CN120242777A

CN120242777A - A method for preparing oriented MFI molecular sieve membrane using a mixed template and its application

Info

Publication number: CN120242777A
Application number: CN202510467158.8A
Authority: CN
Inventors: 刘毅; 丁凯诗; 刘益
Original assignee: Dalian University of Technology
Current assignee: Dalian University of Technology
Priority date: 2025-04-15
Filing date: 2025-04-15
Publication date: 2025-07-04

Abstract

The present invention provides a method and application of preparing an oriented MFI molecular sieve membrane by a mixed template. The preparation method includes: coating a dense and continuous MFI seed layer on the surface of a porous carrier; dissolving a silicon source and a mixed organic template agent in deionized water, stirring and aging to obtain a synthetic mother liquor; placing the carrier coated with the seed layer in the synthetic mother liquor for hydrothermal reaction; after the reaction is completed, washing, drying and roasting are performed to obtain a (h0h)-oriented MFI molecular sieve membrane. The MFI molecular sieve membrane prepared by the present invention has a high degree of orientation, good intergrowth, a small intercrystalline defect density, a controllable film thickness, and can efficiently separate important industrial isomers such as normal/isobutane and ortho/paraxylene mixtures. This type of membrane material is hydrothermally synthesized, has a simple process, is conducive to reducing the preparation cost of the membrane, and can be used for both sheet carriers and tubular carriers and multi-channel carriers while ensuring high separation performance, and is expected to achieve large-scale preparation of oriented MFI molecular sieve membranes, which is convenient for application and promotion.

Description

Method for preparing oriented MFI molecular sieve membrane by using mixed template and application

Technical Field

The invention relates to the technical field of molecular sieve membrane preparation, in particular to a method for preparing an oriented MFI molecular sieve membrane by using a mixed template and application thereof.

Background

In petrochemical industry, separation of hydrocarbons with the same carbon number (such as C4 alkane or C5 alkene isomer) is a key link of light hydrocarbon processing and clean fuel production. The physical and chemical properties of the substances such as molecular size, polarity and the like are very close, and separation and purification are very difficult. Traditional cryogenic rectification technology has high energy consumption and high separation cost, and membrane separation technology is becoming an important way to solve the problem of high-efficiency separation of hydrocarbon mixtures with the same carbon number by virtue of the unique separation mechanism and material characteristics.

The MFI type zeolite molecular sieve membrane becomes an ideal material for realizing high flux-high selectivity cooperative optimization in the field of industrial separation due to an orientation structure capable of being accurately regulated and controlled. Among various zeolite membranes, the research system of the MFI membrane is most perfect, and the performance advantage of the MFI membrane is derived from a unique anisotropic pore structure, namely, a sinusoidal pore in the a-axis directionStraight bore with b-axis directionThree-dimensional interweaving. Through directional assembly of seed crystal and dynamic regulation of synthesis, film layers with different dominant orientations can be prepared. Numerous studies have shown that the straight pore channels of the b-axis oriented MFI zeolite membrane are perpendicular to the support surface, the diffusion path is short, and excellent performance is exhibited in xylene isomer (PX/OX) separation. (h 0 h) -and a-axis The orientation belongs to a sinusoidal pore canal, the pore window size is favorable for butane isomer separation, but an included angle exists between the pore canal of the membrane layer in the a-axis orientation and the surface of the membrane layer, and the pore canal of the membrane layer in the (h 0 h) -orientation is perpendicular to the pore opening direction of the surface of the membrane layer, so that the mass transfer resistance is relatively small, and the MFI zeolite membrane in the (h 0 h) -orientation is more suitable for n/i-C ₄H₁₀ separation.

At present, (h 0 h) -oriented MFI molecular sieve membranes are synthesized by taking TPA ⁺ (tetrapropylammonium ion) as a single organic template agent, and orientation regulation is carried out in the secondary growth process. However, TPA ⁺ preferentially adsorbs to the (010) face of the MFI crystal, promoting the b-axis oriented growth of the film.

Disclosure of Invention

In order to solve the technical problems, the invention provides a method for preparing an oriented MFI molecular sieve membrane by using a mixed template and application thereof, which prepares secondary growth solution by adding quaternary ammonium salt cations with longer alkyl chains as an organic template agent on the basis of tetrapropylammonium ions, promotes the membrane layer to preferentially grow towards (h 0 h) -orientation, improves the compactness of the membrane layer and explores the application thereof in industrial separation. The prepared MFI molecular sieve membrane is continuous and compact, and has excellent heat stability, chemical stability and high orientation. The method has higher separation performance on the mixture of n-isobutane, o-xylene and p-xylene and hydrogen/ammonia, thereby providing good large-scale preparation prospect for preparing the high-performance MFI molecular sieve membrane.

The invention is realized by the following technical scheme:

a method for preparing an oriented MFI molecular sieve membrane by using a mixed template, comprising the following steps:

s1, uniformly coating MFI seed crystals on the surface of a porous carrier to form a compact continuous seed crystal layer, and roasting after drying to solidify the seed crystal layer;

S2, dissolving a mixed organic template agent and a silicon source in deionized water, and stirring and aging to obtain a synthetic mother solution, wherein the mixed organic template agent comprises a tetrapropylammonium cationic compound and a quaternary ammonium salt cationic compound with a longer alkyl chain;

s3, carrying out hydrothermal reaction on the carrier coated with the seed crystal layer obtained in the step S1 and the synthetic mother liquor obtained in the step S2 at 100-200 ℃;

s4, washing, drying and roasting the membrane material obtained in the step S3 to remove the organic template agent, thereby obtaining the (h 0 h) -oriented MFI molecular sieve membrane.

And step S1, the seed crystal is an MFI molecular sieve with the particle size of 50 nm-1 mu m. Further, the seed crystal is an MFI molecular sieve with the particle size of 50-150 nm.

The MFI seed coating method in step S1 is spin coating, dip coating, drop coating, wiping, spray coating or vacuum crystal coating.

And step S1, the roasting temperature of the seed crystal layer is 100-800 ℃, and the roasting time is 10 min-100 h.

The carrier in the step S1 is in a shape of a single-channel tube, a multi-channel tube, a flat plate or a hollow fiber tube, and the carrier is made of ceramics, stainless steel, alumina, titanium dioxide, zirconium dioxide, silicon carbide or silicon nitride, and has a pore diameter of 2-2000 nm.

The quaternary ammonium salt cationic compound of longer alkyl chain of step S2 includes one or more of tetrabutylammonium bromide, tetrabutylammonium hydroxide, tetrabutylammonium chloride, tetrapentylammonium hydroxide, tetrapentylammonium bromide, tetrapentylammonium iodide, methyltributylammonium hydroxide, and ethyltrimethylammonium bromide.

The tetrapropylammonium cationic compound described in step S2 comprises tetrapropylammonium hydroxide, tetrapropylammonium chloride, tetrapropylammonium bromide, tetrapropylammonium sulfate or tetrapropylammonium nitrate.

In the step S2, the mole ratio of the tetrapropylammonium cationic compound, the quaternary ammonium salt cationic compound with a longer alkyl chain, deionized water and a silicon source is (0.05-0.08): (0.03-0.05): (20-400): 1, wherein the silicon source is calculated by SiO ₂.

The silicon source in step S2 includes one or more of methyl orthosilicate, ethyl orthosilicate, water glass, silica sol and silica aerogel.

In the step S2, the aging time is 10 min-100 h.

And in the step S2, the synthetic mother liquor further comprises an aluminum source, wherein the molar ratio of the aluminum source to the silicon source is 0.001-0.5, the silicon source is calculated by SiO ₂, and the aluminum source is calculated by Al ₂O₃.

The aluminum source comprises metallic aluminum, aluminum hydroxide, sodium metaaluminate, aluminum sulfate, aluminum chloride, aluminum isopropoxide or aluminum nitrate.

And in the step S3, the hydrothermal reaction time is 1 h-20 days.

And step S4, the roasting temperature is 100-700 ℃ and the roasting time is 0.2-50 h. Further, the roasting mode is muffle furnace roasting, ozone atmosphere tube furnace roasting or rapid thermal process roasting. Furthermore, the roasting mode is tubular furnace roasting in ozone atmosphere, and the roasting temperature is 150-300 ℃, so that the required roasting temperature is lower, the time is shorter, and meanwhile, the integrity of the membrane is ensured to the greatest extent.

The thickness of the MFI molecular sieve membrane is 0.5-30 mu m.

The invention also provides application of the MFI molecular sieve membrane obtained by the method in the aspect of isomer separation.

The isomer comprises n-isobutane mixed gas, o-xylene mixed liquid or p-xylene mixed liquid or hydrogen gas/ammonia gas mixed gas.

The invention has the beneficial effects that the high-activity synthetic mother liquor is prepared by using the tetrapropylammonium cationic compound and the quaternary ammonium salt cationic compound with longer alkyl chain as the mixed template agent, and the high (h 0 h) -oriented MFI molecular sieve membrane with better cohesiveness is prepared under the hydrothermal reaction. The quaternary ammonium salt cation with longer alkyl chain is used to replace TPA ⁺ to induce formation of larger secondary unit of silicon oxygen, which facilitates the formation of TPA ⁺ and accelerates the nucleation and growth of MFI seed crystal. In addition, quaternary ammonium cations with longer alkyl chains preferentially adsorb at the (100) crystal plane, which competes with the (010) crystal plane adsorption of TPA ⁺, inhibits the growth of the b-axis orientation, and favors the growth of the (h 0 h) -orientation. Meanwhile, the quaternary ammonium salt cations with longer alkyl chains are used for replacing a small amount of TPA ⁺, the thermal decomposition temperature of the quaternary ammonium salt cations with longer carbon chains is far lower than that of TPA ⁺, and the gas is released in a gradient way in the process of removing the template agent through the complementary molecular characteristics and thermal decomposition behaviors, so that the volume shrinkage stress is reduced, the grain boundary crack density is reduced, the occurrence of the inter-crystal defect problem of the MFI zeolite membrane is reduced, and the separation performance and the industrial applicability are synchronously improved. The membrane material prepared by the invention has higher separation performance on the mixture of n-butane gas/isobutane gas (n-butane permeation flux is 6.8X10 ^-8mol·m^-2·s^-1·Pa^-1 and separation factor is 52), the mixture of o-xylene/p-xylene liquid (p-xylene flow rate is 0.8kg.m ^-2·h^-1 and separation factor is 89) and the mixture of hydrogen gas/ammonia gas (ammonia permeation flux is 1.5X10 ^-7mol·m^-2·s^-1·Pa^-1 and separation factor is 135), and has better industrial application prospect.

Drawings

Fig. 1 is (a) SEM image and (b) XRD image of MFI seed crystal prepared in example 1.

Fig. 2 is an SEM image of the MFI seed layer prepared in example 1.

FIG. 3 is an SEM image of the (a) plane and (b) cross-section of the M1 film prepared in example 1.

Fig. 4 is an XRD pattern of the M1 film prepared in example 1.

Fig. 5 is an SEM image of (a) plane and (b) cross-section of the M2 film prepared in example 2.

Fig. 6 is an XRD pattern of the M2 film prepared in example 2.

Fig. 7 is a plan SEM image of the M3 film prepared in example 3.

Fig. 8 is a plan SEM image of the M4 film prepared in example 4.

Fig. 9 is a plan SEM image of the M5 film prepared in example 5.

Fig. 10 is a plan SEM image of the M6 film prepared in example 6.

Fig. 11 is a plan SEM image of the M7 film prepared in example 7.

FIG. 12 is a graph of the long term stability of the separation performance of n-butane/isobutane for the preparation of M1 membranes from example 1.

FIG. 13 is a graph of M1 membrane prepared in example 1 as a function of feed composition for n-butane/isobutane separation.

Fig. 14 is a plan SEM image of the M8 film prepared in comparative example 1.

Fig. 15 is a plan SEM image of the M9 film prepared in comparative example 2.

FIG. 16 is a plan SEM image of the preparation of M10 films of comparative example 3.

Fig. 17 is (a) a planar SEM and (b) XRD pattern of the M11 film prepared in comparative example 4.

Detailed Description

The present invention will be described in further detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the present invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications could be made by those skilled in the art without departing from the inventive concept. These are all within the scope of the present invention.

In the application, the gas separation test conditions are room temperature, and unless otherwise stated, the feed ratio of the mixed gas is 1:1, the permeation side is normal pressure, the transmembrane pressure difference is 1bar, and helium is used as a purge gas. In the application, the pervaporation test temperature is 75 ℃, the molar ratio of ortho-xylene to para-xylene is 1:1, and the transmembrane pressure difference is 1bar.

Example 1

(1) Preparing MFI seed crystal, namely weighing and mixing raw materials according to the mole ratio of tetrapropylammonium hydroxide to tetraethoxysilane to deionized water of 0.36:1:14 to prepare seed crystal synthetic liquid, stirring and ageing the synthetic liquid at room temperature for 12 hours, filtering, transferring the obtained clear and transparent solution into a reaction kettle, crystallizing at 100 ℃ for 72 hours, cooling and taking out, pouring out supernatant, adding equal amount of deionized water, centrifuging to obtain precipitate, and drying at 60 ℃ for 12 hours to obtain the MFI seed crystal;

(2) And coating and solidifying a seed crystal layer, and dispersing the prepared seed crystal in deionized water to prepare a seed crystal liquid with the mass concentration of 0.5 wt.%. The MFI seed crystals were then uniformly coated on the surface of a tubular porous alumina support by a dip coating process in which the support was dipped in a seed dispersion for 30 seconds each time. After the dip-coating is completed, the carrier is placed in a 70 ℃ oven for drying for 12 hours, and then placed in a muffle furnace for roasting for 6 hours at 500 ℃ so as to improve the binding force between the seed crystal and the carrier and solidify the seed crystal layer while removing the organic template agent in the seed crystal.

(3) The synthesis mother liquor is prepared by slowly dripping tetraethyl orthosilicate (TEOS) into a mixed solution of template A tetrapropylammonium hydroxide solution (25 wt.%), template B tetrabutylammonium hydroxide solution (25 wt.%) and deionized water, and stirring and aging for 6h at room temperature. The molar composition of the above synthesis mother liquor was 1TEOS:0.07TPAOH:0.03TBAOH:130H ₂ O.

(4) And (3) preparing the MFI molecular sieve membrane by a secondary growth method, namely placing the carrier tube coated with the seed crystal layer obtained in the step (1) into a synthesis mother solution, sealing, placing into an oven, and reacting for 20 hours at 160 ℃. After the reaction is finished, the membrane tube is taken out, washed to be neutral by deionized water, dried overnight and then put into a tube furnace with ozone atmosphere for roasting at 250 ℃ for 6 DEG C

H, removing the template agent, wherein the temperature rising and reducing rates are 1 ℃ per minute, and the MFI molecular sieve membrane prepared by the process is recorded as M1.

The scanning electron microscope characterization of the MFI seed crystal is shown in figure 1a, the molecular sieve has a regular hexagonal prism shape, and the particle size is uniformly distributed at about 100 nm. XRD results (FIG. 1 b) show that the molecular sieve is a pure phase MFI crystal form, high in crystallinity and free of other impurity peaks. Scanning electron microscope characterization of the prepared seed layer is shown in fig. 2, and the seed crystals are closely arranged to form a continuous and compact seed layer. As can be seen from FIG. 3a, after secondary growth, the MFI molecular sieve membrane layer prepared is uniform, compact and continuous in surface, and the cross-sectional surface Scanning Electron Microscope (SEM) characterization is shown as FIG. 3b, and the thickness of the MFI molecular sieve membrane is 4.8 μm. XRD spectrum (figure 4) shows that the prepared membrane material is of a pure-phase MFI structure. The relative peak intensities representing 101 and 002 orientations reflect the orientation of the MFI membrane layer crystallites, with only (101) diffraction peaks present for the membrane M1, indicating that the MFI molecular sieve membrane is highly (h 0 h) -axis oriented. The membrane M1 was subjected to a nitrogen gas permeation test and a sulfur hexafluoride gas permeation test (normal temperature and normal pressure), respectively, and the results showed that the permeation flux of nitrogen gas was 4.8X10 ^-9mol·m^-2·s^-1·Pa^-1, the permeation flux of sulfur hexafluoride gas was 3.5X10 ^-7mol·m^-2·s^-1·Pa^-1, and the ideal selectivity of the membrane M1 to sulfur hexafluoride/nitrogen gas was 73, which indicates that the membrane layer had good compactness and almost no defects. This suggests that the addition of TBA ⁺ accelerates the induction of the growth of the film layer toward the (h 0 h) -axis orientation and suppresses the generation of defects during film growth. Normal temperature and normal pressure testing of the normal/isobutane mixed component (1:1) of the membrane M1 shows that the permeation flux of normal butane can reach 6.8X10 ^-8mol·m^-2·s^-1·Pa^-1, and the separation factor is 52. The membrane M1 was subjected to an o/p xylene mixed component (1:1) pervaporation test, and the result showed that the p-xylene flow rate of the membrane M1 was 800 g.m ^-2·h^-1, and the separation factor was 89. The membrane M1 is tested at normal temperature and normal pressure by hydrogen/ammonia mixed components (1:1), and the result shows that the permeation flux of ammonia can reach 1.5 multiplied by 10 ^-7mol·m^-2·s^-1·Pa^-1, and the separation factor is 135.

Example 2

The difference from example 1 is that the molar ratio of tetrapropylammonium hydroxide solution, tetrabutylammonium hydroxide solution to silicon source in step 3 is 0.05/0.05/1, and the other steps are the same as in example 1, and the MFI molecular sieve membrane prepared by the process is designated as M2.

The scanning electron microscope characterization of the MFI seed crystal is shown in fig. 5a, the crystal grain on the surface of the film layer is obviously enlarged, the surface of the film layer grows well, and the surface almost has no twin crystal generation. The cross-sectional scanning electron microscope characterization is shown in FIG. 5b, the film layer has better intergrowth and increased thickness of about 11.6 μm. The corresponding XRD results show (fig. 6) that there are (101) diffraction peaks and (002) diffraction peaks over the whole XRD diffraction angle range, the (101) oriented peak intensity is much greater than (002), indicating that the film layer has a higher (h 0 h) orientation advantage. Normal temperature and normal pressure testing of the n-butane mixed component (1:1) of the membrane M2 shows that the permeation flux of n-butane can reach 3.1 multiplied by 10 ^- ⁸mol·m^-2·s^-1·Pa^-1, and the separation factor is 37. The membrane M2 was subjected to an o/p xylene mixed component (1:1) pervaporation test, which revealed that the membrane M2 had a p-xylene flow rate of 420 g.m ^-2·h^-1 and a separation factor of 53.

Example 3

The difference from example 1 is that in step 3, template B was selected from a tetrapentylammonium hydroxide solution having a tetrapentylammonium hydroxide to tetrapropylammonium hydroxide molar ratio of 0.03/0.07. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane prepared by this process was designated M3. The scanning electron microscope characterization of M3 is shown in FIG. 7, the grains on the surface of the film layer become large, and the surface of the film layer grows well. Normal temperature and normal pressure testing of the n-butane mixed component (1:1) of the membrane M3 shows that the permeation flux of n-butane can reach 4.3X10 ^-8mol·m^-2·s^-1·Pa^-1, and the separation factor is 45.

Example 4

The difference from example 1 is that the secondary growth time in step 4 is 15 hours, and the other steps are the same as in example 1, and the MFI molecular sieve membrane prepared by this process is designated as M4. The scanning electron microscope characterization of M4 is shown in FIG. 8, the crystal grains on the surface of the film layer are obviously reduced, and the film layer is continuous and compact and has better intergrowth. Normal temperature and normal pressure testing of the n-butane/isobutane mixed component (1:1) of the membrane M4 shows that the permeation flux of n-butane can reach 1.2 multiplied by 10 ^-7mol·m^-2·s^-1·Pa^-1, and the separation factor is 30.

Example 5

The difference from example 1 is that the molar ratio of deionized water to silicon source in step 3 is H ₂O/SiO₂ =100, and the rest of the procedure is the same as in example 1, and the MFI molecular sieve membrane prepared by this procedure is denoted as M5. The scanning electron microscope characterization of M5 is shown in FIG. 9, the crystal grains on the surface of the film layer are obviously reduced, and the film layer still keeps good cohesiveness. Normal temperature and normal pressure testing of the n-butane mixed component (1:1) of the membrane M5 shows that the permeation flux of n-butane can reach 8.4X10 ^-8mol·m^-2·s^-1·Pa^-1, and the separation factor is 44.

Example 6

The difference from example 1 is that the temperature of the secondary growth in step 4 is 160℃and the time is 20 hours, and the other steps are the same as those in example 1, and the MFI molecular sieve membrane prepared by this process is designated as M6. Scanning electron microscope characterization of M6 is shown in FIG. 10, the film surface has realized intergrowth on the basis of the seed crystal layer, and the crystal gap has been completely closed. Normal temperature and normal pressure testing of the n-isobutane mixed component (1:1) of the membrane M6 shows that the permeation flux of n-butane can reach 5.5 multiplied by 10 ^-8mol·m^-2·s^-1·Pa^-1, and the separation factor is 41.

Example 7

The difference from example 1 is that aluminum chloride is additionally added as an aluminum source in the preparation process of the synthetic mother liquor in the step 3, the molar ratio of aluminum chloride to tetraethoxysilane is Al ₂O₃/SiO₂ =0.02, and the other steps are the same as those in example 1, and the MFI molecular sieve membrane prepared in the process is denoted as M7. The scanning electron microscope characterization of M7 is shown in FIG. 11, the film surface is better in cohesiveness, and the MFI grain growth size is smaller. Normal temperature and normal pressure testing of the n-butane mixed component (1:1) of the membrane M7 shows that the permeation flux of n-butane can reach 1.5 multiplied by 10 ^-7mol·m^-2·s^-1·Pa^-1, and the separation factor is 34.

Example 8 gas separation stability test of the MFI membrane thus prepared

The MFI membrane layer prepared in example 1 was subjected to a n/isobutane gas separation stability test to test its long-term stability. The test conditions were as follows, the test temperature was room temperature, the feed ratio of the mixture was 1:1, the permeation side was normal pressure, and the transmembrane pressure difference was 1bar. The film layer was continuously treated at room temperature for 24 hours to examine its stability (fig. 12). The result shows that the performance of the MFI membrane is always stable and has no obvious change, which indicates that the membrane has better long-term stability and is beneficial to industrial application.

Example 9 separation Performance test of the MFI membrane thus prepared as a function of the feed composition

The MFI membrane layer prepared in example 1 was subjected to a separation performance test according to the composition of n/isobutane feed. The test conditions are that the test temperature is room temperature, the feed ratio of the mixed gas n-butane to the isobutane is 1:9-9:1, the permeation side is normal pressure, the transmembrane pressure difference is 1bar, and the mixed gas n-butane to the isobutane is kept for 6 hours under each feed composition until the membrane layer is stable. The film layers were continuously treated at room temperature to investigate the effect of feed composition changes on separation performance (fig. 13). The results show that as the feed ratio increases, both n-butane and isobutane increase, but the separation selectivity tends to increase and then decrease. The results show that the permeation flux of n-butane can reach 7.5X10 ^-8mol·m^-2·s^-1·Pa^-1 and the separation factor is 58 when the molar ratio of n-butane to isobutane in the feed is 7:3.

Comparative example 1

The difference from example 1 is that in step 3, tetrabutylammonium hydroxide is used as the organic template agent only, and the molar composition of the synthesis mother liquor is 1TEOS:0.1TBAOH:130H ₂ O. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane prepared by this process was designated M8. Scanning electron microscope characterization of M8 as shown in fig. 14, the seed crystal grew slightly and no intergrowth film could be obtained. The membrane M8 is respectively subjected to a nitrogen gas permeation test and a sulfur hexafluoride gas permeation test (normal temperature and normal pressure), and the result shows that the permeation flux of nitrogen gas is 3.4X10 ^-8mol·m^-2·s^-1·Pa^-1, the permeation flux of sulfur hexafluoride gas is 4.7X10 ^-7mol·m^-2·s^-1·Pa^-1, the ideal selectivity of the membrane M8 to sulfur hexafluoride/nitrogen gas is 13.8, and the result is far lower than the result measured by the membrane M1, so that the membrane layer has great inter-crystal defects.

Comparative example 2

The difference from example 1 is that the synthesis mother liquor of step 3 is not subjected to stirring aging for 6 hours at room temperature. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane prepared by this process was designated M9. The scanning electron microscope characterization of M9 is shown in FIG. 15, the seed crystal size is not changed obviously, and the intergrowth of the film surface is extremely poor. The membrane M9 was subjected to a nitrogen gas permeation test and a sulfur hexafluoride gas permeation test (normal temperature and normal pressure), respectively, and the result showed that the permeation flux of nitrogen gas was 9.3×10 ^-8mol·m^-2·s^-1·Pa^-1, the permeation flux of sulfur hexafluoride gas was 1.4×10 ^-6mol·m^-2·s^-1·Pa^-1, and the ideal selectivity of the membrane M8 to sulfur hexafluoride/nitrogen gas was 15.0, which indicates that the membrane layer had defects.

Comparative example 3

The difference from example 1 is that the secondary growth temperature in step 4 is changed to 70℃and the other steps are the same as those in example 1, and the MFI molecular sieve membrane prepared by this process is designated as M10. The scanning electron microscope characterization of M10 is shown in FIG. 16, the seed crystal size is not changed obviously, nucleation is difficult, and film layer coupling is poor. The membrane M10 was subjected to a nitrogen gas permeation test and a sulfur hexafluoride gas permeation test (normal temperature and normal pressure), respectively, and the results showed that the permeation flux of nitrogen gas was 7.6X10 ^-8mol·m^-2·s^-1·Pa^-1, the permeation flux of sulfur hexafluoride gas was 1.2X10 ^-6mol·m^-2·s^-1·Pa^-1, and the ideal selectivity of the membrane M8 to sulfur hexafluoride/nitrogen gas was 15.8, which suggests that the membrane has many defects.

Comparative example 4

The difference from example 1 is that in step 3, only tetrapropylammonium hydroxide is used as the organic template agent, and the molar composition of the synthesis mother liquor is 1TEOS:0.3TPAOH:130H ₂ O. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane produced by this process was designated M11. Scanning electron microscope characterization of M11 is shown in FIG. 17a, and the seed crystal is fully grown to obtain a continuous film layer. The XRD pattern (FIG. 17 b) shows that the membrane M11 has only the (0 k 0) diffraction peak present, which indicates that the MFI molecular sieve membrane is highly b-axis oriented. Normal temperature and normal pressure testing of the n-butane mixed component (1:1) of the membrane M11 shows that the permeation flux of n-butane can reach 2.6X10 ^-7mol·m^-2·s^-1·Pa^-1, and the separation factor is 24.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. It is therefore intended that the following claims be interpreted as including the preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention also include such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

1. A method for preparing an oriented MFI molecular sieve membrane by using a mixed template is characterized by comprising the following steps:

2. The method for preparing an oriented MFI molecular sieve membrane according to claim 1, wherein the seed crystal in the step S1 is an MFI molecular sieve with a particle size of 50nm to 1 μm, and/or,

3. The method for preparing an oriented MFI molecular sieve membrane by using the mixed template according to claim 1, wherein the shape of the carrier in the step S1 comprises a single-channel tube, a multi-channel tube, a flat plate or a hollow fiber tube, and the material of the carrier comprises ceramics, stainless steel, alumina, titanium dioxide, zirconium dioxide, silicon carbide or silicon nitride, and the pore diameter is 2-2000 nm.

4. The method of preparing an oriented MFI molecular sieve membrane with a mixed template according to claim 1, wherein the quaternary ammonium salt cation compound with longer alkyl chain in step S2 comprises one or more of tetrabutylammonium bromide, tetrabutylammonium hydroxide, tetrabutylammonium chloride, tetrapentylammonium hydroxide, tetrapentylammonium bromide, tetrapentylammonium iodide, methyltributylammonium hydroxide and ethyltrimethylammonium bromide.

5. The method for preparing an oriented MFI molecular sieve membrane with a mixed template according to claim 1, wherein in the step S2, the molar ratio of the tetrapropylammonium cationic compound, the quaternary ammonium salt cationic compound with a longer alkyl chain, deionized water and a silicon source is (0.05-0.08): 0.03-0.05): 20-400): 1.

6. The method for preparing an oriented MFI molecular sieve membrane with a mixed template according to claim 1, wherein the silicon source in step S2 comprises one or more of methyl orthosilicate, ethyl orthosilicate, water glass, silica sol and silica aerogel, and/or,

In the step S2, the aging time is 10 min-100 h.

7. The method for preparing an oriented MFI molecular sieve membrane according to claim 1, wherein in step S2, the synthesis mother liquor further comprises an aluminum source, and the molar ratio of the aluminum source to the silicon source is 0.001-0.5.

8. The method for preparing an oriented MFI molecular sieve membrane with a mixed template according to claim 1, wherein the hydrothermal reaction time in the step S3 is 1h to 20 days, and/or,

And step S4, the roasting temperature is 100-700 ℃ and the roasting time is 0.2-50 h.

9. Use of the MFI molecular sieve membrane obtained by the process of any one of claims 1-8 for isomer separation.

10. The use of the MFI molecular sieve membrane of claim 9 for isomer separation, wherein: the isomer comprises n-isobutane mixed gas, o-xylene mixed liquid or p-xylene mixed liquid or hydrogen gas/ammonia gas mixed gas.