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

Abstract

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, non-chlorinated 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.