Electrocatalytic coupling of CO2 to organic molecules for the sustainable production of valuable carboxylic acids

In light of the impact of anthropogenic CO2 emissions on the global climate, capture and re-use of CO2 (CCU) is imperative. For this, electrocatalysis is uniquely suited, driving the direct reduction of CO2 at mild conditions and moderate potentials. However, the commercial viability of such direct CO2 reduction processes is limited, considering the required energy input to produce rather low value fuels. The reductive C-C coupling of CO2 and organic compounds, i.e. electrocarboxylation, presents an interesting pathway to higher value (di)carboxylic acids; intermediates for the synthesis of condensation polymers and pharmaceuticals. To reach a truly sustainable process,  the cathodic electrocarboxylation reaction must be balanced by a waste free and preferably value-adding counterreaction at the anode.

First, we report a novel convergent, paired electrocarboxylation methodology operating in a simple undivided cell with a stable anode, at which tetramethylpiperidine-1-oxyl (TEMPO)-mediated alcohol oxidation takes place. Aromatic alcohols are oxidized with high efficiency to ketones or aldehydes at the anode in the presence of a small amount of water, and then converted to α-hydroxy acids in yields of up to 61 % by coupling with CO2 at the cathode. First, the reaction conditions are optimized for the electrocarboxylation of 1-phenylethanol, followed by benzyl alcohol and benzhydrol. Various cathode materials are evaluated for their selectivity toward the desired C-C coupling between the substrate and CO2. The electrocarboxylation mechanism comprising electron and proton transfers at the cathode is discussed. Electron and proton transfers are influenced by the metal-hydrogen bond strength of the cathode material, which therefore plays an important role in the electrocarboxylation efficiency.

Secondly, we explore the electrocatalytic coupling of CO2 with terminal conjugated dienes in an undivided cell with a stable anode. After optimization, a C6-dicarboxylate yield of over 40 % was reached for the electrocarboxylation of 1,3-butadiene in anhydrous N,N-dimethylformamide at a Ni cathode, using a stable graphite anode. By lowering the reaction temperature below room temperature, we see an almost sixfold increase in selectivity toward the desired dicarboxylates. We hypothesize that by cooling the reaction,  the polymerization of butadiene, of which the chain propagation is not an anodic process, is slowed down. We also show that the addition of a tertiary amine base is crucial in buffering the protons generated by anodic degradation reactions, as these protons cause unwanted cathodic side-reactions. Further attempts to suppress the hypothesized butadiene polymerization were unsuccessful, owing to the difficulty of identifying an anodic counterreaction compatible with the diene electrocarboxylation.

Finally, the electrocarboxylation of 2,3-dimethylbutadiene was investigated in a divided cell equipped with a Nafion-type cation exchange membrane and a stable anode, with the aim of applying Cl2 evolution from aqueous HCl as the counterreaction. We show that certain group 3, 4, 5 and 6 transition metals perform well as cathodes for this reaction in the presence of low amounts of water. For instance, dicarboxylate yields of up to 82 % are obtained using a Ta cathode with 0.5 vol% water present in the anolyte. At elevated anolyte water content, a necessary condition for Cl2 evolution, Hf starts to outperforms Ta. However, even with Hf, the threshold at which water inhibits the electrocarboxylation is lower than 1 vol%. By using a three-compartment setup, we can buffer this water and delay the point at which catholyte water content becomes inhibiting.For example, a dicarboxylate yield of 38 % is obtained in a three compartment reactor equipped with a Hf cathode, where 10 vol% of the anolyte was a concentrated hydrochloric acid solution. We also confirmed the anodic generation of Cl2 in the three-compartment setup.