Abstract

The electrocarboxylation of organic compounds provides a green alternative to the conventional use of cyanides or organometallics. Via a simple electrochemical approach, CO 2 can be directly incorporated into aldehydes or ketones to form αhydroxy acids using electrical energy as a driving force. In this study, propylene carbonate was used as a "green" solvent. Importantly, the use of sacrificial anodes was avoided; instead, cost-efficient and stable graphite anodes were applied. Voltammetric studies on benzaldehyde carboxylation revealed that the presence of CO 2 significantly changes the reduction behavior of benzaldehyde at potentials more negative than the potential of the ketyl radical formation. Bulk electrolysis was performed in various solvents and supporting electrolyte systems, leading to more than 55% mandelic acid yield in propylene carbonate. Among the tested solvents (acetonitrile, N,N-dimethylformamide, dimethyl sulfoxide, and propylene carbonate), propylene carbonate resulted in the highest carboxylate yield. Numerous cathode materials were tested. High carboxylate yields were achieved using graphite, glassy carbon, and lead cathodes. The reaction was successfully carried out for various aromatic aldehydes and ketones as substrates, providing yields of up to 63%.

Copper and its oxides are the main catalyst materials able to promote the formation of hydrocarbons from the electrocatalytic CO2 conversion. Herein, we describe a novel preparation method for carbon-doped copper oxide catalysts based on an oxidative thermal treatment of copper-containing deep eutectic solvents (DES). XRD and EDX analysis of the samples show that thermal treatment at 500 °C in air for a prolonged time (60 min) provides exclusively carbon-doped copper(II) oxide catalysts, whereas shorter calcination time leads to a mixture of less oxidized forms of copper (Cu2O and Cu0), CuO, and a higher carbon content from the DES. Chronoamperometry of the electrode containing the prepared materials in 0.5 M KHCO3 electrolyte show the reduction of CuO to less oxidized copper species. The materials prepared by the use of different DES, copper precursors and calcination times were used as electrocatalysts for the electrochemical CO2 reduction. Chemical analysis of the products reveals an enhanced selectivity toward C2 and C3 products for the catalyst prepared from the DES galactose-urea with copper nanoparticles and calcination for 60 min in air. The electrocatalytic activity of the prepared materials were compared to commercial CuO and showed a higher product concentration at −1.7 V vs. Ag/AgCl, with formation rates of 7.4, 6.0, and 10.4 µmol h−1 cm−2 for ethanol, n-propanol, and ethylene, respectively.