ZIF for CO₂ Capture: Structure, Mechanism, Optimization, and Modeling

Abstract

The requirement to counter carbon emissions is becoming urgent. Zeolitic Imidazolate Frameworks (ZIFs) have been extensively investigated recently for storing and separating gases, especially carbon dioxide. The present review aims to summarise the state of the art of ZIFs for carbon dioxide capture focusing on the structure, mechanism, optimisation, and modelling. The methods utilised for carbon capture are briefly summarized. The morphology of ZIFs with different topologies, N₂-CO₂ adsorption-desorption isotherms, X-ray diffraction patterns, thermo-gravimetric analysis (TGA) results are discussed to give insights into the textural properties, structure-activity relationship and structural-thermal stability of ZIFs. Finally, the experimental optimisation techniques, modelling and simulation studies for improving CO₂ capture by ZIFs are discussed. This review should provide a comprehensive and quick understanding of this research area. It is timely to summarize and review ongoing developments in this growing field to accelerate the research in the right direction.

1. Introduction

The CO₂ emissions estimates of the future are worrying [1]. The rising level of carbon in atmosphere is paving way to snow cover melts, climate change, and the greenhouse effect. Electricity production, industries, construction, transport, and residential contribute to the CO₂ emission. Figure 1a,b shows the global CO₂ emission during the last century and country/region-wise emission levels. CO₂ emissions are the primary source of global climate change, and to avoid the worst effects of climate change, the responsible countries need to urgently reduce CO₂ emissions. It has been observed that the CO₂ emission was relatively slow until mid of 20th century. However, by 1970, this has increased significantly and continuously grow each year. The current level of CO₂ has increased by about 100 ppm compared to the last six decades [1]. Hence, CO₂ capture and utilization are among the most significant environmental challenges in the 21st century. In order to decrease the harmful effects due to CO₂ emissions several countries have committed to policies of decarbonization and net zero emission. Since last decades, countries have been increased the consumption of renewable energies. The renewable energies consumption has been increased by 10.9%, 9.3%, 13.2%, and 17.8% in France, Germany, Italy and UK, respectively. Furthermore, other countries such as Canada, Japan, and USA have also involved in making sound policies for increasing the share of renewable energy. The consumption of renewable energy consumption in Canada, Japan, and USA have been increased by 13.5%, 10.1%, and 13.3%, respectively.

1.1. CO₂ Capture Technologies

CO₂ capture technologies include membrane separation, physical adsorption, and chemical absorption. Membrane separation technology is used for large-scale industrial applications, whereas physical adsorption and chemical absorption are the small to medium-scale commercial CO₂ capture and separation methods. There is a significant energy conservation benefit in physical adsorption because of weak van der Waals forces. Still, limitations include substandard CO₂/N₂ selectivity, less CO₂ capture capacity, and a decline in adsorption performance. In chemical absorption, a large amount of energy is consumed in regeneration due to the covalent bond formulation. The CO₂ capture techniques can be categorized into the post-combustion and pre-combustion capture processes and the oxy-firing process (Figure 2) [2].

In the post-combustion capture process, CO₂ produced from burnt fuel is removed from the air using mainly by organic solvents. The separation of CO₂ from the flue gases is difficult in post-combustion CO₂ capture system due to the dilute concentration of CO₂ and low pressure. CO₂ capture at low pressure has required large secondary power load for compressing it from atmosphere to pipeline pressure. Due to this, the treatment of large volume of gas is required, hence the cost in CO₂ capturing process has increased. Furthermore, the impurities present in the flue gas may degrade the sorbents and reduce the effectiveness of CO₂ capture. However, preliminarily analysis for CO₂ capture with amine scrubbing and low pressure have indicated that this system could raise the cost of electricity by 65%. 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. In this case, CO₂ is captured before fuel is burned. Due to the gasification process, the high partial pressure of CO₂ presents at high concentration (compared to post-combustion system), which further improves the separation and capture technology systems. The pre-combustion is less expensive than post-combustion CO₂ capture system because for same amount of CO₂ capture a smaller volume of gas is needed. Hence, smaller size equipment and capital cost are required. Furthermore, in the oxy-firing process, the fuel is burned in pure oxygen and converted into water vapour and CO₂. After cooling the burning gas, the water vapour is condensed, and CO₂ is captured. Moreover, oxy-combustion CO₂ capture system offers large scale laboratory testing. Both pre- and oxy-combustion have utilized air separation to burn coal in enriched O₂ atmosphere. However, the amount of O₂ needed in oxy-combustion is significantly greater than pre-combustion CO₂ capture system and therefore this system is expensive for CO₂ capture.

Table 1. CO₂-capture materials/technologies with their advantages and disadvantages.

S. No.	CO2 Capture Materials/Technologies	Advantages of Methods	Limitation of Methods
1.	Amine scrubbing (Chemical sorbent)	Simple and common process Low regeneration heat	Oxidative degradation Toxicity High volatility High operational cost Consume high energy
2.	Ionic liquids (Physical solvents)	No volatility High thermal stability Negligible vapour pressure Low energy consumption Continuous operation	High viscosity Toxic Consume high energy for regeneration
3.	Zeolites (Adsorption)	3-D Porous structure Highly crystalline structure High surface area	Smaller pore size Greater affinity to water Requires high temperature for regeneration Low CO2 selectivity
4.	CO2 Membranes (Membranes)	No processing cost during regeneration Requires low maintenance High CO2 selectivity	Corrosion Cannot be used at high temperature Low driving force
5.	Molecular sieves (Membranes)	High adsorption capacity High CO2 selectivity High hydrophobicity High resistance to alkaline and acid media Thermal stability	Cannot be used at high temperature
6.	Covalent organic framework (Adsorption)	Crystalline polymers High porosity Low volumetric density Higher stability in covalent linkage	Inefficient CO2 mass transfer due to low solubility Proton induced catalyst degradation
7.	Metal-organic frameworks (MOFs) (Adsorption)	High porosity Consume less energy for regeneration Convenience for transport and storage Tuneable pore size High CO2 selectivity Thermal and chemical stability.	Low hydrothermal stability Limits industrial applications Adsorption capacity decreases with exposure to the gas mixture. Costly

includes the notable CO₂ capture materials/technologies with their advantages and disadvantages.

The CO₂ has low partial pressure, high temperature, and proportion in exhaust, for which the effectiveness of aqueous amine solutions and amine scrubbers can be limited. There can be stability issues after CO₂ absorption in high CO₂ solubility ionic liquids with tuneable structure and their cost is also very high [3]. Despite challenges such as limited selectivity and adsorption capacity and poor stability in high-temperature and humid conditions, to date, MOFs are among popular choice for carbon-capture [4]. MOFs are collections of compounds in which molecular species link metal ions. MOFs display strong bonding, robustness, linking parts, and well-defined structures. MOFs are porous materials constructed from nodes and organic linkers. These are also known as porous coordination polymers. Some of the advantages of MOFs are high CO₂ uptake, high porosity, good thermal and chemical stability, large surface area, etc. MOFs are the fastest emerging fields in chemistry because of their structural and functional tunability [2,4,5,6,7,8]. Zeolite imidazolate frameworks (ZIFs) are the subclass of MOFs with analogous topology to inorganic porous zeolites. These are attractive candidates for several applications, including storage and separation of gases, due to their outstanding properties such as high CO₂ uptake, large surface area, permanent porosity, high thermal stability, high chemical stability, etc.

1.2. Aim of Present Review

The broad aim of this article is to summarize experimental and simulation studies related to the application of ZIFs for carbon dioxide capture to accelerate research in this growing field. The specific aims of the article include:

• To summarize recent developments in understanding of structure and characterization of ZIF, mechanisms of CO₂ capture using ZIF, and current challenges.
• To highlight opportunities for the researchers by summarizing innovative optimisation methods reported in the literature for ZIF application for CO₂ capture.
• We have not cited any article in open literature that provides a comprehensive review of modelling of ZIF for CO₂ capture.

1.3. Methodology of Present Review

The keywords and phrases including CO₂ emissions, CO₂ capture techniques, MOFs, ZIFs, solid absorbents, mechanism of CO₂ capture, adsorption/desorption of CO₂, CO₂ uptake, characterization of ZIFs, optimization techniques for improving CO₂ capture, modelling methods for ZIFs, simulation studies of ZIFs, etc. have been searched in databases including SCI, SCIE, Elsevier Scopus, and Web of Sciences to construct this review paper. At first, a general literature review has been carried out for ZIFs for CO₂ capture and the requirements for the present review article were recognised to accelerate the research in this evolving field. Figure 3 summarizes the detailed methodology and strategies for the present review article. The selection of publications for this review article were based on the reliability and validity of data, usefulness of the articles, latest research, significant impact factors, number of citations, recognition of researchers in their particular fields, etc.

2. ZIFs for CO₂ Capture

ZIF is a nanoporous material consisting of transition metal ions and imidazolate-type linkers. ZIF is a porous crystal with a 3-D structure of tetrahedral bivalent metal ions linked with organic imidazolates. The bivalent metal ions comprise of transition metals such as Zn or Co atoms. ZIF structure is a combination of nodes (metal ions) which act as connecting points and linkers (organic ligands), which serve as bridging molecules. One of the most prominent and unique properties of ZIF is its hydrophobic characteristic. Researchers are working on the development of ZIF-Based methods for CO₂ capture. Tan et al. have studied the effects of porosity and pore architecture, framework density and network topology, and chemical structure on the mechanical property (elastic modulus and hardness) of ZIFs [9]. Likewise, Phan et al. have reported the synthesis, structure, and CO₂ capture properties of ZIFs. Their paper discusses a comprehensive list of ZIFs, their topology and pore metrics in detail [10]. Yan et al. performed an experimental study on CO₂ capture by ZIFs slurry under normal pressure. The results showed that fine aperture, baffles, low superficial gas velocity, low sorption temperature, high regeneration temperature, and low regeneration pressure were favourable conditions for CO₂ capture [11]. Song et al. have synthesized new composite material by ionic liquid functionalized ZIF and evaluated the CO₂ adsorption behaviour at room temperature under low pressure. The results confirmed that altering porous material with ionic liquids could be an effective method to produce solid sorbent materials for CO₂ adsorption [12]. Morris et al. conducted an experimental and computational investigation on five different series of ZIFs for CO₂ capture. The results confirmed that CO₂ uptake capacity is directly influenced by functionality’s symmetry and polarization ability [13]. Banerjee et al. developed a high throughput protocol for synthesizing ZIFs for CO₂ capture. The results confirmed that these ZIFs structures have a tetrahedral framework, high thermal and chemical stability, high porosity and high selectivity for CO₂ capture [14]. Bhattacharjee et al. have reviewed the synthesis, functionalization, and catalytic/adsorption application of ZIFs [15].

Similarly, Kukkar et al. have reviewed recent advances in synthetic techniques for ZIFs [16]. Lee et al. have reported a new approach to engineer nearly spherical ZIF crystal shape by using leaf-like pseudo-polymorph [17]. Qian et al. have provided a new approach to enhanced water stability in Zn doped ZIF for CO₂ capture. The results revealed that Zn doped ZIF had shown better stability regarding crystal structure, morphology, surface area and CO₂ uptake capacity [18]. Wang et al. have reported the synthesis of functional ZIF-templated porous carbon material for CO₂ capture. The results confirmed that the functional group plays a vital role in improving the surface area, pore area and CO₂ capture of corresponding porous carbon material [19]. Wu et al. demonstrated the synthesis of a porous carbon framework with high CO₂ capture capacity by polyimide/ZIF composite aerogels. The results revealed an optimum CO₂ capture capacity for carbonized aerogel at a loading of a certain percentage of ZIF [20]. Abdelhamid has reviewed the ZIFs for CO₂ removal by adsorption, and catalysis processes such as cycloaddition, carboxylation, photocatalysis, electrocatalysis, etc. (Figure 4). The review has also promoted the industrial applications and commercialization of ZIFs [21].

2.1. ZIF Structure

ZIFs have tetrahedral topologies and are created by connecting 4-coordinated transition metals (M) by imidazolates (Im) with an angle of 145°. The metal atoms are linked through N atoms by ditopic and functionalized Im links to construct neutral frameworks [22]. The M-Im-M (M = Zn or CO and Im = imidazolate) angle is responsible for synthesizing many ZIFs with zeolite-type -tetrahedral topologies. Several factors come into play when approaching the synthesis process of ZIFs, those being geometry, maintenance, functionality, conformation, compatibility, solubility, pH, temperature, etc. The structure of ZIFs mainly depends on the category of imidazolate and solvents used. In addition, the structural diversity in ZIFs is due to the use of functionalized imidazolate ligands in the synthesis process. ZIFs display zeolitic topologies (more than 105) such as sod, rho, or lta [21]. Figure 5 shows the unit cell and crystal structure of ZIF (Figure 5a) [23], along with the common structure of linker used to synthesis ZIFs (Figure 5b) [21].

2.2. Synthesis Techniques for ZIFs

There are a variety of synthetic techniques for ZIFs. Some of them are solvothermal (first used in 1995), electrochemistry (2003), hydrothermal (2004), microwave-assisted (2005), mechanochemical (2005), sonochemical (2006), template (2006), atomic layer deposition (2007), ionothermal (2009), spray dryer (2011), sol-gel (2011), supercritical (2012), flow chemistry (2015), etc. Furthermore, all these synthesis techniques can be divided into solvent technology, solid sorbent technology, membrane technology, and cryogenic distillation. The self-explanatory schematics of all these technologies are shown in Figure 6a–d.

Furthermore, Abdelhamid has reported the general synthesis methods such as in-situ approach, ex-situ approach, and innovative methods for ZIFs-based membranes. The in-situ approach utilizes modified and unmodified supports for ZIF crystal growth. The ex-situ approach consists of physical methods (rubbing and electrospinning), and chemical methods (dip coating, slip coating, and microwave assisted). The modern and innovative methods to synthesize ZIF-based membrane are contra diffusion, rapid thermal diffusion, electrospray deposition, and 3D printing [21].

2.3. Mechanism of CO₂ Capture

The mechanism of CO₂ capture and storage by ZIF mainly depends on the thermodynamic and physiochemical properties of the adsorbent. The CO₂ storage at high pressure is because of the adsorbate-adsorbate interactions, whereas selective CO₂ capture at low pressure is due to the adsorbent-adsorbate interactions. In addition, the adsorbate should have a high contact area and strong polarization interactions. The pores’ design and size have played a significant role, and pores should be large for good interactions in the CO₂ capture process. In addition, the adsorbent should have a large volume and surface areas of pores for high CO₂ adsorption. Furthermore, the adsorbent should display fundamental properties such as high mechanical stability, high thermal stability, high chemical stability, low heat capacity, high adsorption capacity, low cost, etc. [24]. The physical and chemical adsorbents are used for CO₂ capture. Zeolite, MOFs, ZIFs, etc., are promising physical adsorbent materials for CO₂ capture, whereas amine-based and aqueous diamine are chemical adsorbent materials for CO₂ capture.

The mechanism of the CO₂ capture process by solvent technology, solid sorbent technology, membrane technology, and cryogenic distillation are discussed further in this section. The charge, charge dispersion, size, intermolecular hydrogen bonds, degree of solvation, etc., play a significant role in the solvent CO₂ capture process. In this method, the chemical and physical solvents are used according to the characteristic of the gas stream. Examples of physical solvents are glycol ether and methanol, whereas chemical solvents are alkali carbonates, alkanolamines, and aqueous ammonia [25]. In this process, with the help of preferential dissolution, the CO₂ is removed from the multi-component gas stream. The flue gases are cooled down at 40–60 °C by lean CO₂ solvent. The CO₂-rich solvent regenerates steam at a high temperature of 100–140 °C. The CO₂ solubility and the sorption rate are the most significant factors during CO₂ extraction. In addition, the CO₂ solubility depends on the partial pressure of CO₂, operating temperature, solvent concentration, type of solvent, and concentration of other components.

In solid sorbent technology, the CO₂ separation is achieved by using selective adsorption at the surface of the adsorbent or inside its pore structure. The CO₂ sorbent materials should have all the relevant sorption properties such as selectivity, working capacity, heat capacity, sorption rate, stability, etc. For removing CO₂, aluminosilicate zeolite, titano-silicate, and activated carbons are the most prevalent porous solids. MOFs, ZIFs, and porous silica are other solid porous candidates for extracting CO₂. In this process, adsorption can be accomplished in sorption columns with sorbent materials (particles, pellets, or fluidized bed reactors). The sorption columns are operated by pressure and temperature swing adsorption. In pressure swing adsorption, the adsorption and desorption are triggered by an increase or decrease in the pressure. Likewise, in temperature swing adsorption, the adsorption is carried out at atmospheric pressure, and desorption is activated by raising the temperature. The range for adsorption temperature is 55–60 °C and for desorption is 55–150 °C [26].

The currently accessible and functional technology (aqueous amine absorbents) for CO₂ capture incudes 30% energy penalty over power generation. This penalty can be reduced by using solid sorbents. The CO₂ adsorption can be divided as physical, chemical or both adsorptions. The families of solid sorbents are shown in Figure 7 [27]. Several studies have been conducted on solid absorbents for CO₂ capture which are based on characteristics of solid absorbents such as selectivity, adsorption capacity, adsorption-desorption kinetics, mechanical properties, chemical and thermal stability, durability, energy consumption of regeneration, etc.

Wang et al. have reviewed recent advancement and trend in solid sorbents for CO₂ capture. The review focused on low temperature (less than 200 °C), intermediate temperature (200–400 °C), and high temperature (greater than 400 °C) CO₂ sorbents. In addition, the review paper included research of CO₂ sorbents from waste resources (nut shells, residues from wood and food, eggshells, fishbones, etc.) [26]. Similarly other review paper has investigated the porous support materials such as MCM-41, SBA-15, KIT-6, PMMA, and PS for CO₂ adsorption. The review has reported that the development of the solid amine sorbents is very less, and significant research work has to be needed for CO₂ adsorption [28]. Likewise, Dunstan et al. have reviewed the fundamental aspects of solid sorbents based on alkali and alkaline earth metal oxide for CO₂ capture [29]. Samanta et al. have reviewed the usage of solid sorbents for post-combustion CO₂ capture. This review has carried out comparison and comprehensive study on recent progress, techno-economic analysis, and design aspect of solid sorbents [30]. Dao et al. have been studied the CO₂ adsorption/desorption enhancement properties of solid sorbents with Tetraethylenepentaammine/Diethanolamine blends. It has been observed from the study that these solid sorbents have high selectivity and cyclic stability [31]. Dinda et al. have studied zeolite based solid sorbents using monoethanolamine, ethylenediamine, diethylenetriamine, and triethylenetetramine for CO₂ adsorption. The results have shown that these solid absorbents can be used several times without compromising the properties [32]. Fu et al. have studied the MgO modified MCM-41 solid sorbents for CO₂ capture. The results have concluded that MgO modified MCM-41 has possessed good thermal stability and displayed good CO₂ adsorption/desorption properties [33]. Yamada et al. have developed amine-impregnated solid sorbents for CO₂ adsorption. The results confirmed the high performance of solid sorbents with respect to adsorption, desorption, and regeneration energy [34].

Liu et al. have studied the modified MCM-41 impregnated with zeolite A absorbent. The results have confirmed that the adsorption capability of modified sorbent has increased drastically due to the addition of zeolite A [35]. The synthesis and characterization of pore expanded MCM-41 have been carried out to determine CO₂ adsorption. The results have confirmed high CO₂ uptake due to pore expansion [36]. Similarly, Guo et al. have investigated the mechanism, kinetics, and performance of mesoporous silica Fe-MCM-41-A for CO₂ adsorption [37]. Xu et al. developed nanoporous solid sorbent (polyethyleneimine-modified mesoporous molecular sieve of MCM-41 for CO₂ capture. It was reported that the loading of polyethyleneimine into MCM-41 has significantly increased the adsorption capacity [38]. Cao et al. have evaluated the CO₂ adsorption of activated carbon from tire char, chicken waste, commercial activated carbon, and zeolite. The characteristic results have shown that the zeolite has shown the best CO₂ adsorption capacity in comparison to other counterparts [39]. Likewise, the synthesis of porous solid sorbents (activated carbon and MCM-41 from bagasse and rice husk) has been carried out to study the CO₂ adsorption capacity. The results confirmed that activated carbon synthesized from bagasse has shown highest CO₂ adsorption capacity in comparison to other sorbents [40].

In membrane technology, the CO₂ separation is accomplished from flue gas by porous or dense membranes. In a porous membrane, the gas is separated due to the differences in gas diffusion, whereas the differences in reactivity are responsible for gas separation in the dense membrane. In a dense membrane, a solution-diffusion transport mechanism is used to accomplish the gas separation. In this mechanism, the gas molecules initially dissolve into one phase of the membrane and then diffuse across the thickness of the membrane. Membranes are of materials such as polymers, metals, ceramics, hybrids, etc. Polymeric membranes are inexpensive, whereas inorganic materials provide good thermal stability and resist pressure, fouling and chemically aggressive gas steam [41]. The flue gas is dried, cooled, and compressed in cryogenic distillation to capture CO₂. The flue gas is cooled to a temperature somewhat above the point where CO₂ forms a solid. Further cooling results in precipitates of CO₂ as a solid, which depends upon the final temperature. At the end of the expansion process, the efficiency of CO₂ capture depends on the pressure and temperature. In this process, the steps are O₂ fired combustion, which separates O₂ from N₂, combustion process with pure O_2, which produces CO₂ and H₂O, maintaining temperature and heat loads in the boiler by recirculating some fraction of CO₂, condensing H₂O to produce pure CO₂, then the CO₂ compression and CO₂ separation process [42].

2.4. ZIF Characterization

Several tools are used to study the CO₂ capture mechanism and characterize the structural response. The surface area measurements and scanning electron micrograph are used to investigate ZIF’s textural and morphological characteristics for CO₂ capture. N₂-CO₂ adsorption-desorption isotherms are used to characterize textural properties of surface area/pore volume of ZIFs. Furthermore, X-ray diffraction (X-ray absorption of fine structure) has been used to examine the structural activity relationship in ZIFs. Additionally, the X-ray absorption near edge spectroscope and the extended X-ray absorption fine structure can provide oxidation state, charge transfer, bond distance, coordination number, etc. Additionally, TGA investigates the structural or thermal stability of ZIFs. The characterization results are used to compare ZIFs based on crystal structure, pore size, pore geometry, crystallite size, surface area, particle morphology, chemical compositions, particle size distributions, adsorption sites, CO₂ capture capacity, CO₂ sorption rate, CO₂ selectivity, thermal stability, chemical stability, mechanical stability, regeneration ability, etc.

2.4.1. Scanning Electron Microscopy (SEM)

Several studies have been carried out to characterize ZIFs with SEM. Chi et al. have studied the morphology of ZIF-8, and the results have reported the critical characteristic of regulatory product particle size. The range of particle size and average particle of ZIF-8 were found to be 480 to 580 nm and 533 nm, respectively [43]. Similarly, Zhong et al. have discussed the structural characteristic of ZIF-67. The SEM investigation has revealed the shape (polyhedral), the range of particle size (78–385 nm), and the average particle size (228 nm) of ZIF-67 [44]. Likewise, Lin et al. have revealed the morphological structure of ZIF-68 and reported that the ZIF-68 exhibited a soladite structure with an average size of 682 nm [45]. Liu et al. reported SEM characterization of ZIF-69 and concluded that the shape of ZIF-69 is a hexagonal prism and the mean grain diameter is about 20 µm [46]. Lin et al. have explained the shape (hexagonal) and topology (gme) of ZIF-78. In addition, it has been reported that the hexagonal prism shape of ZIF-78 has a similar length and thickness (5 µm) [47]. Yuan et al. have investigated the morphology and particle size of ZIF-90 with SEM characterization. The particle size of ZIF-90 has been mentioned as around 1 to 3 µm, with a hexahedral structure. Furthermore, the SEM characterization has reported that the ZIF-90 has uniform grains with good crystallinity [48]. Ilicak et al. used the SEM image to investigate the microstructure features of ZIF-95. It has been seen from the SEM image that the ZIF-95 has held a tetragonal structure with homogeneous particle size distribution. The report has also confirmed the compatibility of ZIF-95 with other polymers [49]. In another study, the SEM image was used to characterize the morphology and thickness of ZIF-100 [50]. Some of the discussed SEM images, crystal structure, and topologies of ZIFs are given in

Table 2. ZIFs and their observations after SEM.

S. No.	ZIFs	Crystal Structure	SEM	Topology	Ref.
1.	ZIF-8			sod	[43,51]
2.	ZIF-68			sod	[52]
3.	ZIF-69			gme	[46]
4.	ZIF-78			gme	[47]
5.	ZIF-95			poz	[49]

.

2.4.2. Isotherms

The isotherm has been used to investigate the porosity and the architectural stability of ZIFs. Bhattacharjee et al. have characterised the N₂ adsorption and desorption of ZIF-8. The results have displayed a type I isotherm with a microporous network of ZIF-8 [15]. Lee et al. have also discussed the N₂ physisorption of ZIF-8, and the results have indicated the microporous structure of ZIF-8 [17]. Similar works have been carried out for ZIF-8 [53,54,55,56], and the results have reported that the perfect selectivity of CO₂ to N₂ has been obtained by slope ratio and N₂ isotherm. Therefore, type I isotherms (for CO₂ to N₂) have been obtained for ZIF-8. Qian et al. have recorded the N₂ sorption isotherm of ZIF-67 and observed that the ZIF-67 had exhibited reversible type I isotherm with microporous nature [18]. Likewise, Wu et al. have studied the N₂ isotherm of ZIFs-8 and 67 to derive the pore size distribution. The results have revealed that both ZIFs have a steep increase at low relative pressure, i.e., type I isotherm, which further suggests the microporosity [20]. Song et al. have also conducted N₂ adsorption and desorption isotherm test in ZIF-67 to determine surface area and pore size distribution. The results have shown type I isotherm, indicating microporosity [57]. Similarly, Banerjee et al. have studied isotherms and CO₂ capture properties of ZIF-68, 69 and 70. The test results have revealed that all the ZIFs have shown permanent porosity, and ZIF-69 has outperformed their counterparts by showing high affinity and capacity for CO₂ capture [14]. In addition, Liu et al. have studied the N₂ adsorption isotherm of ZIF-69 and confirmed the formation of the framework structure of ZIF-69 [46]. In another study, the CO₂ adsorption isotherm test for ZIF-78 was conducted to determine the gas uptake capacity [58].

Furthermore, the N₂ adsorption and desorption isotherm test has been conducted for ZIF-90, and results have confirmed that ZIF-90 has multiple isotherms, i.e., type I (demonstrate the presence of micropores) and type IV (indicates the mesoporous structures) [48]. Similarly, the CO₂ and N₂ adsorption isotherm at various temperatures and pressure have been calculated for ZIF-90, ZIF-91, and ZIF-91-OLi. The results have confirmed that ZIF-91-OLi has emerged as a promising candidate for selective CO₂ adsorption [59]. Ilicak et al. have studied the N₂ isotherm for ZIF-95. The results have explained that the ZIF-95 has exhibited a type I profile curve, which has confirmed the microporosity in ZIF-95 [49]. Morris et al. have studied the N₂ and CO₂ isotherms of ZIFs. The results have shown that all the ZIFs have shown permanent micro-porosity, and among all the other ZIFs, ZIF-96 and ZIF-71 have shown the highest and lowest CO₂ uptake, respectively [13]. The mixed gas isotherms for ZIF-100 have been studied by molecular simulation, and it has been observed that the ZIF-100 have strong adsorption attraction to CO₂ [50]. Nguyen et al. have studied low pressure (N₂, CO₂, and CH₄) isotherms for ZIF-204. The results have revealed that ZIF-204 has exhibited a typical type I isotherm with microporosity [60]. Some of the isotherms of ZIFs investigated by the adsorption isotherms test are given in

Table 3. ZIFs and their observations after adsorption-desorption isotherms.

S. No.	ZIFs	Isotherms		Textural Property		Ref.
		N2	CO2	SBET (m2/g)	Vpore (cm3/g)
1.	ZIF-8, ZIF-L, ZIF-l@ZIF-8			NA	0.55, 0.06, 0.14	[17]
2.	ZIF-8			1699	0.52	[54]
3.	ZIF-67			2189.6	NA	[18]
4.	ZIF-8 ZIF67			1200	NA	[20]
5.	ZIF-67 and ZIF-67-IL			1716 and 1707	0.72 and 0.65	[57]
6.	ZIF-68, ZIF-69, and ZIF-70			1220 1070 and 1970	NA	[14]
7.	ZIF-69		NA	874	NA	[46]
8.	ZIF-78	NA		NA	NA	[58]
9.	ZIF-95		NA	763.75	0.39	[49]
10.	ZIF-25, ZIF-71, ZIF-93, ZIF-96, and ZIF-97	NA		564, 652, 864, 960 and 1110	NA	[13]
11.	ZIF-204	NA		715	NA	[60]

.

2.4.3. X-ray Diffraction (XRD) Analysis

X-ray Diffraction (XRD) analysis has been used for structural characterization in order to investigate the crystalline size and crystalline phase of ZIFs. The XRD analysis has shown patterns with peak intensities and the corresponding angles. Payra et al. have discussed the XRD analysis of ZIF-8 and concluded that the nanocrystallite diameter for ZIF-8 (MH) and ZIF-8 (CH-NH₃-200) is 27.2 and 51.4 nm, respectively [56]. A similar kind of work has been carried out for ZIF-8 by Chi et al. The results have confirmed the high crystalline structure of ZIF-8 [43]. Furthermore, the XRD patterns of ZIF-67 and ZIF-67-IL have been investigated, and the results have shown a decrease in the peak intensity of ZIF-67-IL compared to ZIF-67. It is due to the introduction of IL into the electron density of nano-realms in ZIF-67 [57]. The XRD patterns for ZIFs-68, 69, and 70 have been examined to confirm the phase purity and crystallinity [61]. XRD has characterized the ZIF-69 membrane, and the pattern has confirmed the pure phase of ZIF-69 [46]. The XRD patterns for ZIF-78 have confirmed its crystalline phase [58]. Similarly, their XRD patterns confirmed the successful syntheses of ZIF-90 and 91 [59].

Additionally, the peak intensity and corresponding angles of ZIF-95 have been investigated by XRD patterns, and the results have confirmed that ZIF-95 has high crystallinity [49]. The powdered XRD analysis has been conducted for ZIF-97/96/93/71/25, and results have confirmed that all ZIFs have a similar topology [13]. Some of the XRD patterns of ZIFs investigated by XRD are given in

Table 4. ZIFs and their observations after XRD.

S. No.	ZIFs	XRD Pattern	Information	Ref.
1.	ZIF-8		■ ZIF series have sodalite structure ■ Sharp peaks confirm the high crystallinity of the ZIF-8.	[43]
2.	ZIF-67/67-IL		■ Ionic liquids (IL) have not affected the crystallinity of ZIF-67.	[57]
3.	ZIF-68		■ Characteristic peaks confirmed the complete conversion of carbonaceous material.	[61]
4.	ZIF-69		■ The sharp, unshifted diffraction peak confirmed the retained structure.	[46]
5.	ZIF-70		■ The crystallinity and phase purity were confirmed for ZIF 70 by comparison with simulated pattern.	[61]
6.	ZIF-78		■ Helps to study the nutrient concentration. ■ Helps to study the ligand concentration.	[58]
7.	ZIF-95		■ Helps to identify the characteristic peaks of ZIF-95.	[49]
8.	ZIF-97/96/93/71/25		■ All the member of ZIF series have rho topology. ■ These structures have shown variation only in the organic functional group.	[13]

.

2.4.4. Thermogravimetric Analysis (TGA)

TGA is used in order to investigate the thermal or structural stability of ZIFs. ZIFs have typically exhibited high thermal and chemical stability. Bhattacharjee et al. conducted TGA of ZIF-8 and found that the sharp weight loss occurred due to the decomposition of the network [15]. Similarly, Payra et al. carried out TGA of ZIF-8 (MH) and ZIF-8 (CH-NH₃-200). The TGA profile of ZIF-8 (MH) has not shown any significant weight loss at low temperatures, whereas the TGA profile of ZIF-8 (CH-NH₃-200) has shown 25% weight loss at low temperatures [56]. The TGA analysis of ZIF-67 has been conducted, and results have concluded that ZIF-67 has shown 2.3% weight loss at temperatures up to 350 °C [12]. Likewise, the TGA profile of ZIF-67 has been discussed for its stability. The results concluded that the ZIF-67 is stable and has not decomposed in low-temperature regions [18]. Furthermore, the thermal stability of ZIF-69 has been explained by TGA. The results have shown two weight loss step decomposition, i.e., 16% weight loss at low temperatures (25–200 °C), and 16% weight loss at temperatures 370–420 °C [46]. Similarly, the TGA of ZIF-90 has been discussed, and the TGA curve has shown three stages of decompositions. In the first stage, 20% weight loss, while in the second stage, there was 10% weight loss, and in the third stage, 40% weight loss occurred [48]. Additionally, the TGA of ZIF-95 confirmed three stages of decompositions, i.e., 350–450 °C, 500–550 °C, and 600–700 °C [49]. Some of the ZIFs and their observations after TGA are given in

Table 5. ZIFs and their observations after TGA.

S. No.	ZIFs	TGA Profile	Weight Loss between (Tem. K)	Ref.
1.	ZIF-8 ZIF67		873–1173 673–823	[20]
2.	ZIF-67		623–773	[12]
3.	ZIF-67		373–403	[18]
4.	ZIF-69		643–693	[46]
5.	ZIF-95		623–1023	[49]

.

3. Optimization Techniques for Improving CO₂ Capture by ZIF

Many studies focus on optimization techniques for improving CO₂ capture by ZIF. According to Adamu et al., suitable process intensification methods based on material, equipment, and the process can significantly enhance the CO₂ capture and conversion. The development of photochemical, biochemical, electrochemical, and thermochemical are leading the way in improving CO₂ Capture and conversion [62]. Hasan et al. have studied the optimization of the adsorption-based process, i.e., pressure swing adsorption and vacuum swing adsorption for CO₂ capture. Both processes are optimized by arranging variable feed concentrations and flow rates [63]. Li et al. have discussed the geological storage of CO₂ and summarize intelligent optimization techniques such as injection, production and well location optimization techniques for improving CO₂ capture and storage [64]. Marquez et al. have introduced an optimized approach to CO₂ capture by ionic liquids. In this, ionic liquids are used as a replacement for conventional solvents, and after computer-aided molecular design optimization, it has been confirmed that ionic solvents are feasible for capturing CO₂ [65]. Lie et al. have optimized the membrane process for improving CO₂ capture, and results confirmed that fixed site career membrane had shown efficient capture of CO₂ in comparison to semi-commercial adsorption selective carbon membrane and in-house tailored carbon molecular sieving membrane [66].

Similarly, He et al. investigated membrane optimization and process condition. The results confirmed that the hybrid fixed site career membrane had shown good CO₂ permeance and CO₂/CH₄ selectivity [67]. Abraha et al. have reported the optimized CO₂ capture of ZIF by modifying the structure of constituent organic ligands. This work confirmed that the CO₂ absorbing capacity had been enhanced by solvent-assisted ligand exchange under ambient conditions [68]. Lai et al. have studied the synthesis of ZIF membrane and its process optimization study by response surface methodology in CO₂ separation [69]. Ghahramaninezhad et al. developed a novel microporous adsorbent by functionalizing ZIF for CO₂ capture. The results confirmed that CO₂ uptake was increased due to the metal/O₂ group [59]. Song et al. have reported the structural manipulation of ZIF crystals and the membrane for improved molecular separation. In addition, ZIF’s pore structure, flexibility, membrane morphology, and interfacial structure have been discussed to deepen the understanding of ZIF-based membrane [70]. Payra et al. have reported the influences of structural and surface modification of ZIFs on CO₂ adsorption efficiencies [56]. Song et al. have synthesized ion exchange ZIF by a novel one-step method to enhance CO₂ adsorption. Several characterization tests have been conducted to investigate the ion exchange method’s effect on the material’s structure, and results confirmed that this method had enhanced the CO₂ uptake capacity [12]. Aniruddha et al. have used a process (temperature, amount of adsorbent, time of carbonation and CO₂ concentration) optimization method for enhanced CO₂ capture [55]. Wang et al. have used a parallel flow-drop solvothermal method to enhance the CO₂ adsorption of ZIF. Polyaniline has played a significant role in structure orientation, and size control of ZIF and premodification of polyaniline improved the CO₂ adsorption [71]. Abdelhamid has discussed the methods to improve adsorption capacity and enhance the selectivity for CO₂ gas. Some of the strategies to improve adsorption capacity and enhance the selectivity are ligand dipole moment, tuneable pore structure, use dual metal nodes, modifications with polymers, etc. [21].

4. Modelling ZIF for CO₂ Capture

There can be several variations of ZIF, and very few of them can be synthesized practically. Searching for the correct ones for intended application could be time-taking. Therefore, computational modelling methods save time, effort, and cost and help synthesize the ideal ones. In literature, several attempts have been made for computational modelling of ZIFs. The ZIF structure is obtained from XRD data, and a force field is applied to the framework atoms. The potential interactions and force fields are used for interaction and modelling purposes. The detailed steps during computational modelling and simulation of ZIFs for CO₂ capture are shown in Figure 8. The Grand Canonical Monte Carlo (GCMC) simulation and Density Functional Theories (DFT) are among the most suitable methods.

The electrostatic potential is an essential property for interaction with the surrounding molecules. It is one electron property easily calculated anywhere in space from a molecular wave function. The electrostatic interactions are responsible for the intermolecular forces between molecules. Therefore, they are significant in molecular modelling and simulations [72]. The different potential interactions in the molecular modelling and simulations are given in

Table 6. The different potential interactions in the simulation of molecular dynamics.

S. No.	Non-Bonded Interactions
1.	Lennard Jones (LJ) potential	Non-electrostatic interaction Two combining rules: Geometric mean and Lorentz-Berthelot Short range interactions
2.	Buckingham potential	Replace the repulsive term in LJ by exponential function of distance. More realistic but expensive to calculate. Only single combining rule: GROMACS
3.	Electrostatic potential	The point charges are allocated to the spots of atomic nuclei Coulomb’s law describes the electrostatic potential Long range interactions
	Bonded interactions
1.	Bond potential	Used to model the interaction between covalent bonds Bond stretch is approximated by harmonic function Poor approximation at extreme stretching
2.	Angle potential	Describes the bond-bending energy Defined for every triplet of bonded atom Bond stretch is approximated by harmonic function
3.	Torsional (dihedral) angle potential	The torsion energy is defined for every 4 sequential bonded atoms Used to keep molecular structure planer

[73,74,75,76,77].

A force field (FF) is a collection of empirical energy functions and parameters which calculates the potential energy of atoms/molecules as a function of the molecular coordinates. The potential energy functions are the empirical functions composed of bonded and non-bonded interactions. FFs can be classified as follows in

Table 7. The different classes of force fields (FF) in the simulation of molecular dynamics.

S. No.	FF	Highlights
1.	Class 1	SHM describes FF dynamics of bond stretching and angle bending The magnitude of restoring force is supposed to be proportionate to the displacement from the equilibrium position FF matrix is diagonal Examples are: OPLS, AMBER, GROMOS, CHARM
2.	Class 2	Added anharmonic cubic and quadric terms to the potential energy Contains higher order terms and cross terms for a better description of interactions More data are needed to identify these additional parameters Examples are: UFF, MMFF94, DREIDING, TraPPE, ReaxFF, etc.
3.	Class 3	Explicitly added special effects of organic chemistry Special effects such as polarization, stereo electronic effect, Jahn-Teller effect, electronegativity effect, etc. Examples are: AMOEBA, DRUDE

[73,74,75,76,77]. Furthermore, from a comprehensive literature survey, it has been observed that specific parameters and techniques are essential for modelling and simulations of different ZIFs. Therefore, an effort has been made to collect these essential parameters and techniques from literature, presented in

Table 8. The popular models for APC.

Models	Theory	Highlights	Ref.
Mulliken	Partitioning the molecular wave function into atomic contribution	Determination of electronic (atomic and overlap) populations analysis Best known model Simple and computationally attractive Unrealistic results in some cases Utilizes non-orthogonal basis sets	[78,79]
Natural population analysis (NPA)	Partitioning the molecular wave function into atomic contribution	General atomic orbital basis sets Alternative to Mulliken population analysis Orthonormal natural atomic functions Improved numerical stability Modest computation cost	[78,80]
Distributed multipole analysis of Gaussian (GDMA)	Partitioning the molecular wave function into atomic contribution	Molecular electron density (MED) distribution is represented by multipoles (positioned on the individual atoms) Gaussian basis function Overcome the conventional basis-set dependency Uses grid-based quadrature for partitioning	[78,81]
Atoms-in-molecules (AIM)	Partition the MED into atomic domains in the physical space	Based on zero flux surfaces of the MED	[78,82]
Hirshfeld	Partition the MED into atomic domains in the physical space	Fragmentation of bonded-atom for describing MED MED is shared with surrounding atoms in direct proportion to their free-atom electron density Can be improved by parameterization	[78,83]
Charge model 5 (CM5)	Partition the MED into atomic domains in the physical space	An extension of Hirshfeld population analysis Utilizes the parameters derived from reference values of gas-phase dipole moments It is appropriate to any charged or uncharged molecules composed of any element in gas or in solution	[78,84]
Merz-Kollman-Singh (MKS)	Reproduction of the molecular electrostatic potential	Simple, analytical, partial charge models for the reproduction of charge distribution of complex molecules Atomic charges derived from semiempirical methods Suited for atom-centred or lone-pair model and for either all-atom or united atom models Points situated at four shells (distance 1.4, 1.6, 1.8, and 2.0 times the van der Walls radii of the atoms)	[78,85]
Charge from electrostatic potentials (CHELP)	Reproduction of the molecular electrostatic potential	Calculation time is shorter than others Charges may be obtainable via optimized geometry, if experimental data are not available Points positioned (distance 2.5, 3.5, 4.5, 5.5 and 6.5 Å from the van der Walls surface)	[78,86]
Charge from electrostatic potentials using a grid (CHELPG)	Reproduction of the molecular electrostatic potential	Improved version of CHELP model Less dependency on molecular orientation Points are selected from regularly spaced grid (between 0 and 2.8 Å from the van der Walls surface)	[72,78,87]
Hu-Lu-Yang (HLY)	Reproduction of the molecular electrostatic potential	Comparable accuracy with better geometry Introduction of the object function in the whole molecular volume space instead of points Object function improves the numerical stability Used to fit electrostatic potentials for molecular mechanics force fields	[78,87]

and

Table 9. The essential parameters and techniques during modelling and simulations of different ZIFs.

S. No.	ZIF	Evaluation of APC	Interacting Potential	Interaction Techniques	Force Field	Simulation	Highlights	Ref.
1.	ZIF-1	NA	LJ	Lorenz Berthelot	TraPPE and DREIDING	GCMC	The molecular simulation has been performed to study ZIF-1’s working capacity and adsorption selectivity	[88]
2.	ZIF-1 to ZIF-3 ZIF-6, ZIF-8,ZIF-10, ZIF-60, ZIF-65, ZIF-67, ZIF-68 ZIF-69, ZIF-78, ZIF-79, ZIF-81, ZIF-90,	CBAC	LJ	Lorenz Berthelot	UFF and DREIDING	GCMC	The results have confirmed that ZIF-2 is the best material for membrane-based separation	[89]
3.	ZIF-1 to ZIF-3 ZIF-6, ZIF-8, ZIF-10 to ZIF-12, ZIF-60, ZIF-65, ZIF-67, ZIF-69, ZIF-78, ZIF-79, ZIF-81, ZIF-90,	DFT	LJ and electrostatic	Lorenz Berthelot	UFF	GCMC and EMD	The gas permeability, working capacity and sorbent selection parameter have been computed to recognise the most appropriate ZIF in adsorption and membrane-based separation	[90]
4.	ZIF-2, ZIF-3 ZIF-6, ZIF-8, ZIF-10 to ZIF-12, ZIF-60, ZIF-65, ZIF-67, ZIF-69, ZIF-78, ZIF-79, ZIF-81, ZIF-90	DFT	LJ and electrostatic	Lorenz Berthelot	UFF	GCMC and EMD	The molecular simulation and continuum modelling have been performed for 360 ZIF-based MMMs The results have shown that the ZIF/polymer pairs have exceeded gas permeability and selectivity than pure polymers	[91]
5.	ZF-2 to ZIF-10	NA	LJ	-	UFF and DREIDING	GCMC	The study has revealed that ZIF-4 is the most promising material among the other counterparts	[92]
6.	ZIF-3 and ZIF-10	DFT	LJ	Lorenz Berthelot	UFF	GCMC and EMD	This work provides understanding and procedures for optimum selection and development of ZIFs as adsorbents and membranes	[93]
7.	ZIF-3, ZIF-8, ZIF-10, ZIF-60, and ZIF-67	NA	LJ	Lorenz Berthelot	UFF and TraPPE	GCMC	This study has provided helpful macro and microscopic information for adsorption and separation properties in ZIFs	[94]
8.	ZIF-4, ZIF-62, and ZIF-77	NA	NA	NA	ReaxFF	NA	This study has compared the ReaxFF with molecular dynamics and DFT The results have revealed that ReaxFF enables more efficient computation design at a low computation cost for ZIFs	[95]
9.	ZIF-8	CBAC	LJ and Coulombic	Ewald Summation	TraPPE and DREIDING	GCMC	This work has investigated the adsorption selectivity and working in ZIF at different temperatures, and the results confirmed that the temperature influenced the working capacity and adsorption	[96]
10.	ZIF-8	DFT	LJ and Coulombic	Lorenz Berthelot	UFF	GCMC	The study has revealed that adding amino functional groups has enhanced the CO2 adsorption ability	[97]
11.	ZIF-8	DFT	LJ	Lorenz Berthelot	AMBER	GCMC	The work has shown a molecular-level picture of the selectivity and the density of gas molecules in ZIF-8 membrane	[98]
12.	ZIF-8	DFT/REPEAT	NA	NA	UFF	GCMC	In this work, the atomic partial charges with fluctuation have been studied for ZIF	[99]
13.	ZIF-8, ZIF-25, ZIF-90, ZIF-93, and ZIF-97	DFT	LJ	Lorenz Berthelot and Ewald Summation	DREIDING	GCMC	The results have shown that ZIF-93 with large pores and lesser density has high adsorption at elevated pressures	[100]
14.	ZIF-8 and ZIF-90	DFT	Electrostatic surface	REPEAT	UFF	GCMC	The study has revealed that the separation property can be enhanced by functionalization and proper selection of solid linker	[101]
15.	ZIF-25, ZIF-71, ZIF-93, ZIF-96, and ZIF-97,	DFT	NA	Lorenz Berthelot	UFF	GCMC	The molecular modelling has revealed that the polarizability and symmetry of the functionalization are significant factors for CO2 uptake	[13]
16.	ZIF-68 and ZIF-69	DFT	LJ and Coulombic	Ewald Summation	UFF and DREIDING	GCMC	The pore topology and accessibility have been analyzed for ZIF-68 and ZIF-69 and results have concluded that these analyses are highly relevant for these materials	[102]
17.	ZIF-68 and ZIF-69	DFT	LJ	Lorenz Berthelot	UFF	GCMC	The computational study has been carried out for ZIF-68 and ZIF-69 and results have demonstrated that diffusion of CO2 in ZIFs is slower than in other counterparts	[103]
18.	ZIF-68 and ZIF-69	DFT	LJ and Coulombic	Lorenz Berthelot	TraPPE	GCMC	This work has demonstrated the important role of electrostatic interaction, which is the key factor during selectivity	[104]
19.	ZIF-68 and ZIF-70	DFT	LJ	Lorenz Berthelot	UFF	GCMC and EMD	The adsorption, membrane selectivity and diffusion have been calculated in ZIF-68 and ZIF-70	[105]
20.	ZIF-68 and ZIF-70	DFT	LJ	Lorenz Berthelot	UFF and DREIDING	GCMC and EMD	The atomistic simulation has been carried out for ZIF-68 and ZIF-70, and adsorption isotherm, self and transport diffusivities have been calculated	[106]

.

5. Concluding Remarks and Future Directions

In this work, a comprehensive review has been carried out on ZIFs for CO₂ capture focusing on the mechanism of CO₂ capture, characterization, optimization, modelling, and simulation studies.

The article summarized ZIFs synthesis techniques, solvent technology, solid sorbent technology, membrane technology, and cryogenic distillation process.

ZIFs characterization, including SEM, XRD, and TGA results from the literature, are discussed in order to give an insight into the current status.

The textural and morphological characteristics of CO₂ capture were reviewed.

The isotherms and the XRD patterns have been discussed to give insights into the textural properties and structure-activity relationship in ZIFs.

State of the art modelling and simulation studies on ZIFs have been summarized, including essential parameters and techniques for modelling and simulations of different ZIFs.

The future scope of work in ZIF-based technologies for CO₂ capture may include the following.

In the future, CO₂ selectivity from ZIF-based methods requires improvement.

A combination of CO₂ capture methods, particularly combined adsorption-absorption, could be helpful.

In addition, the targeted molecular dynamics-based model for associating CO₂ diffusivity with the aperture flexibility–molecular size relation might also be investigated.