Simultaneous boosting visible light-driven benzene hydroxylation reaction rate and phenol selectivity via a novel monolithic polymer photoreactive composite based on N-doped TiO2 supported CuO

Abstract

A novel photoreactive composite consisting of CuO-loaded N-doped TiO₂ (CuNdT) embedded in a syndiotactic polystyrene (sPS) monolithic aerogel was developed and evaluated for the photocatalytic hydroxylation of benzene to phenol under visible light irradiation. The process was identified as biphasic, where the photoreaction occurred within the solid phase of the composite, and the surrounding liquid phase acted as a reservoir for the desired product (phenol). The composite was extensively characterized through WAXD, SEM, BET, and tomographic analyses, confirming the uniform dispersion of CuNdT particles within the porous, hydrophobic sPS matrix. Under acidic conditions (pH = 2), the composite achieved 96 % benzene conversion and >99 % phenol selectivity within 180 min of irradiation, results that surpass those reported in the literature for visible-light-driven benzene hydroxylation. Kinetic modeling, based on experimentally determined partition coefficients for benzene and phenol, revealed that the composite operated entirely in a chemical regime, free from diffusional limitations. The calculated efficiency factors, all equal to 1, confirmed that the reaction was controlled solely by chemical processes occurring on the CuNdT dispersed inside the sPS matrix. The differences in kinetic constants between pH = 7 and pH = 2 highlighted the critical role of pH in driving the photocatalytic process. At pH = 2, the increased tendency of H₂O₂ to form hydroxyl radicals enhanced the rates of benzene consumption and phenol formation. Additionally, the higher affinity of benzene for the hydrophobic sPS polymer matrix at acidic pH, as reflected by its elevated partition coefficient, further facilitated its interaction with the composite, promoting faster oxidation. In contrast, phenol displayed a lower affinity for the hydrophobic sPS matrix at pH = 2, as evidenced by its lower partition coefficient, allowing it to remain in the liquid phase, reducing over-oxidation and contributing to the higher phenol yield (96 %) observed at acidic pH. This synergistic effect between enhanced hydroxyl radical generation, increased benzene affinity and reduced phenol affinity led to a more efficient photocatalytic process under acidic conditions. These findings establish CuNdT/sPS as a highly efficient, selective, and robust photocatalytic material, combining superior performance, structural stability, and excellent reusability for multiple reaction cycles.