Synthesis and characterisation of a polymer-bound rhodium-benzimidazole complex as catalyst for the hydrogenation of nitroarenes

A polymer-anchored rhodium complex was synthesised by sequential attachment of benzimidazole (BzlH) and RhCl3 to chloromethylated poly(styrene–divinylbenzene) co-polymer (PSDVB) with 6.5% cross-linking. The catalyst was characterised by X-ray photoelectron spectroscopy, far-IR, UV–Vis, FTIR, SEM and thermogravimetric analysis. Various physico-chemical properties such as bulk density, surface area and swelling behaviour in different solvents were also studied. The polymer-anchored complex was tested as a catalyst for reduction of nitroarenes, namely o,m,p-nitrobenzoic acid, nitroaniline, nitrophenol and nitrotoluene. Kinetic measurements were carried for o-nitroaniline and p-nitrophenol by varying temperature, catalyst concentration and concentration of substrates. The rate of the reaction was found to be first order with respect to catalyst concentration and also with substrate concentration at low concentrations, becoming independent of substrate at higher concentrations. A plausible mechanism for the reaction is proposed. The energy and entropy of activation calculated from Arrhenius plots indicate high activity of the catalyst on the support. The recycling efficiency of the catalyst has been studied and there was no leaching of metal from the catalyst surface.