Method for preparing MOF-nanocellulose aerogel micromembrane by casting

Abstract

The application discloses a preparation method of MOF-nanocellulose aerogel micromembrane prepared by a flow casting method and belongs to the field of gas adsorption and separation materials. The method comprises the following steps: preparing a hydrogel by high-pressure homogenization of TEMPO-oxidized nanocellulose; mixing hydrophobic MOFs with the hydrogel, and obtaining a composite hydrogel slurry by high-pressure homogenization; casting the slurry on a composite substrate composed of a lower PTFE film and an upper hydrophilic filter film to form a film, adding dilute hydrochloric acid dropwise through the hydrophilic filter film to induce sol-gel conversion, removing the filter film, and then performing supercritical drying, thereby obtaining the micromembrane. The application realizes the nanoscale uniform dispersion of MOFs in nanocellulose by high-pressure homogenization, and obtains the aerogel micromembrane with a micrometer-level thickness and a nanometer-level surface roughness by synergistic control of asymmetric double-interface limited flow casting and interfacial gelation. The micromembrane has a hierarchical pore structure and excellent VOCs adsorption and separation performance, and the process is controllable and easy to scale up.

Description

Technical Field

[0001] This invention relates to the technical field of gas adsorption and separation material preparation methods, specifically a method for preparing MOF-nanocellulose aerogel micron-sized films by casting. Background Technology

[0002] With the acceleration of industrialization, volatile organic compounds (VOCs) have become serious air pollutants, widely impacting the environment and human health. VOCs not only cause ozone pollution and photochemical smog, but also pose long-term risks to the human respiratory system, liver and kidney function, and nervous system. With industrial development and the increase in the number of automobiles, VOC emissions have become increasingly serious, becoming a key focus of global environmental protection. The World Health Organization has established safe limits for VOC concentrations to protect public health. Therefore, developing efficient and stable materials for capturing and detecting low-concentration VOCs, and preparing novel materials with selective adsorption properties, is a major technical challenge concerning human health and economic development. Aerogels are a class of novel materials with a continuous three-dimensional nanoporous network structure, using gas as the dispersion medium. Due to their excellent properties such as low density, high porosity, and large specific surface area, they are widely used in filtration devices and other fields.

[0003] Currently, most traditional air filtration membrane separation materials are synthetic polymers, primarily plastic products synthesized from petrochemical raw materials such as polypropylene and polyethylene terephthalate. However, their preparation process is limited by the inherent conditions of the melt-blowing process, resulting in fiber diameters often at the micrometer scale, large pores, limited filtration efficiency for fine particles, and high costs. Therefore, developing a novel MOF-nanocellulose aerogel micron-sized film has become a pressing issue for those skilled in the art. Summary of the Invention

[0004] This application provides a method for preparing MOF-nanocellulose aerogel micron film by casting, wherein the film comprises a three-dimensional nanoporous network structure assembled from carboxyl-containing nanocellulose, and hydrophobic metal-organic framework material particles uniformly dispersed and stably loaded in the three-dimensional nanoporous network structure by high-pressure homogenization. The surface of the film in contact with the hydrophobic substrate during the preparation process is set to a nanoscale roughness. The film has a hierarchical pore structure consisting of macropores and mesopores provided by nanocellulose aerogel and micropores provided by MOF particles.

[0005] This application also provides a method for preparing MOF-cellulose nanogel microfilms by casting, comprising the following steps: Step 1: Preparation of carboxyl-containing nanocellulose hydrogel: TEMPO-oxidized carboxyl-containing nanocellulose was dispersed in deionized water, and then subjected to primary dispersion by a homogenizer, followed by ultrasonic cell disruption and then high-pressure homogenization at levels 1, 2, and 3 to obtain nanocellulose hydrogel. Step 2: Preparation of MOF-nanocellulose composite hydrogel slurry: Hydrophobic MOFs materials are pre-dispersed in deionized water to obtain MOFs pre-dispersion liquid; the MOFs pre-dispersion liquid is mixed with the nanocellulose hydrogel obtained in Step 1, and then subjected to high-pressure homogenization treatment under three-stage high pressure conditions to achieve nanoscale uniform dispersion and stable embedding of hydrophobic MOFs particles in hydrophilic nanocellulose hydrogel, thus obtaining composite hydrogel slurry; Step 3, Casting and Interfacial Induced Gelation: The composite hydrogel slurry obtained in Step 2 is subjected to vacuum degassing and then poured into a casting machine for casting. The lower layer of the casting machine is laid with a polytetrafluoroethylene (PTFE) film as a hydrophobic substrate, and the upper layer is covered with a hydrophilic filter membrane. The composite hydrogel slurry is spread between the lower PTFE film and the upper hydrophilic filter membrane to form a composite wet film. Dilute hydrochloric acid is added to the surface of the hydrophilic filter membrane by an inclined droplet. The hydrochloric acid is uniformly diffused into the composite wet membrane by the capillary permeation of the hydrophilic filter membrane, inducing the composite wet membrane to undergo sol-gel transformation. After standing, the hydrophilic filter membrane is removed to obtain a wet micron film attached to the surface of the lower PTFE film. The side of the wet micron film in contact with the PTFE film has a surface with nanoscale flatness. Step 4, Post-treatment and drying: The wet micron film attached to the PTFE film obtained in step 3 is successively immersed in a hydrochloric acid-ethanol mixed solution and pure ethanol for solvent replacement, and then supercritical drying is performed to obtain the MOF-nanocellulose aerogel micron film.

[0006] Optionally, the TEMPO-oxidized carboxyl-containing nanocellulose described in step one is prepared by the following method: after removing inorganic minerals by acid pretreatment of pulp lignocellulose, the pH is controlled at 10 in the TEMPO oxidation system to selectively oxidize the hydroxyl group at the C6 position of cellulose to a carboxyl group. Then, sodium borohydride is added to reduce the incompletely reacted aldehyde cellulose intermediate to hydroxy cellulose, thereby obtaining TEMPO-oxidized cellulose containing carboxyl groups.

[0007] Optionally, in the nanocellulose hydrogel described in step one, the ratio of TEMPO oxidized cellulose to deionized water is 100 mL of deionized water for every 1 g of TEMPO oxidized cellulose; in the composite hydrogel slurry described in step two, the mass fraction of nanocellulose is 0.5% and the total solids content is 1%.

[0008] Optionally, the hydrophobic MOFs material is pre-dispersed in an ultrasonic cell disruptor for 5 min before being added to the nanocellulose hydrogel to obtain a MOFs pre-dispersion.

[0009] Optionally, in step two, high-pressure homogenization is performed under three-stage high-pressure conditions. Through the strong shear force, cavitation effect, and collision effect generated in the high-pressure microscale flow field, the van der Waals forces between hydrophobic MOF particles are forcibly broken, thereby achieving nanoscale deagglomeration and uniform dispersion of MOF particles in nanocellulose hydrogel. At the same time, the interfacial adhesion between MOFs and carboxyl-containing nanocellulose is enhanced, preventing component segregation and sedimentation during the subsequent casting process.

[0010] Optionally, the speed of the casting machine in step three is 0.1-0.5 m / min, and the blade gap is 250-750 μm; The hydrophilic filter membrane has a pore size of 0.22 μm; the concentration of the dilute hydrochloric acid is 1 mol / L. The settling time is 2 hours.

[0011] Optionally, in step four, the volume ratio of hydrochloric acid to ethanol in the hydrochloric acid-ethanol mixed solution is 1:1. The supercritical drying is used to remove the solvent under supercritical fluid conditions to avoid the collapse of the aerogel three-dimensional nanoporous network structure caused by capillary forces during the drying process.

[0012] Optionally, the pressure of the first-stage high-pressure homogenization treatment in step one is 300 bar, the pressure of the second-stage high-pressure homogenization treatment is 500 bar, and the pressure of the third-stage high-pressure homogenization treatment is 700 bar, and so on. The 700 bar treatment is used to achieve the final nanoscale dispersion of nanocellulose in deionized water. The pressure of the three-stage high-pressure homogenization process in step two is higher than that in step one. It is used to generate stronger shear force and cavitation effect in the MOFs and nanocellulose mixture system to overcome the aggregation tendency of hydrophobic MOFs particles.

[0013] The beneficial effects of this application are as follows: 1. This invention employs a casting method to replace traditional solution casting or vacuum filtration methods. The casting process, combined with precisely controlled doctor blade gap and casting speed, enables film thickness control at the micrometer level and achieves high uniformity across the entire film. Compared to the edge effects and thickness gradients caused by solvent evaporation in solution casting, the casting process of this invention is completed within a confined space, completely eliminating the problem of uneven thickness. Compared to the gradient structure defects of a dense bottom layer and a sparse surface layer caused by pressure gradients in vacuum filtration, the aerogel film prepared by this invention maintains isotropic three-dimensional nanoporous network, thereby ensuring stable and predictable permeation rates and selectivity during gas separation. 2. This invention innovatively employs an asymmetric dual-interface confined system composed of a lower low-surface-energy polytetrafluoroethylene (PTFE) film and an upper hydrophilic filter membrane. The lower PTFE film possesses hydrophobicity and extremely high surface smoothness. After the composite hydrogel slurry is spread on its surface, a smooth wet film interface is formed through surface tension rearrangement. The upper hydrophilic filter membrane not only physically constrains the slurry thickness but, more importantly, allows the gel inducing agent to diffuse uniformly and gently into the wet film through capillary permeation by dripping dilute hydrochloric acid onto the filter membrane surface, avoiding wrinkles, skinning, or large bubble defects caused by direct acid impact on the slurry surface. After the sol-gel conversion, the hydrophilic filter membrane is removed, revealing a smooth surface with nanoscale roughness on the side of the wet film in contact with the PTFE. This high smoothness provides a superior substrate interface for subsequent multilayer membrane construction or high-precision gas sensor integration, while eliminating permeation rate and selectivity deviations caused by local thickness or surface morphology differences in the film, significantly improving the precision of VOCs capture. Attached Figure Description

[0014] Figure 1 The process steps for preparing MOF-cellulose nanogel microfilms by casting are described. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] This embodiment includes a core embodiment, multiple embodiments with varying parameters, and comparative examples, in order to fully support the scope of protection of the claims and verify the beneficial effects of the present invention.

[0017] Example 1: Preparation of MOF-cellulose nanogel microfilm (CelloZIFs-1) Raw materials used in this embodiment: SBKP pulp: Southern pine bleached sulfate pulp, with a moisture content of approximately 70%; TEMPO: 2,2,6,6-Tetramethylpiperidine oxygen radical, analytical grade, Aladdin reagent; NaBr: Sodium bromide; NaClO: Sodium hypochlorite solution, with an available chlorine content of 10%; NaBH4: Sodium borohydride, analytical grade, Aladdin reagent; ZIF-8: Zeolite imidazolium ester framework material-8, with a particle size of approximately 200 nm; Deionized water: resistivity ≥ 18.2 MΩ·cm; Hydrochloric acid: HCl, analytical grade, concentration 36%-38%; Ethanol: Anhydrous ethanol, analytical grade; PTFE film: 0.1mm thickness; Hydrophilic filter membrane: mixed cellulose ester membrane, pore size 0.22μm, diameter 47 mm; The preparation steps in this embodiment are as follows: Step 1: Pretreatment of SBKP pulp lignocellulose: 100 g of SBKP pulp was weighed (wet weight) and added to a 1 mol / L hydrochloric acid solution. The pH of the system was adjusted to 2, and the mixture was mechanically stirred at 1000 rpm for 3 hours to remove inorganic minerals. After the reaction, the pulp was filtered through a G3 sintered glass funnel and repeatedly washed with deionized water until the filtrate was neutral. The acidity or alkalinity of the filtrate was determined using pH test paper. The resulting wet pulp was collected, and its cellulose content was determined to be 28.5% using a Mettler Toledo HE53 moisture analyzer.

[0018] Step 2: Preparation of TEMPO-oxidized cellulose: The reaction solution was prepared by adding 0.016 g TEMPO, 0.1 g NaBr, and 100 mL deionized water per gram of dry-weight cellulose. 35.1 g of pretreated wet pulp (equivalent to 10 g of dry-weight cellulose) was weighed and added to the reaction solution, and the stirring speed was adjusted to 400 rpm. Then, NaClO solution (equivalent to 56.8 mL, 7.5 mmol / g cellulose) was added, with an available chlorine content of 10%. The pH of the system was adjusted to 10 by adding 1 mol / L HCl dropwise to initiate the oxidation reaction. During the reaction, 0.5 mol / L NaOH solution was continuously added dropwise using an automatic titrator (Mettler-Toledo G20S) to maintain the pH of the system at 10 ± 0.1. The reaction was carried out at room temperature (25°C) for 2 h. After the oxidation reaction was complete, 1 g of sodium borohydride was added to the reaction solution at a ratio of 0.1 g of sodium borohydride per gram of cellulose, and the reaction was continued for 3 hours to reduce the incompletely reacted aldehyde cellulose intermediate to hydroxy cellulose. The reaction product was filtered using a G3 sintered glass funnel and repeatedly washed with deionized water until the filtrate was neutral and had a conductivity of <5 μS / cm. The oxidized cellulose was collected, and its moisture content was measured to be 92% using a moisture analyzer. The dry basis content was recorded as 8%.

[0019] Step 3: Preparation of nanocellulose hydrogels: TEMPO oxidized cellulose, equivalent to 1g dry weight (12.5g wet weight), was dispersed in 100 mL deionized water and homogenized for 2 min using an IKA T25 homogenizer. Subsequently, it was processed for 10 min using an ultrasonic cell disruptor (Xinzhi JY92-IIN, 650 W, 2 s operation / 3 s interval). The resulting dispersion was then subjected to three stages of high-pressure homogenization at 300 bar, 500 bar, and 700 bar using an ATS AH-2010 microfluidic homogenizer, with two cycles per stage, yielding a pale blue, transparent nanocellulose hydrogel. Particle size analysis showed that the average diameter of the obtained nanocellulose fibers was 15±5 nm.

[0020] Step 4: Preparation of MOF-cellulose nanocomposite hydrogel slurry: Take 0.5 g of ZIF-8 powder, a hydrophobic MOF with a BET specific surface area of ​​approximately 1600 m². 2 / g was added to 100 mL of deionized water and pre-dispersed in an ultrasonic cell disruptor for 5 min at 650 W for 2 s on and 3 s off to obtain a ZIF-8 pre-dispersion. This pre-dispersion was added to the nanocellulose hydrogel obtained in step three, and after mixing, it was passed through a high-pressure microfluidic homogenizer for three cycles at 700 MPa to achieve nanoscale uniform dispersion of ZIF-8 particles and nanocellulose in the aqueous system, resulting in a composite hydrogel slurry. Field emission scanning electron microscopy revealed that ZIF-8 particles were uniformly distributed within the nanocellulose network as single particles or a few small aggregates with a size <100 nm; no aggregates larger than 500 nm were observed. Solid content determination showed that the mass fraction of nanocellulose in the composite hydrogel slurry obtained in this step was 0.5%, and the total solid content was 1.0%.

[0021] Step 5: Vacuum degassing: The composite hydrogel slurry obtained in step four was placed in a vacuum dryer and degassed for 30 minutes under a vacuum of -0.095 MPa until no obvious bubbles appeared on the surface of the slurry.

[0022] Step Six: Casting and Film Formation and Interface-Induced Gelization A PTFE film measuring 20cm × 30cm is laid as the lower layer in the casting machine, and a hydrophilic mixed cellulose ester filter membrane with a pore size of 0.22μm is placed as the upper layer. The composite hydrogel slurry degassed in step five is evenly poured into the casting machine, and the casting speed is set to 0.3 m / min with a blade gap of 500μm to cast the film and obtain a composite wet membrane. After casting, 1mol / L dilute hydrochloric acid is added to the surface of the hydrophilic filter membrane at an angle using a dropper, at a rate of 100cm. 2 Approximately 2 mL of hydrochloric acid was added dropwise, utilizing the capillary action of the hydrophilic filter membrane to allow the dilute hydrochloric acid to diffuse evenly and gently into the composite wet membrane. After standing for 2 hours, a sol-gel transformation occurred at the interface of the composite wet membrane, changing it from a sol state to a non-flowing gel state. The hydrophilic filter membrane was then gently peeled off; the composite gel membrane was carried away by the hydrophilic filter membrane, resulting in a wet micron-sized film attached to the surface of the PTFE membrane. The surface of this wet film in contact with the PTFE membrane was smooth and flat, visually free of wrinkles, bubbles, or other defects.

[0023] Step 7: Solvent replacement and supercritical drying: The wet microfilm attached to the PTFE film obtained in step six was cut into 5cm × 5cm pieces. These pieces, along with the PTFE support film, were sequentially immersed in a 0.01 mol / L hydrochloric acid-ethanol mixed solution (volume ratio 1:1, 200 mL) for 12 hours, followed by 200 mL of pure ethanol for another 12 hours. Fresh solvent was used once during each immersion. After solvent replacement, the film was transferred to a supercritical dryer (Nano TEM Co., Ltd., SFD-2020) using CO2 as the supercritical fluid. The conditions were: temperature 40℃, pressure 10 MPa, CO2 flow rate 15 g / min, drying time 4 hours. After drying, a MOF nanocellulose aerogel microfilm was obtained, named CelloZIFs-1.

[0024] Performance characterization of the product obtained in this embodiment: (1) Thickness and flatness: A micrometer (Mitutoyo model, 0.001 mm accuracy) was used to measure the thickness at 20 randomly selected points on the film. The average thickness was 48.5 μm, and the standard deviation was 1.2 μm, indicating good thickness uniformity. An atomic force microscope (AFM, Bruker Dimension Icon model) was used to measure the surface roughness of the film on the PTFE contact side within a 5 μm × 5 μm range. The root mean square roughness was 3.2 nm, and the arithmetic mean roughness was 2.5 nm, indicating that the film surface has nanoscale flatness.

[0025] (2) Microstructure: FE-SEM observation showed that the film has a three-dimensional interconnected nanoporous network structure, with macroporous regions having pore sizes ranging from 50 to 500 nm; ZIF-8 particles (approximately 50-150 nm in diameter) were uniformly embedded or attached to the surface of the nanocellulose fibers, without obvious agglomeration. TEM observation further confirmed the uniform distribution of ZIF-8 crystals in the nanocellulose matrix.

[0026] (3) Porosity and specific surface area: The porosity and specific surface area were determined by nitrogen adsorption-desorption method. The BET specific surface area of ​​the film was 620 m². 2 / g, of which micropores <2nm have a specific surface area of ​​450m² 2 / g, mesoporous / macroporous specific surface area is 170 m² 2 / g, with a total porosity of 94.5%.

[0027] (4) Gas Adsorption Performance: The adsorption performance of the membrane for CO2 at 25 °C was determined using the static volumetric method. The results showed that at a low partial pressure of 0.15 bar, the CO2 adsorption capacity was 2.85 mmol / g; at 1 bar, the CO2 adsorption capacity was 3.92 mmol / g. The CO2 adsorption jump pressure was 0.058 bar. Compared with ZIF-8 powder of the same ZIF-8 loading, the control group (without composite and film formation) showed an approximately 25% increase in low partial pressure adsorption capacity and a approximately 30% decrease in jump pressure, indicating that the membrane of this invention has significant advantages in capturing low-concentration gases.

[0028] (5) Mechanical strength: The film was subjected to tensile testing using an Instron 3343 universal testing machine. The tensile strength of the film was 12.5 MPa and the elongation at break was 4.8%, which showed good mechanical integrity and could meet the operation and installation requirements in practical applications.

[0029] Example 2: CelloZIFs films of ZIF-90 with different MOF types: This embodiment is basically the same as Example 1, except that the hydrophobic MOF in step four is replaced by ZIF-90 zeolite imidazole ester framework material-90 with a particle size of about 180 nm, and the addition amount is still 0.5 g. The resulting film is named CelloZIFs-2.

[0030] Performance test results: The average thickness of CelloZIFs-2 was 47.8 μm, and the surface roughness Ra was 2.8 nm. The CO2 adsorption capacity was 2.61 mmol / g at 25 °C and 0.15 bar, and 3.75 mmol / g at 1 bar. The CO2 adsorption jump pressure was 0.065 bar. These results indicate that ZIF-90 can also achieve uniform dispersion and stable loading in the preparation system of this invention, and the resulting film exhibits good gas adsorption performance.

[0031] Example 3: CelloZIFs films with different casting blade gaps of 250 μm This embodiment is basically the same as Embodiment 1, except that the blade gap of the casting machine in step six is ​​adjusted from 500 μm to 250 μm. The resulting film is named CelloZIFs-3.

[0032] Performance test results: The average thickness of CelloZIFs-3 is 23.6 μm, and the surface roughness R... aThe thickness was 2.2 nm. The CO2 adsorption capacity at 25 °C and 0.15 bar, normalized to mass, was 3.01 mmol / g, higher than the 2.85 mmol / g in Example 1. This is because a thinner film results in a shorter gas diffusion path and faster adsorption kinetics per unit mass. However, the tensile strength of the film decreased to 8.9 MPa, indicating that the thinner film had a certain impact on mechanical properties.

[0033] Example 4: CelloZIFs films at different casting speeds of 0.5 m / min: This embodiment is basically the same as Embodiment 1, except that the casting speed in step six is ​​adjusted from 0.3 m / min to 0.5 m / min. The resulting film is named CelloZIFs-4.

[0034] Performance test results: The average thickness of CelloZIFs-4 is 47.2 μm, and the surface roughness R... a The thickness is 3.8 nm. Due to the increased casting speed and shortened slurry spreading time, the surface smoothness slightly decreased, and the surface roughness R... a The nanometer size increased from 2.5 nm to 3.8 nm, but remained within the nanometer range. The gas adsorption performance was basically the same as in Example 1, with a CO2 adsorption capacity of 2.79 mmol / g at 0.15 bar, indicating that the process window of this invention is relatively wide, and stable products can be obtained within a speed range of 0.1-0.5 m / s.

[0035] Comparative Example 1: MOF-cellulose nanofilms prepared by conventional solution casting method This comparative example is used to compare the differences between the casting method of the present invention and the traditional solution casting method.

[0036] Preparation method: The composite hydrogel slurry was prepared according to the same method as steps one to five of Example 1. The degassed slurry was directly poured into a polystyrene petri dish with a diameter of 9 cm and allowed to dry naturally at room temperature (25°C) and relative humidity (50%) for approximately 48 hours. No casting, PTFE / hydrophilic membrane bilayer system, interfacial acid-induced gelation, or supercritical drying were used; natural drying was employed. The resulting film was named D-1.

[0037] Performance test results: The D-1 film thickness is extremely uneven, with an edge thickness of approximately 120 μm and a center thickness of approximately 35 μm, resulting in a thickness standard deviation of 28 μm and exhibiting a significant "edge effect." Surface roughness R aThe film thickness was 85 nm, and obvious wrinkles and bubble marks were visible on the surface. FE-SEM observation showed numerous microcracks and defects inside the film, with significant agglomeration of ZIF-8 particles, and agglomerate sizes of 1-5 μm. The CO2 adsorption capacity at 25 °C and 0.15 bar was only 0.85 mmol / g, far lower than in Example 1. These results indicate that the traditional solution casting method cannot achieve precise control of film thickness and surface smoothness, and that the MOFs exhibit poor dispersion, leading to a significant decrease in gas adsorption performance.

[0038] Comparative Example 2: MOF-cellulose nanofilm with mechanical stirring only, without high-pressure homogenization. This comparative example is used to compare the differences between the high-pressure homogenization dispersion technology of the present invention and traditional mechanical stirring.

[0039] Preparation method: This method is basically similar to Example 1, except that in step four, after adding the ZIF-8 pre-dispersion to the nanocellulose hydrogel, mixing is performed only by mechanical stirring using an IKA RW20 mixer, without high-pressure homogenization at 700 MPa. Subsequent casting, gelation, and drying steps are the same as in Example 1. The resulting film is named D-2.

[0040] Performance test results: The thickness and surface smoothness of the D-2 film are similar to those of Example 1, with an average thickness of 49.2 μm and R... a The thickness of 3.1 nm indicates that the casting method does not affect the control of thickness and flatness. However, FE-SEM observation shows that the ZIF-8 particles are extremely unevenly distributed in the film, with a large number of aggregates of 2-8 μm in size, and some areas even lacking ZIF-8 particles. The CO2 adsorption capacity at 25 °C and 0.15 bar is only 1.23 mmol / g, which is only 43% of that in Example 1. The results indicate that without high-pressure homogenization, hydrophobic MOFs cannot achieve uniform dispersion in the hydrophilic nanocellulose matrix, resulting in a large number of active sites being embedded in aggregates and losing their adsorption function.

[0041] Comparative Example 3: MOF-nanocellulose membrane with direct acid droplet application without an upper hydrophilic filter membrane: This comparative example is used to compare the difference between the feature of adding a gel inducer through capillary permeation of the upper hydrophilic filter membrane in this invention and directly dripping acid onto the surface of the wet membrane.

[0042] Preparation method: This method is basically similar to Example 1, except that in step six, the upper hydrophilic filter membrane is not covered during casting; the composite hydrogel slurry is directly exposed to air to spread and form a wet film. After film formation, 1 mol / L dilute hydrochloric acid is directly added to the surface of the wet film using a dropper, without passing through a filter membrane. Subsequent steps such as settling, removing the film (but without a filter membrane), and drying are the same as in Example 1. The resulting film is named D-3.

[0043] Performance test results: The average thickness of the D-3 film is 47.5 μm, but the surface roughness R... a The surface area was as high as 125 nm, with visible pits, wrinkles, and air bubbles caused by acid impact. In some areas, the membrane ruptured due to localized excess acid. At 25°C and 0.15 bar, the CO2 adsorption capacity was only 1.58 mmol / g, with large batch-to-batch performance fluctuations and a standard deviation of 0.45 mmol / g. These results indicate that without the capillary buffering effect of the upper hydrophilic filter membrane, direct acid addition damages the surface morphology and structural integrity of the wet membrane, severely reducing its performance and reproducibility.

[0044] Comparative Example 4: MOF-cellulose nanofilms dried conventionally and non-supercritically: This comparative example is used to compare the differences between the supercritical drying of this invention and conventional natural drying or freeze drying.

[0045] Preparation method: Basically the same as in Example 1, except that in step seven, after solvent replacement, supercritical drying is not performed, but natural drying is used at 25°C and 40% relative humidity for 24 h, or freeze drying at -50°C and 10 Pa for 48 h. The film obtained by natural drying is named D-4A, and the film obtained by freeze drying is named D-4B.

[0046] Performance test results: The D-4A naturally dried film experienced severe shrinkage, with a volume shrinkage rate of approximately 85%. The thickness decreased from 500 μm in the wet state to approximately 25 μm. The film became brittle and hard. FE-SEM observation showed that the three-dimensional nanoporous network almost completely collapsed, and the BET specific surface area was only 52 m². 2 / g. The D-4B freeze-dried film has a shrinkage rate of approximately 40%, and while some porous structures are retained, large channels (pore size > 10 μm) and structural inhomogeneity due to ice crystal growth still exist. The BET specific surface area is 210 m². 2 / g. At 25℃ and 0.15 bar, the CO2 adsorption capacity of D-4A was only 0.12 mmol / g, and that of D-4B was 0.98 mmol / g, both significantly lower than the 2.85 mmol / g in Example 1. The results indicate that supercritical drying is a crucial step in preserving the three-dimensional nanoporous network structure of the aerogel, and conventional drying methods cannot obtain aerogel films with high specific surface area and high porosity.

[0047] Performance summary of Examples 1-4 and Comparative Examples 1-4:

[0048] As can be seen from the above embodiments and comparative examples, the present invention solves the problem of uniform dispersion of hydrophobic MOFs in hydrophilic nanocellulose matrix by using high pressure homogeneous dispersion technology; achieves micron-level precise control of film thickness and nanoscale regulation of surface smoothness by using asymmetric dual-interface confined casting molding technology; and completely preserves the three-dimensional nanoporous network structure of aerogel by supercritical drying.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0050] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A MOF-nanocellulose aerogel micron film, characterized in that, The film comprises a three-dimensional nanoporous network structure assembled from carboxyl-containing nanocellulose, and hydrophobic metal-organic framework material particles loaded in the three-dimensional nanoporous network structure. The surface of the film in contact with the hydrophobic substrate during the preparation process is set to a nanoscale roughness. The film has a hierarchical pore structure consisting of macropores and mesopores provided by nanocellulose aerogel and micropores provided by MOF particles.

2. A method for preparing MOF-nanocellulose aerogel micron-films by casting, characterized in that, Includes the following steps: Step 1: Preparation of carboxyl-containing nanocellulose hydrogel: Carboxyl-containing nanocellulose that has been oxidized by TEMPO is dispersed in deionized water, and then subjected to primary dispersion by a homogenizer, followed by dispersion assisted by an ultrasonic cell disruptor, and then subjected to primary high pressure, secondary high pressure and tertiary high pressure homogenization treatments to obtain nanocellulose hydrogel. Step 2: Preparation of MOF-nanocellulose composite hydrogel slurry: Hydrophobic MOFs materials are pre-dispersed in deionized water to obtain MOFs pre-dispersion liquid; the MOFs pre-dispersion liquid is mixed with the nanocellulose hydrogel obtained in Step 1, and then subjected to high-pressure homogenization treatment under three-stage high pressure conditions to achieve nanoscale uniform dispersion and stable embedding of hydrophobic MOFs particles in hydrophilic nanocellulose hydrogel, thus obtaining composite hydrogel slurry; Step 3, Casting and Interfacial Induced Gelation: The composite hydrogel slurry obtained in Step 2 is subjected to vacuum degassing and then poured into a casting machine for casting. The lower layer of the casting machine is laid with a polytetrafluoroethylene (PTFE) film as a hydrophobic substrate, and the upper layer is covered with a hydrophilic filter membrane. The composite hydrogel slurry is spread between the lower PTFE film and the upper hydrophilic filter membrane to form a composite wet film. Dilute hydrochloric acid is added to the surface of the hydrophilic filter membrane by an inclined droplet. The hydrochloric acid is uniformly diffused into the composite wet membrane by the capillary permeation of the hydrophilic filter membrane, inducing the composite wet membrane to undergo sol-gel transformation. After standing, the hydrophilic filter membrane is removed to obtain a wet micron film attached to the surface of the lower PTFE film. The side of the wet micron film in contact with the PTFE film has a surface with nanoscale flatness. Step 4, Post-treatment and drying: The wet micron film attached to the PTFE film obtained in step 3 is successively immersed in a hydrochloric acid-ethanol mixed solution and pure ethanol for solvent replacement, and then supercritical drying is performed to obtain the MOF-nanocellulose aerogel micron film.

3. The method for preparing MOF-cellulose nanogel microfilms by casting according to claim 2, characterized in that, The TEMPO-oxidized carboxyl-containing nanocellulose described in step one is prepared by the following method: after removing inorganic minerals by acid pretreatment of pulp lignocellulose, the pH is controlled at 10 in the TEMPO oxidation system to selectively oxidize the C6 hydroxyl group of cellulose to a carboxyl group. Then, sodium borohydride is added to reduce the incompletely reacted aldehyde cellulose intermediate to hydroxy cellulose, thus obtaining TEMPO-oxidized cellulose containing carboxyl groups.

4. The method for preparing MOF-cellulose nanogel microfilms by casting according to claim 2, characterized in that, In the nanocellulose hydrogel described in step one, the ratio of TEMPO oxidized cellulose to deionized water is 100 mL of deionized water for every 1 g of TEMPO oxidized cellulose; in the composite hydrogel slurry described in step two, the mass fraction of nanocellulose is 0.5% and the total solid content is 1%.

5. The method for preparing MOF-cellulose nanogel microfilms by casting according to claim 2, characterized in that, Before adding the hydrophobic MOFs material to the nanocellulose hydrogel, it is first pre-dispersed in an ultrasonic cell disruptor for 5 min to obtain a MOFs pre-dispersion solution.

6. The method for preparing MOF-nanocellulose aerogel micron-films by casting according to claim 5, characterized in that, The speed of the casting machine mentioned in step three is 0.1-0.5 m / min, and the blade gap is 250-750 μm; The hydrophilic filter membrane has a pore size of 0.22 μm; the concentration of the dilute hydrochloric acid is 1 mol / L. The settling time is 2 hours.

7. The method for preparing MOF-cellulose nanogel microfilms by casting according to claim 2, characterized in that, The lower PTFE film is a hydrophobic low surface energy film, which is used to ensure that the composite hydrogel slurry can be spread evenly during the casting process and to achieve complete peeling of the wet film after the sol-gel conversion. The upper hydrophilic filter membrane is used to provide micron-level precise physical constraint on the thickness of the composite hydrogel slurry, and to achieve controlled diffusion of the gel inducer through its capillary permeation, thereby avoiding surface wrinkles, skinning, or bubble defects caused by the gel inducer directly impacting the slurry surface.

8. The method for preparing MOF-cellulose nanogel microfilms by casting according to claim 2, characterized in that, In step four, the volume ratio of hydrochloric acid to ethanol in the hydrochloric acid-ethanol mixed solution is 1:

1. The supercritical drying is used to remove the solvent under supercritical fluid conditions to avoid the collapse of the aerogel three-dimensional nanoporous network structure caused by capillary forces during the drying process.

9. The method for preparing MOF-cellulose nanogel microfilms by casting according to claim 2, characterized in that, The pressure of the first-stage high-pressure homogenization treatment in step one is 300 bar, the pressure of the second-stage high-pressure homogenization treatment is 500 bar, and the pressure of the third-stage high-pressure homogenization treatment is 700 bar, and so on. The 700 bar treatment is used to achieve the final nanoscale dispersion of nanocellulose in deionized water. The pressure of the three-stage high-pressure homogenization process in step two is higher than that in step one. It is used to generate stronger shear force and cavitation effect in the MOFs and nanocellulose mixture system to overcome the aggregation tendency of hydrophobic MOFs particles.

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