Direct visible-light oxidation of p-xylene to terephthalic acid with molecular oxygen over NH2-functionalized Ce-UiO-66

Selective oxidation of p-xylene (PX) to terephthalic acid (TA) under mild conditions remains challenging because current industrial processes rely on harsh reaction conditions and homogeneous catalysts. Herein, we report a Ce-based metal–organic framework, Ce-UiO-66-NH2, as an efficient visible-light photocatalyst for the direct aerobic oxidation of PX to TA using molecular oxygen as the sole oxidant under ambient pressure and near-room temperature. Comparison among structurally related Ce-based frameworks reveals that photocatalytic activity is not governed by surface area or amino functionalization alone, but reflects the interplay between visible-light-responsive linker chemistry and substrate–photocatalyst interactions. Dark adsorption experiments further support this relationship, showing substantially higher PX uptake for the most active photocatalyst (ca. 27 %) than for the other two frameworks (ca. 11 % and 5 %). Radical scavenger experiments indicate that superoxide and singlet oxygen are involved in the oxidation process, while photogenerated holes and/or oxidized linker species are proposed to participate in substrate activation. Under optimized conditions, Ce-UiO-66-NH2 achieves 95 % PX conversion with 93 % selectivity toward TA (88 % yield) after 5 h of visible-light irradiation. Recycling experiments show sustained photocatalytic activity over consecutive runs following an initial decrease in performance. To the best of our knowledge, this study represents the first example of a metal–organic framework photocatalyst for the direct visible-light oxidation of PX to TA and highlights the combined importance of linker functionalization and favorable substrate adsorption in promoting aerobic oxidation of alkyl aromatics.