Morphological characterization and transport properties of physically reconstructed chromatographic beds

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Quantitative morphology-transport relationships play a key role in the challenge to develop ever more efficient functional materials. Targeted optimization of efficiency requires knowledge on how preparation, morphology, and mass transport of a porous solid are related. In the focus of this project is the characterization of morphological and transport properties of physically reconstructed adsorbents (fixed beds), which represent the latest developments and problems (challenges) in liquid chromatography with small silica-based particles (< 3 µm), silica-based monoliths, and organic polymer-monoliths. The three-dimensional reconstructions are based on experimental particulate and monolithic fixed beds, which have been conventionally prepared regarding the packing process of the particulate beds and the synthesis protocol of the monoliths. This aspect is important as it provides transparency, originality, and genuity to the methodology followed in this project and the obtained results. Simulations of fluid flow and mass transport directly in the reconstructed pore spaces allow the characterization of spatiotemporal dynamics based on the intrinsic morphological heterogeneity of the materials and resulting flow maldistribution - from the microscopic pore scale up to the macroscopic bed scale. The quantification of morphological heterogeneity is achieved with parameters extracted from the geometrical and topological analysis of the pore spaces. We analyze shape, size distribution, and interconnectivity of the pores, in which fluid flow and advective transport dominate, as well as spatial domains, in which diffusive transport and adsorption prevail, like in stagnant regions inside the particles (or a monolith skeleton). The goal of this project is to identify morphological descriptors for effective diffusive transport and especially the chromatographically far more relevant hydrodynamic dispersion through the geometrical and topological analysis of the disordered pore spaces in combination with the detailed pore scale simulations of flow and mass transport. From spatial domains, in which diffusion/adsorption limit mass transport, to regions, in which the flow field dominates, we quantitatively assess their structural heterogeneity with morphological descriptors. This proceeds analogously in the simulations of diffusion/adsorption, flow, and mass transport and derived transport coefficients, which reflect this structural heterogeneity. Consequently, transport properties central to chromatographic practice can be correlated with morphological properties, which themselves reflect the packing process or synthesis protocol of an adsorbent. This complementary approach allows us to identify sensitive morphological descriptors (validated through the detailed transport simulations), which in the future will direct the systematic optimization of material properties from the synthesis to targeted applications based on physical reconstruction.