Efficient acetaldehyde production and recovery upon selective Cu/TiO2-photocatalytic oxidation of ethanol in aqueous solution

The depletion of fossil chemicals and energy sources led to develop processes allowing the conversion of renewable feedstocks into useful products. Bioethanol production is a typical case of biomass conversion into chemical species with attractive uses, such as fuel, fuel additives, or chemical intermediates. In the present work, the possibility to produce acetaldehyde through Cu/TiO2 photocatalytic oxidation in liquid phase with aqueous mixtures at ethanol concentrations comparable to those of biorefinery outlet streams was assessed for the first time. The effect of cupric ion concentration on acetaldehyde selectivity was investigated. A nearly unit value of acetaldehyde selectivity was observed for a starting cupric ion concentration of 120 mM. The recovery and purification of acetaldehyde were performed by removing acetaldehyde from the solution through an inert gas bubbling and feeding the outgoing gaseous mixture to an absorber, in which acetaldehyde was captured by an aqueous stream. The possibility to regenerate the photocatalytic system in situ via zerovalent copper reoxidation was evaluated. In view of full-scale implementations, a careful flammability analysis of the resulting ternary mixture (ethanol/oxygen/nitrogen) along with a feasible process flow sheet were proposed. The possibility to employ bioethanol streams from biorefinery, sunlight irradiation as energy source, water as solvent, mild operating conditions, and low-cost, non-toxic, and reusable photocatalyst, makes the process investigated an attractive example of the eco-green synthesis of valuable commodity chemicals.