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.