Reaction Pathways for the Hydrodeoxygenation of Anisole and Guaiacol over Zn-Pt Bimetallic Catalysts

There is an increasing interest in the use of lignin as a renewable feedstock for the production of high-value aromatic compounds; however, due to its high oxygen content, selective hydrodeoxygenation (HDO) of the aromatic oxygenates produced from lignin depolymerization is required. Metal alloys containing a group 10 metal and a more oxyphillic metal, such as Zn, have been shown to be highly selective for this class of reactions, at least under some conditions. Although, the mechanism by which alloying enhances selectivity is still poorly understood. To provide mechanistic insight, in this study we have investigated the reaction of typical lignin-derived oxygenates, anisole and guaiacol, on single-crystal Pt and Zn-Pt model catalysts by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). For Zn-free Pt it was found that anisole and guaiacol interact strongly with the surface via the aromatic ring, which promotes undesired ring hydrogenation. Adding Zn to the Pt surface was found to inhibit bonding via the aromatic ring and promoted a stronger interaction with the oxygens. This in turn facilitated selective C-O bond scission without hydrogenating the aromatic ring. These results suggest that Zn-Pt alloys may be an effective catalyst for HDO of lignin-derived aromatic oxygenates with low activity for ring hydrogenation. This hypothesis was then tested and verified by investigating the reaction of anisole or guaiacol with H₂ over high surface area carbon-supported Pt and Zn-Pt catalysts.