Surface modification of SiO2 support with N-containing polymer for enantioselective hydrogenation over Pt catalyst

Abstract

The design of highly enantioselective heterogeneous catalysts offers an efficient route for the production of enantiopure chiral molecules, which are crucial intermediates in the pharmaceutical, agrochemical, and fine chemical industries. In this study, we present a strategy to enhance the enantioselectivity of supported Pt catalysts for the hydrogenation of α-keto esters by modifying SiO₂ supports with poly(ethyleneimine) (PEI), a nitrogen-rich polymer. By adjusting the heat-treatment time, the quantity of PEI-derived residues on the support was successfully controlled, thereby fine-tuning the electronic state of Pt through electron-transfer interactions, and resulting in the generation of positively charged Pt species. Catalytic evaluations showed that the Pt/PEI-SiO₂ catalysts exhibited superior enantioselectivities compared to the unmodified Pt/SiO₂, with an optimal performance of 96.1% enantiomeric excess (ee). In addition, the Pt/PEI-SiO₂ catalysts exhibited high enantioselectivities using various α-keto esters, chiral modifiers, and H₂ pressures, as well as an excellent reusability over ten consecutive cycles. The observed correlation between the higher proportion of positively charged Pt species and the increased ee suggests that the improved enantioselectivity of the Pt/PEI-SiO₂ catalyst system is driven by the enhanced adsorption of chiral modifiers, which is facilitated by interactions between the anchoring moiety and the electron-deficient Pt surface. These findings highlight the essential role of interactions between the active metals and the modifying organic species bound to support materials in enhancing the enantioselectivity. Moreover, they provide valuable insights for catalyst design through electronic tuning of the active metal surface with organic-modified supports.