A tubular cobalt-nitrogen co-doped carbon catalyst with a dual mesoporous structure and a preparation method thereof

Abstract

The present application relates to a kind of tubular cobalt-nitrogen co-doped carbon catalysts with double mesoporous structure and its preparation method.The method comprises the following steps: first, low-temperature calcination, nitrogen-rich precursor is obtained by low-temperature calcination under limited air atmosphere to obtain layered nitrogen-rich carrier;Second, self-assembly, soluble cobalt salt is dissolved in deionized water to form solution A, layered nitrogen-rich carrier, imidazole ligand and triethylamine are fully stirred in aqueous solution to form emulsion B, finally, solution A is added dropwise to B, stirring, aging, centrifugation and drying to obtain composite precursor;Third, high-temperature carbon thermal reduction, composite precursor is obtained by high-temperature carbon thermal reduction under nitrogen atmosphere to obtain a kind of double mesoporous and tubular structure rich cobalt-nitrogen co-doped carbon catalyst.The catalyst of the present application is used for acetylpic acid hydrogenation conversion, and the effect is: 180 DEG C and 1.4MPa hydrogen pressure under reaction 2h can be realized in organic solvent acetylpic acid complete conversion, and the selectivity of gamma-valerolactone reaches 100%.

Description

TECHNICAL FIELD

[0001] The present application 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, a preparation method thereof, and a technology of using the catalyst to prepare gamma-valerolactone from acetic propionic acid hydrogenation. BACKGROUND

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

[0003] Porous carbon materials have great potential as catalyst carriers in the field of hydrogenation due to their stable structure and easy control of pore size. Literature (Fuel, 2018, 231:165171.) reports that Cu / AC catalysts can achieve 99% LA conversion and 89.9% GVL selectivity at 220℃ and 2MPa H2 pressure for 5h. Studies have shown that nitrogen doping can effectively adjust the surface electronic structure of the carbon carrier, enhance the interface effect between the carrier and the active metal, and significantly improve the catalytic activity. Literature (Nanotechnology, 2022, 33:355401.) reports the application of Ni-loaded nitrogen-doped graphene in LA hydrogenation. Complete conversion of LA and 100% selectivity of GVL can be achieved at 140℃ and 2MPa H2 pressure for 4h. Chinese patent CN116786167B discloses a Zr-loaded magnetic nitrogen-doped carbon nanotube, which acts as a catalyst. Isopropyl alcohol acts as a solvent and hydrogen donor at the same time. Ethyl levulinate as a raw material can be completely converted at 180℃ for 5h, and the selectivity of GVL is 96.2%. The above cases fully demonstrate that non-noble metal-loaded nitrogen-doped carbon catalysts have excellent activity in catalytic hydrogenation, especially in LA selective hydrogenation.

[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. Studies have shown that by chelating a complex of cobalt salt and organic nitrogen-containing substances, the dispersion of cobalt nanoparticles on the nitrogen-doped carbon carrier can be effectively enhanced during the subsequent carbonization process to form a porous carbon carrier. The LA conversion rate is 99% under the conditions of 180℃ and 1.8MPa H2 pressure for 5h, and the GVL selectivity is close to 100%. This process provides a reference for the subsequent development of high-dispersion nanoparticle-loaded nitrogen-doped carbon catalysts.

[0005] In recent years, in-situ pyrolysis of metal-organic frameworks based on imidazole ligand assembly is considered an excellent method for preparing non-noble metal-loaded nitrogen-doped carbon catalysts, which also has high catalytic activity in the process of LA preparing GVL as a catalyst. Literature (Chemical Engineering Journal, 2022, 450: 138153) reports a tetraethyl orthosilicate combined with a metal-organic framework complex. After removing the SiO2 template by high-temperature pyrolysis, a Zn-Co bimetallic atom-loaded nitrogen-doped carbon catalyst is obtained. With water as the solvent, the LA conversion rate and GVL selectivity are both 99% under the conditions of 180℃ and 4.5MPa H2 pressure for 2h. Literature (Molecular Catalysis, 2022, 533: 112758) reports a Co-loaded nitrogen-doped carbon catalyst obtained by surface-constrained pyrolysis using ZIF-67 as a precursor. With 1,4-dioxane as the solvent, the LA conversion rate is 100% and the GVL selectivity is 97% under the conditions of 200℃ and 2MPa H2 pressure for 6h.

[0006] In summary, Co-N co-doped carbon catalysts exhibit excellent catalytic performance in the process of LA hydrogenation to prepare GVL, but the catalysts and their carrier preparation processes provided by existing technologies are complicated 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

[0007] Based on the above, the purpose of the present application is to provide a preparation method of a cobalt-nitrogen co-doped carbon catalyst with simple process and high activity, which solves the problem of high hydrogen pressure required for high activity, high conversion rate and high selectivity in the process of LA hydrogenation to GVL in the prior art.

[0008] To this end, the present application provides a tubular cobalt-nitrogen co-doped carbon catalyst with a double mesoporous structure and a preparation method thereof, as shown in the preparation flowchart Figure 1 The specific steps include the following steps:

[0009] First step: low-temperature calcination, specifically: the nitrogen-rich precursor is placed in a sealed box furnace and calcined at low temperature under a limited air atmosphere to obtain a layered nitrogen-rich carrier;

[0010] Second step: self-assembly, specifically: the soluble cobalt salt is dissolved in deionized water to form solution A, the layered nitrogen-rich carrier, imidazole ligand and triethylamine are fully stirred in aqueous solution to form emulsion B, and finally solution A is added dropwise to B, stirred, aged, centrifuged and dried to obtain a composite precursor;

[0011] Third step, high-temperature carbon thermal reduction, specifically: the composite precursor is placed in a tube furnace and high-temperature carbon thermal reduction under a nitrogen atmosphere to obtain a double-mesoporous and tubular structure cobalt-nitrogen co-doped carbon catalyst.

[0012] In the above technical solution, preferably, the atmosphere during the first step of low-temperature calcination 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℃; the calcination time is 1-10h; and the heating rate is 2-10℃ / min.

[0013] In the above technical solution, preferably, in the second step of self-assembly, 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-carboxaldehyde; the mass ratio of imidazole ligand to layered nitrogen-rich carrier is 1:0.1-10; the amount of triethylamine is 1-3mL; and the aging time is 1-12h.

[0014] In the above technical solution, preferably, in the third step of high-temperature carbon thermal reduction, the reaction temperature is 600-900℃; the reaction time is 0.5-3h; and the heating rate is 2-10℃ / min.

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

[0016] (1) The specific surface area of the catalyst described in the present application reaches 320m 2 / g and has a double-mesoporous structure (main peaks at 6.8 and 37nm), with a mesopore rate of up to 82%, effectively promoting the adsorption and diffusion of the substrate on the surface and in the pores of the catalyst. Nitrogen further enriches the active sites and regulates the electronic structure of the carrier, making the adsorption and activation of the reaction substrate on the surface of the catalyst easier and effectively improving the efficiency and selectivity of the hydrogenation reaction.

[0017] (2) The catalyst provided by the application is rich in carbon nanotubes, so that the catalyst has good acid resistance, can effectively inhibit active metal leaching, agglomeration and deep oxidation, and exhibits high stability.

[0018] (3) The catalyst provided by the application exhibits excellent activity in the LA hydrogenation reaction for preparing GVL, specifically: compared with the prior art, the catalyst provided by the application can achieve 100% GVL selectivity and keep the LA conversion rate at more than 50% under a relatively wide reaction temperature (160-200℃) and hydrogen pressure (1.0-2.0 MPa). In particular, under the reaction temperature of 180℃ and the hydrogen pressure of 1.4 MPa, the LA can be completely converted (100%) and the GVL selectivity can reach 100% after 2h of reaction. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a preparation flowchart of the catalyst.

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

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

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

[0023] Figure 5 is a pore size distribution curve of the catalyst described in Example 1. DETAILED DESCRIPTION

[0024] The embodiments of the application are described in detail below, and the embodiments are provided to better illustrate the content of the application and are only used to explain the application, and cannot be understood as a limitation of the application.

[0025] Example 1:

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

[0027] (1) Low-temperature calcination: melamine (10g) is calcined at 520℃ for 4h under a limited air atmosphere in a box furnace, the limited air is realized by a large crucible covering a small crucible with a cover, the temperature rising rate is 10℃ / min, and a layered nitrogen-rich carrier is obtained;

[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 the layered nitrogen-rich carrier (1.5765 g) in 90 mL of deionized water to form emulsion B, and maintain a mass ratio of 2-methylimidazole to the layered nitrogen-rich carrier of 1:0.3. Add solution A dropwise to B at room temperature, stir for 10 min, and then age for 3 h. Centrifugal separation and drying yield the composite precursor;

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

[0030] As can be seen from Figure 2 and Figure 3 , the catalyst is rich in carbon nanotubes and the cobalt nanoparticles are uniformly distributed (with a size of about 5 nm), and the rich graphite carbon layer near the cobalt particles is derived from the catalytic graphitization of cobalt at high temperature; the carbon nanotubes in the catalyst are uniform in diameter and long in length, and are connected to each other to form a frame similar to a bamboo joint. As can be seen from Figure 4 , the typical H4-type hysteresis loop in the isotherm indicates that the pore structure of the catalyst is rich in mesopores; as can be seen from Figure 5 , the catalyst has a clear bimodal mesoporous structure, with mesopore size peaks at 6.8 nm and 37 nm.

[0031] The tubular cobalt-nitrogen co-doped carbon catalyst (Co@GNC-800) with a bimodal mesoporous structure obtained in this example was used for LA hydrogenation to synthesize GVL, in the following manner:

[0032] Place the above catalyst (0.05 g), LA (0.5 g), and 1,4-dioxane (20 mL) as a solvent in a 50 mL polytetrafluoroethylene reaction kettle, and pass hydrogen gas in and out of the reaction kettle three times to completely replace the air in the reaction kettle. Increase the hydrogen pressure to 1.4 MPa, and set the reaction temperature to 180℃ to evaluate the hydrogenation activity of the catalyst. Gas chromatography detection results show that the catalyst has excellent performance in the LA hydrogenation reaction, with a LA conversion rate of 100% and a GVL selectivity of 100% after 2 h of reaction.

[0033] Example 2-3:

[0034] The catalyst preparation process in Example 2-3 is the same as in Example 1, except that the reaction temperature for LA hydrogenation is set to 170℃ (Example 2) and 160℃ (Example 3), respectively. The detection results are shown 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 pressure for LA hydrogenation was set at 1.2 MPa (Example 4) and 1.0 MPa (Example 5), respectively. The test results are shown in Table 1.

[0037] Examples 6-8:

[0038] The catalyst preparation process in Examples 6-8 was different from that in Example 1 only in that the carbon thermal reduction temperature was set at 600°C (Example 6), 700°C (Example 7) and 900°C (Example 8), respectively. The prepared catalysts were marked as Co@GNC-600, Co@GNC-700 and Co@GNC-900, respectively. The test results are shown 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. When the reaction is carried out at a hydrogen pressure of 1.4 MPa for 2 h, the LA conversion rate increases from 60.51% to 100% as the reaction temperature increases from 160°C to 180°C, and the GVL selectivity is always 100%. When the reaction temperature is fixed at 180°C, the LA conversion rate increases from 74.65% to 100% as the hydrogen pressure increases from 1.0 MPa to 1.4 MPa and the reaction is carried out for 2 h, and the GVL selectivity is always 100%. This feature shows that the catalyst has excellent GVL selectivity, and the LA conversion rate is affected by the reaction conditions. By improving the reaction operating parameters, the catalytic activity can be further optimized.

[0042] In addition, the catalysts prepared at different carbon thermal reduction temperatures show great differences in catalytic activity. When the carbon thermal reduction temperature is low (Example 6), the LA conversion rate is only 87.2% and the GVL selectivity is 86% when the reaction is carried out at a hydrogen pressure of 1.4 MPa and a temperature of 180°C for 2 h. When the carbon thermal reduction temperature is increased, the reaction activity of the obtained catalyst is increased, and both the LA conversion rate and the GVL selectivity are effectively improved. However, when the carbon thermal reduction temperature is as high as 900°C (Example 8), although the GVL selectivity of the obtained catalyst is still 100%, the LA conversion rate is sharply decreased to 57.6%, which confirms that the active sites of the catalyst are largely destroyed at high temperature, thereby 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 of self-assembly and high-temperature carbon thermal reduction, i.e. no layered nitrogen-rich carrier is used in the self-assembly process, and the catalyst is marked as Co@NC-800. The specific application is the same as Example 1, and the gas chromatography detection result shows 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, and because no layered nitrogen-rich carrier is used in the preparation process, the nitrogen content in the catalyst structure is low, so that the LA hydrogenation activity of the catalyst is lower than that of Example 1. This comparison conclusion fully confirms that the self-assembly of the layered nitrogen-rich carrier to form a composite precursor in Example 1 has a great influence on the subsequent high-temperature carbon thermal reduction to prepare the catalyst, and 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 double mesoporous structure, which is beneficial to the adsorption and diffusion of the reaction substrate on the surface of the catalyst, thereby showing excellent catalytic activity.

[0046] The specific preferred embodiments of the above cases further illustrate the present application, and cannot be regarded as the specific implementation of the present application being limited to these descriptions. The above preferred examples are only a typical example of a double mesoporous structure of a tubular cobalt-nitrogen co-doped carbon catalyst, a preparation method thereof, and a LA hydrogenation reaction to prepare GVL. In addition, the present application can have other various combined embodiments, and any technical solution formed by equivalent replacement or equivalent transformation shall be regarded as belonging to the protection scope of the present application.

Claims

1. A method for preparing a tubular cobalt-nitrogen co-doped carbon catalyst with a dual mesoporous structure, characterized in that, Includes the following steps: Step 1: Low-temperature calcination, specifically: placing the nitrogen-rich precursor in a closed box furnace and calcining it at low temperature in a limited air atmosphere to obtain a layered nitrogen-rich carrier; The second step is self-assembly, which involves dissolving soluble cobalt salt in deionized water to form solution A, stirring layered nitrogen-rich carrier, imidazole ligand and triethylamine in aqueous solution to form emulsion B, and finally adding A dropwise to B, followed by stirring, aging, centrifugation and drying to obtain the composite precursor. The third step is high-temperature carbothermal reduction, which involves placing the composite precursor in a tube furnace and performing high-temperature carbothermal reduction under a nitrogen atmosphere to obtain a cobalt-nitrogen co-doped carbon catalyst with dual mesoporous structures and rich in tubular structures.

2. The catalyst preparation method 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℃; the heating rate is 2-10℃ / min; and the calcination time is 1-10h.

3. The catalyst preparation method 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-methylimidazolium, 2-nitroimidazolium, 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 catalyst preparation method according to claim 1, characterized in that: The high-temperature carbothermic reduction reaction temperature is 600-900℃, the reaction time is 0.5-3h, and the heating rate is 2-10℃ / min.

5. The application of the tubular cobalt-nitrogen co-doped carbon catalyst with a dual mesoporous structure obtained by the catalyst preparation method of claim 1 in the catalytic hydrogenation of levulinic acid (LA) to prepare γ-valerol (GVL).

6. The application according to claim 5, characterized in that, The catalyst to LA mass ratio 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 application according to claim 5, characterized in that, The hydrogen pressure is 1.0-1.6 MPa, the reaction temperature is 160-200℃, and the reaction time is 0.5-4 h.

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