A method for preparing a b-axis oriented MFI type molecular sieve separation membrane in situ

Abstract

This invention discloses a method for preparing b-axis oriented MFI type molecular sieve separation membranes through in-situ crystallization. The method first prepares a graphite transition layer on the surface of a porous support, then directly places the modified porous support into a high-pressure synthesis reactor containing a synthesis solution. After crystallization, the membrane is washed with deionized water and dried to obtain the b-axis oriented MFI type molecular sieve separation membrane product. This invention employs an in-situ crystallization method, avoiding the seed synthesis and orientation layer construction processes in secondary growth methods, thus simplifying the preparation process of b-axis oriented MFI type molecular sieve membranes. Furthermore, the obtained product can be used for the separation of mixed systems such as ethanol / water, and has significant application value in practical chemical production.

Description

Technical Field

[0001] This invention relates to a method for synthesizing zeolite molecular sieve membranes, specifically to a method for in-situ crystallization synthesis of b-axis oriented MFI type molecular sieve separation membranes. Background Technology

[0002] Zeolite molecular sieves have become excellent inorganic film-forming materials due to their sub-nanometer molecular recognition capabilities, easily modulated chemical properties, and diverse framework structures. Among them, MFI-type zeolites possess a microporous structure composed of intersecting a-axis oriented sinusoidal channels and b-axis oriented straight channels, with pore diameters of 0.51 nm × 0.55 nm and 0.53 nm × 0.56 nm, respectively. These pore sizes are similar to the size of many industrial organic molecules, showing promising application prospects in the separation of systems such as ethanol / water, n- and isobutane, and o- and p-xylene. Compared to arbitrary-oriented membranes, the straight channels of b-axis oriented MFI zeolite membranes are perpendicular to the carrier surface, effectively shortening the molecular diffusion path, reducing permeation resistance and grain boundary defects generated during calcination to remove the template agent, thus achieving higher permeation flux and separation selectivity. This demonstrates significant research value and industrial potential.

[0003] There are two main methods for preparing b-axis oriented molecular sieve (MFI) zeolite membranes: secondary growth and in-situ crystallization. The secondary growth method decouples the nucleation and growth processes of zeolite, preparing the oriented membrane by depositing an oriented seed layer on the support surface and controlling the secondary growth conditions. This method can yield MFI zeolite membranes with high separation performance, but it still suffers from limitations such as limited support, complex preparation of the oriented seed layer, difficulty in controlling seed coverage, and low reproducibility. The in-situ crystallization method, on the other hand, has the advantages of simple synthesis steps, no specific requirements on the type and geometry of the support (sheet, tubular, or hollow fiber), and is easy for industrial production. However, it has high standards for the flatness and physicochemical properties of the support surface. Currently, the in-situ crystallization method can prepare continuous b-axis oriented membranes on dense supports such as stainless steel sheets and silicon wafers, but it is still difficult to obtain membranes with high separation performance on porous supports. Therefore, novel in-situ crystallization methods are of great significance for the industrialization of b-axis oriented MFI molecular sieve separation membranes.

[0004] Given the stringent requirements of in-situ crystallization on the support surface, researchers typically employ transition layers to modulate surface properties. Zhang et al. (Adv. Mater., 2006, 18: 3261-3265) prepared a b-axis oriented TS-1 membrane by reducing the surface roughness of the support and introducing a large number of hydroxyl functional groups through the preparation of a chitosan interlayer, but certain defects still existed on the membrane surface. Takahashi et al. (Chem. Mater., 2005, 17: 1167-1173) deposited a mesoporous silica layer on the surface of porous alumina using the sol-gel method, and subsequently obtained a b-axis oriented MFI molecular sieve membrane using in-situ crystallization, but the membrane continuity was poor, and the gas separation performance of the product still needs improvement. Therefore, selecting a low-cost and suitable transition layer material is of great significance for preparing defect-free b-axis oriented MFI molecular sieve membranes. Summary of the Invention

[0005] This invention provides a method for preparing an all-silica b-axis oriented MFI molecular sieve separation membrane by in-situ crystallization. By introducing a graphite transition layer on the surface of the support, the target product is obtained through one-step hydrothermal synthesis, and its separation performance in an ethanol / water system is explored.

[0006] The technical solution of this invention is:

[0007] A method for preparing b-axis oriented MFI type molecular sieve separation membrane by in-situ crystallization includes the following steps:

[0008] 1) Construct a smooth graphite transition layer on the surface of a porous carrier;

[0009] 2) Dissolve the silicon source and organic template agent in deionized water, stir and age at room temperature for 3-4 hours to obtain the synthesis solution, put the carrier with graphite transition layer into the high-pressure synthesis reactor containing the synthesis solution, seal it and hydrothermally synthesize at 150-180℃ for 2-10 hours.

[0010] 3) After crystallization, the membrane material is removed, cleaned, and dried. After high-temperature calcination to remove the template agent, a b-axis oriented MFI type molecular sieve separation membrane is obtained.

[0011] Preferably, the porous carrier in step 1) is a porous metal oxide, porous metal, porous polymer or porous glass, and its shape is tubular, hollow fiber or plate.

[0012] Preferably, the method for constructing the graphite layer in step 1) is as follows: after wetting the porous carrier with an organic solvent, the graphite-based material powder is uniformly coated onto the surface of the carrier; the organic solvent used is one or more of low-carbon alcohols, low-carbon organic acids, low-carbon esters, and low-carbon ketones. Typically, but not limited to, the organic solvent can be one or more of methanol, ethanol, n-propanol, and n-butanol.

[0013] Preferably, the method for constructing the graphite layer in step 1) is as follows: repeatedly rubbing a shaped graphite-based material onto the surface of a porous carrier or a porous carrier already coated with graphite-based material powder to construct a smooth graphite transition layer. Typically, but not limited to, the shaped graphite-based material used can be a graphite rod or a pencil lead.

[0014] Further preferred, the roughness Ra of the graphite transition layer obtained in step 1) is less than 100 nm.

[0015] Preferably, the graphite layer constructed in step 1) can be heat-treated in air at 300-500°C with a heating / cooling rate of 1-10°C / min.

[0016] Preferably, in step 2), the silicon source is tetraethyl orthosilicate and the organic template agent is tetrapropylammonium hydroxide.

[0017] Preferably, in step 2), the molar ratio of the synthesis solution is tetraethyl orthosilicate: tetrapropylammonium hydroxide: water = 1:0.2~0.4:90~200.

[0018] Preferably, in step 3), the high-temperature calcination to remove the template agent is performed by calcining in air at 400–550°C for 6–12 hours.

[0019] Preferably, in step 3), the high-temperature calcination condition for removing the template agent is calcination in ozone at 200–400°C for 2–10 hours.

[0020] This invention further discloses the application of the b-axis oriented MFI molecular sieve membrane prepared by the method in ethanol / water separation.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) This invention provides a method for preparing b-axis oriented MFI molecular sieve membranes through in-situ crystallization of a graphite transition layer, which can rapidly produce membranes with good intergrowth, repeatability, and separation performance. Compared with commonly used secondary growth methods, this preparation process omits the preparation of the orientation seed layer, simplifying the membrane fabrication process. Compared with other in-situ crystallization methods, the graphite transition layer used in this method only requires simple coating to prepare. Compared with the silica or zirconium oxide interlayer prepared by the sol-gel method, this greatly reduces the difficulty of preparing the transition layer. At the same time, the oriented membrane prepared on the graphite transition layer also has better intergrowth and is more likely to be industrialized.

[0023] (2) This invention uses graphite-based materials such as pencil powder, graphite powder, pencil lead, and graphite rods as transition layer construction materials, which have the characteristics of being inexpensive, readily available, and environmentally friendly. At the same time, the erasability and easy oxidation of graphite make the transition layer easy to remove, and the carrier can be reused in the transition layer preparation process.

[0024] (3) The present invention can be implemented on porous carrier surfaces of various shapes. At the same time, the presence of the graphite transition layer can effectively adjust the surface roughness of the porous carrier, reduce the requirements for carrier quality, and thus reduce production costs. Attached Figure Description

[0025] Figure 1 SEM image of the surface of hollow alumina fibers;

[0026] Figure 2 SEM image of the graphite transition layer on the surface of alumina hollow fiber;

[0027] Figure 3 SEM image of the M1 membrane prepared in Example 1;

[0028] Figure 4 The XRD pattern of the M1 film prepared in Example 1;

[0029] Figure 5 SEM image of the surface of stainless steel hollow fiber;

[0030] Figure 6 SEM image of the graphite transition layer on the surface of stainless steel hollow fiber;

[0031] Figure 7 SEM image of the M2 membrane prepared in Example 2;

[0032] Figure 8 SEM image of the M3 membrane prepared in Example 3;

[0033] Figure 9 SEM image of the surface of the macroporous alumina tube;

[0034] Figure 10 SEM image of the graphite transition layer on the surface of the macroporous alumina tube;

[0035] Figure 11 SEM image of the M4 film prepared in Example 4;

[0036] Figure 12 SEM image of the M5 membrane prepared in Comparative Example 1. Detailed Implementation

[0037] To better illustrate the method of preparing b-axis oriented MFI type molecular sieve separation membrane by in-situ crystallization of graphite-based transition layer, some examples of molecular sieve membrane synthesis are given below, but the present invention is not limited to the examples listed.

[0038] Example 1

[0039] Preparation of b-axis oriented MFI type molecular sieve membranes on porous alumina hollow fibers. The surface morphology of the alumina hollow fibers with an average pore size of 0.2 μm is shown below. Figure 1 As shown.

[0040] (1) Preparation of the transition layer: The surface of the alumina hollow fiber carrier was wetted with n-butanol, fixed on the coating device and kept rotating. Graphite powder was evenly coated onto the carrier surface, and then the carrier was placed in a 60℃ oven to remove excess n-butanol. After drying, the carrier with the graphite layer was fixed on the coating device again and kept rotating. The surface of the carrier was repeatedly scraped with an 8B pencil, and excess graphite was wiped off with lens paper to obtain a smooth graphite layer. The carrier coated with the graphite transition layer was placed in a muffle furnace for calcination at 450℃ for 2 hours, with heating and cooling rates set to 1℃ / min. The prepared graphite transition layer was observed under a scanning electron microscope (SEM). Figure 2 As shown, graphite is uniformly covered on the surface of the alumina carrier.

[0041] (2) Membrane preparation: Tetraethyl orthosilicate (TEOS) was added dropwise to a mixed solution of tetrapropylammonium hydroxide (TPAOH) and ultrapure water, and the mixture was stirred and aged at room temperature for 4 hours to obtain the synthesis solution. The final molar composition of the synthesis solution was 1TEOS:0.32TPAOH:165H2O. The support coated with the graphite transition layer was immersed in the synthesis solution, sealed, and placed in an oven for reaction at 165℃ for 3 hours.

[0042] (3) After the reaction was completed, the alumina hollow fiber membrane was removed, washed with deionized water until neutral, dried overnight, and then calcined in a muffle furnace at 400℃ for 10h to remove the template agent. The heating and cooling rates were both 1℃ / min. The molecular sieve membrane prepared in this process is denoted as M1.

[0043] The scanning electron microscopy characterization results of the M1 film are as follows: Figure 3 As shown, the MFI-type molecular sieve membrane prepared by in-situ crystallization exhibits a distinct b-axis orientation and mainly consists of two crystal layers. The lower layer of crystals is well-connected, forming a dense separation layer; however, the upper layer of crystals is not interconnected to form a dense layer. The XRD characterization results of the prepared M1 membrane are as follows: Figure 4 As shown, the membrane exhibits diffraction peaks such as (020) and (040), showing a clear b-axis orientation. Pervaporation tests were conducted on the M1 membrane at 60℃. The results showed that the M1 membrane had a separation factor of 45 for 5% ethanol / water and a permeation flux of 6.70 kg / (m³). 2 / h).

[0044] Example 2

[0045] The remaining steps are the same as in Example 1, except that the carrier used in step (1) is stainless steel hollow fiber with an average pore size of 1 μm. The molecular sieve membrane prepared in this process is denoted as M2. Figure 5 , Figure 6 , Figure 7 The surface structure of stainless steel hollow fibers, the graphite transition layer prepared on their surface, and the morphological characteristics of b-axis oriented MFI molecular sieve membranes are shown respectively. Figure 7 As shown, the surface morphology of the M2 membrane is similar to that of the M1 membrane. Test results indicate that the M2 membrane has a separation factor of 40 for a 5% ethanol / water mixture and a permeation flux of 2.78 kg / (m³). 2 / h).

[0046] Example 3

[0047] The remaining steps are the same as in Example 2, except that the molar ratio of the synthesis solution in step (2) is changed to 1 TEOS: 0.2 TPAOH: 90H2O. The molecular sieve membrane prepared in this process is denoted as M3. Figure 8 The surface morphology of the M3 membrane is shown. Test results indicate that the M3 membrane has a separation factor of 48 for a 5% ethanol / water mixture and a permeation flux of 1.80 kg / (m³). 2 / h).

[0048] Example 4

[0049] The remaining steps are the same as in Example 1, except that the carrier used in step (1) is a macroporous alumina tube with an average pore size of 2 μm. The molecular sieve membrane prepared in this process is designated as M4. Figure 9 , Figure 10 , Figure 11 The surface structure of macroporous alumina tubes, the morphological characteristics of the graphite transition layer prepared on their surface, and the b-axis oriented MFI type molecular sieve membrane are shown. SEM results indicate that the pore size of the macroporous alumina tube surface is 3-5 μm, making it difficult for coffin-shaped crystals to directly spread on the support surface. However, the graphite transition layer can still cover the alumina particles well, providing a smooth surface for the formation of the b-axis oriented MFI type zeolite membrane. At the same time, the morphology of the M4 membrane layer is consistent with that of the hollow fiber support surface, further demonstrating the universality of constructing b-axis oriented MFI type molecular sieve membranes using in-situ crystallization of the graphite transition layer.

[0050] Comparative Example 1

[0051] The remaining steps are the same as in Example 1, except that a graphite transition layer is not constructed on the surface of the alumina hollow fiber; instead, an in-situ crystallization method is used to directly prepare the film on its surface. The molecular sieve membrane prepared in this process is designated as M5. Figure 12 The surface morphology of the M5 film is shown, demonstrating that the film formed on the surface of the carrier without a graphite transition layer is arbitrarily oriented.

[0052] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. However, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention, without departing from the content of the technical solution of the present invention, shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing b-axis oriented MFI type molecular sieve separation membrane by in-situ crystallization, characterized in that: 1) Construct a smooth graphite transition layer on the surface of a porous carrier; the roughness Ra of the obtained graphite transition layer is less than 100 nm. After constructing the graphite transition layer, the porous carrier with the graphite transition layer is heat-treated in air at 300~500℃. The method for constructing the smooth graphite transition layer is as follows: after wetting the porous carrier with an organic solvent, graphite-based material powder is uniformly coated onto the surface of the carrier. The shaped graphite-based material is repeatedly rubbed onto the surface of the porous carrier that has been coated with graphite-based material powder to construct a smooth graphite transition layer. 2) Dissolve the silicon source and organic template agent in deionized water, stir and age at room temperature for 3-4 hours to obtain a synthesis solution. Place the carrier with the graphite transition layer into a high-pressure synthesis reactor containing the synthesis solution, seal it, and perform hydrothermal synthesis at 150-180 °C for 2-10 hours. The silicon source is tetraethyl orthosilicate, and the organic template agent is tetrapropylammonium hydroxide. The molar ratio of the synthesis solution is tetraethyl orthosilicate: tetrapropylammonium hydroxide: water = 1: 0.2-0.4: 90-200. 3) After crystallization, the membrane material is removed, cleaned, and dried. After removing the template agent by high-temperature calcination, a b-axis oriented MFI type molecular sieve separation membrane is obtained.

2. The method according to claim 1, characterized in that: The porous support in step 1) is a porous metal oxide, porous metal, porous polymer or porous glass, and the pore size of the porous support is 0.1~10μm.

3. The method according to claim 1, characterized in that: The organic solvent used in step 1) is one or more of the following: low carbon alcohols, low carbon organic acids, low carbon esters, and low carbon ketones.

4. The method according to claim 1, characterized in that: In step 3), the high-temperature calcination conditions for removing the template agent are calcination at 400~550 ℃ in air for 6~12 hours.

5. The method according to claim 1, characterized in that: In step 3), the high-temperature calcination conditions for removing the template agent are calcination in ozone at 200~400 ℃ for 2~10 hours.

6. The application of a b-axis oriented MFI molecular sieve membrane obtained by the method of claim 1 in ethanol / water separation.

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