Preparation method of C-ZIF-67 derived nanoporous carbon doped PDMS mixed matrix membrane based on tubular ceramic support and its application in pervaporation recovery of ethanol

Abstract

The present invention belongs to the technical field of ethanol recovery and relates to a preparation method of a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic carrier and its application in the pervaporation recovery of ethanol from a low-concentration ethanol aqueous solution; the preparation method comprises synthesizing ZIF-67 crystals and carbonizing the ZIF-67 crystals to obtain C-ZIF-67 filler particles; the present invention prepares ZIF-67-derived nanoporous carbon particles by a high-temperature calcination method, fully carbonizes the particles at a certain calcination temperature, and the disappearance of characteristic groups after carbonization can reduce the influence of the surface electrical properties of the particles, and the particle size shrinkage is reduced, which helps to improve the dispersion of the particles in the matrix membrane; at the same time, the MOF skeleton structure is retained after carbonization, and the pore size is enlarged. The carbon material also has strong hydrophobicity. These hydrophobic large-pore separation channels are more conducive to the passage of ethanol molecules, thereby promoting the improvement of the separation effect. Moreover, the separation factor and total flux remain almost unchanged after long-term operation, showing excellent performance and having good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of ethanol recovery and relates to a mixed matrix membrane for pervaporation recovery of ethanol and a preparation method thereof, in particular to a preparation method of a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic carrier and an application thereof in pervaporation recovery of ethanol from a low-concentration ethanol aqueous solution. Background Art

[0002] The world's over-reliance on fossil fuels has triggered serious energy crises and environmental problems. Renewable biomass has attracted much attention as an alternative resource. Bioethanol, as a valuable biofuel, has great market potential, but its fermentation, production and separation technologies currently face challenges, such as end-product inhibition and high-energy-consuming distillation processes.

[0003] Pervaporation is a promising membrane separation technology with advantages such as ease of operation, low energy consumption, high efficiency, and non-toxicity. Polydimethylsiloxane (PDMS) membranes have been widely studied due to their strong hydrophobicity, good permeability, stability, and low cost. However, the insufficient ethanol separation performance of conventional PDMS membranes has limited their large-scale application.

[0004] Incorporating fillers into PDMS to prepare mixed matrix membranes (MMMs) can improve membrane performance. Metal-organic frameworks (MOFs) are widely used as doping fillers in the preparation of MMMs. For example, ZIF-67 has a high specific surface area, adjustable pore size, small-size structure, strong hydrophobicity, and is easy to synthesize at room temperature, and can be used as a doping filler. However, to date, a large number of studies have focused on the synthesis of MMMs using sheet carriers as supports and the pervaporation separation of ethanol-water solutions, while research on the use of tubular ceramic carriers is still lacking. Tubular ceramic carriers have the advantages of strong chemical stability and high mechanical strength, and are widely used in current industrial production (tubular membranes are used in industry for production).

[0005] ZIF-67 / PDMS MMM was first prepared using ceramic tubes as carriers and applied to the pervaporation separation of low-concentration ethanol aqueous solutions, but there are some shortcomings:

[0006] (1) ZIF-67 particles agglomerated severely on the tubular support, resulting in limited improvement in the separation factor;

[0007] (2) The interfacial compatibility between ZIF-67 and PDMS is not ideal, and the stability is reduced.

[0008] The present invention aims to solve the problems existing in the prior art of PDMS membranes and ZIF-67 / PDMS MMMs in the separation of low-concentration ethanol aqueous solutions, such as ZIF-67 particle agglomeration and unsatisfactory interfacial solubility, and to provide a mixed matrix membrane with better performance and a preparation method thereof. Summary of the Invention

[0009] In order to make up for the shortcomings of the existing technology, the present invention first prepared ZIF-67 / PDMS MMM using ceramic tubes as carriers and applied it to the pervaporation separation of low-concentration ethanol aqueous solution, but there are still some shortcomings:

[0010] (1) ZIF-67 particles agglomerated severely on the tubular support, resulting in limited improvement in the separation factor;

[0011] (2) The interfacial compatibility between ZIF-67 and PDMS is not ideal, and the stability is reduced.

[0012] The reasons for the deficiency were further analyzed:

[0013] ZIF-67 particles themselves have the characteristic of being easy to agglomerate (this phenomenon is more obvious during the synthesis of ZIF-67 in this study). This is due to the electrical properties of the surface groups of the zeolite-imidazole salt skeleton structure. Therefore, they agglomerate severely and have poor dispersion in the PDMS matrix membrane; this also further affects its compatibility with PDMS, resulting in some defects on the membrane surface. After long-term use, some ZIF-67 falls off, and the stability is reduced.

[0014] The solution to this case is to retain the porous skeleton structure of ZIF-67, eliminate the effects of surface groups, and at the same time not reduce its hydrophobic properties; therefore, considering the preparation of ZIF-67-derived nanoporous carbon particles by high-temperature calcination, they are fully carbonized at a certain calcination temperature. The disappearance of characteristic groups after carbonization can reduce the influence of their surface electrical properties, and the particle size shrinks and decreases, all of which contribute to the improvement of their dispersion in the matrix membrane; at the same time, the MOF skeleton structure is still retained after carbonization, and the pore size becomes larger. The carbon material also has strong hydrophobicity. These hydrophobic large-pore separation channels are more conducive to the passage of ethanol molecules, promoting the improvement of its separation effect.

[0015] The technical solution adopted by the present invention to solve the technical problem is: a method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support, comprising the following steps:

[0016] S1: Synthesize ZIF-67 crystals by dissolving Co(NO3)2·6H2O in methanol; then dissolving Hmim in methanol; then mixing the two solutions and stirring them at room temperature for 2 h to obtain purple crystals. The purple crystals were collected by centrifugation at 8000 rpm for 10 min, and repeatedly washed with methanol. Finally, the obtained crystals were vacuum dried at 80°C overnight to obtain ZIF-67 crystals.

[0017] S2: Preparation of C-ZIF-67 filler particles: The synthesized ZIF-67 crystals were carbonized and collected. The collected particles were washed with methanol to remove residues and finally vacuum dried at 80°C overnight to obtain C-ZIF-67 filler particles.

[0018] S3: Treatment of α-alumina ceramic tube carrier: First, polish the ceramic tube with 800# coarse sandpaper and 1500# fine sandpaper respectively, then calcine to remove impurities, and then ultrasonically clean for 5 minutes and use pure water to clean to obtain α-alumina ceramic tube carrier. Soak the α-alumina ceramic tube carrier in pure water before use. When needed, wipe off excess water on the surface of the α-alumina ceramic tube carrier with filter paper and let it stand to dry for use.

[0019] S4: Preparation of PDMS-based mixed matrix membrane: C-ZIF-67 filler particles were added to n-heptane, stirred and ultrasonically treated for 1 hour, PDMS was added and stirred for 4 hours, and PDMS was added to the mixed solution again and stirred for 12 hours; then the crosslinker TEOS and the catalyst DBTOL were added respectively, and the mixture was allowed to stand for degassing. The C-ZIF-67-filled PDMS solution was coated on the outer surface of the α-alumina ceramic tube support by dip coating for 1 minute, allowed to stand at room temperature for 24 hours, and then vacuum dried at 90°C for 12 hours. Finally, a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support was prepared.

[0020] Furthermore, the Co(NO3)2·6H2O in S1 is 0.1-0.4 g; methanol is 10 g-15 g; and Hmim is 0.2-0.5 g.

[0021] Furthermore, the amount of Co(NO3)2·6H2O is 0.29 g; the amount of methanol is 11.8 g; and the amount of Hmim is 0.33 g.

[0022] Furthermore, the carbonization conditions of the ZIF-67 crystals described in S2 are as follows: the initial temperature is room temperature, the temperature is increased in a nitrogen environment, and the heating rate is 5°C·min -1 , heat to 700℃, keep warm for 2h, and then cool down naturally.

[0023] Furthermore, 0.1-0.3 g of C-ZIF-67 filler particles, 20-30 g of n-heptane, and a total amount of PDMS of 1-3 g are added. The first addition amount is 10% of the mass of the filler particles, and the second addition amount is the remaining mass of PDMS.

[0024] Furthermore, the mass of the C-ZIF-67 filler particles is 0.2625 g, the mass of n-heptane is 25 g, and the total mass of PDMS is 1.75 g.

[0025] Furthermore, the added mass ratio of PDMS, cross-linking agent TEOS, and catalyst DBTOL was 1:0.1:0.05.

[0026] The application of pervaporation to recover ethanol is characterized by using a prepared C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support to recover ethanol by pervaporation.

[0027] The beneficial effects of the present invention are as follows:

[0028] The present invention discloses a method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support and its application in pervaporation recovery of ethanol. The prepared C-ZIF-67 / PDMS MMM particles retain the original MOF skeleton structure and have a larger pore size. At the same time, the carbon component has strong hydrophobicity, relatively good dispersibility, and better compatibility with PDMS. These structural and property advantages significantly improve the PV performance of C-ZIF-67 / PDMS MMM in ethanol / water solution compared with pure PDMS membrane and ZIF-67 / PDMSMMM. Moreover, the separation factor and total flux remain almost unchanged after long-term operation, demonstrating excellent performance and having good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below with reference to the accompanying drawings.

[0030] Figure 1 is a flow chart of the method of the present invention;

[0031] Figure 2 Schematic diagram of a pervaporation performance testing device according to the present invention;

[0032] Figure 3 The XRD patterns and TGA curves of ZIF-67 and C-ZIF-67 particles of the present invention are shown;

[0033] Figure 4 is the infrared spectrum of ZIF-67 and C-ZIF-67 particles of the present invention;

[0034] Figure 5 is a scanning electron micrograph of ZIF-67 and C-ZIF-67 particles of the present invention;

[0035] Figure 6 are nitrogen adsorption-desorption isotherms and pore size distribution diagrams of ZIF-67 and C-ZIF-67 particles of the present invention;

[0036] Figure 7are scanning electron micrographs of the surfaces and cross sections of pure PDMS membrane, ZIF-67 / PDMS-15wt% and C-ZIF-67 / PDMS-15wt% MMMs of the present invention;

[0037] Figure 8 The water contact angle diagrams of pure PDMS membrane, ZIF-67 / PDMS-15wt% MMM and C-ZIF-67 / PDMSMMMs with different loading amounts are shown in FIG.

[0038] Figure 9 is a graph of the pervaporation performance of ZIF-67 / PDMS MMMs at different ZIF-67 loadings and C-ZIF-67 / PDMS MMMs at different C-ZIF-67 loadings of the present invention;

[0039] Figure 10 This is a comparison chart of the pervaporation performance of pure PDMS membrane, ZIF-67 / PDMS-15wt% and C-ZIF-67 / PDMS-15wt% MMMs of the present invention;

[0040] Figure 11 This is a graph showing the long-term pervaporation performance of C-ZIF-67 / PDMS-15wt% MMM of the present invention. DETAILED DESCRIPTION

[0041] In order to make the technical means, creative features, objectives and effects of the present invention easier to understand, the following Figures 1-11 The present invention is further described in conjunction with specific embodiments and comparative examples.

[0042] Example:

[0043] like Figure 1 As shown, preparation of C-ZIF-67 derived nanoporous carbon incorporated into PDMS mixed matrix membrane based on tubular ceramic support:

[0044] Synthesis of ZIF-67 crystals: 0.29 g of Co(NO3)2·6H2O was dissolved in 11.8 g of methanol; 0.33 g of Hmim was dissolved in 11.8 g of methanol; the two solutions were then mixed and stirred at room temperature for 2 hours to obtain purple crystals. The product was collected by centrifugation at 8000 rpm for 10 minutes and repeatedly washed with methanol. Finally, the obtained crystals were vacuum dried at 80°C overnight to obtain ZIF-67 crystals.

[0045] Carbonization of ZIF-67 crystals: The synthesized ZIF-67 crystals were heated in a nitrogen environment at an initial temperature of room temperature at a rate of 5 °C min -1, the temperature was raised to 700°C and kept for 2 hours, then naturally cooled. The carbonized particles were collected and washed with methanol to remove the residue. Finally, they were vacuum dried at 80°C overnight to obtain C-ZIF-67 filler particles.

[0046] Treatment of α-alumina ceramic tube carrier: First, polish the ceramic tube with coarse sandpaper (800#) and fine sandpaper (1500#) respectively, then calcined to remove impurities, then ultrasonically cleaned for 5 minutes and cleaned with pure water, and then soaked in pure water for 24 hours. Fill the pores of the carrier with water to prevent the coating polymer solution from penetrating into the pores, and take it out 1 hour before immersing in the coating polymer solution, wipe off excess water on the surface of the ceramic tube with filter paper, and let it stand to dry for use.

[0047] Preparation of PDMS-based mixed matrix membranes: 0.2625 g of C-ZIF-67 filler particles were added to 25 g of n-heptane, stirred, and sonicated for 1 hour. A portion of PDMS (10% of the filler particle mass) was then added and stirred for 4 hours. The remaining PDMS was then added to the mixture and stirred for 12 hours, resulting in a total of 1.75 g of PDMS (7.0 wt% PDMS concentration in n-heptane). Subsequently, the crosslinker TEOS and catalyst DBTOL (PDMS / TEOS / DBTOL ratio of 1 / 0.1 / 0.05) were added, and the mixture was allowed to stand for degassing. The C-ZIF-67-filled PDMS solution was dip-coated onto the outer surface of a treated α-alumina ceramic tube support for 1 minute. The mixture was allowed to stand at room temperature for 24 hours and then vacuum-dried at 90°C for 12 hours to produce a C-ZIF-67-filled PDMS / ceramic tube mixed matrix membrane.

[0048] Comparative Example 1:

[0049] Preparation of ZIF-67 filled PDMS / ceramic tube mixed matrix membrane:

[0050] Synthesis of ZIF-67 crystals: 0.29 g of Co(NO3)2·6H2O was dissolved in 11.8 g of methanol; 0.33 g of Hmim was dissolved in 11.8 g of methanol; the two solutions were then mixed and stirred at room temperature for 2 hours to obtain purple crystals. The product was collected by centrifugation at 8000 rpm for 10 minutes and repeatedly washed with methanol. Finally, the obtained crystals were vacuum dried at 80°C overnight to obtain ZIF-67 crystals.

[0051] Treatment of α-alumina ceramic tube carrier: First, polish the ceramic tube with coarse sandpaper (800#) and fine sandpaper (1500#) respectively, then calcined to remove impurities, then ultrasonically cleaned for 5 minutes and cleaned with pure water, and then soaked in pure water for 24 hours. Fill the pores of the carrier with water to prevent the coating polymer solution from penetrating into the pores, and take it out 1 hour before immersing in the coating polymer solution, wipe off excess water on the surface of the ceramic tube with filter paper, and let it stand to dry for use.

[0052] Preparation of PDMS-based mixed matrix membranes: 0.2625 g of CZIF-67 filler particles were added to 25 g of n-heptane, stirred, and sonicated for 1 hour. A portion of PDMS (10% of the filler particle mass) was then added and stirred for 4 hours. The remaining PDMS was then added to the mixture and stirred for 12 hours, resulting in a total PDMS mass of 1.75 g (7.0 wt% PDMS concentration in n-heptane). Subsequently, the crosslinker TEOS and catalyst DBTOL (PDMS / TEOS / DBTOL ratio of 1 / 0.1 / 0.05) were added, respectively, and the mixture was allowed to stand for degassing. The ZIF-67-filled PDMS solution was dip-coated onto the outer surface of a treated α-alumina ceramic tube support for 1 minute. The mixture was allowed to stand at room temperature for 24 hours and then vacuum-dried at 90°C for 12 hours to produce a ZIF-67-filled PDMS / ceramic tube mixed matrix membrane.

[0053] Comparative Example 2:

[0054] Preparation of pure PDMS membrane: 1.75g ​​PDMS was added to 25g n-heptane and stirred at room temperature for 6h, followed by the addition of crosslinker TEOS and catalyst DBTOL (the ratio of PDMS / TEOS / DBTOL was 1 / 0.1 / 0.05) and allowed to stand for degassing; the PDMS solution was coated on the outer surface of the pretreated tubular ceramic support by dip coating for 1min; the mixture was allowed to stand at room temperature for 24h, and then vacuum dried at 90°C for 12h to finally prepare a pure PDMS membrane.

[0055] The membranes prepared in Example 1, Comparative Example 1 and Comparative Example 2 were subjected to pervaporation experiments as follows:

[0056] like Figure 2 As shown, the tubular membrane was connected to the stainless steel tube with vacuum sealant and then installed in the pervaporation device. The effective membrane area was 0.001695m 2 The feed solution was a 5 wt% ethanol aqueous solution, the temperature was controlled at 60°C, and the vacuum was evacuated by a vacuum pump, and the vacuum degree was maintained below 400 Pa.

[0057] Before collecting samples, the entire pipeline system was preheated for 2 h, and then the permeate was collected in a cold trap immersed in liquid nitrogen; after the experiment, the collected samples were weighed and analyzed by gas chromatography.

[0058] Total permeate flux J, through-equation and partial flux J i Calculated by the following formulas:

[0059]

[0060] J i =y wi J

[0061] Where J is the total flux, in g / m 2 h; Q is the total mass of the permeate, in kg; A is the effective area of ​​the membrane, in m 2 ; t is the pervaporation collection time, in h; y wi is the mass fraction of component i in the permeate;

[0062] The membrane separation factor α is calculated by the ratio of the mass fractions of the two components on the permeate side y and the feed side x, as follows:

[0063]

[0064] Through the above experiments, the conclusions are as follows:

[0065] Characterization of ZIF-67 and C-ZIF-67 particles:

[0066] like Figure 3 As shown, the synthesized ZIF-67 particles before and after carbonization were characterized by XRD. Figure 3 The characteristic diffraction peak in (a) is consistent with the spectrum of ZIF-67 in the prior art, and no other impurities exist, indicating that ZIF-67 particles have been synthesized and the diffraction peak position points to the sodalite structure.

[0067] Figure 3 (b) is the TGA curve of ZIF-67 particles, which is used to analyze its carbonization thermal decomposition process. In the range of 100-200℃, the removal of water adsorbed in the pores causes the weight to gradually decrease; the weight loss above 200℃ may be due to the escape of residual solvent from the pores or the decomposition of unreacted linkers; around 550℃ is the decomposition of -CH3, and after 550-600℃ is the release process of CNH, 600-700℃ is its main decomposition and carbonization temperature, and complete decomposition and carbonization are achieved at 700℃.

[0068] like Figure 3 As shown in (c), the XRD spectrum of C-ZIF-67 is Figure 3Compared with ZIF-67 in (a), the characteristic peaks disappear and it becomes amorphous. The two broad peaks at 24° and 44° are the (002) and (101) phases of graphitic carbon, respectively. It is preliminarily judged that ZIF-67 has been carbonized.

[0069] like Figure 4 As shown, the chemical structures of ZIF-67 and carbonized C-ZIF-67 particles were further investigated by infrared spectroscopy; Figure 4 The absorption peak of ZIF-67 is mainly attributed to the 2-methylimidazole ligand, 600~1500cm -1 The peaks in the range are the result of stretching and bending vibrations of the imidazole group, 1460 cm –1 A small peak at the position corresponds to the stretching mode of C=N bonding. The characteristic absorption peaks of the synthesized particles are consistent with the results in the prior art, further indicating the successful synthesis of ZIF-67. After carbonization, the absorption peaks at the corresponding positions almost completely disappear, indicating that calcination at 700°C causes the decomposition of organic ligands and ZIF-67 is successfully carbonized. The disappearance of characteristic groups after carbonization can reduce the influence of its surface electrical properties, thereby reducing the degree of particle agglomeration in MMM.

[0070] like Figure 5 As shown in Figure 2, the prepared ZIF-67 crystal morphology is highly crystalline rhombic dodecahedron particles with sharp edges and a size of about 300 nm. Figure 5 (b) SEM image of C-ZIF-67. The morphology of the corresponding particles has not changed significantly, and the regular rhombic dodecahedron shape is still retained, indicating that there is no structural damage after carbonization. In this way, the original MOF skeleton structure can be used as MMM filler for the next application. At the same time, Figure 5 It can also be seen in (b) that the particle size is significantly reduced after carbonization, which also helps to improve its dispersion in MMM.

[0071] like Figure 6 As shown in (a), the nitrogen adsorption-desorption curve of ZIF-67 belongs to type I isotherm, indicating that it is a microporous material; Figure 6 (b) is the nitrogen adsorption-desorption curve after carbonization, showing a type IV isotherm with an H3 type hysteresis loop, which reflects that the carbonized particles are mostly mesoporous. Figure 6 The pore size distribution curve of (c) also illustrates this result; as shown in the following table:

[0072]

[0073] Pore ​​structure characteristics of ZIF-67 and C-ZIF-67

[0074] The pore structure characteristics are given. The total pore volume of C-ZIF-67 after carbonization is increased from 0.7443 cm3 g -1 Reduced to 0.3187cm 3 g –1 , the micropore volume is from 0.7039cm 3 g -1 Reduced to 0.1693cm 3 g –1 The pore volume changes from mostly micropore volume to mostly mesopore volume; the average pore diameter increases from 2.18 nm to 3.57 nm, and the pore size distribution becomes wider after carbonization, and has a larger pore size. Combined with the above-mentioned SEM results, it shows that the C-ZIF-67 particles still retain the original MOF skeleton structure, and the pore diameter is further increased compared to ZIF-67.

[0075] Characterization of membrane materials:

[0076] The SEM morphologies of PDMS membrane, ZIF-67 / PDMS-15wt% and C-ZIF-67 / PDMS-15wt% MMMs are shown in Figure 2. Figure 7 As shown; Figure 7 (a) and (b) show that the surface of the PDMS membrane is dense, smooth and defect-free, and the thickness of the membrane is about 7 μm. It can be seen from the surface and cross section of ZIF-MMMs that Figure 7 (c), (d), (e), and (f) show that the surface of the MMMs membrane doped with ZIF-67 and C-ZIF-67 particles becomes rough, which is consistent with the result that the water contact angle of the MMMs surface increases after doping with particles. It is worth noting that there is a certain degree of particle agglomeration in the ZIF-67 / PDMS MMM, which further leads to the inability of some particles to form a good fusion with PDMS. In contrast, the C-ZIF-67 / PDMS MMM particles are less agglomerated, and C-ZIF-67 and PDMS are well soluble, reducing the defects caused by the difficulty of the viscous polymer solution to penetrate the gaps between the agglomerated particles. The cross-section shows that the average thickness of the selective membrane layer of ZIF-MMMs is about 10 μm, which is thicker than the pure PDMS membrane, and there is no obvious peeling from the support layer.

[0077] The surface hydrophobicity of ZIF-67 / PDMS and C-ZIF-67 / PDMS MMMs with different particle loadings was evaluated by water contact angle measurement:

[0078]

[0079] Water contact angles of ZIF-67 / PDMS MMMs

[0080] from Figure 8As can be seen from the table above, the water contact angle of ZIF-67 / PDMS MMM is always higher than that of pure PDMS membrane (115°). This is because the introduction of ZIF-67 particles with a certain hydrophobicity increases the roughness of the membrane surface. Due to the inherent strong hydrophobicity of carbonized particles and their good compatibility with PDMS, the water contact angle of C-ZIF-67 / PDMS MMM is higher than that of ZIF-67 / PDMS at each loading concentration. In addition, the water contact angle increases continuously from 121° to 135° as the filler loading increases from 5% to 25%. This is because the roughness of the membrane surface increases with the increase in loading.

[0081] Comparison of membrane pervaporation performance:

[0082] The pervaporation separation performance of ethanol-water mixtures of ZIF-67 / PDMS and C-ZIF-67 / PDMS MMMs with different particle loadings was further studied; operating conditions: 5.0 wt% ethanol-water mixture, 60 °C; Figure 9 As shown in (a), when the ZIF-67 loading increased from 0 to 25 wt%, the flux decreased from 1.62 to 0.19 kg / m2·h. The main reason is that the tubular support caused the MMM to have different degrees of particle agglomeration during the preparation process (the same is true for C-ZIF-67 / PDMS MMM);

[0083] The preparation of MMM using a sheet carrier: using a scraper to scrape the mixed solution into a film with a thickness of 150μm on a sheet substrate, the particles can be dispersed more evenly and the degree of agglomeration can be greatly reduced; on the other hand, the surface groups of ZIF-67 affect its surface electrical properties, making it easy to agglomerate and resulting in poor dispersion, so the flux gradually decreases and is lower than that of pure PDMS membrane. The separation factor increases from 5.9 to 7.9 with the increase of particle loading, reaching the highest point, which is higher than that of pure PDMS membrane; since ZIF-67 particles have a certain hydrophobicity, the hydrophobicity of the composite membrane increases, so the separation factor gradually increases; but in the range of ZIF-67 loading from 15wt% to 25wt%, the separation factor begins to decrease. This is because the excessive filler loading leads to more serious agglomeration, which causes more interface defects between ZIF-67 and PDMS. Overall, the separation factor is improved by the addition of ZIF-67, but due to the easy agglomeration of ZIF-67 particles themselves and the lack of good fusion with PDMS, the separation selectivity is not greatly improved, and the stability is reduced after long-term use.

[0084] Carbonized ZIF-67 particles were added to the PDMS matrix to replace ZIF-67. The pervaporation performance of C-ZIF-67 / PDMSMMM at different loading amounts was shown in Figure 2. Figure 9As shown in (b), the separation factor increases with the increase of particle loading, and then decreases after reaching a maximum of 15wt%, with a maximum value of 9.2. Compared with ZIF-67, the separation factor of each concentration is improved; the flux also shows a gradual downward trend from 1.62 to 0.21kg / m 2 ·h, and both are lower than PDMS, but higher than ZIF-67 at each concentration; due to the inherent strong hydrophobicity of the carbon component, relatively good dispersibility and better solubility with PDMS, it has better separation selectivity than ZIF-67; at the same time, the large-pore MOF skeleton structure and reduced agglomeration further reduce the solution transmission resistance and improve the flux.

[0085] like Figure 10 As shown, the operating conditions are: 5.0wt% ethanol aqueous solution, 60℃; the pervaporation performance of pure PDMS, ZIF-67 / PDMS-15wt% and C-ZIF-67 / PDMS-15wt% for ethanol recovery is compared. It can be seen that C-ZIF-67 / PDMS-15wt% has the highest separation factor, reaching 9.2; in terms of total flux, MMMs are lower than pure PDMS, but C-ZIF-67 / PDMS-15wt% is better than ZIF-67 / PDMS-15wt%, reaching 1.04kg / m 2 ·h; Overall, C-ZIF-67 / PDMS-15wt%MMM has the best pervaporation separation performance.

[0086] Stability test of C-ZIF-67 / PDMS MMM:

[0087] The long-term stability of the membrane is crucial for industrial applications. Therefore, in order to evaluate the practical application potential, the long-term stability test of C-ZIF-67 / PDMS MMM was carried out in 5.0 wt% ethanol aqueous solution at 60 °C. The results are shown in Figure 2. Figure 11 As shown in the graph, it can be clearly seen that the separation factor and total flux remain almost unchanged over a period of 60 h, with only some small fluctuations that may be caused by concentration deviations of the feed solution; although longer tests are still needed before true industrialization, the above results have preliminarily demonstrated that C-ZIF-67 / PDMS MMM has high operational stability in potential applications.

[0088] in conclusion:

[0089] To improve the separation performance of pure PDMS membranes and ZIF-67 / PDMS MMMs in low-concentration ethanol / water solutions, an effective method of adding C-ZIF-67 to the PDMS matrix was proposed, and C-ZIF-67 / PDMS MMMs were successfully prepared. The carbonized particles retained the original MOF skeleton structure and had larger pores. At the same time, the carbon component was highly hydrophobic, had relatively good dispersibility, and was better soluble in PDMS. These structural and property advantages significantly improved the PV performance of C-ZIF-67 / PDMS MMMs in ethanol / water solutions compared with pure PDMS membranes and ZIF-67 / PDMS MMMs. At 60°C, the MMM with a loading of 15 wt% in a 5 wt% ethanol-water solution had the best PV performance, with a flux of 1.04 kg / m 2 ·h, and the separation factor was 9.2; finally, the stability of C-ZIF-67 / PDMSMMM was evaluated, and the separation factor and total flux remained almost unchanged after long-term operation; therefore, the C-ZIF-67 / PDMS composite membrane has excellent performance and has good application prospects.

[0090] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the foregoing embodiments. The foregoing embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support, characterized in that: The steps include: S1: Synthesize ZIF-67 crystals by dissolving Co(NO3)2·6H2O in methanol; then dissolving Hmim in methanol; then mixing the two solutions and stirring them at room temperature for 2 h to obtain purple crystals. The purple crystals were collected by centrifugation at 8000 rpm for 10 min, and repeatedly washed with methanol. Finally, the obtained crystals were vacuum dried at 80°C overnight to obtain ZIF-67 crystals. S2: Preparation of C-ZIF-67 filler particles: The synthesized ZIF-67 crystals were carbonized and then collected. The collected particles were washed with methanol to remove residues and finally dried under vacuum at 80°C overnight to obtain C-ZIF-67 filler particles; S3: Treatment of the α-alumina ceramic tube support: First, polish the ceramic tube with 800# coarse sandpaper and 1500# fine sandpaper respectively, then calcine to remove impurities, then ultrasonically clean for 5 minutes and rinse with pure water to obtain the α-alumina ceramic tube support. Before use, soak the α-alumina ceramic tube support in pure water. When needed, wipe off excess water on the surface of the α-alumina ceramic tube support with filter paper and let it stand to dry for use; S4: Preparation of PDMS-based mixed matrix membrane: C-ZIF-67 filler particles were added to n-heptane, stirred and ultrasonically treated for 1 hour, PDMS was added and stirred for 4 hours, and PDMS was added to the mixed solution again and stirred for 12 hours; then the crosslinker TEOS and the catalyst DBTOL were added respectively, and the mixture was allowed to stand for degassing. The C-ZIF-67-filled PDMS solution was coated on the outer surface of the α-alumina ceramic tube support by dip coating for 1 minute, allowed to stand at room temperature for 24 hours, and then vacuum dried at 90°C for 12 hours. Finally, a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support was prepared.

2. The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support according to claim 1, characterized in that: The amount of Co(NO3)2·6H2O in S1 is 0.1-0.4 g; the amount of methanol is 10 g-15 g; and the amount of Hmim is 0.2-0.5 g.

3. The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support according to claim 1, characterized in that: The amount of Co(NO3)2·6H2O is 0.29 g; the amount of methanol is 11.8 g; and the amount of Hmim is 0.33 g.

4. The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support according to claim 1, characterized in that: The carbonization conditions of ZIF-67 crystals described in S2 are as follows: the initial temperature is room temperature, the temperature is increased in a nitrogen environment, and the heating rate is 5°C·min -1 , heat to 700℃, keep warm for 2h, and then cool down naturally.

5. The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support according to claim 1, characterized in that: 0.1-0.3 g of C-ZIF-67 filler particles, 20-30 g of n-heptane, and a total amount of PDMS of 1-3 g are used. The first addition amount is 10% of the mass of the filler particles, and the second addition amount is the remaining mass of PDMS.

6. The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support according to claim 1, characterized in that: The mass of the C-ZIF-67 filler particles is 0.2625 g, the mass of n-heptane is 25 g, and the total mass of PDMS is 1.75 g.

7. The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support according to any one of claim 5 or claim 6, characterized in that: The added mass ratio of PDMS, crosslinker TEOS and catalyst DBTOL is 1:0.1:0.

05.

8. Application of pervaporation to recover ethanol, characterized by: The method for preparing a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support described in any one of claims 1 to 7 is used to prepare a C-ZIF-67-derived nanoporous carbon-doped PDMS mixed matrix membrane based on a tubular ceramic support, and ethanol is recovered by pervaporation.

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