Selective Reductive Amination of 5-Hydroxymethylfurfural under Hypobaric H<sub>2</sub> by a Durable Carbon-Coated Magnetic Ni Catalyst
Rong Shang, Hao Zhang, Yulong Li, Bang Gu, Qinghu Tang, Feng Qiu, Qiue Cao, Wenhao Fang
Abstract
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 H 2 from reductive amination of 5-hydroxymethylfurfural (HMF, an important cellulose-derived platform molecule) with NH 3 ·H 2 O. In comparison, a Ni-NiO catalyst obtained by reducing NiO with H 2 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 Ni 0 active sites with a carbon shell was elucidated, demonstrating electron-rich Ni 0 sites for H 2 activation. Moreover, different HMF adsorption modes on Ni-NiO and Ni@C surfaces were illustrated, significantly affecting the product distribution. Subsequently, distinctive NH 3 ·H 2 O 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 Ni 0 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.