Photocatalytic dihydroxylation of light olefins to glycols by water

Aliphatic diols such as ethylene and propylene glycol are the key products in the chemical industry for manufacturing polymers. The synthesis of these molecules usually implies sequential processes, including epoxidation of olefins using hydrogen peroxide or oxygen with subsequent hydrolysis to glycols. Direct hydroxylation of olefins by cheap and green oxidants is an economically attractive and environmentally friendly route for the synthesis of diols. Here, we report a photocatalytic reaction for the dihydroxylation of ethylene and propylene to their glycols at room temperature using water as the oxidant. The photocatalyst contains Pd clusters stabilized by sub-nanometric polyoxometalate with TiO₂ as the host material. Under light irradiation, it results in production rates of ethylene glycol and propylene glycols of 146.8 mmol·g_Pd^-1·h^-1 and 28.6 mmol·g_Pd^-1·h^-1 with liquid-phase selectivities of 63.3 % and 80.0 %, respectively. Meanwhile, green hydrogen derived from water is produced as another valuable product. Combined spectroscopy investigation suggests that the reaction proceeds via π-bonded adsorption of olefins over Pd clusters with hydroxylation by hydroxyl radicals formed by photocatalytic dissociation of water.