Photocatalytic Ammonia Synthesis using Fe-Based MOFs: The Role of Ligand Functionalization

Photocatalytic ammonia (NH₃) synthesis offers a carbon-neutral alternative to the Haber-Bosch process, which generates 42 million metric tons of CO₂ equivalent emissions annually. However, solar-to-ammonia conversion with contemporary photocatalysts remains far from practical requirements, and understanding the limiting factors in systems with well-defined active sites is crucial. Here, we show how the μ₃-oxo-centered trinuclear Fe cluster in MIL-101(Fe) functions as the catalytic motif for N₂-to-NH₃ conversion through combined experimental and computational investigations. Comparative studies with a molecular analogue demonstrate that the cluster is stabilized within the MOF framework, sustaining redox cycling and maintaining high catalytic activity. We systematically functionalized the dicarboxylate ligands of MIL-101(Fe) with -NH₂, -Br, -NO₂, -F, and -CF₃ to probe how ligand chemistry modulates Fe electron density, N₂ adsorption capacity, and proton availability, correlating these properties with catalytic performance using spectroscopic and surface characterization techniques alongside time-resolved infrared to assess excited-state lifetimes. F-functionalization optimally balances N₂ activation, proton availability at Fe active sites, and excited-state lifetimes, boosting NH₃ production by ∼ 60% relative to unmodified MIL-101(Fe). This study of ligand-functionalized MIL-101(Fe) MOFs uncovers the underlying structure-activity relationships and advances design principles for solar-driven NH₃ synthesis.