Directing Reaction Pathways on Supported Metal Catalysts with Low-Density Self-Assembled Monolayers

Abstract

Controlling reactant adsorption on catalyst surfaces is crucial to reaction activity and selectivity. One method for improving selectivity is by imposing steric constraints to bias the reactant binding orientation. In this study, thiol self-assembled monolayers (SAMs) were deposited onto Pt/Al2O3 catalysts as a method for controlling activity and selectivity via steric effects. In addition to a full monolayer, a low-density SAM-coated catalyst was employed. A number of characterization techniques demonstrated the successful deposition of homogeneous low-density SAMs on the metal surface with reduced site-blocking compared to a full high-density monolayer. Reaction kinetic studies showed increased benzyl alcohol hydrodeoxygenation (HDO) selectivity for both SAM-modified catalysts. This was attributed to the inability of the reactant to adsorb on the catalyst with the aromatic ring parallel to the surface, thus preventing decarbonylation and ring hydrogenation reaction pathways. Additionally, SAM density influenced reaction activity significantly, with the low-density-modified SAM catalyst being more active than the catalyst coated with a full monolayer. Moreover, liquid-phase hydrogenation reactions were used to investigate the relationship between SAM density and reactivity for reactant molecules of various sizes. In all cases, the low-density SAM improved reaction rates relative to dense SAMs. The effect of controlling ligand density depended on the type of reaction: high ligand densities greatly diminished ring hydrogenation, while HDO was largely unaffected, suggesting a potential strategy for size-selective reaction rate and selectivity control.

Designing Highly Enantioselective Heterogeneous Pt Catalysts: Selective Exposure of Active Sites via Surface Modification Using Amines

“…In addition to colloidal synthesis methods, treating supported catalysts with organic molecules has also been proposed as an alternative strategy for the facile synthesis of highly selective catalysts. The organic ligands preadsorbed on metal surfaces form an ordered array, where the steric hindrance or molecular interaction of the functional groups in the organic tail with the reactant enhances selectivity by fine-tuning the adsorption geometry of the reactant molecules. − Moreover, they can prevent undesired reactions by inhibiting the adsorption of reactant molecules onto specific sites via metal surface passivation. − Various organic molecules such as thiols, amines, carbenes, and carboxyls showed their potential for surface modification of metal NPs. − …”