CN114471187B - Method for preparing ZIF-8 molecular sieve membrane at low temperature - Google Patents
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Abstract
The invention provides a method for preparing a ZIF-8 molecular sieve membrane at low temperatureThe method comprises the following specific steps: firstly, sequentially filling a porous carrier modified by a zinc oxide precursor layer and precooled reaction liquid into a reaction kettle; then, inhibiting the excessively fast crystallization reaction of the ZIF-8 in a bulk phase through a low-temperature reaction, and ensuring that raw materials are sufficiently supplied at the interface of a reaction liquid and a carrier, so as to ensure the controllable nucleation and growth of a ZIF-8 film layer on the surface of the carrier; and after the reaction is finished, heating the reaction solution to room temperature, and washing and drying to obtain the ZIF-8 molecular sieve membrane. The ZIF-8 molecular sieve membrane prepared by the method has good connectivity, uniformity and thickness, and shows excellent C 3 H 6 /C 3 H 8 Separation Performance with different operating pressures and C 3 H 6 Stability of separation performance under partial pressure ratio conditions. In addition, the ZIF-8 molecular sieve membrane prepared by the method has good repeatability in structure and performance. In a word, the invention aims to solve the problem that the ZIF-8 molecular sieve membrane faces C 3 H 6 /C 3 H 8 The problems of stable operation, large-scale production and the like in industrial separation application provide a feasible solution.
Description
Technical Field
The invention belongs to the field of membrane separation, and particularly relates to a method for preparing a ZIF-8 molecular sieve membrane at low temperature and a molecular sieve membrane prepared by the method at C 3 H 6 /C 3 H 8 Use in gas separation.
Background
C 3 H 6 One of the most important basic raw materials in petrochemical industry is an important raw material for producing fine chemicals such as synthetic resins, synthetic fibers, synthetic rubbers, synthetic detergents, dyes, and intermediates thereof, and is a base stone in modern chemical industry. At present, C 3 H 6 The catalyst is mainly prepared by processes of crude oil cracking, propane dehydrogenation, methanol-to-olefin and the like, wherein various other components such as the same alkane and the like inevitably exist. To obtain polymer grade purity C 3 H 6 The cryogenic rectification technology with great energy consumption is adopted for C in industry 3 H 6 /C 3 H 8 And (5) separating. Statistically, the energy consumption of this term alone accounts for about 0.3% of the total global energy consumption. Therefore, development of C with low energy consumption and high efficiency 3 H 6 /C 3 H 8 Separation technology is considered to be one of seven chemical separation processes that can change the world. And itCompared with the prior art, the membrane separation technology has the advantages of energy conservation, high efficiency, simple operation, environmental protection and the like as a novel separation technology. Thus the development is oriented to C 3 H 6 /C 3 H 8 The separated membrane technology has important practical significance for energy conservation, emission reduction, consumption reduction and efficiency improvement in the chemical production process.
The ZIF-8 (Zeolic Imidazolate Framework-8) molecular sieve membrane is the C with the most potential reported at present 3 H 6 /C 3 H 8 Separating membrane material, which has effective pore diameter between C due to skeleton flexibility caused by ligand rotation 3 H 6 And C 3 H 8 The kinetic diameters of the molecules are in the same range, so that the two can be separated efficiently by strict size screening. C of current ZIF-8 membranes 3 H 6 /C 3 H 8 The separation performance has been competitive in commercial application, but the industrial application of ZIF-8 molecular sieve membranes still faces some urgent problems to be solved: (1) the ZIF-8 film has poor repeatability due to difficulty in ensuring uniformity; (2) ZIF-8 films are difficult to completely eliminate due to intergranular defects resulting in high voltage operation or C 3 H 6 The selectivity sharply decays under partial pressure change conditions. It is particularly emphasized that due to the ubiquitous presence of intergranular defects in ZIF-8 membrane structures, the microscopic defects present intergranular are further amplified under high pressure to form a large number of non-selective diffusion paths, making the reported ZIF-8 membranes very sensitive to pressure, and the increased operating pressure results in a drastic change in membrane separation performance. In addition, different olefin production processes also require ZIF-8 molecular sieve membranes at different C 3 H 6 Has stable separation selectivity under the condition of partial pressure ratio, thereby effectively ensuring the purity of the product.
The fundamental reason for the problems is that the ZIF-8 is crystallized too fast in a bulk phase, so that sufficient supply of raw materials at a carrier interface is difficult to guarantee, the nucleation and growth processes of a ZIF-8 film layer on the surface of a carrier cannot be effectively controlled, and finally the ZIF-8 film structure cannot be accurately regulated. Therefore, the development of a preparation process capable of strictly controlling the nucleation and growth processes of the ZIF-8 is urgently needed, the ZIF-8 membrane structure is accurately regulated and controlled, and a defect-free high-performance molecular sieve membrane is constructed to realize different purposesOperating conditions C 3 H 6 /C 3 H 8 The stable and efficient separation and the repeatable preparation of the high-quality ZIF-8 membrane can finally meet the requirements of practical industrial application.
Disclosure of Invention
The invention utilizes a low-temperature synthesis technology to prepare the ZIF-8 molecular sieve membrane, and solves the specific problems in industrial application. By adopting a low-temperature synthesis strategy (less than or equal to 10 ℃), the nucleation and growth processes of ZIF-8 are conveniently and accurately controlled, so that the ZIF-8 molecular sieve membrane with a good membrane structure is prepared on a commercial porous carrier modified by a ZnO precursor layer. The ZIF-8 molecular sieve membrane prepared by the technology has excellent C 3 H 6 /C 3 H 8 Separation performance, different operating pressures and C 3 H 6 The stability and good repeatability of the separation performance under the condition of partial pressure ratio effectively solve the key problems of stable operation, large-scale production and the like of the ZIF-8 molecular sieve membrane in industrial application.
The technical scheme of the invention is as follows:
a method for preparing a ZIF-8 molecular sieve membrane at low temperature comprises the following steps:
(1) Fixing the ZnO precursor layer modified porous carrier and then placing the fixed ZnO precursor layer modified porous carrier in a reaction kettle;
(2) Respectively precooling Zn to-80-10 DEG C 2+ Rapidly mixing the (zinc ion) solution and the Hmin (2-methylimidazole) solution to form a low-temperature precursor solution, filling the low-temperature precursor solution into the reaction kettle, and carrying out low-temperature reaction at the temperature of-80-10 ℃; the excessively fast crystallization process of the ZIF-8 in a bulk phase is inhibited through low-temperature reaction, and meanwhile, the raw material supply at the interface of a low-temperature precursor solution and a carrier is ensured to be sufficient, so that the nucleation and growth of a ZIF-8 film layer on the surface of the carrier are accurately controlled;
(3) And after the reaction is finished, heating the reaction solution to room temperature, and washing and drying to obtain the continuous and compact ZIF-8 molecular sieve membrane.
Preferably, the ZnO precursor layer modification process in step (1) includes a sol-gel method, a liquid phase growth method, a phase conversion method, a physical vapor deposition method, a chemical vapor deposition method, or an atomic layer deposition method, and further, the ZnO precursor layer modification process includes a sol-gel method or an atomic layer deposition method.
The porous carrier in the step (1) is preferably selected from the group consisting of porous metal, porous carbide, porous metal oxide and porous nonmetal oxide; the structure of the fiber reinforced plastic composite material comprises a flat plate structure, a tubular structure, a roll structure or a hollow fiber structure. Further, the kind of the porous carrier is porous metal oxide (porous alumina, porous zirconia, porous titania, and the like), and the porous carrier structure is a flat plate structure or a tubular structure. Further, the porous support is a tubular porous alumina.
Zn described in the step (2) is preferable 2+ The metal source of the solution is Zn 2+ Inorganic salts and Zn 2+ At least one of the complexes; zn 2+ The solvent of the solution and the Hmin solution is an organic solvent or a mixed solvent of organic solvent/water, wherein the organic solvent is at least one of methanol, ethanol, propanol, N-dimethylformamide and acetone. Further, the Zn 2+ The inorganic salt is one or more of zinc nitrate, zinc chloride, zinc bromide and zinc hydroxide, and the Zn 2+ The complex is one or more of zinc formate, zinc acetate, zinc acetylacetonate and isopropyl zinc. Further, zn 2+ The metal source of the solution is zinc acetate, zn 2+ The solvent of the solution and the Hmin solution is a methanol/water binary mixed solvent.
Preferably Zn in the low-temperature precursor solution in the step (2) 2+ The concentration of (b) is 1-100 mM; hmin and Zn 2+ The molar ratio of (A) to (B) is 2 to 500; the volume ratio of the organic solvent to the water in the solvent is more than or equal to 0.05. Further, zn 2+ The concentration of (A) is 2-10mM, hmin and Zn 2+ The molar ratio of (A) to (B) is 100 to 200; the volume ratio of the organic solvent to the water in the solvent is 0.1-1.
Preferably, in the step (2), the reaction kettle is placed into a low-temperature reactor filled with a coolant liquid which is pre-cooled to minus 80 to 10 ℃ or directly placed into a low-temperature box with the pre-set temperature of minus 80 to 10 ℃ for low-temperature reaction.
Preferably, the single reaction period of the low-temperature reaction in the step (2) is 10 min-120 h. Further, the single reaction period of the low-temperature reaction is 2-24 h.
Preferably, the temperature difference between the solution precooling temperature and the low-temperature reaction temperature in the step (2) is less than or equal to 20 ℃.
The low-temperature reaction temperature is-50 ℃ to 0 ℃.
The invention also provides a ZIF-8 molecular sieve membrane structure synthesized by the method, wherein the ZIF-8 molecular sieve membrane has adjustable grain size and crystallinity and excellent intergrowth state and thickness, the grain size is 30 nm-3 mu m, and the thickness is 50 nm-5 mu m. Further, the ZIF-8 molecular sieve membrane has a crystal grain size of 100nm to 1 μm and a membrane thickness of 100nm to 1 μm.
The invention also provides a ZIF-8 molecular sieve membrane synthesized by the method in C 3 H 6 /C 3 H 8 The application in gas separation, further, the gas feeding pressure is 1-30bar 3 H 6 The partial pressure ratio is 0.01-0.995, and the gas permeability is tested by adopting modes of introducing purge gas or vacuumizing and the like for permeation measurement. C 3 H 6 /C 3 H 8 Has a selectivity of 39 to 218, C 3 H 6 The permeation flux of (A) is 0.5-47 x 10 -8 mol m -2 s -1 Pa -1 And C is 3 H 6 /C 3 H 8 The separation performance has good repeatability.
Compared with the prior art, the invention has the following beneficial effects:
the ultralow temperature synthesis method adopted by the invention can effectively inhibit the ZIF-8 from crystallizing too fast in a bulk phase, ensure the sufficient raw material supply at a carrier interface, control the nucleation and growth of the ZIF-8 film layer on the surface of the carrier, further finely regulate the connectivity, uniformity and thickness of the ZIF-8 film layer, and construct a defect-free high-performance molecular sieve film. Therefore, the ZIF-8 molecular sieve membrane (1) prepared by the method has different operating pressures (1-30 bar) and C 3 H 6 C stable under the condition of partial pressure ratio (0.01-0.995) 3 H 6 /C 3 H 8 Separation performance; (2) has good membrane structure and C 3 H 6 /C 3 H 8 Separation performance repeatability; (3) has excellent C 3 H 6 /C 3 H 8 The separation performance and selectivity are as high as 218,C 3 H 6 The permeation flux is up to 47 multiplied by 10 -8 mol m -2 s -1 Pa -1 。
Drawings
FIG. 1 is an X-ray diffraction (XRD) pattern of a ZnO precursor layer-modified porous alumina support prepared by a sol-gel method in example 1;
FIG. 2 is a Scanning Electron Microscope (SEM) image of a ZnO precursor layer-modified porous alumina support prepared by a sol-gel process in example 1;
FIG. 3 is an energy dispersive X-ray (EDXs) plot of a ZnO precursor layer modified porous alumina support prepared by a sol-gel process in example 1;
FIG. 4 is an SEM image of a ZnO precursor layer-modified porous alumina support prepared by a atomic layer deposition method in example 2;
FIG. 5 is an XRD pattern of a ZIF-8 molecular sieve membrane prepared in example 3;
FIG. 6 is an SEM picture of a ZIF-8 molecular sieve membrane prepared in example 3;
FIG. 7 is an XRD pattern of a ZIF-8 molecular sieve membrane prepared in example 4;
FIG. 8 is an SEM image of a ZIF-8 molecular sieve membrane prepared in example 4;
FIG. 9 is an XRD pattern of a ZIF-8 molecular sieve membrane prepared in example 5;
FIG. 10 is an SEM image of a ZIF-8 molecular sieve membrane prepared in example 5;
FIG. 11 is an XRD pattern of a ZIF-8 molecular sieve membrane prepared in example 6;
FIG. 12 is an SEM image of a ZIF-8 molecular sieve membrane prepared in example 6;
FIG. 13 is an XRD pattern of ZIF-8 molecular sieve membrane prepared in example 7;
FIG. 14 is an SEM image of a ZIF-8 molecular sieve membrane prepared in example 7;
FIG. 15 shows C of ZIF-8 molecular sieve membrane in example 9 3 H 6 /C 3 H 8 Repeatability of separation performance;
FIG. 16 shows the C values of the ZIF-8 molecular sieve membranes of example 10 at different pressures 3 H 6 /C 3 H 8 Separation performance;
FIG. 17 shows ZIF-8 zeolite membranes in example 11 at different C 3 H 6 C at partial pressure ratio 3 H 6 /C 3 H 8 Separation performance;
FIG. 18 is an XRD pattern of a ZIF-8 molecular sieve membrane prepared by the low temperature seed crystal method in comparative example 1;
FIG. 19 is an SEM photograph of a ZIF-8 molecular sieve membrane prepared by a low-temperature seed crystal process in comparative example 1;
FIG. 20 is an XRD pattern for room-temperature preparation of ZIF-8 molecular sieve membrane in comparative example 2;
FIG. 21 is an SEM photograph of a room temperature preparation ZIF-8 molecular sieve membrane of comparative example 2;
FIG. 22 is an XRD pattern for preparation of ZIF-8 molecular sieve membrane at high temperature in comparative example 3;
FIG. 23 is an SEM photograph of a ZIF-8 zeolite membrane prepared at a high temperature in comparative example 3.
Detailed Description
The present invention will be described in further detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.
Example 1: preparation of ZnO precursor layer modified porous alumina carrier by sol-gel method
(1) Dispersing a zinc salt reagent into an organic solvent, heating to a specified temperature, stirring for a period of time, and then adding ethanolamine for glycidyl complexation to form stable ZnO sol. Detailed sol formulation process references a) j. Mater. Chem.a,2013,1,10635. B) nat. Commun.2017,8,406.
(2) Coating a ZnO sol layer on the porous alumina carrier, and then calcining at high temperature to obtain the porous carrier modified by the ZnO precursor layer, wherein the deposition process can be repeated for many times to ensure the compactness, uniformity and continuity of the zinc oxide precursor layer.
XRD (figure 1) shows that after deposition roasting, a ZnO modified layer is introduced on the surface of the porous alumina carrier. The SEM (fig. 2) and EDXS (fig. 3) results show that the ZnO modified layer is dense, uniform and continuous. It should be noted that the sol-gel method for preparing the ZnO precursor layer in this embodiment 1 is a well-established process flow in the industry, and does not belong to the scope of the present invention.
Example 2: preparation of ZnO precursor layer modified porous alumina carrier by adopting atomic layer deposition method
(1) The treated porous alumina support is placed in a deposition zone of an atomic layer deposition apparatus.
(2) Deposition is started after deposition parameters (deposition source type, deposition times, deposition time and the like) are set.
(3) And after the deposition is finished, obtaining the porous alumina carrier modified by the ZnO precursor layer.
The SEM (fig. 4) result shows that the ZnO precursor layer prepared by the atomic layer deposition method is dense, uniform and continuous, and it should be noted that the preparation of the ZnO precursor layer by the atomic layer deposition method in this embodiment 2 is a mature process flow in the industry, and does not belong to the protection scope of the present invention.
Example 3: preparation of ZIF-8 molecular sieve membrane by low-temperature synthesis technology
(1) 7mL of methanol was dispersed in 43mL of water, which was then divided into two equal-volume portions and charged into two reaction vessels, to which 0.051g of Zn (CH) was added 3 COO) 2 ·2H 2 O and 3.421g Hmim, which were then pre-cooled in a low temperature reactor pre-cooled to-5 ℃ for 10min.
(2) Zn (CH) pre-reduced to-5 DEG C 3 COO) 2 ·2H 2 And quickly mixing the O solution and the Hmim solution to obtain a low-temperature precursor solution. Then, the porous alumina carrier modified by the ZnO precursor layer prepared in example 1 was fixed, and was vertically placed in a reaction kettle containing a low-temperature precursor solution, and then was placed again in a low-temperature reactor previously cooled to-5 ℃ for a low-temperature reaction for 24 hours.
(3) And after the reaction is finished, closing the low-temperature reactor. And (3) raising the temperature of the reaction solution and the refrigerant solution (ethanol) to room temperature, and washing and naturally drying to obtain the low-temperature synthesized ZIF-8 molecular sieve membrane.
An XRD (figure 5) spectrum shows a characteristic peak of ZIF-8, and the ZIF-8 phase is generated after low-temperature reaction at the temperature of-5 ℃. SEM (figure 6) shows that the prepared ZIF-8 molecular sieve film has the grain size of 850nm, the film thickness of 620nm, good intergranular intergrowth, compact and continuous film and no obvious defects.
Example 4: preparation of ZIF-8 molecular sieve membrane by low-temperature synthesis technology
The specific implementation steps are the same as those in the embodiment 3, except that: the solvent composition in the step (1) is 13mL of methanol and 37mL of water; the precooling temperature and the low-temperature reaction temperature in the step (1) and the step (2) are changed to-15 ℃.
The intensity of the characteristic peak of ZIF-8 appearing in the XRD (FIG. 7) pattern was weaker than that of example 3, demonstrating that the ZIF-8 phase with reduced crystallinity was formed after the reaction at a low temperature of-15 ℃. SEM (figure 8) shows that the grain size and the film thickness of the prepared ZIF-8 molecular sieve film layer are reduced and are respectively 300nm and 340nm.
Example 5: preparation of ZIF-8 molecular sieve membrane by low-temperature synthesis technology
The specific implementation steps are the same as those in the embodiment 3, except that: the solvent composition in the step (1) is 17mL of acetone and 33mL of water; the precooling temperature and the low-temperature reaction temperature in the step (1) and the step (2) are changed to be-25 ℃. The intensity of the characteristic peak of ZIF-8 appearing in the XRD (FIG. 9) pattern was further weakened as compared with that of example 4, demonstrating that the ZIF-8 phase having a lower crystallinity was formed after the reaction at a low temperature of-25 ℃. SEM (FIG. 10) showed that the crystal grain size and the film thickness of the prepared ZIF-8 molecular sieve film layer were further reduced to 120nm and 180nm, respectively.
Example 6: preparation of ZIF-8 molecular sieve membrane by low-temperature synthesis technology
The specific implementation steps are the same as those in the embodiment 3, except that: changing the solvent in the step (1) into N, N-dimethylformamide; the pre-cooling temperature and the low-temperature reaction temperature in the step (1) and the step (2) are changed to-50 ℃.
The intensity of the characteristic peak of ZIF-8 appearing in the XRD (FIG. 11) pattern was further weakened as compared with that of example 5, demonstrating that the ZIF-8 phase having a lower crystallinity was formed after the reaction at a low temperature of-50 ℃. SEM (FIG. 12) showed that the crystal grain size and the film thickness of the prepared ZIF-8 molecular sieve film layer were further reduced to 60nm and 100nm, respectively.
Example 7: preparation of ZIF-8 molecular sieve membrane by ultralow temperature synthesis technology
The specific implementation steps are the same as those in the embodiment 3, except that: in step (2), the porous alumina support modified with the ZnO precursor layer (atomic deposition method) prepared in example 2 was selected.
The XRD (figure 13) pattern showed the characteristic peak of ZIF-8, which proves that the ZIF-8 phase is generated on the alumina carrier modified by ZnO deposited on the atomic layer. SEM (figure 14) shows that the prepared ZIF-8 molecular sieve film has the grain size of 800nm, the film thickness of 600nm, good intergranular intergrowth, compact and continuous film and no obvious defects.
Example 8: c for preparing ZIF-8 molecular sieve membrane by low-temperature synthesis technology 3 H 6 /C 3 H 8 Separation Performance test
C was successively conducted on the ZIF-8 molecular sieve membranes prepared in examples 3 to 7 3 H 6 /C 3 H 8 And (3) testing the separation performance: at room temperature, the feed pressure on the permeate side was 1bar, the propylene feed content was 0.5 and the purge gas was N 2 And (4) qi.
C of ZIF-8 molecular sieve membrane 3 H 6 /C 3 H 8 The separation performance is shown in table 1, and the gas separation performance result shows that the ZIF-8 molecular sieve membrane prepared by adopting the low-temperature synthesis technology has excellent C 3 H 6 /C 3 H 8 Separability, with a separation factor of 39-218 3 H 6 Has a permeation flux of 1 to 47X 10 -8 mol m -2 s -1 Pa -1 。
Table 1: c for preparing ZIF-8 molecular sieve membrane by low-temperature synthesis technology 3 H 6 /C 3 H 8 Separation Performance
Example 9: c for preparing ZIF-8 molecular sieve membrane by low-temperature synthesis technology 3 H 6 /C 3 H 8 Repeatability test of separation Performance
A series of ZIF-8 molecular sieve membranes, designated M1, M2, M3 and M4, were prepared repeatedly by the low temperature synthesis method described in example 3 and subjected to C in sequence 3 H 6 /C 3 H 8 And (3) testing the separation performance: at room temperature, the feed pressure on the permeate side was 1bar, the propylene feed content was 0.5 and the purge gas was N 2 And (4) qi.
C of ZIF-8 molecular sieve membrane 3 H 6 /C 3 H 8 The separation performance is shown in FIG. 15, and the gas separation performance result shows that the ZIF-8 molecular sieve membrane prepared by the low-temperature synthesis technology has good repeatability, the separation factor is 120-150,C 3 H 6 The permeation flux of (A) is 1.5-4.0X 10 -8 mol m -2 s -1 Pa -1 。
Example 10: c of ZIF-8 molecular sieve membrane prepared by low-temperature synthesis technology under different pressures 3 H 6 /C 3 H 8 Separation Performance test
The ZIF-8 molecular sieve membrane prepared by the ultralow temperature synthesis method in the embodiment 3 is subjected to C treatment under different pressures 3 H 6 /C 3 H 8 And (3) testing the separation performance: at room temperature, the feed pressure on the permeate side was gradually increased from 1bar to 7bar, the propylene content was 0.5 and the purge gas was N 2 。
C of ZIF-8 molecular sieve membrane under different pressures 3 H 6 /C 3 H 8 The separation Performance is shown in FIG. 16, C 3 H 6 /C 3 H 8 The separation performance result shows that the ZIF-8 molecular sieve membrane prepared by the ultralow temperature synthesis technology has good pressure resistance.
Example 11: ZIF-8 molecular sieve membrane prepared by low-temperature synthesis technology at different C 3 H 6 C under partial pressure ratio 3 H 6 /C 3 H 8 Separation Performance test
The ZIF-8 molecular sieve membrane prepared by the ultralow temperature synthesis method in example 3 is subjected to different C conditions 3 H 6 C at partial pressure ratio 3 H 6 /C 3 H 8 And (3) testing the separation performance: at room temperature, C 3 H 6 And C 3 H 8 The total volume flow of (2) is 50mL min -1 ，C 3 H 6 Gradually increasing the partial pressure ratio from 0.1 to 0.9, the feed pressure on the permeate side being 25bar, and the evacuation mode being used.
ZIF-8 molecular sieve membranes in different C 3 H 6 C under partial pressure ratio 3 H 6 /C 3 H 8 The separation Performance is shown in FIG. 17, C 3 H 6 /C 3 H 8 The separation performance result shows that the ZIF-8 molecular sieve membrane prepared by adopting the low-temperature synthesis technology is in different C 3 H 6 Has good separation stability under the condition of partial pressure feeding.
Comparative example 1: preparation of ZIF-8 molecular sieve membrane by low-temperature seed crystal method
The specific implementation steps are the same as those in the embodiment 3, and the differences are that: and (3) selecting a porous alumina carrier modified by a ZIF-8 seed crystal layer in the step (2).
An XRD (figure 18) spectrum shows a characteristic peak of ZIF-8, and the ZIF-8 film is proved to be generated after the epitaxial growth of the low-temperature seed crystal method. SEM (FIG. 19) shows that the prepared ZIF-8 film had a grain size of 250nm and a film thickness of 200nm. However, C of the film was measured by the same procedure as described in example 8 3 H 6 /C 3 H 8 The selectivity was only 8.6, indicating that a large number of intercrystalline defects were present in the ZIF-8 polycrystalline layer, which was not sufficient for commercial applications for C of ZIF-8 films 3 H 6 /C 3 H 8 And (5) separating.
Comparative example 2: preparation of ZIF-8 molecular sieve membrane at room temperature
The specific implementation steps are the same as those in the embodiment 3, except that: the reaction temperature in step (2) was changed to room temperature (30 ℃).
The XRD (figure 20) shows a stronger ZIF-8 characteristic peak, which proves that a ZIF-8 molecular sieve membrane with high crystallinity is generated after room temperature reaction. SEM (FIG. 21) showed that the crystal grain size of the prepared ZIF-8 molecular sieve membrane was 1.3 μm and the membrane thickness was 0.9. Mu.m. The gas separation performance test using the same procedure as that described in example 8 revealed that C in the ZIF-8 molecular sieve membrane prepared by the room temperature reaction was present 3 H 6 The permeate flux was greatly reduced to 2.5×10 -9 mol m -2 s -1 Pa -1 It was shown that the film thickness increase and the overgrowth with ZIF-8 at room temperature severely increases C 3 H 6 The diffusion resistance of (2). Thus, the results of comparative example 2 demonstrate that: in the present invention, a low-temperature synthesis technique is an essential means for preparing a high-quality ZIF-8 film.
Comparative example 3: preparation of ZIF-8 molecular sieve membrane at high temperature
The specific implementation steps are the same as those in the embodiment 3, except that: the reaction temperature in step (2) was changed to a high temperature (150 ℃).
A stronger ZIF-8 characteristic peak appears in an XRD (figure 22) spectrum, and the ZIF-8 molecular sieve membrane with higher crystallinity is generated after high-temperature reaction. SEM (FIG. 23) showed that the grain size and film thickness of the prepared ZIF-8 molecular sieve film were further increased to 1.65 μm and 1.3. Mu.m, respectively. The gas separation performance test was performed by the same procedure as in example 7, and the results showed that the separation performance of the ZIF-8 zeolite membrane prepared by the high-temperature reaction was drastically reduced and the reproducibility was greatly reduced, indicating that the uncontrollable growth of ZIF-8 at high temperature is difficult to ensure the continuity and uniformity of the microstructure of the membrane layer. Thus, the results of comparative example 3 demonstrate that: in the invention, a low-temperature synthesis technology is a necessary means for preparing the high-repeatability ZIF-8 film.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (10)
1. A method for preparing a ZIF-8 molecular sieve membrane at a low temperature is characterized by comprising the following steps:
(1) Fixing the ZnO precursor layer modified porous carrier and then placing the fixed ZnO precursor layer modified porous carrier in a reaction kettle;
(2) Respectively pre-cooling Zn to-80-10 DEG C 2+ The solution and the Hmin solution are quickly mixed to form a low-temperature precursor solution, and the low-temperature precursor solution is put into the reaction kettle and is subjected to low-temperature reaction at the temperature of minus 80-10 ℃;
(3) After the reaction is finished, the temperature of the reaction liquid is increased back to the room temperature, and finally, the continuous compact ZIF-8 molecular sieve membrane is obtained through washing and drying;
the ZIF-8 molecular sieve membrane has adjustable grain size and crystallinity and proper intergrowth state and thickness, wherein the grain size is 30 nm-1 mu m, and the thickness is 50 nm-1 mu m; pair C thereof 3 H 6 /C 3 H 8 The selectivity of (A) is 39-218 3 H 6 The permeation flux of (A) is 1-47 x 10 -8 mol m -2 s -1 Pa -1 。
2. The method for low temperature preparation of ZIF-8 molecular sieve membranes of claim 1, wherein: the ZnO precursor layer modification process in the step (1) comprises a sol-gel method, a liquid phase growth method, a phase conversion method, a physical vapor deposition method, a chemical vapor deposition method or an atomic layer deposition method; the kind of the porous carrier comprises porous metal, porous carbide, porous metal oxide or porous nonmetal oxide; the structure of the porous carrier comprises a flat plate structure, a tubular structure, a roll structure or a hollow fiber structure.
3. The method for preparing ZIF-8 molecular sieve membranes at low temperature according to claim 1, wherein: zn described in step (2) 2+ The metal source of the solution is Zn 2+ Inorganic salts or Zn 2+ At least one of a complex; zn 2+ The solvent of the solution and the Hmin solution is an organic solvent or an organic solvent/water mixed solvent, wherein the organic solvent is at least one of methanol, ethanol, propanol, N-dimethylformamide and acetone.
4. A low-temperature preparation method of ZIF-8 molecular sieve membranes as claimed in claim 1 or 3, wherein: zn in the low-temperature precursor solution in the step (2) 2+ The concentration of (B) is 1 to 100mM; hmin and Zn 2+ The molar ratio of (A) is 2 to 500; the volume ratio of the organic solvent to the water in the solvent is more than or equal to 0.05.
5. The method for preparing ZIF-8 molecular sieve membranes at low temperature according to claim 1, wherein: the single reaction period of the low-temperature reaction in the step (2) is 10 min-120 h.
6. The method for preparing ZIF-8 molecular sieve membranes at low temperature according to claim 1, wherein: the temperature difference between the solution precooling temperature and the low-temperature reaction temperature in the step (2) is less than or equal to 20 ℃.
7. The method of preparing ZIF-8 molecular sieve membranes at low temperature as claimed in claim 1 or 6, wherein: the low-temperature reaction temperature in the step (2) is-50 ℃ to 0 ℃.
8. A ZIF-8 molecular sieve membrane obtained by the process of claim 1.
9. A ZIF-8 molecular sieve membrane as claimed in claim 8, in C 3 H 6 /C 3 H 8 Use in gas separation.
10. The ZIF-8 molecular sieve membrane of claim 9, at C 3 H 6 /C 3 H 8 The application in gas separation is characterized in that: the gas feeding pressure is 1-30 bar, C 3 H 6 And the partial pressure ratio is 0.01-0.995, and a gas permeability test is carried out by adopting a mode of introducing inert purge gas for permeation test or vacuumizing.
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