Multisite CuNi/Al<sub>2</sub>O<sub>3</sub> Catalyst Enabling High-Efficiency Reductive Amination of Biomass-Derived Levulinic Acid (Esters) to Pyrrolidones under Mild Conditions
Xixi Liu, Peng Zhou, Zhihua Zhu, Yimeng Guo, Hang Lv, Zehui Zhang, Liangfang Zhu, Changwei Hu
Abstract
The reductive amination of biomass-derived levulinic acid (LA) and its esters to pyrrolidones over non-noble metal catalysts under mild conditions represents a promising but challenging strategy for valorizing biomass resources into nitrogen-functionalized chemicals. Herein, we report the fabrication of Al 2 O 3 -supported bimetallic CuNi composite catalysts by a simple coprecipitation–calcination–reduction method for high-efficiency reductive amination of LA (esters) into pyrrolidones in ethanol under mild reaction conditions (80 °C, 10 bar H 2, and 6–24 h). We show that the multisite division on the as-prepared Cu 1 Ni 1 /Al 2 O 3 -H 2 catalyst with a Cu/Ni molar ratio of 1:1 enables a distinguished catalytic activity toward the formation of pyrrolidones via an amine-intermediate mechanism. A deliberate structure–activity correlation reveals that the abundant and well-dispersed mesopores, oxygen vacancies, strong Lewis acid sites, and Cu/Ni NPs on the composite catalyst synergistically contribute to the high-efficiency conversion of LA (esters) to pyrrolidones, wherein the oxygen vacancies are responsible for the adsorption/activation of ethyl levulinate (EL), the Lewis acidic coordination-unsaturated Ni species are responsible for the adsorption/activation of butylamine, the Cu/Ni NPs are in charge of the adsorption/dissociation of H 2, and the mesopores provide adequate space for tolerating substrates with higher steric hindrance. Moreover, a remarkable electron transfer from adjacent Ni species to Cu centers occurs, which is conducive to improving the inherent catalytic activity of the non-noble bimetallic composite catalyst by enhancing the electron density of Cu NPs and the Lewis acidic strength. Remarkably, the catalytic system exhibits great performance for converting a variety of biomass-derived LA (esters) with extensive nitrogen sources (e.g., NH 3, aliphatic/aromatic primary amines, nitriles, and nitro compounds), thereby representing an advance toward the valorization of biomass-derived initial platform chemicals into N -containing fine chemicals. It may provide a valuable reference for the rational construction of advanced non-noble metal catalysts in a future biorefinery.