Reticular Chemistry for Highly Porous Metal-Organic Frameworks: The Chemistry and Applications

Current global crises related to clean energy and the environment entail the development of materials that are capable of addressing these challenges. Metal-organic frameworks (MOFs), a class of functional materials assembled from metal-containing nodes and organic ligands via coordination bonds, have been successfully developed for various applications, including catalysis, toxic chemical removal, and gas storage and separation, as a result of their highly tailorable nature and precisely engineered pore structures. In particular, the exceptionally high surface areas and porosities of MOFs are two of their most attractive characteristics and place them among the best porous materials for the storage of clean energy gases, such as hydrogen and methane. Reticular chemistry stands out as a prominent approach to the design of MOFs as this strategy allows for the rational top-down design of frameworks guided by topological nets to afford extended framework structures with precise architectural arrangements at the molecular level. Bridging the gap between reticular chemistry design strategies and highly porous MOFs can facilitate the development of next-generation high-performance materials through state-of-the-art chemical design.In this Account, we summarize our group's efforts over the past few years toward the synthesis and applications of highly porous MOFs inspired by reticular chemistry. First, we describe how we leveraged reticular chemistry to synthesize NU-1500, which is based on the 6-connected edge-transitive acs net, from the assembly of triptycene-based ligands and high-valent metal trimers. This delicate design is amenable to isoreticular expansion, and including an additional phenyl group in the rigid triptycene-based ligand of NU-1500 yields NU-1501. Importantly, NU-1501-Al exhibits both a high gravimetric Brunauer-Emmett-Teller (BET) area of 7310 m² g^-1, which is the current record after satisfying the four BET consistency criteria, and a volumetric BET area of 2060 m² cm^-3. The high porosity and surface area place NU-1501 among the most promising adsorbents for the storage of methane and hydrogen. Second, we illustrate the rational synthesis of highly porous and stable Zr-MOFs based on edge-transitive nets: (1) the successful isoreticular expansions of NU-1000 (a 4,8-connected csq net) form hierarchical mesoporous MOFs with pore sizes of up to 6.7 nm; (2) the assembly of Zr₆ clusters and tetracarboxylates yields the NU-1100 series (4,12-connected ftw net) with BET areas of 4300-6500 m² g^-1; and (3) the use of hexacarboxylates in combination with Zr₆ clusters results in the formation of the NU-1600 series (a 6,12-connected alb net) with BET areas of 2000-4500 m² g^-1. Third, we leveraged a reticular exploration strategy to access mesoporous uranium-based MOFs, NU-1300 (a 3,4-connected tbo net, 2100 m² g^-1) and NU-1301 (a 3-connected nun net, 4750 m² g^-1). In particular, we investigated the structurally complex NU-1301, which formed serendipitously from the combination of uranyl clusters and triangular carboxylates to afford a structure with the largest unit cell among all reported MOFs.Finally, we provide an overview of potential applications of these highly porous MOFs, including water capture, catalysis, methane storage, hydrogen storage, and the separation of organic dyes and biological macromolecules. We hope that this Account may serve as a blueprint and stimulate researchers to develop the next generation of highly porous materials for energy- and environment-related applications and beyond.