Tubular cobalt-nitrogen co-doped carbon catalyst with double-mesoporous structure and preparation method of tubular cobalt-nitrogen co-doped carbon catalyst

Abstract

The invention relates to a tubular cobalt-nitrogen co-doped carbon catalyst with a double-mesoporous structure and a preparation method of the tubular cobalt-nitrogen co-doped carbon catalyst. The method comprises the following steps: step 1, low-temperature calcination: performing low-temperature calcination on a nitrogen-rich precursor in a limited air atmosphere to obtain a layered nitrogen-rich carrier; 2, self-assembly: dissolving a soluble cobalt salt in deionized water to form a solution A, fully stirring the layered nitrogen-rich carrier, an imidazole ligand and triethylamine in an aqueous solution to form an emulsion B, dropwise adding the solution A into the emulsion B, and stirring, aging, centrifuging and drying to obtain a composite precursor; and 3, high-temperature carbon thermal reduction: carrying out high-temperature carbon thermal reduction on the composite precursor in a nitrogen atmosphere to obtain the cobalt-nitrogen co-doped carbon catalyst with double mesopores and rich tubular structures. When the catalyst provided by the invention is applied to hydro-conversion of levulinic acid, the effects are as follows: complete conversion of levulinic acid can be realized in an organic solvent by reacting at 180 DEG C under the hydrogen pressure of 1.4 MPa for 2 hours, and the selectivity of gamma-valerolactone reaches 100%.

Description

Technical Field

[0001] The present invention belongs to the technical field of new chemical materials, and particularly relates to a tubular cobalt-nitrogen co-doped carbon catalyst with a double mesoporous structure and a preparation method thereof, and a technology for using this catalyst in the hydrogenation of levulinic acid to prepare γ-valerolactone. Background Art

[0002] As an important biomass platform molecule, γ-valerolactone (GVL) can be converted into various derivatives such as methyltetrahydrofuran, alkanes, and 1,4-pentanediol. How to efficiently produce GVL has attracted extensive attention from experts in the chemical industry. With the promotion of the concept of green sustainability, the preparation of GVL based on levulinic acid (LA) has become a research hotspot. Since LA can be produced in large quantities by acid catalysis of cellulose or C6 sugars, the conversion from LA to GVL has high atom economy. This process usually requires a catalyst. Commonly used catalysts mainly include noble metals (such as Ru, Pd, Pt) and non-noble metals (such as Cu, Ni, Co, Zr). Although noble metal catalysts have high activity, their high price and limited reserves limit large-scale industrial applications. Therefore, the development of efficient, stable, and low-cost non-noble metal catalysts has become the research focus in this field.

[0003] Due to their stable structure and easy regulation of pore size, porous carbon materials have great potential as catalyst carriers in the field of hydrogenation. The literature (Fuel, 2018, 231: 165 - 171.) reported that the Cu / AC catalyst achieved 99% LA conversion and 89.9% GVL selectivity under the reaction conditions of 220 °C and 2 MPa H 2 pressure for 5 h. Research shows that nitrogen doping can effectively regulate the surface electronic structure of the carbon carrier, enhance the interfacial effect between the carrier and the active metal, and significantly improve the catalytic activity. The literature (Nanotechnology, 2022, 33: 355401.) reported the application of a Ni-loaded nitrogen-doped graphene in the hydrogenation of LA. Complete conversion of LA and 100% GVL selectivity can be achieved under the reaction conditions of 140 °C and 2 MPa H 2 pressure for 4 h. Chinese Patent CN116786167B discloses a Zr-loaded magnetic nitrogen-doped carbon nanotube. When used as a catalyst, isopropanol serves as both a solvent and a hydrogen donor, and ethyl levulinate is used as a raw material. Complete conversion can be achieved at 180 °C for 5 h, and the GVL selectivity reaches 96.2%. The above cases fully confirm that non-noble metal-supported nitrogen-doped carbon catalysts have excellent activity in catalytic hydrogenation, especially in the selective hydrogenation of LA.

[0004] In addition to the common Ni, Cu, and Zr, Chinese Patent CN113546664B discloses a non-noble metal Co-loaded nitrogen-doped fish scale carbon catalyst. Research shows that chelating cobalt salts with organic nitrogen-containing substances to form complexes can effectively enhance the dispersion of cobalt nanoparticles on the nitrogen-doped carbon support during the subsequent carbonization process to form a porous carbon support. The LA conversion rate reached 99% and the GVL selectivity was close to 100% under the reaction conditions of 180 °C and 1.8 MPa H 2 pressure for 5 h. This process provides a reference basis for the subsequent development of highly dispersed nanoparticle-loaded nitrogen-doped carbon catalysts.

[0005] In recent years, the in-situ pyrolysis of metal-organic frameworks assembled based on imidazole ligands has been considered an excellent method for preparing non-noble metal-loaded nitrogen-doped carbon catalysts, which also has high catalytic activity during the process of preparing GVL from LA as a catalyst. The literature (Chemical Engineering Journal, 2022, 450: 138153) reported a tetraethyl orthosilicate combined with a metal-organic framework composite. After high-temperature pyrolysis to remove the SiO 2 template, a Zn-Co dual-atom-loaded nitrogen-doped carbon catalyst was obtained. Using water as a solvent, the LA conversion rate and GVL selectivity both reached 99% under the reaction conditions of 180 °C and 4.5 MPa H 2 pressure for 2 h. The literature (Molecular Catalysis, 2022, 533: 112758) reported a Co-loaded nitrogen-doped carbon catalyst obtained by surface-confined pyrolysis using ZIF-67 as a precursor. Using 1,4-dioxane as a solvent, the LA conversion rate reached 100% and the GVL selectivity reached 97% under the reaction conditions of 200 °C and 2 MPa H 2 pressure for 6 h.

[0006] Based on the existing technologies, Co-N co-doped carbon catalysts exhibit excellent catalytic performance during the process of hydrogenating LA to prepare GVL. However, the preparation processes of the catalysts and their supports provided by the existing technologies are cumbersome or the reaction pressure for catalytic conversion is too high. Therefore, there is an urgent need to develop a multifunctional catalyst with a simple preparation process, high activity, good stability, and low cost. Summary of the Invention

[0007] Based on the above, the purpose of the present invention is to provide a preparation method for a cobalt-nitrogen co-doped carbon catalyst with a simple process and high activity, and to solve the problem that high hydrogen pressure is required for high activity, high conversion rate, and high selectivity in the process of hydrogenating LA to GVL in the existing technology.

[0008] To this end, the present invention provides a tubular cobalt-nitrogen co-doped carbon catalyst with a double mesoporous structure and its preparation method. The preparation process is as Figure 1 shown, and specifically includes the following steps:

[0009] Step 1: Low-temperature calcination, specifically: placing the nitrogen-rich precursor in a closed box furnace and performing low-temperature calcination in a limited air atmosphere to obtain a layered nitrogen-rich support;

[0010] Step 2: Self-assembly, specifically: dissolving the soluble cobalt salt in deionized water to form solution A, fully stirring the layered nitrogen-rich support, imidazole ligand, and triethylamine in an aqueous solution to form an emulsion B, and finally dropwise adding solution A to B and stirring, aging, centrifuging, and drying to obtain a composite precursor;

[0011] Step 3: High-temperature carbothermal reduction, specifically: placing the composite precursor in a tube furnace and performing high-temperature carbothermal reduction in a nitrogen atmosphere to obtain a cobalt-nitrogen co-doped carbon catalyst with a double mesoporous structure and rich tubular structures.

[0012] Preferably, in the above technical solution, during the low-temperature calcination in the first step, the atmosphere is a limited air atmosphere, and the nitrogen-rich precursor is at least one of melamine, thiourea, urea, and dicyandiamide; the calcination temperature is 350 - 560 °C; the calcination time is 1 - 10 h; the heating rate is 2 - 10 °C / min.

[0013] Preferably, in the above technical solution, during the self-assembly in the second step, the soluble cobalt salt is at least one of cobalt nitrate, cobalt acetate, cobalt sulfate, and cobalt chloride; the imidazole ligand is at least one of 2-methylimidazole, 2-nitroimidazole, and imidazole-2-carbaldehyde; the mass ratio of the imidazole ligand to the layered nitrogen-rich support is 1:0.1 - 10; the amount of triethylamine used is 1 - 3 mL; the aging time is 1 - 12 h.

[0014] Preferably, in the above technical solution, during the high-temperature carbothermal reduction in the third step, the reaction temperature is 600 - 900 °C; the reaction time is 0.5 - 3 h; the heating rate is 2 - 10 °C / min.

[0015] Applying the technical solution of the present invention, specifically: using the nitrogen-rich precursor as a raw material (preferably melamine) for low-temperature calcination, self-assembling a complex on the surface of the calcined layered nitrogen-rich support using an imidazole ligand (preferably 2-methylimidazole) and a soluble cobalt salt (preferably cobalt nitrate), and performing high-temperature carbothermal reduction to obtain a tubular cobalt-nitrogen co-doped carbon catalyst with a double mesoporous structure, which can exhibit excellent effects under mild conditions when used in the process of LA hydrogenation to prepare GVL. The specific effects are as follows:

[0016] (1) The specific surface area of the catalyst of the present invention reaches 320 m 2 / g and there is a double mesoporous structure (main peaks are located at 6.8 and 37 nm), and the mesoporous ratio is as high as 82%, effectively promoting the adsorption and diffusion of the substrate on the surface and in the pores of the catalyst. The nitrogen element further enriches the active sites and regulates the electronic structure of the support, making the adsorption and activation of the reaction substrate on the catalyst surface easier, and effectively improving the efficiency and selectivity of the hydrogenation reaction.

[0017] (2) The catalyst provided by the present invention is rich in carbon nanotubes, making the catalyst have good acid resistance, which can effectively inhibit the leaching, agglomeration and deep oxidation of active metals, and show high stability.

[0018] (3) The catalyst shows excellent activity in the hydrogenation reaction of LA to prepare GVL. Specifically, compared with the prior art, the catalyst provided by the present invention can achieve 100% selectivity of GVL and the conversion rate of LA remains above 50% under a relatively wide reaction temperature (160 - 200 °C) and hydrogen pressure (1.0 - 2.0 MPa). In particular, at a reaction temperature of 180 °C and a pressure of 1.4 MPa H 2 Reacting for 2 h under pressure can achieve complete conversion of LA (100%) and the selectivity of GVL reaches 100%. Description of the Drawings

[0019] Figure 1 is a schematic diagram of the preparation process of the catalyst.

[0020] Figure 2 is a scanning electron micrograph of the catalyst described in Example 1.

[0021] Figure 3 is a transmission electron micrograph of the catalyst described in Example 1.

[0022] Figure 4 is the nitrogen adsorption - desorption isotherm of the catalyst described in Example 1.

[0023] Figure 5 is the pore size distribution curve of the catalyst described in Example 1. Detailed Description of the Invention

[0024] The following details the embodiments of the present invention. The examples given are for better illustration of the content of the present invention and are only used to explain the present invention, and should not be construed as a limitation of the present invention.

[0025] Example 1:

[0026] A preparation method of a dual - mesoporous - structured tubular cobalt - nitrogen co - doped carbon catalyst is as follows:

[0027] (1) Low - temperature calcination: Take melamine (10 g) and calcine it in a box furnace at 520 °C for 4 h in a limited air atmosphere. The limited air is realized by putting a small crucible into a large crucible with a lid. The heating rate is 10 °C / min to obtain a layered nitrogen - rich carrier;

[0028] (2) Self-assembly to form a composite precursor: Dissolve cobalt nitrate hexahydrate (2.328 g) in 20 mL of deionized water to form solution A; stir 2-methylimidazole (5.255 g), triethylamine (2 mL), and layered nitrogen-rich support (1.5765 g) in 90 mL of deionized water to form an emulsion B, maintaining a mass ratio of 2-methylimidazole to layered nitrogen-rich support of 1:0.3. Dropwise add solution A to B at room temperature, stir for 10 min, then age for 3 h. After centrifugation and drying, a composite precursor is obtained;

[0029] (3) High-temperature carbothermal reduction: Place the composite precursor (2 g) in a corundum crucible and react in a tubular furnace under a nitrogen atmosphere at 800 °C for 2 h with a heating rate of 4 °C / min. After cooling to room temperature, grind to obtain a tubular cobalt and nitrogen co-doped carbon catalyst with a double mesoporous structure, labeled Co@GNC-800.

[0030] It can be seen from Figure 2 and Figure 3 that the catalyst is rich in carbon nanotubes and the cobalt nanoparticles are evenly distributed (with a size of about 5 nm). The abundant graphite carbon layers near the cobalt particles are derived from the catalytic graphitization of cobalt at high temperature; the carbon nanotubes in the catalyst have a uniform diameter and a long length, connecting with each other to form a framework similar to a bamboo joint shape. It can be seen from Figure 4 that the typical H4-type hysteresis loop in the isotherm indicates that the pore structure of the catalyst is rich in mesopore scale; it can be seen from Figure 5 that the catalyst has an obvious double mesoporous structure, and the main peaks of the mesopore sizes are located at 6.8 and 37 nm respectively.

[0031] Apply the tubular cobalt and nitrogen co-doped carbon catalyst with a double mesoporous structure (Co@GNC-800) obtained in this example to the hydrogenation of LA to synthesize GVL. The specific method is as follows:

[0032] Take the above catalyst (0.05 g), LA (0.5 g), and add 1,4-dioxane (20 mL) as a solvent and place them in a 50 mL polytetrafluoroethylene reaction kettle. Purge the air in the reaction kettle by introducing hydrogen three times back and forth, increase the hydrogen pressure to 1.4 MPa, and set the reaction temperature to 180 °C to evaluate the hydrogenation activity of the catalyst. The gas chromatography detection results show that the catalyst has excellent performance in the hydrogenation reaction of LA. The conversion rate of LA reaches 100% and the selectivity of GVL reaches 100% after 2 h of reaction.

[0033] Example 2-3:

[0034] The preparation process of the catalyst in Example 2-3 is the same as that in Example 1. The difference is that the reaction temperatures for the hydrogenation of LA are set to 170 °C (Example 2) and 160 °C (Example 3) respectively, and the detection results are listed in Table 1.

[0035] Example 4-5:

[0036] The catalyst preparation process in Examples 4 - 5 was the same as that in Example 1, except that the hydrogen pressures for LA hydrogenation were set to 1.2 MPa (Example 4) and 1.0 MPa (Example 5) respectively. The test results are listed in Table 1.

[0037] Examples 6 - 8:

[0038] The difference in the catalyst preparation process between Examples 6 - 8 and Example 1 was only the difference in the carbothermal reduction temperature, which was set to 600 °C (Example 6), 700 °C (Example 7), and 900 °C (Example 8) respectively. The prepared catalysts were labeled Co@GNC - 600, Co@GNC - 700, and Co@GNC - 900 respectively. The test results are listed in Table 1.

[0039] Table 1 LA hydrogenation performance of the catalysts described in Examples 1 - 8

[0040]

[0041] As can be seen from Table 1, LA hydrogenation is greatly affected by hydrogen pressure, reaction time, and reaction temperature. Reacting for 2 h under a hydrogen pressure of 1.4 MPa, when the reaction temperature rises from 160 °C to 180 °C, the conversion rate of LA increases from 60.51% to 100%, and the GVL selectivity always remains 100%. When the reaction temperature is fixed at 180 °C, with the hydrogen pressure increasing from 1.0 MPa to 1.4 MPa and reacting for 2 h, the LA conversion rate increases from 74.65% to 100%, and the GVL selectivity always remains 100%. This characteristic indicates that the catalyst has excellent GVL selectivity, and the LA conversion rate is affected by reaction conditions. By improving the reaction operation parameters, the catalytic activity can be further optimized.

[0042] In addition, the catalytic activities of the catalysts prepared at different carbothermal reduction temperatures vary greatly. When the carbothermal reduction temperature is relatively low (Example 6), the LA conversion rate is only 87.2% and the GVL selectivity is 86% when reacting at a hydrogen pressure of 1.4 MPa and 180 °C for 2 h. As the carbothermal reduction temperature increases, the reaction activity of the obtained catalyst increases, and both the LA conversion rate and GVL selectivity are effectively improved. However, when the carbothermal reaction temperature reaches 900 °C (Example 8), although the GVL selectivity of the obtained catalyst still remains 100%, the LA conversion rate drops sharply to 57.6%, confirming that the active sites of the catalyst are massively damaged at high temperatures, thus affecting the LA hydrogenation activity.

[0043] Comparative Example 1:

[0044] The difference from Example 1 is that the catalyst preparation process only uses two steps: self-assembly and high-temperature carbothermal reduction. That is, a layered nitrogen-rich carrier is not used during the self-assembly process, and the catalyst is labeled Co@NC-800. The specific application is the same as that in Example 1. The gas chromatography detection results show that the LA conversion rate is 98.6% and the GVL selectivity is 98%.

[0045] The results show that the specific surface area of Co@NC-800 is only 200 m 2 / g. At the same time, due to the absence of a layered nitrogen-rich carrier in the preparation process, the nitrogen content in the catalyst structure is relatively low, resulting in lower LA hydrogenation activity than that in Example 1. This comparative conclusion fully confirms that the self-assembly of the composite precursor based on the layered nitrogen-rich carrier in Example 1 has a great influence on the subsequent preparation of the catalyst by high-temperature carbothermal reduction. The layered nitrogen-rich carrier effectively increases the porosity of the catalyst (the specific surface area reaches 320 m 2 / g) and is rich in a double mesoporous structure, which is beneficial to the adsorption and diffusion of reaction substrates on the catalyst surface, thus showing excellent catalytic activity.

[0046] The above specific preferred embodiments of the case further elaborate on the present invention. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. The above preferred examples are only typical examples of a tubular cobalt-nitrogen co-doped carbon catalyst with a double mesoporous structure, its preparation method, and its LA hydrogenation to GVL reaction provided by the present invention. In addition, there can be other various combined implementation manners in this application. Any technical solution formed by equivalent replacement or equivalent transformation should be regarded as falling within the protection scope of the present invention.

Claims

1. A tubular cobalt-nitrogen co-doped carbon catalyst with a dual mesoporous structure and a preparation method thereof, characterized in that: The following steps are involved: The first step is low-temperature calcination, specifically: placing the nitrogen-rich precursor in a closed box furnace and calcining at low temperature in a limited air atmosphere to obtain a layered nitrogen-rich carrier; Step 2: self-assembly, specifically: dissolving a soluble cobalt salt in deionized water to form solution A, fully stirring a layered nitrogen-rich carrier, an imidazole ligand and triethylamine in the aqueous solution to form an emulsion B, and finally adding A dropwise into B, stirring, aging, centrifuging and drying to obtain a composite precursor; The third step is high-temperature carbon thermal reduction, which is specifically: placing the composite precursor in a tubular furnace and performing high-temperature carbon thermal reduction in a nitrogen atmosphere to obtain a double-mesoporous and tubular-structured cobalt-nitrogen co-doped carbon catalyst.

2. The method for preparing a catalyst according to claim 1, characterized in that: The nitrogen-rich precursor in the low-temperature calcination is at least one of melamine, thiourea, urea and dicyandiamide; the calcination temperature is 450-560° C.; the heating rate is 2-10° C. / min; and the calcination time is 1-10 h.

3. The method for preparing a catalyst according to claim 1, characterized in that: The soluble cobalt salt in the self-assembly is at least one of cobalt nitrate, cobalt acetate, cobalt sulfate and cobalt chloride; the imidazole ligand is at least one of 2-methylimidazole, 2-nitroimidazole and imidazole-2-carboxaldehyde; the mass ratio of the imidazole ligand to the layered nitrogen-rich carrier is 1:0.3-3.0, the amount of triethylamine used is 1-3 mL; and the aging time is 1-12 h.

4. The method for preparing a catalyst according to claim 1, characterized in that: The reaction temperature of the high-temperature carbon thermal reduction is 600-900° C., the reaction time is 0.5-3 h, and the heating rate is 2-10° C. / min.

5. Use of the tubular cobalt-nitrogen co-doped carbon catalyst with a dual mesoporous structure as claimed in claim 1 in the catalytic hydrogenation of levulinic acid (LA) to prepare γ-valerolactone (GVL).

6. The use according to claim 5, characterized in that: The mass ratio of the catalyst to LA is 1:5-30, and the solvent is selected from at least one of 1,4-dioxane, ethyl acetate, aniline, methanol, and isopropanol.

7. The use according to claim 5, characterized in that: The hydrogen pressure is 1.0-1.6MPa, the reaction temperature is 160-200°C, and the reaction time is 0.5-4h.

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