A method for preparing a hierarchical pore molecular sieve membrane

Abstract

The application relates to a preparation method of hierarchical pore molecular sieve membrane. Based on coating molecular sieve nanocrystals with different dimensional structures on the surface of a porous carrier to construct a mixed-dimensional crystal seed layer, a hierarchical pore molecular sieve membrane with an ultrathin separation layer and a large-pore diffusion layer is prepared through secondary growth. Compared with a molecular sieve membrane synthesized by using single molecular sieve as a seed layer, the hierarchical pore molecular sieve membrane prepared by the method has lower mass transfer resistance, and separation advantage is very significant.

Description

Technical Field

[0001] This invention relates to a method for preparing a graded pore molecular sieve membrane, belonging to the field of molecular sieve membrane material preparation. Background Technology

[0002] Membrane separation technology is an advanced separation technology with advantages such as low energy consumption, environmental friendliness, and simple operation. Compared with other separation membranes, zeolite molecular sieve membranes have a pore size of less than 1 nm, regular pore channels, high mechanical strength, and good thermochemical stability, making them suitable for molecular-level separation. The flux of a molecular sieve membrane is directly determined by the membrane thickness. Reducing the membrane thickness is beneficial to increasing the flux of the molecular sieve membrane. The preparation of ultrathin molecular sieve membranes with submicron thickness has attracted much attention, but still faces significant challenges.

[0003] MFI molecular sieve membranes have a pore size of approximately 0.55 nm, making them particularly advantageous for separating alkane isomers (such as p / o-xylene and n / isobutane). Most MFI molecular sieve membranes are prepared using a seed-induced secondary growth method. The size and structure of the seed crystals are key factors in controlling the microstructure (thickness, orientation, etc.) of the MFI molecular sieve membrane. Using nanocrystals can effectively reduce the thickness of the molecular sieve membrane; however, nanocrystals easily permeate into macroporous supports, leading to uneven membrane thickness and affecting membrane compactness. Tsapatsis et al. used paraffin to plug the pores of an alumina support, depositing a seed layer with a particle size of 40 nm on the support. They then synthesized an ultrathin MFI molecular sieve membrane with a thickness of 300–400 nm using a secondary hydrothermal method for p / o-xylene / o-xylene (PX / OX) separation. At 150 °C, the PX permeability reached as high as 3.5 × 10⁻⁶. -7 mol m -2 s -1 Pa -1 The PX / OX separation factor is 90–120 [J.Am.Chem.Soc.2011,133,493–502]. Two-dimensional nanosheet molecular sieves possess a high aspect ratio, making them suitable for coating porous supports with seed layers of high coverage and uniform thickness, thereby synthesizing ultrathin molecular sieve membranes with submicron thicknesses or even less than 100 nm. Liu et al. obtained MFI nanosheet seed layers with a thickness of approximately 25 nm using a template etching method, and synthesized ultrathin MFI molecular sieve membranes with a thickness less than 100 nm using a single-mode microwave heating method. The permeability of n-butane reached as high as 2 × 10⁻⁶. -7 mol m -2 s -1 Pa -1[Sci.Adv.,2020,6,eaay5993]. Nair et al. used a vacuum suction method to coat a two-dimensional MFI nanosheet seed layer on a hollow fiber carrier, and synthesized a dense high-throughput MFI molecular sieve membrane through multiple hydrothermal processes [Angew.Chem.Int.Ed.,2019,58,8201-8205].

[0004] Although various methods for preparing ultrathin MFI molecular sieve membranes have been reported in the literature, most synthetic steps are complex and difficult to control. For example, synthesizing ultrathin two-dimensional MFI nanosheets requires designing and synthesizing the template agent dC5 or using template agent etching, resulting in low reproducibility. Choi et al. used TBPOH as a template agent to introduce non-selective micropores (0.6–1.5 nm) and mesopores (2–7 nm) into MFI molecular sieve membranes, achieving twice the xylene permeability of conventional MFI molecular sieve membranes of the same thickness [Angew. Chem. Int. Ed., 2021, 60, 1323-1331]. However, using TBPOH as a template agent to introduce non-selective pores into zeolite membranes requires two synthesis steps and takes more than 160 hours, which is not conducive to scale-up applications. Therefore, designing an MFI molecular sieve membrane with a hierarchical pore structure and a simple synthesis method is of significant research importance. Summary of the Invention

[0005] The technical problem to be solved by this invention is that the existing methods for synthesizing MFI molecular sieve membranes suffer from problems such as seed blockage of the support membrane pores, large molecular sieve thickness, and low separation flux.

[0006] The purpose of this invention is to provide a simple and efficient method for synthesizing hierarchical porous molecular sieve membranes. These membranes possess both an ultrathin microporous separation layer and a rapid diffusion layer rich in mesopores / macropores. Taking a hierarchical porous MFI molecular sieve membrane as an example, two dimensional structures (0D and 2D) of MFI molecular sieves are used as seed crystals. An 0D@2D seed layer (similar to a core-shell structure, with 0D encapsulating 2D) is constructed using dip-coating, followed by secondary growth to prepare a hierarchical porous MFI molecular sieve membrane. This membrane has a dense, ultrathin separation layer and a macroporous intermediate layer, thereby improving the permeation flux of the MFI molecular sieve membrane. This method is simple, easy to implement, and has a short synthesis time.

[0007] The technical solution is:

[0008] A method for preparing a hierarchical pore molecular sieve membrane includes the following steps:

[0009] Step 1: Obtain zero-dimensional and two-dimensional molecular sieve seeds respectively;

[0010] Step 2: Prepare suspensions of zero-dimensional molecular sieve seeds and two-dimensional molecular sieve seeds;

[0011] Step 3: Apply seed crystals to the porous carrier sequentially using a suspension of two-dimensional molecular sieve seed crystals and a suspension of zero-dimensional molecular sieve seed crystals.

[0012] Step 4: The support with seed layer obtained in Step 3 is used to synthesize a molecular sieve membrane in a synthesis solution to prepare a graded pore molecular sieve membrane.

[0013] In step 1, the method for preparing two-dimensional molecular sieve seeds includes the following steps:

[0014] The template agent, silicon source, alkali, and additives are mixed in water, aged, and then subjected to hydrothermal synthesis. After calcination to remove the template agent, two-dimensional molecular sieve seed crystals are obtained.

[0015] The hydrothermal synthesis conditions are 0.1–20 days, and the calcination of the template agent is carried out at a temperature of 400–580℃ for 4–8 hours.

[0016] The template agent is TPAOH, the silicon source is tetraethyl orthosilicate, the base is NaOH, and the additives are isopropanol, ammonium fluoride, ethanol, etc.

[0017] The addition of template agent, silicon source, alkali, water and additives is calculated according to the molar ratio of 1TEOS:0.2-0.4TPAOH:20-50H2O:0.02-0.10NaOH:0.01-0.1 additives.

[0018] In step 1, the method for preparing zero-dimensional molecular sieve seeds includes the following steps:

[0019] The template agent, alkali, and silicon dioxide are mixed in water, aged, and then subjected to hydrothermal synthesis. After calcination to remove the template agent, zero-dimensional molecular sieve seed crystals are obtained.

[0020] The hydrothermal synthesis conditions are 0.1–20 days, and the calcination of the template agent is carried out at a temperature of 400–580℃ for 4–8 hours.

[0021] The template agent is TPAOH, and the silicon source is tetraethyl orthosilicate.

[0022] The addition of template agent, alkali, silica and water is calculated according to the molar ratio 1SiO2:0.1-0.5TPAOH:0.01-0.1NaOH:10-20H2O.

[0023] In step 2, the mass concentration of the suspension of zero-dimensional molecular sieve seeds and the suspension of two-dimensional molecular sieve seeds is 0.0001-10 wt%.

[0024] The average pore size of the porous support is 150-800 nm; the impregnation time is 10-60 s and 5-40 s respectively.

[0025] In step 4, the synthesis solution contains a silicon source, a template agent, an alcohol solvent, and water, and the hydrothermal synthesis parameters are 25–200°C for 1–60 h.

[0026] The silicon source is TEOS, the template agent is TPAOH, and the alcohol solvent is ethanol.

[0027] The amounts of silicon source, template agent, alcohol solvent and water added to the synthesis solution are calculated according to the molar ratio 1TEOS:0.05-0.2TPAOH:2-6EtOH:150-250H2O.

[0028] Applications of MFI molecular sieve membranes in the separation of alkane isomers, liquids, or gases.

[0029] Alkane isomer separation refers to the separation of p- / o-xylene or n-isobutane; liquid separation refers to the separation of alcohol-water mixtures; and gas separation refers to the separation of gaseous components such as CO2 and H2.

[0030] Beneficial effects

[0031] In this patent, 0D and 2D MFI molecular sieves are used together as seed crystals to prepare an MFI molecular sieve membrane with a hierarchical pore structure and an ultrathin separation layer in a short time. The mechanism of secondary growth of zeolite crystals as seed crystals is classified into two types: seed crystal dissolution and crystallization mechanism and seed crystal maintenance and crystallization mechanism, corresponding to the growth mechanisms of 0D and 2D MFI molecular sieves, respectively. A mixed-dimensional seed layer is constructed by sequentially coating 2D and 0D MFI molecular sieves. The 2D MFI molecular sieve seed crystals at the bottom have large gaps between each other, which cannot be effectively filled within a short synthesis time to form a hierarchical pore structure membrane layer with low mass transfer resistance. Furthermore, the 2D MFI molecular sieves can effectively prevent the pore leakage phenomenon of the smaller 0D MFI molecular sieve particles. Simultaneously, the 0D MFI molecular sieves, due to their high surface energy, dissolve first and then rapidly crystallize and grow, with a faster growth rate, filling the gaps between the 2D MFI molecular sieve seed layers to form a dense separation layer. If the two are directly mixed and applied to the surface of the support, the resulting MFI molecular sieve membrane is thicker and has greater mass transfer resistance. This method uses nanocrystal seeds with high synthesis yield and requires no specially designed template agent. Subsequent assembly of the two seeds as a seed layer yields ultrathin MFI molecular sieves. Introducing non-selective macropores or mesopores into the molecular sieve membrane effectively reduces mass transfer resistance and increases membrane flux; furthermore, the membrane preparation can be completed in a single hydrothermal synthesis, offering a simplified process.

[0032] By combining the characteristics of the two seed crystals, it is possible to maintain both an ultra-thin separation layer and a high permeation flux. Attached Figure Description

[0033] Figure 1 These are SEM images of the 0D and 2D MFI molecular sieve seeds prepared in Example 1, where a) is a two-dimensional MFI molecular sieve; b) is a zero-dimensional MFI molecular sieve.

[0034] Figure 2 These are SEM images of the surface and cross-section of the seed layer in Example 1 and Comparative Examples 1-3, where a) e) OD@2D; b) f) OD / 2D; c) g) 2D; d) h) OD

[0035] Figure 3 These are surface and cross-sectional SEM images of the hierarchical pore MFI molecular sieve membrane prepared in Example 1.

[0036] Figure 4 This is a graph showing the PX / OX separation performance of the graded-pore MFI molecular sieve membrane in Example 1. Detailed Implementation

[0037] The method of the present invention is described in detail below:

[0038] (1) Preparation of MFI molecular sieve nanocrystals of different dimensions

[0039] Preparation of zero-dimensional (0D) MFI seed crystals: Nano-sized MFI spherical seed crystals were prepared by in-situ hydrothermal method. Tetrapropylammonium hydroxide (TPAOH), sodium hydroxide (NaOH), tetraethyl orthosilicate (TEOS) and deionized water (H2O) were mixed to form a homogeneous synthesis solution. After aging at room temperature, the solution was poured into a synthesis reactor for synthesis. After washing and high-temperature calcination, 0D MFI molecular sieve seed crystals with a particle size of <100nm were obtained.

[0040] Preparation of two-dimensional (2D) MFI seed crystals: 2D MFI nanosheet seed crystals were prepared by in-situ hydrothermal method. TPAOH, TEOS, NaOH, deionized water and additives (isopropanol, ethanol, ammonium fluoride, etc.) were mixed to form a homogeneous synthesis solution. After aging at room temperature, the solution was poured into a synthesis vessel for synthesis. After washing and high-temperature calcination, 2D MFI nanosheet seed crystals with a thickness of <100nm and a high aspect ratio were obtained.

[0041] (2) Preparation of 0D@2D MFI seed layer: The two nano-seeds prepared in step (1) are dispersed in deionized water to form seed solution; the porous carrier surface is coated with crystals by immersion method in sequence, first coated with 2D seed suspension, then coated with 0D seed suspension, and then dried for later use.

[0042] (3) Preparation of hierarchical pore structure MFI molecular sieve membrane: TPAOH, SiO2, alcohol and deionized water are mixed and stirred evenly and aged. The carrier coated with 0D@2D MFI seed layer in step (2) is placed in hydrothermal synthesis kettle and the above synthesis solution is added. Hydrothermal synthesis is carried out at 25-200℃ for 1-60h. The surface of the prepared molecular sieve membrane is wiped with degreased cotton and finally cleaned, dried and calcined to remove the template agent.

[0043] Preferably, the sizes of the nano-MFI molecular sieve seeds prepared in step (1) are as follows: the average size of the 0D MFI nano-seeds is about 80 nm; and the thickness of the 2D MFI nanosheet seeds is about 90 nm.

[0044] Preferably, the mass fraction of the seed crystals in step (2) is 0.0001-10 wt%.

[0045] Preferably, the composite structure of the multidimensional composite seed layer in step (2) is a combination of 0D@2D, 0D / 2D, 1D@2D, 1D / 2D, etc.

[0046] Preferably, the support carrier used in step (2) is a ceramic carrier, a metal carrier, a polymer carrier, etc. The carrier configuration includes flat plate, tubular, hollow fiber, and the carrier material is one or a combination of several of the following: mullite, alumina, stainless steel, yttrium-stabilized zirconium oxide, and silicon dioxide.

[0047] Preferably, the synthesis temperature in step (3) is 25–200°C.

[0048] Preferably, the synthesis time of the MFI molecular sieve membrane in step (3) is 1 to 60 hours.

[0049] Preferably, the removal of the MFI molecular sieve membrane template agent in step (3) is one or a combination of several methods such as high-temperature calcination and ozone atmosphere removal.

[0050] Coating methods include dip coating, vacuum suction, wiping coating, electrostatic assembly, spraying, etc. Seeds of different dimensions can be coated sequentially or in combination.

[0051] Molecular sieve membranes include types such as MFI, LTA, CHA, MOR, DDR, MCM-22, MWW, SAPO, and AIPO.

[0052] Example 1

[0053] (1) Preparation of MFI molecular sieve nanocrystals of different dimensions

[0054] Preparation of two-dimensional (2D) seed crystals: TPAOH, TEOS, NaOH, and isopropanol were added to deionized water in sequence to form a mixture with a molar ratio of 1TEOS:0.3TPAOH:29.4H2O:0.047NaOH:0.031isopropanol. The mixture was stirred and aged at room temperature for 2 hours, then hydrothermally synthesized at 180℃ for 2 days and dried. Finally, it was calcined at 550℃ for 6 hours to remove the organic template agent inside the MFI molecular sieve.

[0055] Preparation of zero-dimensional (0D) seed crystals: TPAOH, NaOH and deionized water were weighed and mixed evenly, and then silica was added to dissolve them completely to form a sol with a molar ratio of 1SiO2:0.3TPAOH:0.05NaOH:14H2O. After aging at room temperature, the product was hydrothermally synthesized at 60℃ for 15 days. After washing and drying, the product was calcined at 550℃ for 6 hours to remove the organic template agent inside the MFI molecular sieve.

[0056] (2) Preparation of multidimensional composite (OD@2D) seed layer

[0057] The two types of nanocrystal seeds prepared in step (1) are dispersed in deionized water to form a seed solution with a mass fraction of 0.5 wt%. The porous carrier (average pore size 450 nm) is immersed in 2D MFI seed solution and 0DMFI seed solution with a mass fraction of 0.5 wt% for about 30 s and 15 s respectively, and then dried for later use, thereby constructing a multidimensional composite (OD@2D) seed layer.

[0058] (3) Preparation of hierarchical pore structure MFI molecular sieve membranes

[0059] Weigh out the substances with a molar ratio of 1TEOS:0.12TPAOH:4EtOH:180H2O, mix them, and stir them evenly. Place the carrier coated with the 0D@2D MFI seed layer prepared in step (2) into a synthesis vessel containing the synthesis solution and synthesize at 180℃ for 2 hours. Wipe the surface of the prepared graded pore MFI molecular sieve membrane with degreased cotton, and finally remove the template agent by cleaning, drying, and calcination.

[0060] The prepared membrane M1 was used for the separation of xylene isomers (PX / OX) at a separation temperature of 150℃. The PX permeability and PX / OX separation performance of the membrane are shown in Table 1.

[0061] Compare with Example 1

[0062] The operating steps are the same as in Example 1. The difference is in the seed crystals in step (2). First, 2D and 0D MFI molecular sieve seed crystals are mixed in a 1:1 ratio to prepare a (0D / 2D) mixed dimension seed crystal solution with a mass fraction of 0.5wt%. The prepared membrane M2 is used for the separation of xylene isomers (PX / OX) at a separation temperature of 150℃. The PX permeability and PX / OX separation performance of the membrane are shown in Table 1.

[0063] Compare with Example 2

[0064] The operating steps are the same as in Example 1. The difference is in the seed crystals in step (2). Only 2D MFI molecular sieves are used as seed crystals. The prepared membrane M3 is used for the separation of xylene isomers. The separation temperature is 150℃. The PX permeability and PX / OX separation performance of the membrane are shown in Table 1.

[0065] Compare with Example 3

[0066] The operating steps are the same as in Example 1. The difference is in the seed crystals in step (2). Only OD MFI molecular sieves are used as seed crystals. The prepared membrane M4 is used for the separation of xylene isomers. The separation temperature is 150℃. The PX permeability and PX / OX separation performance of the membrane are shown in Table 1.

[0067] Table 1. Xylene separation performance of MFI molecular sieve membranes in Example 1 and Comparative Examples 1-3 under test conditions of 150°C and feed ratio PX:OX = 1:1.

[0068]

[0069] Comparing the SEM images and para-xylene isomer separation performance of MFI molecular sieve membranes synthesized with different seed layers, when using 2D MFI molecular sieve as the seed layer, the MFI molecular sieve membrane synthesized in 2 hours exhibits high para-xylene permeability. However, the SEM images show that the intercrystalline gaps are not completely closed, resulting in poor separation performance of the synthesized MFI molecular sieve membrane for xylene isomers. MFI molecular sieve membranes synthesized using 0D MFI molecular sieve as the seed layer show severe pore permeation and are difficult to synthesize into dense MFI membranes. When 0D and 2D MFI molecular sieves are mixed and used as the seed layer, a relatively dense MFI molecular sieve membrane can be synthesized. However, the SEM images show that the membrane layer is relatively thick, resulting in high mass transfer resistance and poor para-xylene permeability. Finally, 2D and 0D MFI molecular sieves were sequentially dipped into the support. MFI molecular sieves can synthesize MFI molecular sieve membranes with ultrathin and dense separation layers in a short synthesis time. They exhibit high selectivity for xylene isomers. Furthermore, due to the differences in growth kinetics of different seed crystals, the 2D MFI molecular sieves at the bottom have large gaps between each other, ultimately resulting in the synthesis of high-performance MFI molecular sieve membranes with hierarchical pore structures.

Claims

1. The application of hierarchical pore molecular sieve membranes in the separation of p- / o-xylene, characterized in that, The preparation method of hierarchical pore molecular sieve membranes includes the following steps: Step 1: Obtain zero-dimensional and two-dimensional molecular sieve seeds respectively; Step 2: Prepare suspensions of zero-dimensional molecular sieve seeds and two-dimensional molecular sieve seeds; Step 3: Apply seed crystals to the porous carrier sequentially using a suspension of two-dimensional molecular sieve seed crystals and a suspension of zero-dimensional molecular sieve seed crystals. Step 4: The support with seed layer obtained in Step 3 is used to synthesize a molecular sieve membrane in a synthesis solution to prepare a graded pore molecular sieve membrane. In step 1, the method for preparing two-dimensional molecular sieve seeds includes the following steps: The template agent, silicon source, alkali, and additives are mixed in water, aged, and then subjected to hydrothermal synthesis. After calcination to remove the template agent, two-dimensional molecular sieve seed crystals are obtained. The thickness of the two-dimensional molecular sieve seed crystals is 90 nm. The hydrothermal synthesis conditions are 0.1-20 days, and the calcination to remove the template agent is carried out at a temperature of 400-580℃ for 4-8 hours. The template agent used in the preparation process of the two-dimensional molecular sieve seed crystals is TPAOH, the silicon source is tetraethyl orthosilicate, and the alkali is NaOH. The addition of the template agent, silicon source, alkali, and additives is calculated according to the molar ratio of 1 TEOS:0.2-0.4 TPAOH:20-50 H2O:0.02-0.10 NaOH:0.01-0.1 additives. In step 1, the preparation method of zero-dimensional molecular sieve seed crystals includes the following steps: mixing template agent, alkali, and silicon dioxide in water, aging, hydrothermal synthesis, and calcination to remove the template agent to obtain zero-dimensional molecular sieve seed crystals; the average size of the zero-dimensional molecular sieve seed crystals is 80 nm; the hydrothermal synthesis conditions are 0.1-20 days, and the calcination to remove the template agent is carried out at a temperature of 400-580℃ for 4-8 hours; the template agent in the preparation process of the zero-dimensional molecular sieve seed crystals is TPAOH, the silicon source is silicon dioxide, and the addition of template agent, alkali, silicon dioxide, and water is calculated according to the molar ratio of 1 SiO2: 0.1-0.5 TPAOH: 0.01-0.1 NaOH: 10-20 H2O; In step 2, the mass concentration of the suspensions of zero-dimensional molecular sieve seeds and two-dimensional molecular sieve seeds is 0.0001-10 wt%. In step 4, the synthesis solution contains a template agent, a silicon source, an alcohol solvent, and water, and the hydrothermal synthesis parameters are 25-200℃ for 1-60 hours.

2. The application according to claim 1, characterized in that, In step 3, the average pore size of the porous carrier is 150-800 nm; the impregnation time is 10-60 s and 5-40 s respectively.

3. The application according to claim 2, characterized in that, The silicon source in the synthesis solution of step 4 is TEOS, the template agent is TPAOH, and the alcohol solvent is ethanol. The amounts of silicon source, template agent, alcohol solvent, and water added to the synthesis solution are calculated according to the molar ratio of 1 TEOS: 0.05-0.5 TPAOH: 2-6 EtOH: 150-250 H2O.

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