A mixed-ligand-containing moF-801 separation membrane, a preparation method and an alcohol-water separation use

Abstract

The application relates to a mixed-ligand MOF-801 separation membrane, a preparation method and an alcohol-water separation purpose, and belongs to the technical field of membrane separation materials. The application is related to the design and engineering of a MOF-801 membrane for rapidly and selectively permeating water from an alcohol solution through a mixed-ligand strategy. A second ligand 2-methyl fumaric acid (mes) is introduced into the original mother liquor formed by the MOF membrane, and the pore microenvironment is adjusted in situ through a conventional hydrothermal method. The obtained MOF-801 membrane with a required pore diameter shows highly selective water permeation capacity.

Description

Technical Field

[0001] This invention relates to a MOF-801 separation membrane containing mixed ligands, its preparation method, and its application in alcohol-water separation, belonging to the field of membrane separation material technology. Background Technology

[0002] Metal-organic frameworks (MOFs), with their rich functionality, tunable pore systems, and large surface areas, have become promising building blocks for constructing membranes with high separation performance. Although MOF membranes have achieved great success in molecular separation, related research has mainly focused on gas separation. However, the potential of MOF membranes in liquid separation, particularly alcohol dehydration, has not been fully explored, primarily due to insufficient water stability; most MOF materials experience structural collapse in the presence of water. Developing high-valence MOFs (e.g., Ti(IV), Zr(IV), and Al(III)-based MOFs) can effectively address these key issues. They are typically constructed from metal-oxygen cluster nodes (secondary building blocks [SBUs]) and carboxylate ligands through hard acid-hard base interactions, forming a stronger framework. High-valence MOFs often exhibit high connectivity; a typical example is Zr-MOF, which consists of Zr6O4(OH)4(-CO2). 12 The coordination between SBU and 12 ligands results in a face-centered cubic (fcu) topology with a theoretical connectivity of 12. This high connectivity provides spatial shielding and endows the MOF with excellent stability, allowing it to withstand attacks from water molecules. As a typical high-valence Zr-MOF material, MOF-801 possesses excellent water adsorption capacity and a tunable sub-nanopore structure, making it a potential new candidate for constructing MOF-based membranes for liquid separation (such as alcohol / water separation). MOF-801 membranes can separate smaller substances (e.g., ethanol) from water. The potential of this technology has not been fully explored, possibly due to insufficient pore microenvironment (e.g., size).

[0003] It is worth noting that precise control of the pore microenvironment can significantly improve the molecular sieving capacity of membranes. Many strategies for regulating the pore microenvironment have been reported, such as post-synthetic modification and heterostructure construction, but these typically require two steps to achieve the final goal. Summary of the Invention

[0004] This invention relates to the design and engineering of MOF-801 membranes for rapid and selective water permeation from alcohol solutions using a mixed-ligand strategy. A second ligand, 2-methylfumaric acid (mes), is introduced into the initial mother liquor for MOF membrane formation, and the pore microenvironment is situ-regulated using a conventional hydrothermal method. The resulting MOF-801 membrane with the desired pore size exhibits highly selective water permeation. The effects of the second ligand on the physicochemical properties of the MOF membrane are systematically investigated. Water permeation characteristics are observed by separating water / alcohol mixtures through a pervaporation process by controlling the second ligand content, alcohol molecular size, and operating conditions (e.g., feed concentration, temperature, and operating time).

[0005] A MOF-801 separation membrane containing mixed ligands includes a support layer and a selective separation layer, wherein the selective separation layer is composed of MOF crystalline material, wherein the MOF crystalline material is coordinated with Zr as the metal center and fumaric acid and 2-methylfumaric acid as mixed ligands.

[0006] The molar percentage of 2-methylfumaric acid in the total ligands ranges from 10% to 100%, preferably 20% to 40%.

[0007] The preparation method of the above-mentioned MOF-801 separation membrane containing mixed ligands includes the following steps:

[0008] Step 1: Dissolve fumaric acid, 2-methylfumaric acid and zirconium source in a solution;

[0009] Step 2: The solution is brought into contact with the support and a hydrothermal reaction is carried out. After the reaction is completed, the solution is washed with water and dried to obtain the separation membrane.

[0010] The zirconium source is ZrOCl2.

[0011] The solution is obtained by mixing DMF and formic acid in a volume ratio of 20:1-10.

[0012] The hydrothermal reaction conditions are 70-90℃ for 1-30 hours.

[0013] The molar ratio between the zirconium source and the total ligands ranges from 1:0.8 to 1.2.

[0014] The above-prepared MOF-801 separation membrane containing mixed ligands is used in the separation of alcohol-water mixtures.

[0015] The alcohol refers to methanol, ethanol, propanol, or butanol; in the alcohol-water mixture, the mass percentage of water ranges from 10% to 95%.

[0016] Beneficial effects

[0017] This invention constructs a tunable MOF-801 membrane for alcohol dehydration using a mixed ligand strategy. A separation membrane is formed via a conventional solvothermal method by introducing a second ligand, 2-methylfumaric acid (mes), into the original mother liquor. The inserted 2-methylfumaric acid ligand reduces the pore size of the original MOF-801 membrane, significantly improving its size screening capability. Results show that the synthesized MOF-801-30% CH3 membrane exhibits excellent separation performance at a mes loading of 30%, with an ultra-high total flux of 5226 g. -1 ·m -2 ·h -1 The separation factor is 1281. Attached Figure Description

[0018] Figure 1 Schematic diagram of MOF-801 membrane with mixed ligand modulated for rapid selective permeation of water from alcohol-water mixtures.

[0019] Figure 2 SEM surface images of (a) MOF-801, (b) MOF-801-10% CH3, (c) MOF-802-30% CH3, (d) MOF-803-50% CH3, (e) MOF-804-70% CH3, and (f) MOF-701-100% CH3 films. SEM images of cross-sections corresponding to (a1)-(f1).

[0020] Figure 3 : 1H NMR spectrum of MOF-801-CH3(X) membrane.

[0021] Figure 4 N2 adsorption (a), BET surface area and pore size (b), water and ethanol vapor adsorption (c), and water contact angle (d) of MOF-801-CH3(X) membrane.

[0022] Figure 5 (a) Effect of Mes ligand percentage on the pervaporation performance of MOF-801 membrane. (b) Dehydration of MOF-801-30% CH3 membrane in different alcohol / water systems. Water content was 90 wt%, and feed temperature was 40 °C. Detailed Implementation

[0023] Metal-organic framework (MOF) membranes, with their rich functionality and tunable pore size systems, hold promise for molecular separation. However, constructing continuous MOF membranes for liquid separation, especially for water-related mixtures, remains a significant challenge due to insufficient water stability and inadequate pore microenvironment. This invention employs a mixed-ligand strategy to construct a high-valence Zr-MOF membrane with high stability and tunable pore size for alcohol dehydration. 2-Methylfumaric acid (mes) is introduced as a second ligand into the original bulk solution during MOF-801 membrane formation using a conventional solvothermal method, resulting in narrower pore size and enhanced molecular sieving ability. The physicochemical properties of the synthesized MOF-801 membrane were observed using SEM, NMR, XRD, BET, organic vapor adsorption, and water contact angle analysis. The resulting MOF-801-30% CH3 membrane with a 30% mes loading exhibited excellent butanol / water separation performance, with a total flux of 5226 g. -1 ·m -2 ·h -1 The separation factor was 1281, exceeding the upper limit of existing technologies, demonstrating the great potential of solvent dehydration.

[0024] Example 1

[0025] MOF-801 membranes were prepared using a conventional hydrothermal method. The specific steps were as follows: a mixture of fumaric acid (0-50 mmol, 0-5.80 g), 2-methylfumaric acid (0-50 mmol, 0-6.51 g), and ZrOCl2·8H2O (50 mmol, 16.11 g) was dissolved in a mixed solution of DMF / formic acid (200 mL / 70 mL). (For example, the ligand side of the MOF-801-10% CH3 membrane was prepared from 45 mmol, 5.22315 g of fumaric acid and 5 mmol, 0.6505 g of 2-methylfumaric acid.) After stirring for 20 minutes, the prepared α-Al2O3 substrate was placed together with the above solution into a stainless steel autoclave lined with polytetrafluoroethylene. The autoclave was then transferred to an oven and heated at 80 °C for 12 hours. Finally, the resulting membrane was rinsed several times with DMF solution and then immersed in methanol solution for 4 days. Then, the membrane was vacuum dried at 150°C for 12 hours.

[0026] Membrane permeability testing methods:

[0027] The permeation performance of the prepared MOF-801 membrane from alcohol solution was evaluated using a pervaporation apparatus. The feed solution was applied at a flow rate of 100 ml / min. -1 A peristaltic pump circulates between the feed tank and the membrane module. A vacuum pump is used to control the pressure on the permeate side to <200 Pa. Permeate vapor from the permeate side is collected in a cold trap placed in liquid nitrogen. The concentrations of feed and permeate are measured by gas chromatography. The alcohol dehydration performance is evaluated by total flux (J) and separation factor (β).

[0028] MOF-801 is composed of the Zr metal cluster Zr6O4(OH)4(-CO2). 12 It is composed of fumaric acid (fum). First, a pure MOF-801 membrane was prepared using a traditional solvothermal method. Due to its excellent chemical and thermal stability, ceramic alumina was chosen as the porous substrate to support the MOF membrane layer. Figure 2 A well-preserved symbiotic MOF-801 film without visible defects or pinholes was obtained. Cross-sectional photographs show a continuous MOF layer with a thickness of approximately 2 μm firmly anchored to the ceramic support surface. Figure 2 (a1). Different molar percentages (10%-100%) of the second ligand mes (also known as 2-methylfumaric acid) were targeted to the total ligand to prepare corresponding MOF membranes, labeled MOF-801-CH3(X), where X represents the molar percentage of 2-methylfumaric acid. This indicates that the introduction of 2-methylfumaric acid has no effect on the membrane morphology.

[0029] conduct 1 H NMR characterization was used to determine the actual percentage of Mes ligands. The figure shows that, compared to the pure MOF-801 membrane, the MOF-801-CH3(X) membrane exhibits [a certain degree of degradation / conversion]. 1 In the 1H NMR spectrum, the characteristic peak attributed to Mes appears at ~6.35 ppm. Its intensity increases with increasing Mes content, indicating the successful introduction of Mes. According to... 1H The NMR results were used to calculate the percentage of mes. The results of 12%, 29%, 54%, and 72% were consistent with the target values. SEM and 1 1H NMR results indicate that the introduced mes affects the pore microenvironment (e.g., pore size) that exhibits different permeability behaviors during alcohol dehydration.

[0030] The effect of introduced Mes ligands on the pore size of MOF-801 membranes was investigated using nitrogen adsorption. Based on the measured nitrogen adsorption / desorption isotherms, the corresponding BET surface area and pore size distribution were calculated. Figure 4 (a) and (b). As the percentage of mes increases, the amount of nitrogen adsorbed and the BET surface area gradually decrease, because the methyl group of the mes ligand occupies space in MOF-801, resulting in a reduction of adsorption sites. This is further confirmed by the reduction of the pore size in the sub-nanometer scale from 0.64 nm to 0.48 nm, which may lead to enhanced size sieving ability, exhibiting reduced flux and enhanced alcohol / water separation selectivity. The adsorption performance of MOF-801-CH3(X) membrane for ethanol and water molecules was investigated using organic vapor adsorption analysis, such as... Figure 4c) It was found that the adsorption capacity of the MOF membrane for water and ethanol vapor decreased with increasing MES ligand percentage, which is consistent with the trend of BET surface area and pore size changes (Figure 4a and b). The MOF-801 membrane with MES also exhibited >400 cm⁻¹ 3 (STP)·g -1 The extremely high water vapor adsorption capacity is far greater than that of ethanol vapor. This may lead to preferential water absorption behavior, facilitating rapid and selective water transport and achieving high water permeability in alcohol / water mixtures. The water contact angle was also used to evaluate the surface hydrophilicity of MOF-801 membranes with different mes percentages. Figure 4 (d). Due to the introduction of hydrophobic methyl groups, the contact angle of the membrane gradually increases with the increase of the mes percentage, indicating enhanced hydrophobicity. Since the hydrophilicity of the membrane surface mainly determines the water absorption behavior that facilitates rapid selective permeation, optimizing the mes percentage that imparts suitable hydrophilicity and pore size to the MOF-801 membrane is crucial for obtaining excellent alcohol dehydration performance.

[0031] Water permeability test

[0032] The water permeability of the MOF-801 membrane was first evaluated through pervaporation separation of an ethanol / water mixture, a key component in biofuel production. The effect of the percentage of MES introduced on the ethanol dehydration performance was observed. Figure 5 As shown, the flux decreases with increasing mes loading, which is due to the increased transport resistance caused by the narrowing of the associated pore size. With the mes loading increasing to 30%, the corresponding MOF membrane exhibits a flux of 4077 g·m⁻¹. -2 ·h -1 The ideal flux and separation factor of 163 were observed. This separation factor is approximately four times that of the pure MOF-801 membrane. This can be attributed to the narrowed pore size of the MOF-801-30% CH3 membrane, which facilitates rapid and selective water permeation. Further increasing the MES loading resulted in a decrease in the separation factor, likely due to the enhanced hydrophobicity of the membrane from the introduction of MES, allowing more ethanol molecules to pass through the MOF membrane, as confirmed by the increased water contact angle. The optimal MES loading for the MOF-801 membrane during ethanol dehydration is 30%.

[0033] The permeability of the MOF-801-30% CH3 membrane to different alcohol dehydration systems, including methanol, ethanol, isopropanol, and butanol, was further evaluated, with molecular dynamic diameters of 3.8, 4.3, 4.7, and 4.8, respectively. For these four separations, the MOF-801-30% CH3 membrane showed similar fluxes, while the separation factor was positively correlated with the kinetic diameter of the alcohol molecule. For the smallest kinetic diameter... For dehydrated methanol, the membrane exhibits a lower separation factor of 113, slightly lower than that for dehydrated ethanol, which has a larger kinetic diameter. As the kinetic diameter of the alcohol increases... The separation factor was significantly increased to ~1210, exhibiting excellent dehydration performance for butanol / water mixtures. This improvement in separation performance is mainly attributed to the presence of... Compared to water, which has a much smaller kinetic diameter, butanol encounters greater resistance when transported through the narrow aperture created by the introduction of the Mes ligand.

Claims

1. A MOF-801 separation membrane containing mixed ligands, characterized in that, It includes a support layer and a selective separation layer, wherein the selective separation layer is composed of MOF crystal material, wherein the MOF crystal material is coordinated with Zr as the metal center and fumaric acid and 2-methylfumaric acid as mixed ligands.

2. The MOF-801 separation membrane containing mixed ligands according to claim 1, characterized in that, The molar percentage of 2-methylfumaric acid in the total ligands ranges from 20% to 40%.

3. The method for preparing the MOF-801 separation membrane containing mixed ligands as described in claim 1, characterized in that, Includes the following steps: Step 1: Dissolve fumaric acid, 2-methylfumaric acid and zirconium source in a solution; Step 2: The solution is brought into contact with the support and a hydrothermal reaction is carried out. After the reaction is completed, the solution is washed with water and dried to obtain the separation membrane.

4. The method for preparing the MOF-801 separation membrane containing mixed ligands according to claim 3, characterized in that, The zirconium source is ZrOCl2; the molar ratio between the zirconium source and the total ligands is in the range of 1:0.8-1.

2.

5. The method for preparing the MOF-801 separation membrane containing mixed ligands according to claim 3, characterized in that, The solution is obtained by mixing DMF and formic acid in a volume ratio of 20:1-10.

6. The method for preparing the MOF-801 separation membrane containing mixed ligands according to claim 3, characterized in that, The hydrothermal reaction conditions are 70-90℃ for 1-30 hours.

7. Use of the MOF-801 separation membrane containing mixed ligands as described in claim 1 in the separation of alcohol-water mixtures.

8. The use according to claim 7, characterized in that, The alcohol mentioned refers to methanol, ethanol, propanol, or butanol.

9. The use according to claim 8, characterized in that, In alcohol-water mixtures, the mass percentage of water ranges from 10% to 95%.

10. The use according to claim 9, characterized in that, The temperature of the mixture is 10-60℃.

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