Chlorine Radical-Mediated Photocatalytic C─C Coupling of Methanol to Ethylene Glycol with Near-Unity Selectivity

The selective activation of inert C─H bonds in methanol under mild conditions to synthesize high-value C₂ products remains a formidable challenge, primarily due to the competing high reactivity of O─H bonds. Herein, we pioneer a chlorine radical-mediated strategy to redirect the photocatalytic reaction pathway for methanol conversion toward ethylene glycol (EG). Efficient C─H bond activation is achieved by constructing a Z-scheme heterojunction photocatalyst (ZnIn₂S₄/TiO₂-Cl) composed of chlorinated TiO₂ (TiO₂-Cl) and ZnIn₂S₄ with efficient charge separation. Photogenerated holes in this system preferentially oxidize surface-adsorbed Cl^- to chlorine radicals (Cl•). These radicals drive a thermodynamically favorable hydrogen atom transfer via hydrogen abstraction, cleaving the C─H bond of methanol to form hydroxymethyl radicals (•CH₂OH). Subsequent C─C coupling of •CH₂OH intermediates, synergistically combined with a self-sustaining Cl^-/Cl• cycle, produces EG with exceptional selectivity (96.7%) and yield (21.6 mmol g^-1) while suppressing overoxidation. In contrast, nonchlorinated catalysts predominantly utilize photogenerated holes for O─H bond cleavage under identical conditions, yielding only C₁ products (HCHO, HCOOH). This work not only establishes a solar-driven approach for methanol valorization but also advances mechanistic insights into radical-mediated pathway control in heterogeneous photocatalysis.