Tetrazine molecular cage with adjustable structure, synthesis method and application of tetrazine molecular cage in gas separation

Abstract

The invention discloses a tetrazine molecular cage with an adjustable structure, a synthesis method and application of the tetrazine molecular cage in gas separation, and belongs to the technical field of organic synthesis and adsorption separation, the tetrazine molecular cage is obtained through condensation reaction of a dialdehyde building block with tetrazine elements and a scaffold type triamine compound, the synthesis method is simple, the operation is convenient, and the yield is high. A series of tetrazine molecular cages can be obtained by further modifying the tetrazine molecular cage with a modification monomer, the tetrazine molecular cage and the modified tetrazine molecular cage are high in specific surface area and good in permeability and can adsorb various gases at room temperature, and particularly, the modified tetrazine molecular cage can adsorb various gases due to structural distortion caused by the modification process. The gas adsorption capacity is greatly improved, gas separation of acetylene and ethylene can be achieved in a dynamic penetration mode, and the gas separation membrane has wide application prospects in the aspect of gas separation.

Description

Technical Field

[0001] The present invention belongs to the technical fields of organic synthesis and adsorption separation, and particularly relates to a tetrazine molecular cage with adjustable structure, a synthesis method thereof, and an application thereof in gas separation. Background Art

[0002] As a key technology in energy development and green chemical production, gas separation technology plays an irreplaceable role in fields such as fuel gas purification, greenhouse gas capture and separation, and bulk chemical production. The choice of separation technology mainly depends on factors such as product purity, recovery rate, production volume, environment, and economic benefits. Taking ethylene, a very important consumable in the chemical and agricultural fields, as an example, ethylene is widely used in the synthesis of plastics, rubbers, fibers, and their derivatives. Currently, ethylene is mainly produced through the steam cracking process of hydrocarbons, and by-products such as acetylene will be generated during the production process. The presence of acetylene will seriously interfere with the synthesis process of polyethylene and its derivatives. Acetylene will cause catalyst poisoning during the ethylene polymerization reaction, resulting in the termination of the polymerization reaction, thus causing huge economic losses. Nowadays, the industrial separation of acetylene and ethylene mainly uses energy- and cost-intensive industrial purification methods such as cryogenic distillation and acetylene hydrogenation. Therefore, developing a more energy-efficient separation method to purify ethylene is of great significance to the petrochemical industry.

[0003] Due to their unique structures and properties, novel functionalized porous materials have shown great application potential in the field of gas separation. Porous materials such as zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs) have now been deeply studied, and some high-performance porous materials have been successfully widely applied in industrial separation and adsorption fields. For example, MOFs can be used to selectively separate small molecules, especially hydrocarbon small molecules. This is mainly attributed to the fact that MOFs have highly adjustable structures and have various different types of metal centers, enabling them to polarize molecules or bind guest molecules at open metal sites. Porous organic cages (POCs), as a type of zero-dimensional discrete organic molecules with permanent cavities, also have highly ordered pore structures. Although their porous properties are inferior to some high-performance porous organic framework materials, their maximum specific surface area also exceeds 3500 m 2 / g, showing great application potential in gas storage and separation. In addition, POCs are assembled by discrete closed molecules through supramolecular interactions and have dispersibility that most porous organic framework materials do not have, which makes POCs materials have very prominent reprocessing and composite capabilities and a wider range of application scenarios.

[0004] Taking the [4+6] imine cage represented by CC3 as an example, its 1.2 nm window size and 3 nm cavity diameter form hierarchical pores, and this "bottleneck pore" structure can achieve precise sieving based on molecular size. Through precursor molecule design, key parameters such as window size (0.3-1.5 nm) and pore volume (0.3-1.2 cm 3 / g) can be systematically regulated to achieve gradient separation from hydrogen to organic vapor molecules (Barely porous organic cages for hydrogen isotope separation. Science, 2019, 366, 613). In addition, POCs materials show unique advantages in the capture of fuel waste gases such as CO2 and SO2. For example, the new molecular cage developed based on Cage of Cage has a CO2 adsorption capacity of 3.98 mmol / g at 1 bar and 0 °C (Computationallyguided synthesis of a hierarchical[4[2+3]+6]porous organic‘cage of cages’, Nature Synthesis 2024, 3, 825). 6FT-RCC3 developed based on the post-modification of CC3 can adsorb 16.4 SO2 gas molecules per cage molecule at room temperature and atmospheric pressure (SO2Capture Using Porous Organic Cages, Angew.Chem.Int.Ed., 2021, 60, 17556). Another major feature of porous organic cages is their excellent dispersibility with various framework materials, and their solution processability is convenient for the preparation of various forms of membrane materials such as crystal membranes (Asmart and responsivecrystalline porous organic cage membrane with switchable pore apertures forgraded molecular sieving. Nature Materials 2022, 21, 463), mixed matrix membranes (Mixed-matrix membranes with soluble porous organic molecular cage for highlyefficient C3H6 / C3H8 separation, Journal of Membrane Science, 2020, 611, 118288), etc., showing great potential for further industrialization.

[0005] However, compared with MOFs materials, COFs materials, etc., POCs materials usually have a simple structure. The main research focuses on individual and isolated structures rather than a systematic and comprehensive study of their conformational relationships. This inherent limitation stems from the scarcity of building units for organic molecular cages. The assembly process of molecular cages requires high precision, and even minor deviations in the building blocks can significantly change the final structure. Whether using the "bottom-up" direct synthesis or post-synthesis modification strategy to construct functional organic molecular cages, the main obstacle restricting the development of POCs materials for large-scale applications is the lack of active components. Therefore, the development of more efficient and versatile new active building blocks is crucial for the structural expansion and performance exploration of POCs materials. Summary of the Invention

[0006] The present invention provides a tetrazine molecular cage with adjustable structure. This tetrazine molecular cage is constructed based on tetrazine moieties, has a simple synthesis method, a high specific surface area, good permeability, can adsorb various gases at room temperature and achieve rapid separation of ethylene and acetylene, and has good application prospects in the fields of gas storage and purification.

[0007] The specific technical solutions adopted are as follows:

[0008] A tetrazine molecular cage has a structure shown in formula (Ⅰ):

[0009]

[0010] A modified tetrazine molecular cage is obtained by modifying the tetrazine molecular cage with the structure shown in formula (Ⅰ) with a modification monomer. The modification monomer is an unsaturated organic compound with ring strain, including norbornene or its derivatives, morpholine or its derivatives.

[0011] Preferably, the modification monomer is norbornene, and the modified tetrazine molecular cage has a structure shown in formula (II):

[0012]

[0013] Preferably, the molar ratio of the tetrazine molecular cage with the structure shown in formula (Ⅰ) to the modification monomer is 1:15 - 30, the reaction temperature during the modification process is 25 - 40 °C, and the reaction time is 10 - 20 h.

[0014] The present invention also provides a synthesis method of the tetrazine molecular cage, including the following steps:

[0015] (1) Construct a reaction system including 4-cyanobenzaldehyde, p-toluenesulfonic acid, toluene, and ethylene glycol, and react to protect the aldehyde group to obtain a first intermediate;

[0016] (2) Mix sulfur, the first intermediate solution and hydrazine hydrate, and conduct a reflux reaction by heating to obtain a yellow solid. After dissolving the yellow solid, react it with tert-butyl nitrite to obtain a second intermediate;

[0017] (3) Dissolve the second intermediate in a mixed solvent of an acid and an organic solvent, and heat it to undergo a hydrolysis reaction to deprotect the aldehyde group to obtain a third intermediate;

[0018] (4) Use the third intermediate and tris(2-aminoethyl)amine to undergo a condensation reaction to prepare the described tetrazine molecular cage.

[0019] The construction of this tetrazine molecular cage adopts the method of imine condensation dynamic chemistry and is obtained from the condensation reaction of a bis-aldehyde building block with a tetrazine moiety and a scaffolding triamine compound. The bis-aldehyde building block with a tetrazine moiety uses 4-cyanobenzaldehyde as a raw material. First, protect the aldehyde group with ethylene glycol, then obtain the tetrazine moiety through a cyano condensation reaction with the participation of hydrazine hydrate, and finally deprotect the aldehyde group.

[0020] Preferably, in step (1), the reaction temperature is 120 - 150 °C, the reaction time is 10 - 15 h, and more preferably, the reaction temperature is 140 °C and the reaction time is 12 h.

[0021] Preferably, in step (2), the first intermediate solution is an ethanol solution of the first intermediate. The temperature of the reflux reaction by heating is 90 - 110 °C, and the time is 1 - 3 h. More preferably, the temperature is 100 °C and the time is 2 h; dissolve the yellow solid in a mixed solvent composed of ethanol and chloroform, and the temperature of the reaction of the dissolved yellow solid with tert-butyl nitrite is 20 - 80 °C, and the time is 16 - 20 h.

[0022] Preferably, in step (3), dissolve the second intermediate in a mixed solvent of hydrochloric acid and tetrahydrofuran. The temperature of the hydrolysis reaction is 50 - 70 °C, and the time is 20 - 25 h. More preferably, the temperature is 60 °C and the time is 24 h.

[0023] Preferably, in step (4), use the third intermediate and tris(2-aminoethyl)amine to undergo a condensation reaction in a mixed solvent system of methanol and chloroform. The temperature of the condensation reaction is 60 - 80 °C, and the time is 20 - 30 h; the obtained tetrazine molecular cage after the reaction can be completely dissolved in the reaction solvent, and the final tetrazine molecular cage solid is obtained through recrystallization.

[0024] The present invention also provides the application of the described tetrazine molecular cage or the modified tetrazine molecular cage in gas separation. Experimental results prove that the structural distortion brought about by the modification process will improve the adsorption capacity of the tetrazine molecular cage.

[0025] The present invention also provides a method for separating acetylene and ethylene, which utilizes the tetrazine molecular cage or the modified tetrazine molecular cage described above.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) The tetrazine molecular cage provided by the present invention has structural modifiability, and can be further modified to develop a series of organic molecular cages. Moreover, the tetrazine building blocks with post-reactivity provided by the present invention contribute to enriching the assembly modules for the design and synthesis of organic molecular cages and the types of post-modification reactions.

[0028] (2) The tetrazine molecular cage and the modified tetrazine molecular cage provided by the present invention have the ability to adsorb various gases, showing the properties of porous materials.

[0029] (3) The modification process improves the adsorption capacity of the tetrazine molecular cage, and the modified tetrazine molecular cage can achieve the gas separation of acetylene and ethylene through a dynamic penetration method. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a synthetic route diagram of the tetrazine molecular cage p-Cage with the structure shown in formula (I) and the modified tetrazine molecular cage p-Nor with the structure shown in formula (II).

[0031] Figure 2 It is the NMR spectrum diagram of the tetrazine molecular cage p-Cage with the structure shown in formula (I).

[0032] Figure 3 It is the NMR spectrum diagram of the modified tetrazine molecular cage p-Nor with the structure shown in formula (II).

[0033] Figure 4 It is the characterization result diagram of 4-cyanobenzaldehyde p-CN, the third intermediate p-CHO, the tetrazine molecular cage p-Cage and the modified tetrazine molecular cage p-Nor, where (a) is the optical change diagram, (b) is the UV spectrum diagram, and (c) is the IR spectrum diagram.

[0034] Figure 5 It is the gas adsorption data diagram of the tetrazine molecular cage p-Cage and the modified tetrazine molecular cage p-Nor, where a is the nitrogen adsorption result at 77K, b is the CO2 adsorption result at 195K, c is the CO2 adsorption result at 298K, and d is the ethylene and acetylene adsorption result of p-Nor at 298K.

[0035] Figure 6 It is the penetration data diagram of the modified tetrazine molecular cage p-Nor for the dynamic separation of ethylene and acetylene. DETAILED DESCRIPTION OF THE INVENTION

[0036] To make the objectives, features, and advantages of the present invention more apparent and understandable, the following provides a detailed description through specific embodiments. A number of specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflict.

[0037] For the operating methods without specific conditions noted in the following examples, they are generally in accordance with conventional conditions or the conditions recommended by the manufacturer. The content not described in detail in this specification belongs to the prior art well-known to those skilled in the art. The experimental materials used in the following examples, unless otherwise specified, can be obtained from conventional biochemical reagent companies.

[0038] Example 1 Synthesis of Tetrazine Molecular Cage p-Cage

[0039] (1) Synthesis of the First Intermediate (4-(1,3-Dioxolan-2-yl)benzonitrile)

[0040] Add 4-cyanobenzaldehyde p-CN (10 g) and p-toluenesulfonic acid (10 mg) into a 250 mL flask, and then add toluene (150 mL) and ethylene glycol (42.2 mL). The reaction mixture is refluxed and stirred at 140 °C for 12 hours, and the water generated during the reaction is removed through a water separator. When the solution becomes clear, cool the reaction to room temperature, add 40 mL of 5% aqueous NaHCO3 solution, extract the organic phase, wash it successively with water and brine, and dry it with Na2SO4. Concentrate the solution to a clear oil, and after drying under reduced pressure, obtain the first intermediate in the form of a white solid (12.13 g, yield 92.5%).

[0041] (2) Synthesis of the Second Intermediate (3,6-Bis(4-(1,3-dioxolan-2-yl)phenyl)-1,2,4,5-tetrazine)

[0042] Add sulfur (160 mg) to a solution of the first intermediate (1.75 g) in ethanol (3 mL), and then slowly add hydrazine hydrate (3 mL, 50% wt). Heat the reaction mixture to 100 °C and reflux for 2 hours. The color of the solution gradually deepens, and finally a large amount of yellow solid precipitates. Cool the reaction mixture to 0 °C to ensure complete precipitation of the solid, filter and collect the yellow solid, wash it with ice-cold absolute ethanol, and use it directly for the next step of the reaction without further purification;

[0043] Dissolve the yellow solid obtained in the previous step in an ethanol / chloroform mixed solvent (100 mL, v / v = 1:1). Subsequently, add tert-butyl nitrite (3.5 g). The reaction mixture is stirred at room temperature (about 25 °C) for 18 hours and then refluxed at 80 °C for 1 hour. After the reaction is cooled, the insoluble precipitate is filtered out and dried to obtain the second intermediate (1.02 g, two-step yield 53.7%), which is directly used for the next reaction. (3) Synthesis of the third intermediate (4,4'-(1,2,4,5-tetrazine-3,6-diyl)dibenzaldehyde)

[0044] Dissolve the second intermediate in a 3M hydrochloric acid / tetrahydrofuran mixed solvent (100 mL, v / v = 1:1), and heat the resulting solution at 60 °C for 24 hours to carry out a hydrolysis reaction to deprotect the aldehyde group. After the reaction is cooled to room temperature, it is neutralized with saturated aqueous sodium bicarbonate, the solid is filtered out, washed successively with water (3 times) and dichloromethane, the solvent is poured out after centrifugation, and dried under vacuum to obtain the purple powder-like third intermediate (p-CHO, 0.55 g, yield 70.3%).

[0045] (4) Synthesis of the tetrazine molecular cage p-Cage

[0046] Add tris(2-aminoethyl)amine (0.2 mmol) to a methanol / chloroform mixed solution (100 mL, v / v = 1:10) of the third intermediate (0.3 mmol), and mix evenly with stirring. Reflux the entire reaction mixture at 80 °C for 24 hours, then cool to room temperature and filter. Slowly diffuse the ether vapor into the solution. After one week, obtain the purple crystalline powder tetrazine molecular cage p-Cage (85 mg, yield 80.6%).

[0047] The synthetic route diagram of the tetrazine molecular cage p-Cage is as Figure 1 shown, and the NMR spectrum is as Figure 2 shown.

[0048] Example 2 Synthesis of the modified tetrazine molecular cage p-Nor

[0049] Disperse 0.3 g of the tetrazine molecular cage p-Cage prepared in Example 1 in 20 mL of chloroform solvent. Subsequently, add 18 equivalents (molar equivalents) of norbornene. Load the resulting mixture into a glass tube and stir at 40 °C in an oscillator for 16 hours. After the reaction is completed, slowly diffuse the ether vapor into the mother liquor containing the reactants. After one week, obtain the bright yellow crystalline powder modified tetrazine molecular cage p-Nor.

[0050] The synthetic route diagram of the modified tetrazine molecular cage p-nor is as Figure 1 shown, and the NMR spectrum is as Figure 3 shown.

[0051] Sample Analysis

[0052] The aldehyde building blocks all have a very distinct purple color, and the subsequent imine condensation basically does not change their color. This deep color is also a characteristic of the tetrazine molecule. As shown in (a) of Figure 4 , during the synthesis process, the colors of different compounds change significantly, which is mainly caused by the changes in the unsaturated bonds in the molecule. This phenomenon can be recorded by spectroscopy. In the infrared data ( Figure 4 , (c)), the disappearance of the characteristic cyanide peak during the condensation process and the obvious shift of the characteristic tetrazine peak between the synthesis and modification processes can be observed. During the assembly formation process of the tetrazine molecular cage, evidence of the transformation of the aldehyde group into an imine structure can also be shown from the change of the characteristic peak near 1700 cm -1 . During the further modification reaction process, the imine characteristic peak does not change significantly, which also indicates that the Diels - Alder reaction adopted has the ability of precise reaction and will not affect the more reactive imine bond. On the other hand, the ultraviolet spectrum ( Figure 4 , (b)) shows the conjugation changes in the whole system during the molecular cage synthesis process and the post - modification process. After the cyanide condensation of the tetrazine group, the conjugation length of the whole compound increases, and the whole absorption peak shifts significantly to the red. The newly generated absorption peak at 500 nm should correspond to the π - π* transition on the tetrazine functional group, and this characteristic peak disappears during the post - modification process, further proving that this is the characteristic absorption of the tetrazine functional group.

[0053] Before the gas adsorption test, the stability of the samples was investigated by TGA and XRD. It was found that the decomposition temperatures of the tetrazine molecular cage p - Cage and the modified tetrazine molecular cage p - nor are both greater than 250 °C, indicating that the activation temperature of 80 °C will not damage the chemical structure of the molecular cage.

[0054] Gas adsorption test method: The N2, CO2, C2H2, and C2H4 adsorption amounts of solid samples were measured by a Belsorp BSD - 660M volumetric adsorption analyzer. Before the test, the powder samples need to be degassed offline at 80 °C for 10 hours under dynamic vacuum (10 -5 bar). The adsorption amount was measured at a specific temperature (controlled by a cold trap / circulating water cooling device) and 1 bar conditions, and the obtained data were used to calculate the expected adsorption amount of the solid components in the dispersion system. Figure 5 For the gas adsorption data graphs of the tetrazine molecular cage p - Cage and the modified tetrazine molecular cage p - Nor, in the nitrogen adsorption test, both molecular cages show the characteristics of being basically non - porous and insensitive to nitrogen ( Figure 5 , a), and the multi - nitrogen structure may be more suitable for using CO2 as a probe to explore the porous behavior ( Figure 5In b) and c), after the tetrazine molecular cage p-Cage was modified with norbornene, the maximum CO2 adsorption capacity (26.63 mL / g) was higher than 10.79 mL / g before modification. The same phenomenon still occurred at room temperature (p-Nor: 10.72 mL / g, p-Cage: 4.26 mL / g). Most of the change in adsorption capacity during the modification process can be attributed to the greater adsorption capacity brought about by the structural distortion.

[0055] Furthermore, the modified tetrazine molecular cage p-Nor was used for static adsorption tests of ethylene and acetylene. At room temperature and 1 bar, the ethylene adsorption capacity of p-Nor was 6.67 mL / g, while the acetylene adsorption capacity increased to 16.41 mL / g, and the adsorption selectivity of acetylene:ethylene ≈ 2.5:1 ( Figure 5 in d). Using p-Nor to fill the separation column and performing dynamic breakthrough separation with a 1:99 acetylene:ethylene mixed gas, as Figure 6 shown, effective separation of acetylene and ethylene can be achieved at a flow rate of 0.2 mL / min.

[0056] The above-described embodiments have elaborated on the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, supplements, or substitutions in a similar manner within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A tetrazine molecular cage, characterized in that, It has the structure shown in formula (Ⅰ):

2. A modified tetrazine molecular cage, characterized in that, It is obtained by modifying the tetrazine molecular cage with the structure shown in formula (Ⅰ) with a modifying monomer, and the modifying monomer is an unsaturated organic compound with ring strain, including norbornene or its derivatives, morpholine or its derivatives.

3. The modified tetrazine molecular cage according to claim 2, wherein The modifying monomer is norbornene, and the modified tetrazine molecular cage has the structure shown in formula (II):

4. The synthesis method of the tetrazine molecular cage according to claim 1, characterized in that, It includes the following steps: (1) Construct a reaction system including 4-cyanobenzaldehyde, p-toluenesulfonic acid, toluene and ethylene glycol, and react to protect the aldehyde group to obtain a first intermediate; (2) Mix sulfur, the first intermediate solution and hydrazine hydrate, heat and reflux to react to obtain a yellow solid, dissolve the yellow solid, and react with tert-butyl nitrite to obtain a second intermediate; (3) Dissolve the second intermediate in a mixed solvent of an acid and an organic solvent, heat it to carry out a hydrolysis reaction to deprotect the aldehyde group to obtain a third intermediate; (4) Use the third intermediate and tris(2-aminoethyl)amine to carry out a condensation reaction to prepare the tetrazine molecular cage.

5. The synthesis method of the tetrazine molecular cage according to claim 4, wherein In step (1), the reaction temperature is 120-150 °C and the reaction time is 10-15 h.

6. The synthesis method of the tetrazine molecular cage according to claim 4, characterized in that, In step (2), the first intermediate solution is an ethanol solution of the first intermediate, the temperature of the heating reflux reaction is 90-110 °C, and the time is 1-3 h; dissolve the yellow solid in a mixed solvent composed of ethanol and chloroform, and the temperature of the reaction of the dissolved yellow solid with tert-butyl nitrite is 20-80 °C, and the time is 16-20 h.

7. The synthesis method of the tetrazine molecular cage according to claim 4, characterized in that, In step (3), dissolve the second intermediate in a mixed solvent of hydrochloric acid and tetrahydrofuran, and the temperature of the hydrolysis reaction is 50-70 °C and the time is 20-25 h.

8. The synthesis method of the tetrazine molecular cage according to claim 4, wherein, In step (4), use the third intermediate and tris(2-aminoethyl)amine to carry out a condensation reaction in a mixed solvent system of methanol and chloroform, and the temperature of the condensation reaction is 60-80 °C and the time is 20-30 h.

9. Use of the tetrazine molecular cage according to claim 1 or the modified tetrazine molecular cage according to claim 2 or 3 in gas separation.

10. A method for separating acetylene and ethylene, characterized in that, Use the tetrazine molecular cage according to claim 1 or the modified tetrazine molecular cage according to claim 2 or 3.

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