Visible-light-driven selective esterification of benzaldehyde derivatives using strontium-modified 1D titanium dioxide nanotubes

The development of photocatalysts for selective esterification of aldehydes under visible light ‘’solar refinery’’ is greatly promising to solve the environmental and energy issues. Herein, key points were highlighted for the photo-assisted oxidation of benzaldehyde derivatives onto Sr-loaded TiO₂ nanotubes facilitating the selectively aromatic aldehydes conversion to ester under visible light irradiation. Herein, strontium loading was investigated in a 0.2–1.0 wt%. Various spectroscopy techniques such as X-ray diffraction, N₂ adsorption–desorption measurements, scanning electron microscopy coupled with energy-dispersive X-ray analysis, transmission electron microscopy, Raman, UV–vis diffuse reflectance, X-ray photoelectron and photoluminescence spectroscopies were used to fully characterize our samples. Our findings demonstrated that up to a Sr loading of 0.4 wt%, the addition of strontium induces an in-situ formation of high amount of oxygen vacancies Ov and Ti³⁺ entities on the surface of xSr-TiNT samples. However, the formation of segregated SrO species was observed when excess Sr loading was used. Photo-assisted oxidation of benzaldehyde derivatives occurs through competitive oxidation/reduction routes in the presence of photogenerated electron-hole pairs. Controlling the percentage of Ov provides a suitable tuning of the oxidation ability of photogenerated holes, avoiding over reduction to toluene and leading to a high selectivity into ester. Kinetically, oxygen vacancies induced by Sr acts as trap for photogenerated electrons. Moreover, the yield in ester increases following the Hammett constant, the highest yield being obtained for 2-hydroxybenzaldehyde (σ = -0.37).