SCFs and Ionic Liquids

David J. Cole-Hamilton, Thulani E. Kunene, and Paul B. Webb 

The environmentally acceptable nature of supercritical carbon dioxide has already been discussed in this chapter, but another type of solvent which is generating great interest as a possible replacement for volatile organic compoimds is ionic hquids. A detailed account of ionic liquids and their application for catalyst immobilization is the subject of Chapter 5, edited by H. Olivier, in this handbook. In essence, ionic Hquids are involatile, of low toxicity, and very stable, and are therefore seen as having a low environmental impact. The very different properties of supercritical carbon dioxide and ionic liquids makes them ideally suited for use in combination to provide an environmentally acceptable form of two-phase catalysis, which might be carried out as a continuous-flow process. 

Interaction of Supercritical Carbon Dioxide with Ionic Liquids 

The shapes of the liquid-phase composition curves are seen to differ significantly. The N-methylimidazole curve is concave and typical of many organic liquid-C02 systems [6]. This concave correlation implies the presence of a mixture critical 

CO2 dissolution is seen to be largely dependent on the degree offluorination in the anion, following the general trend [BMIM][PFg] [BMIM][BF4] [BMIMJfNOj] [5]. 

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Often a cosolvent is used in order to solubilize particularly polar substrates such as sugars and amino acids. Surfactants or additional solvents may also allow adequate solvation of enzymes. In some cases two-phase systems can be used to conduct bioconversion. For example, Reetz and coworkers employed both SCFs and ionic liquids in a semi-continuous... 

The Mizoroki-Heck reaction is usually performed in polar solvents, and salt additives such as tetrabutylammonium chloride have been shown to activate and stabihze the catalytically active palladium species [19]. Furthermore, the reactions in ionic hquids perform differently in terms of thermodynamic and kinetic properties of the reaction system. Additionally, ionic liquids allow a facile recovery of catalyst and substrates, as well as an easy product separation. Here, another beneficial effect might be used by combination of solvent mixtures for example, of ionic liquids and SCFs. SCFs and ionic liquids have a mixing gap which allows working in two-phase systems, and results in a straightforward phase separation [20]. 

Nevertheless, alternative or nontraditional methods are interesting fields for new applications, due to the need for more selective synthetic and environmentally friendly methods, including the aspect of reaction media (solvent) and energy transfer into a reaction system. The use of supercritical fluids (SCFs) or ionic liquids (IL) as process solvents is well established. From the view point of green chemistry or sustainable chemisfiy, it is SCFs, especially supercritical carbon dioxide (SCCO2), that are the main focus [2, 3]. It is currently under discussion whether ioiuc liquids as alternative process solvents are beneficial. In general, more fundamental problems, like recovery, reuse, purification and ecology (eco-toxicity), have to be solved. 

It would appear that water is a remarkable solvent for Diels-Alder reactions giving both rate and selectivity enhancements. There are now many examples of successful reactions being carried out in this solvent. However, water cannot be used for all reactions. Perfluorinated solvents have also been found to give beneficial rate enhancements over organic solvents as have ionic liquids. Interestingly, both ionic liquids and SCFs can be used to tune the selectivities of these reactions, ionic liquids by varying the solvent used and SCFs by altering the density of the solvent. 

A continuous-flow method for asymmetric catalysis in an SCF/IL system was reported by Leitner s group (144), with the hydrovinylation of styrene [Eq. (31)] as the test reaction. The SCCO2 solution of styrene and ethylene was continuously bubbled up through a column of ionic liquid containing the catalyst. The enantio selectivity was found to be high (in one of the ILs) and catalyst stability was enhanced due to the fact that there was a constant concentration of substrate in the system the catalyst was unstable in ILs in the absence of the olefins. 

Several research groups have reported ionic liquid-containing microanul-sions, but literature on the ionic liquid-in-SCF microemulsion is limited. It includes ionic liquids, 1,1,3,3-tetramethylguanidinium acetate, 1,1,3,3-telra-methylguanidinium lactate, and 1,1,3,3-tetramethylguanidinium trifluoroace-tate in SCCO2 with the aid of surfactant A-ethyl perfluorooctylsulfonamide [40]. 

Reacting lipophilic substrates with hydrophilic compounds, as in the case of most transesteriflcation reactions, is one of the major difficulties in lipase-catalyzed reactions. Several parameters need to be considered to overcome this immiscibility problem. One commonly proposed strategy is the use of a nonaqueous medium. In this chapter, the advantages of using nonaqueous media in biochemical synthesis reactions, over aqueous and solvent-free systems, are discussed. The use of hydrophobic solvents is also discussed, followed by a presentation of the alternatives that can overcome the limitations of solvents. The focus of this chapter is mainly on the use of supercritical fluids (SCFs) as a green alternative reaction medium. The chapter also discusses ionic liquids (ILs) as another alternative. These solvents and the factors affecting their physical properties and their effect on the activity and stability of lipase are also discussed. 

In a continuous flow hydroformyiation with a rhodium phosphite catalyst in an SCF-ionic liquid mixture, after two to three runs the activity and regioselectivity... 

Supercritical fluid (SCF) solvents are unique in that their densities can be varied continuously from gas-like to liquid-like values simply by varying the thermodynamic conditions. Because many of a fluid s solvating properties are strongly dependent on the fluid density, such large changes in density can have dramatic effects on solute reactivity [1,2]. For example, at low pressures supercritical water supports homolytic, free radical reactions, whereas at higher pressures, heterolytic, ionic reactions dominate [3,4]. Thus, thermodynamic control of SCF solvent densities promises to enable us to control reaction outcome and selectively produce desired products. 

As the reduced density is raised, competing formation of 2-naphthol and methanol from methoxy naphthalene, and catechol and methanol from guaiacol becomes apparent polycondensates are suppressed (e.g. at pr(H20) 1.36), charring products are reduced to 6% w/w equivalent of consumed naphthyl. This parallel hydrolysis pathway has been demonstrated also for other hetero-atom-containing organics [67]. The selectivity of hydrolysis increases with pressure and with the addition of salts to increase the polarity of the fluid [6]. Penninger and Kolmschate [65] used the secondary salt effect to test the mechanism of the hydrolytic reaction addition of 1.01% w/w sodium chloride enhanced rates and conversions to hydrolysis products compared with the water-only control at equal density. Protonic catalysis (cf. hydrolysis in liquid water) was also demonstrated, suggesting that the hydrolysis mechanism in SCF water is ionic. 

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