Phosphorus-coordinated metal single-atom catalysts and their use in the selective hydrogenation of alpha, beta-unsaturated imines

Abstract

This invention discloses a phosphorus-coordinated metal single-atom catalyst and its application in the selective hydrogenation reaction of α,β-unsaturated imines. The phosphorus-coordinated metal single-atom catalyst is prepared by the following method: melamine, cyanuric acid, L-alanine, and glyphosate undergo in-situ polycondensation to obtain a supramolecular polymer support; a precursor containing palladium, ruthenium, or iridium metal salts is loaded onto the supramolecular polymer support to obtain a metal-loaded supramolecular polymer; the supramolecular polymer powder precursor is obtained by freeze-drying, followed by high-temperature pyrolysis in an inert atmosphere to obtain the final product. The catalyst of this invention exhibits good conversion rate and selectivity in the selective hydrogenation of α,β-unsaturated imines to β-unsaturated amines; it shows preferential selectivity for the adsorption of C=O polar double bonds, a characteristic that significantly improves the selectivity of the hydrogenation of α,β-unsaturated imines to β-unsaturated amines. Therefore, the catalyst of this invention has significant application value in the selective hydrogenation reaction of α,β-unsaturated imines.

Description

Technical Field

[0001] This invention relates to a phosphorus-coordinated metal single-atom catalyst and its application in the selective hydrogenation reaction of α,β-unsaturated imines, belonging to the field of industrial catalysis technology. Background Technology

[0002] Alkaloids, amino acids, and lactam antibiotics are ubiquitous nitrogen-containing organic compounds. Given the importance of nitrogen-containing heterocycles, numerous stoichiometric and catalytic methods for synthesizing these molecules using various reaction pathways have been established. In recent years, significant progress has been made in the synthesis of such biorelated molecules using imine chemistry. α,β-unsaturated imines, as compounds containing conjugated C=N and C=C double bonds, possess one nucleophilic center and two electrophilic centers. Their hydrogenation reactions can yield a variety of products, such as β-unsaturated amines, saturated amines, and saturated imines. Among these, β-unsaturated amines are key intermediates in the synthesis of β- and γ-amino compounds, which constitute a large proportion of agrochemicals and active pharmaceutical ingredients. Unsaturated terpenoids, intermediates for potential drugs treating neurological diseases, can also be readily prepared from terpenes and unsaturated amines via reductive amination. Currently, β-unsaturated amines are obtained through the amination of alkenes and alkynes in homogeneous systems, driven by homogeneous noble metal / transition metal (Ru, Pd, Au, etc.) catalysts, or by dehydrogenation through the construction of new C-C bonds near the amino group with the aid of highly active butyllithium reagents. However, these methods suffer from difficulties in catalyst separation and severe environmental pollution. Therefore, developing more efficient and environmentally friendly preparation methods has become a research focus. The method of directly reducing α,β-unsaturated imines to β-unsaturated amines with hydrogen as a reducing agent has attracted attention due to its mild reaction conditions and low environmental pollution.

[0003] Single-atom catalysts (SACs) have become the best candidate materials for the selective hydrogenation of α,β-unsaturated imines due to their unique electronic structure, well-defined reaction sites, and 100% atom utilization. Most reported catalysts for the selective hydrogenation of α,β-unsaturated imines are still homogeneous catalysts, with MOFs being the dominant heterogeneous catalyst. Currently, there are no reports of single-atom catalysts catalyzing the selective hydrogenation of α,β-unsaturated imines. The research group of Shengqian Ma (Angew. Chem. Int. Ed. 2019, 58, 7420–7424) anchored Lewis acid-base pairs within metal-organic frameworks (MOFs), obtaining a stable "hindered Lewis acid-base pair" catalytic system. In the selective hydrogenation of (E)-N,3-diphenylprop-2-ene-1-imine, they found that after 48 hours of reaction, the yield and selectivity of the C=N selective hydrogenation product reached 100%. Risto Savela et al. (Catal. Sci. Technol., 2021, 11, 1481-1496) prepared a series of heterogeneous supported Ir, Ru, Pd, and Au catalysts using different metal oxides and carbonitride-based carbon composites, porous carbon, and mesoporous carbon. These catalysts could selectively hydrogenate α,β-unsaturated imines to β-unsaturated amines under mild conditions. However, experimental results showed that the selectivity of the obtained catalysts in the selective hydrogenation reaction of α,β-unsaturated imines was not ideal. Therefore, the preparation of novel SACs to achieve the selective hydrogenation of α,β-unsaturated imines to β-unsaturated amines is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide an iridium single-atom catalyst with a phosphorus coordination structure to solve the problem of selective hydrogenation of α,β-unsaturated imines to β-unsaturated amines. The preparation method of the phosphorus coordination metal single-atom catalyst of this invention has the advantages of low cost, simple operation, no need for acid etching, good reproducibility, controllable loading, and is more conducive to large-scale production.

[0005] The preparation method of the phosphorus coordination metal single-atom catalyst provided by the present invention includes the following steps:

[0006] S1. In the presence of a solvent, melamine, cyanuric acid, L-alanine and glyphosate undergo in-situ polycondensation to obtain a supramolecular polymer carrier.

[0007] S2. A precursor containing a metal salt of palladium, ruthenium, or iridium is loaded onto the supramolecular polymer support to obtain a metal-loaded supramolecular polymer.

[0008] S3. The supramolecular polymer is freeze-dried to obtain a supramolecular polymer powder precursor;

[0009] S4. The supramolecular polymer powder precursor is subjected to high-temperature pyrolysis in an inert atmosphere to obtain the phosphorus-coordinated metal single-atom catalyst.

[0010] In the above preparation method, in step S1, the solvent is selected from at least one of deionized water, ethanol, acetone and ethyl acetate;

[0011] The mass ratio of the melamine, the cyanuric acid, the L-alanine, and the glyphosate is 1:0.5-4g:0.5-4:0.1-1.

[0012] The conditions for the in-situ polycondensation reaction are: temperature of 60–140°C and time of 2–10 h.

[0013] In the above preparation method, in step S2, the precursor containing palladium, ruthenium or iridium metal salt is selected from at least one of Pd(OAc)2, Pd(acac)2, RuCl3, Ru(acac)3, IrCl3, Ir(acac)3;

[0014] The mass ratio of the precursor containing palladium, ruthenium, or iridium metal salt to the melamine is 0.001 to 0.2:1.

[0015] In the above preparation method, the freeze-drying conditions in step S3 are: vacuum conditions, temperature of -40 to -60°C, and time of 24 to 96 hours.

[0016] In the above preparation method, the high-temperature pyrolysis is carried out in a tube furnace;

[0017] The inert gas is at least one of Ar and N2.

[0018] The conditions for the high-temperature pyrolysis are: temperature of 500–1000℃, heating rate of 2–10℃ / min, and time of 1–3h.

[0019] The phosphorus-coordinated metal single-atom catalyst prepared by the method of the present invention has a metal loading of 1% to 20% (mass), preferably 2% to 10%.

[0020] The phosphorus-coordinated metal single-atom catalyst of this invention can be used to catalyze the selective hydrogenation reaction of α,β-unsaturated imines to prepare β-unsaturated amines.

[0021] Based on the aforementioned phosphorus-coordinated metal single-atom catalyst, this invention further provides a method for the selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines, comprising the following steps:

[0022] The α,β-unsaturated imine is obtained by hydrogenation under the catalysis of the phosphorus-coordinated metal single-atom catalyst; the molar ratio of the α,β-unsaturated imine to the phosphorus-coordinated metal single-atom catalyst is 500-5000:1, preferably 1000-3000:1, and more preferably 1000:1;

[0023] The conditions for the hydrogenation reaction are: temperature of 40-140℃, preferably 100-120℃, more preferably 100℃; hydrogen pressure of 0-4MPa, preferably 2MPa; and time of 0-10h, preferably 2h.

[0024] The solvent used in the hydrogenation reaction is at least one of acetonitrile, N,N-dimethylformamide, ethanol, and water.

[0025] The phosphorus-coordinated metal single-atom catalyst provided by this invention exhibits excellent conversion and selectivity in the selective hydrogenation of α,β-unsaturated imines to β-unsaturated amines. This superior performance can be attributed to the high electron density of the iridium center in the phosphorus-coordinated metal single-atom catalyst, a characteristic that enables effective dissociation of hydrogen molecules. Furthermore, the phosphorus-coordinated metal single-atom catalyst shows preferential selectivity for the adsorption of C=O polar double bonds, a feature that significantly improves the selectivity of the hydrogenation of α,β-unsaturated imines to β-unsaturated amines. Therefore, the catalyst of this invention has significant application value in the selective hydrogenation reaction of α,β-unsaturated imines. Attached Figure Description

[0026] Figure 1 The performance of the phosphorus-coordinated metal single-atom catalysts prepared in Examples 1-3 of this invention in the selective hydrogenation reaction of α,β-unsaturated imines is shown.

[0027] Figure 2 The performance of the phosphorus-coordinated metal single-atom catalysts prepared in Examples 1, 4, and 5 of this invention in the selective hydrogenation reaction of α,β-unsaturated imines is shown. Detailed Implementation

[0028] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0029] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0030] Example 1: Phosphorus-coordinated metal single-atom catalysts for the selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines

[0031] (1) Preparation of phosphorus-coordinated metal single-atom catalysts

[0032] In a 500 mL round-bottom flask containing 150 mL of water, 2 g of melamine, 2 g of cyanuric acid, 2 g of L-alanine, and 0.5 g of glyphosate were added sequentially. The mixture was subjected to in-situ polycondensation at 100 °C for 1 hour to generate a precursor for a two-dimensional layered support. Subsequently, a solution containing 8 mg of iridium chloride was added to the flask, and the mixture was stirred continuously until it reached a yogurt-like consistency, thus obtaining an iridium-containing supramolecular polymer. The resulting supramolecular polymer was then transferred to a vacuum desiccator and freeze-dried at -50 °C for 24 hours to remove excess moisture. After freeze-drying, a supramolecular polymer powder containing palladium, platinum, and iridium was obtained.

[0033] Iridium-containing supramolecular polymer powder was placed in a tube furnace and heated to 800°C at a rate of 5°C / min under argon protection, and maintained at this temperature for 2 hours. It was then allowed to cool naturally to room temperature to obtain a phosphorus-coordinated iridium single-atom catalyst. The iridium single-atom loading in this catalyst reached 4.0 wt%.

[0034] (2) Selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines

[0035] In a 100 mL high-pressure reactor with a polytetrafluoroethylene liner, 8 mg of the phosphorus-coordinated iridium single-atom catalyst prepared in step (1), 2 mmol of α,β-unsaturated imine, 15 mL of ethanol, and 15 mL of water were added. H2 at a pressure of 2 MPa was introduced, the reaction temperature was 100 °C, and the reaction time was 1 h. After the reaction was completed, the reaction product was filtered through a filter membrane, and dichloromethane was added to extract the catalytic product. The molar ratio of α,β-unsaturated imine to phosphorus-coordinated iridium single-atom catalyst was 1000:1.

[0036] Example 2: Phosphorus-coordinated metal single-atom catalysts for the selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines.

[0037] β-Unsaturated amines were prepared using the method described in Example 1, except that a solution containing 6 mg of ruthenium chloride was added to the flask to obtain a phosphorus-coordinated ruthenium single-atom catalyst.

[0038] Example 3: Phosphorus-coordinated metal single-atom catalysts for the selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines.

[0039] β-Unsaturated amines were prepared using the method described in Example 1, except that a solution containing 6 mg of palladium chloride was added to the flask to obtain a phosphorus-coordinated palladium single-atom catalyst.

[0040] Example 4: Phosphorus-coordinated iridium single-atom catalyst for the selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines

[0041] β-Unsaturated amines were prepared using the method described in Example 1, with the only difference being that the H2 pressure was 2 MPa and the reaction temperature was 80 °C.

[0042] Example 5: Phosphorus-coordinated iridium single-atom catalyst for the selective hydrogenation of α,β-unsaturated imines to prepare β-unsaturated amines.

[0043] β-Unsaturated amines were prepared using the method described in Example 1, with the only difference being that the H2 pressure was 0.5 MPa and the reaction temperature was 100 °C.

[0044] Effect verification:

[0045] The catalytic products obtained in Examples 1-3 were analyzed using n-tetane as an internal standard by gas chromatography (GC, Shimadzu, GC2010 plus) and gas chromatography-mass spectrometry (GC-MS, Shimadzu, GCMS-QP2010S). The results are as follows: Figure 1 As shown:

[0046] With an H2 pressure of 2 MPa and a reaction temperature of 100 °C, the conversion rate of α,β-unsaturated imines was 98.0%, and the selectivity of β-unsaturated amines was 97.8%.

[0047] A phosphorus-coordinated ruthenium single-atom catalyst was prepared by adding a solution containing 6 mg of ruthenium chloride to a flask. The H2 pressure was 2 MPa, the reaction temperature was 100 °C, the conversion rate of α,β-unsaturated imine was 65.8%, and the selectivity of β-unsaturated amine was 98.7%.

[0048] A phosphorus-coordinated palladium single-atom catalyst was prepared by adding a solution containing 6 mg of palladium chloride to a flask. The H2 pressure was 2 MPa, the reaction temperature was 100 °C, the conversion rate of α,β-unsaturated imine was 16.8%, and the selectivity of β-unsaturated amine was 98.9%.

[0049] The catalytic products obtained in Examples 1, 4, and 5 were analyzed using n-tetane as an internal standard by gas chromatography (GC, Shimadzu, GC2010 plus) and gas chromatography-mass spectrometry (GC-MS, Shimadzu, GCMS-QP2010S). The results are as follows: Figure 2 As shown:

[0050] With an H2 pressure of 2 MPa and a reaction temperature of 100 °C, the conversion rate of α,β-unsaturated imines was 98.0%, and the selectivity of β-unsaturated amines was 97.8%.

[0051] The H2 pressure was 2 MPa, the reaction temperature was 80℃, the conversion rate of α,β-unsaturated imine was 10%, and the selectivity of β-unsaturated amine was 99.7%.

[0052] With an H2 pressure of 0.5 MPa and a reaction temperature of 100 °C, the conversion rate of α,β-unsaturated imine was 48.1%, and the selectivity of β-unsaturated amine was 98.5%.

[0053] As demonstrated by the above examples, the phosphorus-coordinated metal single-atom catalyst can be used for the selective hydrogenation of α,β-unsaturated imines under various reaction conditions. In particular, under H2 pressure of 2 MPa and temperature of 100°C, this catalyst can achieve highly efficient and selective hydrogenation of α,β-unsaturated imines to β-unsaturated amines, maintaining a selectivity of 96.1% for β-unsaturated amines even at a conversion rate of 100.0%. The catalyst of this invention provides an effective solution for the hydrogenation reaction of α,β-unsaturated imines.

Claims

1. A method for preparing a phosphorus-coordinated metal single-atom catalyst, comprising the following steps: S1. In the presence of a solvent, melamine, cyanuric acid, L-alanine and glyphosate undergo in-situ polycondensation to obtain a supramolecular polymer carrier. S2. A precursor containing a metal salt of palladium, ruthenium, or iridium is loaded onto the supramolecular polymer support to obtain a metal-loaded supramolecular polymer. The precursor containing palladium, ruthenium, or iridium is selected from at least one of Pd(OAc)2, Pd(acac)2, RuCl3, Ru(acac)3, IrCl3, and Ir(acac)3; The mass ratio of the precursor containing palladium, ruthenium, or iridium metal salt to the melamine is 0.001 to 0.2:1; S3. The supramolecular polymer is freeze-dried to obtain a supramolecular polymer powder precursor; S4. The supramolecular polymer powder precursor is subjected to high-temperature pyrolysis in an inert atmosphere to obtain the phosphorus-coordinated metal single-atom catalyst. The high-temperature pyrolysis is carried out in a tubular furnace; The conditions for the high-temperature pyrolysis are: temperature of 500-1000℃, heating rate of 2-10℃ / min, and time of 1-3h.

2. The preparation method according to claim 1, characterized in that: In step S1, the solvent is selected from at least one of deionized water, ethanol, acetone and ethyl acetate; The mass ratio of the melamine, the cyanuric acid, the L-alanine, and the glyphosate is 1:0.5~4g:0.5~4:0.1~1. The conditions for the in-situ polycondensation reaction are: temperature of 60~140℃ and time of 2~10h.

3. The preparation method according to claim 1 or 2, characterized in that: In step S3, the freeze-drying conditions are: under vacuum conditions, the temperature is -40~-60℃, and the time is 24~96h.

4. A phosphorus-coordinated metal single-atom catalyst prepared by the method according to any one of claims 1-3; The metal loading in the phosphorus-coordinated metal single-atom catalyst is 1% to 20%.

5. The application of the phosphorus coordination metal single-atom catalyst according to claim 4 in the selective hydrogenation reaction of α,β-unsaturated imines to prepare β-unsaturated amines.

6. A method for selectively hydrogenating α,β-unsaturated imines to prepare β-unsaturated amines, comprising the following steps: The α,β-unsaturated imine is obtained by hydrogenation under the catalysis of the phosphorus-coordinated metal single-atom catalyst described in claim 4. The molar ratio of the α,β-unsaturated imine to the phosphorus-coordinated metal single-atom catalyst is 500~5000:1; The conditions for the hydrogenation reaction are: temperature 40~140℃, hydrogen pressure 0~4 MPa, and time 0~10h; The solvent used in the hydrogenation reaction is at least one of acetonitrile, N,N-dimethylformamide, ethanol, and water.

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