CN117800350A

CN117800350A - Method for preparing MFI molecular sieve membrane at low temperature and application thereof

Info

Publication number: CN117800350A
Application number: CN202310136741.1A
Authority: CN
Inventors: 刘毅; 刘益
Original assignee: Dalian University of Technology
Current assignee: Dalian University of Technology
Priority date: 2023-02-20
Filing date: 2023-02-20
Publication date: 2024-04-02

Abstract

The invention provides a method for preparing an MFI molecular sieve membrane at low temperature and application thereof. The preparation method comprises the following steps: introducing a dense continuous MFI seed layer on the surface of the porous support; dissolving a silicon source and an organic template agent in deionized water, stirring and ageing at 50-120 ℃ to obtain a solution A, dissolving ammonium fluoride in the deionized water to obtain a solution B, slowly dropwise adding the solution B into the solution A, and continuing stirring and ageing to obtain a synthetic mother solution; placing the MFI seed layer in a synthesis mother solution for hydrothermal reaction; and after the reaction is finished, washing, drying and roasting to obtain the high-quality MFI molecular sieve membrane. The MFI molecular sieve membrane prepared by the invention has good intergrowth, small intra-crystalline/inter-crystalline defect density and controllable membrane thickness, and has higher separation performance on industrially important isomers such as n-isobutane and o-xylene and p-xylene mixtures. The preparation process of the membrane material is simple, the synthesis temperature can be reduced to room temperature while ensuring higher separation performance, and the membrane material is far lower than that reported in the prior literature, thereby having better industrial application prospect.

Description

Method for preparing MFI molecular sieve membrane at low temperature and application thereof

Technical Field

The invention relates to the technical field of molecular sieve membrane synthesis, in particular to a method for preparing an MFI molecular sieve membrane at low temperature and application thereof.

Background

Zeolite membranes are an inorganic membrane developed in more than twenty years, and as separation membrane materials, besides the advantages of common inorganic membranes, the zeolite membranes inherit various excellent characteristics of zeolite molecular sieves, such as uniform micropore channels, excellent thermal stability and chemical stability, controllable pore size and hydrophobicity, good catalytic activity and the like, and the characteristics enable the zeolite molecular sieve membranes to realize good separation and catalytic performance at the molecular level.

Among the numerous types of zeolite molecular sieve membranes, zeolite membranes having an MFI-type topology are particularly the most widely studied. The research scope is wide, and benefits from the following points: (1) The MFI molecular sieve has unique pore canal size and structure, the pore canal size is similar to the molecular dynamics diameter of various industrial important substances, and the MFI molecular sieve can be applied to separation of n-xylene and o-xylene isomers, and can be widely applied to separation of acid/water and alcohol/water systems due to adjustable hydrophilicity and hydrophobicity; (2) the morphology and structure of the MFI molecular sieve are diversified; and (3) the membrane material is easy to process, easy to modify and controllable in synthesis.

The synthesis temperature is an important influencing factor of zeolite membrane morphology and separation performance. In order to ensure sufficient growth power of crystals in the secondary growth process, the temperature for preparing the MFI molecular sieve membrane is generally higher (120-180 ℃), so that the reaction is required to be carried out in a closed high-temperature high-pressure reaction kettle. This can undoubtedly increase the difficulty of film forming operation, production energy consumption and equipment cost, and simultaneously, the higher reaction temperature can easily cause uncontrollable film thickness and the generation of defects between film layers. And the nucleation and growth of the MFI molecular sieve membrane can be better controlled by reducing the synthesis temperature, and the generation of membrane layer defects can be well avoided while the membrane thickness is reduced. Through the modes of thermal reflux, single-mode microwave heating and high-activity silicon source, researchers can lower the synthesis temperature of an MFI molecular sieve membrane to 100 ℃ so as to further lower the synthesis temperature, and meanwhile, ensuring good cohesiveness of membrane materials still faces great challenges.

Disclosure of Invention

The invention provides a method for preparing a high-quality MFI molecular sieve membrane under a low-temperature condition, which prepares a high-activity secondary growth solution by regulating and controlling precursor solution to stir and age and introducing a mineralizer so as to reduce the synthesis temperature of the MFI molecular sieve membrane and explore the application of the MFI molecular sieve membrane in industrial separation. The prepared MFI molecular sieve membrane has good coupling property, excellent thermal stability, chemical stability and mechanical stability. Has higher separation performance on both normal/isobutane and o/p-xylene mixtures, thereby providing good industrial application prospect for preparing high-performance MFI molecular sieve membranes.

The invention is realized by the following technical scheme:

a method for preparing an MFI molecular sieve membrane at low temperature, comprising the steps of:

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 silicon source and an organic template agent in deionized water, stirring and ageing at 50-120 ℃ to obtain a solution A, dissolving ammonium fluoride in the deionized water to obtain a solution B, slowly dropwise adding the solution B into the solution A, and continuing stirring and ageing to obtain a synthetic mother solution;

s3, carrying out low-temperature hydrothermal reaction on the seed crystal layer obtained in the step S1 and the synthetic mother liquor obtained in the step S2 at 30-90 ℃;

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

The molar ratio of ammonium fluoride to silicon source in step S2 is NH ₄ F/SiO ₂ ＝0.2～2.0。

The seed crystal in the step S1 is MFI molecular sieve with the particle size of 20 nm-2 mu m.

The specific method of the MFI seed coating in the step S1 is spin coating, dip coating, wiping or spraying; the roasting temperature of the seed crystal layer is 200-800 ℃; the roasting time is 20 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 silicon source in the step S2 is one or more of methyl orthosilicate, ethyl orthosilicate, water glass, silica sol and silica aerogel; the mole ratio of deionized water to silicon source in the step S2 is H ₂ O/SiO ₂ ＝8～100。

The organic template agent in the step S2 is one or more of tetrapropylammonium bromide, tetrapropylammonium hydroxide solution, tetraethylammonium hydroxide solution and methyltributylammonium hydroxide solution, and the molar ratio of the organic template agent to the silicon source is the organic template agent/SiO ₂ ＝0.05～0.4。

In the step S2, the silicon source and the organic template agent are dissolved in deionized water, and the aging time is 4-20 h.

And step S2, slowly dripping the solution B into the solution A, and continuing to age at 50-120 ℃ for 4-20 hours.

The solution A in the step S2 also comprises an aluminum source or a titanium source, wherein the aluminum source is sodium metaaluminate, aluminum sulfate, aluminum chloride or aluminum nitrate, the titanium source is tetrabutyl titanate, titanium chloride or titanium isopropoxide, and the molar ratio of the aluminum source to the silicon source is Al ₂ O ₃ /SiO ₂ =0.001 to 0.05, the molar ratio of the titanium source to the silicon source is TiO ₂ /SiO ₂ ＝0.002～0.1。

The hydrothermal reaction time in the step S3 is 12 h-84 days.

The roasting mode of the MFI molecular sieve membrane in the step S4 is muffle furnace roasting, ozone atmosphere tube furnace roasting or rapid thermal process roasting, and the roasting temperature is 150-700 ℃; the roasting time is 0.3-50 h. Furthermore, the roasting mode is tubular furnace roasting in ozone atmosphere, the roasting temperature is 150-250 ℃, thus the required roasting temperature is lower, the time is shorter, and meanwhile, the integrity of the membrane is ensured to the greatest extent.

The invention also provides the MFI molecular sieve membrane obtained by the method. The thickness of the MFI molecular sieve membrane is 0.6-2 μm.

The invention also provides application of the MFI molecular sieve membrane in separation of n-isobutane mixed gas and separation of o-xylene and p-xylene mixed liquid, and the MFI molecular sieve membrane has good separation performance on both the n-isobutane mixed gas and the o-xylene and p-xylene mixed liquid systems.

The beneficial effects of the invention are as follows: the MFI molecular sieve membrane with better cohesiveness is prepared at a lower synthesis temperature by regulating and controlling stirring aging and introducing mineralizer solution to prepare high-activity synthesis mother liquor. The high-quality MFI molecular sieve membrane with small intra-crystalline/inter-crystalline defect density and controllable thickness and morphology uniformity is obtained by coupling low-temperature synthesis and fluoride ion mineralization. Compared with the traditional MFI molecular sieve membrane preparation process, the synthesis temperature is greatly reduced (the temperature can be reduced to 30 ℃ at room temperature), so that the membrane preparation difficulty and the production cost are greatly reduced. Meanwhile, the membrane material prepared at room temperature of the invention is applied to the n-isobutane gas mixture (n-butane permeation flux is 5.16X10) ^-7 mol·m ^-2 ·s ^-1 ·Pa ^-1 Separation factor 96.7) and an ortho/para-xylene liquid mixture (para-xylene flow rate 350 g.m ^-2 ·h ^-1 Separation factor 37) shows higher separation performance 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 a graph of a one-component gas permeation test for preparing an M1 membrane according to example 1.

FIG. 6 is a graph of the n/isobutane separation performance of the M1 membrane prepared in example 1 as a function of feed pressure.

FIG. 7 is a graph of n/isobutane separation performance as a function of feed composition for the preparation of M1 membranes from example 1.

FIG. 8 is a graph comparing the n/isobutane separation performance of the M1 membrane prepared in example 1 with the values reported in the literature.

FIG. 9 is a graph of the long-term stability of n/isobutane separation performance of the M1 membrane prepared in example 1.

Fig. 10 is an SEM image of MFI seed crystals prepared in example 2.

Fig. 11 is an SEM image of the MFI seed monolayer prepared in example 2.

Fig. 12 is (a) SEM image and (b) XRD image of the M2 film prepared in example 2.

Fig. 13 is an SEM image of the M3 film prepared in example 3.

Fig. 14 is an SEM image of the M4 film prepared in example 4.

Fig. 15 is an SEM image of the M5 film prepared in example 5.

Fig. 16 is an SEM image of the M6 film prepared in example 6.

Fig. 17 is an SEM image of the M7 film prepared in example 7.

Fig. 18 is an SEM image of the M8 film prepared in example 8.

Fig. 19 is an SEM image of the M9 film prepared in comparative example 1.

FIG. 20 is an SEM image of the preparation of M10 films of comparative example 2.

Fig. 21 is an SEM image of the M11 film prepared in comparative example 3.

Fig. 22 is an SEM image of the M12 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 all room temperature, and unless stated otherwise, 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 temperatures are 75 ℃, the molar ratio of ortho-xylene to para-xylene is 1:1, and the transmembrane pressure difference is 1bar.

Example 1

(1) Seed layer preparation: MFI nano-seeds were first prepared by a clear solution method (see reference angel. Chem. Int. Ed.2021,60,7659-7663 for detailed synthesis process) and seed solutions with mass concentrations of 0.6wt.% were prepared by dispersing the prepared seeds in deionized water. And then uniformly coating the MFI seed crystal on the surface of the porous carrier through a suspension coating process, and taking 0.3mL seed crystal liquid drop on the surface of the porous alumina sheet in the suspension coating process. After the suspension coating is finished, 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 550 ℃ so as to improve the binding force between the seed crystal and the carrier while removing the organic template agent in the seed crystal.

(2) Preparing a synthetic mother solution: tetraethyl orthosilicate (TEOS) is slowly added into a mixed solution of tetrapropylammonium hydroxide solution (25 wt.%) and deionized water in a dropwise manner, and the mixture is stirred and aged for 12 hours at 90 ℃ to obtain solution A; then uniformly dissolving ammonium fluoride in deionized water to obtain a solution B; and slowly dripping the solution B into the solution A, and continuously stirring and aging for 12 hours at the temperature of 90 ℃ to obtain a synthetic mother solution. The molar composition of the synthetic mother solution is 1TEOS:0.1TPAOH:0.4NH ₄ F:22H ₂ O。

(3) Preparing an MFI molecular sieve membrane by a secondary growth method: the carrier sheet coated with the seed layer was placed in the synthesis mother liquor, sealed and placed in an oven for reaction at 30 ℃ for 84 days. After the reaction is finished, the membrane is taken out, washed to be neutral by deionized water, dried overnight, and then put into a tube furnace in ozone atmosphere for roasting at 200 ℃ for 3 hours to remove the template agent, wherein the temperature rising and reducing rates are 0.5 ℃/min, 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 250 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. The scanning electron microscope characterization of the prepared seed crystal layer is shown in figure 2, and the seed crystals are continuously and densely distributed on the surface of the carrier. As can be seen from FIG. 3a, after secondary growth, the prepared MFI molecular sieve membrane layer has uniform surface, better inter-crystal growth and compact and continuous membrane layer, and the cross-section scanning electron microscope characterization is shown as FIG. 3b, and the thickness of the MFI molecular sieve membrane is about 910 nm. XRD results (FIG. 4) show that the prepared membrane material is of a pure-phase MFI structure and has higher crystallinity. Subsequently, the membrane M1 was subjected to a gas separation performance test, as shown in FIG. 5, in which the permeation flux gradually decreased with an increase in the molecular size of the single component gas on the raw material side at normal temperature and pressure (25 ℃ C., 1 bar), and in which the permeation flux was increased at normal butane and isobutaneObvious cut-off phenomenon appears between the membrane M1 and the permeation flux of the n-butane is 3.77 multiplied by 10 ^-7 mol·m ^-2 ·s ^-1 ·Pa ^-1 The ideal separation selectivity is 57.6. The membrane M1 was subjected to a feed pressure test (FIG. 6) of the n/isobutane blend component (1:1), which indicated that both the permeation flux and separation factor of n-butane decreased with increasing feed pressure. The feed composition test (fig. 7) of the n/isobutane mix component (1 bar) was performed on membrane M1, and the results showed that both the n-butane permeation flux and the separation factor increased significantly with decreasing n-butane molar concentration in the feed gas. It is worth mentioning that when the molar ratio of n-butane to isobutane in the feed is 1:9, the permeation flux of n-butane can reach 5.16X10 ^-7 mol·m ^-2 ·s ^-1 ·Pa ^-1 The separation factor was 96.7, which exceeds the values reported in all literature (figure 8). Furthermore, the studies showed that the MFI molecular sieve membrane prepared also showed good long-term running stability at 25 ℃ and 1bar, with the n/isobutane separation factor and the n-butane permeability remaining substantially unchanged for 24 hours of continuous testing (fig. 9). Finally, the membrane M1 is subjected to o/p-xylene mixed liquid (1:1) pervaporation test, and the test result shows that the p-xylene flow rate of the membrane M1 is 350 g.m ^-2 ·h ^-1 The separation factor is up to 37.

Example 2

The difference from example 1 is that: the seed layer in step 1 is prepared by a different method, specifically as follows, by first preparing plate-brick MFI seed crystals by a clear solution method (see reference sci.adv.2020,6, eaay5993 for detailed synthesis). Seed layer coating was then performed using a hand coating method as follows: the surface of a porous alumina carrier is pre-modified by using a polyvinyl alcohol solution with the concentration of 6.5wt.%, a proper amount of MFI seed crystal powder is poured on the surface of the carrier, the carrier is coated by fingers with butyronitrile gloves, and the MFI seed crystal single layer with the high b-axis orientation is obtained after roasting in a muffle furnace (550 ℃ for 6 h). Meanwhile, the time for synthesizing the MFI molecular sieve membrane in the step 3 is 56 days, and the other steps are the same as those in the example 1, and the MFI molecular sieve membrane prepared in the process is denoted as M2.

The scanning electron microscope characterization of the MFI seed crystal is shown in figure 10, and the molecular sieve morphology is regular plate brickThe particle size is uniformly distributed at about 1.0 μm, and almost no twins are generated. The scanning electron microscope characterization of the prepared seed crystal layer is shown in fig. 11, and the seed crystals are continuously and densely arranged on the surface of the carrier and are in high b-axis orientation. After the secondary growth, the inter-seed voids have been completely closed, forming a continuous, dense film material on the support with little twinning of the surface (fig. 12 a). The XRD pattern showed (fig. 12 b) that only the (0 k 0) diffraction peak was present over the entire XRD diffraction angle range, indicating that the MFI molecular sieve membrane was highly b-axis oriented. Normal temperature and normal pressure test of n-isobutane mixed component (1:1) is carried out on the membrane M2, and the result shows that the permeation flux of n-butane can reach 3.1X10 ^- ⁷ mol·m ^-2 ·s ^-1 ·Pa ^-1 The separation factor is 42. O/P xylene mixed component (1:1) pervaporation test was performed on membrane M2, and the result showed that the p-xylene flow rate of membrane M2 was 420 g.m ^-2 ·h ^-1 The separation factor is up to 43.

Example 3

The difference from example 1 is that: the temperature of the secondary growth in step 3 was 90℃for 12 hours, and the other steps were the same as in example 1, and the MFI molecular sieve membrane produced in this process was designated as M3. The scanning electron microscope characterization of M3 is shown in FIG. 13, and the film surface is continuous and compact and has good intergrowth. Normal temperature and normal pressure test of n-isobutane mixed component (1:1) is carried out on the membrane M3, and the result shows that the permeation flux of n-butane can reach 2.4x10 ^-7 mol·m ^-2 ·s ^-1 ·Pa ^-1 The separation factor was 36.

Example 4

The difference from example 1 is that: in the step 2, the aging temperature of stirring before and after adding the ammonium fluoride solution is 50 ℃, and the other steps are the same as those of the example 1, and the MFI molecular sieve membrane prepared by the process is denoted as M4. The scanning electron microscope characterization of M4 is shown in FIG. 14, the crystal grains on the surface of the film layer are reduced, and the film layer is continuous and compact and has better intergrowth.

Example 5

The difference from example 1 is that: the mole ratio of deionized water to silicon source in step 2 is H ₂ O/SiO ₂ =100, the rest of the procedure is the same as in example 1, and the MFI molecular sieve membrane prepared by this processDesignated M5. The scanning electron microscope characterization of M5 is shown in FIG. 15, the crystal grains on the surface of the film layer are obviously reduced, and the film layer still keeps good cohesiveness.

Example 6

The difference from example 1 is that: the molar ratio of ammonium fluoride to silicon source in step 2 is NH ₄ F/SiO ₂ =1.3. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane prepared by this process was designated M6. The scanning electron microscope characterization of M6 is shown in FIG. 16, the grains on the surface of the film layer become large, and the surface of the film layer grows well.

Example 7

The difference from example 1 is that: the temperature of the secondary growth in the step 3 is 50 ℃ and the time is 50 days. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane prepared by this process was designated as M7. The scanning electron microscope characterization of M7 is shown in FIG. 17, and the film surface still maintains good cohesiveness.

Example 8

The difference from example 1 is that: step 2, aluminum chloride is additionally added in the preparation process of the solution A, and the molar ratio of the aluminum chloride to the tetraethoxysilane is Al ₂ O ₃ /SiO ₂ =0.02, the remainder of the procedure was as in example 1, and the MFI molecular sieve membrane prepared by this procedure was designated M8. Scanning electron microscope characterization of M8 as shown in fig. 18, the film surface remained well coherent.

Comparative example 1

The difference from example 1 is that: the molar ratio of ammonium fluoride to silicon source in step 2 is NH ₄ F/SiO ₂ =0. 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. 19, the seed crystal is only slightly grown, and the intergrowth of the film surface is poor.

Comparative example 2

The difference from example 1 is that: in the step 2, the mineralizer is NH ₄ Cl. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane produced by this process was designated M10. The scanning electron microscope characterization of M10 is shown in FIG. 20, the seed crystal size is hardly changed obviously, and the intergrowth of the film surface is extremely poor.

Comparative example 3

The difference from example 1 is that: in the step 2, the mineralizer is NaF. The remaining steps were the same as in example 1, and the MFI molecular sieve membrane produced by this process was designated M11. The scanning electron microscope characterization of M11 is shown in FIG. 21, a large number of gaps among the seed crystals are not closed, and the intergrowth of the film surface is poor.

Comparative example 4

The difference from example 1 is that: in the step 2, stirring and aging are not carried out at 90 ℃ for 12 hours before adding the ammonium fluoride solution, and the other steps are the same as those of the example 1, and the MFI molecular sieve membrane prepared by the process is denoted as M12. The scanning electron microscope characterization of M12 is shown in FIG. 22, the seed crystal size is not changed obviously, and the intergrowth of the film is poor.

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 MFI molecular sieve membrane at low temperature, which is characterized in that: the method comprises the following steps:

2. The method for preparing an MFI molecular sieve membrane at low temperature according to claim 1, wherein: the molar ratio of ammonium fluoride to silicon source in step S2 is NH ₄ F/SiO ₂ ＝0.2～2.0。

3. The method for preparing an MFI molecular sieve membrane at low temperature according to claim 1, wherein: the seed crystal in the step S1 is MFI molecular sieve with the particle size of 20 nm-2 mu m.

4. The method for preparing an MFI molecular sieve membrane at low temperature according to claim 1, wherein: the silicon source in the step S2 is one or more of methyl orthosilicate, ethyl orthosilicate, water glass, silica sol and silica aerogel; the mole ratio of the deionized water to the silicon source is H ₂ O/SiO ₂ ＝8～100。

5. The method for preparing an MFI molecular sieve membrane at low temperature according to claim 1, wherein: the organic template agent in the step S2 is one or more of tetrapropylammonium bromide, tetrapropylammonium hydroxide solution, tetraethylammonium hydroxide solution and methyltributylammonium hydroxide solution, and the molar ratio of the organic template agent to the silicon source is the organic template agent/SiO ₂ ＝0.05～0.4。

6. The method for preparing an MFI molecular sieve membrane at low temperature according to claim 1, wherein: and S2, dissolving a silicon source and an organic template agent in deionized water, wherein the aging time is 4-20 h.

7. The method for preparing an MFI molecular sieve membrane at low temperature according to claim 1, wherein: the hydrothermal reaction time in the step S3 is 12 h-84 days.

8. A cryogenic temperature as claimed in claim 1A method of making an MFI molecular sieve membrane characterized by: the solution A in the step S2 also comprises an aluminum source or a titanium source, wherein the aluminum source is sodium metaaluminate, aluminum sulfate, aluminum chloride or aluminum nitrate, the titanium source is tetrabutyl titanate, titanium chloride or titanium isopropoxide, and the molar ratio of the aluminum source to the silicon source is Al ₂ O ₃ /SiO ₂ =0.001 to 0.05, the molar ratio of the titanium source to the silicon source is TiO ₂ /SiO ₂ ＝0.002～0.1。

9. A MFI molecular sieve membrane obtained by the process of claim 1.

10. Use of the MFI molecular sieve membrane of claim 9 for n/isobutane mixed gas separation and ortho/para xylene mixed liquid separation.