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J. Phys. Chem. C 2008, 112, 6450-6461
Adsorption at the Liquid-Liquid Interface in the Biphasic Rhodium-Catalyzed Hydroformylation of 1-Hexene in Ionic Liquids: A Molecular Dynamics Study Nicolas Sieffert and Georges Wipff* Laboratoire MSM, UMR CNRS 7177, Institut de Chimie, 4 Rue B. Pascal, 67 000 Strasbourg, France ReceiVed: January 8, 2008; In Final Form: February 15, 2008
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 1-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-, sulfoxanthphos2and 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 hexeneIL 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 scCO2 to a biphasic system, showing that scCO2 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.
Introduction Biphasic catalysis is a particularly attractive method to efficiently recover the catalyst separately from the reaction products. It consists of the immobilization of an organometallic catalyst in a polar phase (immiscible or weakly miscible with the phase containing the substrate and products), thanks to the design of specific catalyst ligands that are very soluble in polar media and that allow a good reaction efficiency and selectivity. The immobilizing phase traditionally consists of, for example, water,1 perfluorinated solvents,2 or supercritical CO2 (hereafter noted as scCO2).3 In particular, the biphasic hydroformylation reaction has stimulated a large academic and industrial interest and has led to the development of the Ruhr-chemie/RhoˆnePoulenc process, in which the homogeneous rhodium-based catalyst is solubilized in a water phase via its coordinated hydrophilic trisulfonated triphenylphosphine ligands (hereafter noted as TPPTS3-; see Figure 1).4 Further developments concerning this catalytic system showed that ionic liquids (molten salts that melt below 100 °C; hereafter noted as ILs)5,6 can be suitable media to immobilize the negatively charged catalyst since their exceptional tunability, versatility, and solvation characteristics allow us to obtain good reaction yields with substrates of different polarities.7-10 Pioneering studies used the 1-butyl-3-methyl-imidazolium hexafluorophosphate IL (hereafter noted as [BMI][PF6]; see Figure 2), in conjunction with * Corresponding author. E-mail:
[email protected].
Figure 1. Rhodium-catalyzed hydroformylation of 1-hexene. First steps of the generally accepted reaction mechanism (only the path leading to the linear product is represented) with monophosphine ligands (L).
triphenylphosphine and their sulfonated derivatives as rhodium ligands (see Figure 1),11,12 and subsequent developments led to
10.1021/jp800150k CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008
Hydroformylation of 1-Hexene in Ionic Liquids
Figure 2. 1-Hexene, CO2, BMI+, and PF6- species. Bottom: schematic representation of a hexene-IL binary system containing three [RhH(CO)(TPPTS)3]9- complexes at a preformed interface and in a fully mixed system (B5).
an improvement of the catalysis system by designing greener ionic liquid components13 and continuous flow reaction systems.14-17 These are complex heterogeneous systems, and the understanding of what happens at the microscopic level is still limited; more particularly, where precisely the reaction is actually occurring remains unclear. It is generally considered that the reaction occurs in the IL phase (in which the catalyst is solubilized), therefore requiring the transfer of the apolar reaction partners (alkenes and CO/H2 syngas) into the IL polar phase where they are only weakly soluble in general. For instance, by comparing different ionic liquids, Favre et al. found a correlation between the solubility of the 1-hexene substrate in the IL and the hydroformylation reaction turnover frequency (TOF):18 the more soluble (in the IL) the alkene substrate is,
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6451 the more efficient the reaction is. This is not a general rule, however, as the Tf2N--based ILs (Tf2N- is the bis(trifluoromethylsulfonyl)-imide anion) lead to weak TOFs, despite the good solubility of 1-hexene.18 On the other hand, recent developments in supported ionic liquid catalysis19,20 point to the importance of the interface between the IL and the organic phase. For instance, Mehnert et al. observed that an increase in the surface area relative to the IL volume enhanced the hydrogenation21 as well as the hydroformylation22 reactions, and, according to Riisager et al., the reaction occurs “at the interphase or in the diffusion layer of the IL”.17 Molecular dynamics (MD) simulations contribute to our understanding of the nature and characteristics of liquid-liquid nanointerfaces (of typically 5 nm × 5 nm sections),23,24 with relevant applications in, for example, assisted ion extraction25,26 or phase transfer catalysis.27,28 Concerning the rhodiumcatalyzed hydroformylation reaction, we recently reported the first MD studies at the aqueous interface with chloroform or decene, showing that the TPPTS3- ligands and their rhodium complexes are surface active, a feature that favors an interfacial reaction mechanism.29,30 According to the MD results, water soluble cyclodextrins that are known to promote the reaction also are surface active and complex the alkene substrate, as well as a key rhodium-hexene reaction intermediate, right at the interface where the catalytic reaction should thus proceed. Both simulation results concerning the surface activity of the cyclodextrins and the interfacial reaction mechanism were subsequently supported by experiments.31 What happens at the IL interfaces is less clear and led us to recently simulate the 1-hexene-[BMI][PF6] interface32 that was found to be molecularly sharp, as are classical water-fatty olefin interfaces.30 Furthermore, according to the simulations, the partitioning of the precatalyst was found to be mainly determined by the nature of its constitutive phosphine ligand: the neutral [RhH(CO)(PPh3)3] precatalyst partitions to the hexene phase, whereas its charged [RhH(CO)(TPPTS)3]9- analogue prefers the IL phase. However, the interfacial behavior of the key species involved in the reaction was not elucidated clearly, requiring further investigations on the active species involved in the reaction and, in particular, on the catalyst itself and its adduct with the alkene substrate: do they partition in the IL bulk phase (therefore suggesting a reaction occurring in the IL phase) or adsorb at the liquid-liquid interface (suggesting an interfacial reaction mechanism as in the case of classical water-olefin systems)?29,30 In this paper, we report new MD studies on the interfacial behavior of selected reactive species in the hexene-[BMI][PF6] system. We first focus on free phosphine ligands L of different charges, by comparing the PPh3, TPPMS-, sulfoxantphos2-, and TPPTS3- ligands (Figure 1). The free sulfonated ligands were identified in solution by NMR10,33,34 and are used widely to conduct hydroformylation reactions.10,12,15,17-19,22,34-38 Their solvation patterns and their partioning in the biphasic system should critically govern those of their Rh complexes. We thus next consider the [RhH(CO)L2(hexene)] key reaction intermediate (Figure 1) that results from the coordination of the olefin to the catalyst, according to the generally accepted reaction mechanism39,40 (see Figure 1). Indeed, high-pressure IR and NMR studies suggest that the same reaction mechanism is followed in the neat organic phase, in water-oil biphasic systems,33 as well as in IL-oil systems.10,34 The effect of ligand charge on the interfacial behavior of the [RhH(CO)L2(hexene)] complexes thus was investigated, comparing TPPMS- to TPPTS3- as ligands L. Finally, we consider the influence of scCO2 on the catalytic systems because scCO2 has been used
6452 J. Phys. Chem. C, Vol. 112, No. 16, 2008
Sieffert and Wipff
experimentally in continuous flow reactions.15-17,36,38,41 We desired to investigate the influence of CO2 on the interfacial landscape of the catalyst, taking the key [RhH(CO)L2(hexene)] complex as the solute. On the computational side, an important feature should be mentioned. It concerns the sampling, in keeping with the high viscosity and long relaxation times of ILs (typically, on the multinanosecond time scale, according to spectroscopic42-44 and simulation data45,46), as compared to conventional molecular liquids. Thus, to avoid being trapped near an initial state (see, e.g., IL solutions with calixarenes47), we systematically performed demixing simulations of randomly mixed liquids and their solutes. The outcome of the phase separation and partitioning of the solutes will be shown to be markedly dependent on the charge and nature of the ligands and of their rhodium complexes. Materials and Methods Molecular Dynamics. The MD simulations were performed with the modified AMBER7.0 software,48 where the potential energy is described by a sum of bond, angle, and dihedral deformation energies and pairwise additive 1-6-12 (electrostatic + van der Waals) interactions between nonbonded atoms
U)
∑ Kr(r - req)2 + angles ∑ Kθ(θ - θeq)2 + dihedrals ∑ ∑n Vn bonds (1 + cos(nω - γ)) +
∑ i
