Beyond Density: Unveiling the Trade-Off in Catalytic Site Optimization Using Defect-Engineered UiO-66

Enhancing intrinsic activity and increasing catalytic site density are two widely employed strategies to improve catalytic performance. Although typically considered independently, their interplay remains poorly understood. Here, two UiO-66 metal-organic frameworks (MOFs) with distinct catalytic site densities-linker-defective UiO-66L and cluster-defective UiO-66C-are synthesized and systematically compared. Despite a higher density of open Zr catalytic sites, UiO-66L exhibited lower catalytic activity than UiO-66C across four model reactions, performing similarly to defect-free UiO-66. Although defect engineering is expected to enlarge pore connectivity, diffusion-ordered spectroscopy (DOSY) and molecular dynamics (MD) simulations surprisingly reveal that UiO-66C exhibits similar diffusion rates to defect-free UiO-66, while UiO-66L shows significantly slower diffusion. This discrepancy is attributed to self-adsorption of reactants at the high-density catalytic sites, which induces local diffusion resistance even in the presence of expanded channels. These findings reveal a performance trade-off between catalytic site density and intrinsic activity, establishing a critical threshold beyond which further increases in site density can hinder rather than enhance catalysis.