Method for synthesizing molecular sieve membrane by doping molecular sieve fragments into ceramic support transition layer

Abstract

The present invention discloses a method for synthesizing a molecular sieve membrane by doping molecular sieve fragments with a ceramic support transition layer, comprising: using water as a solvent, crushing molecular sieve crystals with a planetary high-energy ball mill and centrifuging to obtain a fragment sol; then mixing the fragment sol with a ceramic sol, uniformly coating the surface of the support, and calcining to obtain a composite structure; placing the composite structure in a molecular sieve synthesis mother liquor for hydrothermal synthesis and removing the template to obtain a molecular sieve membrane. This method directly induces the synthesis of a molecular sieve membrane by preparing a transition layer doped with nano-scale molecular sieve fragments on the surface of the support, which can effectively solve the problem of uniform coating of seed crystals on the surface of the support during the synthesis of the molecular sieve membrane; at the same time, the ceramic transition layer can inhibit the pore permeation of the synthesis mother liquor, reduce the mass transfer resistance of the support, and thus improve the permeability of the molecular sieve membrane. The molecular sieve membrane synthesized using the method of the present invention has high repeatability and excellent performance, and is suitable for large-scale preparation of molecular sieve membranes.

Description

Technical Field

[0001] The invention belongs to the field of membrane separation, and in particular relates to a method for synthesizing a molecular sieve membrane by doping a ceramic support transition layer with nano-scale molecular sieve fragments. Background Art

[0002] Membrane technology is a separation technology that uses selectively permeable membrane materials as its core, and achieves the separation, purification, and concentration of mixtures under the driving force on both sides of the membrane. As a common separation technology with high efficiency and energy saving, membrane technology plays an important role in many key technologies. Among them, zeolite molecular sieve membrane is a membrane material with a regular pore structure (<1nm) that can achieve efficient separation of substances at the molecular scale. It has broad application prospects in industrial fields such as solvent purification, gas separation, and separation of organic mixtures. It can effectively promote energy conservation and emission reduction in related industrial processes and solve the problems of high energy consumption, high pollution, and high emissions in existing related industrial fields.

[0003] At present, the main methods for preparing molecular sieve membranes include in-situ hydrothermal synthesis, secondary growth method, vapor phase conversion method, etc. Among them, the secondary growth method obtains high-performance molecular sieve membranes by pre-coating molecular sieve seeds on the surface of a ceramic support and then performing hydrothermal synthesis. This can effectively improve the yield of molecular sieve membranes and has become one of the most commonly used preparation methods in the current field of molecular sieve membrane research. The surface properties of the ceramic support and the quality of the seed layer are key factors affecting the separation performance of molecular sieve membranes. At present, commercial ceramic supports generally have many surface defects (pinholes, spots), large pore size, and high roughness. Therefore, researchers often modify the surface of the support by using seed crystals to fill the surface defects of the support or by introducing organic functional groups on the surface of the support to obtain high-performance molecular sieve membranes. However, the mutual binding force between the modifier and the support in these methods is weak, which can easily cause problems such as cracking and peeling of the membrane layer during the membrane synthesis process. On the other hand, the quality of the seed layer is crucial to the yield of the molecular sieve membrane. Currently, researchers generally use the dip-coating method to prepare seed layers. This method exploits the capillary effect of the support pores to allow the seed crystals to adhere to the support surface. This can easily lead to uneven seed coating on the support surface, thus affecting the separation performance of the zeolite membrane. Therefore, how to reduce the impact of the surface properties of the ceramic support on the zeolite membrane and improve the quality of the seed layer have become key issues that urgently need to be addressed in the field of zeolite membranes. Summary of the Invention

[0004] In response to the problems of many surface defects, large surface pore size, high roughness and uncontrollable quality of the seed layer on the ceramic support during the preparation of molecular sieve membranes, the present invention carries out surface modification of porous ceramic supports. By preparing a ceramic support transition layer doped with nano-scale molecular sieve fragments, a ceramic support with small surface pore size, low roughness and strong molecular sieve inducing growth ability is obtained, ultimately realizing the preparation of high-performance molecular sieve membranes.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for synthesizing a molecular sieve membrane by doping molecular sieve fragments into a ceramic support transition layer comprises the following steps:

[0007] (1) Preparation of nano-scale molecular sieve fragment sol:

[0008] Using water as solvent, a planetary high-energy ball mill is used to crush the molecular sieve crystals to form nano-scale molecular sieve fragments, and the fragment sol is obtained by high-speed centrifugation;

[0009] (2) Preparation of transition layer:

[0010] The fragment sol and the ceramic sol are uniformly mixed to obtain a transition layer sol; the transition layer sol is then uniformly coated on the surface of the support, dried, and calcined at 200 to 800° C. for 0.5 to 24 hours to obtain a composite structure with a transition layer on the surface;

[0011] (3) Preparation of molecular sieve membrane:

[0012] placing the composite structure in the synthesis mother solution of the molecular sieve, and obtaining a membrane precursor through hydrothermal synthesis;

[0013] (4) Activation of molecular sieve membrane:

[0014] The membrane precursor is calcined at high temperature to remove the template agent, thereby obtaining the molecular sieve membrane.

[0015] Preferably, the molecular sieve is CHA, SSZ-13, NaA, T or DDR.

[0016] Preferably, the rotation speed of the planetary high-energy ball mill is 50 to 500 r / min, and the ball milling time is 1 to 10 hours.

[0017] Preferably, the particle size of the nano-scale molecular sieve fragments described in step (1) is 1 to 100 nm.

[0018] Preferably, the rotation speed of the high-speed centrifugation is 5000-100000 r / min, and the centrifugation time is 5-100 min.

[0019] Preferably, the ceramic sol in step (2) is TiO2 sol, ZrO2 sol or Al2O3 sol.

[0020] Preferably, the support body described in step (2) is a hollow fiber support body, a tubular support body, a multi-channel support body or a flat plate support body.

[0021] Preferably, the support is made of α-Al2O3, mullite or YSZ.

[0022] Preferably, the method for uniform mixing in step (2) is: stirring and ultrasonic treatment.

[0023] Preferably, in the fragment sol in step (1), the mass fraction of the nano-scale molecular sieve fragments is 0.2 to 2 wt%.

[0024] Preferably, the surface pore size of the support in step (2) is 50 to 5000 nm.

[0025] Preferably, the coating method in step (2) is a dipping method, a spraying method or a vacuum suction method.

[0026] Preferably, the coating in step (2) is performed 1 to 8 times in total.

[0027] Preferably, the drying temperature in step (2) is 30-80° C., the drying time is 1-96 h, and the ambient humidity is 50%-80%.

[0028] The beneficial effects of the present invention are:

[0029] The present invention prepares a ceramic transition layer doped with nano-scale molecular sieve fragments on the surface of a support body, and uses the uniformly dispersed nano-scale molecular sieve fragments on the surface of the transition layer to induce the nucleation and intergrowth of the molecular sieve membrane. Compared with traditional molecular sieve membrane synthesis technology, the method of the present invention reduces the process of preparing a seed layer during the hydrothermal synthesis process, which can simplify the synthesis process of the molecular sieve membrane. Secondly, the transition layer prepared by the sol-gel method has a small pore size and a uniform pore size distribution, which can effectively inhibit the pore permeation of the synthetic mother liquor and help improve the permeability of the membrane. In addition, the ceramic transition layer can modify the defects on the surface of the ceramic support body and reduce the roughness of the support body surface, thereby reducing the influence of the surface properties of the support body on the performance of the molecular sieve membrane. Compared with the performance of the molecular sieve membrane synthesized by the method of the present invention and the molecular sieve membrane synthesized by the traditional technology, the molecular sieve membrane synthesized by the method of the present invention has high repeatability and excellent performance, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a TEM photo of the morphology of nanoscale SSZ-13 molecular sieve fragments.

[0031] Figure 2 This is an SEM image of the TiO2 transition layer on the alumina support doped with nano-sized SSZ-13 molecular sieve fragments, where a is its cross-section and b is its surface.

[0032] Figure 3 This is an SEM image of the SSZ-13 molecular sieve membrane prepared in Example 1, where a is its cross-section and b is its surface.

[0033] Figure 4This is an SEM photograph of the composite structure of the alumina support TiO2 transition layer doped with SSZ-13 molecular sieve seeds.

[0034] Figure 5 This is the SEM image of the SSZ-13 molecular sieve membrane prepared in Comparative Example 1.

[0035] Figure 6 This is the SEM image of the TiO2 transition layer on the alumina support.

[0036] Figure 7 This is an SEM picture of the SSZ-13 molecular sieve membrane prepared in Comparative Example 2, where a is its cross section and b is its surface.

[0037] Figure 8 This is an SEM image of the composite structure of the alumina support TiO2 transition layer doped with nano-scale NaA molecular sieve fragments, where a is its surface and b is its cross-section.

[0038] Figure 9 This is an SEM image of the NaA molecular sieve membrane prepared in Example 2, where a is its surface and b is its cross-section. DETAILED DESCRIPTION

[0039] Example 1 Preparation of SSZ-13 Molecular Sieve Membrane

[0040] (1) Preparation of nano-sized SSZ-13 molecular sieve fragment sol:

[0041] (a) Using water as the solvent, a planetary high-energy ball mill was used to crush SSZ-13 molecular sieve crystals at a speed of 450 r / min. After 3 h of ball milling, a suspension was obtained.

[0042] (b) The suspension was centrifuged in a high-speed centrifuge at a speed of 11000 r / min. After centrifugation for 10 min, the supernatant was collected as the fragment sol. The particle size of the SSZ-13 molecular sieve fragments was 20 to 50 nm, and their morphology was as follows: Figure 1 shown.

[0043] (2) Preparation of TiO2 transition layer doped with nano-sized SSZ-13 molecular sieve fragments:

[0044] (a) The fragment sol having a fragment concentration of 0.4 wt% was mixed with TiO2 sol, stirred for 6 h, and then ultrasonicated for 0.5 h for standby use.

[0045] (b) The mixture was evenly coated on the surface of an alumina hollow fiber support with an average pore size of 160 nm using an immersion coating method for 15 seconds over 6 coats. The fiber was dried for 12 hours in a 70% humidity, 60°C, and then calcined in a muffle furnace at 350°C for 2 hours. After natural cooling, a composite structure of an alumina support and a TiO2 transition layer doped with nano-sized SSZ-13 molecular sieve fragments was obtained. Figure 2 The composite structure of alumina support TiO2 transition layer doped with nano-sized SSZ-13 molecular sieve fragments is demonstrated.

[0046] (3) Preparation of SSZ-13 molecular sieve membrane:

[0047] (a) The prepared composite structure was placed in a reactor and the molecular sieve synthesis solution was slowly added. The molar composition of the SSZ-13 molecular sieve membrane synthesis solution was as follows: 20T MAdaOH: 20.4NaOH: 0.45Al2O3: 105SiO2: 4400H2O. The SSZ-13 molecular sieve membrane was prepared by hydrothermal synthesis at 160°C for 24 hours. After synthesis, the SSZ-13 molecular sieve membrane was rinsed with deionized water to remove any residual synthesis solution and dried at 60°C.

[0048] (b) In an ozone atmosphere, the template agent in the SSZ-13 molecular sieve membrane was removed by high-temperature calcination at a temperature of 200°C for 48 h and a heating and cooling rate of 1°C / min. Finally, the SSZ-13 molecular sieve membrane was obtained. The specific morphology is shown in Figure 3 .

[0049] (c) The prepared SSZ-13 molecular sieve membrane was used for the separation of an equimolar CO2 / CH4 system. The separation results are shown in Table 1.

[0050] Table 1 Separation effect of SSZ-3 molecular sieve membrane prepared in Example 1 on equimolar CO2 / CH4 system

[0051] <![CDATA[CO2 permeability / 10 -7 mol m -2 s -1 Pa -1 > <![CDATA[ɑ CO2 / CH4 ]]> Sample 1 1.74 232 Sample 2 1.91 214

[0052] Operating conditions: room temperature, transmembrane pressure difference 0.1 MPa.

[0053] Comparative Example 1 Preparation of SSZ-13 Molecular Sieve Membrane

[0054] (1) Preparation of SSZ-13 molecular sieve seed crystals:

[0055] (a) SSZ-13 molecular sieve crystals were crushed in a planetary high-energy ball mill at a speed of 450 r / min. A suspension was obtained after ball milling for 3 h.

[0056] (b) The nanoparticle suspension was centrifuged in a high-speed centrifuge at a speed of 11,000 r / min for 10 min, and the precipitate was used as SSZ-13 molecular sieve seed crystals.

[0057] (2) Preparation of TiO2 transition layer doped with SSZ-13 molecular sieve seeds:

[0058] (a) The SSZ-13 molecular sieve seed crystals were doped into the TiO2 sol at a final concentration of 0.5 wt%, stirred for 6 h, and then sonicated for 0.5 h for standby use.

[0059] (b) The above solution was evenly coated on the surface of an alumina hollow fiber support with an average pore size of 160 nm using the dip-coating method for 15 seconds over 6 coats. The fiber was dried for 12 hours in a 70% humidity, 60°C, and then calcined in a muffle furnace at 350°C for 2 hours. After natural cooling, a composite structure of an alumina support and a TiO2 transition layer doped with SSZ-13 molecular sieve seeds was obtained. Figure 4 The composite structure of TiO2 transition layer on alumina support doped with SSZ-13 molecular sieve seeds is demonstrated.

[0060] (3) Preparation of SSZ-13 molecular sieve membrane:

[0061] (a) The prepared composite structure was placed in a reactor and the molecular sieve synthesis solution was slowly added. The molar composition of the SSZ-13 molecular sieve membrane synthesis solution was as follows: 20TMAdaOH: 20.4NaOH: 0.45Al2O3: 105SiO2: 4400H2O. The SSZ-13 molecular sieve membrane was prepared by a hydrothermal synthesis method at a temperature of 160°C and a synthesis time of 96 hours. After synthesis, the SSZ-13 molecular sieve membrane was rinsed with deionized water to remove any residual synthesis solution and then dried at 60°C.

[0062] (b) In an ozone atmosphere, the template agent in the SSZ-13 molecular sieve membrane was removed by high-temperature calcination at a temperature of 200°C for 48 h and a heating and cooling rate of 1°C / min. Finally, the SSZ-13 molecular sieve membrane was obtained. The specific morphology is shown in Figure 5 .

[0063] (c) The prepared SSZ-13 molecular sieve membrane was used for the separation of an equimolar CO2 / CH4 system. The separation results are shown in Table 2.

[0064] Table 2 Comparative Example 1 Separation effect of SSZ-3 molecular sieve membrane on equimolar CO2 / CH4 system

[0065] <![CDATA[CO2 permeability / 10 -7 mol m -2 s -1 Pa -1 > <![CDATA[ɑ CO2 / CH4 ]]> Sample 3 2.61 2 Sample 4 1.80 8.6

[0066] Operating conditions: room temperature, transmembrane pressure difference 0.1 MPa.

[0067] Comparative Example 2 Preparation of SSZ-13 Molecular Sieve Membrane

[0068] (1) Preparation of TiO2 transition layer:

[0069] The TiO2 sol was uniformly coated on the surface of an alumina hollow fiber support with an average pore size of 160 nm using the dip-coating method. The coating lasted 15 seconds and was repeated twice. The product was dried for 12 hours in a working environment with a humidity of 70% and a temperature of 60°C. After that, it was calcined in a muffle furnace at 450°C for 2 hours and naturally cooled to obtain the TiO2 transition layer on the alumina support. Figure 6 The microstructure of the TiO2 transition layer on the alumina support is shown.

[0070] (2) Preparation of SSZ-13 molecular sieve membrane:

[0071] (a) Nano-sized SSZ-13 molecular sieve fragments were coated on the surface of the TiO2 transition layer of the alumina support by vacuum suction. The method for obtaining the nano-sized SSZ-13 molecular sieve fragments was the same as that in Example 1. The composite structure was obtained by vacuum suction for 40 minutes under a negative pressure of -0.1 MPa and dried for 12 hours in a working environment with a humidity of 70% and a temperature of 60°C.

[0072] (b) The prepared composite structure was placed in a reactor and the molecular sieve synthesis solution was slowly added. The molar composition of the SSZ-13 molecular sieve membrane synthesis solution was as follows: 20TMAdaOH: 20.4NaOH: 0.45Al2O3: 105SiO2: 4400H2O. The SSZ-13 molecular sieve membrane was prepared using a hydrothermal synthesis method at a temperature of 160°C and a synthesis time of 96 hours. After synthesis, the SSZ-13 molecular sieve membrane was rinsed with deionized water to remove any residual synthesis solution and then dried at 60°C.

[0073] (c) In an ozone atmosphere, the template agent in the SSZ-13 molecular sieve membrane was removed by high-temperature calcination at 200°C for 48 h with a heating and cooling rate of 1°C / min. Finally, the SSZ-13 molecular sieve membrane was obtained. The specific morphology is shown in Figure 7 .

[0074] (d) The prepared SSZ-13 molecular sieve membrane was used for the separation of equimolar CO2 / CH4 system, and there was no separation selectivity.

[0075] Example 2 Preparation of NaA molecular sieve membrane

[0076] (1) Preparation of nanoscale NaA molecular sieve fragment sol:

[0077] (a) NaA molecular sieve crystals were crushed in a planetary high-energy ball mill at a speed of 350 r / min. A suspension was obtained after ball milling for 3 h.

[0078] (b) The suspension was centrifuged in a high-speed centrifuge at a speed of 11,000 r / min for 10 min. The supernatant was collected as the fragment sol. The particle size analyzer showed that the D50 particle size of the supernatant was 80 nm.

[0079] (2) Preparation of TiO2 transition layer doped with nano-scale NaA molecular sieve fragments:

[0080] (a) The fragment sol having a fragment concentration of 1 wt% was mixed with TiO2 sol, stirred for 6 h, and then ultrasonicated for 0.5 h before use.

[0081] (b) The mixture was evenly coated on the surface of an alumina hollow fiber support with an average pore size of 300 nm using the dip-coating method for 15 seconds and two coats. The fiber was then dried in a 60°C oven for 24 hours and calcined in a muffle furnace at 350°C for 2 hours. After natural cooling, a composite structure of an alumina support and a TiO2 transition layer doped with nanoscale NaA molecular sieve fragments was obtained. Figure 8 The composite structure of TiO2 transition layer on alumina support doped with nano-sized NaA molecular sieve fragments is demonstrated.

[0082] (3) Preparation of NaA molecular sieve membrane:

[0083] (a) Sodium metaaluminate and sodium silicate were dissolved in deionized water respectively. After both solutions were clarified, the silicon source solution was slowly added to the aluminum source and stirred until a homogeneous phase was formed to obtain a synthetic mother solution of NaA molecular sieve with a composition of SiO2:Al2O3:Na2O:H2O=1:1.2:2.5:150. The prepared composite structure was placed in a reactor, and the synthetic mother solution was poured in and the reactor was sealed to carry out a hydrothermal synthesis reaction. The reaction time was 4h and the reaction temperature was 100℃. After the reaction was completed, the membrane tube was taken out and the pH value was adjusted to 7 with deionized water. It was placed in a 50℃ oven for drying to obtain a molecular sieve membrane. The morphology is shown in FIG. Figure 9 .

[0084] (b) Repeat the above steps to prepare a total of 5 NaA molecular sieve membranes.

[0085] (c) The synthesized molecular sieve membrane was characterized by pervaporation of an ethanol solution containing 10 wt.% water at an operating temperature of 75°C. The results are shown in Table 3.

[0086] Table 3 Characterization results of the pervaporation separation performance of the NaA molecular sieve membrane synthesized in Example 2

[0087] Separation factor <![CDATA[Flux / kg·m -2 ·h -1 > Sample 5 7752 11.15 Sample 6 >10000 14.14 Sample 7 >10000 10.43 Sample 8 >10000 10.35 Sample 9 >10000 10.44

[0088] Operating conditions: System: 10 wt% ethanol aqueous solution; Operating temperature: 75°C Example 3 Preparation of CHA molecular sieve membrane

[0089] (1) Preparation of nano-scale CHA molecular sieve fragment sol:

[0090] (a) CHA molecular sieve crystals were crushed in a planetary high-energy ball mill at a speed of 400 r / min. A suspension was obtained after ball milling for 2.5 h.

[0091] (b) The suspension was centrifuged in a high-speed centrifuge at a speed of 12,000 r / min for 15 minutes. The supernatant was obtained as the fragment sol. The particle size analyzer showed that the D50 particle size of the supernatant was 50 nm.

[0092] (2) Preparation of TiO2 transition layer doped with nano-sized CHA molecular sieve fragments:

[0093] (a) The fragment sol having a fragment concentration of 0.8 wt% was mixed with TiO2 sol, stirred for 5 h, and then ultrasonicated for 1 h before use.

[0094] (b) The sol was evenly coated onto the surface of a mullite tubular support with an average pore size of 200 nm using the dip-coating method for 10 seconds over four coats. The support was then dried in a 50°C oven for 48 hours and calcined in a muffle furnace at 450°C for 3 hours. The support was then naturally cooled to yield a composite structure of a mullite support doped with nanoscale CHA molecular sieve fragments and a TiO2 transition layer.

[0095] (3) Preparation of CHA molecular sieve membrane:

[0096] (a) Sodium metaaluminate and potassium hydroxide were dissolved in deionized water. Once the solution clarified, the desired aluminum source was obtained. Silica sol was slowly added to the aluminum source and stirred until a homogeneous phase was formed, yielding a CHA molecular sieve synthesis mother solution with a composition of SiO₂:Al₂O₃:K₂O:H₂O = 22:1.2:2.36:4400. The prepared composite structure was placed in a reactor, the synthesis mother solution was poured in, and the reactor was sealed for a hydrothermal synthesis reaction for 16 hours at 140°C. After the reaction was complete, the membrane tube was removed, the pH adjusted to 7 with deionized water, and the membrane was dried in a 50°C oven to yield a molecular sieve membrane.

[0097] (b) Repeat the above steps to prepare a total of 5 CHA molecular sieve membranes.

[0098] (c) The synthesized membrane was characterized by pervaporation of 10 wt% ethanol solution at an operating temperature of 75°C. The results are shown in Table 4.

[0099] Table 4 Characterization results of pervaporation separation performance of CHA molecular sieve membrane synthesized in Example 3

[0100] Separation factor <![CDATA[Flux / kg·m -2 ·h -1 > Sample 10 2237 3.32 Sample 11 3104 4.00 Sample 12 3888 4.07 Sample 13 3370 3.82 Sample 14 2122 4.27

[0101] Operating conditions: System: 10 wt% ethanol aqueous solution; Operating temperature: 75°C.

Claims

1. A method for synthesizing a molecular sieve membrane by doping molecular sieve fragments into a ceramic support transition layer, characterized in that: The steps include: (1) Preparation of nano-scale molecular sieve fragment sol: Using water as solvent, a planetary high-energy ball mill is used to crush the molecular sieve crystals to form nano-scale molecular sieve fragments, and then high-speed centrifugation is performed to obtain the supernatant to obtain the fragment sol; (2) Preparation of transition layer: The fragment sol and the ceramic sol are uniformly mixed to obtain a transition layer sol; the transition layer sol is then uniformly coated on the surface of the support, dried, and calcined at 200 to 800° C. for 0.5 to 24 hours to obtain a composite structure having a transition layer on the surface; (3) Preparation of molecular sieve membrane: placing the composite structure in the synthesis mother solution of the molecular sieve, and obtaining a membrane precursor through hydrothermal synthesis; (4) Activation of molecular sieve membrane: The membrane precursor is calcined at high temperature to remove the template agent, thereby obtaining the molecular sieve membrane.

2. The method according to claim 1, characterized in that The molecular sieve is CHA, SSZ-13, NaA, T or DDR.

3. The method according to claim 1, wherein The rotation speed of the planetary high-energy ball mill is 50-500 r / min, and the ball milling time is 1-10 h.

4. The method according to claim 1, wherein The particle size of the nano-scale molecular sieve fragments described in step (1) is 1 to 100 nm.

5. The method according to claim 1, wherein The high-speed centrifugation speed is 5000-100000 r / min, and the centrifugation time is 5-100 min.

6. The method according to claim 1, characterized in that The ceramic sol in step (2) is TiO2 sol, ZrO2 sol or Al2O3 sol.

7. The method according to claim 1, wherein The support body described in step (2) is a hollow fiber support body, a tubular support body, a multi-channel support body or a flat support body; preferably, the surface pore size of the support body is 50~5000 nm.

8. The method according to claim 1, characterized in that The support is made of a-Al2O3, mullite or YSZ.

9. The method according to claim 1, characterized in that In the fragment sol in step (1), the mass fraction of the nano-scale molecular sieve fragments is 0.2 to 2 wt%.

10. The method according to claim 1, characterized in that The coating method in step (2) is a dipping method, a spraying method or a vacuum suction method.

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