Embracing Lability: Pathways to Hierarchical Porous Frameworks

Abstract

Metal–organic frameworks (MOFs) are porous crystalline materials constructed from organic linkers and metal nodes, which are well-known for their high crystallinity, large surface areas, and excellent structural tunability. In recent years, MOFs have been heavily studied owing to their versatility in separation, catalysis, and biomedicine, originating from their inherent modular structures. Nowadays, research on the evolution of hierarchical MOFs with enhanced diversity and complexity has gained increasing attention, because these materials have the potential to mimic living organisms that can perform complicated functions. Yet, the current level of regulating the hierarchy and functions of artificial framework materials is still far behind the complicated systems found in nature, such as proteins and DNAs.

Lability, originating from the cleavable chemical bonds within MOFs, is historically viewed as one unwanted property or fatal weakness for MOFs, especially when it comes to practical applications exposed to extreme conditions. However, introducing lability into robust MOFs may significantly improve the materials’ structural tunability, uncovering the opportunities to construct hierarchical MOFs with superior structural complexity and diverse pore environments. Chapter I of the dissertation introduces the development pathway of MOFs and highlights several milestones. Discussions have been made on how labile portions in MOFs can be precisely engineered or removed to tailor MOFs’ porosity and compositions, resulting in diverse functional materials. Chapter II describes a laser photolysis method to selectively remove photosensitive ligands in robust MOFs to generate hierarchically porous MOFs with enlarged pore sizes and well-maintained crystallinity. In Chapter III, a bottom-up approach is developed to assemble microporous cages into hierarchically porous frameworks, which is mediated by the non-bonding interactions among cages, such as hydrogen bonding and π-π stacking. The resultant hierarchically porous frameworks feature record-breaking selectivity and outstanding stability in enantioselective separation. Chapter IV explores the structural diversity of rare-earth-based MOFs and reports six unprecedented topologies, because the labile coordination bonds allow metal clusters to feature adaptive coordination modes. Chapter V introduces a modular strategy to orthogonally engineer individual portions within a MOF-on-MOF architecture based on the varied chemical stability of MOFs, generating MOF-on-polymer composites inheriting the structural information of the template. The Chapter VI describes one interesting phenomenon named lattice rearrangement that originated from the lability of the coordination bond of MOFs, enabling the transformation of amorphous solids into highly porous and crystalline materials under evacuation. In the last Chapter VII, a summary is presented to conclude methodologies to construct various hierarchical MOFs, which allow customizing the materials' topology, porosity, and composition. A perspective is also provided to discuss potential solutions to address challenges in developing hierarchical MOFs.