While humans are making great efforts to convert inert CO₂ into something useful, nature has interesting nanomachines in the form of enzymes. The enzyme formate dehydrogenase (FDH) normally converts methanoate (formate) into CO₂, but under certain conditions it does the opposite.

Matías Aguirre, Cristina Ramírez and Yesica Di Iorio from the Universidad Nacional de Mar del Plata wanted to take advantage of this. This had been tried before, for example by using the free enzyme or by binding it to a support, but the results were not satisfactory, so they took a different approach: they bound it to a metal-organic framework (MOF).

The MOF in question is ZIF-8, which is made up of zinc(II) ions and 2-methylimidazole molecules, and can be synthesised in water at room temperature. This comes in handy if you want to trap enzymes – which naturally occur in an aqueous environment – in a MOF. They also added magnetite (Fe₃O₄). During the synthesis process, small pieces of magnetite attach themselves to the MOF, making it possible to attract the MOF with a magnet.

The last thing needed was the biocatalyst NADH. This is because the CO₂ conversion reaction is actually an equilibrium, with the reaction towards CO₂ being favoured. With NADH, this equilibrium can be shifted in the other direction.

The researchers then compared the CO₂ conversion of free FDH with the trapped variant [mZIF-8(FDH)], in which NaHCO₃ (‘dissolved CO₂’) was the CO₂ source. This showed that the trapped mZIF-8(FDH) converted 3.4 times more CO₂ than the free FDH (1015 vs. 300 µmol per litre per hour per milligram).

The magnetite decomposition also allows the whole thing to be recycled after the reaction. After three rounds of reaction, only 5% of the activity had been lost and after five rounds, 86% of the activity remained. All in all, the study provides an efficient and enzyme-friendly framework that could potentially be used for other enzymes too, the authors conclude.

Aguirre, M.E. et al. (2023) Chem. Eur. J. e202301113, DOI: 10.1002/chem.202301113