Mild Photochemical Synthesis of the Antioxidant Hydroxytyrosol via Conversion of Tyrosol

Hydroxytyrosol, a naturally occurred orthodiphenolic antioxidant molecule found in olive oil and olive mill wastewaters, was obtained from the wet hydrogen peroxide photocatalytic oxidation of its monophenolic precursor tyrosol. The liquid-phase oxidation of tyrosol to hydroxytyrosol was performed by use of an iron-containing heterogeneous catalyst (Al-Fe)PILC with the assistance of UV irradiation at 254 nm and at room temperature. The spectroscopic and HPLC data of the synthesized compound proved to coincide fully with those of a pure sample obtained by continuous countercurrent extraction. This reaction was found to be light-induced. The hydroxytyrosol synthesis reaction reached its maximum yield of 64.36% under the optimized operating conditions of 3.6 mM tyrosol, 0.5 g L(-1) catalyst, and 10(-2) M H2O2 with the assistance of UV light. Increasing the initial hydrogen peroxide concentration more than 10(-2) M has a diminishing return on the reaction efficiency. Catalyst can be recuperated by means of filtration and then reused in a next run after regeneration since its activity did not significantly decrease (<10%). The reaction synthesis is operationally simple and could find application for industrial purposes.