Recovery of rhodium-containing catalysts by silica-based chelating ion exchangers containing N and S donor atoms

Two silica-based chelating ion exchangers have been synthesised for the recovery of the model catalysts RhCl3·3H2O and RhCl(PPh3)3. The N,S-donor ligands 2-aminocyclopentene-1-dithiocarboxylic acid and 4-amino-3-methyl-1,2,4-triazole-5-thione have been attached to the bifunctional spacer (3-glycidoxypropyl)trimethoxysilane prior to immobilisation on silica. Both ion exchangers showed a high Rh(III)-uptake capacity and selectivity over Cu(II), even in the presence of an excess of the latter. The Rh(I) complex was adsorbed to a much smaller extent, probably due to the bulky triphenylphosphane groups. Aquation negatively influenced the Rh(III)-uptake kinetics, whereas experiments performed in ethanol significantly increased the kinetic-uptake behaviour. Stripping of the loaded polymers proved to be very difficult, probably due to the strong Rhsulphur coordination. Only with thiourea a reasonable amount of Rh(III) could be stripped from the ion exchangers.