A method for preparing an lta zeolite membrane and applications thereof

Abstract

This invention belongs to the field of membrane separation technology and relates to a method for preparing LTA zeolite membranes and their applications. The method first adds sodium persulfate to the gel synthesis solution of the LTA zeolite membrane. The addition of this chemical increases the heterogeneous nucleation rate in the LTA membrane synthesis solution and reduces aging time. Then, an LTA zeolite seed layer is prepared, and a small amount of the gel synthesis solution is coated onto a support coated with the seed layer. Without adding any additional reaction solution or solvent in the membrane synthesis reactor, the support coated with the seed layer undergoes a crystallization reaction to form a high-performance LTA zeolite membrane solely based on the small amount of membrane gel synthesis solution coated on it. This invention achieves rapid, green, and low-cost preparation of highly selective LTA zeolite membranes on inexpensive, rough, macroporous supports using a "zero-waste gelation strategy," providing an environmentally friendly and economically feasible technical route for nitrogen / propylene industrial separation.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a method for preparing an LTA zeolite membrane and its application. Background Technology

[0002] In the field of nitrogen / propylene separation in the petrochemical industry, traditional cryogenic distillation relies on boiling point differences to achieve nitrogen / propylene separation, requiring high towers and compression refrigeration systems, resulting in high energy consumption, accounting for more than 70% of the total separation cost. LTA zeolite molecular sieve membranes, with their 4.1 Å uniform pore structure, achieve highly efficient separation under low driving pressure differentials through molecular size sieving (nitrogen diffuses easily at 3.6 Å, while propylene is blocked at 4.4 Å) and the synergistic effect of adsorption and diffusion, significantly reducing energy consumption compared to distillation processes. This technological breakthrough is expected to reshape the nitrogen / propylene separation pathway, drive annual emission reductions in million-ton-scale refineries, and provide key technological support for the low-carbon transformation of the refining and chemical industry.

[0003] For example, a method for preparing an LTA-type zeolite membrane and its application in gas separation (CN114307689A) involves the inventors first preparing a synthetic gel, then coating an LTA crystal onto the surface of a tubular carrier as a seed layer, and finally immersing the seed layer in the synthetic gel for crystallization to obtain an LTA-type molecular sieve membrane. Another example is a method for rapidly preparing ultrathin NaA zeolite membranes using direct heating (CN107174954A), which involves directly heating the synthesis solution in a direct-heating crystallization vessel at a certain temperature to rapidly synthesize the NaA zeolite membrane. Most existing LTA membrane preparations utilize hydrothermal synthesis, resulting in low raw material utilization. Even with recycling, a large amount of wastewater is still generated, burdening the environment and resources. Although methods for synthesizing LTA membranes by coating with a small amount of gel have been reported, the LTA zeolite membranes prepared by this method are typically used for ethanol dehydration applications. While exhibiting good water permeation selectivity, the water permeation flux is low, and the crystallinity is low with numerous intercrystalline defects, making it unsuitable for high-precision gas separation where molecular size differences are less than one gram. Summary of the Invention

[0004] This invention provides a method for preparing LTA zeolite membranes and their applications. It is a method for preparing high-performance LTA-type molecular sieve membranes using a gel synthesis solution on a rough, macroporous support. The method first adds sodium persulfate to the gel synthesis solution for the LTA zeolite membrane. The addition of this chemical increases the heterogeneous nucleation rate in the LTA membrane synthesis solution and reduces aging time. Then, an LTA zeolite seed layer is prepared. A small amount of the gel synthesis solution is coated onto the support with the seed layer. The support with the seed layer undergoes a crystallization reaction in the membrane synthesis reactor without any additional reaction solution or solvent, relying solely on the small amount of membrane gel synthesis solution to form a high-performance LTA zeolite membrane.

[0005] This invention innovatively introduces sodium persulfate as a functional additive into the gel system. Persulfate ions can introduce a large number of hydroxyl radicals into the gel, which can accelerate the depolymerization of the silicon source, promote the breaking of Si-O bonds and the formation of Si-O-Si bonds, increase the crystallization nucleation rate, shorten the gel aging time, and ensure the density, continuity, and integrity of the zeolite film, thereby improving the crystallinity of the film and reducing performance degradation caused by intergranular defects. The resulting LTA zeolite film has advantages such as uniform thickness and density without major defects, exhibiting excellent nitrogen / propylene separation performance. Furthermore, the method described in this invention is simple, operates under mild conditions, has high raw material utilization, and allows for efficient use of the synthesis liquid, achieving high atom economy and zero waste liquid discharge, further reducing production costs. Since the entire synthesis process generates almost no highly alkaline waste liquid, the complex post-treatment steps such as neutralization and precipitation required by traditional hydrothermal methods can be eliminated, achieving green synthesis from the source and showing good prospects for industrial application. This not only provides a new, environmentally friendly, and cost-effective route for the preparation of LTA zeolite membranes, but also offers a high-performance, low-cost advanced membrane separation technology solution for the nitrogen / propylene separation process, with significant environmental and economic benefits.

[0006] The technical solution of the present invention: A method for preparing an LTA zeolite membrane includes the following steps: Step (1) Preparation of LTA seed crystals. The LTA seed crystals used can be made in-house or purchased commercially. The preparation process of LTA seed crystals is as follows: the alkali source, silicon source, aluminum source and deionized water are mixed, stirred and aged to obtain a synthesis solution. The synthesis solution is poured into a hydrothermal reactor for crystallization reaction. The product is centrifuged, washed and dried to obtain LTA seed crystals.

[0007] Step (2) Coating a dense, continuous, and defect-free LTA seed layer onto a porous carrier. The LTA seed synthesized in step (1) is uniformly coated onto the surface of the carrier, and the carrier coated with the seed is placed in an oven for curing to obtain the LTA seed layer.

[0008] Step (3) Preparation of gel synthesis solution. Prepare gel synthesis solution by mixing and stirring an alkali source, a silicon source, an aluminum source, sodium persulfate and deionized water.

[0009] Step (4) Preparation of LTA zeolite membrane. The porous support coated with a dense, continuous, and defect-free LTA seed layer obtained in step (2) is immersed in the gel synthesis solution prepared in step (3), and then placed in a membrane reactor for film formation and crystallization reaction. After the reaction, the porous support is removed from the reactor, washed with water, and dried to obtain the LTA zeolite membrane. It is particularly noteworthy that no additional water, synthesis solution, or organic solution is added to the reactor. The film formation and crystallization reaction is completely confined to the surface of the support, avoiding the scientific problem of competition between homogeneous nucleation and crystallization in solution and heterogeneous nucleation and crystallization on the surface of the support in traditional hydrothermal synthesis methods.

[0010] Furthermore, in steps (1) and (3), the silicon source is one or more of tetraethyl orthosilicate, silica sol, fumed silica, silicic acid, and sodium silicate, preferably silica sol.

[0011] Furthermore, in steps (1) and (3), the aluminum source is one or more of sodium aluminate, aluminum isopropoxide, and aluminum oxide, preferably sodium aluminate.

[0012] Furthermore, in steps (1) and (3), the alkali source is sodium hydroxide.

[0013] Further, in step (1), SiO2 is provided by a silicon source, Al2O3 is provided by an aluminum source, and Na2O is provided by an alkali source. The molar ratio of SiO2 to H2O in the solution is (40~150):1, the molar ratio of Al2O3 to H2O is (80~300):1, the molar ratio of Na2O to SiO2 is (1~6):1, the crystallization temperature is 60~200℃, the crystallization time is 3~24h, the drying temperature is 60~100℃, and the drying time is 12~24h.

[0014] Further, in step (2), the porous carrier is made of alumina, zirconium oxide or mullite; the porous carrier is in the shape of a tubular carrier; the pore size of the porous carrier is 0.1~15μm and the carrier length is 50mm~800mm.

[0015] Furthermore, in step (3), the stirring temperature of the gel synthesis solution is 0~80℃, preferably 20~40℃, and the stirring time is 2~24h.

[0016] Further, in step (3), SiO2 is provided by a silicon source, Al2O3 is provided by an aluminum source, and Na2O is provided by both an aluminum source and an alkali source. The molar ratio of each component in the gel synthesis solution is: Na2O:Al2O3:SiO2:H2O:Na2S2O8 is (1.2~3.5):1:2:(80-300):(0.001-0.03).

[0017] Furthermore, in step (4), the crystallization temperature is 80~120℃ and the crystallization time is 3~15h; the drying temperature is 60~100℃ and the drying time is 4~12h.

[0018] Furthermore, in step (4), the time for immersing the porous carrier in the synthetic gel is 1-30 min.

[0019] The LTA zeolite membrane prepared by the method of this invention is used for the separation of nitrogen and propylene.

[0020] The beneficial effects of this invention are: The preparation of dense LTA zeolite membranes on porous supports has long faced technical bottlenecks, including the use of large amounts of reaction liquid leading to low atom economy and difficulty in reducing costs. Furthermore, LTA zeolite membranes prepared by traditional methods often exhibit numerous defects, hindering gas separation. This invention overcomes these bottlenecks with a "gel confinement method": all synthesis solutions are formulated at room temperature into a gel with a certain viscosity that adheres to the support surface. Seed layer preparation and secondary growth are completed within the same gel system, eliminating the need for mother liquor discharge, centrifugation, washing, or solvent recovery. Innovatively, sodium persulfate is added to introduce hydroxyl radicals, which improve membrane crystallinity and reduce intergranular defects, resulting in better gas separation performance. This invention utilizes a "zero-waste gel strategy" to achieve rapid, green, and low-cost preparation of highly selective LTA zeolite membranes on inexpensive, rough, macroporous supports, providing an environmentally friendly and economically feasible technical route for nitrogen / propylene industrial separation. This method requires only about 5 wt% of gel compared to traditional hydrothermal synthesis for preparing LTA membranes of the same area, saving approximately 95 wt% of synthesis liquid, while simultaneously improving nitrogen / propylene separation performance by 50%. Attached Figure Description

[0021] Figure 1 A scanning electron microscope image of the LTA seed crystals prepared in Example 1; Figure 2 X-ray diffraction pattern of the LTA seed crystals prepared in Example 1; Figure 3 A scanning electron microscope image of the surface of the LTA zeolite film prepared in Example 1; Figure 4 A cross-sectional scanning electron microscope image of the LTA zeolite membrane prepared in Example 1; Figure 5 The image shows the X-ray diffraction pattern of the LTA zeolite membrane prepared in Example 1. Detailed Implementation

[0022] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0023] Example 1 Preparation of LTA seed crystals: A solution was prepared by mixing and stirring JN-40 colloidal silica (silicon source), sodium aluminate (aluminum source), sodium hydroxide, and deionized water in a molar ratio of Na₂O:Al₂O₃:SiO₂:H₂O = 5.9:1:2:150, and the solution was aged to obtain seed crystals. The solution was then placed in a hydrothermal reactor and crystallized at 80 °C for 5 h. After the reaction, the product was centrifuged, washed, and dried at 60 °C for 24 h to obtain LTA crystals. The scanning electron microscope (SEM) image and X-ray diffraction (XRD) pattern of the LTA crystals are shown below. Figure 1 and 2 As shown, the average size of LTA crystals is approximately 500 nm.

[0024] Preparation of LTA-type zeolite membranes: 500 nm LTA seed crystals were introduced onto a support (alumina, zirconium oxide, or mullite), the seed crystals were evenly coated, and then transferred to an oven to dry overnight at 80 °C. The preparation steps of the gel synthesis solution were the same as those of the molecular sieve crystal synthesis solution. The formula of the gel synthesis solution was n(Na₂O): n(SiO₂): n(Al₂O₃): n(H₂O): n(Na₂S₂O₈) = 3.0:2:1:210:0.015. The silicon source was JN-40 colloidal silica, the aluminum source was sodium aluminate, the alkali source was sodium hydroxide, the aging time was 9 h, the crystallization temperature was 100 °C, and the crystallization time was 4 h. The prepared smooth LTA seed layer was immersed in the gel synthesis solution for 10 minutes, then placed in a reaction vessel for crystallization to obtain an LTA-type molecular sieve membrane. After the reaction, the LTA-type molecular sieve membrane was removed from the reaction vessel, washed with water, and dried in an 80℃ oven. The surface, cross-section, and XRD pattern of the obtained membrane are shown below. Figure 2 , Figure 3 As shown. The optimal conditions were an aging time of 3 hours, a crystallization temperature of 100 °C, and a crystallization time of 4 hours. The resulting LTA-type zeolite molecular sieve membrane exhibited a permeation flux of 2.15 kg·m³ for 90% ethanol / water at 75 °C. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 1.63 kg·m³. -2 ·h -1 The separation factor is 569, the separation factor for equimolar nitrogen and propylene at room temperature is 58, and the nitrogen flux is 0.5 × 10⁻⁶. -8 mol / (m 2 s Pa).

[0025] Example 2 The operating steps were the same as in Example 1; the silicon source in the synthesis solution was changed to sodium silicate, and the aging stirring time was changed to 3h and 15h respectively to synthesize LTA-type zeolite membranes. When the aging time was 3h, the permeate flux of the membrane to 90% ethanol / water at 75℃ was 2.34 kg·m. -2 ·h-1 The separation factor was 8991, and the permeation flux for 90% methanol / water was 1.71 kg·m³. -2 ·h -1 The separation factor is 357, the separation factor for equimolar nitrogen and propylene at room temperature is 34, and the nitrogen flux is 0.75 × 10⁻⁶. -8 mol / (m 2 When the aging time is 15 h, the permeation flux of the membrane for 90% ethanol / water at 75 °C is 1.68 kg·m³. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 1.22 kg·m³. -2 ·h -1 The separation factor is 702, the separation factor for equimolar nitrogen and propylene at room temperature is 91, and the nitrogen flux is 0.2 × 10⁻⁶. -8 mol / (m 2 s Pa).

[0026] Example 3 The operating steps were the same as in Example 1; the silicon source in the gel was replaced with tetraethyl orthosilicate, and the crystallization temperature of the zeolite membrane was changed to 80℃ and 120℃ respectively to synthesize LTA-type zeolite membranes. When the crystallization temperature was 80℃, the permeation flux of the membrane to 90% ethanol / water at 75℃ was 2.30 kg·m. -2 ·h -1 The separation factor is 5618, and the permeation flux for 90% methanol / water is 1.66 kg·m³. -2 ·h -1 The separation factor is 399, the separation factor for equimolar nitrogen and propylene at room temperature is 58, and the nitrogen flux is 0.5 × 10⁻⁶. -8 mol / (m 2 At a crystallization temperature of 120℃, the membrane's permeation flux for 90% ethanol / water at 75℃ is 1.79 kg·m³. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 1.20 kg·m³. -2 ·h -1 The separation factor is 990, the separation factor for equimolar nitrogen and propylene at room temperature is 70, and the nitrogen flux is 0.41 × 10⁻⁶. -8 mol / (m 2 s Pa).

[0027] Example 4 The operating steps were the same as in Example 1; the aluminum source in the gel was replaced with aluminum isopropoxide, and the crystallization time of the zeolite membrane was changed to 3 h and 15 h to synthesize LTA-type zeolite membranes. When the crystallization time was 3 h, the permeate flux of the membrane to 90% ethanol / water at 75 °C was 2.19 kg·m. -2 ·h -1 The separation factor is 8172, and the permeation flux for 90% methanol / water is 1.87 kg·m³. -2 ·h -1 The separation factor is 314, the separation factor for equimolar nitrogen and propylene at room temperature is 29, and the nitrogen flux is 0.8 × 10⁻⁶. -8 mol / (m 2 (s Pa). With a crystallization time of 3 h, the membrane at 75 °C exhibits a permeation flux of 1.39 kg·m⁻²·h⁻¹ for 90% ethanol / water, with a separation factor >10000; a permeation flux of 1.01 kg·m⁻²·h⁻¹ for 90% methanol / water, with a separation factor of 946; and a separation factor of 69 for equimolar nitrogen-propylene mixtures at room temperature, with a nitrogen flux of 0.28 × 10⁻⁶. -8 mol / (m 2 s Pa) Example 5 Except for the impregnation time, all other conditions were the same as in Example 1; the impregnation time of the carrier in the gel was changed, with 1 min and 30 min used to synthesize LTA-type zeolite membranes, respectively. When the impregnation time was 1 min, the permeation flux of the membrane to 90% ethanol / water at 75°C was 2.60 kg·m. -2 ·h -1 The separation factor is 3419, and the permeation flux for 90% methanol / water is 1.89 kg·m. -2 ·h -1 The separation factor is 203, the separation factor for equimolar nitrogen and propylene at room temperature is 58, and the nitrogen flux is 0.5 × 10⁻⁶. - 8 mol / (m 2 (s Pa), with an impregnation time of 30 minutes, the membrane's permeation flux for 90% ethanol / water at 75°C was 1.54 kg·m³. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 1.49 kg·m³. -2 ·h -1 The separation factor is 835, the separation factor for equimolar nitrogen and propylene at room temperature is 91, and the nitrogen flux is 0.32 × 10⁻⁶. -8 mol / (m 2 s Pa).

[0028] Example 6 Except for the Na₂O / Al₂O₃ molar ratio of the gel, all other conditions were the same as in Example 1. LTA-type zeolite membranes were synthesized by changing the Na₂O / Al₂O₃ molar ratio of the gel to 1 and 3.5. When the Na₂O / Al₂O₃ molar ratio was 1, the membrane's permeation flux to 90% ethanol / water at 75°C was 1.06 kg·m⁻¹. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 0.65 kg·m. -2 ·h -1 The separation factor is 773, the separation factor for equimolar nitrogen and propylene at room temperature is 90, and the nitrogen flux is 0.11 × 10⁻⁶. - 8 mol / (m 2 (s Pa). When the Na2O / Al2O3 molar ratio is 3.5, the membrane permeation flux for 90% ethanol / water at 75℃ is 1.06 kg·m. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 0.65 kg·m. -2 ·h -1 The separation factor is 773, the separation factor for equimolar nitrogen and propylene at room temperature is 90, and the nitrogen flux is 0.11 × 10⁻⁶. -8 mol / (m 2 s Pa).

[0029] Example 7 Except for the water-to-silica ratio of the gel, all other conditions were the same as in Example 2. LTA-type zeolite membranes were synthesized by changing the water-to-silica ratio (molar ratio) of the gel (H2O / SiO2) to 40 and 150, respectively. When the water-to-silica ratio was 30, the membrane's permeation flux for 90% ethanol / water at 75°C was 1.26 kg·m³. -2 ·h -1 The separation factor is >10000, and the permeation flux for 90% methanol / water is 0.84 kg·m. -2 ·h -1 The separation factor is 990, the separation factor for equimolar nitrogen and propylene at room temperature is 90, and the nitrogen flux is 0.17 × 10⁻⁶. -8 mol / (m 2 When the water-to-silica ratio is 160, the membrane permeation flux for 90% ethanol / water at 75℃ is 3.99 kg·m³. -2 ·h -1 The separation factor is 764, and the permeation flux for 90% methanol / water is 2.69 kg·m. -2 ·h -1 The separation factor is 87, the separation factor for equimolar nitrogen and propylene at room temperature is 6, and the nitrogen flux is 1.5 × 10⁻⁶.-8 mol / (m 2 s Pa).

[0030] Comparative Example 1 Except for the amount of sodium persulfate added, all other conditions were the same as in Example 1; sodium persulfate was no longer added to the synthesized gel to synthesize the LTA-type zeolite membrane. Without the addition of sodium persulfate, the membrane's screening ability for methanol and nitrogen / propylene was significantly reduced, and the membrane's permeate flux for 90% ethanol / water at 75°C was 2.32 kg·m. -2 ·h -1 The separation factor is 8172, and the permeation flux for 90% methanol / water is 1.71 kg·m. -2 ·h -1 The separation factor is 83, the separation factor for equimolar nitrogen and propylene at room temperature is 12, and the nitrogen flux is 0.7 × 10⁻⁶. -8 mol / (m 2 s Pa).

Claims

1. A method for preparing an LTA zeolite membrane, characterized in that, Includes the following steps: Step (1) Preparation of LTA seed crystals; The preparation process of LTA seed crystals is as follows: the alkali source, silicon source, aluminum source and deionized water are mixed, stirred and aged to obtain a synthesis solution. The synthesis solution is poured into a hydrothermal reactor for crystallization reaction. The product is centrifuged, washed and dried to obtain LTA seed crystals. Step (2) Coating a dense, continuous, defect-free LTA seed layer onto a porous carrier; The LTA seed crystal synthesized in step (1) is uniformly coated on the surface of the carrier, and the carrier coated with the seed crystal is placed in an oven for curing to obtain an LTA seed crystal layer. Step (3) Preparation of gel synthesis solution; A gel synthesis solution was prepared by mixing and stirring an alkali source, a silicon source, an aluminum source, sodium persulfate, and deionized water. Step (4) Preparation of LTA zeolite membrane; The porous carrier coated with a dense, continuous, and defect-free LTA seed layer obtained in step (2) is immersed in the gel synthesis solution prepared in step (3), and then placed in a membrane reactor for film crystallization reaction. After the reaction is completed, the porous carrier is taken out of the reactor, washed with water, and dried to obtain an LTA zeolite membrane.

2. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In steps (1) and (3), the silicon source is one or more of tetraethyl orthosilicate, silica sol, fumed silica, silicic acid, and sodium silicate; the aluminum source is one or more of sodium aluminate, aluminum isopropoxide, and aluminum oxide; and the alkali source is sodium hydroxide.

3. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In step (1), SiO2 is provided by a silicon source, Al2O3 is provided by an aluminum source, and Na2O is provided by an alkali source. The molar ratio of SiO2 to H2O in the solution is (40~150):1, the molar ratio of Al2O3 to H2O is (80~300):1, the molar ratio of Na2O to SiO2 is (1~6):1, the crystallization temperature is 60~200℃, the crystallization time is 3~24h, the drying temperature is 60~100℃, and the drying time is 12~24h.

4. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In step (2), the porous carrier is made of alumina, zirconium oxide or mullite; the porous carrier is in the shape of a tubular carrier; the pore size of the porous carrier is 0.1~15μm and the carrier length is 50mm~800mm.

5. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In step (3), the stirring temperature for preparing the gel synthesis solution is 0~80℃, preferably 20~40℃, and the stirring time is 2~24h.

6. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In step (3), SiO2 is provided by a silicon source, Al2O3 is provided by an aluminum source, and Na2O is provided by both an aluminum source and an alkali source. The molar ratio of each component in the gel synthesis solution is: Na2O:Al2O3:SiO2:H2O:Na2S2O8 is (1.2~3.5):1:2:(80-300):(0.001-0.03).

7. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In step (4), the crystallization temperature is 80~120℃ and the crystallization time is 3~15h; the drying temperature is 60~100℃ and the drying time is 4~12h.

8. The method for preparing an LTA zeolite membrane according to claim 1, characterized in that, In step (4), the porous carrier is immersed in the synthetic gel for 1-30 min.

9. An LTA zeolite membrane prepared by the method according to any one of claims 1-8 is used for the separation of nitrogen and propylene.

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