Direct air capture of CO2: from insights into the current and emerging approaches to future opportunities

The rapid development of direct air capture (DAC) technologies has become critical in order to remove CO2 from the atmosphere and limit global warming to a maximum of 1.5°C. In this perspective, we provide a mini review of the current research on the emerging liquid- and solid-based sorbent materials to capture CO2, summarize the existing challenges of DAC technologies, and suggest future research directions to accelerate the development of DAC systems. In particular, the desired properties for a breakthrough sorbent that efficiently captures CO2 from the air and releases it for sequestration are described.


. Introduction
The growing global reliance on fossil fuels for energy and material production has resulted in record-high carbon dioxide (CO 2 ) emissions in the atmosphere (Kumar et al., 2015;Sanz-Pérez et al., 2016). To mitigate the potential impact on climate change, ∼10 gigatons of CO 2 /year needs to be removed to limit global warming to a maximum of 1.5 • C by 2050 according to multiple reports (World Resources Institute, 2002;Philander, 2012;Ozkan, 2021;IEA, 2022). Direct air capture (DAC), a process that captures CO 2 directly from air is critical to negative emissions technologies (NETs), and plays a significant role in achieving net zero emissions by the 2050 Scenario, among other CO 2 removal (CDR) approaches (National Academies of Sciences, 2019; IEA, 2021). According to the International Energy Agency (IEA) projection for achieving net-zero emissions by 2050 Scenario, the CDR removal target by DAC is ∼980 Mt CO 2 per year (House et al., 2011). However, capturing CO 2 , particularly from the air is a challenging and energy-intensive process, because CO 2 in the atmosphere is highly diluted, ca. 400 ppm (0.04%), when compared to the CO 2 concentration in a flue gas stream emitted from a power station, cement or natural gas plant (≈4-15%). Further, a significant amount of CO 2 must be removed from the atmosphere and sequestered to have a quantifiable positive impact on the environment, and thus the technological challenge is formidable.
In this perspective, we provide an overview of the current approaches of DAC systems and emerging sorbent materials associated with CO 2 capture from dilute streams. Sorbents typically capture CO 2 at ambient temperatures and are regenerated by releasing the captured CO 2 at elevated temperatures (e.g., by applying convective heat) or using alternative heat sources (e.g., microwave) (Gomez-Rueda et al., 2022), as well as non-thermal techniques such as electrochemical (Stern and Hatton, 2014;Voskian and Hatton, 2018;Rahimi et al., 2020), ultrasound (Ying et al., 2014) and pressure-swing (Wiheeb et al., 2016). Lackner and co-workers introduced a moisture-swing process for DAC where the sorbent absorbs CO 2 when dry and releases it when wet (Lackner, 2009;Wang T. et al., 2011;Shi et al., 2020). Sorbents can be liquid, solid, or a hybrid material that has both liquidlike and solid-like components. The development of sorbents requires the consideration of numerous performance parameters, such as high CO 2 capacity, selectivity, fast sorption/desorption kinetics, and chemical stability under variable temperature and humidity. Furthermore, industry-related challenges such as the operational cost and energy demand can only be addressed when an efficient sorbent material is integrated into a DAC system. Thus, a molecular-level understanding of sorption kinetics, thermodynamics, and structure-property relations are imperative for the discovery of a breakthrough sorbent. Further, research focusing on alternative regeneration techniques is essential to reduce the overall energy consumption and cost of CO 2 removed, i.e., joule per ton of extracted CO 2 in a DAC system.

. Current status and challenges
In general, CO 2 capture or removal from a mixture stream process includes adsorption, absorption, and/or membranebased separation. Adsorption and absorption-based processes are typically considered more promising for directly capturing of CO 2 at ultradilute conditions (McQueen et al., 2021). In an absorptionbased process, CO 2 dissolves in a liquid solvent such as an aqueous amine, aqueous sodium hydroxide, ionic liquid (IL), or a mixture of solvents via physisorption or chemisorption (Mahmoudkhani and Keith, 2009;Kumar et al., 2020). Alternatively, in an adsorptionbased process CO 2 is chemisorbed or physisorbed onto a solid surface that has chemical functionality for CO 2 -philicity, such as in the case of zeolites, metal organic frameworks, or covalent organic frameworks (Choi et al., 2011). CO 2 binding processes are exothermic in nature and requires active cooling to maintain CO 2 capacity. This challenge is mitigated to some extent due to distributed heat of the solid surface material and the density of sites in adsorption. Thermodynamically, the affinity between the sorbent and the target gas molecule is quantified through the heat of absorption or the isosteric heat of adsorption (Builes et al., 2013). Recent studies comparing the energetics of temperature swing vs. pressure swing CO 2 separation processes for a generic adsorbent (with a heat of adsorption of −65 kJ/mol) found the temperature swing adsorption to be more efficient under dilute CO 2 concentrations (i.e., 50% removal from feed and 95% product purity) (Lackner, 2013;Lively and Realff, 2016). Conversely, pressure swing adsorption is more efficient for bulk gas separations, as pressurizing the inlet feed with low CO 2 concentration makes the process energetically costly.
Even with temperature swing processes, both adsorption and absorption processes consume large amounts of thermal energy during the sorbent regeneration, in addition to the energy required to move air through the sorbents. In particular, the trade-off between the strength of CO 2 molecular interactions with the sorbent and the required regeneration energy remains a challenge. An aqueous solution of calcium hydroxide is an effective sorbent for capturing CO 2 from the air due to its high affinity to CO 2 . For example, pilot plant of Carbon Engineering has the capacity to remove 1 Mt of CO 2 /year (requires 8.81 GJ per ton of CO 2 removed) by using an aqueous-sorbent (Keith et al., 2018). First, air is passed through a series of filters to remove any particles and pollutants. Next, the purified air flows into a contactor, a container filled with a solution of potassium hydroxide (KOH). As the air passes through the contactor, the CO 2 in the air reacts with KOH to form potassium carbonate (K 2 CO 3 ) and water. The aqueous solution is then regenerated for reuse by heating at 900 • C to release the captured CO 2 . Table 1 summarizes the key comparisons for DAC plants around the world, including Carbon Engineering (Ozkan et al., 2022).
In contrast to absorption and adsorption processes, membranebased CO 2 separation works on the principle of preferential permeation, where the target gas molecules diffuse across the membrane film under isothermal conditions. The gas molecule can permeate through the membrane via different mechanisms including size sieving, solution diffusion, surface diffusion, ion and facilitated transport. For CO 2 separations from air using a membrane, combination of these diffusion mechanisms is needed since there is a lack of high-pressure driving force to transport CO 2 across the membrane by solution diffusion alone. Few recent studies explored facilitated transport membranes for DAC Matsuoka et al., 2021;Nabity et al., 2021;Lee et al., 2022). Lee et al. (2022) prepared a thin film of poly(ionic liquid) (PIL)-IL impregnated graphene oxide membrane supported on a poly(ethersulfone)/poly(ethylene terephthalate) substrate, and reported excellent CO 2 permanence of 3,923 GPU and CO 2 /N 2 selectivity of 1,200 under 410 ppm CO 2 with a 1 bar feed gas (CO 2 /N 2 /H 2 O mixture) at 40% RH and 22 • C. In this example, helium was used as the sweep gas on the effluent side which limits the purity of the separated CO 2 and the post-process application. For post-sequestering purposes, higher purity CO 2 is needed and the study showed the separation performance of the membrane decreases under vacuum conditions on the effluent side due to the need to reinforce the membrane further. However, the purity requirement does not have to meet prior targets set (i.e., > 95%) for CO 2 removal from post-combustion flue gas. While the recent reports are encouraging to further develop facilitated transport membranes for DAC, they also suggest the need of an integrated approach based on multiple membrane modules or integration with other separation units. Another emerging approach alternative to temperature swing processes is the Faradaic electro-swing process developed by Hatton and co-workers where the CO 2 binding is regulated by electroactive species [e.g., amine sorbents  and quinone (Gurkan et al., 2015) carriers] (Voskian and Hatton, 2019). Recently, they demonstrated a bench scale, .
/frsus. . solid-state Faradaic electro-swing reactive adsorption unit that achieved > 90% Faradaic efficiency with a CO 2 feed < 0.1%, demonstrating a work of 40-90 kJ per mole of CO 2 captured, representing favorable thermodynamics comparison to most other thermally regenerated adsorbents (Voskian and Hatton, 2018). In the following sections, we focus on the conventional and emerging sorbents based on absorption and adsorptionbased processes that have demonstrated promising CO 2 capture capabilities at ultradilute (400 ppm) conditions.

. . Conventional sorbents for CO capture
To date, most of the research on DAC has been focused on hydroxide-or amine-based aqueous sorbents due to their efficacy in capturing CO 2 under ultradilute conditions (Sanz-Pérez et al., 2016;Sodiq et al., 2023). The hydroxide based solvents need 900 • C for regeneration with energy requirement of 6-9 GJ/tCO 2equivalent to the energy utilized by an average household in the US for 2-3 months (Baciocchi et al., 2006;National Academies of Sciences, 2019;McQueen et al., 2021;Lebling et al., 2022). Besides this substantial energy requirement, sorbent loss incurred during the sorption/desorption process via evaporation and degradation is of significant concern. Further, water loss in the air contactor of solvent-based carbon capture systems can also significantly limit their deployment and techonomic effectiveness, particularly in dry climates (Rosa et al., 2021;An et al., 2022). On the other hand, porous solid-supported amines present higher stability under moisture in ambient air, lower heat capacities (≈1-1.5 J/g K compared to the CO 2 -loaded monoethanolamine ≈4 J/g K) (Weiland et al., 1997;McQueen et al., 2021), and mild regeneration temperatures (≈50-120 • C) (Wang et al., 2015). However, when compared to liquid sorbents, amine supported solid sorbents are limited by their slow sorption kinetics, limited CO 2 capacities at low partial pressures, and poor cyclability (absorption-desorption cycles with maintained capacity) (Wang Q. et al., 2011;Lai et al., 2021). The following sections discuss the improvements made over conventional sorbents in consideration of these challenges.

. . Emerging liquid sorbents
Aqueous amino acids have been investigated for carbon capture, which are relatively non-volatile, environmentally friendly, and regenerable using mild heating (≈100 • C). For instance, Custelcean and co-workers reported amino acid-based sorbent with a capacity of 0.7 mol of CO 2 per mol of aqueous solution at ambient air conditions (Custelcean et al., 2019). The CO 2saturated bicarbonate species in the solution were then crystallized using a 2,6-pyridine-bis(iminonoguanidine) (PyBIG) to form a solid hydrated carbonate in an aqueous solution. Heating the solid carbonate produced at a comparatively mild temperature (120 • C) removes most of the bound CO 2 and water via the reaction mechanisms shown in Figure 1A. In general, amino acids with a highly basic functional group are desirable for CO 2 capture (Ramezani et al., 2022), but the reported amino acids present lower cyclic capacities typically in the range of 0.12-0.4 mol CO 2 /mol sorbent (Recker et al., 2022).
Researchers have also explored the use of amino acid derived ILs as an alternative liquid sorbent for CO 2 capture. ILs are unequivocally versatile materials for CO 2 capture with negligible volatility and high chemical stability . For instance, Gurkan et al. synthesized an amino acid-based IL, trihexyl(tetradecyl)phosphonium prolinate, ([P 66614 ][Pro]), which exhibited a CO 2 capacity of ∼0.9 mol CO 2 /mol IL in low partial pressure region at 22 • C (Gurkan et al., 2010a). A significant disadvantage of amine-functionalized ILs for CO 2 capture application is their high viscosity leading to slow sorption kinetics. In this regard, Schneider, Brennecke, and co-workers developed aprotic heterocyclic anion (AHA) ILs for CO 2 capture and showed that the viscosity of ILs remained unchanged before and after the CO 2 absorption while maintaining a 1:1 CO 2 sorption Frontiers in Sustainability frontiersin.org . /frsus. . stoichiometry (Gurkan et al., 2010b;Seo et al., 2014). In addition, the reaction enthalpy (∼ 50 kJ/mol) was lower than conventional amines such that regeneration was possible at temperatures < 100 • C, giving a reasonable working capacity (capacity difference between absorption and desorption). However, the demonstrated AHA ILs had low gravimetric CO 2 capacities (∼9%) due to their large molar mass. Although there has been extensive research on the use of ILs for capture and utilization of pure CO 2 (Aghaie et al., 2018;Shukla et al., 2019), studies on their application in DAC have been limited (Yang and Dai, 2021).
Similar to ILs, deep eutectic solvents (DESs) are known to have significant physisorption capacity for CO 2 (García et al., 2015;Trivedi et al., 2016). DESs are mixtures of two or more components that form stable, low-melting-eutectics and are typically composed of a hydrogen bond acceptor, such as a halide salt, and a hydrogen bond donor (Smith et al., 2014;Hansen et al., 2021). Recent studies demonstrated functionalized DESs and eutectics that demonstrate CO 2 chemisorption capacity, similar to amine functionalized ILs (Zhang et al., 2018;Yan et al., 2020;Klemm et al., 2023). Lee et al. reported a functionalized DES for CO 2 capture that overcomes the limitations of traditional ILs, such as high viscosity and poor gravimetric CO 2 capacity at low partial pressures . In that study, a reactive IL, 1-ethyl-3-methylimidazolium 2-cyanopyrrolide, was used as a hydrogen bond acceptor component and ethylene glycol (EG) was the hydrogen bond donor. The 1:2 molar mixture of IL:EG showed a capacity of 0.85 mol CO 2 /mol solvent at 1 bar of CO 2 and 0.31 mol CO 2 /mol solvent at 410 ppm of CO 2 at 25 • C. Here, it .
/frsus. . is worth noting that the gravimetric CO 2 capacity of the DES (2.7 mol CO 2 /kg sorbent) was higher than that of the neat IL (2.1 mol CO 2 /kg sorbent) at 410 ppm CO 2 . This was attributed to carbonate formation with the deprotonated EG, in the presence of the pyrrolide anion, which was stabilized by the hydrogen bonding network ( Figure 1B). This study demonstrates that while the H-bonding network in DESs may increase viscosity, it can also modulate the CO 2 interactions with the sorbent, leading to an alternative reaction route as compared to neat IL. To tune functionalized DESs for DAC beyond the capture capacity, thermal and oxidative stability upon cycling must be established, since some of the parent compounds are volatile (unlike ILs).

. . Emerging solid sorbents
Traditional amine functionalized sorbents such as zeolites (Fu et al., 2022), mesoporous silicas (Shi et al., 2020) (Figure 2A), and moisture-swing resins/polymers (Wang et al., 2017; have been investigated for CO 2 capture, yet they have considerable shortcomings for DAC. This includes low CO 2 capacities, slow kinetics, diffusion-limited sorption, small surface area, and poor multi-cycle capacity, all of which significantly impact the sorbent material's overall performance. In this context, MOFs are considered superior to conventional solid sorbents because of their highly accessible surface area and adjustable chemical functionality (Sadiq et al., 2020). Choi et al. (2012) developed ethylene diamine (ED) modified ED-Mg/DOBDC ( Figure 2B), which showed a capacity of 1.5 mmol/g under ambient air conditions together with excellent thermal stability and regenerability. The amine groups grafted on the open metal sites introduced additional sites for chemisorption, which improved the CO 2 capacity compared to the parent MOF (1.35 mmol/g). Similarly, McDonald et al. (2012) demonstrated that alkylamine-loaded Mg 2 (dobpdc) have exceptional CO 2 uptake capacity (2 mmol/g) under 390 ppm at 25 • C ( Figure 2C). This improvement in the CO 2 uptake at very low partial pressure was ascribed to the interaction of electrophilic carbon of CO 2 with the nitrogen electron pair in diamine.
Eddaoudi and coworkers synthesized a MOF, NbOFFIVE-1-Ni, which exhibited excellent CO 2 sorption capacity (1.3 mmol/g) under 400 ppm CO 2 at 25 • C due to the favorable interactions of CO 2 with the fluorine centers of the MOF, with a regeneration energy of 54 kJ/mol ( Figures 2D, E) (Bhatt et al., 2016). Alternatively, Darunte et al. illustrated that the CO 2 capacity of a simple MOF [MIL-101(Cr)] could be improved by incorporating amine molecules into the MOF pores (Darunte et al., 2016). Accordingly, tris(2-amino ethyl) (TREN)-loaded MIL-101(Cr) exhibited eight-times higher CO 2 capacity (2.8 mmol/g) than the corresponding pristine MOF (0.35 mmol/g) at 0.4 mbar (400 ppm CO 2 in He) and 25 • C. The composite sample, however, showed a significant loss in cyclic capacity due to the excessive amount of TREN loading (more than the available unsaturated coordination sites). Therefore, future research on MOFs for DAC should focus on stability under various temperature and humidity conditions, cycling capacity, high CO 2 selectivity, and cost-efficient scalability.

. The way forward: key matrix for a breakthrough sorbent
Fundamental studies aiming to develop structure-propertyperformance relations are still needed to further develop sorbents for widespread application in DAC. In addition, computational studies, for example utilizing machine learning, are needed to accelerate sorbent discovery (Mohan et al., 2022). For example, simulations can provide insight into material properties that maximize CO 2 sorption (Guan et al., 2022) as well as CO 2 binding sites and their corresponding thermodynamics and kinetics (Qazvini et al., 2021;Ahmad et al., 2022). In the following sections, we discuss the significance of individual properties of the sorbents that make up the key matrix for a breakthrough sorbent which would significantly expedite the development of DAC technologies to help meet the global goal of decreasing atmospheric CO 2 levels.

. . Sorption, kinetics, and di usion
Sorbents with high basicity can increase affinity to CO 2 which is an acidic gas. However, strong binding of CO 2 , such as with primary and secondary amines relative to tertiary amines for example, also increases the overall energy required to regenerate the sorbent. Therefore, both the enthalpy of the reaction (recommended ≈ −49 to 65 kJ mol −1 ) (Lively and Realff, 2016;Yang and Dai, 2021) and the CO 2 capacity must be considered when designing a sorbent for DAC. Similarly, there is a trade-off between capacity and adsorption rate for solid sorbents. Grafting of CO 2philic moieties such as amine, hydroxy, sulphonate, imidazole, triazine, and imine groups onto solid sorbent pore surfaces to improve CO 2 capacity (Petrovic et al., 2021) can result in blocking of the pore openings and prevents CO 2 diffusion. In a solid adsorbent, gas diffusion can be tuned by modification of pore openings and creating a high surface-to-volume ratio to facilitate the accessibility to the active surface area.
For liquid sorbents, gas diffusion is impacted by the viscosity. Mota-Martinez et al. examined the impact of physiochemical and transport properties of CO 2 capture solvents in terms of the cost of the overall separation process for the emission mitigation purposes. They report that viscosity is a critical parameter determining the absorber size and the associated operational and capital costs (Mota-Martinez et al., 2017). For example, the aqueous amine benchmark solvent with a viscosity of 2.5 cP requires a 50 m tall absorber column whereas a solvent with a viscosity of 16 cP requires 133 m in height for the absorber. For post-combustion CO 2 capture, to maximize the efficiency of CO 2 capture process, it is generally recommended to keep the viscosity of a CO 2capturing solvent under 5 cP when operating in an absorber column (Song et al., 2017). However, similar analysis are not yet available for emerging DAC solvents. In complement to designing novel liquid sorbents with low viscosity, packaging sorbents, e.g., via encapsulation, is an attractive strategy for enhancing performance. Provided a capsule shell that is permeable to CO 2 is used, encapsulation can increase the effective surface area of the liquid sorbent and lead to more rapid gas uptake. Capsules of IL can be prepared by impregnation of a hollow carbon Frontiers in Sustainability frontiersin.org . /frsus. .
shell using a co-surfactant (Moya et al., 2018), extrusion (Zhang and Cai, 2012), or interfacial polymerization (Weiss and Abu-Reziq, 2017). For example, Pentzer, Gurkan, and coworkers used interfacial polymerization in IL-in-water or IL-in-oil emulsions to prepare capsules a core of IL and polyurea-based shell (Huang et al., 2019;Luo et al., 2019;Gaur et al., 2021). The same group encapsulated a task specific IL for the chemisorption of CO 2 and performed breakthrough and regeneration experiments; the capsules were stable under humid conditions, outperformed zeolites at low pressure, and were stable through the multiple absorption-desorption cycles (Lee et al., 2020). Thus, in addition to enhancing the physical properties of liquid sorbents, e.g., by decreasing viscosity, such composites can give access to new structures with various knobs to tune performance related to capacity, selectivity, and regeneration.

. . CO selectivity
Selective capture of CO 2 from atmospheric air is crucial since molecules such as N 2 , H 2 O, and O 2 can also be physisorbed. Further, volatile organic components can permanently bind to active sites that would otherwise be available to bind CO 2 , leading to a decrease in the overall CO 2 capacity of a sorbent. It is important to note that CO 2 capacity of liquid sorbents generally improves under moist conditions, since the interaction between CO 2 and water can also lead to the formation of carbonic acid and increase the CO 2 capacity (Avelar Bonilla et al., 2019). However, the presence of co-adsorbed moisture consumes additional thermal energy during the solvent regeneration. This is because water has a high heat capacity (4.2 J/g K), and the presence of moisture in the sorbent will consume more energy for regeneration (Quang et al., 2015). Additionally, in the context of adsorptive processes, water has higher selectivity compared to CO 2 toward sorption sites, thus hindering CO 2 sorption at the amine sites (Shaik et al., 2022). Recently, Young et al. (2021) proposed a mechanistic isotherm model for amine-containing sorbents that successfully predicted CO 2 co-adsorption performance in humid conditions on Lewatit R VP OC 1065 (a commercially available benchmark sorbent comparable to first-generation sorbent used by Climeworks for DAC process). However, the applicability of such accurate coadsorption models to other types of amine-functionalized sorbents, especially under high relative humidity representing real-world condition remains unclear and requires further development to uncover potential opportunities for enhancing the overall efficiency of a DAC process. Finally, high-purity CO 2 must be obtained so that it can be further sequestered or utilized as a feedstock to produce renewable fuels or value added chemicals. Therefore, to produce highly pure CO 2 from a mixture stream, pore openings of solid sorbent materials should be tuned according to the kinetic diameter of CO 2 (3.3 Å) together with high chemical affinity toward CO 2 .

. . Regeneration energy
The regeneration of sorbents in DAC currently requires energy in the range of 6-10 GJ/tCO 2 and 4-6 GJ/tCO 2 for liquid and solid sorbents, respectively (Ozkan et al., 2022). As mentioned earlier, the regeneration energy is related to the heat capacity and the reaction enthalpy of CO 2 binding. To make DAC cost-effective, developing sorbents that can be regenerated using energy similar to the flue gas CO 2 capture process (≈ 2 GJ/tCO 2 ) is crucial (Zhang et al., 2016). Furthermore, theoretical calculations estimate that the energy requirement to produce a highly concentrated stream of CO 2 (>90% purity) is 20 kJ/mol CO 2 under ≈400 ppm of CO 2 concentration (House et al., 2011). However, DAC technologies are currently not as efficient, with typical energy requirements of ∼400 kJ/mol CO 2 , assuming an efficiency of 5% for DAC system (House et al., 2011). Thus, increasing the CO 2 capture efficiency, ideally above 20%, is a critical target for making DAC more economically sustainable. Additionally, the high regeneration temperature requirement (e.g., 900 • C for hydroxide-based solvents) (Keith et al., 2018; National Academies of Sciences, 2019) poses a challenge in terms of thermal energy and heating rate efficacy; necessitating the need for alternative energy sources and the targeted delivery of energy (i.e., overcome reliance on bulk convective heating). In this context, electrical energy can be utilized to regenerate sorbents through techniques such as microwave (MW) and induction-based heating, which offer rapid dielectric heating rates compared to conventional thermal heating (Wilcox, 2020;Mohd Pauzi et al., 2022). Ozkan et al. (2022) demonstrated that the use of electricity for both liquid and solid sorbent regeneration yields a lower thermal energy equivalent for DAC. In addition, MW based-heating has several potential benefits compared to conductive heating methods, including energy efficiency, rapid heating rates, and the ability to provide instantaneous dielectric heating without heat transfer limitations (Gomez-Rueda et al., 2022). Lee et al. (2023) recently demonstrated the susceptibility of IL sorbents to dielectric heating and rapid CO 2 desorption. These benefits may help to reduce the overall energy consumption, regeneration rate, and cost of DAC operations. However, further research is needed to fully understand the potential of MW assisted regeneration method and its impacts on sorbent stability and absorption-desorption cyclability for DAC.

. . Thermal stability and oxidative degradation
Cyclic capacity is dependent on the thermal and oxidative stability of a sorbent. Oxidative degradation can be particularly problematic in amine-based CO 2 capture systems where the material is exposed to oxygen; amines can undergo chemical reactions in the presence of O 2 resulting in the formation of undesired byproducts, such as ammonia and amine-derived carbonates (Spietz et al., 2018;Vevelstad et al., 2022). Indeed, oxidative and thermal degradation in amines leads to significant CO 2 capacity loss (Vevelstad et al., 2022). In addition, degradation of solvent can increase corrosion, foaming, and fouling in a CO 2 capture unit, thus decreasing the lifetime of a system and increasing the cost of the CO 2 capture process (Saeed et al., 2018). Using a thermally stable solvent can help to minimize the need for frequent solvent replacement and reduce the overall cost of the process, as well as minimize environmental impacts. Recently, we reported on the oxidative and thermal degradation mechanism of a functionalized IL where its superior stability, Frontiers in Sustainability frontiersin.org . /frsus. . compared to conventional CO 2 capture solvents, was shown to maintain its high capacity (Lee et al., 2023). Hence, developing sorbent materials with minimal thermal degradation and excellent oxidative stability is essential for the development of practical DAC technologies. For most of the emerging sorbents, detailed stability and cyclability studies are scarce and thus their practicality cannot be fully assessed.
. . Design and cost DAC technology has an estimated cost of $264-1,000 per tCO 2 assuming 75% air capture efficiency and 95% CO 2 purity (National Academies of Sciences, 2019), which is still significantly higher than the cost of CO 2 capture from flue gas ($50-100/tCO 2 ) (Lebling et al., 2022). It is important to note that the current capture efficiency of DAC technology is <10% (Zeman, 2007;Long-Innes and Struchtrup, 2022), which significantly impacts the cost, making the debate to achieve the highly ambitious target of $100-300/tCO 2 viability skeptical (Küng et al., 2023). Since, DAC is an emerging technology, it is imperative to comprehensively explore the general energy requirements and economic feasibility, as well as the environmental viability of a breakthrough sorbent. An optimized design of an air-sorbent contactor column is critical to efficiently remove CO 2 from the air, especially in the case of powder sorbents. However, driving large volumes of atmospheric air and the associated pressure drop in a sorbent unit incur a substantial operational cost (Zolfaghari et al., 2022). Thus, developing an effective contactor to process the large volume of air will also help lower the overall cost of capturing CO 2 . Recent studies have indicated that fossil fuels remain a cost-effective energy source, but their use may also render the CO 2 capture process ineffective as it releases significant amounts of CO 2 (Terlouw et al., 2021). Therefore, for net zero emissions, in a DAC unit that utilizes fossil fuel-derived thermal energy, the downstream emissions should be captured while controlling the upstream emissions (Ozkan et al., 2022). On the other hand, integrating renewable energy resources such as wind and solar to a DAC unit would enable a cost-effective and environmentally friendly solution for reducing the current atmospheric CO 2 concentration (Zolfaghari et al., 2022).

. . Environmental impacts of sorbents
Environmental assessment of sorbents in DAC technologies is often overlooked. To fully evaluate DAC feasibility, it is crucial to consider the environmental impacts of sorbent production, disposal, and lifecycle costs (Leonzio et al., 2022). For example, sodium hydroxide used in DAC process is corrosive and generates toxic chlorine gas during production (Realmonte et al., 2019). ILs, on the hand, are relatively benign, in particular due to their negligible volatility, but require proper disposal (de Jesus and Maciel Filho, 2022). A recent study linked the presence of high levels of imidazolium-based cations in soil (i.e., landfill site sampled in United Kingdom) to human health issues such as primary biliary cholangitis which is a chronic autoimmune liver disease (Abdelghany et al., 2020).
Likewise, MOF synthesis often involves solvothermal strategies that use toxic solvents (Kumar et al., 2019;He et al., 2022). To address these issues, further research should focus on developing environmental friendly synthesis routes for emerging CDR sorbents that avoid the use of hazardous solvents and explore innovative techniques like microwave-assisted synthesis (Thomas-Hillman et al., 2018), electrosynthesis (Al-Kutubi et al., 2015), or 3D printing (Lieu et al., 2022). For MOF synthesis, these approaches are also shown to reduce synthesis time, promote rapid growth, and enable homogeneous packing assemblies as additional benefits.

. Conclusions
In this perspective, we have highlighted the emerging sorbents to capture CO 2 directly from the air. Most current research focuses on improving CO 2 working capacity sorption kinetics and does not typically address cyclability or the design of scalable DAC infrastructure. In the development of new sorbents, computational studies identifying descriptor for molecular design and approaches that accelerate sorbent discovery and prediction of properties are required. These will likely require multiscale modeling aimed at interrogating both bulk sorbent properties as well as detailed CO 2 binding energetics (Alizadeh et al., 2021;Heydari Dokoohaki and Zolghadr, 2021;Malik et al., 2021). The field is still in search of a breakthrough sorbent that possess high selectivity, high working capacity under varied humidity and temperature conditions, improved thermal and oxidative stability and cyclability. Even with a breakthrough sorbent, the critical factor in ensuring the successful net and sustainable removal of CO 2 is integrating the DAC system with low-carbon energy sources and establishing the lifetime of the sorbent, including appropriate operating conditions and stability to CO 2 uptake-release cycles. Therefore, there is also the need to develop technology platforms that transfer energy efficiently. Taken together, intensive research focus, government policy and support, and private industry funding are needed to overcome the technological challenges of developing viable DAC systems to operate at a net-zero cost.

Data availability statement
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Author contributions
MZ and BG created the first draft of the manuscript. All authors contributed to the revisions.