Hetero‐Coupling of Bio‐Based Medium‐Chain Carboxylic Acids by Kolbe Electrolysis Enables High Fuel Yield and Efficiency

Abstract

Mixtures of n-carboxylic acids (n-CA) as derived from microbial conversion of waste biomass were converted to bio-fuel using Kolbe electrolysis. While providing full carbon and electron balances, key parameters like electrolysis time, chain length of n-CA, and pH were investigated for their influence on reaction efficiency. Electrolysis of n-hexanoic acid showed the highest coulombic efficiency (CE) of 58.9 � 16.4 % (n = 4) for liquid fuel production among individually tested n-CA. Duration of the electrolysis was varied within a range of 0.27 to 1.02 faraday equivalents without loss of efficiency. Noteworthy, CE increased to around 70 % by hetero-coupling when electrolysing n-CA mixtures regardless of the applied pH. Thus, 1 L of fuel could be produced from 12.4 mol of n-CA mixture using 5.02 kWh (< 1 E L À 1 ). Thus, a coupling with microbial processes producing n-CA mixtures from different organic substrates and waste is more than promising.

Amorphous RuO2 Catalyst for Medium Size Carboxylic Acid to Alkane Dimer Selective Kolbe Electrolysis in an Aqueous Environment**

“…The greater hydrophilicity of valeric acid is highly soluble in the aqueous electrolyte, and thus decreases its coverage on the electrode surface. On the other hand, the longer chain C 6+ VFAs had a greater hydrophobicity, and their deprotonated forms improves their adsorptions on the electrode surface [30] . The formation of the hydrophobic carboxylate layer could contribute to the inhibition of water oxidation competition, which improves the oxidative transformation of the adsorbed carboxylate anions.…”

Electrical-energy storage into chemical-energy carriers by combining or integrating electrochemistry and biology

“…For the calculation, we used the This journal is © The Royal Society of Chemistry 2024 experimentally validated 70% and 90% Coulombic efficiency for Kolbe electrolysis at the anode and the hydrogen evolution reaction at the cathode, respectively. 113,114 Considering a carbon efficiency of 50% (mol C mol C À1 ) for the overall conversion of waste substrate into Kolbe product by biological and electrochemical conversions, 112 the system generates 1.1 Â 10 À2 g hydrogen per gram of carbon in the waste substrate (6.4 Â 10 À2 mol hydrogen per mol carbon in the substrate). The produced hydrogen can be utilized as described above.…”

Anodically‐Generated Alkyl Radicals Derived from Carboxylic Acids as Reactive Intermediates for Addition to Alkenes

“…However, the growing economic and environmental benefits of electrosynthesis are making Kolbe electrolysis increasingly appealing to chemists and chemical engineers. For instance, Harnisch et al., in a series of Kolbe electrolysis studies focusing on the impact of supporting electrolytes, electrode design, and electrode dissolution, proposed the reaction as a pathway to biofuel synthesis [27–32] …”

“…For instance, Harnisch et al, in a series of Kolbe electrolysis studies focusing on the impact of supporting electrolytes, electrode design, and electrode dissolution, proposed the reaction as a pathway to biofuel synthesis. [27][28][29][30][31][32] The synthesis of such RR products by radical self-coupling is still not economically attractive, but using the R * intermediates to react with a second reactant may be of interest because higher value product can be pursued. Previous studies have used carboxylic acids as substrates to electrochemically produce alkenes, [33] ethers, [34] cyclic hydrocarbons, [35] lactones, [36] anhydrides, [37] and amides.…”