Designing Carbon‐Based Porous Materials for Carbon Dioxide Capture

Rapid industrialization and urban development around the world have significantly increased carbon dioxide emissions, adversely affecting climate and ecosystems. Therefore, carbon capture and storage emerged as a promising route to reduce environmental CO2 concentration. Among various CO2 capture technologies, adsorption through carbon‐based porous materials has attracted particularly strong attention. This is primarily due to their high specific surface area, selective CO2 adsorption, moderate heat of adsorption, tunable morphology, and reduced degradation in moisture. This review critically examines carbon‐based CO2 sorbents derived from diverse sources. The key factors controlling adsorption performance, including the impact of structural and functional properties are discussed. The future research directions in this rapidly emerging field, contributing to the decarbonization of the global economy and society, are highlighted.


Introduction
World population growth and industrialization have created a high energy demand, which, no doubt is one of the most important prerequisites for continued economic growth. [1]A disadvantage of power generation units is the use of fossil fuels for combustion, which releases large amounts of CO 2 into the atmosphere. [2]In addition to this, the past few decades have also witnessed a tremendous amount of CO 2 build-up, resulting in global climate change.The CO 2 emission data of the Mauna Loa Observatory in Hawaii suggests that the current year's CO 2 concentration is ≈413 ppm, which is much higher than what it was in the middle of last century. [3]The fact that fossil fuels are unlikely to be replaced soon is more worrisome because they are directly DOI: 10.1002/admi.202202290linked to global warming, which can affect all living species on Earth.Furthermore, it has been predicted that CO 2 concentrations will reach 950 ppm by the end of this century, which could drastically change the chemical composition of planet Earth's atmosphere. [4]he combined impact of these effects has caused great concern among governments around the world and established important research in the field of carbon capture.Several research and techno-economic means have been explored for the mitigation of anthropogenic climate alterations due to CO 2 emission, thus building a global model for carbon emission in modern society.
Carbon capture and storage (CCS) is one of the most prominent technologies that could be used to reduce CO 2 emissions from their source of emission such as power plants. [5]CCS comprises capturing CO 2 and transporting it to storage sites/facilities where it can be stored for a longer time. [6]The greatest problem in this whole cycle is the functioning of the CO 2 capture unit as the cost associated with it is very high and limits its introduction in the commercial sector.Currently, the United States is leading the global effort by capturing 7-8.4 Mt.CO 2 per year, followed by China with 0.4-2 Mt.CO 2 per year, and Europe with 0.7-1 Mt.CO 2 per year. [7]he report of Global Carbon Capture and Storage Institute updated the list of operational, suspended, and under-construction CCS facilities throughout the world. [8]A few of the top operational facilities in respective countries are mentioned in Table 1.
Considering that the carbon-oxygen double bond (C═O) has a very high bond energy (750 kJ mol −1 ) and is difficult to cleave in the CO 2 molecule, the liberation of a variety of products (like CH 4 , higher hydrocarbons, oxygenated, etc.), while high energy is required for absorbent regeneration, physical adsorption using porous materials is an effective alternative to traditional chemical adsorption via amine solvents. [9]Porous materials having nanosized pores can be effective in CCS as they offer high adsorption capacity, good selectivity, and low cost.Moreover, the properties of these materials can also be manipulated to achieve the optimal adsorption energy for CO 2 molecules.Major research has also been carried out in the area of meso-and microporous materials for CO 2 capture.These materials are porous polymers, [10] zeolites, [11] porous activated bicarbonates, [12] mesoporous carbon nitrides, [13] metal-organic frameworks (MOFs), [14] and some others.Although different factors like specific surface area, pore size and volume, density, and functional groups contribute toward Table 1.Current CCS facilities and projects across the world.Adapted with permission under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence. [8]Copyright 2021, Global Carbon Capture and Storage Institute Ltd. the adsorption properties of materials, recent studies show that the pore diameter is the key factor that dominates the adsorption properties.Particularly, mesopores and micropores favour CO 2 adsorption at high and low pressures, respectively. [15]Despite the tremendous research in porous materials, the relevant technology has still not reached the commercial level.Hence, it becomes important to continuously advance the knowledge of CCS through collaborations involving industry to explore the potential of porous materials for large-scale CO 2 capture.However, it seems very impractical to completely solve the current issue of CO 2 through porous materials due to several requirements as mentioned in the next section.Still, it would at least minimize the emissions and help in achieving the carbon emission reduction goals.
Another important factor that could contribute to the solution of the above issues is to convert the captured CO 2 into some useful products like clean fuels and chemicals through cost-effective and environment-friendly routes.To convert CO 2 into CO, CH 3 OH, and HCOO-, we need an effective catalyst that can easily transfer charges, has many catalytic centers, has high porosity, and promotes high CO 2 conversion. [16]CO 2 conversion via photocatalysis is reported as a low conversion pathway, while electrocatalysis produces a short chain of products which limits its applicability at a large scale.Thermo-catalysis on the other hand gives exceptionally high efficiency where product generation can also be controlled, thus making it favorable in many industries. [17,18]In the case of porous materials, CO 2 capture, and conversion could be attained in a single system where CO 2 will be captured with the help of porosity, and then the same materials can act as catalysts during the conversion step.
Utilizing captured CO 2 as a resource for generating valuable products beyond its conventional applications such as en-hanced oil recovery or geological storage is yet another attractive topic that has attracted significant attention in research.Numerous catalytic possibilities, such as artificial photosynthesis, photocatalysis, and the production of platform chemical components, energy sources, and pharmaceutical substances, have been proposed. [19]Advancing these technologies to achieve a net reduction in CO 2 emissions poses a significant challenge as there are numerous fundamental and technological hurdles to overcome.For instance, the development of efficient thermal and photo-electrochemical catalytic reaction pathways, understanding the kinetic mechanisms involved in the formation of inorganic carbonates within minerals and industrial waste systems, and improving biological CO 2 conversion processes, are just a few of the key areas that require further exploration. [19]Another important way toward a circular economy relies upon the synthesis of adsorbents from biowaste.This not only provides a CO 2 -free synthesis route but also helps in managing waste produced from various sources like sewage sludge, paper and pulp industry, mining sites, etc. [20][21][22] Waste fuels like industrial nonhazardous waste and municipal waste in the thermal conversion process can be utilized in the production of valuable products, including gaseous energy carriers and solid products for the adsorption process.Research conducted by Karimi et al. [23] shows the potential of producing CO 2 capture adsorbent by using compost derived from municipal solid waste as a resource via chemical activation with sulfuric acid at 800 °C.The results indicate that the CO 2 capture performance is comparable to that of commercial samples. [23]Furthermore, biowaste, consisting of a blend of food and wood waste acts as a promising raw material for producing microporous activated carbon through gasification, particularly when combined with chemical activation using KOH, resulting in superior material porosity, which have potential for CO 2 capture. [24]Apart from this, process integration offers a viable approach in numerous scenarios, allowing for the conversion of CO 2 generated from industrial waste and processes into various organic monomers suitable for polymerization, such as carbonates, carbamates, and urea.Additionally, biowaste can be co-polymerized with other comonomers derived from waste materials to create polymers like polycarbonates, polyurethanes, polyureas, and polyesters. [25]onsidering the importance of CCS and the consecutive development of mesoporous carbon in CO 2 capture, it is necessary to review the recent achievements in this field.Thus, this review paper aims to bring together extensive information regarding CO 2 capture techniques and materials, offering a unified and comprehensive reference.The article achieves this by providing distinct information on critical elements in CO 2 capture, exploring carbon-based advanced materials and effective approaches to enhance their properties.The second section of this paper introduces three prominent methods of CO 2 capture, namely, pre-combustion, post-combustion, and oxyfuel-combustion.Section 3 discussed the properties of an ideal CO 2 adsorbent.In Section 4, first, a general CO 2 adsorption mechanism is discussed, followed by different carbon-based CO 2 adsorbents prepared via synthetic polymers, biomass, and organic frameworks are discussed.Section 5 examines numerous techniques used to improve the CO 2 adsorption properties of these materials.The most common parameters to design an efficient adsorbent are chemical doping, chemical functionalization, structural design, and modulating open metal sites.The conclusion section summarises the current status and possible future developments in porous carbon materials for CO 2 capture technologies.This work not only provides insights into porous carbon material in CCS but also points out potential solutions to the existing problems.

Methods of CO 2 Capture
There are two primary methods used for CO 2 capture, referred to as pre-and post-combustion processes as shown in Figure 1.The pre-combustion process involves the treatment of synthesis gas or syngas comprised primarily of hydrogen and CO, which is generated from the gasification of C-containing feedstock followed by a water-gas-shift reaction (WGS). [26]The rate of precombustion is related to the production of syngas, confiscation of CO 2 , and finally, the combustion of H 2 . [27]All fossil fuels can be partially burned or modified with basic oxygen at an increased pressure of ≈30-70 atm to produce syngas with CO and H 2 predominating.The water vapour is then introduced into the syngas and passed through a packed bed catalyst, where the water gas shift reaction continues to convert CO into CO 2 .In a watergas shift reaction, adding steam and lowering the temperature leads to a better conversion of CO to CO 2 and an increase in H 2 production. [28]In addition, from the CO 2 and H 2 streams, the CO 2 is separated and transferred to the compressor, while the pure H 2 is used as a raw material to generate electricity.The ideal separation process is to use physical solvents such as rectisol (mainly composed of methanol) and selexol (a mixture of dimethyl ethers of polyethylene glycols) due to their low cost.No heat is required in the regeneration of solvent and CO 2 can also be released at above atmospheric pressure conditions.The main drawback of this process is that it requires a chemical plant in front of the turbine.Complicated processes cause extra shutdowns, which results in lower power output.Another drawback is its no-gaseous feedstock and expensive scrubbing for high NOx emission control.A higher concentration of CO 2 in syngas makes it less expensive for pre-combustion capture.Presently, there are few gasification plants whose capital cost is higher than coal-fired power plants. [29]ost-combustion CO 2 capture includes the removal of CO 2 from exhaust (flue) gas coming from the combustion chamber of a power plant.Existing plants utilize air for combustion and produce flue gas at atmospheric pressure with less than 15% CO 2 concentration.Therefore, the capture from flue gas is low, hence, posing a technical challenge for a cost-effective advanced capture process. [26]However, post-combustion CO 2 capture is still more relevant to address climate change because the maximum number of current emission sources like electricity generating units can only work when coupled with post-combustion capture technology.From the gas mixture in the exhaust stream, CO 2 can be chemisorbed by chemisorption with amine solutions such as mono-2 and diethanolamine. [30]Post-combustion is also a topic of interest as it can operate in conditions close to room temperature.Conventional amine-based adsorption technology has currently used for several years in many large-scale commercial plants. [31]However, the adverse effects of toxic volatiles, hazardous side-products, corrosion, solvent loss, and the high cost of the regeneration process are a few limitations of this process. [32]Calcium looping is another promising approach in post-combustion CO 2 capture, where CO 2 is converted into carbonate.However, this method also suffers from the drawback of high operational cost. [33]Among several technologies available for CO 2 capture from flue gas, there is growing interest in the use of adsorption processes as a potential alternate technique.Adsorption using novel solid sorbents efficient for CO 2 capture from flue gas has many advantages in comparison to other techniques.Apart from pre-and post-combustion technologies, oxyfuel combustion is another important area, where the fuel and O 2 are combusted and mixed with recycled flue gas stream of CO 2 and H 2 O to capture CO 2 . [26]

Key Requirements for CO 2 Adsorption Materials
CO 2 adsorption on porous surfaces occurs either via physical or chemical routes, but the mechanism of adsorption highly depends on the surface chemistry, pore conformation, and structural orientation.Specifically, one of the few influential properties of CO 2 capture is the pore/volume filling, which is directly linked to the surface area and pore volume.This significance can be elaborated as follows: a) Pore/volume filling directly relates to the surface area and pore volume of the sorbent.A higher surface area and pore volume provide more active sites for CO 2 molecules to interact, thus resulting in high CO 2 adsorption.b) In microporous structures, such as micropores, the adsorption capacity increases under low-pressure conditions due to molecular interactions in these confined spaces.On the other hand, porous structures like mesoporous, facilitate higher adsorption capacity under high pressure conditions through physical absorption.The detailed mechanism of Cow adsorption in porous materials is explained in Section 3. c) The pore filling/volume also affects the dynamic adsorptiondesorption properties of the adsorbent.A well-designed adsorbent with optimized pore structure ensures efficient mass transfer and faster kinetics, resulting in faster adsorption and desorption cycles, which is important for practical applications.
For instance, zeolites, porous organic polymers, and MOFs have high microporosity and thus exhibit high CO 2 capture at low pressure. [34]On the other hand, nanoporous carbons are known for manipulating their pore geometry to micro-or mesoporous scales, which offers stable CO 2 capture at both low and high pressures.Moreover, mesoporous materials (i.e., porous silica, carbons, and hybrid materials) are well known for highpressure CO 2 adsorption.Besides the pore filling, surface doping is another important factor in improving the CO 2 adsorption properties of porous materials.Introducing basic functional groups and metal centers in porous materials lays out additional space for acid-base, hydrogen-, and electrostatic interactions. [34]37] However, the functionalization of nanoporous carbons usually enhances the CO 2 adsorption due to acid-based interactions, the doping of the nitrogen group can also be advantageous for hydrogen bonding between the surface of carbon and the CO 2 molecule. [38]The nitrogen group present in a material alters the electronic state of hydrogen atoms in -CH and -NH groups, which results in the formation of hydrogen bonds between H and the O atoms of the CO 2 molecule.The requirements for an ideal adsorbent are summarized in Figure 2 and are explained as follows.

Heat of Adsorption
Talking about the first factor, the energetics of the CO 2 capture process must be studied to understand the bonding between porous materials and CO 2 molecules.The CO 2 concentration in post-combustion capture is relatively lower than in precombustion. [39]This implies that the adsorbent which can initiate weaker physisorption-based interactions with CO 2 is better for pre-combustion technology, while the adsorbent capable of binding CO 2 with chemisorption principle is needed for postcombustion.More precisely, the meso-or macro-porous adsorbent would satisfy the requirements of pre-combustion carbon dioxide capture, whereas highly microporous adsorbent and suitable dopant would be more successful for post-combustion carbon dioxide capture.The extent of interactions of porous material and CO 2 molecule is referred to as the isosteric heat of adsorption (Q st ).The high Q st leads to high energy-intensive regeneration of the material.Also, the CO 2 uptake parameters could be highly affected at lower Q st values.From different porous materials, MOFs usually have Q st in the range of 25-35 kJ mol −1 , which could be increased up to 50 kJ mol −1 on modification with suitable functional groups like amines. [40,41]For zeolites, porous carbons, and porous organic polymers, the value is generally between 30-60, 20-30, and 20-50 kJ mol −1 , respectively. [42,43]For an industrial scale application, the Q st value of any new material must be within 35-50 kJ mol −1 with a higher working capacity than the commercially used MEA (monoethanolamine) (1.5 mmol g −1 ). [44]

Selectivity
The second most important property is the high selectivity for CO 2 adsorption out of a gaseous mixture in a flue stream.Porous materials like MOFs, zeolites, and porous organic polymers are well known for their high selectivity in post-combustion CO 2 capture where partial pressure is ≈0.10-0.15bar.This is attributed to the high intrinsic microporosity of the material and the presence of active metal centres.Porous carbon materials with suitable functional groups can show high CO 2 adsorption in lowpressure conditions, but they sacrifice selectivity as compared to other materials in the same conditions.On the contrary, mesoporous materials are more relevant to high-pressure CO 2 adsorption.Another important area where selectivity of CO 2 could be an important parameter is the segregation of CO 2 from CH 4 /N 2 in the natural gas sector.The CO 2 /N 2 and CO 2 /CH 4 selectivity are calculated based on the individual mole fractions in a binary mixture as represented in Equations (1) and (2).
Here, x and y are the mole fractions of respective components in the adsorbed and bulk phases, respectively. [45]The selectivity of well-known Mg-MOF-74 is ≈150 at 50 °C, which almost doubles the selectivity of NaX zeolite (i.e., ≈87) under post-combustion conditions. [46]Due to complete physical adsorption in porous carbons, they show relatively less selectivity than MOFs, zeolites, and porous organic polymers. [47,48]

Chemical, Thermal, and Mechanical Stability
The chemical, thermal, and mechanical stability must be considered.In the post-combustion scenario, the CO 2 in the flue stream is at very low pressure (≈15 bar) and high temperature (≈40-80 °C) conditions.In addition, there is a significant amount of water/moisture present in the flowing stream, which poses a big challenge for its separation.This is mainly because H 2 O competes with CO 2 for adsorption in porous material and results in swelling and structural changes in the sorbent.In addition, the presence of other unwanted components like N 2 , NO 2 , and SO 2 in the flue stream causes major stability issues due to competitive adsorption on the sorbent.In addition, it is important to study the CO 2 adsorption in the presence of gas contaminants to mimic the real working conditions, whether they are in the case of direct air capture or flue gas process. [49]This introduces another criterion for the adsorbent material that it should have a high tolerance to water along with high selectivity.Among the common porous materials, the high surface area MOFs, zeolites, and porous organic polymers with active metal sites can present high CO 2 selectivity but will eventually be degraded due to moisture.

CO 2 Adsorption Capacity
The next important factor is the rate of CO 2 uptake from the mixture of gases.For this purpose, the porous material is expected to feature fast kinetics, which could be sometimes limited due to the existence of many small pores on the surface of materials like MOFs and zeolites.CO 2 molecule would require accessing pores smaller than or equal to its kinetic diameter (0.33 nm) and strictly following the physical adsorption mechanism.Diffusion in micropores is more favourable at low pressures, while at high pressures, CO 2 will spread in the total available volume, which as a result compromises the micro and large pores.In micropores, the diffusion of CO 2 initially varies with the rate-determining step, it is believed any material with high CO 2 uptake will be able to accomplish the complete diffusion in 2-4 min. [50]Further, the presence of amine, alkali metal, nitrogen, or metaloxyhydroxide will encourage the acid-base reactions and enhance the adsorption kinetics. [51]

Material Recyclability
In the context of Carbon Capture and Storage (CCS), the effective separation of CO 2 from the sorbent is a pivotal step that holds equal significance as the initial CO 2 adsorption, thereby completing the entire CCS cycle.In conventional CCS processes utilizing aqueous alkanolamine solvents, CO 2 uptake and separation proceeds through the formation of CO 2 molecular complexes by chemical interactions, followed by regeneration of amine and CO 2 by increasing the temperature of aqueous solution to 100-120 °C.In the process, CO 2 is compressed to 100-150 bar for permanent sequestration. [52]However, such conventional amine-based technologies are energy-intensive and costly, thus raising the need to develop a cost-effective and energyefficient technology for CO 2 capture and separation.Meanwhile, solid nanoporous materials have been emerging for their applications in gas adsorption and separation.The introduction of N-donor groups like pyridine, imidazole, and tetrazole on internal walls of porous material improves CO 2 selectivity for gas separation due to dipole-quadrupole interactions between polarized CO 2 molecules and the accessible nitrogen sites. [53]he N-rich porous adsorbents offer several advantages as compared to the costly regeneration step in the amine-based absorption process.Foremostly, N-rich porous adsorbents display better regeneration properties as they consume less heat since there is no requirement to heat a large amount of water (60-70%) as in an amine-based process.Second, the porous structure of N-rich adsorbents allows them to interact with CO 2 molecules not only on surfaces but also throughout the interior.This results in high CO 2 affinity and more available active sites, leading to high separation efficiency in the CO 2 capture and separation process. [54]In addition to this, the high stability of N-rich porous materials ensures minimum degradation during the regeneration process.Due to these advantages, N-rich porous adsorbents offer cost and energy-effective alternatives for CCS.

Commercialization of CO 2 Capture Process
In January 2021, the US Treasury and IRS (Internal Revenue Service) provided much-awaited regulatory clarity on 45Q tax credits, making significant clarifications on geological storage certification, aggregation of multiple projects, reducing the lookback period for credit reclaim, and broadening the definition of carbon utilization. [55]lberta has been actively promoting CCS policies and the development of a low-carbon hydrogen industry through a Hydrogen Roadmap.Alberta Energy issued Information Letter 2021-19, outlining a planned Carbon Sequestration Tenure Management process to issue carbon sequestration rights for dedicated geological storage hubs.The Greenhouse Gas Pollution Pricing Act 2018 (GGPPA) was upheld as constitutional by the Supreme Court of Canada in March 2021, setting national emission standards and enabling a proposed increase in Canada's carbon price from CA$40 per tonne of CO 2 in 2021 to CA$170 per tonne by 2030. [56,57]he Australian Government's Technology Investment Roadmap prioritized CCS, clean hydrogen, energy storage, low carbon materials, and soil carbon.Funding was allocated for CCS/CCUS projects and clean hydrogen hubs.Additionally, the CCUS Development Fund supported various projects focusing on natural gas processing, cement, DAC, biogas, and CO 2 utilization/mineralization.Australia and Singapore jointly initiated efforts to work on low-emission fuels and technologies, including clean hydrogen and clean ammonia. [8]he process of CCS has the potential to reduce global energy emissions by 20%. [58]As per the report by the Global CCS Institute CO 2 RE database, facilities across the world are doing a combined CO 2 capture of ≈40 MMtpy CO 2 . [59]It is very important to note that the separation of CO 2 from its source constitutes ≈75% of the total cost associated with the CCS system.This makes it very important to adopt a cost-effective system for efficient CO 2 separation.It is important to point out that acquiring up-to-date economic data on certain CO 2 capture processes is challenging.The conventional amine absorption process (CAAP), first used by Econamine FGSM (EFG), a Fluor proprietary used 30 wt% MEA and performs well at low SO 2 (<10 ppmw) and NOx (<20 ppmw) to limit solvent degradation. [60]However, it suffers from high energy consumption of 3.6 GJ/metric t -4 GJ/metric t of CO 2 capture, which leads to high operating costs.Additionally, the tall absorption and desorption towers introduce additional costs associated with the process. [59]o solve this issue of CAAP, Mitsubishi Heavy Industries (MHI) developed a post-combustion CO 2 capture process called KM-CDR (Kansai Mitsubishi Carbon Dioxide Recovery) process.The KM-CDR process utilizes KS-1 solvent, which uses less energy and results in low solvent loss. [61]The KM-CDR can capture more than 90% of CO 2 from the flue stream and can produce CO 2 with more than 99.9% purity.So far, MHI has established 13 CO 2 capture plants to generate fertilizer, methanol, and oil. [62]inde and BASF Group are collaboratively developing CO 2 capture technology by employing BASF's novel amine blend solvent comprised of MEDA (tertiary amine methyl diethanolamine) and 10 wt% cyclic diamine.This technology saved 20% energy input as compared to CAAP and significantly lowered solvent consumption.The pilot plant commercialised in 2015 operated on less energy (i.e., 2.8 GJ/metric of CO 2 ) and captured up to 30 tpd of CO 2 with more than 99.9% pure CO 2 . [59]lean Carbon Solution Ltd. (CCSL) developed another process called CDRMax, which operates at near atmospheric pressure with amine-promoted buffer salt (APBS) as the solvent and validates 95% CO 2 recovery >99% purity. [63]This technology reduced consumption by 27% and loss of solvent by eightfold.The CCSL captures CO 2 at $30/metric t, while other processes range between $45 -$60/metric t.
The CCS process has also been tested over solid sorbents like zeolite 13X, MOFs, activated carbon, etc.The proprietary hydrated sorbent process for CO 2 capture is an alternative that provides the desired features for a commercial plant.The process uses potassium carbonate supported on solid support as a sorbent.The developed sorbent shows high CO 2 adsorption capacity, high selectivity, stability at different temperatures, low degradation toward moisture, fast kinetics, high recyclability, and low energy penalty.There is no requirement for the heat of vaporization of H 2 O.The salient feature of the proprietary process is its CO 2 adsorption capacity at high temperatures of 70-90 °C, and regeneration at 120-130 °C.The process also displayed a low demand for heat for the regeneration process (i.e. from 141 kJ mol −1 CO 2 to 70 kJ mol −1 CO 2 ).Additionally, the overall energy demand per ton of CO 2 was reduced by 60-70%, when compared with the conventional amine process.The high-capacity hydrated sorbent process demonstrates 93% CO 2 removal in a continuous process with 3.2 mmol g −1 CO 2 capture capacity. [59,64]Depending upon the above-mentioned factors, the research on porous adsorbents has significantly increased. [65]irect air capture (DAC) is a growing technology designed to address distributed CO 2 emissions that conventional CCS methods have difficulty capturing.In an extensive study by Barzagli et al., [66] various alkanolamines, commonly used in CCS, were studied for their effectiveness in DAC.The results showed that aqueous primary unhindered amines exhibited a CO 2 capture efficiency comparable to that of aqueous alkaline hydroxides.In addition, these primary amines have the potential to reduce regeneration energy requirements.The cost of DAC systems varies depending on the method and materials used.Abanades et al. [67] reported that a DAC system based on passive CO 2 carbonation of porous Ca(OH) 2 plates could range from USD 140/t CO 2 to USD 340/t CO 2 .However, cost reductions are possible with alternative materials.For instance, Keith et al. [68] estimated a lower CO 2 capture cost, ranging from USD 94/t CO 2 to USD 232/t CO 2 , for a 1 Mt CO 2 /year DAC plant.This system used aqueous NaOH sorbent coupled to a calcium caustic recovery loop with structured PVC packing as a low-cost option for gas-liquid contactors.In the solid sorbent-based approach for DAC, CO 2 is adsorbed onto various solid sorbents, such as zeolites, activated carbons, MOFs, amine-modified materials, silica materials, porous organic polymers, carbon nanotubes, and carbon molecular sieves. [69]This method offers the advantages of lower energy consumption, with as low as 1 GJ/t CO 2 required for regeneration, owing to the use of relatively low temperatures (≈80 °C to 100 °C) for solid sorbent regeneration.Consequently, this results in reduced CO 2 capture costs for the DAC plant. [70,71]Sinha et al. [71] estimated the cost of DAC utilizing solid adsorbents to be in the range of USD 86/t CO 2 to USD 221/t CO 2 , which is lower than the cost estimates mentioned earlier for DAC systems using liquid solvents.While other pathways for DAC, such as electrochemical approaches, mineral carbonation, membranes, and photocatalytic CO 2 conversion, have been proposed, they are not yet as extensively studied as the solid sorbent-based approach. [72]AC is a promising but currently less economical technology for removing CO 2 from the atmosphere.Its high energy requirements, large-scale infrastructure, and low concentration of CO 2 in ambient air contribute to its higher costs compared to traditional carbon capture methods.DAC's relative immaturity and competition from cheaper alternatives like renewable energy also hinder its economic viability.Nonetheless, ongoing research and development may lead to efficiency improvements and cost reductions in the future, making DAC a more feasible option for combating climate change and achieving negative emissions goals.

Carbon-Based Adsorbents for CO 2 Capture
The adsorption of CO 2 uses an adsorbent, which selectively adsorbs target gas.The adsorption process involves two stages: adsorption and desorption.Periodically adsorbing a desorbing is used to complete the CO 2 concentration, followed by regeneration of the sorbent.Based on common adsorption mechanisms proposed throughout time, the method can be categorized into chemical and physical adsorption (Figure 3). [73]hemical adsorption refers to the process where chemical bonds are formed among gas molecules and the adsorbent surface that holds them together.The chemisorption is usually carried by Lewis acid-base interactions and hydrogen bonding.Some studies have claimed that hydroxyl (─OH) and carboxyl (─COOH) groups improve the hydrogen bond interactions. [74]his phenomenon occurs due to the contrasting electronegativities of hydrogen (H) and oxygen (O) atoms.The hydrogen atoms in ─COOH and ─OH groups possess a relatively high electro-positivity, enabling them to form hydrogen bonds with the oxygen atom in CO 2 . [74]Lewis acid-base interactions also occur through N-containing functional groups present on the adsorbent surface.The N-atom is electron-rich due to a lone pair, thus enabling the adsorption of CO 2 .Apart from Lewis acid-base interactions, a few N-containing functional groups can also form hydrogen bonds with CO 2 due to high electro-positivity. [75,76]n physical adsorption, the gas molecules are held at the adsorbent surface by microscopic forces like the Coulomb force and van der Waals force. [77]This forms a stable adsorption without introducing chemical bonds.When gas molecules come in contact with the adsorbent surface, they adhere to the surface due to microscopic forces, leading to the formation of a layer of adsorbent, called the adsorption phase.This layer has a much higher density as compared to the gas density.The remaining gravitational force on the surface and that of gas molecules are combined, which generates an adsorption affinity from the mutual combination, referred to as the adsorptive force.There are usually three sources of adsorptive forces: a) the van der Waals force (both attractive and repulsive) between atoms and molecules, b) the Electrostatic Coulomb force among charged particles, and c) the induced force due to induction of permanent dipole moment. [78]he most notable advantage of physical adsorption is the desorption of adsorbed molecules from the surface of the material can be done under low energy consumption.This enables the reuse of material over multiple cycles, thus making physical adsorption a highly preferred method. [79]O 2 is selectively captured from mixed gases based on differences in size, shape, and molecular interactions.Adsorbents or solvents with specific pore sizes or functional groups preferentially accommodate CO 2 due to their unique properties.CO 2 interacts differently with these materials compared to other gases, resulting in varying levels of adsorption or absorption affinity.The efficiency of the capture process is influenced by temperature, pressure, and the rate of CO 2 interaction with the adsorbent or solvent.Understanding these factors allows researchers to develop customized materials and conditions, enhancing CO 2 capture selectivity and effectiveness for carbon mitigation and sustainable energy production.Following the same, this section will discuss various materials derived from different sources that have been potentially investigated for their CO 2 adsorption properties.

Carbon Materials Derived from Synthetic Polymers
To begin with, it is important to understand the interactions between the adsorbent (host) and CO 2 (guest).Common carbon materials like activated carbons and molecular sieves are synthesized through pyrolysis and physical/chemical activation of organic precursors (e.g., fruit shells, wood, and coal) at raised temperatures.The presence of a broad range of pore sizes in both mesopore and micropore makes carbon even more favorable than other compounds.Therefore, an effectively controlled synthesis route of the carbon framework is highly desired.
Deriving porous carbon precursors from synthetic polymers ensures better chemical composition, precise morphology, and an adjustable pore system to understand surface chemistry.Synthesis methodology plays an important part in maintaining all these aspects.Copyright 2011, American Chemical Society.c) Isosteric heat of adsorption at different CO 2 loadings.Reproduced with permission. [88]Copyright 2013, The Royal Society of Chemistry.CO 2 adsorption isotherms for d) CS-T (nonactivated) at 25 °C, e) CS-T-CD-4 at 25 °C, and f) CS-T-CD-4 at 0 °C.Reproduced with permission. [87]Copyright 2013, American Chemical Society.

Soft-Templating Method
The soft-templating process is receiving great attention for the preparation of porous carbon materials by self-assembly of soft templates (i.e., amphiphilic block copolymers) and soft templates (i.e., phenol resins).The same strategy is applied to the new carbon monolith design with improved functions. [80,81]The highly ordered mesoporous carbons were prepared via the solvent annealing process, accelerated with the self-assembly method.The results showed that trivalent hydroxyl phloroglucinol acts as an excellent precursor in the synthesis of mesoporous carbons with orderly mesostructured due to improved hydrogen bonding with tri-block polymers.At 1 bar and 25 °C, the mesoporous carbon showed an adsorption capacity ranging from 1.38-4.36mmol g −1 .Further, the KOH activation resulted in an enhanced CO 2 capacity of 7 mmol g −1 at 0 °C and 4.4 mmol g −1 at 25 °C. [82]ei et al. [83] synthesized N-doped well-organized mesoporous carbon with up to 13.1 wt% N content through dicyandiamide via evaporation induced self-assembly process.The resultant carbon showed CO 2 capture capacity of 2.8-3.2mmol g −1 at 1 bar and 25 °C.Zhao et al. [84] reported the synthesis of ordered mesoporous carbons with boric acid (B) and/or phosphoric acid (P) doped heteroatoms.Resorcinol-formaldehyde (RF) resin was used as a carbon precursor with Pluronic F127 as the mesoporous framework template.A fast and scalable route was demonstrated for the synthesis of crack-free N-doped carbon monolith with a rapid sol-gel method at 90 °C.The developed material had well-organized mesostructured with large domain interconnected 3D cubic arrangement (Figure 4a,b), obtained via organic base lysin polymerization agent with mesostructure assembly promoter. [85,86]Another study developed a new selfassembly approach utilizing benzoxazine chemistry for adsorption.The final structures displayed exceptional CO 2 capture and separation capacities at room temperature with simple regeneration and high selectivity.At 1 bar pressure, monoliths demonstrated an equilibrium capacity of 2.6-3.3 and 3.3-4.9mmol g −1 at 25 °C and 0 °C, respectively (Figure 4d-f).Whereas the dynamic capacity using 14% (v/v) CO 2 in N 2 came in the range of 2.7-4.1 wt% at 25 °C.From the mixture of CO 2 /N 2 , carbon monoliths showed high selectivity toward CO 2 with a separation component of 13-28.Along with this, the monolith also undergoes a simple release of CO 2 when kept under an Argon stream at 25 °C. [87]o further improve the adsorption capacity and shorten the diffusion path of porous carbon nanosheets, Hao et al. [88] developed a special form of porous carbon nanosheets (PCNs) with precisely controllable thickness of nanometer scale (Figure 4c).This characteristic gave this type of porous carbon a slight preference over micro-sized porous carbons whose structure is difficult to control.The maximum CO 2 adsorption was monitored at 1 bar and 25 °C of 5.67 molecules/nm 3 pore volume and 3.54 molecules/nm 3 surface area.The improved results were mainly due to the large microporosity with 8 Å pore size, the presence of polar surface induced by the residual of heteroatom (i.e., N and O) containing species, and due to the arrangement of neighbouring CO 2 .Zhao and co-workers published the deduction of Ncontaining hollow carbon nanospheres (N-HCSs) by the Stöber method. [89]The formed N-HCSs had a uniform size of ≈200 nm with 14.8 wt% nitrogen content and 767 m 2 g −1 surface area.Reproduced with permission. [90]Copyright 2011, American Chemical Society.CO 2 adsorption isotherms of the sulphur co-doped porous carbon at b) 25 °C and c) 0 °C.Reproduced with permission. [93]Copyright 2018, Elsevier.
Along with this, it showed an exceptional CO 2 capture capacity of 2.67 mmol g −1 and high selectivity of CO 2 in the mixture of N 2 and O 2 gases.
Jeroniec and coworkers synthesized a series of carbon spheres (CS) by carbonizing phenolic resin spheres through the modified Stöber method. [87]The prepared activated carbon spheres exhibited diameters in the range of 200-420 nm and a high surface area of 730-2930 m 2 g −1 .Most significantly, there were the large number of micropores present by volume (i.e., 0.28-1.12cm 3 g −1 ).Furthermore, high CO 2 capture capacities of 4.55 and 8.05 mmol g −1 were obtained at 1 bar/25 °C and 1 bar/0 °C, respectively. [87]A new technique was developed to synthesize highly ordered carbon nanospheres (diameter up to 200 nm) with precise sizes and superior monodispersity (Figure 5a). [90]Monodisperse carbon spheres can be produced by carbonizing polybenzoxazine spheres.These have high porosity and nitrogen groups of an intrinsic nature, resulting in high CO 2 adsorption capacities. [91]Another work reported the synthesis of nitrogen-doped activated carbon through the soft templating method. [92]Among the various samples prepared with different activation temperatures, the catalyst with 800 °C showed a specific surface area of 1900 m 2 g −1 , with < 1 nm micropore volume.This sample exhibited a CO 2 adsorption capacity of 6.25 mmol g −1 at 273 K and 1.13 bar.
It has been observed that co-doping of materials significantly enhances the CO 2 adsorption properties.Jin et al. [93] developed nitrogen and sulphur co-doped porous carbon material from poly benzoxazine and triblock copolymer (Pluronic F127) as a soft template.The highest surface area shown was 2385 m 2 g −1 .Furthermore, the materials prepared with templates displayed fewer sulfur, nitrogen, and oxygen moieties.This is mainly due to the volatilization of individual components during the activation and carbonization process.The maximum CO 2 uptake for templated materials was 4.5 and 6.7 mmol g −1 at 25 and 0 °C and 1 bar, respectively (Figure 5b,c).Kannola et al. [94] developed well-organized mesoporous carbon with nitrogen and sulfur doping from polybenzoxazine using resorcinol, 2-aminothiazole, formaldehyde, and different amphiphilic triblock copolymers.The resultant dual-doped material showed the improved specific surface area of 910 m 2 g −1 , improved mesoporosity, and volume of 0.42 cm 3 g −1 .At 1 bar and 273 K, the CO 2 adsorption capacity was 4.25 mmol g −1 , while it was 3.17 mmol g −1 at 298 K and the same pressure.Carbon dioxide is adsorbed physically and chemically via heteroatom-doped carbonaceous materials due to the ex-istence of basic sulphur and nitrogen groups.This leads to strong interaction forces among acidic CO 2 and basic sulphur/nitrogen components.The type of adsorption mechanism is determined by identifying the adsorbent's isosteric heat of adsorption.Higher Q st values signify porous network and high coordination among CO 2 , S, and N groups.

Hard-Templating Method
Hard-template method (or nano-casting) is a controllable way to get carbon monoliths with adjusted pore size.In nano-casting, a mould (also called a hard template) is permeated with precursor, and the mold is removed after complete processing.Several templates used in the nano-casting process are discussed below.
Crystalline Microporous Material as Templates: It is a molecularsieve-type porous carbon material, prepared from the templates of well-crystalline zeolites and MOF materials.This kind of nanoporous carbon display high specific surface area of up to 4000 m 3 g −1 and 1.8 cm 3 g −1 micropore volume. [95,96]Banerjee et al. [97] experimented with the gas adsorption characteristics of porous carbons through isoreticular zeolitic imidazolate frameworks as a template and furfuryl alcohol as a carbon source.Consequently, Liang et al. [98] reported the synthesis of a series of porous carbons from metal-organic coordination polymers using in situ polymerized phenol resin as carbon precursor.The effect of topological sequence and functional groups of various MOFs on resulting carbon material was reported by ZIF-8, 68, and 69 as precursors. [99]any zeolites have been utilized to prepare zeolite-templated carbons with 6-10 membered rings. [100]These includes LTA, AFI, CHA, IWT, etc. (three letter codes are given by the International Zeolite Association). [101]However, few of these carbon materials do not show promising properties due to poor bonding od micropores and small pore entrances, causing the blockage during carbon filling. [102]Zeolite-templated carbons also have high electrical and thermal conductivity, high stability, corrosive resistance, and tunable wettability. [103]These characteristics have led to further research of zeolite-templated carbons in various fields like fuel cell, [104] gas storage, [105] catalysis, [106] solar evaporation, [107] and capacitors. [108]orous Silica as Template: Well-defined pores are obtained when porous silica is employed as a template.Porous carbon via this method were prepared using mesoporous silica MCM-48 as a template material to prepare the CMK-1 carbon material. [109]Afterward, Hu et al. [110] used mesoporous and microporous silica template to prepare hierarchically porous carbon with high graphite-like carbon content.In the view of the advantage that nano-casted porous carbons provide, Liu et al. [111] synthesized mesoporous nitrogen-doped carbon having 2D hexagonal structures.In the synthesis process, SBA-15 was used as a hard template with diaminobenzene as carbon and nitrogen sources.By modulating the synthesis temperature, pore size could be controlled from 3.4-4.2nm.Moreover, the specific surface area of 26.5 wt% nitrogen-doped mesoporous carbon was tuned between 281.8-535.2m 2 g −1 .
Ammonia and surfactants are commonly utilized in the synthesis routes to act as catalysts and structure directing agents.However, these reagents are responsible for several kinds of pollutions and are considered non-friendly from environment's perspective.Hence, it is very important to design an eco-friendly alternative procedure to develop hollow carbon spheres.Chen et al. [112] proposed a self-catalysing synthesis route where the resorcinol-formaldehyde resin was employed as carbon precursor with amino-functionalized silica (NH 2 -SiO 2 ) as hard template.The catalyst was finally activated with KOH.After activation, the hollow carbon spheres displayed a CO 2 adsorption capacity of 3.65 mmol g −1 at 298 K and 1 bar.The carbon spheres also showed better recyclability and re-adsorption capacity after repeated cycles.One of the most notable points of this study is that this synthesis path is comparatively environmentally friendly.Generally, most of the microporous materials with high specific surface area prepared via chemical activation with NaOH, ZnCl 2 , KOH, and H 3 PO 4 .This sort of synthesis faces several drawbacks like corrosion, high energy intensity, time-consuming and tedious process steps.Consequently, the production of microporous adsorbents with high CO 2 selectivity via an economical pathway becomes a demanding task.
Colloidal Crystals as Template: Colloidal crystals are known for their self-assembling property and are called periodic structures with closely packed uniform particles.The development of colloidal silica/polymer spheres mostly leads to high degree of 3D periodicity.Furthermore, the removal of template results in replica with 3D ordered microporous (3DOM) structures.Although great achievement shave been made in the field of colloidal crystals, herein we will limit our discussion to templating of Adelhelm et al. [113] prepared meso-and macroporous carbons with mesophase pitch as precursor and PS or PMMA as templates.Gierszal et al. [114] carbonized the phenolic resin thin layer on suitable templates to synthesize consistent carbon film with large pore volumes, uniform pore size, and thickness.In a study conducted by Wilke et al., [115] they developed mesoporous ionic liquids (ILs) using a method involving back-filling the interstitial voids of porous colloidal crystal silica with a solution of IL monomers.This was followed by cross-linking and etching processes.The resulting porous ionic liquids (PILs) exhibited remarkable characteristics, including an increased surface area ranging from 150 to 220 m 2 g −1 , well-defined and uniform pore structures, a narrow distribution of pore sizes, and notably improved CO 2 adsorption capacity.The highest CO 2 uptake was reported for mesoporous PIL (≈0.46 mmol g −1 ), with a lower uptake for the bulk PIL (≈0.13 mmol g −1 ) and the monomeric species (≈0.02 mmol g −1 ) at 273 K. Hongkun et al. [116] reported the synthesis of 3DOM materials using colloidal crystals as templates for reversible capture of CO 2 .In the synthesis process, monodisperse poly(methyl methacrylate) (PMMA) spheres were synthesized, forming 3D ordered arrays.Using these colloidal crystal templates, ordered macroporous polymeric materials were created by infiltrating with a mixture of the monomer (vinylbenzyl)trimethylammonium chloride (VBT-MACl) along with a cross-linker and an azo initiator, followed by thermal polymerization and removal of PMMA spheres, resulting in interconnected and highly porous structures.The CO 2 adsorption was conducted at 90% and 20% relative humidity and at a temperature of 20 °C.The KOH-treated 3DOM polymeric material with 10% cross-linking displayed highest CO 2 adsorption rate of 6.2 × 10 −3 mmol min −1 g −1 , which is higher than commercially available Excellion membrane with 4.0 × 10 −3 mmol min −1 g −1 .
Normally, three types of basic precursors are involved in the infiltration of colloidal crystals to synthesize polymer networks.These are polymer solutions, monomer solutions, and monomer-cross-linker solutions.In case of polymer solutions, only evaporation completes the job, whereas for other two types, in situ polymerization is done to get a polymer matrix.In polymer and monomer solutions, the final polymer network is uncrosslinked, however, the cross-linked network is reported beneficial to enhance the mechanical properties and high temperature resistance of solvents. [116]Most of the studies used common precursors and tested the variation of structural and physical properties only.However, to expand the scope and usefulness of 3DOM polymeric materials, the components must be tailored for desired application.

Template-Free Synthesis
Template-free synthesis of porous materials involves the conversion of molecular precursors into heavily cross-linked organogels.Polymer-based monolithic carbons have received a great deal of attention in polymerization and surface functionalization research. [117]Fairen-Jimenez and team proposed carbon aerogels having monolith density from 0.37-0.87g cm −3 . [118]The aerogels were produced by carbonizing RF polymer-based aerogels, prepared in different solvents like water, ethanol, methanol, acetone, and tetrahydrofuran.It was found that the micro-and mesopores are only present in samples with density greater than 0.61 g cm −3 .Chen et al. [119] prepared a series of hierarchical porous carbons (Figure 6a-d) through facile template-free method using polyimide carbon precursor.The cornflower like structured carbon (Figure 6d) showed highest CO 2 uptake of 4.05 mmol g −1 at 273 K and 1 bar.The diffusion and collection of CO 2 molecules in moderate meso-and macropores and abundant micropores let to excellent CO 2 uptake capacity.
The copolymerization between carbon precursor and additional modifiers can also be used to directly prepare functional carbons with improved CO 2 adsorption capacity. [120]Sepehri et al. [121] synthesized nitrogen/boron co-doped carbon cryogels through homogeneous dispersion of NH 3 BH 3 in RF hydrogel.Such nitrogen/boron co-doping improved the porous structure, which in return enhanced the transport properties of molecules  [119] Copyright 2018, Elsevier.CO 2 adsorption properties of porous carbon materials: e) different nitrogen sources, f) different carbon sources.g) Adsorption capacity of E-PTDC at different CO 2 flow rates.(M-BTDC: material with melamine as nitrogen source, E-BTDC: material with ethylenediamine as nitrogen source, H-BTDC: material with hexamethylenetetramine as nitrogen source, E-BTNC: porous carbon with dimethylformamide as solvent, pPDA: porous carbon with p-phenylenediamine as diamine monomer, E-BPDC: porous carbon with 3,3′,4,4′-Biphenyltetracarboxylic dianhydride as dianhydride monomer).Reproduced with permission. [123]Copyright 2020, Elsevier.and ions.Further, Lu et al. [122] reported time efficient synthesis of nitrogen-doped carbon monolith via sol-gel co-polymerization of resorcinol, L-lysine, and formaldehyde.The prepared material displayed a carbon capture capacity of 3.13 mmol g −1 at 25 °C.
Similarly, Zhang et al. [123] synthesized carbon materials with different nitrogen sources (namely, melamine, ethylenediamine, and hexamethylenetetramine) and investigated their CO 2 adsorption properties.The best results were achieved when ethylenediamine was used as a nitrogen source.Further, ethylenediamine was used as a nitrogen source to study the CO 2 uptake behavior of different carbon sources and solvents.The materials prepared with ethylenediamine as the nitrogen source, p-pheylendiamine as the diamine monomer, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride as the dianhydride monomer, and dimethylformamide as the solvent had the best adsorption performance at a CO 2 flow rate of 50 mL min −1 (Figure 6e-g).It was demonstrated that the nitrogen doping and microporous structure increases the performance of material.However, the exact reason behind varying CO 2 adsorption at different flow rates is unclear.Further studies are required to investigate the influence of flow rates and mechanism behind.
Shen et al. [124] synthesized a series of ordered porous carbon fibres with polyacrylonitrile nanofibers as precursors.The highest BET surface area shown by material was 2231 m 2 g −1 with pore volume of 1.16 cm 3 g −1 .The material also had sufficient basic sites and large concentration of nitrogen (≈8.1 wt%), making quick and high CO 2 adsorption as compared to commercial 13X type zeolite used to encapsulate CO 2 in wet conditions.
The template-free synthesis method prohibits the use of any hard or soft template, making the as-prepared carbon material competitive in the economic sense.Furthermore, template-free synthesis of material is expected to enhance research on nanostructure design and synthesis of carbon materials from low-cost polymers.

Conclusion
Three distinct strategies are commonly used for fabricating polymer-derived carbons, each with its advantages and challenges.The CO 2 adsorption capacities and material surface area of few of the materials derived from these methods is listed in Table 2 for easy comprehension.The hard-template method offers precise porosity control, resulting in true negative replicas of templates.However, its extensive usage is hindered by the complex template synthesis process and the use of hazardous chemicals (e.g., HF and NaOH) for template removal.
On the other hand, the soft-template method is relatively simple and versatile, allowing the fabrication of porous carbons with complex structures, various porosities, and surface functionalities.Nonetheless, this approach requires strong selfassembly ability to endure high-temperature thermal treatment (carbonization).The main challenges here are low yields, limited choices of soft-template materials, and relatively higher costs, raising economic and environmental concerns for large-scale applications. [34]inally, the template-free method and soft-template technique typically yield higher carbon conversion.For example, Monte's group achieved a high yield (80%) of carbon monoliths with tailored mesopore diameters using deep eutectic salts as solvents and carbonaceous precursors. [125]Conversely, the hard-template a) Some quantities have been converted/approximated from published units to the given units.
method yields relatively lower carbon conversion due to the carbon collapse after template etching.Overall, each method has its merits and limitations, making it crucial to consider the specific requirements and constraints of a given application when selecting the appropriate technique for fabricating porous carbons.

Carbon-Based Materials Derived from Biomass
Biomass-derived porous carbon materials are highly preferred for CO 2 adsorption due to their superior textural properties, simple synthesis method, high CO 2 uptake, inexpensiveness, and renewable nature, reducing the overall cost of CO 2 capture process.In the synthesis, the biomass is converted into biochar by heating at moderate temperatures without oxygen supply.This process is called pyrolysis. [127][130][131] A wide range of carbon precursors like wood, animals and plants waste, and some non-wooden materials have been used to make biochar.Activated porous carbons also utilize a similar procedure but include the incorporation of on activating agent in the pre-or post-synthesis of biochar.
Although biochar has abundant functional groups, still, activated porous carbons are preferred due to their high porosity and specific surface area.The character and number of functional groups present on the carbon surface depend upon the synthesis method and nature of biomass precursor.Biomass presents two prominent benefits when it comes to CO 2 capture.First, the CO 2 present in environment is fixed with plant biomass through photosynthesis.This biomass is then pyrolyzed to separate carbon as a biochar or activated porous carbons.Second, all these materials can also be utilized as adsorbents in industrial operations like pre-and post-combustion CO 2 capture technologies.
Till now, several biomass-derived activated carbons have been utilized for CO 2 capture purposes (as represented in Table 3).Polysaccharide-derived "Starbons" carbon is one such example with mesoporous texture.Researchers are also focused on developing carbon-containing materials from other sugar as biomass.For instance, Sevilla et al. [132] proposed a wide range of carbonaceous materials produced via hydrothermal treatment of biomass precursors with KOH as an activating agent at different temperatures (Figure 7a-d).The activated materials showed increased micropores in hydrothermally treated carbons.The surface area also varied with activation conditions with the range of 1260-2850 m 2 g −1 .Finally, the activated material showed a high capacity of 4.8 mmol g −1 at 25 °C and 1 atm.It was further concluded that this capture capacity is owed more to the presence of micropores and less to the surface area.Following the same, Sevilla et al. [133] also synthesized N-doped carbon materials from hydrothermal treatment of carbons derived from algae and glucose, followed by its chemical activation.It was further shown that by carefully monitoring the activation temperature and KOH amount, an exclusive microporous material can be generated.These materials show a surface area between 1300 to 2400 m 2 g −1 with 1.2 cm 3 g −1 pore volume.The N content was in the range of 1.1-4.7 wt% with majority of pyridinic-type structures.The sorbent showed an improved CO 2 uptake capacity of 7.4 mmol g −1 at 0 °C and 1 bar, which was owed to their high volume of micropores.
Apart from the hydrothermal treatment, direct pyrolysis is another major activation process to derive porous carbons from a) Some quantities have been converted/approximated from published units to the given units.
waste culture leave and bamboo.The pyrolysis temperature is regarded as the most important parameter in controlling the final texture of the biochar.A few thermal decomposition profiles are represented in Figure 7e.Wang et al. [134] reported that the porous carbons with narrow micropore size and KOH activation of fungibased biomass showed 5.5 mmol g −1 CO 2 adsorption and 27.3 CO 2 /N 2 selectivity at 1 bar and 0 °C.Hence, the carbon materials obtained from biomass will play a major role in designing and developing nanostructured materials.Incorporation of heteroatoms within these materials can significantly enhance the CO 2 adsorption propertied of the materials.The fact that utilization of biomass materials bears a low capital investment and negligible amount of waste makes it an effective option to apply at large scales.Most research have pointed that the existence of high microporous structure and high specific surface area complement each other in pre-and post-combustion CO 2 capture.

Carbon-Based Materials Derived from Organic Frameworks
[153][154][155] Most COFs have pore sizes far larger than gas molecules, increasing gas permeability but decreasing selectivity. [156]The CO 2 capture ability of MOFs is remarkable due to their high surface areas, adjustable pore sizes, and controllable surface properties.However, their practical applications are often limited by poor chemical stability caused by weak coordination bonds.On the other hand, POFs, a class of porous materials composed of organic precursors linked by strong covalent bonds, exhibit relatively high chemical stability.159] Hu et al. [160] proposed the synthesis of carbon material with high porosity and fibre like morphology via direct carbonization of Al-based PCP.The residual Al content was removed using HF (hydrogen fluoride) solution after calcination.The prepared material showed a surface area of 5500 m 2 g −1 and pore volume of 4.3 cm 3 g −1 .The most important aspect about the prepared material was that it was able to retain its fibrous morphology even after undergoing extensive HF treatment so that high surface area and large pore volume are retained.[163] Chaikittisilp et al. [164] and Yang et al. [165] synthesized nanoporous carbonaceous materials though direct carbonization of MOFs.ZIFs (Zeolitic Imidazolate Frameworks) are characterized by a metal-imidazolate-metal angle that bears resemblance to the Si-O-Si angle found in zeolites.and d) 800 °C.Reproduced with permission. [132]Copyright 2011, The Royal Society of Chemistry.e) Thermal decomposition profile of cellulose, hemicellulose, and lignin-based biomass.
ZIFs exhibit expansive surface areas and excellent heat tolerance, making them highly promising candidates for research in the field of carbon capture. [166,167]Chaikittisilp et al. [164] documented nanoporous carbons having high surface area of 1110 m 2 g −1 and a narrow pore size distribution close to the parent ZIF-8.As per the research conducted by Wang et al. [99] three ZIFs, namely ZIF-8, ZIF-68, and ZIF-69 with different topologies and functional imidazolate-derived ligands were directly carbonizing at 1000 °C to prepare porous carbons.After activation with KOH, the respective materials showed a BET surface area of 2437, 1861, and 2264 m 2 g −1 , respectively.Along with it, ZIF-69 represented the highest CO 2 uptake of 4.76 mmol g −1 at 1 atm and 273 K, emerging majorly from its pore structure and its chemical environment.In a study by Ramos-Fernandez et al., [168] a simple synthesis method was developed for ZIF-93 in aqueous solution at room temperature.The obtained ZIF-93 exhibits remarkable thermal and chemical stability.This material has a significant surface area of 604 m 2 g −1 and a pore volume of 0.46 cm 3 g −1 .
In particular, at 303 K, ZIF-93 displayed a CO 2 adsorption capacity of 180 cm 2 g −1 at 30 bar and 35 cm 3 g −1 at 1 bar.Given these promising results, the method used by Ramos-Fernandez et al. has great potential for larger scale applications in the future.It is very important to note that ZIFs are still in the laboratory research stage where few materials outstand than other.However, no exact statement can be made regarding their preference over other class of materials.
A series of nanoporous carbons could also be synthesized via thermal decomposition of dopant-free MOF.Lim et al. [169] discovered that the porosity of carbon materials specifically varies with the Zn/C ratio of MOF.Using this finding, Srinivas et al. [170] reported first ever hierarchically porous carbons (HPC) synthesized from the carbonization of custom-developed MOF.Along with simultaneous high surface area and pore volume, the improvements in synthesis conditions could highly yield MOF-5 in the millimeter range.Further, by carbonizing this MOF, HPCs with high surface area and pore volume of 2734 m 2 g −1 and 5.53 cm 3 g −1 , respectively can be produces.The meso-and micropores present in HPCs come from the defects of the crystals inherited from MOF precursors.The obtained HPCs can have the highest CO 2 adsorption of up to 27 mmol g −1 at 30 bar and 27 °C, being one among the highest reported in literature.
sparked the idea that one-step synthesis of nanoporous carbons adapted form crystalline microporous materials could be highly efficient for CO 2 capture.
Despite the enormous research efforts on MOF adsorbents, the literature discussing adsorption in MOFs is relatively less.The parameters concerning MOFs at industrial scale are mostly summarized in Table 4. Commercialization of MOFs and their subgroups faces significant hurdles in their synthetic procedures, which can be costly and raise environmental concerns.However, a promising alternative approach is to use aqueous solutions instead of solvothermal methods.Aqueous solutions are a cheaper option for MOF synthesis as they pose fewer toxicity issues and offer a more commercially viable option.

Enhancing the Properties of Carbon-Based Adsorbents
Although several factors influence the CO 2 adsorption characteristics, however, only a few most influential parameters shown in Figure 8 are discussed in the following sections.

Chemical Doping
CO 2 molecule is highly quadrupolar and weakly acidic.This slightly acidic molecule will get highly polarized when interacts with a basic functional group.Thus, introducing a metal cation in the adsorbate will trigger the adsorption sites to adjust and arrange the CO 2 molecules in its pore space.The surface properties can be modified not only by pre-introducing the precursor but also by post-modification of some existing carbons.
][200][201] Doping of N atoms mainly follows two approaches, first being utilizing the nitrogencontaining precursors and the second is to proceed with a high temperature transformation of existing carbons with NH 3 , etc. Zhong et al. [202] synthesized N-rich porous carbons through the carbonization of polyacrylonitrile containing block polymer.The prepared sample showed good CO 2 selectivity over N 2 by 7-10 folds of adsorbed CO 2 .Nandi et al. [203] reported N-doped carbons obtained from mesoporous PAN monolith through two steps of thermal treatment.The obtained sample showed high isosteric heat of adsorption (Q st ) value of 65.2 kJ mol −1 .The high initial Q st refers to strong adsorbent-adsorbate interactions among N-containing carbon and the CO 2 molecules.Bai et al. [204] reported a low-cost pathway to prepare nitrogen-rich porous carbons where petroleum coke was employed as a precursor after modifying it with urea and KOH under changing conditions.Improvement by urea generated enough nitrogen groups in the carbon structure and high fraction fine pores with less than 1 nm.The developed porous carbon adsorbed 4.4 and 6.75 mmol g −1 CO 2 at 25 and 0 °C and 1 bar, respectively.On the similar lines, Sevilla et al. [205] chemically activated the porous nitrogen-rich carbons to capture CO 2 .Polypyrrole was used as a precursor, followed by activation at different temperatures, and CO 2 adsorption under pure CO 2 flow of 100 mL min −1 .This research showed that mildly activated porous carbons develop pyridonicand pyridinic-N sites with narrow micropores.These two characteristics leads mildly activated carbons to adsorb more CO 2 at high rates (generally 95% CO 2 is adsorbed within 2 min).The highest CO 2 adsorbed was recorded to be 3.9 mmol CO 2 g −1 at room temperature, attributed to narrow microporosity and high density of N-basic sites.
Treating the already prepared porous carbons with ammonia under high temperatures is another way to prepare N-doped carbons.At elevated temperatures, ammonia decomposes with simultaneous formation of radicals like NH 2 , NH, and H.These radicals then interact with carbon surface to form ─NH 2 ─, ─CN, pyridinic, pyrrolic, and quaternary nitrogen.Due to the presence of groups like C─N and C═N, the CO 2 uptake of high temperature ammonia treated carbons enhances significantly (Figure 9a,b). [206]However, due to high temperature, the micropores may close or their size may get affected.Furthermore, Pevida et al. [207] reported that ammonia treatment above 600 °C substitutes nitrogen mostly in an aromatic ring, B.E. ≈ 533.2-533.8eV (which are thermally more stable), while lowtemperature treatments places it in labile functionalities like amide-like functionalities (B.E.≈399.6-399.9eV).It was concluded that the overall CO 2 adsorption depends more on the functional groups (such as amide-like functionalities) rather than the total nitrogen content in the catalyst.
[210] In a theoretical study of role of oxygen-groups in CO 2 uptake materials, it was indicated that oxygen functional groups on a graphite structure can highly enhance the CO 2 adsorption and its selectivity over CH 4 and N 2 .It was also confirmed that the position of functional groups also influences the gas adsorption properties. [182,185]Lu et al. [211] examined the outcome of edgefunctionalization in nanoporous carbons over CO 2 /CH 4 mixture.As shown in Figure 9c, the adsorption of single gas molecule was shown in the model to study the relative reactivity.At low pressure (say 0.05 MPa) (Figure 9d-i-v), the CO 2 molecules adsorb prior to CH 4 on the material pore surface.While at high pressure (12.00 MPa) (Figure 9d-vi-x), CO 2 molecules accommodate the pore spaces and CH 4 molecules fill up the left-over voids.Xia et al. [212] studied the effect of sulfur-doping on CO 2 adsorption.The 6.5 wt% S content the Q st value of 59 kJ mol −1 , advising high S-CO 2 interactions.215][216] Bhagiyalakshmi et al. [217] documented the one-pot synthesis of Mg-doped ordered mesoporous carbon, which showed an adsorption capacity of 92 mg g −1 at 25 °C.Xu et al. [218] reported Li-doped MOF-5 for selectivity of CO 2 over CH 4 and the enhanced mechanism of adsorption due to introduction of Li atoms (Figure 10a,d).
Li easily loses its electron density, making it highly electropositive (Figure 10b,c).This electrostatic potential and high negative value of aromatic rings prefers CO 2 adsorption over CH 4 .Gao et al. [219] studied doping one/four Ca atoms into pure C60.The results showed the possibility of capturing up to five/sixteen CO 2 molecules, owing to the strong electrostatic interactions among CaC60 system and CO 2 molecules.
While carbons doped with nitrogenous groups have been extensively studied for their ability to capture CO 2 , the adsorption mechanism behind their performance is still a matter of debate.Currently, three main perspectives are used to better understand this issue: a) Acid-base interaction: The presence of electronegative carbons and electronegative oxygen in CO 2 makes it a weak Lewis acid, while the N-doped porous carbons act as Lewis bases due to the negatively charged nitrogen atoms. of them.The interaction between these nitrogenous basic groups and the acidic CO 2 molecules enhances CO 2 adsorption on nitrogendoped carbons.The researchers found that N-doping increased CO 2 absorption and selectivity due to strong acid-base interactions. [220]For example, Wang and Yang observed a linear relationship between the CO 2 adsorption capacity and the surface area, concluding that the N-doped carbons exhibit better CO 2 adsorption capacity than the carbon atoms.undoped carbon atom. [221]) Electrostatic interaction: The CO 2 molecule has a significant electric quadrupole moment due to its strong C═O dipole bonds.N-doping introduces polar groups, which induce local polarization in the carbon structure.This enhances the surface polarization and induces an electrostatic field gradient around the N-doped carbons.The strong interaction between the CO 2 quadrupole moment and the high electrostatic potential of the phase carbons.N impurities enhance CO 2 adsorption capacity.[222,223] Zhuo's group synthesized N-doped microporous carbons, achieving high CO 2 absorption (1.67 mmol g −1 ) and selectivity (CO 2 /N 2 up to 50:1).[224] The strong quadrupole interactions of CO 2 with the N-doped Reproduced with permission.[206] Copyright 2004, Elsevier.c) Interaction model of CO 2 -CH 4 adsorption on edge-functionalized unit.Nomenclature: S: side position; B: position above the bridge; T: top position; H: hcp.d) Equimolar CO 2 -CH 4 fusion in nanoporous carbons at 298 K and pressure 0.05 (i-v) and 12.00 (vi-x) MPa.Reproduced with permission. [211] Copright 2004, The Royal Society of Chemistry.
micropore walls are thought to provide superior performance.Furthermore, Sun's group demonstrated that the negative electrostatic potentials induced by the N-doped sites increased CO 2 adsorption, rather than acid-base interaction.
It was the first time to explicitly present a precise linear relation function to quantitatively illustrate this subject-object interaction. [225]) Hydrogen-bonding interaction: Xing et al. [38] proposed a new adsorption mechanism based on hydrogen-bonding interactions to explain the remarkable CO 2 capture performance of N-doped porous carbons derived from bean dreg biomass waste.They developed these carbons via KOH chemical activation without additional N-containing species, achieving a high CO 2 uptake of 4.24 mmol g −1 at 25 °C at atmospheric pressure. Expeimental and theoretical studies confirmed the presence of hydrogen bonds between CO 2 molecules and the N-doped carbon surface due to the high electronegativity of CO 2 's oxygen atoms.The incorporation of N atoms into the carbon framework significantly enhanced hydrogen-bonding interactions between CO 2 molecules and hydrogen atoms on the carbon surface, leading to outstanding CO 2 capture performance.This finding challenges the conventional acid-base interaction explanation and provides new insights into the role of N-containing groups in CO 2 adsorption.
In contrast, amine immobilization on porous carbons involves a different mechanism, where primary and secondary amines attack CO 2 to form a zwitterion, and tertiary amines catalyze hydration to produce bicarbonate in the presence of water.However, amine impregnation has drawbacks such as pore blockage, amine leaching, thermal instability during long-term cycling, and the need for high-temperature regeneration.Therefore, it may not be the most efficient approach for CO 2 capture under ambient conditions.Considering the potential of N-doped porous carbons and their hydrogen-bonding interactions as a promising alternative, the study offers valuable insights into enhancing Li-purple; Zn-blue; and H-white.Reproduced with permission. [218]Copyright 2010, The Royal Society of Chemistry.
CO 2 capture efficiency and highlights sustainable ways to mitigate CO 2 emissions. [226,227]

Chemical Functionalization
Another major strategy to improve CO 2 capture is to polar functional groups into the adsorbent material to change the affinity of the scaffold.The chemical functionalization enhances the quadrupole CO 2 and functional group interactions by an acid-base chemistry, induced dispersion, and electrostatic chemistry.The NH 2 functional group has been widely studied in CO 2 adsorption properties due to its high affinity toward CO 2 .Apart from this, other popular polar functional groups like F, Cl, Br, OH, COOH, CN, NO 2 , SO 3 , and PO 3 also show similar effects under specified conditions and enhances the CO 2 -scaffold interactions.Several new components like MOFs and metal oxides also have high CO 2 interactions, thus providing active sites for CO 2 binding.However, MOFs suffer from the disadvantage of their hydrophilic nature, an introduction of block co-polymer may come out to be a valid option to restrict the water permeation.Qian et al. [228] reported one such method where the structural characteristics of hierarchical porous carbon monolith were improved by introducing Cu 3 (BTC) 2 MOF via multi-step impregnation.As shown in Figure 11a, octahedral crystals of Cu 3 (BTC) 2 disperse withing HCP matrix, developing a restriction for water permeation (Figure 11b).This composite showed almost twice CO 2 adsorption (22.7 cm 3 STP cm −3 at 1 bar) than the hierarchical porous carbon monolith (12.9 cm 3 STP cm −3 ) at 1 bar (Figure 11c).As seen in Figure 11d, simultaneous regeneration demonstrations showed that the composite retains almost 92-96% intrinsic capture capacity.
Zhao et al. [229] reported an easy method to synthesize impregnated amine-grafted carbon adsorbent via biomass as a precursor.The substrates showed a raspberry type of morphology (Figure 11e), favorable for liquid amines (typically polyethyleneimine or tetraethylenepentamine) loading.The adsorbent showed high CO 2 uptake of 4.3 mmol g −1 and at -20 °C.The obtained CO 2 adsorption capacity of prepared material was found to be in similar range as other materials (i.e., NaX (5.4 mmol g −1 ), ZIF (3.1 mmol g −1 ), MCM-48/AP (2.2 mmol g −1 ), CARiACT/PEI (3.1 mmol g −1 ), etc.).Although the adsorption capacity was highly improved, the impregnation of liquid amines showed some negative effects as well.These were mainly the pore blockage and unstable basic sites during the cyclic use of adsorbent.To study these issues, Hwang et al. [230] proposed a route to prepare polymer-carbon composite via in situ polymerization of amines to generate polyethylenimine and polyvinylamine in the mesocarbon CMK-3 (Figure 12a).An interpenetrating composite structure formed between polymers and porous carbons gave high stability to this composite.The CO 2 adsorption results showed that 39% polyethylenimine -CMK-3 has ≈12 wt% and 37% polyvinylamine-CMK-3 has ≈13 wt% CO 2 uptake at 30 °C and 1 atm.Moreover, the composite also displayed the improved cyclic stability (up to 500 min) and regeneration at 75 °C.
Introducing O-containing groups in porous carbons significantly increases the CO 2 uptake, especially for COOH functional group. [231]Wilcox and co-workers studied the effects of surface chemical functionalization on CO 2 adsorption in graphite slit pores. [74,232]Figure 12b shows the atomic partial charges for O atoms in COOH group, exhibiting strong electronegativities due to electron density from surrounding C atoms. [233]The acidic C atoms take electron densities form Lewis bases, thus more CO 2 molecules are attracted toward pore space, resulting in high CO 2 adsorption capacity.This also helps in improving the CO 2 selectivity over other gases like CH 4 (Figure 12c).Sometimes, the adsorption capacity of a parent material is enhanced by introducing a Lewis basic site on N-containing group. Copyright 2012, American Chemical Society.e) SEM graph of amine rich carbonaceous material synthesized in of acrylic acid.Reproduced with permission. [229]Copyright 2010, John Wiley and Sons.
Among the newly emerged techniques, applying external electric field or charges to polarizable substrates like boron nitride and carbon nanotubes is a prominent one.In this method, electronic structure estimations are combined with van der Waals calculations to study the electronic properties of functionally modified graphitic surface. [182]Carbon nanotubes are pointed as effective materials for selective CO 2 adsorption, owing to their short size and facile functionalization. [74]he integration of functional groups is also a universal strategy toward surface engineering of 2D covalent organic framework pores.The role of electrostatic interfaces on CO 2 selectivity has been promising due to the existence of electronegative atoms.The strong electronegative atoms in chemical functionalization act as basic adsorption sites.Due to this, strong interactions appear between CO 2 molecules and the integrated functional units.In total, this chemical functionalization improves adsorbed and gas-phase equilibrium characteristics like heat of adsorption, pore space, packing pattern, and density.

Structural Design
The textural properties like sizes of micropores, fraction of micropores, pore size distribution, total pore volume, and surface area play a vital in bonding CO 2 molecules to the adsorbent surface.Microporosity is commonly considered among the main factors for efficient CO 2 capture.[240][241][242][243] The topological structural design with physical characteristics is usually done by: a) Adjusting pore size to enhance CO 2 adsorption capacity, [244,245] b) Improving the pore structure to intensify CO 2 -framework interactions, [246] c) Increasing the total surface area to adsorb the maximum number of CO 2 molecules, [247] and d) Designing novel adsorbents with favourable pore sizes and shapes. [248,249]esser et al. [244] investigated a series of carbide-derived carbons for the relationship between CO 2 uptake and pore volume at different pressures.The results showed that CO 2 uptake does not depend on the pore volume, but the pores smaller than specific diameter.At 0.1 and 1.0 bar, the pores smaller than 0.5 and 0.8 nm, respectively, mostly contribute toward the CO 2 uptake.Nearly all porous carbons have large crystal size in all dimensions and thus, CO 2 molecules take longer time to move in and out of inner microporous network.Due to this, there is not complete utilization of inner pores and the total surface porosity.Due to this issue, approaches like construction of hierarchical pore structure, [85,86] improving mesoporosity, [250] and decreasing the size structure size to nanoscale [88,251] have been explored.For an instance, sheet-like carbons show an improved kinetics for CO 2 uptake as compared to spherical counterparts due to their short diffusion paths and highly exposed geometrical area.
In addition, the porosity of porous carbons is another factor to improve CO 2 uptake.The porosity can be easily optimized by selecting activation method and its conditions.Activation of porous carbon is carried out via two methods, namely physical or chemical activation.Qian et al. [252] fabricated microporous carbon materials with distinct Zn species as dynamic molecular porogens to give additional micropores to enhance CO 2 uptake and selectivity (Figure 13a).As shown in Figure 13b, each Zn specie exhibited different surface area.In the carbonization process, the added nano-paths were formed by the evaporation of Zn species and contributes to additional micropore volume.The resulting microporous carbon materials displayed greater micropores of size 0.7-1.0nm and CO 2 update of 5.4 mmol g −1 at 273 K and 3.8 mmol g −1 at 298 K (Figure 13c,d).Skarmoutsos et al. [248] advised a 3-D carbonaceous material with molecular sieves for CO 2 adsorption (Figure 13e,f).The results from molecular dynamics simulation of gas molecular diffusion confirmed an improved adsorption of CO 2 over CH 4 molecules (Figure 13g).The results clearly states that topological structure design is very important parameter to enhance the functioning of adsorbents.
Increasing the specific surface area is also important to enhance the topological design.Huang et al. [253,254] proposed a tetrahedral node diamondyne frameworks by substituting carbon nodes of diamondyne/diamond with acetylenic linkage (C─C ≡ C─C) formed tetrahedron nodes.The synthesized tetrahedral node diamondyne framework showed a specific surface area of 6250 m 2 g −1 , which is much higher than zeolites, mesoporous silica, activated carbons, and marginally higher than MOFs and COFs.The framework also showed high CO 2 adsorption of 2461 mg g −1 at 298 K and 5 MPa with a superior selectivity of CO 2 over H 2 .Liu et al. [255] documented a spherical N-containing polymer and derived microporous carbon materials with high specific surface area of 528-936 m 2 g −1 and 0.6-1.3nm of micropore size.The microporous carbons experienced high CO 2 uptake, mainly due to the presence of N-containing groups and < 0.1 nm sized micropores.

Open/Unsaturated Metal Sites
The open or unsaturated metal sites on the pore surface act as charge-dense binding site, improving the affinity of CO 2 adsorbent like MOF, COF, and zeolites.These metal sites are usually obtained via the incorporation of solvent molecules as terminal ligands, which can immediately be detached after desolvation at high temperature or in a vacuum. [256,257]Many articles confirmed that metal cations can be simply introduced into adsorbents via chemical reduction and cation exchange. [258,259]Such metal sites enable the easy approach of CO 2 molecules toward the pore surfaces and improve the gas-adsorbent interactions.Due to the high polarity and quadrupole moment of CO 2 , the selectivity is also enhanced over high-affinity binding sites. [260]he increased coordination between CO 2 molecule and metal (O═C═O⋯Metal) also results in superior mechanism of selectivity and adsorption of CO 2 . [261]The end-on CO 2 binding to open metal site (Figure 14a) shows a strong correlation between metal and CO 2 binding enthalpies.Different metal atoms like Mg, Ca, Ti, Co, Cu, etc. can be doped in MOF-74 to enhance CO 2 uptake.From Figure 14b, it is evident that Ti-and V-MOF-74 show higher CO 2 affinity as compared to other derivatives of  [252] Copyright 2014, John Wiley and Sons.e) Representation of 3D porous nanotube network.f) Filling up of 3D porous nanotube network with CO 2 /CH 4 mixture.g) Number density versus mole fraction profiles of CO 2 and CH 4 in z-axis.Reproduced with permission. [248]Copyright 2013, American Chemical Society.MOF-74.As shown in Figure 14c,d, the strong binding in this case is coming from the positive charge induced local electric field across open metal spots, which further normalises the lonepair interactions of CO 2 and unoccupied d-levels of metals. [262]part from this, the most outstanding feature of M-MOF-74 is that its CO 2 adsorption increases in the presence of moisture (Figure 14e).For an instance, the computational study of the partition energy of Zn-MPF-74 (Figure 14f) pores implicated higher CO 2 selectivity and increased adsorption capacity in CO 2 /H 2 O mixture. [263]The study revealed that presence of moisture reduces the pore size close to CO 2 molecule diameter, making it vital for improved CO 2 adsorption.However, the regeneration and  with the organic linker and the lone-pair with the metal site (Ti: blue, C, H: white, O: red).Reproduced with permission. [262]Copyright 2005, American Chemical Society.CO 2 /H 2 O binary mixture inside e) Mg-MOF-74 (black), tpt-Mg-MOF-74 (50%) (green), tpt-Mg-MOF-74 (100%) (blue), and tpt-Mg-MOF-74 (100%, crossed) (red) as a function of H 2 O mole fraction at 0.15 bar and 298 K. f) Same as (e) with Zn-MOF-74.Reproduced with permission. [263]Copyright 2017, American Chemical Society.g) Gravimetric and h) volumetric isotherm of CO 2 in Li/COFs at 298 K. Reproduced with permission. [264]Copyright 2010, American Chemical Society.recyclability of material in the presence of moisture is relatively lower and requires further improvement.
Cao and co-workers reported that Li, Sc, and Ti atoms highly increase the CO 2 adsorption properties of COFs. [264,265]i/COF-102 (Figure 14g) showed a maximum adsorption of 1349 mg g −1 while Li/COF-105 (Figure 14h) displayed a capacity of 2266 mg g −1 at 298 K and 40 bar.Karra et al. [266] emphasized that MOFs with open metal sites greatly increases the selectivity of CO 2 uptake owing to different polarities and electrostatic interactions.Generally, developing adsorbent materials with coordinatively unsaturated metal sites is a viable pathway to increase the gas adsorption properties and its selectivity.

Current Status
The sudden evolution of anthropogenic CO 2 release in atmosphere has caused immense global warming and climate change.For this, several developments have been made to sequester CO 2 from the point source.Few of these developed processes are very energy intensive and require large quantity of water to detached CO 2 , adding additional limitations and overall cost to the process.As a result, the research focus largely shifted toward solid carbonbased sorbents for CO 2 capture owing to their easiness of handling, fast kinetics, high stability, high uptake capacity, precise selectivity, low regeneration energy requirements, and comparatively low cost.Currently, the target is to develop solid adsorbent materials with high specific surface area, improved pore characteristics, high adsorption, and high stability at elevated temperatures.Although extensive studies have been published regarding diverse carbonaceous materials for CO 2 adsorption, still, several challenges still needs to be dealt with.This review examined the various promising carbon-based CO 2 adsorbents.The first half of the review elucidated the sorbents, their design methods, and respective properties, while the second half focusses on different strategies to enhance the CO 2 adsorption properties of a material.
One of the common issues with the synthesis of the adsorbent materials is high activation temperature.This issue not only leads to high energy consumption, but also disturbs the spherical morphology of the material.Therefore, a less energy intensive synthesis route is required that retains the physical anatomy of the adsorbent throughout the process.In many cases, a tubular-like furnace is used for sample activation, which have several drawbacks like irregular temperature profiles, longer activation time, high energy consumption, and obstructed release of volatile gases.All these factors affect the quality of the final adsorbent.To solve this issue, microwave treatment is considered as a promising alternative to be used in synthesis process.The microwave-based approach offers better release of volatile gases, resulting in better pore formation.Moreover, it is less energy intensive, takes short time for the operation, and cost effective.
As the many research reports discussed above suggest, ultranarrow micropores are highly desirable for the selective CO 2 capture.They are functionally active and works well in humid conditions.For future carbon-based adsorbents, emphasis should be given on reducing the number of steps in synthesis procedure.This would help in minimizing the structural shrinkage and collapsing of material, improving the mechanical strength, and in-creasing the CO 2 selectivity.Although polymer supports can be added to monoliths to influence the mechanical stability, however, this highly effects the CO 2 capture capacity of the adsorbent.Thus, optimization of polymer percentage is required to achieve better adsorption and stability.
Study of dynamic CO 2 adsorption performance is a crucial factor in commercializing any new adsorbent materials.Consequently, the suitable CO 2 adsorbents must be tested under high gas flow rates of flue gas streams from oil refineries, petrochemical plants, and power plants.This would help in establishing the selectivity of CO 2 in presence of other gases like SO x , NO x , CO, and fly ash particles under humid atmosphere with various pressure and temperature ranges.For a successful adsorption process, the adsorption capacity should be least effected with increasing the cycle number and regeneration steps.Hence, suitable regeneration approaches need to be developed that not only retain the adsorption capacity of CO 2 , but also keep it almost unchanged with more intense cyclic tests.
Selection of suitable raw materials for preparing economical adsorbents is also equally important.Moreover, finding carbon precursors with basic sites like N, S, and metallic constituents is highly desirable to eliminate any post-synthesis modification of adsorbent material.Disposal of used material should also be considered before synthesis such that it does not pose any harm to the environment and human health.For example, resorcinol, the expensive and widely used resin in the synthesis of carbonbased adsorbents, can be replaced by phenol and melamine to reduce the raw materials costs.

Future Outlook
Undoubtedly, carbon-based adsorbents have a long history of research, and the innovations are continuously emerging.However, the functionality and performance of the currently available adsorbents needs to be improved in many aspects like stability, CO 2 selectivity, adsorption, morphology, cyclability, and resistance against various temperature.and pressures.All this could be achieved by better understanding of the carbon chemistry.Moreover, a system should be designed for in situ transformation of adsorbed CO 2 molecules directly from adsorbent to useful fuels.
A very important point to focus here is that every process (including the synthesis of CO 2 capturing material) release some amount of CO 2 to the environment.Hence, in the future, truly sustainable synthesis pathways need to be developed as well.Although we focus on the CCS as a potential solution, but a net-zero goal can never be realistically achieved by the CCS alone, especially when it is linked to highly polluting coal and gas projects.Each industry requires a unique solution to reduce the associated CO 2 emissions.However, there are few steps which are common to limit the greenhouse gas emissions (Figure 15).
Since the road to CCS is still very long, a collaborative and collective efforts from academic research and industry R&D are required to achieve as minimum CO 2 concentrations as possible.Therefore, Since the road to CCS is still very long, a collaborative and collective efforts from academic research and industry R&D are required to achieve as minimum CO 2 concentrations as possible.Therefore, this review paper consolidates a vast body Few steps to limit greenhouse gas emissions. [267] knowledge on CO 2 capture methods and materials into a single resource.The article integrates the key aspects of CO 2 capture by examining advanced materials and strategies.The paper also makes a significant contribution by exploring carbon-based adsorbents for CO 2 capture.It includes materials derived from synthetic polymers, biomass, and structures, providing insight into their unique properties, applications, and challenges.However, it does not cover all available potential adsorbents, which can be found elsewhere in literature.In addition to materials description, the article also explores methods to improve the properties of carbon-based adsorbents, including chemical doping, open metal placement, structural design, and chemical functionality.These innovative techniques add a new dimension to research, providing valuable insights into the future of CO 2 capture materials and technology.In addition, the article reviews the current state of CO 2 capture adsorbents, identifying knowledge gaps and potential research opportunities.This review is invaluable as it will direct future research and investment efforts into more efficient carbon capture technologies.Apart from this, the paper also provides information about current CO 2 capture industries, CO 2 policies of developing nations, and techno-economic considerations of relevant processes.We hope that this approach will be a useful resource for researchers, engineers, and policymakers seeking an in-depth understanding of the topic.This analysis goes beyond conventional methods to include emerging strategies and technologies, giving readers a forward-looking perspective.

Figure 1 .
Figure 1.Illustration of CO 2 capture in post-and pre-combustion technologies.

Figure 2 .
Figure 2. Criteria to design an ideal CO 2 capture material.

Figure 8 .
Figure 8. Few strategies to enhance CO 2 capture capacity of carbon-based adsorbents.

Figure 13 .
Figure 13.a) Synthesis of the microporous carbon material using Zn species as dynamic molecular porogens.b) Correlation between the amount of ZnCl 2 used and the specific surface areas.CO 2 -adsorption isotherms of the microporous carbon materials at c) 273 K and d) 298 K. Reproduced with permission.[252]Copyright 2014, John Wiley and Sons.e) Representation of 3D porous nanotube network.f) Filling up of 3D porous nanotube network with CO 2 /CH 4 mixture.g) Number density versus mole fraction profiles of CO 2 and CH 4 in z-axis.Reproduced with permission.[248]Copyright 2013, American Chemical Society.

Table 2 .
Different carbon materials derived from synthetic polymers and their CO 2 adsorption capacities.