Multiscale Simulation Of Diffusion In Porous Media: From Interfacial Dynamics To Hierarchical Porosity

We present a multiscale simulation approach to derive effective bed diffusion coefficients (D-bed) in hierarchically structured, macro/mesoporous materials employed as fixed beds for chromatographic separation and heterogeneous catalysis. The simulated D-bed values account for the solute dynamics at the interface between the pore surface and mobile-phase as well as for the actual morphology of the material. Molecular dynamics simulations characterize interfacial solute dynamics through mesopore-level distributions of solute density and the diffusion coefficient parallel to the pore surface under explicit consideration of surface chemistry, solute properties, and mobile-phase elution strength. This information is incorporated into Brownian dynamics simulations of the effective diffusion coefficient D-meso in the mesopore space morphology as physically reconstructed by scanning transmission electron microscopy. Mass transfer between pore space hierarchies is simulated using an effective homogeneous medium representation for the mesoporous domain in the macropore space morphology as physically reconstructed by confocal laser scanning microscopy. D-meso and D-bed have immediate value as input parameters for fixed-bed models applied in separation and catalysis, which strengthens the basis for predictive modeling and removes ambiguity regarding adsorption and transport mechanisms. The simulation approach is sensitive to subtle changes in interfacial dynamics (e.g., as induced by adjusting the mobile-phase elution strength) and flexible regarding the employed bed morphologies.