Preparation and application of cobalt / zinc bimetallic organic framework membrane with priority alcohol permeation

Abstract

The application relates to the preparation and application of a cobalt / zinc bimetallic organic framework film with alcohol priority, and belongs to the field of membrane technology. That is, hydroxyl cobalt salt and hydroxyl zinc salt are introduced on the surface of a porous carrier as heterogeneous nucleation sites of a metal organic framework (MOF) film, and then the carrier is placed in an organic ligand precursor solution containing a bimetal source to prepare a cobalt / zinc bimetallic MOF film with good continuity. The competition between cobalt and zinc metals further expands the pore size of the MOFs, and makes the crystal preferentially grow along the (211) crystal face. The expanded pore size is helpful for the transmission of ethanol molecules, and the preferential growth of the (211) crystal face further plays the hydrophobic effect of 2-methyl imidazole ligands in the MOFs to prevent water molecules from entering the pore. The ethanol separation factor is respectively increased by 127.3% and 163.2%. Finally, a solution containing a silicon-containing polymer and a curing agent is gradually coated on the bimetallic MOF layer to enhance the performance and stability of the pervaporation.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and specifically to the preparation and application of a cobalt / zinc bimetallic organic framework membrane that preferentially permeates alcohol. Background Technology

[0002] Bioethanol is an important renewable energy source, obtained from biomass (such as corn, sugarcane, and cellulose) through fermentation. However, during production, solvents and byproducts can hinder microbial metabolism, leading to increased separation costs. Membrane separation technology, with its advantages of low energy consumption, ease of operation, and non-cytotoxicity, is well-suited for the in-situ separation of bioethanol. However, traditional membrane materials have performance limitations, unable to simultaneously improve flux and selectivity. Therefore, the development and optimization of novel materials are necessary to enhance performance.

[0003] Metal-organic frameworks (MOFs), as porous crystalline materials composed of metal ions or clusters and organic ligands, possess properties such as uniform pore size, high specific surface area, and adsorption affinity, making them increasingly widely used in membrane separation. (The text then abruptly shifts to a seemingly unrelated topic: "separately composed of Co...") 2+ or Zn 2+ ZIF-67 and ZIF-8, formed by coordination with hydrophobic 2-methylimidazole (2-MIM), are two common MOF materials that have shown great potential in the pervaporation separation of alcohols preferentially. However, researchers often fail to fully consider the influence of membrane orientation and pore size on the preferential permeability of alcohols during MOF membrane design. Since the kinetic diameter of ethanol molecules (0.44 nm) is larger than that of water molecules (0.28 nm), while the theoretical pore size of ZIF-67 and ZIF-8 (0.34 nm) is smaller, simply increasing the pore size of MOFs may lead to the simultaneous passage of water and ethanol molecules through the membrane. This not only reduces the selectivity of the separation but also weakens the overall separation effect of the membrane. Therefore, while adjusting the pore size, the orientation structure of the MOF membrane must also be carefully designed. By fully utilizing the hydrophobic properties of the MOF surface to block the permeation of some water molecules, the preferential permeability of ethanol can be effectively improved, which is crucial for enhancing the membrane's separation performance.

[0004] Bimetallic organic framework (MOF) membranes are prepared by introducing two different metal nodes into the MOF structure. This method can produce a synergistic effect, enhancing the performance of MOF membranes, such as adsorption selectivity, film formation, and structural stability. In the preparation of bimetallic MOF membranes, the competitive coordination between different metals can further regulate the pore size and channel structure, control the direction of crystal growth, and thus achieve the separation of specific molecules. Summary of the Invention

[0005] The purpose of this invention is to provide the preparation and application of a cobalt / zinc bimetallic organic framework membrane that preferentially pervades alcohol. First, cobalt hydroxyl and zinc hydroxyl salts are constructed on a porous support as heterogeneous nucleation sites. Then, the support with the introduced hydroxyl salt layer is placed in a solution of an organic ligand precursor containing a bimetallic source to prepare a cobalt / zinc bimetallic MOF membrane with good intergrowth properties, which is then used for pervaporation of alcohol. The cobalt / zinc bimetallic elements compete on the support surface; this competition not only promotes the formation of defects in the MOF to increase pore size but also guides crystal growth along a specific orientation, thereby improving ethanol separation efficiency.

[0006] The specific technical solution is as follows:

[0007] A method for preparing a cobalt / zinc bimetallic organic framework membrane with preferential alcohol permeability includes the following steps:

[0008] Step 1: Clean the porous carrier to remove organic matter and contaminants from its surface;

[0009] Step 2: Coating the surface of the porous support with a mixed solution of cobalt salt and zinc salt and heating it to convert it into a porous support modified with bimetallic hydroxyl salt.

[0010] Step 3: Place the support with the bimetallic salt layers modified with cobalt hydroxy and zinc hydroxy in Step 2 into a cobalt / zinc bimetallic organic framework membrane precursor solution, and react at a certain temperature for a period of time to obtain a cobalt / zinc bimetallic organic framework membrane.

[0011] The process further includes step 4, preparing a silicon-containing polymer and its curing agent solution, alternately spin-coating the two onto the surface of the cobalt / zinc bimetallic organic framework membrane described in step 3 and heating to crosslink, thereby obtaining a pervaporation separation membrane.

[0012] Furthermore, the porous support is one of the following sheet membranes: alumina (AAO), polysulfone (PSF), polystyrene (PS), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), etc., and the average pore size is 200 nm to 2 μm.

[0013] Furthermore, in step 2, the mixed solution of cobalt salt and zinc salt that forms the bimetallic hydroxyl salt modified layer is an ethanol solution of cobalt salt and zinc salt, wherein the cobalt salt and zinc salt are cobalt nitrate hexahydrate and zinc nitrate hexahydrate, respectively.

[0014] Furthermore, in step 2, the concentration of the metal salt is 0.1 mol / L to 10 mol / L, and the mass ratio of cobalt nitrate hexahydrate to zinc nitrate hexahydrate is 1:1.

[0015] Further, in step 2, the coating method is one of spin coating, blade coating, or slot coating. The spin coating rotation speed is 1000–5000 rpm, and the spin coating time is 20–100 s. The blade coating and slot coating coating rates are 1–10 mm·s.-1 The coating time is 1 to 10 seconds.

[0016] Furthermore, in step 2, the heating temperature is 80–150°C, and the heating time is 1–6 hours.

[0017] Further, in step 3, the cobalt / zinc bimetallic organic framework membrane precursor solution is a mixed solution obtained by mixing bimetallic salt solution A and ligand solution B and allowing them to stand. Bimetallic salt solution A is a solution prepared by mixing cobalt nitrate hexahydrate and zinc nitrate hexahydrate in methanol and water at a volume ratio of 1:4. Ligand solution B is a solution prepared by mixing 2-methylimidazole in methanol and water at a volume ratio of 1:4.

[0018] Further, in step 3, the concentration of the bimetallic salt solution A is 0.001–0.2 mol / L, and the mass ratio of cobalt nitrate hexahydrate to zinc nitrate hexahydrate is (0.11–9):1, preferably (0.25–1):1. The concentration of the ligand solution B is 0.1 mol / L–2 mol / L.

[0019] Furthermore, in step 3, the reaction time is 30s to 24h, and the reaction temperature is 25 to 65℃.

[0020] Further, in step 4, the silicon-containing polymer is preferably one of hydroxyl-terminated polydimethylsiloxane, vinyl-terminated polydimethylsiloxane, amino-terminated polydimethylsiloxane, and polytrimethylsilylpropyne (PTMSP).

[0021] Further, in step 4, the curing agent includes a crosslinking agent and a catalyst. The crosslinking agent is preferably one of tetraethyl orthosilicate, phenyltrimethoxysilane, octyltrimethoxysilane, and polymethylhydrosiloxane. The catalyst is one of dibutyltin dilaurate, dioctyltin dilaurate, a platinum-containing catalyst, and a nickel-containing catalyst. The solvent is preferably one or more of n-heptane, toluene, dichloromethane, and n-hexane.

[0022] Furthermore, in step 4, the mass concentration of the silicon-containing polymer is 1 wt.% to 30 wt.%, the mass concentration of the crosslinking agent is 10 wt.% to 30 wt.%, and the mass concentration of the catalyst is 0.0005 wt.% to 0.01 wt.%.

[0023] Furthermore, in step 4, the spin coating speed is 1000–5000 rpm, and the spin coating time is 20–100 s. The heating temperature is 80–150℃, and the heating time is 20 min–4 h.

[0024] The beneficial effects of the technical solution provided by the embodiments of the present invention are as follows:

[0025] A cobalt / zinc bimetallic organic framework (MOF) membrane was successfully prepared on a porous support by induction with cobalt hydroxyl and zinc hydroxyl bimetallic hydroxyl salts. The competitive coordination between cobalt and zinc metals further expanded the pore size of the MOFs and caused preferential crystal growth along the (211) crystal plane. This increased pore size allowed for the rapid passage of ethanol molecules with larger kinetic diameters. The preferential growth of the (211) crystal plane also allowed for greater utilization of the hydrophobic effect of the 2-methylimidazolium ligand in the MOFs, preventing water molecules from directly entering the pores and thus achieving more efficient recovery of bioethanol from water. Finally, the surface of the membrane was modified with a cross-linked silicon-containing polymer, enhancing its pervaporation performance and liquid stability. Attached image description:

[0026] To further illustrate the technical embodiments of the present invention, a detailed description will be given below with reference to the accompanying drawings.

[0027] Figure 1 These are SEM images of the surface and cross-section of the cobalt / zinc bimetallic organic framework membrane prepared in Example 3 of this invention;

[0028] Figure 2 This is a pore size distribution diagram of MOF particles taken from the surface of the metal-organic framework membranes prepared in Example 3 and Comparative Example 2 of this invention.

[0029] Figure 3 These are XRD pole figures of the (211) and (110) crystal planes of the metal-organic framework film prepared in Example 3 of this invention. Detailed implementation method:

[0030] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0031] Example 1

[0032] Step 1: Select polyvinylidene fluoride membrane as porous carrier with a molecular weight cutoff of 50,000 Da. Before use, use an ethanol / water mixture to ultrasonically clean the porous carrier to remove organic matter and contaminants from the surface.

[0033] Step 2: Prepare a mixed solution by dissolving cobalt nitrate hexahydrate and zinc nitrate hexahydrate in ethanol at a mass ratio of 1:1, with a metal ion concentration of 0.4 mol / L. Apply the prepared mixed solution to the surface of a polyvinylidene fluoride membrane using spin coating at 3000 rpm for 60 seconds. Then, heat the membrane in an oven at 120°C for 3 hours to convert it into a porous carrier modified with a bimetallic hydroxyl salt.

[0034] Step 3: Dissolve 0.108 g of cobalt nitrate hexahydrate and 0.432 g of zinc nitrate hexahydrate in 125 mL of a mixed solvent of methanol and water (volume ratio 1:4) to obtain bimetallic salt solution A. Dissolve 11.35 g of 2-methylimidazole in 125 mL of a mixed solvent of methanol and water (volume ratio 1:4) to obtain ligand solution B. Mix metal salt solution A and ligand solution B, let stand for 10 min to obtain a mixed solution, and immerse the porous support modified with the bimetallic hydroxyl salt in step 2 in it to grow and prepare a cobalt / zinc bimetallic organic framework membrane.

[0035] Step 4: The silicon-containing polymer used is vinyl polydimethylsiloxane, the crosslinking agent is polydimethylhydrosiloxane, and the catalyst is a platinum-containing catalyst. First, prepare a curing agent solution containing 10 wt.% vinyl polydimethylsiloxane, 25 wt.% polydimethylhydrosiloxane, and 0.01 wt.% platinum-containing catalyst, using dichloromethane as the solvent. Alternately spin-coat the two solutions onto the surface of the cobalt / zinc bimetallic organic framework membrane prepared in Step 3 and heat in an 80℃ oven for 20 min for crosslinking to obtain a pervaporation separation membrane. The spin-coating speed is 3000 rpm, and the spin-coating time is 30 s.

[0036] Example 2

[0037] The preparation method is the same as in Example 1, except that in step 3, 0.216 g of cobalt nitrate hexahydrate and 0.324 g of zinc nitrate hexahydrate are dissolved in 125 mL of a mixed solvent of methanol and water (volume ratio 1:4) to obtain bimetallic salt solution A.

[0038] Example 3

[0039] The preparation method is the same as in Example 1, except that in step 3, 0.27g of cobalt nitrate hexahydrate and 0.27g of zinc nitrate hexahydrate are dissolved in 125mL of a mixed solvent of methanol and water (volume ratio 1:4) to obtain bimetallic salt solution A.

[0040] Example 4

[0041] The preparation method is the same as in Example 1, except that step 4 is omitted.

[0042] Example 5

[0043] The preparation method is the same as in Example 2, except that step 4 is omitted.

[0044] Example 6

[0045] The preparation method is the same as in Example 3, except that step 4 is omitted.

[0046] Comparative Example 1

[0047] A cobalt single-metal organic framework (ZIF-67) film was prepared, and the specific preparation method is as follows:

[0048] Step 1: Select polyvinylidene fluoride membrane as porous carrier with a molecular weight cutoff of 50,000 Da. Before use, use an ethanol / water mixture to ultrasonically clean the porous carrier to remove organic matter and contaminants from the surface.

[0049] Step 2: Dissolve cobalt nitrate hexahydrate in ethanol to prepare a solution with a metal ion concentration of 0.4 mol / L. Apply the prepared solution to the surface of a polyvinylidene fluoride membrane using spin coating at 3000 rpm for 60 seconds. Heat the membrane in an oven at 120°C for 3 hours to convert it into a porous support modified with cobalt hydroxyl salt.

[0050] Step 3: Dissolve 0.54 g of cobalt nitrate hexahydrate in 125 mL of a mixed solvent of methanol and water (volume ratio 1:4) to obtain metal salt solution A. Dissolve 11.35 g of 2-methylimidazole in 125 mL of a mixed solvent of methanol and water (volume ratio 1:4) to obtain ligand solution B. Mix metal salt solution A and ligand solution B, let stand for 10 min to obtain a mixed solution, and immerse the porous support modified with cobalt hydroxyl salt from step 2 in it to grow and prepare ZIF-67 metal-organic framework membrane.

[0051] Step 4: The silicon-containing polymer used is vinyl polydimethylsiloxane, the crosslinking agent is polydimethylhydrosiloxane, and the catalyst is a platinum-containing catalyst. First, prepare a curing agent solution containing 10 wt.% vinyl polydimethylsiloxane, 25 wt.% polydimethylhydrosiloxane, and 0.01 wt.% platinum-containing catalyst, using dichloromethane as the solvent. Alternately spin-coat the two solutions onto the surface of the ZIF-67 metal-organic framework membrane prepared in Step 3 and heat in an 80℃ oven for 20 min for crosslinking to obtain a pervaporation separation membrane. The spin-coating speed is 3000 rpm, and the spin-coating time is 30 s.

[0052] Comparative Example 2

[0053] A zinc single-metal organic framework (ZIF-8) membrane was prepared, and the specific preparation method is as follows:

[0054] The preparation method is the same as that of Comparative Example 1, except that cobalt nitrate hexahydrate is replaced with zinc nitrate hexahydrate in steps 2 and 3.

[0055] Comparative Example 3

[0056] The preparation method is the same as that of Comparative Example 1, except that step 4 is omitted.

[0057] Comparative Example 4

[0058] The preparation method is the same as that of Comparative Example 2, except that step 4 is omitted.

[0059] The pervaporation performance of the metal-organic framework membranes of Examples 1-10 and Comparative Examples 1-4 was tested under the following conditions: a 5 wt.% ethanol / water system with a feed temperature of 60°C and downstream pressure maintained below 300 Pa. The pervaporation performance of each membrane was measured three times under the same manufacturing conditions, and the average value of the three tests was used as the data point.

[0060] The performance test results are shown in the table below.

[0061]

[0062] As shown in the table, compared with the performance of the ZIF-67 or ZIF-8 single metal organic framework membranes prepared in Comparative Examples 1 and 2, the cobalt / zinc bimetallic organic framework membrane prepared in Example 3 showed an increase in permeation flux of 48.7% and 18.4%, respectively, and an increase in separation factor of 24.7% and 54.2%, respectively. Compared with the performance of the membranes prepared in Comparative Examples 3 and 4, the membrane prepared in Example 6 showed an increase in separation factor of 127.3% and 163.2%, respectively, which significantly improved the pervaporation and alcohol permeation performance of the metal organic framework membrane.

Claims

1. A method for preparing a cobalt / zinc bimetallic organic framework membrane with preferential alcohol permeability, characterized in that, Includes the following steps: Step 1: Clean the porous carrier to remove organic matter and contaminants from its surface; Step 2: Coating the surface of a porous support with a mixed solution of cobalt salt and zinc salt and heating it to transform it into a porous support modified with a bimetallic hydroxyl salt; wherein the mixed solution of cobalt salt and zinc salt forming the bimetallic hydroxyl salt modification layer is an ethanol solution of cobalt salt and zinc salt, and the cobalt salt and zinc salt are cobalt nitrate hexahydrate and zinc nitrate hexahydrate, respectively; in Step 2, the concentration of metal salt is 0.1 mol / L~10 mol / L, and the mass ratio of cobalt nitrate hexahydrate to zinc nitrate hexahydrate is 1:1; Step 3: Place the support modified with cobalt hydroxy and zinc hydroxy bimetallic salt layers in Step 2 into a cobalt / zinc bimetallic organic framework membrane precursor solution and react at a certain temperature for a period of time to obtain a cobalt / zinc bimetallic organic framework membrane; wherein the cobalt / zinc bimetallic organic framework membrane precursor solution is a mixed solution obtained by mixing bimetallic salt solution A and ligand solution B and allowing it to stand; bimetallic salt solution A is a solution of cobalt nitrate hexahydrate and zinc nitrate hexahydrate mixed in methanol and water at a volume ratio of 1:4; ligand solution B is a solution of 2-methylimidazole mixed in methanol and water at a volume ratio of 1:4; the concentration of bimetallic salt solution A is 0.001~0.2 mol / L, the mass ratio of cobalt nitrate hexahydrate to zinc nitrate hexahydrate is (0.11~9):1, and the concentration of ligand solution B is 0.1mol / L~2 mol / L; Step 4: Prepare a silicon-containing polymer and its curing agent solution, and alternately spin-coat the cobalt / zinc bimetallic organic framework membrane surface described in Step 3 and heat it for crosslinking to obtain a pervaporation separation membrane.

2. The method according to claim 1, characterized in that, The porous support is one of alumina, polysulfone, polystyrene, polyacrylonitrile, or polyvinylidene fluoride sheet membrane, and the average pore size is 200 nm to 2 μm.

3. The method according to claim 1, characterized in that, In step 2, the coating method is one of spin coating, blade coating, or slot coating; the spin coating speed is 1000~5000 rpm, and the spin coating time is 20~100s; the blade coating and slot coating speeds are 1~10mm·s. -1 The coating time is 1~10s; In step 2, the heating temperature is 80~150°C and the heating time is 1~6 hours.

4. The method according to claim 1, characterized in that, The reaction time is 30s to 24h, and the reaction temperature is 25 to 65°C.

5. The method according to claim 1, characterized in that, In step 3, the mass ratio of cobalt nitrate hexahydrate to zinc nitrate hexahydrate is (0.25~1):

1.

6. The method according to claim 2, characterized in that, In step 4, the silicon-containing polymer is one of hydroxyl-terminated polydimethylsiloxane, vinyl-terminated polydimethylsiloxane, amino-terminated polydimethylsiloxane, and polytrimethylsilylpropyne. In step 4, the curing agent includes a crosslinking agent and a catalyst. The crosslinking agent is one of tetraethyl orthosilicate, phenyltrimethoxysilane, octyltrimethoxysilane, and polymethylhydrosiloxane. The catalyst is one of dibutyltin dilaurate, dioctyltin dilaurate, a platinum-containing catalyst, and a nickel-containing catalyst. The solvent is one or more of n-heptane, toluene, dichloromethane, and n-hexane. In step 4, the mass concentration of the silicon-containing polymer is 1 wt.%~30 wt.%, the mass concentration of the crosslinking agent is 10 wt.%~30 wt.%, and the mass concentration of the catalyst is 0.0005 wt.%~0.01 wt.%. In step 4, the spin coating speed is 1000~5000 rpm, the spin coating time is 20~100s, the heating temperature is 80~150°C, and the heating time is 20min~4h.

7. A cobalt / zinc bimetallic organic framework membrane with preferential alcohol permeability prepared according to any one of claims 1-6.

8. The application of the cobalt / zinc bimetallic organic framework membrane with preferential alcohol permeability prepared by the method according to any one of claims 1-6, as a pervaporation membrane for alcohol-water separation.

9. The application according to claim 8, wherein the alcohol-water is ethanol / water.

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