Hydrodeoxygenation of guaiacol to phenol over ceria-supported iron catalysts

Guaiacol, one of the main components and a model compound of lignin-derived bio-oil, was transformed into phenol and methylated phenols over a series of supported iron catalysts under atmospheric pressure. CeO₂ support was superior to other conventional supports in view of the activity of supported iron catalysts. The effect of iron loading amount on CeO₂ supports showed a linear relationship with an iron loading amount up to 3 wt %, which corresponded to “monolayer” coverage, and then it was almost constant with a further increase of the iron content. The Fe/CeO₂ catalyst with 3 wt % iron loading (Fe(3)/CeO₂) had higher stability than the catalysts with higher iron loadings. The optimal Fe(3)/ CeO₂ catalyst showed a 56% phenol yield and an 87% sum yield of phenol and methylated phenols at 673 K under guaiacol/N₂/H₂ = 1/45/135 flow with W/F = 1.0 g·h·mol_total⁻¹. The X-ray diffraction (XRD), diffuse reflectance UV−visible (DRUV−vis), Raman, and X-ray absorption spectroscopy (XAS) characterizations confirmed that the FeO_x species were finely dispersed on the Fe/CeO₂ catalyst with ≤3 wt % iron, and higher iron loadings led to formation of Fe₂O₃ particles in calcined catalysts. Although other supported iron catalysts with 3 wt % iron loading also contained dispersed FeO_x species, the reducibility of Fe/CeO₂ was higher than that of other supported iron catalysts, which might be related to the higher activity. The XAS characterization of spent catalysts showed that the stable Fe(3)/CeO₂ catalyst contained only Fe³⁺ and Fe²⁺ species during the reaction, while in the Fe(10)/CeO₂ catalyst, the reduction of iron proceeded to a greater extent and Fe₃C crystals grew during the reaction. The structure of this monolayer Fe(3)/CeO₂ was further characterized by extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT) calculations. Rather than a two-dimensional Fe oxide sheet, a structure composed of dispersed Fe₄O₆ clusters is suggested for monolayer coverage. The coordinatively unsaturated site (CUS) in the interface between FeO_x and reducible CeO₂ is suggested to be the active site.