Iron-based catalyst and preparation method thereof

Abstract

The invention provides an iron-based catalyst and a preparation method thereof. The preparation method comprises the following steps: preparing ferric salt and manganese salt into a homogeneous-phase mixed solution; heating the homogeneous-phase mixed solution so that the ferric salt and the manganese salt are subjected to in-situ thermal decomposition to form an iron-manganese oxide matrix; the iron-manganese oxide matrix is placed in an auxiliary agent metal precursor solution for wet impregnation, an impregnation system is subjected to drying and high-temperature roasting, and the iron-based catalyst is obtained, the iron-manganese composite catalyst is prepared by virtue of a one-step co-evaporation pyrolysis process and a dipping roasting process, the preparation steps are simple, the production period and cost are effectively reduced, and unification of preparation simplicity and high catalytic efficiency of the iron-based catalyst is realized.

Description

Technical Field

[0001] This invention relates to the field of CO2 hydrogenation to olefins technology, and particularly to an iron-based catalyst and its preparation method. Background Technology

[0002] In the catalyst system for CO2 hydrogenation to olefins, iron-based catalysts have become a research hotspot and mainstream direction due to their significant advantages. Iron is abundant in the Earth's crust (approximately 5.6% of its mass, ranking fourth), has low preparation costs, and is suitable for large-scale industrial applications. Simultaneously, iron-based catalysts possess a wide operating temperature range, good resistance to poisoning, and can adapt to different hydrogen-to-carbon ratio feed gases. They exhibit high catalytic activity and olefin selectivity in CO2 hydrogenation reactions, especially in Fischer-Tropsch synthesis, where they can effectively promote C-C bond formation, generating target products such as low-carbon olefins. Furthermore, iron-based catalysts also possess strong water-gas shift reaction activity, allowing for flexible adjustment of the hydrogen-to-carbon ratio in the reaction system to further optimize reaction performance. Therefore, they have broad application prospects in the field of CO2 hydrogenation to olefins.

[0003] However, existing iron-based catalyst preparation technologies still have many shortcomings, making it difficult to meet actual production needs in terms of CO2 conversion, olefin selectivity, and industrial applicability. Developing an iron-based catalyst with a simple preparation process, high conversion rate, and high olefin selectivity has become a key breakthrough for promoting the industrialization of CO2 hydrogenation to olefins technology. Summary of the Invention

[0004] In view of the problems existing in the background technology, the present invention provides an iron-based catalyst and its preparation method, aiming to solve the core pain points of existing iron-based catalyst preparation methods, such as complex preparation process, low CO2 conversion rate and poor olefin selectivity.

[0005] The specific details of the invention are as follows: In a first aspect, the present invention provides a method for preparing an iron-based catalyst, the method comprising: Prepare homogeneous mixed solutions from iron and manganese salts; The homogeneous mixed solution is heated to cause the iron salt and manganese salt to undergo in-situ thermal decomposition, forming an iron-manganese oxide matrix. The iron-manganese oxide matrix is ​​placed in a solution of auxiliary metal precursors for wet impregnation. The impregnation system is then dried and calcined at high temperature to obtain the iron-based catalyst.

[0006] Optionally, in the homogeneous mixed solution, the molar ratio of iron to manganese is 1:1 to 8:1.

[0007] Optionally, in the homogeneous mixed solution, the concentration of metal ions is 0.1-5 mol / L.

[0008] Optionally, the heating temperature of the homogeneous mixed solution is 200 ℃-400 ℃.

[0009] Optionally, the auxiliary metal in the auxiliary metal precursor solution is selected from at least one of potassium and sodium, and the mass ratio of iron manganese oxide to auxiliary metal in the impregnation system is 1:0.002-1:0.05.

[0010] Optionally, the wet impregnation time is 2-8 hours.

[0011] Optionally, the high-temperature roasting is carried out in a muffle furnace.

[0012] Optionally, the high-temperature calcination temperature is 400 ℃-800 ℃, and the calcination time is 2 h-10 h.

[0013] In a second aspect, the present invention provides an iron-based catalyst, which is obtained according to the preparation method described in the first aspect above.

[0014] Optionally, the iron-based catalyst is used to catalyze the hydrogenation of carbon dioxide to olefins, wherein the conversion rate of carbon dioxide is greater than 44% and the selectivity of olefins is greater than 50%.

[0015] Compared with the prior art, the present invention has the following advantages: This invention provides a method for preparing an iron-based catalyst, comprising: preparing a homogeneous mixed solution of iron salt and manganese salt; heating the homogeneous mixed solution to cause in-situ thermal decomposition of the iron salt and manganese salt to form an iron-manganese oxide matrix; placing the iron-manganese oxide matrix in a solution of a promoter metal precursor for wet impregnation; and drying and high-temperature calcining the impregnation system to obtain the iron-based catalyst. This invention utilizes a one-step co-evaporation pyrolysis process combined with an impregnation and calcination process to prepare an iron-manganese composite catalyst. The preparation steps are simple, effectively reducing the production cycle and cost, and achieving a balance between the ease of preparation and high catalytic efficiency of the iron-based catalyst. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A flowchart illustrating the preparation method of the iron-based catalyst provided in an embodiment of the present invention is shown; Figure 2 The SEM image of the iron-based catalyst provided in the embodiments of the present invention is shown; Figure 3 The EDS spectrum of the iron-based catalyst provided in the embodiments of the present invention is shown. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0019] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0020] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0021] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] The mainstream preparation methods for existing iron-based catalysts mainly include coprecipitation, impregnation, and sol-gel methods. Among them, coprecipitation is one of the most widely used traditional methods for preparing iron-based catalysts. Its core process involves dissolving iron salts (such as ferric nitrate and ferric chloride) and auxiliary salts (such as potassium and manganese salts) in a certain proportion to form a mixed metal salt solution. A precipitant (such as sodium carbonate or ammonia) is slowly added under stirring conditions, and the pH and temperature of the reaction system are controlled to allow the metal ions to precipitate simultaneously, forming hydroxide or carbonate precursors. The precursors are then filtered, washed multiple times, dried (80-120℃), and calcined (400-600℃) to obtain the iron-based catalyst. German researchers have used this method to prepare iron-based catalysts, achieving certain activity and selectivity in the CO hydrogenation reaction. The active phase structure is relatively uniform, laying the foundation for subsequent research.

[0023] However, the coprecipitation method suffers from problems such as low product purity, poor performance stability and repeatability, and difficulty in precisely controlling the structure of the active phase. The CO2 conversion rate of catalysts prepared by this method is usually less than 40%, and the performance fluctuates greatly between batches. On the one hand, the rapid reaction rate between the precipitant and metal ions during precipitation can easily lead to uneven local concentrations, introduce impurities, or form amorphous precipitates, affecting the purity of the catalyst. On the other hand, this method makes it difficult to precisely control the particle size and distribution of the active phase (such as iron carbide). Agglomeration of the active phase is prone to occur during calcination, resulting in a decrease in the active specific surface area. Furthermore, the poor stability of the active phase crystal structure makes it susceptible to sintering under the influence of temperature and pressure during the reaction, further exacerbating the decline in catalytic performance.

[0024] Impregnation is another mainstream method for preparing iron-based catalysts, especially suitable for the preparation of supported iron-based catalysts. Its core process involves selecting a suitable support (such as SiO2, Al2O3, or SAPO-34 molecular sieve), immersing the support in a mixed solution containing the active component (iron salt) and promoters, and allowing the active component to be loaded onto the support surface through adsorption and diffusion. Subsequently, the catalyst undergoes settling, drying, calcination, and reduction activation treatment to obtain the finished catalyst. A US research team used a negative pressure impregnation method to prepare Fe / SAPO-34 catalysts, significantly improving the selectivity of the target product under specific reaction conditions, verifying the effectiveness of this method in performance regulation.

[0025] However, the impregnation method suffers from problems such as uneven distribution of active components, low utilization of active sites, difficulty in achieving synergistic effects between the active phase and promoters, and limited olefin selectivity. Taking the Fe / SAPO-34 catalyst as an example, even with the Fe loading increased to 25%, the selectivity for low-carbon olefins only reaches 31.9%, and a large amount of oxygen-containing compounds and methane are produced as byproducts. This is because the loading of active components on the support surface depends on physical adsorption and diffusion, which easily leads to a phenomenon of "surface enrichment and internal scarcity," resulting in uneven distribution of active sites and some active components being unable to effectively participate in the reaction. At the same time, the interaction between the support, active components, and promoters is weak, making it difficult to form a stable synergistic system, which cannot effectively suppress side reactions such as secondary hydrogenation and methanation of olefins, and the active phase is prone to detachment from the support, reducing the catalyst's lifespan.

[0026] The sol-gel method is a precision preparation method developed in recent years, aiming to achieve molecular-level control of catalyst structure. Its core process involves using metal alkoxides (such as iron alkoxides) as raw materials, forming a sol system through hydrolysis and condensation reactions in a solvent; the sol is then aged to transform into a gel, followed by supercritical or vacuum drying to remove the solvent, and finally calcined at high temperature to form an iron-based catalyst with high specific surface area and uniform pore size distribution. Iron-based catalysts prepared by Chinese researchers using this method exhibit excellent performance in the CO hydrogenation reaction, with smaller and more uniformly distributed active phase particles.

[0027] However, the sol-gel method is complex, requires stringent conditions, and is costly, hindering large-scale industrial application. This method necessitates strict control of parameters such as temperature, pH, and reaction time, and the raw material, metal alkoxides, is expensive, requiring substantial investment in supercritical drying equipment. This is because the sol-gel conversion process is extremely sensitive to the reaction environment; even minor parameter fluctuations can lead to gel structure defects, affecting catalyst performance. Furthermore, the drying and calcination processes require precise control of the heating rate and atmosphere; otherwise, cracks and pore collapse can easily occur, resulting in the destruction of the active phase structure, further increasing the process difficulty and production cost, thus limiting its industrial application.

[0028] Furthermore, all three preparation methods mentioned above suffer from a core contradiction: either they sacrifice ease of preparation in pursuit of improved performance (such as the sol-gel method), or they prioritize ease of preparation but compromise performance (such as the coprecipitation and impregnation methods), making it difficult to simultaneously achieve the goals of "simple preparation, high conversion rate, and high olefin selectivity." The fundamental reason lies in the fact that existing methods cannot precisely control the particle size distribution and surface chemical properties of the active components in iron-based catalysts, and it is difficult to construct a stable synergistic system of active phase, promoter, and support. This leads to easy deactivation of the active phase and difficulty in suppressing side reactions during the reaction process. Simultaneously, the contradiction between complex processes and performance cannot meet the demands of industrial production.

[0029] Based on the above considerations, this invention abandons the dependence of existing methods on harsh reaction conditions, special equipment, and expensive raw materials. It utilizes a one-step co-evaporation pyrolysis process combined with an impregnation and calcination process to prepare an iron-manganese composite catalyst. The preparation steps are simple, effectively reducing the production cycle and cost, and achieving a balance between the ease of preparation and high catalytic efficiency of iron-based catalysts. The specific embodiments of this invention are described in detail below: In a first aspect, the present invention provides a method for preparing an iron-based catalyst. Figure 1 A flowchart illustrating the preparation method of the iron-based catalyst provided in Example 1 of this invention is shown, as follows: Figure 1 As shown, the preparation method includes: S1. Prepare a homogeneous mixed solution by taking iron salt and manganese salt; S2. Heat the homogeneous mixed solution to cause the iron salt and manganese salt to undergo in-situ thermal decomposition to form an iron-manganese oxide matrix; S3. The iron-manganese oxide matrix is ​​placed in a solution of auxiliary metal precursors for wet impregnation. The impregnation system is then dried and calcined at high temperature to obtain the iron-based catalyst.

[0030] The iron and manganese salts selected in this embodiment are all soluble salts, preferably iron-containing nitrates and manganese-containing nitrates. In this embodiment, the iron and manganese salts are prepared into a homogeneous mixed solution, which is then heated to evaporate the solvent and cause in-situ thermal decomposition of the iron and manganese salts, directly forming an iron-manganese oxide matrix. Manganese acts as a regulator of catalytic performance, on the one hand, through the strong Fe-Mn interaction, directionally transferring electrons to the iron active center, regulating the electron density of active sites, and directionally promoting the formation of a highly active χ-Fe5C2 iron carbide phase. This achieves efficient coupling of the reverse water-gas shift and Fischer-Tropsch synthesis dual active phases, promoting the formation and stability of the active phase (such as iron carbide). On the other hand, it acts as a physical barrier, improving the dispersion of active sites and optimizing the reduction kinetics of iron species, thereby inhibiting the sintering and agglomeration of the active phase. Its regulatory effect exhibits a significant doping threshold effect; excessive doping will cover the active sites, leading to a decrease in activity. Atomically dispersed manganese iron oxide precursors can maximize the regulatory effect of manganese. By optimizing the iron-manganese ratio, the molar ratio of iron and manganese elements in the homogeneous mixed solution is controlled between 1:1 and 8:1; for example, it can be 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, or 8:1, thereby improving catalytic activity and selectivity to meet the needs of different reaction scenarios.

[0031] In this embodiment, the in-situ thermal decomposition process allows iron and manganese ions to decompose and form oxides simultaneously. This avoids the problems of uneven local concentration and amorphous precipitation caused by the rapid reaction between the precipitant and metal ions during the co-precipitation method, and also avoids the defects of surface enrichment and internal depletion of active components during the impregnation method. This results in smaller (micrometer-sized) particle size and more uniform distribution of the active phase (iron oxide) in the iron-manganese oxide matrix, significantly increasing the active specific surface area and providing more active sites for subsequent catalytic reactions.

[0032] In this embodiment, the complete in-situ thermal decomposition reaction can be determined when no more gas escapes during the heating process. The iron-manganese oxide matrix formed by thermal decomposition has a dense structure and a more stable crystal structure compared to the active phase that is prone to agglomeration after co-precipitation calcination. This can effectively suppress the sintering of the active phase caused by temperature and pressure during the reaction, reduce the decay of catalytic performance, and extend the service life of the catalyst.

[0033] Furthermore, in this embodiment, the one-step thermal decomposition to obtain the iron-manganese oxide matrix eliminates the need for multiple filtration and washing steps required in the conventional preparation process, reducing the possibility of impurity introduction and improving catalyst purity. At the same time, the simplified process steps can effectively control performance fluctuations between batches and solve the problem of poor repeatability in existing methods.

[0034] In this embodiment, the obtained iron-manganese oxide matrix is ​​placed in an additive metal precursor solution for wet impregnation. The impregnation time is controlled at 2-8 h. Sufficient impregnation (2-8 h) can ensure that the additive metal is fully adsorbed and diffused into the iron-manganese oxide matrix, avoiding the weakening of synergistic effect due to insufficient impregnation or the blockage of active sites due to excessive impregnation. Moreover, based on the uniform surface structure of the iron-manganese oxide matrix, the additive metal (potassium, sodium, etc.) can be uniformly loaded onto the matrix surface and pores through wet impregnation.

[0035] In this embodiment, the impregnated system is dried and then subjected to high-temperature calcination to allow the auxiliary metal to form a stable chemical bond with the iron-manganese oxide matrix. The high-temperature calcination can be carried out in a muffle furnace at a temperature of 400℃-800℃ for 2-10 hours. Under high temperature, the iron-manganese oxide and the auxiliary metal react fully to form a stable active phase structure. In the resulting product, manganese and the auxiliary metal form a synergistic effect, effectively regulating the surface chemical properties of the catalyst, suppressing side reactions such as secondary hydrogenation and methanation of olefins, and improving the selectivity of the target product (such as low-carbon olefins).

[0036] This embodiment simplifies the preparation process of iron-based catalysts by using a one-step co-evaporation pyrolysis combined with wet impregnation and calcination process, reduces the introduction of impurities, lowers the process difficulty, and improves the batch-to-batch performance repeatability. It solves the problems of poor repeatability and low purity in co-precipitation and impregnation methods. It also resolves the core contradiction of existing methods where performance and simplicity cannot be achieved simultaneously, providing a feasible solution for the large-scale industrial application of iron-based catalysts.

[0037] In one specific embodiment, the concentration of metal ions in the homogeneous mixed solution is controlled at 0.1-5 mol / L; for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, or 5 mol / L.

[0038] In one specific embodiment, the heating temperature of the homogeneous mixed solution is 200 ℃-400 ℃; for example, it can be 200 ℃, 220 ℃, 240 ℃, 260 ℃, 280 ℃, 300 ℃, 320 ℃, 340 ℃, 360 ℃, 380 ℃, or 400 ℃.

[0039] In one specific embodiment, the mass ratio of iron-manganese oxide matrix to auxiliary metal in the impregnation system is controlled at 1:0.002-1:0.05.

[0040] In a second aspect, the present invention provides an iron-based catalyst, which is obtained according to the preparation method described in the first aspect above.

[0041] Optionally, the iron-based catalyst is used to catalyze the hydrogenation of carbon dioxide to olefins, wherein the conversion rate of carbon dioxide is greater than 44% and the selectivity of olefins is greater than 50%.

[0042] To enable those skilled in the art to more clearly understand the present invention, the following embodiments are provided to illustrate in detail an iron-based catalyst and its preparation method.

[0043] Example 1 Weigh 0.0067 mol manganese nitrate and 0.02 mol ferric nitrate into a beaker, stir continuously to dissolve them in 50 ml of deionized water, place the beaker on a heating plate in a fume hood, connect it to a negative pressure exhaust gas treatment device and set the heating temperature to 300℃. After the water is completely evaporated and the reddish-brown gas is completely gone, the iron-manganese oxide matrix is ​​obtained. Weigh 0.0327g of sodium carbonate and dissolve it in an appropriate amount of deionized water. Pour the solution into a beaker containing an iron-manganese oxide matrix, let it stand for 2 hours, and then dry it. Collect the solid product into a crucible and calcine it at 500℃ for 4 hours to obtain a 1.5% Na-Mn1Fe3 catalyst, designated as 1#.

[0044] Figure 2 The SEM image of the iron-based catalyst provided in the embodiments of the present invention is shown, as follows. Figure 1 As shown, the catalyst consists of micron-sized particles with uniform particle size and consistent morphology, demonstrating that the catalyst has a stable morphology and good consistency.

[0045] Figure 3 The EDS spectrum of the iron-based catalyst provided in the embodiments of the present invention is shown, as follows: Figure 3 As shown, the elemental distribution of the fresh catalyst is uniform, with iron, manganese, and sodium concentrated in the same region, proving that the catalyst prepared by the one-step co-evaporation pyrolysis-impregnation calcination method can achieve good elemental dispersion, which is beneficial for the synergistic enhancement of catalytic activity by active components and promoters.

[0046] The catalytic performance test conditions for catalyst #1 are as follows: The feed gas was a mixture of H2 and CO2 in a ratio of 3:1. 4% nitrogen was added as an internal standard for easier calculations. The space velocity was 6000 mL / g. -1 ·h -1 Before the reaction began, the system was activated in situ at atmospheric pressure using hydrogen for 10 hours under a hydrogen atmosphere, and then the atmosphere was changed under pressure for the reaction. The reaction temperature was 350℃ and the reaction pressure was 2MPa. The reaction results are shown in Table 1. The carbon dioxide conversion rate was 51.81%, and the C2-C6 olefin selectivity exceeded 50%, achieving both high conversion rate and high olefin selectivity.

[0047] Example 2 Weigh 0.02 mol manganese nitrate and 0.02 mol ferric nitrate into a beaker, stir continuously until they are dissolved in 100 ml of deionized water, place the beaker on a heating plate in a fume hood, connect it to a negative pressure exhaust gas treatment device and set the heating temperature to 350℃. After the water is completely evaporated and the reddish-brown gas is completely gone, the iron-manganese oxide matrix is ​​obtained. Weigh 0.0333g of sodium carbonate and dissolve it in an appropriate amount of deionized water. Pour the solution into a beaker containing an iron-manganese oxide matrix, let it stand for 6 hours, and then dry it. Collect the solid product into a crucible and calcine it at 600℃ for 6 hours to obtain a 1% Na-Mn1Fe1 catalyst, designated as No. 2.

[0048] The catalytic performance test conditions for catalyst #2 are as follows: The feed gas was a mixture of H2 and CO2 in a ratio of 3:1. 4% nitrogen was added as an internal standard for easier calculations. The space velocity was 6000 mL / g. -1 ·h-1 Before the reaction began, the mixture was activated in situ at atmospheric pressure using hydrogen gas for 10 hours, and then the atmosphere was changed to allow for further pressure-based reaction. The reaction temperature was 300℃, and the reaction pressure was 2 MPa. The reaction results are shown in Table 1. The carbon dioxide conversion rate was 44.38%, and the C2-C6 olefin selectivity was as high as 57%.

[0049] Example 3 Weigh 0.02 mol manganese nitrate and 0.08 mol ferric nitrate into a beaker, stir continuously until they are dissolved in 200 ml of deionized water, place the beaker on a heating plate in a fume hood, connect it to a negative pressure exhaust gas treatment device and set the heating temperature to 400℃. After the water is completely evaporated and the reddish-brown gas is completely gone, the iron-manganese oxide matrix is ​​obtained. Weigh 0.4063g of sodium carbonate and dissolve it in an appropriate amount of deionized water. Pour the solution into a beaker containing an iron-manganese oxide matrix, let it stand for 8 hours, and then dry it. Collect the solid product into a crucible and calcine it at 800℃ for 5 hours to obtain a 5% Na-Mn1Fe4 catalyst, designated as No. 3.

[0050] The catalytic performance test conditions for catalyst #3 are as follows: The feed gas was a mixture of H2 and CO2 in a ratio of 3:1. 4% nitrogen was added as an internal standard for easier calculations. The space velocity was 6000 mL / g. -1 ·h -1 Before the reaction began, the mixture was activated in situ at atmospheric pressure using hydrogen gas for 10 hours, and then the atmosphere was changed under increased pressure for the reaction. The reaction temperature was 300℃, and the reaction pressure was 2MPa. The reaction results are shown in Table 1. The carbon dioxide conversion rate was 43.08%, and the C2-C6 olefin selectivity was as high as 59.43%.

[0051] Comparative Example 1 Weigh 0.05 mol manganese nitrate and 0.05 mol ferric nitrate into a beaker, stir continuously until they are dissolved in 50 ml of deionized water, place the beaker on a heating plate in a fume hood, connect it to a negative pressure exhaust gas treatment device and set the heating temperature to 350℃. After the water has completely evaporated and the reddish-brown gas has completely disappeared, collect the solid product into a crucible and calcine it at 600℃ for 6 h to obtain the Mn1Fe1 catalyst, numbered 4#.

[0052] The test conditions for catalyst #4 were as follows: the feed gas was a mixture of H2 and CO2 in a ratio of 3:1, with 4% nitrogen added as an internal standard for easy calculation; and the space velocity was 6000 mL·g. -1 ·h -1Before the reaction began, the catalyst was activated in situ at atmospheric pressure using hydrogen for 10 hours under a hydrogen atmosphere, and then the atmosphere was changed under pressure for the reaction. The reaction temperature was 300℃ and the reaction pressure was 2MPa. The reaction results are shown in Table 1. The carbon dioxide conversion rate was only 26.16%, the C2-C6 olefin selectivity was 17.22%, and the C2-C6 alkane selectivity was 27.13%. Without the addition of sodium promoter, the catalyst activity and the product olefin / alkane ratio decreased significantly.

[0053] Comparative Example 2 Weigh 0.05 mol manganese nitrate and 0.01 mol ferric nitrate into a beaker and stir continuously until they are dissolved in 50 ml of deionized water. Place the beaker on a heating plate in a fume hood, connect it to a negative pressure exhaust gas treatment device, and set the heating temperature to 380℃. After the water has completely evaporated and the reddish-brown gas has completely disappeared, weigh 0.0515 g sodium carbonate and dissolve it in an appropriate amount of deionized water. Pour the solution into a beaker, let it stand for 4 hours, and then dry it. Collect the solid product into a crucible and calcine it at 800℃ for 5 hours to obtain a 1% Na-Mn5Fe1 catalyst, designated as 5#.

[0054] The test conditions for catalyst #5 were as follows: the feed gas was a mixture of H2 and CO2 in a ratio of 3:1, with 4% nitrogen added as an internal standard for easy calculation; and the space velocity was 6000 mL·g. -1 ·h -1 Before the reaction began, the catalyst was activated in situ at atmospheric pressure using hydrogen for 10 hours under a hydrogen atmosphere, and then the atmosphere was changed under pressure for the reaction. The reaction temperature was 350℃ and the reaction pressure was 2MPa. The reaction results are shown in Table 1. The carbon dioxide conversion rate was 38.26%, and the C2-C6 olefin selectivity was 36.01%. Due to the relatively low content of the main active component, iron, the catalyst activity and olefin selectivity were significantly lower than those in Example 1 under the same reaction conditions.

[0055] Comparative Example 3 Iron-manganese catalyst was prepared using a coprecipitation method. 0.02 mol of ferric nitrate and 0.01 mol of manganese nitrate were weighed and mixed to prepare a solution. Ammonia solution was added dropwise under stirring at 60°C to coprecipitate until the pH reached 10. After filtration, the solution was repeatedly washed with deionized water until neutral. The solution was then dried in an oven for 24 hours, and the solid was collected and calcined at 550°C for 5 hours to obtain the Mn1Fe2 catalyst, designated as #6. The test conditions for catalyst #6 were as follows: the feed gas was a mixture of H2 and CO2 in a ratio of 3:1, with 4% nitrogen added as an internal standard for easy calculation; and the space velocity was 6000 mL·g. -1 ·h -1Before the reaction began, the reactor was activated in situ at atmospheric pressure using hydrogen in a hydrogen atmosphere for 10 hours, and then the atmosphere was changed to be pressurized for the reaction. The reaction results are shown in Table 1. Under the reaction conditions of 350℃ and 20 bar, the CO2 conversion rate of Comparative Example 3 was only 30.58%, the alkene-to-olefin ratio was less than 1, the olefin content was low, and the performance was lower than that of Example 1 under the same test conditions.

[0056] Comparative Example 4 Iron-manganese catalyst was prepared using the sol-gel-impregnation method. 0.04 mol of ferric nitrate, 0.02 mol of manganese nitrate, and 0.06 mol of EDTA were dissolved separately, heated to 50°C, and stirred. The pH of the mixed solution was adjusted to 7 with ammonia water. Heating and stirring continued until gel formation. After standing for 12 hours, the gel was dried in a 120°C oven for 12 hours to obtain a dry gel. This gel was then ground and pre-calcined in a muffle furnace at 400°C for 2 hours. After further grinding, it was calcined in a muffle furnace at 700°C for 4 hours, allowed to cool naturally, and then ground again to obtain Mn1Fe2. 0.01 g of sodium carbonate was prepared into an equal-volume aqueous solution. 1 g of the prepared Mn1Fe was placed in the sodium carbonate solution and impregnated for 6 hours. The solution was then dried in an oven at 80°C and subsequently calcined in a muffle furnace at 550°C for 2 hours. After grinding, 1% Na-Mn1Fe2, designated as #7, was obtained.

[0057] The test conditions for catalyst #7 were as follows: the feed gas was a mixture of H2 and CO2 in a ratio of 3:1, with 4% nitrogen added as an internal standard for easy calculation; and the space velocity was 6000 mL·g. -1 ·h -1 Before the reaction began, the catalyst was activated in situ at atmospheric pressure using hydrogen for 10 hours under a hydrogen atmosphere, and then the atmosphere was changed to allow for further reaction under increased pressure. The reaction results are shown in Table 1. The catalyst exhibited good performance, with a CO2 conversion rate of 41.77% and an olefin selectivity of 49.01% under reaction conditions of 300℃ and 20 bar. Compared to Examples 2 and 3, the methane content was higher. However, considering that the sol-gel-impregnation method for catalyst preparation takes 3-4 days and involves high costs, numerous steps, and complex operations, while the one-step co-evaporation pyrolysis-impregnation-calcination method provided in the examples takes less than one day, is low-cost, and simple to operate, the latter has significant advantages.

[0058] Table 1: Catalyst Evaluation Results of Examples and Comparative Examples

[0059] As shown in Table 1, the catalytic performance test results demonstrate that the catalyst prepared by the method provided in this invention can achieve high olefin selectivity while maintaining high CO2 conversion. Under different operating conditions, the carbon dioxide conversion rate is greater than 44%, and the olefin selectivity is greater than 50%, reaching a maximum of 59.43%. Compared with examples without added additives and different manganese-iron ratios, it exhibits significant advantages in catalytic activity and selectivity.

[0060] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0061] For the sake of simplicity, the method embodiments are described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps can be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and components involved are not necessarily essential to the present invention.

[0062] The present invention provides a detailed description of an iron-based catalyst and its preparation method. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only intended to help understand the method and core ideas of the present invention. At the same time, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing an iron-based catalyst, characterized in that, The preparation method includes: Prepare homogeneous mixed solutions from iron and manganese salts; The homogeneous mixed solution is heated to cause the iron salt and manganese salt to undergo in-situ thermal decomposition, forming an iron-manganese oxide matrix. The iron-manganese oxide matrix is ​​placed in a solution of auxiliary metal precursors for wet impregnation. The impregnation system is then dried and calcined at high temperature to obtain the iron-based catalyst.

2. The method for preparing the iron-based catalyst according to claim 1, characterized in that, In the homogeneous mixed solution, the molar ratio of iron to manganese is 1:1 to 8:

1.

3. The method for preparing the iron-based catalyst according to claim 1, characterized in that, In the homogeneous mixed solution, the concentration of metal ions is 0.1-5 mol / L.

4. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The heating temperature for in-situ thermal decomposition of the iron and manganese salts is 200 ℃-400 ℃.

5. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The auxiliary metal in the auxiliary metal precursor solution is selected from at least one of potassium and sodium, and the mass ratio of iron manganese oxide to auxiliary metal in the impregnation system is 1:0.002-1:0.

05.

6. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The soaking time for the wet impregnation is 2-8 hours.

7. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The high-temperature roasting is carried out in a muffle furnace.

8. The method for preparing the iron-based catalyst according to claim 1 or 6, characterized in that, The high-temperature calcination temperature is 400 ℃-800 ℃, and the calcination time is 2 h-10 h.

9. An iron-based catalyst, characterized in that, The iron-based catalyst is obtained by any one of the preparation methods according to claims 1-8.

10. The iron-based catalyst according to claim 9, characterized in that, The iron-based catalyst is used to catalyze the hydrogenation of carbon dioxide to olefins, wherein the conversion rate of carbon dioxide is greater than 44% and the selectivity of olefins is greater than 50%.

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