Progress in Post-Combustion Carbon Dioxide Capture, Direct Air Capture, and Utilization

Abstract

Carbon dioxide (CO₂) released from natural or even anthropogenic sources may lead to an increase in the average global temperature and ultimately a climate shift. Thus, protecting the environment by reducing CO₂ concentration is a global concern. The recent approach of integrating CO₂ through capture, utilization, and storage seems to be an effective eradication technique. Even though a wide range of CO₂ capture strategies have been successfully adopted, there is an urgent need to compare these based on their advantages and weaknesses to define the development direction for future perspectives. Several materials have been used for capturing CO₂. Thus, we have elaborated and compared the current state of CO₂ capture strategies, including post-combustion CO₂ capture and direct air CO₂ capture. Strategies adopted under post-combustion CO₂ capture, including liquid- and solid-based sorbents, membrane-based separators, and electrochemical reduction, have been explained in detail, including their advantages, weaknesses, and potential risks. Thus, this review presents a thorough analysis of CO₂ capture in terms of material developments and manufacturing techniques, with some research gaps for future development.

1. Introduction

Despite the incipient climate crisis, the world continues to invest in fossil fuels, and by 2030, their use may rise by 2% annually, thus resulting in a temperature rise of 1.5 °C, according to the Production Gap Report 2020 [1]. Climate change patterns and an increase in the average world temperature are both significantly impacted by the growing level of carbon dioxide (CO₂) in the atmosphere [2]. The average atmospheric concentration of CO₂ in 2020 was over 400 ppm, the highest level in the last 800,000 years [3]. The burning of fossil fuels to meet the world’s energy needs is the primary source of CO₂ emissions [4], thus affecting climate change [5]. Significant attempts have been made in recent years by applying various strategies to control CO₂ levels even while supplying the growing energy demand on a worldwide scale. CO₂ capture is one of the most advanced climate mitigation options for reducing aggregated CO₂ concentration in the atmosphere and thereby limiting the detrimental consequences of climate change [6]. In general, CO₂ capture systems comprise adsorption, biochemical, and membrane technologies that can efficiently collect CO₂ from flue gases (post-combustion), capture CO₂ upstream to avoid its release (pre-combustion), or even capture it from the ambient air [7]. Numerous materials have been thoroughly investigated for CO₂ collection, including metal–organic frameworks (MOFs), zeolite, carbon, supported solid amines, ionic liquids, and other sorbents [8].

Amine-based solutions have been widely used due to their high CO₂ absorption capacities and established technological frameworks [9,10,11]. Monoethanolamine (MEA) is the most widely used amine adsorbent for capturing CO₂. Amine-based absorbents have been a staple in CO₂ capture for the past five decades [9]. The mechanism is based on chemical absorption. However, amine-based adsorbents often suffer from high energy requirements for regeneration and from potential degradation over time [10].

Another significant challenge is how to handle the captured CO₂. We can employ it to extract oil and create useful chemicals, fuels, and polymers; we can clean industrial waste using alkaline solutions; or we can infuse captured CO₂ into geological formations and the ocean [12]. Geological storage and CO₂ mineralization are the two primary methods for permanently storing the acquired CO₂ [13]. In the former method, CO₂ is pumped underground to be stored; in the latter method, CO₂ reacts with minerals that contain calcium oxide (CaO) or magnesium oxide (MgO) to produce stable carbonates that can hold CO₂ over millions of years [14]. CO₂ utilization is as crucial for carbon neutrality as for CO₂ capture and storage [15]. Numerous technologies have been applied for CO₂ utilization, including hydrogenation, electrochemical, bio-electrochemical, and photochemical reduction, CO₂ conversion to polymers, and so on [16]. Significant improvements have been made in scientific knowledge and technology development of different CO₂ capture, storage, and usage solutions [17]. This review offers a thorough analysis of CO₂ capture as well as recent developments in materials and manufacturing techniques. It also covers the benefits, drawbacks, and potential risks associated with various technological options and their techno-economics.

2. CO₂ Emissions: Sources and Their Impact on the Environment

CO₂ levels in the atmosphere rise due to a variety of CO₂ emission sources, mainly divided into two categories: natural and human [18]. Natural causes include decomposition, animal and plant respiration, ocean discharge, and respiratory and volcanic activity. In contrast, the production of cement, clearing of forests, electricity, public transit, manufacturing, and the combustion of fossil fuels are all examples of anthropogenic emission sources [19]. Generally, energy sectors are responsible for one-third of the worldwide CO₂ emissions, and approximately 70% of it comes from fossil fuels [20]. Another significant source of CO₂ emissions is wildfires, which release an extra 7.3 to 9.5 gigatons (Gt) of CO₂ into the atmosphere. It is noteworthy that total CO₂ emissions from naturally occurring processes are about 20 times greater than anthropogenic CO₂ emissions [21].

3. CO₂ Capture Strategies

CO₂ capture poses a plethora of challenges due to distinct sources of emission with varied properties, volumes, compositions, temporal or geographical patterns, and technological choices, as well as nanomaterials for CO₂ extraction from flue streams [22]. During the Industrial Revolution (1750–2011), the emission of carbon dioxide into the air was about 2040 ± 310 Gt due to the widespread use of fossil fuels and deforestation [23]. According to the American Physical Society, the collection of CO₂ will lower its atmospheric level by merely 1 ppmv [21]. To address this issue, it is necessary to create a variety of cutting-edge technologies that could both absorb and prevent CO₂ emissions. The development of sophisticated technologies can prevent the CO₂ emission to increase the efficiency of fossil fuel utilization and conversion [24]. A wide range of literature reviews is available describing a comprehensive collection of techniques for CO₂ capture. For example, Mondal et al. presented contemporary CO₂ capture and separation methods from thermal power plant flue gas [25], while Al-Mamoori et al. addressed the most recent achievements in combined CO₂ collection and use, as well as the accompanying obstacles [26]. Similarly, to reduce CO₂ gas emissions, several technologies have been explored, including pre-combustion, chemical looping, post-combustion, and oxyfuel combustion [27]. Modern technologies for CO₂ capture include, but are not limited to, chemical/physical adsorption and absorption, bioremediation, and cryogenic and membrane separation [7]. Briefly, adsorption refers to CO₂ molecules adhering to the surface of solid materials (e.g., zeolites, MOFs, activated carbon). In contrast, absorption involves dissolution of CO₂ in or chemical binding of CO₂ to liquid solvents (e.g., amines, ionic liquids). These mechanisms differ significantly in regeneration energy, operating conditions, and scalability.

The extent of CO₂ in the flue stream is another essential concern in CO₂ capture. It is predicted that, as the CO₂ level in steam drops, the energy and cost burden for CO₂ capture rises [28]. Furthermore, the chemical and thermal composition of other elements in the air stream influences the applicability and performance of the capture technique. Additionally, the energy and cost are also dependent upon fuel type, location, volume, CO₂ level, and the kind of capture device used in the power plant [25]. Advantages and weaknesses are two major factors on which various applied strategies can be compared, and prospects can be adopted. Thus, various strategies, including post-combustion, sorbents, membrane-based separators, molecular sieves, electrochemical reduction, and direct air capture, have been explained, elaborated on, and compared.

3.1. Post-Combustion Strategy

Post-combustion strategy is a widely recognized technique for reducing CO₂ emissions from energy-related sources. The post-combustion process involves removing CO₂ from the flue gas produced by burning fossil fuels in the air. Then, the captured CO₂ is compressed for transportation and geological storage [29]. This process is used on both new and retrofit boilers, heaters, turbines, and natural gas-fired or coal-fired power plants on a small scale. Challenges of post-combustion capture include the need for extensive equipment, concerns about solvent oxidation and thermal degradation, and health and environmental issues due to solvent losses from the absorber. Several different CO₂ collecting systems are used following the combustion of fuel. Experimentally, they differ significantly in how they separate and capture CO₂. They generally involve membrane separation, adsorption, mineralization, absorption, adsorption, and electrochemical separation using fuel cells, as shown in Figure 1. According to the evaluation of the possibilities for post-combustion carbon collection and storage, 57% of the options depended on absorption, 14% on adsorption, 8% on membranes, and 21% on mineralization or bio-fixation [30]. The absorption and desorption of CO₂ by solvents and liquid sorbents are depicted in Figure 2. Regardless of the kind, CO₂ collection and storage significantly increase financial costs.

3.1.1. CO₂ Capture via Liquid Sorbents

Solvents were the first materials utilized in retrofitting existing coal-fired plants for active CO₂ reduction, and are also a significant subgroup of materials devoted to CO₂ capture [31]. Physical absorption or chemical adsorption are both viable options for post-combustion CO₂ collection in liquid [32]. Both methods of absorption use a two-step process that depends upon the interfacial equilibrium among the liquid absorbent and flue gases (Figure 3). By adding a selective solvent in an absorptive tower, the product flue gas is stripped of its CO₂ content on a preferential basis. The solvent is then recycled and reused in the subsequent CO₂ capture procedures. Adsorption of CO₂ usually requires a low temperature, while a rise in temperature leads to desorption due to the inverse relationship between temperature and solubility of gases in a fluid [33]. Thus, low temperatures are maintained. Liquid sorbents are divided into the following categories depending on their properties.

Amine-Based Liquid Sorbents

Amines are recognized as some of the most versatile solvents because of their reactive nitrogen atoms [35]. Additionally, they favor selective and reversible CO₂ reactions. The two main advantages of amines are their affordability and low steam pressure [36]. Amine-based solvents can be divided into two primary groups: organic and inorganic solvents. The organic group mainly consists of polar solvents using methyl diethanolamine, diethanolamine, triethanolamine, and monoethanolamine [37]. The inorganic group mostly includes sodium and potassium carbonates combined with ammonia, with potassium carbonate being the most commonly used. These solvents are more stable, less costly, and have fewer environmental impacts. Because their contact with CO₂ generates less heat compared to primary or secondary amines, they can also lower regeneration energy costs during the desorption process. However, their limited sensitivity to CO₂ in flue gas streams restricts widespread use in absorbers. The most extensively researched and used amine is ethanolamine (MEA), considered the standard for all sorbent-based CO₂ capture processes [38]. The absorption, compression, and refinement of MEA organic amines are commonly used methods to achieve CO₂ capture goals [39]. MEA-based CO₂ capture technology has become the most advanced and economically attractive approach. About 20 to 30% of typical CO₂ capture systems rely on MEA [40]. Nevertheless, it requires significant energy regeneration due to thermal stripping at higher temperatures. Additionally, issues such as corrosion, high operational costs, solvent degradation, and greater power consumption pose challenges for amine-based CO₂ capture. These limitations greatly restrict their widespread application. To address these problems, recent proposals include using amine mixtures and biphasic solvents.

The loading capability of three different CO₂ collection setups, including the testing lab, pilot plant, and extended pilot system, was evaluated by utilizing the MDEA aqueous solution (Figure 4) [41]. In recent times, the usage of benzylamine (BZA) and a primary amine, both alone and in combination with other amines, has attracted great interest. Benzylamine (BZA), which offers attractive properties of large-scale production, like minimal corrosivity and strong resistance toward heat and oxidative degradation, can be coupled with ethanolamine (MEA) and can be used as a standard in the capture technique [42]. Similarly, Figure 5 illustrates the MEA-based deep eutectic solvent with an improved CO₂ absorption capacity of around 0.2715 g g⁻¹ [43]. Additionally, it has been reported that conventional MEA-based absorption operates at ~40–60 °C with regeneration energy of ~3.5 GJ/ton CO₂ and CO₂ loading of 0.4–0.5 mol/mol, while solid sorbents like MOFs exhibit higher loading (1–2 mmol/g) but require elevated regeneration temperatures (~150–200 °C).

Additionally, it has been reported that combining two primary amines may increase the rate of CO₂ absorption; however, it does not significantly reduce the heat required for sorbent renewal [44]. Moreover, in contrast to secondary and tertiary amines, phase separation processes occur more efficiently in primary amine-based absorbents.

Similarly, the phase separation phenomenon was also observed in a mixture of polyamine/1-propanol/H₂O absorbents. Thus, for the creation of novel biphasic solvents, a molecular knowledge of phase separation is essential. Additionally, amine-based CO₂ capture involves a complex equilibrium network of reactions between CO₂, water, and amine molecules. The reaction pathway depends on the type of amine (primary, secondary, tertiary) and the aqueous medium (Equation (1)). Generally, primary and secondary amines capture CO₂ mainly via carbamate formation (Equations (2) and (3)), whereas tertiary amines facilitate bicarbonate formation (Equations (4) and (5)) due to their inability to form carbamates. The mechanism behind the absorption of CO₂ at the amine surface can be explained [36].

CO₂(g) → CO₂(aq)

(1)

CO₂(aq) + 2RNH₂ → RNH₃⁺ + RNHCOO⁻

(2)

RNHCOO⁻ + H₂O → RNH₂ + HCO₃⁻

(3)

CO₂ + H₂O → HCO₃⁻ + H⁺

(4)

R₃N + H⁺ → R₃NH⁺

(5)

Furthermore, recently, the effect of multi-amine solvents with high CO₂ absorption capacity was also assessed. Briefly, in the experiment, two multi-amine solvents, including 2-(2-aminoethylamino)ethanol (AEEA) and diethylenetriamine (DETA), along with the reference solvent monoethanolamine (MEA), were chosen as three amine solvents [45]. Compared to MEA, DETA was able to absorb 2.962 GJ/ton CO₂ while reducing the reboiler duty by 16.8%. Thus, CO₂ collection efficiency can be significantly improved with a multi-amine system. Additionally, the CO₂ absorption/desorption performance of six novel solvent blends—comprising amine, sulfolane, methanol, diethanolamine (DEA), 2-amino-2-methyl-1-propanol (AMP), diisopropylamine (DPA), MDEA, and ethanolamine (MEA)—was evaluated using ATR-FTIR spectroscopy [46]. Results indicate that MEA, DEA, DIPA, and MDEA produce low or negligible amounts of MMC, while AMP has an intermediate performance [47]. These findings suggest that combining amines can somewhat reduce regeneration energy, but there is still a need to develop new absorbents for efficient CO₂ capture. Additionally, piperazine (PZ) and its derivatives are other promising solvents in this category. These compounds outperform MEA in CO₂ capture capacity and thermal stability, despite their higher vapor pressure [48].

Similarly, using a pilot-scale carbon capture process, the performance and features of a new polyamine-based solvent blend for post-combustion CO₂ capture were examined. The proposed solvent was designed to differentiate the roles of a main amine, an auxiliary amine, and a reaction-rate enhancer. It consists of a mixture of three different amine types. Due to its ability to absorb large amounts of CO₂, polyamine 3,3′-iminobis (N, N-dimethylpropylamine) was selected as the primary amine. The auxiliary amine used was 2-amino-2-methyl-1-propanol, while piperazine was included to accelerate the reaction. It was found that the proposed solvent performed steadily and required significantly less reboiler duty than the 30 mass% monoethanolamine (MEA) solvent.

Ammonia-Based Sorbents

Fluid ammonia (NH₃) is considered a promising option because of its unique features, including a high capacity to capture CO₂, low energy consumption, minimal operational complications, and the ability to remove acidic oxides and produce additional beneficial products [49]. Additionally, it does not suffer from corrosion issues like amine-based sorbents. As a result, aqueous ammonia, or NH3, has been widely used for CO₂ capture from industrial gas streams. Both physical and chemical CO₂ absorption can occur simultaneously when using an NH₃ solution [50]. However, physical absorption appears to be the dominant process, which may depend on the gas-sorbent contact area—the process where CO₂ molecules diffuse from the gas phase to the gas–liquid interface before dissolving [51]. Equations (6)–(12) illustrate how NH₃ molecules react with dissolved CO₂ to form CO₂-rich ammonia salts.

CO_{2 (aq)} + H₂O ↔ HCO₃⁻ + H⁺

(6)

HCO₃⁻ ↔ CO₃²⁻ + H⁺

(7)

NH₃ (aq) + H₂O ↔ NH₄⁺ + OH⁻

(8)

NH₃ (aq) + HCO₃⁻ ↔ NH₂COO⁻ + H₂O

(9)

NH₄⁺ + HCO₃⁻ ↔ NH₄HCO₃ (s)

(10)

NH₄⁺ + NH₂COO⁻ ↔ NH₂COONH₄ (s)

(11)

2NH₄⁺ + CO₃²⁻ + H₂ ↔ (NH₄)₂CO₃ H₂O (s)

(12)

Despite heat being needed for the breakdown of NH₄HCO₃ or carbonate during the recycling of the NH₃ solution, it has been claimed that the NH₃ process substantially achieves higher energy efficiency and uses about 60% less energy, on average, than the MEA scrubbing method [52]. The overall estimated energy demand for the NH₃-based process was calculated to be 1147 kJ/kg-CO₂, which was found to be approximately 27% of the 4215 kJ/kg of CO₂ calculated energy required for the MEA-based process utilizing 30% w/w MEA solution [21].

However, the industry appears to be most concerned about NH₃ loss; therefore, the “chilled ammonia process (CAP)” has been created to solve this issue [46]. Ammonium bicarbonate (NH₄HCO₃) and ammonium carbonate ((NH₄)₂CO₃) are prime products of this method, which involves the reaction of a gas stream with an aqueous solution of ammonia in a wet scrubber [53]. Recently, it has been discovered that the biodiesel byproduct glycerol can be effectively used as an additive for reducing NH₃ vaporization. These findings show that increasing the glycerol content from 2.5 to 3.0% leads to a 4.8% increase in the mass transfer coefficient [54]. As a result, adding glycerol to NH₃ raises the mass transfer coefficient, lowers NH₃’s vapor pressure (acting as a green vapor suppressant), and minimizes NH₃ evaporation in the absorption tower.

The addition of Ni(II) and piperazine in NH₃ solution was also analyzed to reduce loss during CO₂ collection (Figure 6) [34]. Furthermore, it was observed that the application of metal ion additives (Cu (II), Zn (II), and Ni (II)) can effectively decrease NH₃ leakage in post-combustion capture [55]. NH₃ emission may decrease to 26.2, 9.7, and 9.6% by adding divalent metal ions Ni, Cu, and Zn, respectively, which can also facilitate the CO₂ desorption process and prevent NH₃ leakage during regeneration. Due to the stronger complexation of metal ions with free NH₃ compared to Cu(II), Zn, and Ni(II), good NH₃ escape inhibition and substantial CO₂ desorption enhancement are observed. Furthermore, aqueous solutions of NH₃ and K₂CO₃ also appear to be viable solvents for CO₂ collection from industrial exhaust emissions, with less NH₃ loss [56].

According to theoretical and experimental research, complexing ammonia with metal ions significantly lowers the amount of free NH₃, which minimizes evaporation loss but also lowers the response to CO₂ [57]. Unfortunately, CO₂ capture by this method may lead to the generation of an NH₃ stream beforehand. It can be due to ammonia’s high energy requirement or high carbon footprint. Therefore, the development of a novel NH₃-based energy-efficient system with less NH₃ leakage along with high CO₂ capture capacity is highly demanded.

Ionic Liquid-Based Sorbents

Ionic liquids (ILs), an evolving class of green solvents, have received a lot of interest recently in the domains of synthesis, catalysis, electrochemistry, biochemistry, and gas separation [58,59]. ILs are molten salts that are liquid under 100 °C and are made completely of organic cations and organic/inorganic anions [60,61]. The comparable solubility of CO₂ and conventional ILs was originally described by Blanchard et al. in 1999 [62], by creating a novel method for the collection and isolation of CO₂. Since then, several thorough investigations on CO₂ capture by ILs have been carried out.

The mechanism of CO₂ absorption by ILs is mainly based on physical absorption, except for the carboxylate-based IL [63,64,65]. The physisorption mechanism relies on the van der Waals force or hydrogen bonding between CO₂ molecules and IL ions. The weak bonds between the CO₂ molecules and IL ions facilitate easier desorption and cycling processes; thus, the CO₂ molecules could be discharged by modifying the pressure or temperature, enabling the reuse of the IL. Figure 7 demonstrates the CO₂ absorption process by ILs, in which the mixture of gases containing CO₂ is inserted into the absorption column containing ILs. Firstly, the CO₂ molecules are absorbed by the ILs, and the CO₂-poor gas is released into the atmosphere. In the second step, the CO₂ is regenerated in the stripper using a higher temperature, pressure swings, or inert gases [66].

Imidazolium-based ILs have attracted far more interest among the wide range of IL compositions due to their extraordinarily high solubility for CO₂. The extent of CO₂ solubility in ILs is largely determined by the reaction between the anion moiety of the ionic compound and CO₂, while the cation plays more of a supporting role [67]. Lewis’s acid–base method is used to define this reaction, with the anion acting as an electron donor and CO₂ as an electron acceptor [68].

The differences reported in CO₂–cation interactions between ILs with hydrogen and methyl substitutions in the temperature gradient between 10 and 50 °C imply that this variation might have an impact. The maximum CO₂ absorption was seen in ionic liquids with the [Tf2N] anion for all temperatures examined. Similar to this, at ambient temperature, a triethylenetetramine (TETA) cation-based IL displays a CO₂ adsorption efficiency of 0.96 mol CO₂/mol-IL, whereas the addition of 40% water boosted the capacity to 2.04 mol CO₂/mol-IL [69]. Furthermore, absorbance ability has been demonstrated to be strongly related to the ionic structure and has followed the order (NO₃) > (BF₄) > (SO₄) for the same cation, with 2(TETAH) and (NO₃)₂ comprising 40% moisture and exhibiting the greatest CO₂ capacity [70]. These ammonia- and imidazolium-based ionic liquids share the same drawbacks in terms of lifespan issues, including high energy costs and huge carbon footprints. The main drawbacks of ILs’ application as a CO₂ capture absorbent are their high viscosity and limited gas solubility at low CO₂ partial pressures [71]. Furthermore, ILs offer solubility for CO₂ capture, which is slightly less than that provided by amines. Thus, to overcome this issue, one option is to functionalize ILs with amine groups that can considerably accelerate the chemisorption process and boost the overall solubility of CO₂. However, the synthesis of functionalized ILs requires stringent purification procedures. Thus, it becomes much more costly than the synthesis of ordinary ILs. Recently, it has been reported that the introduction of a surfactant or some other solvents to an IL sharply reduces its viscosity and boosts its efficiency for absorption [72]. As a result, new techniques are being explored that make use of binary or ternary IL mixtures, including IL blends with amines and IL blends with some other solvents such as ethanol, water, etc. Many workable combinations may be created by mixing more expensive, highly viscous ILs with other liquids or cheap amines [73]. The primary goal of the mixture is to benefit from the advantageous traits of the different solvents. Thus, such combinations can have the desirable CO₂ capture characteristics of task-specific ILs without the disadvantages of high viscosity [74]. Briefly, adding 30–40 wt.% water to imidazolium-based ILs such as [C₂C₁im][NTf₂] has been shown to reduce viscosity by nearly 50% and double the CO₂ uptake capacity. Pinto et al. (2013) reported that the CO₂ solubility increased from approximately 0.96 mol CO₂/mol IL to 2.04 mol CO₂/mol IL at ambient temperature when water was added, which could be attributed to improved mass transfer and reduced diffusional resistance [72]. Ethyl-3-methylimidazolium ethylsulfate ([C2C1im][EtSO₄]), which shares the cation with [C2C1im][NTf2], and an IL with longer alkyl substituent chains in its cation ([C4C2im][EtSO₄]) have been specifically chosen as ILs containing the ethylsulfate anion [75]. Even though ILs have better qualities, such as incredibly low vapor pressure and excellent selectivity for CO₂ compared to other solvents, the price is still 10–20 times higher than the solvents that are readily available in stores. Therefore, the development of ILs with more competitive qualities than other commercial adsorbents is of great interest.

3.1.2. Solid Sorbents for CO₂ Capture

Harmony between high gas stream temperature and CO₂ capture process operating parameters is particularly appealing for better thermal control and improved efficiency [76]. The gas stream must be cooled down to levels that are suitable for the stability of the chosen liquid sorbent. However, solid sorbents, on the other hand, are typically more chemically stable, more cost-effective than liquid solvents, and consistent with the hot temperatures of flue gases generated by industrial operations and power plants. Therefore, solid sorbents have attracted great interest in capturing CO₂. To achieve the best efficiency for CO₂ absorption, various materials have been developed depending on the adsorption state (pre-combustion or post-combustion) [77]. In the past, zeolites and activated carbons (ACs) were two of the earliest solid adsorbents used for CO₂ extraction. But numerous substances, such as metal-–organic frameworks (MOFs), polymers, and metal oxides, have been recently developed to increase the CO₂ adsorption efficiency (Figure 8) [78,79,80]. Common criteria for assessing CO₂ uptake on solid adsorbents include the following [81]:

• High selectivity toward CO₂;
• Moderate renewal requirements;
• Low cost and quick desorption and adsorption kinetics;
• High CO₂ operational capability.

Amine functionalization has often been used to enhance CO₂ adsorption, either through post-synthesis grafting or impregnation during synthesis. Lewis acid/Lewis base interactions strengthen the bond between the adsorbent and CO₂ molecules when nitrogen is added to a carbon surface, increasing the electron density and raising the material’s basicity. It has been shown that doping the network with nitrogen or other atoms offers some advantages over amine functionalization, especially in terms of stability and preventing leaching across regeneration cycles. Here, we discuss various types of solid sorbents [82].

Carbon-Based Materials

With several uses in catalysis, adsorption, and storage devices, carbon-based materials make up a significant fraction of high-performing adsorbents [83]. Carbon-based materials have several benefits, including inexpensive precursor materials and synthesis methods, variable pore size and surface area textural properties, selective adsorption, hydrophobicity, high-temperature stability, and simple recovery [84].

Due to their plentiful feedstock, affordable ingredients, large accessible surface area, and strong adsorptive properties, in numerous studies, activated carbons (ACs) have been demonstrated to be viable adsorbents in the carbon capture process [85]. The origin and make up of the raw materials can be linked to the adsorptive qualities and activation procedure of ACs. Numerous precursors of ACs have been identified, including N-rich chitosan, bagasse, rice husk, palm fiber and shell, coconut shell, waste biomass, and porous organic polymers, among many others [84]. These precursors require high-temperature calcination before being activated physically or chemically. Materials made of physically activated carbon have undergone pyrolysis and treatment with a stream of CO₂, O₂, or steam [86]. However, chemicals like salts, bases, or acids have been used to chemically treat ACs. The majority of the ACs for CO₂ collection have been described for use in nitrogen doping or textural property adjustment, optimizing either one or both separately. Briefly, Sethia and Sayari (2015) created highly selective super-adsorbent ACs (CO₂ capacity 5.39 mmol/g) for CO₂ removal at 25 °C and 1 bar. High adsorption and selectivity are produced by the AC materials’ considerable nitrogen concentration (5.07 mmol/g), high surface area (1317 m²/g), and large pore volume (0.27 cm³/g) [87].

Materials made of ACs have benefits like low susceptibility to moisture, availability, and low cost. Nevertheless, these materials are better suited for high-pressure applications. By synthesizing ordered porous carbons and chemically altering adsorbent surfaces, ACs are created and adjusted to adjust surface areas and pore sizes to optimize low-pressure adsorption (pre-combustion) [83]. The manufacture of ordered mesoporous carbon (OMC) materials employs a variety of techniques. The most effective techniques for achieving high surface areas and narrow pore distributions are the soft template and hard template techniques [88].

The self-assembly of block copolymers, co-condensation, and carbonization are the foundations of the soft template technique. Such a method does away with the requirement to introduce pre-synthesized templates and then remove them. The hard template technique makes use of silica nanostructures as the template, filling them with carbon precursor, carbonizing them, and then removing the template with HF or NaOH [89].

Due to their large specific surface area, uniform pore size distribution, and many micrometer-sized pores, activated carbon fibers (ACFs) have been regarded as potential adsorbents. ACFs have a high adsorption capacity and quick rates of adsorption because of their small pore size dispersion. In contrast to powders and granular materials, ACFs’ fibrous shape makes them easier to handle [90]. For CO₂ capture at atmospheric pressure, commercial ACFs modified by chemical activation using KOH have been tested. The optimal concentration of KOH increased the carbon fibers’ surface area while modifying the textural characteristics of ACFs [91]. Studying the impact of nitrogen doping on ACFs revealed that the porous structure of the ACF plays a major role in controlling CO₂ uptake [92].

Due to its exceptional chemical and physical characteristics, graphene, an allotropic synthetic carbonaceous substance that is relatively new, has received a lot of interest. Large surface area (up to 2630 m²/g), mechanical strength, chemical stability, and strong thermal conductivity are some of these special qualities. Consequently, graphene has a wide range of uses, such as in supercapacitors, transistors, solar cells, touch screens, DNA sequencing, gas sensors, and the absorption of hazardous compounds [93]. However, because graphene layers are restacked during bulk manufacture, their distinctive features are suppressed. According to Chandra et al. (2012), graphene sheets functionalized with polypyrrole were chemically activated to add nitrogen [94]. Nitrogen-doped graphene (NG) has a significant CO₂ adsorption capacity of 4.3 mmol/g, which has been measured under atmospheric pressure and ambient temperature [95]. Additional research on nitrogen-doped reduced graphene was performed by Kemp et al. (2013) employing polyaniline (PANI) to uniformly dope nitrogen in porous graphene structures [96]. The study showed that CO₂ adsorption is controlled by the combination of N concentration and pore size distribution. The key benefits of PANI-doped graphene are its excellent selectivity and degree of reversibility in CO₂ adsorption [95]. Interestingly, nitrogen doping modifies the electronic structure of graphene by offering sites such as pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen that act as Lewis bases, thus enhancing electrostatic interactions and binding affinity against acidic CO₂ molecules and leading to stronger adsorption sites and improving both capacity and selectivity compared to pristine graphene. As a result, current research focuses on precisely controlling the type and distribution of nitrogen species and exploring co-doping strategies to further boost performance. Despite these advances, challenges remain in large-scale synthesis, controlling graphene layers, and maintaining high surface area with structural stability during cyclic adsorption–desorption. Takeuchi et al. (2017)’s investigation of CO₂ adsorption on monolayer graphene on a silicon carbide substrate (001) at 30 K revealed a low required recovery energy (25–30 kJ/mol), demonstrating the physisorption of CO₂ on monolayer graphene [97].

Silica-Based Materials

Being one of the most prevalent substances in the world, silica can be found in a wide variety of minerals and environmental conditions. With a large surface area and high porosity, silica is easily produced. These qualities can be tuned by making the right adjustments during the synthesis process. Particularly, mesoporous silica, with pores ranging from 2 to 50 nm, is employed in numerous applications, such as molecular sieves, drug delivery, catalytic supports, and energy storage [98]. It is a common practice to create mesoporous silica nanoparticles using the Stöber procedure. Tetraethylorthosilicate and water are combined in an alcoholic solvent to create the precursor, which is subsequently hydrolyzed into tiny particles that condense into a bigger structure [99].

The use of silica as a support for CO₂ adsorption devices has demonstrated encouraging efficacy. Silica can be used as a substrate to load with amines to trap and adsorb CO₂, since it is an inexpensive, easily produced porous material [100]. The performance of the combined adsorbent is significantly influenced by the type of porous silica utilized for CO₂ adsorption. Due to its sporadic distribution of pore sizes and shapes, amorphous silica makes an inadequate support for CO₂ adsorption. Functionalizing agents cannot reach all the pore volumes; hence, it is not employed. Not only is a substantially greater portion of the pore volume accessible with ordered mesoporous silica, but also the pore size, shape, and surface area of this material can also be easily modified by changing the synthesis, making it an ideal adsorbent support. The process of creating an amorphous silica framework around an organic template is frequently used to create mesoporous silica [101]. The template is subsequently eliminated by calcination or solvent extraction, leaving behind pores with a predetermined form and size.

Amine groups can be used to functionalize a mesoporous silica substrate and boost CO₂ adsorption efficiency. All primary and secondary amines are capable of adsorbing CO₂ [102]. The hydroxyl groups on the silica surface of the silica template cause a strong reaction with amines. To unravel the amine chains and distribute them uniformly across the silica framework, the hydroxyl groups form hydrogen bonds with the amines [83]. Selecting the proper amine will have an impact on the material’s ability to adsorb CO₂ because different amine types have varied affinities for the gas. Silica loaded with primary or secondary amines will increase the potential loading of CO₂ since primary and secondary amines react with CO₂ more strongly than tertiary amines [103]. The ability to load CO₂ is increased by increasing the loaded amine’s nitrogen weight fraction.

It is typical to create amine-functionalized silica adsorbents in two steps [104]. First, sodium silicate or tetraethylorthosilicate solutions are acidified with an inorganic acid to create porous silica. Second, amines such polyethylenimine (PEI), aminomethylpropanol (AMP), ethanolamine (MEA), and tetraethylenepentamine (TEPA) are supported on the produced porous silica, or amino functional groups are grafted onto it.

Alumina-Based Materials

Aluminum oxide can serve as a model for efficient CO₂ adsorption when functionalized. High surface areas, huge pore volumes, customized mesoporosity, great crystallinity, and high thermal stability are all characteristics of alumina materials [105]. The material has strong potential for use as a CO₂ capture sorbent if it possesses all the qualities listed above. Boehmite, a predecessor to mesoporous alumina, has demonstrated a significant affinity for CO₂ using CO₂ temperature-programmed desorption (TPD) [106]. Even greater CO₂ affinity would result from functionalizing mesoporous alumina. To improve the ability of alumina to adsorb CO₂, Feist et al. (2015) loaded it with La₂O₃. The surface area of the undoped alumina was not considerably changed by the loading of alumina with La₂O₃, showing that La₂O₃ does not obstruct any alumina pores [107].

Zeolite-Based Materials

Zeolite frameworks are networks of interconnected channels or cages that are microporous, having pores of the order of 0.5–1.2 nm. Zeolites are an appealing material to examine CO₂ adsorption because of the resultant structure, which is a very stable material with high crystallinity, a high surface area, and strong adsorption sites [108]. Zeolites have been the subject of in-depth research on CO₂ adsorption. Zeolites are molecular sieves because of their microporosity, which permits selective adsorption based on their pore size [109]. Only molecules that fit through the zeolite’s pores can be adsorbed. Zeolites can be customized for the adsorption of CO₂ based on the molecular sieving principle due to the easily changeable pore size [110]. Zeolites’ basicity is raised by the presence of aluminum atoms in their silicate-based framework. Because aluminum has a lower electronegativity than silicon, this effect is produced [83]. Cations like Li, Na, or K can be added to the zeolite framework to make up for aluminum’s negative framework charges. The basicity of zeolite grows as the electronegativity of the additional cations decreases. The capacity of zeolite to adsorb CO₂ increases with its base content. Additionally, electrostatic interactions between the framework’s cations and CO₂ cause CO₂ to be drawn into the zeolite pores [110].

A phenomenon known as “carbon gating” occurs in some zeolites, allowing CO₂ to pass through the framework but no other substances [111]. Extra framework cations in cationic zeolites can cause narrow pores to get blocked, a phenomenon known as “gating” [112]. Large cages are frequently joined together by these apertures. By passing through the cations and going from cage to cage, only CO₂ can diffuse through the materials. When attempting to adsorb CO₂ from a mixture of gases, the gating effect provides excellent adsorption selectivity for CO₂ over other tiny adsorbates, which is very helpful.

Porous Crystalline Solids

Metal–organic frameworks (MOFs), zeolite imidazolate frameworks (ZIFs), and covalent organic frameworks are examples of porous crystalline materials (COFs) with extraordinary tenability [113]. These substances, either through synthesis or computational design, have become more popular in CO₂ to expedite the search for the most effective adsorbent using the rational adsorbent material design. Electrostatic interactions in MOFs are governed by the metal site type. The most recently researched MOFs with high CO₂ adsorption capability, including HKUST-1 and M-MOF-74 (M = metal, e.g., Mg, Co), have metals like Mg, Zn, and Co in their structure [114]. Open metal sites of different metal ions are necessary for the development of novel structures.

Porous substances provide the opportunity for the development of energy efficiency and cost savings with the post-combustion CO₂ collection technique. As a pure, cyclic option, CO₂ adsorption on porous surfaces is appealing since it consumes less energy as well as a lower enthalpy for adsorption [115]. Adsorption techniques for CO₂ collection and isolation include MOFs, carbon and silicon compounds, zeolites, and COFs [116,117,118,119,120,121,122]. Due to their inexpensive budget, great thermal durability, widespread accessibility, and minimal susceptibility to humidity, ACs have indeed been employed extensively as very effective CO₂-trapping devices. In addition to being effective synthetic sorbents for capturing CO₂, zeolites are also efficient and effective adsorbents for the elimination of CO₂ [123].

Because of their exceptional pore structure, chemical tunability, and thermal stability, MOFs have drawn particular interest in the last two decades [124]. The efficacy of CO₂ capture can be improved by adjusting elements, including open metal sites, the use of Lewis bases, polar functional groups to change organic ligands, post-synthesis modification, and pore size management [124,125,126,127]. The most significant cutting-edge techniques recently described for CO₂ capture in MOFs are the chemistry of defect engineering, hydrophobicity, topology, breathing in MOFs, and functionalization in MOFs. Plenty of zeolite-templated and MOF-derived carbon compounds, as well as MOFs with strong CO₂ capture performance, have been produced and published in recent years. These materials exhibit better CO₂ capture capability than their equivalent counterpart materials [128,129,130]. Some GO/MOF (GO = graphene oxide) composites have been synthesized and evaluated for CO₂ capture capability. Eight different MOF types have received most of the research attention: the MOF-74 series, CuBTC (also known as HKUST-1), UTSA-16, MOF-505, MOF-5, UiO-66, ZIF-8, and the MIL series, including MIL-101 and MIL-53 [131,132,133,134]. The hybridization of the MOF with the proper quantity of GO can considerably increase the CO₂ adsorption capacity at low pressures, according to a general finding from various studies. For instance, a hybrid material based on UiO-66 demonstrated the greatest improvement, attaining a CO₂ capacity 70% more than its equivalent pure MOF [135]. GO/CuBTC, with the maximum GO content (i.e., 65% wt.) and under the assumption of no stacking of GO, exhibits the best results in terms of key performance indicators, even though the increase is around 30–45% for GO/MOF materials employing Mg-MOF-74, CuBTC, or MOF-505 [134], with a selectivity for CO₂/N₂ adsorption of 120 at 313 K, a working capacity of 1.794 mmol/g at a desorption temperature of 443 K, and specific energy consumption of 0.534 GJ/ton-CO_2, similar to amine scrubbing. Lin and co-workers report a highly stable and scalable MOF, termed Calgary Framework-20 (CALF-20), with outstanding high-humidity tolerance for long-lasting CO₂ collection and direct steam regeneration at the pilot scale. Zinc (II) triazolates are stacked in square-grid layers and pillared by chelating oxalate linkers to create the three-dimensional extended framework known as CALF-20. The CO₂ absorption of CALF-20 is 4.07 mmol/g for a v:v = 10:90 gas combination with a realized CO₂/N₂ IAST (ideal adsorbed solution theory) selectivity of 230 due to its intrinsic microporosity at 1.2 pressure (1 bar = 105 Pa) and 293 K [136]. Primary amine-functionalized IRMOF-74 was also studied for effective CO₂ capture under dry and humid conditions (Figure 9) [125].

The IL/MOF composites have strong potential applications in CO₂ collection and conversion [137]. On MOF-808 with high porosity, ionic liquid molecule layers (ILMLs) were constructed for gas adsorption. Due to their higher affinity for CO₂, ILMLs act as an adhesive layer to capture CO₂, while MOF-808 acts as a gas reservoir. At ambient temperatures, the ideal composite adsorbs a significant quantity of CO₂ (3.00 mmol g⁻¹), which is 2.6 times more than MOF-808 (1.15 mmol g⁻¹). Notably, the CO₂ capacity for each gram of ILMLs was 1000 times more than that of the bulk liquid, bypassing the gas–liquid interface constraint [138]. Selective CO₂ capture capabilities were also demonstrated by lanthanide metal–organic frameworks (MOFs) named MOF-590 and -592 based on a benzoimidephenanthroline tetracarboxylate linker (Figure 10). Recently, a combination of MOF and COF was also studied for CO₂ capture. The benefits of MOFs and COFs are combined in the MOF/COF hybrid materials, but they also seem to perform much better due to potential synergistic effects at the MOF/COF interface. The CO₂ capacity of the reported NH₂-UiO-66@Br-COFs hybrid materials was measured to be 169.5 mg/g at 273K and 1.0 bar, outperforming the equivalent single MOF and COF. This hybrid material outperformed the corresponding single MOF and COF. Furthermore, the hybrid materials’ vapor uptake was found to be 3.73 g/g, and it rose as the number of Br-COFs used for coating increased. This creates a new area for successful CO₂ adsorption performance by a novel hybrid MOF/COF interface. Despite a lot of research having been conducted on molecular sieve-based CO₂ capture, its large-scale application is still limited due to lower adsorption capacity at lower CO₂ pressure and stability issues.

3.2. Membranes to Capture CO₂

Many industries use membrane technologies for separation processes, including oxygen enrichment, removing CO₂ and H₂O from natural gas, and purifying H₂. Extraction of CO₂ through membranes is considered as a promising method for CO₂ capture. Additionally, membrane technology offers several advantages, such as low maintenance and energy needs, minimal environmental impact, reliability, and ease of use. However, improving membrane selectivity remains a key challenge for membrane-based systems to compete with other absorption methods. Currently, only a few materials achieve enough CO₂ selectivity to possibly reach 100%, although 50% selectivity is necessary to compete with absorption techniques. However, most of them show either low permeability or low selectivity [140].

Polyethylene oxide (PEO), due to its rubbery properties and intrinsic affinity for CO₂ through interactions between polar ether groups and CO₂, proved to be a potential material to solve this issue. However, PEO is a typical semi-crystalline polymer, with low permeability and high cost [141]. To overcome this challenge, the PEO membrane’s permeability was improved by using crosslinking, copolymerization, and chain branching to break the crystallization and increase chain flexibility. Unfortunately, these polymeric sorbents are not cost-effective for use in CO₂ collection. As a potential sorbent for scalable and affordable CO₂ collection, trimethylammonium-functionalized polyepichlorohydrin membrane (TPp) was created employing cheap, environmentally friendly raw ingredients and simple synthetic methods [142]. Depending on the source of the emission stream, flue gases often contain varying quantities of water vapor, carbon monoxide, sulfur oxides, nitrogen oxides, hydrogen sulfide, ammonia, oxygen, and argon. These components prevent CO₂ separation and have a significant negative effect on the membrane’s lifespan due to plasticization and aging. However, the application of mixed matrix membranes (MMMs) by including certain inorganic elements into the membrane’s polymeric structure enhances the membrane’s mechanical features and helps in addressing the tradeoff issues. In this regard, silica, zeolites, carbon nanotubes, mixed-metal oxides, graphene oxide, carbon molecular sieves, attapulgite, and metal–organic frameworks are the fillers that are most frequently utilized [143].

A set of eight-member-ring zeolite membranes, with pore diameters of 3.6–4.0 Å, was a potential candidate to achieve the separation of CO₂/CH₄ and CO₂/N₂. High CO₂ selectivity was demonstrated using DDR-type zeolite membranes (3.6 Å × 4.4 Å) over N₂ and CH₄, and the CO₂ permeance was 3.5 × 10⁻⁸ mol (m²sPa)⁻¹ [144]. However, the most common porous materials have poor interface compatibility with the polymer matrix, including zeolite and carbon molecular sieves [145]. The inclusion of organic ligands in MOFs improves the interaction with polymers and enables greater inorganic filler loading in the mixed-matrix membrane. This improvement occurs because the organic ligands (e.g., imidazolate or carboxylate groups) on the MOF surface form strong hydrogen-bonding or dipole–dipole interactions with polar functional groups in polymers, thus resulting in enhanced interfacial adhesion and uniform dispersion of fillers [146]. Additionally, this minimizes the interfacial voids that typically reduces the selectivity and enables higher MOF loading without sacrificing mechanical stability. Similarly, La(BTC)(H₂O)(DMF)/Matrimid MMMs showed a 183% increase in CO₂ permeability and improved selectivity from 34 to 48 compared to pristine membranes, confirming the advantage of organic ligand–polymer compatibility [146].

Therefore, the synthesis of MMMs by the inclusion of MOFs into the polymer to improve durability and resolve the permeability and selectivity has become a recent research interest. In this regard, recently, the P84 polymer matrix was doped uniformly with ZIF-302 nanocrystals to create ZIF-302/P84 MMMs for CO₂/N₂ separation. The tradeoff between the polymer membrane’s permeability and selectivity was broken, as seen by the greatly improved CO₂ gas permeability and CO₂/N₂ separation factor, which were 5.2 and 46, respectively. Additionally, the long-term operating stability demonstrated that the ZIF-302/P84 MMMs’ CO₂/N₂ separation performance was maintained for more than 30 h at 3 bar and 60 °C [147].

MMMs created by adding a lanthanide-based MOF called La(BTC)(H₂O)(DMF) as a nano-filler to the Matrimid matrix were also studied. The permeability of CO₂ increased by 183% for the 30 wt.% loaded membranes compared to the pristine membrane. It has been observed that the selectivity of MMMs increased with increasing nano-filler loading. Briefly, rising from 34.1 to 48.45 for CO₂/N₂ and from 36.2 to 54.67 for CO₂/CH₄, the lack of membrane flaws, a better filler/polymer interface, and superior nano-filler dispersion in the polymer matrix were demonstrated [148]. The outcomes demonstrated that these membranes might be utilized further for industrial gas separation applications. However, its non-porous structure limits the further improvement in CO₂ permeability. Porosity can be enhanced by the application of an electro-spun fiber composite membrane. These membranes have inadequate CO₂ permeability, as fibers are made of an impermeable polymer. To overcome this challenge, recently, continuous and quick transport pathways with low resistance for gas molecules in the PEO matrix were built using highly permeable poly (1-trimethylsilyl-1-propyne) (PTMSP) electro-spun fibers [149]. PEO/PTMSP EFCM not only enhances CO₂ permeability and adsorption but also selectivity. Thus, better CO₂ permeability and improved selectivity of CO₂ over N₂ were also observed in ZIF-8@PAN-4/PEO MMM with the same ZIF-8 loading of around 16.6% wt.%; these improvements are boosted by 52.4% and 44.3%, respectively. Due to the low concentrations of CO₂ in flue gases, a significant quantity of flue gas must be treated to achieve successful separation, which is the main drawback of this technique. The requirement of high-pressure gradients, the tradeoff between selectivity and permeability, operational issues, and low durability are limiting factors for large-scale applications. Therefore, there is a need to develop cost-effective membranes that overcome the aforementioned factors and also show stability at high temperatures of flue gases.

3.3. Electrochemical Techniques for CO₂ Capture

Carboxylation processes can also be carried out via electrochemistry techniques, which have received a lot of attention as “greener” alternatives to chemical ones. Due to their benefits of low-temperature operation, excellent energy economy, and flexible plug-and-play operating mode, electrochemical CO₂ capture techniques are receiving extensive attention. Direct redox-active complexation, transition metal-mediated complexation, pH swing, and molten carbonate are major components of the emerging electrochemical CO₂ capture technique [150].

Figure 11 shows the basic parameters for pH-swing-based CO₂ capture. The application of electrochemical CO₂ capture powered by pH swing in carbonate-based capture devices has shown promising results. Proton-coupled electron transfer (PCET) processes were established by Watkins et al. through the creation of a CO₂ separation membrane. To absorb and desorb CO₂, redox-active carriers that engage in PCET reactions transport H⁺ from the anode to the cathode. Aziz et al. utilized a pH swing to take advantage of the CO₂ hydration/dehydration balance generated by a PCET process. The mechanism of electrochemically recovered amine for CO₂ collection, powered by pH swing through a PCET process, is broken down into four phases [151] as shown in Figure 12.

Sharifian et al. [152] and Renfrew et al. [153] review the advanced electrochemical technologies and find that they have acceptable energy consumption at low current densities when run at industrially desired current densities (>50 A/m²) [152]. Electrowinning techniques are used commercially for energy-efficient CO₂ collection. Electrochemical CO₂ capture systems with ammonia serving as the CO₂ capture sorbent and copper serving as the electrochemical medium were studied and achieved a low energy need of 52 kJ/mol CO₂ at anodic and cathodic current densities of 470 A/m² and 2500 A/m², respectively [154]. This energy performance was quite competitive with the most advanced CO₂ capture systems, which typically call for electrical energy of greater than 100 kJ/mol CO₂ at greater than 50 A/m² [150].

A potential electrowinning-coupled CO₂ capture (ECC) system employing ammonia (NH₃) as a CO₂ absorbent and copper (Cu) as an electrochemical mediator was used to develop the electrically driven strategy. To accomplish effective electrochemical CO₂ desorption and NH₃ regeneration at the desired energy consumption, the ECC method utilized the electro-active competitor of Cu(NH₃)₄²⁺/Cu. Using a microkinetics process model, energy needs of 47.1–50.2 kJ/mol CO₂ were attained [155]. An electrochemical CO₂-capture cell with an ideal derivative (7,8-dihydroxyphenazine-2-sulfonic acid, abbreviated as DHPS) was also reported, which shows a superior low-electrolysis energy consumption of 0.49 GJ per ton of CO₂ and an average current efficiency of 95.8% at 10 mA cm² [156].

According to Jens et al., analysis costs can be reduced by about 46% by combining CO₂ capture with utilization. Combined CO₂ collection and conversion utilizing an electrolyte solution that includes both chemical and physical CO₂ absorption solvents has been reported. This highlights the advantage of reducing constant CO₂ feeding as a source of free CO₂. In further detail, it involves converting CO₂ into formate, glycolate, oxalate, and carbon monoxide in a solution of 2-amino-2-methyl-1-propanol (AMP) and propylene carbonate (PC) [157]. The suggested integrated CO₂ capture and conversion system entails three primary stages: CO₂ is first absorbed in the chemical capture solvent, then it is released within the electrolyzer, and finally, it is electrochemically converted on the spot. On the cathode, the freed CO₂ is electrochemically changed into formate, carbon monoxide, or oxalate [158]. Although they have many advantages, electrochemical CO₂ capture systems are still in the early stages of research and need a lot of work to become widely used.

3.4. Direct Capture of CO₂

Lackner first proposed the idea of direct air capture (DAC) of CO₂ in 1999 [159]. The development of DAC is a new method for tackling climate change that could lead to net-negative emissions by reducing atmospheric CO₂ levels [160]. Sanz-Pérez et al. [159], Shi et al. [161], and Kelemen et al. [14] have reviewed the literature on DAC, which can involve various technologies such as mineral carbonation, absorption, and adsorption on membranes. Additionally, Adamu et al. [162] have reported photocatalysis technology and cryogenic separation, respectively. Eisaman et al. [163] and Sabatino et al. [164] have explored other approaches, like electrochemical methods or electrodialysis. Currently, the most developed and well-researched technologies are absorption and adsorption. Because of DAC’s potential to remove CO₂ from the atmosphere, many review papers have been published about the sorbents used and developed for CO₂ capture [165,166,167]. Each of these methods involves two steps: first, capturing CO₂ with an air contactor; second, recycling the liquid or solid sorbent, including molecular sieves. The Clime-Works company in Hinwil, Switzerland, developed the first commercial DAC system based on adsorption. The organization also has pilot projects in Italy (Troia), where CO₂ is used to produce methane; in Iceland (Hellisheidi), where CO₂ is captured and stored for mineralization; and in Germany (Dresden), where CO₂ is captured and used to generate energy resources. Other companies, such as Global Thermostat, Antecy, Hydrocell, and Skytree, also employ adsorption methods for removing CO₂ from the air. A pilot plant based on absorption is operated by Carbon Engineering in Squamish, Canada [168].

Numerous technological and financial obstacles must be overcome by DAC technology for potential application. First, since atmospheric CO₂ levels are very low (currently at 415 ppm), DAC sorbents need to capture CO₂ firmly and to be selectively compared to other airborne components. Second, to impact the climate, DAC technologies must be deployed at a massive scale, potentially capturing up to 10 Gt of CO₂ annually. DAC sorbents must be produced in large quantities (millions of tons per year), be chemically robust, and cost less than USD 10 per kilogram. Currently, only a few CO₂ sorbents meet these strict criteria, most of which are based on alkaline media (such as NaOH or KOH), solid-supported amines, or moisture-swing anion-exchange resins [169]. Crystalline organic materials, such as hydrogen-bonded frameworks and metal–organic frameworks, offer a distinct and promising approach for DAC because of their well-defined, ordered structures. In preliminary tests, HKUST-1, Mg-MOF-74/Mg-dobdc, zeolite-13X, SIFSIX-3-Ni, and TEPA-SBA-15 were evaluated for their ability to extract CO₂ under DAC conditions. These sorbents included HUMs, MOFs, zeolites, and amine-modified mesoporous silicates, representing four different types of porous materials. Under DAC conditions, temperature-programmed desorption (TPD) experiments showed that SIFSIX-3-Ni performed the best. Notably, the excellent behavior of SIFSIX-3-Ni HUM compared to other MOF sorbents suggests that further improvements in pore size and functionality could enhance DAC performance of this group of nanomaterials [170]. A precursor MOF suitable for multiple cycles of CO₂ adsorption and release through chemical adsorption in actual DAC scenarios was created by functionalizing Mg-MOF-74 with ethylenediamine. Testing with simulated dry air indicated a significant CO₂ adsorption capacity of 2.83 mmol/g at 0.39 mbar and 25 °C [171]. Additionally, these materials display a lower CO₂ capture capacity that can be increased through amine functionalization [172]. Extensive research has also explored how different types of amines and their structures influence DAC performance [172]. Composites may also be functionalized without amines by adding Zn-OH groups to the pores, as illustrated in Figure 13. It has been demonstrated that removing CO₂ from highly concentrated sources, such as fossil fuel combustion gases, can be effectively achieved using polymerized fiber contactors. Using PEI-functionalized polymer/silica fiber sorbents for CO₂ collection, as shown in Figure 14, shows promising results.

Furthermore, another good substitute for MOFs is hydrogen-bonded frameworks (HBFs) [173]. Using the concepts of crystal engineering, the structures of HBFs may be accurately built and modified. As a different strategy, CO₂ could be captured in hydrophilic media, provided that the absorption process results in the development of stable, solubilized HBF particles that contain CO₂. In these situations, it is possible to recycle the water-soluble HBFs by first desorbing the CO₂ from the separated particles, followed by filtering to separate the HBF-CO₂ crystals from the mixture. A straightforward amine system, m-xylylenediamine (MXDA), provided DAC by crystallization of HBFs, providing evidence. MXDA-CO₂ insoluble crystals were created when a solution of MXDA was exposed to the air and collected ambient CO₂. This approach effectively blends the features of solid sorbent materials, including structural functionalization and reduced renewal costs, with the merits of liquid media, such as quick kinetics of CO₂ capture, simple scaling, and relatively inexpensive [174,175].

The main disadvantage of this technique for large-scale applications is that most sophisticated sorbents have only been tested in a limited range of moisture and temperature conditions (>20 °C). The average annual temperature around the world ranges from minus thirty to fifty degrees, depending on the region, and almost nowhere do absolute humidity levels reach zero [176]. It will be quite challenging to utilize DAC widely because there are not many studies on it in humid situations and at sub-ambient temperatures (like 20 °C). Utilizing the minute temperature fluctuations made available by this weak chemisorption feature can result in significant energy savings. This research suggests that, to prepare DAC materials for prospective deployment in polar and temperate regions, considerable research on DAC materials that perform at low, sub-ambient temperatures is required.

Table 1. Comparative overview of major CO₂ capture technologies with performance, energy, cost, and maturity (TRL).

Technology	CO2 Capture (%)	Energy (GJ/ton)	Cost (USD/ton)	TRL	Key Insight	Ref.
Amine Absorption (MEA)	85–95	3–4	40–80	7–8	Mature, widely deployed, but energy-intensive	[177]
Solid Sorbents (MOFs, ACs)	70–90	2–3	50–100	3–4	High selectivity, scalability challenges	[178]
Membrane Separation	70–85	2–3	50–120	5–6	Compact design, low flux/fouling issues	[179]
Calcium Looping (CaL)	90–95	2.5–3	30–60	6–7	Pilot-scale maturity, cost-effective potential	[180]
Direct Air Capture (DAC)	50–70	5–8	300–600	4–5	Negative emissions, very high cost	[181]

provides a comparative overview of major CO₂ capture approaches, summarizing capture efficiency, energy demand, cost, maturity (TRL), and key insights.

3.5. Chemical Looping Combustion

Although chemical looping combustion (CLC) is classified among oxyfuel combustion technologies, it can be regarded as a distinct technology [182]. Although the term “chemical looping combustion” was first introduced in 1987 by Ishida, Zheng, et al. [183], the CLC concept was patented by [184], and subsequently utilized to improve the combustion efficiency of power plants by [185]. The CLC process involves using solid oxygen instead of gaseous oxygen for the combustion reaction. The system comprises two reactors: (i) a fuel reactor where fuel is burned, and (ii) an air reactor where the oxygen carrier is reoxidized (Figure 15). CLC can be applied to gaseous, liquid, or solid fuels. It employs a solid oxygen carrier material (metal oxide) that fully oxidizes fuel to CO₂ and H₂O in the fuel reactor by selectively transporting oxygen from the air reactor to the fuel reactor. The metal oxide (MexOy) leaves the fuel reactor in a reduced state and is reoxidized again in the air reactor. Transition metal oxides (such as copper, cobalt, iron, manganese, and nickel) are promising oxygen carrier candidates [186,187]. Fe₂O₃ and Mn₂O₃ possess among the highest theoretical oxygen capacities, providing the highest theoretical ratio of oxygen moles per mole of metal. Recently, various CLC technologies have been developed, including (i) Chemical Looping Air Separation (CLAS) for O₂ separation from air using metal oxide decomposition [188], (ii) Chemical Looping Reforming (CLR) for H₂ production from natural gas [189], (iii) Chemical Looping Gasification (CLG) to convert biomass into syngas [190], (iv) Chemical Looping Oxygen Uncoupling (CLOU) for combusting solid fuels via metal oxide decomposition [191], (v) Self-sufficient Chemical Looping Reforming of Glycerol (CLRG) for H₂ production rather than heat [192], and (vi) Syngas Chemical Looping Gasification (SCL) for producing H₂ and electricity from coal-derived syngas [193]. The CLC technology offers several advantages, including (i) high efficiency [194], (ii) eliminating direct interaction between exhaust gases and combustion air [195], and (iii) avoiding dilution with N₂, which can reduce costs and minimize NOx formation. However, because the reduction reaction of metal oxides is typically endothermic, it may increase overall costs due to the additional energy required.

3.6. Calcium Looping Combustion

Calcium looping (CaL), also known as regenerative carbon cycle (RCC), is one of the highly efficient post-combustion CO₂ capture technologies used for capturing CO₂ from flue gases in various industrial applications, such as cement plants. The CaL process stands out as a promising technology for CO₂ capture, consisting of two main phases: carbonation and calcination. In this process, CaO-based materials serve as CO₂ sorbents, which add value due to their cost-effectiveness and availability. CaL capture relies on two reversible chemical reactions [196]: (i) carbonation, where CaO-based materials react with CO₂, and (ii) calcination, where CaCO₃ decomposes at high temperatures (850–1000 °C) to regenerate CaO and release CO₂ for utilization or storage [197]. The CaO can be reused in the carbonation process. The primary challenge of CaL technology is providing the heat necessary for the calcination process. To address the energy demands of calcination and improve capture efficiency, the CaL process has been integrated with other methods such as solar-assisted CaL, oxy-combustion, steam-assisted CaL, and CLC-assisted CaL [198,199,200,201]. Power plants, cement production facilities, and steel manufacturing plants that incorporate CaL technology benefit from several advantages in CO₂ capture, including (i) directing flue gases to the CaL process, and (ii) utilizing the surplus waste heat generated during combustion, boiler operation, cooling systems, blast furnace activity, clinker production, and kiln operations for both carbonation and calcination stages. Consequently, these integrations enable CO₂ capture with minimal modifications to existing combustion systems, making it a practical retrofit option for conventional industrial plants and playing a vital role in advancing a low-carbon energy future [202,203,204].

4. CO₂ Utilization

The research establishment is averse to carbon dioxide capture and storage (CCS) due to issues with long-term responsibility, a lack of adequate storage capacity, the possibility of leaks, and limited support for coastal storage sites. Deep ocean storage would have an immediate effect on reducing pH, increasing water acidity, and possibly creating environmental imbalance. Implementation of sustainable development and carbon capture and utilization (CCU) is gaining popularity as a feasible alternative for CCS and is considered a long-term remedy to the CO₂-related challenges. CCU involves the utilization of CO₂ for purposes besides storage [205]. For applications that still need carbon sources, CO₂ is a crucial raw material because it contributes to product structure and functionality. Utilizing carbon-free energy sources like electricity or hydrogen would be further justification. CO₂ is one of the few carbon substitutes for fossil fuels. In recent years, CO₂ utilization efforts have included chemical raw materials, coolants, stain removers, solvent media, injection tools, and compressed gas [206].

Among the CO₂ utilization technologies, the conversion of captured CO₂ into value-added products is attaining great attention [207]. CCU technologies consider CO₂ as a raw material for producing numerous products such as methanol, methane, polymers, carbon fibers, fuels, building materials, etc. CO₂ conversion into fuels and chemicals is achieved through several processes, e.g., chemical, biological, or electrochemical processes [208,209]. In chemical conversion, catalysts play a central role in the chemical conversion of CO₂ into a value-added chemicals. Several catalysts, such as CuO/Al₂O₃, ZnO/Al₂O₃, Ru-PNP, Cu/ZnO/Al₂O₃, Ni/Al₂O₃, and Ni/CaAl₂O₄, were widely used in the CO₂ chemical conversion. Briefly, methanol and formic acid are major examples of the chemicals produced through the chemical conversion of CO₂, which could be used as a fuel [208]. In the case of biological conversion, microorganisms or enzymes were used to convert CO₂ into biofuels (i.e., ethanol and butanol) or bioproducts (e.g., bioplastics and proteins) [210]. However, in the case of electrochemical conversion technologies, electric current is applied for the CO₂ reduction to form several valuable end products, e.g., formate, ethylene, and methane [211]. Furthermore, CO₂ mineralization (carbonation) is another CO₂ conversion process, in which CO₂ reacts with naturally occurring minerals (e.g., magnesium silicates and calcium silicates) to form stable carbonate compounds [212]. According to Figure 16, CO₂ utilization can be classified into two categories including conversion and non-conversion. In CCU, the first step is capturing and separating CO₂ at the source. The second step is its use, either directly (as a solvent, operating fluid, or heat-transfer medium) or through conversion into hydrocarbons, polymers, and chemicals [24].

4.1. Direct Utilization of CO₂

Numerous products, including dry ice, carbonated beverages, firefighting equipment, solvents, industrial lubricants, welding media, and algal farms for photosynthesis, can directly employ CO₂ [213,214]. Additionally, it may potentially improve geothermal systems and oil and gas recovery in large-scale operations. Additionally, one of the direct uses of CO₂ is as a heat transfer fluid in many sectors, including the refrigerant and energy sectors [215]. Desalination is another application of CO₂ that does not involve chemical conversion. In this process, CO₂ and brine are mixed at high temperatures and pressures to generate hydrates that might subsequently be removed to produce potable water. In these applications, CO₂ molecules do not interact or further split; they remain intact or dissolved in a solution [216,217]. These direct CO₂ operations, however, are small-scale and have little impact on total CO₂ reduction.

4.2. Utilization of CO₂ by Conversion

The transformation of CO₂ into efficient and environmentally friendly solar fuel offers a potential remedy for environmental problems, including climate change and the depletion of fossil fuels. Practical ways to reduce CO₂ emissions are provided by technologies including CO₂ capture, storage, and use. Although CO₂ can be efficiently separated by CO₂ collection and storage, it is more feasible to transform CO₂ into fuels and chemicals. CO₂ can be converted into important chemicals like carbonate, alcohol, salicylic acid, and other compounds [205,218]. The establishment of a C-C or C-H bond is necessary for the conversion process. Such bond creation requires energy input through the process of activating CO₂ or other substrates, like reductive coupling. In contrast to C-H and C-C chemical bonds, the carbon–oxygen double bond is extremely stable. Chemical fixation of CO₂ often requires significant energy input to carry out substrate activation reactions, as well as high reaction temperatures and pressures [219,220]. There are several methods for converting CO₂. In a nutshell, there are two groups of CO₂ conversion processes. One category involves processes that require little to no external energy, and the CO₂ molecule, therefore, joins the other chemicals, which causes the reaction to take place. Typically, such operations are referred to as carboxylation processes. The resultant generation of lactones, ureas, carboxylates, isocyanates, and carbonates is included in this category [221,222]. The other conversion category involves processes that generate reduced CO₂ and demand a large quantity of external energy. To break the bonds in CO₂, the additional energy required for these processes is provided by temperature, electrons, or radioactive materials [223]. These methods are referred to as thermal, electrochemical, and photochemical reactions, respectively [224]. Therefore, decreasing the potential barrier to the production of C1 building block compounds requires a catalyst as well as extreme-pressure and extreme-temperature conditions. Chemicals, including urea, polymer building blocks, methanol, formic acid, methane, formaldehyde, dimethyl carbonate, salicylic acid, ethylene carbonate, cyclic carbonates, and polycarbonates, are produced during this catalyzed reaction of CO₂ [222,225].

4.3. Enhanced Oil Recovery

The most cost-effective carbon capture, utilization, and storage (CCUS) method is the injection of CO₂ into petroleum reservoirs to boost oil recovery (often known as CO₂-EOR), as shown in Figure 17. Previously, CO₂-EOR operations were intended to maximize the extraction of oil, with CO₂ injection viewed as an expense [226,227]. As a result, various steps were taken to reduce the quantity of CO₂ stored. The co-optimization of oil extraction and CO₂ storage is necessary for CO₂-EOR operations; however, given the increasing worldwide focus on carbon capture and sequestration, the displacement and sweeping capabilities of the inserted CO₂ are crucial for overall oil recovery and CO₂ storage [228]. CO₂ displacement efficiency is excellent because of its relatively high solubility and minimal miscibility pressure with strong oil-expanding elements. However, because of its rapid mobility, gravitational separation, reservoir variability, and limited sweep efficiency, a significant amount of petroleum remains inside the reservoir, which restricts oil recovery and CO₂ storage [229]. CO₂ sweeping performance could be increased by introducing water and CO₂ either simultaneously (SWAG injection) or sequentially (WAG injection) [210]. Oil recovery and CO₂ storage are enhanced by water-saturated CO₂ injection, as the presence of water reduces CO₂ mobility. In this process, water constitutes just under 1% of the fluid. Compared to conventional CO₂ injection, oil recovery increases by 7%, while an additional 1.14 megatons of CO₂ is stored [230].

CO₂ foaming and direct CO₂ thickening agents can be used to restrict CO₂ mobility. Several studies have been conducted on the impact of foam on the CO₂ storage potential [231]. An uninterrupted thin fluid sheet known as lamella separates the discontinuous gas phase that makes up foam. To analyze foam production and degradation, concentrated CO₂ was pumped into a lengthy sandstone core that had first been saturated with each surfactant. In comparison to the control study without a surfactant, the formation of foam and lowered water/CO₂ surface forces during CO₂–foam floods resulted in lower residual water saturations. Along with the better volumetric sweep anticipated by foam, this enhancement in microscopic displacement reveals a greater capability for sequestering CO₂ than previously thought [232].

Figure 17. A general process flow diagram for EOR-CO₂. Reprinted with permission from [233]. Copyright@2016, American Chemical Society.

Figure 17. A general process flow diagram for EOR-CO₂. Reprinted with permission from [233]. Copyright@2016, American Chemical Society.

5. Analysis and Outlooks

CO₂ capture undoubtedly faces a wide variety of obstacles across all frontiers, including difficulties with law and regulation, finances and the economy, the atmosphere and climate, human well-being and quality of life, social and socioeconomic imbalances, and international treaties despite being an accurate and efficient preventive measure. Technically speaking, the variety of methods, procedures, and composites used in CO₂ capture pathways present significant obstacles, research gaps, and problems that must be resolved via basic identification and experimentation combined with vital technological advances. Even though many of these problems are addressed and explained in their appropriate parts, Table 1 briefly summarizes a few of the most obvious difficulties and opportunities.

With a basic understanding of CO₂–sorbent engagement and management of surface binding processes that govern selective absorption of CO₂ from combustion products, research into novel nanomaterials with enhanced sorption capacities should progress more rapidly. Innovative fluid and solid sorbents are needed to increase CO₂ absorption capability while reducing environmental impacts. Although DAC consumes a lot of energy and is unquestionably the most expensive solution, it is nonetheless crucial because it completely removes ambient CO₂ regardless of the type or source of release. The advancements made in post-combustion trapping solvents and nanomaterials will be extremely beneficial to DAC. By creating catalysts that are widely available on Earth and comprehending the basic mechanics for CO₂ surface reduction, this precious carbon source may be utilized as well as recycled to create valuable items. It is now a costly prospect because the cost of CO₂ cleanup is significantly higher than most cap-and-trade rates or carbon pricing. Although this inconsistency is exacerbated by legislation, it does not provide businesses, governments, and the corporate sector with the proper incentives to engage in the development of technological solutions. Instead, it forces sectors to opt for the least expensive production and pollution methods. This hole must be closed.

From

Table 2. Different types of sorbents based on their advantages and limitations.

Technology	Advantages	Limitations	Development and Future Prospects	References
Amine-based sorbents	Most widely used technology for CO2 capture It has a high selectivity, a quick absorption rate, a big cycle capacity, and a high removal efficiency for CO2. Able to collect CO2 from the flue gas stream even at very low partial pressure	It demands significant regeneration energy due to thermal stripping at a higher temperature. Corrosion and deterioration of absorbents are other limiting factors.	Need to develop novel biphasic solvents and polyamine solvents for energy-efficient CO2 capture	[36]
Ammonia-based sorbents	It has a great capacity to collect CO2 with less energy consumption.	Absorption capacity declines because of ammonia leakage.	The application of chilled ammonia and the addition of transition metals can reduce ammonia leakage, but decrease efficiency.	[234,235]
Ionic liquids as adsorbents	It has extraordinarily high solubility for CO2, minimal vapor pressure, a low specific heat capacity, and strong thermal stability. It requires minimal regeneration energy.	Expensive solvents, high viscosity, and reduced fuel absorption at lower CO2 partial pressures	Phase change and blends with other solvents may reduce the effective cost.	[236]
Oxides and minerals	Chemically stable, more cost-effective, and requires low regeneration energy It can collect CO2 at an ambient temperature. They are widely available in nature.	Suffer from rapid loss of reactivity under cyclic operation due to agglomeration, pore structure collapse, and a decrease in active sites	Need to develop a highly stable novel sorbent	[237]
Molecular sieves	They have less energy consumption, lower enthalpy, and tunable structures.	Limited due to a lesser adsorption capacity at lower CO2 pressure	Surface modifications, building interaction between the adsorbent and CO2	[238]
Membranes	Low maintenance and energy input requirements, a small environmental impact, excellent selectivity and dependability, and ease of operation	Require high-pressure gradients, the tradeoff between selectivity and permeability, and the low durability	Tuning and altering the characteristics of materials by surface treatment, pore size and porosity management, process engineering, and doping to enhance permeability	[239]
Electrochemical capturing	The low-temperature operation, excellent energy economy, and flexible plug-and-play operating mode	Still in the early stages of research and needs a lot of work to become widely used	A cost-effective system developed to capture CO2 and give valuable products	[240]
Direct air capture (DAC)	Can capture CO2 from a smaller and distributed point source, with no need for unnecessary transport	High setup cost, lack of experiments at ambient and sub-ambient temperatures	Developing materials that show promising results over a wide range of temperatures	[241]

, it is evident that ammonia-based technologies demonstrate lower energy consumption, while ionic liquids exhibit exceptionally high CO₂ solubility and minimal regeneration energy requirements.

6. Conclusions

Mitigating rising atmospheric CO₂ levels demands urgent deployment of capture, separation, and utilization technologies in a real sense. This review has analyzed the major strategies, including solvent-based absorption, solid sorbents, membranes, and DAC, and explains their mechanisms, performance metrics, and key materials. Solvent-based amines remain the most mature, with capture efficiencies of ~0.4–0.5 mol CO₂ per mol amine but high regeneration energy (3–4 GJ/ton CO₂). On the other hand, solid sorbents such as MOFs and activated carbons offer higher capacities (1–2 mmol/g) and structural tunability but remain limited by thermal stability and scalability. Similarly, membrane-based approaches provide modular, low-temperature operation and integration potential, while DAC offers a complementary pathway with structural–functional advantages but requires major cost reductions to approach the USD 100/ton CO₂ target. A key perspective involves technology readiness levels (TRLs) and real-world deployment. Mature capture processes like calcium looping (CaL) and chemical looping combustion (CLC) have achieved TRL 6 and are supported by operational pilot and demonstration plants, bridging laboratory research and industrial application. In contrast, next-generation materials (e.g., MOFs, ionic liquids) are at TRL 2–3, where scalability, environmental impact, and cost remain unvalidated. Bridging this gap requires not only material innovation but also integration with process engineering, pilot-scale testing, and techno-economic analysis to accelerate the transition from bench to field.

Despite substantial progress, several challenges remain, including degradation under real flue gas conditions, a lack of long-term durability, and poorly characterized environmental footprints. Energy demands for regeneration and system complexity also hinder scale-up. Future research should prioritize AI-assisted design of low-energy, high-capacity sorbents, hybrid capture systems combining the strengths of solids, liquids, and membranes, and integration with renewable energy and CO₂ conversion technologies to create circular carbon systems to guide policy and deployment. In conclusion, advancing CO₂ capture technologies requires a dual focus, such as pioneering new materials and simultaneously maturing processes toward industrial-scale readiness. To achieve this, coordinated efforts between academia, industry, and policymakers will be crucial for achieving carbon neutrality and mitigating the impacts of climate change.