Preparation method and application of MFI zeolite membrane

Abstract

The invention discloses a preparation method and application of an MFI zeolite membrane.The preparation method comprises the steps that an MFI zeolite nanosheet serves as a seed crystal, a uniformly-distributed and compact seed crystal layer is prepared on a carrier through a vacuum filtration method, namely, a nucleation center crystallized into a membrane is loaded on the surface of the carrier, and directional growth and membrane forming of the zeolite nanosheet can be effectively controlled; the highly b-oriented MFI zeolite membrane can be prepared within several hours, the prepared b-oriented MFI zeolite membrane shows a continuous and compact microstructure and almost has no defects, and compared with a traditional hydrothermal method, the method overcomes the defects that the reaction process is too long in time consumption, uneven in heating is prone to occurring, and the like. And therefore, the time required by crystal growth is greatly shortened. The prepared MFI zeolite membrane can be applied to pervaporation separation of xylene isomers, shows good size screening separation performance, and has important industrial application value.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of molecular sieve membrane materials, and particularly relates to a preparation method and application of MFI zeolite membrane. BACKGROUND

[0002] The xylene product in industry includes para-xylene, ortho-xylene and meta-xylene, which often exist in the form of a mixture, and the commercial value of para-xylene dominates, accounting for 86% of the total amount of isomers, and this structural contradiction between supply and demand needs to develop efficient separation and purification technology. The traditional rectification method has inherent defects of low efficiency and large energy consumption; the step-by-step crystallization method is limited by insufficient product yield and purity; and the adsorbent material of the simulated flow bed adsorption process also has obvious shortcomings of poor selective adsorption capacity and poor regeneration performance.

[0003] Compared with the traditional separation technology, the membrane separation technology has significant advantages in the industrial separation field due to its low energy consumption, outstanding environmental friendliness, small occupied area and simple operation. As the third generation of membrane separation technology, the pervaporation breaks through the gas-liquid equilibrium limitation of the traditional distillation process based on the mass transfer characteristics of the dissolution-diffusion mechanism. For the xylene isomer separation system, the physicochemical properties of each xylene molecule are highly similar, and by constructing a pervaporation membrane with molecular recognition function, selective separation based on molecular size difference and shape difference can be realized, and the separation effect will be much better than that of the traditional adsorbent, which is beneficial to industrial application.

[0004] MFI is a molecular sieve membrane material based on a ten-membered ring channel structure, and its crystal structure includes a sinusoidal channel (0.51 nm x 0.55 nm) along the a-axis direction and a straight channel (0.53 nm x 0.56 nm) along the b-axis direction. Since para-xylene has a linear structure and its kinetic diameter is close to the pore size of MFI, it can diffuse preferentially through the b-axis straight channel, while ortho-xylene and meta-xylene have larger steric hindrance due to the branched structure, and their diffusion rates are significantly reduced, so that selective separation can be achieved. In the literature [J.Membr.Sci., 2025, 713, 123304], Banihashemi et al. deposited b-oriented MFI zeolite nanosheets as a seed layer by vacuum filtration, and then grew them again by gel-assisted vapor crystallization to prepare b-oriented MFI zeolite membranes with a thickness of about 600 nm, which exhibited a 2.52 x 10 -7 molm -2 s -1 Pa -1The paraxylene permeate flux and separation factor of 388 were achieved. In the literature [Sep. Purif. Technol., 2023, 315, 123709], Kim et al. further prepared zeolite nanosheets by adding bis-1,5(tripropylammonium) pentamethylene diiodide (dC5) during the zeolite crystal seed preparation process. The average thickness was about 13 nm and the aspect ratio was about 87. Finally, a b-oriented MFI zeolite membrane was prepared, and its PX permeate flux was 1.2×10 -3 molm -2 s -1 , the separation factor reached 795.

[0005] Currently, various methods for preparing b-oriented MFI zeolite membranes have been reported in the literature. However, most of these syntheses involve hydrothermal processes, which are time-consuming and prone to twinning. Furthermore, the separation of xylene isomers using these MFI membranes requires low-pressure vapor phase injection, making them difficult to commercialize. Therefore, designing a b-oriented MFI zeolite membrane for pervaporation separation of xylene isomers is of great research significance. Summary of the Invention

[0006] To overcome the problems of the prior art, one object of the present invention is to provide a method for preparing an MFI zeolite membrane. A second object of the present invention is to provide an MFI zeolite membrane produced by the above-mentioned preparation method. A third object of the present invention is to provide applications of the MFI zeolite membrane.

[0007] The average pore size of the b-oriented MFI zeolite membrane meets the size requirements for screening para-xylene and two other xylenes, and its pores are arranged in a vertical direction, which can reduce the mass transfer resistance of para-xylene diffusion. Therefore, the b-oriented MFI zeolite membrane theoretically has excellent separation performance for the separation of para-xylene isomers. The present invention provides a method for preparing a b-oriented MFI zeolite membrane for the pervaporation separation of xylene isomers. This method uses MFI zeolite nanosheets as seeds, first using vacuum filtration to produce a uniformly distributed and dense seed layer, then using microwaves to promote secondary growth of the zeolite nanosheet seed layer and crystallize it into a membrane. A b-oriented MFI zeolite membrane with a thickness of 1 μm to 1.5 μm is prepared within 1 to 4 hours. Using a pervaporation experimental device, the membrane was used to separate xylene isomers in a liquid feed, demonstrating good para-xylene selectivity and permeability.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] The first aspect of the present invention provides a method for preparing an MFI zeolite membrane, comprising the following steps:

[0010] S1, preparing a sol comprising urea, a silicon source, an organic template and water, performing a hydrothermal crystallization reaction, and calcining to obtain Silicalite-1 zeolite nanosheets;

[0011] S2, dispersing the Silicalite-1 zeolite nanosheets of S1 in water to prepare a seed solution, coating the seed solution on a porous support by vacuum filtration, and obtaining a b-oriented MFI zeolite nanosheet seed layer grown on the porous support after drying and calcining;

[0012] S3. Prepare an ethanol aqueous solution containing a silicon source and an organic template as a secondary growth synthesis liquid, place the porous carrier with a b-oriented MFI zeolite nanosheet seed layer grown in S2 and the secondary growth synthesis liquid in a microwave reactor, carry out a hydrothermal crystallization reaction under microwave assistance, and obtain the MFI zeolite membrane by calcination.

[0013] Preferably, the silicon source is selected from at least one of methyl orthosilicate, ethyl orthosilicate, water glass, silica sol, and silica aerogel.

[0014] Preferably, the organic template is selected from at least one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetrapropylammonium chloride, and methyltributylammonium hydroxide.

[0015] Preferably, in step S1, the molar ratio of the silicon source, the organic template, the urea and the water in the sol is 1:(0.2-0.6):(0.5-1):(20-50).

[0016] More preferably, in step S1, the molar ratio of the silicon source, the organic template, the urea and the water in the sol is 1:(0.3-0.4):(0.6-0.7):(20-50).

[0017] Preferably, in step S1, the reaction temperature of the hydrothermal crystallization reaction is 140° C. to 190° C., and the reaction time is 24 h to 72 h.

[0018] More preferably, in step S1, the reaction temperature of the hydrothermal crystallization reaction is 170° C. to 190° C., and the reaction time is 36 h to 60 h.

[0019] Further preferably, in step S1, the reaction temperature of the hydrothermal crystallization reaction is 175° C. to 185° C., and the reaction time is 42 h to 54 h.

[0020] Preferably, in step S1, the preparation method of the sol comprises: vigorously stirring urea, a silicon source, an organic template and water to prepare the sol.

[0021] Preferably, step S1 further comprises the following steps: collecting the solid product after the hydrothermal reaction, washing, drying and calcining to obtain the Silicalite-1 zeolite nanosheets.

[0022] Preferably, the size of the Silicalite-1 zeolite nanosheets is (1500-1700) nm×(200-250) nm×(80-100) nm.

[0023] Preferably, the mass content of the MFI zeolite nanosheets in the seed solution is 0.5% to 2%.

[0024] More preferably, the mass content of the MFI zeolite nanosheets in the seed solution is 0.8% to 1.2%.

[0025] Preferably, step S2 further comprises the following step: adding a strong acid to the seed solution to adjust the pH thereof to 1-2.

[0026] In the present invention, the pH value of the seed solution is set to 1-2, which can prevent the Silicalite-1 zeolite nanosheets from aggregating.

[0027] More preferably, the strong acid is concentrated nitric acid or concentrated sulfuric acid.

[0028] Preferably, in step S2, the multi-control support is selected from one of porous alumina, porous silica, porous zirconia, porous carbon, and porous stainless steel.

[0029] More preferably, the average pore size of the porous alumina is about 0.4-1 μm, and the porosity is about 38%-40%.

[0030] Preferably, in step S2, the vacuum filtration method specifically comprises: placing the porous carrier in a vacuum filtration device, pouring the seed solution into the porous carrier in multiple times, and spreading the seed solution on the surface and surface pores of the porous carrier by vacuum filtration.

[0031] More preferably, the vacuum filtration device is a Buchner funnel.

[0032] More preferably, the filter paper used for the vacuum filtration is a medium-speed qualitative filter paper with a pore size between 15 μm and 20 μm.

[0033] More preferably, the vacuum degree of the vacuum filtration is 0.02 MPa to 0.05 MPa.

[0034] Preferably, in step S3, the molar ratio of the silicon source, the organic template, ethanol and water in the secondary growth synthesis solution is 1:(0.1-0.2):(0.3-0.7):(150-200).

[0035] More preferably, in step S3, the molar ratio of the silicon source, the organic template, ethanol and water in the secondary growth synthesis solution is 1:(0.1-0.3):(0.4-0.6):(150-200).

[0036] Preferably, in step S3, the reaction temperature of the hydrothermal crystallization reaction is 150° C. to 190° C., and the reaction time is 60 min to 240 min.

[0037] More preferably, in step S3, the reaction temperature of the hydrothermal crystallization reaction is 170° C. to 190° C., and the reaction time is 120 min to 240 min.

[0038] Further preferably, in step S3, the reaction temperature of the hydrothermal crystallization reaction is 175° C. to 185° C., and the reaction time is 160 min to 220 min.

[0039] Preferably, in steps S1-S3, the calcination temperature is 500°C to 650°C, and the calcination time is 4h to 7h.

[0040] Preferably, in step S3, the frequency of the microwave reactor is 300-2000W.

[0041] The second aspect of the present invention provides an MFI zeolite membrane prepared by the preparation method described in the first aspect.

[0042] Preferably, the MFI zeolite membrane has a thickness of 1 μm to 1.5 μm.

[0043] The third aspect of the present invention provides use of the MFI zeolite membrane described in the second aspect in separating xylene isomers.

[0044] Preferably, the xylene isomers include p-xylene, o-xylene and m-xylene.

[0045] The beneficial effects of the present invention are:

[0046] The present invention provides a preparation method of a b-oriented MFI zeolite membrane for pervaporation separation of xylene isomers. The method uses MFI zeolite nanosheets as seeds and first uses a vacuum filtration method to prepare a uniformly distributed and dense seed layer on a carrier, that is, nucleation centers for crystallization and film formation are loaded on the carrier surface, which can effectively control the directional growth and film formation of the zeolite nanosheets, and is conducive to the subsequent acquisition of a continuous and dense MFI membrane; then a microwave method is used to promote the secondary growth of the zeolite nanosheet seed layer and crystallization into a film. By regulating the film growth conditions, the film thickness and pore orientation can be effectively controlled. A highly b-oriented MFI zeolite membrane with a film thickness of 1μm to 1.5μm is prepared within 1h to 4h. Compared with the traditional hydrothermal method, the method overcomes the problems of a long reaction process, prone to uneven heating, and prone to twinning, and greatly shortens the time required for crystal growth.

[0047] The b-oriented MFI zeolite membrane prepared by the present invention was used to separate xylene isomers in a liquid feed through a pervaporation experimental device. It has good para-xylene selectivity and permeability. At a temperature of 90°C and 0.1 MPa, it was used to separate an equimolar para- / o-xylene mixed solution. The permeation flux of para-xylene reached 0.823 gm -2 s -1 , the separation factor is 3.82, with excellent pervaporation separation performance of para- / o-xylene, and the performance is not reduced by shortening the time required for crystal growth. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a scanning electron microscope image of the Silicalite-1 zeolite nanosheet of Example 1;

[0049] Figure 2 For Figure 1 Scanning electron microscopy images at different magnifications;

[0050] Figure 3 is the X-ray diffraction pattern of the Silicalite-1 zeolite nanosheets of Example 1;

[0051] Figure 4 This is a scanning electron microscope image of the seed layer coated on the Al2O3 porous support of Example 1;

[0052] Figure 5 This is a scanning electron microscope image of the surface of the b-oriented MFI zeolite membrane synthesized by the microwave secondary growth method in Example 1;

[0053] Figure 6 This is a scanning electron microscope image of the cross section of the b-oriented MFI zeolite membrane synthesized by the microwave secondary growth method in Example 1;

[0054] Figure 7 is the X-ray diffraction pattern of the b-oriented MFI zeolite membrane of Example 1 (“*” indicates the peak of the porous alumina support);

[0055] Figure 8 This is a scanning electron microscope image of the seed layer coated with the Al2O3 porous support of Comparative Example 2;

[0056] Figure 9 This is a diagram showing the para- / o-xylene separation performance of the b-oriented MFI zeolite membrane of Example 1. DETAILED DESCRIPTION

[0057] The present invention is further described in detail below through specific examples. Unless otherwise specified, the raw materials used in the following examples can be obtained from conventional commercial sources or prepared and isolated by simple synthesis; unless otherwise specified, the processes used are conventional processes in the art.

[0058] The material of the porous support selected in the following examples and comparative examples is porous alumina, the shape of the porous support is a circular flat sheet, the average pore size of the porous alumina is about 0.4 to 1 μm, and the porosity is about 38% to 40%; the instrument used in the vacuum filtration is a Buchner funnel, and the filter paper used for filtration is a medium-speed qualitative filter paper with a pore size between 15 μm and 20 μm.

[0059] Example 1

[0060] This embodiment provides an MFI zeolite membrane, and the preparation method thereof is as follows:

[0061] (1) Preparation of Silicalite-1 zeolite nanosheets: Deionized water, organic template tetrapropylammonium hydroxide (TPAOH), urea (CO(NH2)2), and tetraethyl orthosilicate (TEOS) were added and mixed in sequence, the rotation speed was adjusted to 1000 rpm~2000 rpm, and the mixture was vigorously stirred for 2 h to obtain a clear sol. The molar ratio of the formed sol was 1TEOS:0.35TPAOH:0.69CO(NH2)2:30H2O; the clear sol was transferred to a hydrothermal reactor and crystallized at 180°C for 48 h to obtain a white flocculent precipitate. The product was separated by filtration, washed with deionized water, dried, and then placed in a muffle furnace for calcination at 550°C for 6 h. The heating / cooling rate of the muffle furnace was 1°C / min to obtain Silicalite-1 zeolite nanosheets.

[0062] Figure 1 and Figure 2 The scanning electron microscope images of Silicalite-1 zeolite nanosheets at different magnifications show that the size of the Silicalite-1 zeolite nanosheets is (1500-1700) nm × (200-250) nm × (80-100) nm; Figure 3 The X-ray diffraction patterns of Silicalite-1 zeolite nanosheets are shown, which confirm that Silicalite-1 zeolite nanosheets are prepared in step (1).

[0063] (2) Preparation of b-oriented MFI zeolite nanosheet seed layer: 0.3 g of the Silicalite-1 zeolite nanosheet powder synthesized in step (1) was ultrasonically dispersed in 30 mL of deionized water. The ultrasonic time was 30 min to 90 min and the ultrasonic temperature was 20°C to 40°C to form a 1% suspension. 0.1 mol / L nitric acid solution was added until the pH of the suspension was about 1 to 2 to prevent the aggregation of the seed particles. The solution after ultrasonication was centrifuged at 4000 rpm for 5 min and then poured out. The supernatant liquid is used as the seed liquid; the seed layer is coated by vacuum filtration, the filtration device is assembled, the porous carrier is placed in a Buchner funnel, and all the seed liquid is poured in three times, the vacuum degree of the vacuum pump is controlled to be 0.05 MPa, the filtration time is 3 minutes, and after the vacuum degree returns to zero, it is slowly taken out, dried at 80°C for 1 hour, and then placed in a muffle furnace at 500°C for calcination for 5 hours. The heating / cooling rate of the muffle furnace is 1°C / min, and a b-oriented MFI zeolite nanosheet seed layer with uniform and dense distribution of zeolite nanosheets is obtained.

[0064] Figure 4 It can be shown that the vacuum filtration method can be used to prepare a uniformly distributed and dense seed layer on a porous support.

[0065] (3) Preparation of b-oriented MFI zeolite membrane: organic template TPAOH, deionized water, silicon source TEOS and anhydrous ethanol were added in sequence to obtain a molecular sieve precursor solution with a molar ratio of 1TEOS:0.2TPAOH:0.5EtOH:160H2O, and the rotation speed was controlled to 750rpm and stirred for 4h until the solution was clarified; then the seed layer prepared in step (2) was placed with the carrier coated with the seed layer facing up, and the carrier was clamped with tweezers and placed vertically in a microwave reactor. The frequency of the microwave reactor was 500W, and the upper pressure limit of the microwave reactor was 5.0MPa. Then, the clarified secondary growth synthesis liquid was slowly poured into the reactor until the synthesis liquid covered the surface of the carrier. The reaction temperature was controlled to 180 and crystallized for 3h. After the reaction was completed, the b-oriented MFI zeolite membrane was taken out from the reactor, washed with deionized water, dried, and then placed in a muffle furnace at 550℃ for calcination for 6h. The heating / cooling rate of the muffle furnace was 1℃ / min to remove the organic template.

[0066] The surface and cross-sectional morphologies of the b-oriented MFI zeolite membrane prepared in Example 1 are attached. Figure 5 and 6 It was confirmed that the crystal structure has Figure 7 The X-ray diffraction pattern analysis of the MFI membrane confirmed that when the pore orientation of the MFI membrane is b orientation, the characteristic diffraction peaks corresponding to its XRD spectrum are (0 2 0), (0 4 0), (0 6 0), (0 8 0), and (0 10 0) crystal plane peaks, and can be judged by the peak intensity of the (0 2 0) crystal plane peak and the (1 0 1) crystal plane peak. Figure 7As can be seen from the figure, the XRD spectrum of the MFI zeolite membrane prepared by the microwave method finds the characteristic diffraction peaks of the b orientation, and the peak intensity of the key diffraction peak (0 2 0) crystal face peak of the b orientation is much higher than that of the (1 0 1) crystal face peak, so it can be judged that the pore size orientation of the prepared MFI zeolite membrane is b orientation.

[0067] Example 2

[0068] The present example provides a MFI zeolite membrane, the preparation method of which is referred to Example 1, except that the mass ratio of zeolite nanosheets to deionized water in step (2) of Example 1 is changed from 1:100 to 1:450, and other synthesis conditions remain the same as those of the b-oriented MFI zeolite membrane in Example 1.

[0069] Example 3

[0070] The present example provides a MFI zeolite membrane, the preparation method of which is referred to Example 1, except that the crystallization time of step (3) in Example 1 is changed, and the crystallization into a membrane reaction is carried out at 180℃ for 2h, and other synthesis conditions remain the same as those of the b-oriented MFI zeolite membrane in Example 1.

[0071] Example 4

[0072] The present example provides a MFI zeolite membrane, the preparation method of which is referred to Example 1, except that the crystallization temperature of step (3) in Example 1 is changed, and the crystallization into a membrane reaction is carried out at 160℃ for 3h, and other synthesis conditions remain the same as those of the b-oriented MFI zeolite membrane in Example 1.

[0073] Comparative Example 1

[0074] The present comparative example provides a MFI zeolite membrane, the preparation method of which is referred to Example 1, except that the microwave-assisted secondary growth method in step (3) of Example 1 is changed to a traditional hydrothermal secondary growth, the temperature is 180℃, and the crystallization time is 48h, and other synthesis conditions remain the same as those of the b-oriented MFI zeolite membrane in Example 1.

[0075] Comparative Example 2

[0076] The present comparative example provides a MFI zeolite membrane, the preparation method of which is referred to Example 1, except that the seed coating method in step (2) of Comparative Example 2 is changed, and a finger coating method is used, 2.09g of polyvinyl alcohol and 30.43g of deionized water are weighed, a 6.42% polyvinyl alcohol sol solution is prepared, and the zeolite nanosheets are coated on the porous support by dipping the fingers in the zeolite nanosheets and then coating them on the porous support; other synthesis conditions remain the same as those of the b-oriented MFI zeolite membrane in Example 1.

[0077] The surface structure of the seed layer obtained in Comparative Example 2 is Figure 8 As shown, compared with Example 1, it is not possible to form a uniformly distributed and dense seed layer on the porous support.

[0078] Experimental characterization of xylene isomer separation

[0079] 1. Experimental methods

[0080] The prepared MFI membrane was placed into the membrane assembly, followed by a porous metal gasket to prevent damage to the membrane when fixing the MFI membrane. The screw sleeve was tightened to fix the MFI membrane. Then, equal amounts of p-xylene and o-xylene were weighed and added to a wide-mouth bottle to mix evenly. The membrane assembly was placed into the wide-mouth bottle and connected to the membrane assembly through a PU air tube through the bottle cap. The wide-mouth bottle cap was tightened. The PU air tube was connected to the serpentine cold trap and vacuum pump in sequence to check the air tightness of the system. After the air tightness was good, the serpentine cold trap was placed in a liquid nitrogen cup, a certain amount of liquid nitrogen was poured in, and the vacuum pump was turned on to conduct a pervaporation separation experiment. During the separation process, the raw material content, the product content in the liquid nitrogen cup, and the separation time were recorded to facilitate the evaluation of the separation performance of the MFI membrane.

[0081] 2. Experimental results

[0082] The b-oriented MFI zeolite membrane prepared in Example 1 was used for pervaporation separation of equimolar p-xylene and o-xylene at 90°C and 0.1 MPa. Figure 9 The b-oriented MFI zeolite membrane has a performance diagram for the separation of para-xylene and o-xylene. The experiment shows the effect of changing the pressure difference of the reaction system (achieved by adjusting the vacuum pump) on the membrane separation performance. The permeation flux of para-xylene reaches 0.823 gm -2 s -1 , the separation factor is 3.825.

[0083] The membrane prepared in Example 2 was used for pervaporation separation of equimolar p-xylene and o-xylene at 90°C and 0.1 MPa, and the permeation flux of p-xylene reached 1.982 gm -2 s -1 , the separation factor is 1.133.

[0084] The membrane prepared in Example 3 was used for pervaporation separation of equimolar p-xylene and o-xylene at 90°C and 0.1 MPa. The permeation flux of p-xylene reached 3.362 gm -2 s -1 , the separation factor is 1.414.

[0085] The membrane prepared in Example 4 was used for pervaporation separation of equimolar p-xylene and o-xylene at 90°C and 0.1 MPa. The permeation flux of p-xylene reached 4.406 gm -2 s -1 , the separation factor is 1.209.

[0086] The membrane prepared in Comparative Example 1 was used for pervaporation separation of equal moles of p-xylene and o-xylene at 90°C and 0.1 MPa, and the permeation flux of p-xylene reached 2.689 gm -2 s -1 , the separation factor is 1.407.

[0087] The membrane prepared in Comparative Example 2 was used for pervaporation separation of equal moles of p-xylene and o-xylene at 90°C and 0.1 MPa, and the permeation flux of p-xylene reached 0.425 gm -2 s -1 , the separation factor is 2.378.

[0088] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing an MFI zeolite membrane, characterized in that: The steps include: S1, preparing a sol comprising urea, a silicon source, an organic template and water, performing a hydrothermal crystallization reaction, and calcining to obtain Silicalite-1 zeolite nanosheets; S2, dispersing the Silicalite-1 zeolite nanosheets of S1 in water to prepare a seed solution, coating the seed solution on a porous support by vacuum filtration, and obtaining a b-oriented MFI zeolite nanosheet seed layer grown on the porous support after drying and calcining; S3. Prepare an ethanol aqueous solution containing a silicon source and an organic template as a secondary growth synthesis liquid, place the porous carrier with a b-oriented MFI zeolite nanosheet seed layer grown in S2 and the secondary growth synthesis liquid in a microwave reactor, carry out a hydrothermal crystallization reaction under microwave assistance, and obtain the MFI zeolite membrane by calcination.

2. The method for preparing the MFI zeolite membrane according to claim 1, wherein The silicon source is selected from at least one of methyl orthosilicate, ethyl orthosilicate, water glass, silica sol, and silica aerogel; And / or, the organic template is at least one selected from tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetrapropylammonium chloride, and methyltributylammonium hydroxide.

3. The method for preparing the MFI zeolite membrane according to claim 1, wherein In step S1, the molar ratio of the silicon source, the organic template, the urea and the water in the sol is 1:(0.2-0.6):(0.5-1):(20-50); And / or, in step S3, the molar ratio of the silicon source, the organic template, ethanol and water in the secondary growth synthesis solution is 1:(0.1-0.2):(0.3-0.7):(150-200).

4. The method for preparing the MFI zeolite membrane according to claim 1, wherein In step S1, the reaction temperature of the hydrothermal crystallization reaction is 140° C. to 190° C., and the reaction time is 24 h to 72 h; And / or, in step S3, the reaction temperature of the hydrothermal crystallization reaction is 150° C. to 190° C., and the reaction time is 60 min to 240 min.

5. The method for preparing the MFI zeolite membrane according to claim 1, wherein In step S2, the porous carrier is selected from one of porous alumina, porous silica, porous zirconia, porous carbon, and porous stainless steel.

6. The method for preparing the MFI zeolite membrane according to claim 1, characterized in that: In step S2, the vacuum filtration method specifically includes: placing the porous carrier in a vacuum filtration device, pouring the seed solution into the porous carrier in multiple times, and spreading the seed solution on the surface and surface pores of the porous carrier by vacuum filtration.

7. The method for preparing the MFI zeolite membrane according to claim 1, characterized in that: Step S2 also includes the following steps: adding a strong acid to the seed solution to adjust its pH to 1-2.

8. The method for preparing the MFI zeolite membrane according to claim 1, characterized in that: In steps S1-S3, the calcination temperature is 500°C to 650°C, and the calcination time is 4h to 7h.

9. The MFI zeolite membrane prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the MFI zeolite membrane according to claim 9 in separating xylene isomers.

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