The use of enzymes can be an efficient route for carbon fixation. The enzyme formate dehydrogenase (FDH), for example, reduces carbon dioxide (CO₂) to formic acid, coupled with the oxidation of the cofactor nicotinamide adenine dinucleotide (NADH) to NAD⁺. However, the enzymes are relatively unstable and hard to recycle, which limit the implementation of such processes. Metal-organic frameworks (MOFs), a class of porous crystalline materials, can be used as a stabilizing support for enzymes to prevent structural changes that affect their reactivity.

Omar K. Farha, Northwestern University, Evanston, IL, USA, and colleagues have synthesized a MOF called NU-1006 with large channels which can encapsulate FDH (pictured on yellow background). For the electrocatalytic conversion of NAD⁺ to NADH, a ZrO₂-modified glass electrode (pictured in red) with a coordinated rhodium-based electron mediator (pictured in purple) was used. Drop-casting of the MOF-enzyme composite onto this rhodium-complex-modified electrode resulted in a complete bioelectrochemical reaction system (pictured below).

This system consistently provides FDH with NADH for the conversion of dissolved CO₂ to formic acid. The MOF stabilizes FDH even under unfavorable conditions (for example, in an acidic environment), which can double the formic acid production compared to the free enzyme.

• Stabilization of Formate Dehydrogenase in a Metal-Organic Framework for Bioelectrocatalytic Reduction of CO₂,
  Yijing Chen, Peng Li, Hyunho Noh, Chung-Wei Kung, Cassandra T. Buru, Xingjie Wang, Xuan Zhang, Omar K. Farha,
  Angew. Chem. Int. Ed. 2019.
  https://doi.org/10.1002/anie.201901981