Fe-mediated enhancement of phenol selectivity in syringol hydrodeoxygenation over an in-situ-activated CoFeAg/SiO2 catalyst

The hydrodeoxygenation of lignin-derived syringol to phenol faces challenges in selectively cleaving C_Aryl–O bonds while maintaining aromatic ring and a phenolic -OH group integrity. Herein, we developed a trimetallic CoFeAg/SiO₂ catalyst that achieves 100 % syringol conversion and 52.4 % phenol selectivity, surpassing bimetallic CoAg/SiO₂ (42.2 mol%) and monometallic Co/SiO₂ (7.6 mol%) catalysts. As promoters, Ag facilitates the low-temperature reduction of Co₃O₄ to CoO and further to metallic Co, while Fe prevents excessive reduction of CoO to metallic Co. Through the synergistic effects of Fe and Ag on Co, the CoFeAg/SiO₂ catalyst undergoes in situ reduction under reaction conditions (1 MPa H₂, 330 °C), forming a polycrystalline CoO-Co structure with abundant oxygen vacancy (O_ᴠ) and Lewis acid sites. The CoO-Co-O_v active sites facilitate heterolytic H₂ cleavage while preferentially adsorbing and activating oxygen-containing functional groups over the aromatic ring of syringol. This dual functionality selectively promotes C_Aryl–O bond dissociation while suppressing undesired ring hydrogenation, providing an effective catalyst design strategy for sustainable phenol production from biomass-derived syringol.