Combination of H3PW12O40-TiO2 catalysts for photo-thermal oxidation of cyclohexene to adipic acid by 30% H2O2

In this work, 12-tungstophosphoric acid-TiO2 composite (PTA-TiO2) catalysts were fabricated by a simple method of impregnation-heat treatment. PTA-TiO2 composites were characterized by XRD, XPS, TG/DSC, FTIR, UV−vis diffuse reflectance spectra and N2 adsorption–desorption measurements. The results showed that the heat treatment temperature (200–500 °C) significantly affected the composition and structure of the composite. The higher calcination temperature could destroy the Keggin structure of PTA. The PTA-TiO2 catalyzed oxidation of cyclohexene by H2O2 to adipic acid demonstrated that the Keggin structure of PTA was critical to the catalytic activity of PTA-TiO2. Furthermore, under the photo-thermal catalytic condition, the reaction system showed higher cyclohexene conversion and adipic acid yield. A synergistic effect between the photo- and thermal approach was noticed in the PTA-TiO2 catalyst prepared at 200 °C (PTA-TiO2(200)). Based on the experimental data, a thermocatalysis-dominated but photocatalysis-promoted mechanism was proposed to explain the improvement in catalytic efficiency. The catalyzed oxidation of cyclohexene with photo-thermal synergy enables utilization of sunlight and offers a novel perspective for the manufacture of adipic acid.