Unraveling the Role of CuO in CuxO/TiO2 Photocatalyst for the Direct Propylene Epoxidation With O2 in a Fluidized Bed Reactor

Propylene epoxidation in mild conditions using molecular O2 is a highly desirable reaction that represents a significant challenge in the field of heterogeneous catalysis for the synthesis of oxygenated organic compounds of industrial interest. In this work, CuxO/TiO2 composites with different mominal CuO loadings (in the range of 0.5–8.4 wt %) were used to promote the photocatalytic epoxidation of propylene with molecular oxygen under UV-A irradiation in a fluidized bed system. The photocatalysts were prepared by a straightforward method consisting of thermal annealing of physical mixtures between copper acetate and sol-gel-derived TiO2. Different characterization techniques were employed to assess the influence of CuxO content on the physical-chemical properties of the CuxO/TiO2 composites. The best combination in terms of propylene conversion and selectivity towards propylene oxide (18.1 % and 72 %, respectively) was obtained with CuxO/TiO2 at 1.1 wt % CuO, as shown by photocatalytic tests. The high propylene oxide selectivity is due to the ability of CuO in the CuxO/TiO2 composite to transform molecular O2 into hydrogen peroxide that, in turn, is able to directly oxidize propylene to propylene oxide. By using a UV-A light intensity of 297.2 mW cm−2, the propylene conversion and the epoxide yield were 31.5 and 22.2 %, respectively, significantly higher than that reported in the literature. Moreover, the energy consumption of the reaction system employed in this paper was significantly lower than that of photocatalytic systems studied in the literature dealing with selective propylene epoxidation.