Continuous homogeneous hydroformylation with bulky rhodium catalyst complexes retained by nano-filtration membranes Original Research Article

By employing polymer bound bulky phosphite ligands (synthesized in house) in conjunction with polyimide membranes with appropriate molecular weight cutoff (MWCO), homogeneous Rh complexes were effectively retained in solution. Batch filtration of toluene-based solutions of various phosphorus ligands at constant pressure revealed that Rh complexed with polymer bound bidendate ligands (average molecular weight from 7000 to 10,000) were most effectively retained in solution with Rh leakage in the permeate being on the order of tens of ppb. Continuous filtration of toluene-based solutions containing dissolved polymer bound phosphite ligand at constant pressure and cell hold-up revealed that the Rh and P concentrations in the permeate were steady after several hours at less than 100 ppb. The higher initial concentrations of Rh and P in the permeate suggests the removal of perhaps unbound Rh and P from the initial mixture and also from the fraction of the polymers that are smaller than the MWCO of the membrane. Continuous 1-olefin hydroformylation with Rh complexed with polymer bound bidentate ligand (JanaPhos) was demonstrated at 50 °C in a stirred reactor fitted with a nanofiltration membrane. The catalyst complex was dissolved in a toluene-based reaction mixture and the feed solution (containing the substrate) and syngas were added continuously to maintain a constant flow rate and reactor pressure (3.0 MPa). At optimized conditions, the continuous run reached a steady state characterized by nearly 50% 1-olefin conversion, >98% aldehydes selectivity and a n/i ratio of 3.5, all of which remained constant even after 22 h. The Rh concentrations in the effluent were steady at approximately 20 ppb even after 22 h. The makeup Rh cost required to offset this loss during continuous processing ($0.004/lb aldehyde) exceeds the economic viability criterion ($0.013/lb aldehyde).