A continuous-flow photocatalytic system for highly selective oxidation of p-xylene to terephthalic acid by decatungstate catalyst

The selective oxidation of para-xylene (PX) to terephthalic acid (TPA) has received increasing attention due to the industrial applications of TPA. However, the oxidation of the C(sp³)‒H bond of PX is still a main challenge because of the higher bond dissociation energy. Herein, an efficient photocatalytic system for the oxidation of PX to TPA was developed by using tetrabutylammonium decatungstate (TBADT) photocatalyst using atmospheric oxygen as oxidant and 365 nm LED light irradiation. The resulting TPA product was easily separated from the post-reaction solution through simple filtration treatment with a 93.4% yield in CH₃CN (37.5% 1 mol L⁻¹ HCl) solvent after 19-h photocatalysis. Given the easy separation of TPA and the excellent recycling stability of TBADT, a continuous-flow photoreactor was successfully designed with promising prospect for potential industrial application. Mechanistic studies elucidated that the presence of HCl additive benefits the structural integrity of [W₁₀O₃₂]⁴⁻ anions and the transition from excited states [W₁₀O₃₂]⁴⁻* to wO active species, leading to enhanced photooxidation performance.