Ligand Effects on the Surface Composition of Rh‐Containing Ionic Liquid Solutions Used in Hydroformylation Catalysis

Abstract

In hydroformylation, multiphase catalysis is a well-established and industrially realized method for effective catalyst separation and recycling. Aqueous phase liquid–liquid biphasic catalysis was developed through the pioneering discovery of the highly water-soluble ligand tris(3-sodium sulfonatophenyl)phosphine (tppts, 1) by Kuntz in 1976. This ligand concept enabled the industrial realization of aqueous hydroformylation for propene hydroformylation in the Ruhrchemie–Rh ne–Poulenc process. The process went on stream in 1984, and is still operating today (550000 tons per year). However, the limited solubility of higher olefins (>C4) in water has prompted much research activity into alternative polar catalyst media for liquid–liquid multiphase hydroformylation. Among these endeavors, the use of lowmelting salts, so-called ionic liquids (ILs), has attracted particular interest as many ionic liquids show sufficiently high solubilities for higher olefins to allow reasonable reaction rates. First reports on the application of ionic liquids in Rh-catalyzed hydroformylation were published by Chauvin s group in 1995. Already in this first paper, the use of sulfonated triphenylphosphine ligands was highlighted as a crucial precondition to avoid Rh-leaching into the organic product phase. Later, sulfonated triphenylphosphine ligands were also combined with imidazolium counter ions. ColeHamilton and co-workers suggested, for example, [C3mim]2ACHTUNGTRENNUNG[PhP ACHTUNGTRENNUNG(C6H4SO3)2] as a suitable ligand for hydroformylation reactions in the biphasic system ionic liquid/scCO2. [7] The same ligand system was applied recently to an even more efficient catalytic system using the ionic catalyst solution in the form of a supported ionic liquid phase (SILP). Such SILP catalytic systems have also been very successfully applied in continuous gas-phase reactions where the ionic liquid film supported on a highly porous inorganic support is contacted directly with the gas-phase of the reactants to perform continuous hydroformylation using a continuous fixed-bed reactor. In general, hydroformylation in ionic liquids has produced a huge amount of scientific activity over the last 15 years and particular progress was made by the use of regioselective ionic ligand systems and by the application of ionic liquids carrying halide-free, cheap and hydrolytic stable anions. The field has been recently summarized comprehensively in an excellent review by Haumann and Riisager. In multiphase catalysis, interface processes such as substrate diffusion into the catalyst phase, the reaction rate at the phase boundary (in comparison to reaction rate in the bulk), and product diffusion back into the organic phase play a crucial role for the overall performance of the system. Despite this obvious fact, experimental investigations into the chemical nature of the liquid surface of catalytic systems are lacking so far. This is even more surprising in the light of recent theoretical findings by the group of Wipff, which demonstrated that the composition of the catalytic interface may well be very different from the average chemical composition of two adjacent bulk liquid phases. This fact is of high relevance for the detailed understanding of all above-mentioned interface transport and reaction processes. Moreover, the authors demonstrated that the surface-active character of a given type of an IL soluble complex was similar at the IL–vacuum interface and at the IL interface with weakly polar organic solvents. The [a] Dipl.-Chem. C. Kolbeck, Dipl.-Chem. T. Cremer, Dr. F. Maier, Prof. Dr. H.-P. Steinr ck Chair of Physical Chemistry II Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91054 Erlangen (Germany) Fax: (+49)9131-852-8867 E-mail : steinrueck@chemie.uni-erlangen.de [b] Dipl.-Chem. N. Paape, Dr. P. S. Schulz, Prof. Dr. P. Wasserscheid Lehrstuhl f r Chemische Reaktionstechnik Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) [] These authors contributed equally to this work.