A cobalt carbide composite catalyst for the one-step preparation of light olefins and a preparation method thereof
The preparation of cobalt carbide catalysts by one-step method solved the problems of high CO2 selectivity and product distribution in the process of Fischer-Tropsch synthesis of olefins, and achieved the effects of high olefin selectivity and low CO2 selectivity, simplifying the preparation process and reducing costs.
Patent Information
- Application Number
- CN202310427083.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-04-20
- Publication Date
- 2025-07-25
- Estimated Expiration
- 2043-04-20
AI Technical Summary
The prior art has problems such as high CO2 selectivity and difficult product distribution in the process of Fischer-Tropsch synthesis and olefin production. The traditional cobalt carbide catalyst preparation method leads to uneven dispersion of active components and poor reaction performance.
A one-step method is used to prepare a cobalt carbide catalyst. By mixing cobalt salt, metal salt and electronic additives with sugars and urea, forming a molten liquid, high-temperature heat treatment and calcining, forming uniformly dispersed nanoparticles, and obtaining a highly activated cobalt carbide catalyst.
A catalyst with high olefin selectivity, low CO2 selectivity and high stability is achieved, simplifying the preparation process and reducing the cost of raw materials.
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Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a cobalt carbide composite catalyst for the one-step preparation of light olefins and a preparation method thereof. Background Art
[0002] Light olefins refer to olefins with a carbon number less than 5 such as ethylene and propylene, which are important basic raw materials in the chemical industry. In China, the energy resources are rich in coal and scarce in oil. Therefore, the non-petroleum route is often adopted, that is, syngas is produced from carbon-containing resources such as coal and biomass and then directly or indirectly converted to olefins by Fischer-Tropsch synthesis (FTO). The indirect production of light olefins from syngas mainly goes through the methanol conversion route. CuZnAl is used as a catalyst to convert syngas into methanol, and then zeolite is used as a catalyst for the methanol-to-olefins reaction. However, the traditional indirect preparation technology of light olefins has problems such as high energy consumption of production equipment and low economic benefits; the direct one-step process for converting syngas to olefins has a shorter process path and lower energy consumption, so it has received extensive research.
[0003] Currently, researchers tend to prepare bifunctional catalysts with two active components, that is, by combining methanol synthesis and olefin production in series to simplify the process. The reaction activity of converting syngas to olefins at low temperature is extremely low. Therefore, such bifunctional catalysts usually require high pressure and high temperature to improve the activity of the catalyst. A few years ago, the Bao Xinhe team reported the reaction of directly converting syngas to light olefins by OX-ZEO. CO activation occurs on the surface of the metal oxide catalyst, and then the C-C bond coupling reaction occurs at the acidic sites of the zeolite to generate olefins. However, when scaled up to commercial production, problems such as high reaction temperature and low CO conversion rate will bring huge difficulties to the engineering aspect.
[0004] Directly converting syngas to light olefins through the Fischer-Tropsch synthesis (FTS) reaction without any intermediate steps is called the Fischer-Tropsch to olefins (FTO) process. The traditional FTO catalysts mainly focus on two types: Fe-based and Co-based. Compared with Fe-based catalysts, Co-based catalysts have higher n-alkene selectivity and lower water-gas shift activity in FTO. In the Fischer-Tropsch synthesis reaction, the adsorbed H2* and CO* species react on the Co surface to form surface CHx species, and further carbon-carbon coupling occurs, and olefins and alkanes are formed through dehydrogenation and hydrogenation reactions. By adding an appropriate amount of alkali metal promoter, the adsorption ability of CO can be enhanced, the adsorption of H can be reduced, thereby increasing the chain growth factor, enhancing the desorption of olefins, and reducing the selectivity of methane.
[0005] At present, while obtaining high-selectivity olefins by FTO, there are problems such as high CO2 selectivity and difficult control of product distribution. The traditional preparation methods of cobalt carbide catalysts include the impregnation method and the co-precipitation method. For the catalysts prepared by the impregnation method, the active components cannot be very evenly dispersed on the carrier, resulting in poor reaction performance, often showing high CO2 and CH4 selectivity. For the composite oxide nanoparticles prepared by the co-precipitation method, large agglomerates are easily formed, which is not conducive to reduction carburization, showing high CO2 selectivity. Summary of the Invention
[0006] The main purpose of the present invention is to provide a cobalt carbide catalyst for the one-step preparation of light olefins and its preparation method, aiming at the problems and deficiencies existing in the prior art. This cobalt carbide catalyst exhibits advantages such as high olefin selectivity, low CO2 selectivity, and high stability in Fischer-Tropsch synthesis to olefins; and the involved preparation method is relatively simple, with low raw material costs, and is more suitable for popularization and application.
[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:
[0008] A preparation method of a cobalt carbide composite catalyst for the one-step preparation of light olefins, comprising the following steps:
[0009] 1) Mix sugars and urea in proportion, heat and stir to obtain a molten liquid, and then add a cobalt salt, a metal salt solution, and an electronic additive thereto, and perform secondary heating and stirring to obtain a molten mixed liquid;
[0010] 2) Keep the obtained molten mixed liquid at a constant temperature for heating to perform dehydration carbonization to obtain a black-brown highly porous solid;
[0011] 3) Under the protection of an inert gas, perform high-temperature heat treatment on the obtained black-brown highly porous solid to obtain black precursor solid particles;
[0012] 4) Roast the obtained black precursor solid particles at a high temperature in an air atmosphere to obtain a composite metal oxide catalyst;
[0013] 5) Perform reduction and carbonization treatment on the obtained composite metal oxide catalyst to obtain the cobalt carbide composite catalyst.
[0014] In the above solution, the cobalt salt can be selected from one or several of nitrates, halide salts, carbonates, sulfates, etc.
[0015] In the above solution, the metal element M introduced by the metal salt is one or several of Mg, Ca, Zn, Al, Zr, Mn, La, Ce, etc.
[0016] In the above solution, the metal salt is one or several of nitrates, halide salts, organic acid salts, sulfates.
[0017] In the above solution, the saccharide is one or more of glucose, fructose, sucrose, maltose, lactose, starch, dextrin, etc.
[0018] In the above solution, the electronic assistant can be selected from one or more of sodium carbonate, sodium nitrate, sodium sulfate, etc.
[0019] In the above solution, the mass ratio of the cobalt salt, urea, and saccharide is 1:(1.5 - 5):(1.5 - 7.5).
[0020] In the above solution, the molar ratio of the metal oxide salt to the cobalt salt is (0.1 - 1):1.
[0021] In the above solution, the dosage of the electronic assistant is 0.1 - 1% of the total mass of the cobalt salt and the metal oxide salt.
[0022] Preferably, controlling the heating and stirring temperature of the oil bath at 100 - 120 °C when forming the molten liquid can effectively dissolve glucose and urea.
[0023] In the above solution, the stirring speed is controlled at 500 - 700 rpm, and the molten stirring time is 0.5 - 1 h (rapid melting).
[0024] In the above solution, the secondary heating and stirring temperature adopted in step 1) is 100 - 120 °C, and the time is 4.5 - 6 h.
[0025] In the above solution, the constant temperature heating step includes, preferably drying for 15 - 30 h under an inert gas (such as nitrogen, argon, etc.) at 150 - 200 °C to obtain a fluffy bread - like black - brown solid with high porosity; or subjecting the molten mixture to microwave - assisted treatment for 5 - 30 min under an inert gas (such as nitrogen, argon, etc.), and the microwave power is 0.1 - 5 kW. The present invention conducts constant temperature heating under an inert atmosphere to avoid the incorporation of oxygen in the air, which can effectively reduce the risk of oxidation and combustion of the catalyst.
[0026] In the above solution, the high - temperature heat treatment step includes: roasting in a tubular furnace at 400 - 700 °C for 2 - 4 h, with a heating rate of 2 - 5 °C / min, to remove nitrate and ammonium ions, and obtaining cobalt - metal composite oxide nanoparticles highly dispersed in the graphite carbon pool; the inert gas used is nitrogen, etc., and its flow rate is 1 - 3 L / h. The present invention first conducts high - temperature treatment under an inert gas before high - temperature roasting to prepare the catalyst precursor, which can effectively improve the interaction between Co and Mn bonds, promote the crystallization of Co and Mn to form MnCo₂O 4.5 , which is beneficial to subsequent reduction and carbonization treatment to prepare a cobalt carbide catalyst with uniform size and good dispersion of the nanocomposite material.
[0027] In the above solution, the high-temperature calcination step includes: calcining in static air at 300-500 °C for 2-4 h, which can effectively remove the encapsulated amorphous carbon and reduce the agglomeration of nanoparticles.
[0028] In the above solution, the reducing gas used in the reduction step is hydrogen, carbon monoxide, diluted hydrogen, diluted carbon monoxide or diluted syngas, the diluting gas is an inert gas, and the volume content of the diluting gas is 0-90%; the reduction temperature is 250-650 °C; the reduction time is 1-30 h; the reduction space velocity is 1000-10000 h -1 ; the reduction pressure is 0.1-3 MPa.
[0029] Preferably, the reduction temperature of the precursor catalyst in H2 is 300-500 °C, the reduction pressure is 0.1-1 MPa, and the volume space velocity is 2000-6000 h -1 and the reduction time is 5-15 h;
[0030] Preferably, the reducing gas is H2 or a mixture of hydrogen and an inert gas (the hydrogen content is 10-30%).
[0031] In the above solution, the carbonization gas used in the carbonization step is pure carbon monoxide or diluted carbon monoxide, where the carbon monoxide content is 10-100%, and the rest is an inert gas such as nitrogen or argon; or a mixed gas containing carbon monoxide and hydrogen, where the CO content is 10-90% and the H2 content is 10-90%, and the rest is an inert gas (such as nitrogen, argon, etc.); the carbonization temperature is 160-260 °C, the time is 4-50 h; the carbonization gas space velocity is 2000-5000 h -1 ; the carbonization pressure is 0.1-2 MPa.
[0032] Furthermore, the carbonization process includes: heating at a heating rate of 3-10 °C / min to 190-200 °C, holding for 50-60 min, and then heating at a rate of 1-4 °C / min to the target carbonization temperature for carbonization for 5-10 h.
[0033] The cobalt carbide catalyst prepared according to the above solution has the main phases of cobalt carbide and dissociated metal oxides. The carbonization process involves a short time, is relatively thorough, and has a high atomic utilization rate; the morphology of the obtained cobalt carbide catalyst is mainly spherical, with a particle size of 20-30 nm. The main crystal planes of the catalyst after carbonization analyzed by HRTEM are the (101) and (020) crystal planes of cobalt carbide.
[0034] In the above solution, the reaction conditions in the one-step process for preparing light olefins include: the reaction temperature is 200-280°C, preferably 220-260°C; the pressure is 0.1-2 MPa; the syngas is a H2 / CO mixed gas, where the volume ratio of H2 to CO is 1:2-2:1, preferably 1:1-2:1; the volume space velocity is 2000-10000 h -1 .
[0035] In the above solution, the one-step process for preparing light olefins can be obtained in a fixed-bed reactor or a tubular fixed-bed reactor; preferably, it can be obtained in a tubular fixed-bed reactor; more preferably, it can be obtained in a tubular reactor with a reaction tube diameter of 0.5-2 inches.
[0036] Using the cobalt carbide composite catalyst (catalyst with high Co2C content) obtained from the above solution for the one-step production of light olefins from syngas (mixed gas containing CO and H2) can achieve a total olefin selectivity of over 80%, where the selectivity of light olefins (C2-C4) in the total hydrocarbons reaches 60.29%, the CO2 selectivity is only 11.49%, and the selectivity of C1 (CO2 + CH4) can be controlled within 16.45%.
[0037] The principle of the present invention is:
[0038] For the cobalt carbide catalyst of the present invention, first, a eutectic system is formed through the hydrogen bond interaction between glucose and urea. Through thermodynamic regulation, the hydrolysis of metal salts forms homogeneous nuclei to achieve self-assembly, and nanoparticles with controllable sizes are uniformly dispersed on the carbon template. Then, high-temperature heat treatment in an inert atmosphere and high-temperature calcination in an air atmosphere are carried out in sequence to remove the template, obtaining a composite oxide (MnCo2O 4.5 ) precursor catalyst, and the electronic promoter is uniformly dispersed around the active component, improving the reducibility of the catalyst and exposing more active components. After low-temperature reduction and carbonization, a nano cobalt carbide catalyst with high content and uniform distribution is obtained, and it exhibits excellent properties such as high olefin selectivity and low CO2 selectivity.
[0039] Compared with the prior art, the beneficial effects of the present invention are:
[0040] 1) First, cobalt salts, metal salts, and electronic promoters are introduced into the molten liquid formed by sugars and urea in the present invention, and nanoparticles with controllable sizes are uniformly dispersed on the carbon template. Then, high-temperature heat treatment in an inert atmosphere and high-temperature calcination in an air atmosphere are carried out in sequence to obtain a composite oxide precursor catalyst, and the electronic promoter is uniformly dispersed around the active component, improving the reducibility of the catalyst and exposing more active components. After low-temperature reduction and carbonization, a nano cobalt carbide catalyst with high content and uniform distribution is obtained; the obtained cobalt carbide catalyst can exhibit advantages such as high olefin selectivity, low CO2 selectivity, and high stability in the Fischer-Tropsch synthesis for olefins;
[0041] 2) The preparation method involved in the present invention is relatively simple, with low raw material costs, and is more suitable for popularization and application. Description of the Drawings
[0042] Figure 1 XRD patterns of the precursor catalysts obtained in Examples 1 - 4 of the present invention;
[0043] Figure 2 XRD patterns of the cobalt carbide catalysts obtained in Examples 1 - 4 of the present invention;
[0044] Figure 3 SEM images of the catalyst prepared in Example 1 of the present invention; where a is the product obtained in step 3), b is the product obtained in step 4), and c is the catalyst obtained after carbonization in step 5);
[0045] Figure 4 Particle size statistical distribution diagrams of the product obtained in step 4) and the catalyst obtained after carbonization in step 5) of Example 1 of the present invention;
[0046] Figure 5 TEM and HRTEM images of the product obtained in step 4) and the catalyst obtained after carbonization in step 5) of Example 1 of the present invention;
[0047] Figure 6 XRD patterns of the catalyst prepared in Comparative Example 1 after calcination and after reaction;
[0048] Figure 7 SEM image of the catalyst prepared in Comparative Example 2 after calcination;
[0049] Figure 8 SEM image of the precursor catalyst obtained in step 4) of Comparative Example 3. Detailed Embodiments
[0050] The present invention is not limited to the above - mentioned embodiments. For those of ordinary skill in the art of this technology, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also considered within the protection scope of the present invention. The content not described in detail in this specification belongs to the prior art well - known to those of ordinary skill in the art.
[0051] In the following examples, the specific technologies or conditions are in accordance with those described in the literature of this field or in accordance with the product specifications. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through regular commercial channels.
[0052] Example 1
[0053] A cobalt carbide catalyst for the one - step preparation of light olefins, and its preparation method includes the following steps:
[0054] 1) Mix anhydrous glucose and urea in a mass ratio of 3:5 and place them in a beaker. Stir for 0.5 h under heating at 100 °C to form a molten mixture.
[0055] 2) Sequentially add Co(NO3)2·6H2O, an aqueous solution of Mn(NO3)2 (concentration: 50 wt%) and anhydrous sodium carbonate to the above molten mixture, and stir for 4.5 h (temperature: 110 °C); among them, the mass ratio of the introduced anhydrous glucose to cobalt nitrate is 3:2, the molar ratio of the introduced Co element to the Mn element is 1 / 1, and the mass of sodium carbonate is 0.1 wt% of the total mass of the salts (Co(NO3)2 + Mn(NO3)2).
[0056] 3) Under an Ar atmosphere, place the obtained molten mixture in an oven at 180 °C and heat for 24 h until the sugar is dehydrated and carbonized to obtain a black-brown highly porous solid; then grind the black-brown highly porous solid and place it in a tubular furnace. Remove nitrate ions under the action of Ar protective gas (flow rate: 2 L / h). The high-temperature heat treatment temperature is 400 °C (heating rate: 2 °C / min), and the time is 2 h, thus obtaining a cobalt-manganese composite oxide precursor catalyst supported on a carbon carrier.
[0057] 4) Calcinate the obtained precursor catalyst in a muffle furnace under static air to remove carbon. The calcination temperature is 300 °C (heating rate: 5 °C / min), and the calcination time is 2 h, thus obtaining the catalyst precursor.
[0058] The XRD pattern of the catalyst precursor obtained in step 4) of the present invention is shown in Figure 1 (Co / Mn = 1), which is a single-phase MnCo2O 4.5 ;
[0059] 5) Load the obtained catalyst precursor into a fixed-bed reactor for in-situ reduction. The reduction atmosphere is pure hydrogen, the reduction space velocity is 3000 h -1 , the reduction temperature is 300 °C, the reduction pressure is 0.1 MPa, and the reduction time is 15 h; after the reduction is completed and the temperature is lowered to room temperature, purge with syngas for 30 min; then heat the obtained reduction product to 200 °C at a rate of 5 °C / min and hold for 1 h, and then heat to 240 °C at a rate of 2 °C / min and carbonize for 5 h to obtain the cobalt carbide catalyst; then carry out the reaction of syngas to prepare olefins, where the volume fraction ratio of H2 / CO in the syngas is 2:1, and the remaining 10% N2 is used as the balance gas; the reaction space velocity is 3000 h -1 , the reaction temperature is 240 °C, and the working pressure is 0.1 MPa.
[0060] The specific catalytic reaction results are shown in Table 1, and the XRD pattern of the catalyst after the reaction is shown in Figure 2; The results show that the obtained product is mainly composed of Co2C and MnO.
[0061] Figure 3 SEM images of the product obtained in step 3), the product obtained in step 4), and the catalyst obtained after carbonization in step 5) of this example; it can be seen that: after high-temperature heat treatment in a tubular furnace, the cobalt-manganese composite oxide catalyst precursor obtained in step 3) forms a CoMn nanocomposite, and the spherical metal oxides are uniformly dispersed in the graphite carbon pool. After removing the graphite carbon in static air in step 4), the morphology of the catalyst remains spherical. The nanocomposite prepared by this method has uniform size and good dispersibility; after carbonization in step 5), the morphology of the catalyst changes from the original spherical shape to an ellipsoidal shape, and the surface is also smoother.
[0062] Figure 4 Particle size statistical distribution diagrams of the product obtained in step 4), the catalyst obtained after reduction and carbonization in step 5) of this example; it can be seen that: the catalyst precursor particles obtained in step 4) are evenly distributed, with an average particle size of about 11 nm. After reduction and carbonization in step 5), the particle size distribution of the catalyst is wider, and the average particle size increases to 21 nm.
[0063] Figure 5 TEM and HRTEM images of the product obtained in step 4), the catalyst obtained after reduction and carbonization in step 5) of this example; it can be seen that the lattice spacing of the catalyst precursor obtained in step 4) is corresponding to the (111) crystal plane of MnCo2O 4.5 (111). After reduction and carbonization in step 5), MnCo2O 4.5 (111) is converted to MnO, and the lattice spacing corresponds to the crystal plane (120). In addition, four different cobalt carbide crystal planes are obtained, including elliptical Co2C, with a lattice spacing corresponding to the crystal plane (210), corresponding to the crystal plane (020), and corresponding to the (111) and (101) crystal planes respectively
[0064] Example 2
[0065] A cobalt carbide catalyst for the one-step preparation of light olefins, and its preparation method includes the following steps:
[0066] 1) Mix anhydrous glucose and urea in a mass ratio of 5:3 and place them in a beaker, and stir well for 1 h under heating at 110 °C to form a molten mixture;
[0067] 2) Sequentially add Co(NO3)2·6H2O, an aqueous solution of Mn(NO3)2 (concentration: 50 wt%) and anhydrous sodium carbonate to the above molten mixture, and stir for 6 h (temperature: 120 °C); among them, the mass ratio of introduced anhydrous glucose to cobalt nitrate is 5:2, the molar ratio of introduced Co element to Mn element is 2 / 1, and the mass of the sodium carbonate is 0.3 wt% of the total mass of salts (Co(NO3)2 + Mn(NO3)2);
[0068] 3) Under a N2 gas atmosphere, place the molten mixture in an oven at 180 °C and heat for 15 h until the sugar dehydrates and carbonizes to obtain a black-brown solid; then grind the black-brown solid and place it in a tubular furnace to remove nitrate ions under the action of Ar protective gas (flow rate: 2 L / h), the high-temperature heat treatment temperature is 500 °C (heating rate: 2 °C / min), and the time is 3 h, thus obtaining a cobalt-manganese composite oxide precursor catalyst supported on a carbon carrier;
[0069] 4) Calcinate the obtained precursor catalyst in a muffle furnace under static air to remove carbon, the calcination temperature is 300 °C (heating rate: 5 °C / min), and the calcination time is 3 h, obtaining cobalt-manganese composite oxide nanopowders (catalyst precursors);
[0070] The XRD pattern of the precursor catalyst obtained in step 4) of the present invention is shown in Figure 1 (Co / Mn = 2), single-phase MnCo2O 4.5 ;
[0071] 5) Load the obtained catalyst precursor into a fixed-bed reactor for in-situ reduction, where the reduction atmosphere is pure hydrogen, the reduction space velocity is 2000 h -1 , the reduction temperature is 350 °C, the reduction pressure is 0.1 MPa, and the reduction time is 10 h; after the reduction is completed and the temperature drops to room temperature, purge with syngas for 40 min; then heat the obtained reduction product at a heating rate of 5 °C / min to 200 °C and hold for 1 h, and then heat at a rate of 4 °C / min to 240 °C for carbonization for 10 h to obtain the carbonized cobalt catalyst; then carry out the reaction of syngas to prepare olefins, where the volume fraction ratio of H2 / CO in the syngas is 2:1, and the remaining 10% N2 is used as the balance gas; the reaction space velocity is 2000 h -1 , the reaction temperature is 240 °C, and the working pressure is 0.1 MPa.
[0072] The specific catalytic reaction results are shown in Table 1, and the XRD pattern of the catalyst after the reaction is shown in Figure 2
[0073] Example 3
[0074] A carbonized cobalt catalyst for the one-step preparation of light olefins, and its preparation method includes the following steps:
[0075] 1) Mix anhydrous glucose and urea in a mass ratio of 8:3 and place them in a beaker. Stir well for 0.7 h under heating at 130 °C to form a molten mixture.
[0076] 2) Sequentially add Co(NO3)2·6H2O, an aqueous solution of Mn(NO3)2 (concentration: 50 wt%) and anhydrous sodium carbonate to the above molten mixture, and stir for 5 h (temperature: 110 °C). Among them, the mass ratio of the introduced anhydrous glucose to cobalt nitrate is 4:1, the molar ratio of the introduced Co element to the Mn element is 5 / 1, and the mass of the sodium carbonate is 0.5% of the total mass of the salts (Co(NO3)2 + Mn(NO3)2).
[0077] 3) Place the molten mixture in an oven at 180 °C and heat it in a N2 atmosphere for 30 h until the sugar is dehydrated and carbonized to obtain a black-brown solid. Then, grind the black-brown solid and place it in a tubular furnace. Remove nitrate ions under the action of Ar protective gas (flow rate: 3 L / h). The high-temperature heat treatment temperature is 600 °C (heating rate: 2 °C / min), and the time is 4 h, thus obtaining a cobalt-manganese composite oxide precursor catalyst supported on a carbon carrier.
[0078] 4) Calcinate the obtained precursor catalyst in a muffle furnace under static air to remove carbon. The calcination temperature is 400 °C (heating rate: 5 °C / min), and the calcination time is 4 h, obtaining cobalt-manganese composite oxide nanometer powder (catalyst precursor).
[0079] The XRD pattern of the precursor catalyst obtained in step 4) of the present invention is shown in Figure 1 (Co / Mn = 5), which is a single-phase MnCo2O 4.5 ;
[0080] 5) Load the obtained catalyst precursor into a fixed-bed reactor for in-situ reduction. Among them, the reduction atmosphere is pure hydrogen, the reduction space velocity is 4000 h -1 , the reduction temperature is 400 °C, the reduction pressure is 0.1 MPa, and the reduction time is 5 h. After the reduction is completed and the temperature is lowered to room temperature, purge with syngas for 40 minutes; then heat the obtained reduction product to 200 °C at a rate of 10 °C / min and hold for 1 h, and then heat it to 250 °C at a rate of 2 °C / min for carbonization for 15 h to obtain the carbonized cobalt catalyst; then carry out the reaction of synthesizing olefins from syngas. Among them, the volume fraction ratio of H2 / CO in the syngas is 2:1, and the remaining 10% N2 is used as a balance gas; the reaction space velocity is 4000 h -1 , the reaction temperature is 250 °C, and the working pressure is 0.1 MPa.
[0081] The specific catalytic reaction results are shown in Table 1, and the XRD pattern of the catalyst after the reaction is shown in Figure 2
[0082] Example 4
[0083] A cobalt carbide catalyst for the one-step preparation of light olefins, and its preparation method comprises the following steps:
[0084] 1) Mix anhydrous glucose and urea in a mass ratio of 6:5 in a beaker, and stir well for 0.3 h under heating at 120 °C to form a molten mixture;
[0085] 2) Sequentially add Co(NO3)2·6H2O, an aqueous solution of Mn(NO3)2 (concentration: 50 wt%) and anhydrous sodium carbonate to the above molten mixture, and stir for 5 h (temperature: 100 °C); wherein, the mass ratio of the introduced anhydrous glucose to cobalt nitrate is 3:1, the molar ratio of the introduced Co element to the Mn element is 10 / 1, and the mass of the sodium carbonate is 0.7 wt% of the total mass of the salts (Co(NO3)2 + Mn(NO3)2);
[0086] 3) Place the molten mixture in an oven at 180 °C and heat it in an Ar atmosphere for 20 h until the sugar is dehydrated and carbonized to obtain a black-brown solid; then grind the black-brown solid and place it in a tubular furnace to remove nitrate ions under the action of Ar protective gas (flow rate: 4 L / h), the high-temperature heat treatment temperature is 550 °C (heating rate: 2 °C / min), and the time is 2 h, thus obtaining a cobalt-manganese composite oxide precursor catalyst supported on a carbon carrier;
[0087] 4) Calcinate the obtained precursor catalyst in a muffle furnace under static air to remove carbon, the calcination temperature is 350 °C (heating rate: 5 °C / min), and the calcination time is 3.5 h to obtain a cobalt-manganese composite oxide precursor catalyst (catalyst precursor);
[0088] The XRD pattern of the precursor catalyst obtained in step 4) of the present invention is shown in Figure 1 (Co / Mn = 5), which is a single-phase MnCo2O 4.5 ;
[0089] 5) Pack the obtained catalyst precursor into a fixed-bed reactor for in-situ reduction, wherein the reduction atmosphere is pure hydrogen, the reduction space velocity is 5000 h -1 , the reduction temperature is 450 °C, the reduction pressure is 1 MPa, and the reduction time is 20 h. After the reduction is completed and the temperature is lowered to room temperature, purge with syngas for 1 h; then heat the obtained reduction product to 200 °C at a rate of 5 °C / min and hold for 2 h, and then heat it to 250 °C at a rate of 5 °C / min for carbonization for 20 h to obtain the cobalt carbide catalyst; then carry out the reaction of syngas to prepare olefins, wherein the volume fraction ratio of H2 / CO in the syngas is 2:1, and the remaining 10% N2 is used as a balance gas; the reaction space velocity is 5000 h -1, the reaction temperature is 260 °C and the working pressure is 0.1 MPa.
[0090] The specific catalytic reaction results are shown in Table 1 and Table 2, and the XRD pattern of the catalyst after the reaction is shown in Figure 2 .
[0091] Comparative Example 1
[0092] A cobalt carbide catalyst for the one-step preparation of light olefins, and its preparation method includes the following steps:
[0093] 1) Mix anhydrous glucose and urea in a mass ratio of 3:7 and place them in a beaker. Stir well for 1.2 h under heating at 115 °C to form a molten mixture;
[0094] 2) Add aqueous solutions of Co(NO3)2·6H2O and Mn(NO3)2 (concentration: 50 wt%) to the above molten mixture in sequence and stir for 7 h (temperature: 105 °C); among them, the mass ratio of the introduced anhydrous glucose to cobalt hexahydrate nitrate is 3:2, and the molar ratio of the introduced Co element to the Mn element is 4 / 1. No anhydrous sodium carbonate additive is added during this process;
[0095] 3) Place the molten mixture in an oven at 180 °C and heat it under an Ar atmosphere for 24 h until the sugar is dehydrated and carbonized to obtain a black-brown solid; then grind the black-brown solid and place it in a tubular furnace. Under the action of Ar protective gas (flow rate: 2.5 L / h), remove the nitrate ions. The high-temperature heat treatment temperature is 300 °C (heating rate: 2 °C / min), and the time is 3 h, thus obtaining a cobalt-manganese composite oxide precursor catalyst supported on a carbon carrier;
[0096] 4) Calcinate the obtained precursor catalyst in a muffle furnace under static air to remove carbon. The calcination temperature is 330 °C (heating rate: 5 °C / min), and the calcination time is 3 h to obtain a cobalt-manganese composite oxide precursor catalyst (catalyst precursor);
[0097] The XRD pattern of the precursor catalyst obtained in step 4) of the present invention is shown in Figure 6 (Co / Mn = 4), which is a single-phase MnCo2O 4.5 ;
[0098] 5) Load the obtained catalyst precursor into a fixed-bed reactor for in-situ reduction, where the reduction atmosphere is pure hydrogen and the reduction space velocity is 4000 h -1, the reduction temperature was 320 °C, the reduction pressure was 0.1 MPa, and the reduction time was 12 h. After the reduction was completed and the temperature dropped to room temperature, it was purged with syngas for 1 h; then the obtained reduced product was heated to 200 °C at a rate of 4 °C / min and held for 3 h, and then heated to 250 °C at a rate of 4 °C / min for carbonization for 25 h, where the volume fraction ratio of H2 / CO in the syngas was 2:1, and the remaining 10% N2 was used as the balance gas.
[0099] After passivating the carbonized catalyst, phase analysis was carried out as Figure 6 , the obtained cobalt-manganese composite oxide catalyst was difficult to reduce and separate Co and Mn, and the phase of the catalyst after the reaction was Co x Mn 1-x O (with relatively poor olefin selectivity), and the formation of cobalt carbide was not observed, indicating that the formation of cobalt carbide requires the presence of Na promoter.
[0100] Comparative Example 2
[0101] A cobalt carbide catalyst for the one-step preparation of light olefins, and its preparation method includes the following steps:
[0102] 1) Mix anhydrous glucose and urea in a mass ratio of 2:5 and place them in a beaker, and stir well for 0.8 h (stirring rate is 350 rpm) under heating at 115 °C to form a molten mixture;
[0103] 2) Add Co(NO3)2·6H2O, Mn(NO3)2 solution (concentration of 50 wt%) and anhydrous sodium carbonate to the above molten mixture in sequence, and stir for 3 h (temperature is 105 °C); among them, the mass ratio of the introduced anhydrous glucose to cobalt nitrate hexahydrate is 2:1, the molar ratio of the introduced Co element to the Mn element is 6 / 1, and the mass of the sodium carbonate is 0.2 wt% of the total mass of the salts (Co(NO3)2 + Mn(NO3)2);
[0104] 3) Put the molten mixture into an oven at 160 °C and heat it in an Ar atmosphere for 30 h until the sugar is dehydrated and carbonized to obtain a black-brown solid; then grind the black-brown solid and place it in a tubular furnace to remove nitrate ions under the action of Ar protective gas (flow rate is 1.5 L / h), the high-temperature heat treatment temperature is 360 °C (heating rate is 2 °C / min), and the time is 4 h, that is, a cobalt-manganese composite oxide precursor catalyst supported on a carbon carrier is obtained;
[0105] 4) Calcinate the obtained precursor catalyst to remove carbon in a muffle furnace under static air, the calcination temperature is 700 °C (heating rate is 5 °C / min), and the calcination time is 5 h to obtain a cobalt-manganese composite oxide precursor catalyst.
[0106] The SEM image of the precursor catalyst obtained in step 4) of this comparative example is shown in Figure 7, it can be seen that the obtained metal nanoparticles are sintered together to form larger nanoparticle clusters with non-uniform size, which is not conducive to the reduction and carbonization of the next-step catalyst.
[0107] Comparative Example 3
[0108] A cobalt carbide catalyst for the one-step preparation of light olefins, and its preparation method includes the following steps:
[0109] 1) Mix anhydrous glucose and urea in a mass ratio of 6:5 and place them in a beaker. Stir well for 0.5 h under heating at 115 °C to form a molten mixture;
[0110] 2) Sequentially add Co(NO3)2·6H2O, an aqueous solution of Mn(NO3)2 (concentration 50 wt%) and anhydrous sodium carbonate to the above molten mixture, and stir for 7 h (temperature 100 °C); among them, the mass ratio of the introduced anhydrous glucose to cobalt nitrate is 3:1, the molar ratio of the introduced Co element to the Mn element is 6 / 1, and the mass of the sodium carbonate is 0.6 wt% of the total mass of the salts (Co(NO3)2 + Mn(NO3)2);
[0111] 3) Under an Ar atmosphere, place the obtained molten mixture in an oven at 200 °C and heat for 15 h until the sugar is dehydrated and carbonized to obtain a black-brown solid; then grind the black-brown solid to obtain a precursor catalyst;
[0112] 4) Calcinate the obtained precursor catalyst to remove carbon in a muffle furnace under static air. The calcination temperature is 320 °C (heating rate 5 °C / min), and the calcination time is 4 h to obtain a cobalt-manganese composite oxide precursor catalyst (catalyst precursor).
[0113] The SEM image of the precursor catalyst obtained in step 4) of this comparative example is shown in Figure 8 , it can be seen that: without the treatment of inert gas in the tubular furnace, the outer layer of the obtained catalyst is wrapped, and it is difficult to form uniformly distributed nanoparticles. This is because Co and Mn are easily oxidized to Mn3O4 and Co3O4 in an air atmosphere during the formation of Co-Mn crystallization, and the interaction between cobalt and manganese is weakened, making it difficult to form MnCo2O 4.5 , the structural components change, which is not conducive to further reduction and carbonization treatment.
[0114] Table 1 Reaction results of the catalysts obtained in Examples 1-4 for the process of preparing olefins from syngas
[0115]
[0116] Table 2 Reaction results of the catalysts obtained in Examples 1-4 for the process of preparing olefins from syngas
[0117]
[0118] As can be seen from Table 1, the cobalt carbide catalyst obtained by reducing and carbonizing the catalyst of the present invention can be effectively used for directly preparing olefins from syngas in a fixed-bed reactor. This catalyst exhibits high olefin selectivity and low carbon dioxide and methane selectivity, and its catalytic performance is very stable, having potential industrial application prospects.
[0119] Obviously, the above preferred embodiments are only examples clearly described and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation manners here. And the obvious changes or modifications thus derived are still within the protection scope of the present invention.
Claims
1. A preparation method of a cobalt carbide composite catalyst for one-step preparation of light olefins, characterized in that, It includes the following steps: 1) Mix sugars and urea in proportion, heat and stir to obtain a molten liquid, then add cobalt salt, metal salt solution and electronic additive thereto, and perform secondary heating and stirring to obtain a molten mixture; 2) Keep the obtained molten mixture at a constant temperature for heating, perform dehydration and carbonization to obtain a blackish-brown solid with high porosity; 3) Under the protection of inert gas, perform high-temperature heat treatment on the obtained blackish-brown solid with high porosity to obtain black precursor solid particles; 4) Perform high-temperature calcination on the obtained black precursor solid particles in an air atmosphere to obtain a composite metal oxide catalyst; 5) Perform reduction and carbonization treatment on the obtained composite metal oxide catalyst to obtain the cobalt carbide composite catalyst; The metal element introduced by the metal salt is Mn; The high-temperature heat treatment step includes: calcining in a tube furnace at 400-700°C for 2-4 h; the high-temperature calcination step includes: calcining in static air at 300-500°C for 2-4 h.
2. The preparation method according to claim 1, characterized in that, The cobalt salt is one or more of nitrate, halide, carbonate, and sulfate.
3. The preparation method according to claim 1, characterized in that, The electronic additive is sodium carbonate, sodium nitrate or sodium sulfate.
4. The preparation method according to claim 1, characterized in that, The mass ratio of the cobalt salt, urea and sugars is 1:(1.5-5):(1.5-7.5).
5. The preparation method according to claim 1, wherein, The constant-temperature heating step includes drying at 150-200°C for 15-30 h under inert gas; or subjecting the molten mixture to microwave-assisted treatment for 5-30 min under a protective atmosphere, with a microwave power of 0.1-5 kW.
6. The preparation method according to claim 1, characterized in that, The reducing gas used in the reduction step is hydrogen, carbon monoxide, diluted hydrogen, diluted carbon monoxide or diluted syngas, the diluting gas is an inert gas, and the volume content of the diluting gas is 0-90%; the reduction temperature is 250-650 °C; the reduction time is 1-30 h; the reduction space velocity is 1000-10000 h -1 ; the reduction pressure is 0.1-3 MPa.
7. The preparation method according to claim 1, characterized in that, The carbonization gas used in the carbonization step is pure carbon monoxide or diluted carbon monoxide, where the carbon monoxide content is 10 - 100%, and the rest is inert gas; or a mixed gas containing carbon monoxide and hydrogen, where the CO content is 10 - 90%, the H2 content is 10 - 90%, and the rest is inert gas; the carbonization temperature is 160 - 260 °C, the time is 4 - 50 h; the space velocity of the carbonization gas is 1000 - 5000 h -1 ; the carbonization pressure is 0.1 - 2 MPa.
8. The cobalt carbide composite catalyst prepared by the preparation method according to any one of claims 1-7.