Preparation method and application of cobalt-based catalyst for synthesizing low-carbon alcohol through hydrogenation of carbon dioxide

Abstract

The invention relates to a preparation method and application of a cobalt-based catalyst for synthesizing low-carbon alcohol through hydrogenation of carbon dioxide, and aims to solve the problems of low selectivity and low yield in preparation of low-carbon alcohol through hydrogenation of CO2 and to create a K-modified Mo-doped Co-based composite oxide precursor by adopting a sol-gel method in order to solve the problems of insufficient coupling capability and poor stability of an active phase Co2C. After the precursor is subjected to hydrogen reduction and CO2 + H2 activation, a K-modified Mo-doped Co2C catalyst can be obtained; due to the introduction of the K auxiliary agent, the hydrogenation capacity of the catalyst is weakened, the conversion of a Co2C active phase is inhibited, and the stability of Co2C in the CO2 hydrogenation process is improved; through Mo doping, a Mo-doped Co2C active component is formed, lattice distortion of Co2C is induced, the electron cloud density of Co2C is modulated, and a reaction energy barrier of CHxCO * formed by coupling of key intermediate species such as CHx * and CO * on Co2C is modulated, so that the carbon-carbon coupling capability is greatly improved, and the selectivity and yield of low-carbon alcohol are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of CO2 conversion, and specifically relates to the preparation of a cobalt-based catalyst and its application in the process of preparing low-carbon alcohols by hydrogenating carbon dioxide. Background Art

[0002] CO2 is one of the main components of greenhouse gases. In recent years, large-scale CO2 emissions have caused a serious greenhouse effect and triggered global climate change. "How to achieve the goal of "carbon neutrality" is a major challenge for the current development of the industry. Converting CO2 and renewable hydrogen into high-added products can not only effectively reduce CO2 emissions, but also reduce dependence on fossil fuels, helping to achieve the "carbon neutrality" goal. There are many types of CO2 hydrogenation products, among which low-carbon alcohols (C2OH-C4OH) have a high added value and can be used as fuel additives, detergents, etc. The generation of low-carbon alcohols by CO2 hydrogenation is a very attractive carbon neutrality process.

[0003] However, the process of CO2 hydrogenation to produce low-carbon alcohols has a complex reaction pathway and many competing reactions, resulting in low selectivity and low yield of low-carbon alcohols. How to regulate the reaction pathway and efficiently convert CO2 into low-carbon alcohols is a very challenging process. At present, CO2 hydrogenation to produce low-carbon alcohols can be carried out through the reverse water-gas shift reaction + Fischer-Tropsch synthesis (CO hydrogenation). After the reactant CO2 generates CO through the reverse water-gas shift reaction (CO2+H2→CO+H2O), the generated CO is further catalytically converted in a hydrogen atmosphere to produce low-carbon alcohols. In this process, CO can directly generate CH4 through dissociative adsorption hydrogenation. x * intermediates, or CO* intermediates can be generated on the active site through non-dissociative adsorption. The generated CH x * and CO* intermediates generate CH through carbon-carbon coupling at the active site. x CO* intermediates can be further hydrogenated to generate low-carbon alcohols. How to regulate the dissociative and non-dissociative adsorption capacity of CO and reduce the energy barrier of the carbon-carbon coupling process is extremely challenging. Excessive dissociative adsorption of CO will lead to a large amount of CH x Intermediate formation occurs. In this case, if the coupling ability is weak, methane will be generated, while if the coupling ability is strong, long-chain alkane products will be formed. The efficient production of low-carbon alcohols requires the dynamic coupling of the above three abilities.

[0004] The selection of catalysts for the production of low-carbon alcohols by hydrogenation of CO2 is particularly important. Co-based catalysts exhibit good activity in the hydrogenation of CO and CO2, and compared with precious metal Rh and Pd-based catalysts, Co-based catalysts are relatively low in cost, thus having good application prospects. In the CO2 hydrogenation process, the active components in Co-based catalysts exist in various forms, usually in the form of CoO, Co2C, and metallic Co, among which Co2C usually exhibits carbon-carbon coupling ability. However, when its coupling ability is insufficient, it cannot convert the generated CH x * and CO* intermediates are efficiently converted into precursors for the production of lower alcohols. Therefore, how to improve the coupling ability of Co2C is crucial for the production of lower alcohols by CO2 hydrogenation. In addition, the Co2C phase is unstable in the CO2 hydrogenation reaction atmosphere and is easily converted into other Co components. How to improve the stability of the Co2C phase is also an urgent problem to be solved.

[0005] To address the aforementioned issues with Co2C, the active component in Co-based catalysts, during CO2 hydrogenation to produce lower alcohols, the present invention co-modifies a Co-based catalyst with K and Mo. After hydrogen reduction and CO2+H2 pretreatment, a K-modified Mo-doped Co2C catalyst is obtained, wherein K is highly dispersed and Mo is doped into the Co2C lattice. In this K-modified Mo-doped Co2C catalyst, the introduction of the K promoter weakens the catalyst's hydrogenation capacity, inhibits the transformation of the Co2C active phase, and improves the stability of Co2C during CO2 hydrogenation. At the same time, the introduction of K also modulates the alkalinity of the catalyst surface, facilitating the adsorption and activation of CO2 molecules and promoting the conversion of CO2.

[0006] More importantly, the introduction of Mo components into the Co2C phase by doping induces lattice distortion of Co2C, changes the internal stress of the lattice, strengthens the electron transfer between Mo and Co2C, and modulates the electron cloud density of Co2C. Compared with Co2C without Mo doping, the carbon-carbon coupling ability is greatly improved, and the CH x * and CO* and other key intermediate species to form CH x The reaction energy barrier of CO* regulates its reaction kinetics and thermodynamic behavior, making it more conducive to the production of low-carbon alcohols and significantly improving the selectivity of low-carbon alcohols.

[0007] Therefore, the innovation of the catalyst in the composition and structure of the present invention improves the catalyst's performance in the process of preparing low-carbon alcohols by hydrogenation of carbon dioxide to the key intermediate CH xThe directional coupling ability of * and CO* improves the selectivity for low-carbon alcohols and enhances the stability of the active phase Co2C. Activity tests of the catalyst in carbon dioxide hydrogenation also show that the invented catalyst has excellent activity and selectivity for low-carbon alcohols. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to address the low selectivity and yield of CO2 hydrogenation to produce lower alcohols, as well as the insufficient coupling ability and poor stability of the active phase Co2C. By providing a novel catalyst with stable activity, strong coupling ability, and high selectivity for lower alcohols, the present invention uses Co as the active component and simultaneously introduces K and Mo components. A K-modified, Mo-doped Co-based composite oxide precursor is created using a sol-gel method. This precursor undergoes hydrogen reduction and CO2+H2 activation to produce a K-modified, Mo-doped Co2C catalyst. When the catalyst is used in the CO2 hydrogenation reaction to produce lower alcohols, at a reaction temperature of 320°C, the CO2 conversion exceeds 40% and the lower alcohol selectivity exceeds 10%. Activity testing and phase analysis of the catalyst confirm the catalyst's stable activity and that the Mo-doped Co2C exhibits superior coupling ability and stability compared to undoped Co2C.

[0009] In view of the poor coupling ability and stability of the Co2C active phase in the process of preparing low-carbon alcohols by hydrogenation of carbon dioxide, the present invention prepares a K-modified Mo-doped Co-based composite oxide precursor by a sol-gel method. The composition of the catalyst precursor is (K) a (Co) b (Mo) c The weight percentage composition is: a 0.1%-5.0%, b 65.0%-99.9%, and c 0.0%-30.0%. The preferred weight percentages of the catalyst precursor are: a 2.0%, b 88.0%, and c 10.0%. After H2 reduction and CO2+H2 pretreatment, a K-modified Mo-doped Co2C catalyst, named BJUT-KCM10, is obtained.

[0010] The specific preparation method steps are as follows:

[0011] 1) According to the catalyst (K) a (Co) b (Mo) c The weight percentages of the components are as follows: a is 0.1%-5.0%, b is 65.0%-99.9%, and c is 0.0%-30.0%. Cobalt nitrate, ammonium molybdate, and potassium carbonate are dissolved in deionized water to prepare mixed solution #1;

[0012] 2) Prepare a citric acid and ethylene glycol mixed solution #2 at a total molar ratio of citric acid:ethylene glycol:metal nitrate of 1:1:1. Mix this solution with solution #1 and heat in a water bath at 60°C until a gel forms. Dry at 105°C for 12 hours.

[0013] 3) The sample obtained in step 2) was ground into a powder state, and calcined at a heating rate of 10°C / min from room temperature to 400-700°C for 4h to obtain a K-modified Mo-doped Co-based composite oxide precursor, the crystal structure of which is shown in the attached figure. Figure 1 As shown in the XRD pattern;

[0014] 4) The sample obtained in step 2) was reduced in a hydrogen atmosphere at 400-700°C for 1-4 h, and then pretreated in a CO2+H2 mixed gas atmosphere (v / v=1:1-5) at 0.1-5 MPa and 200-400°C for 1-20 h to obtain the K-modified Mo-doped Co2C catalyst of the present invention, the crystal structure of which is shown in the attached figure. Figure 2 As shown in the XRD pattern of , compared with Co2C without Mo addition, the corresponding Co2C characteristic peak on the catalyst moves toward the high-angle direction, indicating that the Mo component enters the Co2C crystal in the form of doping, while the K species is highly dispersed on the catalyst;

[0015] 4) The catalyst is used for the carbon dioxide hydrogenation reaction via a high-pressure fixed-bed catalyst, with a mixed gas of CO2 / H2=1 / 1-5 by volume introduced, a reaction temperature of 200-400°C, a reaction pressure of 0.1-6 MPa, and a reaction space velocity of 1000-60000 mL / (g-catalyst·h).

[0016] Beneficial effects of the present invention:

[0017] 1) The existing Co2C phase suffers from poor stability in the CO2 hydrogenation reaction atmosphere and is easily converted to other Co components. In the catalyst constructed in this invention, the introduction of a K promoter weakens the catalyst's hydrogenation capacity, inhibits the transformation of the Co2C active phase, and improves the stability of Co2C during CO2 hydrogenation. Furthermore, the introduction of K modulates the alkalinity of the catalyst surface, facilitating the adsorption and activation of CO2 molecules and promoting CO2 conversion.

[0018] 2) The coupling ability of the Co2C component in the process of CO2 hydrogenation to prepare low-carbon alcohols is insufficient, which cannot effectively promote the key intermediate CH x * and CO* to generate CH xThe problem of CO* is solved by Mo doping in this catalyst to form Mo-doped Co2C active component, which induces lattice distortion of Co2C, changes the internal stress of the lattice, strengthens the electron transfer between Mo and Co2C, and adjusts the electron cloud density of Co2C. Compared with Co2C without Mo doping, its carbon-carbon coupling ability is greatly improved, and the CH on Co2C is adjusted. x * and CO* and other key intermediate species to form CH x The reaction energy barrier of CO* regulates its reaction kinetics and thermodynamic behavior, thus being more conducive to the production of low-carbon alcohols.

[0019] 3) The catalyst of the present invention exhibits excellent catalytic activity in the production of lower alcohols from CO2 hydrogenation. At 320°C, 3 MPa, and 6000 mL / (g-catalyst·h), the CO2 conversion rate exceeds 40% and the lower alcohol selectivity exceeds 10%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : XRD pattern of the catalyst precursor of the present invention

[0021] Figure 2 : XRD pattern of the catalyst of the present invention

[0022] Figure 3 : Effect diagram of the catalyst of the present invention DETAILED DESCRIPTION

[0023] Reference example one

[0024] Weigh 15.537g of Co(NO₃)₂·6H₂O and 1.7mL of 0.5mol / L K₂CO₃ solution and add 15mL of deionized water to prepare Solution #1. Weigh 10.411g of C₆H₂O₀·H₂O and 3.075g of (CH₂OH)₂ and add 15mL of deionized water to prepare Solution #2. Mix the two and stir in a 60°C water bath until a purple gel forms. Dry the mixture in a 105°C oven for 12 hours. Calcinate the mixture by heating from room temperature to 500°C at a rate of 10°C / min and holding for 4 hours to obtain the catalyst oxide precursor. The weight percentages of the components in this precursor are: K (2%) and Co (98%). The precursor was reduced at 400°C in a H2 flow at a flow rate of 30 mL / min for 1 h, and then continued to be treated with a gas with a volume ratio of CO2:H2=1:3 at 3 MPa and 400°C for 5 h to obtain a K-modified Mo2C catalyst, named BJUT-KCM0, in which the theoretical Mo loading was 0%.

[0025] The evaluation of the reaction activity of CO2 hydrogenation to produce low-carbon alcohols was carried out in a high-pressure continuous flow fixed-bed reactor. The catalyst was filled in a quartz reaction tube, which was then placed in a stainless steel reactor. The reaction gas with a volume ratio of CO2 / H2 / N2 = 24 / 72 / 4 was introduced, with nitrogen as the internal standard gas. The reaction conditions were 320°C, 3MPa, and a space velocity of 6000mL / (g-catalyst·h). The reaction products were analyzed online by gas chromatograph. The BJUT-KCM0 catalyst showed poor activity in the process of CO2 hydrogenation to produce low-carbon alcohols, with a CO2 conversion rate of less than 30%, a low-carbon alcohol selectivity of about 3%, and a C 2+ The selectivity of hydrocarbon products is 12%, and the remaining products are CO and CH4. This shows that the coupling ability of undoped Co2C is poor and it cannot effectively couple the low-carbon reaction intermediate CH4. x * and CO*, so the selectivity of low-carbon alcohols is low.

[0026] Example 1

[0027] Weigh 13.983gCo(NO3)2·6H2O and 0.490g(NH4)6Mo7O 24 ·4H2O and 1.7mL 0.5mol / LK2CO3 solution, add 25mL of deionized water to prepare solution #1; weigh 10.680gC6H8O7·H2O and 3.155g (CH2OH)2, add 25ml of deionized water to prepare solution #2; mix the two and stir in a constant temperature water bath at 60℃ until a purple gel is formed; then dry in a drying oven at 105℃ for 12 hours; heat from room temperature to 500℃ at a heating rate of 10℃ / min, calcine and maintain for 4h to obtain the oxide precursor of the catalyst. The weight percentage of each component in the precursor is: K is 2%, Mo is 10%, and Co is 88%. XRD ray diffraction experiment was carried out on the precursor, as shown in the attached figure. Figure 1 As shown, it was found that there was a strong signal peak of Co3O4, but no signal peak of K species was observed, indicating that the K species in the precursor was highly dispersed. In addition, compared with the catalyst precursor without Mo added, the diffraction peak corresponding to the catalyst precursor shifted to the right, indicating that the Mo species entered the Co3O4 lattice in the form of doping. The precursor was reduced at a temperature of 400°C and a flow of H2 at a flow rate of 30mL / min for 1h; then it was continued to be treated at 3MPa and 400°C with a gas volume ratio of CO2:H2=1:3 for 5h to obtain a K-modified Mo-doped Mo2C catalyst, which was named BJUT-KCM10, in which the theoretical loading of Mo is 10%. The catalyst was subjected to XRD ray diffraction experiments, as shown in the attached figure. Figure 2As shown in the figure, a highly crystalline Co2C phase was found, while the K species was still highly dispersed and the Mo species still entered the Co2C lattice in the form of doping, forming a Mo-doped Co2C active phase.

[0028] The BJUT-KCM10 catalyst was tested for its activity in the carbon dioxide hydrogenation reaction. Under the reaction conditions of 3MPa, space velocity of 6000mL / (g-catalyst·h), reaction temperature of 320℃, and feed volume ratio of CO2 / H2 / N2=24 / 72 / 4, the catalyst achieved a CO2 conversion rate of 44% with a corresponding selectivity of 13.7% for low-carbon alcohols after a reaction time of 6h. 2+ The selectivity of hydrocarbons is close to 30%, and the selectivity of C1 products CO and CH4 is greatly reduced compared with the Co2C catalyst without Mo doping, indicating that the introduction of Mo greatly improves the carbon-carbon coupling ability of Co2C, can greatly promote the coupling of key intermediates, and improve the selectivity of low-carbon alcohols; at the same time, C 2+ The increase in product selectivity also indirectly confirms the improved carbon-carbon coupling ability of the Mo-doped Co2C catalyst. At the same time, after the reaction, the main phase of the catalyst is still Mo-doped Co2C, and no phase transformation occurs to form phases such as CoO and metallic Co, indicating its excellent stability. As can be seen from the results, the catalyst of the present invention has the advantages of targeted improvement of the carbon-carbon coupling ability of the Co2C component, improved selectivity for lower alcohols, promoted the production of lower alcohols, and high stability in the carbon dioxide hydrogenation reaction.

[0029] Example 2

[0030] Weigh 13.206gCo(NO3)2·6H2O and 0.735g(NH4)6Mo7O 24 4H2O and 1.7mL of 0.5mol / L K2CO3 solution were added to 10mL of deionized water to prepare Solution #1. 10.411g of C6H8O7·H2O and 3.075g of (CH2OH)2 were weighed and added to 10mL of deionized water to prepare Solution #2. The two were mixed and stirred in a 60°C water bath until a purple gel formed. The mixture was then dried in a 105°C oven for 12 hours. The temperature was then increased from room temperature to 500°C at a rate of 10°C / min and calcined for 4 hours to obtain the catalyst oxide precursor. The weight percentages of the components in this precursor were: K (2%), Mo (15%), and Co (83%). The precursor was reduced at 400°C in a H2 flow at a flow rate of 30 mL / min for 1 hour; then, a gas with a volume ratio of CO2:H2 = 1:3 was introduced at 3 MPa and 400°C for 5 hours to obtain a K-modified Mo-doped Mo2C catalyst, named BJUT-KCM15, with a theoretical Mo loading of 15%.

[0031] The activity of the BJUT-KCM15 catalyst was investigated by the carbon dioxide hydrogenation reaction under the reaction conditions of 3 MPa, space velocity of 6000 mL / (g-catalyst·h), reaction temperature of 320 °C, feed volume ratio of CO2 / H 2 / N2=24 / 72 / 4. When the reaction time is 6h, the conversion rate of CO2 by the catalyst can reach about 34.4%, and the selectivity of low-carbon alcohols reaches 15%. 2+ The selectivity of hydrocarbon products exceeded 30%, further demonstrating that the Mo-doped Co2C active phase can significantly enhance the carbon-carbon coupling ability of the catalyst in CO2 hydrogenation and promote the production of low-carbon alcohols.

[0032] The activity test results show that the Mo-doped Co2C active phase formed by the catalyst of the present invention in the reaction of carbon dioxide hydrogenation to prepare low-carbon alcohols can greatly provide carbon-carbon coupling ability and promote the key intermediate CH x * and CO*, promoting the production of low-carbon alcohols; the CO2 conversion rate can exceed 40%, while the selectivity of low-carbon alcohols exceeds 10%. At the same time, the active phase Mo-doped Co2C remains stable before and after the reaction, and no obvious phase transition occurs, indicating that it has good catalytic activity and industrial application potential.

Claims

1. A method for preparing a cobalt-based catalyst for synthesizing lower alcohols by hydrogenation of carbon dioxide, characterized in that: The following steps are involved: Cobalt nitrate, ammonium molybdate, and potassium carbonate were dissolved in deionized water to prepare mixed solution #1. Citric acid and ethylene glycol mixed solution #2 was prepared at a total molar ratio of citric acid:ethylene glycol:metal nitrate of 1:1:

1. Solution #1 and #2 were mixed and stirred in a constant temperature water bath at 60°C until they were in a gel state. The mixture was dried at 105°C for 12 hours and then calcined at 400-700°C for 4 hours to obtain a K-modified Mo-doped Co-based composite oxide precursor having the composition (K) a (Co) b (Mo) c , according to weight percentage, the composition is: a is 0.1%-5.0%, b is 65.0%-99.9%, and c is 0.0%-30.0%; The obtained precursor is reduced in hydrogen at 400-700 degrees for 1-4 hours, and then treated with a mixed gas with a volume ratio of CO2:H2=1:1-5 at 0.1-5MPa and 200-400 degrees for 1-20 hours.

2. Use the catalyst obtained by the method as claimed in claim 1.

3. The catalyst according to claim 2 is used for the reaction of producing low-carbon alcohols by hydrogenation of CO2, characterized in that: The gas composition is CO2 / H2=1 / 1-5, the reaction temperature is 200-400°C, the reaction pressure is 0.1-6MPa, and the reaction space velocity is 1000-60000mL / (g-catalyst·h).

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