Ionic liquids and catalysis

OUvier-Bourbigou H, Magna L, Morvan D (2010) Ionic liquids and catalysis. Appl Catal A Gen 373 1-56... 

Olivier-Bourbigou, H., Magna, L. and Morvan, D., Ionic liquids and catalysis Recent progress from knowledge to applications, App/. Catal. A Gen. 373,1-56 (2010). 

H. Olivier-Bourbigou, L. Magna, D. Morvan, Ionic liquids and catalysis recent progress from knowledge to applications, Appl. Catal. A ... 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

The field of reaction chemistry in ionic liquids was initially confined to the use of chloroaluminate(III) ionic liquids. With the development of neutral ionic liquids in the mid-1990s, the range of reactions that can be performed has expanded rapidly. In this chapter, reactions in both chloroaluminate(III) ionic liquids and in similar Lewis acidic media are described. In addition, stoichiometric reactions, mostly in neutral ionic liquids, are discussed. Review articles by several authors are available, including Welton [1] (reaction chemistry in ionic liquids), Holbrey [2] (properties and phase behavior), Earle [3] (reaction chemistry in ionic liquids), Pagni [4] (reaction chemistry in molten salts), Rooney [5] (physical properties of ionic liquids), Seddon [6, 7] (chloroaluminate(III) ionic liquids and industrial applications), Wasserscheid [8] (catalysis in ionic liquids), Dupont [9] (catalysis in ionic liquids) and Sheldon [10] (catalysis in ionic liquids). 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

Flowever, information concerning the characteristics of these systems under the conditions of a continuous process is still very limited. From a practical point of view, the concept of ionic liquid multiphasic catalysis can be applicable only if the resultant catalytic lifetimes and the elution losses of catalytic components into the organic or extractant layer containing products are within commercially acceptable ranges. To illustrate these points, two examples of applications mn on continuous pilot operation are described (i) biphasic dimerization of olefins catalyzed by nickel complexes in chloroaluminates, and (ii) biphasic alkylation of aromatic hydrocarbons with olefins and light olefin alkylation with isobutane, catalyzed by acidic chloroaluminates. 

The combination of ionic liquids and compressed CO2 - at opposite extremes of the volatility and polarity scales - offers a new and intriguing immobilization technique for homogeneous catalysis. 

The development of phase transfer catalysis, of supercritical fluids, of ionic liquids and of course, new reagents, should also have considerable potential in the labeling area. Furthermore there is the possibility of combining these approaches with energy-enhanced conditions - in this way marked improvements can be expected. 

TABLE 7.6. Process characteristics for optimised nonanal production (using liquid-liquid biphasic catalysis with ionic liquids) and butanal production (using SILP catalysis) on a 100.000 tons/year scale... 

A summary of the research activities of the last four years reveals three different important trends (a) The design of new ionic ligands for excellent catalyst immobilisation in ionic liquids and high regioselectivity (b) the successful application of cheap, halogen-free ionic liquids in the biphasic Rh-catalysed hydroformylation (c) the successful development of unusual multiphasic reaction concepts for Rh-catalysed hydroformylation, namely catalysis in ionic liquid/supercritical C02 and SILP-catalysts. 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

A major step towards applicability of multiphase catalysis in ionic liquids is the development of Supported Ionic Liquid Phase (SLIP) -catalysis by the Wasserscheid group [28,29]. The SLIP concept enables quasi-heterogeneous catalysis in ionic liquids and opens the door to the production of basic chemicals. 

Hintermair, U. and Zhao, G. and Santini, C.C. and Muldoon, M.J. and Cole-Hamilton, D.J. (2007). Supported ionic liquid phase catalysis with supercritical flow. Chem. Commun., 14, 1462-1464. 

There is a wide range of possibilities for adjusting the solubility characteristics of ionic liquids, and this is one of their potential advantages for optimized performance in biphasic or multiphasic catalysis (/). Because of the generally weak coordinating ability of the anions, most catalysts can be isolated in the solvent in a stable state without loss of activity. The product selectivity can sometimes be improved as well by the phase isolation. Because the catalyst is concentrated in the ionic phase, the reaction volume can be much smaller than in classical... 

A number of ionic liquids incorporating new cations and anions have shown decreased viscosity and stability (/). There is no doubt that more applications of ionic liquids in catalysis are yet to be developed. 

According to their miscibility with water, ionic liquids are also frequently classified as hydrophilic or hydrophobic. The hydrophilic ionic liquids are typically salts composed of halide, acetate, nitrate, trifluoroacetate, and, in some cases, tetrafluoroborate anions, in particular their salts with [AMIM] having short alkyl chains, as these ionic liquids are totally miscible with water. The ionic liquids composed of PF and (CF3S02)2N with [AMIM] are immiscible with water in bulk, and are therefore referred to as hydrophobic ionic liquids. The ionic liquids consisting of BF4 and CFsSOi" ions with [AMIM] can be totally miscible or immiscible depending on the substituents on the cation, and they are therefore sometimes called tunable ionic liquids (22). A recent review covers relevant properties of some ionic liquids for catalysis (42,43). 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

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