Multiphasic aqueous-organic reaction

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

The versatility of ILs has driven increasing interest in using them in extraction and multiphasic homogeneous catalytic reactions [104] where one phase is chosen to dissolve the catalyst and be immiscible with the second phase which contains the reactant and products. Such processes occur at the interface between the IL and the overlying aqueous or organic phase, and are dependent on the access of the material to the surface and the transfer of material across the interface. A clearer understanding of the mechanisms behind these processes requires a more detailed examination of the surface properties of the ionic liquids. 

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]. 

Reactions carried in aqueous multiphase catalysis are accompanied by mass transport steps at the L/L- as well as at the G/L-interface followed by chemical reaction, presumably within the bulk of the catalyst phase. Therefore an evaluation of mass transport rates in relation to the reaction rate is an essential task in order to gain a realistic mathematic expression for the overall reaction rate. Since the volume hold-ups of the liquid phases are the same and water exhibits a higher surface tension, it is obvious that the organic and gas phases are dispersed in the aqueous phase. In terms of the film model there are laminar boundary layers on both sides of an interphase where transport of the substrates takes place due to concentration gradients by diffusion. The overall transport coefficient /cl can then be calculated based on the resistances on both sides of the interphase (Eq. 1) ... 

These systems are common in liquid extraction and also in a multiphase reactor with an organic and an aqueous phase. Common sources of pollution are incomplete separation and contamination due to trace organics in the aqueous phase. An example is in alkylation reactions (e.g., n-butane reaction with olefins to form isooctanes). Strong acids, such as sulfuric and hydrofluoric acids, are used as catalysts, and the recovery and the recycle of acid need to be optimized in order to reduce the waste generation. 

The other distinct mode of operation for a tubular reactor occurs in applications where more than one phase is present in the reaction mixture, e.g., gas and liquid reactants. The products from the reaction can be gases, liquids, or solids where the latter can exist as crystalline or amorphous materials. Either aqueous or organic-based solvents are sometimes included in the reaction medium to control the concentrations of reaction species, to provide increased thermal capacity for highly exothermic systems, or to alter solubility properties for subsequent catalyst recovery or product separations and recovery. This type of reaction is often termed a multiphase reaction, owing to the presence of multiple interacting phases in the reaction environment. In most practical applications of this mode, either a soluble organometallic complex or a solid heterogeneous catalyst is utilized to transform the reactants into the desired product or products. 

A further evidence on the acceleration enjoyed by a typical Pd-catalysed reaction, the Heck reaction, in an ionic phase ( V-mcthyl-Y.Y. V.-trioctylammonium chloride or Aliquat 336) is found in a triphasic protocol developed by Tundo and coworkers. 7b.The arylation of electron poor olefins is catalysed by palladium supported on charcoal (Pd/C) and is carried out in the heterogeneous isooctane/Aliquat 336/water system (Figure 27). Under this multiphasic condition, Aliquat 336 forms a third liquid phase between the organic and the aqueous phase that traps the catalyst. The use of phosphines is not necessary. As a matter of fact, Aliquat 336 incorporates the solid-supported catalyst and ensures an efficient mass transfer between the bulk phases resulting in an increase of the reaction rate of an order of magnitude compared to the reaction in the absence of the ionic liquid. A determing role is played by the base while I LN drives the reaction towards the formation of ethyl cinnamate, reaction carried out in the presence of KOH lead to formation of Ullmann dimerisation products. 

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