Mo-MFI molecular sieve membrane as well as preparation method and application thereof

Abstract

The invention relates to a Mo-MFI molecular sieve membrane and a preparation method and application thereof, and belongs to the technical field of molecular sieve membrane material preparation and separation application. The MFI molecular sieve membrane is doped and modified by adopting Mo element, so that the performance of the molecular sieve membrane in pervaporation separation of ethanol / water is remarkably improved. Compared with a traditional MFI molecular sieve membrane and a W-doped MFI molecular sieve membrane, the Mo-doped MFI molecular sieve membrane shows higher permeation flux and separation factor.

Description

Technical Field

[0001] The present invention relates to a Mo-MFI molecular sieve membrane, a preparation method thereof and an application thereof, belonging to the technical field of preparation and separation application of molecular sieve membrane materials. Background Art

[0002] In the field of chemical separation and purification, pervaporation technology, as an efficient and energy-saving separation method, is widely used in the separation process of mixtures of alcohols and water, especially in key industrial processes such as dehydration of ethanol fermentation broth, production of biofuels, and recovery of organic solvents. The pervaporation alcohol permeation technology utilizes the characteristic that specific membrane materials have different permeation rates for alcohol molecules and water molecules to achieve efficient separation of the two. This technology can not only effectively improve the purity of alcohol products, but also significantly reduce energy consumption and production costs. Therefore, it occupies a crucial position in the modern chemical industry. With the increasing global demand for clean energy and sustainable development, the research and application of pervaporation alcohol permeation technology have also received increasing attention and become an important force to promote the green transformation of the chemical industry.

[0003] In the existing pervaporation alcohol permeation technology, molecular sieve membranes are widely used in the separation of ethanol / water mixtures due to their unique pore structures and excellent molecular sieving properties. MFI-type molecular sieve membranes have shown great application potential in the field of alcohol-water separation due to their good thermal stability and chemical stability. Previous studies have shown that by introducing W element during the preparation of Silicalite-1 molecular sieve membranes to dope and modify the molecular sieve membranes, the generation of Si-OH groups on the membrane surface can be effectively reduced, thereby improving the alcohol-water separation performance of the membrane. However, its permeation flux and separation factor are not ideal. Specifically, in the best example 1 disclosed in the early patent (CN114768550A) of the invention, the permeation flux is 3.5 kg×m -2 ×h -1 , and the separation factor is 34. The above performance is difficult to meet the requirements of high-throughput and high-selectivity separation in actual industrial applications.

[0004] Therefore, it is urgent to optimize the existing modification methods of MFI-type molecular sieve membranes so as to improve the separation performance of the membranes on the basis of ensuring stability. Summary of the Invention

[0005] The purpose of the present invention is to provide a Mo-MFI molecular sieve membrane applied to pervaporation alcohol permeation. Compared with the prior art, by incorporating Mo atoms into the framework of the MFI molecular sieve membrane, the hydrophobicity of the membrane layer is increased, thereby further improving the alcohol-water separation performance of the MFI molecular sieve membrane.

[0006] Specifically, the present invention provides a preparation method of a Mo-MFI molecular sieve membrane, which comprises the following steps: (1) Load a layer of Si-MFI seeds on a support; (2) Place the carrier loaded with seeds in a synthesis gel for hydrothermal crystallization. The synthesis gel contains a template agent, water, a molybdenum source, and a silicon source; (3) Wash, dry, and calcine the membrane to obtain the Mo-MFI zeolite membrane.

[0007] Preferably, in step (1), the mass concentration of Si-MFI seeds in the seed solution used to load the Si-MFI seed layer is 0.5-2 wt.%, the coating time of the seeds is 5-30 s, and the method of applying the seed suspension can be one or a combination of several methods such as spin coating, wiping coating, dip coating, or vacuum suction.

[0008] Preferably, in step (2), the template agent is selected from one of tetrabutylphosphonium hydroxide, tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, and tetrapropylammonium hydroxide, preferably tetrapropylammonium hydroxide; the silicon source is selected from silica sol, tetraethyl orthosilicate, or fumed silica, preferably tetraethyl orthosilicate; the molybdenum source is selected from ammonium molybdate or sodium molybdate dihydrate, preferably sodium molybdate dihydrate; wherein, the molar ratio of the amounts of the template agent, water, molybdenum source, and silicon source is SiO2:TPAOH:H2O:Na2MoO4·2H2O = 1:(0.12-0.2):(150-200):(0-0.2), preferably, SiO2:TPAOH:H2O:Na2MoO4·2H2O = 1:(0.12-0.2):(150-200):(0-0.02) (excluding 0).

[0009] Preferably, in step (2), the synthesis gel needs to be aged before hydrothermal crystallization, and the aging time is 2-10 h.

[0010] Preferably, in step (2), the temperature of hydrothermal crystallization is 120-180 °C, and the time is 6-48 h.

[0011] Preferably, in step (3), the calcination environment is air, oxygen, ozone, or an oxygen / ozone mixture, the heating rate is 0.2-0.5 °C / min, the calcination temperature is 200-450 °C, and the calcination time is 24-96 h.

[0012] The present invention also provides a Mo-MFI zeolite membrane prepared by the above preparation method, and the zeolite membrane contains Mo element.

[0013] Application of the Mo-MFI zeolite membrane prepared by the present invention in alcohol-water separation, and its ethanol flux ≥ 5 J / kg×m at 60 °C in a 5 wt.% ethanol / water system -2 ×h -1 and / or the separation factor ≥ 50.

[0014] Compared with the prior art, the present invention has the following advantages: The present invention uses Mo element to dope and modify the MFI zeolite membrane, increasing the hydrophobicity of the membrane layer, thereby significantly improving the performance of the zeolite membrane in the application of alcohol-water systems. Compared with the traditional MFI zeolite membrane and W-doped MFI zeolite membrane, the Mo-doped MFI zeolite membrane exhibits higher permeation flux and separation factor. This improvement benefits from the fact that Mo element is more likely to replace Si and enter the MFI zeolite membrane framework, repair the silanol defects, increase the hydrophobicity of the membrane surface, and enable the obtained zeolite membrane to have higher alcohol-water separation performance.

[0015] The preparation method of the Mo-doped MFI zeolite membrane of the present invention realizes the uniform distribution and efficient doping of Mo element in the zeolite membrane by precisely controlling the doping concentration. The Mo-doped MFI zeolite membrane still exhibits excellent performance in the application of ethanol / water systems under low doping amounts, meeting the requirements for high flux and high separation factor in practical industrial applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 XRD patterns of the zeolite membranes prepared in Example 1 and Comparative Example 1 of the present invention; Figure 2 Surface and cross-section SEM images of the zeolite membrane prepared in Example 1 of the present invention; Figure 3 EDX elemental distribution maps of the zeolite membrane prepared in Example 1 of the present invention; Figure 4 Contact angle diagrams of the zeolite membranes prepared in Example 1 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be specifically introduced below in conjunction with the drawings and examples.

[0018] Example 1 The preparation method of the Mo-MFI zeolite membrane is specifically as follows: (1) Preparation of seed crystals: A synthesis solution was prepared with 1SiO2: 0.12TPAOH: 19.2H2O, aged in a bottle for 6 h according to the molar ratio, and reacted at 150 °C for 48 h. The obtained product Si-MFI was centrifuged and dried.

[0019] (2) The seed crystals prepared in step (1) were uniformly coated on the YSZ support by the dip-coating method. Specifically: The zeolite prepared in the above step (1) was made into a 0.5 wt.% seed crystal suspension, the support was immersed in the seed crystal suspension for 30 s, and then dried at 60 °C for 12 h.

[0020] (3) Preparation of Mo-MFI zeolite membrane: Prepare a synthesis solution according to the molar ratio of 1SiO2: 0.12TPAOH: 180H2O: 0.02Na2MO4·2H2O, and stir and age at room temperature for 6 h.

[0021] Fix the carrier with the seeded layer prepared in step (2), place it in a reaction kettle, and react with the membrane synthesis gel in step (3) at 140 °C for 12 h.

[0022] Wash the reacted membrane with deionized water, dry it at 60 °C for 12 h, then place it in a muffle furnace at 450 °C and calcine it in air for 8 h. The heating and cooling rates are both 1 °C / min to remove the template agent. The prepared zeolite membrane is marked as Mo-0.02-MFI.

[0023] Example 2 Steps (1) and (2) are the same as those in Example 1.

[0024] The preparation process of step (3) is basically the same as that of step (3) in Example 1, except that the molar ratio of each substance in the finally synthesized zeolite membrane gel is 1SiO2: 0.12TPAOH: 180H2O: 0.06Na2MO4·2H2O. The prepared zeolite membrane is marked as Mo-0.06-MFI.

[0025] Example 3 Steps (1) and (2) are the same as those in Example 1.

[0026] The preparation process of step (3) is basically the same as that of step (3) in Example 1, except that the molar ratio of each substance in the finally synthesized zeolite membrane gel is 1SiO2: 0.12TPAOH: 180H2O: 0.08Na2MO4·2H2O. The prepared zeolite membrane is marked as Mo-0.08-MFI.

[0027] Comparative Example 1 Steps (1) and (2) are the same as those in Example 1.

[0028] The preparation process of step (3) is basically the same as that of step (3) in Example 1, except that the membrane synthesis solution formula is 1SiO2: 0.12TPAOH: 180H2O. The prepared zeolite membrane is marked as Si-MFI.

[0029] The pervaporation performance of the membrane is represented by the permeation flux J and the separation factor α. The permeation flux J = W / (A×△t), with the unit kg·m -2 ·h -1 , and the separation factor α = Ye•Xw / (Yw•Xe), where W is the permeation amount of the component passing through the membrane, kg; A is the effective membrane area, m 2; Δt is the operation time, h; Ye and Yw represent the mass fractions of ethanol and water on the permeate side, respectively, and Xe and Xw represent the mass fractions of ethanol and water in the feedstock, respectively.

[0030] Characterization results The XRD patterns of the molecular sieve membranes prepared in Example 1 and Comparative Example 1 are as Figure 1 shown. The surface and cross-section SEM images of the Mo-0.02-MFI molecular sieve membrane prepared in Example 1 are as Figure 2 shown. It can be seen from the figures that the crystals on the membrane surface are all in the regular cuboid morphology, and the crystals grow interactively to form a continuous and dense membrane layer without obvious intercrystalline defects. The membrane thickness is about 3 μm. From Figure 3 the EDX element distribution map, it can be seen that the prepared membrane contains Mo atoms, and there is no obvious aggregation of Mo species clusters in the entire image area, indicating that all the introduced Mo species are highly dispersed in the molecular sieve membrane.

[0031] The influence of Mo element on the surface hydrophobicity of the membrane was evaluated by water contact angle (WCA) test, as Figure 4 shown. The results show that the WCA value of the Si-MFI membrane prepared in Comparative Example 1 is 81.5°, while the Mo-MFI membrane prepared in Example 1 is significantly increased to 124.5°, which is higher than the value of 119° of the W-MFI molecular sieve membrane in Patent CN114768550A, indicating that the Mo-MFI molecular sieve membrane has better hydrophobicity. The Mo-MFI molecular sieve membranes synthesized in Examples 1-3 and the Si-MFI molecular sieve membrane synthesized in Comparative Example 1 were used for pervaporation separation of ethanol / water. The test conditions were as follows: the feed liquid was a 5 wt.% ethanol / water system, the experimental temperature was 60 °C, and the pressure on the membrane permeate side was 200 Pa. The experimental results are shown in Table 1. When the Mo doping amount is 0.02, the performance is the best, the ethanol flux is 5.1, and the separation factor is 58, which are significantly improved compared with the Si-MFI molecular sieve membrane in Comparative Example 1. Moreover, in this Example 1, under the condition of a lower metal doping amount, its permeation flux and separation factor are both higher than the numbers in the inventor's earlier patent application CN114768550A.

[0032] Table 1 Performance test results of different examples and comparative examples Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a Mo-MFI zeolite membrane, characterized in that, It includes the following steps: (1) Loading a layer of Si-MFI seeds on a support; (2) Placing the carrier loaded with seeds into a synthesis gel for hydrothermal crystallization, wherein the synthesis gel contains a template agent, water, a molybdenum source and a silicon source; (3) Washing, drying and calcining the membrane to obtain the Mo-MFI zeolite membrane.

2. The preparation method according to claim 1, characterized in that, In step (1), the support is made of a porous inorganic material; the support configuration is a hollow fiber, a sheet or a tube; the mass concentration of Si-MFI seeds in the seed solution used for loading the Si-MFI seed layer is 0.5-2 wt.%, the coating time of the seeds is 5-30 s, and the method for applying the seed suspension can be one or a combination of several methods among spin coating, wiping coating, dip coating or vacuum suction.

3. The preparation method according to claim 1, characterized in that, In step (2), the template agent is selected from one of tetrabutylphosphonium hydroxide, tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, tetrapropylammonium hydroxide, and tetrapropylammonium hydroxide is preferred; the silicon source is selected from silica sol, tetraethyl orthosilicate or fumed silica, and tetraethyl orthosilicate is preferred; the molybdenum source is selected from ammonium molybdate or sodium molybdate dihydrate, and sodium molybdate dihydrate is preferred; wherein, the molar ratio of the amounts of the template agent, water, molybdenum source and silicon source is SiO2:TPAOH:H2O:Na2MoO4·2H2O = 1:(0.12-0.2):(150-200):(0-0.2).

4. The synthesis method of the Mo-doped pure silicon MFI zeolite membrane according to claim 1, characterized in that, In step (2), the synthesis gel needs to be aged before hydrothermal crystallization, and the aging time is 2-10 h.

5. The synthesis method of the Mo-doped pure silicon MFI zeolite membrane according to claim 1, characterized in that, In step (2), the temperature of hydrothermal crystallization is 120-180 °C and the time is 6-48 h.

6. The synthesis method of the Mo-doped pure silica MFI zeolite membrane according to claim 1, characterized in that, In step (3), the calcination environment is air, oxygen, ozone or an oxygen / ozone mixture, the heating rate is 0.2-0.5 °C / min, the calcination temperature is 200-450 °C, and the calcination time is 24-96 h.

7. A Mo-MFI zeolite membrane prepared by the preparation method according to claim 1, characterized in that , and the zeolite membrane contains Mo element.

8. Use of the Mo-MFI molecular sieve membrane according to claim 7 for alcohol permeation in ethanol / water separation, characterized in that, The water flux in a 5 wt.% ethanol / water system at 60 °C is ≥5 J / kg×m -2 ×h -1 and / or the separation factor is ≥50.

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