This review provides exhaustive insights into material and process design of adsorption-based direct air capture in the past five years.

Supported polyethyleneimine (PEI) adsorbent is one of the most promising commercial direct air capture (DAC) adsorbents with a long research history since 2002. Although great efforts have been input, there are still limited improvements for this material in its CO2 capacity and adsorption kinetics under ultradilute conditions. Supported PEI also suffers significantly reduced adsorption capacities when working at sub‐ambient temperatures. This study reports that mixing diethanolamine (DEA) into supported PEI can increase 46% and 176% of pseudoequilibrium CO2 capacities at DAC conditions compared to the supported PEI and DEA, respectively. The mixed DEA/PEI functionalized adsorbents maintain the adsorption capacity at sub‐ambient temperatures of −5 to 25 °C. In comparison, a 55% reduction of CO2 capacity is observed for supported PEI when the operating temperature decreases from 25 to −5 °C. In addition, the supported mixed DEA/PEI with a ratio of 1:1 also shows fast desorption kinetics at temperatures as low as 70 °C, resulting in maintaining high thermal and chemical stability over 50 DAC cycles with a high average CO2 working capacity of 1.29 mmol g−1. These findings suggest that the concept of “mixed amine”, widely studied in the solvent system, is also practical to supported amine for DAC applications.

The utilization of supported amines as adsorbents in direct air capture (DAC) has been demonstrated to be a promising strategy for the reduction of CO 2 emissions. To improve the performance of amine-based adsorbents, the incorporation of additives has been widely adopted. In the present study, we conduct a comprehensive comparison of seven additives on tetraethylenepentamine-impregnated mesoporous silica as a representative amine-based adsorbent. The results indicate that minor molecular weight additives with hydroxyl groups show improved adsorption-desorption performance and increase oxidative stability. A proposed mechanism for these improvements is the combined physical and chemical promotion effects of hydroxyl groups. Through a comprehensive review of existing literature, it is found that the effects of additives on amine-based adsorbents are dependent on factors, such as additive type, pristine adsorbent properties, incorporation method, and testing conditions. Based on these findings, it is recommended that future DAC systems prioritize the use of hydroxyl-containing additives, whereas higher CO 2 concentration and temperature capture may benefit from the incorporation of additives without hydroxyl groups. These conclusions are expected to contribute to the design of efficient adsorbents for CO 2 capture. K E Y W O R D Sadditives, direct air capture, hydroxyl groups, supported amine adsorbents, tetraethylenepentamineThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The development of efficient adsorbents is one of the key technologies for direct air capture (DAC). Herein, a facile and scalable method to synthesize amine-functionalized layered double hydroxides (LDHs) as efficient DAC adsorbents is presented. Specifically, ultrafine and exfoliated Mg 2 Al−CO 3 LDH nanosheets were generated within 2 min via rapid nucleation and aging followed by organic solvent treatment, and they remained exfoliated on the powders after vacuum drying. The well-dispersed LDH nanosheets were grafted with a high density of monolayered aminosilane (5.51 mmol N g −1 ), leading to a good CO 2 adsorption capacity (0.68 mmol g −1 ) among amine-grafted adsorbents. Additionally, it exhibited good stability with no degradation over 200 adsorption−desorption cycles under ∼20% humid air at 25 °C.