A 1,6-dicarbamate synthesized using M-N4 single-atom solvent and its preparation method

Abstract

The application discloses 1,6-diaminocarbamate synthesized by using M-N4 single-atom solvent and a preparation method, and the method comprises the following steps: mixing a metal salt with a composite nitrogen source composed of melamine and urea, heating to 450-550 DEG C under an inert atmosphere for low-temperature programmed carbonization pyrolysis, obtaining a metal single-atom loaded nitrogen-doped carbon solid material, then mixing and uniformly dispersing the metal single-atom loaded nitrogen-doped carbon solid material with an organic solvent, and obtaining M-N4 single-atom solvent; under a carbon dioxide atmosphere, making hexamethylenediamine and carbamate perform a catalytic reaction in the presence of M-N4 single-atom solvent, and obtaining 1,6-diaminocarbamate. The M-N4 single-atom solvent preparation process has low energy consumption, exhibits better activity and selectivity than high-temperature prepared catalysts in HDC synthesis, the liquid form ensures excellent initial dispersity, and further improves the catalytic efficiency. An extremely attractive technical path is provided for realizing economic and green synthesis of HDC.

Description

Technical Field

[0001] This invention belongs to the field of organic synthetic chemistry and industrial catalysis technology, specifically relating to a 1,6-dicarbamate synthesized using M-N4 single-atom solvent and its preparation method. Background Technology

[0002] The development of green synthesis processes for 1,6-dicarbamate (HDC) is a crucial aspect of the sustainable development of the polyurethane industry. While non-phosgene routes using hexamethylenediamine and carbamates as raw materials hold great promise, they remain limited by the availability of efficient, stable, and economical catalysts. Existing catalyst systems often fail to simultaneously meet the multiple requirements of high activity, high selectivity, low cost, and ease of operation. In recent years, single-atom catalysts, particularly those with an M-N4 structure, have shown great potential in various catalytic reactions due to their extremely high atom economy and tunable electronic properties. However, traditional methods for preparing M-N4 catalysts often rely on high-temperature pyrolysis (>800℃) of mixtures of nitrogen-containing carbon precursors and metal salts, resulting in high energy consumption and a tendency for activity to decrease due to metal migration and aggregation. Furthermore, dispersion and handling issues arising from the solid catalyst morphology persist.

[0003] Currently, reported HDC synthesis routes mainly include: a direct catalytic route using carbon dioxide (CO2) and diamine as raw materials. Chinese patent application CN116693425B discloses a method for one-step synthesis of pentanedicarbamate from pentanediamine and CO2 in the presence of an alcohol using a cerium-based catalyst. This method directly utilizes inexpensive CO2, but the catalyst activity and selectivity still have room for improvement, and the reaction typically requires a large amount of alcohol as a solvent and reactant, increasing the separation burden and cost. Another route uses catalytic aminolysis of carbamate and diamine as raw materials. Chinese patent application CN102989525B discloses a method for preparing a heterogeneous catalyst through simple solvothermal treatment of oxides, and its application in catalyzing the synthesis of HDC from hexanediamine and carbamate. This catalyst is recyclable, but the catalyst precursor (such as PbO2) is expensive or involves precious metals, and the catalytic performance is sensitive to reaction conditions (such as temperature). The overall economic efficiency and versatility of the process need to be improved.

[0004] In summary, while existing HDC synthesis technologies have made progress in terms of greening, their catalytic systems often struggle to simultaneously achieve high catalytic efficiency, excellent selectivity, low raw material and preparation costs, mild reaction conditions, and simple process operation. Therefore, there is an urgent need to find a single-atom solvent, M-N4, obtained through a low-temperature pyrolysis strategy using a composite nitrogen source, for HDC synthesis. Developing a novel, efficient, low-cost, and easy-to-implement HDC synthesis catalytic system that can comprehensively address these issues is of great significance for promoting the green and sustainable development of the polyurethane industry. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent, and to solve the technical problem of how to develop a green synthesis method for 1,6-dicarbamate that is highly active, highly selective, easy to operate and can efficiently utilize CO2, while significantly reducing the energy consumption and cost of catalyst preparation.

[0006] To achieve the above objectives, the present invention employs the following technical solution: This invention discloses a method for synthesizing 1,6-dicarbamate using an M-N4 single-atom solvent, comprising: mixing a metal salt with a composite nitrogen source composed of melamine and urea, heating to 450-550°C under an inert atmosphere for low-temperature programmed carbonization pyrolysis to obtain a nitrogen-doped carbon solid material supported by a metal single atom; then mixing and homogenizing the nitrogen-doped carbon solid material supported by a metal single atom with an organic solvent to obtain an M-N4 single-atom solvent; and catalytically reacting hexamethylenediamine with carbamate in the presence of the M-N4 single-atom solvent under a carbon dioxide atmosphere to obtain 1,6-dicarbamate.

[0007] Preferably, the molar ratio of hexamethylenediamine to carbamate is 1:1 to 1:5; the mass of the M-N4 single-atom solvent is 0.1%-10% of the mass of hexamethylenediamine.

[0008] Preferably, the molar ratio of hexamethylenediamine to carbamate is 1:2 to 1:4; the mass of the M-N4 single-atom solvent is 0.5% of the mass of hexamethylenediamine. 5%.

[0009] Preferably, the temperature of the catalytic reaction is 150°C. At 250℃, the carbon dioxide pressure is 0.5. 3 MPa.

[0010] Preferably, the temperature of the catalytic reaction is 180°C. At 220℃, the carbon dioxide pressure is 0.8. 2 MPa.

[0011] Preferably, the metal salt is a compound of at least one metallic element selected from platinum, palladium, ruthenium, nickel, copper, iron, manganese, silver, cobalt, and cerium.

[0012] Preferably, the mass ratio of melamine to urea in the composite nitrogen source is 1:5 to 5:1; The carbamate is methyl carbamate or ethyl carbamate.

[0013] Preferably, the organic solvent is any one of methanol, ethanol, acetone, isopropanol, N,N-dimethylformamide, and N-methylpyrrolidone; The dispersion concentration of the nitrogen-doped carbon solid material supported by the metal single atom in the organic solvent is 0.5-20 mg / mL.

[0014] Preferably, the heating rate of the low-temperature programmed carbonization pyrolysis is 2-10℃ / min, and the pyrolysis time is 2-6 hours; the inert atmosphere is nitrogen or argon.

[0015] The present invention provides a 1,6-dicarbamate, which is prepared by the above-described method of synthesizing 1,6-dicarbamate using M-N4 single-atom solvent.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for synthesizing 1,6-dicarbamate using an M-N4 single-atom solvent. A single-atom catalyst with a well-defined M-N4 coordination structure, prepared by low-temperature pyrolysis of a composite nitrogen source, is applied to HDC synthesis in a stable liquid solvent. The low-temperature pyrolysis conditions of 450-550℃ are significantly lower than the >800℃ of traditional single-atom catalysts, directly reducing energy consumption. Simultaneously, the composite nitrogen source of melamine and urea works synergistically within this temperature range, effectively promoting the coordination of metal ions and nitrogen species to generate well-structured M-N4 active centers, while maximally suppressing the migration and aggregation of metal atoms. This is the structural basis for achieving high catalytic activity. The liquid solvent solves the problems of uneven dispersion, insufficient exposure of active sites, and cumbersome operation associated with solid catalysts in the reaction system. It ensures that the M-N4 active centers are highly dispersed at the molecular level in the reaction medium, guaranteeing efficient contact between reactant molecules and catalytic sites, significantly improving mass transfer efficiency and catalytic performance. The combination of these two methods ultimately enabled the efficient and selective one-step conversion of inexpensive CO2 and basic chemical raw materials into high-value-added HDC under mild conditions and with a highly atom-economical catalytic approach.

[0017] Furthermore, by optimizing the reactant ratio and catalyst loading, excess carbamate is beneficial for promoting the forward reaction of aminolysis and improving the conversion rate of the target product. The catalyst dosage of 0.1%-10% ensures that high-efficiency catalysis can be achieved with extremely low amounts of precious or transition metals, demonstrating the extremely high atomic utilization rate of single-atom catalysts. It also takes into account the dispersion stability of the catalyst and the economic efficiency of separation and recovery, avoiding insufficient activity due to too low dosage or waste due to too high dosage and possible side reactions caused by excessively high local concentrations.

[0018] Furthermore, a temperature range of 150-250°C is sufficient to activate the C=O bonds of the carbamate and CO2 molecules, enabling them to react effectively at the M-N4 sites, while avoiding catalyst deactivation due to excessively high temperatures or product decomposition. A preferred temperature of 180-220°C further enhances reaction kinetic efficiency. A CO2 pressure of 0.5-3 MPa ensures sufficient CO2 concentration in the reaction system to participate in the reaction or regulate reaction equilibrium, promoting the formation and transformation of intermediates. A preferred pressure of 0.8-2 MPa ensures sufficient driving force for the reaction while reducing the pressure resistance requirements of the equipment, thus improving operational safety and economy.

[0019] Furthermore, a specific ratio of composite nitrogen sources is key to forming an efficient M-N4 structure. Melamine provides abundant pyridine nitrogen and structural rigidity, while urea generates a large number of nitrogen-containing small molecules and gases during pyrolysis, playing a role in pore formation and promoting nitrogen doping. When combined in a ratio of 1:5 to 5:1, the two can synergistically regulate the formation of the carbon-nitrogen framework and the coordination environment of the metal during low-temperature pyrolysis, ensuring the generation of nitrogen-doped carbon supports rich in M-N4 sites. Methyl or ethyl carbamate is used as a reactant because short-chain alkyl (methyl, ethyl) carbamates have high reactivity and volatility, which is beneficial for the forward reaction and the subsequent separation and purification of products. This is an important factor in achieving highly selective synthesis of HDC.

[0020] Furthermore, polar aprotic solvents such as DMF and NMP exhibit excellent wettability and dispersibility for metal-nitrogen-doped carbon materials, effectively preventing the aggregation of single-atom active sites. A concentration range of 0.5-20 mg / mL ensures sufficient catalyst loading to provide ample active sites while avoiding excessively high concentrations that could lead to excessive system viscosity, hindered mass transfer, or decreased dispersion stability. This flowable catalyst form greatly simplifies the feeding, mixing, and separation / recovery processes.

[0021] Furthermore, a slow heating rate of 2-10 °C / min facilitates the gradual and orderly decomposition, condensation, and carbonization of the composite nitrogen source and metal salt precursors during pyrolysis, avoiding structural collapse or rapid aggregation of metal particles caused by violent reactions. A sufficient pyrolysis time of 2-6 hours ensures the full formation and stabilization of the carbon-nitrogen framework, as well as the adequate coordination between metal and nitrogen species. The inert atmosphere of nitrogen or argon prevents the metal from being oxidized at high temperatures, protecting the formation of the M-N4 active centers. This combination of conditions is key to the successful low-temperature preparation of highly active, structurally well-defined M-N4 single-atom catalysts.

[0022] This invention discloses a 1,6-dicarbamate that directly utilizes the greenhouse gas CO2 in its production process, and the catalytic system is recyclable, reducing the use and emission of toxic and harmful substances at the source, resulting in a lower environmental footprint for the final product. The M-N4 single-atom solvent catalyst used exhibits extremely high catalytic selectivity and atomic efficiency (e.g., a yield of up to 92.3% in Example 1), meaning that side reactions and byproducts are minimal during synthesis. Therefore, the obtained HDC product has higher purity and significantly lower impurity content than products prepared using traditional catalytic systems (e.g., Comparative Examples 1 and 2), thus leading to superior product performance and more stable quality in downstream applications such as polyurethane synthesis. Attached Figure Description

[0023] Figure 1 The image shown is a Ni-SAS / NC XRD pattern disclosed in Embodiment 1 of this invention. Figure 2 The image shown is a Ni-SAS / NC SEM image disclosed in Embodiment 1 of this invention. Figure 3 This is a graph showing the effect of different catalysts on the HDC yield of this invention; Figure 4 The Ni disclosed in Embodiment 1 of this invention The recycling performance of SAS / NC / NMP in HDC synthesis. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Unless otherwise specified, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions.

[0026] Unless otherwise specified, all the technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0027] In this invention, unless otherwise specified, percentage (%) or parts refer to weight percentage or parts relative to the composition.

[0028] Unless otherwise specified, the components or preferred components involved in this invention can be combined with each other to form new technical solutions.

[0029] In this invention, unless otherwise specified, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "6~22" indicates that all real numbers between "6~22" have been listed in this document, and "6~22" is simply a shortened representation of these numerical combinations.

[0030] The "scope" disclosed in this invention can be in the form of a lower limit and an upper limit, and can be one or more lower limits and one or more upper limits, respectively.

[0031] In this invention, the term "and / or" as used herein refers to any combination of one or more of the associated listed items, as well as all possible combinations, and includes such combinations.

[0032] In this invention, unless otherwise stated, the various reactions or operation steps may be performed sequentially or in a particular order. Preferably, the reaction methods described herein are performed sequentially.

[0033] Unless otherwise stated, the technical and scientific terms used herein have the same meanings as those familiar to those skilled in the art. Furthermore, any methods or materials similar to or equivalent to those described herein may also be used in this invention.

[0034] This invention provides a preparation method using a composite nitrogen source of melamine and urea, at a significantly reduced temperature (450°C). A well-defined M-N4 single-atom catalyst was successfully prepared by pyrolysis at 550℃, and it can be converted into a stable "M-N4 single-atom solvent". This method has the outstanding advantages of low energy consumption, simple process, and mild conditions. However, the application efficiency and economy of this cost-effective M-N4 single-atom solvent, obtained through a unique low-temperature route, in HDC synthesis, a reaction with stringent requirements for both catalyst cost and performance, have not yet been revealed and verified. There is an urgent need to find an M-N4 single-atom solvent obtained based on a low-temperature pyrolysis strategy using a composite nitrogen source for HDC synthesis, and to systematically evaluate its catalytic performance, stability, and overall economy in the target reaction, thereby providing a new, efficient, low-cost, and easy-to-operate method for HDC synthesis.

[0035] This invention discloses a method for preparing an M-N4 single-atom solvent, comprising the following steps: 1) Dissolve a metal salt in a solvent to form a precursor solution with a concentration of 5-50 mg / mL; the metal salt is a compound containing at least one of the following metallic elements: platinum (Pt), palladium (Pd), ruthenium (Ru), nickel (Ni), copper (Cu), iron (Fe), manganese (Mn), silver (Ag), cobalt (Co), and cerium (Ce); the solvent is any one of methanol, ethanol, isopropanol, acetone, N,N-dimethylformamide, and N-methylpyrrolidone. 2) Add a composite nitrogen source to the precursor solution obtained in step 1), stir and mix evenly, and then dry at 40-100℃ to obtain a solid precursor; The composite nitrogen source is composed of melamine and urea, and the mass ratio of melamine to urea is 1:5 to 5:1; preferably, the mass ratio of melamine to urea in the composite nitrogen source is 1:2 to 2:1. 3) Place the solid precursor obtained in step 2) in an inert atmosphere, which is nitrogen or argon; heat it to 450-550℃ at a heating rate of 2-10℃ / min, and perform low-temperature programmed carbonization pyrolysis at this temperature for 2-6 hours. After cooling, obtain nitrogen-doped carbon solid material supported by metal single atoms. Preferably, the heating rate is 3-7℃ / min; 4) Mix the nitrogen-doped carbon solid material supported by metal single atoms obtained in step 3) with an organic solvent and homogenize and disperse it by stirring or sonication to form an M-N4 single-atom solvent; the organic solvent is any one of methanol, ethanol, acetone, isopropanol, N,N-dimethylformamide and N-methylpyrrolidone; the dispersion concentration of the nitrogen-doped carbon solid material supported by metal single atoms in the organic solvent is 0.5-20 mg / mL.

[0036] To optimize photoelectric generation efficiency, the transmittance of the light source and solvent must be matched: a 365 nm ultraviolet lamp is used for alcohols; and a 450 nm blue LED is used for N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO).

[0037] This invention provides an innovative application of a single-atom solvent with a clearly defined M-N4 active center, prepared based on a low-temperature pyrolysis strategy using a composite nitrogen source, in the catalytic synthesis of the important chemical intermediate 1,6-dicarbamate (HDC).

[0038] The present invention provides a method for synthesizing 1,6-dicarbamate (HDC) using an M-N4 single-atom solvent, comprising: reacting hexamethylenediamine (HDA) with carbamate in the presence of a catalyst under a carbon dioxide atmosphere, wherein the catalyst is an M-N4 single-atom solvent liquid catalytic system.

[0039] The active metal center in the M-N4 single-atom solvent is at least one of cobalt (Co), nickel (Ni), copper (Cu), or platinum (Pt). These metals exhibit a unique activation ability for C=O bonds or CO2 molecules in urethane esters in the M-N4 configuration formed at low temperatures.

[0040] The reaction is carried out in a closed reactor such as an autoclave at a temperature of 150°C. At 250℃, the carbon dioxide pressure is 0.5. 3MPa. The preferred reaction temperature range is 180°C. 220℃; the preferred carbon dioxide pressure is 0.8. 2 MPa.

[0041] Carbamates are methyl carbamate or ethyl carbamate.

[0042] The molar ratio of hexamethylenediamine to carbamate is 1:1 to 1:5, preferably 1:2 to 1:4.

[0043] The amount of M-N4 single-atom solvent used, based on the mass of the solid catalyst it contains, accounts for 0.1% of the mass of the reactant hexamethylenediamine. 10%, preferably 0.5% 5%.

[0044] The M-N4 single-atom solvent can be directly added to the reaction system in liquid form, which simplifies the operation steps. After the reaction is completed, the solid catalyst components can be recovered by centrifugation, or the product can be separated from the M-N4 single-atom solvent by distillation, thus achieving the separation of the catalyst and the product.

[0045] M-N4 single-atom solvent is produced from a composite nitrogen source containing melamine and urea at 450°C. It is prepared by low-temperature pyrolysis at 550℃.

[0046] The catalytically active center in the M-N4 single-atom solvent is obtained through a specific ratio of melamine and urea composite nitrogen source at 450°C. Under low-temperature pyrolysis conditions of 550℃, a distinct M-N4 structure is formed through direct coordination with metal ions. This low-temperature preparation strategy is a core feature that distinguishes it from other single-atom catalyst pathways.

[0047] This invention combines a low-temperature (450-550℃) pyrolysis strategy using a composite nitrogen source (melamine and urea) with the concept of an M-N4 single-atom liquid solvent, successfully applying it to the one-step catalytic synthesis of HDC using CO2 and carbamate as carbonyl sources and hexamethylenediamine as an amine source. At the catalyst preparation level, low-temperature pyrolysis significantly reduces energy consumption, while the composite nitrogen source ensures the controllable formation of high-density, well-defined M-N4 active centers, avoiding metal agglomeration and laying the foundation for high activity. At the catalyst application level, converting it into a liquid single-atom solvent solves the problems of poor dispersibility, limited mass transfer, and cumbersome operation associated with traditional solid catalysts, achieving molecular-level dispersion and efficient utilization of the catalyst in the reaction system. At the catalytic performance level, this method exhibits superior HDC yield (up to 92.3% in Example 1) and excellent selectivity compared to catalysts prepared at high temperatures. Simultaneously, the catalyst (e.g., Ni-based) demonstrates good cycling stability (yield maintained at 89.8% after 5 cycles). In terms of technology and economy, it directly utilizes inexpensive CO2, the reaction conditions are relatively mild, the amount of catalyst used is small and easy to recover, and the whole process is simple and green.

[0048] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0049] Example 1 Using nickel-based M N4 single-atom solvent (Ni SAS / NC / NMP-catalyzed synthesis of HDC (1) Preparation of catalyst: nickel-based M N4 single-atom solvent (Ni Preparation of SAS / NC / NMP.

[0050] First, the precursor was prepared: 0.8 g of nickel nitrate hexahydrate was accurately weighed and dissolved in 50 mL of anhydrous ethanol. The solution was magnetically stirred for 30 minutes until completely dissolved, forming a green transparent solution. Then, 7.5 g of melamine and 7.5 g of urea (mass ratio 1:1) were added to this solution, and the mixture was stirred continuously for 2 hours to form a homogeneous slurry. The slurry was transferred to a petri dish and placed in a forced-air drying oven at 60°C for 12 hours to obtain a light green blocky solid. This solid was then ground into a fine powder to obtain the precursor.

[0051] Next, low-temperature pyrolysis was performed: the ground precursor powder was spread evenly in a ceramic boat and placed in the center of a tube furnace. Under the protection of a continuous flow of high-purity nitrogen (flow rate 50 mL / min), the temperature was programmed to reach 500℃ at a heating rate of 5℃ / min. After reaching 500℃, the temperature was held constant at this temperature (carbonization) for 4 hours. After carbonization, the powder was naturally cooled to room temperature in a nitrogen flow to obtain a fluffy black solid powder, denoted as Ni. SAS / NC.

[0052] Finally, the preparation of the single-atom solvent was carried out: 50.0 mg of Ni was accurately weighed. SAS / NC solid powder was placed in a 20 mL glass bottle. 10.00 mL of N-methylpyrrolidone (NMP) was added. The mixture was then treated with an ultrasonic cell disruptor in pulse mode for 1 hour under ice-water bath conditions to obtain a uniform, dark gray dispersion, i.e., nickel-based M... N4 single-atom solvent (Ni (SAS / NC / NMP), with a concentration of approximately 5 mg / mL.

[0053] (2) Catalytic reaction: 4.0 mL of the above-mentioned nickel-based single-atom solvent dispersion, equivalent to 20.0 mg of solid catalyst material, was accurately added to a 50 mL high-pressure reactor lined with polytetrafluoroethylene using a pipette. Subsequently, 5.0 mmol (0.87 g) of hexamethylenediamine (HDA) and 15.0 mmol (1.35 g) of methyl carbamate (MC) were added sequentially to the reactor. The reactor was sealed, and the air inside was replaced three times with carbon dioxide gas. Finally, CO2 was introduced at room temperature until the initial pressure reached 1.0 MPa. The reactor was placed in an oil bath preheated to 190°C, and magnetic stirring (600 rpm) was started to begin the reaction.

[0054] (3) Post-reaction processing and analysis: After the reaction continued for 3 hours, the reactor was transferred to an ice-water bath to cool to room temperature, and the pressure was slowly released. All the reaction mixture was transferred to centrifuge tubes and centrifuged at 8000 rpm for 10 minutes to separate the supernatant (product liquid phase) and solid catalyst. An appropriate amount of supernatant was taken for gas chromatography (GC) analysis, and the HDC yield was calculated to be 92.3% using the internal standard method.

[0055] (4) Catalyst recovery and recycling: The solid catalyst recovered by centrifugation was washed three times with anhydrous ethanol and dried under vacuum at 80°C. The recovered solid catalyst was redispersed in 4.0 mL of NMP and briefly sonicated to restore the dispersion state for use in the next reaction under the same conditions. After five cycles, the yield remained at 89.8% in the fifth cycle, indicating that the catalyst has excellent cycle stability.

[0056] Example 2 Using cobalt-based M N4 single-atom solvent (Co) SAS / NC / DMF-catalyzed synthesis of HDC (1) Preparation of catalyst: cobalt-based M N4 single-atom solvent (Co) Preparation of SAS / NC / DMF.

[0057] Referring to the unified preparation method described in Example 1, the metal salt was replaced with 0.8 g of cobalt nitrate hexahydrate, and the solvent was replaced with N,N-dimethylformamide (DMF). A fluffy black Co was prepared. SAS / NC solid powder is further dispersed in DMF to form a cobalt-based single-atom solvent with a concentration of approximately 5 mg / mL.

[0058] (2) Catalytic reaction: Take the equivalent of 20.0 mg of solid catalyst material Co SAS / NC / DMF single-atom solvent, along with 5.0 mmol HDA and 15.0 mmol MC, was added to a high-pressure reactor. The reaction was carried out at 1.0 MPa CO2 and 190 °C for 3 hours.

[0059] (3) Post-reaction processing and analysis: The post-reaction processing and analysis methods are the same as in Example 1. The calculated yield of HDC is 80.5%.

[0060] (4) Catalyst recovery and recycling: After recovering and regenerating the catalyst, a recycling test was conducted, and the yield of the fifth test was 79.0%.

[0061] Example 3 Using iron-based M N4 single-atom solvent (Fe) SAS / NC / NMP-catalyzed synthesis of HDC (1) Preparation of catalyst: iron-based M N4 single-atom solvent (Fe) Preparation of SAS / NC / NMP.

[0062] Following the unified preparation method described in Example 1, the metal salt was replaced with 0.8 g of ferric nitrate nonahydrate, while maintaining NMP as the solvent. Black Fe was then prepared. SAS / NC solid powder is further dispersed in NMP to form an iron-based single-atom solvent with a concentration of approximately 5 mg / mL.

[0063] (2) Catalytic reaction: Take Fe equivalent to 20.0 mg of solid catalyst material The HDC synthesis reaction was carried out in the same manner as in Example 1 using SAS / NC / NMP single-atom solvents.

[0064] (3) Post-reaction processing and analysis: The post-reaction processing and analysis methods are the same as in Example 1. The calculated yield of HDC was 73.8%.

[0065] (4) Catalyst recovery and recycling: After recovering and regenerating the catalyst, a recycling test was conducted, and the yield of the fifth test was 71.5%.

[0066] Example 4 Using copper-based M N4 single-atom solvent (Cu) SAS / NC / NMP-catalyzed synthesis of HDC (1) Preparation of catalyst: copper-based M N4 single-atom solvent (Cu) Preparation of SAS / NC / NMP.

[0067] Following the unified preparation method described in Example 1, the metal salt was replaced with 0.8 g of copper nitrate trihydrate, while maintaining NMP as the solvent. Black Cu was then prepared. SAS / NC solid powder is further dispersed in NMP to form a copper-based single-atom solvent with a concentration of approximately 5 mg / mL.

[0068] (2) Catalytic reaction: Take Cu equivalent to 20.0 mg of solid catalyst material The HDC synthesis reaction was carried out in the same manner as in Example 1 using SAS / NC / NMP single-atom solvents.

[0069] (3) Post-reaction processing and analysis: The post-reaction processing and analysis methods are the same as in Example 1. The calculated HDC yield is 65.6%.

[0070] (4) Catalyst recovery and recycling: After recovering and regenerating the catalyst, a recycling test was conducted, and the conversion rate of the fifth cycle was 75.2%.

[0071] Example 5 Using nickel-based M N4 single-atom solvent (Ni SAS / NC / NMP-catalyzed synthesis of HDC (1) Preparation of catalyst: Same as in Example 1, Ni was prepared. SAS / NC / NMP single-atom solvent (5 mg / mL).

[0072] (2) Catalytic reaction: Ni equivalent to 5.0 mg of solid catalyst was added to a 50 mL high-pressure reactor. SAS / NC / NMP dispersion (i.e., 1.0 mL, catalyst / hexamethylenediamine mass ratio approximately 0.57%). 5.0 mmol (0.87 g) of hexamethylenediamine and 5.0 mmol (0.59 g) of ethyl carbamate (EC) (molar ratio 1:1) were added sequentially. The reactor was sealed, purged with air, and then purged with CO2 to an initial pressure of 3.0 MPa. The reactor was placed in an oil bath preheated to 150°C and reacted magnetically (600 rpm) for 10 hours.

[0073] (3) Post-reaction processing and analysis: Same as in Example 1. The calculated yield of HDC (ethyl ester derivative) was 68.5%.

[0074] (4) Catalyst recovery and recycling: The method is the same as in Example 1, showing that the catalyst remains stable under the conditions at this endpoint.

[0075] Example 6 Using cobalt-based M N4 single-atom solvent (Co) SAS / NC / ethanol) catalytic synthesis of HDC (1) Preparation of catalyst: cobalt-based M N4 single-atom solvent (Co) Preparation of SAS / NC / ethanol.

[0076] Precursor preparation: Referring to Example 1, the metal salt was replaced with 0.8 g of cobalt nitrate hexahydrate, the first solvent was still ethanol, and the ratio of composite nitrogen source was changed to melamine:urea = 1:5. The precursor was obtained by drying.

[0077] Low-temperature pyrolysis: Under nitrogen atmosphere, the temperature was increased to 450°C at a rate of 2°C / min, and then isothermal pyrolysis was performed at this temperature for 6 hours. After cooling, Co was obtained. SAS / NC solids.

[0078] Preparation of a single-atom solvent: Weigh 50.0 mg Co SAS / NC solid powder was added to 10.00 mL of anhydrous ethanol and ultrasonically dispersed for 1 hour to obtain a cobalt-based single-atom solvent with a concentration of approximately 5 mg / mL.

[0079] (2) Catalytic reaction: The above dispersion (equivalent to 43.5 mg of solid catalyst material, i.e., 8.7 mL, catalyst / hexamethylenediamine mass ratio of approximately 5.0%) was added to the high-pressure reactor. 5.0 mmol of hexamethylenediamine and 25.0 mmol of methyl carbamate (molar ratio 1:5) were added sequentially. CO2 was introduced to 0.5 MPa. The reaction was carried out at 220°C for 2 hours.

[0080] (3) Post-reaction processing and analysis: Same as in Example 1. The calculated yield of HDC was 71.2%.

[0081] Example 7 Using platinum-based M N4 single-atom solvent (Pt) SAS / NC / NMP-catalyzed synthesis of HDC (1) Preparation of catalyst: platinum-based M N4 single-atom solvent (Pt) Preparation of SAS / NC / NMP.

[0082] Precursor preparation: Referring to Example 1, the metal salt was replaced with 0.8 g of chloroplatinic acid (H2PtCl6·6H2O), and the ratio of composite nitrogen source was changed to melamine:urea = 5:1.

[0083] Low-temperature pyrolysis: Under argon protection, the temperature is increased to 550°C at 10°C / min and pyrolyzed for 2 hours.

[0084] Preparation of a single-atom solvent: Weigh 10.0 mg Pt SAS / NC solid powder was added to 10.00 mL of NMP and ultrasonically dispersed to obtain a platinum-based single-atom solvent with a concentration of approximately 1.0 mg / mL.

[0085] (2) Catalytic reaction: Add the above dispersion (equivalent to 0.5 mg of solid catalyst material, i.e., 0.5 mL, catalyst / hexamethylenediamine mass ratio of approximately 0.057%) to a high-pressure reactor. Then add 5.0 mmol hexamethylenediamine and 10.0 mmol methyl carbamate (molar ratio 1:2). Purge with CO2 to 2.0 MPa. React at 200°C for 5 hours.

[0086] (3) Post-reaction processing and analysis: Same as in Example 1. The calculated HDC yield was 35.4%. This example demonstrates the activity of platinum (Pt) metal and verifies the effect of extremely low catalyst loading.

[0087] Comparative Example 1 Using high-temperature pyrolysis control samples (Ni / NC) 900) as a catalyst (1) Catalyst preparation: Following the method described in the comparative example of the corresponding preparation patent, the same precursor composition (nickel nitrate, melamine, urea) was pyrolyzed at 900°C under nitrogen for 4 hours to obtain Ni / NC containing nickel nanocrystals. 900 solid powder.

[0088] (2) Catalytic reaction: Weigh 20.0 mg Ni / NC 900 solid powder was directly added to the reaction system, and the reaction conditions were the same as in Example 1 (190℃, 1.0 MPa CO2, 3 h).

[0089] (3) Results Analysis: Post-reaction processing analysis. The calculated yield of HDC was 40.8%. Its performance was significantly lower than that of Ni prepared at low temperature (500℃). SAS / NC catalysts have demonstrated the effectiveness of low-temperature pyrolysis in forming highly active M The importance of N4's single-atom structure and avoiding metal aggregation.

[0090] Comparative Example 2 Using commercial NiO nanopowder catalyst (1) Catalyst preparation: using an average particle size of approximately 30 mm 50 nm commercial nickel oxide (NiO) nanoparticles.

[0091] (2) Catalytic reaction: Weigh 20.0 mg of NiO powder and react it under the same conditions as in Example 1.

[0092] (3) Results Analysis: The calculated yield of HDC was 30.2%. Its performance is far lower than that of the nickel-based M prepared at low temperature in this invention. N4 is a single-atom solvent.

[0093] The above examples and comparative examples fully demonstrate that the series of M-type nitrogen compounds prepared by the unified low-temperature pyrolysis method based on a composite nitrogen source claimed in this invention are effective. N4 single-atom solvents (M = Ni, Co, Fe, Cu) exhibit superior overall performance in HDC synthesis reactions. The catalytic efficiency achieved under relatively mild preparation conditions is significantly better than that of nanocrystalline catalysts formed by high-temperature pyrolysis or commercial metal oxides, while also demonstrating the combined advantages of high selectivity, excellent cycle stability, and the ease of handling in liquid form. This technical approach provides a strong candidate for the industrial-scale green synthesis of HDC.

[0094] Figure 1 The image shows the Ni-SAS / NC XRD pattern disclosed in Example 1 of this invention. Three broad diffraction peaks are visible at approximately 13°, 26°, and 43°, which can be attributed to the (100), (002), and (101) crystal planes of amorphous carbon, respectively. No characteristic diffraction peaks associated with nickel crystals were observed across the entire detection angle range. This result clearly indicates that at the relatively low pyrolysis temperature of 450-550°C, the metal species did not undergo significant aggregation and crystallization, consistent with their state of dispersion in single-atom form.

[0095] Figure 2 The image shown is a SEM image of Ni-SAS / NC disclosed in Example 1 of this invention. The microstructure of Ni-SAS / NC was observed using a scanning electron microscope (SEM), and the results are as follows. Figure 2 As shown, the material exhibits a multi-level morphology composed of interconnected and stacked two-dimensional sheet-like structures, with abundant and uniformly distributed micropores. This high-porosity, interconnected sheet structure not only endows the material with a large specific surface area, but its abundant nitrogen defect sites also serve as effective anchoring sites, contributing to the stability of metal single atoms. Furthermore, this structure facilitates mass transfer, allowing reactant molecules to fully contact the metal active sites, thereby effectively improving the catalytic reaction efficiency.

[0096] Figure 3 The graph shows the effect of different catalysts on the yield of HDC in this invention; the series of M-N4 single-atom solvents prepared exhibit significant differences in yield for the catalytic synthesis of 1,6-dicarbamate (HDC) under the same reaction conditions, such as... Figure 3 As shown. Among them, the nickel-based single-atom solvent (Ni SAS / NC / NMP exhibited the best catalytic performance, achieving an HDC yield of 92.3%; cobalt-based single-atom solvent (Co) showed the best catalytic performance. The yield of SAS / NC / DMF was 80.5%; the yield of copper-based single-atom solvent (Cu) was... The yield of SAS / NC / NMP was 73.8%; the yield of iron-based single-atom solvent (Fe) was... The yield of SAS / NC / NMP was 65.6%. The results indicate that the type of metal is the key factor in regulating catalytic activity under the low-temperature pyrolysis preparation method with composite nitrogen source, among which nickel (Ni) exhibits the highest catalytic efficiency in this reaction system.

[0097] Figure 4 The Ni disclosed in Embodiment 1 of this invention The recycling performance of SAS / NC / NMP in HDC synthesis; as shown in the figure, after five cycles of catalysis, the Ni-SAS / NC / NMP catalytic system can still maintain an HDC yield of 89.8%, demonstrating excellent cycling stability. Figure 4 This trend further confirms the important role of the M-N4 single-atom structure in the HDC synthesis reaction and provides direct experimental evidence for optimizing the selection of catalyst metal centers for this reaction.

[0098] In summary, this invention provides a method for synthesizing 1,6-dicarbamate using an M-N4 single-atom solvent. The M-N4 single-atom solvent is used as a catalyst to catalyze the reaction of hexamethylenediamine with carbamate to synthesize HDC with high selectivity under a carbon dioxide atmosphere. The M-N4 single-atom solvent uses a melamine-urea composite nitrogen source and is prepared at 450 °C. Prepared by pyrolysis at a low temperature of 550℃, its active center is a well-defined M-N4 coordination structure formed by metal atoms and nitrogen atoms. The M-N4 single-atom solvent prepared by this low-temperature strategy not only exhibits excellent catalytic activity and selectivity in HDC synthesis, but also has the combined advantages of low preparation cost, low process energy consumption, and excellent cycle stability, providing an innovative and highly competitive catalytic system for the green, economical, and efficient production of HDC.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for synthesizing 1,6-dicarbamate using an M-N4 single-atom solvent, characterized in that, include: A metal salt is mixed with a composite nitrogen source consisting of melamine and urea, and heated to 450-550℃ under an inert atmosphere for low-temperature programmed carbonization pyrolysis to obtain a nitrogen-doped carbon solid material supported by a metal single atom. The nitrogen-doped carbon solid material supported by a metal single atom is then mixed with an organic solvent for homogenization and dispersion to obtain an M-N4 single-atom solvent. Under a carbon dioxide atmosphere, hexamethylenediamine and carbamate are subjected to a catalytic reaction in the presence of the M-N4 single-atom solvent to obtain 1,6-dicarbamate.

2. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The molar ratio of hexamethylenediamine to carbamate is 1:1 to 1:5; the mass of the M-N4 single-atom solvent is 0.1%-10% of the mass of hexamethylenediamine.

3. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The molar ratio of hexamethylenediamine to carbamate is 1:2 to 1:4; the mass of the M-N4 single-atom solvent is 0.5% of the mass of hexamethylenediamine. 5%.

4. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The temperature of the catalytic reaction is 150°C. At 250℃, the carbon dioxide pressure is 0.

5. 3 MPa.

5. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The temperature of the catalytic reaction is 180°C. At 220℃, the carbon dioxide pressure is 0.

8. 2 MPa.

6. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The metal salt is a compound of at least one metallic element selected from platinum, palladium, ruthenium, nickel, copper, iron, manganese, silver, cobalt, and cerium.

7. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The mass ratio of melamine to urea in the composite nitrogen source is 1:5 to 5:

1. The carbamate is methyl carbamate or ethyl carbamate.

8. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The organic solvent is any one of methanol, ethanol, acetone, isopropanol, N,N-dimethylformamide, and N-methylpyrrolidone; The concentration of the nitrogen-doped carbon solid material supported by the metal single atom in the organic solvent is 0.5-20 mg / mL.

9. The method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent according to claim 1, characterized in that, The heating rate of the low-temperature programmed carbonization pyrolysis is 2-10℃ / min, and the pyrolysis time is 2-6 hours; the inert atmosphere is nitrogen or argon.

10. A 1,6-dicarbamate, characterized in that, It was prepared using the method for synthesizing 1,6-dicarbamate using M-N4 single-atom solvent as described in any one of claims 1-9.

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