Spectroscopic Study on the Surface Properties and Catalytic Performances of Palladium Nanoparticles in Poly(ionic liquid)s

Abstract

Palladium nanoparticles in vinylimidazolium-based polymers and poly(ionic liquid)s (PIL)s have been synthesized, systematically characterized, and preliminarily tested in the selective hydrogenation of p-chloronitrobenzene to p-chloroaniline. In both nonionic polymers and PILs the palladium nanoparticles were found to be extremely small (below 2 nm) and hardly detectable by means of transmission electron microscopy (TEM) and X-ray powder diffraction (XRPD), but they have been successfully detected by Fourier transform infrared (FT-IR) spectroscopy of adsorbed CO, which indicated that the available metal surface was approximately the same, as well as the types of exposed sites. In nonionic polymers palladium nanoparticles are stabilized mainly by the interaction with the nitrogen atoms of the imidazole ring, which act as electron donors. In contrast, in absence of available nitrogen species inside PILs, palladium nanoparticles are mainly stabilized by the iodide anions, which determine important electronic effects at the palladium surface. PILs/Pd samples were tested in the selective reduction of p-chloronitrobenzene to p-chloroaniline, under remarkably mild conditions (room temperature, absence of solvents, gaseous H2 below 1 atm). The reaction was followed by FT-IR spectroscopy in operando. All the PILs/Pd samples display an excellent chemoselectivity, whereas nonionic polymers/Pd samples are not selective. Since the morphology and size of the palladium nanoparticles is the same in all the catalysts, it is concluded that the driving force for chemoselectivity is the ionicity of the environment provided by the PIL scaffolds.