NHC-CO2 grafted defective structure MOF catalysts and their applications

Abstract

The present invention discloses an NHC-CO2 grafted defective structure MOF catalyst and its application. First, an imidazole-grafted functional ligand linker is synthesized, and then the linker is reacted with hafnium chloride in the presence of a monocarboxylic acid modulator to obtain a defective structure MOF material. Finally, the defective structure MOF material is reacted with dimethyl carbonate to graft an ionized N-heterocyclic carbene-carbon dioxide adduct group on the side chain of the material, thereby obtaining an NHC-CO2 grafted defective structure MOF catalyst. The catalyst prepared by the present invention has unique advantages of high specific surface area, coordinatively unsaturated metal Lewis acid, and ionized highly active catalytic sites, and can catalyze the conversion of CO2 into cyclic carbonates under green conditions such as without a cocatalyst, without a solvent, at atmospheric pressure, and at a relatively low temperature, and has broad application prospects in realizing the high-value utilization of CO2 in the environment.

Description

Technical Field

[0001] The present invention relates to the fields of material preparation, heterogeneous catalysts, and environmental protection, and particularly relates to a functional design of MOFs ligands for efficiently catalyzing the conversion of CO2 into cyclic carbonates under green conditions. Background Art

[0002] The reaction of CO2 with epoxides to synthesize cyclic carbonates is one of the sustainable development approaches. The atom utilization rate of this reaction is close to 100%, which has high value for reducing CO2 emissions and protecting the environment.

[0003] Cyclic carbonates are important industrial raw materials with wide applications, such as being used as green solvents in the electrochemical field, polymer synthesis, etc. Currently, the materials for catalyzing the conversion of CO2 are divided into homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts have advantages such as high catalytic efficiency and high selectivity, but the disadvantages of being difficult to separate and recycle limit their wide application. Heterogeneous catalysts can be separated from the reaction system by simple centrifugation and recycled, having good application prospects. However, currently, this reaction only shows good catalytic activity under conditions such as high temperature, high pressure, and cocatalysts, and has the disadvantages of high energy consumption and poor safety.

[0004] Imidazole is grafted onto the ligand of MOFs, and then the imidazole is ionized and modified to have more adsorption and catalytic sites; adding a modulator to form a defective structure can further increase the coordination unsaturated sites of MOFs and form a mesoporous structure to increase mass transfer and separation capabilities, making it have higher catalytic performance. Summary of the Invention

[0005] Based on the deficiencies existing in the above-mentioned prior art, the purpose of the present invention is to provide a method for synthesizing a defective structure MOFs catalyst grafted with NHC-CO2 through ligand functionalization and adding a modulator, so as to overcome the respective deficiencies of homogeneous catalysts and heterogeneous catalysts, and achieve efficient catalysis of the conversion of CO2 into cyclic carbonates under solvent-free, cocatalyst-free, normal pressure, and relatively low temperature conditions.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] The invention discloses a preparation method of an NHC-CO2 grafted defective structure MOFs catalyst, which is characterized in that: firstly, an imidazole-grafted functionalized ligand connector is synthesized, and then the connector is reacted with hafnium chloride in the presence of a monocarboxylic acid modulator to obtain a defective structure MOFs material (the monocarboxylic acid forms a metal ion coordination unsaturated defect, which increases Lewis acidity, and at the same time makes MOFs produce a mesoporous structure, which increases the mass transfer and separation capabilities of the MOFs system), and finally the defective structure MOFs material is reacted with dimethyl carbonate to graft an ionized nitrogen heterocyclic carbene-carbon dioxide adduct group (NHC-CO2) on the side group of the material, the purpose of which is to synergistically enhance the reaction activity of catalyzing CO2 to convert cyclic carbonate under green conditions), so as to obtain an NHC-CO2 grafted defective structure MOFs catalyst.

[0008] Furthermore, the imidazole-grafted functionalized ligand connector is 2-(imidazole-1-yl) terephthalic acid prepared from imidazole, 2-bromoterephthalic acid, anhydrous potassium carbonate, and anhydrous copper sulfate. Figure 1 Its crystal morphology.

[0009] Furthermore, the monocarboxylic acid is formic acid, acetic acid, benzoic acid or trifluoroacetic acid.

[0010] The preparation method of the NHC-CO2 grafted defective structure MOFs catalyst of the present invention specifically comprises the following steps:

[0011] Step 1, 14mmol of anhydrous potassium carbonate, 5mmol of 2-bromoterephthalic acid, 25mmol of imidazole and 0.3mmol of anhydrous copper sulfate were put into an agate mortar and ground thoroughly, the obtained mixed powder was poured into a reactor, reacted at 200-210°C for 10-14h, cooled to room temperature, dissolved in 3.33mol of deionized water, filtered, adjusted to pH 2-3 with 6mol / L hydrochloric acid solution, and recrystallized to obtain 2-(imidazole-1-yl)terephthalic acid;

[0012] Step 2, weighing 1.6 mmol of the 2-(imidazol-1-yl)terephthalic acid and 2 mmol of hafnium chloride into a reaction kettle, adding monocarboxylic acid (4 mmol formic acid, 4 mmol acetic acid, 4 mmol benzoic acid or 4 mmol trifluoroacetic acid) and 50 mL of N, N-dimethylformamide into the reaction kettle, sealing, ultrasonically treating for 20-25 min, placing in an oven at 150 ° C for 72 h, cooling to room temperature, washing, filtering and drying to obtain defective structure MOFs material;

[0013] Step 3: Put 0.3000 - 0.3010 g of the defective structure MOFs material and 10 - 20 mL of dimethyl carbonate into a reaction kettle, react at 100 °C for 24 h, cool to room temperature, then filter, wash, and dry to obtain the NHC-CO2 grafted defective structure MOFs catalyst.

[0014] The NHC-CO2 grafted defective structure MOFs catalyst prepared by the present invention can be used in the cycloaddition reaction of CO2 and epoxides to produce the target compound cyclic carbonate. The catalyst prepared by the present invention has unique advantages such as a high specific surface area, coordinatively unsaturated and ionized highly active catalytic sites, and can efficiently catalyze the conversion of CO2 into cyclic carbonate under the conditions of solvent-free, co-catalyst-free, normal pressure, and relatively low temperature. The catalyst has high selectivity and conversion rate for the reaction, and has a stable structure and good recyclability.

[0015] Further, the epoxide is epichlorohydrin, epibromohydrin or styrene oxide.

[0016] Further, the dosage of the NHC-CO2 grafted defective structure MOFs catalyst accounts for 2 - 10% of the mass of the epoxide.

[0017] Further, the temperature of the cycloaddition reaction is 80 - 110 °C and the reaction time is 4 - 24 h.

[0018] Further, after the reaction, the catalyst can be recovered and recycled by centrifugal separation.

[0019] The cycloaddition reaction of CO2 and epoxides in the presence of the catalyst is shown in formula (1):

[0020]

[0021] The beneficial effects of the present invention are as follows:

[0022] 1. The present invention directly prepares colorless rod-shaped crystals of 2-(imidazol-1-yl)terephthalic acid by a simple one-pot method. Using this ligand as a linker, a defective structure MOFs material is synthesized with hafnium chloride under the condition of monocarboxylic acids such as formic acid, acetic acid, benzoic acid, trifluoroacetic acid as modulators, and further reacted with dimethyl carbonate to obtain the MOFs catalyst with NHC-CO2 ionized catalytic sites. This method has a simple preparation process and good reproducibility. The catalyst prepared by the present invention has high thermal stability, acid stability and water stability, and excellent catalytic activity.

[0023] 2. The present invention makes full use of the advantages of the high specific surface area, coordinatively unsaturated and ionized highly active catalytic sites provided by the presence of the modulator and NHC-CO2, which helps to improve the catalytic activity and selectivity of the catalyst under the conditions of co-catalyst-free, solvent-free, normal pressure, and relatively low temperature.

[0024] 3. The catalyst can be recycled and reused through centrifugal separation in the present invention, providing a new green approach for the atom-economic reaction of catalytic synthesis of cyclic carbonates. Brief Description of the Drawings

[0025] Figure 1 It is a microscope image of the colorless rod-shaped crystal of the newly synthesized ligand connector 2-(imidazol-1-yl) terephthalic acid of the present invention;

[0026] Figure 2 It is a schematic diagram of the synthesis route of the NHC-CO2 grafted defective structure MOFs catalyst prepared in Example 1 of the present invention;

[0027] Figure 3 It is a high-resolution scanning electron microscope image of the NHC-CO2 grafted defective structure MOFs catalyst prepared in Example 1 of the present invention;

[0028] Figure 4 It is an infrared spectrum of the NHC-CO2 grafted defective structure MOFs catalyst prepared in Example 1 of the present invention.

[0029] Figure 5 It is the powder X-ray diffraction spectrum of the NHC-CO2 grafted defective structure MOFs catalyst prepared in Example 1 of the present invention after being immersed in different solvents ( Figure 5 (a)) and different pH solutions ( Figure 5 (b)) for one week. Detailed Description of the Embodiments

[0030] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0031] Example 1

[0032] In this example, the NHC-CO2 grafted defective structure MOFs catalyst was synthesized according to the following steps:

[0033] Step 1: Put 14 mmol of anhydrous potassium carbonate, 5 mmol of 2-bromo terephthalic acid, 25 mmol of imidazole, and 0.3 mmol of anhydrous copper sulfate into an agate mortar and grind them thoroughly. Pour the obtained mixed powder into a reaction kettle and react at 200 °C for 10 h. After cooling to room temperature, dissolve the product in 3.33 mol of deionized water, filter, and adjust the pH to 2 - 3 with a 6 mol / L hydrochloric acid solution. Recrystallize to obtain the required ligand: 2-(imidazol-1-yl)terephthalic acid.

[0034] Step 2: Weigh four groups of 0.4640 g of 2-(imidazol-1-yl)terephthalic acid and 0.6400 g of hafnium chloride and put them into a reaction kettle. Add 4 mmol of formic acid, 4 mmol of acetic acid, 4 mmol of benzoic acid, and 4 mmol of trifluoroacetic acid to the four reaction kettles respectively. Then add 50 mL of N,N-dimethylformamide, seal it, and perform ultrasonic treatment for 20 min. Then put it into an oven and react at 150 °C for 72 h. After cooling to room temperature, wash, filter, and dry to obtain the defective structure MOFs material.

[0035] Step 3: Put 0.3000 g of the defective structure MOFs material obtained in Step 2 and 15 mL of dimethyl carbonate into a reaction kettle and react at 100 °C for 24 h. After cooling to room temperature, filter, wash, and dry to obtain the defective structure MOFs catalyst grafted with N-heterocyclic carbene-carbon dioxide adduct (NHC-CO2).

[0036] Figure 1 This is the biological microscope picture of the 2-(imidazol-1-yl)terephthalic acid ligand connector prepared in this example. Colorless rod-shaped crystals are shown in the picture.

[0037] Figure 3 This is the high-resolution scanning electron microscope picture of the defective structure MOFs catalyst grafted with NHC-CO2 prepared with trifluoroacetic acid as the modulator in this example. The picture shows that the product has a near-spherical morphology and a pore structure on the particle surface.

[0038] Figure 4 This is the infrared spectrum of the defective structure MOFs catalyst grafted with NHC-CO2 prepared with trifluoroacetic acid as the modulator in this example. 1134 cm -1 corresponds to the absorption peak of the C-F bond, 1054 cm -1 corresponds to the absorption peak of the imidazole ring, 1672 cm -1 is the absorption peak from the C=O double bond on the MOFs ligand, 1596 - 1376 cm -1 is the vibration peak of the benzene ring skeleton on the ligand.

[0039] Figure 5Powder X-ray diffraction pattern of the NHC-CO2 grafted defective structure MOFs catalyst prepared with trifluoroacetic acid as the modifier in this example. 0.1000 - 0.1500 g of the catalyst was immersed in different pH solutions and different solvents for one week, filtered, and the PXRD pattern obtained after drying is as follows Figure 5 As shown in Figure 5 (a), under different solvent conditions, the structure of the NHC-CO2 grafted defective structure MOFs catalyst did not change, indicating that the catalyst has good solvent stability under different solvent conditions. As shown in

[0040] Example 2

[0041] In this example, the NHC-CO2 grafted defective structure MOFs catalyst synthesized with trifluoroacetic acid as the modifier in Example 1 was used to catalyze the cycloaddition reaction of CO2 and epichlorohydrin as shown in Equation (2). The specific steps are as follows:

[0042]

[0043] 0.1000 g of the MOFs catalyst was added to a 100 mL round-bottom flask, then 2.0000 g of epichlorohydrin was added, and the air in the system was replaced with CO2. The reaction was carried out at 100 °C under atmospheric pressure for 10 h. After the reaction, the product was obtained by filtering the reaction solution through an organic pinhole filter head and detected by 1H NMR. The yield of the product epichlorohydrin carbonate was calculated to be 76.12% and the selectivity was 93.24%.

[0044] Example 3

[0045] In this example, the highly efficient NHC-CO2 grafted defective structure MOFs catalyst synthesized with trifluoroacetic acid as the modifier in Example 1 was used to catalyze the cycloaddition reaction of CO2 and epibromohydrin as shown in Equation (3). The specific steps are as follows:

[0046]

[0047] 0.1000 g of the MOFs catalyst was added to a 100 mL round-bottom flask, then 2.9600 g of epibromohydrin was added, and the air in the system was replaced with CO2. The reaction was carried out at 100 °C under atmospheric pressure for 10 h. After the reaction, the product was obtained by filtering the reaction solution through an organic pinhole filter head and detected by 1H NMR. The yield of the product epibromohydrin carbonate was calculated to be 76.76% and the selectivity was 89.38%.

[0048] Example 4

[0049] In this example, the highly efficient NHC-CO2 grafted defective structure MOFs catalyst synthesized in Example 1 with trifluoroacetic acid as the modulator was used to catalyze the cycloaddition reaction of CO2 and styrene oxide, as shown in Equation (4). The specific steps are as follows:

[0050]

[0051] Add 0.1000 g of the MOFs catalyst to a 100 mL round-bottom flask, then add 2.6000 g of styrene oxide. Replace the air in the system with CO2, and react at 100 °C under atmospheric pressure for 10 h. After the reaction is completed, the product is obtained by filtering the reaction solution through an organic pinhole filter head and detected by 1H NMR. The yield of the product 4-(phenyl)-1,3-dioxolan-2-one is calculated to be 61.83% and the selectivity is 90.72%.

[0052] Example 5

[0053] This example verifies the recyclability of the catalyst: after the cycloaddition reaction is completed, the catalyst is recovered by centrifugation, washed four times with ethanol, and then used for the next catalytic cycle after vacuum drying. The specific method is the same as that described in Example 2. The catalyst can still maintain high catalytic activity after 5 catalytic cycle experiments. The results are shown in Table 1.

[0054] Table 1 Recycling of the catalyst

[0055]

[0056] Note: a The yield was determined using 1H NMR. Yield = conversion * selectivity.

[0057] Although the above content has described the present invention in detail with specific embodiments, based on the present invention, those of ordinary skill in the art can make some modifications or improvements. Any substitution or improvement of the same kind made on the basis of the spirit of the present invention falls within the scope of protection required by the present invention.

Claims

1. Preparation method of NHC-CO2 grafted defective structure MOFs catalyst, characterized in that: Firstly, an imidazole-grafted functionalized ligand connector is synthesized, and then the connector is reacted with hafnium chloride in the presence of a monocarboxylic acid modulator to obtain a defective structure MOFs material, and finally the defective structure MOFs material is reacted with dimethyl carbonate to graft an ionized nitrogen heterocyclic carbene-carbon dioxide adduct group onto the side group of the material, that is, to obtain an NHC-CO2-grafted defective structure MOFs catalyst; The imidazole-grafted functionalized ligand connector is 2-(imidazole-1-yl) terephthalic acid prepared by using imidazole, 2-bromoterephthalic acid, anhydrous potassium carbonate and anhydrous copper sulfate as raw materials; the monocarboxylic acid is formic acid, acetic acid, benzoic acid or trifluoroacetic acid.

2. The preparation method according to claim 1, characterized in that The steps include: Step 1, 14mmol of anhydrous potassium carbonate, 5mmol of 2-bromoterephthalic acid, 25mmol of imidazole and 0.3mmol of anhydrous copper sulfate are put into an agate mortar and fully ground, the obtained mixed powder is poured into a reaction kettle, and the reaction is carried out at 200-210°C for 10-14h, and after cooling to room temperature, the product is dissolved in 3.33mol of deionized water, filtered, and the pH is adjusted to 2-3 with a hydrochloric acid solution with a concentration of 6mol / L, and recrystallized to obtain 2-(imidazol-1-yl)terephthalic acid; Step 2, weighing 1.6 mmol of the 2-(imidazol-1-yl)terephthalic acid and 2 mmol of hafnium chloride into a reaction kettle, adding monocarboxylic acid and 50 mL of N,N-dimethylformamide into the reaction kettle, sealing, ultrasonically treating for 20-25 min, placing in an oven at 150° C. for reaction for 72 h, cooling to room temperature, washing, filtering, and drying to obtain a defective structure MOFs material; Step 3: Place 0.3000-0.3010 g of the defective structure MOFs material and 10-20 mL of dimethyl carbonate into a reactor and react at 100° C. for 24 hours. After cooling to room temperature, filter, wash, and dry to obtain an NHC-CO2 grafted defective structure MOFs catalyst.

3. The preparation method according to claim 2, characterized in that: The monocarboxylic acid in step 2 is 4 mmol formic acid, 4 mmol acetic acid, 4 mmol benzoic acid or 4 mmol trifluoroacetic acid.

4. An NHC-CO2 grafted defective structure MOFs catalyst prepared by the preparation method according to any one of claims 1 to 3.

5. Application of the NHC-CO2 grafted defective structure MOFs catalyst according to claim 4, characterized in that: Used for the cycloaddition reaction of CO2 and epoxide to generate the target compound cyclic carbonate.

6. The application according to claim 5, characterized in that: The epoxide is epichlorohydrin, epibromohydrin or styrene oxide.

7. The application according to claim 5, characterized in that: The amount of the NHC-CO2 grafted defective structure MOFs catalyst is 2-10% of the mass of the epoxide.

8. The application according to claim 5, characterized in that: Centrifugal separation after the cycloaddition reaction can realize catalyst recovery and recycling.

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