Biphasic systems, aqueous

The combination of two mutually incompatible but water-soluble polymers, or the combination of certain water-soluble polymers and specific inorganic salt solutions, produces two immiscible aqueous phases [1,29]. A number of different water-soluble polymers can be utilized in ABSs, with PEGs, dextrans, and Ficolls receiving the most attention [4]. An even wider variety of polymer/salt combinations exist (Table 1), usually with Na , K , or salts of mono- through trivalent anions such as OH , C03 , 

Perhaps the most interesting aspect of aqueous biphasic chemistry, and the origin of its name, is that each of the phases is composed mainly of water. Because both phases are aqueous and the phase-forming components (polymer/polymer or polymer/salt) are water soluble, each component is soluble in the other phase. Hence, in a PEG-ABS, the upper phase contains most, but not all, of the PEG, and the lower phase contains most of the salt [1,29]. Because of the solubility of the PEG and salt in the opposite phase, the upper phase is referred to as the PEG-rich phase and the lower as the salt-rich phase. 

The phase diagram for a given system can be used to interpret phase behavior in an ABS. It gives such useful information as the relative amounts of biphase-forming components needed to maintain a two-phase system, as well as the relative ratios of each component in either phase [1]. How these variables affect the partitioning of solutes in a given ABS is, however, not necessarily well understood. 

A phase diagram (from turbidimetric titration [1]) for the NaOH/ PEG-2000 system is presented in Fig. I [30]. The curve represents the binodal. System compositions below this curve result in a single homogeneous solution, whereas system compositions above the binodal result in two 

The stability of the biphasic system with composition A can be measured by the stability ratio [31], AD/AO, which represents how far the system composition lies from the binodal in the two-phase region. As such, it can be regarded as a measure of phase incompatibility (which increases as AD/AO increases) and can be correlated with partition coefficients. 

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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. 

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]. 

The present section deals with the improvement in the performance of biocatalysis when carried out in organic-aqueous biphasic systems. Such systems are very useful in equilibrium reactions and conversion yield where substrates and products can be dissolved and drawn into different phases. Subsequently the synthesis in the reactive aqueous phase is allowed to continue. 

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). 

For aqueous biphasic systems, the "solvent" is water which shows pronounced solvent... 

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... 

The use of water-soluble ligands was referred to previously for both ruthenium and rhodium complexes. As in the case of ruthenium complexes, the use of an aqueous biphasic system leads to a clear enhancement of selectivity towards the unsaturated alcohol [34]. Among the series of systems tested, the most convenient catalysts were obtained from mixtures of OsCl3 3H20 with TPPMS (or better still TPPTS) as they are easily prepared and provide reasonable activities and modest selectivities. As with their ruthenium and rhodium analogues, the main advantage is the ease of catalyst recycling with no loss of activity or selectivity. However, the ruthenium-based catalysts are far superior. 

The use of water-soluble metal catalysts for the hydrogenation of thiophenes in aqueous biphasic systems has been primarily introduced by Sanchez-Delgado and coworkers at INTEVEP S.A. [61]. The precursors RuHC1(TPPTS)2(L2) (TPPTS=triphenylphosphine trisulfonate L=aniline, 1,2,3,4-tetrahydroquinoline) and RuHC1(TPPMS)2(L2) (TPPMS=triphenylphosphine monosulfonate) were... 

Another environmental issue is the use of organic solvents. The use of chlorinated hydrocarbons, for example, has been severely curtailed. In fact, so many of the solvents favored by organic chemists are now on the black list that the whole question of solvents requires rethinking. The best solvent is no solvent, and if a solvent (diluent) is needed, then water has a lot to recommend it. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enzymatic procedures in water at ambient temperature and pressure can provide substantial environmental and economic benefits. Similarly, there is a marked trend toward the application of organometal-lic catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide and ionic liquids. ... 

The Boots Hoechst Celanese (BHC) ibuprofen process involves palladium-catalyzed carbonylation of a benzylic alcohol (IBPE). More recently, we performed this reaction in an aqueous biphasic system using Pd/tppts as the catalyst (Figure 9.6 tppts = triphenylphosphinetrisulfonate). This process has the advantage of easy removal of the catalyst, resulting in less contamination of the product. 

For the rhodium-catalyzed hydroformylation of propylene in an aqueous biphasic system. Cents et al. have shown that the accurate knowledge of the mass transfer parameters in the gas-liquid-liquid system is necessary to predict and optimize the production rate [180]. Choudhari et al. enhanced the reaction rate by a factor of 10-50 by using promoter Ugands for the hydroformylation of 1-octene in a biphasic aqueous system [175]. 

They are immiscible with some organic solvents, e.g. alkanes, and, hence, can be used in two-phase systems. Similarly, lipophilic ionic liquids can be used in aqueous biphasic systems. 

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. 

Laboratory-scale studies indicate that the aqueous biphasic process is well suited to the recovery of ultrafine, refractory material from soils containing significant amounts of sUt and clay. The main advantages of the aqueous biphasic system in treatment of uranium-contaminated soils are that the process achieves a high removal rate for the uranium contaminant and that such removal is highly selective. Laboratory studies indicate that approximately 99% of the soil is recovered in the clean fraction. 

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. 

The OATS concept was tested on the catalytic hydroformylation of 1-octene, a hydrophobic substrate. This reaction was selected because it has previously been shown to be inactive for traditional aqueous biphasic systems (18). The catalyst used was a Rh/TPPTS complex, an industrial water soluble catalyst (22). The application of the OATS concept increased catalytic efficiency by a factor of 65 (TOP increased from 5 h for biphasic to 325 h for monophasic). 

Organic extractants can be used to complex metal ions and to increase lipophilic-ity. The traditional metal extractants l-(2-pyridylazo)naphthol (PAN) and l-(2-thia-zolyl)-2-naphthol (TAN) have been used in polymer-based aqueous biphasic systems [43] and traditional solvent extraction systems [44]. These are conventional metal extractants widely used in solvent extraction appHcations. When the aqueous phase is basic, both molecules are ionized, yet they quantitatively partition into [HMIM][PF6] over the pH range 1-13. The distribution ratios for Fe, Co, and Cd (Figure 3.3-4) show that the coordinating and complexing abihties of the extractants are dependent on pH and that metal ions can be extracted from the... 

Figure 3.3-9 pH-switchable partitioning of the ionic dye thymol blue in [BMIMKPFd ( ),[HMIM][PFe](H), [OMIM][PFe] (A)/aqueous biphasic systems. From reference [6]. 

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. 

Here E is the solute excess molar refractivity, S is the solute dipolarity/ polarizability A and B are the overall or summation hydrogen-bond acidity and basicity, respectively and V is the McGowan characteristic volume lower-case letters stand for respective coefficients which are characteristic of the solvent, c is the constant. By help of sfafisfical methods like the principal component analysis and nonlinear mapping, the authors determined the mathematical distance (i.e., measure of dissimilarify) from an IL fo seven conventional solvents immiscible with water. It appears that the closest to the IL conventional solvent is 1-octanol. Even more close to IL is an aqueous biphasic system based on PEG-200 and ammonium sulfate (and even closer are ethylene glycol and trifluoroethanol, as calculated for hypofhefical water-solvenf sysfems involving fhese solvenfs). 

Abraham, M.H., Zissimos, A.M., Huddleston, J.G., Willauer, H.D., Rogers, R.D., Acree, W.E., Some novel liquid partitioning systems Water-ionic liquids and aqueous biphasic systems, Ind. Eng. Chem. Res., 42, 413-418,2003. 

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