CN115725103B - ZIF-8 film and preparation method and application thereof - Google Patents

Abstract

The invention belongs to the technical field of MOF membrane preparation, and particularly relates to a ZIF-8 membrane, and a preparation method and application thereof. The preparation method of the ZIF-8 film comprises the following steps: s1, weighing metal salt and ligand, respectively dissolving, and stirring to obtain a metal salt solution and a ligand solution; s2, mixing the metal salt solution prepared in the step S1 with the ligand solution, stirring to obtain a reaction mother solution, then placing a polyacrylonitrile substrate into the reaction mother solution, and standing at room temperature to obtain a ZIF-8 film crude product; s3, soaking the crude product of the ZIF-8 film in methanol, and drying in vacuum to obtain the ZIF-8 film. Compared with the prior art, the ZIF-8 membrane with excellent gas separation performance is simpler and more convenient in operation steps and mild in reaction conditions, and in addition, the water is selected as a solvent, so that the whole reaction is more environment-friendly and safer, and the large-scale production is facilitated.

Description

A ZIF-8 membrane and its preparation method and application

Technical Field

The invention belongs to the technical field of MOF film preparation, and specifically relates to a ZIF-8 film and a preparation method and application thereof.

Background Art

As a green energy source, _H2 has the advantages of high energy density, clean combustion process and good sustainability. In recent years, the market demand for _H2 has been increasing. However, _H2 always coexists with other light gases such as _CO2 , _N2 , _CH4, etc., which are by-products in different industrial production processes. Effective separation and purification of _H2 from the mixture of these gases is very important in industry. At present, polymer membranes are mainly used in industrial membrane separation technology. However, the trade-off effect between gas permeability and selectivity in polymer membranes has greatly inhibited their widespread application. Porous materials have been widely used in the preparation of membrane materials due to their low density, high porosity, large specific surface area and inherent adsorption properties. Among them, metal-organic frameworks (MOFs) with highly diverse structures and pore sizes and specific adsorption affinity have become new crystalline microporous materials for the preparation of advanced molecular sieve membranes. Polycrystalline membranes prepared based on metal-organic framework materials can effectively break through the Robertson upper limit and show great potential in gas separation applications.

Due to its small pore size, good chemical stability and simple synthesis process, ZIF-8 polycrystalline membranes are one of the most widely studied MOF membranes for gas separation. Hou et al. prepared an ultrathin ZIF-8 membrane with a layer thickness of 400 nm on a relatively cheap and flexible polyvinylidene fluoride (PVDF) hollow fiber support by coating TiO ₂ nanoparticles on the modified PVDF support and then treating it with APTES. The H ₂ permeability reached 60,000 GPU (GPU = gas permeation unit) and the ideal H ₂ /CO ₂ selectivity reached 7. Wang et al. prepared an ultrathin ZIF-8 membrane on a flexible polypropylene (PP) substrate by fast current-driven synthesis, with an ideal H ₂ /N ₂ selectivity of 4.4 and a selectivity of 122 for a binary gas mixture of C ₃ H ₆ /C ₃ H ₈ . Recently, Jiang et al. used phytic acid (PA) to modify the polymer substrate and soaked it in metal ion solution, and then prepared MOF membranes such as ZIF-8, ZIF-7, and CuBTC in a methanol system by the reverse diffusion method, among which the ZIF-8 membrane had an ideal H ₂ /N ₂ selectivity of 11.9 and a C ₃ H ₆ /C ₃ H ₈ binary gas mixture selectivity of 150. In order to promote its industrial implementation, it is crucial to develop a reliable and environmentally friendly membrane manufacturing method to produce high-quality ZIF-8 membranes with good mechanical strength.

Summary of the invention

The present invention aims to provide a ZIF-8 membrane and a preparation method and application thereof. Compared with the prior art, the ZIF-8 membrane preparation method is easier to operate, has simple equipment, mild and environmentally friendly reaction conditions, short synthesis time, high production efficiency, and the prepared ZIF-8 membrane has uniform crystal size, high flux, excellent membrane stability and _H2 separation and purification performance.

In order to achieve the above object, the present invention adopts the following technical scheme: a method for preparing a ZIF-8 membrane, comprising the following steps:

S1. Weigh a metal salt and a ligand, dissolve them separately, and stir to obtain a metal salt solution and a ligand solution;

S2, mixing the metal salt solution and the ligand solution obtained in step S1, stirring to obtain a reaction mother solution, then placing the polyacrylonitrile substrate into the reaction mother solution, and standing at room temperature to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol and vacuum dry it to obtain the ZIF-8 membrane.

Furthermore, the dissolving solvent of the metal salt and the ligand is water.

Furthermore, the metal salt is zinc nitrate hexahydrate, and the ligand is 2-methylimidazole.

Furthermore, the concentration of the metal salt solution is 0.0093-0.014 mmol/mL; the concentration of the ligand solution is 0.69-1.04 mmol/mL.

Furthermore, the stirring in step S1 is performed at room temperature at 200 to 500 r/min for 8 to 15 min.

Furthermore, in the step S2, the stirring time is 20 to 50 seconds; and the standing time is 12 to 24 hours.

Furthermore, the immersion time in methanol in step S3 is 12 to 36 hours; the vacuum drying temperature is 60 to 80° C., and the vacuum drying time is 8 to 12 hours.

The invention also provides a ZIF-8 membrane prepared by the preparation method.

The present invention also provides an application of the ZIF-8 membrane prepared by the preparation method in gas separation, wherein the gas is a mixed gas containing _H2 .

The present invention uses water as a reaction solvent at room temperature and uses a polyacrylonitrile material as a substrate. Since the polyacrylonitrile substrate contains a large number of highly electronegative cyano groups, metal ions can be attracted to anchor the metal ions on the substrate as nucleation sites, inducing heterogeneous nucleation of ZIF-8 on the substrate. At the same time, 2-methylimidazole deprotonates faster in the water system, which is conducive to the formation of ZIF-8 and coordinates with the metal ions on the substrate, so that a continuous and dense (1,1,1) oriented ZIF-8 film is obtained on the polyacrylonitrile substrate in the system, exposing the six-membered ring window of ZIF-8, and obtaining a better gas flux while maintaining the molecular sieving performance.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The present invention uses an inorganic metal as the center, 2-methylimidazole as a ligand, water as a solvent, and polyacrylonitrile as a supporting substrate at room temperature to synthesize a ZIF-8 membrane in situ using a one-step method. The prepared ZIF-8 membrane has uniform crystal size and strong stability, and has excellent separation performance for _H2 , wherein the selectivity for _H2 / _CO2 , _H2 / _N2 , and _H2 / _CH4 gases can reach 15.13, 8.72, and 3.72, respectively.

(2) The present invention adopts a one-step method to prepare a ZIF-8 membrane with excellent gas separation performance. Compared with the existing ZIF-8 membrane preparation method, the operation steps are simpler and more convenient, and the reaction conditions are mild. In addition, the present invention uses water as the reaction solvent, making the entire reaction process more green, environmentally friendly and safe, which is conducive to large-scale industrial production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM image of the ZIF-8 membrane prepared in Example 1 of the present invention.

FIG. 2 is a SEM image of the ZIF-8 membrane prepared in Example 2 of the present invention.

FIG. 3 is a SEM image of the ZIF-8 membrane prepared in Example 3 of the present invention.

FIG4 is a cross-sectional SEM image of the ZIF-8 membrane prepared in Example 2 of the present invention.

FIG5 is an XRD spectrum of the ZIF-8 film prepared in Example 2 of the present invention.

FIG6 is a graph showing the single gas permeability performance of the ZIF-8 membrane prepared in Example 2 of the present invention (left) and a graph showing the separation selectivity data (right).

DETAILED DESCRIPTION

The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

In the embodiments and comparative examples, the experimental methods used are conventional methods unless otherwise specified, and the materials, reagents, etc. used are all commercially available unless otherwise specified.

Example 1: ZIF-8 membrane of the present invention and preparation method thereof

S1. Weigh 0.18 g of zinc nitrate hexahydrate and 3.7 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 200 r/min for 8 min to obtain a metal salt solution and a ligand solution;

S2, adding the metal salt solution to the ligand solution, stirring with a glass rod for 20 seconds to mix evenly, obtaining a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 12 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 12 hours, and vacuum dry it at 60° C. for 8 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane is continuous and dense, with good symbiosis.

Example 2: ZIF-8 membrane of the present invention and preparation method thereof

S1. Weigh 0.18 g of zinc nitrate hexahydrate and 3.7 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 300 r/min for 10 min to obtain a metal salt solution and a ligand solution;

S2, adding the above metal salt solution to the ligand solution, stirring with a glass rod for 30 seconds to mix evenly, to obtain a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 18 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 24 hours, and vacuum dry it at 60° C. for 10 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane is continuous and dense, with good symbiosis.

Example 3: ZIF-8 membrane of the present invention and preparation method thereof

S1. Weigh 0.27 g of zinc nitrate hexahydrate and 5.55 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 500 r/min for 15 min to obtain a metal salt solution and a ligand solution;

S2, adding the metal salt solution to the ligand solution, stirring with a glass rod for 50 seconds to mix evenly, obtaining a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 24 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 36 hours, and vacuum dry it at 80° C. for 12 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane is continuous and dense, with good symbiosis.

Example 4: ZIF-8 membrane of the present invention and preparation method thereof

S1. Weigh 0.18 g of zinc nitrate hexahydrate and 3.7 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 500 r/min for 15 min to obtain a metal salt solution and a ligand solution;

S2, adding the above ligand solution to the metal salt solution, stirring with a glass rod for 50 seconds to mix evenly, obtaining a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 24 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 36 hours, and vacuum dry it at 80° C. for 12 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane is continuous and dense, with good symbiosis.

Comparative Example 1

S1. Weigh 0.18 g of zinc nitrate hexahydrate and 3.7 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 300 r/min for 10 min to obtain a metal salt solution and a ligand solution;

S2, adding the metal salt solution to the ligand solution, stirring with a glass rod for 30 seconds to mix evenly, obtaining a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 6 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 24 hours, and vacuum dry it at 60° C. for 10 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane had defects, was not dense, and leaked in the gas permeation test.

Compared with Example 2, the difference is that the standing time at room temperature is 6 hours.

Comparative Example 2

S1. Weigh 0.36 g of zinc nitrate hexahydrate and 7.4 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 300 r/min for 10 min to obtain a metal salt solution and a ligand solution;

S2, adding the above metal salt solution to the ligand solution, stirring with a glass rod for 30 seconds to mix evenly, to obtain a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 18 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 24 hours, and vacuum dry it at 60° C. for 10 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane had defects, was not dense, and leaked in the gas permeation test.

Compared with Example 2, the difference is that the concentration of the metal salt solution is not in the range of 0.0093 to 0.014 mmol/mL, and the concentration of the ligand solution is not in the range of 0.69 to 1.04 mmol/mL.

Comparative Example 3

S1. Weigh 0.18 g of zinc nitrate hexahydrate and 3.7 g of 2-methylimidazole, respectively, and dissolve them in 32 mL of methanol. Stir for 10 min at room temperature and a stirring intensity of 300 r/min to obtain a metal salt solution and a ligand solution.

S2, adding the above metal salt solution to the ligand solution, stirring with a glass rod for 30 seconds to mix evenly, to obtain a reaction mother solution, then placing the pre-cleaned polyacrylonitrile substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 18 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 24 hours, and vacuum dry it at 60° C. for 10 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane had defects, was not dense, and leaked in the gas permeation test.

Compared with Example 2, the difference is that the solvent of the metal salt solution and the ligand solution is methanol.

Comparative Example 4

S1. Weigh 0.18 g of zinc nitrate hexahydrate and 3.7 g of 2-methylimidazole and dissolve them in 32 mL of deionized water respectively, and stir them at room temperature and a stirring intensity of 300 r/min for 10 min to obtain a metal salt solution and a ligand solution;

S2, adding the above metal salt solution to the ligand solution, stirring with a glass rod for 30 seconds to mix evenly, obtaining a reaction mother solution, then placing the pre-cleaned polypropylene substrate on a mold and vertically placing it into the reaction mother solution, and standing at room temperature for 18 hours to obtain a ZIF-8 membrane crude product;

S3. Soak the crude ZIF-8 membrane product in methanol for 24 hours, and vacuum dry it at 60° C. for 10 hours to obtain a ZIF-8 membrane.

The obtained ZIF-8 membrane had defects, was not dense, and leaked in the gas permeation test.

Compared with Example 2, the difference is that the base material selected is polypropylene.

Test Example 1: Single gas permeation and mixed gas separation experiment

The ZIF-8 membranes prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to single gas permeation and mixed gas separation tests according to the following methods.

Before the permeation test of various gases or mixed gases, the ZIF-8 membrane needs to be activated at 80°C for 12 hours to remove various impurities adsorbed on the membrane and ensure the reliability of the permeation data obtained. In order to measure the permeability of a single gas and its binary mixture, the prepared ZIF-8 membrane was sealed in the permeation module using an O-ring at room temperature. The pressure on the feed side and the permeate side was maintained at 1 bar, and the total gas flow rate was controlled at 40 mL min ^-1 . Ar was used as a purge gas on the permeate side to keep the concentration of the permeate gas at a low level, thereby providing a driving force for permeation.

In the permeation test of a single component gas, the permeability of the membrane, Pi, is defined as:

where _Ni is the permeation rate of component i (mols ^-1 ), A is the effective area of the membrane being tested ( ^m2 ), and _pi is the transmembrane pressure (Pa).

The separation selectivity is defined by calculating the single component permeability:

In the separation selectivity test of two-component mixed gas, the gas concentration on the permeate side is measured by a calibrated gas chromatograph (SHIMADZU GC-2014C). In this system, the separation factor of the gas is defined as:

Wherein, α _i,j represents the separation factor of component i to component j; Yi _(j) and Xi _(j) represent the content of gas component i or j on the permeate side and the feed side, respectively.

Table 1 Summary of gas separation data

The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical ideas disclosed by the present invention shall still be covered by the claims of the present invention.

Claims (4)

1. An application of a ZIF-8 membrane in gas separation, characterized in that the ZIF-8 membrane is prepared by the following steps: S1. Weigh 0.18 g of metal salt and 3.7 g of ligand, dissolve them in 32 mL of solvent respectively, and stir to obtain a metal salt solution and a ligand solution; Alternatively, 0.27 g of metal salt and 5.55 g of ligand are weighed and dissolved in 32 mL of solvent respectively, and stirred to obtain a metal salt solution and a ligand solution; S2, mixing the metal salt solution and the ligand solution obtained in step S1, stirring to obtain a reaction mother solution, then placing the polyacrylonitrile substrate into the reaction mother solution, and standing at room temperature for 12 to 24 hours to obtain a ZIF-8 membrane crude product; S3, soaking the crude ZIF-8 membrane product in methanol, and vacuum drying to obtain a ZIF-8 membrane; The solvent used in the metal salt solution and the ligand solution is water; The metal salt is zinc nitrate hexahydrate, and the ligand is 2-methylimidazole; The ZIF-8 membrane is used to separate H ₂ /CO ₂ , H ₂ /N ₂ or H ₂ /CH ₄ . 2. Application of ZIF-8 film in gas separation as claimed in claim 1, characterised in that the stirring in step S1 is stirring at room temperature 200 ~ 500 r / min for 8 ~ 15min. 3. Application of ZIF-8 film in gas separation as claimed in claim 1, characterised in that the stirring time in the step S2 is 20 to 50 s. 4. The use of the ZIF-8 membrane in gas separation as claimed in claim 1, characterized in that the immersion time in methanol in step S3 is 12 to 36 hours; the vacuum drying temperature is 60 to 80°C, and the vacuum drying time is 8 to 12 hours.