Combining photocatalytic and adsorption units for the partial oxidation of tyrosol to hydroxytyrosol

Photocatalytic partial oxidation of tyrosol (Tyr) to hydroxytyrosol (Htyr) which is a valuable antioxidant compound, was carried out in a fully recirculated flow photoreactor under UVA light irradiation by using as photocatalyst bare or fluorinated TiO₂ immobilized on glass beads. The maximum selectivity towards Htyr obtained under optimized conditions in the presence of fluorinated TiO₂ at pH 8.7 was approximately 12 %. Reducing the residence time did not result in any further improvement of reaction selectivity due to the fast overoxidation of Htyr.

The photocatalytic reactor was then coupled with an adsorption unit containing a home-made adsorbent exposing amine-stabilized boronate moieties, which showed negligible interaction with Tyr but was capable of selectively binding Htyr on its surface. This allowed for the complete recirculation of the reactant Tyr to the reactor, while the desired product Htyr could be selectively removed from the reaction medium, significantly limiting its parasitic overreaction. Consequently, performing the reaction in the coupled system doubled the selectivity towards Htyr for all configurations, providing a maximum selectivity of 25 % under optimized conditions, which is twice the highest value ever reported for this photocatalytic reaction.

Finally, the kinetic experimental data were modeled using the Copasi software package to elucidate the reaction pathway and estimate the apparent kinetic parameters of the photocatalytic process, both alone and in combination with the adsorption unit. The simulated results showed excellent agreement with the experimental data.