Carbonaceous materials as catalyst supports for the enantioselective hydrogenation of (E)-alpha-phenylcinnamic acid: Effect of the support acidity

Abstract

A systematic comparative study of carbonaceous materials as catalyst supports for the asymmetric heterogeneous hydrogenation of (E)-alpha-phenylcinnamic acid (PCA) was performed to investigate the influence of the supports on the enantioselectivity. A series of Pd-based catalysts supported on activated carbon (AC), graphene oxide (GO), or carbon nanotubes (CNTs) was prepared using deposition methods. The hydrogenation reactions of PCA were conducted with/without the pretreatment of the prepared catalysts, and the results showed a significant drop of enantiomeric excess (ee) when the Pd/GO catalyst was used (27%) compared to that obtained from the Pd/AC (70%) and Pd/CNTs (66%) catalysts. This drop of ee values was assigned to the abundance of acidic sites present on the Pd/GO catalyst, as revealed by TGA, FT-IR, and TPD-NH3 analyses. The effect of the support acidity on the ee was ascribed to the preferential adsorption mode of cinchonidine (CD) on the GO surface via electrostatic interactions between the negatively charged oxygen functional groups present on the GO surface and the positively protonated amine groups of the CD molecule.