MFI type molecular sieve membrane and controllable preparation method and application thereof

Abstract

The invention discloses an MFI type molecular sieve membrane as well as a controllable preparation method and application thereof, and relates to the technical field of inorganic membrane material preparation, the residual amount of an organic template agent in membrane pores is adjusted by controlling ozone calcination conditions, the pore diameter of the membrane is accurately regulated and controlled, the organic template agent in the pores is gradually oxidized and removed along with the prolonging of calcination time, and the pore diameter of the membrane is reduced. The preparation method comprises the following steps: preparing a seed crystal reaction gel, preparing an MFI type molecular sieve seed crystal, preparing a support body seed crystal, preliminarily preparing the MFI type molecular sieve membrane, and preparing the MFI type molecular sieve membrane. The prepared MFI type molecular sieve membrane has excellent performance on separation of small-size molecules and large-size molecules and can be used for separation of system gases such as hydrogen / iso-butane, methane / iso-butane and n-butane / iso-butane, the preparation process is simpler and more convenient, and the separation effect on the small-size molecules and the large-size molecules is more efficient; therefore, the separation requirements under different conditions are met.

Description

Technical Field

[0001] The present invention relates to the technical field of inorganic membrane material preparation, in particular to a controllable preparation method of an MFI type molecular sieve membrane, the MFI type molecular sieve membrane obtained by the preparation method, and its application in separation of hydrogen and isobutane, methane and isobutane, and normal butane and isobutane. Background Art

[0002] The separation process is an indispensable unit operation in many industries. Compared with traditional separation technologies such as distillation, adsorption and evaporation, membrane separation has the advantages of low separation energy consumption, high efficiency and small footprint. Membrane separation technology has gradually become an alternative to traditional separation units. Zeolite molecular sieve membranes have uniform pore size, excellent separation performance and good chemical stability, and are considered to be good candidates for separating related mixtures. Among them, MFI-type zeolite molecular sieve membranes have received widespread attention and have been used to separate various molecular mixtures, such as the separation of n / isobutane and p / m-xylene systems based on molecular sieving mechanisms. The preparation process of zeolite molecular sieve membranes usually requires two steps: hydrothermal synthesis of membranes and removal of organic templates in membrane pores. For a specific zeolite molecular sieve membrane, the pore size of the membrane is fixed. In order to obtain excellent separation performance, it is usually necessary to modify and regulate the pore size of the membrane.

[0003] Common methods for regulating the pore structure of zeolite molecular sieve membranes include chemical vapor deposition and ion exchange. Chemical vapor deposition effectively reduces the pore size of the molecular sieve membrane by depositing amorphous materials (such as SiO2, TiO2) on the zeolite pores / pores, thereby improving the separation selectivity of the membrane. For example, Langmuir, 2009, 25.9: 4848-4852 showed that the H2 / CO2 selectivity of the MFI molecular sieve membrane modified by vapor deposition was significantly increased, while the unmodified membrane only showed Knudsen selectivity. However, in practical applications, this method is difficult to ensure the uniformity of species on the surface and in the pores of the deposited membrane, and the process is complex and has high requirements for equipment. The ion exchange method combines ions in the molecular sieve framework with different metal ions (such as Li + , K + 、Ag + and Ca 2+ ) is used to exchange metal ions to adjust the pore size and adsorption characteristics of the molecular sieve membrane. The regulation of membrane pore size by ion exchange may be limited by the types of available metal ions, resulting in a limited regulation range. In addition, some metal ions may migrate or lose under the conditions of use, affecting the long-term stability of the membrane.

[0004] In addition, encapsulating or wrapping metal species in the pores of molecular sieves can also reduce the pore size of molecular sieves. For example, a self-assembled Fe-containing mordenite was prepared in Science, 2021, 373 (6552), 315-320. Due to the presence of Fe species, the molecular sieve channels were reduced, and good adsorption and screening selectivity were shown in the study. However, the method of encapsulating metal species in molecular sieves is mainly used for adsorption, and its application in membrane separation is still being explored.

[0005] In addition, a Chinese invention patent (authorization announcement number CN114804139B) discloses a method for preparing a multi-channel MFI molecular sieve membrane, the process of which is as follows: (1) preparing short columnar MFI molecular sieve seeds; (2) coating the inner wall of the multi-channel carrier with a uniform and dense MFI molecular sieve seed layer; (3) preparing a continuous and dense MFI molecular sieve membrane on the inner surface of each channel of the multi-channel carrier by a hydrothermal synthesis method. The molecular sieve membrane prepared by the invention using a multi-channel carrier has a high surface area to volume ratio, which can greatly increase the packing density of the membrane and reduce the volume of the membrane assembly. Moreover, the membrane has good surface area and volume ratio in n-butane / isobutane (n-C4H 10 / i-C4H 10 ) mixed system. The innovation of this invention is mainly to provide a preparation method for a multi-channel MFI type zeolite membrane for n-butane / isobutane separation, and no innovation is proposed for the microporous structure and pore size control preparation of the MFI type zeolite membrane; in addition, this invention cannot separate small molecular gases using the inherent macroporous structure (b-axis and a-axis) of the MFI zeolite, and has little reference value for the controllable preparation of zeolite membranes.

[0006] In view of the shortcomings of the prior art, the present invention provides an MFI-type zeolite molecular sieve membrane that can be used to separate gases of different molecular sizes, and a controllable preparation method and application thereof. By controlling the degree of template removal and adjusting the residual amount of organic template in the membrane pores, the inherent large pores of the MFI molecular sieve are effectively reduced to adjust the membrane pore size and obtain excellent gas separation performance. Summary of the invention

[0007] 1. Technical issues to be resolved

[0008] In view of the shortcomings of the prior art, the present invention provides an MFI zeolite molecular sieve membrane with a simple preparation process and high separation efficiency, and a controllable preparation method and application thereof. During the preparation process of the MFI zeolite membrane, the membrane pore size is effectively adjusted by controlling the degree of template removal and adjusting the residual amount of organic template in the membrane pores, thereby obtaining excellent separation performance.

[0009] (II) Technical solution

[0010] To achieve the above object, the present invention provides the following technical solutions: A controllable preparation method of an MFI type molecular sieve membrane, by controlling the degree of template removal and adjusting the residual amount of organic template in the membrane pores, effectively adjusts the membrane pore size and obtains excellent separation performance. The specific steps are as follows: S1. Preparation of seed crystal reaction gel: According to the silicon source: organic template SDA: alkali source (OH -1 Prepare the raw materials in a molar ratio of ): distilled water = 10: (1.6-2.8): (0.44-1.3): (100-150), add them into a stirring tank and mix them well to form a solution, and continue stirring and aging at room temperature for 6-48 hours to obtain a first reaction gel; In this step, during the stirring and aging process, when the synthesis reaction is carried out at room temperature, the aging time can be extended to increase the crystal nucleus density and promote the crystal growth during synthesis; S2. Preparation of MFI type molecular sieve seed crystals: pouring the obtained first reaction gel into a reaction kettle, hydrothermally synthesizing at 80-120° C. for 48-96 hours, centrifuging the reaction product after the reaction is completed, washing until the pH is neutral, and freeze-drying under vacuum to obtain highly dispersed MFI molecular sieve seed crystals; S3, preparing a seeded support: adding the prepared MFI molecular sieve seed crystals to ethanol or an aqueous solution for ultrasonic dispersion to prepare a seed solution with a concentration of 0.06-2%, coating the seed solution on a porous support, and drying at 30-80° C. for 2 h to form a seed layer, thereby preparing a seeded support; S4, preliminary preparation of MFI type molecular sieve membrane: adding silicon source, organic template SDA and distilled water in a molar ratio of 25: (2-8): (1000-1500) into a stirring tank and mixing evenly to form a solution, stirring continuously at room temperature for 6-24 hours to obtain a second reaction gel, placing the second reaction gel and the seed support prepared in step S3 into a reaction kettle, synthesizing at 75-140° C. for 24-85 hours, cooling to room temperature after the reaction to obtain an MFI type molecular sieve membrane, taking out the membrane, washing and drying; S5. Regulating the pore size of the MFI type molecular sieve membrane: placing the MFI type molecular sieve membrane in step S4 in a flowing gas atmosphere furnace for calcination to remove the organic template in the membrane layer, thereby obtaining an MFI type molecular sieve membrane with high separation performance.

[0011] Preferably, the silicon source in step S1 and step S4 is one including but not limited to tetraethyl silicate, ammonium fluorosilicate, silica sol and fumed silica.

[0012] Preferably, the organic template agent SDA in step S1 and step S4 is one including but not limited to tetrapropylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium bromide.

[0013] Preferably, the alkali source in step S1 is one including but not limited to sodium hydroxide and potassium hydroxide.

[0014] Preferably, the porous support in step S3 is a ceramic support made of one of silicon carbide, alumina, and mullite, and has a tubular, sheet, or disc shape.

[0015] Preferably, the seed coating method in step S3 includes but is not limited to one of a dip coating method and a vacuum coating method; wherein the vacuum coating method has a coating pressure of a negative pressure of 5-20 kPa and a coating time of 10-60 s.

[0016] Preferably, in step S4, the membrane is taken out and washed 3-5 times, and dried at 50-100° C. for 2-4 hours.

[0017] Preferably, the atmosphere of the atmosphere furnace in step S5 is a mixed gas containing ozone, the ozone concentration is 60-100 mg / L, the calcination temperature is 30-250° C., and the calcination time is 6-60 h.

[0018] In addition to ozone, the other components of the ozone mixed gas can be nitrogen, oxygen or air. Since ozone is more stable at lower temperatures but less reactive, the rate of ozone decomposition increases with increasing temperature, producing more free radicals, thereby enhancing oxidation activity; but since the half-life (lifespan) of ozone is shorter, the calcination time can be extended at lower temperatures and shortened at higher temperatures to achieve the same effect. If the temperature exceeds 250°C, ozone will decompose instantly (1~2s), seriously reducing the effect of calcination oxidation to remove the template.

[0019] Another object of the present invention is to provide an MFI type molecular sieve membrane prepared by the above controllable preparation method.

[0020] Another object of the present invention is to provide an application of an MFI type molecular sieve membrane prepared by the above-mentioned controllable preparation method for the separation of small-sized molecules from large-sized molecules, including but not limited to the separation of hydrogen and isobutane, methane and isobutane, and n-butane and isobutane.

[0021] Among them, the sizes of n-butane and isobutane molecules are different. The size of n-butane molecule is 0.47nm, and the size of isobutane molecule is 0.53nm. The difference in their molecular sizes is large, so they can be effectively separated.

[0022] (III) Beneficial effects

[0023] Compared with the prior art, the preparation method of the present invention has the following beneficial effects: (1) The present invention effectively adjusts the pore size of the MFI molecular sieve membrane by controlling the degree of template removal to meet different separation requirements, thereby making the separation performance of the membrane more excellent and being able to achieve precise separation for specific gas mixtures.

[0024] (2) Compared with the traditional membrane preparation method, the present invention adopts simple and efficient steps to achieve membrane pore size regulation in the process of removing the organic template, thereby reducing the complex requirements for equipment and the difficulty of the process, thereby improving the production efficiency of MFI type molecular sieve membranes, effectively reducing production costs, and being easy to promote.

[0025] (3) The MFI molecular sieve membrane of the present invention can be widely used in the separation of small-sized molecules and large-sized molecules, such as hydrogen and isobutane, methane and isobutane, n-butane and isobutane, etc. It has important industrial application value and helps to improve resource utilization and reduce energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a SEM characterization image of the MFI type molecular sieve seed crystal prepared in Example 1 of the present invention.

[0027] Figure 2 1 is the XRD diagram of the MFI type molecular sieve seed crystal and the MFI type molecular sieve membrane prepared in Example 1 and Example 2 of the present invention.

[0028] Figure 3 This is a SEM characterization image of the seeded support prepared in Example 1 of the present invention.

[0029] Figure 4 This is a SEM characterization image of the MFI type molecular sieve membrane prepared in Example 1 of the present invention.

[0030] Figure 5 This is a SEM characterization image of the seeded support prepared in Example 2 of the present invention.

[0031] Figure 6 This is a SEM characterization image of the MFI type molecular sieve membrane prepared in Example 2 of the present invention.

[0032] Figure 7 The ATR-FTIR spectra of MFI molecular sieve and tetrapropylammonium hydroxide under different calcination conditions corresponding to the embodiments of the present invention.

[0033] Figure 8 This is a chart showing the separation results of the MFI type molecular sieve membrane prepared in the present invention for small-sized molecules and large-sized molecules. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0035] Example 1 The controllable preparation method of MFI type molecular sieve membrane, the specific steps are as follows: Step S1, preparing a seed reaction gel: adding silica sol, tetrapropylammonium hydroxide, sodium hydroxide and water in a molar ratio of SiO2:SDA:NaOH:H2O=10:2.4:0.87:119 into a stirring tank and mixing evenly, and continuously stirring and aging the solution at room temperature for 12 hours to obtain a first reaction gel; Step S2, preparing MFI molecular sieve seed crystals: pouring the first reaction gel prepared in step S1 into a reaction kettle, and subjecting the reaction product to hydrothermal synthesis at 100° C. for 72 hours, and after the reaction is completed, centrifuging and washing the reaction product until the pH is neutral, and then placing the product under vacuum freeze drying to obtain highly dispersed MFI molecular sieve seed crystals; Step S3, preparing a seeded support: adding the MFI molecular sieve seed crystals prepared in step S2 to ethanol or an aqueous solution for ultrasonic dispersion to prepare a seed solution with a mass concentration of 0.6%; coating a seed layer on a porous support (a tubular support structure made of an alumina material) by a vacuum coating method, wherein the coating pressure is negative 5 kPa, the coating time is 20 s, and then drying at 80° C. for 2 h to form a seed layer, thereby preparing a seeded support; Step S4, preliminary preparation of MFI type molecular sieve membrane: tetraethyl silicate, tetrapropylammonium hydroxide and water are added into a stirring tank at a molar ratio of 25:3:1450 and mixed evenly, and the solution is continuously stirred at room temperature for 8 hours to obtain a second reaction gel, and the second reaction gel and the seed support prepared in step S3 are placed in a reactor and synthesized at 90°C for 70 hours. After the reaction is completed, the reactor is cooled to room temperature, and the membrane is taken out, cleaned 4 times, and dried at 50°C for 4 hours; Step S5, regulating the pore size of the MFI type zeolite membrane: placing the MFI type zeolite membrane preliminarily prepared in step S4 in a flowing gas atmosphere furnace, the calcination temperature is 200°C, the calcination time is 36h, the ozone concentration is 80mg / L, and the organic template in the membrane layer is partially removed to obtain an MFI type zeolite membrane M1 with high separation performance.

[0036] The SEM results of the prepared MFI molecular sieve seed crystals are shown in the attached figure. Figure 1 As shown in FIG. 1 , the MFI molecular sieve seed crystal has a spherical shape and a diameter of 40-100 nm; the XRD results of the seed crystal are shown in FIG. Figure 2 As shown in the figure, the prepared MFI type molecular sieve seed crystal is pure phase MFI; the surface and cross-section SEM images of the prepared seeded support are shown in the attached Figure 3 As shown, it can be seen that the seed layer has a high stacking density and a small porosity between the seeds. The seed layer is very dense and has a thickness of 1.57µm.

[0037] The surface and cross-sectional SEM images of the final membrane product, MFI molecular sieve membrane M1, prepared in Example 1 are shown in the attached figure. Figure 4 As shown in the figure, the surface crystals of the membrane intergrow with each other without macroporous defects, the membrane layer is relatively dense and the thickness is about 1.98μm (increased by 0.41um); the XRD spectrum of the MFI molecular sieve membrane M1 is shown in the attached Figure 2 As shown in the figure, the diffraction peaks of the prepared MFI molecular sieve membrane M1 are consistent with the standard peaks, indicating that the prepared membrane is a pure phase MFI molecular sieve membrane; the prepared MFI molecular sieve membrane M1 is used to separate equimolar n-butane / isobutane mixed gas and other systems at 60°C, and the separation experimental results are shown in the attached figure. Figure 8 shown.

[0038] Example 2 Compared with Example 1, the preparation conditions in Example 2 of the present invention are different in that: In step S3, a seed layer is coated on the porous support by vacuum coating, wherein the coating pressure is minus 15 kPa and the coating time is 10 s, to obtain a seeded support; In step S4, the synthesis temperature and time of the MFI type molecular sieve membrane are 80° C. and 80 h, respectively, to obtain the MFI type molecular sieve membrane M2.

[0039] In Example 2 of the present invention, the surface and cross-sectional SEM images of the prepared seeded support are shown in the attached Figure 5 As shown in the figure, it can be seen that the seed layer has a high stacking density, the porosity between the seeds is small, the seed layer is dense and the thickness is 0.86µm. The surface and cross-section SEM images of the MFI molecular sieve membrane M2 are shown in the attached figure. Figure 6 As shown in the figure, the surface of the membrane is very smooth, the membrane crystals are interlaced with each other without macroporous defects, the membrane layer is dense and the thickness is about 1.54μm; the XRD spectrum of the MFI molecular sieve membrane M2 is shown in the attached figure. Figure 2 The membrane M1 prepared at 60 ° C was used to separate the equimolar n-butane / isobutane mixed gas and other systems. The experimental results are shown in the attached Figure 8 shown.

[0040] Example 3 Compared with Example 1, the preparation conditions in Example 3 of the present invention are different in that: The ozone calcination time in step S5 is 24 hours, and the MFI type molecular sieve membrane M3 is obtained.

[0041] The prepared membrane M3 has a thickness of about 1.98 µm. The membrane can separate an equimolar mixture of n-butane and isobutane (50 / 50) from other systems at 60 °C. The experimental results are shown in the attached figure. Figure 8 shown.

[0042] Example 4 Compared with Example 1, the preparation conditions in Example 4 of the present invention are different in that: The ozone calcination time in step S5 is 48 hours, and the MFI type molecular sieve membrane M4 is obtained.

[0043] The prepared membrane M4 has a thickness of about 1.98 µm. The membrane can separate an equimolar mixture of n-butane and isobutane (50 / 50) from other systems at 60 °C. The experimental results are shown in the attached figure. Figure 8 shown.

[0044] Example 5 Compared with Example 1, the preparation conditions in Example 5 of the present invention are different in that: The ozone calcination time in step S5 is 60 hours, in order to completely remove the organic template in the membrane pores and obtain the MFI type molecular sieve membrane M5.

[0045] The prepared membrane M5 has a thickness of about 1.98 µm. The membrane can separate an equimolar mixture of n-butane and isobutane (50 / 50) from other systems at 60 °C. The experimental results are shown in the attached figure. Figure 8 shown.

[0046] Example 6 Compared with Example 1, the preparation conditions in Example 6 of the present invention are different in that: In step S1, the seed reaction gel is prepared by mixing silica sol, tetrapropylammonium hydroxide, sodium hydroxide and water in a molar ratio of SiO2:SDA:NaOH:H2O=10:2.4:0.44:119 to obtain a first reaction gel; and an MFI type molecular sieve membrane M6 is obtained.

[0047] In this Example 6, the diameter of the prepared MFI molecular sieve seed crystals was 180 nm; the seed layer thickness of the prepared seeded support was 2.65 μm; the prepared membrane layer thickness was about 3 μm (increased by 0.35 μm); the prepared membrane M6 was used to separate an equimolar n-butane / isobutane mixed gas and other systems at 60°C, and the separation experimental results are shown in the attached figure. Figure 8 shown.

[0048] Example 7 Compared with Example 1, the preparation conditions in Example 7 of the present invention are different in that: In step S1, the seed reaction gel is prepared by mixing silica sol, tetrapropylammonium hydroxide, sodium hydroxide and water in a molar ratio of SiO2:SDA:NaOH:H2O=10:2.4:1.3:119 to obtain a first reaction gel; and an MFI type molecular sieve membrane M7 is obtained.

[0049] In this Example 7, the diameter of the prepared MFI molecular sieve seed crystal is 30nm; the seed layer thickness of the prepared seeded support is 1.4µm; the prepared membrane layer thickness is about 2.3µm (increased by 0.9um); the prepared membrane M7 is used to separate equimolar n-butane / isobutane mixed gas and other systems at 60°C, and the separation experimental results are shown in the attached figure. Figure 8 shown.

[0050] Example 8 Compared with Example 1, the preparation conditions in Example 8 of the present invention are different in that: In step S4, the synthesis temperature of the membrane is 75° C. for 85 h; an MFI type molecular sieve membrane M8 is obtained.

[0051] The thickness of the prepared MFI molecular sieve membrane is about 1.8µm. The prepared membrane M8 can separate equimolar n-butane / isobutane mixed gas and other systems at 60°C. The separation experimental results are shown in the attached figure. Figure 8 shown.

[0052] Example 9 Compared with Example 1, the difference of Example 9 of the present invention is that: In step S4, the synthesis temperature of the membrane is 140° C. for 24 hours; and an MFI type molecular sieve membrane M9 is obtained.

[0053] The thickness of the prepared MFI molecular sieve membrane is about 2.2µm. The prepared membrane M9 can separate equimolar n-butane / isobutane mixed gas and other systems at 60℃. The separation experimental results are shown in the attached figure. Figure 8 shown.

[0054] Example 10 Compared with Example 1, the preparation conditions in Example 10 of the present invention are different in that: In step S3, the concentration of the seed solution is 1.03%, and finally the MFI type molecular sieve membrane M10 is obtained.

[0055] The thickness of the prepared MFI molecular sieve membrane is about 4.5µm. The prepared membrane M8 can separate equimolar n-butane / isobutane mixed gas and other systems at 60°C. The separation experimental results are shown in the attached figure. Figure 8 shown.

[0056] Embodiment 11 Compared with Example 1, the preparation conditions in Example 10 of the present invention are different in that: In step S4, the concentration of the seed solution is 2%, and finally an MFI type molecular sieve membrane M11 is obtained.

[0057] The thickness of the prepared MFI molecular sieve membrane is about 6.3µm. The prepared membrane M8 can separate equimolar n-butane / isobutane mixed gas and other systems at 60°C. The separation experimental results are shown in the attached figure. Figure 8 shown.

[0058] Example 12 Compared with Example 1, the preparation conditions in Example 12 of the present invention are different in that: In step S4, the molar ratio of tetraethyl silicate, tetrapropylammonium hydroxide and water is 25:2:1000, and finally an MFI type molecular sieve membrane M12 is prepared.

[0059] The thickness of the prepared MFI molecular sieve membrane is about 1.85µm. The prepared membrane M8 can separate equimolar n-butane / isobutane mixed gas and other systems at 60°C. The separation experimental results are shown in the attached figure. Figure 8 shown.

[0060] Example 13 Compared with Example 1, the preparation conditions in Example 13 of the present invention are different in that: In step S4, the molar ratio of tetraethyl silicate, tetrapropylammonium hydroxide and water is 25:8:1500, and finally an MFI type molecular sieve membrane M13 is prepared.

[0061] The thickness of the prepared MFI molecular sieve membrane is about 3.9µm. The prepared membrane M8 can separate equimolar n-butane / isobutane mixed gas and other systems at 60℃. The separation experimental results are shown in the attached figure. Figure 8 shown.

[0062] Comparative Example 1 Compared with Example 1, the preparation conditions in Comparative Example 1 are different in that: The ozone calcination time in step S5 is 72 hours, that is, to completely remove the organic template in the membrane pores to obtain the MFI type molecular sieve membrane B1.

[0063] The prepared membrane B1 was used to separate an equimolar mixture of n-butane / isobutane (50 / 50) and other systems at 60°C. The experimental results are shown in the attached figure. Figure 8 shown.

[0064] Comparative Example 2 Compared with Example 1, the preparation conditions in Comparative Example 2 are different in that: In step S1, the seed reaction gel is prepared by mixing silica sol, sodium tetrapropylammonium hydroxide and water in a molar ratio of SiO2:SDA:H2O=10:2.4:119 to obtain a first reaction gel, thereby obtaining an MFI type molecular sieve membrane B2.

[0065] The diameter of the MFI molecular sieve seed prepared in Comparative Example 2 is 300nm; the seed layer thickness of the prepared seeded support is 3.9µm; the prepared membrane layer thickness is about 4.15µm (increased by 0.25um); the prepared MFI molecular sieve membrane B2 exhibits a high permeation rate for separation of equimolar n-butane / isobutane mixed gas and other systems at 60°C, but exhibits Knudsen selectivity, indicating that the membrane layer has many defects and no separation performance. The separation test results are shown in the attached Figure 8 shown.

[0066] It should be pointed out that we cannot list all the embodiments, and the specific embodiments of the present invention are only for the purpose of illustrative description. Among them, the synthesis temperature and synthesis time for preparing the seed crystals in step S2 are 80°C and 96h, and the synthesis temperature and synthesis time are 120°C and 48h to obtain the MFI molecular sieve seed crystals with the same morphology and size range of 40-100nm, and the separation effect is also similar, which is the optimal hydrothermal synthesis condition in the experimental research process.

[0067] By the attached Figure 7 It can be seen that with the extension of ozone calcination time, the content of organic template in the pores of MFI zeolite molecular sieve gradually decreases. After 60 hours of ozone calcination, the organic template is completely removed. That is, the residual amount of template in the pores of MFI molecular sieve can be precisely controlled by adjusting the calcination conditions.

[0068] By the attached Figure 8 (Examples 1, 6, 7 and Comparative Example 2) It can be seen that the size of the MFI molecular sieve seed crystals decreases with the increase of the alkali concentration, but it also needs to be controlled within an appropriate range. Too low or too high alkali concentration will affect the crystal size, and smaller seed crystals have higher activity and can promote the rapid growth of the membrane layer.

[0069] By the attached Figure 8(Examples 1, 3, 4, 5 and Comparative Example 1) It can be seen that as the ozone calcination time increases, the content of the organic template in the pores of the MFI zeolite molecular sieve gradually decreases, resulting in a gradual increase in the pore size. Initially, the removal of the template makes the size match between the pore size of the membrane and the n-butane separation more ideal. The molecular size of n-butane is smaller and it is easier to pass through the membrane pores, thereby increasing the permeation rate of n-butane and the separation factor of n- / isobutane mixed gas. However, when the calcination time continues to increase, excessive removal of the template causes the membrane pores to expand excessively, destroying the size matching. At this time, although the permeation rate of n-butane continues to increase, due to the expansion of the pore size, the selectivity of the MFI membrane for the molecules of n-butane and isobutane decreases, resulting in a decrease in the separation factor. At the same time, by extending the calcination time in the ozone environment, the essence is to control the ozone to produce more free radicals, thereby removing the organic template SDA. As the ozone concentration increases, the oxidation reaction is accelerated, thereby shortening the active oxidation time of ozone. Therefore, the concentration of ozone has little effect on the performance of the molecular membrane sieve, and is closely related to the calcination time in the ozone environment. Therefore, it is sufficient to control the reasonable concentration of ozone.

[0070] By the attached Figure 8 (Examples 1, 10 and 11) It can be seen that the higher the concentration of the seed solution, the more seed crystals are deposited on the surface of the membrane substrate, forming a higher crystal nucleus density, which helps to promote the growth of membrane layer crystals, making the membrane layer thicker and the n / isobutane separation factor tends to increase.

[0071] By the attached Figure 8 (Examples 1, 2, 7, 8, 9, 10, 11, 12, 13 and Comparative Example 1) It can be seen that although the MFI molecular sieve membranes with different microstructures (different membrane thicknesses), the partial removal of the organic template in the pores can obtain excellent separation performance, and different proportions of the organic template SDA have little effect on the performance of the obtained MFI molecular sieve membrane, so only the optimal organic template SDA ratio is provided in the examples. Figure 8 It can be seen that the MFI molecular sieve membrane prepared by the present invention also has good separation performance for hydrogen / isobutane and methane / isobutane.

[0072] It can be seen from the experimental results that compared with the shortcomings of traditional molecular sieve membrane pore structure control methods, the preparation method of the MFI molecular sieve membrane of the present invention can be precisely controlled, its preparation process is simpler, the permeation analysis is more efficient, and the separation effect of small-sized molecules and large-sized molecules is better.

Claims

1. A controllable preparation method of an MFI type molecular sieve membrane, characterized in that: By controlling the degree of template removal and adjusting the residual amount of organic template in the membrane pores, the membrane pore size can be effectively adjusted to obtain excellent separation performance. The specific steps are as follows: S1. Preparation of seed crystal reaction gel: According to the silicon source: organic template SDA: alkali source (OH -1 Prepare the raw materials in a molar ratio of ): distilled water = 10: (1.6-2.8): (0.44-1.3): (100-150), add them into a stirring tank and mix them well to form a solution, and continue stirring and aging at room temperature for 6-48 hours to obtain a first reaction gel; S2. Preparation of MFI type molecular sieve seed crystals: pouring the obtained first reaction gel into a reaction kettle, hydrothermally synthesizing at 80-120° C. for 48-96 hours, centrifuging the reaction product after the reaction is completed, washing until the pH is neutral, and freeze-drying under vacuum to obtain highly dispersed MFI molecular sieve seed crystals; S3, preparing a seeded support: adding the prepared MFI molecular sieve seed crystals to ethanol or an aqueous solution for ultrasonic dispersion to prepare a seed solution with a concentration of 0.06-2%, coating the seed solution on a porous support, and drying at 30-80° C. for 2 h to form a seed layer, thereby preparing a seeded support; S4, preliminary preparation of MFI type molecular sieve membrane: adding silicon source, organic template SDA and distilled water in a molar ratio of 25: (2-8): (1000-1500) into a stirring tank and mixing evenly to form a solution, stirring continuously at room temperature for 6-24 hours to obtain a second reaction gel, placing the second reaction gel and the seed support prepared in step S3 into a reaction kettle, synthesizing at 75-140° C. for 24-85 hours, cooling to room temperature after the reaction to obtain an MFI type molecular sieve membrane, taking out the membrane, washing and drying; S5. Regulating the pore size of the MFI type molecular sieve membrane: placing the MFI type molecular sieve membrane in step S4 in a flowing gas atmosphere furnace for calcination to remove the organic template in the membrane layer, thereby obtaining an MFI type molecular sieve membrane with high separation performance.

2. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: The silicon source in step S1 and step S4 is one of, but not limited to, tetraethyl silicate, ammonium fluorosilicate, silica sol and fumed silica.

3. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: The organic template agent SDA in step S1 and step S4 is one including but not limited to tetrapropylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium bromide.

4. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: The alkali source in step S1 includes but is not limited to one of sodium hydroxide and potassium hydroxide.

5. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: The porous support in step S3 is a ceramic support made of one of silicon carbide, aluminum oxide, and mullite, and is in the shape of a tube, a sheet, or a disc.

6. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: The seed coating method in step S3 includes but is not limited to one of a dip coating method and a vacuum coating method; wherein the vacuum coating method has a coating pressure of 5-20 kPa negative pressure and a coating time of 10-60 s.

7. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: In the step S4, the membrane is taken out and washed 3-5 times, and dried at 50-100° C. for 2-4 hours.

8. The controllable preparation method of the MFI type molecular sieve membrane according to claim 1, characterized in that: In step S5, the atmosphere of the atmosphere furnace is a mixed gas containing ozone, the ozone concentration is 60-100 mg / L, the calcination temperature is 30-250° C., and the calcination time is 6-60 hours.

9. An MFI type molecular sieve membrane, characterized in that: The invention is prepared by the controllable preparation method described in any one of claims 1 to 9.

10. Use of the MFI type molecular sieve membrane prepared by the controllable preparation method according to any one of claims 1 to 8, characterized in that: Used for the separation of small molecules from large molecules, including but not limited to the separation of hydrogen and isobutane, methane and isobutane, and n-butane and isobutane.

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