A method for preparing an iron-based catalyst purified by magnetic field separation and a method for preparing low-carbon olefins using the same
By separating and purifying iron-based catalysts using magnetic fields, the problems of high energy consumption and environmental pollution in traditional Fischer-Tropsch synthesis have been solved. This has enabled high selectivity and low CO2 generation of low-carbon olefins, promoting the sustainable development of the petrochemical industry.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HUAZHONG NORMAL UNIV
- Filing Date
- 2024-01-06
- Publication Date
- 2026-06-23
AI Technical Summary
In the traditional Fischer-Tropsch synthesis process, iron-based catalysts lead to high energy consumption and environmental pollution under high temperature and pressure, and it is difficult to suppress the generation of by-product CO2, which affects the selectivity of low-carbon olefins.
By employing magnetic field separation and purification technology, high-performance iron-based catalysts are prepared by controlling the surface active components of iron-based catalysts and utilizing the physical magnetism of the catalysts for separation and purification. These catalysts can be used for photothermal or thermocatalytic Fischer-Tropsch synthesis reactions.
Under low-carbon emission conditions, it achieves high selectivity for low-carbon olefins and low CO2 generation. The catalyst is easy to produce on a large scale, with low energy consumption and is environmentally friendly.
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Figure CN117943078B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of iron-based catalyst preparation and application, specifically to a method for preparing an iron-based catalyst purified by magnetic field separation and a method for preparing low-carbon olefins using the catalyst. Background Technology
[0002] Against the backdrop of current global climate change and energy security concerns, developing low-carbon energy and reducing carbon emissions have become urgent tasks worldwide. The Fischer-Tropsch process, also known as FT synthesis, is a process that uses syngas (a mixture of carbon monoxide and hydrogen) as a raw material to synthesize liquid hydrocarbons or hydrocarbons under catalyst and appropriate conditions. However, traditional Fischer-Tropsch synthesis involves high-temperature and high-pressure reaction conditions (200-400℃, 2-5 MPa), leading to severe energy consumption (traditional Fischer-Tropsch synthesis is powered by the combustion of fossil fuels) and environmental pollution (mainly the emission of large amounts of the greenhouse gas carbon dioxide). Therefore, finding a low-cost, sustainable, and environmentally friendly Fischer-Tropsch synthesis route is of great significance.
[0003] Iron-based catalysts have attracted widespread attention due to their abundant resource reserves, low cost, and excellent Fischer-Tropsch synthesis catalytic activity and selectivity for low-carbon olefins. Furthermore, photothermal catalysis, as an emerging catalytic technology, utilizes clean and sustainable solar energy to provide energy for the reaction through photothermal conversion on the catalyst surface, achieving the conversion of solar energy into chemical energy. Therefore, the use of iron-based catalysts for photothermal Fischer-Tropsch synthesis as a novel Fischer-Tropsch synthesis route for the preparation of low-carbon olefins holds great potential. According to research reports, the Fischer-Tropsch synthesis performance of iron-based catalysts is related to the active components at their interface. Iron (Fe), iron carbides (Fe5C2, Fe2C, Fe7C3, Fe3C), etc., are considered active components in Fischer-Tropsch synthesis, facilitating the conversion of syngas (CO and H2) into hydrocarbon products. However, due to the reactive chemical properties of iron and iron carbide, partial oxidation of the catalyst is unavoidable during synthesis and reaction, generating iron oxides, including Fe2O, Fe3O4, and Fe2O3. These iron oxides are the active components in the catalytic water-gas reaction (CO + H2O → CO2 + H2O) and are the cause of the carbon dioxide (CO2) byproduct in Fischer-Tropsch synthesis. Therefore, by controlling the surface active components of iron-based catalysts, the selectivity of the target product, low-carbon olefins, can be improved while suppressing the formation of the byproduct CO2. This will help promote the sustainable development of the petrochemical industry and make a positive contribution to the sustainable development of humankind.
[0004] The method of purifying iron-based catalysts using magnetic fields can utilize the physical magnetism of the catalyst to separate and purify its surface and interfacial structures, thereby controlling the surface-active components of the iron-based catalyst. Magnetic field separation and purification of iron-based catalysts requires extremely low energy consumption and is environmentally friendly. This technology holds promise for Fischer-Tropsch synthesis with low carbon emissions and for achieving excellent selectivity for low-carbon olefins. Summary of the Invention
[0005] The purpose of this invention is to provide a method for preparing iron-based catalysts using magnetic field separation and purification, and to explore the application prospects of the obtained iron-based catalysts in the Fischer-Tropsch synthesis for the preparation of low-carbon olefins. This invention is the first to propose using a magnetic field to separate and purify catalysts synthesized by wet chemical methods, and then applying the obtained catalysts to the Fischer-Tropsch synthesis for the preparation of low-carbon olefins. The high-performance iron-based catalyst purified by the magnetic field exhibits excellent selectivity for low-carbon olefins under low carbon emission conditions.
[0006] To achieve the above objectives, this invention proposes a method for preparing iron-based catalysts using magnetic field separation and purification, comprising the following steps:
[0007] (1) Mix the iron source, solvent, carbon source, inducer and carrier thoroughly under magnetic stirring, heat to 320-360℃ under an inert atmosphere and keep warm for 10-60 minutes;
[0008] (2) Under the action of a magnetic field with a surface magnetization of 40-90 mT, the iron-based catalyst obtained in step (1) has a part of the weakly magnetic iron-based catalyst dispersed in the solution and another part of the strongly magnetic iron-based catalyst adsorbed on the magnetic field. The weakly magnetic iron-based catalyst dispersed in the solution is directly poured out and collected. The strongly magnetic iron-based catalyst adsorbed on the magnetic field is washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties is achieved.
[0009] (3) The iron-based catalysts with different magnetic properties collected in step (2) are washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. The catalyst is then washed with ethanol once and then dried under vacuum to obtain the iron-based catalyst separated and purified by magnetic field.
[0010] Furthermore, in step (1), the iron source can be nonacarbonyl diferric, pentacarbonyl iron, or nano iron; the solvent and carbon source can be octadecylamine; the inducing agent can be hexadecyltrimethylammonium bromide or ammonium bromide; the support can be alumina or silicon dioxide, or no support may be added.
[0011] Furthermore, in step (2), the catalyst needs to be separated in a solution at 50 to 150°C. Due to their different magnetic properties, the iron-based catalyst can be separated under the action of a magnetic field. The rotation speed of the magnetic stirring is 400 to 2000 revolutions.
[0012] In this invention, the prepared iron-based catalysts consist of 85%–100% iron carbide (chemical formula Fe5C2) and a small amount of iron oxides. The catalysts have a core-shell structure, with an amorphous carbon layer or iron oxide as the shell and Fe5C2 as the core. The differences in the overall magnetic properties of the iron-based catalysts originate from the differences in the surface shell region structure of individual catalyst particles: the iron-based catalysts with stronger magnetic properties have a bulk composition of Fe5C2, while the surface region contains abundant iron oxides; the iron-based catalysts with weaker magnetic properties have a bulk composition of Fe5C2, with only a small amount of iron oxides on the surface, mainly amorphous carbon.
[0013] The present invention also provides a method for preparing low-carbon olefins using the above-mentioned iron-based catalyst, the specific steps of which are as follows:
[0014] S1, place the iron-based catalyst in the Fischer-Tropsch synthesis reaction system and evacuate the reaction system;
[0015] S2, syngas is introduced into a closed or flow-through reaction system. The syngas comprises a mixture of carbon monoxide (CO), hydrogen (H2), and inert gases, wherein the volume ratio of CO to H2 is 1:1 to 1:5, and the remaining gases are inert gases. The Fischer-Tropsch synthesis reaction is driven by photothermal or thermal catalysis. The iron-based catalyst catalyzes the Fischer-Tropsch synthesis reaction at a temperature of 200-400°C. Gas chromatography is used to detect the products of the Fischer-Tropsch synthesis.
[0016] In this invention, the surface structure of the iron-based catalyst can significantly affect the conversion rate and product selectivity of Fischer-Tropsch synthesis. The weakly magnetic iron-based catalyst obtained in step (2), with Fe5C2 as the main active component, exhibits excellent activity, low-carbon olefin selectivity and stability in Fischer-Tropsch synthesis, while greatly suppressing the generation of CO2. The strongly magnetic iron-based catalyst obtained in step (2) is not conducive to the formation of unsaturated hydrocarbons due to the abundant iron oxide component on its surface, and the iron oxide component on the surface, as the active component of the water-gas reaction (CO+H2O→CO2+H2), will lead to the generation of a large amount of byproduct CO2.
[0017] In this invention, the separation and purification of iron-based catalysts with different magnetic properties can be achieved by adjusting the magnetic field strength and the number of purification cycles, in order to obtain an iron-based catalyst with Fe5C2 as the main active component. The obtained magnetically purified iron-based catalyst can achieve high selectivity for low-carbon olefins under low CO2 production conditions in Fischer-Tropsch synthesis.
[0018] The beneficial effects of this invention are as follows:
[0019] 1. This invention employs a clean, efficient, and pollution-free magnetic field separation and purification technique during or after catalyst synthesis for the preparation of iron-based catalysts with different magnetic properties, which facilitates large-scale production of catalysts.
[0020] 2. The high-performance iron-based catalyst obtained by magnetic field separation and purification catalyzes the Fischer-Tropsch synthesis reaction, with a CO conversion rate of 25-35%, a selectivity of over 50% for the target product low-carbon olefins, and a selectivity of less than 5% for the by-product CO2.
[0021] 3. When photothermal catalysis is used, since the energy driving the catalytic reaction comes from sunlight and very little CO2 is produced in the reaction, ultra-low carbon emissions of Fischer-Tropsch synthesis catalyzed by iron-based catalysts have been achieved for the first time, making Fischer-Tropsch synthesis an effective carbon reduction technology. Attached Figure Description
[0022] Figure 1 This is a schematic diagram of magnetic separation during the preparation process of this invention.
[0023] Figure 2 The images show the XRD patterns of iron-based catalysts with different magnetic properties obtained in Example 1 of this invention.
[0024] Figure 3 These are high-resolution transmission electron microscope images of iron-based catalysts with different magnetic properties obtained in this invention. Detailed Implementation
[0025] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further clarifies the invention. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.
[0026] Unless otherwise specified, the preparation methods in this invention are all conventional methods. Unless otherwise specified, all raw materials used are available from publicly available commercial sources.
[0027] This embodiment provides a method for preparing an iron-based catalyst using magnetic field separation and purification, such as... Figure 1 As shown, it includes the following steps:
[0028] (1) The iron source, solvent, carbon source, inducer, and carrier are thoroughly mixed under magnetic stirring. Under an inert atmosphere, the temperature is raised to 320–360°C and held for 10–60 minutes;
[0029] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated from the magnetic field due to the difference in magnetic properties. Some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0030] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then the catalyst is washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation.
[0031] Figure 3 The image shows high-resolution transmission electron microscopy (HRTEM) images of the iron-based catalysts with different magnetic properties. In the image, a and b correspond to the weaker magnetic iron-based catalyst (Fe5C2-s) and the stronger magnetic iron-based catalyst (Fe5C2-m), respectively. The difference in the surface magnetic iron oxide content of the catalysts leads to the difference in overall magnetic properties, which provides a basis for the technique of magnetic field separation and purification of iron-based catalysts.
[0032] The obtained iron-based catalyst was applied to a photothermal catalytic reaction system or a flow-type thermocatalytic reaction system. Syngas (CO, H2, and N2 in a volume ratio of 20:60:20) was introduced into the reaction system, the reaction temperature was 340℃, and the reaction pressure was 0.18 MPa. Under these conditions, the performance of the iron-based catalyst in Fischer-Tropsch synthesis was measured, and the changes in Fischer-Tropsch synthesis products over time were detected using gas chromatography.
[0033] Example 1
[0034] A method for preparing an iron-based catalyst purified by magnetic field separation includes the following steps:
[0035] (1) Mix 0.655g of Fe2(CO)9, 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide evenly under magnetic stirring (the surface magnetic field strength of the magnetic stirrer is about 55mT); under an inert atmosphere, heat to 320-360℃ and keep warm for 10-60 minutes;
[0036] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated from the magnetic field due to the difference in magnetic properties. Some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0037] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then the catalyst is washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation.
[0038] The iron-based catalyst (Fe5C2-s) dispersed in solution prepared by the above method exhibits weak magnetism, while the iron-based catalyst (Fe5C2-m) adsorbed on magnetic particles exhibits strong magnetism. The obtained iron-based catalysts were applied to a photothermal Fischer-Tropsch synthesis reaction. 50 mg of the iron-based photothermal catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:60:40, volume ratio) was introduced. The iron-based Fischer-Tropsch synthesis reaction was driven by a full-spectrum xenon lamp, and the catalyst activity was determined by detecting the changes in products over time using gas chromatography. The Fe5C2-s catalyst showed a CO conversion rate of 30.1%, a selectivity for low-carbon olefins of 54.3%, and a CO2 byproduct selectivity of 2.6%; the Fe5C2-m catalyst showed a CO conversion rate of 63.3%, a low-carbon olefin selectivity of 11.5%, and a CO2 byproduct selectivity of 33.9%.
[0039] Figure 2 The XRD patterns of the iron-based catalysts with different magnetic properties obtained in Example 1 are shown. Curves a and b in the figure correspond to the weaker magnetic iron-based catalyst (Fe5C2-s) and the stronger magnetic iron-based catalyst (Fe5C2-m), respectively. Under these conditions, the synthesized Fe5C2-s and Fe5C2-m catalysts have characteristic peaks at 43.6° and 44.6°, respectively, which are attributed to the (021) and (510) crystal planes of Fe5C2. Both catalysts exhibit good crystallinity.
[0040] Example 2
[0041] A method for preparing an iron-based catalyst purified by magnetic field separation includes the following steps:
[0042] (1) Mix 0.655g of Fe2(CO)9, 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide evenly under magnetic stirring (the surface magnetic field strength of the magnetic stirrer is about 55mT); under an inert atmosphere, heat to 320-360℃ and keep warm for 10-60 minutes;
[0043] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated from the magnetic field due to the difference in magnetic properties. Some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0044] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then the catalyst is washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation.
[0045] The iron-based catalyst (Fe5C2-s) dispersed in solution prepared by the above method has weak magnetic properties, while the iron-based catalyst (Fe5C2-m) adsorbed on magnetic particles has strong magnetic properties. The obtained iron-based catalyst was used in a flow-type thermocatalytic Fischer-Tropsch synthesis system. 100 mg of the iron-based catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:60:20, volume ratio) was introduced. Gas chromatography was used to detect changes in the product over time, and the catalyst activity was determined. In the thermocatalytic Fischer-Tropsch synthesis reaction, the high-performance iron-based catalyst (Fe5C2-s) purified by magnetic field separation showed a CO conversion rate of 35.2%, a selectivity of 49.1% for the target product low-carbon olefins, and a selectivity of 3.8% for the byproduct CO2.
[0046] Example 3
[0047] A method for preparing an iron-based catalyst purified by magnetic field separation includes the following steps:
[0048] (1) Mix 0.655g of Fe2(CO)9, 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide evenly under magnetic stirring (the surface magnetic field strength of the magnetic stirrer is about 55mT); under an inert atmosphere, heat to 320-360℃ and keep warm for 10-60 minutes;
[0049] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated from the magnetic field due to the difference in magnetic properties. Some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0050] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then the catalyst is washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation.
[0051] The iron-based catalyst (Fe5C2-s) dispersed in solution prepared by the above method has weak magnetic properties, while the iron-based catalyst (Fe5C2-m) adsorbed on magnetic particles has strong magnetic properties. The two types of iron-based catalysts were thoroughly ground and mixed in a mortar according to a mass ratio before being applied to the Fischer-Tropsch synthesis reaction. 50 mg of the iron-based catalyst was added to the reactor, and diluted synthesis gas (CO:H2:N2 = 20:60:20, volume ratio) was introduced. Gas chromatography was used to detect changes in the product over time, and the catalyst activity was determined. With increasing Fe5C2-m component content in the catalyst, the CO conversion rate increased from 30.1% to 63.3%, the selectivity for the target product low-carbon olefins decreased from 54.3% to 11.5%, and the selectivity for the byproduct CO2 increased from 2.6% to 33.9%.
[0052] Example 4
[0053] A method for preparing an iron-based catalyst purified by magnetic field separation includes the following steps:
[0054] (1) Mix 0.655g of Fe2(CO)9, 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide under magnetic stirring (magnetic stirrers with surface magnetic field strengths of 25mT, 32mT and 55mT respectively); heat to 320-360℃ under an inert atmosphere and keep warm for 10-60 minutes;
[0055] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated by magnetic field. Due to the difference in magnetic properties of the iron-based catalyst, some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0056] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then the catalyst is washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation.
[0057] The iron-based catalyst (Fe5C2-s) dispersed in solution prepared by the above method exhibits weak magnetic properties, while the iron-based catalyst (Fe5C2-m) adsorbed on magnetic particles exhibits strong magnetic properties. 50 mg of Fe5C2-s catalyst was added to the reactor, and diluted syngas (CO:H2:N2 = 20:60:20, volume ratio) was introduced. Gas chromatography was used to detect changes in the product over time, and the catalyst activity was determined. With increasing magnetic field strength on the magnetic particle surface, the CO conversion rate of the Fe5C2-s catalyst decreased from 37.7% to 30.1%, the selectivity for the target product (low-carbon olefins) increased from 48.8% to 54.3%, and the selectivity for the byproduct CO2 decreased from 6.6% to 2.6%.
[0058] Example 5
[0059] A method for preparing an iron-based catalyst purified by magnetic field separation includes the following steps:
[0060] (1) Mix 0.655g of Fe2(CO)9, 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide evenly under magnetic stirring (the surface magnetic field strength of the magnetic stirrer is about 55mT); under an inert atmosphere, heat to 320-360℃ and keep warm for 10-60 minutes;
[0061] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated by magnetic field. Due to the difference in magnetic properties of the iron-based catalyst, some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0062] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then, the catalyst is washed with ethanol once and then vacuum dried to obtain the iron-based catalyst separated and purified by magnetic field.
[0063] (4) Disperse the catalyst (Fe5C2-m) adsorbed on the magnetic pole with ethanol for 10-60 minutes using ultrasonication. Then, stir the catalyst with the magnetic pole, collect the unadsorbed catalyst from the ethanol and dry it. Repeat the above operation to control the number of times the magnetic separation and purification is performed.
[0064] According to the above method, iron-based catalysts undergoing different numbers of magnetic field separation and purification operations were prepared and applied to the Fischer-Tropsch synthesis reaction. 50 mg of iron-based catalyst was added to the reactor, followed by the introduction of diluted syngas (CO:H2:N2 = 20:60:20, volume ratio). Gas chromatography was used to detect changes in product yield over time, and the catalyst activity was determined. With increasing magnetic separation cycles, the CO conversion rate of the Fe5C2-m catalyst decreased from 63.3% to 45.5%, the selectivity for the target product (low-carbon olefins) increased from 11.5% to 36.5%, and the selectivity for the byproduct CO2 decreased from 33.9% to 15.4%.
[0065] Example 6
[0066] A method for preparing an iron-based catalyst purified by magnetic field separation includes the following steps:
[0067] (1) Mix 0.655g of Fe2(CO)9, 14.5g of octadecylamine and 0.113g of hexadecyltrimethylammonium bromide evenly under magnetic stirring (the surface magnetic field strength of the magnetic stirrer is about 55mT); under an inert atmosphere, heat to 320-360℃ and keep warm for 10-60 minutes;
[0068] (2) Under the action of a magnetic field with a surface magnetization of about 55 mT, the iron-based catalyst obtained in step (1) will be separated from the magnetic field due to the difference in magnetic properties. Some of the weaker iron-based catalysts will be dispersed in the solution, while the other part of the stronger iron-based catalysts will be adsorbed on the magnetic field. First, the weaker iron-based catalysts dispersed in the solution will be poured out and collected. Then, the stronger iron-based catalysts adsorbed on the magnetic field will be washed off with n-hexane and collected. By following the above operation, the separation of iron-based catalysts with different magnetic properties can be achieved.
[0069] (3) The iron-based catalyst collected in step (2) is washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. Then the catalyst is washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation.
[0070] The weakly magnetic iron-based catalyst (Fe5C2-s) dispersed in solution, prepared using the above method, was applied to an outdoor solar-driven Fischer-Tropsch synthesis reaction. 50 mg of the iron-based catalyst was added to the reactor, and diluted syngas (CO:H2:N2 = 20:60:40, volume ratio) was introduced. Gas chromatography was used to detect changes in the products over time, and the catalyst activity was determined. The Fe5C2-s catalyst showed a CO conversion rate of 36.8%, a selectivity for low-carbon olefins of 48.6%, and a CO2 byproduct selectivity of 5.4%.
[0071] In summary, suppressing CO2 emissions in the iron-based catalytic conversion of syngas to produce low-carbon olefins is challenging in existing technologies. Compared to existing technologies, this invention employs magnetic field separation and purification technology, utilizing the magnetic differences of iron-based catalysts to obtain a high-performance iron-based catalyst with Fe5C2 as the main active component. This catalyst can achieve highly selective production of low-carbon olefins while reducing the generation of the byproduct CO2. Furthermore, in addition to using thermocatalytically driven iron-based catalysts for Fischer-Tropsch synthesis, a clean and sustainable photothermal catalytic Fischer-Tropsch synthesis can also be employed, further reducing carbon emissions and thus contributing to environmental protection and effective utilization of solar energy.
[0072] 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 low-carbon olefins using iron-based catalysts purified by magnetic field separation, characterized in that, The preparation method of the iron-based catalyst includes the following steps: (1) Mix the iron source, solvent, carbon source and inducer thoroughly under magnetic stirring, and heat to 320~360°C under an inert atmosphere. o C, and keep warm for 10 to 60 minutes; (2) Under the action of a magnetic field with a surface magnetic induction intensity of 40-90 mT, the iron-based catalyst obtained in step (1) has a part of the weakly magnetic iron-based catalyst dispersed in the solution and another part of the strongly magnetic iron-based catalyst adsorbed on the magnetic field. The weakly magnetic iron-based catalyst dispersed in the solution is poured out and collected. The strongly magnetic iron-based catalyst adsorbed on the magnetic field is washed off with n-hexane and collected. By following the above operation, the iron-based catalysts with different magnetic properties are separated. The strongly magnetic iron-based catalyst has a bulk phase composition of Fe5C2 and a surface region of iron oxide. The weakly magnetic iron-based catalyst has a bulk phase composition of Fe5C2 and a surface region of amorphous carbon. (3) The iron-based catalysts with different magnetic properties collected in step (2) are washed by alternating centrifugation with hexane and ethanol until the upper washing liquid after washing the catalyst with hexane is colorless and transparent. The catalyst is then washed with ethanol once and dried under vacuum to obtain the iron-based catalyst purified by magnetic field separation. The iron-based catalyst composition contains 85% to 100% iron carbide. The content of iron carbide is not 100%. The catalyst has a core-shell structure. The shell is an amorphous carbon layer or iron oxide, and the core is Fe5C2. The difference in the overall magnetic properties of the iron-based catalyst comes from the difference in the surface shell region structure of the single particle catalyst. The weakly magnetic iron-based catalyst is used to catalyze the Fischer-Tropsch synthesis reaction to prepare low-carbon olefins.
2. The method according to claim 1, characterized in that: In step (1), the iron source is nonacarbonyl diferric or pentacarbonyl iron; the solvent and carbon source are octadecylamine; and the inducing agent is hexadecyltrimethylammonium bromide or ammonium bromide.
3. The method according to claim 1, characterized in that: In step (1), a carrier is added, wherein the carrier is alumina or silicon dioxide.
4. The method according to claim 1, characterized in that, The method includes the following steps: S1, a weakly magnetic iron-based catalyst is placed in the Fischer-Tropsch synthesis reaction system and the reaction system is evacuated; S2, the syngas is introduced into a closed or flow-through reaction system. The syngas comprises a mixture of carbon monoxide, hydrogen, and inert gases, wherein the volume ratio of CO to H2 is 1:1 to 1:5, and the remaining gases are inert gases. The Fischer-Tropsch synthesis reaction is driven by photothermal or thermal catalysis. The weakly magnetic iron-based catalyst catalyzes the Fischer-Tropsch synthesis reaction at a temperature of 200-400°C. o C, The products of Fischer-Tropsch synthesis were detected using gas chromatography.
5. The method according to claim 4, characterized in that: The Fischer-Tropsch synthesis reaction was catalyzed by a weakly magnetic iron-based catalyst, with a CO conversion rate of 25-35%, a selectivity of over 50% for the target product low-carbon olefins, and a selectivity of less than 5% for the byproduct CO2.