Mesoporous metal-organic frameworks (MOFs) with high surface area, large pore sizes (2-50 nm), and tunable structures have found widespread applications in fields such as catalysis, adsorption, and separation. However, most reported MOFs to date are microporous (with pore sizes smaller than 2 nm). Among the restricted pool of mesoporous MOFs, a significant proportion encounters stability challenges. On the other hand, the synthesis of stable MOFs usually requires high temperature and acidic conditions, which prevents the in-situ encapsulation of labile functional species (e.g. enzymes, metal-oxo clusters, metal-sulfide quantum dots, and coordination complexes). Therefore, developing new strategies for synthesizing stable mesoporous MOFs under mild conditions (i.e. room temperature, acid-free) is an urgent and challenging task.
Recently, Professor Shuai Yuan and Professor Jing-Lin Zuo from the School of Chemistry and Chemical Engineering of Nanjing University collaborated with Dr. He Zheng from the State Key Laboratory of NBC Protection for Civilian to overcome the room-temperature synthesis of stable mesoporous MOFs. They further encapsulated labile functional species into stable mesoporous MOFs for synergistic catalysis.
Figure 1. Dynamic bond-directed synthesis of stable mesoporous MOFs for synergistic catalysis.
The synthetic conditions of MOFs are usually determined by the strength of coordination bonds: MOFs based on high-valent metals (such as Zr-MOFs) typically require high-temperature and acidic modulators to generate crystalline products, while MOFs based on low-valent metals can form single crystals at room temperature in neutral solvents. Therefore, the team developed a dynamic bond-directed synthetic method that can obtain single crystals of stable mesoporous Zr-MOFs under mild conditions. They first constructed MOF templates with stable Zr-carboxylate coordination bonds and labile Cu-pyridine moieties. Subsequently, the unstable Cu-pyridine ligand was replaced by stable linear organic ligands with similar lengths through the single crystal-to-single crystal transformation, forming single crystals of stable mesoporous MOFs. The low synthetic temperature allows the isolation of mesoporous MOFs as kinetic products and avoids the thermodynamically stable microporous MOFs. Furthermore, acid-sensitive species can be in-situ encapsulated within the frameworks during MOF synthesis. By encapsulating polyoxometalates (POMs) into Zr-MOFs, they observed high catalytic activity for VX degradation by the POM@Zr-MOF catalysts as a result of the synergy between redox-active POMs and Lewis-acidic Zr sites. The dynamic bond-directed synthetic method will not only lead to the discovery of novel MOFs with enhanced stability and porosity but also facilitate the synthesis of MOF-based hybrid composites.
Figure 2. Ligand exchange to form stable mesoporous MOFs.
This work was published in the Journal of the American Chemical Society with the title Dynamic Bond-Directed Synthesis of Stable Mesoporous Metal−Organic Frameworks under Room Temperature (DOI:10.1021/jacs.3c01219). The first author of the paper is the Ph.D. candidate, Youcong Li, from Nanjing University. This work was supported by the National Natural Science Foundation of China and the Natural Science Foundation of Jiangsu Province.