Design of Hierarchical Architectures in Metal-Oganic Frameworks for Catalysis and Adsorption

A key factor to improve the performance of metal-organic frameworks (MOFs) is the design and synthesis of hierarchical interconnected porosity at different length scales. The presence of secondary mesopore systems, in addition to the intrinsic framework micropores, avoids inherent mass-transfer constraints that occur in multiple examples of liquid phase chemical processes. Different strategies to introduce ordered void spaces in the MOF materials are reviewed here, based on either bottom-up or top-down approaches. A thorough discussion of the different bottom-up synthesis protocols, based on the interactions of the molecular building blocks with soft or hard templates, is followed by presenting synthesis concepts relating to multidimensional MOF transformations, interfacial and modulated synthesis. The holy grail of defect engineering MOF crystals with controlled porosity is tackled through several top-down approaches in the nanoscale regime. The resultant hierarchical MOFs with interconnected porosity represent the next generation of advanced catalysts and adsorbents, as is presented here by using a vast number of examples where the hierarchical porous structure is key to the MOF performance. A combination of tailor-made pore shape, size, and functionality leads to a synergistic effect between the MOF structures and functions, which offers an improvement on catalysis and adsorption while preserving their porosity, reactivity, and stability. The hierarchical MOFs presented in the different sections of this Review are compared with the purely microporous parent MOFs, used as benchmark in challenging applications where the performance is boosted in the case of hierarchical MOFs.