Flexible metal-organic framework material with humidity response as well as preparation method and application of flexible metal-organic framework material

Abstract

The invention relates to a flexible metal-organic framework material with humidity response, the chemical formula is {Cd (TPB) (NDC) (NO3)} n, in the formula, TPB is 1, 3, 5-tris (4-pyridyl) benzene, NDC is 1, 4-naphthalic acid; the minimum asymmetric unit comprises a Cd (II) ion, a TPB molecule, a dehydrogenated NDC ion and a nitro group. The preparation method comprises the following steps: mixing Cd (NO3) 2.4 H2O, NDC and TPB, dispersing in a N, N-dimethylacetamide (DMA) solvent, reacting the mixed suspension obtained by ultrasonic treatment in a solvothermal manner, cleaning with DMA, carrying out solvent exchange, and carrying out vacuum activation and drying to obtain the target product. The flexible metal-organic framework can realize the reverse adsorption separation of propane / propylene under the condition of 298 K along with humidity.

Description

Technical Field

[0001] This invention belongs to the field of inorganic-organic hybrid material synthesis technology, specifically relating to a humidity-responsive flexible metal-organic framework material, its preparation method, and its special adsorption separation mechanism and application in adsorption separation, namely, a humidity-responsive flexible metal-organic framework material, its preparation method, and its application. Background Technology

[0002] Propylene (C3H6) is one of the most important basic raw materials in modern chemical engineering. Currently, industrial purification of propylene mainly relies on cryogenic distillation. This process depends on extremely small boiling point differences, resulting in the need for very high trays and huge reflux ratios, leading to extremely high energy consumption. Therefore, developing efficient and energy-saving alternative separation technologies has significant strategic and economic importance. Adsorption separation technologies based on porous materials, especially pressure swing adsorption (PSA), are considered one of the most promising alternatives due to their mild operating conditions and low energy consumption potential. Among them, metal-organic frameworks (MOFs) have become star materials for light hydrocarbon separation adsorbents due to their extremely high specific surface area, tunable pore size, and functionalizable pore wall chemistry. To date, most research has focused on designing MOFs with stronger adsorption capacity for olefins, with the main strategies including: unsaturated metal sites: utilizing M... +Strong interactions with olefin π bonds; pore wall functionalization: introducing groups that specifically interact with olefins; pore size sieving: designing rigid micropores that only allow diffusion of smaller molecules. However, these strategies generally face a fundamental challenge: under thermodynamic equilibrium, the high selectivity of strong adsorption sites for olefins often comes at the cost of adsorption capacity, and adsorbent regeneration (olefin desorption) is difficult, increasing process energy consumption. More importantly, they follow the same "adsorption-desorption" pathway for olefins, and separation efficiency is limited by the adsorption isotherm itself. To overcome these limitations, a disruptive "reverse adsorption separation" strategy has emerged, which designs materials that preferentially adsorb alkanes rather than olefins. Its core advantages are: the adsorbate phase is enriched with alkanes, and the effluent gas directly yields high-purity olefin products, simplifying the process; alkanes typically have low adsorption enthalpy, making them easy to desorb and regenerate; avoiding strong π-complexation with olefins may improve cycle stability. However, achieving "reverse adsorption" is extremely challenging because alkane molecules are less polar, making the design of interactions with the pore walls more difficult. Flexible metal-organic frameworks (MOFs) offer a unique platform for addressing this challenge. Their dynamically responsive channels (such as breathing and gating effects) can generate differentiated structural responses to molecules with minute differences in size, shape, or forces, thereby achieving adsorption-induced "molecular recognition" rather than traditional thermodynamic equilibrium adsorption. This provides a novel mechanism for breaking the conventional "olefin-first" paradigm. Therefore, constructing flexible MOFs offers a new approach for one-step purification of propylene gas and is of great significance. Although flexible MOFs have broad application prospects in adsorption separation, the "reverse" adsorption separation of propane (C3H8) / propylene (C3H6) using flexible MOFs remains extremely challenging. Furthermore, in practical industrial purification of propylene gas, it is difficult to avoid the influence of humidity. Therefore, finding a simple and efficient adsorbent that can purify propylene gas in one step at both room temperature and ambient humidity is a crucial research area. Summary of the Invention

[0003] The present invention aims to provide a humidity-responsive flexible metal-organic framework material, its preparation method, and its application. It can effectively solve the problem of one-step purification of propylene (C3H6) gas. When the humidity is greater than 20%, the framework adsorbs water molecules, changing its crystal structure and pore size. Based on this, it exhibits special adsorption and separation behavior of propane (C3H8) / propylene (C3H6) gas.

[0004] The technical solution of this invention is: a flexible metal-organic framework material with humidity response, the chemical formula of which is: {Cd(TPB)(NDC)(NO3)} n In the formula, TPB is 1,3,5-tris(4-pyridyl)benzene, NDC is 1,4-naphthalenedicarboxylic acid; the smallest asymmetric unit includes a Cd(II) ion, a TPB molecule, a dehydrogenated NDC ion, and a nitro group.

[0005] The metal-organic framework crystal belongs to the triclinic crystal system, space group P-1, and has the following cell parameters: a=14.7080(3) Å, b=18.3069(4) Å, c=27.1502(4) Å, α=74.424(2) °, β=76.637(1) °, γ=69.982(2) °.

[0006] The cadmium in the metal-organic framework is seven-coordinated, coordinating with the three pyridine nitrogen atoms on the three TPBs, the two oxygen atoms on the NDC, and the two oxygen atoms on the nitro group.

[0007] The flexible metal-organic framework is a layer-column type metal-organic framework, in which two-dimensional meshes are connected together by column ligand 1,4-naphthalenedicarboxylic acid to form a Bey topological framework structure.

[0008] A method for preparing the above-mentioned humidity-responsive flexible metal-organic framework material includes the following steps: (1) Cd(NO3)2·4H2O, NDC and TPB were mixed and dispersed in N,N-dimethylacetamide (DMA) solvent and placed in a sealed container; after ultrasonic treatment, a uniform milky white suspension was obtained. (2) The mixed suspension obtained in step (1) is reacted by solvothermal means. After the reaction is completed, a mixture containing pale yellow blocky crystals is generated. (3) Cool to room temperature, wash with DMA and collect pale yellow blocky crystals, then perform solvent exchange; (4) Finally, after vacuum activation and drying, the target product, flexible metal-organic framework material, is obtained.

[0009] In step (1), the molar ratio of Cd(NO3)2·4H2O, NDC and TPB is 2:2:1.

[0010] The amount of N,N-dimethylacetamide solvent used in step (1) is 2 mL of N,N-dimethylacetamide per 0.05 mmol Cd(NO3)2·4H2O, 0.05 mmol NDC and 0.025 mmol TPB.

[0011] The ultrasonic treatment time in step (1) is 30 minutes.

[0012] The solvothermal reaction in step (2) is carried out at 92 °C for 72 hours.

[0013] In step (3), an ultra-dry acetonitrile solvent is used for solvent exchange.

[0014] The solvent exchange step in step (3) involves exchanging the solvent three times a day with ultra-dry acetonitrile solvent for a total of three days, which means nine solvent exchanges are performed using ultra-dry acetonitrile solvent.

[0015] The conditions for vacuum activation drying in step (4) are vacuum activation drying at 80 °C for 12 hours.

[0016] An application of the above-mentioned humidity-responsive flexible metal-organic framework material, wherein the activated flexible metal-organic framework material is used for the adsorption of water (H2O), propane (C3H8) and propylene (C3H6) gases.

[0017] The activated flexible metal-organic framework material achieves reverse adsorption separation of propane (C3H8) / propylene (C3H6) due to the action of water molecules under conditions of 298 K and humidity greater than 20%.

[0018] The activated flexible metal-organic framework material shrinks under humid conditions due to the action of water molecules and occupies the adsorption sites of propylene molecules, thereby achieving reverse adsorption and separation of propane (C3H8) / propylene (C3H6) gases.

[0019] Advantages and beneficial effects of this invention: 1. After activation, the flexible metal-organic framework material of this invention exhibits dynamic flexible behavior under the influence of water molecules, propane (C3H8), propylene (C3H6), etc. Under different humidity conditions, the flexible metal-organic framework will shrink to different degrees, thereby affecting the adsorption behavior of propane (C3H8) and propylene (C3H6). That is, as the humidity gradually increases, the flexible metal-organic framework has a stronger adsorption capacity for propane (C3H8). Based on this special adsorption behavior, the material can achieve reverse adsorption separation of propane / propylene at 298 K with humidity. 2. Due to the interaction between different guest molecules and the host framework after entering the pores, the framework undergoes structural transformation. The flexible metal-organic framework material of this invention exhibits special adsorption behavior for gases such as water (H2O), propane (C3H8), and propylene (C3H6), which can effectively separate propane (C3H8) / propylene (C3H6) and achieve one-step purification of propylene (C3H6). 3. This invention utilizes its preferential adsorption of water molecules and the "gating" and "breathing" effects of its dynamic channels to achieve a specific response to the weak physical properties of propane molecules (such as polarizability and quadrupole moment). That is, propane can be strongly adsorbed by "pushing open" the channels under specific pressure, while propylene cannot, thereby achieving reverse adsorption separation based on adsorption-induced phase transition. Attached Figure Description

[0020] Figure 1This is an asymmetric unit of the flexible metal-organic framework, consisting of a Cd(II) ion, a TPB molecule, a dehydrogenated NDC ion, and a nitro group; Figure 2 The image shows the crystal structure of the flexible metal-organic framework. The bilayer structure is constructed from cadmium ions and TPB, and the bilayer is connected by NDC as a pillar ligand. Figure 3 PXRD diffraction patterns of solvent synthesis and single-crystal simulation of the flexible metal-organic framework; Figure 4 Thermogravimetric analysis (TGA) spectra of the flexible metal-organic framework are shown below. Figure 5 The specific surface area (BET) diagram of the flexible metal-organic framework is shown below. Figure 6 The adsorption isotherms at 298 K for propane (C3H8) and propylene (C3H6) of the flexible metal-organic framework under humidity-free conditions are shown. Figure 7 The 298 K water adsorption isotherm of the flexible metal-organic framework; Figure 8 The breakthrough curves of the flexible metal-organic framework for propane (C3H8) / propylene (C3H6) under different humidity conditions are shown. Detailed Implementation

[0021] Example: A humidity-responsive flexible metal-organic framework material with the chemical formula: {Cd(TPB)(NDC)(NO3)} n In the formula, TPB is 1,3,5-tris(4-pyridyl)benzene, and NDC is 1,4-naphthalenedicarboxylic acid; the smallest asymmetric unit includes a Cd(II) ion, a TPB molecule, and a dehydrogenated NDC ion.

[0022] The cadmium in the metal-organic framework is seven-coordinated, coordinating with the three pyridine nitrogen atoms on the three TPBs, the two oxygen atoms on the NDC, and the two oxygen atoms on the nitro group.

[0023] The flexible metal-organic framework is a layer-column type metal-organic framework, in which two-dimensional meshes are connected together by column ligand 1,4-naphthalenedicarboxylic acid to form a Bey topological framework structure.

[0024] A method for preparing the above-mentioned humidity-responsive flexible metal-organic framework material includes the following steps: (1) Mixing 0.05 mmol (15 mg) Cd(NO3)2·4H2O, 0.05 mmol (10.8 mg) NDC and 0.025 mmol (7.7 mg) TPB and dissolving them in 2 mL of DMA solvent, and placing them in a 10 mL sealed glass bottle. After ultrasonic treatment for 30 minutes, a uniform milky white suspension is obtained; (2) Keeping the mixed suspension obtained in step (1) at 92 °C in an oven for 72 hours, reacting by solvothermal means, and generating a mixture containing pale yellow blocky crystals after the reaction is completed; (3) Washing with DMA multiple times and collecting the pale yellow blocky crystals, and performing 9 solvent exchanges using ultra-dry acetonitrile solvent for three days, 3 times a day; (4) Finally, vacuum activating and drying at 80 °C for 12 hours to obtain the target product, flexible metal-organic framework material.

[0025] An application of the above-mentioned humidity-responsive flexible metal-organic framework material, wherein the activated flexible metal-organic framework material is used for the adsorption of water (H2O), propane (C3H8) and propylene (C3H6) gases.

[0026] The activated flexible metal-organic framework material achieves reverse adsorption separation of propane (C3H8) / propylene (C3H6) due to the action of water molecules under conditions of 298 K and humidity greater than 20%.

[0027] The activated flexible metal-organic framework material shrinks under humid conditions due to the action of water molecules and occupies the adsorption sites of propylene molecules, thereby achieving reverse adsorption and separation of propane (C3H8) / propylene (C3H6) gases.

[0028] The flexible metal-organic framework material was characterized using the following instruments and methods: 1. Crystals of suitable size and good crystal quality were selected for single-crystal structure analysis. Single-crystal X-ray diffraction (SCXRD) data were obtained at 100 K using Rigaku XtaLAB Synergy-R (Cu Kα, λ = 1.5418 Å). The obtained compound belongs to the triclinic crystal system, space group P-1, with cell parameters: a = 14.7080(3) Å, b = 18.3069(4) Å, c = 27.1502(4) Å, α = 74.424(2) °, β = 76.637(1) °, γ = 69.982(2) °. The crystal structure is shown in [reference needed]. Figure 2 (Drawn using Diamond software).

[0029] 2. Powder X-ray diffraction (PXRD) was performed on a Rigaku MiniFlex 600 diffractometer equipped with a 40 kV, 15 mA copper target and a graphite monochromator. The corresponding scanning speed and step size were 10° / min and 0.02°, respectively. The powder sample was prepared from ground crystals. Figure 3 As shown, the measured PXRD image is consistent with the single-crystal simulated PXRD, indicating that the synthesized flexible metal-organic framework material has good phase purity.

[0030] 3. Thermogravimetric analysis (TGA) was performed using a Rigaku TG8121 thermal analyzer under argon protection, with the temperature increased from room temperature to 850 °C at a rate of 10 °C / min, using empty Al₂O₃ ceramic as a reference. Figure 4 As shown, the weight loss process is divided into two stages. The first stage (room temperature ~ 200 °C) corresponds to the desorption and evaporation of the DMA solvent. Since the solvent is located in the channels, its evaporation temperature is slightly higher than the boiling point of free DMA (approximately 165 °C), and the weight loss in this stage is approximately 20%. The second stage (approximately 350 ~ 600 °C) corresponds to the collapse and carbonization of the flexible metal-organic framework.

[0031] 4. BET testing was performed on a Micrometrics ASAP 2460 gas adsorption analyzer. After exchanging the above-mentioned flexible metal-organic framework material nine times with ultra-dry acetonitrile solvent, a sufficient amount of the activated sample was placed in a sample tube and heated in situ to 80 °C on a Micrometrics Smart VacPrep degassing station for 12 hours to remove acetonitrile molecules. Measurements were performed using more than 100 mg of desolventized sample. The N2 adsorption isotherm of the activated sample was measured at 77 K. Figure 5 As shown, the nitrogen adsorption isotherm of this flexible metal-organic framework material at 77 K exhibits typical type I isotherm characteristics. In the extremely low relative pressure region (P / P0 < 0.01), the adsorption capacity increases rapidly, indicating the presence of accessible microporous structures and micropore filling within the material. As the relative pressure increases (P / P0 = 0.01~0.3), the adsorption capacity continues to increase, but the slope gradually slows down. When P / P0 > 0.3, the adsorption capacity essentially reaches saturation and forms a flat plateau throughout the medium-high pressure region (P / P0 = 0.3~1.0). The adsorption and desorption branches almost completely overlap across the entire pressure range; no hysteresis loop was observed, nor were the common gate pressure or step-like adsorption behaviors seen in flexible metal-organic framework materials. This indicates that the interaction force between nitrogen molecules and the framework is weak, and the low temperature inhibits the dynamic response of the framework. The saturated nitrogen adsorption capacity of this flexible metal-organic framework material, obtained from the isotherm data, is 177 cm⁻¹. 3 / g, specific surface area is 770m² 2 / g, total pore volume is 0.27cm³ 3 / g.

[0032] 5. Adsorption tests of propane (C3H8) / propylene (C3H6) were performed on a Micrometrics ASAP 2020 gas adsorption analyzer. After exchanging the above-mentioned flexible metal-organic framework material nine times with ultra-dry acetonitrile solvent, a sufficient amount of activated sample was placed in a sample tube and heated in situ to 80 °C on a Micrometrics Smart VacPrep degassing station for 12 hours to remove acetonitrile molecules. Measurements were performed using more than 100 mg of desolventized sample. The adsorption isotherms of activated propane (C3H8) and propylene (C3H6) were measured at 298 K. Figure 6 As shown in the adsorption isotherms, the adsorption capacity of this flexible metal-organic framework material for propane and propylene is relatively low in the low-pressure region, but the adsorption capacity for propylene is higher than that for propane. At a pressure of 0.13 bar, the framework opens to propane, and the adsorption capacity rapidly increases to a maximum of 62 cm⁻¹. 3 / g; At a pressure of 0.18 bar, the frame opens to propylene, and the adsorption capacity rapidly increases to a maximum of 67 cm⁻¹. 3 / g. The above results indicate that, regardless of whether it is in the low-pressure region (P < 0.18 bar) or the high-pressure region, the flexible metal-organic framework material cannot complete the reverse adsorption separation of propane / propylene under 0 humidity conditions.

[0033] 6. Water adsorption tests were performed on a Bel sorb max vapor adsorption instrument. After exchanging the above-mentioned flexible metal-organic framework material nine times with ultra-dry acetonitrile solvent, a sufficient amount of the activated sample was placed in a sample tube and heated in situ to 100 °C on a Micrometrics Smart VacPrep degassing station for 12 hours to remove acetonitrile molecules. Measurements were taken using 50 mg of the activated sample. The water vapor adsorption isotherm was measured at 298 K. Figure 7 As shown in the water adsorption isotherm, the water adsorption amount is not high when the humidity is less than 80%, indicating that the flexible metal-organic framework material further shrinks after absorbing water in this humidity range and remains a closed phase. However, after the humidity reaches 80%, the water adsorption amount shows a steep step, and obvious "open-door" adsorption occurs. Water molecules induce a structural transformation of the framework, changing from a closed phase to an open phase.

[0034] 7. Transmission curve testing was performed using a dynamic gas transmission test apparatus (BSD-MAB, Beijing Best Instrument Technology Co., Ltd.). This apparatus is equipped with a stainless steel column (2.0 mm inner diameter, 100 mm length). The column was filled with activated sample powder (0.8326 g). Before testing, the packed column was incubated at 100 °C at a rate of 20 mL / min. -1 Argon gas was purged for 12 hours at a flow rate that was no longer detectable by the mass spectrometer. Subsequently, a propane (C3H8) / propylene (C3H6) mixture (50 / 50 volume ratio) was introduced. Different humidity conditions could be achieved by adjusting the flow rate of the mixed gas through the deionized water bubbler. The outlet gas flow was continuously monitored using an online mass spectrometer (MASS-0108-100F). After each breakthrough experiment, the gas was purged at 100 °C at a flow rate of 20 mL / min. -1 Argon gas flow rate is used to purge the packed column, achieving its regeneration. For example... Figure 8 As shown in the breakthrough curves under different humidity levels, the flexible metal-organic framework achieved short-term forward separation of propane / propylene at 0 humidity. Correspondingly, at 0 humidity, the adsorption isotherm showed that neither propane nor propylene was open before the pressure reached 0.15 bar, but propane adsorption was less than propylene, thus achieving a brief forward separation. Subsequently, with increasing pressure, the framework transitioned from a closed phase to an open phase, and the forward separation disappeared. However, at 20% humidity and above, due to the effect of water molecules, the flexible metal-organic framework exhibited the ability to reverse adsorption and separation of propane / propylene, even in extremely high humidity environments (100%). This flexible metal-organic framework not only achieved the transition from forward to reverse adsorption separation of propane / propylene but also demonstrated good stability and reverse adsorption separation capability under high humidity conditions.

Claims

1. A flexible metal-organic framework material having a humidity response, characterized by having the chemical formula: {Cd(TPB)(NDC)(N03)} n where TPB is 1,3,5-tris(4-pyridyl)benzene and NDC is 1,4 naphthalene dicarboxylic acid; the minimum asymmetric unit includes one Cd(II) ion, one TPB molecule, one deprotonated NDC ion, and one nitrate group.

2. The flexible metal-organic framework material with humidity response according to claim 1, wherein The flexible metal-organic framework crystal belongs to the triclinic crystal system, space group P-1, and has the following cell parameters: a=14.7080(3) Å, b=18.3069(4) Å, c=27.1502(4) Å, α=74.424(2) °, β=76.637(1) °, γ=69.982(2) °.

3. The flexible metal-organic framework material with humidity response according to claim 1, wherein The cadmium in the flexible metal-organic framework is seven-coordinated, coordinating with the three pyridine nitrogen atoms on the three TPBs, the two oxygen atoms on the NDC and the two oxygen atoms on the nitro group, respectively. The flexible metal-organic framework is a layer-column type metal-organic framework, in which two-dimensional meshes are connected together by column ligand 1,4-naphthalenedicarboxylic acid to form a Bey topological framework structure.

4. A method for preparing the humidity-responsive flexible metal-organic framework material according to claim 1, characterized in that... Includes the following steps: (1) Cd(NO3)2·4H2O, NDC and TPB were mixed and dispersed in N,N-dimethylacetamide (DMA) solvent and placed in a sealed container; after ultrasonic treatment, a uniform milky white suspension was obtained. (2) The mixed suspension obtained in step (1) is reacted by solvothermal means. After the reaction is completed, a mixture containing pale yellow blocky crystals is generated. (3) Cool to room temperature, wash with DMA and collect pale yellow blocky crystals, then perform solvent exchange; (4) Finally, after vacuum activation and drying, the target product, flexible metal-organic framework material, is obtained.

5. The method for preparing a humidity-responsive flexible metal-organic framework material according to claim 4, characterized in that... In step (1), the molar ratio of Cd(NO3)2·4H2O, NDC and TPB is 2:2:1; The amount of N,N-dimethylacetamide solvent used in step (1) is: 2 mL of N,N-dimethylacetamide per 0.05 mmol Cd(NO3)2·4H2O, 0.05 mmol NDC and 0.025 mmol TPB. The ultrasonic treatment time in step (1) is 30 minutes.

6. The method for preparing a humidity-responsive flexible metal-organic framework material according to claim 4, characterized in that... The solvothermal reaction in step (2) is carried out at 92 °C for 72 hours.

7. The method for preparing a humidity-responsive flexible metal-organic framework material according to claim 4, characterized in that... In step (3), an ultra-dry acetonitrile solvent is used for solvent exchange. The solvent exchange step in step (3) involves exchanging the solvent three times a day with ultra-dry acetonitrile solvent for a total of three days, which means nine solvent exchanges are performed using ultra-dry acetonitrile solvent.

8. The method for preparing a humidity-responsive flexible metal-organic framework material according to claim 4, characterized in that... The conditions for vacuum activation drying in step (4) are vacuum activation drying at 80 °C for 12 hours.

9. An application of the humidity-responsive flexible metal-organic framework material of claim 1, characterized in that... The activated flexible metal-organic framework material is used for the adsorption of water (H2O), propane (C3H8) and propylene (C3H6) gases.

10. An application of the humidity-responsive flexible metal-organic framework material of claim 1, characterized in that... The activated flexible metal-organic framework material was used for reverse adsorption separation of propane (C3H8) / propylene (C3H6) under conditions of 298 K and humidity greater than 20%.

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