Molecular sieve coating modification method of molecular sieve ZIF-8 composite material

Abstract

The invention belongs to the technical field of molecular sieve modified synthesis, and provides a molecular sieve coating modification method of a molecular sieve (at) ZIF-8 composite material in order to solve the problem of low methane / nitrogen selectivity of the existing molecular sieve. The method comprises the following steps: pretreating a molecular sieve with a surfactant, and centrifugally separating; dissolving Zn (NO3) 26H2O in an ethanol / water mixed solution, and adding the treated molecular sieve to anchor Zn < 2 + >; then adding an ethanol / water mixed solution of 2-methylimidazole, ensuring that the size of the ZIF-8 is relatively small by excessive ligands, and continuously stirring for two hours at normal temperature to finish the final growth of the ZIF-8 shell layer; and washing the reacted product with an ethanol / water mixed solution, filtering, and drying to obtain the final product. According to the invention, a material with molecular sieves 13X and 5A as a core and ZIF-8 as a shell layer is obtained. According to the technical scheme, after modification treatment, the adsorption ratio becomes 2.79, the selectivity is improved to 3.54, and good adsorption performance is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of molecular sieve modification and synthesis, and specifically relates to a method for molecular sieve coating modification of a molecular sieve@ZIF-8 composite material. Background Art

[0002] The efficient utilization of low-concentration coalbed methane can effectively alleviate global environmental pollution and greenhouse effect, and help improve the energy structure. In the process of coalbed methane enrichment, pressure swing adsorption separation technology has the characteristics of low energy consumption, simple process and high efficiency, and is considered to be a very efficient industrial application technology. The selection of PSA technology adsorbent is particularly important. At present, the research on low-concentration CH4 adsorbents mainly focuses on materials such as activated carbon, molecular sieves and MOFs (Metal Organic Frameworks). However, in actual applications, there are some disadvantages of a single material that hinder its application.

[0003] ZIF-8, as one of the most widely studied typical ZIF compounds, is a hydrophobic material. Its core-shell structure composite materials have attracted widespread attention in recent years due to their low price and easy synthesis. They can be used in a variety of application research such as gas separation, heterogeneous catalysis, electronic and optical sensors, etc.

[0004] For example, “Preparation of adsorbent material ZIF-8 and its methane storage performance” Petrochemicals, 2015.44.6 disclosed that “ZIF-8” was prepared by hydrothermal synthesis and can adsorb and store methane under 10 MPa.

[0005] The core-shell structure consists of a guest particle enclosed in the outer layer of another guest particle. It is widely regarded as the most convenient and effective way to utilize the synergistic effect between two materials. As a class of commercial adsorbents with high chemical / thermal stability and low cost, ZIF-8 molecular sieve materials have been used in gas separation for a long time and are an adsorbent with great industrial application potential. Unfortunately, most molecular sieves have low selectivity in the methane / nitrogen gas adsorption separation process and are generally hydrophilic materials.

[0006] The prior art (Insight into construction and growth process of core-shell KCHA@ZIF-8 composites, Microporous and Mesoporous Materials, Micropor. Mesopor. Mater. Pub Date : 2024-04-11) discloses that KCHA (zeolite) is used as the core and coated with ZIF-8 (MOF), and the KCHA@ZIF-8 composite material is successfully prepared by a two-step temperature control and surfactant pretreatment method. The adsorption performance of KCHA core for CH4 / N2 separation is confirmed. This study will provide a useful reference for the synthesis of zeolite@MOF composites in the future.

[0007] The document states that KCHA (zeolite) has good gas (CH4 and N2) adsorption performance, but poor hydrophobic performance. If in an environment with a lot of water, the adsorption sites of KCHA (zeolite) are easily occupied by water molecules, resulting in poor gas adsorption performance. Therefore, by coating ZIF-8 (MOF), the hydrophobic performance of the material is improved while maintaining the gas adsorption performance.

[0008] In actual production, KCHA (zeolite) is difficult to obtain, while molecular sieves such as 5A and 13X are industrial products with large quantities and are easy to obtain.

[0009] 5A molecular sieve is a calcium aluminosilicate. The uniform pore size is about 5×10 -7 mm, bulk density is 700~800 kg / m 3 , with a specific surface area of ​​750-800 m 2 / g, porosity is 47%, mechanical strength is greater than 95%, the adsorption capacity for water is about 21.5%, the adsorption capacity for carbon dioxide is 1.5%, and it has a co-adsorption effect on acetylene and other hydrocarbons while adsorbing water and carbon dioxide. 13X is a sodium aluminosilicate with a uniform pore size of about 10×10 -7 mm, bulk density is 600~700 kg / m 3 , with a specific surface area of ​​800~1000 m 2 / g, the porosity is 50%, the mechanical strength is greater than 90%, the adsorption capacity for water is about 28.5%, the adsorption capacity for carbon dioxide is 2.5%, and it has a co-adsorption effect on acetylene and other hydrocarbons while adsorbing water and carbon dioxide.

[0010] Compared with the two adsorbents, the adsorption performance of 13X molecular sieve is better than that of 5A molecular sieve. Studies have shown that the adsorption performance of these two molecular sieves for CH4 and N2 is average. Figure 5As shown, the methane-nitrogen adsorption ratio of the original molecular sieve is 1.74, and the selectivity is 1.80. The adsorption data proves that the adsorption performance is general. In industrial production, the gas adsorption selectivity must be higher than 3 to be used, so the existing molecular sieve must be modified. Summary of the invention

[0011] In order to solve the problem of low methane / nitrogen selectivity of existing molecular sieves, the present invention provides a method for molecular sieve coating modification of molecular sieve@ZIF-8 composite material. Our research goal is to combine the two to improve the methane / nitrogen selectivity of the molecular sieve and obtain hydrophobic properties at the same time.

[0012] The core-shell material proposed in the present invention is a molecular sieve as a base material, and a ZIF-8 shell is grown on the outer surface of the molecular sieve through a method of pretreatment with an amphoteric surfactant. The ZIF-8 shell material can promote the molecular sieve to improve the methane / nitrogen selectivity, can improve the methane / nitrogen selectivity of the composite material, and at the same time bring excellent hydrophobic properties.

[0013] The present invention is achieved through the following technical solution: A method for molecular sieve coating modification of a molecular sieve@ZIF-8 composite material, comprising the following steps: (1) Pretreatment of molecular sieve materials: The molecular sieve particles are first stirred in an aqueous solution of an amphoteric surfactant to pretreat the molecular sieve with the surfactant, and then centrifuged; (2) Growth of ZIF-8 shell material: Zn(NO3)2•6H2O was dissolved in an ethanol / water mixed solution, and the treated molecular sieve was added. The mixture was stirred at room temperature for half an hour. 2+ To anchor; A mixed solution of ethanol / water containing 0.43 mol / L 2-methylimidazole was added to the Zn 2+ In the anchoring solution, the excess ligand ensures that the size of ZIF-8 is small, and stirring is continued for two hours at room temperature so that a sufficient amount of ZIF-8 grows on the outer surface of the molecular sieve to complete the final growth of the ZIF-8 shell; The product after the reaction is washed with an ethanol / water mixed solution, filtered, and dried at 80° C. overnight to obtain the final product.

[0014] The amphoteric surfactant is one of dodecyl ethoxy sulfobetaine, dodecyl dimethyl hydroxypropyl sulfobetaine, dodecyl dimethyl sulfopropyl betaine, tetradecyl amide propyl hydroxypropyl sulfobetaine, and octadecyl dihydroxyethyl amine oxide.

[0015] The ratio of molecular sieve particles to the aqueous solution of amphoteric surfactant is: 1-4 g of molecular sieve is added into 20 mL of solution, and the solution is centrifuged after stirring without drying to obtain the pretreated molecular sieve.

[0016] The molecular sieves are 13X, 5A and the like.

[0017] The ZATE potential of the molecular sieve is negative. After being treated with amphoteric surfactants, the cationic end is connected to the molecular sieve, the long chain is wrapped around the outside of the molecular sieve, and the anionic end is used for subsequent connection with Zn 2+ .

[0018] In step (2), the mass ratio of the molecular sieve to the zinc precursor Zn(NO3)2•6H2O is in the range of 1:1 to 4:1. In step (2), the volume ratio of ethanol to water is in the range of 1:0 to 0:1.

[0019] The concentration of the amphoteric surfactant in the aqueous solution of the amphoteric surfactant is 0.3 wt %, and the stirring time is 10-30 minutes.

[0020] The present invention also provides a molecular sieve@ZIF-8 composite material obtained by using the method.

[0021] The present invention also provides the use of the molecular sieve@ZIF-8 composite material as a CH4 adsorbent in the separation and recovery of CH4.

[0022] like Figure 6 As shown, the ZIF-8 shell obtained by the above process has a looser structure than the KCHA@ZIF-8 composite material, which makes the subsequent gas separation kinetics better.

[0023] Through the above steps, the present invention obtains a material with molecular sieves 13X and 5A as the core and ZIF-8 as the shell, which has good (CH4 and N2) adsorption performance.

[0024] The innovation of the present invention is that the original molecular sieve 13X, 5A methane nitrogen adsorption ratio is 1.74, and the selectivity is 1.80. After the modification treatment of the technical solution of the present invention, the adsorption ratio becomes 2.79, and the selectivity is improved to 3.54. In industrial production, the adsorbent selectivity is required to be higher than 3 before it can be used. The selectivity is higher than 3, which means that the adsorbent has a significantly stronger adsorption capacity for the target gas than other gases, and can economically and effectively purify the gaseous mixture. In the prior art disclosed in the background technology, the ZIF-8 shell structure is dense, and only the hydrophobic property is improved, and there is no further improvement in the adsorption performance. In the present invention, a loosely structured ZIF-8 shell is obtained by a specific process modification, and we have made molecular sieves 13X and 5A have the opportunity to be used in industrial production of low-concentration methane enrichment. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0026] Figure 1 This is the XRD diffraction pattern of 13X@ZIF-8 prepared in Example 1; Figure 2 is the SEM image of Example 1; Figure 3 is the EDS spectrum of Example 1; Figure 4 The water contact angle images of each sample in Example 1 at 30 s; Figure 5 For Example 1, the adsorption isotherms of CH4 and N2 for each material at 298 K and the IAST selectivity of CH4 / N2 (50% / 50%, v / v) at 298 K were calculated by fitting the Dual-Site Langmuir model; Figure 6 It is a comparison diagram of the surface structure of the product mentioned in the background technology and the product of this solution. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0028] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs, and the disclosure and materials cited therein are hereby incorporated by reference.

[0029] Technical equivalents to the specific embodiments described that are apparent to those skilled in the art using no more than routine experimentation are intended to be encompassed by this application.

[0030] The experimental methods in the following examples are all conventional methods unless otherwise specified. The instruments and equipment used in the following examples are all conventional laboratory instruments and equipment unless otherwise specified; the experimental materials used in the following examples are all purchased from conventional biochemical reagent stores unless otherwise specified.

[0031] Example 1: Preparation method of 13X@ZIF-8 core-shell material 1.0 g of 13X particles were first stirred in an aqueous solution of 0.3 wt % dodecyl sulfopropyl betaine for 20 minutes for surfactant pretreatment.

[0032] Dissolve 1.0216 g of Zn(NO3)2•6H2O in 31.6 mL of ethanol / water solution, add the treated 13X, stir at room temperature for half an hour, then add 40 mL of ethanol / water solution containing 1.65 g of 2-methylimidazole, and continue stirring at room temperature for two hours. Filter, wash with ethanol, and dry at 80°C overnight.

[0033] XRD was conducted using a German Bruker D8 ADVANCE (λ=1.5418Å) with Cu-Kα as the target source. The working voltage and current were 40 kV and 40 mA respectively, and the range of 2θ = 5°-40° was selected with a step size of 0.02°. Figure 1 It can be seen that the composite material has diffraction peaks of both 13X and ZIF-8, which proves that the ZIF-8 structure is successfully grown in the preparation of 13X@ZIF-8 composite material in Example 1.

[0034] SEM and EDS were performed using a Hitachi SU8010 field emission scanning electron microscope with operating voltages of 3 kV and 15 kV respectively. The samples were sprayed with gold before testing. Figure 2 We can intuitively see that a large number of ZIF-8 particles grow on the outer surface of the molecular sieve and completely cover the molecular sieve, which proves that our scheme can successfully prepare core-shell 13X@ZIF-8 composite materials. Figure 3 It can be observed that the core molecular sieve containing mainly Si and Al elements in 13X@ZIF-8 is well concentrated inside, and Zn and N as the basic elements of ZIF-8 are evenly distributed in the whole material with a smaller concentration, which is more widely distributed than the molecular sieve. This is because ZIF-8 exists as a shell and accounts for a smaller proportion.

[0035] The water contact angle was measured by a water contact angle analyzer DSA100S. The sample was dried before testing and then pressed into a flat plate. Figure 4 It can be seen that the original 13X is a hydrophilic material. The composite material obtained by growing ZIF-8 on its outer surface has excellent hydrophobic properties, which also provides evidence for the successful preparation of the composite material.

[0036] The single-component gas adsorption isotherms of CH4 and N2 were measured by Micromeritics ASAP 2460. The sample was vacuum activated for 5 h at 423 K before testing, and then the single-component gas adsorption test of CH4 and N2 was carried out at 298 K. The pressure range was 0-100 KPa, and the sample mass was about 0.1 g. The IAST selectivity of CH4 / N2 (50% / 50%, v / v) at 298 K was then calculated by fitting the Dual-Site Langmuir model. Figure 5 It can be seen that the single-component adsorption capacity of CH4 and N2 of the composite material is between that of the original molecular sieve and ZIF-8, the adsorption capacity of the composite material is slightly lower than that of the original 13X, and the methane / nitrogen selectivity is improved compared with the original molecular sieve. These results also further prove the successful synthesis of the composite material.

[0037] Example 2: 4.0 g of 13X particles were first stirred in an aqueous solution of 0.3 wt % dodecyldimethylhydroxypropylsulfobetaine for 20 minutes for surfactant pretreatment.

[0038] Dissolve 1.0216 g of Zn(NO3)2•6H2O in 31.6 mL of aqueous solution, add the treated 13X, stir at room temperature for half an hour, then add 40 mL of aqueous solution containing 1.65 g of 2-methylimidazole, continue stirring at room temperature for two hours. Filter, wash with water, and dry at 80°C overnight.

[0039] Example 3: 2.0 g of 13X particles were first stirred in an aqueous solution of 0.3 wt % dodecyldimethylsulfonpropyl betaine for 20 minutes for surfactant pretreatment.

[0040] Dissolve 1.0216 g of Zn(NO3)2•6H2O in 31.6 mL of ethanol / water (1:1) solution, add the treated 13X, stir at room temperature for half an hour, then add 40 mL of ethanol / water (1:1) solution containing 1.65 g of 2-methylimidazole, and continue stirring at room temperature for two hours. Filter, wash with ethanol / water (1:1), and dry at 80 °C overnight.

[0041] Example 4: Preparation method of 5A@ZIF-8 core-shell material 1.0 g of 5A particles were first stirred in an aqueous solution of 0.3 wt % myristylamidopropyl hydroxypropyl sulfobetaine for 20 min for surfactant pretreatment.

[0042] Dissolve 1.0216 g of Zn(NO3)2•6H2O in 31.6 mL of ethanol solution, add the treated 5A, stir at room temperature for half an hour, then add 40 mL of ethanol solution containing 1.65 g of 2-methylimidazole, and continue stirring at room temperature for two hours. Filter, wash with ethanol, and dry at 80 °C overnight.

[0043] Example 5: 3.0 g of 5A particles were first stirred in an aqueous solution of 0.3 wt % octadecyl dihydroxyethyl amine oxide for 20 minutes for surfactant pretreatment.

[0044] Dissolve 1.0216 g of Zn(NO3)2•6H2O in 31.6 mL of ethanol / water (3:1) solution, add the treated 5A, stir for half an hour at room temperature, then add 40 mL of ethanol / water (3:1) solution containing 1.65 g of 2-methylimidazole, and continue stirring for two hours at room temperature. Filter, wash with ethanol / water (1:0.25) solution, and dry at 80 °C overnight.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for molecular sieve coating modification of molecular sieve@ZIF-8 composite material, characterized in that: The steps include: (1) Pretreatment of molecular sieve materials: The molecular sieve particles are first stirred in an aqueous solution of an amphoteric surfactant to pretreat the molecular sieve with the surfactant, and then centrifuged; (2) Growth of ZIF-8 shell material: Zn(NO3)2•6H2O was dissolved in an ethanol / water mixed solution, and the treated molecular sieve was added. The mixture was stirred at room temperature for half an hour. 2+ To anchor; A mixed solution of ethanol / water containing 0.43 mol / L 2-methylimidazole was added to the Zn 2+ In the anchoring solution, the excess ligand ensures that the size of ZIF-8 is small, and stirring is continued for two hours at room temperature so that a sufficient amount of ZIF-8 grows on the outer surface of the molecular sieve to complete the final growth of the ZIF-8 shell; The product after the reaction is washed with an ethanol / water mixed solution, filtered, and dried at 80° C. overnight to obtain the final product.

2. The method for molecular sieve coating modification of the molecular sieve@ZIF-8 composite material according to claim 1, characterized in that: The amphoteric surfactant is one of dodecyl ethoxy sulfobetaine, dodecyl dimethyl hydroxypropyl sulfobetaine, dodecyl dimethyl sulfopropyl betaine, tetradecyl amide propyl hydroxypropyl sulfobetaine, and octadecyl dihydroxyethyl amine oxide.

3. The method for molecular sieve coating modification of the molecular sieve@ZIF-8 composite material according to claim 1, characterized in that: The ratio of molecular sieve particles to the aqueous solution of amphoteric surfactant is: 1-4 g of molecular sieve is added into 20 mL of solution, and the solution is centrifuged after stirring without drying to obtain the pretreated molecular sieve.

4. The method for molecular sieve coating modification of the molecular sieve@ZIF-8 composite material according to claim 1, characterized in that: The molecular sieve is 13X or 5A.

5. The method for molecular sieve coating modification of the molecular sieve@ZIF-8 composite material according to claim 1, characterized in that: In step (2), the mass ratio of the molecular sieve to the zinc precursor Zn(NO3)2•6H2O is in the range of 1:1 to 4:

1.

6. The method for molecular sieve coating modification of the molecular sieve@ZIF-8 composite material according to claim 1, characterized in that: The volume ratio of ethanol to water in step (2) is 1:0 to 0:

1.

7. The method for molecular sieve coating modification of the molecular sieve@ZIF-8 composite material according to claim 1, characterized in that: The concentration of the amphoteric surfactant in the aqueous solution of the amphoteric surfactant is 0.3 wt %, and the stirring time is 10-30 minutes.

8. A molecular sieve@ZIF-8 composite material obtained by the method described in any one of claims 1 to 7.

9. Use of the molecular sieve@ZIF-8 composite material according to claim 8 as a CH4 adsorbent in the separation and recovery of CH4.

Images