Heterogeneous bismuth catalyst for the selective hydrogenation of nitroarenes to arylamines using molecular hydrogen

Abstract

Bismuth is renowned for its exceptional safety, highlighted by numerous examples demonstrating its low toxicity. Considering this, we present a novel, active, and reusable heterogeneous bismuth catalyst (Bi-NC/γ-Al₂O₃) for chemoselective hydrogenation of nitroarenes to aryl amines using molecular hydrogen. The Bi-NC/γ-Al₂O₃ catalyst was synthesized through a simple impregnation method followed by pyrolysis at 700°C, for 2 h under a nitrogen atmosphere. The physicochemical properties of fresh and spent Bi-NC/γ-Al₂O₃ catalysts were comprehensively characterized by powder X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS), High-resolution transmission electron microscopy (HR-TEM), N₂ adsorption-desorption, H₂-temperature programmed reduction (H₂-TPR), NH₃-temperature programmed desorption (NH₃-TPD), and thermogravimetric analysis (TGA). The bismuth content in the Bi-NC/γ-Al₂O₃ catalyst was estimated by inductively coupled plasma optical emission spectroscopy (ICP-OES). The catalyst demonstrated exceptional performance and selectivity in reducing a wide variety of nitroarenes to their corresponding aryl amines, achieving good to excellent yields (36 examples). Furthermore, the Bi-NC/γ-Al₂O₃ catalyst was applied to the selective hydrogenation of nitro groups in three commercially available drugs. Additionally, five pharmaceutical intermediates, including linezolid, Norfloxacin, Carbamazepine, paracetamol, and tizanidine, were synthesized up to a ∼1 g scale with a quantitative yield, showcasing the catalyst activity and selectivity. Further, the reaction mechanism was elucidated by tracking the intermediates (through GC-FID & GC-MS) and employing Density Functional Theory (DFT). Notably, the Bi-NC/γ-Al₂O₃ catalyst is stable, recovered, and reused for four cycles without losing significant activity and selectivity.