The high material and energy costs associated with direct air capture (DAC) of CO₂ hinder its widespread deployment. Negative Emission Technologies (NETs), including DAC, are projected to remove significant amounts of CO₂ by mid-century to limit global warming. However, DAC faces challenges due to the ultra-low partial pressure of CO₂ in air (40 Pa), resulting in slow CO₂ uptake rates compared to natural processes like photosynthesis and ocean absorption. While artificial DAC materials offer significantly faster kinetics, challenges remain in designing effective adsorbents. Amine-based sorbents are widely studied, but the impregnation process can lead to pore blockage, limiting CO₂ diffusion. In situ polymerization methods are preferred, aiming to retain mesoporous structure. This research focuses on chemically grafting quaternary ammonium (QA) groups onto mesoporous materials for DAC. QA groups enable moisture swing adsorption (MSA), offering potentially lower energy consumption than temperature-swing adsorption (TSA). The use of mesopores promotes molecular diffusivity, avoids functional group overlap, and provides high surface area for efficient CO₂ capture. The behavior of water in these mesopores, including diffusion and capillary condensation, significantly impacts MSA performance.

The literature review discusses existing challenges in direct air capture (DAC) technology, highlighting the slow kinetics of CO2 uptake under atmospheric conditions. It explores various functional groups used in DAC adsorbents, such as alkali hydroxides and solid amines, emphasizing the importance of porous structures for efficient CO2 adsorption. Studies on amine impregnation show that amines tend to fill smaller pores first, leading to pore blockage and limited CO2 diffusion, especially in microporous adsorbents. To overcome this limitation, researchers have explored approaches like low amine loading and in situ polymerization methods using amine-containing monomers. The paper also examines previous work on quaternary ammonium (QA) groups for DAC, emphasizing their ability to perform moisture swing adsorption (MSA) for energy-efficient desorption, and explores the role of pore structure in enhancing the performance of MSA adsorbents. Prior research has shown that the pore structure of the support material can significantly impact the efficiency of CO2 adsorption and the overall performance of the moisture swing adsorption process. The literature review sets the stage for the current research by highlighting the need for a DAC adsorbent with high kinetics and efficient MSA capability.

Quaternary ammonium functionalized mesoporous adsorbents (QMPRs) were synthesized through a three-step process. First, commercial mesoporous polystyrene resins (MPRs) were treated with chloroacetyl chloride and aluminum chloride to introduce chloroacetyl groups. Dichloromethane and methanol were used as swelling agents to improve reagent penetration. Next, trimethylamine (TMA) was reacted with the chloroacetyl intermediate to form quaternary ammonium groups. Finally, ion exchange with Na₂CO₃ solution yielded the QMPRs. Three types of QMPRs were prepared using different MPRs. Characterization included N₂ adsorption/desorption isotherms (for pore structure analysis), FTIR spectroscopy (for chemical structure identification), elemental analysis (for nitrogen content), SEM (for surface morphology), and Mohr titration (for charge density). CO₂ adsorption isotherms and kinetics were measured using a custom-built system. Langmuir, Freundlich, and Temkin isotherm models were applied to fit the adsorption data, while pseudo-first-order, pseudo-second-order, and mixed 1,2-order kinetic models were used to analyze the adsorption kinetics. The water adsorption capacity of the MPRs and QMPRs was determined using a gravimetric method. Kelvin's equation was used to analyze capillary condensation in the pores.

The synthesized QMPRs exhibited type IV N₂ adsorption isotherms with hysteresis loops, indicating mesoporous structures. QMPR-2, with cylindrical pores, showed the highest CO₂ capacity (0.28 mmol g⁻¹ at 21% RH and 400 ppm CO₂) and the highest functional group efficiency (92%). The study confirmed that the pore structure significantly impacts the CO₂ adsorption capacity and kinetics. QMPR-2 showed an adsorption half-time of 2.9 min, the fastest reported for DAC adsorbents. The results suggest that uniform cylindrical mesopores lead to more uniform distribution of QA groups, resulting in higher ion exchange efficiency and CO₂ capacity. Capillary condensation of water in the mesopores affects the local humidity and CO₂ binding energy; this behavior was modeled using Kelvin's equation. The MSA capacity of QMPR-2 was demonstrated by cycling tests, showing a stable swing capacity of 0.26 mmol g⁻¹ by adsorbing at 21% RH and desorbing at 100% RH. The adsorption kinetics of QMPRs were shown to be governed by surface area, with QMPR-3 (smallest surface area) having the lowest adsorption rate. The adsorption rate of QMPR-1 decreased more rapidly due to its ink-bottle-like pore structure, which traps released water during CO₂ adsorption and reduces kinetics. The study established a clear link between pore structure and both CO2 adsorption capacity and kinetics, demonstrating the superior performance of QMPRs.

The findings demonstrate that the carefully designed mesoporous structure of the QMPRs is crucial for their superior CO₂ adsorption kinetics. The high kinetics of QMPR-2, with its adsorption half-time of 2.9 min, greatly surpasses that of previously reported DAC adsorbents. This enhanced performance stems from the optimal combination of mesoporosity and quaternary ammonium functionalization, which allows for efficient diffusion of both the functional groups and CO2 molecules within the material. The influence of pore structure on water behavior and its consequent impact on CO2 binding energy were experimentally verified and theoretically explained by Kelvin's equation. This work underscores the importance of rational design of the adsorbent materials to achieve ultra-high kinetics in DAC applications, paving the way for more efficient and cost-effective CO2 capture from ambient air. The results suggest that future research could focus on optimizing the pore size and distribution to further enhance the adsorption capacity and kinetics of MSA adsorbents.

This study successfully demonstrated a novel mesoporous adsorbent with significantly enhanced CO₂ capture kinetics for DAC applications. The use of tailored mesoporous polymers and effective functionalization strategies led to an adsorption half-time of 2.9 min, the fastest reported. Future work could explore modifying the mesoporous matrix with hydrophobic groups to further enhance both capacity and kinetics, potentially leading to even more efficient and cost-effective DAC systems.

The study focused on a limited number of MPRs and QMPRs, and further investigation with a wider range of materials is needed to fully generalize the findings. The CO₂ adsorption measurements were conducted under controlled laboratory conditions, and further experiments are required to assess the performance under real-world, fluctuating conditions. The current study primarily focused on the adsorption kinetics; long-term stability and durability under repeated adsorption-desorption cycles require further investigation.