Distribution and Transport of CO₂ in Hyperbranched Poly(ethylenimine)-Loaded MCM-41: A Molecular Dynamics Simulation...

Fossil fuel use is accelerating climate change, driving the need for efficient CO₂ capture technologies. Solid adsorption-based direct air capture (DAC) of CO₂ has emerged as a promising mode for CO₂ removal from the atmosphere due to its potential for scalability. Sorbents based on porous supports incorporating oligomeric amines in their pore spaces are widely studied. In this study, we investigate the intermolecular interactions and adsorption of CO₂ and H₂O molecules in hyperbranched poly(ethylenimine) (HB-PEI) functionalized MCM-41 systems to understand the distribution and transport of CO₂ and H₂O molecules. Density Functional Theory (DFT) is employed to compute the binding energies of CO₂ and H₂O molecules with HB-PEI and MCM-41 and to develop force field parameters for molecular dynamics (MD) simulations. The MD simulations are performed to examine the distribution and transport of CO₂ and H₂O molecules as a function of the HB-PEI content. The study finds that an HB-PEI content of approximately 34 wt % is thermodynamically favorable, with an upper limit of HB-PEI loading between 45 and 50 wt %. The distribution of CO₂ and H₂O molecules is primarily determined by their adsorptive binding energies, for which H₂O molecules dominate the occupation of binding sites due to their strong affinity with silanol groups on MCM-41 and amine groups of HB-PEI. The HB-PEI content has a considerable impact on the diffusion of CO₂ and H₂O molecules. Furthermore, a larger number of water molecules (higher relative humidity) reduces the correlation of CO₂ with the MCM-41 pore surface while enhancing the correlation of CO₂ with the amine groups of the HB-PEI. Overall, the presence of H₂O molecules increases the CO₂ correlation with the amine groups and also the CO₂ transport within HB-PEI-loaded MCM-41, meaning that the presence of H₂O enhances the CO₂ capture in the HB-PEI-loaded MCM-41. These findings are consistent with experimental observations of the impact of increasing humidity on CO₂ capture while providing new, molecular-level explanations for the macroscopic experimental findings.