XRP and PXCT are enabling technologies to understand complex synthesis pathways of porous materials. Hierarchical structures are omnipresent in our environment and are particularly found in connection with transport processes, for example, the evolved human respiratory system for oxygen delivery in the lungs, or modern designed road networks for managing traffic flow. Similarly to the above examples, mass transport properties strongly influence the performance of porous materials used in heterogeneous catalysis, particularly in terms of catalytic activity, product selectivity, and material stability. Diffusion limitations of reactants or products are highly relevant both in normal operating conditions and following changes induced by catalyst deactivation. Therefore, strategies have to be found for a knowledge-based design of porous catalysts with optimized mass transport properties and improved catalyst performance. One promising approach is to design catalysts with hierarchical rather than uniform pore structures, generally including some combination of solids with micro- (50 nm) pore size distribution. Here, we focus on meso-/macroporous systems with the predominant transport processes of Knudsen diffusion in mesopores and molecular diffusion or convective flow in macropores.
Introducing macropores into a uniform mesoporous system can provide higher mass transfer velocity by promoting convective flow over diffusion. The last two decades have brought significant progress in synthesis and design of meso-/macroporous materials with potential applications in heterogeneous catalysis or adsorption.
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