Preparation of Noble-metal Nanoparticles by Microwave-assisted Chemical Reduction and Evaluation as Catalysts for Nitrile Hydrogenation under Ambient Conditions

Abstract

Herein, we report a protocol for rapid preparation of uniformly sized Ru, Rh, Pd, Ir, and Pt nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone) via microwave-assisted chemical reduction with ethanol. Although the boiling point of ethanol, which has high reduction ability, is much lower than the temperature required for metal cation reduction, this solvent could be used as a reductant because the preparation was carried out in a sealed vessel. All metal nanoparticles showed similar sizes and amounts of poly(N-vinyl-2-pyrrolidone). The crystallite size of the Pd and Pt nanoparticles could be controlled by changing the ethanol concentration. The catalytic performance of the prepared metal nanoparticles was evaluated for the hydrogenation of benzonitrile under ambient conditions (25 °C, 1 bar H2). Rh nanoparticles showed the highest benzonitrile conversion and highest selectivity for secondary imine product. Interestingly, the particle-size dependence of the catalytic activity of the Rh nanoparticles showed volcano-type behavior; that is, the second smallest Rh nanoparticles (3.3 nm) showed the highest activity, which was attributed to high metal surface area and high turnover frequency. Notably, this is the first report of nitrile hydrogenation to afford a secondary imine under ambient conditions.

Recent Studies on the Application of Microwave-Assisted Method for the Preparation of Heterogeneous Catalysts and Catalytic Hydrogenation Processes

“…In addition, a Rh catalyst with a Rh particle size of 2.7 nm showed high activity in the hydrogenation of benzonitrile to a secondary imine and showed reusability in the hydrogenation of nitriles (30 • C, 3 bar H 2 ) [45]. Ru, Rh, Pd, Ir, and Pt nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) with a uniform size were prepared by chemical reduction with ethanol in the microwave oven [46]. All metallic nanoparticles had a similar size and the same amount of PVP.…”

Nitrile hydrogenation to secondary amines under ambient conditions over palladium–platinum random alloy nanoparticles

“…STEM-EDXS maps of the resulting nanoparticles showed that the Pd-L and Pt-L signals were localized at the interior and exterior of the particles, respectively (Figure S11b-f), which indicates the formation of a Pd-rich core and a Pt-rich surface phase. Furthermore, the small particles produced by heterogeneous heating from the mantle heater 34 were composed mainly of Pt species. On the basis of these experimental results, we propose that the alloying mechanism involves rapid and simultaneous reduction of the readily reducible [PdCl 4 ] 2− and [PtCl 4 ] 2− ions by microwave heating.…”

“…Recently, we demonstrated for the first time that Rh nanoparticles stabilized by PVP effectively catalyze the selective formation of secondary imines from nitriles by hydrogenation under ambient conditions (25 °C and 1 bar). 33,34 Herein, we report an effective method for nitrile hydrogenation that affords secondary amines under ambient conditions over newly developed PdPt-PVP nanoparticle catalysts. Random alloying by chemical reduction generated PdPt-PVP nanoparticles, despite the fact that Pd and Pt are classically immiscible below 700 °C.…”