(532l) Enhancing Activity of Ligand-Modified Supported Metal Catalysts for Hydrodeoxygenation

Organic ligands have been deposited onto supported metal catalysts as a method for altering catalytic performance, primarily selectivity, through mechanisms related to steric and/or electronic effects. Specifically, SAMs have been shown to improve catalytic performance for hydrodeoxygenation (HDO), an important reaction in biomass upgrading. However, SAMs bound directly to the catalyst metal reduce the availability of active sites, lowering overall activity.¹ In this work, we investigated methods for synthesizing catalysts with minimal site blocking to improve activity of SAM-modified catalysts.

The location of phosphonic acid (PA) ligands on Pd/Al₂O₃ catalysts was controlled by varying the deposition sequence during catalyst synthesis. Modified catalysts were then tested for benzyl alcohol HDO. “Metal-first” catalysts, analogous to typical SAM-modified catalysts, improved selectivity to toluene with modest improvements in activity through additional Brønsted acidity, as seen in prior work. “SAM-first” catalysts showed improvements in toluene selectivity similar to metal-first catalysts with significantly improved HDO rates. The improvements in activity were attributed to reduced site blocking (as evidenced by CO chemisorption) and smaller average particle size (as evidenced by TEM measurements).

Low-density thiol SAMs were synthesized on Pt/Al₂O₃ catalysts using an ion exchange method to create ligands with bulky endgroups. Characterization methods including ICP, XPS, and DRIFTS confirmed lower sulfur loadings and greater conformational freedom for low-density SAMs. The reduced density was hypothesized to allow greater accessibility to the catalyst surface while still restricting the orientation of bound reactants, thereby producing catalysts with high activity and selectivity. Specifically, full thiol monolayers on Pt/Al₂O₃ force benzyl alcohol to bind to the surface via the hydroxyl group, increasing HDO selectivity to produce toluene. Low-density thiol catalyst yielded similar improvements in selectivity with activity an order of magnitude greater.

References

(1) Blanchette, Z.; et al., Catal. Sci. Technol. 2022, 12 (7), 2306–2314.