Iron-based catalyst for preparing low-carbon alcohol through carbon dioxide hydrogenation as well as preparation method and application of iron-based catalyst

Abstract

The invention belongs to the technical field of carbon dioxide resource utilization, and particularly relates to an iron-based catalyst for preparing low-carbon alcohol through carbon dioxide hydrogenation as well as a preparation method and application of the iron-based catalyst. The iron-based catalyst provided by the invention takes iron as a main active component and alkali metal and a sulfur element as co-assistants, and is prepared by adopting a co-precipitation method: mixing ferric sulfate with the alkali metal assistants, aging, washing, drying and roasting, thereby obtaining the iron-based catalyst. Or adopting a later impregnation method: after the drying step, carrying out impregnation treatment on the catalyst by using a salt solution containing sulfate radicals. The catalyst can efficiently catalyze a carbon dioxide hydrogenation reaction under the conditions of high temperature and high pressure, 35.7% of carbon dioxide conversion per pass and 18.7% of C2 + alcohol selectivity can be achieved, and the proportion of C2 + alcohol in total alcohol is as high as 90% or above. In addition, the catalyst disclosed by the invention is simple in preparation process, low in cost and easy to realize industrial amplification, and has a wide application prospect in the field of carbon dioxide resource utilization.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of carbon dioxide resource utilization, and particularly relates to an iron-based catalyst for synthesizing low-carbon alcohol through carbon dioxide hydrogenation and a preparation method and application thereof. BACKGROUND

[0002] Under the global background, carbon dioxide resource utilization has become a research hotspot. Among them, the catalytic hydrogenation of carbon dioxide into high-value-added low-carbon mixed alcohol (C 2+ alcohol) not only helps carbon emission reduction, but also produces important chemical raw materials, and has significant environmental and economic benefits. Low-carbon alcohol (such as ethanol, propanol, butanol, etc.) has high energy density and is widely used, and is not only an excellent solvent and fuel additive, but also a key intermediate for the synthesis of pharmaceuticals, plasticizers, fragrances, and food industry. However, the direct hydrogenation of carbon dioxide to synthesize low-carbon alcohol has not yet been scaled up for application, mainly because of the lack of efficient catalysts, which makes it difficult to achieve high carbon dioxide conversion and high low-carbon alcohol selectivity.

[0003] The carbon dioxide hydrogenation process usually requires high energy input, and the reaction path is complex, and the activity and product selectivity regulation are the main challenges currently faced. At present, the catalysts for catalyzing the hydrogenation of carbon dioxide to synthesize low-carbon alcohol mainly include four types: one is a noble metal catalyst such as Rh and Pd, which has high cost and poor stability, and the product is mainly ethanol; two is a Mo-based catalyst, and the overall activity and alcohol selectivity of which need to be greatly improved; three is a modified methanol catalyst (mainly a copper-based catalyst), which usually needs to introduce a second component to improve the activity and carbon chain growth ability of the catalyst, and still has the problems of low conversion rate, poor stability, and alcohol product mainly being methanol; four is a modified Fischer-Tropsch synthesis catalyst, mainly including Fe-Cu-based and Co-Cu-based catalysts, which also has difficulty in realizing effective and stable intermetallic synergistic effect. Therefore, it is urgent to develop a carbon dioxide hydrogenation synthesis catalyst which is cheap in raw materials, simple in preparation, and has high catalytic activity and low-carbon alcohol selectivity. SUMMARY

[0004] In order to overcome the shortcomings of the prior art, the application provides an iron-based catalyst for synthesizing low-carbon alcohol through carbon dioxide hydrogenation and a preparation method thereof. The catalyst selects cheap and readily available raw materials, and has a simple and convenient preparation process, and at the same time has excellent catalytic activity and high low-carbon alcohol selectivity in the synthesis of low-carbon alcohol through carbon dioxide hydrogenation.

[0005] In order to achieve the above purpose, the technical scheme adopted by the application is: The iron-based catalyst for carbon dioxide hydrogenation to produce low-carbon alcohols provided by the first aspect of the present application is mainly composed of iron (Fe) as the main active metal component, and contains an alkali metal additive and a sulfur additive, and the phase composition thereof is mainly iron oxide; the alkali metal additive is selected from at least one of lithium (Li), sodium (Na), potassium (K), rubidium (Rb) or cesium (Cs), and the sulfur additive exists in the form of sulfate (SO4 2- ). Preferably, the content of iron element is 64.8% by element mass, the content of alkali metal is 2.4% by element mass, and the content of sulfur element is 0.6% by element mass, based on the total mass of the catalyst.

[0006] Preferably, the alkali metal additive is sodium (Na).

[0007] Preferably, the content of iron element is 64.8% by element mass, the content of alkali metal is 2.4% by element mass, and the content of sulfur element is 0.6% by element mass, based on the total mass of the catalyst.

[0008] Preferably, the catalyst further contains copper (Cu) as the second active metal component; the content of Cu is 1%-20% by element mass, based on the total mass of the catalyst.

[0009] The second aspect of the present application provides a preparation method of the iron-based catalyst of the first aspect, that is, the iron-based catalyst is prepared by a co-precipitation method or a post-impregnation method. The preparation method of the iron-based catalyst by the co-precipitation method comprises the following steps: S11, precipitation reaction: dissolving iron sulfate in a solvent to prepare an iron salt solution, mixing the iron salt solution with the alkali metal additive at 60-90°C, and controlling the pH value of the reaction system to be 9-11 to perform a precipitation reaction, so that the active component precursor is precipitated and separated out; S12, aging: aging the slurry obtained in S11 at 60-90°C for 0.5-3 hours to make the crystal structure more stable; S13, washing and separation: performing solid-liquid separation on the product after aging in S12, and washing with water; S14, drying: placing the product after washing in S13 at 60-120°C for drying for 6-24 hours to remove the physically adsorbed water; S15, calcination: placing the solid product obtained in S14 at 300-500°C for calcination for 2-8 hours to convert it into a metal oxide form with catalytic activity, so as to obtain the iron-based catalyst; The method uses iron sulfate as the iron source in the precipitation step, and retains part of the sulfate by controlling the washing degree in the washing step.

[0010] The late-stage impregnation method for preparing the iron-based catalyst comprises the following steps: S21, precipitation reaction: dissolving iron salt without sulfur element in a solvent to form an iron salt solution, mixing the iron salt solution with an alkali metal additive at 60-90℃, and controlling the pH value of the reaction system to be 9-11 to perform a precipitation reaction; S22, aging: aging the slurry obtained in S21 at 60-90℃ for 0.5-3 hours; S23, washing and separation: performing solid-liquid separation on the product after aging in S22, and washing with water; S24, drying: placing the product after washing in S23 at 60-120℃ for drying for 6-24 hours; S25, impregnation: impregnating the solid product obtained after drying in S24 with a salt solution containing sulfate to introduce sulfate into the product; S26, calcination: placing the product after impregnation in S25 for drying, and then calcining at 300-500℃ for 2-8 hours to obtain the iron-based catalyst.

[0011] After the drying step, the method performs impregnation treatment on the catalyst with a salt solution containing sulfate such as sodium sulfate or potassium sulfate.

[0012] Preferably, the solvent in S11 is a mixed solution of concentrated sulfuric acid and water.

[0013] Preferably, in S21, the iron salt without sulfur element is ferric nitrate or ferric chloride, and the solvent is a mixed solution of concentrated nitric acid and water.

[0014] Preferably, in S25, the salt containing sulfate is at least one of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), or cesium (Cs) sulfate.

[0015] The third aspect of the present application further provides another preparation method of the iron-based catalyst of the first aspect, i.e., adding a copper salt to the iron salt solution in step S11 or S21 by using the preparation method of the iron-based catalyst of the second aspect; the copper salt is at least one of copper sulfate, copper nitrate, or copper chloride.

[0016] The fourth aspect of the present application further provides an application of the iron-based catalyst of the first aspect in catalyzing carbon dioxide hydrogenation to prepare low-carbon alcohols.

[0017] Preferably, the method for catalyzing carbon dioxide hydrogenation to prepare low-carbon alcohols is specifically: under catalytic reaction conditions, contacting the iron-based catalyst of any one of claims 1-3 with a raw gas containing CO2 and H2, and generating a product containing C 2+The product is alcohol; the catalytic reaction conditions are: reaction temperature 250-350 DEG C, reaction pressure 1.0-5.0 MPa, raw gas space velocity 2000-12000 mL h -1 , the volume ratio of H2 to CO2 in the raw gas is 1:1-5:1.

[0018] Compared with the prior art, the present application has the following advantages: The application discloses an iron-based catalyst for carbon dioxide hydrogenation to prepare low-carbon alcohol, which takes iron as a main active component, is supplemented with alkali metal and sulfur element as co-assistants, and is prepared by a coprecipitation method, namely, after mixing iron sulfate and alkali metal assistants, the mixture is subjected to aging, washing, drying and calcination steps to obtain the catalyst; or a post-impregnation method, namely, after the drying step, the catalyst is impregnated with a salt solution containing sulfate. 2+ The catalyst can efficiently catalyze the carbon dioxide hydrogenation reaction under high-temperature and high-pressure conditions, can realize 35.7% carbon dioxide single-pass conversion rate and 18.7% C 2+ Alcohol selectivity, and the proportion of C 2+ Alcohol in total alcohol is as high as 90% or more. The core advantage of the application lies in that the catalyst preparation process is simple, low in cost and easy to realize industrial amplification, and therefore, the application has wide application prospects in the field of carbon dioxide resource utilization.

[0019] Specifically, the application has the following advantages: (1) excellent catalytic performance: through the synergistic effect of Fe, alkali metal and sulfate, the application breaks through the limitation that traditional iron-based catalysts mainly generate hydrocarbons in the CO2 hydrogenation reaction, and realizes the selective synthesis of C 2+ Alcohol.

[0020] (2) clear assistant effect and flexible introduction method: the important role of sulfate as a key assistant in regulating product distribution is clarified, and two controllable introduction methods, namely, coprecipitation and post-impregnation, are provided to adapt to different preparation requirements.

[0021] (3) component adjustment: the type and content of alkali metal in the catalyst, whether Cu is added or not, can be adjusted according to requirements, so as to further optimize the catalytic performance and adapt to different process requirements.

[0022] (4) low cost: taking inexpensive and readily available iron as the main active component, the use of noble metal is avoided, the catalyst cost is greatly reduced, and the application has good industrial application prospects.

[0023] (5) simple preparation method: the classical precipitation method is adopted, the process is mature, and the production is easy to scale up. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1For the XRD patterns of Example 1 and Comparative Example 1, the phase composition is mainly iron oxide. DETAILED DESCRIPTION

[0025] The specific embodiments of the present application will be further described below. It should be noted that the description of these embodiments is used to help understand the present application, but does not constitute a limitation on the present application. In addition, the technical features involved in each of the embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0026] The experimental methods in the following examples are all conventional methods unless otherwise specified. The experimental materials used in the following examples are all commercially available unless otherwise specified.

[0027] Example 1: Preparation of FeNaS catalyst (1) About 5.0 g of Fe2(SO4)3xH2O powder (the actual amount of Fe2(SO4)3xH2O used is 22.85 g; at the same time, due to the hygroscopicity and instability of hydrated iron sulfate, the purchased iron sulfate reagent is marked as xH2O) was accurately weighed by Fe, dissolved in a mixed solution composed of 0.8 mL of 98wt% concentrated sulfuric acid and 150 mL of deionized water to obtain an iron salt solution. The solution was placed in a 500 mL three-necked flask, heated in a 60°C constant temperature water bath and continuously stirred. Then 1.5 mol / L NaOH solution was added at a constant speed under stirring until the pH value of the suspension was stabilized at about 10.0. After the addition was completed, the constant temperature aging was continued at 60°C and under stirring for 1 hour.

[0028] (2) After the reaction was completed, the slurry was centrifuged and washed with 800 mL of deionized water, and then dried in an 80°C oven for 12 hours.

[0029] (3) Finally, the dried sample was placed in a muffle furnace and calcined at 400°C in air for 2 hours to obtain the FeNaS catalyst. The mass fraction of the elements was analyzed by elemental analysis, which was: Fe 64.8%, Na 2.4%, S 0.6%. At the same time, as shown in the XRD pattern of Figure 1 , the phase composition of the obtained FeNaS catalyst is mainly iron oxide.

[0030] Example 2: Preparation of FeLiS catalyst The preparation process is the same as Example 1, only the NaOH precipitant is replaced by LiOH in equimolar amount, and finally the FeLiS catalyst is obtained. The content of the additive is analyzed by elemental analysis, which is: Li 0.3%, S 0.02%.

[0031] Example 3: Preparation of FeKS catalyst The preparation process is the same as that of Example 1, except that the NaOH precipitant is replaced by KOH in equimolar amount. The final FeKS catalyst is obtained. The content of the promoter is K 3.0% and S 0.6% respectively by elemental analysis.

[0032] Example 4: Preparation of im-FeNaS catalyst by late introduction of sulfate First, Fe(NO3)3·9H2O is used as the iron source, and concentrated nitric acid is used to replace concentrated sulfuric acid to create an acidic environment in the early stage of the reaction. NaOH is used as the precipitant. The precipitation, aging, washing and drying are carried out in the same way as steps (1) and (2) of Example 1 (the intermediate obtained after this step does not contain sulfur). Then, the above-mentioned intermediate is impregnated with 0.2 mol / L Na2SO4 aqueous solution by equal volume impregnation method, so that the theoretical sulfur content of the final catalyst is 0.6%. After impregnation, it is dried at 80℃ for 12h and calcined at 400℃ for 2h to obtain the im-FeNaS catalyst. The content of the promoter is Na 2.7% and S 0.5% respectively by elemental analysis.

[0033] Example 5: Preparation of Cu-modified FeNaS catalyst The preparation process is the same as that of Example 1, except that when preparing the iron salt solution, a certain amount of CuSO4·5H2O is added to make the atomic ratio of Fe / Cu 10. Then, the same method is used to carry out precipitation, aging, washing, drying and calcination to obtain the Cu-FeNaS catalyst. The mass fraction is Fe 58.3%, Cu 6.4%, Na 1.9%, and S 0.7% by elemental analysis.

[0034] Comparative Example 1: Sulfur-free catalyst The preparation process is the same as that of Example 1, except that the iron source Fe2(SO4)3·xH2O is replaced by Fe(NO3)3·9H2O, and concentrated nitric acid is used to replace concentrated sulfuric acid to create an acidic environment in the early stage of the reaction. Figure 1 As shown in the XRD pattern, the phase composition of the obtained catalyst is mainly iron oxide.

[0035] Comparative Example 2: Alkali-free catalyst The preparation process is the same as that of Example 1, except that the precipitant NaOH is replaced by ammonia.

[0036] Comparative Example 3: Pure iron catalyst without any promoter The preparation process is the same as that of Example 1, except that the iron source Fe2(SO4)3·xH2O is replaced by Fe(NO3)3·9H2O, and concentrated nitric acid is used to replace concentrated sulfuric acid to create an acidic environment in the early stage of the reaction. At the same time, the precipitant NaOH is replaced by ammonia.

[0037] Experimental Example: Performance test of catalyst The catalysts obtained in Examples 1-5 and Comparative Examples 1-3 were pressed into tablets, crushed, and 20-40 mesh particles were sieved. 0.5 g of the catalyst was loaded into a fixed bed reactor for a carbon dioxide hydrogenation reaction to prepare lower alcohols to evaluate its CO2 hydrogenation performance. After loading the catalyst, hydrogen was first passed to reduce it, and after reduction was completed, a mixed gas of carbon dioxide and hydrogen was switched to perform the reaction. The reduction conditions were: normal pressure, pure H2 atmosphere, reduction at 350°C for 8 hours. The reaction conditions were: reaction temperature 320°C, reaction pressure 3.0 MPa, molar ratio of raw material gas H2 / CO2 3:1, space velocity 8000 mL·g -1 -1 The reaction products were analyzed online by chromatography, and the carbon dioxide conversion rate and product selectivity were calculated by carbon molar balance, and the results are shown in Table 1.

[0038] As can be seen from Table 1, the various iron-based catalysts (Examples 1-4) provided by the present application all exhibit excellent CO2 hydrogenation activity and lower alcohol selectivity, and the C 2+ alcohol accounts for a dominant position in the total alcohol. Among them, the late introduction of sulfur additives can also achieve good catalytic performance; but the addition of Cu can achieve a carbon dioxide single-pass conversion rate of 35.7% and a C 2+ alcohol selectivity of 18.7%, and the distribution of C 2+ alcohol in the total alcohol is as high as more than 90%. This shows that the technical solution of the present application has good flexibility and reliability. In contrast, the C 2+ alcohol selectivity of Comparative Example 1 (sulfur-free catalyst) is significantly lower than that of Example 1, proving that the sulfur additive (sulfate) is a key and essential component for the present catalyst system to achieve the selective synthesis of lower alcohols. In addition, the replacement of alkali metals (Comparative Example 2) and the lack of additives (Comparative Example 3) will greatly reduce the catalytic performance, further proving that alkali metals and additives play an important role in the present catalyst system to achieve the selective synthesis of lower alcohols.

[0039] Table 1 Evaluation results of CO2 hydrogenation performance of various catalysts The above describes the embodiments of the present application in detail, but the present application is not limited to the described embodiments. For those skilled in the art, various changes, modifications, replacements and variations of these embodiments can be made without departing from the principles and spirits of the present application, and still fall within the protection scope of the present application.​

Claims

1. An iron-based catalyst for the hydrogenation of carbon dioxide to produce lower alcohols, characterized in that, The catalyst has iron as the main active metal component and contains alkali metal promoters and sulfur promoters, and its phase composition is mainly iron oxide; the alkali metal promoters are selected from at least one of lithium, sodium, potassium, rubidium or cesium, and the sulfur promoters exist in the form of sulfate ions; Based on the total mass of the catalyst, the content of iron is 30%-70% by elemental mass; the content of alkali metals is 0.3%-5%; and the content of sulfur is 0.02%-3%.

2. The iron-based catalyst for the hydrogenation of carbon dioxide to lower alcohols according to claim 1, characterized in that, The alkali metal auxiliary agent is sodium.

3. The iron-based catalyst for the hydrogenation of carbon dioxide to lower alcohols according to claim 1, characterized in that, The catalyst also contains copper as a second active metal component; based on the total mass of the catalyst, the copper content is 1%-20% by elemental mass.

4. The method for preparing the iron-based catalyst according to claim 1 or 2, characterized in that, Iron-based catalysts can be prepared by co-precipitation or by post-impregnation. The co-precipitation method for preparing iron-based catalysts includes the following steps: S11. Precipitation reaction: Iron sulfate is dissolved in a solvent to prepare an iron salt solution. The iron salt solution is then mixed with an alkali metal auxiliary agent at 60-90℃, and the pH of the reaction system is controlled to be 9-11 to carry out the precipitation reaction. S12. Aging: Aging the slurry obtained in S11 at 60-90℃ for 0.5-3 hours; S13. Washing and Separation: The product after S12 aging is subjected to solid-liquid separation and washed with water; S14. Drying: Place the product washed in S13 at 60-120℃ and dry for 6-24 hours; S15. Calcination: The solid product obtained in S14 is calcined at 300-500℃ for 2-8 hours to obtain the iron-based catalyst. The preparation of iron-based catalysts by the post-impregnation method includes the following steps: S21. Precipitation reaction: Iron salt without sulfur is dissolved in a solvent to prepare an iron salt solution. The iron salt solution is then mixed with an alkali metal auxiliary agent at 60-90℃, and the pH of the reaction system is controlled to be 9-11 to carry out the precipitation reaction. S22. Aging: The slurry obtained in S21 is aged at 60-90℃ for 0.5-3 hours; S23. Washing and separation: The product after S22 aging is subjected to solid-liquid separation and washed with water; S24. Drying: Place the product washed in S23 at 60-120℃ and dry for 6-24 hours; S25. Impregnation: The solid product obtained after drying S24 is impregnated with a salt solution containing sulfate to introduce sulfate ions into the product. S26. Calcination: After drying the product obtained by impregnation of S25, it is calcined at 300-500℃ for 2-8 hours to obtain the iron-based catalyst.

5. The method for preparing the iron-based catalyst according to claim 4, characterized in that, The solvent described in S11 is a mixed solution of concentrated sulfuric acid and water.

6. The method for preparing the iron-based catalyst according to claim 4, characterized in that, In S21, the sulfur-free iron salt is ferric nitrate or ferric chloride, and the solvent is a mixed solution of concentrated nitric acid and water.

7. The method for preparing the iron-based catalyst according to claim 4, characterized in that, In S25, the sulfate-containing salt is at least one of the sulfates corresponding to lithium, sodium, potassium, rubidium, or cesium.

8. The method for preparing the iron-based catalyst according to claim 3, characterized in that, Using the iron-based catalyst preparation method according to claim 4, copper salt is added to the iron salt solution in step S11 or S21; the copper salt is at least one of copper sulfate, copper nitrate or copper chloride.

9. The application of the iron-based catalyst according to any one of claims 1-3 in the catalytic hydrogenation of carbon dioxide to produce lower alcohols.

10. The application according to claim 9, characterized in that, The method for catalytic hydrogenation of carbon dioxide to prepare lower alcohols specifically involves contacting the iron-based catalyst described in any one of claims 1-3 with a feed gas containing CO2 and H2 under catalytic reaction conditions, resulting in the formation of a product containing C. 2+ The product of the alcohol; the catalytic reaction conditions are: reaction temperature 250-350 ℃, reaction pressure 1.0-5.0 MPa, and feed gas space velocity 2000-12000 mL·h. -1 The volume ratio of H2 to CO2 in the raw gas is 1:1 to 5:1.

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