Mechanistic Insights into the Reductive Amination of 5-HMF to BAMF on the Ni Catalyst and Design of Bimetallic Alloys

Density Functional Theory (DFT) simulations were used to elucidate the complete reaction pathway for the reductive amination of 5-hydroxymethyfurfural (5-HMF) to 2,5-bis(aminomethyl)furan (BAMF) on the Ni (111) surface. In the most stable configuration, the 5-HMF ring was centered over a hollow site and adsorbed parallel to the Ni (111) surface, with a binding energy of −215.5 kJ/mol. At first, the aldehyde group of 5-HMF underwent hydrogenation via the ammonia-assisted pathway, which was found to be kinetically favorable (Ea = 66.1 kJ/mol) compared to the surface-assisted hydrogenation pathway (Ea = 80.7 kJ/mol). The aldehyde hydrogenation of 5-HMF was followed by nucleophilic addition of NH (Ea = 78.3 kJ/mol) and dehydroxylation (Ea = 50.2 kJ/mol) to produce an imine intermediate, hydroxymethylfurfurylimine (HMFI), which, upon subsequent hydrogenation, produces 5-(hydroxymethyl) furfurylamine (HMFA). In the next step, the hydroxymethyl group of HMFA underwent reductive amination to produce another imine intermediate, 2-iminomethylfurfurylamine (IMFA) (Ea = 48.3 kJ/mol), which upon hydrogenation produced BAMF. Among all these intrinsic steps, hydrogenation of the carbon atom of both the imine intermediates (HMFI and IMFA) was found to be the rate-determining steps with high activation barriers of 94 and 108.3 kJ/mol, respectively. DFT calculations further showed that N atom bound strongly with the Ni (111) surface (Eb = −531 kJ/mol), which correlates well with the experimentally observed catalyst deactivation caused due to the formation of Ni3N under reductive amination reaction conditions. Furthermore, DFT calculations showed that alloying Ni with Au will destabilize the N adsorbed on the surface, which in turn would suppress the nitride formation and stabilize the active Ni0 sites required for the amination and hydrogenation steps. This electronic tuning provided a rational strategy for designing stable Ni–Au bimetallic catalysts for the reductive amination of 5-HMF to BAMF.