Fe3C catalyst as well as preparation method and application thereof

Abstract

The invention provides a preparation method of a Fe3C catalyst. The preparation method comprises the following steps: preparing a ferric salt solution; transferring the ferric salt solution into a crystallization kettle for hydrothermal aging; centrifugally separating and collecting solids, washing and drying to obtain a catalyst precursor; and carrying out reduction carbonization on the catalyst precursor, and then carrying out decarbonization treatment to obtain the Fe3C 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 an Fe3C catalyst 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 an iron or cobalt catalyst. 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 the precipitation method and the melting method. Among them, the catalyst prepared by the melting method has a lower specific surface area and lower activity, but has high mechanical strength and is suitable for the high-temperature Fischer-Tropsch synthesis process using a fluidized bed reactor. The catalyst prepared by the precipitation method has a high specific surface area and high activity, but slightly poor mechanical strength and is suitable for the low-temperature Fischer-Tropsch synthesis process using a fixed bed reactor and a slurry bed reactor.

[0004] In the Fischer-Tropsch synthesis reaction system under the action of an iron-based catalyst, in addition to the main reaction of generating hydrocarbons (CO + 2H2 = -CH2- + H2O), there are also the water-gas shift reaction (CO + H2O = CO2 + H2) and the 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. And 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 hoped 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 and melts iron salts and these two substances, and calcines 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 selectivities 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 an Fe3C catalyst and its preparation method, which has high CO conversion and low CO2 and CH4 selectivities in Fischer-Tropsch synthesis.

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

[0010] Step 1: Prepare an iron salt solution;

[0011] Step 2: Transfer the iron salt solution to a crystallization kettle for hydrothermal aging;

[0012] Step 3: Centrifuge and separate to collect the solid, wash it, and the filter cake is dried to obtain catalyst precursor 1 (Fe2O3 catalyst precursor 1);

[0013] Step 4: Redisperse the filter cake after centrifugation and washing in step 3, and then mix it with potassium silicate solution or silica sol and stir evenly. After drying and calcination, catalyst precursor 2 (Fe2O3 catalyst precursor 2) is obtained;

[0014] Step 5: Perform reduction carbonization on catalyst precursor 1 or catalyst precursor 2, and then perform decarbonization treatment to obtain the Fe3C catalyst.

[0015] According to the preparation method of the present invention, in step 1, the iron salt used to prepare the iron salt solution is FeCl3·6H2O, anhydrous FeCl3, Fe(NO3)3·9H2O, etc.

[0016] According to the preparation method of the present invention, in step 1, the iron salt is dissolved in a mixed solution of ethanol and deionized water, stirred evenly, and then sodium acetate is added to obtain the iron salt solution.

[0017] According to the preparation method of the present invention, in step 1, the mass ratio of ethanol to deionized water is 11 - 19:1 (g / g).

[0018] According to the preparation method of the present invention, in step 1, based on the Fe element, the mass ratio of the iron salt to sodium acetate is 4 - 7.5:100 (g / g), preferably 6.5 - 7.2:100 (g / g).

[0019] According to the preparation method of the present invention, in step 1, in the iron salt solution, the concentration of Fe 3+ is 0.37 - 0.65 wt%, preferably 0.55 - 0.63 wt%.

[0020] According to the preparation method of the present invention, in step 1, the pH of the iron salt solution is 5 - 8.

[0021] According to the preparation method of the present invention, in step 2, the temperature of the hydrothermal aging is 170°C - 220°C, and the time is 6 h - 24 h.

[0022] According to the preparation method of the present invention, in step 3, the washing is carried out with deionized water.

[0023] According to the preparation method of the present invention, in step 3, the drying temperature is 100°C - 150°C.

[0024] According to the preparation method of the present invention, in step 4, 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.

[0025] According to the preparation method of the present invention, based on the Fe element, the mass ratio of the iron salt to SiO2 in the potassium silicate solution or silica sol in step 4 is 70:5 - 30 (g / g), preferably 70:10 - 25.

[0026] According to the preparation method of the present invention, in step 4, the drying is spray drying, and the conditions are: the inlet air temperature is 180°C - 300°C, and the outlet air temperature is 105°C - 125°C.

[0027] According to the preparation method of the present invention, in step 4, the calcination conditions are: heating to 110°C - 150°C (for example, 110°C, 120°C, 130°C, 140°C or 150°C) at a rate of 5 - 20°C / min, maintaining at this temperature for 4 - 16 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 maintaining at this temperature for 4 h - 10 h.

[0028] According to the preparation method of the present invention, in step 5, the conditions for reduction carbonization are as follows: 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, and performing reduction carbonization at 320°C - 370°C for 2 - 24 h.

[0029] According to the preparation method of the present invention, in step 5, after reduction carbonization, the temperature is lowered to 200°C - 280°C (for example, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C or 280°C), and then decarbonization treatment is carried out.

[0030] According to the preparation method of the present invention, in step 5, the conditions for decarbonization treatment are as follows: a mixed gas of 5%-100% hydrogen and an inert gas, heating to 450°C - 540°C at a rate of 1°C / min - 5°C / min, and performing decarbonization treatment for 1 h - 12 h.

[0031] According to the preparation method of the present invention, in step 5, the inert gas is nitrogen, argon or helium.

[0032] Another object of the present invention is to provide an Fe₃C catalyst, which is prepared according to the method described above.

[0033] According to the Fe₃C catalyst of the present invention, the Fe₃C catalyst is a θ-Fe₃C crystal phase catalyst.

[0034] Another object of the present invention is to provide the use of the above Fe₃C catalyst in the Fischer-Tropsch synthesis reaction.

[0035] Beneficial effects

[0036] In the present invention, hexagonal nanosheets or nanopyramidal Fe₂O₃ catalyst precursors (catalyst precursor 1 and catalyst precursor 2) with special crystal plane orientations are synthesized, and after high-temperature reduction carbonization, Fe₅C₂ phase iron carbide is formed, and then decarbonization is carried out in a hydrogen atmosphere at 450 - 540°C to prepare an iron carbide catalyst with a pure phase Fe₃C structure.

[0037] In the present invention, by using a catalyst precursor with a special morphology and preparing the Fe₃C catalyst by the method of carbonization first and then decarbonization, the surface area carbon deposition of the active phase of the Fe₃C catalyst is greatly reduced, and the reaction activity and stability of the catalyst are well maintained, and high activity can be achieved at about 300°C. The present invention can be applied to a high-temperature Fischer-Tropsch system of 280 - 350°C (preferably 280°C - 300°C), and has good Fischer-Tropsch reaction performance. Description of the drawings

[0038] Figure 1The XRD pattern of catalyst precursor 1 of the hexagonal nanosheets prepared in Example 1 of the present application is shown.

[0039] Figure 2 The SEM pattern of catalyst precursor 1 of the hexagonal nanosheets prepared in Example 1 of the present application is shown.

[0040] Figure 3 The XRD pattern of the iron carbide phase obtained by reducing and carbonizing catalyst precursor 1 of the hexagonal nanosheets prepared in Example 1 of the present application at 330 °C under 50% CO-N2 conditions and then decarbonizing with hydrogen at 470 °C is shown, where the phase is the Fe3C crystal phase.

[0041] Figure 4 The XRD pattern of the Fe2O3 catalyst precursor of the nanocones prepared in Example 5 of the present application is shown.

[0042] Figure 5 The SEM pattern of catalyst precursor 1 of the nanocones prepared in Example 5 of the present application is shown.

[0043] Figure 6 The XRD pattern of the iron carbide phase obtained by reducing and carbonizing catalyst precursor 1 of the nanocones prepared in Example 5 of the present application at 330 °C under 50% CO-N2 conditions and then decarbonizing with 50% H2-N2 at 460 °C is shown, where the phase is the Fe3C crystal phase.

[0044] Figure 7 The XRD pattern of the iron carbide phase obtained by reducing and carbonizing catalyst precursor 2 of the hexagonal nanosheets prepared in Example 9 of the present application at 340 °C under 50% CO-N2 conditions and then decarbonizing with hydrogen at 490 °C is shown, where the phase is the Fe3C crystal phase.

[0045] Figure 8 The XRD pattern of the iron carbide phase obtained by reducing and carbonizing the catalyst precursor prepared in Comparative Example 3 at 330 °C under 50% CO-N2 conditions and then decarbonizing with hydrogen at 440 °C is shown, where the phase is the Fe crystal phase.

[0046] Figure 9 The XRD pattern of the iron carbide phase obtained by reducing and carbonizing the catalyst precursor prepared in Comparative Example 4 at 330 °C under 50% CO-N2 conditions and then decarbonizing with hydrogen at 420 °C is shown, where the phase is a mixed crystal phase of Fe and Fe5C2.

[0047] Figure 10 The XRD pattern of the iron carbide phase obtained by reducing and carbonizing the catalyst precursor prepared in Comparative Example 5 at 330 °C under synthesis gas H2:CO = 5:1 conditions and then decarbonizing with hydrogen at 470 °C is shown, where the phase is a mixed crystal phase of Fe3C and Fe5C2. Detailed implementation manners

[0048] 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.

[0049] The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. 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.

[0050] Unless otherwise specified, the raw materials used in the following examples are all purchased conventionally from the market.

[0051] Example

[0052] Example 1

[0053] 8.0 g of FeCl3·6H2O was dissolved in a mixed solution of 231.1 g of ethanol and 20.5 g of deionized water, stirred evenly, and then 23.4 g of NaAc was added. After stirring evenly, the pH was adjusted to 7.8. Then the mixture was transferred to a 600 mL stainless steel autoclave with a polytetrafluoroethylene liner and maintained at 180 °C for 12 h. The obtained product was collected by centrifugation at 5000 rpm, then washed several times with deionized water and dried at 100 °C to obtain catalyst precursor 1. This catalyst precursor 1 is a hexagonal nanosheet with a preferential orientation of the (110) crystal plane. The XRD pattern is shown in Figure 1 , and the SEM pattern is shown in Figure 2 .

[0054] 1 g of catalyst precursor 1 was placed in a fixed bed, and a mixed gas of 50% CO and N2 was used as the reduction atmosphere. It was reduced and carbonized at 330 °C for 8 h. After carbonization, the temperature was lowered to 250 °C, and then hydrogen was switched. The temperature was raised to 470 °C at a rate of 1 °C / min and decarbonized for 6 h to obtain a catalyst with a θ-Fe3C crystal phase. The XRD pattern is shown in Figure 3 .

[0055] Example 2

[0056] 8.0 g of anhydrous FeCl3 was dissolved in a mixed solution of 373.6 g of ethanol and 19.7 g of deionized water, stirred evenly, and then 40.5 g of NaAc was added. After stirring evenly, the pH value was adjusted to 7.5. Then the mixture was transferred to a 600 mL stainless steel autoclave with a polytetrafluoroethylene liner and maintained at 180 °C for 12 h. The obtained product was collected by centrifugation at 5000 rpm, then washed several times with deionized water and dried at 120 °C to obtain catalyst precursor 1. Its XRD pattern and SEM pattern are the same as those of the catalyst precursor 1 prepared in Example 1. Figure 1 Consistent.

[0057] Put 1 g of catalyst precursor 1 into a fixed bed, use CO as the reducing atmosphere, reduce and carbonize at 320 °C for 16 h. After carbonization, cool down to 250 °C, switch to a 50% hydrogen and N2 mixture, and heat up to 480 °C at a rate of 1 °C / min for decarbonization treatment for 3 h to obtain a catalyst with a θ-Fe3C crystal phase. Its XRD pattern is consistent with that of the catalyst finally prepared in Example 1. Figure 1 Consistent.

[0058] Example 3

[0059] Dissolve 8.0 g of anhydrous FeCl3 in a mixed solution of 373.6 g of ethanol and 19.7 g of deionized water, stir evenly, add 40.5 g of NaAc, adjust the pH value to 7.5 after stirring evenly, and then transfer the mixture to a 600 mL stainless steel autoclave lined with polytetrafluoroethylene and keep it at 180 °C for 6 h. Centrifuge to collect the obtained product at 5000 rpm, then wash it several times with deionized water and dry it at 120 °C to obtain catalyst precursor 1. Its XRD pattern and SEM pattern are consistent with those of catalyst precursor 1 prepared in Example 1. Figure 1 Consistent.

[0060] Put 1 g of catalyst precursor 1 into a fixed bed, use a 20% CO and N2 mixture as the reducing atmosphere, reduce and carbonize at 340 °C for 6 h. After carbonization, cool down to 250 °C, switch to a 30% hydrogen and N2 mixture, and heat up to 460 °C at a rate of 5 °C / min for decarbonization treatment for 12 h to obtain a catalyst with a θ-Fe3C crystal phase. Its XRD pattern is consistent with that of the catalyst finally prepared in Example 1. Figure 1 Consistent.

[0061] Example 4

[0062] Dissolve 8.0 g of anhydrous FeCl3 in a mixed solution of 373.6 g of ethanol and 19.7 g of deionized water, stir evenly, add 40.5 g of NaAc, adjust the pH value to 7.0 after stirring evenly, and then transfer the mixture to a 600 mL stainless steel autoclave lined with polytetrafluoroethylene and keep it at 180 °C for 6 h. Centrifuge to collect the obtained product at 5000 rpm, then wash it several times with deionized water and dry it at 120 °C to obtain catalyst precursor 1. Its XRD pattern and SEM pattern are consistent with those of catalyst precursor 1 prepared in Example 1. Figure 1 Consistent.

[0063] Put 1 g of catalyst precursor 1 into a fixed bed. Use a 50% CO and N2 mixed gas to reduce and carbonize at 350 °C for 4 h. After carbonization, cool down to 250 °C, switch to a 50% hydrogen and N2 mixed gas, and heat up to 470 °C at a rate of 1 °C / min for decarbonization treatment for 4 h, then a catalyst with θ-Fe3C crystal phase can be obtained, and its XRD pattern is the same as that of the catalyst finally prepared in Example 1. Figure 1 Consistent.

[0064] Example 5

[0065] Dissolve 8.0 g of FeCl3·6H2O in a mixed solution of 239.2 g of ethanol and 16.24 g of deionized water, stir evenly, then add 24.91 g of NaAc, and after stirring evenly, add 37 wt% concentrated hydrochloric acid dropwise until the pH value reaches 6. Then transfer the mixture to a 600 mL stainless steel autoclave lined with polytetrafluoroethylene and keep it at 180 °C for 12 h. Centrifuge to collect the obtained product at 5000 rpm, then wash it several times with deionized water and dry it at 150 °C to obtain catalyst precursor 1. The XRD pattern of this catalyst is shown in Figure 4 , and the SEM pattern is shown in Figure 5 .

[0066] Put 1 g of catalyst precursor 1 into a fixed bed. Use a 50% CO and N2 mixed gas to reduce and carbonize at 330 °C for 6 h. After carbonization, cool down to 250 °C, switch to a 50% hydrogen and N2 mixed gas, and heat up to 460 °C at a rate of 1 °C / min for decarbonization treatment for 6 h, then a catalyst with θ-Fe3C crystal phase can be obtained, and its XRD pattern is shown in Figure 6 .

[0067] Example 6

[0068] Dissolve 8.0 g of FeCl3·6H2O in a mixed solution of 239.2 g of ethanol and 16.24 g of deionized water, stir evenly, then add 24.91 g of NaAc, and after stirring evenly, add 37 wt% concentrated hydrochloric acid dropwise until the pH value reaches 5. Then transfer the mixture to a 600 mL stainless steel autoclave lined with polytetrafluoroethylene and keep it at 180 °C for 12 h. Centrifuge to collect the obtained product at 5000 rpm, then wash it several times with deionized water and dry it at 120 °C to obtain catalyst precursor 1, and its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 5. Figure 1 Consistent.

[0069] Put 1 g of catalyst precursor 1 into a fixed bed, use a 50% CO and N2 mixed gas, reduce and carbonize at 340 °C for 4 h. After carbonization, cool down to 250 °C, switch to a 50% hydrogen and N2 mixed gas, and heat up to 490 °C at a rate of 1 °C / min for decarbonization treatment for 3 h, then a catalyst with a θ-Fe3C crystal phase can be obtained, and its XRD pattern is the same as that of the catalyst finally prepared in Example 5 Figure 1 consistent.

[0070] Example 7

[0071] Use the method of Example 1 to scale up the preparation of 40 g of Fe2O3. Up to the washed filter cake, add 20 g of water for pulping and dispersion, then add 10 g of silica sol containing 20% SiO2 and stir evenly. Spray drying is carried out 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, 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 catalyst precursor 2, and its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 1 Figure 1 consistent.

[0072] Put 1 g of catalyst precursor 2 into a fixed bed, use a 50% CO and N2 mixed gas, reduce and carbonize at 350 °C for 4 h. After carbonization, cool down to 250 °C, switch to a 50% hydrogen and N2 mixed gas, and heat up to 460 °C at a rate of 1 °C / min for decarbonization treatment for 8 h, then a catalyst with a θ-Fe3C crystal phase can be obtained, and its XRD pattern is the same as that of the catalyst finally prepared in Example 9 Figure 1 consistent.

[0073] Example 8

[0074] Use the method of Example 2 to scale up the preparation of 40 g of Fe2O3. Up to the washed filter cake, add 1800 g of water for pulping and dispersion, then add 20 g of potassium silicate solution containing 20% SiO2 and stir evenly. Dropwise add 10% nitric acid solution until the pH reaches 6, filter, re-pulp the filter cake, and 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 to 110 °C at a rate of 15 °C / min in a muffle furnace, kept at this temperature for 8 h, and then heated to 500 °C at a rate of 10 °C / h and kept at 500 °C for 5 h to obtain catalyst precursor 2, and its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 1 Figure 1 consistent.

[0075] Put 1 g of catalyst precursor 2 into a fixed bed. Use a 50% CO and N2 mixed gas to carry out reduction carbonization at 340 °C for 4 h. After carbonization, cool down to 250 °C, switch to hydrogen, and raise the temperature to 480 °C at a rate of 1 °C / min for decarbonization treatment for 6 h, then a catalyst with a θ-Fe3C crystal phase can be obtained. Its XRD pattern is the same as that of the catalyst finally prepared in Example 9 Figure 1 consistent.

[0076] Example 9

[0077] Use the method of Example 3 to scale up the preparation of 40 g of Fe2O3. For the filter cake after washing, add 40 g of silica sol containing 20% SiO2 and stir evenly. 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 to 110 °C at a rate of 15 °C / min in a muffle furnace, kept at this temperature for 8 h, then heated to 600 °C at a rate of 10 °C / h, and kept at 600 °C for 5 h to obtain catalyst precursor 2. Its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 1 Figure 1 consistent.

[0078] Put 1 g of catalyst precursor 2 into a fixed bed. Use a 50% CO and N2 mixed gas to carry out reduction carbonization at 340 °C for 4 h. After carbonization, cool down to 250 °C, switch to hydrogen, and raise the temperature to 490 °C at a rate of 1 °C / min for decarbonization treatment for 5 h, then a catalyst with a θ-Fe3C crystal phase can be obtained. Its XRD pattern is shown in Figure 7 .

[0079] Example 10

[0080] Use the method of Example 4 to scale up the preparation of 40 g of Fe2O3. For the filter cake after washing, add 1800 g of water for pulping and dispersion, then add 60 g of potassium silicate solution containing 20% SiO2 and stir evenly. Dropwise add 10% nitric acid solution until the pH reaches 6.5, age for 1 h, filter, re-pulp the filter cake, and carry out spray drying under the conditions of an inlet air temperature of 180 °C and an outlet air temperature of 105 °C. The obtained microsphere catalyst is heated to 110 °C at a rate of 15 °C / min in a muffle furnace, kept at this temperature for 8 h, then heated to 500 °C at a rate of 10 °C / h, and kept at 500 °C for 5 h to obtain catalyst precursor 2. Its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 1 Figure 1 consistent.

[0081] Put 1 g of catalyst precursor 2 into a fixed bed. Use a 50% CO and N2 mixed gas to reduce and carbonize at 330 °C for 10 h. After carbonization, cool down to 250 °C, switch to hydrogen, and heat up to 490 °C at a rate of 1 °C / min for decarbonization treatment for 5 h, then a catalyst with a θ-Fe3C crystal phase can be obtained. Its XRD pattern is the same as that of the catalyst finally prepared in Example 9. Figure 1 consistent.

[0082] Example 11

[0083] Use the method of Example 5 to scale up the preparation of 40 g of Fe2O3. Up to the washed filter cake, add 35 g of silica sol containing 20% SiO2 and stir evenly. 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 110 °C at a rate of 15 °C / min in a muffle furnace, held at this temperature for 8 h, then heated to 500 °C at a rate of 10 °C / h, and held at 500 °C for 5 h to obtain catalyst precursor 2. Its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 5. Figure 1 consistent.

[0084] Put 1 g of catalyst precursor 2 into a fixed bed. Use a 50% CO and N2 mixed gas to reduce and carbonize at 340 °C for 5 h. After carbonization, cool down to 250 °C, switch to hydrogen, and heat up to 520 °C at a rate of 1 °C / min for decarbonization treatment for 5 h, then a catalyst with a θ-Fe3C crystal phase can be obtained. Its XRD pattern is the same as that of the catalyst finally prepared in Example 9. Figure 1 consistent.

[0085] Example 12

[0086] Use the method of Example 6 to scale up the preparation of 40 g of Fe2O3. Up to the washed filter cake, add 35 g of silica sol containing 20% SiO2 and stir evenly. 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 110 °C at a rate of 15 °C / min in a muffle furnace, held at this temperature for 8 h, then heated to 500 °C at a rate of 10 °C / h, and held at 500 °C for 5 h to obtain catalyst precursor 2. Its XRD pattern and SEM pattern are the same as those of catalyst precursor 1 prepared in Example 5. Figure 1 consistent.

[0087] Put 1 g of catalyst precursor 2 into a fixed bed. Use a 50% CO and N2 mixed gas to reduce and carbonize at 340 °C for 5 h. After carbonization, cool down to 250 °C, switch to hydrogen, and heat up to 540 °C at a rate of 1 °C / min for decarbonization treatment for 3 h, then a catalyst with a θ-Fe3C crystal phase can be obtained. Its XRD pattern is the same as that of the catalyst finally prepared in Example 9. Figure 1To.

[0088] Comparative Example

[0089] Comparative Example 1

[0090] Place 4 g of maltose and 6 g of pyridine in a 100 mL beaker. After mechanically stirring to mix them evenly, place the beaker in a heating mantle 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 at 750 °C for 2 h under a nitrogen atmosphere to obtain a comparative catalyst. The storage of the catalyst and the loading of the sample before the fixed-bed evaluation are both completed in a glove box.

[0091] Comparative Example 2

[0092] Mix a 1.2 mol / L ferric nitrate solution with a 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 roast it at 400 °C for 10 h to obtain a catalyst precursor.

[0093] Reduce the catalyst precursor with H2 at a pressure of 2.6 atm, a H2 flow rate of 22000 mL / h / g, and a temperature of 470 °C for 12 h; cool the above reduction product from 470 °C to 400 °C at a rate of 1.5 °C / min, switch to a H2 and CO mixed gas (molar ratio 60:1), a pressure of 20 atm, and a total gas flow rate of 20000 mL / h / g, and treat it for 24 h to obtain a comparative catalyst, which is a composite crystal phase of various iron carbides.

[0094] Comparative Example 3

[0095] Mix a 1.2 mol / L ferric nitrate solution with a 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 roast it at 400 °C for 10 h to obtain a catalyst precursor.

[0096] Place 1 g of the catalyst precursor in a fixed bed, use a 50% CO and N2 mixed gas, reduce and carbonize it at 330 °C for 8 h. After carbonization, cool it to 250 °C, switch to hydrogen, and heat it to 440 °C at a rate of 1 °C / min for decarbonization treatment for 3 h to obtain a comparative catalyst. After XRD testing, the catalyst is an α-Fe phase. The XRD pattern is shown in Figure 8 .

[0097] Comparative Example 4

[0098] Mix ferric nitrate with a concentration of 1.2 mol / L and sodium carbonate solution with a concentration of 0.9 mol / L 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 catalyst precursor.

[0099] Put 1 g of the catalyst precursor into a fixed bed, use a 50% CO and N2 mixed gas, reduce and carbonize it at 330 °C for 8 h. After carbonization, cool it down to 250 °C, switch to hydrogen, and raise the temperature to 420 °C at a rate of 1 °C / min for decarbonization treatment for 3 h to obtain a comparative catalyst. After XRD testing, the catalyst is a mixed phase of α-Fe and χ-Fe5C2. The XRD pattern is shown in Figure 9 。

[0100] Comparative Example 5

[0101] Dissolve 8.0 g of FeCl3·6H2O in a mixed solution of 231.1 g of ethanol and 20.5 g of deionized water, stir evenly, and then add 23.4 g of NaAc. Then transfer the mixture to a 600 mL stainless steel autoclave with a PTFE liner and keep it at 180 °C for 12 h. Centrifuge and collect the obtained product at 5000 rpm, then wash it with deionized water several times and dry it at 100 °C to obtain a catalyst precursor.

[0102] Put 1 g of the catalyst precursor into a fixed bed, use a H2 and CO mixed gas (molar ratio 5:1) as the reducing atmosphere, reduce and carbonize it at 330 °C for 8 h. After carbonization, cool it down to 250 °C, switch to hydrogen, and raise the temperature to 470 °C at a rate of 1 °C / min for decarbonization treatment for 6 h to obtain an iron carbide catalyst. After XRD testing, the catalyst is a mixed phase of θ-Fe3C and χ-Fe5C2. The XRD pattern is shown in Figure 10 。

[0103] Comparative Example 6

[0104] Mix ferric nitrate with a concentration of 1.2 mol / L and sodium carbonate solution with a concentration of 0.9 mol / L 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.

[0105] Reduce the precursor with H2 at a pressure of 2.6 atm, a H2 flow rate of 22000 mL / h / g, and a temperature of 470 °C for 12 h;

[0106] The above reduction product was cooled from 470 °C to 400 °C at a rate of 1.5 °C / min and contacted with a mixed gas of H2 and CO at this temperature for the preparation of precipitated carbide. The conditions were as follows: pressure 20 atm, total gas flow rate 20000 mL / h / g, molar ratio of H2 to CO 60:1, treatment time 24 h, and precipitated iron carbide was obtained, denoted as iron carbide 1;

[0107] Under Ar gas protection, 97 parts by mole of precipitated iron carbide 1 was mixed with 3 parts by mole of ferrous oxide (i.e., Fe-containing impurities). The mixed product was the comparative catalyst.

[0108] Test Example

[0109] Test conditions:

[0110] After putting the catalyst precursor equivalent to 1 g of Fe mass in terms of reduced calculation into a 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.

[0111] The process conditions for Fischer-Tropsch synthesis were as follows:

[0112] Fischer-Tropsch synthesis: After the reduction and carbonization were completed, the reaction temperature was lowered 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 marked as the reaction starting point 0.

[0113] Data calculation: By measuring and analyzing the molar number of CO in the feed at the feed inlet of the stirred tank and the molar 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:

[0114] CO conversion rate % = [(molar number of CO in the feed - molar number of CO in the product) / molar number of CO in the feed] × 100%;

[0115] CO2 selectivity % = [molar number of CO2 in the product / (molar number of CO in the feed - molar number of CO in the product)] × 100%;

[0116] CH4 selectivity % = [molar number of CH4 in the product / (molar number of CO in the feed × CO conversion rate %

[0117] (1 - CO2 selectivity %))] × 100%.

[0118] The deactivation rate of the catalyst refers to the rate of decrease in the reaction activity (carbon monoxide conversion rate) of the catalyst from the stable reaction period to the end of the reaction, with the unit of % / h.

[0119] Table 1 Catalyst Performance Evaluation Results

[0120]

[0121]

[0122] As can be seen from Table 1, the CO conversion rate of the catalyst provided by the present invention is significantly better than that of the catalyst of the comparative example. Therefore, the catalyst provided by the present invention has excellent catalytic activity; the deactivation rate of the catalyst provided by the present invention is better than that of the catalyst of the comparative example. Therefore, the catalyst provided by the present invention has excellent stability.

Claims

1. A preparation method of Fe3C catalyst, comprising the following steps: Step 1: Prepare an iron salt solution; Step 2: Transfer the iron salt solution to a crystallization kettle for hydrothermal aging; Step 3: Centrifuge to separate and collect the solid, wash it, and dry the filter cake to obtain catalyst precursor 1; Step 4: Redisperse the filter cake after centrifugation and washing in Step 3, mix it with potassium silicate solution or silica sol, stir evenly, and dry and calcine it to obtain catalyst precursor 2; Step 5: Perform reduction carbonization on catalyst precursor 1 or catalyst precursor 2, and then perform decarbonization treatment to obtain Fe3C catalyst.

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

3. The preparation method according to claim 1 or 2, wherein, In Step 1, dissolve the iron salt in a mixed solution of ethanol and deionized water, stir evenly, and then add sodium acetate to obtain the iron salt solution.

4. The preparation method according to claim 3, wherein, In Step 1, the mass ratio of ethanol to deionized water is 11-19:1 (g / g).

5. The preparation method according to claim 3, wherein, In Step 1, based on the Fe element, the mass ratio of the iron salt to sodium acetate is 4-7.5:100 (g / g), preferably 6.5-7.2:100 (g / g).

6. The preparation method according to claim 1 or 2, wherein In step 1, the concentration of Fe 3+ in the ferric salt solution is 0.37 - 0.65 wt%, preferably 0.55 - 0.63 wt%.

7. The preparation method according to claim 1 or 2, wherein In Step 1, the pH of the iron salt solution is 5-8.

8. The preparation method according to claim 1 or 2, wherein In Step 2, the temperature of hydrothermal aging is 170°C - 220°C, and the time is 6h - 24h.

9. The preparation method according to claim 1 or 2, wherein In Step 3, the washing is performed with deionized water.

10. The preparation method according to claim 1 or 2, wherein, In Step 3, the drying temperature is 100°C - 150°C.

11. The preparation method according to claim 1 or 2, wherein, In Step 4, the added potassium silicate solution or silica sol contains 15wt% - 25wt% of SiO2.

12. The preparation method according to claim 1 or 2, wherein Based on the Fe element, the mass ratio of the iron salt to SiO2 in the potassium silicate solution or silica sol in Step 4 is 70:5 - 30.

13. The preparation method according to claim 1 or 2, wherein, In Step 4, the drying is spray drying, and the conditions are: the inlet air temperature is 180°C - 300°C, and the outlet air temperature is 105°C - 125°C.

14. The preparation method according to claim 1 or 2, wherein In Step 4, the calcination conditions are: heat up to 110°C - 150°C at a rate of 5 - 20°C / min, keep the temperature constant at this temperature for 4 - 16h, 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.

15. The preparation method according to claim 1 or 2, wherein In Step 5, the reduction carbonization conditions are: introduce a mixed gas of 5% - 100% CO and an inert gas, and perform reduction carbonization at 320°C - 370°C for 2 - 24h.

16. The preparation method according to claim 1 or 2, wherein In Step 5, after reduction carbonization, cool down to 200°C - 280°C and then perform decarbonization treatment.

17. The preparation method according to claim 1 or 2, wherein In Step 5, the decarbonization treatment conditions are: a mixed gas of 5% - 100% hydrogen and an inert gas, heat up to 450°C - 540°C at a rate of 1°C / min - 5°C / min, and perform decarbonization treatment for 1h - 12h.

18. An Fe3C catalyst, which is prepared by the preparation method according to any one of claims 1 - 17.

19. The use of the Fe3C catalyst prepared by the preparation method according to any one of claims 1 - 17 or the Fe3C catalyst according to claim 18 in the Fischer-Tropsch synthesis reaction.

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