Catalysis Employing Ionic Liquids

Multiphase catalysis performed in I Ls can lead to various phase systems in which the catalyst should reside in the IL. Before the reaction starts, and where there is no involvement of gaseous reactants, two systems can usually be formed a single phase, in which the substrates are soluble in the ionic liquid or biphasic, in which one or all of the substrates reside preferentially in an organic phase. If a gaseous reactant is involved, two-phase and three-phase systems can be formed. At the end 

The separation of the products from the IL catalytic mixture can be performed in various cases by simple decanting and phase separation or by product distillation. In this respect, a continuous-flow process using toluene as extractant has been appHed for the selective Pd-catalyzed dimerization of methyl acrylate in ILs [136]. However, in cases where the products are retained in the IL phase, extraction with supercritical carbon dioxide can be used instead of classical liquid-liquid extractions that necessitate the use of organic solvents, which may result in cross-contamination of products. This process was successfully used in catalyst recycling and product separation for the hydroformylation of olefins employing a continuous-flow process in supercritical carbon dioxide-IL mixtures [137]. Similarly, free and immobilized Candida antarctica lipase B dispersed in ILs were used as catalyst for the continuous kinetic resolution of rac-l-phenylethanol in supercritical carbon dioxide at 120°C and 150°C and 10 Mpa with excellent catalytic activity, enzyme stability and enantioselectivity levels (Fig. 3.5-11). 

The potential of ILs in liquid-liquid biphasic catalysis for the generation of clean technology has became a reality with the announcement of a commercial process for the dimerization of butenes to isooctenes (Difasol process) by IFF (France) based on nickel complexes immobilized in organo-aluminate imidazolium ILs [138]. 

This new process provides significant benefits over the exishng homogeneous Di-mersol X process, which is currently in operation in five industrial plants, producing nearly 200000 tonnes per year of isooctenes [139]. 

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It is mostly complexes of ruthenium and rhodium that have been used to conduct hydrogenation reactions in ionic liquids and little attention has so far been paid to modifying the employed catalysts to improve their performance in the ionic environment. The majority of the catalysts used are identical to those employed in conventional homogeneous catalysis conducted in molecular solvents like, for example, RhCl(PPh3)3 and RuCl2(PPh3)3. 

No detailed studies on catalyst degradation pathways in ionic liquids have yet been reported. Due to the reaction of [PF6] with water, transformation of P(OR)3 to P(OR)nF3.n has been observed/471 In the case of aqueous biphasic catalysis with TPPTS as ligand, it is known that migration of a phenyl ring and ultimately elimination of sulfonated phenyl derivatives can take place/59,601 However, when suitable (ionic) ligands are employed, the catalyst can remain active for more than 10 cycles and may be stable for more than 14 days under air/4 1... 

In the case of homogeneous catalysis, employing supercritical fluids enables complete recovery of the expensive transition metal species. Also, these species may have environmentally unfriendly effects if not recovered completely. The combination of ionic liquids with the supercritical fluid can lead to product isolation as well as catalyst immobilization. Thus, catalysts can be recycled batchwise. ... 

Other interesting systems have been employed, such as CO/HjO (water gas) or CO/Hj (syngas) as reducing mixtures [49, 50], phase transfer catalysis [37], and more recently, aqueous [46, 47] and non-aqueous ionic liquid [48] biphasic catalysis which offer more promise for practical uses. Some interesting examples of metal complexes grafted onto oxides [55, 56] or supported metals [38, 39] as arene hydrogenation catalysts have been provided. 

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