Aqueous biphasic separation systems

Recently, a new aqueous biphasic catalytic system based on the cloud point of nonionic tensioactive phosphine, termed thermoregulated phase-transfer catalysis (TRPTC) has been developed [13]. The concept ofTRPTC as a missing link could not only provide a meaningful solution to the problem of catalyst/product separation, but also extricate itself from the limitation of low reaction rates of water-immiscible substrates. 

Extractions and separations in two-phase systems require knowledge of the miscibilities and immiscibilities of ILs with other solvents compatible with the process. These are most usually IL/aqueous biphase systems in which the IL is the less polar phase and organic/IL systems in which the IL is used as the polar phase. In these two-phase systems, extraction both to and from the IL phase is important. 

The major advantage of the use of two-phase catalysis is the easy separation of the catalyst and product phases. FFowever, the co-miscibility of the product and catalyst phases can be problematic. An example is given by the biphasic aqueous hydro-formylation of ethene to propanal. Firstly, the propanal formed contains water, which has to be removed by distillation. This is difficult, due to formation of azeotropic mixtures. Secondly, a significant proportion of the rhodium catalyst is extracted from the reactor with the products, which prevents its efficient recovery. Nevertheless, the reaction of ethene itself in the water-based Rh-TPPTS system is fast. It is the high solubility of water in the propanal that prevents the application of the aqueous biphasic process [5]. 

When the products are partially or totally miscible in the ionic phase, separation is much more complicated (Table 5.3-2, cases c-e). One advantageous option can be to perform the reaction in one single phase, thus avoiding diffusional limitation, and to separate the products in a further step by extraction. Such technology has already been demonstrated for aqueous biphasic systems. This is the case for the palladium-catalyzed telomerization of butadiene with water, developed by Kuraray, which uses a sulfolane/water mixture as the solvent [17]. The products are soluble in water, which is also the nucleophile. The high-boiling by-products are extracted with a solvent (such as hexane) that is immiscible in the polar phase. This method... 

In comparison with classical processes involving thermal separation, biphasic techniques offer simplified process schemes and no thermal stress for the organometal-lic catalyst. The concept requires that the catalyst and the product phases separate rapidly, to achieve a practical approach to the recovery and recycling of the catalyst. Thanks to their tunable solubility characteristics, ionic liquids have proven to be good candidates for multiphasic techniques. They extend the applications of aqueous biphasic systems to a broader range of organic hydrophobic substrates and water-sensitive catalysts [48-50]. 

An important problem in emulsified organic-aqueous systems is that of scale-up, which is concerned with the realization of stable emulsions and the separation of phases after the reaction. The use of biphasic membrane systems that contain the enzyme and keep the two phases separated is likely to solve this problem. In the case of 5-naproxen an ee of 92% has been demonstrated without any decay in activity over a period of two weeks of continuous operation. A number of examples of biocatalytic membrane reactors have been provided by Giorno and Drioli (2000) and include the conversion of fumaric acid to L-aspartic acid, L-aspartic acid to L-alanine, and cortexolone to hydrocortisone and prednisolone. 

These alternative processes can be divided into two main categories, those that involve insoluble (Chapter 3) or soluble (Chapter 4) supports coupled with continuous flow operation or filtration on the macro - nano scale, and those in which the catalyst is immobilised in a separate phase from the product. These chapters are introduced by a discussion of aqueous biphasic systems (Chapter 5), which have already been commercialised. Other chapters then discuss newer approaches involving fluorous solvents (Chapter 6), ionic liquids (Chapter 7) and supercritical fluids (Chapter 8). 

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

As outlined in Chapter 5, Section 5.2.3.2 various approaches to overcoming the low rates of the hydroformylation of long chain alkenes in aqueous biphasic systems have been proposed. Some of these, such as the use of microemulsions [24-26] or pH dependent solubility [27], have provided improvements often at the expense of complicating the separation process. Perhaps the most promising new approaches involve the introduction of new reactor designs where improved mixing allows for... 

An aqueous biphasic system consisting of two immiscible liquid phases (i.e., two separate distinct layers) can be used to separate a particular component such as certain heavy metals from contaminated soil. A combination of phases such as a water-soluble polymer (e.g., polyethylene glycol) phase and a concentrated aqueous salt solution (e.g., sodium carbonate, sodium sulfate, or sodium phosphate) phase can comprise a biphasic system. Aqueous biphasic systems are... 

Aqueous biphasic systems offer the potential for highly selective and low-cost separations. Aqueous biphasic extraction for soil decontamination is based on the selective partitioning of either dissolved solutes or ultrafine particulates between two immiscible aqueous phases. Both soluble and particulate uranium contaminants can be separated from soil using this technique. Aqueous biphasic extraction may also have application for separation of plutonium and thorium from soil or waste. 

Aqueous biphasic systems have been used commercially for protein separations, separation of metal ions, ultrafine particles, and organics. Application of the technology for soil decontamination has only been demonstrated in laboratory-scale studies. 

Supercritical media, in general, have the potential to increase reaction rates, to enhance the selectivity of chemical reactions and to facilitate relatively easy separations of reactants, products, and catalysts after reaction (3). However reactions involving CO2 and water are typically conducted as biphasic processes, with the organic substrate dissolved mostly in the C02-rich phase and the water-soluble catalysts and/or oxidant dissolved in the aqueous phase. Such systems suffer from inter-phase mass-transfer limitations (4). 

Visser, A.E. et al. Metal ion separations in aqueous biphasic systems and room temperature ionic liquids, PhD thesis. University of Alabama, Tuscaloosa, AL, USA, 2002. 

Gutowski, K.E., Broker, G.A., Willauer, H.D., Huddelston, R.R, Swatloski, J.D., Holbrey, J.D., Rogers, R.D., Gontrolling the aqueous miscibility of ionic liquids Aqueous biphasic systems of water-miscible ionic liquids and waterstructuring salts for recycle, metathesis and separations, /. Am. Chem. Soc., 125, 6632-6633, 2003. 

Aqueous systems have been used in separations, coatings, and synthesis. Aqueous biphasic systems have been developed for hquid-liquid extractions using water-soluble polymers such as polyethylene glycol, and inorganic salts such as ammonium chloride or potassium phosphate (Rogers et al., 1998), (Sherman et al., 1998). These systems have been investigated for use in the separation and recovery of heavy metals from mixed wastes and in the recovery of colored impurities from textile waste streams (Sherman et al., 1998). 

Similarly, a water-soluble palladium complex of a sulfonated phenanthroline ligand catalyzed the highly selective aerobic oxidation of primary and secondary alcohols in an aqueous biphasic system in the absence of any organic solvent (Figure 1.8) [40]. The liquid product could be recovered by simple phase separation, and the aqueous phase, containing the catalyst, used with a fresh batch of alcohol substrate, affording a truly green method for the oxidation of alcohols. 

The latest development in industrial alkene hydroformylation is the introduction by Rurhchemie of water-soluble sulfonated triphenylphosphine ligands.94 Hydroformylation is carried out in an aqueous biphasic system in the presence of Rh(I) and the trisodium salt of tris(m-sulfophenyl)phosphine (TPPTN). High butyraldehyde selectivity (95%) and simple product separation make this process more economical than previous technologies. 

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