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·moltotal–1. The X-ray diffraction (XRD), diffuse reflectance UV–visible (DRUV–vis), Raman, and X-ray absorption spectroscopy (XAS) characterizations confirmed that the FeOx 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 FeOx 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 FeOx and reducible CeO2 is suggested to be the active site.