Hollow-structured metal-organic frameworks (MOFs) often exhibit higher catalytic activity than their solid counterparts, but the mechanism behind this enhancement remains controversial due to the lack of direct experimental evidence. Using ZIF-8 as a model system, this study employs in situ fluorescence imaging to monitor the catalytic process in real time, providing direct visual evidence for the diffusion-separation mechanism in hollow MOF particles. The results show that the hollow structure facilitates inward diffusion of products, thereby separating products from reactants, which accounts for the improved catalytic performance. By systematically investigating the relationship between shell thickness and catalytic performance, we identify three critical size ranges, indicating that shell thickness directly influences catalytic behavior by regulating product permeation through the MOF shell. Furthermore, catalytic experiments with a series of molecules of different sizes confirm the generality of this enhancement mechanism. This work transforms theoretical hypotheses into visualized mechanistic understanding, resolves a long-standing controversy in the field, and lays a foundation for the rational design of high-performance hollow catalytic materials. Why hollow porous crystals speed reactions has remained unclear. Real-time imaging in a model metal organic framework shows that hollow particles separate products from reactants, and shell thickness controls this transport and catalytic gain.
Ma et al. (Mon,) studied this question.