Method for preparing olefin by catalyzing carbon dioxide hydrogenation

Abstract

The invention discloses a method for preparing olefin by catalyzing hydrogenation of carbon dioxide, which comprises the following steps: adopting a fixed bed reactor, reducing a prepared Fe3O4 catalyst by using hydrogen before reaction, and then introducing carbon dioxide and hydrogen for reaction. According to the catalyst, the strategy that an alkali metal accelerant is added to an iron oxide precursor, a transition metal structure additive is introduced or multi-element ferrite is synthesized in the past is changed, by changing the preparation method of the catalyst, CO2 hydrogenation for olefin preparation is efficiently catalyzed under the condition that no functional additive is added, the CO2 conversion rate is 40.3% under the reaction conditions that the temperature is 320 DEG C, the pressure is 1 MPa, the pressure is 8100 mL / (gcat.h) and the ratio of H2 to CO2 is 3, and the conversion rate of CO2 is 40.3%. When the product comprises CO, the selectivity of C2 + olefin reaches 74.2%, and the selectivity of by-products CO and CH4 is respectively as low as 10.8% and 3.8%. The catalyst is low in preparation cost, the method is simple and convenient, meanwhile, the catalyst has excellent reaction performance and good stability of olefin preparation through CO2 catalytic hydrogenation, and the catalyst has good industrial application prospects.

Description

Technical Field

[0001] The invention belongs to the field of catalytic technology, and specifically relates to a method for preparing olefins by catalytic hydrogenation of carbon dioxide (CO2+H2). Background Art

[0002] As an important industrial raw material, olefins have a wide range of application values ​​and can be used to produce a variety of value-added products, such as synthetic lubricants, high-octane gasoline, biodegradable detergents, new polymers, agricultural chemicals, coatings and corrosion inhibitors, etc. However, the main source of olefins in industry is still petroleum cracking and reforming. With the increasing scarcity of petroleum resources and the aggravation of climate problems such as global warming caused by the large amount of CO2 emitted during its utilization, the development of technology for preparing olefins by catalytic hydrogenation of CO2 can not only solve the problem of high CO2 emissions and promote the country's energy strategic security, but also is a solution to the potential threat of oil energy depletion, which is of great significance to achieving carbon neutrality and sustainable development.

[0003] The catalytic hydrogenation of CO2 to olefins can be achieved through two pathways: Fischer-Tropsch synthesis (FTS) and methanol intermediates. Compared with the methanol intermediate pathway, the thermodynamically favorable reaction temperatures of the two reactions of CO2 activation by reverse water gas reaction (RWGS) and hydrogenation to olefins in the FTS pathway are relatively matched, the reaction CO2 conversion rate and olefin yield are high, and the formation of C1 by-products is effectively suppressed. At present, Fe-based catalysts have made a series of important research progress in the CO2 hydrogenation to olefins reaction due to their good RWGS activity and high olefin selectivity in the product, but there are still problems such as low catalyst activity and low selectivity of target products. Therefore, current researchers mainly focus on discussing and studying how to further improve the reaction performance by adding alkali metal promoters to iron oxide precursors, introducing transition metal structural additives, or synthesizing multi-ferrites to regulate the electronic properties, surface alkalinity and structural flexibility of the catalyst.

[0004] Sun Jian's team (Catalysis Science & Technology, 2016, 6, 4786-4793) used a simple one-pot precipitation method to synthesize Fe3O4 with different Na contents, proving that the presence of Na has a significant effect on the structural properties of Fe-based catalysts. Compared with Fe3O4 catalysts without Na, the activity of the catalyst was significantly improved after the introduction of Na, which led to the formation of more C2-C4 olefins. The catalyst with the best Na content (Na / Fe mass ratio of 1.18 / 100) was 1.2 times higher than that of Fe3O4 at 320°C, 3MPa and 2000mL / (g cat h) for 60 h, the CO2 conversion rate reached 40.5%, the C2-C4 olefin selectivity was 46.6%, and the C 5+The selectivity is 30.1%, and the selectivities of CO and CH4 are as low as 13.5% and 15.8%, respectively. Han Yifan's research group (ACS Catalysis, 2021, 11, 2121-2133) prepared Fe5C2-ZnO bimetallic catalyst (denoted as Fe-Zn) for direct preparation of long-chain olefins by CO2 hydrogenation. The study found that the introduction of ZnO is beneficial to CC coupling and the generation of more olefins. At the same time, it also inhibits the oxidation of by-products H2O and CO2 to iron-carbon active species during the reaction, thereby enhancing the stability of the catalyst. Finally, the Fe2Zn1 catalyst was tested at 330°C, 1.5MPa and 45000mL / (g cat ·h) reaction conditions, the reaction time is up to 200h, and the CO2 conversion rate, C2-C7 olefin selectivity and CO selectivity are maintained at about 35.4%, 57.8% and 15.5%, respectively. Zeng Jie's research group (Nature Communications, 2022, 13, 2396) reported a Cu-Fe catalyst composed of copper, iron oxide and iron carbide. The iron oxide component enables CO2 to produce CO through reverse water gas conversion. The synergistic effect between the carbide path on the iron carbide component and the CO insertion path at the interface of copper and iron carbide leads to efficient CC coupling to generate C 4+ Olefins, at 320°C, 0.1 MPa and 2400 mL / (g cat h), the CO2 conversion rate was 27.3%. When the product calculation method included CO, its C 4+ The olefin selectivity is 37.6%, and the C2-C3 olefin selectivity is 10.6%. Thongthai Witoon et al. (Chemical Engineering Journal, 2022, 428, 131389) synthesized Fe-Co-K-Al catalysts using different preparation methods and found that the catalyst prepared by precipitation reduction significantly enhanced the reducibility of metal oxides. In addition, this catalyst also has a large number of medium-strength CO2 adsorption sites and a small number of weak H2 adsorption sites, which can effectively promote the conversion of CO2 and improve the selectivity of olefin products. Under the reaction conditions of 350°C and 3MPa, the CO2 conversion rate is 42.2%, and the C2-C4 olefin selectivity is 39.3%.

[0005] In summary, the CO2 hydrogenation to olefins reaction has made important research progress by adding alkali metal promoters to the iron oxide precursor, introducing transition metal structural additives or synthesizing multi-component iron oxides. However, current research ignores the effect of the preparation method of the iron oxide precursor on its catalytic performance. By simply changing the preparation method of the iron oxide precursor, the high catalyst preparation cost, complex process and unstable catalytic effect that are common in current research can be reasonably improved. At the same time, by regulating the structural shape, surface properties, adsorption behavior of reactants and intermediate molecules and the generation of reactive species of the catalyst, the generation of C1 by-products can be inhibited, and the selectivity of olefins can be improved. Therefore, it is of great research significance to design a catalyst that is simple in preparation and effectively inhibits the generation of by-products, and produces the target product olefins with high selectivity. Summary of the invention

[0006] The purpose of the present invention is to provide a method for efficiently catalyzing the hydrogenation of CO2 to produce olefins. The Fe-based catalyst used in the method does not require any additives, has a simple preparation method, and has excellent reaction performance and good stability for the catalytic hydrogenation of CO2 to produce olefins.

[0007] In view of the above purpose, the technical solution adopted by the present invention is: Fe3O4 catalyst is loaded into the constant temperature section of a fixed bed reactor; before the reaction, the Fe3O4 catalyst is first reduced with hydrogen, and the reduction conditions are: temperature 350-400°C, time 2-10h, pressure 0.1MPa-1MPa, and mass space velocity 12000-24000mL / (h·g); then carbon dioxide and hydrogen are introduced for reaction, and the reaction conditions are: temperature 300-360°C, pressure 0.5-2.5MPa, H2 / CO2 molar ratio 1-4, and mass space velocity 6000-12000mL / (h·g). The preparation method of the Fe3O4 catalyst is as follows: dissolving ferric chloride in ethylene glycol, adding a dispersant and sodium hydroxide, stirring at 40-60°C until the sodium hydroxide is completely dissolved, and then transferring the resulting solution to a stainless steel high-pressure reactor equipped with a polytetrafluoroethylene liner, sealing the reactor and heating it to 160-180°C for reaction for 8-12 hours; after the reaction is completed, the reactor is cooled to room temperature, the product is magnetically separated, and the product is fully washed with deionized water and ethanol to remove the Na element and then dried to obtain the Fe3O4 catalyst.

[0008] In the above-mentioned method for preparing the Fe3O4 catalyst, the dispersant is polyvinyl pyrrolidone or polyethylene glycol.

[0009] In the above-mentioned preparation method of Fe3O4 catalyst, the mass ratio of ferric chloride to dispersant and sodium hydroxide is preferably 1: 0.008-0.012: 1.5-2.0.

[0010] In the above-mentioned method for preparing the Fe3O4 catalyst, it is further preferred that the reactor is sealed and heated to 180°C for reaction for 12 hours.

[0011] In the above method for preparing olefins by catalytic hydrogenation of carbon dioxide, the Fe3O4 catalyst is tableted, crushed, passed through a 20-40 mesh sieve, and then loaded into the constant temperature section of a fixed bed reactor.

[0012] In the above method for preparing olefins by catalytic hydrogenation of carbon dioxide, the reduction conditions are preferably: temperature 340-360°C, time 8-10 h, pressure 0.1 MPa-0.2 MPa, and mass space velocity 20000-24000 mL / (h·g).

[0013] In the above-mentioned method for preparing olefins by catalytic hydrogenation of carbon dioxide, the preferred reaction conditions are: temperature 320-330°C, pressure 1-1.5 MPa, H2 / CO2 molar ratio 3-4, and mass space velocity 8000-9000 mL / (h·g).

[0014] The beneficial effects of the present invention are as follows:

[0015] 1. The present invention changes the previous strategy of adding alkali metal promoters to ferrite precursors, introducing transition metal structural additives or synthesizing multinary ferrites. By changing the catalyst preparation method, ethylene glycol with good fluidity and reducibility is used as a solvent, and high molecular materials such as polyvinyl pyrrolidone and polyethylene glycol are used as dispersants. The Fe3O4 catalyst is prepared by a solvothermal method without adding any functional additives. The absolute content of Na element remaining in the catalyst bulk phase obtained by ICP characterization is only 0.001wt.%. The catalyst has low cost and a simple preparation method. More importantly, this preparation method enables the catalyst surface to have the most abundant oxygen defects, showing excellent adsorption and activation ability for the reaction molecule CO2. During the reaction, the carbonization degree of Fe species is strong, the highest proportion of FTS active species χ-Fe5C2 is formed, and the highly active crystal plane χ-Fe5C2 (510) is exposed, making the CC coupling reaction more likely to occur and significantly reducing the energy barrier required for olefin formation, thereby achieving efficient catalytic CO2 hydrogenation to olefins. The catalyst was evaluated in a fixed bed reactor at 320°C, 1 MPa, 8100 mL / (g cat ·h) and H2 / CO2=3 for 48h, the CO2 conversion rate was 40.3%. When the product calculation method includes CO, C2 + The olefin selectivity reached 74.2%, the C2-C4 olefin selectivity was 12.4%, and the C 5+ The olefin selectivity is 70.8%, the by-product CO and CH4 selectivities are 10.8% and 3.8% respectively, and the catalyst maintains excellent stability during the long-term evaluation process, showing good prospects for industrial application.

[0016] 2. The olefin products generated by the catalytic hydrogenation of CO2 by the catalyst of the present invention are intermediates, and the product distribution can be further regulated by coupling different molecular sieves, which has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the stability evaluation result of the Fe3O4 catalyst in Example 1 for catalyzing CO2 hydrogenation to olefins. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below in conjunction with the embodiments, but the protection scope of the present invention is not limited to these embodiments.

[0019] Example 1

[0020] 1. Preparation of Fe3O4 catalyst

[0021] 0.405 g of FeCl3·6H2O was dissolved in 10 mL of ethylene glycol, 0.004 g of polyvinyl pyrrolidone and 0.640 g of NaOH were added, and the mixture was stirred at 50°C for 60 min to completely dissolve the sodium hydroxide. The resulting solution was then transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene, and the reactor was sealed and heated to 180°C for reaction for 12 h. After the reaction was completed, the reactor was cooled to room temperature, and the obtained black product was separated by a magnet. The black product was fully washed with a large amount of deionized water and ethanol to remove the Na element, and then dried at 80°C for 6 h to obtain a Fe3O4 catalyst.

[0022] 2. CO2 hydrogenation to olefins

[0023] The Fe3O4 catalyst was tableted, crushed, and passed through a 20-40 mesh sieve before being filled in the constant temperature section of a fixed bed reactor. Before the reaction, the Fe3O4 catalyst was first reduced with hydrogen under the following reduction conditions: temperature 350°C, time 10h, pressure 0.1MPa, and mass space velocity 24000mL / (h·g); then carbon dioxide and hydrogen were introduced for reaction under the following reaction conditions: temperature 320°C, pressure 1MPa, H2 / CO2 molar ratio of 3, and mass space velocity 8100mL / (h·g). The reaction results are shown in Table 1.

[0024] Example 2

[0025] In this embodiment, the preparation method of Fe3O4 catalyst is the same as that of Example 1. The pressure of CO2 hydrogenation to olefins is 2.5 MPa, and other conditions are the same as those of Example 1. The reaction results are shown in Table 1.

[0026] Example 3

[0027] In this embodiment, during the preparation of the Fe3O4 catalyst, the reactor was sealed and heated to 180°C for 8 hours, and the other steps were the same as those in Example 1. The conditions for hydrogenating CO2 to olefins were the same as those in Example 1, and the reaction results are shown in Table 1.

[0028] Example 4

[0029] In this embodiment, during the preparation of the Fe3O4 catalyst, the reactor was sealed and heated to 160°C for 12 hours, and the other steps were the same as those in Example 1. The conditions for CO2 hydrogenation to olefins were the same as those in Example 1, and the reaction results are shown in Table 1.

[0030] Example 5

[0031] In this embodiment, the preparation method of Fe3O4 catalyst is the same as that of Example 1. The reaction conditions of CO2 hydrogenation to olefins include a temperature of 340°C, and other conditions are the same as those of Example 1. The reaction results are shown in Table 1.

[0032] Table 1 Results of the catalyst of the present invention for catalytic CO2 hydrogenation conversion (the calculation of product selectivity includes CO)

[0033]

[0034] As shown in Table 1, the Fe3O4 catalyst prepared by the method of the present invention is used for fixed bed catalytic CO2 hydrogenation reaction, and the CO2 conversion rate is high. 2+ High olefin selectivity, with C being the main product 5+ Olefins, by-products CO and CH4 have low selectivity, and have excellent reaction performance and good stability for CO2 catalytic hydrogenation to olefins.

[0035] The stability of the Fe3O4 catalyst in Example 1 for catalytic CO2 hydrogenation to olefins was further evaluated. The reaction was continued for 200 h under the reaction conditions of temperature 320°C, pressure 1 MPa, H2 / CO2 molar ratio of 3, and mass space velocity of 8100 mL / (h·g). Figure 1 .Depend on Figure 1 It can be seen that the CO2 conversion rate and product selectivity of Fe3O4 catalyst remained basically unchanged until 200h after the reaction reached a stable state, and there was no catalyst deactivation. 2+ The selectivity of olefin products was kept above 70%.

Claims

1. A method for preparing olefins by catalytic hydrogenation of carbon dioxide, characterized in that: The Fe3O4 catalyst is loaded into the constant temperature section of the fixed bed reactor. Before the reaction, the Fe3O4 catalyst is first reduced with hydrogen, and the reduction conditions are: temperature 350-400°C, time 2-10h, pressure 0.1MPa-1MPa, and mass space velocity 12000-24000mL / (h·g); then carbon dioxide and hydrogen are introduced for reaction, and the reaction conditions are: temperature 300-360°C, pressure 0.5-2.5MPa, H2 / CO2 molar ratio 1-4, and mass space velocity 6000-12000mL / (h·g); The preparation method of the Fe3O4 catalyst is as follows: dissolving ferric chloride in ethylene glycol, adding a dispersant and sodium hydroxide, stirring at 40-60° C. until the sodium hydroxide is completely dissolved, then transferring the obtained solution to a stainless steel high-pressure reactor equipped with a polytetrafluoroethylene liner, sealing the reactor and heating it to 160-180° C. for reaction for 8-12 hours; after the reaction is completed, waiting for the reactor to be cooled to room temperature, magnetically separating the product, washing it with deionized water and ethanol to remove the Na element, and then drying it to obtain the Fe3O4 catalyst.

2. The method for preparing olefins by catalytic hydrogenation of carbon dioxide according to claim 1, characterized in that: The dispersant is polyvinyl pyrrolidone or polyethylene glycol.

3. The method for preparing olefins by catalytic hydrogenation of carbon dioxide according to claim 1 or 2, characterized in that: The mass ratio of the ferric chloride to the dispersant and the sodium hydroxide is 1:0.008-0.012:1.5-2.

0.

4. The method for preparing olefins by catalytic hydrogenation of carbon dioxide according to claim 2, characterized in that: The reactor was sealed and heated to 180°C for 12 h.

5. The method for preparing olefins by catalytic hydrogenation of carbon dioxide according to claim 1, characterized in that: The Fe3O4 catalyst is tableted, crushed, passed through a 20-40 mesh sieve and then loaded into the constant temperature section of the fixed bed reactor.

6. The method for preparing olefins by catalytic hydrogenation of carbon dioxide according to claim 1, characterized in that: The reduction conditions are: temperature 340-360° C., time 8-10 h, pressure 0.1 MPa-0.2 MPa, and mass space velocity 20000-24000 mL / (h·g).

7. The method for preparing olefins by catalytic hydrogenation of carbon dioxide according to claim 1, characterized in that: The reaction conditions are: temperature 320-330° C., pressure 1-1.5 MPa, H 2 / CO 2 molar ratio 3-4, and mass space velocity 8000-9000 mL / (h·g).

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