Selective ligand removal to improve accessibility of active sites in hierarchical MOFs for heterogeneous photocatalysis

While metal organic frameworks offer highly tunable materials for photocatalytic applications, catalytic site accessibility is a crucial parameter. Here, authors demonstrate selective ligand removal to introduce new active sites and mesopores and to enhance photocatalytic H-2 evolution. Metal-organic frameworks (MOFs) are commended as photocatalysts for H-2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. For dynamic processes in liquid phase, the accessibility of active sites becomes a critical parameter as reactant diffusion is limited by the inherently small micropores. Our strategy is to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Here we report photoactive MOFs of the MIL-125-Ti family with two distinct mesopore architectures resembling either large cavities or branching fractures. The ligand removal is highly selective and follows a 2-step process tunable by temperature and time. The introduction of mesopores and the associated formation of new active sites have improved the HER rates of the MOFs by up to 500%. We envision that this strategy will allow the purposeful engineering of hierarchical MOFs and advance their applicability in environmental and energy technologies.

Automated summary

In this study, the authors demonstrate that selective ligand removal in metal-organic frameworks (MOFs) can introduce new active sites and mesopores, leading to enhanced photocatalytic hydrogen evolution. This strategy allows for the purposeful engineering of hierarchical MOFs, advancing their applicability in environmental and energy technologies.