An MFI-type molecular sieve membrane and its preparation method, its application in separating mixtures of normal and isomeric alkanes, and the separation method.

Abstract

This invention relates to an MFI-type molecular sieve membrane and its preparation method, its application in separating mixtures of normal and isomeric alkanes, and the separation method. The preparation method includes: (1) subjecting a seed crystal synthesis solution to a first crystallization by microwave heating; (2) loading molecular sieve seeds onto a carrier; (3) subjecting the carrier loaded with the molecular sieve seed layer to a second crystallization by microwave heating in a membrane synthesis solution to obtain an MFI-type molecular sieve membrane; the temperature of the second crystallization is 150-180°C, and the time is 1-4 hours. This invention prepares a short b-axis sheet-like MFI-type molecular sieve membrane by microwave heating, adding additives to the seed crystal synthesis solution and the membrane synthesis solution respectively. It exhibits excellent separation and permeation performance in the separation of mixtures of normal and isomeric alkanes, greatly shortening the entire preparation cycle time, significantly improving the preparation efficiency of zeolite molecular sieve membranes, and reducing pollution.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve technology, specifically to an MFI type molecular sieve membrane and its preparation method, its application in separating mixtures of normal and isomeric alkanes, and the separation method thereof. Background Technology

[0002] Refining and chemical enterprises possess abundant liquefied petroleum gas (LPG) resources, primarily consisting of n-butane, isobutane, and propane. Separating these components using traditional distillation equipment is energy-intensive. Membrane separation can significantly reduce energy consumption. MFI-type molecular sieves possess one set of 0.53 × 0.56 nm linear channels parallel to the b-axis and another set of 0.51 × 0.55 nm sinusoidal channels parallel to the a-axis. The pore size falls within the kinetic diameter range of many important industrial raw materials, resulting in excellent separation performance for n- and isobutane.

[0003] Chinese patent CN113184875A discloses a method for preparing an all-silica short b-axis plate-type ZSM-5 zeolite molecular sieve. The method is simple to prepare and the synthesized molecular sieve crystals have short b-axis. However, the traditional heating method used in this method results in a long synthesis time. Furthermore, this patent only discusses the preparation method of molecular sieve seed crystals, not the preparation of molecular sieve membranes.

[0004] How to control the process of preparing MFI molecular sieve membranes to achieve good separation performance, and how to easily and quickly prepare high-performance molecular sieve membranes composed of structurally complete, size-uniform, short b-axis plate-type MFI molecular sieves, is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] This invention provides an MFI-type molecular sieve membrane and its preparation method, its application in separating mixtures of normal and isomeric alkanes, and the separation method. The aim is to rapidly prepare a dense, high-performance molecular sieve membrane composed of short b-axis plate-type MFI molecular sieves.

[0006] In a first aspect, the present invention relates to a method for preparing an MFI type molecular sieve membrane, the method comprising the following steps: (1) subjecting a seed crystal synthesis solution to a first crystallization by microwave heating, and then separating the solid phase product to obtain molecular sieve seed crystals; (2) loading the molecular sieve seed crystals onto a carrier to obtain a carrier loaded with a molecular sieve seed layer; (3) subjecting the carrier loaded with the molecular sieve seed layer to a second crystallization by microwave heating in a membrane synthesis solution, separating the solid phase product and drying and calcining it to obtain an MFI type molecular sieve membrane; the temperature of the second crystallization is 150-180°C and the time is 1-4 hours; the seed crystal synthesis solution comprises a first silicon source, a first template agent, a first auxiliary agent and water, and the membrane synthesis solution comprises a second silicon source, a second template agent, a second auxiliary agent and water; the first auxiliary agent and the second auxiliary agent are each independently selected from urea, polyol and amino acid.

[0007] Optionally, the first silicon source and the second silicon source are tetraethyl silicate, the first template agent and the second template agent are tetrapropylammonium hydroxide, the polyol is glycerol, and the amino acid is lysine.

[0008] Optionally, in the seed crystal synthesis solution, the molar ratio of the first silicon source to the first template agent is 1:(0.2-0.4), the molar ratio of the first silicon source to the first auxiliary agent is 1:(0.1-0.5), and the molar ratio of the first silicon source to water is 1:(10-60); and / or, in the sieve membrane synthesis solution, the molar ratio of the second silicon source to the second template agent is 1:(0.2-0.4), the molar ratio of the second silicon source to the second auxiliary agent is 1:(0.1-0.5), and the molar ratio of the second silicon source to water is 1:(10-60).

[0009] Optionally, in step (1), the temperature of the first crystallization is 100-180°C and the time is 0.5-1 hour.

[0010] Optionally, in step (2): the carrier is a sheet-like porous alumina carrier, and loading the molecular sieve seed crystals onto the carrier includes: placing the sheet-like porous alumina carrier into the molecular sieve seed crystal suspension for 10-20 seconds, and then removing and drying the sheet-like porous alumina carrier to obtain the carrier loaded with the molecular sieve seed crystal layer.

[0011] Optionally, in step (2): the carrier is a tubular porous alumina carrier, and loading the molecular sieve seed crystals onto the carrier includes: coating the tubular porous alumina carrier with the molecular sieve seed crystal suspension by vacuum extraction for 10-20 seconds, and then drying it to obtain the carrier loaded with the molecular sieve seed crystal layer.

[0012] Optionally, the molecular sieve seed suspension comprises a solvent and the molecular sieve seed obtained from step (1), wherein the solvent is selected from one or more combinations of methanol, ethanol and water, and the solid-liquid mass ratio of the molecular sieve seed suspension is (0.1-0.2):100.

[0013] In a second aspect, the present invention relates to an MFI type molecular sieve membrane, which is prepared by the preparation method described in the first aspect, and the MFI type molecular sieve membrane is composed of short b-axis MFI type molecular sieve crystals.

[0014] Optionally, the short b-axis MFI type molecular sieve crystals have a length of 200–1500 nm, a width of 100–600 nm, and a thickness of 40–120 nm.

[0015] Thirdly, the present invention relates to the application of the MFI-type molecular sieve membrane prepared by the preparation method described in the first aspect or the MFI-type molecular sieve membrane described in the second aspect in the separation of mixtures of normal and isomeric alkanes.

[0016] Fourthly, the present invention relates to a method for separating a mixture of normal and isomeric alkanes, the method comprising: passing the mixture of normal and isomeric alkanes through the MFI-type molecular sieve membrane prepared by the preparation method described in the first aspect or the MFI-type molecular sieve membrane described in the second aspect.

[0017] Beneficial effects:

[0018] This invention prepares short b-axis sheet-like MFI molecular sieve membranes by microwave heating. Additives are added to both the seed crystal synthesis solution and the membrane synthesis solution to allow the seed crystals to grow on the surface of a porous ceramic support into a continuous and dense short b-axis sheet-like MFI molecular sieve membrane. This membrane exhibits excellent separation and permeation performance in the separation of mixtures of normal and isomeric alkanes. The entire preparation cycle is significantly shortened, greatly improving the preparation efficiency of zeolite molecular sieve membranes while minimizing pollution. Attached Figure Description

[0019] Figure 1 Here is a SEM image of the molecular sieve membrane M1 prepared in Example 1;

[0020] Figure 2-1 This is a SEM image of the molecular sieve seed crystals prepared in Comparative Example 1;

[0021] Figure 2-2 This is a SEM image of the molecular sieve membrane M7 prepared in Comparative Example 1;

[0022] Figure 3 This is a SEM image of the molecular sieve membrane M8 prepared in Comparative Example 2;

[0023] Figure 4 This is the XRD diffraction pattern of the molecular sieve membrane M1 prepared in Example 1. Detailed Implementation

[0024] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. Through these descriptions, the features and advantages of the present application will become clearer and more apparent.

[0025] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments. Although various aspects of embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated otherwise.

[0026] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0027] In a first aspect, the present invention relates to a method for preparing an MFI type molecular sieve membrane, the method comprising the following steps: (1) subjecting a seed crystal synthesis solution to a first crystallization by microwave heating, and then separating the solid phase product to obtain molecular sieve seed crystals; (2) loading the molecular sieve seed crystals onto a carrier to obtain a carrier loaded with a molecular sieve seed layer; (3) subjecting the carrier loaded with the molecular sieve seed layer to a second crystallization by microwave heating in a membrane synthesis solution, separating the solid phase product and drying and calcining it to obtain an MFI type molecular sieve membrane; the temperature of the second crystallization is 150-180°C and the time is 1-4 hours; the seed crystal synthesis solution comprises a first silicon source, a first template agent, a first auxiliary agent and water, and the membrane synthesis solution comprises a second silicon source, a second template agent, a second auxiliary agent and water; the first auxiliary agent and the second auxiliary agent are each independently selected from urea, polyol and amino acid.

[0028] It should be noted that in the preparation method of the present invention, in step (1), the seed crystal synthesis liquid is subjected to a microwave heating crystallization process to obtain molecular sieve seed crystals. In step (2), the molecular sieve seed crystals are loaded onto the surface of the carrier. In step (3), the molecular sieve seed crystals loaded on the carrier are grown into MFI type molecular sieve membranes in the membrane synthesis liquid by a microwave heating crystallization process.

[0029] Firstly, unlike traditional heating methods, microwave heating under the influence of an electromagnetic field heats the reaction medium in bulk through dielectric loss, offering advantages such as rapid and uniform heating, selective heating, and high energy efficiency. This significantly reduces the time required to prepare MFI molecular sieve membranes, thereby greatly improving the preparation efficiency of molecular sieve membranes.

[0030] Secondly, the first and second additives act as growth aids. In particular, by adding the first and second additives to the seed crystal synthesis solution in step (1) and the membrane synthesis solution in step (3), respectively, it is possible to better prepare seed crystals and grow them into a dense short b-axis MFI type molecular sieve membrane. In other words, by adding growth aids, the size of the molecular sieve crystals can be adjusted, and a uniform short b-axis sheet-like molecular sieve membrane can be prepared, which greatly shortens the preparation time of the molecular sieve membrane. The prepared molecular sieve membrane has excellent permeation and separation performance in the separation of mixtures of normal and isomer alkanes.

[0031] Thirdly, in step (3), the second crystallization temperature is set to 150–180°C and the time is 1–4 hours by microwave heating. During the second crystallization process, the carrier loaded with molecular sieve seeds and the sieve membrane synthesis solution are rapidly crystallized at a suitable temperature of 150–180°C for 1–4 hours by microwave heating, so that the molecular sieve seeds can be rapidly grown into short b-axis plate-like molecular sieves, and the connection between the molecular sieve crystals is more compact and continuous. When the sieve membrane grown in this way is used for the separation of mixtures of normal and isomer alkanes, the separation performance and permeation performance are further significantly improved.

[0032] It should be noted that, as Figure 1 As shown, the b-axis refers to the thickness direction of the molecular sieve crystals, the a-axis refers to the width direction of the molecular sieve crystals perpendicular to the thickness direction of the b-axis, and the c-axis refers to the length direction of the molecular sieve crystals perpendicular to the thickness direction of the b-axis.

[0033] According to a specific embodiment of the preparation method described in the first aspect of the present invention, the first silicon source and the second silicon source are tetraethyl silicate, the first template agent and the second template agent are tetrapropylammonium hydroxide, the polyol is glycerol, and the amino acid is lysine.

[0034] It should be noted that in the preparation method of the present invention, by adding the first and second auxiliary agents to the synthesis solution of the seed crystal and the sieve membrane, and simultaneously selecting the silicon source and template agent as described above, a molecular sieve membrane composed of short b-axis plate-like molecular sieves with a high aspect ratio can be better prepared. Meanwhile, the crystallization process employs microwave heating, enabling the short b-axis molecular sieve crystals to grow into a dense MFI molecular sieve membrane more quickly.

[0035] According to a specific embodiment of the preparation method described in the first aspect of the present invention, in the seed crystal synthesis solution, the molar ratio of the first silicon source to the first template agent is 1:(0.2-0.4), the molar ratio of the first silicon source to the first auxiliary agent is 1:(0.1-0.5), and the molar ratio of the first silicon source to water is 1:(10-60); and / or, in the sieve membrane synthesis solution, the molar ratio of the second silicon source to the second template agent is 1:(0.2-0.4), the molar ratio of the second silicon source to the second auxiliary agent is 1:(0.1-0.5), and the molar ratio of the second silicon source to water is 1:(10-60).

[0036] It should be noted that in the molar ratio of silicon source to other substances, the molar amount of silicon source can refer to the molar amount of silicon element in the silicon source. By controlling the molar ratio between substances in the seed crystal synthesis solution and the membrane synthesis solution as described above, it is possible to better prepare densely grown short b-axis MFI molecular sieve membranes, thereby obtaining excellent separation and permeation performance.

[0037] As a preferred embodiment, the additives in the seed crystal synthesis solution in step (1) and the sieve membrane synthesis solution in step (3) are the same substances, such as urea. As a further preferred embodiment, the composition and component ratio of the seed crystal synthesis solution and the sieve membrane synthesis solution are the same, that is, the molar ratio of the first silicon source to the first template agent and the molar ratio of the second silicon source to the second template agent are equal, the molar ratio of the first silicon source to the first additive and the molar ratio of the second silicon source to the second additive are equal, and the molar ratio of the first silicon source to water and the molar ratio of the second silicon source to water are equal. This is beneficial for preparing a sieve membrane composed of short b-axis molecular sieves with more uniform size, and the sieve membrane has better separation and permeation performance.

[0038] It should be noted that in the membrane synthesis solution, when the molar ratio of the second silicon source to the second auxiliary agent is in the range of 1:(0.1 to 0.5), for example, by controlling the molar ratio of the second auxiliary agent, such as urea, and the second silicon source, such as tetraethyl silicate, the relationship of the axial dimensions of the MFI molecular sieve crystals can be regulated. When the molar ratio of urea to tetraethyl silicate is in the above range and is larger, the size ratio of the a-axis / b-axis and the c-axis / b-axis of the MFI molecular sieve crystals is larger, and the b-axis of the molecular sieve crystals is relatively smaller, and the molecular sieve crystals exhibit a larger aspect ratio.

[0039] According to another specific embodiment of the preparation method described in the first aspect of the present invention, in step (1), the temperature of the first crystallization is 100-180°C and the time is 0.5-1 hour.

[0040] It should be noted that in the preparation method of the present invention, microwave heating is used to control the temperature for crystallization in steps (1) and (3), and the temperature and time of the first crystallization and the second crystallization are controlled, which significantly improves the speed of the entire preparation process, and the prepared MFI type molecular sieve membrane is densely grown and composed of short b-axis molecular sieves.

[0041] According to a first specific embodiment of the preparation method described in the first aspect of the present invention, in step (2): the carrier is a sheet-like porous alumina carrier, and loading the molecular sieve seed crystals onto the carrier includes: placing the sheet-like porous alumina carrier into the molecular sieve seed crystal suspension for 10-20 seconds, and then removing and drying the sheet-like porous alumina carrier to obtain the carrier loaded with the molecular sieve seed crystal layer.

[0042] It should be noted that, as mentioned above, when the support is a sheet-like porous alumina support, molecular sieve seeds can be loaded onto the support using methods such as the Czochralski method.

[0043] According to a second specific embodiment of the preparation method described in the first aspect of the present invention, in step (2): the carrier is a tubular porous alumina carrier, and loading the molecular sieve seed crystals onto the carrier includes: coating the tubular porous alumina carrier with the molecular sieve seed crystal suspension by vacuum extraction for 10-20s, and then drying to obtain the carrier loaded with the molecular sieve seed crystal layer.

[0044] It should be noted that, as mentioned above, when the support is a tubular porous alumina support, molecular sieve seeds can be loaded onto the support using methods such as vacuum extraction.

[0045] According to a first or second specific embodiment of the preparation method described in the first aspect of the present invention, the molecular sieve seed suspension comprises a solvent and the molecular sieve seed obtained from step (1), wherein the solvent is selected from one or more combinations of methanol, ethanol and water, and the solid-liquid mass ratio of the molecular sieve seed suspension is (0.1-0.2):100.

[0046] It should be noted that a seed crystal suspension can be prepared before proceeding to step (2). 1 g to 2 g of the dried molecular sieve seed crystals obtained in step (1) can be dispersed in 1000 g of solvent to obtain a molecular sieve seed crystal suspension. Specifically, the molecular sieve seed crystals are dispersed in the solvent, preferably by shaking and ultrasonic treatment, so that the molecular sieve seed crystals are uniformly dispersed in the solvent to obtain a molecular sieve seed crystal suspension.

[0047] It should be noted that in step (2), the molecular sieve seed crystals in the molecular sieve seed suspension are coated onto the carrier, and then placed in an oven for drying treatment to form a continuous and dense molecular sieve crystal layer on the surface of the carrier or support, and then the subsequent step (3) is carried out.

[0048] It should be noted that in step (3), the carrier loaded with molecular sieve seed layer and the sieve membrane synthesis liquid can be placed together in a reaction vessel suitable for microwave heating, so that the liquid level of the sieve membrane synthesis liquid is 2 to 6 cm higher than the carrier loaded with molecular sieve seed layer, and then microwave heating crystallization is performed. The solid product is washed with deionized water, dried and calcined to obtain a short b-axis MFI type molecular sieve membrane.

[0049] The preparation method of the MFI-type molecular sieve membrane of the present invention is simple, adjustable, and has a short preparation time. Molecular sieve crystals with high aspect ratios are more likely to form a dense membrane on the support surface, thereby obtaining excellent separation performance. In the preparation method of the present invention, the first microwave heating and the second microwave heating can be carried out in a microwave synthesizer with a set temperature, so as to facilitate the control of the temperature and time of the microwave heating process.

[0050] In a second aspect, the present invention relates to an MFI type molecular sieve membrane, which is prepared by the preparation method described in the first aspect of the present invention, and the MFI type molecular sieve membrane is composed of short b-axis MFI type molecular sieve crystals.

[0051] It should be noted that the MFI-type molecular sieve membrane of the present invention is a dense molecular sieve membrane composed of short b-axis plate-like MFI molecular sieve crystals.

[0052] According to a specific embodiment of the MFI type molecular sieve membrane of the second aspect of the present invention, the short b-axis MFI type molecular sieve crystals have a length of 200-1500 nm, a width of 100-600 nm, and a thickness of 40-120 nm.

[0053] Thirdly, the present invention relates to the application of the MFI-type molecular sieve membrane prepared by the preparation method described in the first aspect or the MFI-type molecular sieve membrane described in the second aspect in the separation of mixtures of normal and isomeric alkanes.

[0054] Fourthly, the present invention relates to a method for separating a mixture of normal and isoparaffins, the method comprising: passing the mixture of normal and isoparaffins through the MFI-type molecular sieve membrane prepared by the preparation method described in the first aspect of the present invention or the MFI-type molecular sieve membrane described in the second aspect of the present invention.

[0055] It should be noted that the MFI-type molecular sieve membrane prepared by the preparation method described in the first aspect of the present invention, or the MFI-type molecular sieve membrane described in the second aspect, exhibits excellent permeation rate and separation effect when used for the separation of normal and isomeric alkanes. That is, the MFI-type molecular sieve membrane of the present invention has excellent permeation and separation performance in the separation of mixtures of normal and isomeric alkanes. Specifically, in the separation method of the present invention, during the separation process of a mixture of normal and isomeric alkanes, such as a mixture of n-butane and isobutane, flowing through the MFI-type molecular sieve membrane, the temperature can be 50–70°C and the pressure can be 8–12 kPa.

[0056] The present invention will be further described in detail below through examples, but this does not limit the present invention. Unless otherwise specified, all reagents used in the following examples are commercially available finished reagents.

[0057] The sheet-like porous alumina used in the following examples or comparative examples can be prepared by the following preparation examples, or the sheet-like porous alumina used in the following examples or comparative examples can also be a commercially available finished carrier, and the tubular porous alumina carrier used in the following examples or comparative examples can be prepared by referring to the method in "Journal of Membrane Science, 503 (2016), 69-80".

[0058] Example of preparation of sheet-like porous alumina:

[0059] Mix 50g of alumina powder with 2mL of deionized water until uniform, grind the mixture, weigh 3g of uniformly ground alumina powder into a mold, press it under 10MPa pressure for 30s to form the shape, and then calcine it at 1200℃ for 4h to obtain an alumina support with an average pore size of 200nm and a porosity of 30%, with a diameter of 25mm and a thickness of 2mm.

[0060] Example of preparation of tubular porous alumina support:

[0061] Alumina powder, polyvinylpyrrolidone, N-methyl-2-pyrrolidone, and polyethersulfone were mixed evenly in a certain proportion and stirred for 48 hours to prepare a spinning solution. The spinning solution was extruded into a hollow fiber preform through a spinneret under nitrogen pressure. The preform passed through a 15cm air gap into a tap water tank and remained submerged for 10–20 hours to allow for sufficient exchange between the solvent and non-solvent. Then, it was straightened and dried at room temperature. The dried hollow fiber matrix was then placed in a high-temperature calcination furnace and calcined at 1350℃–1450℃ for 4 hours. Finally, it was cooled to room temperature with the furnace to obtain a tubular support.

[0062] Example 1

[0063] (1) Seed preparation: 12g of tetraethyl silicate was dissolved in 10g of tetrapropylammonium hydroxide aqueous solution (mass fraction 25%) and 20g of deionized water solution, and then 1g of urea was added. The mixture was stirred and aged at room temperature and 150rpm for 24h. The mixture was synthesized at 120℃ for 40min using a microwave synthesizer. The prepared crystals were washed with deionized water and centrifuged to obtain molecular sieve seed crystals.

[0064] (2) Coating the support with seed crystals: Weigh 1.0g of molecular sieve seed crystals (prepared in step (1)) and add them to 1000g of ethanol. After ultrasonic and vibration treatment, the crystals are uniformly dispersed in the ethanol to form a uniform molecular sieve suspension. The sheet-like porous alumina support prepared in the above preparation example is placed in the molecular sieve suspension for 20s and then taken out of the suspension at a uniform speed. After drying in an oven, a continuous and dense molecular sieve crystal layer is formed on the surface of the support.

[0065] (3) Synthesis of MFI type molecular sieve membrane: 12g of tetraethyl silicate was dissolved in 10g of tetrapropylammonium hydroxide solution (mass fraction 25%) and 20g of deionized water solution. Then, 1g of urea was added to the synthesis solution. The mixture was stirred and aged at room temperature and 150rpm for 24h. The support coated with molecular sieve crystals from step (2) was placed into the synthesis solution and then microwave heated. The mixture was crystallized at 180℃ for 1h using a microwave synthesizer. After the reaction was complete, the solid product was separated, washed with deionized water for 15 minutes, dried in an oven at 50℃, and calcined at 450℃ for 4 hours to obtain a short b-axis plate type MFI molecular sieve membrane, denoted as M1.

[0066] Example 2

[0067] (1) Seed preparation: The preparation process is the same as step (1) in Example 1, except that the auxiliary agent urea is replaced with an equal mass of glycerol;

[0068] (2) Seed coating on support: The preparation process is the same as step (2) in Example 1;

[0069] (3) Synthesis of MFI type molecular sieve membrane: The preparation process is the same as step (3) in Example 1, except that the auxiliary agent urea is replaced with an equal mass of glycerol;

[0070] The prepared molecular sieve membrane is denoted as M2.

[0071] Example 3

[0072] (1) Seed preparation: The preparation process is the same as step (1) in Example 1;

[0073] (2) Support coated with seed crystals: The preparation process is the same as step (2) in Example 1. The difference is that the carrier is the tubular porous alumina carrier prepared in the above preparation example. One end of the porous alumina carrier is sealed and the other end is connected to a vacuum device. The molecular sieve seed crystal suspension is coated onto the surface of the carrier by vacuum extraction for 10-20s, and then dried to obtain a tubular porous alumina carrier coated with molecular sieve seed crystals.

[0074] (3) Synthesis of MFI type molecular sieve membrane: The preparation process is the same as step (3) in Example 1;

[0075] The prepared molecular sieve membrane is designated as M3.

[0076] Example 4

[0077] (1) Seed preparation: The preparation process is the same as step (1) in Example 1, except that 12g of tetraethyl silicate is dissolved in 10g of tetrapropylammonium hydroxide solution (mass fraction 25%) and 10g of deionized water solution, and 1g of urea is added;

[0078] (2) Seed coating on support: The preparation process is the same as step (2) in Example 1;

[0079] (3) Synthesis of MFI type molecular sieve membrane: The preparation process is the same as step (3) in Example 1, except that 10g of tetraethyl silicate is dissolved in 10g of tetrapropylammonium hydroxide solution (mass fraction 25%) and 9g of deionized water solution, and then 1g of urea is added.

[0080] The prepared molecular sieve membrane is designated as M4.

[0081] Example 5

[0082] (1) Seed preparation: The preparation process is the same as step (1) in Example 1;

[0083] (2) Seed coating on support: The preparation process is the same as step (2) in Example 1;

[0084] (3) Synthesis of MFI type molecular sieve membrane: The preparation process is the same as step (3) in Example 1, except that the crystallization conditions are different. A microwave synthesizer is used to crystallize at 160°C for 2 hours.

[0085] The prepared molecular sieve membrane is designated as M5.

[0086] Example 6

[0087] (1) Seed preparation: The preparation process is the same as step (1) in Example 1, except that the amount of urea added is different, with 0.5g of urea added;

[0088] (2) Seed coating on support: The preparation process is the same as step (2) in Example 1;

[0089] (3)MFI type molecular sieve membrane synthesis: The preparation process is the same as step (3) in Example 1, except that the amount of urea added is different, with 0.5g of urea added.

[0090] The prepared molecular sieve membrane is designated as M6.

[0091] Comparative Example 1

[0092] (1) Seed preparation: The preparation process is the same as step (1) in Example 1, except that the growth aid urea is not added;

[0093] (2) Seed coating on support: The preparation process is the same as step (2) in Example 1;

[0094] (3) Synthesis of MFI type molecular sieve membrane: The preparation process is the same as step (3) in Example 1; the difference is that the auxiliary agent urea is not added;

[0095] The prepared molecular sieve membrane is designated as M7.

[0096] Comparative Example 2

[0097] (1) Seed preparation: The preparation process is the same as step (1) in Example 1;

[0098] (2) Seed coating on support: The preparation process is the same as step (2) in Example 1;

[0099] (3) Synthesis of MFI type molecular sieve membrane: The preparation process is the same as step (3) in Example 1; the difference is that the crystallization conditions are different, crystallization at 120℃ for 40 min;

[0100] The prepared molecular sieve membrane is designated as M8.

[0101] Test Example 1

[0102] The molecular sieve membranes prepared in the above examples and comparative examples were used at 60℃ and 10KPa under n- / i-C4H 10 The gas separation performance was characterized in a (40 / 60) system, and the test results are shown in Table 1. The gas separation performance of the prepared MFI molecular sieve membrane was tested by measuring the separation performance of a 4:6 molar ratio of n- / isobutane mixed gas, and evaluated using two indicators: gas permeation rate P and separation factor SF. Gas permeation rate P represents the total amount of gas permeating through a unit area of ​​membrane per unit time and unit pressure, P = N / (A×t×ΔP), with units of mol / (m²). 2 ·s·pa), where N is the molar amount of gas permeating the membrane, A is the area, t is the time, and ΔP is the pressure; the separation factor SF is used to evaluate the separation efficiency of the membrane, SF=(y 1，p / y2, p ) / (y 1，f / y 2，f ); where: y 1，p y represents the mass fraction of n-butane on the permeate side. 2，p y represents the mass fraction of isobutane on the permeate side. 1，f y represents the mass fraction of n-butane in the raw material; 2，f This represents the mass fraction of isobutane in the raw material.

[0103] Table 1

[0104]

[0105] As can be seen from the data in Table 1, the molecular sieve membrane prepared by the method of the present invention in the examples has a good separation effect on n- / isobutane.

[0106] Test Example 2

[0107] The molecular sieve membranes prepared in the above examples and comparative examples were analyzed for morphology using scanning electron microscopy. Figure 1 This is a surface SEM image of the short b-axis plate-type MFI molecular sieve membrane M1 prepared by microwave crystallization in step (3) of Example 1. The average size of the molecular sieve grains is approximately 1100 nm × 500 nm × 80 nm. Figure 1 It can be seen that the short b-axis plate-type MFI molecular sieve crystals grow continuously and densely on the support surface. Upon examination, the SEM images of the molecular sieve membranes prepared in Examples 2 to 6 show that... Figure 1 Similarly, the molecular sieve membranes of Example 1 have essentially the same morphological characteristics, and both membranes are composed of short b-axis molecular sieve crystals.

[0108] Figure 2-1 The image shows a SEM image of the molecular sieve seed crystals prepared in Comparative Example 1. Figure 2-2 The image shows a SEM image of the molecular sieve membrane prepared in Comparative Example 1. It can be seen from the image that, under the same conditions, the molecular sieve seed crystals and the molecular sieve membrane prepared without the addition of the auxiliary agent urea have a cylindrical morphology with an average size of about 210×180nm×140nm and a large thickness.

[0109] Figure 3 The image shows a surface SEM image of the molecular sieve membrane M8 prepared in Comparative Example 2. It can be seen from the image that the average size of the molecular sieve grains is about 350nm×250nm×80nm. The molecular sieve grains grow alternately on the surface of the support, but there are gaps in the molecular sieve membrane.

[0110] Test Example 3

[0111] XRD analysis was performed on the molecular sieves prepared in the above examples and comparative examples. The XRD pattern of molecular sieve membrane M1 prepared in Example 1 is shown in [reference needed]. Figure 4 .pass Figure 4 It can be seen that the molecular sieve membrane prepared in Example 1 exhibits typical characteristic peaks of MFI molecular sieves. The XRD patterns of the molecular sieves prepared in other examples are similar to... Figure 4 The positions of the diffraction peaks are basically the same, indicating that the membrane prepared is also an MFI molecular sieve membrane.

[0112] Test Example 4

[0113] The yields of the molecular sieve seeds prepared in the above embodiments and comparative examples were calculated using the following method: Where m1 is the theoretical mass of the molecular sieve seed crystals that can be prepared by the silicon source actually used in the seed crystal synthesis solution of step (1) as SiO2, and m2 is the mass of the molecular sieve seed crystals actually prepared in step (1). The results are shown in Table 2 below.

[0114] Table 2

[0115] Molecular sieve seed yield, % Example 1-M1 90.4 Example 2-M2 72.3 Example 3-M3 90.4 Example 4-M4 90.8 Example 5-M5 90.4 Example 6-M6 90.2 Comparative Example 1-M7 79.2 Comparative Example 2-M8 90.4

[0116] As can be seen from the data in Table 2, the examples show that when molecular sieve seeds are prepared using the method of this application and urea is used as an auxiliary agent, the yield is high.

[0117] Test Example 5

[0118] The dimensions of the a-axis, b-axis, and c-axis of the molecular sieve grains in the molecular sieve membranes prepared in the above embodiments and comparative examples were detected by scanning electron microscopy, and the size ratios of the a-axis and b-axis, as well as the c-axis and b-axis, were calculated. Some results are shown in Table 3 below:

[0119] Table 3

[0120]

[0121] Research has shown that the axial dimensions of MFI molecular sieve crystals can be controlled by adjusting the molar ratio of additives such as urea and silicon sources such as tetraethyl silicate in the sieve synthesis solution. When the molar ratio of silicon source to additive is in the range of 1:(0.1~0.5), the smaller the molar ratio of silicon source to additive, that is, the larger the molar ratio of additives such as urea to silicon sources such as tetraethyl silicate, the larger the a-axis / b-axis and c-axis / b-axis ratios of the MFI molecular sieve crystals. Consequently, the b-axis of the molecular sieve crystals is relatively smaller, and the molecular sieve crystals exhibit a larger aspect ratio.

[0122] In summary, the method for preparing the MFI-type molecular sieve membrane of this application uses a microwave synthesizer to synthesize molecular sieve seeds and molecular sieve membranes by microwave heating, which is faster and more efficient. By comprehensively controlling the composition of the seed synthesis solution and the membrane synthesis solution, as well as the temperature and time conditions during the membrane growth process, a densely grown MFI-type molecular sieve membrane with good separation effect was successfully prepared.

[0123] The present application has been described above with reference to preferred embodiments; however, these embodiments are merely exemplary and illustrative. Various substitutions and modifications can be made to the present application based on these embodiments, all of which fall within the protection scope of the present application.

Claims

1. A method for preparing an MFI-type molecular sieve membrane, wherein, The preparation method includes the following steps: (1) The seed crystal synthesis solution is subjected to first microwave heating for first crystallization, and then the solid product is separated to obtain molecular sieve seed crystals; (2) Load the molecular sieve seed crystals onto the carrier to obtain a carrier loaded with a molecular sieve seed crystal layer; (3) The carrier loaded with the molecular sieve seed layer is subjected to a second crystallization in the sieve membrane synthesis solution by a second microwave heating, the solid phase product is separated and dried and calcined to obtain an MFI type molecular sieve membrane; the temperature of the second crystallization is 150-180℃ and the time is 1-4 hours. The seed crystal synthesis solution comprises a first silicon source, a first template agent, a first auxiliary agent, and water; the membrane synthesis solution comprises a second silicon source, a second template agent, a second auxiliary agent, and water; the first auxiliary agent and the second auxiliary agent are each independently selected from urea, polyol, and amino acid.

2. The preparation method according to claim 1, wherein, The first silicon source and the second silicon source are tetraethyl silicate, the first template agent and the second template agent are tetrapropylammonium hydroxide, the polyol is glycerol, and the amino acid is lysine.

3. The preparation method according to claim 1, wherein, In the seed crystal synthesis solution, the molar ratio of the first silicon source to the first template agent is 1:(0.2-0.4), the molar ratio of the first silicon source to the first auxiliary agent is 1:(0.1-0.5), and the molar ratio of the first silicon source to water is 1:(10-60); and / or, In the membrane synthesis solution, the molar ratio of the second silicon source to the second template agent is 1:(0.2-0.4), the molar ratio of the second silicon source to the second auxiliary agent is 1:(0.1-0.5), and the molar ratio of the second silicon source to water is 1:(10-60).

4. The preparation method according to claim 1, wherein, In step (1), the temperature of the first crystallization is 100-180°C and the time is 0.5-1 hour.

5. The preparation method according to claim 1, wherein, In step (2): The carrier is a sheet-like porous alumina carrier. Loading the molecular sieve seed crystals onto the carrier includes: placing the sheet-like porous alumina carrier into a molecular sieve seed crystal suspension for 10-20 seconds, and then removing and drying the sheet-like porous alumina carrier to obtain the carrier loaded with the molecular sieve seed crystal layer.

6. The preparation method according to claim 1, wherein, In step (2): The carrier is a tubular porous alumina carrier. Loading the molecular sieve seed crystals onto the carrier includes: coating the tubular porous alumina carrier with the molecular sieve seed crystal suspension by vacuum extraction for 10-20 seconds, and then drying it to obtain the carrier loaded with the molecular sieve seed crystal layer.

7. The preparation method according to claim 5 or 6, wherein, The molecular sieve seed suspension comprises a solvent and the molecular sieve seed obtained from step (1), wherein the solvent is selected from one or more combinations of methanol, ethanol and water, and the solid-liquid mass ratio of the molecular sieve seed suspension is (0.1-0.2):

100.

8. An MFI type molecular sieve membrane, wherein, The MFI-type molecular sieve membrane is prepared by the preparation method according to any one of claims 1 to 7, and the MFI-type molecular sieve membrane is composed of short b-axis MFI-type molecular sieve crystals.

9. The MFI type molecular sieve membrane according to claim 8, wherein, The short b-axis MFI type molecular sieve crystals have a length of 200–1500 nm, a width of 100–600 nm, and a thickness of 40–120 nm.

10. The use of the MFI type molecular sieve membrane prepared by the preparation method according to any one of claims 1 to 7, or the MFI type molecular sieve membrane according to claim 8 or 9, in the separation of a mixture of normal and isomeric alkanes.

11. A method for separating a mixture of normal and isomeric alkanes, wherein, The separation method includes passing a mixture of normal isoalkanes through the MFI-type molecular sieve membrane prepared by any one of claims 1 to 7 or the MFI-type molecular sieve membrane as described in claim 8 or 9.

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