Distribution and Transport of CO2 in Hyperbranched Poly(Alkylene Imine)-Loaded MCM-41: A Molecular Dynamics Simulation Approach

Abstract

Direct air capture (DAC) using amine-functionalized solid sorbents has emerged as a promising strategy for mitigating atmospheric CO2 emissions; however, the coupled influences of polymer architecture, hydration, and mesopore confinement on molecular adsorption and transport remain poorly understood. In this study, a multiscale computational framework combining density functional theory (DFT) and long-time scale molecular dynamics (MD) simulations was employed to investigate hyperbranched poly(ethylenimine) (HB-PEI)- and hyperbranched poly(propyleneimine) (HB-PPI)-loaded MCM-41 under humid DAC conditions. Thermodynamic analysis identified energetically favorable polymer loadings of approximately 34 wt % for HB-PEI and 37 wt % for HB-PPI, corresponding to an optimal balance among amine-site density, polymer organization, and mesopore accessibility. Spatial distribution analyses revealed that CO2 preferentially accumulates near polymer–void interfacial regions, whereas H2O strongly associates with silanol groups and amine functionalities, inducing competitive adsorption and humidity-dependent structural reorganization. Pair correlation analyses further demonstrated that secondary amines serve as the dominant local coordination sites for both CO2 and H2O under mesopore confinement. Transport analysis revealed pronounced anisotropic diffusion along the mesopore axis together with a nonmonotonic humidity dependence of CO2 mobility arising from the competing effects of polymer plasticization and H2O-induced steric obstruction. Although HB-PPI exhibits greater intrinsic segmental mobility, HB-PEI maintains higher CO2 diffusivity under humid conditions. This behavior is consistent with reduced transient steric obstruction arising from its more compact polymer organization. These findings demonstrate that intrinsic polymer mobility alone cannot predict CO2 transport under mesoporous confinement. Instead, CO2 transport emerges from the coupled effects of polymer architecture, confinement, hydration, and humidity-induced structural reorganization of the polymer matrix.