Visible‐Light‐Driven CO ₂ Photoreduction Assisted by Reduced Methyl Viologen Using MIL‐125‐NH ₂ as a Light Harvester and Scaffold for FDH Immobilization

Abstract Metal‐organic frameworks (MOFs)‐enzyme composites are promising catalysts for energy production and climate change mitigation through CO 2 reduction. However, many biocatalysts are limited by their dependence on reduced nicotinamide adenine dinucleotide (NADH) as a cofactor. Here, we report a simple system using formate dehydrogenase (FDH) adsorbed onto MIL‐125‐NH 2 , enabling visible‐light‐driven formic acid (HCOOH) production without NADH. The MOF serves as both support and photoactive material, achieving a maximum enzyme loading of 3.84% (w/w) at neutral pH. Upon excitation at 470 nm and using triethanolamine (TEOA) as electron donor, the system produced 134.9 µmol HCOOH g FDH −1 h −1 . This increased to 655 µmol HCOOH g FDH −1 h −1 using photoreduced methyl viologen (MV +● ), which transfers electrons 10.8 times more efficiently than NADH. pH significantly influences enzyme loading and activity; while lower pH enhances loading, it causes irreversible deactivation. Optimal results were achieved by immobilizing FDH at pH 7.0 and performing the reaction at pH 8.0, yielding 2005 µmol HCOOH g FDH −1 in 4 h. Reduced activity in acidic media was attributed to protonation of TEOA and MV +● , rather than enzyme denaturation, suggesting a protective effect from the MOF scaffold. These results highlight pH tuning as a key factor for optimizing catalytic efficiency in MOF‐enzyme systems.