The promotion effect of π-π interactions in Pd NPs catalysed selective hydrogenation

The utilization of weak interactions to improve the catalytic performance of supported metal catalysts is an important strategy for catalysts design, but still remains a big challenge. In this work, the weak interactions nearby the Pd nanoparticles (NPs) are finely tuned by using a series of imine-linked covalent organic frameworks (COFs) with different conjugation skeletons. The Pd NPs embedded in pyrene-COF are ca. 3 to 10-fold more active than those in COFs without pyrene in the hydrogenation of aromatic ketones/aldehydes, quinolines and nitrobenzene, though Pd have similar size and surface structure. With acetophenone (AP) hydrogenation as a model reaction, systematic studies imply that the pi-pi interaction of AP and pyrene rings in the vicinity of Pd NPs could significantly reduce the activation barrier in the rate-determining step. This work highlights the important role of non-covalent interactions beyond the active sites in modulating the catalytic performance of supported metal NPs. The non-covalent interactions play an important role in biocatalysis, but they have not been fully explored in metal-catalysed reactions. Herein, the authors present advanced understanding of the pi-pi interactions of covalent organic frameworks in Pd catalysed selective hydrogenation.