Direct Conversion of Ethanol to Ethyl Acetate by Dynamic Polyoxometalate/Carbon Nanohorn Electrocatalytic Interfaces

Abstract

The electrosynthesis of ethyl acetate (EtOAc) by oxidative esterification of aqueous (up to 20% water) ethanol (EtOH) is performed by employing the tetraruthenate polyoxometalate [Ru4(μ-O)4(μ–OH)2(H2O)4(γ-SiW10O36)2]10– (Ru4POM) electrocatalyst with carbon nanohorns (CNHs) as a heterogeneous support. This strategy involves a voltage-gated electro-adsorption of Ru4POM on CNH-modified glassy carbon anodes with favorable interfacial dynamics, maintained under electrocatalytic conditions. These conditions are reached through the continuous reconstruction of the organic/inorganic interface (catch-and-release), as probed by converging thermal, microscopic, and electrochemical analyses. In fact, control experiments reveal that both pristine and N-doped CNHs display a Ru4POM loading in the range 13–18 nmol mg–1 with a remarkable ∼100 mV onset potential anticipation and current enhancement in the range 400–700% compared to the homogeneous conditions. By adopting the “catch-and-release” protocol, electro-esterification of aqueous EtOH features long-term stability of the productive current in the mA range (Jchrono ≈ 2 mA cm–2 at +1.2 V vs Ag/AgCl probed up to 18 h), with Faradaic efficiencies, FEEtOAc, of >90%. This effect is attributed to the crucial role of CNHs hydrophobicity to control hydrolysis equilibria, thus outperforming the solution-phase behavior, which levels off at FEEtOAc < 60%.