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.