Synthesis routes of zeolitic imidazolate framework-8 for CO₂ capture: A review

1.   Introduction

CO₂ is widely recognized as a greenhouse gas and the principal cause of global warming, which has been amplified by the usage of fossil fuels ^[1]. A report suggests that the level of CO₂ has increased by about 100 ppm compared to the last six decades and is predicted to increase further because of the continuous usage of fossil fuels as the primary energy source ^[2,3]. The reduction in CO₂ emission and adsorption/capturing of CO₂ are challenging issues around the globe as the CO₂ pollution may increase the greenhouse effects, climate changes, snow cover melts, etc. ^[4,5]. The advancement of CO₂ capture and separation systems is ongoing. Recently, membrane separation, physical adsorption, and chemical absorption have been used to adsorb/separation of CO₂. Among the existing techniques, membrane separation (metal-organic framework) is one of the efficient techniques to adsorb/separate CO₂ due to its low energy consumption, easy industrialization, high economic benefits, and high efficiency ^[4,6]. The metal-organic framework (MOF) is porous crystalline compounds with metal ions linked by organic linkers ^[7]. MOFs are the fastest emerging fields in chemistry because of their structural and functional tunability. MOFs are used for CO₂ uptake/capture due to their high porous structure, thermal and chemical stability, and large surface area ^[8,9]. Among the MOFs, zeolitic imidazolate frameworks (ZIFs), a sub-family of MOFs, are popular due to their advanced features like high surface area, porous structure, and flexibility towards permeability. ZIFs are porous hybrid materials with zeolitic-like structures ^[10]. ZIFs are formed of a divalent metal cation (e.g., Zn²⁺ or CO²⁺) connected to nitrogen atoms in an anionic imidazolate molecule, resulting in tetrahedral frameworks ^[11]. They are composed of a Zn²⁺ metal cation that is linked with molecules of the organic linker 2-methylimidazole (2-mIm), resulting in huge cavities of 1.16 nm interconnected with windows of approximately 0.34 nm ^[12].

ZIF-8 has been widely used as a filter material for gas separation (CO₂, H₂, and CH₄) due to its higher chemical and thermal stability and flexibility ^[13]. ZIF-8 is a potential gas storage material. It was claimed that the adsorption and desorption process for ZIF-8 was reversible with no hysteresis, indicating a rapid adsorption and desorption process. ZIF-8's porous nature and wide surface area make it an excellent material for gas storage applications ^[14]. Despite ZIF-8 being well-known for its flexibility, the presence of H₂ (kinetic diameter of 0.29 nm) is expected to be preferred over that of CO₂ (0.33 nm) ^[15]. ZIF-8 has shown excellent chemical resistance, remaining unchanged after seven days of immersion in boiling methanol, benzene, and water, as well as 24 h of immersion in concentrated sodium hydroxide at 100 ℃ ^[16].

ZIF-8 is simple to synthesize and exhibits a molecular sieving action, resulting in exceptional CO₂ selectivity and permeability ^[17]. It has been reported that in the separation of CO₂/CH₄, ZIF-8 demonstrates the highest permeability (240 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) and selectivity (~7) among all the reported ZIFs (ZIF-7, ZIF-67, ZIF-90, and ZIF-9-67) ^[18]. ZIFs are based on tetrahedral topologies and are created by connecting 4-coordinated transition metals by imidazolate with an angle of 145°. The M-Im-M (M = Zn or CO and Im = imidazolate) angle is responsible for the synthesis of a large number of ZIFs with Zeolitic-type-tetrahedral topologies ^[19]. ZIF-8 is synthesized using different routes; however, the CO₂ uptake or capture by ZIF-8 depends on the chosen synthesis route ^[20]. Several factors come into play when approaching the synthesis process of ZIFs; some are geometry, maintenance, functionality, conformation, compatibility, solubility, pH, temperature, etc. ^[21,22]. The structure of ZIFs mainly depends on the category of imidazolate and solvents used. Moreover, the structural diversity in ZIFs is due to the use of functionalized imidazolate ligands in the synthesis process ^[23,24]. In view of the above, our objective of this review is to summarize the synthesis routes of ZIF-8 and their outcomes in terms of CO₂ adsorption. We also focus on factors affecting CO₂ capture/adsorption. The techniques used for CO₂ adsorption are also discussed briefly.

2.   Current technologies for CO₂ capture

The capture/storage of CO₂ is a process in which CO₂ is captured from different sources such as industry, fossil fuels, and wastes before its interaction with the atmosphere. The emitted CO₂ is compressed and injected deep enough for everlasting storage ^[25,26]. Three CO₂ capturing techniques were developed: (ⅰ) the post-combustion capture process, (ⅱ) the pre-combustion capture process, and (ⅲ) the oxy-combustion process ^[27].

In the post-combustion capture process, CO₂ is produced in a regular way from burnt fuel, and then organic solvents are used to remove CO₂ from the air ^[28]. In the pre-combustion technique, oxygen and steam are used to convert fuel into hydrogen and CO₂ and then capture CO₂ with the help of solvents or solid absorbents ^[29]. In this process, CO₂ is captured before fuel is burned. Further, in the oxy-firing process, the fuel is burned in pure oxygen and converted into water vapor and CO₂. After cooling the burning gas, the water vapor is condensed, and CO₂ is captured ^[30].

The most significant method of CO₂ removal is sorption, which includes independent adsorption and absorption ^[31]. In the literature, a number of CO₂ adsorbing/absorbing materials have been documented ^[32]. The adsorbent material used for absorption has (a) high selectivity; (b) high CO₂ adsorption capacity at high temperature; (c) good adsorption/desorption kinetics; (d) stable adsorption capacity after repeated cycles; and (e) adequate mechanical strength of adsorbent particles for cyclic exposure ^[33].

Furthermore, carbon-based adsorbents and metal oxide adsorbents such as CaO, MgO, hydrotalcite-like compounds (HTIcs), and zeolites, are employed for CO₂ capture ^[34]. It has been observed that adsorbents that change with chemical reagents have boosted the CO₂ sorption capacity; for example, chemical modification with K₂CO₃ has significantly increased CO₂ adsorption capacity for two hydrotalcite-like compounds at 400 ℃ ^[35]. Using these modified HTIcs, CO₂ adsorption values in the 0.9 mol/kg range are obtained. This improvement could be attributed partly to CO₂ absorption by the chemically changed material ^[36].

Dolomites, silicates, and synthetic absorbent materials have been employed to change the balance of hydrocarbon steam reforming toward H₂ creation via selective CO₂ removal from the system ^[37]. Some of the most common CO₂ removal techniques are cryogenic separation, dry adsorption, wet absorption, and membrane separation. However, the power inputs required are rather high, making the benefits in terms of total CO₂ emissions insignificant ^[38]. Materials/technologies were developed/adopted for CO₂ capture/separation; some are discussed with their advantages and disadvantages in Figure 1.

2.1.   CO₂ adsorption techniques used in industries

Adsorption of CO₂ from industrial sources is a critical focus area in efforts to mitigate greenhouse gas emissions. Extensive research has been undertaken over the last few decades for the development of strategies for CO₂ adsorption from various sectors ^[39]. Carbon dioxide adsorption techniques play a crucial role in various industries, particularly in addressing environmental concerns and complying with emission regulations. Several methods are employed for CO₂ adsorption, and some commonly used techniques in industries are discussed in Table 1 with their advantages and limitations.

The mentioned methods/technologies have been applied for CO₂ capture/separation in industries. Although, these methods are good enough for capturing CO₂, the development of new materials and methods is ongoing to overcome the limitations of the processes mentioned in Table 1. Among the discussed methods/materials, the solid sorbent method has been pointed as one of the efficient techniques to capture CO₂.

2.2.   Carbon-based absorbent for CO₂ capture

Carbon-based absorbents are materials that contain carbon and are designed to capture CO₂ from gas streams. These materials typically have high surface areas and can adsorb CO₂ through physical or chemical interactions. Here are some common carbon-based absorbents used for CO₂ capture ^[47]. Table 2 shows the advantages and limitations of carbon-based absorbent for CO₂ capture.

3.   Metal-organic frameworks (MOFs) for CO₂ capture

The choice of a MOF for CO₂ capture depends on factors such as pore size, chemical composition, and stability ^[53]. Some MOFs exhibit a high affinity for CO₂ due to their porous nature and the presence of specific functional groups within the framework. For instance, MOFs studied for CO₂ capture include ZIF-8, HKUST-1, UiO-66, and MIL-101. These MOFs have demonstrated promising results in terms of adsorption capacity and selectivity for CO₂ ^[54].

ZIF-8 stands out among MOFs due to its remarkable stability, tunable pore size, and large surface area. Its high stability ensures structural integrity across conditions, while the ability to adjust pore dimensions enables selective gas adsorption ^[55]. The simplicity of its synthesis process, coupled with cost-effectiveness, enhances its practicality for industrial applications. The materials used for its synthesis can be economically advantageous ^[56]. Overall, ZIF-8's unique combination of properties, including ease of synthesis, tunable structure, and adaptability to various applications, positions it as a promising MOF with distinct advantages over others in gas storage, separation, and catalysis. Table 3 shows the comparison of key MOFs for gas storage, separation, and catalysis.

Over the past two decades, research on ZIF-8 has undergone remarkable advancements, particularly in its application for CO₂ capture. The unique properties of ZIF-8, including high porosity, tuneable structure, and exceptional chemical stability, have made it an attractive material for gas separation and storage.

To provide a comprehensive view of the evolution of ZIF-8 research, a graphical timeline (Figure 2) is presented, summarizing major breakthroughs in the field. These milestones include the synthesis of ZIF-8, its molecular sieving properties, membrane integration, surface modifications, composite development, and direct air capture applications.

Figure 2 illustrates the significant progress in utilizing ZIF-8 for CO₂ separation, highlighting key achievements that have shaped the research landscape. The timeline underscores how continuous improvements in synthesis methods, functionalization, and composite integration have enhanced the adsorption efficiency, permeability, and selectivity of ZIF-8-based materials for CO₂ capture applications.

With increasing global concerns over rising CO₂ levels, direct air capture (DAC) has gained attention as a viable strategy for reducing atmospheric CO₂ concentrations. Unlike conventional capture methods that focus on industrial flue gases, DAC operates at ultra-dilute CO₂ levels (~400 ppm), making the development of highly selective and efficient adsorbents essential. MOFs, particularly ZIF-8 and its derivatives, have emerged as promising materials for DAC due to their high porosity, tuneable surface chemistry, and selective CO₂ adsorption properties ^[47].

Among MOFs, ZIF-8 exhibits exceptional thermal and chemical stability, making it an attractive choice for low-concentration CO₂ capture applications. However, its hydrophobic nature and pore aperture (~0.34 nm) present challenges in adsorbing CO₂ under ambient conditions. To enhance DAC performance, researchers have explored ligand modifications and functionalized ZIF-8 derivatives, improving CO₂ affinity through amine-functionalization and pore engineering ^[23,24]. Furthermore, ZIF-8-based composite sorbents and hybrid membranes have demonstrated enhanced selectivity and faster adsorption kinetics, making them more efficient for air purification and carbon capture applications.

Beyond ZIF-8, other MOFs such as HKUST-1, UiO-66, and MOF-74 have been studied for DAC applications, with MOF-74 showing particularly high CO₂ uptake at low pressures. Studies also indicate that MOF-based solid sorbents, when integrated with monolithic supports or structured adsorbents, can significantly reduce energy costs and enhance CO₂ removal efficiency in DAC systems. As research advances, the optimization of MOF-based materials continues to focus on improving CO₂ uptake capacity, stability, and regeneration efficiency, ensuring their practical viability in large-scale direct air capture applications ^{[19,20,21,22,23,24]}.

4.   Various routes to synthesize ZIF-8

ZIF-8 has been synthesized using various methods, some of which are briefly mentioned below.

4.1.   Solvothermal method

The solvothermal synthesis route has been used to synthesize high-quality single-crystal ZIF-8 ^[57]. The simplicity and low-temperature are very suitable for mass production ^[11]. The main disadvantage of this technique is the extensive ZIF-8 macro crystal preparation ^[58]. In this approach, ZIF-8 was prepared using two solvent systems: dimethylformamide (DMF) and methanol (MeOH). A total of 2 mmol of zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O, 98%), 2-methylimidazole (MeIM, 98%), and N, N-dimethylformamide were suspended in 50 mL of N, N-dimethylformamide (DMF, industrial grade) and rapidly agitated until a clear solution was formed ^[59]. Following this, the substrate mixture was transferred to a 100 mL Duran bottle with a Teflon taped screw cap, and the solution was heated in a convection oven at 140 ℃ for 24 h ^[60]. The product was filtered, washed with DMF several times, kept in MeOH for 3 days, and dried at room temperature (Figure 3). The same composition of the precursors was mixed in 80 mL of methanol (MeOH), which was left at room temperature for 24 h without stirring, to prepare ZIF-8 using methanol solvent. The product was washed several times with MeOH, centrifuged, and dried overnight in a vacuum oven at room temperature. The dried powder was collected for characterization.

4.2.   Microwave-assisted method

The microwave-assisted method is a solvent-free route to synthesize ZIF-8 particles. In this route, synthesis is completed in ~4 h, which is five times less than the conventional synthesis route ^[61]. The ZIF-8 obtained by this method has a large surface area and microprobe volume compared to those produced by a conventional method. The main disadvantage of this route is its processing temperature, due to which a large particle size ZIF-8 is produced ^[62]. The nucleation and crystallization have been enhanced at a higher temperature, which increases the particle size of the produced ZIF-8 ^[63]. The microwave-assisted method is similar to the solvothermal synthesis route except for the temperature and time for the autoclave. In this method, 17 mL of the substrate mixture obtained from the solvothermal method (DMF) was put into a 35 mL tube, sealed with a septum, and placed in a microwave oven (Discover S-class, CEM, Maximum power is 300 W). The resulting mixture was heated to 120–160 ℃ at 80 W, held for 3 h, and then cooled to room temperature ^[64]. The dark yellow powder was separated by centrifuging. The product was washed with DMF several times and dried at 100 ℃ (Figure 4).

4.3.   Sonochemical method

Sonochemical synthesis is based on applying high-energy ultrasound to a reaction mixture. Ultrasound in this synthesis is applied to speed up chemical reactions ^[65]. The ZIF-8 produced by the sonochemical route has a small particle size and narrow particle distribution. This method can be applied for mass production due to its short time processing compared to the traditional method ^[66]. Although, this method is advantageous for industrial applications, this technique has certain limitations: The reaction parameters can be more effectively controlled before being used in systematic studies of MOFs ^[67]. The synthesis of ZIF-8 by the sonochemical route can be done using Zn(NO₃)₂·6H₂O and triethylamine as precursors. Initially, 0.67 g (2 mmol) of Zn(NO₃)₂·6H₂O and 0.167 g (2 mmol) of MeIM were mixed in 50 mL of DMF and stirred until a clear solution was obtained. Then, 0.5 mL of triethylamine (TEA, (C₂H₅)₃N, 99%) was added. The obtained mixture was then transferred to a 70 mL horn-type custom-made reactor and connected to a sonicator bar (VCX500,500 W at 20 kHz) for 1 h. After the synthesis at 60% power, the sample was washed with DMF three times and placed in methanol, which was decanted and replenished four times for 48 hrs. After filtration, the sample was dried in a vacuum oven at 80 ℃ ^[55]. The powder was then collected for further characterization (Figure 5).

4.4.   Mechanochemical method

In this method (Figure 6), grinding or mechanochemical force can be used to initiate chemical reactions by mechanically breaking chemical bonds, which is then followed by some chemical change ^[68]. Pichon et al. ^[69] studied this route in 2006 in their solvent-free approach to MOF synthesis. In this route, metal, salt, and the ligand are ground together for a few minutes without using any heat, resulting in the formation of the MOF ^[70]. One of the most important reasons is the method's environmental benefit, which enables large-scale green production while avoiding using large quantities of unpleasant solvents ^[71]. This makes it safer than the traditional solvothermal route, which employs organic solvents and high temperatures. Furthermore, the absence of solvent molecules in the framework or pores of the MOF structures is advantageous ^[72]. The reaction times are also typically very short, ranging from 10 to 60 min, which is significantly less than the solvothermal synthesis procedures. Furthermore, the material is subjected to physical grinding conditions, forming small fragments from large particles. This process offers readily compact nano-MOFs ^[73]. While employing this strategy, it is crucial to ensure the reaction's completion, retain the original MOF's established pore metrics (size, shape, and distribution), and apply a mechanical force consistently throughout the reaction mixture ^[74]. This process has challenges with reproducibility because the uniformity of the acquired crystal's size and shape varies greatly depending on the strength and length of the grinding ^[75]. ZIF-8 was synthesized using zinc oxide and ammonium nitrate as a precursor. Initially, a 25 cm³ steel jar with five 10 mm-diameter steel ball bearings was filled with 0.064 g (0.79 mmol) of zinc oxide (ZnO, 99.9%), 0.128 g (1.56 mmol) of MeIM, and 0.020 g (0.25 mmol) of ammonium nitrate (NH₄NO₃, 98%, Sigma-Aldrich). The combination was milled for 45 min at 25 Hz in the Retsch MM200 mill. The product was washed three times with DMF and MeOH before being dried in a vacuum oven at room temperature ^[76]. The product was dried and collected for further characterization.

4.5.   Dry-gel conversion (DGC) method

In this route, materials were placed on a sieve or porous support on top of an autoclave container, with a small amount of solvent at the bottom ^[77]. The solvent can be recovered with little contamination and reused for subsequent reaction runs by separating the solvent and reactant mixture ^[78]. Solvent reuse is important because the industry prefers such environmental and economic improvements in product synthesis. Other advantages of DGC techniques include lower solvent amounts, higher yields, and a smaller reactor size ^[79]. Initially, 0.11 g (0.5 mmol) of zinc acetate dehydrate (Zn(OAc)₂·2H₂O, 99%) was mixed with 0.41 g (5 mmol) of MeIM. The mixture of substrates was placed on a holed Teflon-plate inside a 30 mL Teflon-lined stainless-steel autoclave. H₂O (2.0 mL) was added at the bottom of the autoclave, and the assembly was heated at 120 ℃ for 24 h. After the reaction, the autoclave was rapidly cooled in cold water, and the light yellow-colored product from the Teflon-plate inside the autoclave was separated by filtration and washed with H₂O three times ^[80]. Finally, it was dried in a vacuum at 150 ℃ for 6 h ^[81]. The sample was dried and collected for further characterization (Figure 7).

4.6.   Microfluidic synthesis method

In microfluidic synthesis, continuous-flow micro reactors have been used for the production of nanoparticles with narrow size distributions due to their precise reaction control ^[82]. In this method, the Zn(NO₃)₂ (669 mg, 2.25 mmol) and MeIM (12.930 g, 157.5 mmol) were separately prepared in 45 mL deionized (DI) water and then transferred using a syringe pump to a T-junction in the reactor setup ^[83], where the substrate solution droplets were formed in a continuous flow of fluorinated oil (Figure 8) ^[84]. The substrate solution droplets surrounded by oil entered a 1/16-inch tube and extended through an oil bath at 150 ℃. The flow rates for both solution and oil were adjusted to 0.5 mL h⁻¹. ZIF-8 crystals separated from the oil phase were washed with DMF three times and once with methanol ^[85]. The filtered product was dried before it was subjected to further characterization. The microfluidic synthesis route (similar chemicals and procedures) has also been used by researchers in their work ^[86,87,88].

4.7.   Hydrothermal method

The hydrothermal method is a low-cost and ecologically benign method of producing ZIF-8 crystals ^[89]. This method replicates synthesis by simulating chemical processes in an aqueous solution at temperatures above the boiling point of water ^[90]. Researchers believe that hydrothermal synthesis of coordination frameworks will lead to a shift toward commercial manufacturing at high production rates by lowering associated production costs and environmental consequences. In comparison to the solvothermal approach, this method yields smaller ZIF-8 particles ^[91]. Zinc nitrate hexahydrate and 2-methyl imidazolate are the key reagents employed in the synthesis. However, hexadecyltrimethylammonium chloride (CTAC), trimethylstearylammonium chloride (STAC), sodium dodecyl sulfate, cetyltrimethylammonium bromide (CTAB), and tetrapropylammoniumbromide (TPABr) are the surfactants used ^[92]. The stoichiometric amounts of zinc salt and 2-methylimidazole were first dissolved in separate beakers of DI water. In a typical synthesis, 0.29 g of zinc nitrate hexahydrate was first dissolved in 10 mL DI water (solution A), followed by 4.54 g of 2-methylimidazole in 70 mL DI water (solution B), and whenever possible, the surfactant was dissolved in solution B first. Solution A was swiftly poured into solution B, which was agitated at 300 rpm in a larger beaker, and these two solutions were allowed to react at a specific temperature for a certain period. Following the completion of the synthesis, the products were collected by repeated centrifugation (8000 rpm, 30 min), washed three times with DI water and methanol, and dried ^[93]. The schematic representation of hydrothermal method for ZIF-8 is shown in Figure 9.

4.8.   Rapid synthesis of ZIF-8

This is a non-solvothermal route to synthesize ZIF-8 at room temperature. It is a rapid synthesis route (Figure 10) with an easy variation of particle size ^[94]. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 98%) and 2-Methylimidazole (MeIM, 98%) were employed as precursors for the production of ZIF-8 in this approach. Initially, 1.17 g of Zn(NO₃)₂·6H₂O and 22.70 g of 2-methylimidazole were dissolved separately in 8 g of zinc nitrate hexahydrate and 80 ml DI water. These two solutions were stirred for 5 min at room temperature, and pH was monitored. The resulting nanoparticle was separated from milky dispersion by centrifugation at 7000 rpm for 5 min. The precipitate was washed with DI water three times. The resulting nanoparticles wewre dried in an oven overnight at 85–90 ℃ in a Teflon vessel using an autoclave. These steps were repeated to synthesize ZIF-8 of different particle sizes by varying stirring time from 5 to 60 min and at different pH values at room temperature. Extensive experimentation would be required by attempting different combinations to find the most suitable method, particle size, and structure of ZIF-8 ^[11,23,95].

This method has advantages over the mentioned methods because the particle size of synthesized ZIF-8 can easily vary. The particle size of ZIF-8 is an influential parameter in capturing/adsorbing the CO₂. A comparative analysis of advantages and limitations of all synthesis routes are shown in Table 4. Table 5 presents the quality assessment matrix for ZIF-8 based on synthesis parameters.

5.   Various routes of ZIF-8 synthesis and their outcomes

The ZIF-8 particles were synthesized using various routes and reported with their outcomes. Table 6 shows the summary of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of ZIF-8 prepared by different routes. The XRD pattern obtained from all the routes is similar, and the highest peak was obtained at 2θ = 7.20º and (011) plane. It has been reported that ZIF-8 has the ability to absorb CO₂, N₂ CH₄, etc., due to its highly porous structure and larger specific area ^[96]. It has been reported that water and methanol have been used as solvents in the solvothermal route. The XRD and SEM analyses of ZIF-8 prepared using both (water and methanol) solvents have been similar ^[97]. However, the structure of ZIF-8 is cubic and hydrangea-like, while using water and methanol as a solvent. ZIF-8s synthesized by the solvothermal route are highly porous but larger and lower the Brunauer, Emmett and Teller (BET) surface area. ZIF-8 produced by microwave-assisted and sonochemical synthesis routes have a smaller particle size compared to those produced by the solvothermal route. However, some of the described synthesis routes (hydrothermal, rapid synthesis, etc.) can produce nano-size ZIF-8s ^[98,99]. The particle size of ZIF-8 is the influential factor in the preparation of the cellulose filter used to capture CO₂ ^[100]. Therefore, the preparation of ZIF-8 for CO₂ adsorption/capture is method-specific. Among the described methods, ZIF-8 prepared by rapid synthesis and the hydrothermal route has a smaller particle size than those obtained from other routes ^[94]. It has been reported that CO₂ uptake by the filters/composites prepared by ZIF-8 obtained from hydrothermal routes is higher than that obtained from other routes due to a small particle size (nano-size), higher pore volume, and specific surface area ^[11,101]. The ZIF-8 prepared by mechanochemical routes has a rhombic dodecahedron-like structure but a larger particle size compared to those obtained through the hydrothermal and rapid synthesis route. The CO₂ uptake also depends on the structure of ZIF-8. It has been reported that the hexagonal structure of ZIF-8 demonstrates ~52%, 35%, and 65%, CO₂, N₂, and CH₄ uptake, respectively ^[102,103]. In another study, it was reported that the leaf-like structure of ZIF-8 demonstrates ~56% higher CO₂ adsorption than normal ZIF-8. The microwave-assisted route of synthesis has been reported as a solvent-free technique for ZIF-8 synthesis. The ZIF-8s produced using the microwave-assisted synthesis route are hexagonal; however, the BET surface area is lower compared to ZIF-8s obtained from the solvothermal route. The microwave-assisted technique has a complication and it is not cost effective for industrial purposes ^[75]. Overall, it is concluded that the ZIF-8 prepared by rapid synthesis and the hydrothermal route is more efficient (small particle size) than other routes. Enhancing CO₂ adsorption involves choosing the right synthesis route for ZIF-8 and optimizing its particle size. Furthermore, membrane composition and processing also contribute to the improvement of CO₂ adsorption ^[62].

While ZIF-8-based composite membranes and aerogels have emerged as promising materials for CO₂ capture, several challenges hinder their large-scale deployment. One of the primary issues is poor interfacial adhesion between ZIF-8 and polymer matrices in mixed matrix membranes (MMMs), often leading to non-uniform dispersion and defect formation, which adversely affects gas separation performance. Additionally, the fragile nature of ZIF-8 aerogels makes their scalable fabrication complex, as they are highly sensitive to synthesis conditions such as temperature, pressure, and solvent selection. Another critical limitation is the permeability-selectivity trade-off, where membranes with high permeability often suffer from lower selectivity for CO₂ separation, requiring further functionalization and hybridization to enhance efficiency. Moreover, real-world industrial CO₂ capture conditions expose these materials to moisture, impurities (SO₂, NO_x), and variable pressure/temperature environments, which may lead to degradation and loss of adsorption capacity over time. Addressing these challenges requires advancements in scalable synthesis techniques, improved polymer compatibility, and cost-effective production routes, all of which are actively being explored to enhance the practical feasibility of ZIF-8 composites and aerogels for industrial CO₂ capture applications.

Table 6 highlights ZIF-8 synthesis methods and their outcomes.

For ZIF-8 to be commercially viable in large-scale carbon capture applications, the development of scalable and cost-effective synthesis methods is critical. While traditional synthesis approaches such as solvothermal, hydrothermal, and mechanochemical routes have demonstrated high purity and tunability, their high solvent usage, long reaction times, and low batch yields hinder industrial scalability.

Several strategies have been explored to enable tonne-scale production of ZIF-8:

1. Solvent-free (mechanochemical) synthesis—This method eliminates organic solvents, significantly reducing production costs and environmental impact. It has been successfully demonstrated for gram-to-kilogram scale production, showing promise for further scale-up.

2. Continuous flow synthesis—Unlike batch methods, continuous flow synthesis offers higher yields, better reproducibility, and reduced reaction times, making it suitable for industrial-scale production.

3. Spray drying and ultrasonic spray pyrolysis (USP)—These approaches enable mass production of uniform ZIF-8 particles with controlled morphology and rapid processing.

4. Green synthesis approaches—The use of water-based synthesis and biomass-derived precursors is being investigated to reduce raw material costs and make ZIF-8 production more sustainable.

The cost of ZIF-8 synthesis remains a key barrier to its widespread deployment in carbon capture technologies. However, the ongoing development of scalable synthesis methods is expected to reduce production costs. Based on current progress, a potential commercialization pathway could involve:

● Short-term (1–3 years): Optimization of continuous flow and mechanochemical synthesis to achieve kilogram-scale production at lower cost.

● Mid-term (3–5 years): Integration of green synthesis approaches and expansion into industrial-scale batch production.

● Long-term (5+ years): Establishment of fully commercial production plants capable of tonne-scale ZIF-8 synthesis, with further reduction in production cost per gram to make it competitive with existing CO₂ adsorbents.

The development of scalable, cost-effective, and environmentally sustainable ZIF-8 synthesis methods remains a key focus area for researchers and industries looking to commercialize MOFs for CO₂ capture.

6.   CO₂ measurement conditions and techniques

The conditions (temperature, pressure, exhaust gas, and normal gas) of CO₂ play a significant role in CO₂ measuring. The determination of CO₂ levels is influenced by the methods and techniques employed in measurements ^[23]. Researchers have documented CO₂ measurements under elevated temperatures and pressure conditions ^[113,114]. Conversely, few researchers have conducted CO₂ measurements under ambient temperature and pressure conditions, including assessments for exhaust and flue gases ^[100]. Wu et al. ^[115] measured CO₂ using BELSORP-MAX, Micromeritics Instrument Corp. at 25 ℃ temperature and up to 1 bar pressure and found that CO₂ measuring capacity of carbon aerogels (20 wt.% ZIF-8 in carbon aerogels) increased by ~2.23 mmol/g. In another study, the CO₂ measurement was carried out using the Wicke-Kallenbach technique at 70–80 ℃ for 60 min. The maximum selectivity of CO₂ was ~20.29 at 70 ℃ measuring for 55 min ^[116]. Martin et al. ^[117] measured CO₂ at 2 bar pressure and 25–150 ℃ temperature using chromatography (PerkinElmer, Clarus 400) equipped with a TCD detector and found that the selectivity of H₂/CO₂ gas mixture (ZIF-8 membrane) was ~7.8 at 100 ℃ and 2 bar pressure. The CO₂ adsorption measured at ambient pressure and the room temperature was carried out using Pyris Diamond TG/DTA Perkin Elmer Analysis, and CO₂ uptake was ~0.4 mmol/g for the ZIF-8 membrane ^[118]. Similarly, Ding et al. ^[119] measured CO₂ uptake using the volumetric determination method at ambient temperature and pressure and found that CO₂ uptake of ZIF-L was ~1.56 mmol/g. In another report, the CO₂ uptake was ~0.94 mmol/g ^[110]. However, in another study, the measurement of CO₂ from flue gas was carried out using gas chromatography (GC) equipped with thermal conductivity detectors (TCDs) (Agilent Technologies, Palo Alto, CA). The permeance and selectivity of CO₂ (flue gases) on polymer/zeolite Y (ZY) (Pebax/PEG-DME500 25/75) composite membranes were ~661 GPU and 30 ^[120]. Table 7 summarizes the CO₂ measuring methods with the condition.

7.   Factors affecting the capture/adsorption of CO₂ by ZIF-8

7.1.   Synthesis method

The synthesis method used to produce ZIF-8 can have a significant impact on its properties, including its performance in CO₂ capture applications. The synthesis method affects the particle size, crystallinity, surface area, and other structural features of ZIF-8, which in turn influence its ability to adsorb CO₂ ^[129]. Here are some key considerations regarding the effect of synthesis methods on CO₂ capture by ZIF-8:

7.1.1.   Particle size

The particle size of synthesized ZIFs/ZIF-8 in CO₂ capturing plays an important role, as the particle size of ZIFs reduces the performance of the filter membrane increases ^[127]. It has been reported that incorporation of ZIF-8 (45 nm) as filler in polybenzimidazole (PBI) based MMMs increased the membrane performance ~ by 46% ^[128]. Chen et al. ^[108] reported that the incorporation of 20 wt.% of ZIF-8 (50 nm) in PBI-based MMMs increased the H₂ approximately six times, and the H₂/CO₂ selectivity increased nearly by 55% compared to the bare PBI polymer membrane ^[129]. The permeability and selectivity of CO₂/CH₄ depend on the dispersion of ZIF-8 crystal in MMMs. It was demonstrated that the incorporation of 5 wt.% ZIF-8 (45–450 nm) in the polymer of intrinsic microporosity (PIM) (to form MMMs) enhanced the permeability and selectivity of CO₂ by approximately 43% and ~6.4%, respectively, for ZIF-8 (120 nm) due to homogeneous dispersion ^[130]. Table 8 summarizes the effect of particle size of ZIF-8 on the permeability and selectivity of CO₂.

Overall, it has been concluded that the particle size of ZIF-8 plays a significant role in membrane performance. However, a few results suggested that dispersion of ZIF-8 particles in MMMs can alter the results, i.e., proper dispersion of ZIF-8 can improve the permeability as well as selectivity of CO₂.

7.1.2.   Structure and surface area

The ZIF-8 structure is highly effective for CO₂ capturing ^[132]. The strong chemical affinity of the zinc and imidazolate groups in the ZIF-8 structure for CO₂ also contributed to the high efficiency of CO₂ capture ^[133]. The unique structure of ZIF-8, which consists of zinc ions and organic linkers arranged in a tetrahedral arrangement (Figure 11) ^[134], enables a large surface area ^[135]. This enables a high affinity for CO₂ molecules, which can be easily adsorbed onto the surface of the MOF. One of the key benefits of using ZIF-8 for CO₂ capture is its high selectivity for CO₂ unlike other MOFs, which may also adsorb other gases such as nitrogen or methane; ZIF-8 has been shown to have a strong preference for CO₂ ^[136]. This means that it is more effective at separating CO₂ from other gases in the atmosphere, enabling more efficient capture and storage. Another advantage of ZIF-8 is its stability in harsh conditions. Unlike other MOFs, which can lose their porosity and surface area over time, ZIF-8 has been shown to maintain its structure even under high temperatures and pressures ^[137]. This means that it can be used in a wide range of applications, from capturing CO₂ from flue gases to purifying air in enclosed spaces; the unique structure of ZIF-8 has a significant impact on its ability to effectively capture CO₂. Its high porosity and selectivity for CO₂ make it a promising candidate for use in CO₂ capture and storage technologies.

Yuel et al. ^[136] reported that normal ZIFs (without modification) may have certain limitations, low solubility of metal salts, and formation of unwanted topologies. To overcome such a challenging issue, solvent-assisted ligands/linker exchange (SALE) has been used. This is a post-synthesis modification on the structure of ZIFs where the organic linker is exchanged with another suitable solvent medium. This structural modification increases the CO₂ adsorption capacity of ZIFs. The CO₂ uptake of NO₂ and SALE modified increases by approximately 32% and 100%. However, after 120 h of SALE modification, the CO₂ uptake is 4.23 mmol/g ^[136].

In another study, it has been demonstrated that the shape (cavity) of fabricated ZIFs can enhance the CO₂ uptake. Chen et al. ^[108] reported that two-dimensional ZIF with a cushion-shaped cavity (leaf-like crystal structure) demonstrates approximately 54% higher CO₂ adsorption than ZIF-8 nanocrystalline due to smaller pore size and high density of ZIF-L than ZIF-8. Table 9 summarizes the CO₂ uptake and specific surface area obtained from previous reports. In view of the above results, it can be concluded that the structural modification in ZIFs increases the CO₂ uptake.

7.1.3.   Effect of porosity

The porosity of ZIF-8 is a critical factor that can be controlled and optimized during the synthesis process and it significantly influences its performance in CO₂ capture. There are some effects of porosity as a synthesis parameter for ZIF-8 in the context of CO₂ capture ^[23]. Notably, zeolitic-type porous materials have gained attention for their advanced technological and industrial applications ^[139]. Among these, MOFs, a type of hybrid material, have been a subject of considerable effort, with their application spanning various industrial uses, including separation, catalysis, and sensing ^[140]. A specific example is the attention given to ZIFs in the context of CO₂ adsorption. The porosity of ZIFs plays a crucial role in this process, as illustrated in Figure 12 ^[141], where the impact of porosity on membrane support is depicted. Studies indicate that ZIFs with larger pore sizes exhibit favorable interactions in CO₂ capture ^[125]. Additionally, effective CO₂ adsorption requires adsorbents with significant volume and surface areas of pores. It is essential that these adsorbents exhibit fundamental properties such as high mechanical, thermal, and chemical stability, low heat capacity, high adsorption capacity, and cost-effectiveness ^[129].

7.1.4.   Cost and scalability

The choice of synthesis method can also impact the cost and scalability of ZIF-8 production. Some methods such as the stoichiometric molar ratios of zinc ions and 2-methylimidale precursor and rapid synthesis routes are more cost-effective and suitable for large-scale production ^[94,142].

The reproducibility of the synthesis method is crucial for the consistent production of ZIF-8 with desired properties. Moreover, variability in synthesis conditions may lead to variations in material performance ^[143].

Researchers explore various synthesis routes, including solvothermal, microwave-assisted, hydrothermal, and room-temperature methods, to tailor the properties of ZIF-8 for optimal CO₂ capture. The choice of synthesis method depends on the specific requirements of the application and the desired characteristics of the material ^[144]. Optimization of synthesis conditions is an ongoing area of research to improve the efficiency of ZIF-8 for CO₂ capture.

7.2.   Metal linker and ligands

The CO₂ capture performance by MOFs, particularly the ZIF-8 depends on the choice of metal linker and ligands. Here are some considerations for the effect of metal linkers and ligands on CO₂ capture in ZIF-8.

7.2.1.   Metal linker

The metal ion in the linker plays a crucial role. Different metal ions have different affinities for CO₂. Zinc is commonly used in ZIF-8, and its coordination properties influence the adsorption capacity and selectivity for CO₂ ^[145]. Other metal ions with different Lewis acidities may alter the interaction between the framework and CO₂ molecules.

In addition, the coordination environment around the metal ion can influence the structural stability of ZIF-8 and its ability to adsorb CO₂. It can also affect the energy of interaction between the metal ion and CO₂.

7.2.2.   Ligands

The organic ligands in ZIF-8 contain nitrogen-containing imidazolate groups. The presence of specific functional groups, such as amino or alkyl amino groups, in the ligands can enhance the interaction with CO₂ through hydrogen bonding or other interactions, thereby improving the capture efficiency ^[97]. The choice of ligands can influence the size of the pores in the MOF. Tuning the pore size is crucial for optimizing the adsorption capacity and selectivity for CO₂. Larger or smaller pores may be more suitable for different CO₂ capture applications ^[136]. It has been reported that functional ligands with specific properties, such as open metal sites or polar functional groups, can enhance the interaction with CO₂ and improve the overall performance of ZIF-8 for carbon capture ^[146].

The combination of specific metal ions and ligands can result in synergistic effects that enhance CO₂ capture properties. Optimizing the pairing of metal ions and ligands can lead to MOFs with improved performance.

7.3.   Impurities in a gas stream

The presence of gas impurities in the feed gas stream can have various effects on the performance of ZIF-8 for CO₂ capture. Impurities may interact with ZIF-8, affecting its adsorption capacity, selectivity, and overall efficiency ^[147]. Here are some considerations regarding the effects of gas impurities on CO₂ capture by ZIF-8: Flue gas comprises not only CO₂ and N₂ but also significant impurities like H₂O, O₂, and SO₂, negatively impacting traditional amine-based CO₂ scrubbing ^[148]. Research on CO₂ capture from flue gas using ZIFs has predominantly encompassed pure CO₂ adsorption or its capture from CO₂/N₂ mixtures ^[148,149].

7.3.1.   Interactions with functional groups

Gas impurities may interact with the functional groups present in ZIF-8's structure. These interactions can influence the adsorption behavior and stability of the material, potentially leading to changes in its performance ^[150].

7.3.2.   Humidity effects

Moisture in the gas stream can also be considered an impurity. ZIF-8 and other MOFs can be sensitive to water, potentially leading to structural changes or decreased adsorption capacity ^[151]. The adsorption capacity of CO₂ for activated carbon (AC) based ZIF-8 (AC@ZIF-8) decreased by ~16.27% while relative humidity increased by ~61% ^[152]. To address the challenges posed by gas impurities, researchers often conduct comprehensive studies to understand the interaction mechanisms and develop strategies to enhance the robustness of ZIF-8.

8.   CO₂ absorption by various ZIF filter/composite systems

The membrane's filter materials and the design of the filter play an important role in the capture of CO₂. Generally, nanoporous ZIF-8 can be used to prepare the polymer membrane, which enhances the performance of the membrane ^[153]. In addition, various substrates such as copper net, titanium support tubular α-alumina support, Zn-related nanofibers, and Nylon membrane were used with ZIFs/ZIF-8 for CO₂ capture. It has been reported that the ZIF-8 membrane was prepared on alumina hollow fiber alumina porous support, and the various membrane filters were prepared for CO₂ capturing using ZIF-8. The uptake of CO₂ by various filter/composite systems are summarized in Table 10.

In the past few decades, ZIFs have been given attention due to their high porosity and chemical and thermal stabilities. Various ZIFs are developed as membrane filter material as well as composites for CO₂ capture ^[113]. It has been reported that various polymeric membranes have been modified with ZIFs to prepare MMMs ^[114]. It has been demonstrated that the permeability of CO₂ and H₂ can be improved with the addition of ZIFs in membrane/composite ^[99]. Moreover, the incorporation of 15 wt.% ZIF-90 in 6FDA-DFM increases the permeability of CO₂ ~45% compared to pure 6FDA-DFM ^[157]. In another study, it was reported that pure NH₂-MIL-53(Al) demonstrates ~34% CO₂ selectivity while incorporating 25 wt.% PSF increases the CO₂ selectivity ~56% ^[158]. Table 11 summarizes various ZIF based-membranes/composites for CO₂ capture.

Overall, it can be concluded that incorporating ZIFs in MMMs/composites increases the selectivity/permeability of CO₂. Among the ZIFs, ZIF-8 has good CO₂ uptake to other prepared ZIFs.

ZIF-8-based materials have demonstrated promising CO₂ adsorption properties in laboratory environments; however, their performance in real-world flue gas conditions remains a critical factor for industrial deployment. Flue gas streams typically contain CO₂ (10%–15%), N₂, H₂O vapor, O₂, NO_x, SO₂, and other trace contaminants that influence adsorption selectivity, stability, and regeneration efficiency. ZIF-8 exhibits moderate CO₂ uptake (0.88–1.62 mmol/g at ambient conditions), making it suitable for low-to-moderate CO₂ concentrations in flue gas. Its hydrophobic nature provides relative stability in humid environments, but modifications such as amine-functionalization and hybrid composites enhance CO₂ selectivity and adsorption performance in competitive gas conditions. Additionally, when integrated into mixed-matrix membranes, ZIF-8 has been shown to improve CO₂/N₂ selectivity, reducing energy losses in gas separation processes.

Long-term stability is another key aspect of practical deployment. While pure ZIF-8 remains stable under moderate operating conditions, it may degrade under prolonged exposure to high temperatures (>550 ℃) and acidic gases such as SO₂ and NO_x. However, composite formulations incorporating polymers or hybrid MOFs have demonstrated enhanced thermal, chemical, and hydrolytic stability, enabling sustained adsorption-desorption cycles. Studies indicate that ZIF-8 can maintain stable CO₂ uptake over 50+ adsorption-desorption cycles, making it a promising material for pressure swing adsorption (PSA) and temperature swing adsorption (TSA) processes.

A major advantage of ZIF-8 is its low regeneration energy compared to traditional amine-based solvents, requiring significantly lower energy for desorption (~50 kJ/mol CO₂ versus ~140 kJ/mol CO₂ for amines). Its low heat of adsorption makes it suitable for vacuum and mild temperature swing adsorption processes, reducing the overall energy footprint of CO₂ capture operations. Recent advances in ZIF-8 hybrid materials have optimized CO₂ affinity while maintaining low regeneration costs, improving the economic feasibility of industrial-scale carbon capture. Despite these advantages, challenges remain, particularly in terms of competitive adsorption with H₂O and other flue gas components, which can reduce CO₂ uptake efficiency. Hydrophobic surface modifications and functional group tuning are active research areas to mitigate this issue. Furthermore, large-scale synthesis of ZIF-8 remains a challenge, requiring cost-effective production strategies for broader industrial adoption. Future research should consist of hybridizing ZIF-8 with other MOFs or adsorbents to enhance selectivity, stability, and long-term performance in flue gas conditions, ensuring its viability as an efficient and scalable CO₂ capture material.

9.   Advantages of ZIF-8 for CO₂ capture

ZIF-8 is a highly porous, thermally, and chemically stable material with a large surface area and pore volume, which is best suitable for the preparation of membrane/composite material for CO₂ capture. It has been reported that the permeability of H₂ on ZIF-8 grown hollow ceramic tube is ~ 67% ^[106]. In another study, it was reported that the permeabilities of H₂ and CO₂ on the ZIF-8/rGO (graphene oxide) membrane are ~ 68% and 65%, respectively ^[24]. In another study, it was reported that the CO₂ capture capacities of ZTF-1, MGMOF-74, ZIF-177, and ZIF-67 are ~5.4, 4.3, 6.2, 8.3, respectively ^[166,167]. It has been demonstrated that ZIF-8 can increase the selectivity and permeability of gas separation ^[168]. Therefore, ZIF-8 can be used as membrane material for the development of a filter to absorb the CO₂. In view of the above, the advantages of ZIF-8 over other developed materials (for CO₂ capture) are listed as follows ^[169]:

● ZIF-8 has a high affinity for CO₂, making it highly efficient at capturing and retaining the gas.

● Its porous structure enables large amounts of CO₂ to be captured within a small volume, making it a space-efficient option for CO₂ capture.

● ZIF-8 is stable and robust, making it suitable for use in various conditions and environments.

● The material can be easily regenerated for reuse, enabling a more sustainable and cost-effective approach to CO₂ capture.

● ZIF-8 is relatively low-cost compared to other materials used for CO₂ capture, making it an affordable option for large-scale applications.

● The material has a high surface area, enabling more efficient CO₂ absorption and increasing its overall effectiveness.

● ZIF-8 has a high selectivity for CO₂, meaning it effectively captures the gas while minimizing the capture of other gases or impurities.

● The material can be synthesized in a controlled manner, enabling the production of high-quality, consistent batches of ZIF-8 for use in CO₂ capture applications.

The practical processability of ZIFs remains a critical challenge in their large-scale application for CO₂ capture, as their powder form limits direct industrial use. One of the most promising approaches to improve processability is the integration of ZIFs into polymers, leading to the development of three-dimensional (3D) sorbents such as membranes and aerogels. ZIF-8-based MMMs have gained attention due to their enhanced gas separation performance, where the incorporation of ZIF-8 into polymeric matrices improves selectivity, permeability, and mechanical strength ^[170]. Similarly, ZIF-8 aerogels, with their high porosity and ultra-lightweight structure, have demonstrated superior CO₂ adsorption capacity and rapid diffusion kinetics, making them suitable for direct air capture and industrial gas separation applications ^[123]. However, challenges such as heterogeneous dispersion, polymer-ZIF interfacial adhesion issues, and long-term stability under industrial conditions remain critical barriers to their commercialization. Advancements in polymer functionalization, ligand modification, and hybrid aerogel structures have been explored to address these limitations, paving the way for more efficient and scalable ZIF-8-based CO₂ capture systems ^[170].

While ZIF-8 is widely recognized for its exceptional structural properties and gas separation efficiency, its biocompatibility and cytotoxicity are also important considerations, particularly for applications where environmental and biological exposure is relevant. In the context of ZIFs, cytotoxicity refers to how these materials interact with living cells—whether they are toxic to human or environmental cells when inhaled, ingested, or exposed to biological systems. This toxicity is often linked to zinc ion (Zn²⁺) release, oxidative stress, or cellular uptake of nanoparticles. The biocompatibility and cytotoxicity of ZIFs are key considerations for their industrial, biomedical, and environmental applications.

ZIF-8, the most extensively studied ZIF, is generally regarded as biocompatible under moderate exposure levels, exhibiting low inherent toxicity due to its chemical stability and minimal interaction with biological systems ^[170]. However, potential cytotoxicity concerns arise from zinc ion release, which can occur under acidic or highly reactive environments, leading to cellular stress and oxidative effects. Studies indicate that ZIF-8 degradation in biological fluids can influence its toxicity profile, with prolonged exposure or high concentrations potentially impacting cell viability.

Structural modifications, such as surface functionalization and polymer coatings, have been explored to enhance biocompatibility by preventing excessive ion leaching and improving stability ^[170]. In environmental applications, ZIFs are considered to have low ecotoxicity, and their controlled degradation pathways make them suitable for long-term use in gas separation and adsorption technologies. The continued development of engineered ZIF-based materials focuses on balancing structural integrity, controlled degradation, and minimal biological impact, ensuring their safe and effective integration into practical applications.

ZIF-8, like most MOFs, is typically synthesized as a fine crystalline powder, which is highly porous but difficult to handle in adsorption-based processes. For practical CO₂ capture applications, ZIF-8 needs to be shaped into structurally stable forms that maintain high adsorption capacity while ensuring mechanical durability and efficient gas transport. Various shaping techniques have been explored to transform ZIF-8 powders into pellets, granules, monoliths, and composite structures, each offering distinct advantages and challenges. Pelletization is a commonly used method, where ZIF-8 powders are compressed with binders or additives to form mechanically stable adsorbents, though excessive binder content can reduce porosity and adsorption efficiency. Monolithic structures, synthesized through direct templating or additive manufacturing, provide improved gas diffusion properties and reduce pressure drop in adsorption columns, making them suitable for PSA, TSA, and vacuum swing adsorption (VSA) processes. Bead-based ZIF-8 composites, formed via spray-drying or gelation techniques, offer enhanced handling properties and can be optimized for cyclic adsorption-desorption performance. The shaping process must balance mechanical strength, gas diffusion, and adsorption efficiency to ensure that ZIF-8-based materials perform optimally under industrial CO₂ separation conditions. By optimizing shaping processes, ZIF-8 can be effectively adapted for industrial-scale adsorption, offering an alternative to traditional sorbents in PSA, TSA, and TVSA-based CO₂ separation systems. Researchers have focused on binder-free shaping techniques, hybrid MOF-polymer composites, and 3D-printed MOF architectures to achieve scalable, structurally stable, and high-performance CO₂ adsorbents.

10.   Conclusions and future scope

In conclusion, we delved into the synthesis methods/routes employed for creating ZIF-8, specifically for CO₂ capture applications. We encompassed an overview of various synthesis techniques and their corresponding characterization results, elucidating the impacts of particle size, porosity, and structural attributes on CO₂ capture efficiency. Furthermore, the investigation extended to the utilization of materials/composites for enhancing CO₂ adsorption, along with a comprehensive discussion on the diverse techniques/equipment utilized for precise CO₂ measurement.

A critical focus was placed on the distinct advantages of ZIF-8 over alternative materials for CO₂ capture. Despite its immense potential, the challenge of achieving reproducibility in most fabrication methods remains significant. Addressing this issue is pivotal to ensuring consistent performance, and the exploration of a rapid synthesis route emerged as a standout option due to its efficiency in terms of production rate, time, and cost-effectiveness of precursors. While membranes for CO₂ capture have demonstrated promise, their predominance at the lab scale necessitates further research to facilitate a transition to industrial implementation.

In summary, the potential of ZIF-8 and its composites for CO₂ capture is undeniable, positioning them as emerging materials of great significance. For eventual commercialization, continued refinement in fabrication techniques is imperative. Among these techniques, the rapid synthesis route is particularly promising, showcasing efficiency in producing ZIF-8 materials with remarkable CO₂ capture capabilities. This avenue holds the key to transforming these materials from a realm of potential into a practical reality, offering a sustainable solution for addressing the pressing challenges of CO₂ capture and contributing to a greener future.

Use of AI tools declaration

The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.

Acknowledgments

This project has received funding from the European Union's Horizon 2020 research and innovation Programme under the Marie Skłodowska-Curie Grant Agreement No 847639, PASIFIC Postdoctoral Fellowship Programme. The content of this article does not reflect the official opinion of the European Union. Responsibility for the information and views expressed herein lies entirely with the author(s).

Author contributions

Conceptualization: A.V.; methodology: A.V., A.S. (Aman Singh) and A.S. (Angaraj Singh); software: A.V.; validation: A.S. (Aman Singh), K.K. and A.V.; formal analysis: A.V., A.S. (Aman Singh) and A.S. (Angaraj Singh); investigation: A.V., A.S. (Aman Singh) and A.S. (Angaraj Singh); resources: A.V. and M.W.; data curation: A.V., A.S. (Aman Singh) and K.K.; writing—original draft preparation: A.V., A.S. (Aman Singh), A.S. (Angaraj Singh) and K.K.; writing—review and editing: K.K., M.W. and A.V.; visualization: A.V.; supervision: A.V. and M.W.; project administration: A.V. and M.W.; funding acquisition: A.V. and M.W. All authors have read and agreed to the published version of the manuscript.

Conflict of interest

The authors declare no conflict of interest.