Exploring the Mechanisms and Kinetic Modeling of Phenol Amination Using Pd and Rh-Based Catalysts

Producing biomass-derived chemicals to substitute their petrochemical counterparts has long been an aspiration of the green chemistry research community. However, synthesizing secondary amines from biomass precursors presents several challenges related to catalyst nature and the mechanistic understanding of reaction systems. Here, we unravel the mechanistic and kinetic implications of the reductive amination of phenol with cyclohexylamine over Pd/C and Rh/C. A competitive Langmuir-Hinshelwood reaction model well interpreted the kinetic data, suggesting that support-metal interfaces serve as active sites for H2, & horbar;NH2 and & boxH;NH activation. The apparent activation energies for imine hydrogenation were 87.6 kJ mol-1 (Pd/C) and 34.5 kJ mol-1 (Rh/C), while Delta Hads and Delta Sads values confirmed the physicochemical consistency of the model. Moreover, the catalysts demonstrated their high stability to operate for several catalytic cycles, with minor activity losses due to metal leaching and partial sintering of Pd nanoparticles. Despite phenol reductive amination following similar mechanisms on Rh/C and Pd/C, they show differences in selectivity because the hydrogenation of imine is more efficient on Rh0 than on Pd0. This is the first mechanism-oriented kinetic study for phenol reductive amination; thus, it provides valuable information for process design and scale-up. There is an increasing interest in the green chemistry community to produce lignocellulosic-based amines to provide sustainable alternatives to petro-based amines. This article looks at mechanism-oriented kinetic modeling of the direct reductive amination of phenol over Pd/C and Rh/C. It also investigates how stable these catalysts are over sequential operational cycles. image