Response of Metal Sites toward Water Effects on Postcombustion CO2 Capture in Metal-Organic Frameworks

Coordinatively unsaturated metal (CUM) sites of metal organic frameworks (MOFs) play an important role in water adsorption and its effects on CO2 capture. To explore the trends among various metal identities toward water, M-HKUST-1 analogous to HKUST-1 based on different metal sites M (M = Zn, Co, Ni, and Mg) are chosen as model frameworks and their water effects on CO2 capture are evaluated. Our results demonstrate that Mg-based framework shows diverse effects by water coordination from Zn-, Co-, and Ni-based frameworks. Water coordination lowers CO2 uptake dramatically in Mg-HKUST-1 whereas improves CO2 adsorption in Zn-, Co-, and Ni-HKUST-1 at most of the pressure ranges we studied. The detailed evaluation of interaction energy indicates that with the increased loading of CO2 molecules both the weaker interactions between CO2 and the framework and CO2-CO2 contribute to the lower CO2 uptakes of the hydrated Mg-HKUST-1. These findings could benefit CUMs screening in MOFs' design for CO2 capture and other applications.