(309d) Investigating CO2 Fixation Via Electrochemical Carboxylation of Organic Halide Substrates

To combat the threat of escalating anthropogenic climate change, it is critical to simultaneously decarbonize conventional chemical production and durably remove carbon dioxide (CO₂) from the atmosphere. We can advance this goal by leveraging captured CO₂ as a nontoxic, sustainable C₁ feedstock. Specifically, organic electrosynthesis powered by abundant renewable energy sources can displace toxic, energy-intensive reducing agents such as Grignard reagents with green electrons. Electrochemical carboxylation (EC) is one such reaction which produces myriad carboxylic acids from a variety of substrates such as alcohols, halides, olefins, and epoxides. Despite prior work on EC, the structure-reactivity relationships of the heterogeneous electrocatalyst as well as the impact of local reaction environments driven by electrolyte composition have not been thoroughly examined and are poorly understood. Furthermore, halohydrins are an underexplored substrate that are particularly well-suited to achieve high-value carboxylic acid products via EC and integrate recent advances in halogen-mediated electrooxidation of olefins.

In this work, we present our investigation into CO₂ coupling with ethylene halohydrin substrates over a heterogeneous electrocatalyst in polar aprotic electrolytes. We use transition and post-transition metal electrocatalysts as pure foils to elucidate reactivity trends. We select metals based on (1) their activity for direct substrate reduction and (2) whether they favor CO₂ reduction (CO₂R) towards CO/CO₃^2- (e.g., Ag, Cu) or oxalate (e.g., Pb, Pt) in aprotic media. In this way, we elucidate how the metal identity tunes the selectivity of EC vs. CO₂R and substrate reduction without CO₂ coupling. We also confirm that the reduction potential of the halohydrin species is significantly modulated by halogen identity (i.e., Br, Cl) in aprotic media and that ethylene and CO are major electrolysis products over a Ag electrocatalyst, as shown in Figure 1.

Figure 1: (a) Representative cyclic voltammograms (CVs) for Ar-purged electrolyte with and without 0.1 M halohydrin substrates (2-bromoethanol or 2-chloroethanol) added. (b) Gas product quantification in terms of Faradaic efficiency (FE) using gas chromatography (GC). Gas purge flow rate = 20 sccm.