A flexible MOF material with lone pair electron regulated gas recognition and application in propylene / propane separation

Abstract

The application belongs to the technical field of metal organic framework (MOF) materials, and particularly relates to a preparation method of a flexible columnar layered MOF adsorption material with lone pair electron participation in gas identification and application of the MOF adsorption material in selective separation of propylene / propane. The MOF material is of a columnar layered structure, and a skeleton of the MOF material is composed of metal nodes of divalent copper ions and first organic ligands and second organic ligands which are connected with the metal nodes in coordination. The first organic ligands are used for coordinating with the metal nodes to construct a two-dimensional layered structure, and the second organic ligands are used for connecting adjacent two-dimensional layered structures to form a three-dimensional skeleton structure.

Description

Technical Field

[0001] This invention relates to a MOF material with lone pair electron regulation of gas recognition flexibility and its application in propylene / propane separation, belonging to the field of MOF-based adsorption material technology. Background Technology

[0002] Propylene is a critical chemical intermediate essential for the production of plastics and a wide range of materials. Global demand for it is soaring, with over 60% used in the production of polypropylene, which requires ultra-high purity (99.5%) propylene. However, propylene is primarily associated with propane, and its separation presents a significant industrial challenge due to their nearly indistinguishable molecular size and boiling point. Traditional methods—cryogenic distillation—are extremely energy-intensive. They require distillation columns with over 200 trays and operate under conditions of very low relative volatility, resulting in enormous heating and cooling energy consumption. Indeed, olefin / alkane separation accounts for a considerable portion of the energy footprint of the petrochemical industry. Therefore, the development of adsorption separation materials capable of distinguishing C3H6 and C3H8 at ambient temperatures and moderate pressures has urgent scientific and industrial significance. Metal-organic frameworks (MOFs), as one of the most promising porous materials, have shown great advantages in gas separation due to their modular building blocks that can form adaptive pore geometries and surface properties for target gases. The excellent controllability of MOFs in their synthesis and functionalization, particularly in rigid structures, has led to their extensive exploration in olefin / alkane separation. Strategies include constructing unsaturated metal sites, introducing π-electron ligands, or introducing polar functional groups to preferentially bind olefins. For metal-organic frameworks constructed with flexible ligands, their structural transformation behaviors (e.g., gating effects, breathing behavior, or phase transitions) under the influence of guest molecules or external stimuli have been applied to gas separation. In such systems, the framework is typically closed or contracted in the absence of guest molecules; however, under the induction of specific guest molecules, the framework can reversibly expand or open, thereby altering pore size and accessibility to achieve selective adsorption of target gas molecules. This dynamic adaptability provides a pathway to amplify minute molecular differences (size, electronic structure, polarizability) into considerable separation performance. However, achieving precise, robust, and selective flexible behavior in MOFs for olefin / alkane separation remains challenging. A key challenge lies in the precise control of the framework's flexibility: excessive rigidity hinders structural responsiveness, while excessive compliance can lead to structural collapse or irreversible phase transitions accompanied by hysteresis. Another process-related issue is that when the partial pressure of the target gas falls below the gating threshold, it can affect the purity of the final product. Therefore, an ideal framework requires a comprehensive balance of affinity, mechanical deformability, and kinetic accessibility. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention proposes an imidazole-based flexible Cu-based MOF material, wherein the uncoordinated nitrogen atom on the imidazole ring can act as an anchor point with lone pair electrons (LPEs) to interact with hydrogen bond donors or molecules with positive charge centers, thereby inducing the contraction / expansion of the framework.

[0004] A metal-organic framework adsorbent material has a columnar layered structure, the crystal framework of which includes a divalent copper ion metal center and a first organic ligand and a second organic ligand covalently bonded to the metal center; the metal center is a tetranuclear copper cluster [Cu4( μ [3-OH)2N4(COO)6]; The first organic ligand is N,N',N''-tris(carboxymethyl)-1,3,5-benzenetricarboxamide, used to coordinate with the metal center to form a two-dimensional layered structure; the second organic ligand is 1,4-di(imidazol-1-yl)butane, used to column-connect adjacent two-dimensional layered structures; wherein, the imidazolium group of the second organic ligand contains nitrogen atoms that do not participate in the framework coordination, and the nitrogen atoms have lone pairs of electrons, which can form specific interactions with propylene molecules, thereby inducing a reversible structural transformation of the adsorbent material. The adsorbent material is in a closed-pore phase after vacuum activation, with a porosity of 5-10%; and it can undergo a gate-opening effect when adsorbing propylene molecules, transforming into an open-pore phase, increasing the porosity to 25-30%; preferably, the gate-opening effect is accompanied by an increase in the dihedral angle of the second organic ligand from 80-90° to 120-130°, and an extension in the distance between adjacent copper clusters (Cu···Cu) from 13-14 Å to 14-15 Å; preferably, the ratio of the amount of propylene adsorbed to the amount of propane adsorbed by the adsorbent material is greater than 30 under the conditions of 298 K and 0.5 bar.

[0005] A method for preparing the adsorbent material includes the following steps: a) In water, a copper source, a first organic ligand, and a second organic ligand are mixed evenly and reacted in a closed reaction vessel at 90-110 °C for 3-24 hours to obtain the metal-organic framework; b) The metal-organic framework is subjected to solvent exchange treatment in an alcohol solvent for 2-4 days; c) The material after solvent exchange treatment is subjected to stepwise thermal vacuum activation, first vacuum treatment at 25 °C for 2 hours, and then vacuum treatment at 100-120 °C for 12 hours to obtain activated adsorbent material.

[0006] In step a), the molar ratio of the copper source, the first organic ligand, and the second organic ligand is 1:(0.8-1.2):(1.5-2.5); the copper source is copper chloride.

[0007] The aqueous solvent mentioned in step a) is pure deionized water, or a mixed solvent of deionized water and methanol; the reaction time is 3-24 h.

[0008] The alcohol solvent mentioned in step b) is anhydrous methanol, and the solvent is replaced with fresh solvent every 6-10 hours during the solvent exchange process; the vacuum condition mentioned in step c) is dynamic high vacuum condition.

[0009] The application of adsorbent materials in gas separation, wherein the gas separation is the selective adsorption of propylene from a mixed gas containing propylene and propane; wherein the selective adsorption is achieved by forming multiple CH···N hydrogen bonds between the uncoordinated nitrogen atom on the second organic ligand and the acidic vinyl hydrogen of the propylene molecule, thereby triggering the gating opening effect of the adsorbent material through synergistic π···π interactions.

[0010] The selective adsorption is carried out at a temperature of 273-298 K; preferably, the selective adsorption is carried out at a pressure of 0.0001-1.0 bar, wherein the adsorbent material is induced by propylene to undergo a gate opening effect in a low pressure range of 0.1-0.2 bar.

[0011] The mixed gas also contains water vapor, with a relative humidity of 65%-85%.

[0012] The volume ratio or molar ratio of propylene to propane in the mixed gas is 1:1. The mixed gas is passed into an adsorption bed containing the activated adsorption material for dynamic breakthrough separation, thereby achieving complete dynamic separation of propylene and propane.

[0013] The beneficial effects of this invention are: In the material of this invention, the imidazole group of the second organic ligand contains nitrogen atoms that are not involved in framework coordination. The lone pair electrons on the uncoordinated nitrogen atoms on the imidazole ring can form specific interactions with propylene molecules, thereby inducing a reversible structural transformation (from closed-pore phase to open-pore phase) in the adsorbent material. By controlling the length of the column ligand, the flexibility of the framework and the structural response behavior can be finely tuned, enabling the material to preferentially adsorb propylene under ambient temperature and pressure conditions, thus achieving efficient and selective separation of propylene / propane.

[0014] By precisely controlling the column ligand length, molecular-level control over the flexibility of the framework was achieved, solving the problem that traditional MOF materials cannot simultaneously achieve high selectivity and fast response. The material can achieve efficient separation of propylene / propane at room temperature and pressure. The continuous flexible change from the original rigidity to a highly dynamic one is triggered by lone pair nitrogen electrons and combined with alkyl chain extension, realizing the gating opening transition. Under low pressure, the transition is preferentially triggered by C3H6. π-π interactions and multiple CH···N / O interactions jointly stabilize C3H6 in the expanded channels, while the binding of C3H8 is relatively weak. Attached Figure Description

[0015] Figure 1 Adsorption isotherms of C3H6 and C3H8 in Comparative Example 1 (a), Comparative Example 2 (b) and Example 1 (c) at 298 K.

[0016] Figure 2 Comparative Example 1: In-situ pressure-switching powder X-ray diffraction (PXRD) patterns and corresponding adsorption isotherms under C3H6(a) and C3H8(b) atmospheres.

[0017] Figure 3 Comparative Example 2: In-situ pressure-switched powder X-ray diffraction (PXRD) patterns and corresponding adsorption isotherms under C3H6(a) and C3H8(b) atmospheres.

[0018] Figure 4 Example 1: In-situ pressure-switched powder X-ray diffraction (PXRD) patterns and corresponding adsorption isotherms under C3H6(a) and C3H8(b) atmospheres.

[0019] Figure 5 Example 1: Dynamic breakthrough curve of C3H6 / C3H8 (50:50, v / v) gas mixture at 298 K and 1 bar. Detailed Implementation Example

[0022] Copper chloride dihydrate (CuCl2·2H2O, 17 g) and TCMBT ligand (N,N',N″-tris(carboxymethyl)-1,3,5-benzenetricarboxamide, 38.1 g) were added to 1 L of deionized water and sonicated for 30 minutes until completely dissolved, yielding a clear blue solution. This solution was transferred to a 2.5 L blue-capped reagent bottle with a screw cap. Separately, bib ligand (1,4-bis(imidazol-1-yl)butane, 38 g) was dissolved in 1 L of deionized water and sonicated until completely dissolved. This solution was then added to the above reaction vessel. The reaction mixture was heated at 100 °C overnight. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The resulting crystalline product was filtered and washed successively with N,N-dimethylformamide (DMF) and deionized water to obtain 43.14 g of blue crystals.

[0023] Comparative Example 1 The synthesis method is similar to that in Example 1, except that bpp (1,3-bis(4-pyridyl)propane) is used instead of bib.

[0024] Comparative Example 2 The synthesis method is similar to that in Example 1, except that bib is replaced with bip (1,3-bis(imidazol-1-yl)propane).

[0025] Solvent replacement and sample activation treatment The newly synthesized sample was immersed in anhydrous methanol for solvent exchange treatment. The supernatant was decanted every 8 hours and fresh anhydrous methanol was added. The exchange was repeated for three days to obtain a solvent-exchanged crystalline sample. Subsequently, the solvent-exchanged sample was heated at 25 °C for 2 hours; then, it was heated at 100-120 °C for 12 hours under dynamic high vacuum conditions to obtain a fully activated sample.

[0026] Single-component adsorption experiment The adsorption of Ar, N2, CO2, CH4, SF6, C3H6, and C3H8 was measured using a gas adsorption analyzer. Before the experiment, appropriate amounts of each sample were placed in sample tubes and activated under vacuum at 120 °C for 12 h to remove guest solvent molecules. Adsorption isotherms for C3H6 and C3H8 were collected at 273 K and 298 K for each sample. For other gases, adsorption measurements were performed at the following temperatures: N2 at 77 K, Ar at 87 K, CO2 at 195 K, SF6 at 298 K, and CH4 at both 273 K and 298 K.

[0027] The material of Example 1 provided by this invention exhibits excellent guest-response gating advantages in the adsorption and separation of propylene (C3H6) and propane (C3H8), which is fully confirmed by comparing the isothermal adsorption curves with those of the isomorphic rigid material of Comparative Example 1 and the confined flexible material of Comparative Example 2. At 298 K, the rigid material exhibits rapid adsorption of both C3H6 and C3H8 under low pressure, with adsorption capacities of 50.90 cm⁻¹ for both at 0.5 bar. 3 g -1 and 43.75 cm 3 g -1 The adsorption selectivity was only 1.16, making effective separation impossible. While confined flexible materials exhibited almost no adsorption of C3H8, their response to C3H6 was kinetically severely hampered; even with limited pore opening at 0.85 bar, the C3H6 adsorption at room temperature remained below 10 cm⁻¹. 3 g -1 It lacks practical application value. In contrast, such as Figure 1 As shown in Figure c, the material of Example 1 of the present invention exhibits clear and highly selective gating behavior: within a low pressure range, C3H6 can trigger pore expansion, with adsorption capacities reaching 0.37, 1.86, 42.62, and 47.18 cm⁻¹ at 0.01, 0.1, 0.5, and 1 bar, respectively. 3 g -1 C3H8 is hardly adsorbed below approximately 0.6 bar (only 1.24 cm at 0.5 bar). 3 g -1 Therefore, a C3H6 / C3H8 adsorption ratio of up to approximately 34.36 was achieved at 0.5 bar. Thanks to the greater rotational freedom brought about by its ligand extension and the precise recognition of vinyl hydrogen by its lone pair electrons, the shortcomings of the two comparative materials mentioned above were successfully overcome.

[0028] Comparison of object-induced gating selectivity mechanisms based on in-situ PXRD During gas adsorption, in-situ pressure-switching powder X-ray diffraction directly revealed significant differences in the guest-induced gating response of the three materials. For the rigid comparative example 1 material, throughout the entire C3H6 ( Figure 2 a) and C3H8 ( Figure 2 b) Within the adsorption pressure range, its structure remains intact, with only minor and completely reversible peak shifts observed, such as the (011) crystal plane shifting from 7.58° to 7.34°, exhibiting elastic expansion rather than a phase transition, and lacking the ability to recognize C3H6 / C3H8 molecules. For the confined flexible material of Comparative Example 2, the diffraction peak positions remain almost unchanged upon exposure to C3H6 or C3H8. Figure 3a) and 3b) indicate that it is in a metastable, non-responsive state, lacking sufficient flexibility to allow for guest-induced structural transformations. In contrast, the material of Example 1 exhibits a highly selective gating mechanism for C3H6. Figure 4 As shown in Figure a, in C3H6 adsorption, diffraction features associated with the transient mesophase are observed in the extremely low pressure range of 0 to 0.1 bar, capturing the initial expansion from the closed-pore phase to the open-pore phase; when the pressure is above 0.1 bar, the PXRD pattern rapidly evolves into that of the almost fully expanded open-pore phase. Conversely, as... Figure 4 As shown in b, C3H8 fails to induce any significant structural changes below approximately 0.6 bar, with multiple new diffraction peaks appearing only at 0.8–1.0 bar, indicating a slow and multi-step structural transformation. This structural transformation, rapidly triggered by C3H6 within an extremely narrow low-pressure window (0–0.1 bar), contrasts sharply with the slow high-pressure response of C3H8 and the unresponsive / rigid single-component gas adsorption behavior of the other two comparative materials, directly revealing the microstructural basis of its excellent gating selectivity.

[0029] Dynamic penetration experiment: The breakthrough was performed in a dynamic gas breakthrough apparatus equipped with a gas mixing system and an online gas chromatograph (GC). A pre-activated sample (0.4877 g by weight) was tightly packed into a stainless steel column (φ = 0.30 cm, L = 20 cm). Prior to the breakthrough measurement, the column was reactivated and then purged with a helium stream until no other gases were detected by the GC. Then, a C3H6 / C3H8 gas mixture (50 / 50, v / v) was introduced at a rate of 0.5 mL / min. -1 The flow rate was introduced into the column. The breakthrough point was determined by GC analysis. Regeneration between cycles was achieved by vacuum aspiration of the column at 100 °C for 60 minutes. The feed gas pressure was maintained at 1 bar. As revealed by the aforementioned static adsorption results, the rigid material in Comparative Example 1 could not achieve effective separation due to its extremely low adsorption selectivity (only 1.16), while the confined flexible material in Comparative Example 2 resulted in extremely low adsorption at room temperature (<10 cm⁻¹) due to kinetic hindrance. 3 g -1 Neither of these two methods has practical application value in separation. In contrast, thanks to its excellent object response gating mechanism, only the material in Example 1 exhibited superior practical separation capability in macroscopic dynamic penetration experiments. Figure 5 As shown, under conditions of 298 K and 1 bar, a C3H6 / C3H8 mixture with an equimolar ratio (50:50, v / v) was used at a flow rate of 0.5 mL / min. -1 Dynamic penetration tests were conducted on the flow rate of C3H8 at 40.2 min g. -1It immediately penetrates and flows out rapidly, while C3H6 continues to flow until 144.2 min g. -1 Penetration occurs only after penetration, creating an extremely wide effective separation window between the two. Furthermore, this flexible framework exhibits extremely high structural toughness and cycling stability during repeated adsorption-desorption operations; after five consecutive penetration cycles, the retention time of C3H6 remains as high as approximately 70 minutes. -1 More importantly, it maintains its excellent separation performance even under humid conditions of 65% and 85% relative humidity, consistent with its ultra-high hydrolytic stability demonstrated in water vapor adsorption tests. This moisture resistance, extremely rare in flexible MOF materials, not only provides extremely high separation efficiency but also allows it to perfectly adapt to industrial mixed gas conditions containing moisture.

Claims

1. A metal-organic framework adsorbent material, said adsorbent material having a columnar layered structure, characterized in that, Its chemical framework comprises a divalent copper ion metal center, and a first organic ligand and a second organic ligand covalently bonded to the metal center; the metal center is a tetranuclear copper cluster [Cu4( μ [3-OH)2N4(COO)6]; The first organic ligand is N,N',N''-tris(carboxymethyl)-1,3,5-benzenetricarboxamide (TCMBT), used to coordinate with the metal center to form a two-dimensional layered structure; the second organic ligand is 1,4-bis(imidazol-1-yl)butane (bib), used to column-connect adjacent two-dimensional layered structures; wherein, the imidazolium group of the second organic ligand contains nitrogen atoms that do not participate in the framework coordination, and the nitrogen atoms have lone pairs of electrons, which can form specific interactions with propylene molecules, thereby inducing a reversible structural transformation of the adsorbent material.

2. The adsorbent material according to claim 1, characterized in that, The adsorbent material, after vacuum activation, is in a closed-pore phase with a porosity of 5-10%, and can transform into an open-pore phase through a gated opening effect when adsorbing propylene molecules, increasing the porosity to 25-30%. Preferably, the gated opening effect is accompanied by an increase in the dihedral angle of the second organic ligand from 80-90° to 120-130°, and an extension in the distance between adjacent copper clusters (Cu···Cu) from 13-14 Å to 14-15 Å. Preferably, under conditions of 298 K and 0.5 bar, the ratio of the amount of propylene adsorbed to the amount of propane adsorbed by the adsorbent material is greater than 30.

3. A method for preparing the adsorbent material according to claim 1 or 2, characterized in that, Includes the following steps: a) In water, the copper source, the first organic ligand, and the second organic ligand are mixed evenly and reacted in a closed reaction vessel at 90-110 °C for 3-24 hours to obtain the target metal-organic framework material; b) The metal-organic framework material is subjected to solvent exchange treatment in an alcohol solvent for 2-4 days; c) The material after solvent exchange treatment is subjected to stepwise thermal vacuum activation. First, it is vacuum treated at 25 °C for 1-3 hours, and then vacuum treated at 100-120 °C for 12-15 hours to obtain activated adsorbent material.

4. The method according to claim 3, characterized in that, In step a), the molar ratio of the copper source, the first organic ligand, and the second organic ligand is 1:(0.8-1.2):(1.5-2.5); the copper source is copper chloride.

5. The method according to claim 3, characterized in that, The aqueous solvent mentioned in step a) is pure deionized water, or a mixture of deionized water and methanol; the reaction time is 3-24 hours.

6. The method according to claim 3, characterized in that, The alcohol solvent mentioned in step b) is anhydrous methanol, and the solvent is replaced with fresh solvent every 6-10 hours during the solvent exchange process; the vacuum condition mentioned in step c) is dynamic high vacuum condition.

7. The application of the adsorbent material according to claim 1 or 2 in gas separation, characterized in that, The gas separation is a selective adsorption of propylene from a mixed gas containing propylene and propane; wherein, the selective adsorption is achieved by forming multiple CH···N hydrogen bonds between the uncoordinated nitrogen atom on the second organic ligand and the acidic vinyl hydrogen of the propylene molecule, triggering the gating opening effect of the adsorption material through synergistic π···π interactions.

8. The application according to claim 7, characterized in that, The selective adsorption is carried out at a temperature of 273-298 K; preferably, the selective adsorption is carried out at a pressure of 0.0001-1.0 bar, wherein the adsorbent material is induced by propylene to undergo a gate opening effect in a low pressure range of 0.1 bar.

9. The application according to claim 7, characterized in that, The mixed gas also contains water vapor, and its relative humidity is preferably 65-85%.

10. The application according to claim 7, characterized in that, The volume ratio or molar ratio of propylene to propane in the mixed gas is 1:

1. The mixed gas is passed into an adsorption bed containing the activated adsorption material for dynamic breakthrough separation, thereby achieving complete dynamic separation of propylene and propane.

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