Unprecedented catalytic performance in amine syntheses via Pd/g-C3N4 catalyst-assisted transfer hydrogenation

Abstract

The preparation of amine compounds is very important for both the chemical industry and renewable feedstock processing. Nevertheless, difficulties remain in finding a catalytic system that is sufficiently active and environmentally benign for producing amine compounds. In this work, we report that g-C₃N₄ nanosheets as support materials can significantly boost the efficiency of Pd nanoparticles for the reduction of nitro compounds to primary amines. Using formic acid as a hydrogen donor and water as a solvent, the optimized 5 wt% Pd/g-C₃N₄ catalyst exhibited an unprecedented performance in the conversion of nitrobenzene into aniline (achieving almost full conversion with an extremely high turnover frequency of 4770 h⁻¹ at room temperature), yielding the best activity ever reported for heterogeneously catalyzing nitro compound reduction. Pd/g-C₃N₄ catalyst was also active for the one-pot reductive amination of carbonyl compounds with nitro compounds to obtain the corresponding secondary amines with excellent selectivity (>90%). We proposed that the protic N–H⁺ and hydridic Pd–H⁻ on Pd/g-C₃N₄ are the active species for the transfer hydrogenation reaction of nitro compounds. Furthermore, Pd/g-C₃N₄ catalyst was highly stable with a wide scope in the syntheses of various amine compounds. This work will open up a new approach for the transfer hydrogenations of nitro compounds to produce primary or secondary amines in green chemistry.