(177c) Exploring Pt-Wox Interactions in Hydrodeoxygenation Catalysts for Lignin Upgrading

The valorization of biomass-derived lignin holds great potential as a potential route for the sustainable production of primary chemicals such as benzene, toluene, and xylenes (BTX), but requires selective hydrodeoxygenation (HDO). To date, a catalytic system of acceptable selectivity and activity without also catalyzing over-hydrogenation to lower value alkanes has yet to be discovered. The most promising catalysts for this process are inverse oxide-on-metal MO_x/Pt (M = W, Mo) catalysts, though the active site remains unidentified.

To elucidate the unique active site for HDO on these materials, Pt nanoparticles size and subsequent WO_x coverage are varied on an inert silica support. CO probe molecule DRIFTS and CO chemisorption measurements give insight into the behavior of sub-monolayer oxide structures on Pt under reducing conditions, showing WO_x migrated to preferentially decorate well-coordinated Pt sites, leaving under-coordinated Pt sites free for CO adsorption and presumably reactivity.

To connect this behavior to selective HDO, a series of Pt and WO_x/Pt catalysts with Pt crystallite size from 1.1 nm to 5+ nm are investigated in the HDO reaction of a lignin model compound, dihydroeugenol (DHE). The results over monometallic Pt catalysts indicate that small Pt particles without WO_x modification effectively catalyze deoxygenation, whereas large particles preferentially catalyze hydrogenation of the aromatic ring. This implicates that under-coordinated Pt sites, the same sites exposed at low coverages of WO_x, are the likely active site for deoxygenation, whereas well-coordinated Pt sites are primarily active for hydrogenation.

HDO results over WO_x/Pt catalysts varying both Pt size and WO_x coverage elucidate 1.) the influence of Pt particle size on WO_x-Pt interactions and 2.) the influence of WO_x on HDO chemistry. This will shed light on whether the unique WO_x/Pt reactivity in HDO stems from blocking hydrogenation sites or creating novel reactivity at WO_x-Pt interfaces, driving C-O cleavage.