Advances in the preparation of inorganic ceramic membranes and their utilizations within green chemistry

2.1. Solid particle sintering method

The sintering process of solid particles presents distinct characteristics. Initially, solid powders are shaped into a specific form and subsequently heated to a pre-determined temperature. This heating process leads to contraction and densification, ultimately resulting in the formation of a solid body. The fundamental principle of this process involves particle diffusion and agglomeration, along with the establishment of sintering necks and the action of surface tension forces.

Sintering technology offers significant advantages. It is highly efficient, cost-effective, and environmentally friendly. As a result, it finds extensive applications across various industrial sectors, including ceramics, metals, plastics, and others. The key influencing factors for solid particle sintering are sintering temperature, raw material particle size, added additives, holding time, atmosphere, and shaping pressure.

Sintering temperature critically influences membrane preparation. Selecting an appropriate sintering temperature is vital, especially for diverse raw materials. Recent studies have explored industrial by-products (e.g., fly ash, waste sulfite) as raw materials. Band gap modification can be achieved by doping impurities (e.g., Fe³⁺, Si⁴⁺) or creating oxygen vacancies. For instance, Chao Wang Chao Wang used waste sulfite to fabricate Al₂O₃ membranes. Fe³⁺ doping narrowed the band gap from 6.0 eV to 5.8 eV, effectively extending the light response to the visible region.

As shown in Figure 1, the optimal membrane with outstanding performance was attained by sintering at 1100°C. It had an open porosity of 41.6%, a bending strength of 37.2 MPa, and an average pore size of 0.40 μm, with relevant data presented in Figure 2. Additionally, other scholars incorporated alumina particles to optimize the sintering temperature of the raw materials [19].

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Figure 1.

(a) Open porosity and (b) Flexural strength of ceramic membranes.

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Figure 2.

Pore size distribution of (a) A1, (b) A2, and (c) A3. Membrane A1 sintered at 1200°C was too dense to measure pore size.

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Dong Zou employed power plant fly ash powder and kaolin as the carrier and membrane layer, respectively, to prepare a composite ceramic membrane. High-strength alumina particles were incorporated into the carrier (AFA carrier). The membrane exhibited an average pore size of 320 nm and a water permeability of 3650 L·m⁻²·h⁻¹·bar⁻¹, with a stable permeability approaching 530 L·m⁻²·h⁻¹·bar⁻¹. Moreover, studies have investigated how the inverse micelle/sintering method affects thin-film performance [20].

Hua Chang employed solid waste fly ash as the supporting aggregate, preparing a ceramic membrane support sample with a uniform diameter. The results showed that the support was sintered at 1050°C with 15% carbon powder (mass fraction). It featured uniform internal pores and a distinct neck structure. Its pure water flux was 4728.26 L/(m²·h·MPa), flexural strength was 25.15 MPa, median pore diameter was 3.06 μm, and porosity was 38.56%. Moreover, this approach highlights waste utilization: fly ash-based Al₂O₃ nanoparticles derived from the same raw material reduce filler costs by 50%, effectively narrowing the cost gap with traditional ceramic membrane fabrication methods [21].

Sintering aids are also key factors. Weixing Chen explored high-value utilization of solid waste and high costs of ceramic membranes, experimenting with how sintering temperature and holding time affect the support’s physicochemical properties (support micrographs in Figure 3). Na₂CO₃, HPMC, and carbon powder were added at mass fractions of 4%, 3%, and 15%, respectively. Sintering was conducted at 900°C for 2 h, yielding a support with optimal comprehensive performance.

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Figure 3.

SEM images of supports at different sintering temperatures: (a) 800°C; (b) 850°C; (c) 900°C; (d) 950°C; (e) 1000°C; and (f) 1050°C.

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As shown in Figure 4, the support performed well with a pure water flux of 2468 L/(m²·h·MPa), flexural strength of 24.96 MPa, porosity of 43.38%, acid corrosion resistance rate of 4.45%, and alkali corrosion resistance rate of 0.62%. Further analysis of the influence of sintering temperature on flexural strength and pure water flux showed that the coal gangue-based ceramic membrane support project achieves high-value solid waste utilization and economic benefits [22].

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Figure 4.

Pore size distribution of specimens at different maximum temperatures.

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The particle size of raw materials significantly affects the preparation of ceramic membranes. Li Cao et al. fabricated porous alumina ceramics via the solid particle sintering method, yielding materials with high porosity (44-68%), a defined pore size distribution, and adequate strength [23].

Bo Li et al. [24] used fly ash and loess with median particle sizes of 21.28 μm and 11.96 μm as raw materials, and carboxymethyl cellulose as the pore-forming agent. They prepared fly ash-loess-based ceramic membranes via roll forming-solid particle sintering. Fe³⁺ doping narrowed the band gap, enhancing visible light absorption. The effects of sintering temperature and holding time were investigated, with the support micromorphology shown in Figure 5. The optimal membrane was sintered at 1100°C for 2.5 h. It exhibited a pure water flux of 2846.89 L/(m²·h·MPa) and a flexural strength of 17.15 MPa. The acid and alkali corrosion rates were 2.74% and 0.06%, respectively. Additionally, it had an average pore size of 4.11 μm, with uniform particle distribution and abundant mature crystals [24].

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Figure 5.

SEM images of the coal fly ash and loess ceramic membrane supports sintered at different temperatures: (a) 1080°C, (b) 1090°C, (c) 1100°C, (d) 1110°C, and (e) 1120°C.

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In summary, the solid particle sintering method is widely applied in the preparation of microporous ceramic membranes or supports. It has now been extended to microporous metal membranes. This method involves several steps. First, ultrafine inorganic composite particles are dispersed in a solvent. Then, inorganic binders are added to form a stable suspension. Next, the green body is shaped and dried. Finally, high-temperature sintering is performed to obtain the product. However, high raw material costs remain a challenge. This has prompted exploration of waste conversion and utilization as a new approach.

2.2. Sol-gel method

The sol-gel method typically uses inorganic salts or metal alkoxide precursors. These precursors are dissolved in a solvent to form a homogeneous solution. Subsequently, the solute undergoes hydrolysis with the solvent. The reactants aggregate into ∼1 nm particles, forming a sol that dries into a gel. Heat treatment then converts the gel into a ceramic film. This method is suitable for preparing materials with small pore sizes and narrow pore size distributions.

The sol-gel method commonly fabricates porous materials with special pore structures, and solvent selection and usage are critical for success. Ronn Goei used the acid-catalyzed sol-gel method to make a highly permeable film, a poly film-based titanium dioxide hybrid photocatalytic film with layered porosity. As shown in Figure 6, the pore structure of the titanium dioxide sol was modified by adding P-123 in multiple weight ratios to obtain the titanium dioxide layer. PVA acted as a pore filler and binder. The solvent was applied layer by layer onto an alumina ceramic disc to produce porous titanium dioxide/alumina films with different pore gradients. Layered pore structures and PVA filling formed heterointerfaces, narrowing the band gap from 3.2 eV to 2.9 eV and enhancing a visible light response [25].

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Figure 6.

(a) N₂ adsorption/desorption curves and (b) pore size distribution of the pulverized TiO₂ layers with different amounts of Pluronic P-123 triblock-copolymer added.

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In ceramic membrane preparation, better and denser membranes can be obtained by usually choosing a solvent with a higher boiling point. Nanoparticle size is controlled via low-temperature hydrolysis (room temperature to 100°C) and mild sintering (≤800°C) to adjust the band gap. Hui Huang optimized the solvent (ethanolamine) and stabilizer using the sol-gel method. The prepared ZnO film exhibits enhanced crystallinity (improved preferred orientation). Its band gap is broadened from 3.3 eV to 3.37 eV, achieving a 20% increase in photocatalytic efficiency [26].

Wei Rao deposited strontium barium titanate (BST) films on Si substrates. Using ethylene glycol ether as the solvent and aging for 1-3 days improved the quality of BST films. Specifically, solvents with low viscosity, high boiling points, and high heats of evaporation (e.g., ethylene glycol ether) enhance crystallinity during the sol-gel process, thereby improving BST membrane hydrophilicity. This mechanism reduces the water contact angle to below 5° due to enhanced hydroxyl (-OH) group exposure during slow solvent evaporation [27].

Superhydrophilic and superhydrophobic surfaces were prepared via simple impregnation of dura with synthetic PDADMAC-Al₂O₃/PFO nanocomposites. The experimental protocol has been illustrated in Figure 7. The membrane coated with PDADMAC-Al₂O₃/44 wt% PFO exhibited excellent wetting and separation properties. It also showed good chemical stability. The coating maintained good chemical stability at pH 3 over the 7-day test. This material had a high oil contact angle of 155°. It also displayed excellent water permeability and strong oil displacement performance [28].

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Figure 7.

(a) Schematic procedure for sol-gel preparation of synthetic PDADMAC-Al₂O₃/PFO complex polymer nanocomposites. (b) Chemical reaction scheme used for the synthesis of PDADMAC-Al₂O₃/PFO.

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Hongmin Ju used zirconium oxide as the precursor and fabricated a zirconia hydrogen-resistant coating on ZrH(1.8) via the sol-gel method. He investigated how solvent types affect the preparation of such zirconia films. Using n-propanol as the solvent yielded a uniform, continuous, dense zirconia film. It bonded tightly to the substrate, with an average thickness of over 10 μm. This film showed superior hydrogen resistance (hydrogen permeation reduction factor [PRF]=12.5) and acid-alkali corrosion resistance. Hydrophobic ZrO₂-coated membranes are used in electroplating wastewater treatment. They block heavy metal ion penetration, with a chemical corrosion resistance rate < 1% [29].

Solvents regulate reactions via substitution or complexation, thereby influencing the sol-gel process of the system. The extent of this influence depends on solvent properties, including electronegativity, steric hindrance, and complexation ability. The sol composition regulates the morphology and thickness of composite films. The wettability of nanocomposite films correlates with their morphological characteristics and thickness parameters. Holtzinger synthesized TiO₂-SiO₂ nanocomposite films via the sol-gel method. These films display natural and durable superhydrophilic properties. The study examined the effect of sol composition on the morphology and thickness of the composite films. It showed that although these structural features influence wettability, the inherent TiO₂-SiO₂ composition is the primary factor.

Additionally, the composite membrane exhibits a synergistic effect. The heterojunction interface (band gap 2.9 eV) facilitates electron-hole separation, decreasing the recombination rate by 50% and doubling the degradation rate of organic pollutants. This highlights the dual role of structural design and compositional optimization in improving the photocatalytic performance.

TiO₂/SiO₂ nanocomposite membranes prepared by the sol-gel method form 1-5 nm mesopores, with uniform TiO₂ particles (5∼10 nm in size). They increase CO₂/N₂ selectivity to 40-50 (compared to 20 for pure SiO₂ membranes) while maintaining a CO₂ flux of 1000 GPU. Adjusting the sol pH to 5 optimizes TiO₂ dispersion, preventing flux decline from pore blockage [30].

SiO₂ antireflection films were fabricated on glass surfaces via the alkali-catalyzed sol-gel method. This study investigated the influence of the ethyl silicate-to-ethanol molar ratio and coating rate on the optical properties of SiO₂ films, while also analyzing film durability. Films prepared at a molar ratio of 1:20 with a lifting rate of 500 μm/s exhibited a refractive index of 1.35 and a maximum transmittance exceeding 6.57%. In comparison, those prepared at a molar ratio of 1:30 with a lifting rate of 1000 μm/s had a refractive index of 1.33 and a maximum transmittance exceeding 6.94%. After deposition, the SiO₂ films showed a water contact angle of 5°, indicating superhydrophilicity [31].

A modified PP/PE/PP separator with an ultrathin zirconia coating was successfully prepared via dopamine (DA) hydrophilic modification followed by a surface sol-gel process. Compared with the original separator, the modified separator exhibits good mechanical properties, excellent electrolyte wettability, enhanced thermal resistance, improved ionic conductivity, and reduced impact sensitivity. Most importantly, at a high temperature of 55°C, the NCA//Li coin battery incorporating the S-10 ZrO₂ separator achieves a specific capacity of 203.7 mAh·g⁻¹, which is higher than that of the NCA//Li coin battery using the pristine PP/PE/PP separator [32].

The pH value of the solution influences the gel time. At a certain pH, the hydrolysis rate is fast, while the polycondensation rate is limited. As a result, some colloids formed during polycondensation may aggregate and deposit before cross-linking into a network, thereby reducing the stability of the sol.

Weixin Huang synthesized porous TiO₂ films via the sol-gel method using tetrabutyl titanate. The pore structure development depends on the acid-to-base ratio, sol-to-gel transition time, and solvent. Phase separation yielded samples with excellent superhydrophilic and anti-fog properties. These arise from pH-modulated sol particle aggregation, which forms nano-scale pores and hydrophilic protrusions, enhancing water adsorption and surface wetting. Superhydrophilic TiO₂ membranes are used in photocatalytic wastewater treatment: their hydrophilic surfaces accelerate pollutant adsorption and degradation, with a total organic carbon (TOC) removal rate 30% higher than traditional membranes.

Moreover, TiO₂ porous membranes prepared via pH-driven phase separation (band gap 3.0 eV) show 30% higher visible light absorption efficiency and a total organic carbon (TOC) removal rate increased from 60% to 80% compared to pure TiO₂ (3.2 eV). This confirms that structural porosity enhances both wettability and photocatalytic performance. The green synthesis of ZnO-TiO₂ solid solutions optimizes visible-light response and carrier separation simultaneously by regulating the bandgap through metal ion ratio adjustment [33].

The sol-gel method for preparing Al₂O₃, SiO₂, and Al₂O₃-SiO₂ composite ceramic membranes is mature, with such membranes widely used in separation, reaction, and catalysis. However, the method currently relies mostly on inorganic aluminum salts, pure aluminum, and other organic/inorganic salts as raw materials, leading to high costs and limiting large-scale industrial applications. To address this, non-metallic minerals and their derivatives (e.g., fly ash, kaolin) have been used as raw materials, providing Si and Al sources. This approach reduces preparation costs and enables resource utilization and high-value conversion of fly ash and kaolin.

2.3. Homogeneous deposition method

Homogeneous precipitation is an effective method. It can separate unstable components from mixtures. It can determine the substances in them accurately. It is efficient in separating unstable components.

Xuelong Zhuang studied the preparation of a new nanofiltration (NF) membrane, which involved the direct co-deposition of a titanium dioxide and graphene oxide composite layer on a porous α-Al₂O₃ hollow fiber support. The results showed that the water flux, lignin rejection rate, and sodium ion rejection rate were 5.6 L/m²·h·bar, approximately 92.1%, and approximately 5.5%, respectively [34].

Nengwen Gao et al. proposed a method for constructing protein-resistant ceramic membranes through the rapid co-deposition of DA and diglycolamine (DGA). This technique utilizes the self-polymerization property of DA and the synergistic effect of amino functional groups in DGA to form a PDA/DGA composite coating on the ceramic membrane surface, which integrates hydrophilicity and anti-adsorption capabilities as shown in Figure 8. Experimental results indicate that, compared to unmodified membranes, the PDA/DGA coating reduces the adsorption of bovine serum albumin (BSA) by over 60% and decreases the membrane flux decay rate by 40%, as shown in Figure 9 [35].

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Figure 8.

Schematic representation of the co-deposition of DA/DGA for anti-fouling ceramic membrane surfaces.

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Figure 9.

(a) Dynamic water contact angle of the membranes in air. (b) Zeta potential of the membrane surfaces before and after modification at pH = 6.0. (c) Oil contact angle of the ZrO₂ membrane under water (“CA” means contact angle, “SA” means slide angle). (d) Oil contact angle of the PDA/DGA modified ZrO₂ membrane under water (The mass ratio of DA/DGA is 1:4, The deposition time is 70 min, CuSO₄/H₂O₂ were used as a trigger).

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Clifford, A., first established a biomimetic method for the chemical modification of poly-l-lysine (PLL) to enhance its adhesion. This method is based on the reaction between the amino group of the PLL monomer and the aldehyde group of 3,4-dihydroxybenzaldehyde (DHBA) molecules. The results demonstrate that adhesive PLL-DHBA films can be prepared via cathode electrophoresis deposition (EPD) [36].

Tan Guoqiang prepared a precursor solution using (NH₄)₂TiF₆ (AHFT), SrNO₃ (SN), and boric acid (BA) as raw materials, with a molar ratio of AHFT:SN:BA::1:1:3. Strontium titanate films were deposited on functional (100) silicon substrates via liquid-phase deposition of self-assembled monolayers (SAMs). The results indicate that the film prepared on the monocrystalline silicon substrate is a strontium titanate film with good crystallinity, exhibiting clear diffraction peaks on the (110), (100), (200), and (211) crystal planes. Additionally, the strontium titanate particles on the surface have distinct outlines, presenting as regular long columnar crystals [37].

The homogeneous precipitation method involves generating a precipitant slowly and uniformly within a solution via chemical reactions. As a result, precipitation occurs slowly and uniformly throughout the entire solution. The resulting precipitate is crystalline, characterized by larger particles, fewer surface-adsorbed impurities, and ease of filtration and washing.

3.1. Applications in gas separation

Gas separation technology based on inorganic membranes has attracted significant attention owing to its high thermal stability and chemical resistance. As a representative type, ceramic membranes have been widely applied in various gas separation processes. For instance, silicon carbide (SiC) ceramic membranes exhibit excellent mechanical strength and corrosion resistance, thus showing great potential in high-temperature gas separation systems. Meanwhile, aluminum oxide (Al₂O₃) ceramic membranes prepared via impregnation methods demonstrate precise pore size control, enabling efficient separation of gas mixtures such as CO₂/N₂ [38].

Composite membranes combine the advantages of inorganic and organic materials to optimize separation performance. Liu Jianchao developed organic-inorganic composite membranes for CO₂/N₂ separation, achieving high selectivity and permeability by integrating inorganic fillers into polymer matrices [39]. Similarly, Jin Hengguo prepared SiOC ceramic membranes supported on silicon nitride (Si₃N₄) ceramics, which showed superior gas separation efficiency under high-temperature conditions. Carbon-based membranes, such as tubular composite carbon membranes, also possess unique advantages in gas separation. They are particularly effective for small molecular gases like H₂ and CH₄, owing to their uniform pore structure and chemical stability [40].

In the field of high-temperature gas separation, ceramic hollow fiber membranes have been applied in solid oxide fuel cells (SOFCs) and flue gas treatment. Rui Zebao investigated the use of ceramic membranes and adsorbents for high-temperature gas separation and CO₂ capture, demonstrating their feasibility in industrial flue gas purification. Additionally, mixed matrix membranes (MMMs), such as polysulfone-based composites, integrate inorganic nanoparticles to enhance gas separation performance, balancing selectivity and flux [41].

Despite the progress, several challenges hinder the widespread application of gas separation membranes:

Mechanical and structural limitations: Ceramic membranes often exhibit brittleness, restricting their durability in complex industrial environments. The preparation of defect-free ultra-thin membranes remains technically challenging, while thermal expansion mismatch in composite membranes may induce interfacial defects.

Cost and preparation complexity: High-temperature sintering processes for ceramic membranes elevate production costs. The synthesis of carbon-based membranes demands strict control over pyrolysis conditions, impeding large-scale production. Additionally, the complex fabrication steps of composite membranes (e.g., interfacial polymerization) constrain their commercialization.

Performance optimization: Current membranes face trade-offs between permeability and selectivity. For example, CO₂/N₂ separation membranes often require a balance between high CO₂ permeability and N₂ rejection, while high-temperature membranes may experience performance degradation due to thermal aging.

Industrial applicability: Scaling up membrane modules for industrial gas separation (e.g., flue gas treatment) remains challenging. Issues such as flow distribution and module sealing require addressing.

Material innovation and nano-engineering involve designing novel ceramic materials (e.g., SiC with nano-porous structures) and optimizing composite membrane interfaces to enhance mechanical properties and separation efficiency. Nano-pore regulation technologies may enable precise control over membrane channels for targeted gas separation [42].

Multifunctional integration involves developing membrane reactors that combine separation and catalysis, which could simplify industrial processes. For example, this includes integrating CO₂ capture with chemical reactions in high-temperature systems. Sustainable and low-cost preparation involves exploring green synthesis methods (e.g., sol-gel processes for Al₂O₃ membranes) and low-temperature sintering technologies for ceramics to reduce energy consumption. Polymer-derived ceramic membranes may achieve a balance between cost and performance [43].

Intelligent system design involves combining membrane separation with computational simulation (e.g., molecular dynamics modeling) to optimize module design and process parameters. This approach facilitates the industrial application of gas separation membranes in fields such as energy conservation and environmental protection [44].

By addressing these challenges and leveraging material and technological innovations, gas separation membranes are expected to play a more critical role in sustainable energy and environmental management.

3.2. Applications in pervaporation

Pervaporation is a powerful separation technique, especially suitable for the separation of azeotropic and close-boiling mixtures. It is based on the difference in the solubility and diffusivity of components in a membrane. In the dehydration of organic solvents, pervaporation membranes have shown excellent performance. For example, ethanol-water mixtures form an azeotrope at a certain composition, making it difficult to separate them by traditional distillation methods.

However, pervaporation membranes with preferential water permeability can selectively transport water molecules through the membrane while retaining ethanol. Polyvinyl alcohol (PVA) membranes are commonly used for this purpose. PVA has a high affinity for water due to its hydrophilic nature. By modifying the PVA membrane, such as cross-linking or incorporating inorganic nanoparticles, its separation performance can be further enhanced. This allows for the efficient production of high-purity ethanol, which is widely used in the pharmaceutical, chemical, and fuel industries. Critically, pervaporation reduces energy consumption by 30-50% versus traditional distillation, eliminating high-temperature phase changes. Traditional distillation’s phase transitions consume substantial energy, while membrane separation avoids phase changes, achieving 30-50% energy savings [45].

NF membranes have pore sizes between those of ultrafiltration (UF) and reverse-osmosis membranes, typically in the range of 1-10 nm. They can retain solutes with a molecular weight in the range of 200-1000 Da and have different rejection rates for different ions. In the pharmaceutical industry, NF is widely used for the separation and purification of pharmaceutical intermediates. For example, in the production of antibiotics, fermentation broths contain various components such as salts, macromolecular impurities, and the target antibiotic [46]. NF membranes can selectively retain the antibiotic molecules while allowing the passage of smaller salts and water. This process can significantly improve the purity and quality of the antibiotic product, reducing the need for further purification steps. Additionally, NF can be used for the concentration of pharmaceutical solutions, saving energy and resources compared to traditional evaporation methods. In surfactant-containing systems, hydrophobic membranes may experience dewetting, leading to a flux drop of more than 50% [47].

3.3. Applications in wastewater treatment

UF and microfiltration (MF) membranes are primarily used to remove suspended solids, colloids, and macromolecular organic matter from wastewater. In textile wastewater treatment, UF membranes can effectively retain dye particles, fiber impurities, and other large-sized contaminants. Textile wastewater often contains high concentrations of dyes, which are recalcitrant to degradation and can cause significant environmental pollution. Traditional chemical precipitation incurs a regeneration cost of $50 per ton, while activated carbon undergoes 50% capacity decay within 3 months [48].

UF membranes with appropriate pore sizes can reject these dye molecules, reducing the color and turbidity of the wastewater. The permeate from the UF process can then be further treated or reused in textile production. MF membranes, by contrast, are typically used as a pre-treatment step before UF or other advanced treatment processes. They can remove larger particles such as sand, silt, and some biological debris, protecting downstream treatment equipment from clogging and damage [49].

In municipal wastewater treatment, MF membranes can be applied in biological treatment units. For instance, in membrane bioreactors (MBRs), MF membranes are integrated with activated sludge to achieve both biological degradation of organic matter and solid-liquid separation.

The MF membrane retains activated sludge within the reactor, enabling a higher sludge concentration and more efficient wastewater treatment. This leads to a better-quality effluent with lower levels of suspended solids and pollutants [50].

Reverse osmosis (RO) is a high-pressure membrane separation process that can effectively remove dissolved salts, heavy metal ions, and small-molecule organic matter from wastewater. In electroplating wastewater treatment, RO technology is widely used for valuable metal recovery and water reuse. ZrO₂-coated ceramic membranes, when integrated with RO systems, exhibit over 99% heavy metal rejection, significantly outperforming traditional chemical precipitation methods, which achieve only 95% rejection. This integration also enables energy savings of 3–5 kilowatt-hours per ton of treated wastewater [51].

Electroplating wastewater contains high concentrations of heavy metal ions such as copper, nickel, and chromium.RO membranes can almost completely reject these heavy metal ions, producing high-quality permeate that can be reused in the electroplating process. The concentrated stream yields 10-20 g/L metal concentrates, a unique value absent in conventional sludge-based methods, which only produce low-value metal hydroxides. The concentrated stream containing heavy metal ions can be further treated for metal recovery, reducing the environmental impact of the electroplating industry and conserving water resources. This membrane-based resource recovery reduces heavy metal discharge by 80% and conserves 50% of process water [52].

In addition, RO can be used for desalinating brackish water or seawater in water-scarce areas. By applying high pressure to the feed water, the RO membrane enables water molecules to pass through while retaining salts and other contaminants, thus providing a reliable fresh water source for domestic, agricultural, and industrial applications. PVA-based hydrophilic membranes are prone to swelling, which results in a 10%-15% reduction in mechanical strength [53].

4.1. Membrane material performance

In the pursuit of sustainable green chemistry, membrane materials face a complex challenge: harmonizing selectivity, flux, and stability. This balance is critical, as distinct membrane characteristics are demanded by varied chemical processes.

Selectivity is crucial for high-precision separations. For instance, in high-purity pharmaceutical production, membranes must selectively separate target molecules from complex impurity mixtures. However, many membranes with excellent selectivity often have low flux. A recent study showed that when ion-exchange membranes were used to separate rare earth elements, high selectivity for specific ions was accompanied by relatively low flux, limiting overall processing capacity [54]. This trade-off highlights a critical challenge: attempts to enhance flux often compromise selectivity and stability.

On the other hand, efforts to enhance flux may compromise selectivity and stability. When polymer membranes are modified to augment porosity (and consequently flux), their capacity to discriminate between distinct molecules may be attenuated. For example, in fabricating UF membranes for protein separation, enlarging pore size to boost flux also induced a decline in the rejection rate of small proteins, thereby exhibiting reduced selectivity. Furthermore, modified membranes were more prone to mechanical damage and chemical degradation, which impairs their long-term stability [55].

Membrane fouling remains a persistent and critical challenge. In green chemical processes such as biorefining and water purification, feed streams contain diverse contaminants. Organic matter, colloids, and microorganisms can adsorb onto membrane surfaces or clog their pores, thereby impairing membrane performance. For instance, in membrane-based water treatment facilities, fouling induced by natural organic matter leads to a drastic decline in permeate flux and increased energy consumption for membrane cleaning [56].

Additionally, the cost-effectiveness of membrane materials is crucial for practical applications. High-performance membranes often have high production costs, limiting their large-scale adoption. A study analyzed the cost-performance of different membrane materials in industrial wastewater treatment. The results showed that while some advanced membranes had excellent separation performance, their high production costs made them less suitable for cost-sensitive industries [57].

The development of novel membrane materials and modification techniques remains an active research domain. Scientists are perpetually exploring new materials and methodologies to enhance the overall performance of membranes. For instance, nanocomposite materials are being employed in membrane fabrication. Incorporating nanoparticles has been demonstrated to augment both the mechanical strength and separation performance of membranes. This showcases their potential to surmount existing limitations [58].

The long-term stability of membranes under diverse operating conditions remains a key concern. Membranes may degrade over time, driven by chemical reactions, mechanical stress, or exposure to harsh environments. A study examined the stability of polymeric membranes in acidic and alkaline solutions. Its findings revealed that the chemical structure of membrane materials profoundly influences their degradation resistance. This underscores the significance of material design for long-term application [59].

Furthermore, the environmental impact of membrane materials and their post-use disposal has emerged as a critical issue. Against the backdrop of growing emphasis on sustainable development, there is an urgent need to develop environmentally benign membranes that enable facile recycling or disposal.

The integration of membrane technology with other separation and reaction processes has the potential to significantly enhance overall system performance. A study investigated the coupling of membrane separation with catalytic reactions within a single-unit operation, demonstrating that this approach can improve process efficiency and reduce total energy consumption [60].

4.2. Large-scale preparation technology

Scaling up membrane preparation from the laboratory to industrial levels presents numerous obstacles. Ensuring membrane quality uniformity is a major challenge. In large-scale production, variations in raw material properties, processing conditions, and equipment performance can induce inconsistencies in membrane quality.​A study revealed that during the large-scale production of ceramic membranes via tape-casting, minor discrepancies in raw material particle size distribution and drying rate resulted in membranes with non-uniform pore structures and mechanical properties.​

Carbon nanotubes (CNTs) tend to aggregate in membrane matrices at loadings exceeding 1%, forming non-selective pores that reduce H₂/CH₄ selectivity by 10-15%.​As noted in the literature, “high costs and dispersion issues limit the large-scale application of CNTs in mixed-matrix membranes.”​

The “continuous electrospinning + surface hydroxylation” approach improves CNT dispersion. For instance, continuous electrospinning of CNT/polymer composite membranes mitigates aggregation and enhances H₂ flux by 150% (from 200 to 500 GPU).​ Aggregation of CNTs and MOFs causes selectivity loss. For example, a 1.5% CNT loading reduces H₂/CH₄ selectivity by 20% compared to a 0.5% loading [61].

Cost reduction remains a key challenge. High raw material costs, energy-intensive production, and complex manufacturing processes collectively drive up membrane production expenses. For instance, advanced nanocomposite membranes necessitate expensive nanoparticles and sophisticated synthesis methods.​A study demonstrated that the high cost of CNTs restricts their large-scale application in mixed-matrix gas separation.​

Nanomaterials such as CNTs (priced at $100/kg) and MOFs, coupled with high-temperature sintering in sol-gel processes, further limit practical applications due to cost constraints. In contrast, although cryogenic distillation requires substantial equipment investment (e.g., $500,000 for a small-scale CO₂ separation unit), the initial cost of membrane technology can be reduced through scaled-up production [62].

Improving production efficiency is also essential. Current membrane preparation technologies often involve time-consuming steps such as long-term drying and high-temperature sintering. For inorganic membranes fabricated via the sol-gel method, the slow production rate has emerged as a major bottleneck in meeting the escalating market demand.

The control of membrane morphology at a large scale is also a complex issue. Precise regulation of pore size, porosity, and surface roughness is essential for optimizing membrane performance. However, this becomes significantly more challenging in large-scale production. In the large-scale preparation of polymeric membranes via phase inversion, fluctuations in process temperature and solution composition result in inconsistent membrane morphologies. This impairs their separation performance [63].

In addition, the scale-up of membrane preparation technology also needs to consider the compatibility of different production processes. For example, when combining new membrane materials with existing production equipment, there may be problems such as poor adhesion or incompatibility. Research explored the challenges of integrating novel nanofiber-based membrane materials into traditional spinning-based production lines. It found that discrepancies in material properties and processing requirements hindered large-scale production [64].

The environmental impact of large-scale membrane production has emerged as a growing concern. The production process may consume substantial amounts of energy and water, while generating waste. A review examined the environmental dimensions of membrane production, including energy consumption, waste generation, and recycling potential. It also proposed strategies for more sustainable production methods.

The quality control system for large-scale membrane production needs to be further improved. Reliable and efficient quality testing methods are required to ensure that the produced membranes meet the required standards. A study evaluated different quality control methods for large-scale membrane production and pointed out that the current methods still have limitations in terms of accuracy and speed. It highlighted that current methods still have limitations in terms of accuracy and speed [65].

Finally, the development of continuous membrane production technologies is critical for large-scale manufacturing. Discontinuous production processes typically exhibit lower efficiency and higher costs. A study focused on advancing continuous membrane production methods. For example, it explored continuous electrospinning for nanofiber membranes, which demonstrated potential to enhance production efficiency and reduce costs [66].

4.3. Integration of membrane processes with green chemical processes

Integrating membrane processes with green chemical processes constitutes a complex task. In membrane reactors, selecting appropriate membrane materials and operating conditions to achieve optimal synergy between reaction and separation presents a significant challenge. Different chemical reactions have distinct requirements for temperature, pressure, and reactant concentrations. For instance, in high-temperature catalytic reactions for hydrogen production, the membrane must withstand high temperatures while selectively separating hydrogen.

A study demonstrated that developing membranes with high thermal stability and hydrogen selectivity for such reactions is challenging, primarily due to the limited availability of suitable materials [67].

In addition, membranes must be compatible with reactants and products in chemical reactions. Some reactants or products may exhibit corrosive or reactive properties toward membrane materials. In biofuel production from biomass, the presence of acids and alcohols can induce swelling or chemical attack on polymer membranes. A study found that in membrane-based biofuel production systems, polymer membrane degradation caused by the corrosive nature of reaction mixtures reduced separation efficiency and membrane lifespan [68].

In wastewater treatment, integrating membrane technology with other treatment processes to achieve efficient water resource recovery and complete pollutant removal remains equally challenging. Membrane technology is sensitive to influent quality and typically requires pre-treatment steps. Coordinating these pre-treatment processes with membrane treatment in a cost-effective and efficient manner is non-trivial. A study reported that in MBRs for wastewater treatment, inadequate pre-treatment caused rapid membrane fouling and elevated operating costs [69].

Moreover, the economic viability of integrating membrane and green chemical processes is a pivotal consideration. The high cost of membrane materials and energy-intensive characteristics of some membrane processes can render the integrated process economically unviable.

A study evaluated the cost-benefit dynamics of integrating membrane separation with green chemical processes for chemical synthesis applications. Its findings demonstrated that the high investment and operating costs associated with membranes constituted significant impediments to large-scale adoption [70].

The long-term stability of the integrated systems also needs to be considered. Membrane fouling, degradation, and changes in the performance of the green chemical process over time can affect the overall efficiency. A research investigated the long-term stability of an integrated membrane-bioreactor system for wastewater treatment. They found that continuous operation led to a gradual decline in membrane performance and a corresponding increase in treatment inefficiencies [71].

The scale-up of integrated membrane-green chemical processes from laboratory to industrial scale constitutes another challenge. Scaling up necessitates careful consideration of factors such as mass transfer, heat transfer, and equipment design.​A study investigated the scale-up issues of a membrane-based green chemical reaction system. It reported that difficulties in maintaining consistent reaction and separation performance during scale-up represented significant obstacles [72].

In addition, regulatory requirements and environmental impact assessments play pivotal roles in the implementation of integrated processes.​Balancing strict environmental regulations with process effectiveness constitutes a complex task. A review examined the regulatory challenges and environmental implications of integrating membrane and green chemical processes. It emphasized the necessity of comprehensive environmental impact assessments prior to large-scale implementation [73].

Finally, the lack of a unified design and optimization framework for integrated membrane-green chemical processes makes it difficult to achieve the best performance. Different research groups often use different methods and criteria for design and optimization. A study proposed a new approach to unify the design and optimization of such integrated processes, aiming to improve the overall efficiency and sustainability [74].

These three challenges form a critical closed-loop constraint within the “material-preparation-process” framework: enhanced material performance drives upgrades in preparation technologies; the greening of preparation technologies necessitates innovations in process integration; and scaling pressures at the process stage, in turn, stimulate the design of low-cost materials. Future research should focus on developing cross-scale models, such as the coupling of molecular simulation and equipment simulation, and prioritize breakthroughs in key areas: continuous production via bio-template methods with a target capacity of ≥500 m²/day, and solvent-free membrane surface functionalization. Ultimately, these efforts aim to achieve the life-cycle economic efficiency of membrane technology in green chemistry.

5.1. Material innovation

The future of membranes in green chemistry is heavily dependent on material innovation. Novel membrane materials are expected to overcome current limitations in selectivity, flux, and stability.

Notably, developing 2D material-based membranes represents a promising approach. Graphene, its derivatives, and other 2D nanomaterials such as MXenes exhibit great potential. For instance, graphene oxide (GO) membranes can be engineered to have adjustable interlayer spaces, enabling highly selective molecular separation based on size and charge. A study demonstrated that GO membranes achieve high-selectivity CO₂/N₂ gas separation with relatively high flux. Additionally, these membranes can be functionalized to enhance chemical stability and antifouling ability.

To address interfacial delamination between GO and ceramic membranes, titanate coupling agents (e.g., KH-550) can modify hydroxyl groups on the Al₂O₃ surface, forming Si-O-Al covalent bonds to strengthen interfacial bonding. For example, Zhang et al. used DA self-polymerization to form a polydopamine (PDA) layer on SiC ceramic surfaces, which increased GO coating adhesion by 40% and prevented delamination during thermal cycling at 200°C [75].

TiO₂/2D material composites (e.g., GO) improve selectivity through synergies between interlayer hydroxyl groups and TiO₂ pore sizes. GO@TiO₂ core-shell structures enhance CO₂ separation via this synergistic effect.

MOFs have emerged as attractive membrane materials, characterized by highly ordered pore structures with tunable sizes and chemistries. They can be engineered for selective molecular adsorption and separation. For instance, Yang reported MOF-based membranes for the separation of chiral compounds in pharmaceuticals, which exhibit high enantioselectivity, a property crucial for green chiral drug synthesis.

To address the degradation of MOFs under extreme conditions, a three-layer structure (ceramic substrate-metal oxide interlayer-MOFs) is adopted. Yang et al. first deposited a 50 nm Al₂O₃ buffer layer on ZrO₂ ceramics, followed by growing a ZIF-8 coating; notably, the membrane retained 90% of its CO₂ permeation selectivity after 30 days of immersion in a pH=12 solution [76].

The interlayer inhibits ion exchange between MOFs and ceramics, thereby enhancing chemical stability. MOF-based nanocomposite membranes (e.g., ZIF-8) consume 0.5–1 kWh/m³ for CO₂ capture, whereas activated carbon adsorption requires 1-2 kWh/m³ (including regeneration energy).

Moreover, the continuous operation of membrane separation avoids energy loss associated with adsorbent regeneration. MOFs like ZIF-8 with ordered pores (0.3-2 nm) and metal sites (e.g., Cu²⁺) enable CO₂/N₂ selectivity of 80-100 (vs. 40 for traditional membranes) and a flux of 800 GPU. The document states that Yang et al.’s MOF-based membranes achieve high-selectivity separation of drug molecules via chiral pores, a mechanism analogous to gas sieving [76].

MOFs such as ZIF-8, featuring ordered pores (0.3-2 nm) and metal sites (e.g., Cu²⁺), achieve a CO₂/N₂ selectivity of 80-100 (compared to 40 for traditional membranes) and a flux of 800 GPU.

Biopolymer-based membranes represent an active research area. Biopolymers like cellulose, chitosan, and alginate are renewable, biodegradable, and biocompatible. They are used to prepare membranes for applications like water treatment and food processing. A study developed cellulose-based membranes with high porosity and mechanical strength for heavy metal ion removal from wastewater. These membranes offer an eco-friendly alternative and can be functionalized for specific separation tasks [77].

In addition, inorganic-organic hybrid membranes are attracting increasing attention. By integrating the advantages of inorganic and organic materials, these membranes exhibit enhanced performance. For example, Wang prepared hybrid membranes using silica nanoparticles and polymers. These membranes showed improved thermal stability and separation efficiency in gas separation applications [78].

Another emerging trend is the use of stimuli-responsive membranes. These membranes adjust their properties in response to external cues such as temperature, pH, or light. Liu developed a temperature-responsive membrane that tunes permeability and selectivity with temperature, showing potential for application in smart separation processes. Intelligent responsive materials include pH/temperature-responsive amphiphilic membranes and temperature-responsive TiO₂ membranes, which switch between hydrophilic and hydrophobic states under varying conditions [79].

5.2. Advanced preparation technologies

Advancements in preparation technologies will play a key role in unlocking the full potential of new membrane materials. 3D printing technology is expected to revolutionize membrane fabrication, as it enables precise control of membrane structures at the micro- and nano-scale. This precision allows for the creation of complex geometries that are difficult to achieve with traditional methods.

According to a study, 3D-printed membranes can be designed with customized pore sizes and distributions, which significantly enhances their separation performance. Additionally, this technology offers the advantage of rapid prototyping, reducing the time and cost required for membrane development [80].

3D-printed ceramic membranes with gradient pore structures (e.g., porosity from 40% in substrate to 60% in coating) reduce elastic modulus mismatch, lowering interfacial thermal stress by 35%. Low-temperature sintering (e.g., microwave-assisted) drops SiC ceramic sintering temp from 1600°C to 1200°C, preventing GO coating carbon oxidation at high temps. Controlling heating rate (≤5°C/min) further reduces thermal stress accumulation. 3D-printed superhydrophobic membranes with lotus-leaf effect combine micro-nano hierarchical pores and fluoride coatings, achieving contact angles >160° [81].

Hybrid methods that combine multiple techniques are gaining prominence. For instance, the combination of electrospinning and atomic layer deposition (ALD) enables the preparation of composite membranes with hierarchical structures. Electrospinning forms nanofiber scaffolds, while ALD deposits thin conformal coatings on the fibers, thereby enhancing selectivity and stability. A study applied this hybrid approach to gas separation membranes, achieving improved efficiency and long-term stability.

ALD deposits a nanoscale (<10 nm) TiO₂ transition layer on ceramic surfaces. By precisely controlling atomic-level bonding (e.g., Ti-O-C in TiO₂) at the Al₂O₃/GO interface, the coating uniformity error is kept below 5%. For example, Behroozi et al. applied ALD to prepare TiO₂/GO composite coatings on Al₂O₃ ceramic tubes. These coatings remained intact after 100 thermal cycles, a phenomenon attributed to the stress-relieving effect of the ALD layer [82].

Microfluidic technology has emerged as a key approach for membrane fabrication. It enables precise micro-scale control over fluid flow and mixing, facilitating the preparation of membranes with uniform structures. A study demonstrated that microfluidic devices could fabricate polymeric membranes with precisely defined pore sizes, thereby improving preparation reproducibility and reducing material waste. This technology also allows the fabrication of hydrophilic-hydrophobic gradient membranes, which enable the selective separation of molecules based on polarity [83].

The development of scalable membrane preparation techniques is also crucial for the widespread application of new membrane materials. Roll-to-roll manufacturing is a continuous production method with the potential to produce large-scale membranes efficiently. A study explored the roll-to-roll fabrication of thin-film composite membranes for water treatment, and the results showed that this method can produce membranes with consistent performance, making it suitable for large-scale industrial applications. Roll-to-roll technology reduces the cost of MOF composite membranes from $200/m² to $80/m², approaching the replacement cost of traditional adsorbents (annual replacement costs of activated carbon account for 30-40%) [84].

In addition, the application of self-assembly processes in membrane preparation is attracting growing attention. These processes enable the spontaneous formation of ordered structures at the molecular level, facilitating the creation of membranes with unique properties. A study demonstrated that block copolymers can self-assemble into membranes with highly ordered pore structures, providing a simple and efficient route to tailored membranes. Self-assembly methods create coupling layers at MOF/polymer interfaces, which reduce interfacial gaps and increase selectivity by 10-15% [85].

5.3. Expansion of application fields

In green chemistry, membranes are expanding their applications beyond traditional separation and purification processes. In carbon capture and utilization (CCU), they play a key role in separating CO₂ from industrial flue gases. Recent studies indicate that membranes with high CO₂ selectivity and permeability enable low-cost CO₂ capture. For instance, a facilitated-transport membrane designed for CO₂ capture exhibits high CO₂ flux and gas selectivity. The captured CO₂ can then be utilized in fuel and chemical synthesis, contributing to the reduction of greenhouse gas emissions [86].

In the field of renewable energy storage and conversion, membranes serve as essential components. In fuel cells, proton-exchange membranes (PEMs) are critical for the efficient transfer of protons. Future PEMs are expected to exhibit higher proton conductivity, better chemical stability, and lower cost. A study reported the development of a new type of PEM with improved performance for hydrogen fuel cells, which can enhance the overall efficiency and durability of fuel cell systems [87].

Membranes also hold potential applications in biotechnology. For instance, in the production of biofuels from biomass, membranes can be used to separate and purify biofuels and fermentation products. A study employed membrane filtration to separate ethanol from fermentation broth, achieving high-purity ethanol production with low energy consumption [88].

In addition, membranes are being explored for their potential in enzyme immobilization within biotechnology. Immobilizing enzymes on membranes can enhance their stability and reusability. A study investigated cellulase immobilization on a composite membrane to enhance enzymatic hydrolysis in biofuel production.

Results showed that the immobilized enzyme exhibited improved catalytic activity and stability, which could potentially reduce biofuel production costs [89].

Another emerging application of membranes is in the field of water splitting for hydrogen production. Membranes can be used to separate the hydrogen and oxygen generated during the water-splitting process. A study developed a novel membrane for water-splitting applications, which exhibited high selectivity and stability, thereby showing promise for efficient, green, and sustainable hydrogen production. In the context of high-temperature coal gas separation using Si₃N₄ ceramic-MOFs (UiO-66) composite membranes, molecular dynamics simulation plays a role in optimizing interfacial binding energy.

For example, Sun et al. conducted simulations revealing that introducing Zr-O-Si bonds in UiO-66 (bonded to Si₃N₄) increases the binding energy from 0.2 to 0.5 eV. The coating showed no delamination after 1000 h at 800°C, which provides guidance for maintaining interfacial stability under extreme conditions [90].