Adsorption at the liquid-liquid interface in the biphasic rhodium-catalyzed hydroformylation of 1-hexene in ionic liquids: A molecular dynamics study

We report a molecular dynamics study of the phase separation of binary 1-hexene/[BMI][PF6] ionic liquid (IL) random mixtures containing four widely used phosphine ligands and their key reaction intermediates involved in the biphasic rhodium-catalyzed hydroformylation of I-hexene. In all cases, the organic and IL phases separate during the dynamics, leading to different partitioning of the solute species, depending on their charge and constitution. The most important finding concerns the surface activity of the ligands and their complexes. The neutral unsubstituted triphenylphosphine ligand prefers the organic phase over the IL phase, but displays transient contact with the IL at the interface. The charged TPPMS-, sulfoxanthphos(2-) and TPPTS3- ligands prefer the IL over the hexene phase, but can adsorb at the IL side of the interface in an amphiphilic manner, i.e., with their sulfonate group toward the IL phase and their aryl groups toward hexene. In this series, the most charged ligand has the lowest surface activity. Next, we simulated the [RhH(CO)(TPPMS)(2)(hexene)](2-) and [RhH(CO)(TPPTS)(2)(hexene)](6-) key reaction intermediates in hexene-IL binary systems and found that both complexes can adsorb at the interface in an amphiphilic manner, thus displaying direct contacts with hexene molecules. The [RhH(CO)(TPPMS)(2)(hexene)](2-) complex is more surface active than its more charged [RhH(CO)(TPPTS)(2)(hexene)](6-) analogue. We finally investigated the effect of added scCO(2) to a biphasic system, showing that scCO(2) enhances the diffusion of all species, leading to a faster phase separation process and presumably to a faster reaction kinetics. It does not modify, however, the surface activity of the reaction intermediate. The simulation results point to the importance of the interfacial activity of phosphine ligands and of their rhodium complexes for the efficient catalytic hydroformylation of heavy alkenes. Efficient ligands should be sufficiently polar to avoid leaching and loss of their rhodium complexes in the organic phase but not too much charged, however, to avoid being trapped in the bulk ionic phase, far from the interface.