Bifunctional design of Pd single-atom catalyst on acidic alumina to promote reductive amination towards tertiary amines

The reductive amination of aldehydes with secondary amines and hydrogen is a crucial route to high-value tertiary amines, yet the development of efficient heterogeneous catalysts remains challenging due to the competing reduction of aldehydes prior to imine formation and the high steric hindrance during imine hydrogenation. To overcome these obstacles, we report a bifunctional Pd single-atom catalyst supported on commercial acidic Al₂O₃, synthesized via an effective melamine–formaldehyde resin adhesion method. The acidic sites on Al₂O₃ facilitate aldehyde adsorption and condensation with amines, thereby avoiding prior aldehyde reduction. Meanwhile, the atomic Pd centers offer enhanced spatial accessibility for the facile hydrogenation of the sterically hindered imine intermediates. Consequently, tertiary amine production is significantly accelerated, achieving high yields up to 94% without generating alcohol by-products. The turnover frequency reaches 3133 h⁻¹, substantially outperforming the Al₂O₃-supported Pd nanocatalyst, commercial Pd-based catalysts, and state-of-the-art heterogeneous catalysts reported in the literature. Detailed control experiments confirm the bifunctional role of the Pd single-atom catalyst in this reaction. This work provides new guidance for the multifunctional design of high-performance heterogeneous catalysts tailored to the characteristics of target reaction pathways.