Micellar enhanced ultrafiltration of a rhodium catalyst

In homogenous catalysis, the retention and reuse of the catalyst, which is extremely important for the feasibility of the process, is more complicated than in heterogeneous catalysis. Micellar enhanced ultrafiltration of Rh(cod)2CF3SO3, a rhodium catalyst precursor for homogeneous hydrogenation reactions, was carried out in a stirred ultrafiltration cell using micellar solutions of Marlophen NP9 without and with the addition of TPP, a phosphine ligand. For two different membranes with a molecular weight cut-off of 5 kDa, made from regenerated cellulose (C) and polyethersulfone (PES), the rhodium retentions without TPP were below 30% while the micelle retentions were almost 100%. In the presence of TPP, the rhodium retentions significantly increased and values of 80% for the PES membrane and 90% for the C membrane were obtained, respectively. The enhanced rhodium retentions are driven by the high micelle–water partition coefficient of TPP that draws the newly formed Rh–P-complex into the micelles. The partition coefficient of TPP was calculated by the enhanced solubility method to be log KMW=5.4±0.1. The C membrane does not only show better rhodium retentions, but also a better performance regarding the flux. In comparison to pure water, the flux for the micellar solutions (cNP9=10 g/L) decreases slightly to a value of 80% for the C-membrane and to a value of 8%–9% for the PES membrane, respectively.