Phosgene-Free Method for Diphenyl Carbonate Synthesis at the Pd⁰/Ketjenblack Anode

Electrocarbonylation of phenol (PhOH) with CO to diphenyl carbonate (DPC) at a Pd⁰-supported Ketjenblack electrocatalyst (Pd/KB) was studied at P(CO) = 1 atm and 25 °C. Electrocarbonylation was conducted by galvanostatic electrolysis at 1 mA in an electrolyte containing PhOH, sodium phenoxide (PhONa), LiCl, and CH₃CN. The electrocatalytic activity of DPC formation was strongly affected by the reduction temperature of a PdCl₂/KB electrocatalyst with H₂. The Pd/KB electrocatalyst reduced at 393 K showed the highest electrocatalytic activity, while that reduced at 673 K showed less activity. Pd/KB was characterized by X-ray diffraction, transmission electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Pd particle sizes are strongly related to the electrocatalytic activity of DPC formation. Pd⁰ particles less than 2 nm were active, whereas those greater than 6 nm were inactive. To reveal the electrocarbonylation mechanism, electrocarbonylations using p-cresol, p-chlorophenol, and sodium phenoxides were studied. Cyclic voltammetry studies were conducted using a palladiumized palladium-wire electrode. These results strongly indicated that sodium phenoxides did not incorporate into diaromatic carbonates and also functioned as proton acceptors during electrocarbonylation.