Ionic liquids catalytic leaching

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

In conventional solid-liquid or solid-gas heterogeneous catalytic systems, the catalyst is conveniently separated from the fluid-phase reaction product. When an ionic liquid is used as a phase to isolate a catalyst, the catalyst is fully dispersed and mobile and may be fully involved in the reaction. When a homogeneous catalyst is isolated by anchoring onto the surface of a solid support (e.g., by reaction with OH groups), the result may be a stable catalyst that is not leached into the reactant... 

Leaching of the ionic liquid from the support was negligible. However, deactivation was observed, caused by moisture present during product separation (which can be minimized with proper care) and by oligomerization of dodecene on the catalyst surface. Unfortunately, both factors cause irreversible loss of catalytic activity. 

Water soluble Rh/tppts and Rh/tppms complexes dissolved in nonaqueous media such as the ionic liquids, l-ethyl-3-methylimidazolium or l-n-butyl-3-methylimidazolium salt have also been used as catalysts in the hydroformylation of 1-pentene employing a two phase system.15,16 The yields obtained were 16-33% (TOF=59-103 h 1) without any leaching of the rhodium from the ionic liquid to the aldehydes/feedstock phase. Rh/PPh3 catalysts exhibited higher rates (TOF=333 h 1) for the same biphasic reaction albeit with leaching of rhodium due to the uncharged nature of the catalytic system.15... 

The third case shows the immobilisation of Lewis-acidic ionic liquids. The resulting catalysts, named Novel Lewis-Acidic Catalysts (NLACs), are highly active in the Friedel-Crafts alkylation of aromatic compounds with dodecene. Conversions and selectivities to the desired monoalkylated products were excellent. No leaching of the catalytically active component could be observed. The isomer distribution of the monoalkyated products is very similar to that obtained over pure aluminum(III)chloride. The main drawback of the NLACs is that thy are very sensitive towards water, which leads to irreversible deactivation. A second problem is the deactivation after long reaction times. The most likely cause is olefin oligomerisation. 

The synthesis, characterization and catalytic properties of an ionic liquid supported Pd (0.3 wt.%) catalyst ([NB4MPy ][BFT]Pd (acac)/active charcoal) are discussed. The oxidation state of Pd in a fresh, reduced catalyst was corresponding to Pj, whereas after three-phase citral hydrogenation it was close to Pdf. Obtained results, including negligible Pd leaching, demonstrated the feasibility of stable ionic liquid supported Pd catalysts preparation, which can be applied in liquid phase hydrogenation reactions. 

Silica gels and controlled-pore glass, which were covered with thin films of polar phases such as water, ethylene glycol or ionic liquids, were used as polar solid supports. These systems are limited to very polar, usually ionic catalysts and non-polar reaction media in order to prevent catalyst leaching. This in turn, can be limiting to the range of substrates. Existing catalytic processes in common liquid-liquid biphasic systems can be easily transferred to supported liquid-phase conditions. At the same time the interfacial area between the... 

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