern B in).

[0086] Cool down the above catalyst to 280 °C, switch to H2:CO = 5:1 syngas, and hold for 6 h. The active phase of the catalyst is a catalyst with a θ-Fe3C and χ-Fe5C2 composite structure. Its XRD pattern is the same as that of the catalyst with an activated θ-Fe3C and χ-Fe5C2 composite structure prepared in Example 4 Figure 2 consistent with ( Figure 2 XRD pattern A in).

[0087] Example 6

[0088] Weigh 100 g of FeCl3·6H2O and dissolve it in deionized water with stirring to prepare an iron salt solution with a concentration of 0.5 wt% of Fe 3+ ;

[0089] Weigh 75 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with 0.5 wt% of NH3, and then add 37.5 g of a sol solution with 20% SiO2 content and mix evenly;

[0090] Mix the iron salt solution and the precipitant solution in a parallel flow for coprecipitation reaction at a temperature of 40 °C. Control the pH value in the reaction kettle to 6 by adjusting the pump speeds of the two pumps respectively, and the reaction time is 120 min;

[0091] After the coprecipitation reaction, obtain the precipitate slurry and wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm. After obtaining the filter cake and slurrying it, put the precipitate slurry into a crystallization kettle and hydrothermally age it at 220 °C for 5 h. After the slurry cools down, filter to obtain the filter cake;

[0092] After slurrying the above filter cake, perform spray drying under the conditions of an inlet air temperature of 200 °C and an outlet air temperature of 115 °C. The obtained microsphere catalyst is heated to 120 °C at a rate of 15 °C / min in a muffle furnace, held at this temperature for 8 h, and then heated to 500 °C at a rate of 10 °C / min and held at 500 °C for 5 h to obtain a catalyst precursor. Its XRD pattern is the same as that of the catalyst precursor prepared in Example 1 Figure 1 consistent;

[0093] Put the catalyst equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixed gas of H2:CO = 5:1 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, then cool down to 250 °C after carbonization, switch to a 70 vol% H2-N2 mixed gas, and heat up to 600 °C at a rate of 1 °C / min for decarburization treatment for 2 h to obtain a catalyst with an activated θ-Fe3C and α-Fe composite structure, and its XRD pattern is as Figure 2 shown ( Figure 2 XRD pattern B in

[0094] Cool the above catalyst to 280 °C, switch to a synthesis gas of H2:CO = 5:1, and hold for 6 h. The active phase of the catalyst is a catalyst with a θ-Fe3C and χ-Fe5C2 composite structure, and its XRD pattern is the same as that of the activated θ-Fe3C and χ-Fe5C2 composite structure catalyst prepared in Example 4 Figure 2 consistent ( Figure 2 XRD pattern A in

[0095] Example 7

[0096] Prepare a composite catalyst containing Fe3C using the catalyst precursor prepared in Example 1.

[0097] Put the catalyst equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixed gas of H2:CO = 20:1 as the reduction atmosphere, reduce and carbonize at 320 °C for 8 h, then cool down to 250 °C after carbonization, switch to hydrogen, and heat up to 450 °C at a rate of 1 °C / min for decarburization treatment for 6 h to obtain a catalyst with an activated θ-Fe3C and χ-Fe5C2 composite structure, and its XRD pattern is the same as that of the activated θ-Fe3C and χ-Fe5C2 composite structure catalyst prepared in Example 4 Figure 2 consistent ( Figure 2 XRD pattern A in

[0098] Comparative Example

[0099] Comparative Example 1

[0100] Place 4 g of maltose and 6 g of pyridine in a 100 ml beaker, mechanically stir to mix them evenly, then place them in a heating jacket at 165 °C and continue stirring for 10 min until both are completely melted. Add 6 g of ferric nitrate nonahydrate and continue to stir vigorously for 5 min until completely miscible. Quickly transfer the beaker to an oven at 180 °C, keep it for 20 h, then take out the product and grind it into powder. Roast the ground powder in a tubular furnace under a nitrogen atmosphere at 750 °C for 2 h to obtain a comparative catalyst.

[0101] Comparative Example 2

[0102] Weigh 150 g of Fe(NO3)3·9H2O and dissolve it by stirring in deionized water to prepare an iron salt solution with an Fe 3+ concentration of 1.5 wt%;

[0103] Weigh 75 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with 3 wt% NH3, and then add 30 g of potassium silicate solution with 20% SiO2 content and mix evenly;

[0104] Mix the iron salt solution and the precipitant solution in a parallel flow and carry out a co-precipitation reaction at a temperature of 40 °C. Control the pH value in the reaction kettle to 7 by adjusting the pump speeds of two pumps respectively, and the reaction time is 40 min;

[0105] Wash and filter the above slurry with deionized water repeatedly until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake;

[0106] After slurrying the above filter cake, carry out spray drying under the conditions of an inlet air temperature of 200 °C and an outlet air temperature of 115 °C. The obtained microsphere catalyst is heated in a muffle furnace to 120 °C at a rate of 15 °C / min, kept at this temperature for 8 h, and then heated to 400 °C at a rate of 10 °C / min and kept at 400 °C for 5 h to obtain a catalyst precursor;

[0107] Put the catalyst equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixture of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, cool down to 250 °C after carbonization, switch to hydrogen, and heat up to 440 °C at a rate of 1 °C / min for decarbonization treatment for 6 h to obtain an activated α-Fe and χ-Fe5C2 mixed-phase catalyst.

[0108] Cool the above catalyst to 280 °C, switch to H2:CO = 5:1 syngas, and keep it for 6 h. The active phase of the catalyst is a χ-Fe5C2 phase catalyst, and its XRD pattern is as Figure 3 shown.

[0109] Comparative Example 3

[0110] Weigh 150 g of Fe(NO3)3·9H2O and dissolve it by stirring in deionized water to prepare an iron salt solution with an Fe 3+ concentration of 1.5 wt%;

[0111] Weigh 75 g of 25 wt% concentrated ammonia water, add deionized water and stir evenly to obtain a precipitant solution with 3 wt% NH3, and then add 30 g of potassium silicate solution with 20% SiO2 content and mix evenly;

[0112] The iron salt solution and the precipitant solution are mixed in a co-current manner to carry out a co-precipitation reaction at a temperature of 40 °C. The pH value in the reaction kettle is controlled at 7 by adjusting the speeds of two pumps respectively, and the reaction time is 40 min.

[0113] The above-mentioned slurry is washed and filtered repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain a filter cake.

[0114] After the above filter cake is slurried, it is spray-dried under the conditions that the inlet air temperature is 200 °C and the outlet air temperature is 115 °C. The obtained microsphere catalyst is heated to 120 °C at a rate of 15 °C / min in a muffle furnace, kept at this temperature for 8 h, then heated to 400 °C at a rate of 10 °C / min, and kept at 400 °C for 5 h to obtain a catalyst precursor.

[0115] Put the catalyst equivalent to 1 g of Fe mass after conversion calculation into a fixed-bed device, use a mixed gas of 50% CO and N2 as the reduction atmosphere, reduce and carbonize at 330 °C for 8 h, then cool down to 250 °C after carbonization, switch to hydrogen, and heat up to 600 °C at a rate of 1 °C / min for decarburization treatment for 6 h to obtain an activated α-Fe phase catalyst.

[0116] Cool the above catalyst to 280 °C, switch to a synthesis gas of H2:CO = 5:1, and keep it for 6 h. The active phase of the catalyst is a χ-Fe5C2 phase catalyst, and its XRD pattern is as Figure 4 shown.

[0117] Comparative Example 4

[0118] Mix 1.2 mol / L iron nitrate and 0.9 mo1 / L sodium carbonate solution at 55 °C and pH = 6.2 to obtain a precipitate slurry. Wash it with deionized water, filter to obtain a filter cake, dry it at 110 °C for 24 h, and calcine it at 400 °C for 10 h to obtain a precursor.

[0119] Reduce the precursor with H2 at a pressure of 2.6 atm, a flow rate of H2 of 22000 mL / h / g, and a temperature of 470 °C for 12 h;

[0120] Cool the above reduction product from 470 °C to 400 °C at a rate of 1.5 °C / min, and contact it with a mixed gas of H2 and CO at this temperature to prepare a precipitated carbide. The conditions are: pressure 20 atm, total gas flow rate 20000 mL / h / g, molar ratio of H2 to CO 60:1, treatment time 24 h, to obtain a precipitated iron carbide, denoted as iron carbide 1;

[0121] Under Ar gas protection, mix 97 parts by mole of precipitated iron carbide 1 with 3 parts by mole of ferrous oxide (i.e., containing Fe impurities). After mixing, a comparative catalyst is obtained.

[0122] Test example

[0123] The performance of the catalysts prepared in each example and comparative example was tested.

[0124] The samples were isolated from air and sealed in liquid paraffin, and the iron phase composition of the catalysts was analyzed by Mossbauer spectroscopy.

[0125] Table 1 Analysis results of the iron phase composition of the catalysts by Mossbauer spectroscopy

[0126]

[0127]

[0128] Test conditions:

[0129] After putting the catalyst precursor equivalent to 1 g of Fe mass after conversion calculation into the fixed-bed device, it was pretreated according to the reduction and carbonization conditions in the corresponding example. After reduction and carbonization, it was switched to the reaction conditions of Fischer-Tropsch synthesis for reaction.

[0130] The process conditions for Fischer-Tropsch synthesis are as follows:

[0131] Fischer-Tropsch synthesis: After the reduction and carbonization were completed, the reaction temperature was adjusted to 290 °C and kept stable. The H2 / CO ratio of the syngas was adjusted to 2:1, and the space velocity was 15000 mL / g-Fe / h. After stabilizing for a period of time until the gas-phase composition in the reaction tail gas basically did not change with time, it was recorded as the reaction starting point.

[0132] Data calculation: By measuring and analyzing the mole number of CO in the feed at the feed inlet of the stirred tank and the mole numbers of CO, CO2, and CH4 in the product at the product outlet, the CO conversion rate %, CO2 selectivity %, and CH4 selectivity % were calculated through the following formulas:

[0133] CO conversion rate % = [(mole number of CO in the feed - mole number of CO in the product) / mole number of CO in the feed] × 100%;

[0134] CO2 selectivity % = [mole number of CO2 in the product / (mole number of CO in the feed - mole number of CO in the product)] × 100%;

[0135] CH4 selectivity % = [mole number of CH4 in the product / (mole number of CO in the feed × CO conversion rate %

[0136] (1 - CO2 selectivity %))] × 100%.

[0137] The deactivation rate of the catalyst refers to the decline rate of the reaction activity (carbon monoxide conversion rate) of the catalyst from the reaction stable period to the end of the reaction, and the unit is % / h.

[0138] Table 2 Catalyst Performance Evaluation Results

[0139]

[0140]

[0141] As can be seen from Table 2, the catalyst provided by the present invention is significantly superior to the comparative catalyst in terms of the selectivity of CO2 and CH4 and the deactivation rate. Therefore, the catalyst provided by the present invention has excellent catalytic activity and stability.

Claims

1. A preparation method of a composite catalyst containing Fe3C, comprising the following steps: Step 1: Prepare an iron salt solution; Step 2: Prepare a precipitant solution; Step 3: Mix the iron salt solution and the precipitant solution in a co-current manner for coprecipitation reaction; Step 4: After the coprecipitation reaction is completed, put the coprecipitation slurry into a crystallization kettle for hydrothermal aging. After the slurry cools, wash and filter it repeatedly with deionized water to obtain a filter cake; Or After the coprecipitation reaction is completed, wash and filter the coprecipitation slurry repeatedly with deionized water to obtain a filter cake. Pulp the filter cake, put the obtained slurry into a crystallization kettle for hydrothermal aging, and centrifuge to obtain a filter cake after the reaction ends; Step 5: After slurryifying the above filter cake, perform spray drying and then calcine it in a muffle furnace to obtain a catalyst precursor; Step 6: Reduce and carbonize the catalyst precursor, and then perform decarburization treatment to obtain a composite catalyst containing Fe3C.

2. The preparation method according to claim 1, wherein, In Step 1, the iron salt is Fe(NO3)3·9H2O, FeCl3·6H2O or Fe2(SO4)3·xH2O.

3. The preparation method according to claim 1 or 2, wherein In step 1, the concentration of Fe 3+ in the iron salt solution is 0.5-3 wt%.

4. The preparation method according to claim 1 or 2, wherein In Step 2, add water to the inorganic base, stir evenly, and then add potassium silicate solution or silica sol, and mix evenly to obtain a precipitant solution; wherein, the inorganic base is concentrated ammonia water, sodium carbonate, sodium bicarbonate, potassium carbonate or potassium bicarbonate.

5. The preparation method according to claim 1 or 2, wherein In Step 2, the concentration of the inorganic base in the precipitant solution is 0.5-4wt%.

6. The preparation method according to claim 1 or 2, wherein In Step 2, the added potassium silicate solution or silica sol contains 15wt%-25wt% of SiO2.

7. The preparation method according to claim 1 or 2, wherein, In Step 2, the molar ratio of the inorganic base in the precipitant solution to the silicon dioxide in the potassium silicate solution or silica sol is 3.7-40:

1.

8. The preparation method according to claim 1 or 2, wherein In Step 3, the flow rate ratio of the iron salt solution to the precipitant solution is 5-25:

10.

9. The preparation method according to claim 1 or 2, wherein In Step 3, the pH of the mixed solution of the iron salt solution and the precipitant solution is 5-9.

10. The preparation method according to claim 1 or 2, wherein, In Step 3, the temperature of the coprecipitation reaction is: 20°C-60°C; the time is 30-120 min.

11. The preparation method according to claim 1 or 2, wherein, In Step 4, after the coprecipitation reaction is completed, wash the coprecipitation slurry with deionized water, filter it to obtain a filter cake, pulp the filter cake to obtain a slurry with a solid content of 0.5wt%-4wt%, put the slurry into a crystallization kettle for hydrothermal aging, and centrifuge to obtain a filter cake after the reaction ends.

12. The preparation method according to claim 1 or 2, wherein In Step 4, the hydrothermal aging temperature is 160°C-240°C, and the hydrothermal aging time is 2h-16h.

13. The preparation method according to claim 1 or 2, wherein In Step 4, wash and filter repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm.

14. The preparation method according to claim 1 or 2, wherein, In Step 5, the conditions for spray drying are: the inlet air temperature is 180-300°C, and the outlet air temperature is 100-150°C.

15. The preparation method according to claim 1 or 2, wherein In Step 5, the conditions for calcination are: heat up to 100-150°C at a rate of 5-15°C / min, keep the temperature constant at this temperature for 5-20 h, and then heat up to 300°C-600°C at a rate of 5-15°C / min, and keep the temperature constant at this temperature for 4h-10h.

16. The preparation method according to claim 1 or 2, wherein, In step 6, the conditions for reductive carbonization are as follows: introducing a mixed gas of 5%-100% CO and an inert gas, heating to 320°C-370°C at a rate of 1°C / min-15°C / min, and carrying out reductive carbonization at this temperature for 2-24 h; or introducing a reducing atmosphere with H2:CO=(20-0):1, heating to 320°C-370°C at a rate of 1°C / min-15°C / min, and carrying out reductive carbonization at this temperature for 2-24 h.

17. The preparation method according to claim 1 or 2, wherein In step 6, after reductive carbonization, the temperature is lowered to 200°C-300°C, and then decarburization treatment is carried out.

18. The preparation method according to claim 1 or 2, wherein, In step 6, the conditions for decarburization treatment are as follows: a mixed gas of 5%-100% hydrogen and an inert gas, heating to 420°C-650°C at a rate of 1°C / min-5°C / min, and carrying out decarburization treatment for 1 h-12 h.

19. A composite catalyst containing Fe3C, which is prepared by the preparation method according to any one of claims 1-18.

20. Use of the composite catalyst containing Fe3C prepared by the preparation method according to any one of claims 1-18 or the composite catalyst containing Fe3C according to claim 19 in the Fischer-Tropsch synthesis reaction.

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