ZIF-67 / PDMS mixed matrix pervaporation membrane, preparation method thereof and application of ZIF-67 / PDMS mixed matrix pervaporation membrane in food aroma backfilling

Abstract

The invention discloses a preparation method of a ZIF-67 / PDMS mixed matrix pervaporation membrane, which comprises the following steps: dissolving 2-methylimidazole and docusate sodium in methanol to obtain a 2-methylimidazole solution; the preparation method comprises the following steps: uniformly mixing a methanol solution of cobalt nitrate hexahydrate and a 2-methylimidazole solution, standing to obtain a ZIF-67 precursor suspension, carrying out solid-liquid separation to obtain ZIF-67 crystals, uniformly dispersing the ZIF-67 crystals in ethanol, adding polydimethylsiloxane into the ZIF-67 crystals, uniformly dispersing the ZIF-67 crystals to obtain a suspension, adding a cross-linking agent and a catalyst into the suspension, uniformly stirring to obtain a ZIF-67 / PDMS coating solution, and carrying out vacuum drying on the ZIF-67 / PDMS coating solution. And uniformly coating the surface of a PVDF substrate membrane with the obtained product, crosslinking at normal temperature, and drying to obtain the product. Compared with a pure PDMS pervaporation membrane, the ZIF-67 / PDMS mixed matrix membrane has the advantages that the separation factor of the ZIF-67 / PDMS mixed matrix membrane on a linalool and water mixed system is obviously improved, and the membrane flux is up to 600-700 mgm <-2 > h <-1 >. The method is applied to a pervaporation technology to separate aromatic substances in food, and after a to-be-intercepted side solution is subjected to series processing to form a product, a permeation side solution rich in the aromatic substances is backfilled into the product, so that the aroma of the product is repaired.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a ZIF-67 / PDMS mixed matrix pervaporation membrane, its preparation method, and its application. Background Technology

[0002] Fruit juice is widely popular due to its rich nutritional value, making it a top beverage choice for consumers worldwide. With increasing health consciousness, the demand for high-quality fruit juice has gradually surpassed that for sugary and carbonated drinks. This shift is primarily attributed to the natural nutrients in fruit juice, such as minerals and vitamins. In the beverage industry, heat treatment techniques, especially vacuum concentration, are frequently used to extend the shelf life of fruit juice and reduce transportation costs. However, these heat processing methods often negatively impact the sensory quality and nutritional components of fruit juice, leading to problems such as pigment degradation, loss of fresh fruit flavor, and the development of a "ripe" taste. Therefore, researchers have been actively exploring new concentration technologies to overcome the negative effects of traditional heat processing methods while preserving the nutritional value and delicate flavor of fruit juice.

[0003] Pervaporation (PV) is a novel separation technology based on functional membrane materials. Its separation mechanism relies on the selective dissolution and diffusion characteristics of target substances within the membrane. This process utilizes the differences in affinity and mass transfer rates of the membrane material for different components to achieve efficient separation. Compared to traditional methods, PV technology operates under milder conditions, avoiding structural damage to heat-sensitive substances (such as natural fragrances and active ingredients). It also offers significant advantages such as low energy consumption, a simple operating procedure (no reagents or auxiliary media required), and continuous operation. Therefore, it shows significant application potential in various fields including chemical production, energy recovery, environmental protection, biomedicine, and food processing.

[0004] Based on their separation characteristics, pervaporation membranes can be classified into three types: water-preferential permeability, alcohol-preferential permeability, and organic-organic separation. Among them, alcohol-preferential permeation membranes exhibit high selectivity for organic molecules and are considered a promising alternative technology in the field of plant aromatic component recovery. In recent years, this technology has been widely applied in the concentration of various fruit juices (such as apple juice, pomegranate juice, blackberry juice, strawberry juice, and lemon juice) and the recovery of aromatic substances in food raw materials such as tea, demonstrating good separation effects. However, pervaporation membranes are usually dense structures, and the diffusion rate of organic matter within the membrane is not only affected by the physicochemical properties of the target molecules, such as boiling point, polarity, and molecular weight, but also closely related to the structure and properties of the membrane itself. Volatile aromatic components of plants are mostly alcohols, acids, ketones, aldehydes, esters, ethers, and hydrocarbons, generally possessing high molecular weights and boiling points, resulting in low permeation flux within the membrane. Therefore, existing pervaporation membranes currently cannot meet the requirements of industrial applications.

[0005] Hybrid matrix membranes (MMMs) combine the material properties of inorganic fillers as the dispersed phase matrix and polymers as the continuous phase, effectively integrating the material characteristics of both. The addition of inorganic fillers typically improves gas permeability while maintaining the economics and processability of polymers, thus exceeding the performance of single-polymer films. Although MMMs have proven advantageous in improving separation performance, the incompatibility between the inorganic and polymer phases leads to non-selective interfacial voids, often limiting the practical application of MMMs. To address these challenges, researchers have conducted extensive explorations in membrane structure design and performance optimization. Introducing a polydimethylsiloxane (PDMS) interlayer into composite membranes not only effectively prevents pore penetration but also makes the membrane surface smoother, facilitating the uniform preparation of the selective layer. Furthermore, the interlayer can optimize gas flow distribution, improving the utilization efficiency of the selective layer and further enhancing the overall membrane performance. In addition, by introducing inorganic filler components—such as nanomaterials like nano-silica, carbon nanotubes (multi-walled / single-walled) or metal-organic frameworks (MOFs)—the intramembrane mass transfer channels can be effectively regulated, thereby simultaneously improving the permeation flux and separation selectivity of organic matter, which has attracted widespread attention from academia and industry. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a ZIF-67 / PDMS hybrid matrix pervaporation membrane to address the shortcomings of the prior art.

[0007] Another technical problem to be solved by the present invention is to provide a method for preparing the ZIF-67 / PDMS hybrid matrix pervaporation membrane.

[0008] The final technical problem to be solved by this invention is to provide the application of the ZIF-67 / PDMS mixed matrix pervaporation membrane in food aroma refilling.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing a ZIF-67 / PDMS hybrid matrix pervaporation membrane includes the following steps:

[0011] Step 1: Dissolve cobalt nitrate hexahydrate in methanol to obtain a cobalt nitrate hexahydrate solution; dissolve 2-methylimidazole and the surfactant sodium docusate (AOT) in methanol to obtain a 2-methylimidazole solution.

[0012] Step 2: Mix cobalt nitrate hexahydrate solution and 2-methylimidazole solution evenly, let stand to obtain ZIF-67 precursor suspension, separate solid and liquid, collect solid, wash, dry to obtain ZIF-67 crystals.

[0013] Step 3: Disperse the ZIF-67 crystals obtained in Step 2 uniformly in ethanol to obtain a dispersion. Add polydimethylsiloxane to the dispersion and disperse it evenly to obtain a suspension. Add a crosslinking agent and a catalyst to the suspension and stir to obtain a ZIF-67 / PDMS coating solution.

[0014] Step 4: Coat the ZIF-67 / PDMS coating solution evenly onto the surface of the PVDF substrate membrane, crosslink it at room temperature, and then dry it to obtain the final product.

[0015] In step 1, the concentration of the cobalt nitrate hexahydrate solution is 0.005 ~ 0.02 g / mL, preferably 0.015 g / mL; in the 2-methylimidazole solution, the concentration of the 2-methylimidazole solution is 0.01 ~ 0.03 g / mL, preferably 0.02 g / mL, and the concentration of docusate sodium (AOT) is 0.001 ~ 0.002 g / mL, preferably 0.002 g / mL.

[0016] In step 2, the cobalt nitrate hexahydrate solution and the 2-methylimidazole solution are mixed at a volume ratio of 1:1 to 2, preferably at a volume ratio of 1:1.

[0017] Preferably, in step 2, the settling time is 30 to 60 minutes.

[0018] In some embodiments, in step 2, before mixing the cobalt nitrate hexahydrate solution and the 2-methylimidazole solution, the cobalt nitrate hexahydrate solution and the 2-methylimidazole solution are ultrasonically dispersed for 10 to 30 minutes to ensure that the cobalt nitrate hexahydrate solution and the 2-methylimidazole solution are uniformly dispersed in methanol.

[0019] In step 2, the solid-liquid separation is performed by centrifugation at 7000-8000 rpm for 8-10 minutes; the washing agent used is methanol; and the drying temperature is 60-80℃.

[0020] In step 3, the mass ratio of ZIF-67 crystals to ethanol is 1:13 to 100, preferably 1:20 to 40, and most preferably 1:40; the mass ratio of polydimethylsiloxane to ZIF-67 dispersion is 15 to 20:1, preferably 18:1.

[0021] In step 3, the crosslinking agent is selected from any one of tetraethyl orthosilicate, 3-methacryloyloxypropylmethyldimethoxysilane, vinyltrimethoxysilane and phenyltriethoxysilane, preferably tetraethyl orthosilicate; the catalyst is dibutyltin dilaurate or dioctyltin dilaurate, preferably dibutyltin dilaurate.

[0022] In step 3, the mass ratio of polydimethylsiloxane to the crosslinking agent is 10 to 15:1, preferably 10:1; the amount of catalyst added is 5% to 6% of the mass of polydimethylsiloxane, preferably 5%.

[0023] In step 3, the stirring is carried out until the viscosity of the ZIF-67 / PDMS coating solution is 5000 ~ 10000 cp.

[0024] In step 4, the coating thickness of the ZIF-67 / PDMS coating solution on the PVDF substrate film surface is 10 ~ 25 μm, preferably 15 μm.

[0025] In step 4, the drying temperature is 60-80℃.

[0026] Furthermore, the ZIF-67 / PDMS hybrid matrix pervaporation membrane prepared by the above method is also within the scope of protection of this invention.

[0027] Furthermore, this invention also claims the application of the ZIF-67 / PDMS mixed matrix pervaporation membrane prepared by the above method in food aroma refilling.

[0028] Preferably, the food is fruit juice, vegetable juice, tea product, or tea beverage.

[0029] More preferably, the food is fruit juice.

[0030] Most preferably, the food is strawberry juice.

[0031] Preferably, when the ZIF-67 / PDMS mixed matrix pervaporation membrane is applied to food aroma backfilling, the aromatic compounds in the food are separated under the following process conditions: the liquid temperature is 30-50°C, the vacuum degree is 300-600 Pa, and the feed rate is 0.16 L / min-0.40 L / min.

[0032] More preferably, when the ZIF-67 / PDMS mixed matrix pervaporation membrane is applied to food aroma backfilling, the aromatic compounds in the food are separated under the following process conditions: the liquid temperature is 50°C, the vacuum degree is 400~600 Pa, and the feed rate is 0.32 L / min.

[0033] The ZIF-67 / PDMS hybrid matrix pervaporation membrane prepared using the method of this invention is used for pervaporation separation of volatile aromatic components in food. The permeate-side solution rich in aromatic compounds is collected. After the retentate-side solution undergoes a series of processing steps to form the product, the permeate-side solution rich in aromatic compounds is backfilled into the product, thus achieving aroma restoration. Utilizing the ZIF-67 / PDMS hybrid matrix pervaporation membrane to restore product aroma avoids the loss of aromatic compounds during product processing and maximizes the preservation of the product's natural aroma.

[0034] Zeolite imidazole ester framework material (ZIF) is based on Zn 2+ or Co 2+ ZIF-67 is a porous crystalline material formed by the coordination of a central ion with imidazole ester linkers. The metal-linker-metal bonding angle (-145°) in ZIF is similar to the bonding angle of TOT in zeolites, thus forming a zeolite-like topology. ZIF-67 is a three-dimensional porous material with nanoscale pores, formed by the connection of cobalt ions with 2-methylimidazolium to create a zeolite-like SOD-like topological network structure. Compared to many other MOF materials, ZIF-67 exhibits good stability in water and common organic solvents and can withstand certain high temperatures. Furthermore, ZIF-67 possesses abundant microporous structure, high specific surface area, and high hydrophobicity, which enhances the adsorption capacity and selectivity of the membrane. The nitrogen and oxygen functional groups on its surface can interact with the PDMS matrix, providing additional channels for mass transfer and diffusion of organic molecules within the membrane.

[0035] Beneficial effects:

[0036] ZIF-67 possesses abundant microporous structure, high specific surface area, and high hydrophobicity. This invention utilizes sodium docusate (AOT) surfactant to coat ZIF-67 particles, reducing their surface energy and preventing aggregation. This allows ZIF-67 to be uniformly distributed in a PDMS-containing coating solution. The resulting ZIF-67 / PDMS hybrid matrix membrane exhibits strong hydrophobicity and an extremely high specific surface area (up to 1500 m² / g), enhancing the membrane's adsorption capacity and selectivity. Simultaneously, the two biporous structures (0.34 nm and 1.1 nm) of the ZIF-67 crystals provide additional permeation channels for organic molecules, improving the membrane separation efficiency of organic aromatic compounds. Compared to a simple PDMS pervaporation membrane, the ZIF-67 / PDMS hybrid matrix membrane significantly improves the separation factor (i.e., separation coefficient) for the linalool and water mixture, achieving a membrane flux of 600–700 mg·m³. -2 ·h -1 This technology is applied to the pervaporation process to separate aromatic compounds from food. After the retentate solution undergoes a series of processing steps to form the product, the aromatic-rich pervaporate solution is then backfilled into the product, thus restoring its aroma. Attached Figure Description

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer.

[0038] Figure 1 A scanning electron microscope image of the surface of the ZIF-67 crystal prepared for implementation 1.

[0039] Figure 2 The image shows a scanning electron microscope (SEM) mapping of the ZIF-67 / PDMS hybrid matrix pervaporation membrane II prepared in Example 1.

[0040] Figure 3 Scanning electron microscope image of a cross section of the ZIF-67 / PDMS hybrid matrix pervaporation membrane II prepared in Example 1.

[0041] Figure 4 Scanning electron microscope image of the surface of the PDMS pervaporation membrane PDMS-15 prepared in Example 2.

[0042] Figure 5 This is a flowchart of the process of using a pervaporation membrane to refill the aroma of strawberry juice in Example 6.

[0043] Figure 6 The images show the total ion chromatograms of volatile substances in the strawberry juice stock solution, the retentate side solution, and the strawberry concentrate in Example 6. Detailed Implementation

[0044] The present invention will be further described below with reference to the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0045] Example 1

[0046] Preparation of ZIF-67 / PDMS hybrid matrix pervaporation membrane:

[0047] (1) Dissolve 2.91 g Co(NO3)2·6H2O in 200 mL of methanol to obtain a cobalt nitrate hexahydrate solution; dissolve 3.85 g 2-methylimidazole and 0.385 g AOT in 200 mL of methanol to obtain a 2-methylimidazole solution; place the two solutions in an ultrasonic cleaner and ultrasonically disperse for 20 min to ensure that Co(NO3)2·6H2O and 2-methylimidazole are uniformly dispersed in methanol. Mix the two solutions evenly and let them stand for 30 min to obtain a ZIF-67 precursor suspension.

[0048] (2) Centrifuge the ZIF-67 precursor suspension obtained in step (1) at 8000 rpm for 10 min, collect the solid, wash it three times with methanol, and dry it overnight at 60℃ to obtain ZIF-67 crystals. Figure 1 This is a scanning electron microscope image of a ZIF-67 crystal.

[0049] (3) Mix 10 mg, 25 mg, 50 mg, and 75 mg of ZIF-67 crystals with 1 g of ethanol solution, respectively, and sonicate for 1 h to uniformly disperse ZIF-67 in the ethanol solution, obtaining ZIF-67 dispersions of 1% w / w, 2.5% w / w, 5% w / w, and 7.5% w / w, respectively, for later use. Take 0.167 g of each of the 1% w / w, 2.5% w / w, 5% w / w, and 7.5% w / w ZIF-67 dispersions and mix them with 3 g of PDMS, respectively, and place them in an ultrasonic cleaner to disperse for 10 min to ensure uniform dispersion of the mixed solution. After magnetic stirring for 30 min, suspensions I, II, III, and IV are obtained, respectively.

[0050] (4) Add 0.3 g of tetraethyl orthosilicate and 0.15 g of dibutyltin dilaurate to the suspensions I, II, III and IV obtained in step (3), respectively, and stir continuously until the mixture becomes viscous (viscosity value is about 10000 cp) to obtain ZIF-67 / PDMS coating solution. Slowly pour the coating solution onto the surface of PVDF membrane (pore size is 0.1 μm), and use an adjustable coating applicator to coat it evenly (coating thickness is 15 μm); let it stand at room temperature to fully crosslink and solidify, and then place it in an oven at 80℃ for 24 h to obtain ZIF-67 / PDMS mixed matrix pervaporation membranes I, II, III and IV, respectively. Figure 2 The image shows a scanning electron microscope mapping of the ZIF-67 / PDMS hybrid matrix pervaporation membrane II. It can be seen from the image that the Co element is relatively uniformly distributed on the membrane surface. Figure 3 The image shows a cross-section of the ZIF-67 / PDMS hybrid matrix pervaporation membrane II. It can be seen that the ZIF-67 / PDMS separation layer and the support layer are tightly bonded and the interface is relatively clear. ZIF-67 crystals are uniformly dispersed in PDMS.

[0051] Example 2

[0052] ZIF-67 crystal addition amount optimized.

[0053] Linalool-water separation experiments were conducted using ZIF-67 / PDMS hybrid matrix pervaporation membranes I, II, III, and IV (hereinafter referred to as ZIF-67 / PDMS-I, ZIF-67 / PDMS-II, ZIF-67 / PDMS-III, and ZIF-67 / PDMS-IV) prepared in Example 1. The feed temperature during pervaporation was 50°C, the feed rate was 0.32 L / min, and the pervaporation side pressure was maintained at 400–600 Pa using a vacuum pump. The feed solution to be separated was a mixture of linalool and water at a ratio of 50 mg: 1 L, and the processing time was 2 h. The pervaporated vapor was collected in a liquid nitrogen cold trap and liquefied. The concentration of linalool was determined by liquid chromatography.

[0054] In comparison, linalool-water separation experiments were conducted using PDMS pervaporation membranes, and the experimental procedures were the same as those for the ZIF-67 / PDMS mixed matrix pervaporation membranes I, II, III, and IV described above.

[0055] The preparation method of PDMS pervaporation membrane is as follows: 0.167 g of ethanol and 3 g of PDMS are uniformly mixed and dispersed in an ultrasonic cleaner for 10 min to ensure uniform dispersion. After magnetic stirring for 30 min, a suspension is obtained. 0.3 g of tetraethyl orthosilicate and 0.15 g of dibutyltin dilaurate are added to the suspension, and stirring is continued until the mixture becomes viscous to obtain the PDMS coating solution. The coating solution is slowly poured onto the surface of a PVDF membrane with a pore size of 0.1 μm, and uniformly coated using an adjustable coating applicator to a thickness of 15 μm. After standing at room temperature for full cross-linking and solidification, the membrane is then placed in an oven at 80℃ for 24 h to obtain PDMS pervaporation membrane-15 (abbreviated as PDMS-15). Scanning electron microscope images of the PDMS-15 surface are shown below. Figure 4 As shown in the figure, the PDMS-15 membrane surface is flat and smooth, with no obvious defects.

[0056] The pervaporation performance of a membrane is characterized by membrane flux and separation factor (separation coefficient). J ) and separation factor ( α The calculation method for ) is as follows:

[0057]

[0058] in, J —Total flux of membrane material (g·m) -2 ·h -1 ); W —Mass of the liquid on the permeate side (g); A —Effective area of ​​the membrane (m²) 2 ); Δt —Pervaporation time (h).

[0059]

[0060] in, Y A , Y B These represent the concentrations of substances A (linalool) and B (water) in the osmotic liquid, respectively, in mg / L. X A , X B The values ​​represent the concentrations of substance A (linalool) and substance B (water) in the feed solution, respectively, in mg / L.

[0061] The experimental results are shown in Table 1.

[0062] Table 1. Pervaporation performance of different membranes

[0063]

[0064] As shown in Table 1, when the mass percentage of ZIF-67 crystals to ethanol was 2.5% during the preparation of the coating solution, the separation factor and permeate flux of the mixed matrix pervaporation membrane were the highest. This is likely because when the ZIF-67 crystal loading was low, it could effectively provide permeate channels for PDMS, thereby accelerating the diffusion of aroma components in the membrane. However, with further increases in the ZIF-67 loading, ZIF-67 crystals would form particle aggregates on the membrane surface, leading to a reduction in the porosity within the polymer and decreasing the mass transfer channels for aroma components, thus causing a gradual decrease in the membrane's permeate flux and separation factor. Therefore, the mass percentage of ZIF-67 crystals to ethanol was set at 2.5% during the preparation of the coating solution.

[0065] Since the linalool-water mixture and strawberry juice contain similar volatile aromatic components, and both linalool and strawberry juice have high volatility, they are suitable for separation using pervaporation technology. Secondly, both strawberry juice and the linalool-water system use water as a solvent, and their molecular sizes and polarities are similar, resulting in similar permeation behavior and separation patterns. Therefore, the experimental conditions of the linalool-water system, such as membrane selection, temperature, concentration, and feed rate, can provide a reference for the pervaporation of strawberry juice, helping to optimize experimental design and improve the extraction efficiency of aromatic components. Therefore, this invention uses the linalool-water mixture instead of strawberry juice for optimizing pervaporation conditions.

[0066] Example 3

[0067] Experiment on optimizing the temperature of the feed liquid during pervaporation.

[0068] Linalool-water separation experiments were conducted using the ZIF-67 / PDMS hybrid matrix pervaporation membrane II prepared in Example 1 to investigate the separation performance of ZIF-67 / PDMS-II at different feed temperatures. The feed temperatures during pervaporation were set at 30℃, 35℃, 40℃, 45℃, and 50℃, with a feed rate of 0.32 L / min. The pervaporation side pressure was maintained at 400–600 Pa using a vacuum pump. The feed solution to be separated was a mixture of linalool and water at a ratio of 50 mg: 1 L, and the processing time was 2 h. The pervaporation vapor was collected in a liquid nitrogen cold trap and liquefied, and the concentration of linalool was determined by liquid chromatography. The pervaporation performance of the membrane was characterized by flux and separation coefficient. The experimental results are shown in Table 2.

[0069] Table 2. Separation performance of pervaporation membranes at different feed temperatures

[0070]

[0071] Table 2 shows that the flux and separation factor of the ZIF-67 / PDMS mixed matrix pervaporation membrane for linalool gradually increase with increasing operating temperature. At 50℃, both the separation factor and permeate flux are highest, at 26.48 and 628.02 mg·m⁻², respectively. -2 ·h -1 (Data from Example 2) This may be because the vapor pressure on the feed liquid side increases with increasing temperature, enhancing the driving force for adsorption and permeation, accelerating its transport speed, and thus continuously increasing the permeation flux. However, at higher temperatures (above 50°C), the evaporation rate of water accelerates, which may cause an imbalance in the ratio of linalool to water, thereby affecting the solution concentration and the final permeation effect. Furthermore, excessively high temperatures may lead to thermal degradation of linalool, affecting its chemical structure and causing changes in its aroma components or loss of its original aroma. Therefore, the feed liquid temperature should not be increased further; selecting a moderate temperature can both increase the permeation flux and maximize the retention of linalool components.

[0072] Example 4

[0073] Experiment on optimizing feed concentration during pervaporation.

[0074] Linalool-water separation experiments were conducted using the ZIF-67 / PDMS hybrid matrix pervaporation membrane II prepared in Example 1 to investigate the separation performance of ZIF-67 / PDMS-II under different feed concentrations. The feed temperature during pervaporation was 50℃, the feed rate was 0.32 L / min, and the permeate-side pressure was maintained at 400–600 Pa using a vacuum pump. The feed solutions to be separated were aqueous linalool solutions with concentrations of 20 mg / L, 30 mg / L, and 50 mg / L, respectively, and the processing time was 2 h. The permeate-side vapor was collected in a liquid nitrogen cold trap and liquefied. The concentration of linalool was determined by liquid chromatography. The pervaporation performance of the membrane was characterized by flux and separation coefficient. The experimental results are shown in Table 3.

[0075] Table 3. Separation performance of pervaporation membranes at different feed concentrations

[0076]

[0077] Table 3 shows that, using the ZIF-67 / PDMS mixed matrix pervaporation membrane as the separation medium in the pervaporation experiment of the linalool-water binary system, both the linalool flux and the separation factor increased with increasing feed concentration, reaching their maximum at 50 mg / L. This is likely because the increased adsorption of aromatic compounds in the membrane at high concentrations increases the driving force for permeation, thus leading to a continuous increase in permeation flux.

[0078] Example 5

[0079] Experiment on optimizing feed rate during pervaporation.

[0080] Linalool-water separation experiments were conducted using the ZIF-67 / PDMS hybrid matrix pervaporation membrane II prepared in Example 1 to investigate the separation performance of ZIF-67 / PDMS-I under different feed rates. The feed temperature during pervaporation was 50℃, the feed concentration was 50 mg / L, and the pervaporation side pressure was maintained at 400–600 Pa using a vacuum pump. The feed rates during pervaporation were 0.16 L / min, 0.24 L / min, 0.32 L / min, and 0.40 L / min, respectively, and the processing time was 2 h. The pervaporated vapor was collected in a liquid nitrogen cold trap and liquefied. The concentration of linalool was determined by liquid chromatography. The pervaporation performance of the membrane was characterized by flux and separation coefficient. The experimental results are shown in Table 4.

[0081] Table 4. Separation performance of pervaporation membranes at different feed rates

[0082]

[0083] Table 4 shows that, using the ZIF-67 / PDMS mixed matrix pervaporation membrane as the separation medium in the pervaporation experiment of the linalool-water binary system, the linalool flux and separation factor exhibited a trend of first increasing and then decreasing. Both the pervaporation flux and separation factor reached their maximum values ​​at a circulation flow rate of 0.32 L / min. This phenomenon can be analyzed based on concentration polarization and the gel model: when flux is controlled by concentration polarization, increasing the flow rate helps improve membrane flux, but excessive flow rate leads to a decrease in the pressure difference across the membrane, which in turn causes a decrease in flux.

[0084] Example 6

[0085] Using the ZIF-67 / PDMS hybrid matrix pervaporation membrane II prepared in Example 1 as the separation medium, volatile aromatic compounds in the feed liquid (strawberry juice) were separated by pervaporation. The feed liquid temperature during pervaporation was 50°C, the feed rate was 0.32 L / min, the pervaporation side pressure was maintained at 400-600 Pa using a vacuum pump, and the processing time was 9 h. The pervaporated vapor was collected in a liquid nitrogen cold trap and liquefied. The aromatic components and their contents in the feed liquid and the retentate side solution were detected using solid-phase microextraction combined with GC-MS (SPME-GC-MS) (both the retentate side solution and the feed liquid (strawberry juice) were diluted by 1-fold during detection).

[0086] Simultaneously, the retentate side solution was concentrated, and then the pervaporation side solution rich in aromatic components collected during pervaporation was added to it to achieve strawberry aroma refill, thus preparing the final strawberry concentrate. The aromatic components and their contents in the aroma-refilled strawberry concentrate were detected by solid phase microextraction combined with GC-MS (SPME-GC-MS) method (the strawberry concentrate was diluted to the same volume as the raw strawberry juice and then diluted by 1 time before detection).

[0087] Figure 5 This is a flowchart illustrating the use of a pervaporation membrane for aroma refilling of strawberry juice in this embodiment.

[0088] The experimental results are shown in Table 5 and Figure 6 As shown.

[0089] The results showed that after 9 hours of continuous pervaporation, the content of volatile aromatic compounds in the retentate solution was significantly reduced compared to the original strawberry juice, indicating that pervaporation has a good separation and recovery effect on volatile aromatic compounds in strawberry juice. Notably, no ketones were detected in the retentate solution, suggesting that ZIF-67 / PDMS-II has high selectivity for ketone separation. Furthermore, four new compounds were detected in the retentate solution. These may be due to the selective adsorption of the membrane, causing insignificant substances in the feed solution to be adsorbed by the membrane material or forming new compounds under the membrane's action, ultimately appearing in the retentate solution. Alternatively, temperature changes during pervaporation may have caused chemical reactions or decomposition reactions of certain components in the feed solution, or these compounds may have already existed in the feed solution but were masked by other substances with similar properties during detection due to their low concentration. After the pervaporation solution was backfilled, the composition and content of volatile aromatic compounds in the strawberry concentrate were restored. This indicates that the backfilling application after pervaporation technology can effectively reduce the loss and damage of aromatic components in strawberry juice during the concentration process, and better ensure the flavor quality and nutritional and health benefits of the strawberry concentrate product.

[0090] Table 5. Volatile aromatic compounds and their contents in the raw material liquid, retentate side solution, and strawberry concentrate after aroma refilling.

[0091]

[0092] This invention provides a ZIF-67 / PDMS mixed matrix pervaporation membrane, its preparation method, and its application in food aroma refilling. Many methods and approaches exist to achieve this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method for preparing a ZIF-67 / PDMS mixed matrix pervaporation membrane, characterized in that, Comprising the following steps: Step 1, dissolving cobalt nitrate hexahydrate in methanol to obtain a cobalt nitrate hexahydrate solution; dissolving 2-methylimidazole and sodium docusate in methanol to obtain a 2-methylimidazole solution; Step 2, mixing the cobalt nitrate hexahydrate solution and the 2-methylimidazole solution obtained in step 1 uniformly, standing to obtain a ZIF-67 precursor suspension, solid-liquid separation, collecting the solid, washing, drying to obtain ZIF-67 crystals; Step 3, dispersing the ZIF-67 crystals obtained in step 2 in ethanol uniformly to obtain a ZIF-67 dispersion, adding polydimethylsiloxane to the ZIF-67 dispersion to disperse uniformly to obtain a suspension, adding a crosslinking agent and a catalyst to the suspension, stirring to obtain a ZIF-67 / PDMS coating solution; Step 4, uniformly coating the ZIF-67 / PDMS coating solution on the surface of a PVDF base film, crosslinking at room temperature and drying to obtain the product.

2. The method of claim 1, wherein, In step 1, the concentration of the cobalt nitrate hexahydrate solution is 0.005 ~ 0.02 g / mL; in the 2-methylimidazole solution, the concentration of the 2-methylimidazole solution is 0.01 ~ 0.03 g / mL, and the concentration of sodium docusate is 0.001 ~ 0.002 g / mL.

3. The method of claim 1, wherein, In step 2, the cobalt nitrate hexahydrate solution and the 2-methylimidazole solution are mixed in a volume ratio of 1:1 ~ 2.

4. The method of claim 1, wherein, In step 3, the mass ratio of the ZIF-67 crystals to ethanol is 1:30 ~ 50; the mass ratio of the polydimethylsiloxane to the ZIF-67 dispersion is 15 ~ 20:

1.

5. The method of claim 1, wherein, In step 3, the crosslinking agent is selected from any one of tetraethyl orthosilicate, 3-methacryloxypropylmethyldimethoxysilane, vinyltrimethoxysilane and phenyltriethoxysilane; the catalyst is dibutyltin dilaurate or dioctyltin dilaurate.

6. The method of claim 1, wherein, In step 3, the mass ratio of the polydimethylsiloxane to the crosslinking agent is 10 ~ 15:1; the addition amount of the catalyst is 5% ~ 6% of the mass of the polydimethylsiloxane.

7. The method of claim 1, wherein, In step 3, the stirring is stirring until the viscosity value of the ZIF-67 / PDMS coating solution is 5000 ~ 10000 cp.

8. The method of claim 1, wherein, In step 4, the coating thickness of the ZIF-67 / PDMS coating solution on the surface of the PVDF base film is 10 ~ 25 μm.

9. The ZIF-67 / PDMS mixed matrix pervaporation membrane prepared by the method of any one of claims 1 ~ 8.

10. The application of the ZIF-67 / PDMS mixed matrix pervaporation membrane of claim 9 in food aroma backfilling.

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