Remarkably fast and selective recovery of a rhodium-containing catalyst with silica-based (poly)amine ion exchangers

Abstract

Three silica-based ion exchangers, containing monoamine (1), ethylenediamine (2) and diethylenetriamine (3) functionalities, have been examined for their capacity to recover the catalyst RhCl₃, with emphasis on industrially important factors, such as kinetics and re-usability of the ion exchange materials. Ligand occupations have been found to lie between 40% and 55% (some 40–50 mg Rh per g ion exchanger), suggesting the predominant formation of complexes of the type ML₂. The recovery proved to be most efficient with diluted mineral acids, and the initially adsorbed Rh³⁺ has been removed almost quantitatively (>95%) from 1. All ion exchangers display excellent recovery kinetics, with t_1/2 values of adsorption never exceeding 60 s. Successive Rh recovery experiments indicate stable recovery percentages after the first recovery cycle, but show a gradual build-up of residual Rh with ion exchangers 2 and 3. Rh³⁺ has been extracted with high selectivity up to pH values as high as pH 4 in the presence of other transition metal ions, like Cu²⁺ and Ni²⁺.