An alkali metal bulk phase doped induced phase transformation iron-based catalyst, its preparation method and application

Abstract

The application provides an alkali metal bulk phase doping induced crystal phase transition iron-based catalyst and a preparation method and application thereof, and belongs to the technical field of catalytic materials. The preparation method comprises the following steps: dissolving an alkali metal salt and an iron precursor in a solvent in proportion to obtain a mixed solution containing alkali metal ions and iron ions; a weak base precipitant solution is prepared and added into the mixed solution, a precipitate is obtained through a coprecipitation reaction, and the alkali metal bulk phase doping induced crystal phase transition iron-based catalyst is obtained through calcination. The method for preparing the alkali metal bulk phase doping iron-based catalyst through one-step coprecipitation enables the alkali metal to directly enter the interior of the iron oxide crystal lattice during the preparation of the catalyst, realizes bulk phase doping, and spontaneously induces the generation of Fe3O4 crystal phase, thereby significantly improving the catalytic performance of CO2 hydrogenation to olefins, the CO2 conversion rate can reach more than 50%, the olefin selectivity is as high as 79.5%, and the performance is stable during continuous operation for 520 hours.

Description

Technical Field

[0001] This invention belongs to the field of catalytic materials technology, specifically relating to an iron-based catalyst induced by alkali metal bulk doping for crystal phase transformation, its preparation method, and its application. Background Technology

[0002] The use of fossil fuels has greatly promoted the development of modern society, but it has also generated a large amount of CO2, leading to a sharp rise in global temperatures. Hydrogenating captured carbon dioxide (CO2) into high-value-added chemicals has become a research hotspot. Among these methods, the production of olefins (ethylene, propylene, butene, etc.) through CO2 hydrogenation can reduce carbon emissions and partially replace the petroleum cracking route for olefin production, making it of significant strategic importance.

[0003] Iron-based catalysts are widely used in CO2 hydrogenation reactions due to their low cost and high activity. However, traditional iron-based catalysts face two major challenges: (1) high methane selectivity: iron easily catalyzes the deep hydrogenation of CO2 to produce CH4, reducing carbon utilization efficiency; (2) low olefin selectivity: alkanes, especially methane, dominate the products, and the olefin yield is difficult to meet industrial requirements. Therefore, the development of catalysts for the hydrogenation of carbon dioxide to produce olefins is the key to realizing the industrialization of the hydrogenation of carbon dioxide to produce low-carbon olefins.

[0004] In the prior art, for example, invention patent CN111905737A discloses a method for preparing a single iron catalyst and an alkali metal modified catalyst, as well as their applications. The sample preparation sequence involves slowly adding a weak base substance or solution to a salt solution containing iron ions to form a precipitation system and obtain a precipitate; grinding and calcining the precipitate to obtain a single iron catalyst; and then impregnating the single iron catalyst with an alkali metal solution to obtain an alkali metal modified catalyst. This method essentially involves first preparing iron oxides (mostly α-Fe₂O₃), then impregnating, drying, and calcining them with an alkali metal salt solution. Its essence is to achieve surface loading through impregnation. The alkali metal is mainly distributed on the outer surface and pores of the support, making it difficult to penetrate the crystal lattice and prone to agglomeration. More importantly, the surface-loaded alkali metal achieved by this sample preparation sequence cannot effectively induce the phase transformation of the iron oxide from α-Fe₂O₃ to Fe₃O₄, thus limiting the improvement in olefin selectivity.

[0005] For example, Reference 1 (The evolutions of carbon and iron species modified by Na and their tuning) The paper "Effect on the hydrogenation of CO2 to olefins [J], Applied Surface Science, 525, 1-13" discloses a method for preparing an iron-based catalyst. The method involves first preparing iron oxide, then impregnating it with an alkali metal salt solution, drying it, and calcining it. Essentially, it still uses the impregnation method to achieve surface loading, which has limited effect on improving the selectivity of olefins.

[0006] Studies have shown that the crystal phase of iron oxides has a decisive influence on catalytic selectivity. The α-Fe₂O₃ phase tends to generate methane, while the Fe₃O₄ phase is more conducive to carbon chain growth and olefin formation. Therefore, controlling the crystal phase of iron-based catalysts to transform them into the Fe₃O₄ phase is one of the key strategies for improving olefin selectivity. Summary of the Invention

[0007] To address the shortcomings of existing technologies where alkali metals can only be surface-loaded and are difficult to induce favorable crystal phase transformations, the present invention aims to provide an iron-based catalyst for bulk doping of alkali metals to induce crystal phase transformations, along with its preparation method and applications. The method employs a one-step co-precipitation process to prepare the alkali metal-doped iron-based catalyst, allowing the alkali metal to directly enter the iron oxide lattice during catalyst preparation, achieving bulk doping and spontaneously inducing the formation of the Fe3O4 crystal phase, thereby significantly improving the catalytic performance of CO2 hydrogenation to olefins.

[0008] The technical solution adopted by this application to solve the above problems is as follows: This invention provides a method for preparing an iron-based catalyst induced by alkali metal bulk doping and crystal phase transformation, comprising the following steps: (1) Dissolve the alkali metal salt and iron precursor in a solvent in proportion to obtain a mixed solution containing alkali metal ions and iron ions; (2) Prepare a weak base precipitant solution and add it to the mixed solution. Obtain the precipitate through co-precipitation reaction, and then obtain the iron-based catalyst with alkali metal bulk doping-induced crystal phase transformation by calcination.

[0009] To address the two major bottlenecks of surface loading and uncontrollable crystal phase in existing impregnation methods, this invention employs a one-step co-precipitation method to dissolve alkali metal salts and iron precursors together in the early stages of catalyst preparation. This allows alkali metal ions to directly enter the iron oxide lattice during precipitation, forming a bulk doped structure. During calcination, the alkali metal bulk doping spontaneously induces a crystal phase transformation of the iron oxide from α-Fe2O3 (hematite) to Fe3O4 (magnetite), and the degree of transformation is positively correlated with the amount of alkali metal doping.

[0010] On the other hand, bulk doped alkali metals not only induce the formation of the Fe3O4 phase, but also act as electron traps to synergize with Fe active sites, promoting CO2 adsorption activation and inhibiting deep hydrogenation (alkane formation). Based on the dual effects of crystal phase transformation and alkali metal electronic effects, high olefin selectivity and high stability are synergistically achieved, providing a high-performance and industrially feasible catalyst solution for CO2 hydrogenation to olefins.

[0011] Preferably, the alkali metal salt includes one or more halides, acetates, or nitrates of sodium, potassium, and cesium; The iron source is selected from one or more of ferric chloride, ferric nitrate, and ferric acetylacetone.

[0012] More preferably, the alkali metal salt is a sodium halide, acetate, or nitrate.

[0013] Further preferably, the ratio of the sodium element to the iron element in the alkali metal salt and the iron source dissolved in the solvent is 1 to 4:1.

[0014] More preferably, the ratio of sodium to iron in the solvent of the alkali metal salt and iron source is 2:1.

[0015] Preferably, the solvent is selected from one or more of water and alcohol solvents.

[0016] Different solvents may affect the precipitation rate and particle morphology, but this does not change the core concept of the present invention, and therefore it is not limited to the solvents described.

[0017] More preferably, the alcohol solvent is selected from one or more of methanol, ethylene glycol, and glycerol.

[0018] Preferably, the weak base precipitant is ammonium carbonate, ammonia monohydrate, ammonium acetate, or ammonium chloride. The aforementioned weak base precipitant hydrolyzes to produce OH-. - Slow precipitation is beneficial for uniform sedimentation.

[0019] More preferably, the mass percentage of the weak base precipitant in the weak base precipitant solution is 5-7 wt%.

[0020] Preferably, the volume ratio of the weak base precipitant solution to the mixed solution is 1.5 to 2:1.

[0021] More preferably, the volume ratio of the weak base precipitant solution to the mixed solution is 1.5:1.

[0022] Preferably, the precipitate needs to be washed and dried before calcination; wherein the drying temperature is 60℃~100℃ and the drying time is 2~6 h.

[0023] Preferably, the roasting temperature is 250~550℃ and the roasting time is 2~8 h.

[0024] On the other hand, the present invention also provides an iron-based catalyst for alkali metal bulk doping-induced crystal phase transformation prepared based on the preparation method described above.

[0025] In the iron-based catalyst prepared by the above method, alkali metals not only distribute on the surface of iron oxide particles, but also enter the interior of the iron oxide lattice, inducing the iron oxide to transform from the Fe2O3 crystal phase to the Fe3O4 crystal phase or a mixed crystal phase of Fe2O3 and Fe3O4, forming a bulk doped structure, promoting electron transfer modification of the iron-based catalyst, and the effect increases with the amount of alkali metal doping.

[0026] Preferably, the microstructure of the iron-based catalyst induced by alkali metal bulk doping to undergo crystal phase transformation is predominantly Fe3O4 crystal phase.

[0027] More preferably, the alkali metal in the iron-based catalyst for alkali metal bulk doping-induced crystal phase transformation has a mass ratio of 1.5 to 3.0 wt%.

[0028] More preferably, the alkali metal in the iron-based catalyst for alkali metal bulk doping-induced crystal phase transformation is 1.6 to 3.0 wt%.

[0029] The above-mentioned mass proportions of alkali metals can all be used to prepare alkali metal bulk doped iron-based catalysts. Since the (311) diffraction peak of alkali metal doped Fe3O4 is shifted to a lower angle relative to standard Fe3O4, and the shift increases with the increase of alkali metal doping amount.

[0030] On the other hand, the present invention also provides the application of the iron-based catalyst with alkali metal bulk doping-induced crystal phase transformation in the catalytic hydrogenation of carbon dioxide to prepare olefins.

[0031] Preferably, the reaction for the catalytic hydrogenation of carbon dioxide to olefins is carried out in a fixed-bed reactor, and the iron-based catalyst is pretreated at 300°C to 400°C for 2 to 10 h under a reducing atmosphere before the reaction. The reaction temperature was 280℃~340℃, the reaction pressure was 0.5~3.0 MPa, the feed gas was a CO2 / H2 mixture, the reaction process used an inert gas as the equilibrium gas, and the equilibrium gas space velocity was 4500~22500 mL·g. -1 ·h -1 .

[0032] Furthermore, the reducing atmosphere is hydrogen.

[0033] Compared with the prior art, the present invention has the following beneficial effects: (1) The one-step co-precipitation method embeds the alkali metal into the iron oxide lattice in the early stage of preparation, eliminating the need for multiple impregnation, drying and calcination processes. It has low equipment requirements, simple operation and low cost, making it suitable for large-scale production. At the same time, since the alkali metal is doped into the lattice rather than attached to the surface, it effectively avoids loss and agglomeration during high-temperature reactions. The CO2 conversion rate and olefin selectivity are almost unaffected, and it has a strong resistance to deactivation, meeting the actual needs of long-term stable operation of industrial plants.

[0034] (2) The catalyst prepared by alkali metal bulk doping to induce crystal phase transformation has a product distribution that reverses from methane to olefins. The alkali metal doped into the crystal lattice can both spontaneously induce crystal phase transformation and achieve electronic regulation. The two work synergistically to significantly improve olefin selectivity.

[0035] (3) The catalyst can directly convert captured CO2 and green hydrogen into high-value-added olefins (ethylene, propylene, etc.), partially replacing the traditional route of petroleum cracking to olefins, and has both carbon emission reduction benefits and economic benefits. Attached Figure Description

[0036] Figure 1 The images show the XRD patterns of the catalysts prepared in Examples 1 and 2, and Comparative Examples 1 and 3.

[0037] Figure 2 The XRD patterns of the catalysts prepared in Examples 1, 2 and Comparative Examples 1 and 3 are shown in the diffraction angle range of 34-37°.

[0038] Figure 3 This is a TEM image of the catalyst prepared in Example 1.

[0039] Figure 4 This is a particle size distribution chart of the catalyst prepared in Example 1.

[0040] Figure 5 This is a TEM image of the catalyst prepared in Comparative Example 1.

[0041] Figure 6 The particle size distribution of the catalyst prepared in Comparative Example 1 is shown in the figure.

[0042] Figure 7 The image shows the catalytic stability of the catalyst prepared in Example 1. Detailed Implementation

[0043] The specific implementation methods of this application will be further described in detail below with reference to the accompanying drawings and embodiments. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0044] The iron source used in the examples and comparative examples was ferric chloride hexahydrate (FeCl3·6H2O, content ≥99%), the alkali metal salt was sodium chloride (NaCl, content ≥99.5%), and the weak base precipitant was ammonium carbonate ((NH4)2CO3, reagent grade). Example 1 A method for preparing an iron-based catalyst induced by alkali metal bulk doping for crystal phase transformation includes the following steps: (1) Disperse 5.4 g of FeCl3·6H2O and 2.4 g of NaCl in 60 mL of ethylene glycol and stir magnetically for 30 minutes. This mixture is called solution A. (2) Add 7.7 g of ammonium carbonate to a mixed solution of 50 mL of deionized water and 50 mL of ethylene glycol, and sonicate until completely dissolved. This solution is called mixed solution B. (3) Add the mixed solution B slowly and completely to all the mixed solutions A, stir magnetically for 30 minutes at room temperature, and let stand for 12 hours to obtain the mixed turbid liquid C; (4) The above mixed turbid liquid C was washed with deionized water and the precipitate was collected by centrifugation. The precipitate was then dried in an oven at a temperature of 60°C to 100°C for 2 to 6 hours. The resulting solid was ground and calcined in a muffle furnace at 450°C for 4 hours to obtain the Na-doped iron-based catalyst, labeled as 4Na-FeO. x The Na doping amount is 1.6 wt%.

[0045] Example 2 A method for preparing an iron-based catalyst induced by alkali metal bulk doping for crystal phase transformation includes the following steps: (1) Disperse 5.4 g of FeCl3·6H2O and 4.7 g of NaCl in 60 mL of ethylene glycol and stir magnetically for 30 minutes. This mixture is called solution A. (2) Add 7.7 g of ammonium carbonate to a mixed solution of 50 mL of deionized water and 50 mL of ethylene glycol, and sonicate until completely dissolved. This solution is called mixed solution B. (3) Add the mixed solution B slowly and completely to all the mixed solutions A, stir magnetically for 30 minutes at room temperature, and let stand for 12 hours to obtain the mixed turbid liquid C; (4) The above mixed turbid liquid C was washed with deionized water and the precipitate was collected by centrifugation. The precipitate was then dried in an oven at a temperature of 60℃~100℃ for 2~6 h. The resulting solid was ground and calcined in a muffle furnace at 450℃ for 4 h to obtain the Na-doped iron-based catalyst, labeled as 8Na-FeO. x The Na doping amount is 3.0 wt%.

[0046] Comparative Example 1 The preparation method of the catalyst in Comparative Example 1 includes the following steps: (1) Disperse 5.4 g of FeCl3·6H2O in 60 mL of ethylene glycol and stir magnetically for 30 minutes. This mixture is called solution A. (2) Add 7.7 g of ammonium carbonate to a mixed solution of 50 mL of deionized water and 50 mL of ethylene glycol, and sonicate until completely dissolved. This solution is called mixed solution B. (3) Add the mixed solution B slowly and completely to all the mixed solutions A, stir magnetically for 30 minutes at room temperature, and let stand for 12 hours to obtain the mixed turbid liquid C; (4) The above mixed turbid liquid C was washed with deionized water and the precipitate was collected by centrifugation. The precipitate was then dried in an oven at a temperature of 60℃~100℃ for 2~6 h. The resulting solid was ground and calcined in a muffle furnace at 450℃ for 4 h to obtain the iron oxide sample, labeled as 0Na-FeO. x .

[0047] Comparative Example 2 Comparative Example 2: Commercial Fe3O4 was calcined in a muffle furnace at 450ºC for 4 h to obtain a ferrooxide sample, labeled as Fe3O4.

[0048] Comparative Example 3 A method for preparing an iron-based catalyst induced by alkali metal bulk doping for crystal phase transformation includes the following steps: (1) Disperse 5.4 g of FeCl3·6H2O and 1.2 g of NaCl in 60 mL of ethylene glycol and stir magnetically for 30 minutes. This mixture is called solution A. (2) Add 7.7 g of ammonium carbonate to a mixed solution of 50 mL of deionized water and 50 mL of ethylene glycol, and sonicate until completely dissolved. This solution is called mixed solution B. (3) Add the mixed solution B slowly and completely to all the mixed solutions A, stir magnetically for 30 minutes at room temperature, and let stand for 12 hours to obtain the mixed turbid liquid C; (4) The above mixed turbid liquid C was washed with deionized water and the precipitate was collected by centrifugation. The precipitate was then dried in an oven at a temperature of 60℃~100℃ for 2~6 h. The resulting solid was ground and calcined in a muffle furnace at 450℃ for 4 h to obtain the Na-doped iron-based catalyst, labeled as 2Na-FeO. x The Na doping amount is 0.7 wt%.

[0049] Figure 1 and Figure 2 The 4Na-FeO prepared in Examples 1 and 2 and Comparative Examples 1 and 3, respectively. x 8Na-FeO xand 0Na-FeO x 8Na-FeO x The X-ray diffraction (XRD) pattern of the prepared 4Na-FeO was confirmed by comparison with a standard database. x and 8Na-FeO x The sample was a typical magnetite-type Fe3O4,0Na-FeO x The sample was hematite-type Fe2O3, while 2Na-FeO x The sample was a mixed phase of Fe3O4 and Fe2O3, indicating that the incorporation of the alkali metal Na induces a phase transformation in the iron-based catalyst. With increasing Na content, the diffraction peaks continuously shifted to lower angles. + Entering the Fe3O4 lattice causes cell expansion, achieving bulk doping.

[0050] Figure 3 The 4Na-FeO prepared in Example 1 x TEM images and their particle size distribution ( Figure 4 ), Figure 5 The 0Na-FeO prepared for Comparative Example 1 x TEM images and their particle size distribution ( Figure 6 Particle size analysis revealed that the average particle size of the catalyst particles in Example 1 was 35.9 nm, while the average particle size of the catalyst particles in Comparative Example 1 was 33.4 nm. The particle size of the iron-based catalyst doped with alkali metal Na bulk phase increased slightly, but the increase was limited.

[0051] Application Example 1 100 mg of the catalyst prepared in the examples and comparative examples were weighed and crushed to 40-80 mesh, and filled into the middle of a fixed-bed stainless steel reaction tube with an inner diameter of 8 mm. The reaction tube was placed in a programmable tube furnace.

[0052] The catalyst was pre-activated at 350°C for 5 h in a hydrogen atmosphere, and then the tubular furnace was cooled for activity testing.

[0053] The mixed reaction gas consisted of 20% CO2, 60% H2, and N2 as a balance gas. The reaction proceeded at 300℃ to test the activity of CO2 hydrogenation to olefins. The test results of thermocatalytic carbon dioxide hydrogenation are shown in Table 1.

[0054] Table 1. Test results of CO2 hydrogenation to olefins in the Examples and Comparative Examples at 300℃

[0055] As shown in Table 1, and in both the examples and comparative examples, bulk doping with Na (Example 1) transformed the catalyst from the α-Fe₂O₃ phase to the pure Fe₃O₄ phase, resulting in a surge in olefin selectivity from 1.6% to 79.5%, while CO₂ conversion also increased from 34.3% to 50.5%. This demonstrates that the synergistic effect of bulk doping and phase transformation is the fundamental reason for the performance improvement. The pure Fe₃O₄ catalyst (Comparative Example 2) exhibited an olefin selectivity of only 5.7%, far lower than the 79.5% of Example 1. This indicates that the Fe₃O₄ phase itself is insufficient to achieve high olefin selectivity; bulk doping with Na is necessary to utilize the electronic effects of Na to suppress excessive hydrogenation in order to achieve high olefin selectivity.

[0056] In addition, the catalyst prepared in Example 1 underwent a 520-hour stability test, and the results are as follows: Figure 7 As shown, the CO2 conversion rate remained at around 49.8%, the olefin selectivity remained at around 80.5%, and the catalytic performance remained stable over a long period.

[0057] Compared with the comparative examples, the preferred alkali metal Na content in Examples 1 and 2 exhibits higher stable conversion rate and selectivity, making it suitable for long-term industrial operation.

[0058] In summary, this invention utilizes alkali metal bulk doping, which induces a spontaneous phase transformation in iron oxides during calcination. Alkali metals, acting as effective electron traps, synergistically enhance the adsorption and activation of CO2 molecules with Fe active species, significantly improving the CO2 single-pass conversion rate. Simultaneously, they suppress alkane formation, thereby greatly improving olefin selectivity.

[0059] Furthermore, the one-step co-precipitation method for catalyst preparation in this invention requires no complex equipment or multiple steps, resulting in low cost and suitability for industrial production. This highly efficient and stable catalyst facilitates the coupling of green electricity to produce green hydrogen and CO2 capture to produce green fuels, providing a promising technical route for CO2 resource utilization and renewable energy storage under dual-carbon objectives.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing an iron-based catalyst induced by alkali metal bulk doping and crystal phase transformation, characterized in that, Includes the following steps: (1) Dissolve the alkali metal salt and the iron source in a solvent in proportion to obtain a mixed solution containing alkali metal ions and iron ions; (2) Prepare a weak base precipitant solution and add it to the mixed solution. Obtain the precipitate through co-precipitation reaction, and then obtain the iron-based catalyst with alkali metal bulk doping-induced crystal phase transformation by calcination.

2. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The alkali metal salts include any one or more halides, acetates, or nitrates of sodium, potassium, and cesium; The iron source is selected from one or more of ferric chloride, ferric nitrate, and ferric acetylacetone.

3. The method for preparing the iron-based catalyst according to claim 2, characterized in that, The ratio of sodium to iron in a solvent containing alkali metal salts and iron sources is 1 to 4:

1.

4. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The weak base precipitant is ammonium carbonate, ammonia monohydrate, ammonium acetate, or ammonium chloride; The mass percentage of the weak base precipitant in the aforementioned weak base precipitant solution is 5-7 wt%.

5. The method for preparing the iron-based catalyst according to claim 4, characterized in that, The volume ratio of the weak base precipitant solution to the mixed solution is 1.5~2:

1.

6. The method for preparing the iron-based catalyst according to claim 1, characterized in that, The roasting temperature is 250~550℃, and the roasting time is 2~8 h.

7. An alkali metal bulk doping-induced crystal phase transformation iron-based catalyst prepared by the method of any one of claims 1-6.

8. The iron-based catalyst for alkali metal bulk doping-induced crystal phase transformation according to claim 7, characterized in that, The main microstructure is Fe3O4 crystal phase.

9. The iron-based catalyst for alkali metal bulk doping-induced phase transformation according to claim 7, characterized in that, The alkali metal in the iron-based catalyst for alkali metal bulk doping-induced crystal phase transformation is 1.5~3.0 wt%.

10. The application of the alkali metal bulk doping-induced phase transformation iron-based catalyst according to any one of claims 7-9 in the catalytic hydrogenation of carbon dioxide to olefins, characterized in that, The reaction of catalytic carbon dioxide hydrogenation to prepare olefins is carried out in a fixed-bed reactor, and the iron-based catalyst is pretreated at 300℃~400℃ for 2~10 h under a reducing atmosphere before the reaction. The reaction temperature was 280℃~340℃, the reaction pressure was 0.5~3.0 MPa, the feed gas was a CO2 / H2 mixture, the reaction process used an inert gas as the equilibrium gas, and the equilibrium gas space velocity was 4500~22500 mL·g. -1 ·h -1 .

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