Hydrothermal Carbonization of Renewable Natural Plants as Superior Metal-Free Catalysts for Aerobic Oxidative Coupling of Amines to Imines

摘要

Seeking low-cost sustainable catalysts with high stability and excellent catalytic activity to substitute traditional metal or petroleum-based catalysts is a desirable strategy for catalytic reactions. Here, we report a simple fabrication of biomass-derived hydrochar catalysts with various oxygen functional groups by hydrothermal carbonization of renewable natural plants using water at 190 degrees C. These prepared hydrochars from maple and straw are found to be very effective in the gram-scale aerobic oxidation of a variety of different amines. The results show that most of the imine yields are over 90% (4 h, 100 degrees C), and the cycling performance of the catalyst can be tested seven times. Mode studies and density functional theory calculations reveal that the surface carboxylic acid groups (adsorption energy = -25.0 kJ/mol for amine) and quinone moieties (adsorption energy = -22.7 kJ/mol for amine or 9.0 kJ/mol for O-2) are the dominant active sites of the hydrochar catalyst, which may participate in substrate binding and facilitate amine oxidation. This study provides a simple chemical processing step using natural plants to prepare an ecofriendly and highly active biochar catalyst, allowing the premises of sustainable biomass-derived carbocatalysis to be explored for industrial applications.