Cobalt-based catalyst for preparing ethanol through hydrogenation of carbon dioxide and preparation method of cobalt-based catalyst

Abstract

The invention aims to overcome the problems that the existing form of cobalt species in a cobalt-based catalyst is difficult to regulate and control and the selectivity of ethanol in a carbon dioxide hydrogenation reaction is low in the prior art, and provides a cobalt-based catalyst for preparing ethanol through carbon dioxide hydrogenation and a preparation method thereof. By regulating the range of the silica-alumina ratio of the Na-type molecular sieve to be 40-57 and adding a specific lysine ligand in an environment with water, the prepared cobalt-based catalyst realizes improvement of ethanol selectivity in a reaction for preparing ethanol through hydrogenation of carbon dioxide; the selectivity of ethanol in a carbon dioxide hydrogenation reaction on a fixed bed reactor under mild reaction conditions (250 DEG C, 2MPa) is improved by 30%. In the invention, by reducing the silica-alumina ratio of the carrier molecular sieve and regulating and controlling the type and adding sequence of the ligand, the metal-carrier interaction can be enhanced, the Co species are stabilized under the reaction conditions of high temperature and high pressure in the presence of hydrogen, so that the Co species are not easy to transform, and meanwhile, the dispersion of cobalt particles is promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalyst preparation, and relates to a preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol and a cobalt-based catalyst prepared by this method. Background Art

[0002] The excessive emission of carbon dioxide has caused non-negligible environmental problems such as global warming and ocean acidification worldwide. Direct catalytic conversion of carbon dioxide into high-value-added products is an ideal means of carbon dioxide emission reduction. Among all products, ethanol has a wide market demand and can be used as a clean fuel and a chemical synthesis intermediate. Therefore, the reaction of hydrogenating carbon dioxide to ethanol is a carbon dioxide conversion method that has received much attention. However, the chemical inertness of carbon dioxide molecules makes them difficult to be activated, and there are many reaction paths in the carbon dioxide hydrogenation reaction, resulting in an increase in the selectivity of non-target products. Therefore, catalyst activity and ethanol selectivity are still a huge challenge. However, the product distribution of the catalytic hydrogenation reaction can be adjusted by adjusting the structural characteristics of the catalyst. Therefore, it is of great significance to develop highly efficient catalysts with high catalytic performance.

[0003] At present, the catalysts for hydrogenating carbon dioxide to ethanol mainly include noble metal catalysts, cobalt-based catalysts, and copper-based catalysts. Noble metal catalysts can usually achieve relatively high ethanol selectivity, but their conversion rates are often not ideal, which will directly lead to the problem of low ethanol yield. In addition, considering the high cost and scarcity of noble metals, inexpensive non-noble metals seem to be a better choice for realizing the industrialization of carbon dioxide hydrogenation. Among non-noble metal catalysts, the carbon dioxide conversion rate of copper-based catalysts is generally higher than that of noble metal catalysts. However, copper-based catalysts need to add many additives to improve their selectivity, resulting in a complex preparation process, and the proportions of various additives often need to be accurately matched, which undoubtedly limits the application of copper-based catalysts. Another non-noble metal catalyst, cobalt-based catalyst, has excellent abilities of C-O bond dissociation activation and hydrogenation, and has a low price and rich reserves. Therefore, it has become a non-noble metal catalyst with great R & D potential. However, the too strong hydrogenation ability of cobalt-based catalysts will also cause methane to become the main by-product, thereby reducing the selectivity of ethanol.

[0004] Studies have shown that the existence form of Co species in cobalt-based catalysts plays an important role in inhibiting the methanation reaction and improving ethanol selectivity (Angew. Chem., Int. Ed. 2018, 57, 6104). By regulating the interaction strength between cobalt particles and the support in cobalt-based catalysts, the product distribution can be adjusted (J. Catal. 2020, 382, 86). Currently, there are various strategies to regulate the existence form of Co species, including constructing strong metal-support interactions, partial oxidation, and adjusting the reduction temperature. Compared with partial oxidation and adjusting the reduction temperature, constructing strong metal-support interactions is beneficial to stabilizing the active Co species under high-pressure, high-temperature, and hydrogen-containing conditions, and is also beneficial to the dispersion of cobalt particles, thereby improving the catalytic activity. However, how to effectively construct metal-support interactions to precisely regulate the existence form of Co species to improve the selectivity of ethanol in the carbon dioxide hydrogenation reaction has not been fully studied.

[0005] CN115445669A discloses a preparation method of a highly efficient catalyst for carbon dioxide hydrogenation. By acid-treating Y zeolite, dealumination forms silanol groups, reducing the acid density and increasing the acid strength while providing dispersion loading sites for metal components, improving the dispersion of metal components. The aluminum components falling off the zeolite framework combine with the added metal components to finally form a metal solid solution, enhancing the synergistic effect between metals and between metals and acidic sites, and improving the activity and stability of carbon dioxide hydrogenation. CN116550374A discloses a preparation method of a carbon dioxide hydrogenation catalyst, which is obtained by calcining a nano-metal oxide and a modified ZSM-5 zeolite after being treated with a silane coupling agent, wherein the silica-alumina ratio of the ZSM-5 zeolite is 80-300. CN114904569B discloses a water-vapor-tolerant methane combustion catalyst, which is obtained by stirring and mixing a zeolite with an acid solution to obtain a modified zeolite, and the silica-aluminum molar ratio in the modified zeolite is 21-100. Summary of the Invention

[0006] The object of the present invention is to overcome the problems in the prior art that it is difficult to regulate the existence form of Co species in cobalt-based catalysts and the selectivity of ethanol in the carbon dioxide hydrogenation reaction is low. A cobalt-based catalyst for carbon dioxide hydrogenation to ethanol and its preparation method are provided. By regulating the silica-aluminum ratio of Na-type zeolite, using a specific ligand, and modulating the ligand addition sequence, the control of the interaction strength between Co species and the support is realized, and then the existence form of Co species is regulated, thereby significantly improving the selectivity of ethanol in the carbon dioxide hydrogenation reaction, which is beneficial to the industrialization of the carbon dioxide hydrogenation to ethanol reaction.

[0007] To achieve the above object, in the first aspect of the present invention, a preparation method of a cobalt-based catalyst for carbon dioxide hydrogenation to ethanol is provided, which includes the following specific steps:

[0008] (1) dissolving cobalt nitrate hexahydrate in deionized water to prepare an impregnation solution;

[0009] (2) adding the impregnation liquid dropwise to the Na-type molecular sieve carrier having a silicon-aluminum ratio of not less than 30 to impregnate the carrier in equal volumes, stirring the mixture to form an ice cream-like state, and allowing it to stand;

[0010] (3) grinding the mixture and lysine ligand powder thoroughly;

[0011] (4) drying the ground mixture;

[0012] (5) calcining the dried mixture to obtain the cobalt-based catalyst for hydrogenating carbon dioxide to ethanol.

[0013] The second aspect of the present invention provides a cobalt-based catalyst for producing ethanol by hydrogenating carbon dioxide obtained by the above method.

[0014] The third aspect of the present invention provides the use of the above-mentioned cobalt-based catalyst for producing ethanol by hydrogenating carbon dioxide in the reaction of producing ethanol by hydrogenating carbon dioxide.

[0015] Through the above technical scheme, the cobalt-based catalyst for hydrogenating carbon dioxide to ethanol, its preparation method and application have the following beneficial effects:

[0016] (1) The ligand-assisted impregnation method used in the present invention regulates the interaction between metal carriers, is simple to operate, reduces energy consumption, and does not require the addition of metal additives.

[0017] (2) The reduction of the silicon-aluminum ratio of the carrier molecular sieve is conducive to the enhancement of the metal-carrier interaction, which helps to stabilize the Co species under high temperature and high pressure hydrogen reaction conditions, making it less likely to undergo transformation.

[0018] (3) Reducing the Si / Al ratio of the carrier molecular sieve can not only regulate the existence form of Co species but also facilitate the dispersion of cobalt particles.

[0019] (4) The strength of the metal-support interaction can be easily controlled by the type of ligand and the order of addition.

[0020] (5) The cobalt-based catalyst prepared by the present invention achieves improved ethanol selectivity and ethanol yield in the carbon dioxide hydrogenation reaction. By regulating the existence form of the Co species and the strength of the interaction between the metal and the carrier, the selectivity of ethanol in the carbon dioxide hydrogenation reaction under mild reaction conditions (250°C, 2MPa) in a fixed bed reactor reached more than 33%. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1It is the hydrogen temperature-programmed reduction curve of each catalyst involved in the present invention (S-1 is a pure silicon molecular sieve with an MFI topological structure, ZSM-5 is a silicon-aluminum molecular sieve with an MFI structure, and the suffix number is the framework silicon-aluminum ratio).

[0022] Figure 2 It is the CO adsorption infrared of each reduced catalyst involved in the present invention.

[0023] Figure 3 is the content of Co species in each reduced catalyst involved in the present invention. DETAILED DESCRIPTION

[0024] In the prior art, the Co species obtained by changing the reduction conditions or partial oxidation methods are unstable and easily change under high temperature, high pressure and hydrogen conditions. Moreover, changing the reduction conditions or partial oxidation is usually carried out at a higher temperature, which is easy to cause metal agglomeration and is not conducive to metal dispersion. The methods reported so far to control the existence form of Co species by constructing metal-support interactions are only rough controls of the existence form of Co species (for example, controlling the Co species to CoO x Status or Co 0 and CoO x In addition, the relationship between the existence form of Co species and ethanol selectivity is not fully understood.

[0025] To this end, in the present invention, the inventors specifically proposed for the first time a method for preparing a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol, by controlling the silicon-aluminum ratio of the carrier molecular sieve (i.e., the aluminum content in the molecular sieve) to regulate the metal-carrier interaction. The stronger the interaction, the more difficult it is for the Co species to be completely reduced, resulting in different forms of the Co species after reduction. Different forms of the Co species lead to different catalytic properties. Therefore, by accurately regulating the silicon-aluminum ratio of the carrier molecular sieve, the selectivity of ethanol in the carbon dioxide hydrogenation reaction can be finally achieved.

[0026] The first aspect of the present invention provides a method for preparing a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol, comprising the following specific steps:

[0027] (1) dissolving cobalt nitrate hexahydrate in deionized water to prepare an impregnation solution;

[0028] (2) adding the impregnation liquid dropwise to the Na-type molecular sieve carrier with a silicon-aluminum ratio ranging from ∞ to 30 for equal volume impregnation, stirring the mixture to form an ice cream shape, and letting it stand;

[0029] (3) grinding the mixture and lysine ligand powder thoroughly;

[0030] (4) drying the ground mixture;

[0031] (5) Calcinate the dried mixture to obtain the cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol.

[0032] Under the condition of the molecular sieve with the specific silicon-aluminum ratio mentioned above, the regulation of the metal-support interaction is realized. The decrease of the silicon-aluminum ratio of the support molecular sieve is beneficial to the enhancement of the metal-support interaction, which helps to stabilize the Co species under the reaction conditions of high temperature, high pressure and hydrogen, making it difficult to be completely reduced, and realizing the regulation of the existence form of Co species. Different existence forms of Co species lead to different catalytic performances. Therefore, the silicon-aluminum ratio of the molecular sieve can realize the regulation of the ethanol selectivity in the hydrogenation reaction of carbon dioxide.

[0033] The Na-type molecular sieve described in the present invention is ZSM-5 molecular sieves with different silicon-aluminum ratios synthesized by conventional steps, and the silicon-aluminum ratio range of the ZSM-5 molecular sieve support is regulated to be 40-57.

[0034] In the present invention, the preferred silicon-aluminum ratio is 50. The inventors have studied and experimentally confirmed that reducing the silicon-aluminum ratio of the support molecular sieve can enhance the metal-support interaction. As the silicon-aluminum ratio of the molecular sieve decreases from ∞ to 40, the cobalt-based catalyst gradually has more CoO x species, and the ratio of CoO x species to Co 0 species increases. The selectivity of ethanol shows a trend of first increasing and then decreasing. When the silicon-aluminum ratio of the molecular sieve is 50, the ratio of CoO x species to Co 0 species is most suitable for the production of ethanol.

[0035] The silicon-aluminum ratio described in the present invention refers to the molar ratio of silicon to aluminum elements in the molecular sieve. The molecular sieve with a silicon-aluminum ratio of ∞ described in the present invention is a pure silicon molecular sieve, also known as S-1.

[0036] Furthermore, in the present invention, on the basis of the traditional impregnation method, a ligand is added to assist in the preparation of the cobalt-based catalyst, thereby realizing the regulation of the ethanol selectivity. The inventors have studied and found that there are various functional groups on the organic ligand, which can interact with the support while forming a complex with the metal. Different ligand functional groups are different, and the steric hindrance of coordination with the metal is also different. In the presence and absence of water, the coordination mode will also be different. Therefore, by adjusting the type of ligand and the addition timing of the ligand, the simple regulation of the interaction between the metal and the support is realized, and the selectivity of ethanol in the hydrogenation reaction of carbon dioxide is improved.

[0037] According to the present invention, the ligand is an organic ligand, and the organic ligand is lysine.

[0038] According to the present invention, adding an organic ligand in an aqueous environment and performing a grinding operation helps to enhance the metal-support interaction, and ultimately improves the ethanol selectivity. And the organic ligand is removed in the subsequent calcination stage and does not affect the reaction result itself.

[0039] In the present invention, by mass percentage, in the above preparation method, the dosage ratio of cobalt nitrate hexahydrate, molecular sieve, and organic ligand is (10 - 20):(40 - 50):(5 - 20). Preferably, 0.52 - 1.04 g of cobalt nitrate hexahydrate, 2 - 2.5 g of molecular sieve, and 0.65 - 0.75 g of lysine.

[0040] In the present invention, the static temperature range in step (2) is 10 - 30 °C, and the static time range is 0 - 48 h.

[0041] In the present invention, in step (3), the grinding method is manual grinding or ball mill grinding, and the grinding time is 10 - 30 min.

[0042] In the present invention, in step (4), the drying temperature is 110 °C and the drying time is 2 h.

[0043] In the present invention, in step (5), the calcination is carried out in an air atmosphere, the calcination temperature is 550 °C, and the calcination time is 4 - 6 h.

[0044] In the present invention, on the basis of screening a support molecular sieve with a suitable silicon-aluminum ratio, adjusting the type of ligand and the addition timing of the ligand, the two means cooperate synergistically and are used in combination to achieve more precise control of the metal-support interaction, further improving the ethanol selectivity, up to 33%, and the highest is 34.5%.

[0045] The second aspect of the present invention provides a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol prepared by the above method. The cobalt-based catalyst obtained after calcination in the above step (5) only contains cobalt tetroxide and a molecular sieve support, wherein cobalt tetroxide accounts for 5% - 20% of the catalyst mass fraction; the cobalt-based catalyst is loaded on a micro fixed-bed reactor and reduced with pure H2 at 300 °C and atmospheric pressure for 2 h to obtain a reduced catalyst. In the reduced catalyst, cobalt only exists in two forms, Co and CoO, and on a molecular sieve support with a silicon-aluminum ratio of 57 - 40, the CoO / Co 0 molar ratio is 0.77 - 1.12, and on a molecular sieve support with a silicon-aluminum ratio of 50, the CoO / Co 0 molar ratio is 1.06. Co 0 represents the elemental Co form.

[0046] The third aspect of the present invention provides the application of the above cobalt-based catalyst in the reaction of hydrogenating carbon dioxide to ethanol. The cobalt-based catalyst is loaded on a micro fixed-bed reactor. First, it is reduced with pure H2 at 300 °C and atmospheric pressure for 2 h, and then a mixed gas with a volume ratio of CO2:H2 = 3:1 is continuously fed under the conditions of 250 °C and 2 MPa for the hydrogenation reaction of carbon dioxide. Finally, the selectivity of ethanol is higher than 33%.

[0047] The present invention will be described in detail below with reference to specific examples. However, they should not be construed as limiting the scope of protection of the present invention. The raw materials used in the following examples and comparative examples are all commercially available products.

[0048] The carbon dioxide conversion rate is calculated by the formula, and the specific calculation formula is as follows:

[0049]

[0050] The ethanol selectivity is calculated by the formula, and the specific calculation formula is as follows:

[0051]

[0052] Example 1

[0053] A preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol includes the following specific steps:

[0054] (1) Dissolve 0.52 g of cobalt nitrate hexahydrate in an appropriate amount of deionized water to prepare an impregnation solution;

[0055] (2) Dropwise add the impregnation solution to 2 g of S-1 molecular sieve support. After stirring, the mixture becomes ice cream-like and is left standing at room temperature for 24 h;

[0056] (3) Dry the mixture at 110 °C for 2 h;

[0057] (5) Calcinate the dried mixture at 550 °C for 4 - 6 h to obtain the cobalt-based catalyst.

[0058] Example 2

[0059] Prepare a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to the method of Example 1, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 97.

[0060] Example 3

[0061] Prepare a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to the method of Example 1, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 57.

[0062] Example 4

[0063] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 1, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 50.

[0064] Example 5

[0065] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 1, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 40.

[0066] Example 6

[0067] A preparation method of a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol, comprising the following specific steps:

[0068] (1) Dissolve 0.52 g of cobalt nitrate hexahydrate in an appropriate amount of deionized water to prepare an impregnation solution;

[0069] (2) Dropwise add the impregnation solution to 2 g of S-1 molecular sieve support. After stirring, the mixture becomes ice cream-like and is left standing at room temperature for 24 h;

[0070] (3) Grind the mixture thoroughly with 0.65 g of lysine powder;

[0071] (4) Dry the ground mixture at 110 °C for 2 h;

[0072] (5) Calcinate the dried mixture at 550 °C for 4 - 6 h to obtain the cobalt-based catalyst.

[0073] Example 7

[0074] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 6. The difference is that 0.27 g of glycine is used to replace 0.65 g of lysine.

[0075] Example 8

[0076] A preparation method of a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol, comprising the following specific steps:

[0077] (1) Dissolve 0.52 g of cobalt nitrate hexahydrate in an appropriate amount of deionized water to prepare an impregnation solution;

[0078] (2) Dropwise add the impregnation solution to 2 g of S-1 molecular sieve support. After stirring, the mixture becomes ice cream-like and is left standing at room temperature for 24 h;

[0079] (3) Dry the mixture at 110 °C for 2 h;

[0080] (4) Grind the dried mixture thoroughly with 0.65 g of lysine powder;

[0081] (5) Calcinate the ground mixture at 550 °C for 4 - 6 h to obtain a cobalt-based catalyst.

[0082] Example 9

[0083] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 8, except that 0.27 g of glycine is used to replace 0.65 g of lysine.

[0084] Example 10

[0085] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 6, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 57.

[0086] Example 11

[0087] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 6, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 50.

[0088] Example 12

[0089] Prepare a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to the method of Example 6, except that the S-1 molecular sieve support used in step (2) is replaced with a ZSM-5 molecular sieve with a silica-alumina ratio of 40.

[0090] Test Example: Catalyst Structure and Performance Evaluation

[0091] 1. Perform hydrogen temperature-programmed reduction characterization on the cobalt-based catalysts prepared in Example 1 and Examples 6 - 9, as Figure 1 shown.

[0092] As Figure 1 can be seen, all the catalyst samples mainly have two reduction peaks, corresponding to Co3O4 → CoO and CoO → Co respectively 0The strength of the metal-support interaction can be analyzed by the shift of the reduction peak position. Using glycine as the ligand (Examples 7 and 9), compared with the cobalt-based catalyst prepared by the traditional impregnation method (Example 1), whether it is ground before drying (Example 7) or after drying (Example 9), the temperature shift of the metal reduction peak is not significant. Using lysine as the ligand and grinding after drying (Example 8), the temperature of the reduction peak also shows no obvious shift, while using lysine to grind before drying (Example 6) will cause the reduction peak to move significantly to a higher temperature, indicating an enhanced metal-support interaction. The above analysis shows that using lysine ligand before drying (Example 6) will enhance the metal-support interaction. In the method of the present invention, cobalt-based catalysts with different metal-support interactions can be prepared by different ligand types and ligand addition sequences.

[0093] 2. Perform in-situ carbon monoxide adsorption diffuse reflectance infrared spectroscopy characterization ( Figure 2 ) and in-situ X-ray photoelectron spectroscopy analysis ( Figure 3 ) on the cobalt-based catalysts prepared in Examples 1 to 5.

[0094] As Figure 2 can be seen, the two absorption peaks at 1895 cm -1 and 1963 cm -1 correspond to the CO bridged adsorption of larger particles of Co 0 and Co δ+ respectively. The absorption peak at 2022 cm -1 is the linear adsorption of CO on smaller Co 0 . The absorption peak at about 2137 cm -1 is the absorption peak of gaseous CO, while the absorption peak at 2163 cm -1 is the adsorption of CO by smaller-sized Co δ+ . The little change in the absorption peak intensities at 1895 cm -1 and 2022 cm -1 indicates that the change in the number of Co 0 species is not obvious. It should be noted that as the silica-alumina ratio decreases, the intensity of the adsorption peak of Co -1 for CO at 2163 cm δ+ gradually increases, indicating that the content of Co δ+ gradually increases. Combining the analysis of H2-TPR, as the Al content in the support increases, the temperature required for the complete reduction of Co3O4 gradually increases, and at the same reduction temperature, it shows that the ratio of Co δ+ to Co 0 gradually increases. That is to say, as the silica-alumina ratio of the molecular sieve support framework decreases from ∞ to 40 (from Example 1 to Example 5), the amount of CoO species in the cobalt-based catalyst gradually increases, and the ratio of CoO species to Co 0The proportion of the species gradually increases. The results of CO adsorption infrared analysis indicate that changing the silica-alumina ratio can achieve precise regulation of the existence form of Co species.

[0095] It can be seen from Figure 3 that after reduction, all the catalyst samples showed signal peaks attributed to Co 0 and Co 2+ at 778.2 and 780.4 eV, indicating that both CoO and Co 0 species coexist on all the catalysts. By fitting the two peaks and determining the relative proportion of the two through the peak area, as the silica-alumina ratio of the molecular sieve framework decreases, the metal-support interaction increases, and the metal oxide is more difficult to be reduced. From Example 1 to Example 5, the CoO / Co 0 proportion gradually increases from 0.54 to 1.12, indicating that the regulation of the proportion of CoO species and Co 0 species is achieved by using supports with different silica-alumina ratios.

[0096] 3. The cobalt-based catalysts prepared in Examples 1-9 were used for the reaction of hydrogenation of carbon dioxide to ethanol. The catalysts were loaded on a micro fixed-bed reactor. First, they were reduced with pure H2 at 300 °C and atmospheric pressure for 2 h, and then reacted continuously with a mixed gas of CO2:H2 = 3:1 (volume ratio) fed under the conditions of 250 °C and 2 MPa.

[0097] Table 1 Catalytic performance evaluation of catalysts prepared by ligand-assisted impregnation method

[0098]

[0099] In the table, dry represents that the ligand is added to the mixture, dried and then ground with the ligand, and wet represents that the ligand is added to the mixture, ground and then dried; gas and las represent glycine and lysine respectively; "X" in ZSM-5-X represents the silica-alumina ratio.

[0100] By comparing Examples 1-5 in Table 1, as the silica-alumina ratio of the support decreases from ∞ to 50, the selectivity of ethanol in the product gradually increases. When the silica-alumina ratio continues to decrease to 40, the selectivity of alcohols decreases slightly again. This indicates that the ratio of CoO to Co 0 is a key factor affecting the catalyst performance. Regulating the silica-alumina ratio of the molecular sieve support is an effective means to regulate the existence form of the ratio of CoO to Co 0 species in the cobalt-based catalyst, and can further achieve the regulation of ethanol selectivity, so as to achieve the best catalytic effect. As the silica-alumina ratio of the molecular sieve decreases from ∞ to 40, the cobalt-based catalyst gradually has more CoO species, the ratio of CoO species to Co 0 species increases, and the selectivity of ethanol shows a trend of first increasing and then decreasing. When the silica-alumina ratio of the molecular sieve is 50, the ratio of CoO species to Co0 The proportion of the species is most suitable for ethanol production, and the ethanol selectivity is 16.3% at this time.

[0101] As can be seen from Table 1, by comparing the catalytic performances of Examples 1, 6-9, the ligand-assisted method can regulate the selectivity of ethanol. The ethanol selectivity is closely related to the interaction between the metal and the support. Adding ligands during the impregnation process is beneficial to improving the ethanol selectivity. Among them, reacting lysine (Example 6) with the metal precursor before drying is beneficial to significantly improving the metal-support interaction and thus significantly improving the ethanol selectivity. At this time, the ethanol selectivity reaches 32.7%.

[0102] As can be seen from Examples 10-12 in Table 1, by combining the two methods of jointly regulating the silica-alumina ratio of the molecular sieve support and adding ligand assistance, more precise regulation of the interaction between the metal and the support can be achieved, further increasing the ethanol selectivity to be higher than 33%, up to 34.5%.

Claims

1. A preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol, characterized in that, It includes the following specific steps: (1) Dissolve cobalt nitrate hexahydrate in deionized water to prepare an impregnation solution; (2) Dropwise add the impregnation solution to a Na-type molecular sieve support with a silica-alumina ratio in the range of ∞ - 30 for isovolumetric impregnation. After stirring, the mixture becomes ice-cream-like and is allowed to stand; (3) Thoroughly grind the mixture with lysine ligand powder; (4) Dry the ground mixture; (5) Calcinate the dried mixture to obtain the cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol.

2. The preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to claim 1, characterized in that, The Na-type molecular sieve is ZSM-5 molecular sieve, and the silica-alumina ratio of the ZSM-5 molecular sieve support is adjusted to be in the range of 40 - 57.

3. The preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to claim 2, characterized in that, The silica-alumina ratio of the ZSM-5 molecular sieve is 50.

4. The preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to claim 1, characterized in that, By mass percentage, the dosage ratio of cobalt nitrate hexahydrate, Na-type molecular sieve, and lysine is (10 - 20):(40 - 50):(5 - 20).

5. The preparation method of a cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to claim 4, characterized in that, The amount of cobalt nitrate hexahydrate is 0.52 - 1.04 g, the amount of Na-type molecular sieve is 2 - 2.5 g, and the amount of lysine is 0.65 - 0.75 g.

6. The preparation method of a cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol according to claim 1, characterized in that, The standing temperature range in step (2) is 10 - 30 °C, and the standing time range is 0 - 48 h; In step (3), the grinding method is manual grinding or ball mill grinding, and the grinding time is 10 - 30 min; In step (4), the drying temperature is 110 °C and the drying time is 2 h; In step (5), the calcination is carried out in an air atmosphere, the calcination temperature is 550 °C, and the calcination time is 4 - 6 h.

7. A cobalt-based catalyst for hydrogenating carbon dioxide to ethanol, characterized in that, Prepared by the method according to any one of claims 1 - 6, the cobalt-based catalyst only contains cobalt tetroxide and the molecular sieve support, wherein cobalt tetroxide accounts for 5% - 20% of the catalyst mass fraction.

8. According to the cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol as claimed in claim 7, load the cobalt-based catalyst on a micro fixed-bed reactor. First, reduce it with pure H2 at 300 °C and atmospheric pressure for 2 h to obtain a reduced catalyst. Then, continuously feed a mixture with a volume ratio of CO2:H2 = 3:1 at 250 °C and 2 MPa for the hydrogenation reaction of carbon dioxide, and finally the selectivity of ethanol is higher than 33%.

9. The cobalt-based catalyst for hydrogenating carbon dioxide to ethanol according to claim 8, wherein only two forms of Co and CoO exist for cobalt in the reduced catalyst, on a molecular sieve support with a silica-alumina ratio of 57-40, the CoO / Co 0 molar ratio is 0.77-1.12, and on a molecular sieve support with a silica-alumina ratio of 50, the CoO / Co 0 molar ratio is 1.

06.

10. Application of the cobalt-based catalyst for hydrogenation of carbon dioxide to ethanol prepared by the method according to any one of claims 1 - 6 in the reaction of hydrogenation of carbon dioxide to ethanol.

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