Electrocarboxylation in supercritical CO2 and CO2-expanded liquids

In this study, the electrocarboxylation of benzyl chloride in pressurized CO2, or pressurized mixtures of dimethylformamide (DMF) and CO2, was investigated in order to synthesize phenylacetic acid. A stainless steel cathode was used as the working electrode, whereas a sacrificial massive magnesium rod or a platinized platinum grid was used as the anode, tetrabutylammonium perchlorate (TBAP) or tetrakis(decyl)ammonium tetraphenylborate (TDATPhB) being the supporting electrolyte. The electrocarboxylation was carried out at 40 °C, at operating pressures of 1, 6, 7, 8, 9 and 12 MPa, using current densities ranging from 0.1 to 150 mA cm−2. It was found that a small amount of DMF was necessary to ensure the solubility of the supporting electrolyte, to obtain sufficient electrical conductivity of the medium. The best results were obtained using the magnesium sacrificial anode, at 6 MPa. After consumption of the theoretical amount of electrical current (2F mol−1), 65.7% benzyl chloride conversion was reached, together with an 82.4% phenylacetic acid selectivity and a 54.2% faradaic yield. Detected by-products were toluene, bibenzyle, benzyl alcohol and benzaldehyde.