Method for co-hydrogenation of co2 to ethanol and liquid hydrocarbons and iron-based catalyst therefor

Abstract

The application provides a method for synthesizing ethanol and liquid hydrocarbon by CO2 hydrogenation and an iron-based catalyst thereof, and the method comprises the following steps: taking carbon dioxide and hydrogen as raw materials, and under the conditions of 250-330 DEG C, 3-5 MPa and the presence of a catalyst, ethanol and liquid hydrocarbon are generated by reaction; the catalyst is a supported iron-based catalyst, the active phase composition of the catalyst comprises Fe, Fe3O4, Fe7C3 and Fe5C2, the liquid hydrocarbon is a hydrocarbon with carbon number greater than or equal to 5, and the volume ratio of the carbon dioxide to the hydrogen is 1:2-1:5. Based on the single system iron-based catalyst, the carbon dioxide can be converted into high value-added ethanol and liquid hydrocarbon products by one-step hydrogenation, the CO2 conversion rate is greater than 40%, the total selectivity of ethanol+liquid hydrocarbon is greater than 50%, and the selectivity of CO by-product is less than 10%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of CO2 hydrogenation conversion, in particular to a method for synthesizing ethanol and co-producing liquid hydrocarbons from CO2 hydrogenation and an iron-based catalyst thereof. BACKGROUND

[0002] Converting carbon dioxide into high-value fuels and chemicals is of great significance for reducing carbon emissions to alleviate global warming, improving the ecological environment and solving the problem of the increasingly exhausted fossil resources. Direct hydrogenation of CO2 into ethanol is a promising technical approach for ethanol production and hydrogen storage. However, there are still challenges in technology. Activating CO2 requires a high energy barrier and is usually accompanied by high CH4 selectivity and high by-product CO selectivity. For example, when using Cu-based catalysts, there are problems such as low CO2 conversion rate, high CO selectivity, and easy deactivation of the catalyst.

[0003] Iron and cobalt-based catalysts have good application prospects in CO2 hydrogenation conversion systems, not only because of their relatively high conversion rate, but also because their active components are relatively inexpensive, which is conducive to future scaling up and industrialization of the catalyst. However, this catalytic system is based on the traditional Fischer-Tropsch synthesis reaction route, and its reaction products are extremely complex, with different selectivities for different metal active sites. For example, existing iron-based catalysts for catalyzing CO2 hydrogenation to produce ethanol have the defects of high by-product CO output and low selectivity for ethanol. There are also related technologies that use Fe-based catalysts and Cu-based catalysts in combination. For example, CN113908840A provides a Fe-based multifunctional catalyst, which realizes high selectivity of CO2 hydrogenation to synthesize ethanol and efficient synergistic effect between different catalytically active components by optimizing the coupling mode between different catalytically active components of the multifunctional catalyst and considering the reverse water gas shift reaction and Fischer-Tropsch synthesis carbon chain growth in the reaction process. However, its catalyst system is complex (serial catalysis), and the spatial distance between the two catalytic centers (regulated by physical mixing) is crucial to the reaction performance. Therefore, the physical mixing of two catalysts poses a great challenge to the engineering scaling up of this technology. SUMMARY

[0004] In view of this, the main purpose of the present application is to provide a method for synthesizing ethanol and co-producing liquid hydrocarbons from CO2 hydrogenation and an iron-based catalyst thereof. The method is based on a single iron-based catalyst system, which can realize one-step hydrogenation conversion of carbon dioxide into high-value ethanol and liquid hydrocarbon products, with CO2 conversion rate > 40%, total selectivity of ethanol + liquid hydrocarbons > 50%, and selectivity of CO by-product < 10%.

[0005] To achieve the above-mentioned application purposes, the present application adopts the following technical solutions: the present application provides a preparation method of an iron-based catalyst in the first aspect, which comprises the following steps:

[0006] 1) washing and pretreating the foamed iron;

[0007] 2) mixing the foamed iron treated in step 1) with iron salt, lye, hydrothermal additive and water to perform a first hydrothermal synthesis reaction, washing and drying the foamed iron after the reaction;

[0008] 3) mixing the foamed iron treated in step 2) with water and potassium permanganate to perform a second hydrothermal synthesis reaction, washing and drying the foamed iron after the reaction;

[0009] 4) immersing the foamed iron treated in step 3) in an additive metal salt solution, and drying the foamed iron after the immersion;

[0010] 5) performing high-temperature carbonization treatment on the foamed iron treated in step 4) to obtain an iron-based catalyst.

[0011] According to the method of the present application, in step 1), the washing and pretreatment includes: sequentially immersing the foamed iron in a hydrochloric acid solution, an anhydrous ethanol solution and deionized water, and drying; in a specific embodiment, the foamed iron can be first immersed in a hydrochloric acid solution for ultrasonic cleaning to remove surface impurities such as oxidation layers, then immersed in an anhydrous ethanol solution for ultrasonic cleaning to remove surface grease, and finally immersed in deionized water for ultrasonic cleaning, and then dried. In another specific embodiment, the foamed iron can be first immersed in an anhydrous ethanol solution for ultrasonic cleaning to remove surface grease, then immersed in a hydrochloric acid solution for ultrasonic cleaning to remove surface oxidation layers, and finally immersed in deionized water for ultrasonic cleaning, and then dried. Preferably, the concentration of the hydrochloric acid solution can be selected to be 0.05-0.3M, the time for ultrasonic cleaning with the hydrochloric acid solution, the anhydrous ethanol solution or the deionized water is 3-8min, and the drying temperature of the foamed iron is 60-150℃, and the drying time is 2-4h.

[0012] According to the method of the present application, the pore size of the foamed iron ranges from 0.5 to 20μm, and the specific surface area ranges from 1 to 50m 2 / g.

[0013] According to the method of the present application, in step 2), the temperature of the first hydrothermal synthesis reaction is 100-140℃, the reaction time is 8-12h, the drying temperature is 60-150℃, and the drying time is 10-14h; preferably, the mass ratio of the foamed iron: iron salt: lye: hydrothermal additive: water is (2-8):(1-5):(0.5-2):(0.1-1):60. Through the first hydrothermal synthesis reaction, an iron oxide layer is formed on the surface of the foamed iron.

[0014] According to the method of the present application, in step 2), the iron salt is selected from one or more of ferric nitrate, ferric chloride, ferric sulfate, ammonium citrate ferric or ferric oxalate; the alkali solution is selected from one or more of urea, ammonia, sodium hydroxide or potassium hydroxide to facilitate the hydrolysis of the precursor salt. The hydrothermal additive can be selected as NH4F to adjust the distribution of metal ions in the hydrothermal synthesis process.

[0015] According to the method of the present application, in step 3), the temperature of the secondary hydrothermal synthesis reaction is 120-160℃, the reaction time is 20-28h, the drying temperature is 70-150℃, and the drying time is 10-14h. Through the secondary hydrothermal synthesis reaction, Mn and K are introduced onto the surface of the foam iron-iron oxide layer in the form of an element or an oxide; preferably, the mass ratio between the potassium permanganate and the foam iron is 0.1-0.5:1.

[0016] According to the method of the present application, in step 4), the auxiliary metal salt is selected from one or more of trisodium citrate, sodium nitrate, sodium carbonate; preferably, the auxiliary metal salt further comprises one or more of zinc nitrate, zinc acetate, copper nitrate, copper sulfate, cobalt nitrate, cobalt sulfate, magnesium nitrate, manganese nitrate, and the impregnation time is 20-40min. In this context, the solvent in the metal salt solution is not limited as long as the substance can be fully dissolved. Through the impregnation of the metal salt solution, one or more of Na, Zn, Co, Cu, Mn, K, Mg, S is introduced onto the surface of the foam iron-iron oxide layer in the form of an element or an oxide.

[0017] According to the method of the present application, the high-temperature carbonization treatment comprises: carbonizing the foam iron at 220-360℃ in the presence of one or more of CO, CO2, H2, and the carbonization treatment time is 3-20h, preferably 8-12h at 300-340℃. The above preferred can obtain a better mass ratio of Fe, Fe3O4, Fe7C3 and Fe5C2. Through the high-temperature carbonization treatment, a carbonized iron layer is formed on the surface of the iron oxide layer of the foam iron.

[0018] Another aspect of the present application also provides an iron-based catalyst prepared by the above method.

[0019] According to the catalyst of the present application, the active phase is composed of Fe, Fe3O4, Fe7C3 and Fe5C2 in a mass ratio of (3-5):1:(0.2-0.5):(0.5-1).

[0020] The catalyst according to the application, wherein the catalyst further comprises an auxiliary agent, the auxiliary agent being one or more of Na, Mn, K in elemental or oxide form; preferably, the auxiliary agent further comprises one or more of Zn, Co, Cu, Mg, S in elemental or oxide form; and / or, the auxiliary agent accounts for 0.5%-10% of the mass of the catalyst.

[0021] The application also provides a method for synthesizing ethanol and liquid hydrocarbon from CO2 and H2, comprising the following steps:

[0022] The reaction is carried out at 250-330 DEG C, 3-5 MPa and in the presence of the above-mentioned iron-based catalyst, using CO2 and H2 as raw materials to generate ethanol and liquid hydrocarbon; the liquid hydrocarbon is hydrocarbon with carbon number >=5; the volume ratio of CO2 to H2 is 1:2-1:5.

[0023] According to the method of the application, the reaction is carried out in a continuous fixed-bed reactor.

[0024] Compared with the prior art, the application has the following beneficial effects:

[0025] The application uses foamed iron as a carrier, forms an iron oxide layer on the surface of the foamed iron through a one-step hydrothermal synthesis reaction, introduces Mn and K on the surface of the iron oxide layer through a two-step hydrothermal synthesis reaction, introduces one or more of Na, Zn, Co, Cu, Mn, Mg and S on the surface of the iron oxide layer through impregnation, and forms a carburized iron layer near the metal auxiliary agent through a gas high-temperature carbonization method, thereby obtaining the supported iron-based catalyst of the application.

[0026] The reaction system for synthesizing ethanol and liquid hydrocarbon from CO2 and H2 according to the application mainly comprises the following main reactions:

[0027] 1) CO2+H2→CO+H2O

[0028] 2) nCO+2nH2→C n H 2n +nH2O

[0029] 3) 2CO+4H2→C2H5OH+H2O

[0030] The process for producing liquid hydrocarbon is that reaction 1) is carried out on the active site of the iron oxide, and then reaction 2) is carried out on the active site of the carburized iron. The process for producing ethanol is that reaction 1) is carried out on the active site of the iron oxide, and then reaction 3) is carried out on the active site of the carburized iron-auxiliary agent. Therefore, by forming the interface structure of Fe3O4+FeCx+auxiliary agent, the efficient cooperation between reaction 1) and reaction 2) / 3) can be promoted, the reaction equilibrium can be moved to the right, and thus the CO2 conversion rate can be improved and the generation of CO by-product (CO not participating in reactions 2) and 3) can be reduced.

[0031] Therefore, the supported iron-based catalyst of the present application provides multiple active sites, improves the specific surface area of the material, and thus improves the catalytic activity. In addition, by forming an interface structure of Fe3O4+FeCx+adjuvant, the synergistic effect between the double active phases (Fe3O4+FeCx) and between the double active phases and the adjuvant is strengthened, and the reaction performance is improved. It has excellent heat transfer, mass transfer performance and high temperature mechanical stability. The present application uses a single iron-based catalyst to realize the production of ethanol and liquid hydrocarbon coproducts by CO2 hydrogenation. DETAILED DESCRIPTION

[0032] The present application will be further described below in conjunction with examples, but the present application is not limited to the listed examples, and should also include equivalent improvements and modifications of the technical solutions defined in the claims attached to the present application.

[0033] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges and values should be interpreted as approximately between the stated values and other values near the stated values. For ranges, the endpoints are included between the values of each range, between the endpoints of each range, and between each individual point value, and new ranges can be created by combining the endpoints and individual point values with each other. These new ranges are to be considered disclosed herein.

[0034] In the following examples and comparative examples:

[0035] (1) Main raw materials

[0036] The foamed iron raw material was obtained from a commercial channel, and the specific surface area was determined to be 14.8 m 2 / g.

[0037] Unless otherwise specified, the raw materials of the present application can be obtained from a commercial channel, and are analytical pure reagents.

[0038] (2) Characterization and testing

[0039] Conversion rate and selectivity acquisition process:

[0040] The reaction products were analyzed on-line and off-line by gas chromatography (GC-2014C, Shimadzu, Japan) with a hydrogen flame ionization detector (FID). The on-line separation of C1 to C5 hydrocarbon products was performed by a fixed phase Porapak-Q column, and the off-line separation of C6 to C20 hydrocarbon components was performed by a SE-30 column. The H2, CO, CH4, and CO2 in the reaction products were analyzed on-line by gas chromatography (GC-2014C, Shimadzu, Japan) with a thermal conductivity detector (TCD), and the fixed phase was an activated carbon column. The selectivity of various alkenes and alkanes was calculated by the external standard method according to the detection results of the FID. The carbon balance of all reactions was calculated to be maintained above 95%.

[0041] The specific surface area measuring instrument or method is as follows: the specific surface area of the catalyst is determined by nitrogen physical adsorption, and the instrument used is a Micromeritics ASAP 2020 (Micromeritics, USA).

[0042] The mass of Fe, Fe3O4, Fe7C3, and Fe5C2 is obtained by Mossbauer spectrum test analysis, and the specific test method is as follows: a VIS-1170-MO constant acceleration transmission mode is used The spectrometer (Cryodynamics, USA) is used at room temperature, and the measurement is performed at a constant temperature of 300 K. 57 Co / Pd is used as a radiation source. The data analysis uses a non-linear least squares method, and the spectrum fitting is based on a Lorentz linear sextet fitting. The component identification is based on isomer shift (IS), quadrupole splitting (QS), and hyperfine magnetic field (Hhf). The hyperfine magnetic field is calibrated at room temperature using α-Fe (330-kOe).

[0043] The following examples 1-10 are used to illustrate the supported iron-based catalyst and the preparation method thereof.

[0044] Example 1

[0045] 1) 3.0 g of foamed iron was weighed into a beaker, 30 mL of 0.1 M HCl was added, and the foamed iron was taken out after 3 min of treatment; 30 mL of anhydrous ethanol was added, and the foamed iron was taken out after 5 min of ultrasonic cleaning; 30 mL of deionized water was added, and the foamed iron was taken out after 5 min of ultrasonic cleaning, and then the foamed iron was washed with deionized water for 3 times; finally, the foamed iron was placed in an 80 ℃ oven for drying for 3 h.

[0046] 2) Take 0.82 g Fe(N03)3*9H20 (2 mmol), 0.606 g urea (10 mmol), 3.0 g of foamed iron into the hydrothermal synthesis kettle, add 0.308 g NH4F (8 mmol), then add 36 mL of deionized water, mix well, seal, and place it in a 120 °C rotary oven for 10 h; remove the sample that has reacted completely, wash it with deionized water several times, and dry the foamed iron in a 60 °C vacuum oven for 12 h.

[0047] 3) Put the sample obtained in 2) into the hydrothermal kettle, add 1.0 g of KMn04 solid, add 36 mL of deionized water, mix well, seal, and place it in a 140 °C rotary oven for 24 h, and let it cool naturally to room temperature. Remove the sample and wash it with deionized water, and dry it in an 80 °C vacuum oven for 12 h.

[0048] 4) Soak the foamed iron treated in step 3) in a trisodium citrate solution for 30 min, and then dry the foamed iron;

[0049] 5) Carbonize the foamed iron treated in step 4) at 320 °C under the introduction of 50% CO / 50% H2 (volume percentage) for 10 h to obtain a supported iron-based catalyst. The specific surface area is 42 m 2 / g. The active phase composition is: 51.2% Fe, 13.8% Fe304, 13.1% Fe7C3, 21.9% Fe5C2.

[0050] Example 2

[0051] 1) Take 2.0 g of foamed iron into a beaker, add 30 mL of 0.05 M HC1, treat for 3 min, and then remove the foamed iron; add 30 mL of anhydrous ethanol, ultrasonic clean for 5 min, and then remove; add 30 mL of deionized water, ultrasonic clean for 5 min, and then remove, and then wash it with deionized water 3 times; finally, dry it in an 80 °C oven for 3 h.

[0052] 2) Take 0.6 g Fe(N03)3*9H20, 0.3 g urea, 1.2 g of foamed iron into the hydrothermal synthesis kettle, add 0.06 g NH4F, then add 36 mL of deionized water, mix well, seal, and place it in a 120 °C rotary oven for 10 h; remove the sample that has reacted completely, wash it with deionized water several times, and dry the foamed iron in a 60 °C vacuum oven for 12 h.

[0053] 3) Put the sample obtained in 2) into the hydrothermal reactor, add 0.4 g of KMnO4 solid, add 36 mL of deionized water, mix thoroughly, seal, and place in a rotating oven at 140°C for 24 h. Allow it to cool naturally to room temperature. Take out the sample, wash it with deionized water, and place it in a vacuum oven at 80°C for 12 h.

[0054] 4) After the foam iron treated in step 3) is immersed in a trisodium citrate solution for 30 min, dry the foam iron;

[0055] 5) The foam iron treated in step 4) is carbonized by passing synthetic gas (H2 / CO volume ratio of 2) at 340°C for 8 h to obtain a supported iron-based catalyst. The specific surface area is 38 m 2 / g. The active phase composition is: 48.9% Fe, 21.2% Fe3O4, 10.2% Fe7C3, 19.7% Fe5C2.

[0056] Example 3

[0057] 1) Weigh 5.0 g of foam iron into a beaker, add 30 mL of 0.3 M HCl, and treat for 3 min. Take out the foam iron, add 30 mL of anhydrous ethanol, and ultrasonically clean for 5 min. Take out, then add 30 mL of deionized water, ultrasonically clean for 5 min, then take out, and then wash with deionized water 3 times. Finally, place it in an 80°C oven and dry for 3 h.

[0058] 2) Weigh 3.0 g of Fe(NO3)3·9H2O, 1.2 g of urea, and 4.8 g of foam iron into a hydrothermal synthesis reactor, add 0.6 g of NH4F, then add 36 mL of deionized water, mix thoroughly, seal, and place in a rotating oven at 120°C for 10 h. Take out the sample that has reacted, wash it with deionized water several times, and place the foam iron in a vacuum oven at 60°C for 12 h.

[0059] 3) Put the sample obtained in 2) into the hydrothermal reactor, add 0.4 g of KMnO4 solid, add 36 mL of deionized water, mix thoroughly, seal, and place in a rotating oven at 140°C for 24 h. Allow it to cool naturally to room temperature. Take out the sample, wash it with deionized water, and place it in a vacuum oven at 80°C for 12 h.

[0060] 4) After the foam iron treated in step 3) is immersed in a trisodium citrate solution for 30 min, dry the foam iron;

[0061] 5) The foamed iron treated in step 4) is carbonized in H2 and synthetic gas at 240℃ and 280℃ for 3h and 10h respectively to obtain the supported iron-based catalyst. The specific surface area is 51 m 2 / g. The active phase composition is: 61.2% Fe, 20.1% Fe3O4, 7.9% Fe7C3, 10.8% Fe5C2.

[0062] Example 4

[0063] Based on example 1, the sodium citrate solution in step 4) is replaced by sodium nitrate solution and zinc nitrate solution, and the impregnation time is 40 min. The specific surface area of the iron-based catalyst is 44 m 2 / g.

[0064] Example 5

[0065] Based on example 1, the sodium citrate solution in step 4) is replaced by sodium nitrate solution and manganese nitrate solution, and the impregnation time is 20 min. The specific surface area of the iron-based catalyst is 37 m 2 / g.

[0066] Example 6

[0067] Based on example 1, the amount of KMnO4 in step 3) is changed to 0.3g. The specific surface area of the iron-based catalyst is 46 m 2 / g.

[0068] Example 7

[0069] Based on example 1, the amount of KMnO4 in step 3) is changed to 1.5g. The specific surface area of the iron-based catalyst is 35 m 2 / g.

[0070] Example 8

[0071] Based on example 1, the temperature of the first hydrothermal synthesis reaction in step 2) is changed to 100℃, the reaction time is 12h, the drying temperature is 60℃, and the drying time is 14h. The temperature of the second hydrothermal synthesis reaction in step 3) is changed to 120℃, the reaction time is 28h, the drying temperature is 70℃, and the drying time is 14h. The specific surface area of the iron-based catalyst is 39 m 2 / g.

[0072] Example 9

[0073] On the basis of Example 1, the temperature of the first hydrothermal synthesis reaction of step 2) is changed to 140°C, the reaction time is 8h, the drying temperature is 150°C, and the drying time is 10h; the temperature of the second hydrothermal synthesis reaction of step 3) is changed to 160°C, the reaction time is 20h, the drying temperature is 150°C, and the drying time is 10h. The specific surface area of the iron-based catalyst obtained is 51m 2 / g.

[0074] Example 10

[0075] On the basis of Example 1, the temperature of the carbonization treatment of step 5) is changed to 300°C, and the carbonization time is 12h. The specific surface area of the iron-based catalyst obtained is 44m 2 / g.

[0076] Comparative Example 1 (first hydrothermal synthesis)

[0077] Take 3.0g of foamed iron in a beaker, add 30mL of 0.1M HCl, and treat for 3min before taking out the foamed iron; add 30mL of anhydrous ethanol, ultrasonic clean for 5min, then take out; add 30mL of deionized water, ultrasonic clean for 5min, then take out, and then wash with deionized water for 3 times; finally, place it in an 80°C oven to dry for 3h.

[0078] Take 0.82g of Fe(NO3)3·9H2O (2mmol), 0.606g of urea (10mmol), and 3.0g of foamed iron in a hydrothermal synthesis kettle, add 0.308g of NH4F (8mmol), then add 36mL of deionized water, mix well, seal, and place it in a 120°C rotary oven for 10h; take out the reacted sample, wash with deionized water several times, and place the foamed iron in a 60°C vacuum oven to dry for 12h to obtain the catalyst. The specific surface area is 38.1m 2 / g.

[0079] Comparative Example 2 (first hydrothermal synthesis + calcination + second hydrothermal synthesis)

[0080] Take 3.0g of foamed iron in a 50mL beaker, add 30mL of 0.1M HCl, and treat for 3min; add 30mL of anhydrous ethanol, ultrasonic clean for 5min; add 30mL of deionized water, ultrasonic clean for 5min, then wash with a large amount of deionized water several times; finally, place it in an 80°C oven to dry for 3h.

[0081] Take 0.82 g Fe(NO3)3·9H2O (2 mmol), 0.606 g urea (10 mmol), 3.0 g of foamed iron in a hydrothermal kettle, add 0.308 g NH4F (8 mmol), then add 36 mL of deionized water, mix thoroughly, seal, and place it in a 120°C rotary oven for 10 h; remove the sample that has reacted completely, wash it with deionized water several times, and place it in a 60°C vacuum oven for 12 h, and finally place the sample in a 350°C muffle furnace for 3 h, and remove it when the temperature drops to room temperature.

[0082] Put all the above samples into the hydrothermal kettle, add 0.2 g of KMnO4 solid, add 36 mL of deionized water, mix thoroughly, seal, and place it in a 140°C rotary oven for 24 h, and allow it to cool naturally to room temperature. Remove the sample, wash it with deionized water several times, and place it in a 80°C vacuum oven for 12 h to obtain the catalyst. The specific surface area is 27.2 m 2 / g.

[0083] The catalysts prepared in Examples 1-10 and Comparative Examples 1-2 of the present application are used to synthesize ethanol and liquid hydrocarbons (hydrocarbons with carbon number ≥5) by hydrogenation of CO2 to test their catalytic performance, and the specific application steps are as follows:

[0084] In a continuous fixed-bed reactor, carbon dioxide and hydrogen are used as raw materials to generate ethanol and liquid hydrocarbons in the presence of the above catalyst at 300°C, 3.5 MPa, and for 50 h; wherein the volume ratio of carbon dioxide to hydrogen is 1:3. The reaction results are shown in Table 1 below.

[0085] Table 1

[0086]

[0087]

[0088] As can be seen from the data in Table 1 above, the catalyst is prepared by using commercially available foamed iron as the carrier, and using multi-step hydrothermal, wet chemical impregnation, and high-temperature carbonization in sequence in the present application. The supported iron-based catalyst of the present application provides multiple active sites, increases the specific surface area of the material, and thus improves the catalytic activity and the CO2 conversion rate. In addition, by forming an interface structure of Fe3O4+FeCx+adjuvant, the synergistic effect between the double active phases (Fe3O4+FeCx) and between the double active phases and the adjuvant is strengthened, and the reaction performance is improved. The base material used in the examples of the present application is foamed iron (elemental iron metal), and therefore has excellent heat transfer and mechanical stability. In addition, the foamed iron material has an open macroporous structure, which is beneficial to strengthening the mass transfer performance of the reactants and products on the foamed iron catalyst.

[0089] The specific surface area of the catalyst obtained by the preparation method in Comparative Example 1-2 is low, resulting in poor catalytic effect, low CO2 conversion rate, and excessively high selectivity of byproduct CH4. It can be seen that, according to the process cooperation between the multi-step hydrothermal synthesis, impregnation and high-temperature carbonization process set in the embodiments of the present application, the phase composition and ratio of the structure can be adjusted according to the requirements, the catalytic reaction performance is improved, and the iron-based catalyst with high activity and high selectivity is obtained.

[0090] Obviously, the above embodiments of the present application are only examples for clearly illustrating the present application, and are not intended to limit the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. It is impossible to enumerate all the embodiments here. Any obvious changes or variations derived from the technical solutions of the present application are within the scope of the spirit of the present application.

Claims

1. A method for preparing an iron-based catalyst, characterized in that, include: 1) Clean and pre-treat the foamed iron; 2) Mix the foamed iron treated in step 1) with iron salt, alkaline solution, hydrothermal additive and water to carry out a hydrothermal synthesis reaction. After the reaction, wash the foamed iron and dry it. 3) Mix the foamed iron treated in step 2) with water and potassium permanganate to carry out a second hydrothermal synthesis reaction. After the reaction is completed, wash the foamed iron and dry it. 4) Immerse the foamed iron treated in step 3) in an auxiliary metal salt solution, and then dry the foamed iron. 5) The foamed iron treated in step 4) is subjected to high-temperature carbonization to obtain an iron-based catalyst; An iron oxide layer was formed on the surface of the foamed iron through a primary hydrothermal synthesis reaction. Mn and K were then introduced onto the surface of the iron oxide layer through a secondary hydrothermal synthesis reaction. One or more of Na, Zn, Co, Cu, Mn, Mg, and S were introduced onto the surface of the foamed iron oxide layer through impregnation. Finally, an iron carbide layer was formed near the metal additives through a high-temperature gas carbonization method, thus yielding Fe3O4 + FeC. x +Interfacial structure of the promoter; The active phase composition of the catalyst includes Fe, Fe3O4, Fe7C3 and Fe5C2.

2. The method for preparing the iron-based catalyst according to claim 1, characterized in that: In step 1), the cleaning pretreatment includes: placing the foamed iron in hydrochloric acid solution, anhydrous ethanol solution and deionized water in sequence for ultrasonic treatment and drying; wherein, the concentration of hydrochloric acid solution is 0.05-0.3M, the ultrasonic cleaning time of hydrochloric acid solution, anhydrous ethanol solution or deionized water is 3-8min, the drying temperature of foamed iron is 60-150℃, and the drying time is 2-4h.

3. The method for preparing the iron-based catalyst according to claim 1, characterized in that: In step 2), the temperature of the first hydrothermal synthesis reaction is 100-140℃, the reaction time is 8-12h, the drying temperature is 60-150℃, and the drying time is 10-14h; and / or, the mass ratio of foamed iron: iron salt: alkali solution: hydrothermal additive: water is (2-8):(1-5):(0.5-2):(0.1-1):

60.

4. The method for preparing the iron-based catalyst according to claim 1, characterized in that: In step 3), the temperature of the secondary hydrothermal synthesis reaction is 120-160℃, the reaction time is 20-28h, the drying temperature is 70-150℃, and the drying time is 10-14h; and / or, the mass ratio between potassium permanganate and foamed iron is 0.1-0.5:

1.

5. The method for preparing the iron-based catalyst according to claim 1, characterized in that: In step 4), the auxiliary metal salt is selected from one or more of trisodium citrate, sodium nitrate, and sodium carbonate.

6. The method for preparing the iron-based catalyst according to claim 5, characterized in that: In step 4), the auxiliary metal salt also includes one or more of zinc nitrate, zinc acetate, copper nitrate, copper sulfate, cobalt nitrate, cobalt sulfate, magnesium nitrate, and manganese nitrate, and the immersion time is 20-40 minutes.

7. The method for preparing the iron-based catalyst according to claim 1, characterized in that: In step 5), the high-temperature carbonization treatment includes: carbonizing the foamed iron at 220-360°C in the presence of CO and H2 for 3-20 hours.

8. An iron-based catalyst prepared by any one of claims 1-7.

9. The iron-based catalyst according to claim 8, characterized in that: The active phase composition of the iron-based catalyst, Fe, Fe3O4, Fe7C3 and Fe5C2, has a mass ratio of (3-5):1:(0.2-0.5):(0.5-1).

10. A method for synthesizing ethanol by CO2 hydrogenation and co-producing liquid hydrocarbons, characterized in that: Using carbon dioxide and hydrogen as raw materials, under conditions of 250-330℃, 3-5MPa and the presence of the iron-based catalyst described in any one of claims 8-9 or the iron-based catalyst prepared by any one of claims 1-7, ethanol and liquid hydrocarbons are produced; the liquid hydrocarbons are hydrocarbons with ≥5 carbon atoms; the volume ratio of carbon dioxide to hydrogen is 1:2-1:5.

Images