Highly Stable Porous-Carbon-Coated Ni Catalysts for the Reductive Amination of Levulinic Acid via an Unconventional Pathway

The catalytic conversions of biomass and its derivatives into fuels and chemicals require active and stable catalysts. Non-noble-metal catalysts typically suffer from deactivation due to the leaching and sintering of the metal species in liquid-phase reactions. In this work, we report a facile synthesis of porous-carbon-coated Ni catalysts supported on carbon nanotubes (CNF_x@Ni@CNTs) by atomic layer deposition for the reductive amination of levulinic acid (LA) with amines to pyrrolidones. Under the protection of porous carbon with a moderate thickness, the optimized CNF₃₀@Ni@CNTs catalyst showed a 99% yield of pyrrolidones and recyclability of up to 20 runs without the leaching and sintering of Ni nanoparticles. On the basis of verification experiments and density functional theory calculations, we determined that our Ni-catalyzed reductive amination of LA with amines underwent an unconventional pathway via amides as the first intermediate, followed by tandem cyclization, intramolecular dehydration, and hydrogenation to the desired pyrrolidones. This pathway was completely different from the reported imine-intermediated route in Pt-catalyzed systems. This work provides insights into the design of active and stable heterogeneous catalysts for liquid-phase reactions as well as into switching reaction pathways to realize the replacement of noble metals for the transformation of biobased multifunctional substrates.