(632g) Directing Reaction Pathways on Supported Metal Catalysts with Low-Density Self-Assembled Monolayers

Organic monolayers have been shown to be useful surface modifiers for controlling catalyst selectivity, but they often result in substantially lower reaction rates due to their high surface densities. In this work, an ion-exchange technique was employed to generate a low-density SAM coated catalyst capable of improving selectivity based on geometric effects with significantly higher activity compared to a full monolayer due to reduced site-blocking. Multiple characterization techniques including DRIFTS, ICP-OES, and CO chemisorption provided evidence for successful deposition of a homogeneous SAM layer at approximately 1/3^rd of the coverage of a full monolayer. Catalyst performance was tested using benzyl alcohol hydrogenation. A drastic reduction in activity was observed for a full monolayer catalyst compared to unmodified Pt/Al₂O₃, which was attributed to significant site-blocking. However, toluene selectivity increased, likely due to steric interactions. CO DRIFTS data suggested low amounts of contiguous metal sites due to the ligands. As a result, reactant adsorption onto Pt was restricted to an upright configuration where hydrodeoxygenation (HDO) producing toluene is favored while flat-lying adsorption was limited, resulting in less decarbonylation and ring hydrogenation. The low-density SAM catalyst showed similar improvements in selectivity with drastically increased activity. The low-density SAM used in this study paired well with benzyl alcohol, but SAM density and gap size could presumably be tailored to target other reactions as a potential strategy for size-selective reaction control.