Characterization of microscopic disorder in reconstructed porous materials and assessment of mass transport-relevant structural descriptors

Abstract

The targeted optimization of the functional properties of porous materials includes the understanding of their transport properties and thus requires knowledge about the relationship between material synthesis, resulting in three-dimensional material morphology, and relevant transport properties. In this Perspective, we present our views and results on the characterization of microscopic disorder in functional porous materials, which are widely used today as fixed beds in adsorption, separation, and catalysis. This allows us to identify structural parameters that impact their mass transport properties and eventually their overall performance in technological operations. We address this complex topic at the following levels: (i) computer-generation of disordered packings allows the systematic investigation of the bed porosity (packing density) and degree of packing heterogeneity. These studies are complemented by the physical reconstruction of real packed and monolithic beds, which resolves the salient features of the packing process and monolith synthesis that are under the control of the experimentalist. (ii) Once reconstructed packed-bed and monolith morphologies are available, they are analysed by statistical methods to derive structural descriptors for their mass transport properties. Spatial tessellation schemes and chord length distributions are shown to be suitable for that purpose. They lead us to sensitive correlations of the degree of pore-environment heterogeneity and packing-scale disorder with the dynamics of (random) diffusion and (flow-field dependent) hydrodynamic dispersion, respectively. (iii) Direct or pore-scale numerical simulations are implemented on a high-performance computing platform to quantify the relevant transport properties of the materials. This complementary approach highlights the morphological descriptors of mass transport efficiency. They are validated by the simulations and in the future could direct the rational design of materials from their synthesis to targeted applications based on physical reconstruction.