Selective Synthesis of Secondary Imines via Reductive Coupling of Aldehydes with Ammonia over Encapsulated NiFe Alloy Catalysts

Abstract

Secondary imines (Schiff bases) are versatile intermediates in organic synthesis and pharmaceutical manufacturing.However, their selective synthesis directly from aldehydes and ammonia under reductive conditions is challenging, as the imine functionality is thermodynamically unstable and prone to over-hydrogenation to amines. Herein, we present a robust strategy for the selective re-ductive coupling of biomass-derived aldehydes into secondary imines using nickel-iron alloy nanoparticles encapsulated in chitosan-derived nitrogen-doped carbon shells (NiFe@CS). Advanced characterization reveals that the metal nanoparticles are confined within distinct graphitic carbon layers, which modulates the electronic structure and steric environment of the active sites.Under optimized conditions (100 °C, 2 MPa H₂), the NiFe@CS catalyst demonstrates exceptional performance for benzaldehyde conversion (>99%) with high selectivity (>98%) toward the target secondary imine (N-benzylidene benzylamine). Kinetic studies and characterization suggest a tandem pathway: the catalyst promotes the initial formation of a primary imine, which undergoes partial hydrogenation and subsequent condensation. Crucially, high ammonia concentrations were found to stabilize the imine intermediate via competitive adsorption, effectively inhibiting further hydrogenation to the secondary amine. The catalyst exhibits remarkable stability, maintaining >90% selectivity over five consecutive cycles, attributed to the protective carbon shell preventing metal leaching and sintering. This work provides a sustainable, atom-efficient protocol for constructing C=N bonds from renewable resources, offering new insights into controlling selectivity in reductive transformations.