Adsorption at the Liquid-Liquid Interface in the Biphasic Rhodium-Catalyzed Hydroformylation of 1-Hexene in Ionic Liquids: A Molecular Dynamics Study
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- 作者：Nicolas Sieffert ; Georges Wipff
- 刊名：Journal of Physical Chemistry C
- 出版年：2008
- 出版时间：April 24, 2008
- 年：2008
- 卷：112
- 期：16
- 页码：6450 - 6461
- 全文大小：1543K
- 年卷期：v.112,no.16(April 24, 2008)
- ISSN：1932-7455
文摘
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 intermediatesinvolved in the biphasic rhodium-catalyzed hydroformylation of 1-hexene. In all cases, the organic and ILphases separate during the dynamics, leading to different partitioning of the solute species, depending ontheir charge and constitution. The most important finding concerns the surface activity of the ligands andtheir complexes. The neutral unsubstituted triphenylphosphine ligand prefers the organic phase over the ILphase, but displays transient contact with the IL at the interface. The charged TPPMS-, sulfoxanthphos2-and TPPTS3- ligands prefer the IL over the hexene phase, but can adsorb at the IL side of the interface in anamphiphilic 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, thusdisplaying direct contacts with hexene molecules. The [RhH(CO)(TPPMS)2(hexene)]2- complex is more surfaceactive than its more charged [RhH(CO)(TPPTS)2(hexene)]6- analogue. We finally investigated the effect ofadded scCO2 to a biphasic system, showing that scCO2 enhances the diffusion of all species, leading to afaster phase separation process and presumably to a faster reaction kinetics. It does not modify, however, thesurface activity of the reaction intermediate. The simulation results point to the importance of the interfacialactivity of phosphine ligands and of their rhodium complexes for the efficient catalytic hydroformylation ofheavy alkenes. Efficient ligands should be sufficiently polar to avoid leaching and loss of their rhodiumcomplexes in the organic phase but not too much charged, however, to avoid being trapped in the bulk ionicphase, far from the interface.