On the Importance of the Aqueous Interface in the Multiphasic Rhodium Catalyzed Hydroformylation of Propene: a Molecular Dynamics Study

Abstract

We report a molecular dynamics study of propene/water biphasic systems involved in the hydroformylation of propene catalyzed by water soluble rhodium complexes with TPPTS3- ligands (where TPPTS3− is the tris(m-sulfonatophenyl)phosphine ligand), neutralized by Na+ counterions). The neat biphasic system is found to display a quite thin and irregular interface, involving some propene partitioning to the bulk aqueous phase. Interestingly, when propene is diluted (at low presssures), propene molecules tend to somewhat condense at the interface. The most important finding concerns the interfacial activity of key reaction partners (the free TPPTS3− ligands, the active catalyst [RhH(CO)(TPPTS)2]6−, and the reaction intermediate [RhH(CO)(TPPTS)2(propene)]6−). This is found first by following the outcome of the phase separation of randomly mixed water/propene solutions with these different solutes, and further supported by potential of mean force (PMF) calculations on the migration of the [RhH(CO)(TPPTS)2(propene)]6− complex across the interface. Added butyraldehyde molecules (the product of the reaction) are shown to increase the surface activity the [RhH(CO)(TPPTS)2(propene)]6− complex, due to increased attractions of the complex with the organic phase. These results point to the importance of the aqueous interface in the multiphasic catalyzed reaction, and suggest that a significant amount of propene reacts “right at the interface”, i.e., in a region of less than 1 nm width of the solution.