Preparation method and application of CHA type zeolite molecular sieve membrane

Abstract

The invention belongs to the technical field of membrane separation, and relates to a preparation method and application of a CHA type zeolite molecular sieve membrane, in particular to a method for preparing the CHA type zeolite molecular sieve membrane through wet gel conversion by taking an FAU type molecular sieve as a heterogeneous seed crystal. The preparation method comprises the following steps: firstly, taking synthesized multilevel structure FAU as a large seed crystal, taking ball-milled FAU as a small seed crystal, inducing growth of a CHA phase in a film layer by using SBU (d6r) which is the same as that of an FAU type molecular sieve and a CHA type molecular sieve, and preparing a compact CHA type zeolite film through a gel conversion method; the membrane has excellent separation performance when being used for dehydration of aqueous solutions such as ethanol by pervaporation. The m-FAU molecular sieve is directly used as a seed crystal to be converted into the CHA membrane, so that the synthesis time and cost are greatly shortened, and industrialization is facilitated; compared with a method for directly preparing the CHA type molecular sieve, the method has the advantage that the use of an organic template agent is avoided.

Description

Technical Field

[0001] The present invention belongs to the field of membrane separation technology and relates to a preparation method and application of a CHA type zeolite molecular sieve membrane, specifically a method for preparing a CHA type zeolite molecular sieve membrane by wet gel conversion using a FAU type molecular sieve as a heterogeneous crystal seed. Background Art

[0002] The pervaporation membrane process is a separation technology that has gradually been applied with the development of membranes. This membrane separation is driven by the difference in vapor partial pressure (or concentration) of the components in a volatile mixed liquid, relying on the different osmotic diffusion rates of each component in the membrane to achieve separation. During this process, due to the low partial vapor pressure on the permeate side, the permeating component (liquid) is converted into the vapor phase using the enthalpy of evaporation. During operation, a vacuum pump is usually placed on the permeate side of the membrane to generate low vapor pressure. Pervaporation can be used for separation tasks that are difficult to achieve using traditional separation technologies, and is particularly suitable for the separation of constant boiling and near-boiling systems. Compared with traditional separation technologies such as distillation, absorption, and extraction, this separation technology has the advantages of low energy consumption and environmental protection. Pervaporation is mainly used for the separation of organic matter, the removal of small amounts (trace amounts) of water from organic matter, and the removal of small amounts of organic matter from water.

[0003] CHA zeolite molecular sieves, also known as chabazite, have a structure consisting of cage columns composed of alternating d6r and CHA cages. Based on the elemental content of the framework, CHA zeolites are classified as either phosphate-alumino or silica-alumino. Silica-alumino CHA zeolites are composed of framework silicon, aluminum, and oxygen, with a tunable Si / Al ratio between 2 and ∞. Based on the Si / Al ratio, they can be divided into three categories: all-silica (Si-CHA), high-silica (SSZ-13) (Si / Al > 5), and low-silica (Si / Al = 2-5). The CHA framework has a three-dimensional octahedral pore system along the crystallographic a-, b-, and c-axes, with a diameter of 0.38 nm x 0.38 nm. The pore opening diameter is larger than that of an H2O molecule (0.265 nm) but smaller than that of an organic molecule. Adjusting the Si / Al ratio allows for tuning hydrophilicity, hydrophobicity, and acid resistance, making CHA zeolite membranes ideal for organic dehydration.

[0004] At present, the main method for preparing CHA zeolite membranes is the homogeneous seed secondary hydrothermal growth method, that is, pre-coating a CHA seed layer on a carrier and then placing the pre-coated carrier into a synthesis liquid to prepare the membrane. This method separates the formation of the crystal nucleus from the growth process, which is conducive to controlling the directional growth of the membrane. However, this method also has certain limitations: the preparation of seed crystals usually requires an organic template agent TMAdaOH, and the use of organic template agents will cause environmental pollution and increase costs; the need for a large amount of synthesis liquid is likely to cause waste of resources, which is not in line with the concept of green environmental protection. CHA zeolite molecular sieve seeds are difficult to synthesize directly. Usually, the intercrystalline conversion method is used to prepare CHA-type zeolite molecular sieves using other zeolite molecular sieves. At present, FAU-type molecular sieves are more often used to prepare CHA-type zeolite molecular sieves through intercrystalline conversion as seeds, and then the converted CHA-type molecular sieves are used to prepare CHA-type zeolite molecular sieves. Yamanaka et al. (Microporous and Mesoporous Materials, 2012, 158:141-147) used benzyltrimethylammonium as a structure-directing agent and FAU zeolite molecular sieves for intercrystalline conversion. They synthesized high-silicon CHA zeolite molecular sieve membranes with varying Si / Al ratios on alumina support tubes. These membranes exhibited high acetic acid stability. PV testing in a 50 wt.% aqueous solution of acetic acid at 75°C yielded a separation factor of 2500 and a permeate flux of 8 kg·m -2 ·h -1 . This process avoids the use of organic templates, but the increase in steps increases costs, which is not conducive to the actual industrialization of the process. Chinese invention patent CN202011072131.2 proposes to prepare CHA-type zeolite molecular sieve membranes using T-type zeolite molecular sieves as heterogeneous crystal seeds, but there are certain limitations, namely, the preparation of T-type zeolite molecular sieves requires the organic template TMAOH. Summary of the Invention

[0005] To solve the above problems, the present invention provides a method for preparing a CHA-type zeolite molecular sieve membrane for efficient organic dehydration. The method utilizes heterogeneous seed FAU-type molecular sieves and CHA-type molecular sieves having the same SBU (d6r) to induce the growth of the CHA phase in the membrane layer. First, a FAU seed layer is pre-coated on an inexpensive macroporous support, and a heterogeneous seed wet gel conversion method is used to prepare a CHA-type zeolite molecular sieve membrane in a shorter crystallization time, which has certain separation stability.

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

[0007] A method for preparing a CHA-type zeolite molecular sieve membrane comprises the following steps:

[0008] Step (1) Preparation of FAU molecular sieve large seed crystals: Using colloidal silica as a silicon source and NaAlO2 as an aluminum source, NaOH, the aluminum source, and deionized water are mixed and stirred thoroughly to clarify. The silicon source is then added dropwise to the clarified solution, stirred continuously, and aged to obtain a synthetic solution. The aged synthetic solution is placed in a hydrothermal reactor for high-temperature crystallization. The product is centrifuged, washed, and dried to obtain FAU molecular sieve large seed crystals.

[0009] Step (2) Preparation of FAU type zeolite molecular sieve small seed crystals: wet ball milling the FAU type molecular sieve, subjecting the ball-milled FAU type molecular sieve to sedimentation treatment, taking the supernatant for centrifugation, drying, and grinding to obtain FAU type zeolite molecular sieve small seed crystals.

[0010] Step (3) Preparation of FAU seed layer: The FAU type molecular sieve prepared in steps (1) and (2) is dispersed in deionized water as a large seed solution and a small seed solution, respectively, and the large and small seed crystals are coated on the surface of the porous carrier tube in turn by two-step variable temperature hot impregnation. Specifically, the large seed crystal solution is ultrasonicated in an ultrasonic machine, and plugs are plugged at both ends of the carrier tube for preheating. The preheated carrier is vertically immersed in the large seed crystal solution by a pulling method. After holding for a period of time, it is pulled out at a uniform speed, dried at room temperature, and cured at high temperature; after wiping off the excess large seed crystals on the surface of the carrier tube with absorbent cotton, the small seed crystals are coated using the same steps as for coating the large seed crystals, dried at room temperature, and cured at high temperature to obtain a flat and defect-free FAU seed crystal layer.

[0011] Step (4) preparing a sol-gel system: using colloidal silica as a silicon source, NaAlO2 as an aluminum source, NaOH and KOH as alkali sources, and NaF and KF as fluorine sources, dissolving the alkali source and aluminum source in deionized water, and stirring and clarifying in a water bath to obtain a mixed aqueous solution of the alkali source and aluminum source, dissolving the fluorine source in deionized water and ultrasonically dissolving it, and dripping the completely dissolved fluorine source into the continuously stirred aqueous solution of the mixed alkali source and aluminum source. After complete stirring, continuously dripping the silicon source, stirring and aging to form a stable sol-gel system.

[0012] Step (5) Preparation of a CHA-type zeolite molecular sieve membrane: Plug both ends of the carrier tube coated with the seed layer, immerse it in deionized water using the Czochralski method, and then air-dry it at room temperature until no obvious water marks are left on the surface. Subsequently, the carrier tube coated with the gel layer is placed in an aged sol-gel system using the Czochralski method and immersed vertically for a certain period of time. The carrier tube is then placed in a reactor for high-temperature crystallization. After the reaction is completed, the zeolite membrane is washed with deionized water until neutral and dried to obtain a CHA-type zeolite molecular sieve membrane.

[0013] In the step (1), the molar ratio of the components in the synthetic liquid is Na2O:Al2O3:SiO2:H2O=(7-20):1.0:(10-18):(200-1000).

[0014] In the step (1), the aging temperature is 20 to 30° C., and the aging time is 12 to 24 hours.

[0015] In the step (1), the hydrothermal crystallization temperature is 60-80° C., and the hydrothermal crystallization time is 3-4 days, preferably the hydrothermal crystallization temperature is 60° C., and the hydrothermal crystallization time is 3.5 days.

[0016] In the step (1), the centrifuge speed is 8000-12000 rpm, the centrifugation time is 5-10 min, the drying temperature is 60-80° C., and the drying time is 12-24 h.

[0017] In the step (1), the obtained FAU type zeolite molecular sieve large crystal seed size is 380 to 420 nm.

[0018] In the step (2), the ball mill is set to a rotation speed of 300-500 r / min and a time of 2-4 days.

[0019] In the step (2), the sedimentation time is 5 to 15 days, preferably 15 days; the centrifuge speed is 8000 to 15000 rpm, the centrifugation time is 5 to 10 minutes; the drying temperature is 60 to 80° C., and the drying time is 12 to 24 hours.

[0020] In the step (2), the FAU type zeolite molecular sieve small crystal seeds obtained by ball milling have a size of 50 to 250 nm.

[0021] In the step (3), the porous carrier is made of alumina, zirconia or mullite, and the pore size of the porous carrier is 0.02 to 50 μm. Preferably, the carrier is made of alumina, and the average pore size is 3 μm.

[0022] In the step (3), the concentration of the prepared large crystal seed solution is 0.5-2 wt.%, and the concentration of the small crystal seed solution is 0.1-1.0 wt.%, preferably the large crystal seed concentration is 2 wt.%, and the small crystal seed concentration is 0.2 wt.%.

[0023] In the step (3), the ultrasonic time of the large and small seed solutions is 8 to 24 hours, the preheating temperature when coating the large seed solution is 100 to 200°C, the preheating time is 2 to 8 hours, and the curing temperature is 100 to 200°C, preferably the ultrasonic time is 24 hours, the preheating temperature when coating the large seed solution is 150°C, the preheating time is 3 hours, and the curing temperature is 150°C; the preheating temperature when coating the small seed solution is 60 to 120°C, the preheating time is 2 to 8 hours, and the curing temperature is 60 to 120°C, preferably the preheating temperature when coating the small seed solution is 100°C, the preheating time is 3 hours, and the curing temperature is 100°C.

[0024] In the step (3), the immersion time is 20 to 30 seconds when coating large seed crystals, and the immersion time is 20 to 30 seconds when coating small seed crystals. Preferably, the immersion time of large seed crystals is 20 seconds, and the immersion time of small seed crystals is 20 seconds.

[0025] In the step (4), the molar ratio of the components in the sol-gel system is SiO2:Al2O3:Na2O:K2O:MF:H2O=1:(0.005-0.5):(0.05-5):(0.015-1.5):(0-0.70):(20-2500).

[0026] In the step (4), the water bath aging temperature is 20-35° C., and the water bath aging time is 16-24 hours. Preferably, the aging temperature is 25° C., and the aging time is 24 hours.

[0027] In the step (5), the immersion time in deionized water is 10 to 60 seconds, and the drying time at room temperature is 10 to 30 minutes. Preferably, the immersion time in deionized water is 30 seconds, and the drying time at room temperature is 15 minutes.

[0028] In the step (5), the immersion time in the aged synthetic liquid is 10 to 30 seconds, preferably 30 seconds.

[0029] In the step (5), the crystallization temperature is 130-170° C., and the crystallization time is 2-5 hours. Preferably, the crystallization temperature is 150° C., and the crystallization time is 4 hours.

[0030] In the step (5), the drying temperature is 40 to 60° C., and the drying time is 10 to 24 hours.

[0031] The CHA type zeolite molecular sieve membrane prepared by the above-mentioned preparation method of the CHA type zeolite molecular sieve membrane is used for separation of aqueous solutions of organic substances, such as ethanol, propanol, methanol, etc.

[0032] Beneficial effects of the present invention:

[0033] This method eliminates the need to separately convert FAU molecular sieves into CHA zeolite molecular sieves through intercrystalline conversion. The simplification of the process reduces costs and is conducive to industrialization. In addition, compared with the preparation of CHA zeolite membranes by the heterogeneous seed conversion method using T-type molecular sieves as seeds and the direct preparation of CHA molecular sieves to prepare CHA zeolite membranes, the preparation of FAU molecular sieves in the present invention does not require the use of organic templates and is environmentally friendly. The combination of the wet gel method significantly reduces the crystallization time, prepares CHA zeolite membranes with thinner film thickness, and greatly improves the utilization rate of the synthesis liquid. To synthesize CHA zeolite membranes of the same length, the synthesis liquid required by the wet gel method is only 5-6% of the membrane synthesis mother liquor required by the traditional hydrothermal method, which is in line with the concept of green environmental protection. The prepared CHA zeolite molecular sieve membrane is continuous and dense, and has excellent pervaporation performance for ethanol / water solution separation. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the pervaporation test device, where a-magnetic stirrer, b-water bath, c-membrane tube, d-membrane assembly, e-raw material storage tank, f-cold trap, g-liquid nitrogen, h-buffer bottle, i-vacuum gauge, and j-vacuum pump.

[0035] Figure 2 (a) and (b) are scanning electron microscope images of FAU type molecular sieve large seeds and FAU type zeolite molecular sieve small seeds, respectively.

[0036] Figure 3 These are X-ray diffraction patterns of FAU type molecular sieve large crystals and FAU type zeolite molecular sieve small crystals.

[0037] Figure 4 This is the XRD pattern of the synthesized CHA type zeolite molecular sieve membrane.

[0038] Figure 5 (a) and (b) are scanning electron micrographs of the surface and cross-section of the synthesized CHA zeolite molecular sieve membrane, respectively. DETAILED DESCRIPTION

[0039] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0040] Example 1

[0041] (1) Preparation of large FAU zeolite molecular sieve seeds: A synthetic solution was prepared according to a molar ratio of Na2O:Al2O3:SiO2:H2O=9.5:1.0:14:288, and the solution was stirred and aged in a 30°C water bath for 20 h. The aged synthetic solution was placed in a reactor and crystallized in a 60°C oven for 3.5 days. After centrifugation, drying, and grinding, a multi-level FAU molecular sieve with a size of 380-420 nm was obtained.

[0042] (2) Preparation of small seed crystals of FAU type zeolite molecular sieve: FAU type molecular sieve purchased on the market was wet-milled at a speed of 300-500 r / min and a ball-milling time of 2-4 days. The FAU type molecular sieve after ball milling was subjected to sedimentation treatment for 15 days. The supernatant was centrifuged, dried, and ground to obtain small seed crystals of FAU type molecular sieve with a size of 50-250 nm.

[0043] (3) Preparation of FAU seed layer: The FAU molecular sieve prepared in steps (1) and (2) is dispersed in a certain amount of deionized water to form a 2 wt.% large seed solution and a 0.2 wt.% small seed solution, wherein the crystal structures of the FAU large and small seed crystals are as follows: Figure 2 As shown, XRD Figure 3 As shown, a two-step variable temperature hot dip process is used to sequentially coat the surface of a porous carrier tube with large and small seed crystals: the large seed crystal solution is ultrasonicated in an ultrasonic machine for 8 hours, the carrier tube is plugged at both ends and preheated at 150°C for 3 hours, the preheated carrier is vertically immersed in the large seed crystal solution for 30 seconds using the Czochralski method, then withdrawn at a constant speed, dried at room temperature overnight, and cured at 150°C for 3 hours. After wiping off excess large seed crystals from the carrier tube surface with absorbent cotton, the small seed crystals are coated using the same steps as for coating the large seed crystals. The carrier tube is plugged at both ends and preheated at 100°C for 3 hours, the preheated carrier is vertically immersed in the small seed crystal solution for 30 seconds using the Czochralski method, withdrawn at a constant speed, dried at room temperature overnight, and cured at 100°C for 3 hours, resulting in a uniform, dense, and defect-free FAU seed layer.

[0044] (4) Preparation of synthetic solution: Prepare the synthetic solution according to the molar ratio of SiO2:Al2O3:Na2O:K2O:MF:H2O=1:0.05:0.5:0.15:0.10:24 and a certain order of addition. Weigh a certain amount of NaOH, KOH, and NaAlO2 using an analytical balance, add deionized water to dissolve, and stir in a constant temperature stirrer at 25°C to clarify, which is recorded as solution A. Then, weigh a certain amount of KF or NaF using an analytical balance, add deionized water and ultrasonically dissolve, which is recorded as solution B. Add solution B dropwise to the stirred clarified solution A, stir for 10 minutes, and then add silica sol AS-40 dropwise. Stir continuously in a water bath at 25°C for 24 hours to obtain the synthetic solution.

[0045] (5) Preparation of CHA zeolite molecular sieve membrane: The carrier tube into which the seed layer was introduced in step (3) was plugged at both ends, immersed in deionized water for 30 seconds using the Czochralski method, and then aired at room temperature for 15 minutes until no obvious water marks were left on the surface. Subsequently, the carrier tube was vertically immersed in an aged sol-gel system for 30 seconds. The carrier tube coated with the gel layer was then placed in a reactor and crystallized at 150°C for 5 hours. After the reaction, the zeolite membrane was washed with deionized water until neutral and dried to obtain a CHA zeolite molecular sieve membrane.

[0046] The crystal structure of the CHA type zeolite molecular sieve membrane is confirmed by the analysis of the attached figure. The membrane layer prepared in this embodiment is relatively dense and smooth, with an average thickness of 4 μm. Figure 5 As shown, the characteristic peaks of CHA are shown as Figure 4 As shown, it is confirmed that the prepared membrane layer is a CHA type zeolite membrane. The dense membrane layer makes it have excellent pervaporation performance. The pervaporation test was carried out on the CHA type molecular sieve membrane prepared in Example 1, wherein the pervaporation device is as shown in FIG. Figure 1 As shown in Figure 2, at a temperature of 75°C, the water permeate flux obtained by dehydrating a 90 wt.% ethanol / water solution is 2.50 kg·m -2 ·h -1 , the separation factor is 7325.

[0047] Example 2

[0048] The formula of the synthetic solution in step (4) of Example 1 was changed to SiO2:Al2O3:Na2O:K2O:MF:H2O=1:0.05:0.5:0.15:0.10:(20, 26, 2500). The remaining steps were the same as in Example 1. The CHA type zeolite molecular sieve membranes were synthesized by changing the water-silicon ratio in the synthetic solution to 20, 26, and 2500, respectively. The performance of the membranes was obtained after pervaporation test of 90 wt.% ethanol / water at 75°C: when the water-silicon ratio was 20, the flux and separation factor of the membrane were 2.56 kg·m -2 ·h -1 , 2531; when the water-silicon ratio is 26, the flux and separation factor of the membrane are 3.35 kg·m -2 ·h -1 , 2278; when the water-silicon ratio is 2500, the flux and separation factor of the membrane are 3.65 kg·m -2 ·h -1 , 358.

[0049] Example 3

[0050] The formula of the synthetic solution in step (4) of Example 1 was changed to SiO2:Al2O3:Na2O:K2O:MF:H2O=1:(0.005, 0.055, 0.5):0.5:0.15:0.10:24. The remaining steps were the same as in Example 1. CHA type zeolite molecular sieve membranes were synthesized by changing the aluminum source in the synthetic solution formula to 0.005, 0.055, and 0.5, respectively. The performance of the membranes was obtained after pervaporation test of 90 wt.% ethanol / water at 75°C: when the aluminum source in the synthetic solution formula was 0.005, the flux and separation factor of the membrane were 2.39 kg·m -2 ·h -1 , 3193; when the aluminum source in the synthetic solution formula is 0.055, the flux and separation factor of the membrane are 3.28 kg·m-2 ·h -1 ,>10000; when the aluminum source in the synthetic solution formula is 0.5, the flux and separation factor of the membrane are 2.83kg·m -2 ·h -1 , 317.

[0051] Example 4

[0052] The formula of the synthetic solution in step (4) of Example 1 was changed to SiO2:Al2O3:Na2O:K2O:MF:H2O=1:0.05:(0.05, 0.45, 5):0.15:0.10:24. The remaining steps were the same as in Example 1. CHA type zeolite molecular sieve membranes were synthesized by changing the Na2O in the synthetic solution formula to 0.005, 0.055, and 0.5, respectively. The performance of the membranes was obtained after pervaporation test of 90 wt.% ethanol / water at 75°C: when the Na2O in the synthetic solution formula was 0.05, the flux and separation factor of the membrane were 2.29 kg·m -2 ·h -1 , 4716; when the aluminum source in the synthetic solution formula is 0.45, the flux and separation factor of the membrane are 3.14 kg·m -2 ·h -1 ,>10000; when the aluminum source in the synthesis solution formula is 5, the flux and separation factor of the membrane are 2.41kg·m -2 ·h -1 , 461.

[0053] Example 5

[0054] The formula of the synthetic solution in step (4) of Example 1 was changed to SiO2:Al2O3:Na2O:K2O:MF:H2O=1:0.05:0.5:(0.015, 0.2, 1.5):0.10:24. The remaining steps were the same as in Example 1. CHA type zeolite molecular sieve membranes were synthesized by changing the K2O in the synthetic solution formula to 0.015, 0.2, and 1.5, respectively. The performance of the membranes was obtained after pervaporation test of 90 wt.% ethanol / water at 75°C: when the K2O in the synthetic solution formula was 0.015, the flux and separation factor of the membrane were 2.62 kg·m -2 ·h -1 , 1395; when the K2O content in the synthetic solution was 0.2, the flux and separation factor of the membrane were 3.19 kg·m -2 ·h -1 ,>10000; when the K2O in the synthetic liquid formula is 1.5, the flux and separation factor of the membrane are 2.41kg·m -2 ·h -1 , 5379.

[0055] Example 6

[0056] The formula of the synthetic solution in step (4) of Example 1 was changed to SiO2:Al2O3:Na2O:K2O:MF:H2O=1:0.055:0.486:0.15:(0,0.7):24. The remaining steps were the same as those in Example 1. CHA type zeolite molecular sieve membranes were synthesized and subjected to a 90 wt.% ethanol / water pervaporation test at 75°C to obtain their performance: when there was no fluorine source, the flux and separation factor of the membrane were 2.71 kg·m -2 ·h -1 ,>10000; when MF in the formula is 0.7, the flux and separation factor of the membrane are 2.58kg·m -2 ·h -1 , 5417.

[0057] Example 7

[0058] The crystallization time in step (4) of Example 1 was changed to 2 h, 3 h, and 4 h. The remaining steps were the same as in Example 1. CHA-type zeolite molecular sieve membranes were synthesized at different crystallization times. The performance of the membranes was obtained after pervaporation test of 90 wt.% ethanol / water at 75°C. When the crystallization time was 2 h, the flux and separation factor of the membrane were 3.41 kg·m -2 ·h -1 ,700; when the crystallization time was 3h, the flux and separation factor of the membrane were 2.90kg·m -2 ·h -1 , 3213; when the crystallization time was 4 h, the flux and separation factor of the membrane were 2.60 kg·m -2 ·h -1 ,>10000.

[0059] Example 8

[0060] The immersion time of the large seed crystals in step (3) of Example 1 was changed to 10 s and 20 s, and the immersion time of the small seed crystals was changed to 20 s. The remaining steps were the same as those in Example 1. CHA type zeolite molecular sieve membranes were synthesized and subjected to a 90 wt.% ethanol / water pervaporation test at 75°C to obtain their performance: when the large seed crystal immersion time was 10 s, the flux and separation factor of the membrane were 3.37 kg·m -2 ·h -1 , 3838; when the large seed immersion time is 20s, the flux and separation factor of the membrane are 3.01kg·m -2 ·h -1 ,>10000.

[0061] Example 9

[0062] The CHA zeolite molecular sieve membrane synthesized by changing the immersion time of the large seed crystals to 20 s and the immersion time of the small seed crystals to 20 s in Example 8 was subjected to a 90 wt.% isopropanol / water pervaporation test at 75°C. The membrane performance was obtained: the flux and separation factor were 4.56 kg·m -2 ·h -1 ,>10000.

[0063] Comparative Example 1

[0064] The microstructure of the small seed crystals was changed in the preparation of the FAU seed layer in step (3) of Example 1. The remaining steps were the same as in Example 1. The effect of the structure of the FAU type molecular sieve on the formation of the CHA type zeolite molecular sieve membrane was explored. The CHA type zeolite molecular sieve membrane was prepared by crystallizing at 150°C for 4 hours using 200nm, 250nm, and 300nm multi-level structure FAU and FAU with a size of 50-250nm after ball milling as small seed crystals. The membrane layer prepared when the regular multi-level structure FAU was used as the small seed crystal was not a CHA crystal phase and did not have separation performance. Only when the irregular FAU after ball milling was used as the small seed crystal could the CHA type zeolite molecular sieve membrane be prepared. The pervaporation test of 90wt.% ethanol / water at 75°C is shown in Table 1 below:

[0065] Table 1 Pervaporation performance of zeolite membranes prepared with different small seed crystals

[0066]

[0067] Comparative Example 2

[0068] The large seed FAU preparation formula was changed to Na2O:Al2O3:SiO2:H2O=7:1.0:10:200 and 20:1.0:18:1000 in step (1) of Example 1 to prepare a multi-level FAU and a non-multi-level FAU molecular sieve as large seed to prepare a CHA type zeolite membrane. The remaining steps were the same as those in Example 1. The CHA type zeolite molecular sieve membrane was crystallized at 150°C for 4h and subjected to a 90wt.% ethanol / water pervaporation test at 75°C to obtain its performance: the flux and separation factor of the membrane were 2.84kg·m -2 ·h -1 、3.59kg·m -2 ·h -1 ,>10000,3219.

[0069] Comparative Example 3

[0070] The gel method in Example 1 was replaced by a hydrothermal method, and a CHA zeolite membrane was prepared on a 5 cm long alumina carrier tube using the hydrothermal method and the wet gel method respectively. In step (5), the mass of the gel synthesis liquid coated on the 5 cm long carrier tube by vertical immersion for 30 seconds in the sol-gel system after aging in the synthesis liquid was about 0.28 g by weighing; the mass of the synthesis liquid required to prepare the CHA zeolite membrane on the 5 cm long carrier tube using the hydrothermal method was about 25 g, which was converted into a gel mass of about 5 g. It can be seen that for synthesizing a CHA zeolite membrane of the same length, the synthesis liquid required by the wet gel method is only 5-6% of the membrane synthesis mother liquid required by the traditional hydrothermal method.

[0071] The results show the importance of heterogeneous seeds in the preparation of CHA zeolite membranes by the heterogeneous seed wet gel method. Using ball-milled FAU molecular sieves as small seeds and multi-level structure FAU molecular sieves as large seeds is more conducive to the preparation of pure phase CHA zeolite membranes. The combination of wet gel method can shorten the crystallization time to 2 to 4 hours, prepare CHA zeolite membranes with thinner membrane thickness, greatly improve the utilization rate of synthesis liquid, and for synthesizing CHA zeolite membranes of the same length, the synthesis liquid required by the wet gel method is only 5 to 6% of the membrane synthesis mother liquid required by the traditional hydrothermal method, which is in line with the concept of green environmental protection.

Claims

1. A method for preparing a CHA type zeolite molecular sieve membrane, characterized in that: The steps include: Step (1) Preparation of FAU type molecular sieve large seed crystals: using colloidal silica as a silicon source and NaAlO2 as an aluminum source, NaOH, the aluminum source and deionized water are mixed and stirred thoroughly to clarify, the silicon source is added dropwise to the clarified solution, and the solution is continuously stirred and aged to obtain a synthetic solution; the aged synthetic solution is placed in a hydrothermal reactor for crystallization, and the product is centrifuged, washed and dried to obtain FAU type molecular sieve large seed crystals; Step (2) Preparation of FAU type zeolite molecular sieve small seed crystals: wet ball milling the FAU type molecular sieve, subjecting the ball-milled FAU type molecular sieve to sedimentation treatment, taking the supernatant and performing centrifugation, drying, and grinding to obtain FAU type zeolite molecular sieve small seed crystals; Step (3) Preparation of FAU seed layer: The FAU type molecular sieve prepared in steps (1) and (2) is dispersed in deionized water as a large seed solution and a small seed solution, respectively, and the large and small seed crystals are coated on the surface of the porous carrier tube in sequence by two-step variable temperature hot impregnation, specifically: the large seed crystal solution is ultrasonicated in an ultrasonic machine, and plugs are plugged at both ends of the carrier tube for preheating, and the preheated carrier is vertically immersed in the large seed crystal solution by a pulling method, and after holding for a period of time, it is pulled out at a uniform speed, dried at room temperature, and then solidified; after wiping off the excess large seed crystals on the surface of the carrier tube with absorbent cotton, the small seed crystals are coated with the same steps as for coating the large seed crystals, dried at room temperature, and then solidified to obtain a flat and defect-free FAU seed crystal layer; Step (4) preparing a sol-gel system: using colloidal silica as a silicon source, NaAlO2 as an aluminum source, NaOH and KOH as alkali sources, and NaF and KF as fluorine sources, dissolving the alkali source and aluminum source in deionized water, and stirring and clarifying in a water bath to obtain a mixed aqueous solution of the alkali source and aluminum source, dissolving the fluorine source in deionized water and ultrasonically dissolving it, and dripping the completely dissolved fluorine source into the continuously stirred aqueous solution of the mixed alkali source and aluminum source. After complete stirring, continuously dripping the silicon source, stirring and aging the mixture to form a stable sol-gel system; Step (5) Preparation of CHA type zeolite molecular sieve membrane: plug the two ends of the carrier tube coated with the seed layer, immerse it in deionized water by the pulling method, and then air it at room temperature until there are no obvious water marks on the surface. Then, use the pulling method to put it into the aged sol-gel system and immerse it vertically for a certain period of time. Then, put the carrier tube coated with the gel layer into the reactor for crystallization; after the reaction is completed, use deionized water to wash the zeolite membrane until it is neutral, and dry it to obtain a CHA type zeolite molecular sieve membrane.

2. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, wherein: In the step (1), the molar ratio of the components in the synthetic liquid is Na2O:Al2O3:SiO2:H2O=(7-20):1.0:(10-18):(200-1000); in the step (4), the molar ratio of the components in the sol-gel system is SiO2:Al2O3:Na2O:K2O:MF:H2O=1:(0.005-0.5):(0.05-5):(0.0 15~1.5):(0~0.70):(20~2500); in the step (3), the concentration of the prepared large seed solution is 0.5~2wt.%, and the concentration of the small seed solution is 0.1~1.0wt.%; in the step (1), the obtained FAU type zeolite molecular sieve large seed size is 380~420nm; in the step (2), the FAU type zeolite molecular sieve small seed size obtained by ball milling is 50~250nm.

3. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, wherein: In the step (1), the aging temperature is 20-30°C, the aging time is 12-24h; the hydrothermal crystallization temperature is 60-80°C, the hydrothermal crystallization time is 3-4d, the centrifuge speed is 8000-12000rpm, the centrifugation time is 5-10min, the drying temperature is 60-80°C, and the drying time is 12-24h.

4. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, wherein: In step (2), the ball mill is set to a rotation speed of 300-500 r / min and a time of 2 to 4 days; the sedimentation time is 5 to 15 days; the centrifuge speed is 10,000 to 12,000 rpm and the centrifugation time is 5 to 10 minutes; the drying temperature is 60 to 80° C. and the drying time is 12 to 24 hours.

5. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, wherein: In the step (3), the material of the porous carrier is alumina, zirconia or mullite, and the pore size of the porous carrier is 0.02 to 50 μm.

6. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, characterized in that: In the step (3), the ultrasonic time of the large and small seed solutions is 8 to 24 hours, the preheating temperature when coating the large seed solution is 100 to 200°C, the preheating time is 2 to 8 hours, and the curing temperature is 100 to 200°C. Preferably, the preheating temperature when coating the large seed solution is 150°C, the preheating time is 3 hours, and the curing temperature is 150°C; the preheating temperature when coating the small seed solution is 60 to 120°C, the preheating time is 2 to 8 hours, and the curing temperature is 60 to 120°C. Preferably, the preheating temperature when coating the small seed solution is 100°C, the preheating time is 3 hours, and the curing temperature is 100°C; the immersion time when coating the large seed solution is 20 to 30 seconds, and the immersion time when coating the small seed solution is 20 to 30 seconds.

7. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, characterized in that: In the step (4), the water bath aging temperature is 20-35° C., and the water bath aging time is 16-24 hours. Preferably, the aging temperature is 25° C., and the aging time is 24 hours.

8. The method for preparing a CHA type zeolite molecular sieve membrane according to claim 1, characterized in that: In the step (5), the immersion time in deionized water is 10 to 60 seconds, and the drying time at room temperature is 10 to 30 minutes; the immersion in the aged synthetic liquid is 10 to 30 seconds; the crystallization temperature is 130 to 170° C., the crystallization time is 2 to 5 hours, and the crystallization time is 4 hours; the drying temperature is 40 to 60° C., and the drying time is 10 to 24 hours.

9. Application of the CHA type zeolite molecular sieve membrane prepared by the preparation method of the CHA type zeolite molecular sieve membrane according to any one of claims 1 to 8 in the separation of organic aqueous solutions.

10. Use according to claim 9, characterized in that The organic matter is ethanol, propanol or methanol.

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