Multiphase catalysis

It is important to make the distinction between the multiphasic catalysis concept and transfer-assisted organometallic reactions or phase-transfer catalysis (PTC). In this latter approach, a catalytic amount of quaternary ammonium salt [Q] [X] is present in an aqueous phase. The catalyst s lipophilic cation [Q] transports the reactant s anion [Y] to the organic phase, as an ion-pair, and the chemical reaction occurs in the organic phase of the two-phase organic/aqueous mixture [2]. 

Further progress in multiphasic catalysis will rely on the development of alternative techniques that allow the reactivity of a broader range of substrates, the efficient separation of the products, and recovery of the catalyst, while remaining economically viable. 

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. 

Multiphasic Catalysis with Ionic Liquids in Combination with Compressed CO2 5.4... 

MULTIPHASIC CATALYSIS WITH IONIC LIQUIDS - ENGINEERING ASPECTS... 

Liquid-liquid multiphasic catalysis with the catalyst present in the ionic liquid phase relies on the transfer of organic substrates into the ionic liquid or reactions must occur at the phase boundary. One important parameter for the development of kinetic models (which are crucial for up-scaling and proper economic evaluation) is the location of the reaction. Does the reaction take place in the bulk of the liquid, in the diffusion layer or immediately at the surface of the ionic liquid droplets ... 

As with classical multiphase catalysis, the organometallic catalyst is retained here in a liquid phase that is immiscible with the second phase containing substrates and/or products. For hydrogenation, the liquid/SCF system is always biphasic, whereas conventional systems are usually triphasic (liquid-1 /liquid-2/ H2). The liquid phase must provide a stable environment for the organometallic catalyst and should be insoluble in the SCF phase. Water, ILs and PEG have been used successfully for this purpose, together with scC02 as the mobile phase. Again, the products must not be too polar in order to be effectively extracted if C02 is used as the SCF. 

The use of ionic liquids has been successfully studied in many transition metal-catalyzed hydrogenation reactions, ranging from simple alkene hydrogenation to asymmetric examples. To date, almost all applications have included procedures of multiphase catalysis with the transition-metal complex being immobilized in the ionic liquid by its ionic nature or by means of an ionic (or highly polar) ligand. 

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. 

The idea to use solvent systems enabling homogeneous reaction conditions at elevated temperatures and liquid/liquid phase separation at lower— preferably room—temperature seems to be obvious. Nevertheless, it is only recently that thermomorphic solvent systems gain attention [30-33] for product separation or multiphase catalysis [34,35]. The main reasons for the delayed engagement is that an efficient choice of a useful solvent system is not easy to achieve. There is also a lack of experience with thermomorphic systems in general. Reactions are optimized to be carried out in solvents having certain distinct solubility and polarity characteristics. A thermomorphic solvent system of choice will have to fulfill these requirements and to show the thermomorphic effect in addition. 

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