A method for the preparation of mfi molecular sieve membranes for carbon capture

Abstract

The application discloses a preparation method of MFI molecular sieve membrane for carbon capture, and particularly relates to the following steps: first, preparing MFI molecular sieve seeds and configuring the MFI molecular sieve seeds into a seed suspension; second, loading an MFI molecular sieve seed layer on the surface of a porous carrier which is surface-modified with a gaseous SiO2 layer by using an impregnation and pulling method; and third, introducing W element on the surface of the porous carrier loaded with the MFI molecular sieve seed layer by using a dry gel conversion method, a hydrothermal synthesis method or an ultra-dilute solution preparation method, and then performing calcination to prepare a W-doped MFI molecular sieve membrane. The introduction of the metal element W in the MFI molecular sieve membrane has better adsorption performance on CO2, and the advantage of the adsorption-diffusion mechanism in separating CO2 mixture is more obvious. Meanwhile, the introduction of the W element also eliminates the hydrophilic group Si-OH on the surface of the membrane, and then increases the hydrophobicity of the membrane, so that even under the condition of the presence of water vapor, the W-MFI molecular sieve membrane still has high separation selectivity and stability on the CO2 mixture.

Description

Technical Field

[0001] This invention relates to a method for preparing an MFI molecular sieve membrane for carbon capture, belonging to the field of molecular sieve membrane material preparation and gas separation technology. Background Technology

[0002] As is well known, the greenhouse effect is caused by the excessive emission of greenhouse gases, and CO2 accounts for about 77% of global greenhouse gases [Journal of Materials Chemistry A, 2015, 3(23):12500-6]. Therefore, CO2 is one of the main factors causing climate change, and because of its large proportion in human social life, it has received widespread attention from the scientific community at home and abroad. The Paris Agreement stipulates that the global average temperature rise should be limited to well below 2°C compared with the pre-industrial level, and actively strives to limit the global average temperature rise to well below 1.5°C [Advances in Climate Change Research, 2016, 12(1):61-67].

[0003] Currently, the main CO2 separation and capture processes include cryogenic distillation, absorption, adsorption separation, and membrane separation. Among them, membrane separation [Journal of Membranes Science, 2008, 307(1):88-95] is an emerging low-energy separation technology with advantages such as low investment cost, small footprint, and easy scale-up, thus its application is becoming increasingly widespread. Gas separation membranes are energy-saving, highly efficient, and environmentally friendly, playing a crucial role in the separation of typical industrial small molecule gases (such as CO2 / N2).

[0004] Various membranes have been developed for CO2 separation. Among them, zeolite molecular sieve membranes have a regular pore structure and better thermal stability, chemical stability and structural stability [Chem Soc Rev, 2015, 44(20):7128-54]. In addition, zeolite molecular sieve membranes can be used at high temperature and high pressure and can operate in harsh chemical environments, and are expected to be used for separation through molecular sieving and adsorption-diffusion mechanisms. The most common zeolite molecular sieves are MFI type, CHA type, DD3R type, LTA type, FAU type zeolite molecular sieves, and mordenite. Among them, MFI type zeolite molecular sieves have become an important zeolite membrane material due to their unique pore structure. The basic structure of MFI molecular sieve consists of 8 five-membered rings with D2d symmetry. These unit structures are connected by sharing one side to form a five-silicon chain parallel to the c-axis, and then further linked between the network layers to form a three-dimensional structure. MFI molecular sieve has two sets of intersecting channels: one set is a ten-membered ring channel parallel to the a-axis direction, with a pore size of The other group consists of ten-element ring channels parallel to the b-axis, with a diameter of [missing information].

[0005] MFI molecular sieves have pore sizes similar to the kinetic diameters of many commonly used gas and solvent molecules. Therefore, MFI molecular sieves and MFI molecular sieve membranes can be used for the separation of various important substances, such as organic isomers, H2 and CO2, H2 and CH4, and organic matter and water [Microporous and Mesoporous Materials, 2019, 289:1-7]. CH. Nicolas [Ind. Eng. Chem. Res. 2012, 51, 10451-10461] et al. investigated in detail the separation of water and NO. x The effects of light hydrocarbons on the separation of CO2 / N2 using MFI molecular sieve membranes were investigated, with a CO2 / N2 separation selectivity of 3. Wang et al. [Journal of Membrane Science 636, 2021, 119565] used a bilayer membrane (inner SSZ-13 membrane, outer MFI membrane) to study the separation of CO2 / CH4 and CO2 / N2, finding that the optimal CO2 permeability was 1.5 × 10⁻⁶. -6 mol·m -2 ·s -1 ·Pa -1 and 1.4×10 -6 mol·m -2 ·s -1 ·Pa -1 The separation factors were 153 and 22, respectively. MPBernal et al. [AIChE Journal. 2004, 50] prepared MFI molecular sieve membranes (ZSM-5) on alumina and stainless steel tube supports and tested their separation performance for CO2 / N2 mixtures. The best separation performance was achieved with Na-ZSM5 membrane, which had a separation selectivity as high as 13.7.

[0006] In recent years, researchers have successfully incorporated various metal elements into the framework of MFI molecular sieve membranes, exploring new research areas. Pera-Titus et al. [Microporous and Mesoporous Materials, 2010, 133(1-3):18-26] synthesized B-MFI molecular sieve membranes, which showed improved separation performance for p-xylene compared to traditional aluminum-based MFI hollow fiber membranes. Zhang et al. [Separation and Purification Technology, 2001, 25:269-74] synthesized crack-free Fe-MFI molecular sieve membranes, in which Fe was incorporated into the molecular sieve framework. The synthesized Fe-MFI molecular sieve membrane had an H2 permeability of 3.0 × 10⁻⁶. -6 mol·m-2 ·s -1 ·Pa -1 The separation coefficient of H2 / C3H8 is 25.8.

[0007] Therefore, how to further improve the separation performance of CO2 / N2 by introducing metal elements remains a problem that needs to be addressed. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing an MFI molecular sieve membrane for carbon capture. Compared to all-silica MFI molecular sieve membranes, W-MFI molecular sieve membranes, due to the introduction of the metal element W into their framework, exhibit better CO2 adsorption performance, thus demonstrating a more significant advantage in separating CO2 mixtures via adsorption-diffusion mechanisms. Simultaneously, the introduction of W eliminates the hydrophilic Si-OH groups on the membrane surface, thereby increasing the membrane's hydrophobicity. This allows the W-MFI molecular sieve membrane to maintain high selectivity and stability for CO2 mixtures even in the presence of water vapor.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing an MFI molecular sieve membrane for carbon capture includes the following steps:

[0011] S1. Prepare MFI molecular sieve seed crystals and prepare a seed crystal suspension; load the MFI molecular sieve seed layer on the surface of a porous support modified with a vapor phase SiO2 layer using the dip-coating method;

[0012] S2. W element is introduced into the surface of a porous carrier loaded with MFI molecular sieve seed layer by means of dry gel conversion method, hydrothermal synthesis method or ultra-dilute solution preparation method, and then calcined to obtain W-doped MFI molecular sieve membrane.

[0013] Preferably, in step S1, the method for preparing MFI molecular sieve seeds is as follows:

[0014] According to the molar ratio of H2O:TPAOH:SiO2:C2H5OH in the synthesis solution of (10-200):(0.1-0.5):(0.5-1.5):(2-6), water, tetrapropylammonium hydroxide and tetraethyl orthosilicate are mixed and stirred for 1-5 hours, and then hydrothermally synthesized at 100-200℃ for 1-72 hours.

[0015] After the reaction is complete, the mixture is cooled, centrifuged, washed, dried, and then calcined at 500-750℃ for 5-8 hours.

[0016] Preferably, in step S1, the concentration of the seed crystal suspension is 0.2-2 wt%; the porous carrier is hollow fiber Al2O3.

[0017] Preferably, in step S1, the operation time of the immersion lifting method is 5-60 seconds, and after completion, it is dried at 120-160℃.

[0018] Preferably, before the dip-coating process, a sintering step is performed on the porous support with a vapor-phase SiO2 layer on its surface. The sintering conditions are: 450-700℃, 5-20h.

[0019] Preferably, in step S2, the specific method for preparing W-doped MFI molecular sieve membranes by dry gel conversion is as follows:

[0020] H2O, template agent, silicon source, and tungsten source are mixed in a molar ratio of (30-80):(0.1-0.3):(0.5-1.5):(0.02-0.07) and then stirred and aged for 2-5 hours to obtain the synthesis solution.

[0021] The porous support loaded with MFI molecular sieve seed layer was placed in the synthesis solution for 0.5-3 min, and then dried at 120-160℃ for 0.5-2 h; then placed in a reaction vessel containing 0.5-3 g of water and dried at 160-200℃ for 20-25 h, quenched with cold water, the membrane was taken, washed with water and dried.

[0022] The membrane is then placed in an air, oxygen, or ozone atmosphere and calcined at 200-260℃ for 40-60 hours; the heating rate is 0.5-3℃ / min.

[0023] Preferably, in step S2, the specific method for preparing W-doped MFI molecular sieve membranes by hydrothermal synthesis is as follows:

[0024] H2O, template agent, silicon source, and tungsten source are mixed in a molar ratio of (100-250):(0.1-0.3):(0.5-1.5):(0.04-0.09) and then stirred and aged for 4-7 hours to obtain the synthesis solution.

[0025] The porous support loaded with MFI molecular sieve seed layer was placed in the synthesis solution and reacted at 120-160℃ for 40-60h. The reaction was then quenched with cold water, the membrane was taken, washed with water and dried.

[0026] The membrane is calcined at 200-260℃ for 40-60 hours in an ozone atmosphere; the heating rate is 0.5-3℃ / min.

[0027] Preferably, in step S2, the specific method for preparing the W-doped MFI molecular sieve membrane using the ultra-dilute solution preparation method is as follows:

[0028] H2O, template agent, silicon source, and tungsten source are mixed in a molar ratio of (1000-2500):(0.1-0.3):(0.5-1.5):(0.02-0.07) and then stirred and aged for 4-7 hours to obtain the synthesis solution.

[0029] The porous support loaded with MFI molecular sieve seed layer was placed in the synthesis solution and reacted at 120-160℃ for 40-60h. The reaction was then quenched with cold water, the membrane was taken, washed with water and dried.

[0030] The membrane is calcined at 200-260℃ for 40-60 hours in an ozone atmosphere; the heating rate is 0.5-3℃ / min.

[0031] Preferably, the template agent is tetrapropylammonium hydroxide; the silicon source is at least one of tetraethyl orthosilicate, fumed SiO2, and silica sol; and the tungsten source is at least one of sodium tungstate dihydrate and ammonium tungstate.

[0032] Application of MFI molecular sieve membranes for carbon capture prepared by any of the above methods in CO2 / N2 separation.

[0033] The beneficial effects of this invention are as follows:

[0034] The prepared W-doped MFI molecular sieve membranes all conform to the crystal form of MFI molecular sieves, have good crystallinity, and exhibit good inter-crystal growth. However, the membranes prepared by the dry gel conversion method are thinner, which can significantly improve gas permeability. At the same time, the introduction of W element can enhance the adsorption of CO2 by the membrane. On the other hand, the introduction of W eliminates the hydrophilic Si-OH groups on the membrane surface, increasing the hydrophobicity of the membrane. This allows the W-MFI molecular sieve membrane to still have high separation selectivity and stability for CO2 mixtures even in the presence of water vapor. Attached Figure Description

[0035] Figure 1 XRD patterns of the molecular sieve membranes prepared in Example 1 and Comparative Example 1;

[0036] Figure 2 XRD patterns of the molecular sieve membranes prepared in Example 2 and Comparative Example 2;

[0037] Figure 3 XRD patterns of the molecular sieve membranes prepared in Example 3 and Comparative Example 3;

[0038] Figure 4 SEM images of the molecular sieve membranes prepared in Example 1 and Comparative Example 1;

[0039] Figure 5 SEM images of the molecular sieve membranes prepared in Example 2 and Comparative Example 2;

[0040] Figure 6 SEM images of the molecular sieve membranes prepared in Example 3 and Comparative Example 3;

[0041] Figure 7 The N2 adsorption-desorption isotherms of W-MFI and Si-MFI molecular sieves are shown.

[0042] Figure 8 The surface water contact angle diagrams of the molecular sieve membranes prepared in Example 1 and Comparative Example 1 are shown.

[0043] Figure 9 The adsorption isotherms of CO2 by W-MFI molecular sieve at 273 K and 298 K are shown.

[0044] Figure 10 The adsorption isotherms of CO2 by Si-MFI molecular sieve at 273 K and 298 K are shown.

[0045] Figure 11 The adsorption isotherms of N2 by W-MFI molecular sieve at 273 K and 298 K are shown.

[0046] Figure 12 The adsorption isotherms of Si-MFI molecular sieve for N2 at 273 K and 298 K are shown.

[0047] Figure 13 The graphs show the CO2 / N2 / H2O stability test results of the molecular sieve membranes prepared in Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0048] This application obtains a W-MFI molecular sieve membrane by coating a layer of all-silicon MFI molecular sieve onto the surface of an Al2O3 support modified with SiO2 as an inducing seed crystal, then preparing a membrane synthesis solution containing tungsten and aging it, and finally coating the surface of the crystallized support with the membrane synthesis solution containing tungsten, followed by drying, loading into a reactor, adding water to the bottom of the reactor, and high-temperature crystallization.

[0049] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. In the following embodiments and comparative examples, TPAOH is tetrapropylammonium hydroxide, TEOS is tetraethyl orthosilicate, and C2H5OH is ethanol.

[0050] Example 1

[0051] The specific steps for preparing MFI molecular sieve membranes using the dry gel conversion method are as follows:

[0052] (1) Preparation of MFI molecular sieve seeds:

[0053] In-situ hydrothermal synthesis was used to prepare MFI molecular sieves as inducing seeds. The molar ratio of the synthesis solution was H2O:TPAOH:SiO2:

[0054] The C2H5OH ratio is 19.2:0.2:1:4. First, deionized water was weighed and placed in a polytetrafluoroethylene bottle. Tetrapropylammonium hydroxide and tetraethyl orthosilicate were added sequentially, and the mixture was stirred and aged at room temperature for 4 hours. The resulting synthesis solution was poured into a reaction vessel and placed in a 150°C oven for hydrothermal synthesis for 2 days. After the reaction was complete, the mixture was cooled to room temperature, and the molecular sieve was removed, centrifuged and washed until neutral, then dried in an oven. Finally, it was calcined at 550°C for 6 hours to remove the organic template agent.

[0055] (2) Preparation of W-MFI molecular sieve seeds:

[0056] The preparation steps are the same as in step (1), except that Na2WO4·2H2O needs to be added to the synthesis solution. The final molar ratio of the synthesis solution is H2O:TPAOH:SiO2:C2H5OH:Na2WO4·2H2O=19.2:0.2:1:4:0.04.

[0057] (3) Preparation of seed layer:

[0058] Hollow fiber Al2O3 (outer diameter 4.06 mm; inner diameter 1.20 mm; porosity 45%; pore size 0.368 μm) with a surface-modified fumed SiO2 layer (manual wiping method, particle size 300 nm) was used as a support. The support was calcined at 550 °C for 10 h to sinter SiO2 onto the support, washed with deionized water, and dried in an oven overnight. The seed crystals prepared in step (1) were prepared into a 0.5 wt.% seed crystal suspension, and then the support was placed in the seed crystal suspension for 30 s using the dip-coating method. Finally, it was dried in an oven at 140 °C for later use.

[0059] (4) Preparation of W-MFI molecular sieve membranes:

[0060] Deionized water, TPAOH, TEOS, and Na2WO4·2H2O were prepared in a molar ratio of 60:0.12:1:0.04. The prepared membrane synthesis solution was stirred and aged at room temperature for 4 hours. The support coated with seed crystals in step (2) was placed in the membrane synthesis solution for 1 minute to coat the surface of the support with a layer of membrane synthesis gel. Then, it was dried in an oven at 140℃ for 1 hour. After drying, it was placed in a reaction vessel containing 1.5g of water and placed in an oven at 180℃ for 1 day. After the reaction was completed, the reaction vessel was quenched with cold water, the membrane was taken, washed with water, and dried for later use. Then, the membrane was calcined in an ozone atmosphere to remove the template agent. The calcination temperature was 230℃, the calcination time was 2 days, and the heating rate was 1℃ / min to obtain the W-MFI molecular sieve membrane.

[0061] Comparative Example 1

[0062] Steps (1) and (2) are the same as steps (1) and (3) in Example 1.

[0063] Step (3): Preparation of Si-MFI molecular sieve membrane:

[0064] The preparation steps are basically the same as step (4) of Example 1. The difference is that Na2WO4·2H2O is not added to the membrane synthesis solution, and the membrane prepared is an all-silicon MFI molecular sieve membrane.

[0065] Example 2

[0066] The conventional hydrothermal method for preparing MFI molecular sieve membranes is as follows:

[0067] Steps (1) and (2) are the same as steps (1) and (3) in Example 1.

[0068] Step (3): Preparation of W-MFI molecular sieve membrane:

[0069] First, deionized water, TPAOH, TEOS, and Na2WO4·2H2O were prepared in a molar ratio of 180:0.12:1:0.06. The prepared membrane synthesis solution was stirred and aged at room temperature for 6 hours. After the solution became clear, it was poured into a reaction vessel, and a carrier with a seed layer was placed on it. The reaction vessel was then placed in a 130℃ oven for 2 days. After the reaction was completed, the reaction vessel was quenched with cold water, the membrane was taken, washed with water, and dried for later use. The membrane was then calcined in an ozone atmosphere at 230℃ for 2 days at a heating rate of 1℃ / min to remove the template agent, thus obtaining the W-MFI molecular sieve membrane.

[0070] Comparative Example 2

[0071] Steps (1) and (2) are the same as steps (1) and (3) in Example 1.

[0072] Step (3): Preparation of Si-MFI molecular sieve membrane:

[0073] The preparation steps are basically the same as step (3) in Example 2. The difference is that Na2WO4·2H2O is not added to the membrane synthesis solution, and the membrane obtained is an all-silicon MFI molecular sieve membrane.

[0074] Example 3

[0075] The specific steps for preparing MFI molecular sieve membranes from ultradilute solutions are as follows:

[0076] Steps (1) and (2) are the same as steps (1) and (3) in Example 1.

[0077] Step (3): Preparation of W-MFI molecular sieve membrane:

[0078] First, deionized water, TPAOH, TEOS, and Na2WO4·2H2O were prepared in a molar ratio of 1500:0.12:1:0.4. The prepared membrane synthesis solution was stirred and aged at room temperature for 6 hours. After the solution became clear, it was poured into a reaction vessel, and a carrier with a seed layer was placed on it. The reaction vessel was then placed in a 140℃ oven for 2 days. After the reaction was completed, the reaction vessel was quenched with cold water, the membrane was taken, washed with water, and dried for later use. The membrane was then calcined in an ozone atmosphere at 230℃ for 2 days at a heating rate of 1℃ / min to remove the template agent, thus obtaining the W-MFI molecular sieve membrane.

[0079] Comparative Example 3

[0080] Steps (1) and (2) are the same as steps (1) and (3) in Example 1.

[0081] Step (3): Preparation of Si-MFI molecular sieve membrane:

[0082] The preparation steps are basically the same as step (3) in Example 3, except that Na2WO4·2H2O is not added to the membrane synthesis solution, and the membrane obtained is an all-silicon MFI molecular sieve membrane.

[0083] The XRD patterns of the molecular sieve membranes prepared in Examples 1 and 1, 2 and 2, and 3 and 3 are shown below. Figure 1 , Figure 2 and Figure 3 As shown, the membranes prepared by the three methods all conform to the crystal form of MFI molecular sieves. The surface and cross-sectional SEM images of the molecular sieve membranes prepared in Examples 1 and 2, 2 and 3, and 3 are shown below. Figure 4 , Figure 5 and Figure 6 As shown in the figure, the molecular sieve membranes prepared by the three methods provided in Examples 1-3 have better crystallinity and better inter-crystal growth, but the membrane prepared by the dry gel conversion method is thinner.

[0084] N2 adsorption-desorption isotherm ( Figure 7 This indicates that the BET surface area and micropore volume (377.02 m²) of the W-MFI molecular sieve membrane prepared in Example 1 are... 2 / g and 0.17m 3 The BET surface area and micropore volume (501.45 m² / g) of the Si-MFI molecular sieve membrane prepared in Comparative Example 1 were lower than those of the membrane prepared in Comparative Example 1. 2 / g and 0.21m 3 / g), which indicates that introducing W into the framework of the MFI molecular sieve membrane will reduce the pore size of the membrane.

[0085] Surface water contact angle ( Figure 8The results show that the water contact angle of the W-MFI molecular sieve membrane prepared in Example 1 is greater than that of the Si-MFI molecular sieve membrane prepared in Comparative Example 1. This indicates that the introduction of W into the framework of the MFI molecular sieve membrane will improve the hydrophobicity of the membrane.

[0086] The gas separation performance of a membrane is represented by two parameters: the gas permeation rate P and the separation coefficient a. The gas permeation rate P represents the total amount of gas passing through a unit area of ​​membrane per unit time and unit pressure, P = N / (A × ΔP), with units of mol·m³. -2 ·s -1 ·Pa -1 The separation coefficient α is used to evaluate the efficiency of membrane separation; α = P A / P B .

[0087] The CO2 / N2 gas separation performance of the W-MFI and Si-MFI molecular sieve membranes prepared in the examples and comparative examples was tested. The test conditions were: temperature 25℃, permeate pressure maintained at 0.1 MPa, feed gas composition of 60:60 mL / min, and total feed gas flow rate of 170 mL / min. The gas flow rate on the permeate side was measured using a soap bubble flow meter; the gas composition on the permeate side was analyzed using gas chromatography; the test results are shown in Table 1.

[0088] Table 1. Gas separation performance test results

[0089]

[0090] As shown in Table 1, compared to Si-MFI molecular sieve membranes, the W-MFI molecular sieve membranes prepared in the examples and comparative examples exhibit better CO2 / N2 separation performance when used for CO2 / N2 separation. Furthermore, the W-MFI molecular sieve membrane prepared in Example 1 has a CO2 / N2 separation factor as high as 38, which is better than membranes prepared by other methods.

[0091] Based on the Langmuir equation, the single-component adsorption data obtained at different temperatures were fitted ( Figure 9 , 10 (11, 12), and then the adsorption heat data were obtained. The fitting correlation data are shown in Table 2.

[0092] Table 2 Adsorption heat fitting data

[0093]

[0094] As shown in Table 2, the introduction of W increases the membrane's adsorption capacity for CO2. Furthermore, the CO2 / N2 / H2O stability test results ( Figure 13It can be seen that even in the presence of water vapor, the W-MFI molecular sieve membrane still exhibits high separation selectivity and stability for CO2 mixtures.

[0095] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An application of an MFI molecular sieve membrane for carbon capture in CO2 / N2 separation, characterized in that, The preparation method of MFI molecular sieve membrane includes the following steps: S1. Prepare MFI molecular sieve seed crystals and prepare a seed crystal suspension; load the MFI molecular sieve seed layer on the surface of a porous support modified with a vapor phase SiO2 layer using the dip-coating method; S2. W element is introduced into the surface of a porous carrier loaded with MFI molecular sieve seed layer by means of dry gel conversion method, hydrothermal synthesis method or ultra-dilute solution preparation method, and then calcined to obtain W-doped MFI molecular sieve membrane. The specific method for preparing W-doped MFI molecular sieve membranes via dry gel conversion is as follows: H2O, template agent, silicon source, and tungsten source are mixed in a molar ratio of (30-80):(0.1-0.3):(0.5-1.5):(0.02-0.07) and then stirred and aged for 2-5 hours to obtain the synthesis solution. The porous support loaded with MFI molecular sieve seed layer was placed in the synthesis solution for 0.5-3 min, and then dried at 120-160℃ for 0.5-2 h; then placed in a reaction vessel containing 0.5-3 g of water and dried at 160-200℃ for 20-25 h, quenched with cold water, the membrane was taken, washed with water and dried. The membrane is then placed in an air, oxygen, or ozone atmosphere and calcined at 200-260℃ for 40-60 hours; the heating rate is 0.5-3℃ / min. The specific method for preparing W-doped MFI molecular sieve membranes via hydrothermal synthesis is as follows: H2O, template agent, silicon source, and tungsten source are mixed in a molar ratio of (100-250):(0.1-0.3):(0.5-1.5):(0.04-0.09) and then stirred and aged for 4-7 hours to obtain the synthesis solution. The porous support loaded with MFI molecular sieve seed layer was placed in the synthesis solution and reacted at 120-160℃ for 40-60h. The reaction was then quenched with cold water, the membrane was taken, washed with water and dried. The membrane is calcined at 200-260℃ for 40-60 hours in an ozone atmosphere; the heating rate is 0.5-3℃ / min. The specific method for preparing W-doped MFI molecular sieve membranes using the ultra-dilute solution preparation method is as follows: H2O, template agent, silicon source, and tungsten source are mixed in a molar ratio of (1000-2500):(0.1-0.3):(0.5-1.5):(0.02-0.07) and then stirred and aged for 4-7 hours to obtain the synthesis solution. The porous support loaded with MFI molecular sieve seed layer was placed in the synthesis solution and reacted at 120-160℃ for 40-60h. The reaction was then quenched with cold water, the membrane was taken, washed with water and dried. The membrane is calcined at 200-260℃ for 40-60 hours in an ozone atmosphere; the heating rate is 0.5-3℃ / min.

2. The application according to claim 1, characterized in that, In step S1, the method for preparing MFI molecular sieve seeds is as follows: According to the molar ratio of H2O: TPAOH: SiO2: C2H5OH in the synthesis solution of (10-200):(0.1-0.5):(0.5-1.5):(2-6), water, tetrapropylammonium hydroxide and tetraethyl orthosilicate are mixed and stirred for 1-5 hours, and then hydrothermally synthesized at 100-200℃ for 1-72 hours. After the reaction is complete, the mixture is cooled, centrifuged, washed, dried, and then calcined at 500-750℃ for 5-8 hours.

3. The application according to claim 1, characterized in that, In step S1, the concentration of the seed crystal suspension is 0.2-2 wt%; the porous support is hollow fiber Al2O3.

4. The application according to claim 1, characterized in that, In step S1, the operation time of the immersion lifting method is 5-60 seconds, and after completion, it is dried at 120-160℃.

5. The application according to claim 1, characterized in that, Before the dip-coating process, a sintering step was performed on the porous support with a vapor phase SiO2 layer on its surface. The sintering conditions were 450-700℃ for 5-20 hours.

6. The application according to claim 1, characterized in that, The template agent is tetrapropylammonium hydroxide; the silicon source is at least one of tetraethyl orthosilicate, fumed SiO2, and silica sol; the tungsten source is at least one of sodium tungstate dihydrate and ammonium tungstate.

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