Water multiphase catalysis

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. 

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) ... 

Bhanage BM, Shirai M, Arai M et al (1999) Multiphase catalysis using water-soluble metal complexes in supercritical carbon dioxide. Chem Commun 14 1277-1278... 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

Notably, the use of liquid polymers in multiphase catalysis with SCCO2 is not restricted to transition metal catalysts. Biocatalysts can also be used in this environment, as demonstrated by the yeast-catalyzed reduction of a (i-ketoester that gave excellent (99%) enantioselectivity in PMPS-710 and the ionic liquid [P(Me)(Bu)3][ Bu)3][03SCgH4pMe] [Eq. (8)]. The product was isolated by extraction with water or SCCO2, respectively [54]. 

Cairns JR, Na TY. (1969) Optimum design of water jet pump. Trans. ASME, 91(l) 62-68. Chaudhari RV, Mills PL. (2004) Multiphase catalysis and reaction engineering for emerging pharmaceutical processes. Chem. Eng. Sci., 59 5337-5344. 

Multiphase catalysis occurs in C02/water by adding surfactants, which form micelles that disperse catalysts in pressurised C02/water mixtures. This is known for homogeneous toluene oxidation,toluene oxidation on immobilised micelles, oxybromination of phenol and aniline derivatives, or enzyme catalysis. 

Nowadays, there are several alternatives under investigation as fluid for multiphase catalysis, including the resurgence of water, perfluorinated hydrocarbons, and supercritical fluids, in particular CO2. Indeed, the advent of water-soluble organo-metallic complexes, especially those based on sulfonated phosphorus-containing ligands, has enabled various biphasic catalytic reactions to be conducted on an industrial scale, in particular, for the hydroformylation of olefins. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

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. 

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