Post-Combustion Capture of Carbon Dioxide by Natural and Synthetic Organic Polymers

Abstract

The elevation of carbon dioxide (CO₂) levels in the atmosphere is responsible for global warming which in turn causes abrupt climate change and consequently poses a threat to living organisms in the coming years. To reduce CO₂ content in the atmosphere CO₂ capture and separation is highly necessary. Among various methods of CO₂ capture post-combustion capture is very much useful because of its operational simplicity and applicability in many industries and power sectors, such as coal-fired power plants. Polymers with high surface area, high volume and narrow pores are ideal solid sorbents for adsorption-driven post-combustion CO₂ capture. Natural polymers, such as polysaccharides are cheap, abundant, and can be modified by various methods to produce porous materials and thus can be effectively utilized for CO₂ capture while the surface area and the pore size of synthetic porous organic polymers can be tuned precisely for high CO₂ capturing capacity. A significant amount of research activities has already been established in this field, especially in the last ten years and are still in progress. In this review, we have introduced the latest developments to the readers about synthetic techniques, post-synthetic modifications and CO₂ capture capacities of various biopolymer-based materials and synthetic porous organic polymers (POPs) published in the last five years (2018–2022). This review will be beneficial to the researchers to design smart polymer-based materials to overcome the existing challenges in carbon capture and storage/sequestration.

1. Introduction

A major threat to the inhabitants of the earth is global warming which is primarily responsible for climate change. Global warming is mainly caused by increasing concentrations of carbon dioxide (CO₂) gas in the atmosphere with time. CO₂ emissions rose to a historic high in recent years mostly due to the burning of fossil fuels [1]. To prevent the increase in the concentration of CO₂ in the atmosphere, carbon capture and storage/sequestration (CCS) is highly necessary. In general, the strategies used to capture CO₂ are pre-combustion, post-combustion, oxy-fuel combustion and direct air capture. Out of these strategies, post-combustion capture of CO₂ is operationally simple and useful in many industries and power sectors, such as coal-fired power plants [2]. Adsorption of gas onto the surface of solid material is considered an effective method for post-combustion CO₂ capture. Among various types of solid sorbents polymer and polymer-based materials are found to be highly promising. The use of non-toxic, cheap and widely available biopolymers such as cellulose, lignin, cyclodextrin, chitosan etc to capture CO₂ is an effective and highly sustainable carbon capture strategy while low density, high porosity, large surface area and high stability of synthetic organic polymers make them a good choice of materials for post-combustion carbon dioxide capture as well [3]. Surface modification of the POPs can also be performed easily to enhance CO₂ capture capacities. Porous biopolymer-based materials are usually prepared by carbonization and physical or chemical activation of the chosen biopolymer such as cellulose, chitosan, lignin, cyclodextrin, etc. Synthetic organic polymers are generally obtained by connecting monomers by covalent bonds utilizing various types of reactions. The pore size and the nature of the surface can be tuned precisely by the selection of monomers and synthetic procedures to enhance CO₂ adsorption capacity and selectivity of CO₂ over other gases [3].

As CCS is an important and frontier area of research, a huge number of articles have been published in the last two decades. Development of new types of POPs and novel porous materials derived from them is going on rapidly worldwide. Some good review articles have been reported in the last decade on the syntheses and applications of POPs on CCS [3,4,5,6]. In this scenario, a comprehensive report is required to introduce to the readers the latest developments on the synthesis of various types of POPs and new materials derived from bio and synthetic polymers and their applications towards CCS. For this purpose, we have summarized synthetic techniques, post-synthetic modifications and CO₂ capture capacities of various biopolymer-based materials and synthetic POPs published in the last five years (2018–2022). Furthermore, we briefly mentioned the factors which influence CO₂ capture capacity and selectivity.

2. Mechanism of Adsorption and Related Thermodynamic Factors

There are two processes of adsorption, namely physical adsorption or physisorption and chemical adsorption or chemisorption. Physisorption occurs on the surface of the adsorbent via non-covalent interactions (Coulombic, Van der Waals, etc.) between adsorbate molecules and the surface. Desorption of the gas molecules in this case is a low-energy consumption process. The major advantage of physisorption is adsorbents can be reused easily while the disadvantages include less selectivity and low adsorption capacity of the adsorbent at high temperatures. In chemisorptions, covalent bonds are formed between gas molecules and the surface of the adsorbent. Common adsorbents contain basic functional groups such as amine on the surface of the material. Basic functional groups react with acidic CO₂ molecules producing salts. The major advantage of chemisorption is high adsorption capacity and good selectivity of the adsorbents. A common drawback of this process is the regeneration of sorbents consumes a lot of energy [7].

Isosteric heat of adsorption (Q_st) is indicative of the strength of interaction between adsorbents and CO₂ molecules. Q_st is calculated by fitting adsorption isotherms by using the Clausius–Clapeyron equation. A low Q_st value suggests predominant physisorption, whereas, a high Q_st value indicates a strong interaction between gas molecules and the material’s surface leading to predominant chemisorption. Good adsorbents should selectively adsorb CO₂ over all other gases to achieve effective separation. Thus CO₂/N₂ selectivity is an important index for CO₂ capture by adsorbents. In practical applications, CO₂/N₂ selectivity is calculated by the initial slope method (Henry’s law) and ideal adsorption solution theory (IAST). Flue gas released from power stations consists of 10–15% CO₂ and a large quantity of N_2. Therefore, effective CO_2-capturing materials should have a high affinity for CO₂ at low pressure and high CO₂/N₂ selectivity. The porosity and surface area of the adsorbents are also important factors which affect adsorption performance. The surface areas of the adsorbents are typically analyzed using Brunauer–Emmett–Teller (BET) theory [8].

3. Polysaccharide-Based Biopolymers for CO₂ Capture

Biopolymers are crowned with biocompatibility and biodegradability which can be utilized for designing biomaterials for various applications such as packaging materials in the food industry, fuel cells, drug delivery, membrane and medical implants organ preperation, tissue engineering and many more [9,10,11,12,13,14]. Polysaccharides are carbohydrate-based biopolymers which act as the most important source of energy in animal and plant biosystems. Among multiple applications of polysaccharides [15,16,17,18,19], one of the most important is CO₂ capture. Considering from an environmental and economic perspective polysaccharides are cheap and abundant [3,20,21,22] which perfectly outfits sustainable development goals (SDGs) by the implementation of bio-based materials for environment cleaning and healthy living.

3.1. Cellulose

Cellulose is a linear polysaccharide consisting of repeated D-glucose units with the formula of (C₆H₁₀O₅)_n. In a recent study, bottom-up ecosystem simulation is coupled with models of cellulosic biofuel production, carbon capture and storage to track ecosystem and supply chain carbon flows for current and future biofuel systems. This approach could have climate mitigation and stabilization potential [20]. Qaroush et al. have discussed in detail different types of polysaccharides for CO₂ capture describing the reversible reaction between cellulose and CO₂, their subsequent dissolution, regeneration and CO₂ capturing using functionalised cellulosic materials [3]. One interesting approach is converting cellulose to sustainable porous carbon materials which is further applied for CO₂ capture [21]. Porous carbonaceous materials are usually prepared by carbonization and activation [21]. The carbonization process again can be of two types, namely (i) conventional pyrolytic approach which is heating the sample at elevated temperatures of 400–1000 °C in an inert atmosphere (e.g., N₂, Ar). This involves dehydration, condensation and isomerization, which eliminates most of the hydrogen and oxygen atoms to form H₂O, H₂, CH₄, and CO gases. (ii) Hydrothermal carbonization (HTC) is the other approach which is usually performed at moderate temperatures (<300 °C) and is advantageous due to reduced energy consumption, sample does not need to be dry and gives carbon-rich hydrochars in high yields. Thus in recent times, the HTC method is considered an energy-saving and environmentally friendly approach for carbonization [21]. Two activation methods are being reported which produce porous carbons with large differences in porosity. In general, physical activation processes create porous carbons with moderate surface areas (1000 m²/g) and narrow micropores that can be beneficial for, e.g., CO₂/N₂ and CO₂/CH₄ separation [21]. In contrast, chemical activation significantly increases the surface area (up to >3000 m²/g) and pore volume of the porous carbons which can be useful for gas storage [21]. Here we discuss the CO₂ adsorption capacity of some cellulose-derived materials derived by the carbonization process. In this regard, Heo et al. reported a series of porous carbons derived from commercial cellulose fibres in three steps and described that steam molecules played a key role in the pore-opening process and induced an increase in the surface area of the porous carbon materials formed. The cellulose fibres were carbonized under N₂ atmosphere followed by physical activation with steam under gauge pressure. Ultramicropores (pore size < 0.8 nm) were resulted by physical activation process which significantly contributed to the increase in surface areas from 452 to 540 m²/g for pre-activated samples to 599–1018 m²/g for steam-activated samples causing CO₂-over-N₂ adsorption selectivity and increase in CO₂ adsorption capacity [22]. This study was followed by Zhuo et al. who reportedly prepared hierarchically porous carbons by carbonization/activation of cellulose aerogels under CO₂ and N₂ atmosphere with improved surface area and volume for CO₂ adsorption and further showed that steam activation is an efficient process to prepare cellulose-based porous carbons with high CO₂ adsorption capacities [23].

Chemically activated carbonaceous materials have grabbed attention because of much higher surface areas resulting in much higher CO₂ adsorption capacities. Sevilla et al. reported on the chemical activation of cellulose by KOH thus designing microporous carbon materials with a very high surface area of 2370 m²/g and CO₂ adsorption capacity of 5.8 mmol g⁻¹ at 1 bar and 273 K at a high adsorption rate and excellent adsorption recyclability. They designed the material by hydrothermal carbonization of potato starch, cellulose and eucalyptus sawdust followed by chemical activation using potassium hydroxide [24]. Xu et al. have applied algae-extracted nanofibrous chemically modified cellulose carbonized under N₂ and CO₂ atmosphere and activated in CO₂ with significantly higher surface areas (832–1241 m²/g) and higher volumes of ultramicropores (0.24–0.29 cm³/g) for CO₂ adsorption [25]. Recent years have seen a promising application of cellulose aerogels in carbon storage. Ho et al. have reported a review [26] depicting a chemical modification of nanocellulose aerogels that led to a large surface area which improved selectivity towards CO₂ adsorption. Kamran et al. have developed highly porous carbons by hydrothermal carbonization method and chemical activation using acetic acid as an additive. The cellulose-based materials showed high specific surface area (SSA) (1260–3019 m² g⁻¹), microporosity in the range of 0.21–1.13 cm³ g⁻¹ with CO₂ adsorption uptake of 6.75 mmol g⁻¹ and 3.96 mmol g⁻¹ at 273 K and 298 K at 1 bar, respectively, and CO₂ selectivity. They reported that micropores between 0.68 nm and 1 nm exhibit high CO₂ adsorption potential [27].

However non-carbonized cellulose-derived materials have also been reported for efficient CO₂ adsorption capacities. In this regard, cross-linking of nanocellulose enhances the surface area and CO₂ adsorption was observed by Wang et al. and Sun et al. [28,29]. Amino-functionalized nanocellulose aerogels although had a reduced surface area still displayed chemisorption of CO₂ with a capacity of more than 2 mmol g⁻¹ [26]. In some other reports, cellulose hybrids were designed without any carbonization with inorganic fillers such as silica, zeolite and metal–organic frameworks which improved the surface area and physisorption of CO₂ [26]. Sepahvand et al. have reported nano filters by a combination of cellulose nanofibers (CNF) and chitosan (CS) at varied loading compositions. They reported that increasing the concentration of modified CNFs increases the adsorption rate of CO₂ and the highest adsorption of CO₂ belonged to the 2% modified CNF [30]. A most recent study by Chen et al. reports epoxy-functionalized polyethyleneimine modified epichlorohydrin-cross-linked cellulose aerogel having a rich porous structure with a specific surface area in the range of 97.5–149.5 m²/g. This material showed good adsorption performance, with a maximum CO₂ adsorption capacity of 6.45 mmol g⁻¹ [31]. Material type and composition, BET surface area (m² g⁻¹), pore size (nm)/total pore volume (cm³ g⁻¹), mechanism of adsorption, CO₂ capture capacity (mmol g⁻¹) and special features of cellulose-based materials have been tabulated in

Table 1. Summary of material type and composition, BET surface area (m² g⁻¹), pore size (nm)/total pore volume (cm³ g⁻¹), mechanism of adsorption, CO₂ capture capacity (mmol g⁻¹) and special features of cellulose-based materials.

Material Type and Composition	BET Surface Area (m2 g−1)	Pore Size (nm)/Total Pore Volume (cm3 g−1)	Mechanism of Adsorption	CO2 Capture Capacity (mmol g−1)	Special Features	Ref
Porous carbons derived from commercial cellulose fibres	540 and 1018	<0.8 nm–/0.234 and 0.429	for pre-activated samples for steam-activated samples	3.776 at 298 K	CO2-over-N2 adsorption selectivity	[22]
Carbonized and activated cellulose from cotton linter	1364	1.42	physisorption	3.42	-	[23]
Chemically activated cellulose	2200–2400	1.1	physisorption	4.8	CO2-over-N2 adsorption selectivity	[24]
Algae extracted nanofibrous chemically modified cellulose activated in CO2	832–1241	0.24–0.29	physisorption	2.29 at 0.15 bar, 5.52 at 1 bar; 273 K	CO2-over-N2 adsorption selectivity	[25]
Silica/Cellulose Nanofibril aerogel functionalized with 3-aminopropyl triethoxysilane	11	0.05	Chemisorption via reaction between amine group and CO2	2.2 at humid condition	high chemisorption of CO2 with reduced surface area	[26]
Highly porous cellulose by hydrothermal method and chemical activation using acetic acid as an additive.	1260–3019	0.21–1.13	physisorption	6.75 at 273 K, 1 bar and 3.96 at 298 K, 1 bar	CO2 selectivity	[27]
polyethyleneimine-crosslinked cellulose (PCC) aerogel sorbent	234.2	-	Surface diffusion and intra-particle diffusion mechanism	2.31 at 25 °C under pure dry CO2 atm	Adsorption-desorption recyclability	[28]
Cellulose nanofiber (CNF) surface was functionalized using chitosan (CS), poly [β-(1, 4)-2amino-2-deoxy-Dglucose]	~360	~4 nm	Physisorption	4.8	Increasing the concentration of modified CNFs increases the adsorption rate of CO2	[30]
Epoxy-functionalized polyethyleneimine modified epichlorohydrin-cross-linked cellulose aerogel	97.5–149.5	-	Chemisorption	6.45	Material showed preferable rigidity and carrying capacity	[31]

.

One important class of nanocellulose-based materials and their subsequent application involves membrane separation of CO₂. Ansaloni et al. reported micro fibrillated cellulose/Lupamin membrane which showed very good CO₂ permeability but the selectivity CO₂/N₂ and CO₂/CH₄ (in the order of 500 and 350, respectively, for pure micro cellulose) was compromised thus decreasing the overall membrane performance [32]. Venturi et al. later investigated the influence of doping in nanocellulose-based materials and performed a systematic study of CO₂ permeability by the newly designed membranes. They blended the commercial Polyvinylamine solution Lupamin^® 9095 (BASF) with Nano Fibrillated Cellulose (NFC) which resulted in films. They observed that increasing water vapour and a higher presence of Lupamin in the film resulted in an improvement of both CO₂ gas permeability and selectivity. NFC content of 70 wt% Lupamin showed a selectivity of 135 for the separation of CO₂/CH₄ and 218 for CO₂/N₂. The maximum permeability in the order of 187 Barrer was reached at 80% RH [33]. In a later study, the addition of l-arginine to a matrix of carboxymethylated nano-fibrillated cellulose (CMC-NFC) resulted in a mobile carrier facilitated transport membrane for CO₂ separation. They reported that l-arginine (45 wt% loading) greatly improved CO₂ permeability by 7-fold from 29 to 225 Barrer and selectivity with respect to N₂ from 55 to 187 compared to pure carboxymethyl nanocellulose matrix [34] [Figure 1]. Hussain et al. have worked with pure and mixed matrix membranes (MMMs) to capture carbon from natural gas. They used polyethylene glycol (PEG), Multi-walled carbon nanotubes (MWCNTs) and cellulose acetate (CA) to prepare membranes of pure CA, CA/PEG blend of different PEG concentrations (5%, 10%, 15%) and CA/PEG/MWCNTs blend of 10% PEG with different MWCNTs concentrations (5%, 10%, 15%). They observed that CO₂/CH₄ selectivity is enhanced 8 times for pure membranes containing 10% PEG and 14 times for MMMs containing 10% MWCNTs and in mixed gas experiments, the CO₂/CH₄ selectivity is increased 13 times for 10% PEG and 18 times for MMMs with 10% MWCNT [35]. Mubashir et al. have developed composite membranes using non-stoichiometric ZIF-62 MOF glass and cellulose acetate (CA) which exhibited pore size (7.3 Å) and significant CO₂ adsorption on the unsaturated metal nodes [36]. In more recent studies, Rehman et al. have reported the synthesis of mixed matrix membranes (MMMs) by incorporating (1–5 wt%) Cu-MOF-GO composites as filler into cellulose acetate (CA) polymer matrix by adopting the solution casting method. They reported 1.79 mmol g⁻¹ and 7.98 wt% of CO₂ uptake at 15 bar [37]. Some other foam-like cellulose composites have also been reported to capture CO₂. Wang et al. designed a strong foam-like composite by the in situ growth of thermally stable and microporous metal–organic frameworks (MOFs) including copper benzene-1,3,5-tricarboxylate [Cu₃(BTC)₂], zinc 2-methylimidazolate, and aluminium benzene-1,3,5-tricarboxylate in a mesoporous cellulose template derived from balsa wood. The TO-wood/Cu₃(BTC)₂ composite shows high durability during the temperature swing cyclic CO₂ adsorption/desorption process and a high CO₂ adsorption capacity of 1.46 mmol g⁻¹ at 25 °C and atmospheric pressure [38].

3.2. Chitosan

Natural biopolymer chitosan (CS) is a marine waste material that may be used in CO₂ adsorption because of its ease of processability, low maintenance and energy necessity. CS is inexpensive, abundantly available, renewable, environmentally friendly and biodegradable polysaccharide which is the second most abundant natural polysaccharide after cellulose [39]. CS chains have a large number of basic amine groups which facilitate adsorption of the acidic CO₂ molecule on the surface of the adsorbents [40,41]. However, pure chitosan suffers from low carbon dioxide adsorption due to the lower surface area thus most of the studies reporting chitosan-derived sorbents aim to fabricate the surface properties and maximize the CO₂ adsorption capacity [42]. Like cellulose materials, carbonization to chitosan-based materials has also resulted in efficient CO₂ adsorbent which are discussed herein. Hierarchical porous nitrogen-containing activated carbons (N-ACs) were prepared with LiCl-ZnCl₂ molten salt as a template derived from cheap chitosan via simple one-step carbonization under Ar atmosphere. The obtained N-ACs with the highest specific surface area of 2025 m² g⁻¹ and a high nitrogen content of 5.1 wt% were obtained using a low molten salt/chitosan mass ratio (3/1) and moderate calcination temperature (1000 °C). Importantly, using these N-ACs as CO₂ solid-state adsorbents, the maximum CO₂ capture capacities could be up to 7.9/5.6 mmol g⁻¹ at 0 °C/25 °C under 1 bar pressure, respectively. These CO₂ capture capacities of N-ACs were the highest compared to reported biomass-derived carbon materials, and these values were also comparable to most of porous carbon materials. Moreover, these N-ACs also showed good selectivity for CO₂/N₂ separation and excellent recyclability [43]. Chagas et al. reported a green method for CO₂ capture by showing the effects of hydrothermal carbonization (HTC) on chitosan’s chemical properties and its potential. HTC allows changes in chitosan’s surfaces and structural properties, increasing the CO₂ adsorption capacity by 4-fold compared to the non-HTC treated chitosan [44]. Kamaran et al. developed acetic acid-mediated chitosan-based porous carbons by a combination of hydrothermal carbonization treatment and chemical activation with KOH and NaOH under a flowing stream of nitrogen which developed. They noticed that would increase the CO₂ uptake to 8.36 mmol g⁻¹ for KOH samples and 7.38 mmol g⁻¹ for the NaOH sample. These synthesized carbon adsorbents also exhibited regenerability after four consecutive adsorption–desorption cycles and also high CO₂ capture selectivity over N₂ gas [45].

A non-carbonized chitosan–bleaching earth clay composite (Chi–BE) has been reported by Azharul Islam et al. as an efficient adsorbent for CO₂. They showed that temperature, adsorbent loading and CO₂ concentration exerted significantly positive effects on CO₂ adsorption by Chi–BE within the ranges and levels studied, whereas the interaction of adsorbent loading and CO₂ concentration only affected CO₂ adsorption. The optimised factors were 38.13 °C, adsorbent loading of 0.72 g and CO₂ concentration of 25%, which produced the adsorption capacity of 7.84 mmol g⁻¹ using the desirability function which was very close to the validation study and the composite can also be recycled which shows its cost-effectiveness [46]. Material type and composition, BET surface area (m² g⁻¹), pore size (nm)/total pore volume (cm³ g⁻¹), mechanism of adsorption, CO₂ capture capacity (mmol g⁻¹) and special features of chitosan-based materials have been tabulated in

Table 2. Summary of material type and composition, BET surface area (m² g⁻¹), pore size (nm)/total pore volume (cm³ g⁻¹), mechanism of adsorption, CO₂ capture capacity (mmol g⁻¹) and special features of chitosan-based materials.

Material Type and Composition	BET Surface Area (m2 g−1)	Pore Size (nm)/Total Pore Volume (cm3 g−1)	Mechanism of Adsorption	CO2 Capture Capacity (mmol g−1)	Special Features	Ref
N-doped Atcivated carbon from chitosan char by KOH activation	907	0.39	Physisorption	1.86	High CO2/N2 selectivity and excellent recyclability	[40]
N-doped carbonized chitosan	849	0.5–1.0 nm, 1.0–1.5 nm and 1.5–2.5 nm with maximum pore volume of 0.68	Physisorption	3.2	Can be used as an electrode material and adsorbent	[41]
Pyrolyzed chitosan- and chitosan-periodic mesoporous organosilica (PMO)-based porous materials	376	~2 nm, 0.346	Physisorption	1.9 at 500 kPa	Best selectivity for CO2/CH4 separation at 1.5% (m/v) of chitosan solution dried under supercritical CO2	[42]
N containing activated carbons (N-ACs) with LiCl-ZnCl2 molten salt as a template derived from cheap chitosan by carbonization.	2025	1.15	Physisorption	7.9 mmol g−1 at 0 °C/25 °C, 1 bar	Selectivity for CO2/N2 separation, excellent recyclability	[43]
Hydrothermal carbonized (HTC) of chitosan	2	-	Adsorption by the acid−base reaction between the CO2 molecule and the basic sites of the materials, associated with the presence of nitrogen atoms	0.45	-	[44]
Acetic acid-mediated chitosan-based highly porous carbon adsorbents	4168	1.386	Physisorption	8.36	CO2 selectivity over N2	[45]
Chitosan-Bleaching earth	71.26	0.19	Physisorption	7.65	Recyclable	[46]

.

3.3. Lignin

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark. Chemically, lignins are polymers made by cross-linking phenolic precursors. Zhao et al. have reported the synthesis of multiscale carbonized carbon supraparticles (SPs) by soft-templating lignin nano- and microbeads bound with cellulose nanofibrils (CNFs) which were well suited for CO₂ capture (1.75 mmol g⁻¹), while presenting a relatively low pressure drop (~33 kPa·m⁻¹ calculated for a packed fixed-bed column). Moreover, the carbon SPs did not require doping with heteroatoms for effective CO₂ uptake and also showed regeneration after multiple adsorption/desorption cycles [47]. While non-carbonized lignin-based materials have been reported by Shao et al. and Liu et al. [48,49]. They have reported lignin depolymerization selecting six aromatic units from lignin and developed O-rich hyper-cross-linked polymers (HCPs) by one-pot Friedel–Crafts alkylation reaction for CO₂ capture. Lignin-modified hyper-cross-linked nanoporous resins for efficient CO₂ capture have been described in a recent report. The resins were synthesized from lignin, 4-vinylbenzyl chloride, and divinylbenzene by free radical polymerization reaction followed by Friedel–Crafts reaction which displayed excellent CO₂ capture (1.96 mmol g⁻¹) at 273 K and 1 bar and reusability [49].

3.4. Cyclodextrins

Cyclodextrins are glucopyranosides bound together in various ring sizes renowned for their structural, physical and chemical properties. They are widely used in industrial applications due to their unique ability to encapsulate other molecules [50]. The cyclodextrin (CD)/graphene composite aerogel synthesized by hydrothermal carbonized reaction at 80 °C for 18 h exhibits an adsorption capacity of CO₂ at 1.02 mmol g⁻¹ [51]. Cyclodextrin-based non-carbonized materials have been also reported to be efficient CO₂ adsorbent [52,53,54]. Two isostructural cyclodextrin-based CD-MOFs (CD-MOF-1 and CD-MOF-2) are demonstrated to have an inverse ability to selectively capture CO₂ from C₂H₂ by single-component adsorption isotherms and dynamic breakthrough experiments. These two MOFs exhibit excellent adsorption capacity and benchmark selectivity (118.7) for CO₂/C₂H₂ mixture at room temperature [52]. In addition, a new solid acid adsorbent for CO₂ capture derived from β-cyclodextrin has been obtained and achieves a capacity of 39.87 cm³/g at 3.5 bar [53]. For thermal activation, a rapid temperature-assisted synthesis has been reported to improve the porous structure of the cyclodextrins for CO₂ adsorption [54]. A third category of cyclodextrin-based materials involves CO₂ adsorption by thermal activation under N₂ atmosphere [55,56]. The adsorption thermodynamics of CO₂ on β-CD-derived adsorbent by thermal activation have been reported recently [55]. However, the pore formation mechanism during the thermal activation of β-CD for CO₂ uptake was systematically investigated by the same group in another study [56].

4. Synthetic Porous Organic Polymers (POPs) for CO₂ Capture

Synthetic porous organic polymers are a good choice of materials for post-combustion carbon dioxide capture because of their low density, high porosity, large surface area, and high stability. Various types of chemical reactions, such as Suzuki–Miyaura coupling, Sonogashira–Hagihara coupling, Buchwald–Hartwig amination, Schiff-base reaction, Friedel–Crafts reaction, etc., are used to synthesize organic polymers. The desired pore size of the polymers is obtained by selecting monomer building blocks and linkers. Affinity for CO₂ can also be enhanced by post-synthetic modification of the polymers. Various kinds of porous organic polymers have been reported in the literature. Out of them, hyper-cross-linked polymers (HCPs), covalent organic frameworks (COFs), conjugated microporous polymers (CMPs) and covalent triazine-based frameworks (CTFs) were used extensively for CO₂ capture [57]. Syntheses and CO₂ capture capacities of these four major types of POPs published in the last five years are summarised here. In Table 3, synthetic methods, measured parameters related to CO₂ capture, CO₂ capture capacities and CO₂/N₂ selectivities of the POPs mentioned in this review are shown.

4.1. Hyper-Crosslinked Polymers (HCPs)

Friedel–Crafts and simple condensation reactions are usually employed to prepare HCPs from monomers. Extensive cross-linking between the monomers creates permanent porosity in the HCPs. In general, HCPs displayed good CO₂ capture capacity though the surface area of the majority of these types of polymers is low. Hu et al. synthesized a microporous polymer, termed PIM-1, by one-step condensation of 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane and 2,3,5,6-tetrafluoroterephthalonitrile. Hydrolysis of PIM-1 using NaOH produced the hydrolyzed form hPIM-1. Measured BET surface areas of the polymers were found to be 970 and 780 m²/g for PIM-1 and hPIM-1, respectively. Hydrophilic hPIM-1 exhibited slightly higher CO₂ uptake capacity (1.73 mmol g⁻¹ at 298 K and 1 bar) than that of PIM-1. Both polymers are also effective in the uptake of CO₂ at low partial pressure such as 0.15 bar. PIM-1 exhibited high competitive adsorption of CO₂ over N₂ at a high total pressure of 1 bar and a certain level of moisture resistance. PIM-1 can be solution-reprocessed keeping its CO₂ uptake capacity intact [58]. Fayemiwo et al. synthesized a series of nitrogen-rich HCPs, poly[methacrylamide-co-(ethylene glycol dimethacrylate)] by copolymerisation of methacrylamide (MAAM) and ethylene glycol dimethacrylate (EGDMA) in different molar ratio via radical initiated bulk polymerizations. Three polymers, termed HCP-MAAM-1, -2, -3 were obtained by increasing the concentrations of MAAM with respect to EGDMA by 1:2:3, respectively. These polymers exhibited high affinities towards CO₂ at both high and low pressure due to the presence of polar amide groups within the polymer network. CO₂ adsorption capacities were found to be 1.56, 1.45, and 1.28 mmol g⁻¹ for HCP-MAAM-1, -2, and -3, respectively, at 273 K and 1 bar. The reduction in the adsorption capacities with an increase in the concentration of MAAM with respect to EGDMA in the polymers is due to a decrease in the specific surface areas [59]. A series of hydroxyl-based HCPs was synthesized by Liu et al. via one-pot Friedel–Crafts alkylation of benzyl alcohol (BA) using formaldehyde dimethyl acetal (FDA) as external cross-linker and anhydrous FeCl₃. Pore volumes of the synthesized HCPs were found to be very sensitive to the reaction time and amounts of FeCl₃ and FDA. The HCPs obtained by use of optimized amounts of FeCl₃ and FDA in the reactions possess high BET-specific surface areas up to 1101 m²/g and exhibit high CO₂ uptake capacities up to 3.03 mmol g⁻¹ at 273 K and 1 bar [60]. Abdelnaby et al. synthesized an HCP, termed KFUPM-1, by acid-catalyzed condensation of pyrrole, 1,4-benzenediamine and p-formaldehyde. The presence of a high concentration of amine groups in the backbone of this meso-/macroporous polymer resulted in high selectivity (141) for CO₂ over N₂ and moderate CO₂ uptake capacity of 1.04 mmol g⁻¹ at 298 K and 1 bar [61]. In a similar manner, another HCP, termed KFUPM-2, was synthesized by Friedel−Crafts alkylation polymerization of phenothiazine and pyrrole (1:3 ratio) using p-formaldehyde as a cross-linker in the presence of FeCl₃ as a catalyst. This microporous polymer showed a moderate CO₂ uptake capacity of 1.04 mmol g⁻¹ with CO₂/N₂ selectivity of 51 at 298 K and 1 bar [62]. A novel ynone-linked porous organic polymer, named y-POP, was synthesized by Kong et al. by Sonogashira coupling of 1,3,5-triethynylbenzene with terephthaloyl chloride. Post-modification of y-POP by tethering alkyl amine species produced y-POP-NH₂. Increase in amine loading, the CO₂ adsorption capacity of y-POP-NH₂ gradually increased up to 1.95 mmol g⁻¹ at 273 K and 1 bar from the corresponding value of 1.34 mmol g⁻¹ of y-POP under the same conditions [63]. Cross-linking of a copolymer polydivinylbenzenechloride (PDV) was performed by reaction with anhydrous FeCl₃ and CCl₄ to produce methylene cross-linked HCP, named PDV-pc-1. Another carbonyl cross-linked HCP, named PDV-pc-2 was obtained in a similar reaction without using CCl₄. Studies showed that PDV-pc-1 has a higher BET-specific surface area (686 m²/g) than PDV-pc-2 (635 m²/g). However, the CO₂ uptake capacity of PDV-pc-2 (1.95 mmol g⁻¹) was found to be higher than PDV-pc-1 (1.45 mmol g⁻¹) at 273 K and 1 bar. Higher porosity and the presence of a large number of carbonyl functional groups made PDV-pc-2 a better CO₂-capturing agent [64]. Sharma et al. synthesized a heptazine-based microporous polymeric network, termed HMP-TAPA, by nucleophilic substitution of trichloroheptazine (TCH) by tris-(4-aminopenyl) amine (TAPA). The presence of a large number of CO₂-philic -N-, -NH, and -NH₂ groups on the surface enhanced CO₂ sorption capacity of HMP-TAPA, which exhibited a CO₂ uptake capacity of 2.42 mmol g⁻¹ at 273 K and 1 bar. In addition, this polymer catalyzed the cycloaddition of CO₂ with epoxides under mild conditions to generate cyclic carbonates with high yield and selectivity [65]. A series of N-containing HCPs was synthesized from triphenylamine (TPA) and/or carbazole (Cz) monomers by one-step cross-coupling reactions including Scholl coupling and solvent knitting Friedel–Crafts reactions. Among these microporous polymers, HCP1, HCP2, and HCP3, prepared by Scholl coupling exhibited high CO₂ uptake capacities (2.38–2.64 mmol g⁻¹ at 273 K and 1 bar) due to high porosity though measured surface areas were found to be low. On the other hand, HCP4, HCP5, and HCP6, prepared by 1,2-dichloroethane knitting Friedel–Crafts reactions were found to be meso/macroporous in nature and they exhibited comparatively low CO₂ uptake capacities (0.9–1.52 mmol g⁻¹ at 273 K and 1 bar) due to their low porosity [66]. Mohamed et al. synthesized two microporous HCPs, named TPE-CPOP1 and TPE-CPOP2, by AlCl₃ catalysed Friedel–Crafts reactions of tetraphenylethene (TPE) monomer with and without cyanuric chloride, respectively. CO₂ adsorption capacities were found to be 0.89 and 1.15 mmol g⁻¹ at 298 K and 1 bar for TPE-CPOP1 and TPE-CPOP2, respectively. Higher CO₂ uptake capacity of TPE-CPOP2 is caused by the presence of triazine units in the framework. The carbonization and KOH activation process of these HCPs produced porous carbon materials which exhibited high CO₂ uptake capacities (Figure 2a) [67]. Qiao et al. prepared two HCPs, named P0 and P1, by one-step reaction of each p-terphenyl and 4-amino-p-terphenyl monomers with AlCl₃ catalyst in dichlorometane, respectively. Further reaction of nitrobenzene and P1 in 1:3 ratios afforded another HCP, named P2. CO₂ uptake capacities of P0, P1 and P2 were found to be 3.79, 4.24, and 3.02 mmol g⁻¹ at 273 K and 1.13 bar. The remarkable CO₂ uptake capacity of P1 was attributed due to the presence of the amine groups in the polymeric network [68]. Zhou et al. synthesized a series of microporous HCPs by Friedel−Crafts polymerization of each hexaphenylbenzene (HPB), triphenylbenzene (TPB), spirobisfluorene (SBF), and triptycene (Trip) monomers catalyzed by AlCl₃ in the presence of dichloromethane, which acts as both the solvent and as a cross-linker. The polymers were functionalized further by covalently attaching −NO₂, −NH₂, and −SO₃H groups. Generally, surface functionalization of the polymers causes loss of porosity but nitro- and sulfonic acid-containing polymers retained a good amount of initial porosity. Sulfonated polymers showed high BET surface areas in the range of 1145 to 1390 m²/g and highest CO₂ uptake capacity reached to 6.77 mmol g⁻¹ at 273 K and 1 bar [69]. Abdelnaby et al. synthesized two azo-linked porous organic polymers, termed man-Azo-P1 and man-Azo-P2, by diazotization reactions of 4,4′-diaminobiphenyl (benzidine) and 4,4′-diaminodianiline, respectively, with phloroglucinol in aqueous medium at 0 °C. The CO₂ uptake capacities were found to be 1.43 and 0.89 mmol g⁻¹ at 273 K and 1 bar for man-Azo-P1 and man-Azo-P2, respectively. The former exhibited better CO₂ uptake capacity due to the presence of polar azo and hydroxy functional groups in the framework [70].

4.2. Covalent Organic Frameworks (COFs)

Precise control of the structure and pore size of COFs is performed by choosing the monomer building blocks and reaction conditions. Gao et al. synthesized two 2D-COFs by condensation of amine and aldehyde functionalized tetraphenylethane (TPE). Solvent controlled [4 + 4] condensation produced TPE-COF-I and an unusual [2 + 4] condensation pathway produced TPE-COF-II. TPE-COF-II exhibited higher CO₂ capture capacity (5.3 mmol g⁻¹ at 273 K and 1 atm) than TPE-COF-I (3.06 mmol g⁻¹) due to the presence of unreacted-CHO groups in the framework [71]. Li et al. designed and synthesized a metalloporphyrin-containing COF by solvothermal reaction of cobalt(II)-5,10,15,20-tetrakis(4-aminophenyl)porphyrin (Co(II)@TAPP) and tetrakis(4-formylphenyl)pyrene (TFPPy). The COF captures CO₂ and catalytically converts it into cyclic carbonates under mild conditions. BET surface area of the microporous COF was found to be 1076 m²/g and the pore size was 1.6 nm. High surface area, good stability and the presence of a single type of micropores made it a good catalyst. The COF exhibited a strong CO₂ adsorption capacity of 3.84 mmol g⁻¹ (16.9 wt%) at 298 K and 1 bar. Co(II)@TAPP units in the COF are alternately stacked perpendicular to the porphyrin planes with a slipped distance of 1.7 Å which fits with the size of CO₂. Adsorbed CO₂ molecules interact effectively with the metal centres (catalytic sites) and facilitate catalytic reactions (Figure 2b) [72]. Lyu et al. established a new synthetic strategy to covalently attach aliphatic amines to construct COFs. First, an imine-linked COF, named COF-609-Im, was synthesized through imine condensation between 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) and 4,4′-diaminobenzanilide (DABA). Crystallization of COF-609-Im, followed by conversion of its imine linkage to base-stable tetrahydroquinoline (THQ) linkage through aza-Diels−Alder cycloaddition produced COF-609-THQ-Im. Finally, the covalent incorporation of tris(3-aminopropyl)amine (TRPN) into the framework produced COF-609. All three COFs are porous amorphous in nature and the BET surface area of COF-609-Im was found to be 724 m²/g. CO₂ capture capacity of COF-609 was found to be 6.8 cm³/g (0.304 mmol g⁻¹) which is 1360 times higher compared to that of COF-609-THQ-Im at 0.4 mbar CO₂ at 273 K. Further 29% increase in CO₂ capture was observed in the presence of humidity. This condition is comparable to direct air capture of CO₂. Strong chemisorptions of CO₂ by aliphatic amines incorporated into COFs made these sorbents such efficient capturing agents at low CO₂ pressures. These three COFs also exhibited excellent CO₂ capture capacities at 40 mbar (comparable to post-combustion capture from natural gas burned flue gas) and 150 mbar (comparable to post-combustion capture from coal-fired flue gas) pressures of CO₂ [73].

4.3. Conjugated Microporous Polymers (CMPs)

Usually, CMPs are prepared by coupling/cross-coupling reactions of aromatic monomers by many well-known reactions, such as, Suzuki, Sonogashira, Yamamoto cross-coupling reactions. Pore size, surface areas and CO₂-philic nature of the polymers can be tuned by proper selection of monomers, reaction type and post-synthetic modifications [74]. Wang et al. synthesized three novel biphenylene-based CMPs, termed CMP-LS1, CMP-LS2 and CMP-LS3, by palladium-catalyzed Suzuki and Sonogashira–Hagihara cross-coupling reactions of 3,4′,5-tribromobiphenyl (TBBP) with each 1,4-phenylenediboronic acid, 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene and 1,3,5-triethynylbenzene, respectively. BET surface areas of these porous polymers were found to be 493, 1576 and 643 cm²/g for CMP-LS1, CMP-LS2 and CMP-LS3, respectively. Among the three CMPs, CMP-LS2 exhibited highest CO₂ adsorption capacity of 3.88 mmol g⁻¹ at 273 K and 1 bar due to its large surface area [75]. By oxidative homocoupling of 1,3,6,8-tetraethynylpyrene monomer using Pd(II)-Cu(I) catalysts Ren et al. prepared a CMP, named LKK-CMP-1. This 1,3-diyne linked CMP exhibited moderate CO₂ uptake capacity (2.22 mmol g⁻¹ at 273 K and 1 bar) (Figure 2c) [76]. Saber et al. synthesized two azo-linked CMPs, termed Azo-Cz-CMP and Azo-Tz-CMP, by reduction of the corresponding monomers 3,6-dinitro-9-(4-nitrophenyl) carbazole (Cz-3NO₂) and 3,7-dinitro-10-(4-nitrophenyl)-10H-phenothiazine (Tz-3NO₂), respectively, using sodium borohydride (NaBH₄). BET surface areas of both CMPs were found to be low and pore sizes centered at 0.79 and 1.18 nm for Azo-Cz-CMP and Azo-Tz-CMP, respectively. The former exhibited higher CO₂ capture value of 2.13 mmol g⁻¹ at 298 K and 1 bar than the later which showed 1.36 mmol g⁻¹ CO₂ capture capacity under same conditions [77].

Table 3. Summary of synthetic process, surface area, pore size, total pore volume, CO₂ uptake capacity, CO₂/N₂ selectivity and heat of adsorption of the synthetic POPs.

POPs	Synthetic Process	SBET a	Pore Size b	Vtot c	CO2 Capture Capacity d		CO2/N2 Selectivity e		Qst f	Ref.
					273 K	298 K	273 K	298 K
PIM-1	One-step condensation in presence of K2CO3	970	<2, 2–50	0.70	-	1.66	-	19.3	20.8	[58]
hPIM-1	Hydrolyzation of PIM-1 using NaOH	780	<2, 2–50	0.49	-	1.73	-	11.7	32.8	[58]
HCP-MAAM-1	Radical initiated bulk copolymerization	298	2–40	0.47	1.56	0.92	45–86	38–48	28–35	[59]
HCP-MAAM-2	Radical initiated bulk copolymerization	142	2–40	0.87	1.45	0.85	50–99	38–63	28–35	[59]
HCP-MAAM-3	Radical initiated bulk copolymerization	83	2–40	0.24	1.28	0.79	52–104	45–72	28–35	[59]
BAHCP-7	Friedel-Crafts alkylation polymerization	1101	1.7	1.15	3.03	1.96	35	-	26–28	[60]
KFUPM-1	Acid catalyzed polycondensation	305	-	-	1.52	1.04	-	141	34	[61]
KFUPM-2	Friedel−Crafts alkylation polymerization	352	-	0.21	1.75	1.04	-	51	34	[62]
y-POP	Sonogashira coupling	226	0.74, 1.2, 34	-	1.34	-	20	-	29	[63]
y-POP-A1	Amine modification of y-POP	145	-	-	1.50	-	239	-	46.8	[63]
PDV	Radical polymerzation	364	1–2	0.20	0.66	0.25	31.3	-	36.9	[64]
PDV-pc-1	Friedel−Crafts reaction of PDV	686	1–2	0.37	1.45	0.59	16.4	-	34.3	[64]
PDV-pc-2	Friedel−Crafts reaction of PDV	635	1–2	0.33	1.95	0.80	46.8	-	39.7	[64]
HMP-TAPA	Polymerization via nucleophilic substitution reaction	424	0.7–1.2, 2–4	-	2.42	-	26.27	30.79	32.8	[65]
HCP1	Scholl coupling	534.5	-	0.32	2.64	1.57	23.6	-	46.7	[66]
HCP2	Scholl coupling	215.7	-	0.11	2.38	1.51	30.2	-	28.0	[66]
HCP3	Scholl coupling	199.9	-	0.12	2.47	1.46	26.7	-	36.7	[66]
HCP4	Friedel−Crafts alkylation polymerization	10.8	-	0.023	1.05	0.72	8.6	-	26.2	[66]
HCP5	Friedel−Crafts alkylation polymerization	34.8	-	0.065	1.52	0.72	15.4	-	39.8	[66]
HCP6	Friedel−Crafts alkylation polymerization	30.3	-	0.061	0.90	0.42	7.0	-	33.0	[66]
TPE-CPOP1	Friedel-Crafts polymerization	489	1.49, 1.82	0.269	0.99	0.89	-	-	-	[67]
TPE-CPOP2	Friedel-Crafts polymerization	146	2.57	0.1	1.26	1.15	-	-	-	[67]
TPE-CPOP1-800	Carbonization and KOH activation of TPE-CPOP1	1177	1.04, 2.99	0.48	3.19	1.74	-	-	-	[67]
TPE-CPOP2-800	Carbonization and KOH activation of TPE-CPOP2	1165	1.02, 2.29	0.62	2.93	1.72	-	-	-	[67]
P0	Friedel-Crafts polymerization	1062	5.65	0.69	3.79	-	-	18.28	24–32	[68]
P1	Friedel-Crafts polymerization	447	1.91	0.21	4.24	-	-	20.97	24–32	[68]
P2	Condensation polymerization using base	242	1.94	0.12	3.02	-	-	34.52	24–32	[68]
PIM-TPB	Friedel−Crafts polymerization	2540	0.35, 0.56, 0.86	1.300	5.00	2.57	-	14.1	25.2	[69]
PIM-TPB-NO2	-NO2 functionalization of PIM-TPB using HNO3	950	0.35, 0.56, 0.86	0.553	5.13	3.11	-	24.7	32.1	[69]
PIM-TPB-NH2	-NH2 functionalization by Na2S2O4 treatment of PIM-TPB-NO2	710	0.35, 0.56, 0.86	0.333	4.45	2.98	-	26.1	31.7	[69]
PIM-TPB-HSO3	-SO3H functionalization of PIM-TPB using H2SO4	1585	0.35, 0.56, 0.86	0.852	6.77	4.07	-	17.9	29.0	[69]
man-Azo-P1	Diazotization of aromatic diamines followed by coupling with aromatic alcohol	290	-	0.33	1.43	-	80	-	40	[70]
man-Azo-P2	Diazotization of aromatic diamines followed by coupling with aromatic alcohol	78	-	0.15	0.89	-	110	-	23	[70]
TPE-COF-I	Acid catalysed condensation	1535	-	1.65	3.06	1.69	-	-	-	[71]
TPE-COF-II	Acid catalysed condensation	2168	-	2.14	5.30	2.70	-	-	-	[71]
Co(II)@TA-TF COF	Solvothermal reaction	1076	1.6	-	-	3.84	-	-	-	[72]
COF-609-Im	Acid catalysed condensation	724	-	-	1.5	-	-	-	-	[73]
COF-609	aza-Diels−Alder cycloaddition of COF-609-Im followed by amination	-	-	-	0.076	-	-	-	-	[73]
CMP-LS1	Suzuki coupling	493	0.4–1.4	0.32	1.38	0.76	23.2	-	30.2	[75]
CMP-LS2	Suzuki coupling	1576	0.4–1.4	1.06	3.88	2.1	27.9	-	31.6	[75]
CMP-LS3	Sonogashira-Hagihara coupling	643	0.4–1.4	0.37	1.88	1.07	19.8	-	30.4	[75]
LKK-CMP-1	Oxidative homocoupling	467	0.59	0.371	2.22	1.38	-	44.2	35	[76]
Azo-Cz-CMP	One-pot reductive reaction using NABH4	315	0.79	-	2.13	0.91	-	-	32.08	[77]
Azo-Tz-CMP	One-pot reductive reaction using NABH4	225	1.18	-	1.36	0.64	-	-	18.36	[77]
TrzPOP-1	Polycondensation	995	1.7	-	6.19	3.53	108.4	42.1	29	[78]
TrzPOP-2	Polycondensation	868	1.5	-	7.51	4.52	140.6	75.7	34	[78]
TrzPOP-3	Polycondensation	772	1.4	-	8.54	5.09	167.4	94.5	37	[78]
NT-POP-5	Suzuki cross-coupling	8	-	-	0.78	-	-	-	25.4–19.4	[79]
NT-POP@800-4	Pyrolysis of NT-POP-1-6 at 800 oC	736	-	0.463	3.96	3.25	36.9	-	25.4–19.4	[79]
CTF1	ZnCl2 catalyzed ionothermal reaction	1654	-	1.06	5.23	3.32	-	11	34.0	[80]
CTF4	ZnCl2 catalyzed ionothermal reaction	784	-	0.41	4.39	3.83	-	46	21.5	[80]
CTF-DCE	ZnCl2 catalyzed ionothermal reaction	1355	0.6, 1.2, 2–4	0.93	4.34	3.59	54	-	24.9	[81]
CTF-PF-4	ZnCl2 catalyzed ionothermal reaction	889	1.7–1.9	0.58	2.0	1.27	-	-	>33	[82]
ICTF-Cl	ZnCl2 catalyzed ionothermal reaction	751	-	0.458 (approx.)	2.36	1.41	119.1	68.74	-	[83]
ICTF-SCN	ZnCl2 catalyzed ionothermal reaction	1000 (approx.)	-	0.458 (approx.)	2.48	1.40	39.28	24.82	-	[83]
CTF-N4	ZnCl2-mediated cyclotrimerization	701	-	0.31	3.4	2.2	45	-	44	[84]
CTF-N6	ZnCl2-mediated cyclotrimerization at high temperature	1236	-	0.51	5.0	3.4	36	-	26	[84]
CTF-hex4	ZnCl2-mediated ionothermal reaction	609	-	0.31	3.4	-	-	-	29	[85]
CTF-hex6	ZnCl2-mediated ionothermal reaction	1728	-	0.87	3.1	-	-	-	37	[85]
An-CTF-20-500	ZnCl2-mediated ionothermal reaction	700	1.06, 1.66	-	5.25	2.69	-	-	-	[86]

^a BET surface area (m² g⁻¹). ^b Pore size (nm). ^c Total pore volume (cm³ g⁻¹). ^d CO₂ capture capacity (mmol g⁻¹) at 1 bar. ^e IAST (ideal adsorbed solution theory) for the mixture including 85% of N₂ and 15% of CO₂ at 1 bar. ^f Heat of absorption (kJ mol⁻¹) of CO₂ (calculated using Clausius-Clapeyron equation at low CO₂ loading).

Table 3. Summary of synthetic process, surface area, pore size, total pore volume, CO₂ uptake capacity, CO₂/N₂ selectivity and heat of adsorption of the synthetic POPs.

POPs	Synthetic Process	SBET a	Pore Size b	Vtot c	CO2 Capture Capacity d		CO2/N2 Selectivity e		Qst f	Ref.
					273 K	298 K	273 K	298 K
PIM-1	One-step condensation in presence of K2CO3	970	<2, 2–50	0.70	-	1.66	-	19.3	20.8	[58]
hPIM-1	Hydrolyzation of PIM-1 using NaOH	780	<2, 2–50	0.49	-	1.73	-	11.7	32.8	[58]
HCP-MAAM-1	Radical initiated bulk copolymerization	298	2–40	0.47	1.56	0.92	45–86	38–48	28–35	[59]
HCP-MAAM-2	Radical initiated bulk copolymerization	142	2–40	0.87	1.45	0.85	50–99	38–63	28–35	[59]
HCP-MAAM-3	Radical initiated bulk copolymerization	83	2–40	0.24	1.28	0.79	52–104	45–72	28–35	[59]
BAHCP-7	Friedel-Crafts alkylation polymerization	1101	1.7	1.15	3.03	1.96	35	-	26–28	[60]
KFUPM-1	Acid catalyzed polycondensation	305	-	-	1.52	1.04	-	141	34	[61]
KFUPM-2	Friedel−Crafts alkylation polymerization	352	-	0.21	1.75	1.04	-	51	34	[62]
y-POP	Sonogashira coupling	226	0.74, 1.2, 34	-	1.34	-	20	-	29	[63]
y-POP-A1	Amine modification of y-POP	145	-	-	1.50	-	239	-	46.8	[63]
PDV	Radical polymerzation	364	1–2	0.20	0.66	0.25	31.3	-	36.9	[64]
PDV-pc-1	Friedel−Crafts reaction of PDV	686	1–2	0.37	1.45	0.59	16.4	-	34.3	[64]
PDV-pc-2	Friedel−Crafts reaction of PDV	635	1–2	0.33	1.95	0.80	46.8	-	39.7	[64]
HMP-TAPA	Polymerization via nucleophilic substitution reaction	424	0.7–1.2, 2–4	-	2.42	-	26.27	30.79	32.8	[65]
HCP1	Scholl coupling	534.5	-	0.32	2.64	1.57	23.6	-	46.7	[66]
HCP2	Scholl coupling	215.7	-	0.11	2.38	1.51	30.2	-	28.0	[66]
HCP3	Scholl coupling	199.9	-	0.12	2.47	1.46	26.7	-	36.7	[66]
HCP4	Friedel−Crafts alkylation polymerization	10.8	-	0.023	1.05	0.72	8.6	-	26.2	[66]
HCP5	Friedel−Crafts alkylation polymerization	34.8	-	0.065	1.52	0.72	15.4	-	39.8	[66]
HCP6	Friedel−Crafts alkylation polymerization	30.3	-	0.061	0.90	0.42	7.0	-	33.0	[66]
TPE-CPOP1	Friedel-Crafts polymerization	489	1.49, 1.82	0.269	0.99	0.89	-	-	-	[67]
TPE-CPOP2	Friedel-Crafts polymerization	146	2.57	0.1	1.26	1.15	-	-	-	[67]
TPE-CPOP1-800	Carbonization and KOH activation of TPE-CPOP1	1177	1.04, 2.99	0.48	3.19	1.74	-	-	-	[67]
TPE-CPOP2-800	Carbonization and KOH activation of TPE-CPOP2	1165	1.02, 2.29	0.62	2.93	1.72	-	-	-	[67]
P0	Friedel-Crafts polymerization	1062	5.65	0.69	3.79	-	-	18.28	24–32	[68]
P1	Friedel-Crafts polymerization	447	1.91	0.21	4.24	-	-	20.97	24–32	[68]
P2	Condensation polymerization using base	242	1.94	0.12	3.02	-	-	34.52	24–32	[68]
PIM-TPB	Friedel−Crafts polymerization	2540	0.35, 0.56, 0.86	1.300	5.00	2.57	-	14.1	25.2	[69]
PIM-TPB-NO2	-NO2 functionalization of PIM-TPB using HNO3	950	0.35, 0.56, 0.86	0.553	5.13	3.11	-	24.7	32.1	[69]
PIM-TPB-NH2	-NH2 functionalization by Na2S2O4 treatment of PIM-TPB-NO2	710	0.35, 0.56, 0.86	0.333	4.45	2.98	-	26.1	31.7	[69]
PIM-TPB-HSO3	-SO3H functionalization of PIM-TPB using H2SO4	1585	0.35, 0.56, 0.86	0.852	6.77	4.07	-	17.9	29.0	[69]
man-Azo-P1	Diazotization of aromatic diamines followed by coupling with aromatic alcohol	290	-	0.33	1.43	-	80	-	40	[70]
man-Azo-P2	Diazotization of aromatic diamines followed by coupling with aromatic alcohol	78	-	0.15	0.89	-	110	-	23	[70]
TPE-COF-I	Acid catalysed condensation	1535	-	1.65	3.06	1.69	-	-	-	[71]
TPE-COF-II	Acid catalysed condensation	2168	-	2.14	5.30	2.70	-	-	-	[71]
Co(II)@TA-TF COF	Solvothermal reaction	1076	1.6	-	-	3.84	-	-	-	[72]
COF-609-Im	Acid catalysed condensation	724	-	-	1.5	-	-	-	-	[73]
COF-609	aza-Diels−Alder cycloaddition of COF-609-Im followed by amination	-	-	-	0.076	-	-	-	-	[73]
CMP-LS1	Suzuki coupling	493	0.4–1.4	0.32	1.38	0.76	23.2	-	30.2	[75]
CMP-LS2	Suzuki coupling	1576	0.4–1.4	1.06	3.88	2.1	27.9	-	31.6	[75]
CMP-LS3	Sonogashira-Hagihara coupling	643	0.4–1.4	0.37	1.88	1.07	19.8	-	30.4	[75]
LKK-CMP-1	Oxidative homocoupling	467	0.59	0.371	2.22	1.38	-	44.2	35	[76]
Azo-Cz-CMP	One-pot reductive reaction using NABH4	315	0.79	-	2.13	0.91	-	-	32.08	[77]
Azo-Tz-CMP	One-pot reductive reaction using NABH4	225	1.18	-	1.36	0.64	-	-	18.36	[77]
TrzPOP-1	Polycondensation	995	1.7	-	6.19	3.53	108.4	42.1	29	[78]
TrzPOP-2	Polycondensation	868	1.5	-	7.51	4.52	140.6	75.7	34	[78]
TrzPOP-3	Polycondensation	772	1.4	-	8.54	5.09	167.4	94.5	37	[78]
NT-POP-5	Suzuki cross-coupling	8	-	-	0.78	-	-	-	25.4–19.4	[79]
NT-POP@800-4	Pyrolysis of NT-POP-1-6 at 800 oC	736	-	0.463	3.96	3.25	36.9	-	25.4–19.4	[79]
CTF1	ZnCl2 catalyzed ionothermal reaction	1654	-	1.06	5.23	3.32	-	11	34.0	[80]
CTF4	ZnCl2 catalyzed ionothermal reaction	784	-	0.41	4.39	3.83	-	46	21.5	[80]
CTF-DCE	ZnCl2 catalyzed ionothermal reaction	1355	0.6, 1.2, 2–4	0.93	4.34	3.59	54	-	24.9	[81]
CTF-PF-4	ZnCl2 catalyzed ionothermal reaction	889	1.7–1.9	0.58	2.0	1.27	-	-	>33	[82]
ICTF-Cl	ZnCl2 catalyzed ionothermal reaction	751	-	0.458 (approx.)	2.36	1.41	119.1	68.74	-	[83]
ICTF-SCN	ZnCl2 catalyzed ionothermal reaction	1000 (approx.)	-	0.458 (approx.)	2.48	1.40	39.28	24.82	-	[83]
CTF-N4	ZnCl2-mediated cyclotrimerization	701	-	0.31	3.4	2.2	45	-	44	[84]
CTF-N6	ZnCl2-mediated cyclotrimerization at high temperature	1236	-	0.51	5.0	3.4	36	-	26	[84]
CTF-hex4	ZnCl2-mediated ionothermal reaction	609	-	0.31	3.4	-	-	-	29	[85]
CTF-hex6	ZnCl2-mediated ionothermal reaction	1728	-	0.87	3.1	-	-	-	37	[85]
An-CTF-20-500	ZnCl2-mediated ionothermal reaction	700	1.06, 1.66	-	5.25	2.69	-	-	-	[86]

^a BET surface area (m² g⁻¹). ^b Pore size (nm). ^c Total pore volume (cm³ g⁻¹). ^d CO₂ capture capacity (mmol g⁻¹) at 1 bar. ^e IAST (ideal adsorbed solution theory) for the mixture including 85% of N₂ and 15% of CO₂ at 1 bar. ^f Heat of absorption (kJ mol⁻¹) of CO₂ (calculated using Clausius-Clapeyron equation at low CO₂ loading).

4.4. Covalent Triazine-Based Frameworks (CTFs)

High nitrogen content of the aromatic triazine (C₃N₃) rings in the CTFs enhances affinity for CO₂. Furthermore, high stability and abundant micropores in the surface made CTFs potential CO₂ capturing agents [5]. Das et al. synthesized three CTFs, namely TrzPOP-1, -2 and -3, via polycondensation of two tetraamine bearing triazine ring and three different dialdehydes (two of them contain phenolic –OH groups). TrzPOP-1, -2 and -3 possess high BET surface areas of 995, 868 and 772 m²/g, respectively, and they exhibited high CO₂ uptake capacities of 6.19, 7.51 and 8.54 mmol g⁻¹, respectively, at 273 K and 1 bar. Though BET surface areas of TrzPOP-2 and TrzPOP-3 are comparatively low still they exhibited high CO₂ uptakes because of the presence of phenolic –OH groups [78]. Yao et al. synthesized a series of six CTFs, termed NTPOP-1 to -6, by Suzuki cross-coupling driven polycondensation of N2, N4, N6-tris(4-bromophenyl)-1,3,5-triazine-2,4,6-triamine (TPTT) and a number of benzeneboronic monomers or ethynyl monomers. BET surface areas of the NTPOPs were found to be in the lower side and highest CO₂ uptake was 0.78 mmol g⁻¹ at 273 K and 1.05 bar. Carbonization of these NTPOPs at 800 °C produced pore-tunable porous carbon materials which exhibited excellent CO₂ adsorption capacity of 2.83–3.96 mmol g⁻¹ at 273 K and 1.05 bar [79]. A set of five CTFs (CTF1–5) were prepared by ionothermal reactions of dicyano-aryl or heteroaryl monomer and molten ZnCl₂ in 1:5 molar ratio at temperature 400 °C (first 10 h) and 600 °C (next 10 h). Obtained CTFs were found to be bimodal micro-mesoporous in nature and they displayed high specific surface areas (up to 1860 m²/g). Selected polymers of this series displayed excellent CO₂ uptake and highest uptake value was found to be 5.23 and 3.83 mmol g⁻¹ at 273 and 298 K, respectively, at ambient pressure [80]. In a similar way, Dang et al. synthesized a CTF, termed CTF-DCE, via ZnCl₂ catalyzed ionothermal trimerization of di(4-cyanophenyl)ethyne. CTF-DCE displayed high BET surface area of 1355 m²/g and excellent CO₂ capture capacity of 4.34 mmol g⁻¹ at 273 K and 1 bar [81]. Utilizing the similar strategy, a series of four CTFs based on imidazolium salts were synthesized by Xu et al. via ionothermal reactions of nitriles and ZnCl₂ in different ratios at 400 °C. The obtained CTFs, were termed as CTF-Cl-1, CTF-Cl-2, CTF-PF-3, and CTF-PF-4 based on the type and number of counterions (Cl⁻ and PF₆⁻) present. These CTFs displayed high BET surface areas. Pore volumes and sizes of the CTFs can be controlled by simply exchange of counterions. CTF-PF-4 containing highest PF₆⁻ content, showed highest CO₂ adsorption of 2.0 mmol g⁻¹ [82]. Zhu et al. reported synthesis of a series of bipyridinium-based ionic covalent triazine frameworks (ICTFs) with anions Cl⁻ and SCN⁻ through ZnCl₂ catalyzed ionothermal polymerization. High specific surface area, microporous structure, ionic nature and high nitrogen content made them excellent CO₂ capturing agents. The surface area and porosity can be regulated by adjusting the anions. Both ICTF-SCN and ICTF-Cl showed high CO₂ uptake capacities of 2.48 and 2.36 mmol g⁻¹ at 273 K and 1 bar, respectively [83]. Three PhNH-, PhO-, and PhS-linked CTFs were synthesized by Liao et al. via ZnCl₂ mediated cyclotrimerization of nitrile-containing monomers including 2,4,6-tris(4-cyanophenylamino)-1,3,5-triazine (TAT), 2,4,6-tris(4-cyanophenoxy)-1,3,5-triazine (TOT), and 2,4,6-tris(4-cyanobenzenesulfenyl)-1,3,5-triazine (TST) by stepwise heating method. These microporous CTFs possess high BET surface areas and found to be excellent CO₂ sorbents. The PhNH-linked CTF prepared at high temperature (600 °C) displayed very high CO₂ adsorption capacity (5.0 mmol g⁻¹ at 273 K and 1 bar). The CO₂ capture performances of the three CTFs were found to be in the order of PhNH- > PhO- > PhS-linked CTF [84]. Wessely et al. reported synthesis of a series of CTFs using pseudo-octahedral hexanitrile 1,4-bis(tris(4′-cyanophenyl) methyl) benzene (BTB-nitrile) monomer. Among these, CTF-hex6 was prepared under ionothermal reaction conditions with ZnCl₂ at 400 °C and CTF-hex1 was prepared under mild reaction conditions with the strong Brønsted acid trifluoromethanesulfonic acid at room temperature. In addition, the BTB-nitrile was combined with different di-, tri-, and tetranitriles as a second linker under ionothermal reactions under the same conditions produced mixed-linker CTFs, named CTF-hex2-6. These CTFs displayed BET surface areas in a wide range of 493 m²/g to 1728 m²/g. They exhibited CO₂ capture capacities in the range 2.5 to 3.4 mmol g⁻¹ at 273 K and 1 bar (Figure 2d) [85]. A series of porous covalent triazine framework (An-CTFs) based on 9,10 dicyanoanthracene (An-CN) units was prepared by Mohamed et al. via ionothermal reactions of AnCN and molten ZnCl₂ in two different molar ratios (1:10 and 1:20) at two different temperatures (400 °C and 500 °C). These microporous highly stable An-CTFs possessing moderate BET surface areas ranging from 406 to 751 m²/g exhibited high CO₂ adsorption capacity up to 5.65 mmol g⁻¹ at 273 K and 1 bar [86].

Figure 2. Types of synthetic porous organic polymers (POPs) with selected examples: (a) TPE-CPOP1. Adapted with permission from [67]. (b) Co(II) @ TA-TF COF. Adapted with permission from [72]. (c) LKK-CMP-1. Adapted with permission from [76]. (d) CTF-hex1–6. Adapted with permission from [85].

Figure 2. Types of synthetic porous organic polymers (POPs) with selected examples: (a) TPE-CPOP1. Adapted with permission from [67]. (b) Co(II) @ TA-TF COF. Adapted with permission from [72]. (c) LKK-CMP-1. Adapted with permission from [76]. (d) CTF-hex1–6. Adapted with permission from [85].

5. Conclusions and Outlook

In this review, we have summarized synthesis, CO₂ capture capacities and influential factors behind the CO₂-philicity of some novel natural and synthetic POPs reported in the literature in the last five years. Though CO₂ capture capacities of biopolymers are generally low but microporous and nanoporous materials derived from them exhibited superior adsorption capacity. Particularly, nanocellulose-based membranes were found to be potential candidates for large-scale capture of CO₂ and separation from flue gas. On average, synthetic POPs including HCPs, COFs, CMPs and CTFs showed higher CO₂ uptake capacity and selectivity than porous carbonaceous materials. POPs played a major role in the research of developing new materials for post-combustion CO₂ capture and separation. In general, POPs with high surface area (>1000 m²/g), micropore, and presence of CO₂-philic functional groups (such as -NH₂, -OH, etc.) on the surface are proven to be promising candidates for CO₂ capture. A potential solid adsorbent for large-scale CO₂ capture should show >2 mmol g⁻¹ CO₂ uptake, >100 CO₂/N₂ selectivity and good moisture resistivity. At the same time, large-scale production of the adsorbent must be cost-effective.

Although good progress has been achieved so far in CO₂ capture using POPs, there are still a lot of challenges. The limited solubility of many biopolymers (cellulose, chitin, lignin, etc.) in common solvents hinders surface modification and processability. As a result, difficulties arise in the preparation of POPs membranes which are highly useful for large-scale CO₂ uptake and separation. Many POPs were synthesized using costly monomers and metal catalysts which is a concern for their large-scale applications. The partial pressure of CO₂ in flue gas is as low as 3–15 kPa and the temperature is in the range of 80–90 °C. Hence, the CO₂ capture capacities of POPs at low pressure and high temperature should be improved.

The development of new-generation CO₂ capture materials with POPs and biopolymers requires fine-tuning the thermodynamics of the interaction between CO₂ and the adsorbent to improve the energy efficiency of CO₂ capture. In most cases, the CO₂ capture efficiency of the adsorbents was evaluated by single-component CO₂ uptake isotherms or breakthrough experiments using a CO₂/N₂ mixed gas. The presence of other minor gases (O₂, CO, SO_x, NO_x) and water vapour in the flue gas may have significant consequences on the performance of the materials. This issue must be taken into account during the development of new-generation adsorbents. To decrease the cost of scaling up the materials for industrial applications new building blocks, synthetic routes and simple ways of post-functionalization should be explored. To improve the CO₂ capture capacities of POPs a lot of effort has already been made. The very fast progress in this field of research strongly indicates that synthetic and biopolymer-based materials will play a major role in developing next-generation CO₂ capturing agents to achieve the sustainable development goals (SDGs).