Preparation method of carbon cobalt catalyst, dimethyl carbonate synthesis method and reaction device

Abstract

The application discloses a high-performance cobalt-based catalyst for synthesizing dimethyl carbonate by methanol carbonylation oxidation and a matching reaction system thereof. 0 @Co 3+ ‑N x The active center with a core-shell structure is precisely constructed by using a defective carbon nitride containing a high-density nitrogen vacancy as a carrier through an integrated strategy of defect regulation, interface assembly and phase reconstruction. The structure realizes a dimethyl carbonate selectivity of greater than or equal to 99% in the reaction through a spatially separated oxidation-reduction synergistic mechanism, significantly improves the deactivation resistance of the catalyst by virtue of a stable coordination structure, and is completely free of chlorine, thereby fundamentally solving the equipment corrosion problem caused by the chlorine contained in the catalyst in a traditional process. The matching reaction system adopts a multi-stage gas-lifting reactor and a cross-flow separator, realizes efficient and mild separation and continuous circulation of the catalyst, and thereby jointly ensures efficient, stable and long-period operation of the whole process.

Description

Technical Field

[0001] This invention belongs to the field of chemical reaction engineering technology, specifically relating to a method for preparing a cobalt carbon catalyst, a method for synthesizing dimethyl carbonate, and a reaction apparatus. Background Technology

[0002] Dimethyl carbonate (DMC), as an important green chemical, continues to see growing market demand. Among numerous synthetic routes, the methanol oxidative carbonylation method is considered a highly promising industrial process due to its high atom economy; however, its industrialization is largely limited by the core performance bottleneck of the catalyst.

[0003] Catalysts for the methanol oxidative carbonylation synthesis of DMC have seen continuous research progressing towards addressing catalyst corrosion, deactivation, and performance improvement. While first-generation chloride-containing copper-based catalysts (such as cuprous chloride) exhibited acceptable catalytic activity, the presence of chloride ions led to corrosion of the reaction equipment, increasing equipment costs and maintenance expenses, and limiting the product's application in high-purity fields. To eliminate the corrosive effects at the source, second-generation chloride-free copper-based catalysts have been developed. For example, patent CN116726922B discloses a chloride-free copper-based catalyst (CuO) derived from a copper-metal-organic framework (Cu-MOF) structure. x This catalyst ( / PC) achieves 100% dimethyl carbonate selectivity in the reaction, fundamentally avoiding chlorine corrosion. However, its methanol single-pass conversion is low (approximately 2%), and the long-term operational stability of the catalyst has not been verified.

[0004] To simultaneously achieve both corrosion-free operation and high selectivity, researchers further explored cobalt-based catalytic systems. Cobalt possesses suitable electronegativity and multivalence, theoretically offering greater potential for regulating oxygen-containing intermediates. However, existing studies on cobalt-based catalysts lack systematic strategies for the stable anchoring of active sites, valence state evolution, and interfacial electronic structure, resulting in significant limitations in long-term stability and selectivity.

[0005] Among the preparation routes of cobalt-based catalysts, the method represented by ZIF-67-derived Co-NC materials is advantageous because it can form Co-N with a well-defined structure. xThe active sites have attracted attention, but this material generally suffers from problems such as framework shrinkage and metal particle agglomeration during independent pyrolysis, which severely limits performance improvement. Patent CN116571262B discloses a cobalt-based catalyst derived from ZIF-67, which achieves a high selectivity of over 99.5% and a methanol conversion rate of approximately 21.2% in the methanol oxidative carbonylation reaction by constructing a hierarchical porous structure, significantly surpassing traditional copper-based catalysts. However, the preparation of this catalyst requires a high-temperature carbonization process of 600-1000℃, and cobalt nanoparticles are prone to agglomeration at high temperatures, posing challenges to its ability to control the fine structure of the active sites and its long-term stability.

[0006] In terms of reaction processes and equipment, existing technologies commonly employ stirred tank reactors, which suffer from inherent drawbacks such as difficulties in mechanical sealing, significant scale-up effects, and high operating energy consumption. Therefore, developing a novel catalytic system that can fundamentally solve equipment corrosion problems while possessing high activity, high selectivity, and excellent stability, along with a highly reliable, low-energy-consumption reactor and system designed to be highly compatible with it, is a pressing issue in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the problems of active center sintering and decay, uncontrollable microstructure, and difficulty in achieving both high activity and high stability in existing cobalt-based catalysts. This invention provides a method for preparing a cobalt carbon catalyst, a method for synthesizing dimethyl carbonate, and a reaction apparatus. The catalyst prepared by this method possesses the core functions of stable electron gradient and spatially separated oxidation-reduction, and also has the advantages of good metal dispersion, stable framework structure, controllable interface bonding, and tunable local electronic structure. When using this catalyst for the methanol oxidative carbonylation synthesis of dimethyl carbonate, it exhibits high methanol conversion rate, high dimethyl carbonate selectivity, and high space-time yield. When this reaction is carried out in the reaction apparatus provided by this invention, it can significantly reduce catalyst recovery costs and achieve continuous, efficient separation and immediate reuse of the catalyst.

[0008] This invention is achieved through the following technical solution: The first aspect of the present invention is to provide a method for preparing a cobalt carbon catalyst, comprising the following steps: S1, Defect control: Graphite-phase carbon nitride (g-C3N4) is etched in a hydrogen atmosphere to obtain a carrier H2-g-C3N4 with controlled defects on its surface. S2, Interface Assembly: The carrier H2-g-C3N4 with surface-controlled defects is dispersed in methanol, and then cobalt salt and organic ligand are added. After coordination reaction, the solid product after the reaction is the precursor composite material CN-ZIF-67. S3, Phase transformation and reconstruction: The precursor composite material CN-ZIF-67 is heat-treated under an inert atmosphere to obtain the cobalt carbon catalyst (Co-NC catalyst).

[0009] In the above technical solution, in step S1, the etching temperature of the etching process is 500~700℃, and the etching time is 0~9h, for example, 0.5, 1, 2, 3, 4, 5, 6, 7, 8 or 9 hours; preferably, the etching temperature is 550℃ and the etching time is 7h.

[0010] In the above technical solution, in S2, the mass ratio of the carrier H2-g-C3N4 with surface-controlled defects, cobalt salt and organic ligand is 1:(1~6):(4~5); the reaction time of the coordination reaction is 12~48 h, preferably 24 h.

[0011] In the above technical solution, the cobalt salt is cobalt nitrate, and the organic ligand is 2-methylimidazole.

[0012] In the above technical solution, in step S2, the H2-g-C3N4 support with surface-controlled defects is dispersed in methanol by ultrasonic treatment to obtain a first mixture; cobalt nitrate is dissolved in methanol to obtain a second solution; the second solution and the first mixture are mixed and ultrasonically treated to obtain a third mixture; 2-methylimidazole is dissolved in methanol to obtain a fourth solution; the fourth solution is added to the third mixture and stirred continuously to carry out a coordination reaction. This technical solution first ensures that the support is fully exfoliated and prevents agglomeration; then, the cobalt nitrate methanol solution is added and ultrasonicated again, so that cobalt ions are preferentially anchored at the defect sites on the support surface, establishing strong interactions; finally, the 2-methylimidazole ligand solution is added to achieve in-situ nucleation and growth of ZIF-67 on the support surface. The core advantage of preparing according to this sequence is that it achieves the adsorption and anchoring of cobalt species on the support surface first, followed by in-situ crystallization, ensuring the tight binding of the active component and the support, avoiding the self-aggregation of ZIF-67 in bulk, and obtaining a highly dispersed and structurally stable Co-NC catalyst after pyrolysis.

[0013] In the above technical solution, in step S3, the heat treatment temperature is 600~900℃ and the heat treatment time is 60~180min; preferably, the heat treatment temperature is 700℃ and the heat treatment time is 120min; preferably, the inert atmosphere is a nitrogen atmosphere.

[0014] This invention introduces carrier defect engineering as a pre-control step in interface construction, proposing an integrated construction strategy of "defect control - interface assembly - phase transition reconstruction" to overcome the fundamental bottleneck of uncontrollable interfaces in traditional composite systems. First, g-C3N4 is controllably etched in an H2 atmosphere to construct high-density, tunable nitrogen vacancies and edge defects within its two-dimensional framework, thereby modulating the local electronic structure of the carrier and providing spatially selective metal anchoring points. Based on this, defect-induced in-situ assembly achieves directional nucleation and confined distribution of ZIF-67 on the g-C3N4 surface, fundamentally avoiding the metal agglomeration and framework shrinkage problems that occur during independent MOF carbonization. Finally, after synergistic pyrolysis, the precursor undergoes an interface-guided phase transition evolution along the defect distribution, ultimately constructing a composite system composed of metallic Co. 0 Core and Rich Electronics Co 3+ -N x Co with shell composition, stable electron gradient and spatially separated redox function 0 @Co 3+ -N x structure.

[0015] The Co-NC catalyst for the methanol oxidative carbonylation synthesis of DMC of the present invention is obtained by using a g-C3N4 support with surface defects regulated by hydrogen etching as a substrate, growing a ZIF-67 structure of cobalt salt and organic ligand on its surface in situ, and then subjecting it to high-temperature pyrolysis treatment under an inert atmosphere.

[0016] Another object of the present invention is to provide a method for synthesizing dimethyl carbonate, wherein the method is carried out by liquid-phase oxidative carbonylation with methanol, and the catalyst is a cobalt carbon catalyst prepared by the method described in the above technical solution.

[0017] Another object of the present invention is to provide a reaction apparatus for the above-described method for synthesizing dimethyl carbonate, the reaction apparatus comprising an airlift external circulation reactor, a condenser, and a gas-liquid two-phase separator; a raw material gas phase feed pipe connected to the bottom gas phase inlet of the airlift external circulation reactor, a raw material liquid phase feed pipe connected to the lower liquid phase inlet of the airlift external circulation reactor, an oxygen feed pipe connected to the middle oxygen inlet of the airlift external circulation reactor, a gas phase outlet at the top of the airlift external circulation reactor connected to the gas phase inlet pipe of the condenser, a gas phase outlet of the condenser connected to the inlet pipe of the gas-liquid two-phase separator, a gas phase outlet of the gas-liquid two-phase separator connected to the raw material gas phase feed pipe via a gas phase circulation pipe, a liquid phase outlet of the airlift external circulation reactor connected to a product collection pipe, a product collection pipe connected to the raw material liquid phase feed inlet pipe via a liquid phase circulation pipe, and a liquid phase outlet of the gas-liquid two-phase separator connected to the product collection pipe via a pipe.

[0018] In the above technical solution, the reaction device includes n airlift external circulation reactors connected in series, where n ≥ 2; The raw material gas phase feed pipe is connected to the bottom gas phase inlet of the first-stage airlift external circulation reactor, and the raw material liquid phase feed pipe is connected to the liquid phase inlet at the bottom of the first-stage airlift external circulation reactor. The gas phase outlet at the top of the first-stage airlift external circulation reactor is connected to the gas phase inlet pipe at the bottom of the second-stage airlift external circulation reactor; the liquid phase outlet of the first-stage airlift external circulation reactor is connected to the liquid phase inlet pipe at the bottom of the second-stage airlift external circulation reactor. Up to this principle, The gas phase outlet at the top of the (n-1)th stage airlift external circulation reactor is connected to the gas phase inlet pipe at the bottom of the nth stage airlift external circulation reactor; the liquid phase outlet of the (n-1)th stage airlift external circulation reactor is connected to the liquid phase inlet pipe at the bottom of the nth stage airlift external circulation reactor. The gas phase outlet at the top of the nth stage airlift external circulation reactor is connected to the gas phase inlet pipe of the condenser. The gas phase outlet of the condenser is connected to the inlet pipe of the gas-liquid two-phase separator. The gas phase outlet of the gas-liquid two-phase separator is connected to the raw material gas phase feed pipe through a gas phase circulation pipe. The liquid phase outlet of the nth stage airlift external circulation reactor is connected to the product collection pipe. The product collection pipe is connected to the raw material liquid phase feed pipe through a liquid phase circulation pipe. The liquid phase outlet of the gas-liquid two-phase separator is connected to the product collection pipe through a pipe. The oxygen feed pipe is connected to the oxygen inlet in the middle of each stage of the airlift external circulation reactor.

[0019] In the above technical solution, the gas phase circulation pipe connecting the gas phase outlet of the gas-liquid two-phase separator and the raw material gas phase feed pipe is also equipped with a booster pump. A circulation pump is also installed on the liquid phase circulation pipe that connects the product collection pipe and the raw material liquid phase feed pipe.

[0020] In the above technical solution, the airlift external circulation reactor includes a reactor body and a cross-flow separator, wherein the top of the cross-flow separator is connected to the upper side wall of the reactor body through a downcomer, and the bottom of the cross-flow separator is connected to the bottom of the reactor body through a circulation return pipe. The lower part of the reactor body is provided with a gas phase inlet for gas phase reactants to enter the reactor body, and the density difference formed by the introduction of gas in the rising and falling zones of the reactor body drives the slurry in the reactor body to generate a stable directional circulation flow; the top of the reactor body is provided with a gas phase outlet for gas phase products to be discharged from the reactor body. The cross-flow separator is equipped with a cross-flow filtration unit for separating the solid and liquid phases in the material entering the cross-flow separator. The liquid phase is discharged through the liquid product outlet located on the side wall of the cross-flow separator. The solid phase and the undischarged liquid phase are returned to the reactor body through the concentrate outlet at the bottom of the cross-flow separator via a circulation return pipe.

[0021] In the above technical solution, the cross-flow filtration unit in the cross-flow separator is a tubular filter assembly arranged along the main material movement direction. During operation, the material entering the cross-flow separator further enters the interior of the tubular filter assembly, where the liquid phase with the solid phase removed flows out of the tubular filter assembly and is discharged through the liquid phase product outlet. Meanwhile, the solid phase flows downward along the interior of the tubular filter assembly with the undischarged liquid phase and is discharged through the concentrate outlet. Cross-flow is formed between the main material flow direction and the discharged liquid phase flow direction, which can promptly remove the solid phase adhering to the tubular filter assembly from the cross-flow separator and prevent it from clogging.

[0022] In the above technical solution, the cross-flow filtration unit adopts a tubular ceramic membrane module; this module is composed of multiple single-tube membrane elements arranged in parallel in a bundle within the cross-flow separator. This bundled structure can provide the maximum effective membrane area within a limited space, meeting the high-flux requirements of industrial production.

[0023] In the above technical solution, the filtration accuracy configuration of the tubular ceramic membrane assembly within the cross-flow filtration unit is selected based on the average particle size of the solid catalyst in the reaction system, and the average pore size of the ceramic membrane (d) m ) represents the average particle size of the catalyst (d 50 1 / 10 to 1 / 3 of ), that is, d m = (1 / 10 ~ 1 / 3) × d 50 This ratio range is established based on the combined mechanism of sieving and surface filtration: when d m d 50 At / 10, although the rejection rate is extremely high, the initial flux of the membrane is too low, and the pores are more easily blocked by tiny impurities; when d m d 50 At a pore size ratio of 1 / 3, although the initial flux is high, particles easily become embedded in the membrane pores, leading to irreversible blockage and a decrease in retention rate. Choosing a pore size ratio of (1 / 10 ~ 1 / 3) allows most catalyst particles to be retained on the membrane surface, while a small amount of submicron-sized fine powder is adsorbed and retained by the inner wall of the membrane pores, thus achieving efficient separation (retention rate...). Achieving an optimal balance between 99.5% and sustainable flux.

[0024] In the above technical solution, the reactor body is a vertical cylindrical structure with a height-to-diameter ratio of 5~20, preferably 8~15; The diameter of the equipment is 0.5~10m, preferably 1~5m; The ratio of the diameter of the downcomer to the diameter of the reactor body is 0.1 to 0.5, preferably 0.2 to 0.3.

[0025] In the above technical solution, a gas distributor is provided at the lower part of the reactor body and is connected to the gas phase inlet; The reactor body has an expansion section at the top, the diameter of which is 1.2 to 1.5 times the diameter of its lower cylinder. This expansion structure significantly reduces the apparent gas velocity, providing ample space for the catalyst particles entrained in the gas stream to settle due to gravity, thereby effectively reducing the amount of solids entrained in the gas stream.

[0026] In the above technical solution, the separation material of the tubular ceramic membrane module is preferably a porous ceramic formed by high-temperature sintering of alumina (Al2O3), zirconium oxide (ZrO2), or titanium oxide (TiO2). This type of material possesses excellent high-temperature resistance, corrosion resistance, and mechanical strength, and can adapt to the high-temperature and high-pressure environment of the reaction system.

[0027] The gaseous product from the nth-stage airlift external circulation reactor is condensed and separated by heat exchange. The resulting gaseous mixture is recycled back to the first-stage airlift external circulation reactor after being mixed with fresh gaseous feedstock. Part of the liquid product from the nth-stage airlift external circulation reactor is recycled back to the first-stage airlift external circulation reactor after being mixed with fresh liquid feedstock. The remainder is then subjected to subsequent purification. The liquid product mixture obtained by cross-flow filtration is combined with the liquid product obtained after gas-liquid separation for further purification. Carbon monoxide, methanol, and catalyst are supplied from the first-stage airlift external circulation reactor, while oxygen is supplied from each stage of the airlift external circulation reactor. The ratio of oxygen supply from the first stage to each subsequent stage of the airlift external circulation reactor is (1~1.5):1. In the above technical solution, the reaction temperature in each stage of the airlift reactor is 100~150℃, and the reaction pressure is 1.0MPa~5.0MPa; In the above technical solution, the recycled gas phase mixture is mixed with fresh gas phase raw material to obtain mixed reaction raw material gas, and the molar concentration ratio of carbon monoxide, oxygen and carbon dioxide in the mixed reaction raw material gas is (50~99):(0.5~15):(0~30). Preferably, the recycled liquid product contains dimethyl carbonate, which is mixed with fresh methanol to obtain a mixed reaction feed liquid, wherein the mass ratio of methanol to dimethyl carbonate in the mixed reaction feed liquid is (50~90):(5~30).

[0028] The advantages and beneficial effects of this invention are as follows: The Co-NC catalyst of this invention is prepared by in-situ growth of the ZIF-67 precursor (CN-ZIF-67) on a hydrogen-controlled defect g-C3N4 support followed by pyrolysis. Compared with existing copper-based or conventional cobalt-based catalysts for the liquid-phase oxidative carbonylation of methanol to synthesize dimethyl carbonate, it has the following substantial features and advancements: (1) The Co-NC catalyst prepared in this invention exhibits significantly improved catalytic selectivity. While maintaining a high methanol conversion rate, it can improve the selectivity of dimethyl carbonate (DMC) to over 98%, effectively suppressing the formation of byproducts such as dimethyl ether (DME), dimethoxymethane (DMM), and methyl formate (MF), and has a high space-time yield of dimethyl carbonate.

[0029] (2) The continuous cyclic operation of the device is achieved by relying on the easy separation and long-term stability of the catalyst: The catalyst of this invention has a solid heterogeneous structure, which is easy to separate by filtration, and Co... 0 @Co 3+ -N x The catalyst exhibits a structure resistant to aggregation and deactivation. In the reaction process, the reactor body is directly connected to a cross-flow filter. After gentle separation (0.1-0.4 MPa), the catalyst is immediately returned to the reactor. This utilizes the catalyst's "easy separation" advantage, avoiding the energy consumption and mechanical sealing problems associated with traditional stirred tank reactors. Furthermore, the "gentle separation" protects the catalyst's anti-aggregation structure, synergistically achieving continuous operation for 500 hours without deactivation, significantly reducing catalyst recovery costs and the difficulty of subsequent product separation and purification. The preparation process is simple, the raw materials are readily available, and it has excellent potential for process scale-up.

[0030] (3) The catalyst is chlorine-free, solving the corrosion problem of traditional equipment: The catalyst of this invention is completely chlorine-free, eliminating the risk of chlorine corrosion from the source. For this reason, the equipment abandons the expensive materials and complex anti-corrosion design of traditional chlorine-resistant equipment, and adopts conventional chemical materials with cross-flow filtration and separation system (special ceramic membrane module), which reduces the manufacturing cost of the equipment and solves the core problem of severe corrosion and high maintenance cost of traditional chlorine-containing catalyst supporting equipment.

[0031] (4) Matching the catalyst oxidation-reduction synergistic mechanism to optimize reaction mass transfer and oxygen control: The catalyst of this invention utilizes Co 0 @Co 3+ -Nx Achieving spatially separated redox synergistic reactions using a core-shell structure requires precise control of the contact mode between the gaseous feedstock and the active sites, as well as the oxygen concentration distribution. To this end, the device employs a multi-stage series airlift reactor with a staged oxygen supply strategy (O2 ratio of 1-1.5:1 between the first stage and subsequent stages), and enhances gas-liquid mass transfer through an external airlift circulation. This staged oxygen supply avoids localized excessive O2 levels that could lead to Co-oxidation. 0 Core oxidation deactivation, and airlift circulation ensure CO / O2 and Co 3+ -N x Oxidation sites, Co 0 Uniform contact at reduction sites enables highly efficient synergistic redox reactions, which, combined with the high selectivity (≥98%) of the catalyst, achieves efficient synthesis of DMC. Furthermore, a staged oxygen supply strategy keeps the oxygen concentration within the primary reactor within a safe range, reducing the risk of explosion from the process source. The integrated cross-flow filter, employing a tangential flow design, achieves continuous, efficient separation and immediate reuse of the catalyst and reaction liquid, ensuring long-term stability of catalyst concentration and activity within the reaction zone. Attached Figure Description

[0032] Figure 1. (a) K-space Co K-edge XANES spectrum; (b) R-space EXAFS spectrum after Fourier transform; (c, d) R-space EXAFS fitting results for Co-NC-7 and Co foil, respectively; (e, f) Wavelet transform (WT) EXAFS analysis results for the two, respectively; (g) Co 2p XPS spectrum; (h) Quantitative comparison of surface Co valence state distribution obtained based on XPS; (i) N 1s XPS spectrum.

[0033] Figure 2. (a) HAADF-STEM image; (b) Elemental distribution map; (c) Edge and interior regions of EELS; (d) EELS spectrum of Co L edge and (e) Comparison of Co L3 / L2 ratio.

[0034] Figure 3 This is a schematic diagram of the reaction apparatus for the synthesis method of dimethyl carbonate in Example 12 of the present invention.

[0035] in: 1-First-stage airlift external circulation reactor, 2-Second-stage airlift external circulation reactor, 3-Raw material gas phase feed pipe, 4-Raw material liquid phase feed pipe, 5-Oxygen feed pipe, 6-Condenser, 7-Gas-liquid two-phase separator, 8-Booster pump, 9-Gas phase circulation pipe, 10-Product collection pipe, 11-Circulation pump, 12-Liquid phase circulation pipe.

[0036] For those skilled in the art, other related figures can be obtained from the above figures without any creative effort. Detailed Implementation

[0037] To enable those skilled in the art to better understand the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments.

[0038] Example 1:

[0039] The catalyst was prepared with a mass ratio of g-C3N4 to Co(NO3)2·6H2O of 1:1.

[0040] Melamine was placed in a muffle furnace and heated to 550°C at a heating rate of 2.5°C / min, held at that temperature for 4 h, and then cooled to obtain pale yellow lumps of g-C3N4.

[0041] 720 mg of the above g-C3N4 precursor was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 720 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0042] The dried sample was transferred to a tube furnace and heated at 2 °C / min. It was then carbonized at 700 °C for 2 h under an inert N2 atmosphere. After carbonization, the resulting solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as CN / Co-1.

[0043] Example 2:

[0044] The catalyst was prepared with a mass ratio of g-C3N4 to Co(NO3)2·6H2O of 1:2.

[0045] Melamine was placed in a muffle furnace and heated to 550°C at a heating rate of 2.5°C / min, held at that temperature for 4 h, and then cooled to obtain pale yellow lumps of g-C3N4.

[0046] 720 mg of the above g-C3N4 precursor was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 1440 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0047] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2h under an inert N2 atmosphere. After carbonization, the resulting solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as CN / Co-2.

[0048] Example 3:

[0049] The catalyst was prepared with a mass ratio of g-C3N4 to Co(NO3)2·6H2O of 1:3.

[0050] Melamine was placed in a muffle furnace and heated to 550°C at a heating rate of 2.5°C / min, held at that temperature for 4 h, and then cooled to obtain a pale yellow blocky g-C3N4 precursor.

[0051] 720 mg of the above g-C3N4 precursor was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0052] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2h under an inert N2 atmosphere. After carbonization, the resulting solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as CN / Co-3.

[0053] Example 4:

[0054] The catalyst was prepared with a mass ratio of g-C3N4 to Co(NO3)2·6H2O of 1:4.

[0055] Melamine was placed in a muffle furnace and heated to 550°C at a heating rate of 2.5°C / min, held at that temperature for 4 h, and then cooled to obtain a pale yellow blocky g-C3N4 precursor.

[0056] 720 mg of the above g-C3N4 precursor was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2880 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0057] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2 hours under an inert N2 atmosphere. After carbonization, the obtained solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as CN / Co-4.

[0058] Example 5:

[0059] The catalyst was prepared with a mass ratio of g-C3N4 to Co(NO3)2·6H2O of 1:6.

[0060] Melamine was placed in a muffle furnace and heated to 550°C at a heating rate of 2.5°C / min, held at that temperature for 4 h, and then cooled to obtain a pale yellow blocky g-C3N4 precursor.

[0061] 720 mg of the above g-C3N4 precursor was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 4320 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0062] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2h under an inert N2 atmosphere. After carbonization, the resulting solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as CN / Co-5.

[0063] Examples 6-11 show that different catalysts were prepared by adding hydrogen etching with a mass ratio of H2-g-C3N4-X and Co(NO3)2·6H2O of 1:3 and different etching times.

[0064] Example 6

[0065] Melamine was heated to 550°C in a muffle furnace at a heating rate of 2.5°C / min and held for 4 h. After cooling, a pale yellow, blocky g-C3N4 precursor was obtained. The ground sample was transferred to a tube furnace and etched at 550°C under a hydrogen atmosphere (flow rate 20 mL / min) at a heating rate of 2°C / min for 0 h (this example essentially did not involve a hydrogen etching process). After cooling to room temperature, the product was collected. The powder color gradually changed from the original pale yellow to orange-yellow and then brownish-yellow, indicating a structural change. The sample was named: H2-g-C3N4-0.

[0066] 720 mg of H2-g-C3N4-O was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0067] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2 hours under an inert N2 atmosphere. After carbonization, the obtained solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as Co-NC-0.

[0068] Example 7

[0069] Melamine was heated to 550°C in a muffle furnace at a heating rate of 2.5°C / min and held for 4 h. After cooling, a pale yellow, blocky g-C3N4 precursor was obtained. The ground sample was transferred to a tube furnace and etched at 550°C under a hydrogen atmosphere (flow rate 20 mL / min) at a heating rate of 2°C / min for 1 h. After cooling to room temperature, the product was collected. The powder color gradually changed from the original pale yellow to orange-yellow and then brownish-yellow, indicating a structural change. The sample was named: H2-g-C3N4-1.

[0070] 720 mg of H2-g-C3N4-1 was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0071] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2 hours under an inert N2 atmosphere. After carbonization, the obtained solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as Co-NC-1.

[0072] Example 8

[0073] Melamine was heated to 550°C in a muffle furnace at a heating rate of 2.5°C / min and held for 4 h. After cooling, a pale yellow, blocky g-C3N4 precursor was obtained. The ground sample was transferred to a tube furnace and etched at 550°C under a hydrogen atmosphere (flow rate 20 mL / min) at a heating rate of 2°C / min for 3 h. After cooling to room temperature, the product was collected. The powder color gradually changed from the original pale yellow to orange-yellow and then brownish-yellow, indicating a structural change. The sample was named: H2-g-C3N4-3.

[0074] 720 mg of H2-g-C3N4-3 was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0075] The dried sample was transferred to a tube furnace and heated at 2 °C / min. It was then carbonized at 700 °C for 2 h under an inert N2 atmosphere. After carbonization, the solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as Co-NC-3.

[0076] Example 9

[0077] Melamine was heated to 550°C in a muffle furnace at a heating rate of 2.5°C / min and held for 4 h. After cooling, a pale yellow, blocky g-C3N4 precursor was obtained. The ground sample was transferred to a tube furnace and etched at 550°C under a hydrogen atmosphere (flow rate 20 mL / min) at a heating rate of 2°C / min for 5 h. After cooling to room temperature, the product was collected. The powder color gradually changed from the original pale yellow to orange-yellow and then brownish-yellow, indicating a structural change. The sample was named: H2-g-C3N4-5.

[0078] 720 mg of H2-g-C3N4-5 was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0079] The obtained dried sample was transferred to a tube furnace and heated at 2℃ / min. It was then carbonized at 700℃ for 2 hours under an inert N2 atmosphere. After carbonization, the obtained solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as Co-NC-5.

[0080] Example 10

[0081] Melamine was heated to 550°C in a muffle furnace at a heating rate of 2.5°C / min and held for 4 h. After cooling, a pale yellow, blocky g-C3N4 precursor was obtained. The ground sample was transferred to a tube furnace and etched at 550°C under a hydrogen atmosphere (flow rate 20 mL / min) at a heating rate of 2°C / min for 7 h. After cooling to room temperature, the product was collected. The powder color gradually changed from the original pale yellow to orange-yellow and then brownish-yellow, indicating a structural change. The sample was named: H2-g-C3N4-7.

[0082] 720 mg of H2-g-C3N4-7 was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0083] The dried sample was transferred to a tube furnace and heated at 2 °C / min. It was then carbonized at 700 °C for 2 h under an inert N2 atmosphere. After carbonization, the solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as Co-NC-7.

[0084] Example 11

[0085] Melamine was heated to 550°C in a muffle furnace at a heating rate of 2.5°C / min and held for 4 h. After cooling, a pale yellow, blocky g-C3N4 precursor was obtained. The ground sample was transferred to a tube furnace and etched at 550°C under a hydrogen atmosphere (flow rate 20 mL / min) at a heating rate of 2°C / min for 9 h. After cooling to room temperature, the product was collected. The powder color gradually changed from the original pale yellow to orange-yellow and then brownish-yellow, indicating a structural change. The sample was named: H2-g-C3N4-9.

[0086] 720 mg of H2-g-C3N4-9 was dispersed in 180 mL of methanol and sonicated for 30 min to obtain a homogeneous suspension A. 2160 mg of Co(NO3)2·6H2O was dissolved in 180 mL of methanol to form solution B. After stirring thoroughly, solution B was added to solution A, and the mixture was sonicated again for 30 min to obtain solution C. 3420 mg of 2-methylimidazole was dissolved in 180 mL of methanol to form solution D. After stirring thoroughly, solution D was slowly added to solution C and mixed completely. The mixture was stirred at room temperature for 24 h, and the final product was collected by centrifugation. The obtained product was washed with methanol, centrifuged, and dried overnight at 60°C to obtain a purple powder.

[0087] The dried sample was transferred to a tube furnace and heated at 2 °C / min. It was then carbonized at 700 °C for 2 h under an inert N2 atmosphere. After carbonization, the solid product was cooled and collected to obtain the cobalt-based catalyst, denoted as Co-NC-9.

[0088] The above catalyst evaluation conditions are as follows: The catalyst was mixed with methanol and added to a 25 mL reactor. After sealing, the reactor was flushed three times with carbon monoxide to replace the air. A mixture of CO and O2 (volume ratio = 2:1) was then introduced until the total pressure reached 3.0 MPa. The heating system was then activated to raise the reaction temperature to 110°C and maintained for 2 h. After the reaction, the product composition was analyzed using a Nexis GC-2030 gas chromatograph with a flame ionization detector (FID). The cyclic stability of the catalyst was tested under the same conditions as the catalytic activity evaluation. The main reaction byproducts included dimethoxymethane (DMM) and methyl formate (MF). The catalytic performance was characterized by the following parameters: methanol conversion (C... MeOH， %), dimethyl carbonate selectivity (S) DMC， %), space-time yield of dimethyl carbonate (STY) DMC， g·g -1 ·h -1 ), dimethoxymethane selectivity (S DMM， %) and methyl formate selectivity (S) MF， %)

[0089] Table 1 Comparison of the performance of unetched catalysts with different mass ratios in the catalytic synthesis of DMC via methanol oxidative carbonylation.

[0090] Table 2 Comparison of the performance of catalysts at different etching times in the synthesis of DMC via methanol oxidative carbonylation.

[0091] Table 1 shows that the mass ratio of g-C3N4 to Co(NO3)2·6H2O significantly affects catalyst performance. At a mass ratio of 1:3, the cobalt species exhibit suitable dispersion on the support surface, achieving a good balance between the number and structure of active sites. This results in high methanol conversion and DMC selectivity, demonstrating the best overall performance. However, the space-time yield of DMC is relatively low at this ratio, mainly due to mass transfer limitations imposed by the pore structure. Insufficient reactant diffusion efficiency or hindered product desorption limits the amount of product generated per unit time. This indicates that while maintaining the number of active sites, further optimization of the pore structure is needed to improve mass transfer efficiency. In summary, the 1:3 ratio achieves a good balance between activity and selectivity and is the preferred ratio under current experimental conditions.

[0092] Table 2 shows that g represents the surface defect control after hydrogen etching. The catalyst obtained using C3N4 support (H2-g-C3N4) as a substrate significantly improved methanol conversion and dimethyl carbonate space-time yield compared to catalysts without hydrogen etching, exhibiting excellent reactivity and selectivity. Specifically, it achieved high methanol conversion while increasing DMC selectivity to over 99% and realizing excellent space-time yield.

[0093] Table 3 Catalytic performance in continuous cycle tests

[0094] Cyclic performance testing was conducted using the catalyst prepared in Example 10, and the results are shown in Table 3. With increasing reaction cycle number, methanol conversion and DMC space-time yield gradually decreased: in five cycles, C... MeOH It decreased from 10.74% to 6.18%, and STY decreased from 4.86 g·g -1 ·h -1 Reduced to 2.38 g·g -1 ·h -1 (Table 3). Conversely, DMC selectivity remained at a high level throughout the cycle ( (97%), indicating that the active phase retains its inherent selectivity even after partial loss of activity. Cyclic performance tests demonstrate that the catalyst exhibits stable selectivity and controllable activity decay, further validating the robustness of the cobalt-based catalyst structure constructed with the defect-regulated support and its long-term applicability in the methanol oxidative carbonylation reaction.

[0095] To further investigate the reasons for the improved catalyst performance, the following characterization was performed: XPS: Surface chemical composition and valence states were analyzed using X-ray photoelectron spectroscopy (XPS) with Al Kα radiation as the light source, as measured by an ESCALAB 250Xi spectrometer. Binding energy was calibrated using the C 1s peak at 284.8 eV, and quantitative analysis was performed using a semi-quantitative method based on narrow-scan spectroscopy (results are shown in [link to results]). Figure 1 (Table 5) XAFS: Co K-edge X-ray absorption fine structure (XAFS) spectra were acquired in transmission mode at room temperature, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Standard procedures were used for data analysis and fitting, and wavelet transform (WT) analysis was employed to distinguish different coordination environments (results see [see details]). Figure 1 (Table 4).

[0096] STEM-EELS: Aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were performed on a JEOL ARM-200F transmission electron microscope equipped with a probe-side spherical aberration corrector and a cold field emission electron gun. The microscope's operating accelerating voltage was 200 kV. Under these conditions, the STEM probe current was approximately 17 pA, the probe size was approximately 0.133 nm, and the incident beam convergence half-angle was 20.6 mrad. High-angle annular dark field (HAADF) images were acquired using a detector with an acquisition angle range of 54–220 mrad.

[0097] The EELS testing system is equipped with a Gatan energy filter and a high-resolution EELS spectrometer, possessing a large solid angle acquisition capability (~0.97 sr). At 200 kV, the system's energy resolution is better than 0.5 eV. STEM-EELS data is acquired using spectral imaging mode, enabling spatially resolved analysis of elemental distribution and electronic structure at the single-particle scale. The obtained EELS data is simultaneously used for elemental imaging and quantitative analysis of the fine structure of the cobalt L-edge (results are shown in...). Figure 2 ).

[0098] Table 4. EXAFS fitting parameters for Co-K edges of different samples (s0 = 0.77)

[0099] a CN , coordination number; b R The distance between the absorbing atom and the backscattering atom; c σ 2The Debye-Waller factor is used to characterize the combined effect of thermal disorder and structural disorder. d ΔE 0, Internal potential correction value R Factors are used to characterize goodness of fit; Fitting range: 3 k ( / Å) 12 and 1 R (Å) 3.

[0100] Table 5. Co and N species distribution in Co-NCX series catalysts

[0101] a Measured by XPS (Co 2p3 / 2); b Measured by XPS (N 1s) test.

[0102] The Co-NCX catalysts in Examples 6-11 exhibited excellent catalytic performance in the methanol oxidative carbonylation reaction, achieving high methanol conversion while simultaneously increasing the selectivity of dimethyl carbonate (DMC) to over 99% and obtaining excellent space-time yields. To explore the underlying reasons for this significant performance improvement, bulk-sensitive XAFS, surface-sensitive XPS, and high spatial resolution STEM-EELS techniques were used to systematically investigate the valence state distribution and coordination environment of Co at the single-particle scale. The study revealed a Co valence state gradient structure within the particles, characterized by a "metallic core – high-valence surface," which is identified as the key to the enhanced catalytic performance.

[0103] First, the bulk average structure of the sample was detected using Co K-edge XAFS technology, confirming that the catalyst particles are predominantly composed of metallic Co. 0 The primary oxidation was not observed. The XANES spectra of Co K-edge in Figure 1a show that the absorption edge position and white line intensity of all samples are between those of metallic Co foil and CoO, and after H2 etching, the spectra did not show a shift towards higher valence Co. 3+ The systematic shift trend indicates that the bulk electronic structure is altered by the metallic Co. 0 Dominant, no large amount of high-valent Co in the bulk phase 3+Species. The R-space Fourier transform spectra in Figures 1b–d all show that the main scattering peaks are located at ~2.15 Å, corresponding to the Co-Co metallic bond scattering path; while no obvious Co-N or Co-O coordination signals were detected in the 1.4–1.9 Å range. The quantitative fitting data in Table 4 show that the Co-Co coordination number of Co-NC-7 is 8.24±1.22, significantly lower than 12 for the bulk metallic Co foil, and the bond length is consistent with that of metallic Co (2.49 Å), confirming that the bulk phase is low-coordination metallic Co. 0 Nanoclusters, not oxides or coordination compounds. Wavelet transform analysis in Figures 1e–f further shows that Co-NC-7 exhibits characteristic signals of metallic Co only in the high-k region, with no Co-O or Co-N characteristic peaks in the low-k region, completely ruling out the possibility of oxides or coordination structures in the bulk phase that are masked by conventional EXAFS. In other words, the catalyst bulk phase consistently maintains a metallic Co composition. 0 The dominant coordination environment provides a metallic core for the valence gradient structure.

[0104] Surface-sensitive XPS analysis revealed significant electronic structure reconstruction on the catalyst surface, resulting in the enrichment of high-valent Co. 3+ The quantitative data in Figure 1g–h and Table 5 show that the unetched sample Co-NC-0 contains Co. 2+ The majority (52.96%) of the surface Co content increased with increasing H2 etching. 3+ The proportion of Co increased significantly in the core sample Co-NC-7. 3+ Co accounted for 60.77%, becoming the dominant valence state on the surface. Co was present in moderately etched samples. 0 The signal (22.92% in Co-NC-7), combined with the bulk metallic state conclusions shown by XAFS, indicates that the signal originates from the metallic core within the XPS detection depth, while the sample surface is actually composed of Co. 3+ The enrichment was the dominant feature. Figure 1i and Table 5 show that H2 etching significantly increased the proportion of pyridine N (59.81% in Co-NC-7) and significantly decreased the proportion of metal-coordinated N (21.46%), indicating that the original saturated Co-N on the surface was due to this enrichment. x When the network is disrupted, the resulting defective N-anchor points can stabilize high-priced Co. 3+ -N x Structure, with surface Co 3+ The presence of [a specific element] provides structural support. Co is enriched on the catalyst surface after defect engineering reconstruction. 3+ -N x With bulk metal Co 0The formation of significant valence state differences constitutes the surface feature of the valence state gradient structure.

[0105] To directly verify the existence of spatial valence state separation within the particles, aberration-corrected HAADF-STEM and STEM-EELS analyses were further conducted. The HAADF-STEM image in Figure 2a shows that the catalyst particles have a dense and continuous structure, without hollow areas, multiphase splicing, or core-shell separation, confirming that the valence state differences truly exist within a single continuous particle. The EELS elemental imaging in Figure 2b shows that the Co L-edge signal is clearly distributed within the particles, while the OK-edge signal is concentrated only at the particle edges, with a significant decrease in the internal O signal. Combined with the result of no obvious Co-O signal in bulk XAFS, this proves that oxidation is only a localized surface behavior, and the formation of high-valence Co on the surface originates from local electronic reconstruction, rather than bulk oxidation. EELS radial line scan analysis of the Co L-edge fine structure in Figures 2c-e, after unified background correction and integration windowing to eliminate thickness effect interference, revealed that the Co L3 / L2 intensity ratio at the particle edges is approximately 1.39, corresponding to a higher average valence state, consistent with the Co intensity measured by XPS. 3+ The enrichment layer is consistent; when scanning into the interior of the particles, the L3 / L2 ratio gradually decreases to ≈1.29, the valence state decreases and tends towards the metallic state, consistent with the Co measured by XAFS. 0 The proportions are consistent. The continuous change in this ratio directly confirms the existence of a single-particle memory originating from the kernel Co. 0 To surface Co 3+ A clear valence gradient.

[0106] In summary, this study successfully prepared a series of Co-NCX catalysts using hydrogen-etched g-C3N4 as a support and employing an in-situ growth and pyrolysis strategy for ZIF-67. Characterization results from XAFS, XPS, and STEM-EELS show that the "Co" formation within individual catalyst particles in this series of catalysts... 0 Metal core + Co 3+ -N x The "continuous valence gradient" structure on the surface allows for precise control of the radial electronic environment, leading to a comprehensive improvement in the catalytic performance of the methanol oxidative carbonylation reaction: Surface Co 3+ -N x The site exhibits intrinsic selectivity for the target reaction, enabling efficient catalysis of oxidative elementary reactions; Core Co 0Serving as an electron reservoir and transport channel, it provides support for reduction-based elementary reactions. The spatial separation and electronic connectivity between the two effectively avoid competition for active sites and quenching of reaction intermediates. Simultaneously, the defect-rich N sites on the support can modulate the electronic structure of cobalt, enhancing both the activation ability of reactants and stabilizing catalytic active sites. Through the synergistic effect of these multiple advantages, this series of catalysts ensures high methanol conversion while simultaneously improving the selectivity of dimethyl carbonate (DMC) to over 99%, and also achieves excellent space-time yield.

[0107] Example 12 The Co-NC catalyst of the present invention is used in a liquid-phase synthesis reaction apparatus for dimethyl carbonate. The method uses methanol as liquid-phase raw material, carbon monoxide and oxygen as gas-phase raw materials, and the above-mentioned Co-NC catalyst as a liquid-phase chlorine-free cobalt carbon catalyst. As shown in the figure, it includes a reaction apparatus and a product removal apparatus. The reaction apparatus includes two interconnected first-stage airlift external circulation reactors 1 and 2, which enable catalyst separation and reflux. The first-stage airlift external circulation reactor 1 has a gas phase inlet 1-1 at the bottom and a gas phase outlet 1-5 at the top. A liquid phase inlet 1-2 is located at the lower part of the first-stage airlift external circulation reactor 1, and a liquid phase outlet 1-4 is located in the upper middle cross-flow filter of the first-stage airlift external circulation reactor 1. The gas phase outlet 1-5 of the first-stage airlift external circulation reactor 1 is connected to the gas phase inlet 2-1 of the second-stage airlift external circulation reactor 2 via a pipe. The liquid phase outlet 1-4 of the first-stage airlift external circulation reactor 1 is connected to the liquid phase inlet 2-2 of the second-stage airlift external circulation reactor 2 via a pipe. An oxygen feed pipe 5 is connected to the oxygen inlet 1-3 of the first-stage airlift external circulation reactor 1.

[0108] The product removal device includes a condenser 6 and a gas-liquid two-phase separator 7; the gas phase outlet 2-5 of the second-stage airlift external circulation reactor 2 is connected to the heat medium inlet pipe of the condenser 6, and the heat medium outlet of the condenser 6 is connected to the inlet pipe of the gas-liquid two-phase separator 7; the liquid phase outlet 2-4 of the second-stage airlift external circulation reactor 2 is connected to the product collection pipe 10 or connected to the liquid phase inlet 1-2 of the first-stage airlift external circulation reactor 1 through the liquid phase circulation pipe 12; The gas phase outlet 7-1 of the gas-liquid two-phase separator 7 is connected to the gas phase inlet 1-1 of the first-stage airlift external circulation reactor 1 via a gas phase circulation pipe 9; the liquid phase outlet 7-2 of the gas-liquid two-phase separator 7 is connected to the product collection pipe 10.

[0109] The cross-flow filter uses a high-temperature resistant special ceramic membrane module with a membrane pore size of 5-10 micrometers, a total membrane area of ​​1.0 m², an operating pressure of 0.1-0.4 MPa, and a catalyst single-pass separation efficiency of over 99.5%.

[0110] Using the dimethyl carbonate liquid-phase synthesis apparatus described in this embodiment, and in conjunction with a cobalt carbon catalyst, the methanol conversion rate is 8-12%; the dimethyl carbonate selectivity is 98-99.7%; and the dimethyl carbonate space-time yield is 2.84-4.86 g DMC / g.cat.h. After continuous operation for 500 hours without deactivation, the methanol conversion rate becomes 6-10%, the dimethyl carbonate selectivity is 98.5%, and the DMC space-time yield is 2.76-4.57 g DMC / g.cat.h.

[0111] Preferably, the first-stage airlift external circulation reactor 1 is provided with an oxygen inlet 1-3 in the middle, and the second-stage airlift external circulation reactor 2 is provided with an oxygen inlet 2-3 in the middle. The oxygen inlet is connected to an oxygen feed pipe 5 so that oxygen can be supplied separately to the first-stage airlift external circulation reactor 1 and the second-stage airlift external circulation reactor 2. Preferably, a booster pump 8 is installed on the gas phase circulation pipe 9 to provide circulation power; Preferably, a circulation pump 11 is installed on the liquid phase circulation pipe 12 to provide circulation power.

[0112] Preferably, the cross-flow filter on the reactor is configured with a membrane area according to the catalyst separation requirements, and a suitable power circulation pump is provided on the external circulation pipe.

[0113] For ease of explanation, relational terms such as “first” and “second” are used in the embodiments only to distinguish one component from another that has the same name, and do not necessarily require or imply any such actual relationship or order between the components.

[0114] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a cobalt carbon catalyst, characterized in that, Includes the following steps: S1, etch the graphitic carbon nitride (g-C3N4) under a hydrogen atmosphere to obtain a carrier H2-g-C3N4 with controlled defects on the surface; S2, the carrier H2-g-C3N4 with surface-regulated defects is dispersed in methanol, and then cobalt salt and organic ligand are added. After coordination reaction, the solid product after the reaction is the precursor composite material CN-ZIF-67. S3, the precursor composite material CN-ZIF-67 is heat-treated under an inert atmosphere to obtain the carbon-cobalt catalyst.

2. The method for preparing the carbon-cobalt catalyst according to claim 1, characterized in that, In step S1, the etching temperature of the etching process is 500~700℃ and the etching time is 0~9h; preferably, the etching temperature is 550℃ and the etching time is 7h.

3. The method for preparing the carbon-cobalt catalyst according to claim 1, characterized in that, In S2, the mass ratio of the carrier H2-g-C3N4 with surface-regulated defects, cobalt salt, and organic ligand is 1:(1~6):(4~5); the reaction time of the coordination reaction is 12~48 h, preferably 24 h.

4. The method for preparing the carbon-cobalt catalyst according to claim 3, characterized in that, The cobalt salt is cobalt nitrate, and the organic ligand is 2-methylimidazole.

5. The method for preparing the carbon-cobalt catalyst according to claim 3, characterized in that, The carrier H2-g-C3N4 with surface-regulated defects was dispersed in methanol by ultrasonic treatment to obtain a first mixture; cobalt nitrate was dissolved in methanol to obtain a second solution; the second solution and the first mixture were mixed and ultrasonically treated to obtain a third mixture; 2-methylimidazole was dissolved in methanol to obtain a fourth solution; the fourth solution was added to the third mixture and stirred continuously to carry out a coordination reaction.

6. The method for preparing the carbon-cobalt catalyst according to claim 1, characterized in that, In step S3, the heat treatment temperature is 600~900℃ and the heat treatment time is 60~180min; preferably, the heat treatment temperature is 700℃ and the heat treatment time is 120min; preferably, the inert atmosphere is a nitrogen atmosphere.

7. A method for synthesizing dimethyl carbonate, characterized in that, The synthesis method employs methanol liquid-phase oxidative carbonylation, and the catalyst is a cobalt carbon catalyst prepared according to any one of claims 1 to 6.

8. A reaction apparatus for the synthesis of dimethyl carbonate according to claim 7, characterized in that, The reaction apparatus comprises an airlift external circulation reactor, a condenser, and a gas-liquid two-phase separator. The raw material gas phase feed pipe is connected to the bottom gas phase inlet of the airlift external circulation reactor; the raw material liquid phase feed pipe is connected to the lower liquid phase inlet of the airlift external circulation reactor; the oxygen feed pipe is connected to the middle oxygen inlet of the airlift external circulation reactor; the gas phase outlet at the top of the airlift external circulation reactor is connected to the gas phase inlet pipe of the condenser; the gas phase outlet of the condenser is connected to the inlet pipe of the gas-liquid two-phase separator; the gas phase outlet of the gas-liquid two-phase separator is connected to the raw material gas phase feed pipe via a gas phase circulation pipe; the liquid phase outlet of the airlift external circulation reactor is connected to the product collection pipe; the product collection pipe is connected to the raw material liquid phase feed pipe via a liquid phase circulation pipe; and the liquid phase outlet of the gas-liquid two-phase separator is connected to the product collection pipe via a pipe.

9. The reaction apparatus according to claim 8, characterized in that, The reaction apparatus includes n airlift external circulation reactors connected in series, where n ≥ 2; The raw material gas phase feed pipe is connected to the bottom gas phase inlet of the first-stage airlift external circulation reactor, and the raw material liquid phase feed pipe is connected to the liquid phase inlet at the bottom of the first-stage airlift external circulation reactor. The gas phase outlet at the top of the first-stage airlift external circulation reactor is connected to the gas phase inlet pipe at the bottom of the second-stage airlift external circulation reactor; the liquid phase outlet of the first-stage airlift external circulation reactor is connected to the liquid phase inlet pipe at the bottom of the second-stage airlift external circulation reactor. Up to this principle, The gas phase outlet at the top of the (n-1)th stage airlift external circulation reactor is connected to the gas phase inlet pipe at the bottom of the nth stage airlift external circulation reactor; the liquid phase outlet of the (n-1)th stage airlift external circulation reactor is connected to the liquid phase inlet pipe at the bottom of the nth stage airlift external circulation reactor. The gas phase outlet at the top of the nth stage airlift external circulation reactor is connected to the gas phase inlet pipe of the condenser. The gas phase outlet of the condenser is connected to the inlet pipe of the gas-liquid two-phase separator. The gas phase outlet of the gas-liquid two-phase separator is connected to the raw material gas phase feed pipe through a gas phase circulation pipe. The liquid phase outlet of the nth stage airlift external circulation reactor is connected to the product collection pipe. The product collection pipe is connected to the raw material liquid phase feed pipe through a liquid phase circulation pipe. The liquid phase outlet of the gas-liquid two-phase separator is connected to the product collection pipe through a pipe. The oxygen feed pipe is connected to the oxygen inlet in the middle of each stage of the airlift external circulation reactor.

10. The reaction apparatus according to claim 8 or 9, characterized in that, The gas phase circulation pipe connecting the gas phase outlet of the gas-liquid two-phase separator and the raw material gas phase feed pipe is also equipped with a booster pump. A circulation pump is also installed on the liquid phase circulation pipe that connects the product collection pipe and the raw material liquid phase feed pipe.

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