Introduction

The solvents discussed here provide simple and efficient vehicles for conducting reactions and separations. This is accomplished by exposing the solvent system to an external physical or chemical stimulus which results in a dramatic change in its physical and chemical properties. Tunable solvents are defined as solvents that change properties continuously upon application of an external stimulus. Supercritrical fluids (SFC), (Dillow et al. [1], Brown et al. [2], Thompson et al. [3], Brown et al. [4], Nolen et al. [5], Eckert et al. [6], Koch et al. [7], Furstner et al. [8], Solinas et al. [9], Maayan et al. [10]) nearcritical liquids (Eckert et al. [6], Chandler et al. [11], Chandler et al. [12], Lesutis et al. [13], Patrick et al. [14], Nolen et al. [15]) and gas-expanded liquids (GXLs) (Jessop and Subramaniam [16], Ablan et al. [17], Jessop et al. [18]) are examples of tunable solvents. It has been demonstrated that these solvents systems are useful for coupling reactions and separations. (Dillow et al. [1], Eckert et al. [6], Daintree et al. [19], Eckert et al. [20], Eckert and Chamber [21], Eckert et al. [22, 23], Jessop [24], Ramsey et al. [25], Rayner [26], Rezaei et al. [27], Tang et al. [28]) In contrast, switchable solvents are solvents that change physical properties abruptly. In other words, they can be switched "on" and "off'. This unique property is a consequence of a reversible reaction (i.e. addition/elimination reactions) in response to an external stimulus such as adjusting temperature and/or the addition or removal of a gas. Because of the reversibility of the reaction, the changed solvent can be brought back to its original state.

Figure 1: Solvent power and transport ability of various solvents.

Figure 1 relates qualitatively the solvent power (in terms of the Kamlet-Taft polarizability/dipolarity, n*) to transport ability (in terms of diffusion coefficient, DA) for the relevant solvents. Most liquids are strong solvents but they have low diffusion coefficients (about four to six orders of magnitude smaller than those for gases, Hines and Maddox [29], Bird et al. [30]) which may lead to mass transfer limitations. In contrast, gases have far better transport properties but are

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much weaker solvents. Supercritical fluids such as CO2 are much stronger solvents than gaseous CO2 and have much high diffusion coefficients than liquids. Gas-expanded liquids (GXLs), mixtures of organics and dissolved gases, are stronger solvents than supercritical fluids and have better diffusion coefficients than liquids. For any given process, the choice of solvent system depends on the physical and chemical properties of the reactants and products, the processing requirements, and environmental considerations.

It is important to develop reaction processes in a holistic manner. Just having a high yield reaction is not enough. The ease of separation of the desired product from the reaction system is also of paramount importance. As a consequence, our focus is placed on solvent systems that combine the benefits of homogeneous reactions and heterogeneous separations. Homogeneous reactions are usually superior to heterogeneous reactions in terms of reaction rates, process control, and selectivity. However, the difficulty of separating and recycling the catalyst limits economic applications of homogeneous catalysis. Heterogeneous catalysis is widely used industrially since it has a built-in separation of the product from the catalyst. The alternative solvent systems combine homogeneous reactions with heterogeneous separations. Herein, we discuss applications benefiting from the use of tunable and switchable solvents.

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