Sustainable economies may one day switch from using fossil feedstocks to producing chemicals and fuels from bio-based resources. This is a fascinating application of electrochemistry, especially when using green power. Example: Kolbe electrolysis, or the anodic decarboxylation of n-carboxylic acids, could be used in electrobiorefineries to produce valuable chemicals.

Since the Kolbe electrolysis may be carried out in aqueous solutions at room temperature, it has a little impact on the environment. Kolbe electrolysis of low concentrations of short-chain n-carboxylic acids has been the primary focus of research thus far. Microbial conversions can be used to produce medium-chain acids (MCCA) with a chain length of four to eight carbon atoms from waste and biobased feedstocks. Converting MCCA at greater concentrations yields hydrocarbon mixtures that can be utilised as fuel additives, making it a good candidate for use in electrobiorefineries.

Researchers led by Falk Harnisch from Germany’s Helmholtz Centre for Environmental Research in Leipzig showed that electrolyzing a mixture of MCCA at concentrations similar to broths produced by bioconversion outperformed electrolyzing single acids. A mixture of n-butanoic (C4), n-hexanoic (C6), and n-octanoic (C8) acids were electrochemically converted using Kolbe electrolysis.

The researchers determined that dimerization, with selectivities of up to 70%, was the most favourable reaction route. However, in the case of a mixture of acids, it is necessary to differentiate between homo- and hetero-coupling for this type of reaction. A whopping 66% of the hexanoic acid conversions resulted in homo-coupled dimers of the resultant radicals, whereas just 8-10% of the butanoic and octanoic acid conversions did so. Hetero-coupling with hexanoic acid radicals was the preferred method of reaction for butanoic and octanoic acids. Accordingly, in comparison to the electrolysis of single acids, hetero-coupling considerably boosted the overall conversion.