A method for preparing an inorganic iron salt catalyst for synthesizing iron carbide and a method for using the catalyst to prepare low-carbon olefins.

Abstract

This invention relates to the field of Fischer-Tropsch synthesis, providing a method for preparing an iron carbide catalyst using inorganic iron salts and its application in the Fischer-Tropsch synthesis for the preparation of low-carbon olefins. The method for preparing the iron carbide catalyst includes the following steps: mixing an aqueous solution of an inorganic iron salt with a reducing agent to form a crude product of nano-zero-valent iron, followed by reduction with hydrogen to obtain nano-zero-valent iron particles, and further preparing the iron carbide catalyst using a liquid-phase carbonization method. This catalyst serves as the active component in the Fischer-Tropsch synthesis reaction, exhibiting good selectivity for low-carbon olefins. This invention uses inorganic iron salts instead of carbonyl iron to synthesize the iron carbide catalyst, reducing the synthesis cost by nearly 73% and being environmentally friendly, thus contributing to the large-scale production of iron carbide catalysts.

Description

Technical Field

[0001] This invention relates to the fields of thermocatalysis and photothermal catalysis, and particularly to a method for preparing an inorganic iron salt catalyst for synthesizing iron carbide and its application in the Fischer-Tropsch synthesis for preparing low-carbon olefins. Background Technology

[0002] my country's primary energy structure is characterized by abundant coal, scarce oil, and limited natural gas. With my country's economic development, its dependence on imported oil continues to rise. Olefins, as raw materials for many high-value-added chemical products, are mainly produced from petroleum. Therefore, there is increasing interest in alternative processes for non-petroleum-based carbon resources, and Fischer-Tropsch synthesis is a key technology for coal-to-olefins and coal-to-oil. Using coal as a raw material to produce syngas, and then converting the syngas into olefins and light oils through Fischer-Tropsch synthesis, is one of the best ways to address future renewable energy needs. Generally, traditional Fischer-Tropsch synthesis reactions require high temperatures (200-300℃) and high pressures (2-3 MPa) to overcome the energy barriers of CO activation and CC coupling reactions; however, this often consumes a large amount of energy. In recent years, utilizing solar energy to replace traditional thermal energy to drive CO hydrogenation for hydrocarbon production has proven to be a very promising new approach. Using solar photocatalysis technology to convert solar energy into chemical energy is considered one of the best ways to address future renewable energy needs.

[0003] Currently, transition metal (Fe, Co, Ni, Ru) catalysts are commonly used in Fischer-Tropsch synthesis reactions. Among them, iron-based catalysts are frequently used in the Fischer-Tropsch synthesis process for producing low-carbon olefins due to their low cost, wide availability, and high selectivity for low-carbon olefins. Mechanistic studies have shown that the reactivity of iron-based catalysts is related to the properties of iron carbide. Reported production technologies for producing iron carbide have yielded mixtures of various iron carbide phases (such as Fe2C, Fe3C, Fe7C3, and Fe5C2). Among these, Fe5C2 is generally considered the active phase due to its relatively good stability, inherent catalytic activity, and high selectivity for light olefins.

[0004] The main methods for synthesizing Fe5C2 are gas-solid phase carbonization and liquid phase carbonization. Gas-solid phase carbonization is simple to operate, but the composition is uneven, making it difficult to prepare pure-phase Fe5C2, and particle size is also difficult to control. Liquid phase carbonization can solve these problems well; however, the iron source used for liquid phase carbonization to prepare Fe5C2—carbonyl iron—is expensive (approximately 100 yuan / gram) and highly toxic, limiting the large-scale application of Fe5C2. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing synthetic raw materials, such as high cost and high toxicity, and to provide a method for preparing iron carbide catalysts using inorganic iron salts and its application in Fischer-Tropsch synthesis to prepare low-carbon olefins. This invention is based on nano-zero-valent iron obtained by inorganic iron salt reduction as a precursor, and prepares Fe5C2 catalysts through liquid-phase carbonization for application in Fischer-Tropsch synthesis. The synthesis cost is low and it has good selectivity for low-carbon olefins, which is beneficial to environmental protection and effectively utilizes solar energy.

[0006] To achieve the above objectives, the present invention adopts the following technical solution.

[0007] This invention provides a method for preparing an inorganic iron salt catalyst for synthesizing iron carbide, comprising the following steps:

[0008] (1) Prepare an aqueous solution of inorganic iron salt, slowly add sodium borohydride aqueous solution, stir thoroughly at room temperature, wash and dry to obtain crude product of nano zero-valent iron;

[0009] (2) Heat the nano-zero-valent iron obtained in step (1) to 300-400℃ in a hydrogen atmosphere, keep it for 1-2 hours, and then let it cool down to room temperature to obtain the nano-zero-valent iron precursor with the surface oxide layer reduced.

[0010] (3) The carbon source, solvent, carbonization inducer and the precursor obtained in step (2) are thoroughly mixed under stirring. Under an inert atmosphere, the temperature is raised to 330-350℃ and kept at that temperature for a certain time. After the reaction system is cooled, the solid and liquid are separated. The collected solids are washed sequentially with non-polar solvent and polar solvent, dried, and the iron carbide particle product is collected.

[0011] Furthermore, in step (1), the amount of iron in the inorganic iron salt is 0.004 mol, which is completely dissolved in deionized water; the amount of sodium borohydride added should be excessive, which is 0.01 to 0.02 mol, and is completely dissolved in deionized water; the washing method is to wash with deionized water until the solution is clear; the drying method is to freeze-dry under vacuum overnight at a drying temperature of -50 to -40°C.

[0012] Furthermore, in step (3), the carbon source is octadecylamine; the solvent is octadecylamine; the carbonization inducing agent is hexadecyltrimethylammonium bromide or ammonium bromide; the inert atmosphere is nitrogen, argon or helium; the nonpolar solvent is selected from n-hexane; the polar solvent is selected from anhydrous ethanol; the washing method is to wash with ethanol and hexane alternately until the solution is clear; the drying method is to freeze-dry under vacuum overnight at a drying temperature of -50 to -40°C.

[0013] Furthermore, in step (3), the obtained iron carbide particles have Fe5C2 as the main active component and a size of 70-90 nm, which can exhibit high catalytic activity and are especially suitable for catalytic Fischer-Tropsch synthesis.

[0014] Furthermore, in step (3), the iron carbide catalyst can be activated by providing heat or by absorbing and utilizing the full solar spectrum.

[0015] Furthermore, in step (3), the iron carbide catalyst exhibits good catalytic activity and stability in both batch reaction systems and flow reaction systems.

[0016] (1) The use of a batch reaction system specifically includes the following steps:

[0017] The iron carbide catalyst was placed in the reaction system and the reaction system was evacuated.

[0018] Syngas is introduced into the reaction system. The syngas is a mixture of CO, H2 and inert gases, wherein the volume ratio of CO to H2 is 1:1 to 1:3, and the remainder is inert gas. The reaction pressure is 1-2 bar, the reaction temperature is 300-340°C, and a high-temperature heating element or a full-spectrum xenon lamp is used to provide heat energy. The reaction time is 30-120 min, and the products are detected by gas chromatography.

[0019] (2) The use of a flow reaction system includes the following steps:

[0020] The iron carbide catalyst is placed in the reaction system, and the synthesis gas is introduced into the reaction system until the air remaining in the reaction gas is exhausted. The synthesis gas includes a mixture of CO, H2 and inert gas, wherein the volume ratio of CO and H2 is 1:1-1:3, and the remainder is inert gas.

[0021] The synthesis gas is controlled to pass through at a certain flow rate using a mass flow meter, and heat is provided by a high-temperature heating element or a full-spectrum xenon lamp. The reaction temperature is 300-340℃, and the products are detected by gas chromatography every 30 minutes.

[0022] In the batch reaction system described above, the optimal catalyst CO conversion rate can reach 40.3%-45.2%, and the low-carbon olefin selectivity is 50.9%-55.4%; in the flow reaction system described above, the optimal catalyst CO conversion rate can reach 37.5%-41.2%, and the low-carbon olefin selectivity is 49.8%-52.6%.

[0023] The present invention has the following beneficial effects:

[0024] 1. This invention prepares Fe5C2 catalyst using inorganic iron salts as raw materials. Compared with using carbonyl iron as raw materials, the synthesis cost is reduced to 27% of the original cost, which is beneficial for the mass production of Fe5C2 nanoparticles.

[0025] 2. This invention uses inorganic iron salts as the raw material for synthesis, replacing toxic carbonyl iron, which is more environmentally friendly.

[0026] 3. After optimizing the catalyst preparation conditions, the CO conversion rate can reach 45.2%, and the selectivity for low-carbon olefins can reach as high as 50.9%. Attached Figure Description

[0027] Figure 1 The XRD pattern of the iron carbide catalyst obtained in Example 1 of the present invention is shown.

[0028] Figure 2 The image shown is a transmission electron microscope (TEM) image of the optimal iron carbide catalyst obtained in Example 1 of the present invention. Detailed Implementation

[0029] Example 1

[0030] A method for preparing an inorganic iron salt catalyst for synthesizing iron carbide includes the following steps:

[0031] (1) Dissolve 1.112 g FeSO4·7H2O in 20 mL of deionized water at room temperature, and add dropwise 0.0378 g·mL sodium borohydride solution to 10 mL of water. -1 The resulting precipitate will be washed and dried to obtain a crude product of zero-valent iron.

[0032] (2) Grind the zero-valent iron crude product obtained in step (1), heat it to 300℃, 350℃ and 400℃ respectively in H2 / Ar atmosphere, keep it for 2h, and cool it naturally to room temperature to obtain nano zero-valent iron material.

[0033] (3) Mix 0.29g of nano-zero-valent iron obtained in step (2) with 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide, heat to 330-350℃ in an inert gas atmosphere, hold for 10min, and cool naturally to room temperature. The collected crude products are recorded as Fe5C2, Fe5C2-300, Fe5C2-350 and Fe5C2-400, respectively.

[0034] (4) The crude product obtained in step (3) is washed and dried to obtain the iron carbide particles for the photothermal catalytic Fischer-Tropsch synthesis to prepare low-carbon olefins, the main component of which is Fe5C2.

[0035] The iron carbide catalyst prepared according to the above method was applied to the photothermal Fischer-Tropsch reaction in a batch reaction system. 50 mg of iron carbide catalyst was added to a light-transmitting sealed reactor, diluted synthesis gas (CO:H2:N2 = 20:60:20, volume ratio) was introduced, and full-spectrum sunlight was concentrated for illumination. The product changes over time were detected by gas chromatography to determine the catalyst activity. The reaction temperature was 340 °C and the reaction pressure was 0.18 MPa.

[0036] Figure 1 The XRD pattern of the iron carbide catalyst obtained in Example 1 of the present invention is shown. Figure 2 The image shown is a transmission electron microscope (TEM) image of the iron carbide catalyst obtained in Example 1 of the present invention.

[0037] Depend on Figure 1 It can be seen that under these conditions, a Fe5C2 catalyst with good crystallinity was successfully prepared, and its (021) and (510) crystal plane characteristic peaks were very obvious. Figure 2 It is known that the main active component of iron carbide is Fe5C2, with a particle size of 70-90 nm. The catalytic activity and selectivity of the catalyst after 0.5 h of full-spectrum irradiation are shown in Table 1. The increase in hydrogen reduction temperature correlates with the catalyst's CO conversion rate in a volcano-like pattern. The optimal CO conversion rate and low-carbon olefin selectivity are achieved at a reduction temperature of 350 °C, with a low-carbon olefin selectivity of 24.3% and a reactant selectivity of 54.2%.

[0038] Table 1. Catalytic performance of iron carbide in Fischer-Tropsch synthesis

[0039]

[0040]

[0041] The optimal catalyst prepared according to the above method was applied to a flow reaction system for Fischer-Tropsch synthesis under pure heat conditions. 50 mg of iron carbide catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:60:20, volume ratio) was introduced until residual air in the reaction gas was exhausted. The synthesis gas flow rate was controlled by a mass flow meter. Under pure heat conditions, an electric heating base was used to provide heat for the reaction. Gas chromatography was used to detect changes in the products over time and to determine the catalyst activity. The reaction temperature was 340℃, and the reaction pressure was 0.18 MPa.

[0042] The optimal catalyst prepared in Example 1 still maintains good catalytic performance in the flow reaction system. Under pure heat and photothermal conditions, there is no significant difference in the activity of catalyzing the Fischer-Tropsch synthesis to prepare low-carbon olefins. Within 5 hours, the CO conversion rate remains at about 22.0%, and the low-carbon olefin selectivity remains at about 53.0%, showing good stability.

[0043] Example 2

[0044] A method for preparing an inorganic iron salt catalyst for synthesizing iron carbide includes the following steps:

[0045] (1) Dissolve 1.112 g FeSO4·7H2O in 20 mL of deionized water at room temperature, and add dropwise 0.0378 g·mL sodium borohydride solution to 10 mL of water. -1 The resulting precipitate will be washed and dried to obtain a crude product of zero-valent iron.

[0046] (2) Grind the crude product of zero-valent iron obtained in step (1), heat it to 350°C in H2 / Ar atmosphere, keep it for 2 hours, and cool it down to room temperature to obtain nano zero-valent iron material.

[0047] (3) Mix 0.29g of nano-zero-valent iron obtained in step (2) with 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide and different supports (α-Al2O3, γ-Al2O3, HB-SiO2, HL-SiO2), heat to 330-350℃ in an inert gas atmosphere, hold for 10min, and cool naturally to room temperature. Collect the crude products and record them as Fe5C2 / α-Al2O3, Fe5C2 / γ-Al2O3, Fe5C2 / HB-SiO2, Fe5C2 / HL-SiO2, respectively.

[0048] (4) The crude product obtained in step (3) is washed and dried to obtain the iron carbide particles for the photothermal catalytic Fischer-Tropsch synthesis to prepare low-carbon olefins, the main component of which is Fe5C2.

[0049] The iron carbide catalyst prepared according to the above method was applied to photothermal catalytic Fischer-Tropsch synthesis. 50 mg of iron carbide catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:20:60, volume ratio) was introduced. Full-spectrum sunlight was concentrated, and gas chromatography was used to detect changes in the products over time to determine the catalyst activity.

[0050] Example 3 shows that the iron carbide photothermal catalytic Fischer-Tropsch synthesis reaction obtained using different supports exhibits good performance. In the FTO process, Al2O3 is more effective than SiO2 in treating Fe5C2. Among them, the Fe5C2 / α-Al2O3 catalyst achieves the best conversion and selectivity. The interaction between the active phase Fe5C2 and the support γ-Al2O3 is stronger than that between α-Al2O3 and the support. The CO conversion rate is 32.8%, and the selectivity for low-carbon olefins is 48.5%.

[0051] Example 3

[0052] A method for preparing an inorganic iron salt catalyst for synthesizing iron carbide includes the following steps:

[0053] (1) Dissolve 1.112g FeSO4·7H2O in 20mL of deionized water at room temperature, and add 10 drops of water.

[0054] 0.0378 g / mL sodium borohydride solution -1 The resulting precipitate will be washed and dried to obtain a crude product of zero-valent iron.

[0055] (2) Grind the crude product of zero-valent iron obtained in step (1), heat it to 350°C in H2 / Ar atmosphere, keep it for 2 hours, and cool it down to room temperature to obtain nano zero-valent iron material.

[0056] (3) Mix 0.29g of nano-zero-valent iron obtained in step (2) with 14.5g of octadecylamine, 0.113g of hexadecyltrimethylammonium bromide and support α-Al2O3 (70-150mg), heat to 330-350℃ in an inert gas atmosphere, hold for 10min, cool naturally to room temperature, and collect the crude product;

[0057] (4) The crude product obtained in step (3) is washed and dried to obtain the iron carbide particles for the photothermal catalytic Fischer-Tropsch synthesis to prepare low-carbon olefins, the main component of which is Fe5C2.

[0058] The iron carbide catalyst prepared according to the above method was applied to photothermal catalytic Fischer-Tropsch synthesis. 50 mg of iron carbide catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:20:60, volume ratio) was introduced. Full-spectrum sunlight was concentrated, and gas chromatography was used to detect changes in the products over time to determine the catalyst activity.

[0059] The amount of α-Al2O3 support added showed a direct correlation with the CO conversion rate of the catalyst. When the amount of support added was 130 mg, the CO conversion rate and the selectivity of low-carbon olefins reached the optimal levels, with a CO conversion rate of 45.2% and a low-carbon olefin selectivity of 50.9%.

[0060] Example 4

[0061] A method for preparing an inorganic iron salt catalyst for synthesizing iron carbide includes the following steps:

[0062] (1) n Fe =0.004 mol of inorganic iron salts (such as FeCl3, FeCl2, Fe2(SO4)3 and FeSO4) are fully dissolved in 20 mL of deionized water at room temperature, and 0.0567 g·mL of sodium borohydride solution is added dropwise. -1 The resulting precipitate will be washed and dried to obtain a crude product of zero-valent iron.

[0063] (2) Grind the crude product of zero-valent iron obtained in step (1), heat it to 350°C in H2 / Ar atmosphere, keep it for 2 hours, and cool it down to room temperature to obtain nano zero-valent iron material.

[0064] (3) Mix 0.29g of nano-zero-valent iron obtained in step (2) with 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide, heat to 330-350℃ in an inert gas atmosphere, hold for 10min, and cool naturally to room temperature to collect the crude product.

[0065] (4) The crude product obtained in step (3) is washed and dried to obtain the iron carbide particles for the photothermal catalytic Fischer-Tropsch synthesis to prepare low-carbon olefins, the main component of which is Fe5C2.

[0066] The iron carbide catalyst prepared according to the above method was applied to a batch reaction system for Fischer-Tropsch synthesis under pure heat conditions. 50 mg of iron carbide catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:40:40, volume ratio) was introduced. The pure heat conditions were achieved by electric heating from a heating base. Gas chromatography was used to detect changes in the products over time, and the catalyst activity was determined. The reaction temperature was 340℃, and the reaction pressure was 0.18 MPa.

[0067] Example 5 shows that the iron carbide photothermal catalytic Fischer-Tropsch synthesis reaction obtained using different inorganic iron salts exhibits good performance and high selectivity for low-carbon olefins. Specifically, when FeCl3·6H2O is used as the raw material, the CO conversion rate is 28.4%, and the low-carbon olefin selectivity is 53.8%. This indicates that the inorganic iron salts used in the iron carbide catalyst obtained in this invention have certain versatility and are promising for industrial application.

[0068] In summary, this invention provides a method for preparing an iron carbide catalyst using inorganic iron salts instead of carbonyl iron. This method significantly reduces synthesis costs and is environmentally friendly. Applying the catalyst to Fischer-Tropsch synthesis can produce low-carbon olefins with high selectivity.

[0069] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

[0070] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for preparing a catalyst for synthesizing iron carbide using inorganic iron salts, characterized in that... Includes the following steps: (1) Prepare an aqueous solution of inorganic iron salt, slowly add sodium borohydride aqueous solution, stir thoroughly at room temperature, wash and dry to obtain crude product of nano-zero valent iron; the amount of iron in the inorganic iron salt is 0.004 mol, completely dissolved in deionized water; the amount of sodium borohydride added should be excessive, 0.01 ~ 0.02 mol, completely dissolved in deionized water, and washed with deionized water until the solution is clear, then dried overnight at a drying temperature of -50 ~ -40°C. o C; (2) The nano-zero valent iron obtained in step (1) is heated to 300~400°C in a hydrogen atmosphere. o C, keep for 1~2 h, and let it cool naturally to room temperature to obtain a nano-zero-valent iron precursor with reduced surface oxide layer; (3) The carbon source, carbonization inducer, solvent and the precursor obtained in step (2) are thoroughly mixed under stirring, and the mixture is heated to 330~350°C under an inert atmosphere. o C, keep at this temperature for a certain time, and after the reaction system cools, separate the solid and liquid phases. The collected solids are washed sequentially with a non-polar solvent and a polar solvent, then dried to obtain iron carbide powder. The carbon source is octadecylamine; the solvent is octadecylamine; the carbide inducing agent is hexadecyltrimethylammonium bromide or ammonium bromide; the inert atmosphere is nitrogen, argon, or helium; the non-polar solvent is selected from n-hexane; the polar solvent is selected from anhydrous ethanol; the washing method involves alternating washing with ethanol and hexane until the solution is clear; vacuum freeze-dry overnight at a temperature of -50 to -40°C. o C; The obtained iron carbide powder has Fe5C2 as the main active component and a particle size of 70~90 nm. It can exhibit high catalytic activity and product selectivity, and is suitable for catalytic Fischer-Tropsch synthesis.

2. A method for preparing low-carbon olefins using the iron carbide catalyst synthesized by the method of claim 1, characterized in that: The iron carbide catalyst is heated by external electrical energy or by absorbing the full solar spectrum to achieve catalyst-driven catalytic reactions.

3. The method for preparing low-carbon olefins using an inorganic iron salt-synthesized iron carbide catalyst according to claim 2, characterized in that, The iron carbide catalyst exhibits good catalytic activity and stability in both batch and flow reaction systems. (1) The use of a batch reaction system specifically includes the following steps: The iron carbide catalyst was placed in the reaction system and the reaction system was evacuated. Syngas is introduced into the reaction system. The syngas comprises a mixture of CO, H2, and inert gases, wherein the volume ratio of CO to H2 is 1:1 to 1:3, and the remainder is inert gases. The reaction pressure is 1 to 2 bar, and the reaction temperature is 300 to 340 °C. o C. Use a heated base or concentrated sunlight to provide heat energy. The reaction time is 30~120 min. Use gas chromatography to detect the products. (2) The use of a flow reaction system specifically includes the following steps: The iron carbide catalyst is placed in the reaction system, and the synthesis gas is introduced into the reaction system until the air remaining in the reaction gas is exhausted. The synthesis gas includes a mixture of CO, H2 and inert gas, wherein the volume ratio of CO and H2 is 1:1 to 1:3, and the rest are inert gases. The synthesis gas is controlled to flow through at a certain rate using a mass flow meter, and heat is provided by a heating base or by direct sunlight. The reaction temperature is 300~340°C. o C. The products were detected by gas chromatography every 30 minutes.

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