5-Hydroxymethyl Furfural Oxidation by Perylene Diimide-Sensitized Electrodes Boosted by Photoinduced Doping.

We explored the electrochemical behavior of antimony-doped tin oxide (ATO) and perylene diimide (PDI)-sensitized ATO (ATO-PDI) for the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) mediated conversion of 5-hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a value-added substrate for alternative polymer synthesis. We first showed that ATO displayed good electrocatalytic properties towards TEMPO, affording a quasi-reversible response with a heterogeneous rate constant on the order of 2×10^-4 cm s^-1. We then evaluated the performance of ATO under exhaustive electrolysis of HMF in basic aqueous electrolyte, reaching 80 % Faradaic Efficiency (FE) for FDCA production. Interestingly, a significantly enhanced current (up to 2.5 mA cm^-2) was recorded over time when ATO-PDI was exposed to prolonged visible illumination in a Dye-Sensitized Photoelectrochemical Cell (DSPEC) configuration, which we ascribed to the photoinduced doping of ATO resulting from the oxidative quenching of PDI excited states. The proposed system enabled the production of FDCA with ca. 75 % FE in <2 h reaction time, and an almost quantitative HMF conversion when both the mono- and di-acid products were considered. To the best of our knowledge, this is the first example of a molecular dye-sensitized interface used for the TEMPO-mediated oxidation of HMF.