Carbon nanofiber-supported ReOx catalysts for the hydrodeoxygenation of lignin-derived compounds

Abstract

The effect of ReO_x loading (2–13 wt%) and H₂ pressure (0–5 MPa) for the hydrodeoxygenation of phenol has been studied for carbon nanofiber-supported ReO_x catalysts in a batch reactor at 573 K. Characterization of the supports and catalysts has been obtained from N₂ physisorption, TPD, FTIR, XRD, potentiometric titration, TPR and XPS measurements, which revealed the presence of a crystalline and surface ReO_x phase whose particle size and surface coverage increased with loading. The reactivity of the catalysts was linked to the in situ partial reduction of ReO₃ to form Re⁴⁺ and Re^6–7+ sites, whose presence and relative amounts were determined by post-reaction XPS analysis. The reaction rate increased with ReO_x loading up to 10 wt%, attributed to the increase in Re surface coverage; a decrease in reaction rate at higher loading was ascribed to the formation of aggregates. The study revealed a strong affinity for direct cleavage of the C–O bond to form benzene. The similar relative abundance of the Re species is responsible for the similar trend in product distribution of the catalysts. The dependence of activity and product distribution with respect to H₂ pressure has been related to kinetics and thermodynamics. The reactivity of the best catalyst for the HDO of guaiacol (2-methoxyphenol), anisole (methoxybenzene), phenol and o-cresol (2-methylphenol) further demonstrated the catalyst's preference for C–O bond scission.