Adsorption of Chiral Modifiers from Solution onto Supported Platinum Catalysts: The Effect of the Solvent, Other Coadsorbates, and the Support

Abstract

The adsorption from a solution of s-1-(1-naphthyl)­ethylamine (s-NEA), cinchonidine, and cinchonine on Pt/SiO2, Pt/Al2O3, and a Pt disk was probed in situ by infrared absorption spectroscopy. Adsorption on the supported catalysts shows similar behavior to that seen on flat polycrystalline Pt surfaces, with the adsorption geometry transitioning from the aromatic ring lying close to parallel to the metal surface at low coverages to a more tilted geometry at monolayer saturation. The extent of the adsorption of the chiral modifiers and its degree of irreversibility both vary with the nature of the liquid (CCl4, toluene, ethanol), tracking solubility. Carbon monoxide can be coadsorbed with s-NEA, a coadsorption that leads to a compression and an increase in ordering of the s-NEA layer (aided by exposing the surface to H2). In contrast, s-NEA adsorption dominates when competing with quinoline. Finally, a few small differences were seen in the IR spectra of the chiral modifiers adsorbed on Pt/SiO2 (compared to the other solids) because of the smaller size of the Pt nanoparticles associated with that catalyst. In general, it is crucial to characterize the adsorption of chiral modifiers in situ in the presence of the liquid phase, but the contribution from the catalyst support appears to be minor.

Influence of N-Substituents on the Adsorption Geometry of OH-Functionalized Chiral N-Heterocyclic Carbenes

“…It should be noted that exposure to reaction conditions involves the addition of various factors that can also affect the adsorption geometry of the ligands and their interaction with the prochiral reactant. It was demonstrated that the surface adsorption of hydrogen, , solvent molecules, , coadsorption of modifiers, , and reactants − can alter the reaction mechanism and the adsorption geometry of surface ligands. It cannot be excluded that similar factors will also modify the adsorption geometry of NHC ligands during the enantioselective α-arylation reaction.…”

Enhancing Effect of Residual Capping Agents in Heterogeneous Enantioselective Hydrogenation of α-keto Esters over Polymer-Capped Pt/Al2O3

“…9,28 Thus, both the reaction rate and enantioselectivity strongly depend on the size of the Pt NPs and the exposed Pt sites. 11,12,29 To exclude size effects on the catalytic performance, Pt NPs of uniform size were synthesized using two different capping agents, PVP and PVA. TEM images of the Pt Although capping agents are useful for the size control of NPs, the adsorbed capping agents on the metal surface can block the active sites, significantly decreasing the catalytic performance of the nanocatalysts.…”

Comparative Density Functional Theory Study of Cinchonidine and Hydrogen Coadsorption on Platinum Group Metals (Rh, Ir, Pd, and Pt) and Its Implications on Surface Chiral Site Formation

“…Finally, it should be kept in mind that during asymmetric hydrogenation, that is, in the presence of substrate, product and solvent, further changes are expected. Previous experimental investigations showed that the enantiodifferentiation of cinchona-modified Pt can also be affected by the interaction of the adsorbed modifier with other reaction components, substrate, 62 solvent, 63,64 product, 65 and other coadsorbates. 64 Thus, the present calculations only mimic the coadsorption behavior of CD and hydrogen, that is, the stability and spatial orientation of CD after the chiral modification procedure and hydrogen admission.…”

“…Previous experimental investigations showed that the enantiodifferentiation of cinchona-modified Pt can also be affected by the interaction of the adsorbed modifier with other reaction components, substrate, 62 solvent, 63,64 product, 65 and other coadsorbates. 64 Thus, the present calculations only mimic the coadsorption behavior of CD and hydrogen, that is, the stability and spatial orientation of CD after the chiral modification procedure and hydrogen admission. The behavior of adsorbed CD under reaction conditions is expected to be more complex due its interaction with the substrate, and possible interactions with solvent and product.…”