Dynamic Confinement Approach for High Metal Loading Single- Atom Catalysts based on Covalent Organic Frameworks

Single-atom catalysts (SACs) offer stable, well-defined active sites by anchoring individual metal atoms on stable organic or inorganic supports, though achieving high metal loadings without clustering or leaching remains a major challenge. Here, we report a synthetic strategy for developing ultra-high metal loading SACs based on palladium polyphthalocyanine covalent organic frameworks (COFs) synthesized via a mixed metal ionothermal approach, which involves the cyclization of tetracyanobenzene and tetracyanopyrazine as precursors in the molten salt mixtures of PdCl2/ZnCl2 or PdCl2/ZnCl2/NaCl. This approach effectively combines the formation of crystalline polymeric hosts with metal impregnation in a single step, yielding COFs with atomically distributed Pd ions and metal contents of up to 22.2 wt%. Theoretical simulations reveal that the crystalline framework dynamically confines Pd atoms between different binding sites within the pores, which has a barrier of only 0.1 eV, corresponding to an approximate frequency of 1012 events per second, preventing dimerization and ensuring long-term catalyst stability. The synthesized catalysts were evaluated under continuous flow conditions, exhibiting stable performance with yields as high as 90% and maintaining stability over a 24-hour time-on-stream under low-conversion conditions. These results establish a new benchmark for SACs and underscore the importance of dynamic confinement approach in achieving high metal loadings on crystalline organic supports.