Aqueous Photocatalytic Glycerol Oxidation to Formic Acid Coupled to H2O2 Production with an Anthraquinone Dye

The photocatalytic oxidation of glycerol into formic acid (FA) is reported employing a 9,10-anthraquinone-2,6-disulphonate disodium salt (AQDS) photocatalyst. The system operates in water, in the absence of additives, using O-2 as the oxidant and irradiating with blue light (lambda = 415 nm). In 22 h, conversion of glycerol up to 79% leads to 30% yield of FA (turnover number of 15 for AQDS), with 79% selectivity among the products in solution and a quantum yield of 1.2%. The oxidation of glycerol is coupled to the reduction of oxygen to hydrogen peroxide (up to 16 +/- 5 mm), a high-added value photosynthetic product. A mechanistic investigation combining electron paramagnetic resonance (EPR) spectroscopy, transient absorption spectroscopy (TAS), and time-dependent density-functional theory (TD-DFT) calculations reveals a photoinduced hydrogen atom abstraction involving the triplet excited state 3*AQDS and the glycerol substrate (k = 1.02(+/- 0.03)x10(7) m(-1)s(-1), H/D kinetic isotope effect = 2.00 +/- 0.16). The resulting ketyl radical of AQDS follows fast deprotonation to the radical anion AQDS center dot-, that further reacts with oxygen (k = 1.2x10(8) m(-1)s(-1)), ultimately leading to the production of H2O2.