Selective Reductive Amination of 5-Hydroxymethylfurfural under Hypobaric H2 by a Durable Carbon-Coated Magnetic Ni Catalyst

The synthesis of primary amines from renewable biobased carbonyl compounds via reductive amination has received much attention. Modifying the surface structure of a heterogeneous catalyst may offer prospects to address selectivity maneuvering in this complex amination process. In this work, a metal–organic framework (MOF)-derived carbon-coated strategy was adopted to prepare the Ni@C catalysts with different thicknesses of carbon shells. This type of Ni catalyst achieved 98% yield of (5-(aminomethyl)furan-2-yl)methanol (AMF) at 100 °C under 2 bar of H2 from reductive amination of 5-hydroxymethylfurfural (HMF, an important cellulose-derived platform molecule) with NH3·H2O. In comparison, a Ni-NiO catalyst obtained by reducing NiO with H2 would mainly overhydrogenate AMF to (5-(aminomethyl)tetrahydrofuran-2-yl)methanol (THAMF, 52% in yield) as a byproduct. The modification of the electronic structure of Ni0 active sites with a carbon shell was elucidated, demonstrating electron-rich Ni0 sites for H2 activation. Moreover, different HMF adsorption modes on Ni-NiO and Ni@C surfaces were illustrated, significantly affecting the product distribution. Subsequently, distinctive NH3·H2O activation processes were proposed for both Ni catalysts to understand selective conversion of the Schiff base (i.e., the critical reaction intermediate). The developed Ni@C catalyst with suitable carbon thickness, abundant Ni0 centers, and defective carbon species showed high AMF yield under mild reaction conditions, facile separation–regeneration, and satisfying reusability. In addition, this Ni catalyst can also provide a relatively good substrate scope toward bioamine synthesis via reductive amination.