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Anion exchange materials (AEMs) containing quaternary ammonium groups with charge balancing alkaline anions have shown promise for CO2 direct air capture (DAC), particularly under low-humidity conditions. These materials can be regenerated by increasing water activity, leveraging the moisture swing (MS) effect. The regeneration step releases heat due to water sorption, providing an opportunity to develop an autothermal Vacuum Moisture Swing (aVMS) process that utilizes both a change in CO2 affinity due to moisture and the heat of water sorption for efficient atmospheric CO2 capture. In this work, the moisture-driven CO2 sorption was studied for the first time using dynamic column breakthrough (DCB) experiments and subsequent modeling of the obtained sorption isotherms. The results confirm that humidity significantly affects the shape and capacity of the CO2 isotherms. CO2 uptake increased sharply at lower relative humidity (RH), while temperature had a less pronounced effect, especially at higher RH. At 15 % RH, the CO2 loading saturates at 200 ppm, with maximum loads of 0.82 mmol/g at 25 °C and 0.64 mmol/g at 45 °C. However, at 80 % RH, the CO2 partial pressure required for saturation increases significantly, reaching 60,000 ppm, and the maximum loading drops below 0.4 mmol/g. Interestingly, under certain conditions, partial water desorption was observed during CO2 sorption, suggesting a complex interplay between the two molecules and the MS sorbent. In addition, the influence of sorbent form factor, flow rate and column geometry on the separation performance was investigated. These findings not only advance the understanding of the complex interaction between CO2 and water during moisture swing processes but also provide a basis for the engineering of a cost-effective aVMS process for CO2 DAC.
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Tailoring Hydrophobicity and Pore Environment in Physisorbents for Improved Carbon Dioxide Capture under High Humidity
CALF-20, a Zn-triazolate-based metal-organic framework (MOF), is one of the most promising adsorbent materials for CO2 capture. However, competitive adsorption of water severely limits its performance when the relative humidity (RH) exceeds 40%, limiting the potential implementation of CALF-20 in practical settings where CO2 is saturated with moisture, such as post-combustion flue gas. In this work, three newly designed MOFs related to CALF-20, denoted as NU-220, CALF-20M-w, and CALF-20M-e that feature hydrophobic methyl-triazolate linkers are presented. Inclusion of methyl groups in the linker is proposed as a strategy to improve CO2 uptake in the presence of water. Notably, both CALF-20M-w and CALF-20M-e retain over 20% of their initial CO2 capture efficiency at 70% RH – a threshold at which CALF-20 shows negligible CO2 uptake. Grand canonical Monte Carlo (GCMC) simulations reveal that the methyl group hinders water network formation in the pores of CALF-20M-w and CALF-20M-e and enhances their CO2 selectivity over N2 in the presence of high moisture content. Moreover, calculated radial distribution functions indicate that introducing the methyl group into the triazolate linker increases the distance between water molecules and Zn coordination bonds, offering insights into the origin of the enhanced moisture stability observed for CALF-20M-w and CALF-20M-e relative to CALF-20. Overall, this straightforward design strategy has afforded more robust sorbents that can potentially meet the challenge of effectively capturing CO2 in practical industrial applications.
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- Award ID(s):
- 2119433
- PAR ID:
- 10528471
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- Volume:
- 146
- Issue:
- 6
- ISSN:
- 0002-7863
- Page Range / eLocation ID:
- 3943 to 3954
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jacs.3c11671
