Materials for Direct Air Capture and Integrated CO2 Conversion: Advancement, Challenges, and Prospects

Direct air capture and integrated CO2 conversion (DACC) technologies have emerged as promising approaches to mitigate the increasing concentration of carbon dioxide (CO2) in the Earth’s atmosphere. This Perspective provides a comprehensive overview of recent advancements in materials for capturing and converting atmospheric CO2. It highlights the crucial role of materials in achieving efficient and selective CO2 capture as well as catalysts for CO2 conversion. The paper discusses the performance, limitations, and prospects of various materials in the context of sustainable CO2 mitigation strategies. Furthermore, it explores the multiple roles DACC can play in stabilizing atmospheric CO2.


INTRODUCTION
Direct air capture (DAC) is a technology that aims to capture carbon dioxide (CO 2 ) directly from the atmosphere.The rising levels of CO 2 contribute significantly to climate change and global warming.Addressing the impacts of these emissions requires effective strategies for both capturing and converting CO 2 .−3 Materials play a crucial role in the development of efficient DACC systems. 4These materials serve as catalysts, adsorbents, membranes, or electrodes, enabling the selective capture and conversion of CO 2 into valuable products or its safe sequestration.The search for innovative materials with enhanced properties has intensified in recent years, driven by the urgency to find sustainable solutions for CO 2 mitigation. 5n the field of DAC, materials capable of selectively adsorbing and absorbing CO 2 from ambient air are highly desirable.These sorbents should possess a high affinity for CO 2 , excellent selectivity over other gases, a high adsorption capacity, and good stability.Metal−organic frameworks (MOFs), zeolites, and amine-functionalized materials, oxides, and hydroxides have shown promising results in CO 2 capture from air.However, there is still a need for advanced materials with improved performance and lower energy requirements. 1,2n this field, the direct conversion of captured CO 2 to valuable products is an excellent strategy to overcome some regeneration problems.CO 2 valorization technologies aim to transform this gas into chemicals, fuels, or other value-added materials, thereby closing the carbon cycle.Catalysts are key components for those processes, as they facilitate the activation and transformation of CO 2 molecules.In this field, various types of catalysts, including homogeneous, heterogeneous, electro and photocatalysts have been explored. 6,7Advancements in materials science have led to the development of novel catalysts with improved activity, selectivity, and stability.However, limited examples using atmospheric air to directly reuse the CO 2 are observed.Thus, far, the products observed from the reutilization of atmospheric CO 2 include cyclic carbonate, 8 CO, 9 formate, 10−12 methanol, 13−16 and more recently methane. 7ccordingly, this Perspective aims to provide a comprehensive overview of the recent advancements in materials for DACC.It will discuss the key materials used in DAC systems as well as the catalysts employed for the conversion.The focus will be on highlighting the performance, limitations, and prospects of these materials in the context of sustainable CO 2 mitigation strategies.This paper will be organized into the subsequent sections: (i) Exploration of materials for DAC; (ii) examination of materials and reactions in DACC; and (iii) f uture perspectives and concluding remarks.

MATERIALS FOR DIRECT AIR CAPTURE
Given the relatively low concentration of CO 2 in ambient air, approximately 425 ppm (0.04%), the energy consumption of sorbents tends to be high.This is largely dictated by the type of material used, its sorption capacities, and selectivity toward the CO 2 .−3 The mechanisms for capturing CO 2 predominantly involve adsorption and absorption.Adsorption is a surface phenomenon where the adsorbate, here, CO 2 , adheres to the surface of an adsorbent either via weak van der Waals forces (physisorption) or through stronger chemical bonds (chemisorption).Absorption, conversely, is a volume-based process where the absorbate permeates and uniformly disperses throughout the volume of the absorbent. 17A more encompassing term is sorption, which includes both adsorption and absorption.In this context, the terms 'sorption' and 'sorbent' will be used as the standard terminology.
Numerous sorbents, such as zeolite, MOF, and alkali oxides, have been extensively studied.Sorbents are generally categorized into physical and chemical types depending on their respective sorption mechanisms.Physisorption entails a lower energy cost for separating CO 2 from the sorbent but involves higher expenses for activating and reusing the gas.The carbon atom of the CO 2 molecule is sp hybridized, and the strong overlap of the bonding orbitals limits reactivity.In contrast, chemically captured CO 2 adopts a more reactive configuration (trigonal planar sp 2 hybridization) in the form of bicarbonate and carbamates.Between the both, the bicarbonate (−45 kJ/ mol) is more reactive than carbamate (−80 kJ/mol) facilitating the reuse, 18 suggesting that the formation of bicarbonate is a way to activate the CO 2 . 19This section provides a summary of the state-of-the-art sorbents for DAC (Figure 1).

MOFs
MOFs have attracted significant attention in the field of DAC due to their unique properties.They are highly porous materials composed of metal ions coordinated with organic ligands.Their large surface areas and tailored pore sizes make them ideal candidates for efficient CO 2 adsorption.The selective capture of CO 2 from ambient air requires adsorbents with high affinity, excellent selectivity over other gases, high adsorption capacity, and good stability.MOFs can meet these requirements, and their performance can be further enhanced by incorporating specific functional groups, mainly amines, to improve the CO 2 affinity and selectivity.Regarding the metal center, while Cu, Zn, Ni, Co, and Mg have been tested, the most promising results were obtained with Mg. 20−22 Despite the great advance in this area, the state-of-the-art DAC using MOF is around 2.83 mmolCO 2 /g mat . 21,22This result was achieved using a diaminefunctionalized MOF, en-Mg 2 (dobpdc) (en = ethylenediamine; dobpdc = 4,4′-dioxi-dobiphenyl-3,3′-dicarboxylate).The primary hurdle associated with MOFs is that their predominant mechanism of capture is physisorption, which inherently restricts the quantity of CO 2 that can be absorbed. 21

Zeolites
Zeolites are crystalline aluminosilicate materials known for their well-defined nanoporous structures.Their unique pore sizes and shapes make them suitable for selective CO 2 adsorption.Aminemodified zeolites, in which amine groups are grafted onto the zeolites, exhibit enhanced CO 2 affinity and selectivity, making them promising materials for DAC systems.By tailoring the properties of zeolites, improved performance in terms of the CO 2 capture efficiency has been observed.Yang and co-workers focused on optimizing zeolite structures and developing tailored synthesis methods to meet the requirements of large-scale DAC applications, resulting in sorption capacity of 1.34 mmolCO 2 / g mat . 23Currently, numerous researchers have presented a variety of zeolites that exhibit improved CO 2 capture performance, accomplished through cation exchange and amine modification processes.However, the presence of water vapor is a significant factor impacting the CO 2 adsorption efficacy of zeolites, as it can compete with CO 2 for the active adsorption sites, which limited the use of these materials to capture CO 2 directly from the air. 24

Amine-Functionalized Materials
Amines, characterized by their strong affinity for CO 2 , have been widely studied for CO 2 capture applications, especially aqueous solutions of primary and secondary amines, such as mono-and diethanolamine.They react with CO 2 to form carbamates, which can further transform into bicarbonate species in the presence of water.However, they are typically only used in 20− 30% concentration in water due to corrosion and degradation issues.A significant downside of these solution-state CO 2 capture methods is their high heat capacity, making the regeneration step energy-intensive and expensive.Furthermore, these amines are better suited for capturing CO 2 from oxygenfree or low-oxygen gas mixtures as they tend to degrade over time.To mitigate energy costs, amines and polyamines on solid supports have been suggested as alternatives.Amine-functionalized materials, such as modified silica, polymers, and solid sorbents, chemically react with CO 2 to form stable carbamate compounds.This chemisorption process enables efficient CO 2 capture from the air, thus far yielding a sorption capacity of 6.85 mmolCO 2 /g mat .However, challenges associated with the regeneration of amine-based sorbents and their susceptibility to degradation remain areas of active research. 13,25The development of stable and regenerable amine-based materials is crucial for the practical implementation of DAC technologies.

Metal Oxides and Hydroxide
Metal oxides, such as calcium oxide (CaO) and magnesium oxide (MgO), offer an alternative approach to CO 2 capture.These materials capture CO 2 through the formation of carbonates, which undergo reversible reactions under specific conditions.This reversibility allows for the release of captured CO 2 for storage or utilization.Metal oxide-based sorbents have shown promise in terms of their capacity for CO 2 capture and subsequent release, making them potential candidates for DAC systems.These materials exhibit remarkable sorption capacity (17 mmol of CO 2 /g mat ), particularly due to their low molecular weight and basicity, which enables them to produce metal carbonates.However, challenges exist in terms of the energy requirements for regeneration and the optimization of capture/ release cycles. 26lkali hydroxide solutions also present remarkable interest for CO 2 scrubbing, especially focusing on the further reuse of CO 2 .Recent reports in this regard have been published by Prakash and co-workers. 14,15The true value of this methodology stems from the fact that the hydrogenation of ester and bicarbonate intermediates, are significantly more efficient than that of formamide or carbamate intermediates resulting in the reaction between CO 2 and amines. 14

Ionic Liquids (ILs)
ILs are described as organic salts with melting points below 100 °C and are predominantly solids at room temperature.Their potential in DAC is based on their versatility to combine various cations and anions, which offers the possibility of fine-tuning the chemical and physical attributes of the absorbent.Furthermore, particularly given their low volatility, minimal corrosiveness, exceptional chemical and thermal stability, nonflammable nature, and reduced vapor pressure are key features for sustainable CO 2 capture process. 27,28In addition, basic ILs can activate water molecules, forming a guest@host complex that can react with CO 2 to produce bicarbonate. 29This presents a significant advantage compared to other materials, as they can achieve a high sorption capacity even under humid conditions.Yang and co-workers showed that using atmospheric CO 2 conditions pyrrolidinium-based IL with a borohydride anion was able to capture 7.76 mmol CO 2 /g IL. 30,31 The authors demonstrate not only the capture but the possibility to transform this CO 2 into formate. 31

Moisture Swing Materials
Moisture swing materials have properties that change with the presence or absence of moisture, allowing them to alternate between absorbing and releasing CO 2 .This "moisture swing" between dry and wet states drives the cyclical process of CO 2 capture (in the dry state) and release (in the wet state).In the dry state, the sorbent exhibits an affinity for CO 2 , allowing them to capture the gas from the surrounding environment.This is achieved through the formation of weak chemical bonds between the sorbent material and the CO 2 molecules.Once the sorbent has reached its CO 2 saturation point, the introduction of moisture alters the physical properties of the sorbent.The presence of water molecules disrupts the sorbent-CO 2 interaction, leading to the desorption or release of previously captured CO 2 .
Unlike other carbon capture methods, such as thermal or pressure swing adsorption, which necessitate substantial energy input in the form of heat or pressure alterations, the moisture swing process capitalizes on ambient changes in humidity, thereby presenting a potentially energy-efficient and environmentally benign alternative.However, so for limited sorption capacity has been demonstrated (0.82 mmol CO 2 /g mat ). 32

Others
Porous carbon materials, such as activated carbon and carbon nanotubes, have shown promise in DAC applications.These materials possess high surface areas and porosities, enabling them to absorb CO 2 molecules through physical adsorption.However, challenges remain in terms of optimizing adsorption capacity and reducing energy requirements for regeneration processes. 1,2hile substantial progress has been made in the field of DAC, these materials still present significant challenges.Specifically, their high energy demands, particularly for the regeneration process, pose a considerable problem.This process accounts for approximately 70% of the total budget of a CO 2 capture system, as it requires temperatures exceeding 100 °C. 28

DIRECT AIR CAPTURE AND INTEGRATED CO 2 CONVERSION
The catalysts are a central point to transform the CO 2 into added-value products.While there have been significant advances in materials science leading to the creation of improved catalysts primarily for pure CO 2 , there is limited work on using atmospheric CO 2 . 2,10−14, 30,33 The pursuit of developing a singular material with the combined abilities of simultaneous or sequential sorption and catalysis for both CO 2 capture and conversion is undeniably appealing, yet it presents considerable challenges.The process of extracting CO 2 directly from the atmosphere holds significant climate-related advantages, but it is accompanied by substantial costs, as does the subsequent sequential conversion. 33To date, the examples of DACC are based on two-step reactions, first performing the capture and second, the conversion (Figure 2), yielding: cyclic carbonate, 8 CO, 9 formate, 10−12 methanol 13−16 and methane. 7In this section, each of these products will be described in detail.

Cyclic Carbonate
The formation of cyclic carbonate using CO 2 captured from air has been a focus of attention recently.In a pioneering work, Mg(II)-based MOFs demonstrated an efficient catalyst for directly converted CO 2 from the atmospheric air into cyclic carbonates under mild conditions (60 °C, 48 h, balloon loaded with air), resulting in 92% of conversion for epichlorohydrin (ECH). 34ore recently, our research group presented a groundbreaking methodology for DACC that enables the efficient transformation of atmospheric CO 2 into cyclic carbonates.This novel technique leverages readily available basic ILs, eliminating the requirement for complex and expensive cocatalysts or sorbents, while operating under mild reaction conditions.Our methodology demonstrates exceptional performance, with the IL solution efficiently capturing CO 2 from the atmospheric air (0.98 mol CO 2 /mol IL by bubbling air with 425 ppm of CO 2 ), and subsequently achieving complete conversion into cyclic carbonates using substrates derived from epoxides or halohydrins, potentially sourced from biomass.Mild condition was employed in this work, 40 °C; atmospheric CO 2 (0.04%) and 16 h, resulting in >99% yield. 8

Formate
In 2018, Guan et al. introduced the use of Ru(II) PN 3 P pincer complexes for the purpose of hydrogenating CO 2 .These complexes exhibit remarkable selectivity and catalytic activity, with a high turnover frequency (TOF) of up to 13 000 h −1 and a turnover number (TON) of up to 33000, particularly in a biphasic system comprising tetrahydrofuran (THF) and water.One notable achievement of the study is the successful conversion of carbon dioxide from air into formate (69%) in the presence of an amine at 110 °C and 27 bar of H 2 .Importantly, the catalytic system employed in this study combines the advantages of both homogeneous and heterogeneous systems.This process enables separation of the product and recycling of the catalyst. 11eller's group, in 2021, presented an amino-acid-based reaction system for DACC to generate formates.The system incorporates naturally occurring amino acid L-lysine.By employing a specific Ru-based catalyst, mainly RuMACHO-BH, 80 bar H 2 , 145 °C, they achieved good activity by converting 46% of captured CO 2 into formate, and TON > 50 000. 10n 2022, Choudhury and co-workers developed an efficient catalytic system based on phosphine-free Ir(III)-NHC (Nheterocyclic carbene) for DACC to generate formate.Tetramethylguanidine was used as a capturing agent, followed by conversion to formate using H 2 gas (25−40 bar, 120 °C), and both steps were conducted in water.The system demonstrates impressive product yields of up to 85% and TON around 19 000 in 12 h of reaction.The utilization of a phosphine-free Ir(III)-NHC catalyst in this system offers a promising alternative for efficient and sustainable CO 2 utilization. 12

Methanol
The process of creating methanol can entail the hydrogenation of CO 2 , in which CO 2 undergoes a sequence of reactions to be transformed into methanol.Various catalysts and methods have been studied for this process.Commercial catalysts often consist of CuO and ZnO supported on Al 2 O 3 with stabilizing additives and promotors.Amine and hydroxide solutions have been used for CO 2 capture and in situ hydrogenation to methanol, allowing for the separation of amine and catalyst after the reaction. 13,35n 2016, Prakash and co-workers introduced significant advancements in the field of CO 2 conversion.They demonstrated for the first time the development of a methodology for DACC to produce methanol from atmospheric CO 2 , achieving a yield of 79%.The catalyst system utilizes pentaethylenehexamine (PEHA) and Ru-MACHO-BH in a solvent, operating at temperatures ranging from 125 to 165 °C and a H 2 pressure of 50 bar.The methanol separation from the reaction mixture was demonstrated through distillation.The catalyst could be recycled over five runs without significant loss of activity; however, the sorbent was not. 13This work initiates a string of publications on this subject from this research group.
In 2018, following capture in an aqueous amine solution, CO 2 from the air was converted to methanol in a high yield (89%) within a MeTHF/H 2 O biphasic system.This system also facilitates separation and partial recycling of both the amine and the catalyst for multiple reaction cycles, retaining 87% of the initial cycle's methanol productivity.The method consists of the use of Ru-MACHO-BH as catalyst and the polyamine PEHA as a sorbent. 36nother significant breakthrough was achieved in 2020 by the same group, with the establishment of the first alkali hydroxidebased system for capturing CO 2 from the air and converting it into methanol.The study demonstrates that bicarbonate and formate salts can be efficiently hydrogenated to methanol with high yields in a solution of ethylene glycol.The researchers developed an integrated one-pot system, where CO 2 is captured by an ethylene glycol solution containing the alkali hydroxide base.Subsequently, the captured CO 2 is hydrogenated to produce methanol using Ru-PNP catalysts, performed at 140 °C and 70 bar H 2 .The resulting methanol was separated from the reaction mixture through distillation.Notably, the study also observed partial regeneration of hydroxide bases at low temperatures, which was an advance from previous works.The researchers suggest that the high capture efficiency and stability of hydroxide bases make them superior to the existing aminebased routes for DACC to methanol.They propose that this novel approach using hydroxide bases could be implemented in a scalable process for efficient and sustainable CO 2 capture and methanol production. 14he same group demonstrated a similar system for methanol production using a heterogeneous commercial Cu/ZnO/Al 2 O 3 catalyst.Among the evaluated solvents, glycols demonstrated a notable effect in promoting methanol formation at a temperature range of 170−200 °C utilizing molecular H 2 .Methanol yields of up to 90% were achieved.The catalytic process and potential reaction pathways were examined through control experiments, suggesting that hydrogenation in the presence of an alcohol proceeds through the formation of a formate ester as an intermediate.Lastly, a demonstration of DACC was showcased as a novel process for methanol synthesis, utilizing the combination of heterogeneous catalysis and air as a renewable carbon source. 15

Methane
Recently, DACC into methane has been first reported, with yields reaching up to 100% using both Ni/Al 2 O 3 and Ni/ CaAl 2 O 4 catalysts.The methodology is based on the formation of metal carbonate through the sorption of CO 2 from air into inorganic hydroxide.The conversion step was performed under 50 bar of H 2 , 48 h, and 225 °C.The authors demonstrated that water enhances the conversion of the carbonate salt to methane.The Ni/Al 2 O 3 catalyst preserved 99% of its activity in the alkaline medium after five consecutive hydrogenation cycles 29

Other Chemicals
A proof-of-concept experimental setup where CO 2 is captured from air in the form of a carbonate/bicarbonate solution via direct air capture and then converted to formate and CO, has been demonstrated by Breugelmans and co-workers.The findings presented open a new possibility for scaling up the electrochemical conversion of CO 2 . 9ombardo et al. have demonstrated the capability of tetraalkylammonium borohydrides to effectively capture substantial amounts of CO 2 and convert it into formic acid and Nformylated compound under ambient conditions.Their study revealed that these materials exhibit impressive CO 2 absorption capacities since each BH 4 − anion could react with three CO 2 molecules, resulting in the formation of triformatoborohydride ([HB(OCHO) 3 ]).The researchers accomplished direct capture and reduction of CO 2 from the air using various tetraalkylammonium borohydrides, including tetraethyl, -propyl, and -butylammonium borohydrides.Interestingly, they observed that the alkyl chain length in these compounds played a significant role in the reaction kinetics and thermodynamics.Additionally, they achieved the transfer of formate for the Nformylation of an amine. 30igure 3 demonstrates a summary of the reports related to the DACC mentioned above.

CHALLENGES AND PERSPECTIVES
To enable the practical implementation of DACC technologies, the development of materials should consider factors such as scalability, cost-effectiveness, and environmental sustainability.Materials with abundant and low-cost precursors offer advantages in terms of the cost reduction and availability for large-scale deployment.Furthermore, environmentally friendly synthesis methods that minimize energy consumption and waste generation are essential for sustainable materials production.The integration of these considerations into materials design and synthesis processes will contribute to the viability and widespread adoption of DACC technologies.
By capturing atmospheric CO 2 , the goal is to create neutral emissions and lose the carbon cycle by producing synthetic f uels.CO 2 must be constantly removed from the atmosphere, oceans, and terrestrial biomass, thereby reducing excess CO 2 and helping to mitigate climate change.The captured CO 2 can be used as a feedstock for various carbon-based materials.It can be used in the production of cement, plastics, carbon fibers, and other industrial applications.This helps keep carbon stored in the infrastructure, preventing its release into the atmosphere for extended periods.
However, for this to transition from concept to reality, the DACC must transpose five critical challenges (Figure 4): 1. Slow kinetics of sorption: When contrasting the capture process using pure CO 2 with that of atmospheric CO 2 , the total time can stretch from under 15 min 37 to a lengthy 16 to 72 h, 8,13,36 respectively.In the combination of capture and conversion, the rate-limiting step is the initial sorption, especially when it is arduous to capture the gas from such a diluted source, as the atmospheric air that contains around 0.04% of CO 2 .A significant advancement lies in the development of materials and processes that can reduce this time.This necessitates further studies in this field to comprehend the process better and propose innovative alternatives.2. Ef f icient and cost-ef fective sorbent regeneration: Typically, high energy is required, particularly for the regeneration process, which accounts for approximately 70% of the total budget of a CO 2 capture system, given the necessity for temperatures exceeding 100 °C. 28,38Within this realm, the direct conversion of captured CO 2 into valuable products emerges as a superb strategy to sidestep the energy costs associated with desorption and compression, thereby facilitating the closure of the carbon cycle.However, the existing examples are limited by the partial regeneration of the sorbent, which reduces around 10% of its capacity in each cycle.

Rigorous conditions for converting CO 2 into value-added
−16 These stringent parameters pose challenges in terms of operational feasibility and safety.Maintaining high temperatures and pressures over extended periods not only increases energy demands but also heightens the risk of system failures and potential hazards.Therefore, developing techniques that can convert CO 2 under milder conditions is still a challenge.4. Product isolation: The most efficient catalysts reported to date for the DACC process are homogeneous, which complicates the final separation of the formed product.A strategy that has been employed is the use of a biphasic system, which can aid in catalyst regeneration. 13,14,36owever, in certain instances, such as the production of formate, separation of the product remains a challenging task. 10Another alternative is the distillation process for product isolation, as demonstrated in the case of methanol.While this method is effective, it is also energy-intensive, posing an additional challenge to overall process efficiency. 5. Scalable and continuous process: Lastly, for DACC process to become a part of everyday reality, a substantial challenge lies in developing a process that can be easily scaled up or modified to fit existing industrial plants or even everyday applications.A scalable process would allow for greater capacity and flexibility, facilitating the integration of DACC technology into diverse sectors.Furthermore, the process needs to operate continuously to constantly capture CO 2 from the atmospheric air and convert it into useful products.The continual operation would ensure a steady and reliable output, crucial for meeting ongoing demands and achieving the desired impact in terms of carbon capture and sequestration.

CONCLUSION
The development of advanced materials holds great promise for enhancing the efficiency and viability of DACC technologies.Through the integration of materials science advancements, researchers have made significant progress in developing adsorbents, membranes, sorbents, and catalysts with improved performance for CO 2 capture and conversion.To date, sophisticated and expensive sorbents and catalysts have been necessary for these tasks as well as the limited number of addedvalue products generated.Further innovation in this field is crucial for the development of multifunctional materials capable of capturing, activating, and transforming CO 2 .This progress requires an interdisciplinary approach, particularly in integrating insights from chemistry, materials science, and engineering.

Figure 1 .
Figure 1.State-of-the-art materials for DAC.

Figure 2 .
Figure 2. Generic route for DACC is described in the literature.