Method for preparing b-axis orientation MFI type molecular sieve separation membrane through in-situ crystallization

Abstract

The invention discloses a method for preparing a b-axis oriented MFI type molecular sieve separation membrane through in-situ crystallization. The method comprises the following steps: firstly, preparing a graphite transition layer on the surface of a porous carrier, then directly putting the modified porous carrier into a high-pressure synthesis kettle filled with synthesis liquid, crystallizing, washing with deionized water, and drying to obtain a b-axis oriented MFI type molecular sieve separation membrane product. According to the present invention, with the in-situ crystallization method, the seed crystal synthesis in the secondary growth method and the construction process of the carrier surface orientation seed crystal layer are avoided, the preparation process of the b-axis orientation MFI type molecular sieve membrane is simplified, and the obtained product can be used for the separation of the ethanol / water and other mixed systems, and has important application value in the actual chemical industry production field.

Description

Technical Field

[0001] The present invention relates to a method for synthesizing a zeolite molecular sieve membrane, in particular to a method for synthesizing a b-axis oriented MFI type molecular sieve separation membrane by in-situ crystallization. Background Art

[0002] Zeolite molecular sieves have become an excellent inorganic membrane-forming material due to their subnanometer molecular recognition capabilities, easily adjustable chemical properties, and diverse skeleton structures. MFI zeolites, in particular, have a microporous structure composed of a-axis oriented sinusoidal channels and b-axis oriented straight channels. Their pore diameters are 0.51nm×0.55nm and 0.53nm×0.56nm, respectively. These pore sizes are similar to the molecular sizes of many industrial organic compounds and have promising application prospects in systems such as the separation of ethanol / water, n-isobutane, and o-paraxylene. Compared to randomly 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, and can achieve higher permeation flux and separation selectivity. These membranes have considerable research value and industrial prospects.

[0003] There are two main methods for preparing b-axis oriented MFI zeolite membranes: the secondary growth method and the in-situ crystallization method. The secondary growth method decouples the nucleation and growth processes of zeolite, and prepares an oriented membrane by depositing an oriented seed layer on the surface of the support and regulating the secondary growth conditions. This method can obtain MFI zeolite membranes with high separation performance, but it still has problems such as limited support, complex preparation of the oriented seed layer, difficult to control seed coverage, and low reproducibility. The advantage of the in-situ crystallization method is that the synthesis steps are simple, there are no specific requirements for the type and geometric morphology of the support (sheet, tubular or hollow fiber), and it is easy to industrialize. However, it has very high standards for the flatness and physical and chemical properties of the support surface. At present, the in-situ crystallization method can prepare continuous b-axis oriented membranes on the surfaces of dense supports such as stainless steel sheets and silicon wafers, but it is still difficult to obtain membrane layers with high separation performance on the surface of porous supports. Therefore, the new in-situ crystallization method is 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 usually use a transition layer to adjust the surface properties. Zhang et al. (Adv. Mater., 2006, 18:3261-3265) reduced the surface roughness of the support and introduced a large number of hydroxyl functional groups by preparing a chitosan intermediate layer, thereby preparing a b-axis oriented TS-1 membrane by in-situ crystallization, 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 a sol-gel method and then used in-situ crystallization to obtain a b-axis oriented MFI zeolite membrane, but the membrane layer continuity was poor and the gas separation performance of the product still needed to be improved. Therefore, selecting an inexpensive and suitable transition layer material is of great significance for the preparation of defect-free b-axis oriented MFI zeolite membranes. Summary of the Invention

[0005] The present invention provides a method for preparing an all-silicon b-axis oriented MFI molecular sieve separation membrane by in-situ crystallization. A graphite transition layer is introduced on the surface of a carrier to obtain the target product by a one-step hydrothermal synthesis, and its separation performance in an ethanol / water system is explored.

[0006] The technical solution of the present invention is:

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

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

[0009] 2) dissolving a silicon source and an organic template in deionized water, stirring and aging at room temperature for 3 to 4 hours to obtain a synthesis solution, placing the support with the graphite transition layer into a high-pressure synthesis autoclave filled with the synthesis solution, sealing the autoclave, and conducting hydrothermal synthesis at 150 to 180° C. for 2 to 10 hours;

[0010] 3) After the crystallization is completed, the membrane material is taken out, cleaned, dried, and calcined at a high temperature to remove the template agent to obtain a b-axis oriented MFI type molecular sieve separation membrane.

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

[0012] Preferably, the method for constructing the graphite layer in step 1) is: after wetting the porous support with an organic solvent, the graphite-based material powder is evenly applied to the support surface by rubbing; the organic solvent used is one or more of a low-carbon alcohol, a low-carbon organic acid, a low-carbon ester, and a low-carbon ketone. Typically, but not limiting, the organic solvent can be one or more of methanol, ethanol, n-propanol, and n-butanol.

[0013] Preferably, the graphite layer in step 1) is formed by repeatedly rubbing a shaped graphite-based material onto the surface of a porous support or a porous support coated with graphite-based material powder to form a smooth graphite transition layer. Typically, but not limiting, the shaped graphite-based material can be a graphite rod or pencil lead.

[0014] Further preferably, 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 at 300-500° C. in air at a heating and cooling rate of 1-10° C. / min.

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

[0017] Preferably, in step 2), the molar ratio of the synthesis liquid is ethyl 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 carried out at 400-550° C. in air for 6-12 hours.

[0019] Preferably, in step 3), the high-temperature calcination to remove the template is carried out at 200-400° C. in ozone for 2-10 hours.

[0020] The invention further discloses application of the b-axis oriented MFI type 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) The present invention provides a method for preparing a b-axis oriented MFI-type molecular sieve membrane by in-situ crystallization of a graphite transition layer, which can quickly prepare a membrane layer with good continuity, repeatability, and separation performance. Compared with the commonly used secondary growth method, this preparation process omits the preparation process of the oriented seed layer, simplifying the membrane production process. Compared with other in-situ crystallization methods, the graphite transition layer used in this scheme can be prepared by simple coating. Compared with the silica intermediate layer or zirconium oxide intermediate layer prepared by the sol-gel method, the difficulty of preparing the transition layer is greatly reduced. At the same time, the oriented film prepared on the graphite transition layer also has better continuity, which is more promising for industrial production.

[0023] (2) The present invention uses graphite-based materials such as pencil powder, graphite powder, pencil lead, and graphite rods as transition layer construction materials, which are inexpensive, readily available, and environmentally friendly. Furthermore, the erasability and oxidizability of graphite make the transition layer easily removable, and the carrier can be reused in the transition layer preparation process.

[0024] (3) The present invention can be implemented on the surface of porous carriers of various shapes. At the same time, the presence of the graphite transition layer can effectively adjust the surface roughness of the porous carrier, reducing the demand for carrier quality and thus reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

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

[0031] Figure 7 is a SEM image of the M2 film prepared in Example 2;

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

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

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

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

[0036] Figure 12 This is the SEM image of the M5 film prepared in Comparative Example 1. DETAILED DESCRIPTION

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

[0038] Example 1

[0039] Preparation of b-axis oriented MFI molecular sieve membrane on porous alumina hollow fiber. The surface morphology of alumina hollow fiber with an average pore size of 0.2 μm is as follows Figure 1 shown.

[0040] (1) Preparation of transition layer: After wetting the surface of the alumina hollow fiber carrier with n-butanol, fix it on the coating device and keep it rotating, evenly rub graphite powder on the surface of the carrier, and then place the carrier in a 60°C oven to remove excess n-butanol. After drying, fix the carrier with the graphite layer on the coating device again and keep it rotating. Use an 8B pencil to repeatedly scrape the surface of the carrier, and use lens paper to wipe off excess graphite to obtain a smooth graphite layer. Place the carrier coated with the graphite transition layer in a muffle furnace for calcination. The calcination temperature is 450°C, the calcination time is 2h, and the heating and cooling rates are all set to 1°C / min. The prepared graphite transition layer is placed under a scanning electron microscope (SEM) for observation. As Figure 2 As shown, graphite is evenly covered on the surface of the alumina support.

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

[0042] (3) After the reaction is complete, the alumina hollow fiber membrane is removed, washed with deionized water until neutral, dried overnight, and then calcined in a muffle furnace at 400°C for 10 h to remove the template, with a heating and cooling rate of 1°C / min. The molecular sieve membrane prepared in this process is designated M1.

[0043] The scanning electron microscopy characterization results of M1 film are shown in Figure 2. Figure 3 As shown in Figure 2, the MFI molecular sieve membrane prepared by in-situ crystallization has a clear b-axis orientation and is mainly composed of two layers of crystals. The crystals in the lower layer are well connected to form a dense separation layer; while the crystals in the upper layer are not connected to form a dense layer. The XRD characterization results of the prepared M1 membrane are shown in Figure 2. Figure 4 As shown, the membrane layer has diffraction peaks such as (020) and (040), showing obvious b-axis orientation. The M1 membrane was subjected to pervaporation test at a temperature of 60°C. The test results showed that the separation factor of the M1 membrane for 5% ethanol / water was 45, and the permeation flux was 6.70 kg / (m 2 / h).

[0044] Example 2

[0045] The remaining steps were the same as those in Example 1, except that the carrier used in step (1) was a stainless steel hollow fiber with an average pore size of 1 μm. The molecular sieve membrane prepared in this process was designated as M2. Figure 5 、 Figure 6 、 Figure 7 The surface structure of stainless steel hollow fiber, the graphite transition layer prepared on its surface and the morphology of b-axis oriented MFI type molecular sieve membrane are respectively shown. Figure 7 As shown in Figure 2, the surface morphology of the M2 membrane is similar to that of the M1 membrane. The test results show that the separation factor of the M2 membrane for 5% ethanol / water is 40, and the permeation flux is 2.78 kg / (m 2 / h).

[0046] Example 3

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

[0048] Example 4

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

[0050] Comparative Example 1

[0051] The remaining steps were the same as those in Example 1, except that no graphite transition layer was constructed on the surface of the alumina hollow fiber, but instead a membrane layer was directly prepared on the surface by in-situ crystallization. The molecular sieve membrane prepared in this process was designated M5. Figure 12 The surface morphology of the M5 film is shown, indicating that the film formed on the support surface without a graphite transition layer is randomly oriented.

[0052] The above-described embodiments merely represent several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art may make several modifications and improvements without departing from the concept of the present invention. However, any simple modifications, equivalent changes, and modifications made to the above-described 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 a b-axis oriented MFI molecular sieve separation membrane by in-situ crystallization, characterized in that: 1) constructing a smooth graphite transition layer on the surface of the porous support; 2) dissolving a silicon source and an organic template in deionized water, stirring and aging at room temperature for 3 to 4 hours to obtain a synthesis solution, placing the support with the graphite transition layer into a high-pressure synthesis autoclave filled with the synthesis solution, sealing the autoclave, and conducting hydrothermal synthesis at 150 to 180° C. for 2 to 10 hours; 3) After the crystallization is completed, the membrane material is taken out, cleaned, dried, and calcined at a high temperature to remove the template agent to obtain a b-axis oriented MFI type molecular sieve separation membrane.

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

3. The method according to claim 1, wherein: The method for constructing the graphite transition layer in step 1) is: after wetting the porous carrier with an organic solvent, the graphite-based material powder is evenly coated on the carrier surface; the organic solvent used is one or more of low-carbon alcohols, low-carbon organic acids, low-carbon esters, and low-carbon ketones.

4. The method according to claim 1, wherein: The method for constructing the graphite transition layer in step 1) is: repeatedly rubbing the shaped graphite-based material on the surface of the porous carrier or the porous carrier coated with graphite-based material powder to construct a smooth graphite transition layer.

5. The method according to claim 1, characterized in that: The roughness Ra of the graphite transition layer obtained in step 1) is less than 100 nm.

6. The method according to claim 1, characterized in that: After the graphite transition layer is constructed in step 1), the porous carrier with the graphite transition layer is heat-treated at 300-500° C. in air.

7. The method according to claim 1, characterized in that: The silicon source in step 2) is tetraethyl orthosilicate, and the organic template is tetrapropylammonium hydroxide; the molar ratio of the synthesis liquid in step 2) is tetraethyl orthosilicate: tetrapropylammonium hydroxide: water = 1:0.2-0.4:90-200.

8. The method according to claim 1, characterized in that: Step 3) calcining at a medium to high temperature to remove the template is carried out at a temperature of 400 to 550° C. in air for 6 to 12 hours.

9. The method according to claim 1, characterized in that: Step 3) calcining at a medium to high temperature to remove the template agent is carried out in ozone at 200 to 400° C. for 2 to 10 hours.

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

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