The marriage of porous cages and metal clusters for advanced catalysis

Metal clusters (MCs), a special species of ultrafine metal nanoparticles with dimensions below 2 nm, serve as highly active catalysts for a broad spectrum of chemical reactions, but usually suffer from serious aggregation due to their high surface energy. A balance between the activity and stability of MCs is greatly challenging in designing efficient catalysts. Cage-bearing materials such as organic molecular cages and metal-organic cages, as another promising category of porous materials, are attracting significant research attention. Thanks to their intrinsic cavity, such materials can serve as ideal confined templates for the size-controlled synthesis of MCs without blocking their active sites. Moreover, benefiting from the easy-to-modify architecture, the cage polyhedrons can be further functionalized to obtain advanced composite catalysts in combination with the hosted MCs. As such, the multiple active sites are spatially organized and compartmentalized by the cage skeleton, which therefore avoids undesired mutual quenching. With the synergy of multi-catalytic centers, the integrated cage-bearing nanocomposite catalysts have advanced as another burgeoning candidate to perform accurate and efficient multistep cascade reactions by mimicking cell metabolism and biological synthesis. In this review, we will introduce the most recent adopted confined synthetic methodologies for MCs enabled by cage materials on the one hand, and their applications in advanced catalysis on the other hand.