(594i) Understanding the Role of Charge Distribution and Pore Size for Water Vapor Adsorption in Idealized Nanoporous Materials

Since water vapor is present in major gaseous streams such as industrial flue gas, air, etc.,separating it efficiently can lead to augmenting water supply, increasing efficiency for proton con-duction, better CO₂capture performance in presence of water and many others [1, 2]. In this pursuit, nanoporous materials such as metal-organic frameworks (MOFs), having a high pore volume and surface area, can act as a media for separating water vapor from these mixtures. Recent studies have shown that MOFs are capable of separating water vapor from air in various capacities including the case of atmospheric water harvesting (AWH) units [3, 4]. Computational studies have revealed that water vapor adsorption on nanopores are highly dependent on pore geometry, functional groups and topology [5]. We seek to provide further understanding on the role of electrostatic interactions in water vapor adsorption in nanoporous materials.

To do so, we designed idealized carbon-based cylinders (ICC) and performed Monte Carlo simulations to understand the role of electric potential and pore size in the water vapor adsorption. We found that the maximum water vapor uptake was independent of the charge distribution but was proportional to pore size. Our study also confirmed that charge arrangement on pore surface plays a significant role in influencing the hydrophilicity of ICCs. We observed that a surface was found to be more hydrophilic when charges are alternated (from positive and negative) along the circumference than when they were alternated along the length of the cylinder. Also, through wa-ter vapor adsorption density profiles, we found the water adsorption sites were also dependent on charge distribution even at the highest relative humidity. We also formed the electric potential maps for these ICCs and found a strong relationship between the electric potential inside a pore and the water vapor adsorption sites. We hypothesize that MOFs having similar electrostatic characteristics to the ICCs would also exhibit identical water vapor adsorption behavior and they can be potentially used for AWH technology and other water vapor separation processes. Further, novel MOFs may be designed by adding chemical moieties to existing MOFs to induce a desired water vapor adsorption and separation behavior as observed in the ICCs.

References

[1] Song Li, Yongchul G. Chung, and Randall Q. Snurr. “High-Throughput Screening of Metal–Organic Frameworks for CO2 Capture in the Presence of Water”. In:Langmuir32.40 (2016). PMID: 27627635, pp. 10368–10376.doi: 10.1021/acs.langmuir.6b02803. eprint:https://doi.org/10.1021/acs.langmuir.6b02803.url:https://doi.org/10.1021....

[2] Stefan K. Henninger, Hesham A. Habib, and Christoph Janiak. “MOFs as Adsorbents for Low Temperature Heating and Cooling Applications”. In: Journal of the American ChemicalSociety131.8 (2009). PMID: 19206233, pp. 2776–2777.doi:10.1021/ja808444z. eprint:https://doi.org/10.1021/ja808444z.url:https://doi.org/10.1021/ja808444z.

[3] Hyunho Kim et al. “Water harvesting from air with metal-organic frameworks powered by natural sunlight”. In:Science356.6336 (2017), pp. 430–434.issn: 0036-8075.doi:10.1126/science.aam8743. eprint: https://science.sciencemag.org/content/356/6336/430.full.pdf.url:https:/....

[4] Xingyi Zhou et al. “Atmospheric Water Harvesting: A Review of Material and Structural De-signs”. In: ACS Materials Letters2.7 (2020), pp. 671–684.doi:10.1021/acsmaterialslett.0c00130. eprint: https://doi.org/10.1021/acsmaterialslett.0c00130.url:https://doi.org/10.....

[5] Aurélie U. Ortiz et al. “What makes zeolitic imidazolate frameworks hydrophobic or hydrophilic? The impact of geometry and functionalization on water adsorption”. In: Phys.Chem. Chem. Phys.16 (21 2014), pp. 9940–9949.doi:10.1039/C3CP54292K.url:http://dx.doi.org/10.1039/C3CP54292K.