Adsorption of CO2 by Amine-Functionalized Metal–Organic Frameworks Using GCMC and ReaxFF-Based Metadynamics Simulations

Starting from 20 amine-functionalized metal–organic framework (MOF) candidates, we have downselected a few for CO2 adsorption applications. We developed ReaxFF force fields for these MOFs and applied both Grand canonical Monte Carlo calculation and metadynamics simulations to measure CO2 adsorption both physically and chemically. On one hand, the CO2 uptake at high CO2 concentration is found mostly contributed by physisorption. On the other hand, chemisorption is dominant at low CO2 concentration (e.g., air with 2500 ppm of CO2). The metadynamics simulation results have shown that the energy barrier for CO2 chemisorption is high for these MOFs and can be significantly reduced with assistance of water or –NH2. We found that a mixture of ZIF-8 with polyethylenimine shows a much higher initial CO2 chemisorption, yet its adsorption capability degrades after repeated uses. In contrast, the utilization rates of amine for the amine-functionalized MOFs are low in adsorption experiments using humid air. While CO2 chemisorption remains a challenging topic for theoretical study, here we have provided an example where multiple techniques can be used to downselect MOF candidates, facilitate force field development, and simulate materials with potential applications in CO2 adsorption.