A mixed matrix membrane of mxene nanoplatelets / polyvinyl alcohol, preparation method and application

Abstract

The application relates to a kind of MXene nanosheet / polyvinyl alcohol mixed matrix pervaporation membrane, preparation method and application, and relates to the field of membrane separation technology and new materials, and the preparation method comprises the preparation of mixed matrix casting solution, high-speed centrifugal coating, drying and heat crosslinking steps.MXene nanosheet in PVA matrix spreads horizontally under low speed of centrifugal field, migrates to the bottom of PVA matrix under high speed, and is finally stacked layer by layer at the bottom of PVA mixed matrix separation layer.Therefore, the compatibility and binding force of PVA mixed matrix separation layer and hydrophobic support membrane are improved.Meanwhile, the self-assembly MXene nanosheet stacked layer by layer constructs interlayer hydrophilic permeation channel, and further improves the alcohol / water separation performance of PVA membrane.The preparation method of the mixed matrix pervaporation membrane is optimized, the process is simple, easy to control, and has industrial production prospect.

Description

Technical Field

[0001] This invention relates to the fields of membrane separation technology and new materials, specifically to an MXene nanosheet / polyvinyl alcohol mixed matrix pervaporation membrane, its preparation method, and its application. Background Technology

[0002] The concentration of aqueous ethanol solutions and the preparation of anhydrous ethanol have always been among the most energy-intensive steps in the application of fuel ethanol. Traditional distillation processes suffer from high energy consumption and the tendency to form azeotropic systems, which limits further increases in ethanol concentration.

[0003] Pervaporation membrane separation technology is a novel ethanol dehydration separation technology characterized by low energy consumption, high selectivity, and ease of coupling with other separation methods. Polyvinyl alcohol (PVA), as a hydrophilic organic polymer, is one of the materials used to prepare pervaporation membranes and has received widespread attention in the field of pervaporation-based ethanol dehydration. Pure PVA membranes are easily soluble in hot water and exhibit poor long-term operational stability. Therefore, it is necessary to thermally or chemically crosslink PVA membranes to construct a network structure of PVA molecular chains and improve the stability of the PVA membrane. However, crosslinking of PVA molecular chains increases the density of the PVA membrane and reduces its permeability. Chinese patent [CN 1459326A] utilizes small molecule additives to regulate the free volume of PVA membrane molecular chains, increasing the membrane's porosity and improving its permeability. Chinese patents [CN103331109A] and [CN 104759212A] introduce hyperbranched polymers into PVA membranes to increase their permeation flux. In addition, novel hydrophilic nanomaterials, such as graphene oxide (GO), have been used to improve the permeability of PVA membranes and prepare PVA hybrid matrix membranes. Chinese patent [CN108479423A] describes adding hydrophilic GO nanosheets to PVA membranes to improve their permeability. Chinese patent [CN112588118A] describes adding modified SiO2 nanoparticles to PVA membranes to improve their hydrophilicity.

[0004] Hydrophilic nanoparticles in PVA membranes not only enhance the membrane's hydrophilicity but also construct hydrophilic permeation channels, further improving the membrane's alcohol / water separation performance. Therefore, the microstructure and distribution of hydrophilic nanoparticles in PVA membranes influence the construction of hydrophilic permeation channels and are a crucial factor determining membrane separation performance. Currently, no research has been published on the directional distribution and permeation channel construction of nanoparticles in PVA mixed matrix membranes, and no related domestic or international invention patents have been published. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides an MXene nanosheet / polyvinyl alcohol hybrid matrix pervaporation membrane, its preparation method, and its application. This MXene nanosheet / polyvinyl alcohol hybrid matrix pervaporation membrane exhibits high permeation flux and separation coefficient.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing an MXene nanosheet / polyvinyl alcohol hybrid matrix pervaporation membrane includes the following steps: preparation of a mixed matrix casting solution, high-speed centrifugal coating, drying, and thermal crosslinking. In the casting solution preparation stage, MXene nanosheets interact with crosslinking agent molecules to form crosslinking agent-modified MXene nanosheets, which are then uniformly dispersed in a PVA solution. Under the action of a high-speed centrifugal field, the PVA hybrid matrix casting solution is uniformly coated onto a hydrophobic support membrane. The thickness of the liquid film layer is controlled by changing the solution viscosity and centrifugal speed, thereby regulating the self-assembly process of the MXene nanosheets, allowing MXene nanosheets to stack layer by layer at the bottom of the liquid film layer. Finally, the obtained support membrane and its surface PVA liquid film are dried and thermally crosslinked together to prepare the MXene nanosheet / polyvinyl alcohol hybrid matrix pervaporation membrane.

[0008] As a preferred technical solution of the present invention, the preparation steps of the mixed matrix casting solution are as follows: water, PVA, MXene nanosheets and crosslinking agent are mixed, and a uniform PVA mixed matrix casting solution is prepared by mechanical stirring, ultrasonic dispersion and shear dispersion.

[0009] A further preferred technical solution is as follows: In the mixed matrix casting solution, the mass fraction of PVA is 1%–10%, preferably 3%–5%; the mass fraction of MXene nanosheets is 0.1%–1%, preferably 0.2%–0.5%; the mass fraction of crosslinking agent is 0.5%–5%, preferably 1%–3%; and the remainder is water. The PVA has a molecular weight of 70,000–85,000 g / mol, a degree of alcoholysis of 98%–100%, and an average degree of polymerization of 1700–1850; the MXene nanosheets are Ti3C2T x The material is MXene nanosheets; the crosslinking agent is one of sulfosuccinic acid, glycerol, triethanolamine, polyacrylic acid, caprolactam, and butanediol. Water, PVA, MXene nanosheets, and the crosslinking agent are mixed and then sequentially mechanically stirred at 500 rpm for 2 h, ultrasonically dispersed at 400 W for 4 h, and dispersed in a shear mill at 5 Kr / min for 4 h. Finally, after standing for degassing for 24 h, a mixed matrix casting solution is obtained. The viscosity of the mixed matrix casting solution is measured using a rotational viscometer, and its range is 2000–5000 mPa·s, preferably 3000–3800 mPa·s.

[0010] As a preferred embodiment of the present invention, the high-speed centrifugal coating step is as follows: the support membrane is fixed on the horizontal tray of the high-speed centrifugal coating machine, and a certain amount of casting liquid is poured onto the surface of the support membrane; the coating machine is turned on, and the support membrane is run stably at a low speed of 100-2000 rpm, preferably 500-1000 rpm for 5s-2min, preferably 30s-1.5min, and then at a high speed of 3000-6000 rpm, preferably 3000-4500 rpm for 10s-1min, preferably 10s-30s, to obtain a support membrane with a uniform PVA mixed matrix casting liquid layer on the surface.

[0011] A further preferred technical solution is that the material of the support membrane is polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), with an average pore size of 0.22 μm and an average thickness of 90–120 μm.

[0012] As a preferred embodiment of the present invention, the drying and thermal crosslinking steps are as follows: a uniformly coated support film is placed in an electric heating drying oven, the oven temperature is set to 80-120°C, and the drying time is 12-24 hours to obtain a dried PVA mixed matrix composite film; then the dried PVA mixed matrix composite film is placed in an electric heating oven for thermal crosslinking, the oven temperature is set to 150-250°C, preferably 150-170°C, and the crosslinking time is 1-10 hours, preferably 2-4 hours to obtain an MXene / polyvinyl alcohol mixed matrix pervaporation membrane.

[0013] Another object of the present invention is to provide a pervaporation membrane of MXene nanosheets / polyvinyl alcohol mixed matrix prepared by the above method, and its application in ethanol dehydration.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] 1. The MXene nanosheet / polyvinyl alcohol (PVA) mixed matrix pervaporation membrane prepared in this invention comprises a crosslinking agent molecule that modifies the MXene nanosheets through interactions such as hydrogen bonding, thereby increasing surface charge and steric hindrance to promote uniform dispersion of the MXene nanosheets in the PVA mixed matrix solution. At low rotational speeds in a centrifugal field, the MXene nanosheets spread horizontally and uniformly in the PVA mixed matrix solution layer; at high rotational speeds in a centrifugal field, the uniformly spread MXene nanosheets migrate towards the bottom of the PVA liquid film layer and eventually stack layer by layer at the bottom of the PVA mixed matrix separation layer.

[0016] 2. Due to the abundance of -F functional groups on the surface of MXene nanosheets, they can interact strongly with the surfaces of PTFE and PVDF support membranes, thereby improving the compatibility between the hydrophilic PVA mixed matrix separation layer and the hydrophobic support membrane. The crosslinking agent that migrates with the MXene nanosheets increases the degree of crosslinking at the bottom of the PVA mixed matrix separation layer, thereby further enhancing the bonding force between the separation layer and the support membrane. More importantly, the layer-by-layer stacked self-assembled MXene nanosheets improve the hydrophilicity of the PVA mixed matrix separation layer and construct hydrophilic permeation channels between layers, thereby further improving the alcohol / water separation performance of the PVA membrane.

[0017] 3. The preparation method of the MXene nanosheet / polyvinyl alcohol mixed matrix pervaporation membrane proposed in this invention optimizes the traditional preparation method of mixed matrix pervaporation membrane. The process is simple, easy to control, and has the prospect of industrial production. Attached Figure Description

[0018] Figure 1 (a) is a scanning electron microscope image of the MXene nanosheets used, and (b) is an X-ray diffraction pattern of the MXene nanosheets used.

[0019] Figure 2 (a) is a photograph of the MXene nanosheet / PVA hybrid matrix film, (b) is a scanning electron microscope (SEM) image of the surface of the MXene nanosheet / PVA hybrid matrix film, and (c) is a cross-sectional SEM image of the MXene nanosheet / PVA hybrid matrix film.

[0020] Figure 3 The results are from long-term pervaporation tests of the MXene nanosheet / PVA hybrid matrix membrane. Detailed Implementation

[0021] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way. Unless otherwise specified, the experimental methods described in the embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0022] Example 1

[0023] The preparation steps of the MXene nanosheet / polyvinyl alcohol hybrid matrix pervaporation membrane are as follows:

[0024] ① Preparation of the mixed matrix casting solution: First, PVA powder (3% by mass) was dissolved in boiling water at 100℃, mechanically stirred until homogeneous, and then cooled. Next, MXene powder (0.2% by mass) and sulfosuccinic acid (1% by mass) were added sequentially, followed by mechanical stirring at 500 rpm for 2 hours, ultrasonic dispersion at 400 W for 4 hours, and dispersion in a shear mill at 5 Kr / min for 4 hours. Finally, the mixture was allowed to stand for 24 hours to remove bubbles, yielding the mixed matrix casting solution. The viscosity of the mixed matrix casting solution was measured using a rotational viscometer, and the range was 3000–3500 mPa·s.

[0025] ② High-speed centrifugal coating: Fix a PVDF membrane with an average pore size of 0.22μm and an average thickness of 120μm on the horizontal tray of a high-speed centrifugal coating machine, and pour a certain amount of casting liquid onto the surface of the PVDF membrane; turn on the coating machine and let the PVDF membrane run stably at a low speed of 500rpm for 30s, and then run stably at a high speed of 3000rpm for 10s to obtain a PVDF membrane with a uniform PVA mixed matrix casting liquid layer on the surface.

[0026] ③ Drying and thermal crosslinking: The uniformly coated PVDF membrane was placed in an electrically heated drying oven at 100℃ for 12 hours to obtain a dried PVA mixed matrix composite membrane. The dried PVA mixed matrix composite membrane was then placed in an electrically heated oven for thermal crosslinking at 150℃ for 2 hours to obtain an MXene nanosheet / PVA mixed matrix pervaporation membrane.

[0027] Figure 1 (a) is a scanning electron microscope image of the MXene nanosheets used, and (b) is an X-ray diffraction pattern of the MXene nanosheets used. Figure 1 It can be seen that the MXene used in this invention is Ti3C2T. x It is an MXene nanosheet material.

[0028] Figure 2 (a) is a photograph of the MXene nanosheet / PVA hybrid matrix film; (b) is a scanning electron microscope (SEM) image of the surface of the MXene nanosheet / PVA hybrid matrix film; (c) is a cross-sectional SEM image of the MXene nanosheet / PVA hybrid matrix film. Figure 2 It can be seen that the crosslinking agent molecules modify MXene nanosheets through interactions such as hydrogen bonding, promoting the uniform dispersion of MXene nanosheets in the PVA mixed matrix solution by increasing surface charge and steric hindrance. At low rotational speeds in the centrifugal field, MXene nanosheets spread horizontally and uniformly in the PVA mixed matrix solution layer; at high rotational speeds, the uniformly spread MXene nanosheets migrate towards the bottom of the PVA liquid film layer and eventually stack layer by layer at the bottom of the PVA mixed matrix separation layer. Figure 2 c).

[0029] The test method for the pervaporation performance of the membrane is as follows: MXene nanosheet / PVA hybrid matrix membrane is cut into circular sheets and loaded into a membrane cell, with an effective membrane area of ​​7 cm². 2 The flow rate of the 95% ethanol / water solution, kept at a constant temperature of 30℃, was set to 20 L / h. Liquid nitrogen was poured into the cold trap, and the vacuum pump was turned on to maintain a vacuum of 0.1 MPa. The feed liquid circulation pump was turned on to start collecting the permeate in the cold trap. After the device had been running stably for 1 hour, the experiment was stopped, the cold trap was removed and weighed, and the content of each component was determined by gas chromatography (Agilent 7890GC). At this time, the water flux and separation factor of the membrane can be calculated by formulas (1), (2) and (3) respectively:

[0030]

[0031]

[0032] J w =J·α(3)

[0033] In the formula, J is the total flux (kg / m³). 2 ·h), M is the total mass of the permeate (kg), and A is the effective permeate area of ​​the membrane (m²). 2 ), where t is the infiltration time (h); J w Let Y be the water flux, α be the separation factor, and Y be the water flux. e Y represents the mass percentage of ethanol in the raw material. w X represents the mass percentage of water in the raw material. e X represents the mass percentage of ethanol in the permeate. w This represents the mass percentage of water in the permeate.

[0034] The method for testing the swelling performance of the membrane is as follows: cut three dry membrane sheets of the same size (4cm × 4cm) and weigh them to obtain m. d The membrane was then immersed in anhydrous ethanol solution at 30°C for 48 hours; after removing the membrane, the mass of the wet membrane was quickly weighed as m. w The swelling degree D of the membrane is calculated using formula (4). s :

[0035]

[0036] The method for testing the adhesion between the separation layer and the support membrane is as follows: First, the membrane is loaded into a nano-scratcher (NanoTest, MicroMaterials, UK). The maximum scratch load of the nanoindenter is controlled at 0.5 mN, the loading rate is 0.05 mN / s, and the scratch length is 500 μm. The load at which the separation layer breaks, i.e., the critical load (mN), can be measured. Under the same conditions, the larger the critical load, the greater the adhesion between the separation layer and the support membrane.

[0037] Examples 2-6

[0038] Using the crosslinking agent selected in Table 1 below, repeat the method of Example 1 to conduct a pervaporation experiment with 95% ethanol / water solution. The test results are listed in Table 1 below.

[0039] Table 1

[0040]

[0041]

[0042] Examples 7-10

[0043] Using the PVA powder mass fraction selected in Table 2 below, repeat the method of Example 1 to conduct a pervaporation experiment with 95% ethanol / water solution. The test results are listed in Table 2 below.

[0044] Table 2

[0045]

[0046] Examples 11-15

[0047] Using the MXene powder mass fraction selected in Table 3 below, repeat the method of Example 1 to conduct a pervaporation experiment with 95% ethanol / water solution. The test results are listed in Table 3 below.

[0048] Table 3

[0049]

[0050] Examples 16-20

[0051] Using the mass fraction of sulfosuccinic acid selected in Table 4 below, repeat the method of Example 1 to perform a pervaporation experiment with 95% ethanol / water solution. The test results are listed in Table 4 below.

[0052] Table 4

[0053]

[0054] Examples 21-26

[0055] Repeat the method of Example 1 to perform a pervaporation experiment of 95% ethanol / water solution at the low rotation speed of high-speed centrifugal coating selected in Table 5 below. The test results are listed in Table 5 below.

[0056] Table 5

[0057]

[0058] Examples 27-31

[0059] The pervaporation experiment of 95% ethanol / water solution was repeated using the high rotation speed of high-speed centrifugal coating selected in Table 6 below, and the test results are listed in Table 6 below.

[0060] Table 6

[0061]

[0062]

[0063] Based on the comparative data from the above embodiments, combined with Figure 3 It can be seen that MXene nanosheets interact strongly with the surface of the PVDF support membrane, thereby improving the compatibility between the hydrophilic PVA mixed matrix separation layer and the hydrophobic support membrane. As the crosslinking agent migrates with the MXene nanosheets, it increases the degree of crosslinking at the bottom of the PVA mixed matrix separation layer, thereby further enhancing the bonding force between the separation layer and the support membrane. More importantly, the layer-by-layer stacked self-assembled MXene nanosheets improve the hydrophilicity of the PVA mixed matrix separation layer and construct hydrophilic permeation channels between layers, thereby further improving the alcohol / water separation performance of the PVA membrane.

[0064] The above description is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined in the claims, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing a MXene nanoplatelet / polyvinyl alcohol mixed matrix pervaporation membrane, characterized in that, The method comprises the following steps: preparing a mixed matrix casting solution, high-speed centrifugal coating, drying and heat crosslinking. The preparation step of the mixed matrix casting solution: in the casting solution preparation stage, the MXene nanosheet interacts with the crosslinking agent molecules to form the MXene nanosheet modified by the crosslinking agent molecules and uniformly disperses in the PVA solution; the PVA has a molecular weight of 70000-85000 g / mol, an alcoholysis degree of 98%-100%, and an average polymerization degree of 1700-1850; the MXene nanosheet is a Ti3C2T x layer material; the crosslinking agent is one of sulfosuccinic acid, glycerol, triethanolamine, polyacrylic acid, caprolactam, and butanediol; in this step, after the water, the PVA, the MXene nanosheet, and the crosslinking agent are mixed, mechanical stirring is sequentially performed at 500 rpm for 2 h, dispersion is performed in ultrasonic waves of 400 W for 4 h, and dispersion is performed in a shear machine of 5 Kr / min for 4 h; finally, after standing and defoaming for 24 h, the mixed matrix casting solution is obtained; the viscosity of the mixed matrix casting solution is determined by a rotary viscometer and is in the range of 2000-5000 mPa·s; in the mixed matrix casting solution, the mass fraction of the PVA is 1%-10%, the mass fraction of the MXene nanosheet is 0.1%-1%, the mass fraction of the crosslinking agent is 0.5%-5%, and the rest is water. The high-speed centrifugal coating step: under the action of a high-speed centrifugal field, the PVA mixed matrix casting solution is uniformly coated on the hydrophobic support film, and the thickness of the liquid film layer is controlled by changing the viscosity of the solution and the centrifugal speed to regulate the self-assembly process of the MXene nanosheets, so that the MXene nanosheets are stacked layer by layer at the bottom of the liquid film layer; in this step, the support film is fixed on the horizontal tray of a high-speed centrifugal coating machine, and a certain amount of casting solution is poured onto the surface of the support film; the coating machine is turned on, and the support film is stably run at a low speed of 100-2000 rpm for 5-2 min, and then at a high speed of 3000-6000 rpm for 10-1 min, to obtain a support film with a uniform PVA mixed matrix casting solution layer on the surface. The drying and heat crosslinking step: the obtained support film and the PVA liquid film on the surface thereof are dried and heat crosslinked to prepare a MXene nanosheet / polyvinyl alcohol mixed matrix pervaporation membrane.

2. The production method according to claim 1, wherein In the preparation step of the mixed matrix casting solution, the viscosity of the mixed matrix casting solution is measured by a rotary viscometer, and the viscosity is 3000-3800 mPa·s.

3. The production method according to claim 1, wherein In the prepared mixed matrix casting solution, the mass fraction of PVA is 3%-5%, the mass fraction of MXene nanosheets is 0.2%-0.5%, the mass fraction of the crosslinking agent is 1%-3%, and the rest is water.

4. The production method according to claim 1, wherein In the high-speed centrifugal coating step, the support film is fixed on the horizontal tray of a high-speed centrifugal coating machine, and a certain amount of casting solution is poured onto the surface of the support film; the coating machine is turned on, and the support film is stably run at a low speed of 500-1000 rpm for 30-1.5 min, and then at a high speed of 3000-4500 rpm for 10-30 s, to obtain a support film with a uniform PVA mixed matrix casting solution layer on the surface.

5. The production method according to claim 1, wherein The material of the support film is polytetrafluoroethylene or polyvinylidene fluoride, the average pore size is 0.22 μm, and the average thickness is 90-120 μm.

6. The production method according to claim 1, wherein The drying and heat crosslinking step is as follows: the uniformly coated support film is placed in an electric heating air drying oven, the oven temperature is set to 80-120 ℃, and the drying time is 12-24 h to obtain a dried PVA mixed matrix composite membrane; then the dried PVA mixed matrix composite membrane is placed in an electric heating oven for heat crosslinking, the oven temperature is set to 150-250 ℃, and the crosslinking time is 1-10 h to obtain a MXene / polyvinyl alcohol mixed matrix pervaporation membrane.

7. The production method according to claim 6, wherein The heat crosslinking step is as follows: the dried PVA mixed matrix composite membrane is placed in an electric heating oven for heat crosslinking, the oven temperature is set to 150-170 ℃, and the crosslinking time is 2-4 h to obtain a MXene / polyvinyl alcohol mixed matrix pervaporation membrane.

8. The MXene nanosheet / polyvinyl alcohol mixed matrix pervaporation membrane prepared by the method of any one of claims 1-7.

9. Use of the MXene nanoplatelet / polyvinyl alcohol hybrid matrix pervaporation membrane according to claim 8 for the dehydration of ethanol, characterized in that, The concentration of the ethanol aqueous solution to be separated is 85%-99.5%, and the membrane separation treatment temperature is 30-70 ℃.

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