Correlated bifunctionality in heterogeneous catalysts: selective tethering of cinchonidine next to supported Pt nanoparticles

Abstract

A strategy has been devised to add molecular functionality to heterogeneous catalysts in a spatially correlated fashion. The idea was tested for a specific system where enantioselectivity was added to a Pt/SiO₂ catalyst by tethering cinchonidine (Cd) selectively to silica sites adjacent to the metal nanoparticles. Our methodology relies on the selective strong adsorption of the cinchona alkaloid on the platinum surface, the same interaction responsible for the chiral modification; the catalyst was first exposed to a Cd molecule to which a propyltriethoxysilane moiety was added, the excess was then washed away with a pure solvent, and the click chemistry used to link the chiral modifier to the surface of the silica support was finally triggered via thermal activation. The resulting samples were thoroughly characterized by using several titration and spectroscopic techniques, and their catalytic performance for the hydrogenation of ethyl pyruvate was contrasted with that seen for similar catalysts where Cd tethering is done in a random fashion. Superior performance was seen with the correlated-tethering samples, mainly in terms of activity but also with respect to enantioselectivity. In fact, the best correlated-tethering samples show performance comparable to that obtained by adding the cinchona in the solution under similar conditions. The effects of the location within the molecular structure of cinchonidine where tethering is performed, the Cd : Pt molar ratio, and the solvent used were evaluated.