Sequential hydrogenation of nitroaromatics to alicyclic amines via highly-dispersed Ru–Pd nanoparticles anchored on air-exfoliated C3N4 nanosheets

Abstract

Developing efficient and green catalytic systems is highly desired in the syntheses of alicyclic amines via hydrogenation of nitroaromatics. Herein, we developed Ru–Pd dual active site catalysts in which Ru and Pd species were anchored and highly dispersed on air-exfoliated carbon nitride (Ru–Pd/C₃N₄-air). As-prepared catalysts were employed in the hydrogenation of nitrobenzene (NB) to cyclohexylamine (CHA). Compared with single Ru or Pd based catalysts, Ru–Pd dual active site catalysts obtained a higher CHA production rate of 26.7 mol CHA mol⁻¹ Ru·Pd h⁻¹ at 80 °C and 3 MPa H₂. The activation energy for the hydrogenation of the nitro group and benzene ring was calculated as 26.26 kJ mol⁻¹ and 66.30 kJ mol⁻¹, respectively. Intrinsic kinetic studies demonstrated that Pd was the dominant metal for hydrogenation of nitro group, while Ru was dominant for benzene ring. Thereinto, the corresponding non-dominant metals enhanced activation and dissociation of H₂, thereby improving catalytic activity significantly. This excellent performance of Ru–Pd catalysts could be attributed to highly dispersed Ru–N_x and Pd–N_x at a nanoscale distance, which was conducive to metal-assisted hydrogenation. Stability investigation showed that the performance of Ru–Pd catalysts could be essentially maintained at a high level. Additionally, the substrate scope could be successfully extended to hydrogenation of other nitroaromatics with different substituents.