Preparation method of multilayer molecular sieve membrane and membrane reactor application

Abstract

This invention discloses a method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology and its application in membrane reactors. It relates to materials research and development technology and mainly includes the following steps: after synthesizing a first layer of macroporous molecular sieve membrane on a support, a treatment solution is used to in-situ transform the surface layer of the molecular sieve membrane into a second layer of microporous molecular sieve membrane. Subsequently, a repair solution is used to repair structural defects in the formed second layer of molecular sieve membrane, ultimately obtaining a multilayer molecular sieve membrane. This invention solves the technical problem of the trade-off between permeability and selectivity currently faced by molecular sieve membrane materials. The resulting multilayer molecular sieve membrane possesses both high permeability and high selectivity, with the microporous molecular sieve membrane acting as an interceptor and the macroporous molecular sieve membrane acting as a dehydrator, thus enhancing the membrane's role in the reaction process. Furthermore, the multilayer molecular sieve membrane exhibits high coupling between its layers and a stable membrane structure, making it suitable for harsh industrial conditions and possessing significant application potential in the field of permeable membrane reactors.

Description

Technical Field

[0001] This invention relates to a method for preparing a multilayer molecular sieve membrane and its application in a membrane reactor. Background Technology

[0002] Molecular sieve membranes are inorganic membrane materials composed of molecular sieves. They possess high chemical stability and mechanical strength, enabling stable operation in harsh industrial environments. The porous structure characteristic of molecular sieve membranes provides advantages such as high flux, uniform pore size, and tunable structural properties. Currently, molecular sieve membranes of various types, including MFI, LTA, MOR, and FAU, are used in pervaporation, gas separation, and membrane reactors.

[0003] Molecular sieve membrane reactors are a type of reactor that couples molecular sieve membranes with catalytic reactions. They can simultaneously perform catalytic reactions and product separation, promoting a forward shift in reaction equilibrium and thus enhancing the reaction process. Based on the separation performance of the molecular sieve membrane, molecular sieve membrane reactors include several types, such as water-permeable membrane reactors, organic-permeable membrane reactors, and hydrogen-permeable membrane reactors. The reactions involved cover dehydration condensation reactions, isomerization reactions, and dehydrogenation reactions. Among these, water-permeable membrane reactors have been the most extensively studied. Their main working mechanism utilizes the affinity of hydrophilic molecular sieves for water molecules, preferentially drawing water molecules into the sieve and removing them from the reaction system through the pores of the sieve. Simultaneously, the ordered microporous structure of the molecular sieve prevents other reactants or products from entering the pores, avoiding reaction losses. Huang et al. (Angewandte Chemie International Edition, Vol. 60, No. 33, 2021, pp 18289-18294) reported the use of an LTA-type molecular sieve membrane reactor for CO2-to-methanol reaction, breaking the thermodynamic equilibrium limitation of the reaction by rapidly removing the byproduct water. Compared with a fixed-bed reactor, under the same reaction conditions, this molecular sieve membrane reactor increased the CO2 conversion rate from 21.9% to 36.1%. Miao Yu et al. (Science, Vol. 367, No. 6478, 2020, pp 667-671) reported that the LTA-type molecular sieve membrane reactor can increase the CO2-to-methanol conversion rate to 60.1%, and the advantage of this membrane reactor lies in its higher water permeability. Currently, the main challenge facing permeable membrane reactors lies in their application to reaction systems containing small molecules. Using a single large-pore molecular sieve membrane leads to reactant leakage, while using a single small-pore molecular sieve membrane increases water diffusion resistance and reduces membrane permeability. Therefore, molecular sieve membranes that combine high permeability and high selectivity are the key focus and challenge in the research and development of permeable membrane reactors.

[0004] Multilayer molecular sieve membranes with a sandwich structure offer a novel design approach. Nanjing University of Technology (patent application number 202310122622.0) reported a method for preparing a bilayer CHA molecular sieve membrane. This membrane consists of a support and two CHA membrane layers with different silicon-to-aluminum ratios, serving the separation and structural defect repair functions, respectively. Huang Aisheng et al. (Angewandte Chemie International Edition, Vol. 55, No.41, 2016, pp 12678-12682) reported a FAU / LTA coupled molecular sieve membrane reactor for enhancing the methanol dehydration to dimethyl ether reaction, where the FAU and LTA membrane layers serve catalytic and dehydration functions, respectively. However, existing multilayer molecular sieve membranes with sandwich structures are all prepared using a layer-by-layer synthesis method, i.e., the first molecular sieve membrane is synthesized on a support first, and then the second molecular sieve membrane is synthesized on top of it. This results in a significant two-phase interface between the two molecular sieve membranes, which is prone to cracking during calcination, leading to a loss of membrane separation performance. In addition, to ensure the continuity of each molecular sieve membrane, each layer of the multilayer molecular membrane has a certain thickness, which leads to an increase in the overall membrane thickness and is not conducive to the permeation of substances.

[0005] In-situ phase inversion technology for molecular sieves involves transforming metastable molecular sieves into a phase with higher structural stability under alkaline hydrothermal conditions. The inverted molecular sieve typically possesses the same secondary structural units as the original sieve, but with higher framework density, smaller pore size, and a lower framework silica-to-alumina ratio. This technology is currently commonly used to prepare pure-phase molecular sieves. Summary of the Invention

[0006] To address the problems existing in water-permeable molecular sieve membranes in the background art, this invention provides a method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology. The multilayer molecular sieve membrane obtained by this method has tightly connected molecular sieve layers without clear separation interfaces, thus ensuring both water permeability and small molecule interception performance.

[0007] This invention discloses a method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology, the technical solution of which is as follows:

[0008] S1 prepares molecular sieve seeds with an average particle size of less than 100 nm;

[0009] S2 The molecular sieve seed crystals prepared in step S1 are coated on a porous support and placed in a primary synthesis solution L1. The first molecular sieve membrane is formed by hydrothermal crystallization.

[0010] S3 After washing and drying the molecular sieve membrane obtained in step S2, the surface layer of the first molecular sieve membrane is placed in the secondary treatment solution L2, which is a NaOH solution or KOH solution with a mass fraction of 5-35%. The surface layer of the first molecular sieve membrane undergoes in-situ phase inversion through alkaline treatment, thereby forming the second molecular sieve membrane.

[0011] S4 After washing and drying the molecular sieve membrane obtained in step S3, place the second molecular sieve membrane in the three-stage repair solution L3 and hydrothermally crystallize to repair the structural defects formed in the second molecular sieve membrane in step S3.

[0012] S5 After washing and drying the molecular sieve membrane obtained in step S4, a multilayer molecular sieve membrane is obtained.

[0013] The molecular sieve pore size of the first layer of molecular sieve membrane is greater than 0.4 nm, and the molecular sieve pore size of the second layer of molecular sieve membrane is less than or equal to 0.4 nm; the molecular sieves of the multilayer molecular sieve membrane are closely connected and there is no clear layering interface.

[0014] Preferably, the first molecular sieve membrane is a FAU type or EMT type molecular sieve, and the second molecular sieve membrane is a SOD type, LTA type or GIS type molecular sieve, and the silica-alumina ratio of the framework of the two molecular sieves is in the range of 1-5.

[0015] Preferably, in step S1, the molecular sieve seed crystals are FAU type or EMT type molecular sieves.

[0016] Preferably, in step S2, the molar ratio of the primary synthesis solution L1 is 5-25 SiO2:Al2O3:2.5-75 Na2O:800-5000 H2O, the hydrothermal crystallization temperature is 40-100 ℃, and the hydrothermal crystallization time is 12-48 h.

[0017] Preferably, in step S4, the molar ratio of the three-stage repair solution L3 is 5-18 SiO2:Al2O3:1.5-50 Na2O (K2O): 900-1500 H2O, the hydrothermal crystallization temperature is 50-120 ℃, and the hydrothermal crystallization time is 12-24 h.

[0018] Compared with existing technologies, the multilayer molecular sieve membrane prepared by in-situ phase inversion technology provided by this invention has the following beneficial effects when applied to membrane reactors:

[0019] 1. In-situ phase inversion technology enables macroporous molecular sieves with metastable structures to be converted in situ into microporous molecular sieves with more stable structures. The molecular sieve membrane has high coupling between layers and no obvious interface, which can avoid cracking between layers of the molecular sieve membrane.

[0020] 2. The small-pore molecular sieves located on the surface have high sieving performance, which can hinder the penetration of small molecules and prevent the leakage of reactants.

[0021] 3. The macroporous molecular sieve in the middle layer has high water permeability and low diffusion resistance, which can promote the diffusion of water molecules, enable the product water to quickly leave the reaction system, and enhance the reaction process.

[0022] 4. Multilayer molecular sieve membranes have high tolerance to high temperatures and reaction substrates, and can adapt to catalytic reaction conditions;

[0023] 5. Under conditions of 150-280 ℃, the water permeability of multilayer molecular sieve membranes can reach 1.5-4×10⁻⁶. -7 mol·m - 2 Pa -1 The selectivity of H2O / H2 is greater than 40, and the selectivity of H2O / CO2 is greater than 60. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the specific embodiments will be briefly described below:

[0025] Figure 1 This is a schematic diagram of the process flow for preparing multilayer molecular sieve membranes using in-situ phase inversion technology in Example 1 of the present invention;

[0026] Figure 2 This is a schematic diagram of the structure of the multilayer molecular sieve membrane obtained in Example 1 of the present invention;

[0027] Figure 3 The XRD patterns of the first FAU-type molecular sieve membrane and the second SOD-type molecular sieve membrane obtained in Example 2 of the present invention are shown.

[0028] Figure reference numerals: 1-carrier; 2-carrier coated with seed crystals; 3-hydrothermal crystallization in primary synthesis solution; 4-first molecular sieve membrane; 5-in-situ phase transformation in secondary treatment solution; 6-multilayer molecular sieve membrane with structural defects; 7-hydrothermal crystallization in tertiary repair solution; 8-multilayer molecular sieve membrane; 9-EMT type molecular sieve membrane layer; 10-GIS type molecular sieve membrane layer. Detailed Implementation

[0029] To make the technical solutions of the present invention clearer, the technical solutions of the embodiments of the present invention will be described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Preparation of molecular sieve seeds, reference: Eng-Poh Ng, Daniel Chateigner, Thomas Bein, Valentin Valtchev, Svetlana Mintova, Science, Vol. 335, 2012, pp 70-73; Hussein Awala, Jean-Pierre Gilson, Richard Retoux, Philippe Boullay, Jean-Michel Goupil, Valentin Valtchev, Svetlana Mintova, Nature Materials, Vol.14,2015, pp 447-451.

[0031] Example 1

[0032] EMT-type seed crystals were coated onto a porous ceramic substrate and then hydrothermally crystallized in a primary synthesis solution. The molar ratio of the primary synthesis solution was 15 SiO2:Al2O3:70 Na2O:1500 H2O, the hydrothermal crystallization temperature was 40 ℃, and the hydrothermal crystallization time was 36 h.

[0033] The obtained molecular sieve membrane was removed, rinsed with deionized water, and dried at 60 °C for 12 h to obtain the first molecular sieve membrane, which was an EMT type molecular sieve. Its surface was then subjected to in-situ phase inversion treatment in a 12% (w / w) NaOH secondary treatment solution. The in-situ phase inversion treatment temperature was 100 °C, and the treatment time was 12 h.

[0034] The obtained molecular sieve membrane was removed, rinsed with deionized water, and dried at 60 °C for 12 h to obtain a second molecular sieve membrane, which was a GIS-type molecular sieve. Its surface was then placed in a tertiary repair solution for hydrothermal crystallization. The molar ratio of the tertiary repair solution was 7.5 SiO2:Al2O3:45 Na2O:1200 H2O, the hydrothermal crystallization temperature was 100 °C, and the hydrothermal crystallization time was 15 h.

[0035] The molecular sieve membrane obtained above was taken out, rinsed with deionized water, and dried at 60 °C for 12 h to obtain a multilayer molecular sieve membrane.

[0036] The multilayer molecular sieve membrane obtained above was used in a membrane reactor. At 220 °C, the water permeability of the membrane was 2.2 × 10⁻⁶. -7 mol·m -2 Pa -1 The selectivity of H2O / H2 is 41, and the selectivity of H2O / CO2 is 62.

[0037] Figure 1 A schematic diagram of the process flow for preparing multilayer molecular sieve membranes using in-situ phase inversion technology in Example 1 is shown.

[0038] Figure 2 A schematic diagram of the structure of the multilayer molecular sieve membrane obtained in Example 1 is shown.

[0039] Example 2

[0040] FAU-type seed crystals were coated onto a porous ceramic substrate and then hydrothermally crystallized in a primary synthesis solution. The molar ratio of the primary synthesis solution was 20 SiO2:Al2O3:70 Na2O:2200 H2O, the hydrothermal crystallization temperature was 80 ℃, and the hydrothermal crystallization time was 48 h.

[0041] The obtained molecular sieve membrane was removed, rinsed with deionized water, and dried at 80 °C for 8 h to obtain the first molecular sieve membrane, which was a FAU type molecular sieve. Its surface was then subjected to in-situ phase inversion treatment in a 32% (w / w) NaOH secondary treatment solution. The in-situ phase inversion treatment temperature was 80 °C, and the treatment time was 36 h.

[0042] The obtained molecular sieve membrane was removed, rinsed with deionized water, and dried at 80 °C for 8 h to obtain a second molecular sieve membrane, which was an SOD-type molecular sieve. Its surface was then subjected to hydrothermal crystallization using a three-stage repair solution. The molar ratio of the three-stage repair solution was 5SiO2:Al2O3:50Na2O:1005H2O, the hydrothermal crystallization temperature was 80 °C, and the hydrothermal crystallization time was 24 h.

[0043] The molecular sieve membrane obtained above was taken out, rinsed with deionized water, and dried at 80 °C for 12 h to obtain a multilayer molecular sieve membrane.

[0044] The multilayer molecular sieve membrane obtained above was used in a membrane reactor. At 260 °C, the water permeability of the membrane was 1.7 × 10⁻⁶. -7 mol·m -2 Pa -1 The selectivity of H2O / H2 is 52, and the selectivity of H2O / CO2 is 78.

[0045] Figure 3 The XRD patterns of the first FAU-type molecular sieve membrane and the second SOD-type molecular sieve membrane obtained in Example 2 are shown.

[0046] Please note that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above embodiments only illustrate several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent.

Claims

1. A method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology, characterized in that, Includes the following steps: S1 prepares molecular sieve seeds with an average particle size of less than 100 nm; S2 The molecular sieve seed crystals prepared in step S1 are coated on a porous support and placed in a primary synthesis solution L1. The first molecular sieve membrane is formed by hydrothermal crystallization. S3 After washing and drying the molecular sieve membrane obtained in step S2, the surface layer of the first molecular sieve membrane is placed in the secondary treatment solution L2, which is a NaOH solution or KOH solution with a mass fraction of 5-35%. The surface layer of the first molecular sieve membrane undergoes in-situ phase inversion through alkaline treatment, thereby forming the second molecular sieve membrane. S4 After washing and drying the molecular sieve membrane obtained in step S3, place the second molecular sieve membrane in the three-stage repair solution L3 and hydrothermally crystallize to repair the structural defects formed in the second molecular sieve membrane in step S3. S5 After washing and drying the molecular sieve membrane obtained in step S4, a multilayer molecular sieve membrane is obtained. The molecular sieve pore size of the first layer of molecular sieve membrane is greater than 0.4 nm, and the molecular sieve pore size of the second layer of molecular sieve membrane is less than or equal to 0.4 nm; the molecular sieves of the multilayer molecular sieve membrane are closely connected and there is no clear layering interface.

2. The method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology according to claim 1, characterized in that: The first molecular sieve membrane is a FAU type or EMT type molecular sieve, and the second molecular sieve membrane is a SOD type, LTA type or GIS type molecular sieve. The silica-alumina ratio of the framework of the two molecular sieves ranges from 1 to 5.

3. The method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology according to claim 1, characterized in that: In step S1, the molecular sieve seed crystals are FAU type or EMT type molecular sieves.

4. The method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology according to claim 1, characterized in that: In step S2, the molar ratio of the primary synthesis solution L1 is 5-25 SiO2:Al2O3:2.5-75 Na2O:800-5000 H2O, the hydrothermal crystallization temperature is 40-100 ℃, and the hydrothermal crystallization time is 12-48 h.

5. The method for preparing multilayer molecular sieve membranes using in-situ phase inversion technology according to claim 1, characterized in that: In step S4, the molar ratio of the three-stage repair solution L3 is 5-18 SiO2:Al2O3:1.5-50 Na2O (K2O): 900-1500H2O, the hydrothermal crystallization temperature is 50-120 ℃, and the hydrothermal crystallization time is 12-24 h.

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