A high-throughput MOF / polyamide composite membrane based on inkjet printing technology and a preparation method thereof

Abstract

The application belongs to the technical field of nanofiltration composite membranes, and particularly relates to a high-flux MOF / polyamide composite membrane based on inkjet printing technology and a preparation method thereof. In the MOF nucleation stage, triethylamine is added to compete for coordination, 30-50 nm monodisperse MOF precursor ink is prepared in situ, and composite ink can be obtained by compounding 0.04-0.16 wt% CNC. Two-step precise printing is adopted by using piezoelectric inkjet printing, MOF precursors are first deposited, a non-agglomerated intermediate layer is formed by self-assembly through solvent gradient volatilization, water-phase monomers are then loaded by inkjet printing, and a polysulfone support layer / MOF intermediate layer / polyamide separation layer three-layer integrated sandwich structure composite membrane is obtained through interfacial polymerization and thermal crosslinking. The application inhibits MOF agglomeration from the source, reduces material loss through picoliter-level micro-printing, fills the interface defects of the membrane layer with CNC, and simultaneously improves the pure water flux and the separation selectivity of mono- / di-valent salt; the process has strong adaptability, excellent batch stability, and is suitable for the technical field of industrial wastewater, dye wastewater, water quality softening, and seawater desalination pretreatment nanofiltration.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration composite membrane technology, and particularly relates to a high-throughput MOF / polyamide composite membrane based on inkjet printing technology and its preparation method. Background Technology

[0002] Membrane separation technology is a core technology for fine water purification and resource utilization of industrial wastewater. With its advantages of high efficiency, energy saving, and no secondary pollution, it is widely used in dye wastewater treatment, industrial wastewater purification, water softening, and seawater desalination pretreatment. Polyamide thin-film composite membranes possess excellent film-forming and processing properties, mechanical structural stability, and precise solute sieving capabilities, making them the most widely used and best-performing core functional membrane material in current nanofiltration separation systems.

[0003] As the industrial water treatment industry develops towards higher precision, lower energy consumption, larger scale, and greater stability, the market demands for membrane materials in terms of permeate flux, salt separation selectivity, uniformity of large-area preparation, and batch operation stability. The inherent defects of traditional pure polyamide thin-film composite membranes are becoming increasingly apparent, including low permeate flux, numerous interface defects, weak antifouling ability, and limited precision in separating mono / divalent salts. These shortcomings make it difficult to meet the current industrial separation requirements of high precision, large scale, and low material consumption.

[0004] Metal-organic frameworks (MOFs) possess excellent properties such as large specific surface area, tunable pore size, rich structure, and regular mass transfer channels, making them ideal functional nanofillers for the modification of polyamide composite membranes. Introducing MOF materials into polyamide separation systems can effectively optimize the microporous structure of the membrane layer, construct efficient water molecule transport channels, and simultaneously improve membrane permeate flux and pollutant retention performance. Currently, MOF / polyamide thin film composite membranes have become a research hotspot in the field of high-performance nanofiltration membranes. Existing preparation processes mostly employ traditional methods such as physical blending, dip coating, blade coating, spin coating, and conventional spray molding, directly incorporating finished MOF powder particles into aqueous monomer solutions or organic phase solutions, and completing the composite membrane preparation based on conventional interfacial polymerization reactions.

[0005] The current traditional MOF / polyamide composite membrane preparation process suffers from several inherent technical defects: First, commercially available MOF nanoparticles have abundant surface active sites and poor interfacial compatibility with organic polymer matrices, making them prone to irreversible aggregation in aqueous solutions. This results in a large number of non-selective interfacial voids and structural defects within the membrane layer, significantly reducing the salt separation selectivity and long-term operational stability of the membrane. Second, traditional molding methods such as dip coating, blade coating, and conventional spraying have strong randomness in liquid application, making it impossible to achieve quantitative, directional, and uniform loading of MOF fillers on the polysulfone-based membrane surface. During large-area membrane fabrication, problems such as uneven membrane thickness, disordered filler distribution, and large differences in microstructure easily occur, causing significant batch-to-batch performance fluctuations in membrane products and extremely poor mass production and engineering applicability. Third, traditional extensive molding processes cannot precisely control microscopic processes such as droplet spreading and solvent evaporation, making it difficult to match the dynamic synergistic relationship between MOF crystal nucleation and growth kinetics and polyamide interfacial polymerization kinetics. This makes it impossible to construct an ultra-thin, uniform, and dense polyamide separation layer, and the technical contradiction of mutual constraint between flux and selectivity always exists.

[0006] Existing research employs electrostatic spraying to achieve low-feed film formation, which to some extent alleviates the problems of excessive coating volume and excessively thick films in traditional coating processes. However, this process still has significant technical limitations. Conventional electrostatic spraying produces droplets with a wide particle size distribution, and the droplets carrying the same charge are prone to electrostatic repulsion and dispersion. This only enables coarse, irregular low-feed forming, and cannot achieve pixel-level precise deposition at specific points, quantities, and directions. It is also difficult to finely control the ink droplet spreading and fusion process and the solvent gradient evaporation process, and cannot fundamentally solve the core problems of MOF particle agglomeration, numerous interfacial defects, and uncontrollable microstructure. At the same time, existing technologies lack dedicated stable MOF ink systems adapted for precision printing, have not established controllable stabilization strategies for MOF precursors, and have not formed a synergistic modification mechanism for MOF and functional nanomaterials. Significant technical gaps exist in the degree of film formation refinement, structural control precision, and performance optimization.

[0007] While existing patents disclose various preparation methods for MOF-modified polyamide composite films, there is still significant room for improvement in areas such as refined molding, microstructure control, and synergistic performance optimization. A detailed comparative analysis is as follows:

[0008] Patent CN202010187027.1 discloses a conventional preparation process for MOF-modified polyamide composite membranes. This process involves physically blending finished MOF powder into a reaction solution and combining it with a traditional dip-coating process to achieve membrane modification. This technical solution uses direct doping of finished MOF particles, without constructing a stable coordination system for small MOF precursor molecules. This results in limited inhibition of MOF particle aggregation and generally poor overall ink dispersion stability. Furthermore, this process lacks precise low-feed molding techniques and the concepts of droplet kinetic control and layered self-assembly molding. It struggles to coordinate the dynamic reaction processes of MOF crystallization and interfacial polymerization, leading to limitations in the uniformity and batch consistency of large-area membranes. It can only achieve basic performance improvements and cannot simultaneously achieve the comprehensive effects of low-loss preparation, precise control of membrane structure, and synergistic improvement in flux and selectivity.

[0009] Patent CN202511029934.2 discloses a high-flux porous polyolefin-based reinforced PA / MOF composite organic solvent nanofiltration membrane and its preparation method. This method achieves simple, low-feed film formation through spraying, which to some extent optimizes the problem of excessive coating liquid volume in traditional methods. However, this scheme still uses a direct doping process with finished MOF particles, without introducing small-molecule regulators to stabilize and control the MOF precursor, making it difficult to address the issues of particle aggregation and insufficient interfacial compatibility at the source. Furthermore, it employs a single-spray integrated molding mode, failing to form a layered structure of MOF intermediate layer and aqueous monomer, and lacking a dedicated ink system suitable for high-precision molding. This limits its ability to control droplet spreading and fusion, solvent evaporation, and self-assembly processes, and the controllability of membrane thickness and microstructure needs further improvement. The problem of balancing flux and selectivity remains.

[0010] Patent CN202211301624.8 discloses a nanofiltration membrane composed of MOF materials and its preparation method. It utilizes a single nanofiller to modify and optimize a polyamide membrane, which can improve membrane separation performance to some extent. However, this modification system is relatively simple, failing to construct a synergistic modification system combining MOF and functional nanofillers, thus limiting its effectiveness in repairing membrane interface defects and strengthening the structure. Its molding process still relies mainly on traditional scraping and spraying, resulting in insufficient controllability of liquid supply, low material utilization, and a lack of ability for precise micro-liquid supply at the pixel level. More importantly, this technology does not address micro-control mechanisms such as MOF precursor coordination stability, droplet kinetic matching, and solvent evaporation-induced ordered self-assembly. The optimization dimensions for inorganic-organic interface compatibility, filler distribution regularity, and long-term membrane stability are limited, leaving room for further exploration in overall process controllability and performance improvement.

[0011] In summary, current MOF / polyamide composite membrane preparation technologies still have several areas for optimization and improvement: First, the common practice of physical doping with finished MOF powder lacks a precursor stabilization mechanism, leading to MOF agglomeration and insufficient ink dispersion stability. Second, the molding process mainly relies on traditional coating and conventional spraying methods, lacking pixel-level precise low-liquid-feed control capabilities, resulting in high material loss and poor uniformity and batch consistency in large-area membrane fabrication. Third, the lack of a synergistic control mechanism between ink droplet kinetics and MOF crystallization and interfacial polymerization kinetics makes it difficult to accurately construct the membrane microstructure, hindering simultaneous optimization of flux and selectivity. Fourth, the modified formulation system is singular, lacking multi-filler compound synergistic design, limiting the ability to repair membrane interface defects, and the overall separation performance and mass production stability of the composite membrane still have significant room for improvement. Current preparation technologies cannot simultaneously meet the multiple requirements of high performance, low loss, controllable structure, and large-scale mass production. Based on this, developing an inkjet printing-assisted MOF / polyamide composite film preparation technology that can stably prepare ultra-small MOF precursors, achieve precise low-feed molding, dynamically control the precursor self-assembly and interfacial polymerization process, and has the advantage of multi-filler synergistic modification has important research value and engineering application significance. Summary of the Invention

[0012] To address the shortcomings of existing technologies, the technical problem to be solved by this invention is to provide a high-throughput MOF / polyamide composite membrane based on inkjet printing technology, which features good precursor stability, high droplet control precision, strong molding controllability, effective suppression of MOF agglomeration and interface defects, precise matching of the dual dynamic processes of MOF crystallization and interface polymerization, and effective improvement of the problems of uneven membrane structure, performance imbalance, and high material consumption in traditional processes. This membrane achieves controllable microstructure, synergistic improvement of throughput and selectivity, low liquid consumption, and stable batch performance.

[0013] This invention relates to a refined, low-loss, and controllable inkjet printing modification technology for nanofiltration thin film composite membranes, overcoming the limitations of traditional physical blending and coarse coating methods. Existing technologies lack precise control over the dispersion stability, deposition uniformity, and dynamic film formation process of MOF fillers, and have not yet formed a stable ink system and layered forming structure suitable for precision inkjet printing. The core innovative concept of this invention lies in: introducing a small molecule competitive coordination regulation mechanism in the early nucleation and growth stage of MOF crystals to stabilize ultra-small MOF precursors and construct a highly stable ink system suitable for inkjet printing, thereby improving the problem of MOF particle aggregation from the source; based on the pixel-level and picoliter-level precise low-liquid-feed deposition advantages of piezoelectric inkjet printing, establishing a two-step layered molding system of MOF intermediate layer pre-printing and controllable loading of aqueous monomers, precisely controlling the entire process of ink droplet spreading, fusion, and solvent gradient evaporation, inducing the MOF precursor to achieve ordered self-assembly; at the same time, introducing CNC nanofillers for compound synergistic modification, optimizing the micro-interface structure of the membrane layer and the water molecule mass transfer channels, precisely matching the synergistic relationship between MOF crystallization kinetics and polyamide interfacial polymerization kinetics, and comprehensively improving the overall separation performance of the composite membrane.

[0014] The core technology system of this invention includes: coordination stabilization of small molecule precursors, precise low-feed forming via two-step inkjet printing, synergistic control of droplet dynamics and dual-reaction dynamics, and interface modification of MOF / CNC composites. This invention abandons the traditional physical blending and extensive coating preparation mode, improving upon the problems of disordered filler dispersion, strong randomness in film formation, and frequent interface defects in traditional processes. It effectively solves the technical shortcomings of existing technologies, such as poor uniformity in large-area film formation, difficulty in synergizing flux and selectivity, high raw material loss, and insufficient batch stability. It establishes a standardized preparation system for MOF / polyamide composite films that is structurally controllable, performance-synergistic, and low-consumption for mass production.

[0015] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology, comprising the following steps:

[0016] (1) Base membrane pretreatment: Immerse the polysulfone ultrafiltration base membrane in deionized water for at least 3 hours, remove it, remove the surface water, and fix it on the printing workbench for later use.

[0017] (2) Preparation of MOF precursor printing ink: Triethylamine is added during the MOF crystal nucleation stage for coordination regulation, and MOF precursor is prepared in situ. The MOF precursor is then dispersed in the corresponding solvent to prepare MOF precursor printing ink.

[0018] (3) Preparation of aqueous printing ink: Dissolve the aqueous monomer, surfactant and co-solvent in pure water, stir evenly, and prepare an aqueous ink suitable for piezoelectric inkjet.

[0019] (4) Preparation of organic phase reaction solution: Dissolve pyromellitic chloride in n-hexane to prepare an organic phase solution for interfacial polymerization;

[0020] (5) Inkjet printing to construct MOF intermediate layer: Piezoelectric inkjet printing equipment is used to output micro-droplets to quantitatively deposit MOF precursor printing ink on the surface of pretreated polysulfone base film; a multi-stage printing method is adopted, and the printing is naturally dried in the printing gap. By precisely controlling the ink droplet spreading and fusion rate and solvent evaporation rate, the MOF precursor is orderly self-assembled on the base film surface to form a defect-free, continuous and dense MOF intermediate layer.

[0021] (6) Inkjet controllable load aqueous phase monomer: Keep the same inkjet equipment parameters unchanged, switch the aqueous phase printing ink, and perform low liquid supply precision inkjet printing on the surface of the formed MOF intermediate layer.

[0022] (7) Interfacial polymerization and thermal crosslinking: The membrane loaded with aqueous monomers is immersed in an organic phase solution for interfacial polymerization. After the reaction is completed, excess organic phase is removed from the surface of the membrane, and then crosslinked and cured by heating to finally obtain a high-throughput MOF / polyamide thin film composite membrane with a sandwich structure of polysulfone support layer / MOF intermediate layer / polyamide separation layer.

[0023] In the above-mentioned method for preparing high-throughput MOF / polyamide composite film based on inkjet printing technology, in step (2), the MOF precursor is at least one of ZIF-8, CuBTC, and UiO-66-NH2, with a particle size of 30-50 nm; the reaction solvent for preparing the MOF precursor is anhydrous methanol, the dispersion solvent for the MOF precursor printing ink is anhydrous ethanol or pure water, and the ink mass fraction is 0.01wt%-2wt%.

[0024] In the above-mentioned method for preparing high-throughput MOF / polyamide composite film based on inkjet printing technology, in step (3), the aqueous monomer is at least one of piperazine and polyethyleneimine; the surfactant is at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and polyvinylpyrrolidone; the cosolvent is at least one of ethanol and dimethyl sulfoxide; the mass concentration of the aqueous monomer is 0.1wt% to 2wt%, the mass concentration of the surfactant is 0.1wt% to 5wt%, and the mass concentration of the cosolvent is 1wt% to 10wt%; in step (4), the mass concentration of trimesoyl chloride in n-hexane is 0.05wt% to 0.2wt%.

[0025] In the above-mentioned method for preparing high-throughput MOF / polyamide composite film based on inkjet printing technology, in step (5), the volume of the piezoelectric inkjet output droplets is 3-10 pL, the nozzle diameter is 0.01-1 mm, the jetting pressure is 0.1-0.5 MPa, the distance between the nozzle and the base film is 0.5-10 mm, the number of printing times is 1-5 times, and the natural drying time between two adjacent printing times is 1-5 min.

[0026] In the above-mentioned method for preparing high-throughput MOF / polyamide composite film based on inkjet printing technology, the parameters of the piezoelectric inkjet equipment in steps (5) and (6) are controlled as follows: the needle rise time is 0.1 to 3s, the needle fall time is 0.1 to 0.5s, the valve opening time is 0.1 to 5s, and the needle stroke is 25% to 90%; the printing mode is single-dot mode or line drawing mode, the single-dot / line segment spacing is 0.1 to 5mm, the printhead moving speed is 0 to 300mm / s, and the single adhesive removal time is 0.001 to 1s.

[0027] In the above-mentioned method for preparing high-throughput MOF / polyamide composite film based on inkjet printing technology, the interfacial polymerization reaction time in step (7) is 1-5 min, the thermal crosslinking temperature is 50-70℃, and the thermal crosslinking duration is 7-15 min.

[0028] In the above-mentioned method for preparing high-throughput MOF / polyamide composite membrane based on inkjet printing technology, when preparing MOF precursor printing ink in step (2), cellulose nanocrystals (CNC) are added to form MOF / CNC composite printing ink. The mass fraction of cellulose nanocrystals (CNC) in the composite printing ink is 0.04wt% to 0.16wt%. The CNC is uniformly interspersed inside the MOF skeleton to eliminate MOF particle aggregation defects and improve the salt separation selectivity and pure water flux of the membrane.

[0029] This invention provides a high-throughput MOF / polyamide composite membrane based on inkjet printing technology, prepared by the above-mentioned method. The composite membrane has a three-layer integrated sandwich structure, with a porous polysulfone support layer at the bottom, a continuous porous intermediate layer formed by the ordered self-assembly of inkjet-printed MOF precursors, and an ultra-thin dense polyamide nanofiltration separation layer on the surface. The composite membrane is free from MOF particle agglomeration and interlayer interface void defects, and the membrane uniformity, flux, and separation stability are significantly improved.

[0030] The aforementioned high-throughput MOF / polyamide composite film based on inkjet printing technology has an intermediate layer that is a composite reinforcing layer formed by the self-assembly of MOF precursor and CNC through inkjet printing, with CNC uniformly interspersed and anchored inside the MOF skeleton.

[0031] The present invention also provides the application of the above-mentioned high-throughput MOF / polyamide composite membrane based on inkjet printing technology in industrial wastewater purification, dye wastewater treatment, water softening, and seawater desalination pretreatment.

[0032] The advantages of this invention, which relates to a high-throughput MOF / polyamide composite film based on inkjet printing technology and its preparation method, are as follows: This invention constructs an integrated preparation system of "precursor stabilization modification + precise low-feed inkjet deposition + droplet dynamics control + multi-filler synergistic self-assembly," overcoming the challenges of uncontrollable filler dispersion, strong film formation randomness, and multiple core defects in traditional processes. It possesses the following technical advantages:

[0033] First, it inhibits MOF aggregation at the source, significantly improving the stability of printing ink. This invention abandons the traditional physical doping method of finished MOF powder, and introduces triethylamine small molecules during the MOF nucleation stage. By competitively coordinating and passivating the active sites of MOF nanoclusters, it inhibits excessive crystal growth and particle aggregation, and stably prepares a 30-50nm highly dispersed MOF precursor. This constructs a highly stable ink system suitable for piezoelectric inkjet printing, solving the defects of traditional inks such as easy sedimentation, easy particle aggregation, and poor printing uniformity.

[0034] Secondly, it achieves precise low-dispensing molding, reduces material loss, and is suitable for large-scale mass production. Utilizing a piezoelectric inkjet two-step layered printing process, based on the pin-level ink droplet's point-to-point, quantitative, and directional deposition characteristics, printing process parameters can be precisely controlled. This achieves uniform and controllable loading of MOF precursors and aqueous monomers on the base film surface, effectively overcoming the drawbacks of traditional dip coating, scraping coating, and conventional spray coating processes, such as random dispensing, high material loss, uneven film formation over large areas, and large batch fluctuations. It is suitable for standardized continuous production.

[0035] Third, this invention achieves precise and controllable microstructure of the film layer, matching the synergistic molding of dual reaction kinetics. By regulating the ink droplet spreading and fusion process and the gradient evaporation of solvent and regulator, this invention induces the orderly self-assembly of MOF precursors to form a continuous, uniform, and defect-free MOF interlayer. It precisely matches the MOF crystallization kinetics with the polyamide interfacial polymerization kinetics, controllably constructing an ultrathin, dense, and low-defect polyamide separation layer, effectively improving the integrity and performance stability of the film layer structure.

[0036] Fourth, the technical challenge of flux-selectivity trade-off is overcome through MOF / CNC composite modification. This invention introduces CNC nanofiller for synergistic modification, utilizing its rigid support and hydrophilic properties to further suppress MOF aggregation, fill interfacial micro-defects, and optimize water molecule mass transfer channels. While ensuring ultra-high dye retention performance, it simultaneously improves the pure water flux and salt separation selectivity of the composite membrane, significantly enhancing the overall separation performance and long-term operational stability of the membrane.

[0037] In summary, this invention achieves innovation from multiple dimensions, including material formulation, molding process, micro-control, and synergistic modification, and constructs a refined composite membrane preparation system with controllable structure, few defects, low loss, superior performance, and batch stability. It effectively breaks through the technical difficulties of traditional membrane manufacturing industry, has good engineering application and industrialization prospects, and is worthy of widespread promotion and application. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the overall three-layer sandwich structure of the composite membrane of the present invention;

[0039] Figure 2 A schematic diagram of the complete process flow for inkjet printing to prepare MOF / polyamide composite films according to the present invention;

[0040] Figure 3 This is a schematic diagram of the microscopic process of the self-assembly of the MOF precursor inside the ink droplet into the MOF intermediate layer as the solvent and regulator evaporate in gradient;

[0041] Figure 4 A schematic diagram comparing the microstructure of a MOF precursor stabilized by a triethylamine modifier with that of conventional unregulated crystallized MOFs.

[0042] Figure 5 The particle size distribution test curve of the ZIF-8 precursor ink prepared in Example 1 of this invention is shown.

[0043] Figure 6 This is a line graph showing the separation performance test of the pure PA thin film composite membrane in Example 1 of the present invention.

[0044] Figure 7 The images show the surface and cross-sectional SEM morphology of the pure PA thin film composite film in Example 1 of this invention.

[0045] Figure 8 This is a line graph showing the separation performance test of a single ZIF-8 modified MOF / polyamide composite membrane in Example 2 of the present invention.

[0046] Figure 9 The images show the surface and cross-sectional SEM morphology of the single ZIF-8 modified MOF / polyamide composite film in Example 2 of this invention.

[0047] Figure 10 This is a line graph showing the separation performance test of a single CNC-modified polyamide composite membrane in Example 3 of the present invention.

[0048] Figure 11 The images show the surface and cross-sectional SEM morphology of the single CNC modified polyamide composite film in Example 3 of this invention.

[0049] Figure 12 This is a line graph showing the separation performance test of the ZIF-8 / CNC compound modified high-performance composite membrane in Example 4 of the present invention.

[0050] Figure 13 The images show the surface and cross-sectional SEM morphology of the ZIF-8 / CNC compound modified high-performance composite membrane in Example 4 of this invention. Detailed Implementation

[0051] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] In this invention, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in its actual use or working state, specifically the drawing directions in the accompanying drawings; while "inner" and "outer" refer to the outline of the device. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". The terms first, second, third, etc., are used merely as illustrative purposes and do not impose numerical requirements or establish an order. The term "multiple" means "two or more". It should be noted that the following embodiments are merely preferred embodiments of this application and are not intended to limit the scope of protection of this application. Any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of this invention are all within the scope of protection of this invention.

[0053] This invention provides a method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology, comprising the following steps:

[0054] (1) Base membrane pretreatment: Immerse the polysulfone ultrafiltration base membrane in deionized water for at least 3 hours to fully remove residual protective agent and impurities on the base membrane surface. After soaking, take out the base membrane, thoroughly remove the surface water, and fix it flat on the inkjet printing worktable to ensure that the base is flat, wrinkle-free, and free of water accumulation.

[0055] (2) Preparation of MOF precursor printing ink: Triethylamine is added during the early nucleation stage of MOF crystals for coordination regulation. A highly stable MOF precursor is prepared in situ through competitive coordination of small molecules. The obtained MOF precursor is then uniformly dispersed in a solvent to prepare a stable and uniform MOF precursor printing ink. The MOF precursor is at least one of ZIF-8, CuBTC, and UiO-66-NH2, and the precursor particle size is stably controlled to be 30-50 nm. Anhydrous methanol is used as the solvent for the synthesis reaction of the MOF precursor, and anhydrous ethanol or pure water is used as the dispersion solvent for the MOF precursor printing ink. The ink mass fraction is controlled to be 0.01 wt%-2 wt%.

[0056] In the preparation of MOF precursor printing ink, cellulose nanocrystals (CNCs) are added for blending and compounding to form MOF / CNC composite printing ink; the mass fraction of the cellulose nanocrystals (CNCs) in the composite printing ink is 0.01wt% to 0.16wt%. The CNCs are uniformly interspersed within the MOF framework, which can further eliminate MOF particle aggregation defects, optimize the microstructure of the membrane layer, and simultaneously improve the salt separation selectivity and pure water flux of the membrane.

[0057] (3) Preparation of aqueous printing ink: The aqueous monomer, surfactant and co-solvent are dissolved together in pure water and stirred thoroughly to obtain an aqueous printing ink with low viscosity, high stability and suitable for precise piezoelectric inkjet printing; wherein, the aqueous monomer is at least one of piperazine and polyethyleneimine; the surfactant is at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate and polyvinylpyrrolidone; the co-solvent is at least one of ethanol and dimethyl sulfoxide; the mass concentration of the aqueous monomer is 0.05wt% to 2wt%, the mass concentration of the surfactant is 0.1wt% to 5wt%, and the mass concentration of the co-solvent is 1wt% to 10wt%.

[0058] (4) Preparation of organic phase reaction solution: Dissolve pyromellitic chloride fully in n-hexane solvent, stir until completely dissolved, and prepare an organic phase solution for interfacial polymerization reaction, wherein the mass concentration of pyromellitic chloride in n-hexane is 0.01wt%~0.2wt%, and seal and let stand for later use.

[0059] (5) Construction of MOF intermediate layer by inkjet printing: A piezoelectric inkjet printer outputs micro-droplets to quantitatively and directionally deposit MOF precursor printing ink onto the surface of a pretreated polysulfone film. A multi-stage printing process is used, with natural drying between adjacent prints. By precisely controlling the droplet spreading and fusion process and the solvent gradient evaporation process, the MOF precursor is induced to self-assemble in an orderly manner, forming a continuous, uniform, and defect-free MOF functional intermediate layer on the surface of the base film. Specifically, the piezoelectric inkjet output droplet volume is 3–10 pL, the nozzle diameter is 0.03–1 mm, the jetting pressure is 0.1–0.4 MPa, the distance between the nozzle and the base film is 1–10 mm, the number of prints is 1–5, and the natural drying time between adjacent prints is 1–5 min.

[0060] (6) Controllable loading of aqueous monomers in inkjet printing: Keep the operating parameters of the inkjet printing equipment unchanged, switch to loading aqueous printing ink, and perform low-liquid-feed precision inkjet printing on the surface of the formed MOF intermediate layer to achieve uniform, quantitative and controllable loading of aqueous monomers on the surface of the intermediate layer, providing a basis for the formation of a uniform and dense polyamide separation layer.

[0061] In steps (5) and (6) above, the uniform parameter control of the piezoelectric inkjet equipment is as follows: the needle rise time is 0.1 to 3s, the needle fall time is 0.1 to 0.5s, the valve opening time is 0.1 to 5s, and the needle stroke is 25% to 90%; the printing mode is single-dot mode or line drawing mode, the single-dot / line segment spacing is 0.1 to 5mm, the printhead moving speed is 0 to 300mm / s, and the single glue opening time is 0.001 to 1s.

[0062] (7) Interfacial polymerization and thermal crosslinking: The membrane loaded with aqueous monomers is stably immersed in an organic phase solution to carry out interfacial polymerization reaction. After the reaction is completed, the excess organic phase solution on the surface of the membrane is removed, and the membrane is cured by heating to crosslink and finally obtain a MOF / polyamide thin film composite membrane with a sandwich structure of polysulfone support layer / MOF intermediate layer / polyamide separation layer. The interfacial polymerization reaction time is 2 to 5 min, the thermal crosslinking temperature is 50 to 70 °C, and the thermal crosslinking time is 7 to 15 min.

[0063] This invention also protects the high-flux MOF / polyamide composite membrane prepared by the above-described method. The unmodified composite membrane has a regular three-layer sandwich structure: a bottom layer of porous polysulfone support with excellent mechanical strength and substrate load-bearing capacity; a middle layer of continuous porous intermediate layer formed by the ordered self-assembly of inkjet-printed MOF precursors, exhibiting a uniform structure and no particle agglomeration; and an outer layer of ultrathin, dense polyamide nanofiltration separation layer. The overall membrane layer has no obvious interfacial voids or structural defects. The invention also protects the CNC-modified composite membrane, whose middle layer is a composite functional layer formed by the inkjet self-assembly of MOF precursors and CNC. The CNC is uniformly dispersed and interwoven within the MOF framework, effectively optimizing the membrane microstructure and repairing interfacial defects, resulting in superior flux and selectivity synergy, and significantly improved overall structural stability and operational durability.

[0064] Unless otherwise specified, all raw materials and reagents used in this invention are commercially available conventional industrial or analytical grade reagents. The equipment used is conventional piezoelectric inkjet printing equipment, oven, and magnetic stirring device. There are no special limitations. The preparation process is simple, highly controllable, and suitable for large-scale mass production.

[0065] The present application will be specifically described below through specific embodiments. The following embodiments are only some embodiments of the present application and are not intended to limit the present application.

[0066] Example 1: Preparation of pure polyamide thin film composite film (PA TFC)

[0067] In this embodiment, a pure polyamide film composite membrane without filler modification was prepared as a basic control sample. The specific preparation steps are as follows:

[0068] (1) Pretreatment of base membrane: Soak the PSf polysulfone ultrafiltration base membrane in deionized water for 3 hours to fully remove surface impurities and protective agents. After taking it out, wipe off the surface water for later use.

[0069] (2) Preparation of aqueous printing ink: Accurately weigh 0.05g piperazine (PIP) and 0.2g polyvinylpyrrolidone (K10), dissolve them together in 10mL of pure water, stir thoroughly to dissolve, and prepare a uniform and stable aqueous printing ink with a mass concentration of 0.5wt%.

[0070] (3) Preparation of organic phase solution: Accurately weigh 0.1 g of trimesoyl chloride (TMC), dissolve it in 100 g of n-hexane solvent, stir thoroughly to prepare 0.1 wt% organic phase interfacial polymerization reaction solution, and seal for later use.

[0071] (4) Inkjet equipment parameter settings: Fill the prepared aqueous printing ink into the inkjet printer's ink tank; install a flat-head tungsten carbide printhead with a diameter of 0.03mm; fix the piezoelectric printing parameters: voltage rise time 0.50ms, fall time 0.30ms, valve opening time 0.30ms, striker stroke 50%, delay 1.00ms. Set the printing program to line drawing mode, x-direction line segment length 100mm, y-direction line segment spacing 1mm, number of line segments 80; nozzle distance from base film 3mm, trigger time 0.01s, nozzle retraction height 10mm, printhead moving speed 100mm / s, jet air pressure 0.2MPa.

[0072] (5) Base film fixation and inkjet printing: Cut the pretreated PSf base film into a rectangle of 20cm×15cm, lay it flat on the glass plate surface, use a rubber roller to thoroughly scrape off the residual moisture on the film surface, fix the base film flat with a plastic frame and clamps, place the whole thing on the inkjet printer workbench, wait for the program to run, and complete the uniform spraying of water phase ink.

[0073] (6) Interfacial polymerization and thermal crosslinking: After spraying, take 20 mL of organic phase solution and slowly pour it to cover the surface of the membrane. Let the interfacial polymerization reaction stand for 3 min. After the reaction, pour off the excess organic phase solution and place the membrane in a 60℃ oven for thermal crosslinking for 10 min. After natural cooling, a pure polyamide film composite membrane (PA TFC) is obtained.

[0074] The pure PA thin film composite membrane prepared in this embodiment has the following properties: pure water flux 34.82 LMH / bar, Na2SO4 rejection rate 95.9%, NaCl rejection rate 21.5%, separation factor 19.1; Congo red rejection rate 99.9%, methyl orange rejection rate 98.3%.

[0075] Example 2: Preparation of a single ZIF-8 modified MOF / polyamide composite membrane (ZIF-8 / PA TFC)

[0076] In this embodiment, a single MOF-modified polyamide composite film was prepared using triethylamine as a stabilized ZIF-8 precursor via a two-step inkjet printing process. The specific steps are as follows:

[0077] (1) Base film pretreatment: Immerse the PSf polysulfone base film in deionized water for at least 3 hours, and then take it out for use.

[0078] (2) Preparation of ZIF-8 precursor and printing ink: 2.06 g of dimethylimidazole and 1.26 g of triethylamine (TEA) were dissolved in 100 mL of anhydrous methanol and mixed with magnetic stirring. The mixture was then quickly poured into 100 mL of anhydrous methanol solution containing 3.72 g of Zn(NO3)2·6H2O and stirred at a constant temperature for 10 min. The ZIF-8 ultrasmall precursor was stabilized by competitive coordination of triethylamine. After the reaction was completed, the precipitate was collected by centrifugation and the precipitate was evenly dispersed in anhydrous ethanol to prepare 0.4 wt% ZIF-8 precursor printing ink.

[0079] (3) Preparation of aqueous printing ink: Dissolve 0.05g PIP, 0.015g sodium dodecylbenzenesulfonate (SDBS) and 0.25g dimethyl sulfoxide (DMSO) in 10mL of pure water and stir thoroughly to obtain 0.5wt% uniform and stable aqueous printing ink.

[0080] (4) Preparation of organic phase solution: Same as in Example 1, prepare 0.1wt% TMC n-hexane organic phase solution.

[0081] (5) ZIF-8 intermediate layer inkjet printing: Fill the printer liquid tank with ZIF-8 precursor printing ink and install a 0.05mm flat-head tungsten carbide printhead; fix the piezoelectric parameters: voltage rise time 0.50ms, fall time 0.30ms, valve opening time 0.30ms, striker stroke 50%, delay 1.00ms. Set the dual-line printing mode: line mode 1 is 100mm in length in the x direction, 3mm in spacing between lines in the y direction, and 40 lines; line mode 2 is 3mm in spacing between lines in the x direction, 33 lines, and 120mm in length in the y direction; nozzle distance from the base film is 3mm, trigger time is 0.01s, gun retraction height is 10mm, printhead moving speed is 300mm / s, and jet air pressure is 0.2MPa. After the pretreated PSf base film is cut and fixed, place it on the worktable, run the printing program in batches, and allow it to air dry for 3-5 minutes after each printing to complete the uniform deposition and self-assembly of the ZIF-8 precursor.

[0082] (6) Water phase inkjet printing: Replace the 0.03mm flat-head tungsten steel printhead, load water phase printing ink, and use the water phase printing parameters and scribing program of Example 1 to complete the precise spraying of water phase monomers on the ZIF-8 intermediate layer surface.

[0083] (7) Interfacial polymerization and thermal crosslinking: After spraying, take 20 mL of organic phase solution to cover the membrane surface, let it stand for 3 min, remove the residual liquid, and then thermally crosslink it in an oven at 60℃ for 10 min. After cooling, ZIF-8 / PA TFC composite membrane is obtained.

[0084] The composite membrane prepared in this embodiment has the following performance characteristics: pure water flux 55.32 LMH / bar, Na2SO4 rejection 95.3%, NaCl rejection 11.3%, separation factor 18.9; Congo red rejection 99.9%, methyl orange rejection 98.2%. Compared with pure polyamide membranes, the ZIF-8 interlayer can significantly improve the pure water permeate flux of the membrane while maintaining excellent dye rejection performance.

[0085] Example 3: Preparation of a single CNC-modified polyamide composite film (CNC / PA TFC)

[0086] This embodiment uses pure CNC nanofiller inkjet printing modification to prepare CNC reinforced polyamide composite film. The specific steps are as follows:

[0087] (1) Base film pretreatment: Immerse the PSf polysulfone base film in deionized water for at least 3 hours, then take it out, flatten it and fix it for later use.

[0088] (2) Preparation of CNC printing ink: Take 2 mL of cellulose nanocrystal (CNC) aqueous solution with a mass fraction of 4 wt%, add 100 mL of pure water to dilute and mix well to prepare a uniform CNC printing ink with a mass fraction of 0.08 wt%.

[0089] (3) Preparation of aqueous printing ink and organic phase solution: The formulation and preparation process of aqueous ink and organic phase solution are completely consistent with those in Example 2.

[0090] (4) CNC functional layer inkjet printing: Load CNC printing ink, install a 0.05mm flat-head tungsten steel printhead, and use the same piezoelectric parameters, double-line printing mode and drying regime as in Example 2 to complete the uniform spraying of the CNC functional layer on the base film surface.

[0091] (5) Aqueous phase monomer spraying, interfacial polymerization and thermal crosslinking: The subsequent aqueous phase inkjet printing, organic phase interfacial polymerization and thermal crosslinking process parameters are consistent with those in Example 2, and finally CNC modified polyamide film composite film is prepared.

[0092] The composite membrane prepared in this embodiment has the following properties: pure water flux 40.79 LMH / bar, Na2SO4 rejection rate 94.9%, NaCl rejection rate 27.3%, separation factor 14.3; Congo red rejection rate 99.9%, methyl orange rejection rate 96.8%. The introduction of CNC can effectively optimize the membrane interface structure and improve the hydrophilicity and structural stability of the membrane.

[0093] Example 4: Preparation of ZIF-8 / CNC modified high-performance composite membrane (MOF / CNC / PA TFC)

[0094] This embodiment uses ZIF-8 precursor and CNC composite synergistic modification, and prepares a composite modified film with optimal flux and selectivity through two-step inkjet precision molding. The specific steps are as follows:

[0095] (1) Base film pretreatment: Immerse the PSf polysulfone base film in deionized water for at least 3 hours, then take it out, flatten it and fix it for later use.

[0096] (2) Preparation of high-concentration ZIF-8 precursor ink: ZIF-8 precursor was prepared using the same precursor synthesis process as in Example 2. It was dispersed in anhydrous ethanol to prepare a 0.8 wt% ZIF-8 precursor dispersion.

[0097] (3) Preparation of MOF / CNC composite printing ink: Take 4 mL of 4 wt% CNC aqueous solution and dilute it in 100 mL of pure water to prepare 0.16 wt% CNC solution; take 5 mL of ZIF-8 precursor dispersion and 5 mL of CNC solution in equal volumes, stir and mix them thoroughly to prepare ZIF-8 / CNC composite printing ink with uniform dispersion.

[0098] (4) Preparation of aqueous printing ink and organic phase solution: The formula and preparation process are completely consistent with those in Example 2.

[0099] (5) MOF / CNC composite intermediate layer printing: loaded with compound printing ink, using a 0.05mm printhead, using the piezoelectric parameters, double-line printing program and interval drying regime of Example 2, a uniform and dense MOF / CNC composite intermediate layer is constructed on the surface of the base film. The CNC interpenetrating support effect is used to suppress MOF agglomeration and optimize the microstructure of the film layer.

[0100] (6) Precision spraying of aqueous phase monomers: Replace the 0.03mm nozzle and use standard aqueous phase printing parameters to complete the uniform loading of surface aqueous phase monomers.

[0101] (7) Interfacial polymerization and thermal crosslinking: The interfacial polymerization reaction and thermal crosslinking curing process parameters are the same as those in the previous examples, and the high-performance polyamide composite film with MOF / CNC synergistic modification is finally prepared.

[0102] The performance of the modified composite membrane prepared in this embodiment is as follows: pure water flux 48.47 LMH / bar, Na2SO4 rejection rate 98.8%, NaCl rejection rate 16.9%, separation factor 69.2; Congo red rejection rate 99.9%, methyl orange rejection rate 99.1%.

[0103] Example 5: Preparation of UiO-66-NH2 / CNC modified composite membrane

[0104] This embodiment verifies the compatibility of the small molecule coordination stabilization strategy and two-step inkjet process of the present invention with the UiO-66-NH2 system. The steps are basically the same as in Example 4, except that the raw materials for MOF precursor preparation are replaced:

[0105] (1) The preparation of the base film pretreatment, CNC ink, aqueous ink, and organic phase solution is the same as in Example 4;

[0106] (2) Preparation of UiO-66-NH2 precursor: Terephthalic acid, aminoterephthalic acid and triethylamine were co-dissolved in anhydrous methanol, and zirconium tetrachloride methanol solution was added and stirred at a constant temperature for 12 min. The excessive growth of UiO-66-NH2 crystals was inhibited by competitive coordination of triethylamine. The precursor with a particle size of 40-60 nm was obtained by centrifugation and dispersed in anhydrous ethanol to prepare a 0.8 wt% precursor dispersion.

[0107] (3) Mix UiO-66-NH2 dispersion and 0.04wt% CNC aqueous solution in equal volumes and stir until homogeneous to obtain composite printing ink;

[0108] (4) The process parameters for inkjet layer printing, interface polymerization, and thermal crosslinking are completely the same as those in Example 4.

[0109] The membrane performance in this embodiment is as follows: pure water flux 46.91 LMH / bar, Na₂SO₄ rejection 98.3%, NaCl rejection 17.5%, mono / divalent salt separation factor 66.5; Congo red rejection 99.9%, methyl orange rejection 98.9%. The results indicate that the preparation system of this invention is also applicable to amino-modified zirconium-based MOF materials, achieving synergistic optimization of flux and salt separation performance.

[0110] Example 6: Preparation of CuBTC / CNC Modified Composite Membrane

[0111] This embodiment verifies the versatility of the process of the present invention in the copper-based MOF system. The preparation process is the same as in Example 4, with the MOF precursor synthesis raw materials adjusted:

[0112] (1) The base membrane, CNC, aqueous phase, and organic phase systems are all consistent with those in Example 4;

[0113] (2) Preparation of CuBTC precursor: Tristyric acid and triethylamine were dissolved in anhydrous methanol, and copper acetate methanol solution was added dropwise and stirred for 8 min. Triethylamine passivated the active sites on the surface of CuBTC nanoclusters. The 30-45 nm precursor was collected by centrifugation and dispersed in ethanol to obtain a 0.8 wt% dispersion.

[0114] (3) The composite printing ink is prepared by mixing it with an equal volume of CNC solution, and the subsequent inkjet, polymerization and thermal crosslinking parameters remain unchanged.

[0115] The membrane performance in this embodiment is as follows: pure water flux 47.55 LMH / bar, Na2SO4 rejection 98.5%, NaCl rejection 17.2%, mono / divalent salt separation factor 67.8; Congo red rejection 99.9%, methyl orange rejection 99.0%. It is evident that the small molecule stability control and inkjet printing scheme of this invention are not limited by the MOF metal center or ligand type, and possesses good process scalability.

[0116] Comparative Example 1: Preparation of MOF / CNC composite membranes without the addition of triethylamine (no precursor stabilizing system)

[0117] The preparation process was the same as in Example 4, except that the triethylamine addition step in the MOF synthesis stage was omitted; the remaining raw material ratios, printing, and polymerization parameters were completely identical. The resulting ZIF-8 particles showed significant agglomeration, and the ink stratified and settled after standing for 30 minutes. Membrane performance: pure water flux 36.15 LMH / bar, Na₂SO₄ rejection rate 67.2%, separation factor 2.6, and dye rejection rate decreased to 84.7%.

[0118] The comparison shows that the lack of triethylamine small molecule coordination stabilization mechanism leads to severe MOF aggregation, a significant increase in intramembrane defects, and a marked decline in both flux and separation performance, verifying that small molecule regulation is a necessary condition to ensure the overall performance of the membrane.

[0119] Comparative Example 2: Preparation of MOF / CNC composite film using conventional dip coating process (precise low-feed molding without inkjet printing)

[0120] Using the same MOF / CNC mixture as in Example 4, an intermediate layer was loaded onto the surface of a polysulfone-based membrane using a conventional dip-coating method. The immersion time was 30 seconds, followed by air drying at room temperature. Subsequent aqueous monomer loading was also achieved using dip-coating, with the interfacial polymerization and thermal crosslinking parameters remaining unchanged. The chemical consumption during the molding process was 6.2 times that of the inkjet process of this invention. The pure water flux of the membranes prepared in multiple batches fluctuated within a range of ±8.7 LMH / bar, and the sodium sulfate rejection rate fluctuated within a range of ±4.1%, indicating poor batch consistency. The average membrane performance was: pure water flux 41.23 LMH / bar, Na2SO4 rejection rate 93.6%, and separation factor 32.5.

[0121] Data proves that the upgraded piezoelectric inkjet coating for precise layered printing can reduce material consumption, improve the uniformity of large-area film formation and batch stability, which is superior to the traditional dip coating method.

[0122] Comparative Example 3: Gradient control of different CNC doping levels (only the CNC content in the composite ink was changed, while the rest remained the same)

[0123] With the ZIF-8 precursor printing ink concentration fixed at 0.8 wt%, and all other formulation parameters and preparation processes kept identical to Example 4, only the doping ratio of CNC filler in the composite ink was adjusted. Four groups of composite printing inks with CNC mass fractions of 0.01 wt%, 0.08 wt%, 0.16 wt%, and 0.22 wt% were prepared, and composite films were fabricated. The influence of CNC doping on the film separation performance was investigated. The specific performance results are as follows:

[0124] 1. CNC content 0.01wt%: The composite membrane has a pure water flux of 43.61 LMH / bar, a sodium sulfate rejection rate of 95.1%, and a mono / divalent salt separation factor of 37.2. At this content, the CNC filler content is relatively low, and its supporting effect on the gaps in the MOF skeleton and its filling effect on micro-defects at the membrane interface are both limited, resulting in no significant synergistic modification improvement effect.

[0125] 2. CNC dosage of 0.04wt%: The composite membrane has a pure water flux of 45.28 LMH / bar, a sodium sulfate rejection rate of 97.4%, and a mono / divalent salt separation factor of 52.7. With a moderate increase in CNC dosage, membrane interface defects are somewhat repaired, the mass transfer channel structure is initially optimized, and the overall separation performance of the composite membrane is steadily improved.

[0126] 3. CNC doping at 0.08 wt%: The composite membrane has a pure water flux of 48.47 LMH / bar, a sodium sulfate rejection rate of 98.8%, and a mono / divalent salt separation factor of 69.2. At this doping level, CNC can be uniformly intercalated within the MOF framework, effectively inhibiting MOF nanoparticle aggregation, repairing interfacial micro-defects, and optimizing membrane hydrophilicity and water molecule mass transfer channels. The synergistic optimization of flux and selectivity is optimal, making it the preferred doping range for this application.

[0127] 4. CNC doping content of 0.16wt%: The pure water flux of the composite membrane decreased to 34.16 LMH / bar, the sodium sulfate rejection rate decreased to 96.3%, and the mono / divalent salt separation factor decreased to 44.1. When the CNC doping content is too high, it will increase the viscosity of the printing ink system, affecting the uniform spreading and fusion of ink droplets on the base membrane surface. At the same time, excessive nanofillers can easily cause blockage of the internal pores of the membrane and reduce the effective mass transfer channels, thus leading to a decline in the overall separation performance of the composite membrane.

[0128] The gradient control experiments described above show that CNC doping has a significant impact on the microstructure and separation performance of the composite membrane, and it can only exert a good synergistic modification effect within a suitable doping range. The CNC doping range of 0.02–0.16 wt% in this application can better balance the ink printing performance and the microstructure control effect of the membrane layer, effectively improving the sieving selectivity of the membrane material without sacrificing permeation flux. Excessively high or low CNC doping ratios make it difficult to achieve simultaneous optimization of flux and selectivity.

[0129] Comprehensive performance analysis of the examples:

[0130] Based on the test results of the above-mentioned basic embodiments, gradient control embodiments, and extended embodiments of the multi-MOF system, it can be seen that different modification methods and process conditions have a significant impact on the microstructure and separation performance of the composite membrane. Pure polyamide film composite membranes can achieve basic dye retention and salt separation, but suffer from low pure water permeate flux and insufficient salt separation selectivity, resulting in limited overall separation performance. Single ZIF-8MOF modification can effectively construct porous mass transfer channels and significantly improve the pure water flux of the membrane, but its ability to repair defects at the membrane interface is limited, making it difficult to effectively optimize the sieving performance of mono / divalent salts, and the flux-selectivity trade-off remains. Single CNC modification can optimize the stability of the membrane structure and improve the hydrophilicity of the membrane surface to a certain extent, but its effect on constructing water molecule mass transfer channels is weak, and its improvement on flux and separation selectivity is relatively limited.

[0131] Compared to single-modification systems, the ZIF-8 / CNC composite synergistic modification scheme adopted in this invention can fully combine the high porosity mass transfer characteristics of MOF materials with the rigid support and interface defect repair advantages of CNC, achieving complementary performance optimization. While maintaining an ultra-high dye rejection rate of 99.9% and ensuring excellent pure water permeation flux, it significantly improves the selectivity of mono / divalent salt separation in the membrane, effectively alleviating the technical shortcomings of the mutual constraint between flux and selectivity in traditional polyamide composite membranes. At the same time, the results of extended experiments with multiple MOF systems such as UiO-66-NH2 and CuBTC show that the small molecule precursor stabilization strategy and inkjet printing process of this invention have good versatility and adaptability, and are not limited to a single MOF material system.

[0132] Furthermore, compared to traditional, extensive molding processes such as dip coating, the skin-upgraded, precise, low-feed inkjet printing process employed in this invention effectively reduces material loss during preparation. Simultaneously, it achieves uniform and controllable membrane formation, exhibits strong process replicability, and minimizes batch-to-batch performance fluctuations. This effectively addresses the problems of traditional processes, such as high randomness in membrane formation, poor large-area uniformity, and insufficient mass production stability. In summary, through synergistic optimization of material formulation and refined innovation in molding processes, this invention produces composite membranes with excellent overall separation performance, broad process adaptability, and strong mass production feasibility. It possesses significant research value and promising industrial application prospects in the field of fine water treatment separation.

[0133] Core mechanism of action:

[0134] This invention achieves comprehensive optimization of the microstructure and macroscopic separation performance of the composite membrane through the synergistic effect of four core mechanisms: small molecule coordination stabilization mechanism, controllable droplet dynamics formation mechanism, ordered self-assembly mechanism of precursor, and MOF / CNC synergistic modification mechanism. It solves the technical problems of traditional MOF / polyamide composite membranes, such as easy agglomeration of fillers, poor ink stability, numerous membrane defects, weak formation controllability, flux selectivity constraints, and poor batch consistency. The specific core mechanisms are as follows:

[0135] 1. Triethylamine small molecule competitive coordination stabilization mechanism of MOF precursor: Triethylamine small molecule regulator is introduced in the early nucleation and growth stage of MOF crystals. Through competitive coordination with metal ions, it seizes the active sites of MOF metals and slows down the rapid growth of MOF crystals. At the same time, triethylamine molecules are physically adsorbed on the surface of MOF nanoclusters to form a molecular protective layer, passivating highly active sites and inhibiting disordered stacking and aggregation of particles. This stabilizes the preparation of 30-50nm monodisperse ultrasmall MOF precursors, greatly improving the dispersion stability of inks in water and alcohol solvents, and adapting to the micro-volume inkjet printing needs of leather-upgraded products.

[0136] 2. Controllable Formation Mechanism of Inkjet Droplet Dynamics: The uniform and controllable spreading of ink droplets on the porous substrate surface is crucial for the fabrication of controllable composite films using inkjet printing. This is influenced by multiple factors, including the physicochemical properties of the ink droplets, the surface characteristics of the substrate film, and the printing conditions. This invention precisely optimizes the surface tension, wettability, and dispersion stability of ink droplets by adapting and controlling the ratio of co-solvent and surfactant in the ink, and matching the optimal printing process parameters. This allows for precise control of the size, distribution, and spreading state of individual ink droplets, enabling the uniform spreading of micro-droplets on the porous substrate surface. As adjacent ink droplets gradually spread and merge to form stable liquid bridges, the MOF precursor can achieve uniform arrangement and full-coverage deposition over a large scale. Simultaneously, combined with piezoelectric inkjet 3-10 pL micrometer-level precise non-contact deposition technology, the printhead parameters, printing spacing, travel speed, and wet / dry interval are programmed to achieve targeted, quantitative, and uniform loading of the MOF precursor and aqueous monomers on the substrate surface, completely solving the problems of random liquid feeding, high material loss, and poor film consistency over large areas in traditional processes.

[0137] 3. Solvent and regulator gradient evaporation-induced ordered self-assembly mechanism: After printing, the solvent and triethylamine regulator slowly evaporate in a gradient, the passivation protective layer on the MOF precursor surface desorbs in an orderly manner, the active sites are uniformly exposed, and the MOF precursor nanoclusters are induced to crosslink and stack in an orderly manner to form a continuous, dense, non-agglomerated, and void-free MOF porous intermediate layer. This precisely controls the rate of subsequent interfacial polymerization reaction and provides a stable substrate for the molding of ultrathin, low-defect polyamide separation layer.

[0138] 4. MOF / CNC Synergistic Interface Optimization Mechanism: CNC possesses excellent hydrophilicity, nano-size effect, and rigid support characteristics, which can be uniformly interpenetrated and filled in the gaps of the MOF framework, physically preventing MOF aggregation and filling micro-interface defects; at the same time, it optimizes the hydrophilicity and mass transfer channel structure of the membrane layer, reduces the resistance to water molecule transport, and significantly improves the pure water flux while ensuring high retention accuracy, breaking through the technical problem of flux-selectivity balance in traditional membrane materials.

[0139] Performance and morphology comprehensive characterization description:

[0140] 1. Microscopic morphology analysis

[0141] The surface and cross-sectional micromorphology of the membrane were observed by scanning electron microscopy (SEM): the pure polyamide membrane had few wrinkles on its surface and a large number of interfacial voids inside; the single ZIF-8 modified membrane had local particle agglomeration and micropores; the single CNC modified membrane had a slight reduction in interfacial defects, but the number of mass transfer channels was limited; the MOF / CNC composite modified membrane of this invention had a smooth and uniform surface with no obvious particle agglomeration, and the interfacial voids inside the membrane were fully filled by CNC to form continuous and interconnected water molecule mass transfer channels. The three-layer structure had a tight interface bond without delamination defects.

[0142] 2. Permeability test

[0143] Under the same operating pressure, the pure PA membrane has a pure water flux of only 34.82 LMH / bar; the single ZIF-8 modification increases the flux to 55.32 LMH / bar by relying on porous channels; the MOF / CNC composite system takes into account both pore transport and interface defect repair, and maintains a flux of more than 48 LMH, which is far higher than that of the traditional dip-coated composite membrane, and has the best overall water permeability performance.

[0144] 3. The selective polyamide membrane for salt separation has a separation factor of only 19.1 for monovalent and divalent salts; the improvement of the separation factor of a single modified membrane is limited; the separation factor of the MOF / CNC compound modified membrane of this invention can reach up to 69.2, which greatly improves the screening accuracy of monovalent and divalent sulfates and is suitable for water softening and wastewater salt separation.

[0145] 4. Long-term operation and batch stability: Traditional dip-coating process membranes exhibit large fluctuations in batch flux and rejection rate; the inkjet printing process parameters of this invention are standardized and can be programmed for replication, resulting in minimal performance differences between different batches of membranes; at the same time, the MOF / CNC composite intermediate layer structure is stable, and flux decay is slow under long-term continuous filtration conditions, with superior anti-fouling and long-term operation performance compared to traditional modified nanofiltration membranes.

[0146] Compared with existing MOF / polyamide composite films prepared by traditional physical blending, blade coating, dip coating, and conventional spraying, this invention constructs a triethylamine small molecule precursor stabilization system, a two-step inkjet pixel-level low-feed-solvent molding mechanism, a solvent gradient evaporation-induced self-assembly control strategy, and a MOF / CNC dual-filler synergistic modification system. It overcomes the bottlenecks of traditional film-making technologies from multiple dimensions, including material formulation, molding process, microstructure control, and performance optimization. Based on performance data from multiple sets of comparative examples, this invention possesses the following outstanding innovative advantages and beneficial effects compared to existing technologies:

[0147] (1) Improve MOF aggregation problem and optimize membrane microstructure and sieving accuracy.

[0148] Traditional processes often involve direct physical doping with finished MOF particles. However, these nanoparticles are prone to aggregation and sedimentation within the system, easily forming non-selective pore defects within the membrane and affecting the separation selectivity of the membrane material to some extent. This invention introduces a triethylamine small molecule competitive coordination regulation mechanism during the MOF nucleation stage, enabling in-situ preparation of MOF precursors with uniform size (30–50 nm) and good dispersibility, effectively alleviating MOF particle aggregation. Simultaneously, the use of CNC nanofibers interspersed in the gaps between MOF framework particles further improves interfacial micro-defects and optimizes the membrane pore structure. Test results show that, compared to pure polyamide membranes and single-modified membranes, this invention can increase the sodium sulfate rejection rate to 98.8% while maintaining a dye rejection rate of 99.9%, effectively addressing the problems of numerous structural defects and insufficient sieving performance in traditional modified membranes.

[0149] (2) Achieve precise and controllable micro-scale molding, improve raw material utilization, and adapt to large-scale preparation conditions.

[0150] Traditional molding methods such as dip coating, blade coating, and spray coating apply liquid in a coarse manner, resulting in weak controllability of the molding process and relatively limited material utilization, making it difficult to achieve micro-volume, targeted, and uniform film construction. This invention, based on the 3-10 pL picoliter micro-deposition characteristics of piezoelectric inkjet printing, achieves quantitative and directional loading of MOF precursors and aqueous monomers on the substrate film surface by rationally matching process parameters such as droplet size, printing spacing, travel speed, and layer drying. Compared to traditional processes, this invention offers greater controllability of molding liquid volume, reducing material consumption of MOF fillers, aqueous monomers, and organic solvents to a certain extent, saving preparation costs, and is more suitable for standardized and large-scale preparation scenarios.

[0151] (3) Optimize film formation kinetics matching to improve film uniformity and batch stability.

[0152] Traditional preparation processes struggle to simultaneously manage the dynamic matching between the MOF crystallization process and the polyamide interfacial polymerization process, easily leading to uneven film thickness, dispersed filler distribution, and batch-to-batch performance fluctuations. This invention, by controlling the droplet spreading and fusion process and the gradient evaporation of solvents and regulators, guides the orderly cross-linking and self-assembly of the MOF precursor, resulting in a structurally continuous and uniformly thick MOF intermediate functional layer. This provides a good substrate for the stable growth of subsequent polyamide functional layers. Furthermore, the process parameters of this invention can be programmed and replicated, effectively improving the inherent randomness and insufficient controllability of traditional film formation methods, thus enhancing batch-to-batch consistency and operational stability of the composite film.

[0153] (4) Alleviate the contradiction between flux and selectivity and improve the overall separation performance of the membrane.

[0154] Conventional MOF-modified polyamide membranes often suffer from a trade-off between flux and selectivity, making it difficult to simultaneously optimize both properties. This invention leverages the porous channel structure of MOF, the hydrophilic properties of CNC, and the synergistic repair effect of interfacial defects to effectively optimize the intramembrane mass transfer pathway and reduce water molecule transport resistance, achieving a synergistic improvement in separation performance. Data from related examples show that compared to the pure membrane's pure water flux of 34.82 LMH / bar, single ZIF-8 modification can increase the flux to 55.32 LMH / bar. Furthermore, the MOF / CNC composite system constructed on this basis can optimize the mono / divalent salt separation factor from 19.1 in the pure membrane to 69.2 while maintaining excellent flux and dye retention performance, effectively alleviating the performance limitations of traditional membrane materials and significantly improving overall separation performance.

[0155] (5) It has wide process adaptability and good potential for expanded application.

[0156] The small-molecule precursor stabilization strategy employed in this invention is not limited to the ZIF-8 material system; it is also applicable to the modification and preparation of various common MOF materials such as CuBTC and UiO-66-NH2. Furthermore, the ink solid content, filler ratio, printing parameters, and interfacial polymerization conditions can all be adaptively adjusted according to actual separation requirements, enabling the preparation of functional composite membranes with different performance focuses. These membranes are suitable for various nanofiltration applications, including industrial wastewater treatment, dye retention, water softening, and seawater desalination pretreatment, demonstrating good process versatility and engineering application potential.

[0157] (6) The overall process system is different from the traditional technical path and has a certain degree of technological advancement.

[0158] Most existing related technologies focus on modifying finished MOFs through physical blending combined with extensive coating processes, with less emphasis on integrated process approaches such as precise precursor stabilization, controllable micro-deposition via inkjet printing, dynamic self-assembly regulation, and synergistic modification with dual fillers. This invention constructs an integrated preparation system comprising "small molecule coordinated stable precursor + two-step inkjet precise deposition + gradient evaporation self-assembly + MOF / CNC synergistic defect repair," which differs from existing conventional technologies in material control, molding process, and microstructure optimization, forming a differentiated technical solution with significant innovation and technological advancement.

[0159] In summary, this invention addresses the problems existing in the preparation of MOF / polyamide composite membranes, such as filler agglomeration, numerous membrane defects, limited molding controllability, low material utilization, poor flux-selectivity matching, and the need to improve batch stability. It provides a controllable, replicable, and mass-production-ready preparation method. The prepared composite membrane exhibits a uniform structure and excellent overall separation performance, demonstrating significant research value and promising industrial application prospects in the field of fine separation in water treatment.

[0160] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should be protected by the present invention.

Claims

1. A method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology, characterized in that, Includes the following steps: (1) Base membrane pretreatment: Immerse the polysulfone ultrafiltration base membrane in deionized water for at least 3 hours, remove it, remove the surface water, and fix it on the printing workbench for later use. (2) Preparation of MOF precursor printing ink: Triethylamine is added during the MOF crystal nucleation stage for coordination regulation, and MOF precursor is prepared in situ. The MOF precursor is then dispersed in the corresponding solvent to prepare MOF precursor printing ink. (3) Preparation of aqueous printing ink: Dissolve the aqueous monomer, surfactant and co-solvent in pure water, stir evenly, and prepare an aqueous ink suitable for piezoelectric inkjet. (4) Preparation of organic phase reaction solution: Dissolve pyromellitic chloride in n-hexane to prepare an organic phase solution for interfacial polymerization; (5) Inkjet printing to construct MOF intermediate layer: Piezoelectric inkjet printing equipment is used to output micro-droplets to quantitatively deposit MOF precursor printing ink onto the surface of pretreated polysulfone film. By using a multi-stage printing method and allowing natural drying during the printing intervals, the MOF precursor is able to self-assemble in an orderly manner on the base film surface through precise control of the ink droplet spreading and fusion rate and the solvent evaporation rate, forming a defect-free, continuous and dense MOF intermediate layer. (6) Inkjet controllable load aqueous phase monomer: Keep the same inkjet equipment parameters unchanged, switch the aqueous phase printing ink, and perform low liquid supply precision inkjet printing on the surface of the formed MOF intermediate layer. (7) Interfacial polymerization and thermal crosslinking: The membrane loaded with aqueous monomers is immersed in an organic phase solution for interfacial polymerization. After the reaction is completed, excess organic phase is removed from the surface of the membrane, and then crosslinked and cured by heating to finally obtain a high-throughput MOF / polyamide thin film composite membrane with a sandwich structure of polysulfone support layer / MOF intermediate layer / polyamide separation layer.

2. The method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology according to claim 1, characterized in that: In step (2), the MOF precursor is at least one of ZIF-8, CuBTC, and UiO-66-NH2, with a particle size of 30-50 nm; the reaction solvent for preparing the MOF precursor is anhydrous methanol, the dispersion solvent for the MOF precursor printing ink is anhydrous ethanol or pure water, and the ink mass fraction is 0.01wt%-2wt%.

3. The method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology according to claim 1, characterized in that: In step (3), the aqueous monomer is at least one of piperazine and polyethyleneimine; the surfactant is at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and polyvinylpyrrolidone; the cosolvent is at least one of ethanol and dimethyl sulfoxide; the mass concentration of the aqueous monomer is 0.1wt% to 2wt%, the mass concentration of the surfactant is 0.1wt% to 5wt%, and the mass concentration of the cosolvent is 1wt% to 10wt%; in step (4), the mass concentration of trimesoyl chloride in n-hexane is 0.05wt% to 0.2wt%.

4. The method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology according to claim 1, characterized in that: In step (5), the volume of the piezoelectric inkjet output droplets is 3-10 pL, the nozzle diameter is 0.01-1 mm, the jetting pressure is 0.1-0.5 MPa, the distance between the nozzle and the base film is 0.5-10 mm, the number of prints is 1-5, and the natural drying time between two adjacent prints is 1-5 min.

5. The method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology according to claim 1, characterized in that: In steps (5) and (6), the parameters of the piezoelectric inkjet equipment are controlled as follows: the needle rise time is 0.1 to 3s, the needle fall time is 0.1 to 0.5s, the valve opening time is 0.1 to 5s, and the needle stroke is 25% to 90%. The printing mode is single-dot mode or line drawing mode, the single-dot / line segment spacing is 0.1 to 5mm, the printhead moving speed is 0 to 300mm / s, and the single glue opening time is 0.001 to 1s.

6. The method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology according to claim 1, characterized in that: In step (7), the interfacial polymerization reaction time is 1 to 5 minutes, the thermal crosslinking temperature is 50 to 70°C, and the thermal crosslinking time is 7 to 15 minutes.

7. The method for preparing a high-throughput MOF / polyamide composite film based on inkjet printing technology according to claim 1, characterized in that: In step (2), when preparing MOF precursor printing ink, cellulose nanocrystals (CNC) are added to form MOF / CNC composite printing ink. The mass fraction of cellulose nanocrystals (CNC) in the composite printing ink is 0.04wt% to 0.16wt%.

8. A high-throughput MOF / polyamide composite membrane prepared by the method according to any one of claims 1 to 7, characterized in that: The composite membrane has a three-layer integrated sandwich structure. The bottom layer is a porous polysulfone support layer, the middle layer is a continuous porous intermediate layer formed by the ordered self-assembly of inkjet-printed MOF precursors, and the surface layer is an ultra-thin dense polyamide nanofiltration separation layer.

9. The high-flux MOF / polyamide composite membrane according to claim 8, characterized in that: The intermediate layer is a composite reinforcement layer formed by the self-assembly of MOF precursor and CNC inkjet, with CNC uniformly inserted and anchored inside the MOF skeleton.

10. The application of the high-flux MOF / polyamide composite membrane according to claim 8 or 9 in industrial wastewater purification, dye wastewater treatment, water softening, and seawater desalination pretreatment.

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