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Ind. Eng. Chem. Res. 2009, 48, 4254–4265
Understanding Biphasic Ionic Liquid/CO2 Systems for Homogeneous Catalysis: Hydroformylation Azita Ahosseini, Wei Ren, and Aaron M. Scurto* Department of Chemical & Petroleum Engineering and NSF-ERC Center for EnVironmentally Beneficial Catalysis, UniVersity of Kansas, Lawrence, Kansas 66045
A biphasic ionic liquid (IL) and compressed carbon dioxide system has a number of advantages for efficient homogeneous catalysis. The hydroformylation of 1-octene to nonanal catalyzed by a rhodium-triphenylphosphine complex was used as a model reaction to illustrate the effects of carbon dioxide in a biphasic ionic liquid/CO2 system using the model ionic liquid, 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][Tf2N]). Detailed phase equilibrium studies were conducted to determine volume expansion of the IL phase and the multiphase equilibria and mixture critical points between the reactant, product, and IL with CO2 and syngas (CO/H2). These data ultimately affect the concentration of the reactant and, thus, the apparent reaction rate. The viscosity of the IL with CO2 pressure was measured and demonstrates the dramatic decrease with increasing CO2 pressure. The selfdiffusion coefficient of the ionic liquid and 1-octene were measured and indicate a large increase with CO2 pressure (solubility). With an understanding of the kinetics, phase behavior, and mass transport, biphasic IL/CO2 reaction systems may be properly understood and designed. 1. Introduction Homogeneous catalysis with organometallic complexes has been utilized for efficient chemical transformations with high chemo-, regio-, and enantioselectivity.1,2 In most cases, the practical feasibility of a catalytic system is determined by the activity, selectivity, and, equally important, by the ability to separate and recycle the often costly catalyst and ligands. A variety of scenarios have been developed to address this issue including reaction in a single fluid phase followed by separation, multiple fluid phases for reactant delivery and selective extraction, or solid support of the metal complex. In multiphase systems, one phase immobilizes or sequesters the catalyst and the other phase acts as a mobile phase to deliver reactants and to remove products. The idealized biphasic system would have complete immiscibility between phases (no cross contamination); no catalyst partitioning (leaching); and reactants would partition into the catalytic phase, and products would partition out of the catalytic phase. While solid-support or “heterogenizing” the metal complexes would result in the most facile separations of the catalyst from the products, often the solid support decreases the activity and, importantly, selectivity of the catalyst. In addition, catalyst processing and ultimate recovery of these materials requires complete shutdown of a reactor. One of the largest examples of biphasic homogeneous catalysis is the Ruhrchemie/Rhoˆne-Poulenc2 process for short-chain olefin hydroformylation, which uses water to sequester and recycle a rhodium catalyst with sulfonate-modified triphenylphosphine ligands. However, its application to higher olefins is limited by the solubility of the olefin (
