Photocatalytic Anaerobic Conversion of m-Xylene to Xylenols With Co-Production of Hydrogen.

Mild photocatalytic valorization of aromatic hydrocarbons is promising for phenolic synthesis, but reliance on oxidants (O2 or H2O2) often leads to the oxidation of alkyl groups rather than stable phenyl rings, thus producing aromatic aldehydes or ketones instead of targeted phenols. In this work, we employed a photocatalytic anaerobic reaction pathway for directly synthesizing the targeted high-value xylenol and H2 from m-xylene and water over a palladium single atom-loaded TiO2 with zinc modification (Pd1-TiO2(Zn)). In situ infrared spectroscopy and electron paramagnetic resonance, in combination with first-principles simulations, revealed that the photocatalytic anaerobic conversion of m-xylene proceeds via surface lattice oxygen-mediated hydroxylation. Lattice oxygen coordinated with Pd acts as a recyclable oxygen source for phenolic hydroxyl groups, regenerated by the rate-determining water oxidation. Isotope labeling confirms hydroxyl hydrogen originates from the benzene ring, not water; water only provides oxygen. The zinc modification could significantly reduce the reaction barrier of the water-oxidation step from 0.85 to 0.22 eV at the Pd-O site. Therefore, the phenolic production rate over Pd1-TiO2(Zn) reached 376.9 µmol g-1 h-1, accompanied by an exceptionally high phenolic selectivity of 98.5% and a hydrogen production rate of 380.6 µmol g-1 h-1, 2-fold higher than that of unmodified Pd1-TiO2.