Method for preparing oriented mfi zeolite membrane in a wider temperature range and application thereof

Abstract

The application provides a method for preparing an oriented MFI molecular sieve membrane in a wide temperature range and application. The preparation method comprises the following steps: firstly, coating an oriented MFI seed monolayer on the surface of a carrier; then, taking an organic template agent, an alkali source, a silicon source, a fluorine-containing crystallization regulator and deionized water as raw materials, introducing a synthesis mother liquor by controlling alcohol evaporation and water supplementing in the stirring aging process of a precursor liquid and the fluorine-containing crystallization regulator; finally, placing the oriented MFI seed layer in the synthesis mother liquor to perform a hydrothermal reaction at 30-210 DEG C; after the reaction, washing, drying and calcining are performed to obtain a high-quality oriented MFI molecular sieve membrane. The MFI molecular sieve membrane prepared by the application has high orientation, good intergrowth, almost no twin crystals on the surface and controllable membrane layer thickness, and exhibits excellent separation performance in electrochemical ion sieve separation and n-butane / isobutane separation. Meanwhile, the orientation and surface morphology are not limited by the synthesis temperature (30-210 DEG C constant temperature or variable temperature can be maintained), and the application has good industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve membrane synthesis technology, and relates to a method for preparing oriented MFI molecular sieve membranes over a wide temperature range and its application. Background Technology

[0002] Zeolite molecular sieve membranes are aluminosilicate crystals with a regular pore structure, possessing characteristics such as precise molecular recognition, high temperature resistance, high mechanical strength, and resistance to chemical corrosion. They offer unique advantages in separation, membrane catalysis, and corrosion-resistant materials. Numerous studies have shown that the internal crystallite orientation of zeolite molecular sieve membranes significantly affects the microstructure of the membrane material, thus significantly influencing its application performance. Therefore, the preparation of highly oriented zeolite molecular sieve membranes has become a hot topic in this research field. Taking zeolite molecular sieve membranes with an MFI-type topology as an example, due to their anisotropic two-dimensional pore structure, the pore structure within the membrane varies greatly when the crystals grow in different directions, thus significantly affecting its mass transfer characteristics. Compared to randomly oriented MFI zeolite membranes, when the crystals grow perpendicular to the support surface along the b-axis, the intergranular defects within the membrane are significantly reduced, and the membrane thickness and mass transfer resistance are correspondingly lowered. This plays a crucial role in improving its permeate flux and separation selectivity.

[0003] The seed-oriented method is a relatively effective way to construct oriented zeolite films. This process usually includes: (1) preparation of seed crystals with regular morphology; (2) deposition of seed-oriented crystal layers; and (3) control of secondary growth. By separating the nucleation and growth steps, the microstructure of the MFI zeolite film can be controlled more precisely. At the same time, since the seed-oriented crystal layer is pre-coated, it is easier to form a continuous and dense b-axis oriented MFI zeolite film during secondary growth.

[0004] The key to controlling the secondary growth process lies in maintaining the epitaxial growth of zeolite molecular sieve seed crystals along their original orientation direction. The key to solving the above problems lies in rationally controlling the epitaxial growth rate of zeolite molecular sieve seed crystals both inside and outside the crystal plane, while effectively suppressing the large number of twins that may be generated on the surface of the seed layer. In this regard, scholars have carried out the following three aspects of research: (1) Using special organic structure guiding agents to reduce the bulk nucleation rate of zeolite crystals while modulating the growth rate of zeolite crystals along different crystal axes; (2) Finely modulating the secondary growth reaction parameters, such as pretreatment of the synthesis solution, introduction of crystallization regulators, finely modulating the composition of the synthesis solution, and microwave-assisted hydrothermal synthesis. This method suppresses the large-scale generation of zeolite crystal nuclei in the synthesis liquid phase without significantly changing the growth characteristics of zeolite seed crystals; (3) Using solid-phase reaction systems (such as gel conversion, gel-free vapor phase conversion, and solvent-free growth) for secondary growth. This strategy effectively avoids direct contact between the seed crystals and the bulk solution during the growth process, and effectively avoids bulk nucleation without significantly changing the growth characteristics of zeolite seed crystals.

[0005] However, in most cases, the effective suppression of twinning during the preparation of alignment films places extremely high demands on the crystallization temperature. Even slight fluctuations in the crystallization temperature range can easily lead to the formation of surface twins, thereby affecting the application performance of the film material. This is extremely detrimental to the large-scale production of alignment films in actual industrial processes. Summary of the Invention

[0006] This invention provides a method for preparing oriented MFI molecular sieve membranes over a wide temperature range. The synthesis mother liquor is prepared by controlling the alcohol distillation and water replenishment during the stirring and aging process of the precursor solution, as well as introducing a fluorinated crystallization regulator. This ensures that the MFI molecular sieve membrane maintains good intergrowth within a wide crystallization temperature range (30-210℃, either isothermal or variable-temperature), while effectively suppressing the excessive formation of crystalline twins on the membrane surface. The application of this method in industrial separation and electrochemistry is also explored. The prepared MFI molecular sieve membrane exhibits high orientation, good intergrowth, almost no twinning on the surface, and controllable membrane thickness. It demonstrates excellent separation performance in both n-butane / isobutane separation and electrochemical ion sieving. Therefore, it provides promising prospects for the industrial application of high-performance MFI molecular sieve membranes.

[0007] This invention is achieved through the following technical solution:

[0008] A method for preparing oriented MFI molecular sieve membranes over a wide temperature range includes the following steps:

[0009] S1 The carrier modified with polymer layer is cured, and regular brick-shaped MFI zeolite powder is poured onto the surface of the carrier for coating to obtain a continuous and dense oriented MFI seed monolayer. The obtained seed monolayer is calcined to enhance the bonding force between the seed and the carrier while removing the polymer layer.

[0010] S2 involves sequentially adding the organic structure-directing agent and the alkaline source to deionized water, where they are fully dissolved through the impact and shearing effects of mechanical movement. Then, a silicon source is added dropwise to the solution, and the mixture is vigorously stirred and aged at 60–120°C for 6–12 hours to remove the ethanol produced by the hydrolysis of the silicon source. Water is then added to bring the volume back to the original level to obtain solution A. A fluorinated crystallization regulator is dissolved in deionized water to obtain solution B. Solution B is then slowly added dropwise to solution A, and the mixture is stirred and aged for another 6–12 hours to obtain the synthesis mother liquor.

[0011] S3 involves placing the seed layer obtained in S1 into the synthesis mother liquor obtained in S2 and carrying out a hydrothermal crystallization reaction at 30–210 °C.

[0012] S4 involves washing, soaking, drying, and calcining the membrane material obtained in S3 to prepare an oriented MFI molecular sieve membrane.

[0013] In step S1, the curing temperature is 60–120°C and the curing time is 1–6 hours.

[0014] In step S1, the coating method is to manually coat the seed layer using fingers wearing nitrile gloves.

[0015] The carrier mentioned in step S1 is a glass sheet, a platinum sheet, or a porous alumina sheet.

[0016] The seed crystals mentioned in step S1 are uniform, regular, brick-shaped MFI molecular sieves with a particle size of 400 nm to 2 μm.

[0017] The silicon source mentioned in step S2 is one or more of methyl orthosilicate, ethyl orthosilicate, and propyl orthosilicate, and the molar ratio of deionized water to silicon source is H2O / SiO2 = 5 to 100, and the silicon source is SiO2.

[0018] The fluorine-containing inorganic salt mentioned in step S2 is one or more of ammonium fluoride, sodium fluoride, potassium fluoride, and ammonium hydrofluoride, and the molar ratio of the fluorine-containing crystallization regulator to the silicon source is F. - / SiO2 = 0.1~10, the fluorine-containing crystallization regulator is F - remember.

[0019] The organic structure directing agent mentioned in step S2 is one or more of tetrapropylammonium hydroxide solution, tetraethylammonium hydroxide solution, tetrabutylammonium hydroxide solution and methyltributylammonium hydroxide solution, and the molar ratio of organic structure directing agent to silicon source is organic template agent / SiO2 = 0.02 to 0.5.

[0020] The alkali source mentioned in step S2 is one or more of sodium hydroxide and potassium hydroxide.

[0021] The hydrothermal crystallization process described in step S3 is either constant temperature or variable temperature, and the hydrothermal crystallization time is 0.8 hours to 54 days.

[0022] The calcination method described in step S4 is muffle furnace calcination, ultraviolet irradiation, oxygen plasma treatment, tube furnace calcination in an ozone atmosphere, or rapid thermal process calcination, with a calcination temperature of 50–700°C and a calcination time of 0.2–100 h. Furthermore, the calcination method is tube furnace calcination in an ozone atmosphere, with a calcination temperature of 140–280°C. This requires a lower calcination temperature and a shorter time, while maximizing the integrity of the film layer.

[0023] The present invention also provides an MFI molecular sieve membrane obtained by the above method. The thickness of the MFI molecular sieve membrane is 420 nm to 450 μm.

[0024] The present invention also provides an application of the MFI molecular sieve membrane in electrochemical ion sieving or separation of n-butane / isobutane mixed gas.

[0025] The MFI molecular sieve membrane exhibits superior separation performance in both ion sieving and separation of n-butane / isobutane mixed gases.

[0026] The beneficial effects of this invention are as follows: This invention prepares the synthesis mother liquor by regulating the alcohol evaporation and water replenishment during the stirring and aging process of the precursor solution, and by introducing a fluorinated crystallization regulator. This allows for the preparation of MFI molecular sieve membranes with good intergrowth, no twins, and high b-axis orientation within a relatively wide crystallization temperature range (30-210℃, either isothermal or variable temperature). Through synergistic aging, alcohol evaporation and water replenishment, and fluoride mineralization, a good balance can be achieved between the intergrowth of the MFI molecular sieve membrane and the formation of twins on the membrane surface. Compared to traditional MFI molecular sieve membrane preparation processes, the crystallization temperature range is significantly increased, which greatly reduces the difficulty of membrane preparation and production costs. Simultaneously, the membrane material prepared by this invention within a wider crystallization temperature range exhibits excellent electrochemical ion sieving ([Ru(NH3)6)) performance. 3+ and [Fe(phen)3] 2+ (Electrical signal response) and separation of n-butane / isobutane gas mixtures (n-butane permeation flux 3.4 × 10⁻⁶) -7 mol·m -2 ·s -1 ·Pa -1 It exhibits high separation performance in terms of separation factor 43, and has good prospects for industrial application. Attached Figure Description

[0027] Figure 1 These are (a) SEM images and (b) XRD images of the MFI seed crystals prepared in Example 1.

[0028] Figure 2 These are (a) planar and (b) cross-sectional SEM images of the MFI seed layer prepared in Example 1.

[0029] Figure 3 These are (a) planar and (b) cross-sectional SEM images of the M1 membrane prepared in Example 1.

[0030] Figure 4 This is the XRD pattern of the M1 film prepared in Example 1.

[0031] Figure 5 These are planar SEM images of the M2-M7 films prepared in Examples 2-7.

[0032] Figure 6 These are cross-sectional SEM images of the M2-M7 membranes prepared in Examples 2-7.

[0033] Figure 7 These are the XRD patterns of the M2-M7 films prepared in Examples 2-7.

[0034] Figure 8 These are planar SEM and XRD images of the M8-M10 films prepared in Examples 8-10.

[0035] Figure 9 These are planar SEM and XRD images of the M11 film prepared in Example 11.

[0036] Figure 10 This is a schematic diagram of the three-electrode system used in the cyclic voltammetry test of Example 11.

[0037] Figure 11 The working electrode was a platinum sheet modified with MFI molecular sieve membrane, and cyclic voltammetry was used to test [Ru(NH3)6]. 3+ and [Fe(phen)3] 2+ Electrochemical response signal diagram.

[0038] Figure 12 These are planar SEM and XRD images of the M12 film prepared in Example 12.

[0039] Figure 13 This is a schematic diagram of the gas permeation test apparatus in Example 12.

[0040] Figure 14 This is a planar SEM image of the M13 film prepared in Example 13.

[0041] Figure 15 This is a planar SEM image of the M14 film prepared in Example 14. Detailed Implementation

[0042] The present invention will be further described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0043] In this application, the electrochemical ion sieving test and the n-butane / isobutane separation test are conducted at room temperature. Unless otherwise specified, the ion sieving test uses the cyclic voltammetry method. In the gas separation test, the mixed gas feed ratio is 1:1, the permeate side is at atmospheric pressure, the transmembrane pressure difference is 1 bar, and helium is used as the purge gas.

[0044] Example 1

[0045] (1) Seed layer preparation: First, brick-shaped MFI seed crystals were prepared using a rotary oven method (for detailed synthesis procedures, please refer to Chem. Asian J. 2020, 15, 1277-1280). Before coating the seed layer, a 6.5 wt.% polyvinyl alcohol (PVA) aqueous solution was prepared as follows: First, a certain amount of PVA powder was slowly poured into a sample bottle containing deionized water and stirred vigorously at room temperature for 3 hours to allow it to fully swell. Then, the temperature was increased to 80℃ at a rate of 1℃ / min, and stirring was continued at this temperature for 2 hours to allow the PVA to fully dissolve until the solution was clear. Finally, stirring and heating were stopped, and the PVA solution was allowed to cool to room temperature and allowed to stand at room temperature for 2 hours to allow unreacted flocculent matter to settle to the bottom of the sample bottle. A glass slide was horizontally clamped on the sample stage of a spin coater, and a certain amount of PVA solution was dropped onto the carrier to completely cover the surface of the carrier. The spin coater was set with an acceleration time of 4s, a rotation speed of 3000rpm, and a spin coating time of 60s. After spin coating, the PVA-coated glass slide was quickly transferred to a 100℃ oven for curing for 1 hour. After curing, 0.02g of MFI seed powder was poured onto the carrier surface and hand-coated using fingers wearing nitrile gloves. Finally, the resulting seed monolayer was calcined at 550℃ for 6 hours to remove the PVA layer and enhance the adhesion between the seed and the carrier. It is important to note that the hand-coating process should be carried out with moderate pressure to minimize the presence of a second seed layer on the surface, and the ambient humidity should be controlled at around 40%.

[0046] (2) Preparation of the synthesis mother liquor: Tetraethyl orthosilicate (TEOS) was slowly added dropwise to a mixed solution of tetrapropylammonium hydroxide (25 wt.%), sodium hydroxide, and deionized water. The mixture was stirred and aged at 80°C for 10 h to remove the ethanol produced by the hydrolysis of the silicon source, and water was added to the original volume to obtain solution A. Ammonium fluoride was dissolved in deionized water to obtain solution B. Solution B was slowly added dropwise to solution A, and the mixture was stirred and aged at 90°C for another 10 h to obtain the synthesis mother liquor. The molar composition of the above synthesis mother liquor was 1 TEOS: 0.15 TPAOH: 0.01 NaOH: 0.55 NH4F: 24H2O.

[0047] (3) Preparation of MFI molecular sieve membrane by secondary growth method: The glass slide coated with the seed layer was placed in the synthesis mother liquor, sealed and placed in an oven, and reacted at 120℃ for 2.5h. After the reaction was completed, the membrane was taken out, washed with deionized water until neutral, dried overnight and then placed in a tube furnace under ozone atmosphere at 200℃ for 6h to remove the template agent. The heating and cooling rates were both 1℃ / min. The MFI molecular sieve membrane prepared by this process is denoted as M1.

[0048] Scanning electron microscopy characterization of MFI seeds, as follows: Figure 1As shown in figure a, the molecular sieve morphology is a regular brick shape with uniform particle size and height, almost no twinning, and the particle size is uniformly distributed at around 1.0 μm. XRD results ( Figure 1 b) The molecular sieve is shown to be a pure-phase MFI crystal form with high crystallinity and no other impurity peaks. Scanning electron microscopy characterization of the prepared seed layer is as follows: Figure 2 As shown, the seed crystals are continuously and densely arranged on the surface of the support, with a thickness of only one layer and exhibiting a high degree of b-axis orientation. After secondary growth, the voids between the seed crystals are completely filled, forming a continuous and dense film material on the support, with almost no twinning on the surface. Figure 3 a), while the density is only 430nm ( Figure 3 b). XRD spectrum shows ( Figure 4 In the entire XRD diffraction angle range, only the (0k0) diffraction peak exists, which indicates that the MFI molecular sieve membrane is highly b-axis oriented.

[0049] Examples 2-7

[0050] The difference from Example 1 is that the secondary growth conditions in step 3 of Examples 2-7 are 30℃-54 days (M2), 60℃-120h (M3), 90℃-12h (M4), 150℃-1.5h (M5), 180℃-1h (M6), and 210℃-0.8h (M7), respectively. The remaining steps are the same as in Example 1. The MFI molecular sieve membranes prepared by this process are designated as M2-M7. Scanning electron microscopy (SEM) of M2-M7... Figure 5 and 6 ) and XRD characterization ( Figure 7 The results showed that, despite a wide crystallization temperature range, all prepared films exhibited high b-axis orientation, continuous and dense surfaces, and no twinning. Furthermore, the film thickness could be precisely controlled within the range of 420–450 nm.

[0051] Examples 8-10

[0052] The difference from Example 1 is that the crystallization temperature program in step 3 of Examples 8-10 is a linear temperature variation, with temperature programs of 30℃-10h-120℃ (9℃ / h, M8), 30℃-4.5h-150℃ (26.7℃ / h, M9), and 30℃-3h-180℃ (50℃ / h, M10), respectively. The remaining steps are the same as in Example 1. The MFI molecular sieve membranes prepared by this process are designated as M8-M10. SEM and XRD characterization of M8-M10 are described below. Figure 8 The results showed that, despite the use of a linear heating program, all prepared films were highly b-axis oriented, with continuous and dense surfaces and no twins present.

[0053] Example 11

[0054] The difference from Example 1 is that the support used in step 1 is an inert platinum sheet, while the remaining steps are the same as in Example 1. The MFI molecular sieve membrane prepared by this process is designated as M11. SEM and XRD characterization of M11 are described below. Figure 9 The results showed that the film prepared on the platinum sheet exhibited a high b-axis orientation, a continuous and dense surface, and no twins. Cyclic voltammetry (CV) electrochemical performance tests were then performed to investigate the film material's ability to sieve different ions. The cyclic voltammetry tests were conducted using a three-electrode system. Figure 10 In this design, a platinum sheet modified with an MFI molecular sieve membrane serves as the working electrode (WE), a platinum wire as the counter electrode (CE), and a saturated Ag / AgCl electrode as the reference electrode (RE). [The following is a partial translation of the original text:] Selected... and As a probe ion. Characterization results show that for [Ru(NH3)6] 3+ Although the peak current value is significantly reduced compared to a bare platinum electrode, a distinct redox peak can still be observed. Figure 11 a). And when the complexed ions become [Fe(phen)3]... 2+ At that time, the electrochemical response completely disappeared. Figure 11 b). The above results confirm that the prepared membrane material has excellent ion sieving properties.

[0055] Example 12

[0056] The difference from Example 1 is that the support used in step 1 is a porous alumina sheet, while the remaining steps are the same as in Example 1. The MFI molecular sieve membrane prepared in this process is designated as M12. M12 was characterized by scanning electron microscopy and XRD. Figure 12 The results showed that the film prepared on the alumina sheet was highly b-axis oriented, with a continuous and dense surface and no twins present. Using... Figure 13 The gas permeation testing apparatus shown conducted ambient temperature and pressure tests on membrane M12 with a 1:1 mixture of n-butane and isobutane. The results showed that the permeation flux of n-butane reached 3.4 × 10⁻⁶. -7 mol·m -2 ·s -1 ·Pa -1 The separation factor is 43.

[0057] Comparative Example 1

[0058] The difference from Example 1 is that no fluorine-containing crystallization regulator is added in step 2. The remaining steps are the same as in Example 1, and the MFI molecular sieve membrane prepared by this process is designated M13. The scanning electron microscopy characterization of M13 is as follows: Figure 14 As shown, the seed size hardly changes, and the film surface has extremely poor intergrowth.

[0059] Comparative Example 2

[0060] The difference from Example 1 is that, in step 2, the precursor solution is not subjected to alcohol evaporation and water replenishment before the ammonium fluoride solution is added; the remaining steps are the same as in Example 1. The MFI molecular sieve membrane prepared in this process is designated as M14. The scanning electron microscopy characterization of M14 is as follows: Figure 15 As shown, a large number of twins appear on the surface of the seed crystal, and the orientation of the film layer is poor.

[0061] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0062] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing oriented MFI molecular sieve membranes over a wide temperature range, characterized in that: Includes the following steps: S1 The carrier modified with polymer layer is cured, and regular brick-shaped MFI zeolite powder is poured onto the surface of the carrier for coating to obtain a continuous and dense oriented MFI seed monolayer. The obtained seed monolayer is calcined to enhance the bonding force between the seed and the carrier while removing the polymer layer. S2. Organic structure directing agent and alkaline source are added to deionized water in sequence. The solution is fully dissolved by the impact and shearing action of mechanical movement. Then, silicon source is added dropwise to the above solution. The solution is stirred vigorously at 60~120 °C for 6~12 h to remove the ethanol produced by the hydrolysis of silicon source. Water is added to the original volume to obtain solution A. The fluorine-containing crystallization regulator was dissolved in deionized water to obtain solution B; solution B was slowly added dropwise to solution A and the mixture was stirred and aged for 6-12 hours to obtain the synthesis mother liquor; S3 The seed layer obtained in S1 is placed in the synthesis mother liquor obtained in S2 and subjected to hydrothermal crystallization reaction at 30~210 °C; S4 The membrane material obtained in S3 is subjected to rinsing, soaking, drying and calcination to obtain an oriented MFI molecular sieve membrane.

2. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: In step S1, the curing temperature is 60~120℃ and the curing time is 1~6 h; the carrier is a glass sheet, platinum sheet or porous alumina sheet.

3. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: The seed crystals mentioned in step S1 are uniform, regular, brick-shaped MFI molecular sieves with a particle size of 400 nm to 2 μm.

4. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: The silicon source mentioned in step S2 is one or more of methyl orthosilicate, ethyl orthosilicate, and propyl orthosilicate; the molar ratio of deionized water to silicon source is H2O / SiO2 = 5~100.

5. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: The fluorinated crystallization regulator mentioned in step S2 is one or more of ammonium fluoride, sodium fluoride, potassium fluoride, and ammonium hydrofluoride; the molar ratio of the fluorinated crystallization regulator to the silicon source is F. - / SiO2=0.1~10.

6. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: The organic structure directing agent mentioned in step S2 is one or more of tetrapropylammonium hydroxide solution, tetraethylammonium hydroxide solution, tetrabutylammonium hydroxide solution and methyltributylammonium hydroxide solution; the molar ratio of the organic structure directing agent to the silicon source is organic structure directing agent / SiO2 = 0.02~0.

5.

7. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: The hydrothermal crystallization process described in step S3 is either constant temperature or variable temperature, and the hydrothermal crystallization time is 0.8 h to 54 days.

8. The method for preparing oriented MFI molecular sieve membranes over a wide temperature range as described in claim 1, characterized in that: The roasting temperature in step S4 is 50~700 °C; the roasting time is 0.2~100 h.

9. An MFI molecular sieve membrane obtained by the method of claim 1.

10. The application of the MFI molecular sieve membrane of claim 9 in electrochemical ion sieving or separation of n-butane / isobutane mixed gas.

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