Mfi molecular sieve membrane, preparation method and application thereof in separating normal and iso-paraffin mixture

Abstract

The application relates to an MFI molecular sieve membrane and a preparation method and application in separating normal and iso-paraffin mixtures, the preparation method comprising the following steps: (1) performing first crystallization on a first molecular sieve synthesis solution, then separating a solid-phase product to obtain a molecular sieve seed; (2) loading the molecular sieve seed on a carrier to obtain a carrier loaded with a molecular sieve seed layer; (3) sequentially performing second crystallization at a second temperature and third crystallization at a third temperature on the carrier loaded with the molecular sieve seed layer in a second molecular sieve synthesis solution, separating a solid-phase product and performing drying and calcination to obtain the MFI molecular sieve membrane. The MFI molecular sieve membrane prepared by the preparation method has parallel upper surfaces of flaky molecular sieve crystals, the molecular sieve crystals are tightly connected, a dense MFI molecular sieve membrane is formed, and excellent separation performance of normal and iso-paraffin is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of molecular sieve, in particular to a MFI molecular sieve membrane, a preparation method thereof and application thereof in separating normal and isomeric alkane mixture. BACKGROUND

[0002] MFI type molecular sieve membrane has regular pore structure, pore size is between the kinetic diameters of many important industrial raw materials and excellent thermal stability, and has important application value in the separation field. It has a group of 0.53*0.56 nm straight channels parallel to the b axis and another group of 0.51*0.55 nm sinusoidal channels parallel to the a axis. When the crystal grows in different directions on the carrier, the pore structure in the membrane is very different, and the mass transfer characteristics are also different. According to the related literature, the diffusion speed of molecules in the straight channel is faster than that in the sinusoidal track.

[0003] Therefore, in order to construct a high-performance molecular sieve membrane, we need smaller thin sheet-like molecular sieve crystals to grow on the surface of the carrier to obtain a dense molecular sieve membrane. Compared with the traditional micron-sized MFI molecular sieve crystals, when the nanosheet crystals grow on the surface of the carrier, the intercrystalline defects in the membrane will be significantly reduced, the membrane thickness and mass transfer resistance will be correspondingly reduced, and the molecular sieve membrane will not be easily affected by the intercrystalline stress caused by the thermal shrinkage of the crystal during the removal of the template by high-temperature calcination, and thus the molecular sieve membrane can be more dense to more effectively improve the permeation flux and separation selectivity.

[0004] Chinese patent CN113184875A discloses a preparation method of ZSM-5 zeolite molecular sieve. The method is simple to prepare, but the patent only discusses the preparation method of the molecular sieve seed crystal, and not the preparation of the molecular sieve membrane.

[0005] In the process of preparing the MFI molecular sieve membrane, how to make the molecular sieve crystals grow in a direction to obtain a dense molecular sieve membrane so as to have good permeation performance and separation performance in the separation of normal and isomeric alkane mixture is a technical problem to be solved at present. SUMMARY

[0006] The present application provides a MFI molecular sieve membrane, a preparation method thereof and application thereof in separating normal and isomeric alkane mixture. The purpose of the present application is to make the upper surfaces of the thin sheet-like molecular sieve crystals grow in parallel to obtain a dense MFI molecular sieve membrane, and thus to improve the permeation performance and separation performance of the molecular sieve membrane in the separation of normal and isomeric alkane mixture.

[0007] In a first aspect, the present application relates to a method for preparing a MFI zeolite membrane, comprising the following steps: (1) subjecting a first zeolite synthesis solution to a first crystallization, and then separating a solid-phase product to obtain a zeolite seed crystal; (2) loading the zeolite seed crystal onto a carrier to obtain a carrier loaded with a zeolite seed crystal layer; (3) subjecting the carrier loaded with the zeolite seed crystal layer to a second crystallization at a second temperature and a third crystallization at a third temperature in sequence in a second zeolite synthesis solution, separating a solid-phase product and drying and calcining the solid-phase product to obtain the MFI zeolite membrane; the second temperature is 100-120°C, the second crystallization lasts for 0.1-8h, the third temperature is 160-180°C, and the third crystallization lasts for 6-14h; the first zeolite synthesis solution and the second zeolite synthesis solution each independently comprise a silicon source, a MFI zeolite template agent, an auxiliary agent, and water.

[0008] Optionally, the auxiliary agent is selected from a combination of one or more of urea, a polyhydric alcohol, and an amino acid.

[0009] Optionally, the silicon source is tetraethyl orthosilicate, the MFI zeolite template agent is tetrapropylammonium hydroxide, the polyhydric alcohol is glycerol, and the amino acid is lysine.

[0010] Optionally, in the first zeolite synthesis solution and the second zeolite synthesis solution: the molar ratio of the silicon source to the MFI zeolite template agent is each independently 1:(0.2-0.4), the molar ratio of the silicon source to the auxiliary agent is each independently 1:(0.1-0.5), and the molar ratio of the silicon source to water is each independently 1:(10-60).

[0011] Optionally, in step (1): the first crystallization is performed at a first temperature, and the first temperature is 100-180°C, and the first crystallization lasts for 4-8h.

[0012] Optionally, in step (2): the carrier is a sheet-shaped porous alumina carrier, and loading the zeolite seed crystal onto the carrier comprises: placing the sheet-shaped porous alumina carrier in a zeolite seed crystal suspension for 10-20s, and then taking out and drying the sheet-shaped porous alumina carrier to obtain the carrier loaded with the zeolite seed crystal layer; or the carrier is a tubular porous alumina carrier, and loading the zeolite seed crystal onto the carrier comprises: coating the tubular porous alumina carrier with the zeolite seed crystal suspension by vacuum extraction for 10-20s, and then drying to obtain the carrier loaded with the zeolite seed crystal layer.

[0013] Optionally, the molecular sieve seed crystal suspension comprises a solvent selected from the group consisting of methanol, ethanol and water in combination of one or more thereof and the molecular sieve seed crystals from step (1), and the solid-liquid mass ratio of the molecular sieve seed crystal suspension is (0.1-0.2):100.

[0014] In a second aspect, the present application relates to a MFI molecular sieve membrane, which is prepared by the preparation method of the first aspect; the MFI molecular sieve membrane is a dense molecular sieve membrane, and the MFI molecular sieve crystals in the MFI molecular sieve membrane are in a sheet shape.

[0015] Optionally, the length of the MFI molecular sieve crystals is 100-800 nm, the width is 100-200 nm and the thickness is 40-80 nm.

[0016] In a third aspect, the present application relates to the application of the MFI molecular sieve membrane prepared by the preparation method of the first aspect or the MFI molecular sieve membrane of the second aspect in separating a normal-isomer alkane mixture.

[0017] Advantages:

[0018] The MFI molecular sieve membrane prepared by the preparation method of the present application has parallel upper surfaces of the sheet-shaped molecular sieve crystals, and the molecular sieve crystals are tightly connected to form a dense MFI molecular sieve membrane, which has excellent normal-isomer alkane permeability and separation performance. BRIEF DESCRIPTION OF DRAWINGS

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

[0020] Figure 2 is a SEM photo of the molecular sieve membrane M6 prepared in Comparative Example 1;

[0021] Figure 3 is a SEM photo of the molecular sieve membrane M7 prepared in Comparative Example 2;

[0022] Figure 4 is a SEM photo of the molecular sieve membrane M9 prepared in Comparative Example 4;

[0023] Figure 5 is an XRD chart of the molecular sieve membrane M1 prepared in Example 1 and the molecular sieve membrane M6 prepared in Comparative Example 1. DETAILED DESCRIPTION

[0024] The present application will be further described in detail by the accompanying drawings and examples. Through these descriptions, the features and advantages of the present application will become more apparent.

[0025] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any implementation described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other implementations. Unless specifically indicated otherwise, the drawings are not necessarily to scale.

[0026] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as there is no conflict.

[0027] In a first aspect, the present application relates to a preparation method of MFI zeolite membrane, comprising the following steps:

[0028] (1) crystallizing a first zeolite synthesis solution, and then separating the solid product to obtain zeolite seeds; (2) loading the zeolite seeds onto a carrier to obtain a carrier loaded with a layer of zeolite seeds; (3) sequentially subjecting the carrier loaded with the layer of zeolite seeds to second crystallization at a second temperature and third crystallization at a third temperature in a second zeolite synthesis solution, separating the solid product and drying and calcining to obtain the MFI zeolite membrane; the second temperature is 100-120℃, the time length of the second crystallization is 0.1-8h, the third temperature is 160-180℃, and the time length of the third crystallization is 6-14h; the first zeolite synthesis solution and the second zeolite synthesis solution each independently comprise a silicon source, an MFI zeolite template, an auxiliary agent, and water.

[0029] It should be noted that the first zeolite synthesis solution and the second zeolite synthesis solution can also be referred to as a seed synthesis solution and a membrane synthesis solution, respectively. After mixing and aging the silicon source, the MFI zeolite template, the auxiliary agent, and the water, the first zeolite synthesis solution or the second zeolite synthesis solution can be obtained, wherein the aging time can be 12-24h.

[0030] It should be noted that in the preparation method of the present application, the molecular sieve seed crystals are obtained through the first crystallization of step (1), and the first crystallization in step (1) can be carried out at one temperature for one-stage crystallization, or can be carried out at two temperatures for two-stage crystallization, or can be carried out at multiple temperatures for multiple-stage crystallization, respectively. After the first crystallization in step (1) is completed, the solid-phase product is separated, and then deionized water washing and centrifugation are carried out to obtain the molecular sieve seed crystals. In step (2), the molecular sieve seed crystals obtained from step (1) can be dispersed in a solvent to obtain a molecular sieve seed crystal suspension, and then the molecular sieve seed crystals are coated or loaded on the carrier or support; the method for coating or loading the seed crystals on the carrier in step (2) can use a variety of methods such as the pull-up method. In step (3), the molecular sieve seed crystals are subjected to two-stage temperature crystallization, and the upper surfaces of the molecular sieve crystals grow in parallel, for example, can be parallel to the surface of the carrier, and through the comprehensive adjustment of the temperature and time of the two-stage crystallization, the molecular sieve crystals are tightly connected, and a dense molecular sieve membrane is prepared. In summary, through steps (1)-(3) of the present application, the thin flake-shaped molecular sieve crystals are directionally grown, and a dense MFI molecular sieve membrane with good separation effect is obtained.

[0031] According to an embodiment of the preparation method of the first aspect of the present application, the auxiliary agent is selected from the combination of one or more of urea, polyhydric alcohol and amino acid.

[0032] It should be noted that in the preparation method of the present application, the addition of the auxiliary agent in the synthesis solution of the seed crystals and the sieve membrane is beneficial to making the obtained molecular sieve crystals thin flake-shaped, and in addition, in the crystallization process of the molecular sieve membrane, the temperature and time of the two-stage crystallization are controlled to make the upper surfaces of the molecular sieve crystals substantially parallel, and the molecular sieve crystals are tightly connected, and then a dense molecular sieve membrane is obtained. That is, by introducing the auxiliary agent in the first molecular sieve synthesis solution and the second molecular sieve synthesis solution, and according to the above steps (1)-(3), especially in step (3), the sieve membrane is subjected to two-stage crystallization at two temperatures, and then the thin flake-shaped molecular sieve crystals are directionally grown into a dense MFI molecular sieve membrane.

[0033] According to an embodiment of the preparation method of the first aspect of the present application, the silicon source is tetraethyl silicate, the MFI molecular sieve template agent is tetrapropyl ammonium hydroxide, the polyhydric alcohol is glycerol, and the amino acid is lysine.

[0034] It should be noted that in the preparation method of the MFI molecular sieve membrane of the present application, the molecular sieve seed crystals are coated on the surface of the carrier through steps (1) and (2), and the seed crystals on the carrier are grown into a molecular sieve membrane through step (3), and through the adjustment of the composition of the first molecular sieve synthesis solution and the second molecular sieve synthesis solution, and the selection of the silicon source, the template agent and the auxiliary agent, and the comprehensive adjustment of the multiple-stage crystallization temperature and time during the growth of the seed crystals into the sieve membrane, the molecular sieve crystals are directionally grown, and then a dense molecular sieve membrane is prepared.

[0035] According to an embodiment of the preparation method of the first aspect of the present application, in the first and second molecular sieve synthesis solutions:

[0036] The molar ratio of the silicon source to the MFI molecular sieve template is independently 1:(0.2-0.4), the molar ratio of the silicon source to the assistant is independently 1:(0.1-0.5), and the molar ratio of the silicon source to water is independently 1:(10-60).

[0037] It should be noted that in the molar ratio of the silicon source to other substances, the amount of the silicon source refers to the amount of silicon element in the silicon source. By controlling the molar ratio between various component substances in the first and second molecular sieve synthesis solutions, the growth direction of the molecular sieve membrane can be well controlled, and a dense MFI molecular sieve membrane with directional growth can be obtained to achieve a good separation effect.

[0038] According to an embodiment of the preparation method of the first aspect of the present application, in step (1):

[0039] The first crystallization is performed at a first temperature of 100-180°C, and the first crystallization time is 4-8h.

[0040] It should be noted that in step (1), by comprehensively controlling the composition of the first molecular sieve synthesis solution, the first temperature, and the first crystallization time, the obtained molecular sieve seed can be well prepared to grow a dense molecular sieve membrane in subsequent steps (2) and (3).

[0041] According to an embodiment of the preparation method of the first aspect of the present application, in step (2):

[0042] The carrier is a sheet-shaped porous alumina carrier, and loading the molecular sieve seed onto the carrier comprises: placing the sheet-shaped porous alumina carrier in a molecular sieve seed suspension for 10-20s, then taking out the sheet-shaped porous alumina carrier and drying to obtain the carrier loaded with a molecular sieve seed layer; or,

[0043] The carrier is a tubular porous alumina carrier, and loading the molecular sieve seed onto the carrier comprises: coating the tubular porous alumina carrier with the molecular sieve seed suspension by vacuum extraction for 10-20s, and then drying to obtain the carrier loaded with a molecular sieve seed layer.

[0044] It should be noted that in step (3), the carrier with the seed layer of the molecular sieve and the second synthesis solution of the molecular sieve can be put into a reaction kettle together, the liquid level of the second synthesis solution of the molecular sieve is higher than the carrier with the seed layer of the molecular sieve by 2-6 cm, then two-stage temperature crystallization is carried out, the solid product is separated out, dried in an oven at 50-100 ℃, calcined at 450 ℃ for 4 hours, and the nanosheet MFI type molecular sieve membrane is obtained.

[0045] According to an embodiment of the preparation method of the first aspect of the present application, the seed suspension of the molecular sieve comprises a solvent and the seed of the molecular sieve obtained from step (1), the solvent is selected from a combination of one or more of methanol, ethanol and water, and the solid-liquid mass ratio of the seed suspension of the molecular sieve is (0.1-0.2):100. That is, when the seed of the molecular sieve is dispersed into the solvent, 1-2 g of the dried seed of the molecular sieve can be dispersed into 1000 g of the solvent.

[0046] It should be noted that in step (2), the seed of the molecular sieve is dispersed into the solvent, and preferably, oscillation and ultrasonic treatment are carried out to uniformly disperse the seed of the molecular sieve in the solvent to obtain the seed suspension of the molecular sieve. In step (2), the seed of the molecular sieve in the seed suspension of the molecular sieve is coated on the carrier, and then can be placed in an oven for drying treatment to form a continuous and dense seed crystal layer on the surface of the carrier or support, and then the subsequent step (3) is carried out.

[0047] The preparation method of the oriented growth MFI molecular sieve membrane of the present application is simple, adjustable and can be produced on a large scale, and the thin sheet-shaped molecular sieve crystals are more easily oriented and grown on the surface of the carrier to form a dense membrane.

[0048] In the second aspect, the present application relates to a MFI molecular sieve membrane prepared by the preparation method of the first aspect of the present application.

[0049] The MFI molecular sieve membrane is a dense molecular sieve membrane, and the MFI molecular sieve crystals in the MFI molecular sieve membrane are thin sheet-shaped.

[0050] It should be noted that the direction of the oriented growth of the MFI molecular sieve membrane of the present application can be described as vertical growth to the surface of the carrier, or the upper surface of the molecular sieve membrane is parallel to the surface of the carrier.

[0051] According to an embodiment of the MFI molecular sieve membrane of the second aspect of the present application, the length of the MFI molecular sieve crystal is 100-800 nm, the width is 100-200 nm, and the thickness is 40-80 nm.

[0052] In a third aspect, the present application relates to the use of the MFI molecular sieve membrane prepared by the preparation method of the first aspect or the MFI molecular sieve membrane of the second aspect in separating a normal-isomeric alkane mixture.

[0053] It should be noted that the MFI molecular sieve membrane prepared by the preparation method of the first aspect or the MFI molecular sieve membrane of the second aspect has a very good permeation rate and a very good separation effect when used in the separation of normal-isomeric alkanes. That is, the MFI molecular sieve membrane of the present application has very good permeation performance and separation performance in the separation of a normal-isomeric alkane mixture.

[0054] The present application will be further described in detail by the following examples, but the present application is not limited by the following examples. The reagents in the following examples are commercially available reagents unless otherwise specified.

[0055] The sheet-shaped porous alumina used in the following examples or comparative examples can be prepared by the following preparation example, or the sheet-shaped porous alumina used in the following examples or comparative examples can also be a commercially available product carrier, and the tubular porous alumina carrier used in the following examples or comparative examples can be prepared by referring to the method of Journal of Membrane Science, 503 (2016), 69-80.

[0056] Preparation example of sheet-shaped porous alumina:

[0057] 50 g of alumina powder was mixed with 2 mL of deionized water, ground, and 3 g of the ground alumina powder was weighed into a mold. The mold was pressed at a pressure of 10 MPa for 30 s, and then calcined at 1200℃ for 4 h to obtain an alumina support with an average pore size of 200 nm and a porosity of 30%, a diameter of 25 mm, and a thickness of 2 mm.

[0058] Preparation example of tubular porous alumina carrier:

[0059] The alumina powder, polyvinylpyrrolidone, N-methyl-2-pyrrolidone, and polyether sulfone were mixed in a certain proportion and stirred for 48 hours to prepare a spinning solution. The spinning solution was extruded from a spinneret to form a hollow fiber blank under the push of nitrogen, and then passed through a 15 cm air gap into a tap water tank. The hollow fiber blank was kept immersed in the tap water tank for 10-20 hours to allow sufficient exchange between the solvent and the non-solvent. Then, it was straightened and air-dried at room temperature. The air-dried hollow fiber matrix was placed in a high-temperature calcination furnace and calcined at a temperature of 1350℃-1450℃ for 4 hours, and finally cooled to room temperature with the furnace to obtain a tubular support.

[0060] Example 1

[0061] (1) Seed preparation: 12 g of tetraethyl orthosilicate was dissolved in 12 g of tetrapropylammonium hydroxide solution (25% by mass) and 25 g of deionized water solution, 1 g of urea was added to the synthesis solution, and the mixture was stirred at room temperature at 150 rpm for 24 h, then transferred to a muffle furnace for crystallization, and crystallized at 120°C for 6 h. The prepared crystals were washed with deionized water and centrifuged to obtain the molecular sieve seed crystals;

[0062] (2) Seed coating on the support: 1.0 g of the molecular sieve seed crystals was weighed and added to 1000 g of ethanol, and the mixture was ultrasonically treated and shaken to uniformly disperse the crystals in the ethanol to form a uniform molecular sieve suspension. The porous support prepared in the above preparation example was placed in the molecular sieve suspension for 20 s and then taken out at a uniform speed. After drying in an oven, a continuous and dense layer of molecular sieve crystals was formed on the surface of the support;

[0063] (3) Synthesis of nanosheet MFI-type molecular sieve membrane: 12 g of tetraethyl orthosilicate was dissolved in 12 g of tetrapropylammonium hydroxide solution (25% by mass) and 25 g of deionized water solution, 1 g of urea was added to the synthesis solution, and the mixture was stirred at room temperature at 150 rpm for 24 h. The synthesis solution and the support coated with the molecular sieve crystals in step 2 were placed in a reaction kettle, then crystallized at 120°C for 6 h and at 180°C for 8 h. After the reaction was completed, the molecular sieve membrane was washed with deionized water for 15 min, dried in an oven at 50°C, and calcined at 450°C for 4 h to obtain a nanosheet MFI-type molecular sieve membrane, which was denoted as M1.

[0064] Example 2

[0065] (1) Seed preparation: The preparation process was the same as that in step (1) of Example 1, except that the additive urea was replaced with an equal amount of lysine;

[0066] (2) Seed coating on the support: The preparation process was the same as that in step (2) of Example 1;

[0067] (3) Synthesis of nanosheet MFI-type molecular sieve membrane: The preparation process was the same as that in step (3) of Example 1, except that the additive urea was replaced with an equal amount of lysine;

[0068] The prepared molecular sieve membrane was denoted as M2.

[0069] Example 3

[0070] (1) Seed preparation: The preparation process was the same as that in step (1) of Example 1;

[0071] (2) Support seed coating: the preparation process is the same as step (2) of Example 1, except that the carrier is the tubular porous alumina carrier prepared in the above preparation example, one end of the porous alumina carrier is blocked, one end is connected to a vacuum device, and the molecular sieve seed suspension is coated onto the surface of the carrier by vacuum extraction for 10-20 s, then dried, to obtain a tubular porous alumina carrier coated with molecular sieve seeds;

[0072] (3) MFI type molecular sieve membrane synthesis: the preparation process is the same as step (3) of Example 1;

[0073] The prepared molecular sieve membrane is denoted as M3.

[0074] Comparative Example 1

[0075] (1) Seed preparation: 12 g of tetraethyl orthosilicate was dissolved in a solution of 12 g of tetrapropylammonium hydroxide (25% by mass) and 25 g of deionized water, 1 g of urea was then added to the synthesis solution, which was stirred at room temperature at 150 rpm for 24 h, and then transferred to a muffle furnace for crystallization at 120°C for 6 h. The prepared crystals were washed with deionized water and centrifuged to obtain molecular sieve seeds;

[0076] (2) Support seed coating: 1.0 g of the molecular sieve seeds prepared in step (1) was added to 1000 g of ethanol, and the crystals were uniformly dispersed in the ethanol to form a uniform molecular sieve suspension after ultrasonic and shaking treatment. The sheet-shaped porous carrier prepared in the above preparation example was placed in the molecular sieve suspension for 20 s and then removed uniformly from the suspension. After drying in an oven, a continuous and dense layer of molecular sieve crystals was formed on the surface of the support;

[0077] (3) MFI type molecular sieve membrane synthesis: 12 g of tetraethyl orthosilicate was dissolved in a solution of 12 g of tetrapropylammonium hydroxide (25% by mass) and 25 g of deionized water, 1 g of urea was then added to the synthesis solution, which was stirred at room temperature at 150 rpm for 24 h, and then transferred to a muffle furnace for crystallization at 120°C for 6 h. The prepared crystals were washed with deionized water and centrifuged to obtain molecular sieve seeds;

[0078] Comparative Example 2:

[0079] (1) Seed preparation: the preparation process is the same as step (1) of Example 1, except that the crystallization temperature is 180°C for 8 h;

[0080] (2) Support seed coating: the preparation process is the same as step (2) of Example 1;

[0081] (3) Nanosheet MFI zeolite membrane synthesis: the preparation process was the same as step (3) of Example 1, except that the crystallization temperature was 180°C for 8h, and the nanosheet MFI zeolite membrane was obtained after calcination at 450°C for 4h, which was recorded as M7.

[0082] Comparative Example 3

[0083] (1) Seed preparation: the preparation process was the same as step (1) of Example 1, except that no seed crystal was added;

[0084] (2) Seed-coated support: the preparation process was the same as step (2) of Example 1;

[0085] (3) Nanosheet MFI zeolite membrane synthesis: the preparation process was the same as step (3) of Example 1, except that no seed crystal was added;

[0086] The prepared zeolite membrane was recorded as M8.

[0087] Comparative Example 4

[0088] (1) Seed preparation: the preparation process was the same as step (1) of Example 1;

[0089] (2) Seed-coated support: the preparation process was the same as step (2) of Example 1;

[0090] (3) MFI zeolite membrane synthesis: the preparation process was the same as step (3) of Example 1, except that the crystallization time was different in two stages, 6h at 120°C and 4h at 180°C;

[0091] The prepared zeolite membrane was recorded as M9.

[0092] Test Example 1

[0093] The gas separation performance of the above prepared zeolite membranes was characterized in n- / i-C4H 10 (40 / 60) system at 60°C and 10KPa, and the test results are shown in Table 1. The gas separation performance of the prepared nanosheet MFI zeolite membrane was tested by the separation performance of the n-butane / isobutane mixed gas with a molar ratio of 4:6, and was evaluated by two indexes of gas permeation rate P and separation factor SF. The gas permeation rate P represents the total amount of gas moles per unit area per unit time, P=N / (AxtAP), unit: mol / (m 2 ·s·pa), wherein N is the gas moles permeating the membrane, A is the area, t is the time, and AP is the pressure; the separation factor SF is used to evaluate the separation efficiency of the membrane, SF=(y 1，p / y 2，p ) / (y 1，f / y2，f ) ; wherein: y 1，p is the mass fraction of n-butane on the permeation side; y 2，p is the mass fraction of isobutane on the permeation side; y 1，f is the mass fraction of n-butane in the raw material; y 2，f is the mass fraction of isobutane in the raw material.

[0094] Table 1

[0095]

[0096] As can be seen from the data in Table 1, the molecular sieve membrane prepared by the method of the present application in the examples has a very good separation effect on n-butane / isobutane.

[0097] Test Example 2

[0098] The molecular sieve membranes prepared in the above examples and comparative examples were analyzed for morphology by scanning electron microscopy. Figure 1 is a surface SEM image of the nanosheet MFI molecular sieve membrane M1 prepared by two-stage temperature swing crystallization in step (3) in Example 1, and the molecular sieve crystal size is about 200 nm x 150 nm x 60 nm. As can be seen from the image, the upper surface of the nanosheet MFI molecular sieve crystal is parallel to the support surface, and is a dense membrane. Figure 1 The SEM image of the molecular sieve membrane M2 prepared in Example 2 is basically the same as that of Example 1, and has the same growth direction and morphological characteristics as the molecular sieve membrane of Example 1.

[0099] Figure 2 is a surface SEM image of the molecular sieve membrane M6 prepared in Comparative Example 1. As can be seen from the image, the molecular sieve crystal size is about 460 nm x 180 nm x 70 nm, and the crystals grow on the support surface in disorder.

[0100] Figure 3 is an SEM image of the molecular sieve membrane M7 prepared in Comparative Example 2. As can be seen from the image, the molecular sieve crystal size is about 420 nm x 200 nm x 100 nm, and the nanosheet MFI crystals grow on the support surface in disorder.

[0101] Figure 4 is a surface SEM image of the molecular sieve membrane M9 prepared in Comparative Example 4. As can be seen from the image, the molecular sieve crystal size is about 200 nm x 150 nm x 60 nm, and as can be seen from the image, the upper surface of the MFI molecular sieve crystal is parallel to the support surface, but there are gaps in the sieve membrane.

[0102] In Comparative Example 1 and Comparative Example 2, the crystallization process of the molecular sieve membrane in step (3) was only carried out at 180°C for one stage, and Figure 2 andFigure 3 It can be seen that the crystals grow disorderly on the support surface, but the crystals are close to each other, and thus the data in Table 1 show that the separation performance is achieved to some extent. Figure 4 It can be seen that in Comparative Example 4, the crystallization of the molecular sieve membrane in step (3) is carried out at 120℃ and 180℃ respectively, and the crystals can grow in the direction that the upper surface of the crystals is parallel to the surface of the support, but the crystallization time at 180℃ is too short, and thus there are many gaps between the crystals in the obtained molecular sieve membrane, and thus the separation factor of the molecular sieve membrane in Comparative Example 4 in Table 1 is very small, and the separation effect cannot be achieved. In Comparative Example 3, no additive such as urea is added in the preparation of the molecular sieve membrane, and thus the separation factor of the prepared molecular sieve membrane is very small, and the separation function cannot be achieved well.

[0103] Test Example 3

[0104] The molecular sieve prepared in the above examples and comparative examples is subjected to XRD analysis, and the XRD spectrum of the molecular sieve membrane M1 prepared in Example 1 is shown in FIG. 1. Figure 5 It can be seen from the figure that the molecular sieve membrane M1 prepared in Example 1 has diffraction peaks at 9.23°, 17.71°, 26.92°, 36.76° and 45.56°, which are respectively attributed to the (020), (040), (060), (080) and (0100) crystal planes of the MFI crystal, indicating that the molecular sieve membrane M1 is a MFI molecular sieve membrane with thin sheet shape and in a flat arrangement. The spectrum of the molecular sieve membrane M6 prepared in Comparative Example 1 can see the (200), (400) crystal planes of a-axis orientation and the (020), (040), (060) crystal planes of b-axis orientation, because the crystals in the molecular sieve membrane prepared in Comparative Example 1 are arranged in a disorderly manner on the support. The characteristic peak positions in the XRD spectrum of the molecular sieve membranes prepared in Example 2 and Comparative Example 4 are basically the same as those of the molecular sieve membrane M1 in Example 1 in Figure 5 , indicating that the MFI molecular sieve membrane is also a molecular sieve membrane with thin sheet shape and in a flat arrangement. The characteristic peak positions in the XRD spectrum of the molecular sieve membranes prepared in the other comparative examples are basically the same as those of the molecular sieve membrane M6 prepared in Comparative Example 1 in Figure 5 , indicating that the molecular sieve membrane is a MFI crystal, which is not a molecular sieve membrane with thin sheet shape and in a flat arrangement.

[0105] In the description of the present application, it should be noted that the terms "upper", "lower", "inner", "outer", "front", "back", "left", "right" and the like indicate the orientation or positional relationship based on the working state of the present application, and are only for the convenience of describing the present application and simplifying the description, and thus cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus cannot be understood as limiting the present application.

[0106] In the description of the application, it needs to be explained that, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connection" should be understood in a broad sense. The specific meanings of the above terms in the application can be understood by the person skilled in the art according to the specific circumstances.

[0107] The above describes the application in combination with the preferred embodiments, but these embodiments are only exemplary and are only for illustrative purposes. On this basis, various substitutions and improvements can be made to the application, and these all fall within the protection scope of the application.

Claims

1. A method for producing an MFI molecular sieve membrane, characterized by, The preparation method comprises the following steps: (1) subjecting a first molecular sieve synthesis solution to first crystallization, and then separating a solid-phase product to obtain a molecular sieve seed crystal; (2) loading the molecular sieve seed crystal onto a carrier to obtain a carrier loaded with a molecular sieve seed crystal layer; (3) subjecting the carrier loaded with the molecular sieve seed crystal layer to second crystallization at a second temperature and third crystallization at a third temperature in a second molecular sieve synthesis solution in sequence, separating a solid-phase product and then drying and calcining to obtain the MFI molecular sieve membrane; the second temperature is 100-120℃, the second crystallization is performed for 0.1-8h, the third temperature is 160-180℃, and the third crystallization is performed for 6-14h; the first molecular sieve synthesis solution and the second molecular sieve synthesis solution each independently comprises a silicon source, an MFI molecular sieve template agent, an auxiliary agent and water; wherein the auxiliary agent is selected from a combination of one or more of urea, a polyhydric alcohol and an amino acid.

2. The production method according to claim 1, characterized by, the silicon source is tetraethyl orthosilicate, the MFI molecular sieve template agent is tetrapropylammonium hydroxide, the polyhydric alcohol is glycerol, and the amino acid is lysine.

3. The preparation method according to claim 1, characterized in that, in the first molecular sieve synthesis solution and the second molecular sieve synthesis solution: the molar ratio of the silicon source to the MFI molecular sieve template agent is each independently 1:(0.2-0.4), the molar ratio of the silicon source to the auxiliary agent is each independently 1:(0.1-0.5), and the molar ratio of the silicon source to water is each independently 1:(10-60).

4. The method of claim 1, wherein, in step (1): the first crystallization is performed at a first temperature, the first temperature is 100-180℃, and the first crystallization is performed for 4-8h.

5. The preparation method according to claim 1, characterized in that, in step (2): the carrier is a sheet-shaped porous alumina carrier, and loading the molecular sieve seed crystal onto the carrier comprises: placing the sheet-shaped porous alumina carrier in a molecular sieve seed crystal suspension for 10-20s, and then taking out and drying the sheet-shaped porous alumina carrier to obtain the carrier loaded with the molecular sieve seed crystal layer; or the carrier is a tubular porous alumina carrier, and loading the molecular sieve seed crystal onto the carrier comprises: 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.

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

100.

7. An MFI molecular sieve membrane characterized by, the MFI molecular sieve membrane is obtained by the preparation method of any one of claims 1-6; the MFI molecular sieve membrane is a dense molecular sieve membrane, and the MFI molecular sieve crystal in the MFI molecular sieve membrane is in a sheet shape.

8. The MFI molecular sieve membrane of claim 7, wherein, the length of the MFI molecular sieve crystal is 100-800nm, the width is 100-200nm, and the thickness is 40-80nm.

9. Application of the MFI molecular sieve membrane obtained by the preparation method of any one of claims 1-6 or the MFI molecular sieve membrane of claim 7 or 8 in separating a normal-isomer alkane mixture.

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