Biphasic systems fluid phases

In suspension polymerization the reaction mixture consists of a liquid-liquid biphasic system, one phase (typically water) of which is inert while the other phase is monomer. With intense mixing of the reactor, small droplets of monomer form in the aqueous phase. The added initiator is soluble in the monomer phase and starts the polymerization in the monomer droplets that are fully surrounded by the water, which acts as heat transfer fluid. The process results in spherical polymer beads of the size of the original monomer droplet if coagulation of the monomer droplets during mixing and polymerization can be avoided. In many suspension polymerization processes dispersants are added to achieve this. Important technical polymers produced by suspension polymerization are PVC and PMMA. In addition to these, spherical ion-exchange resins (typically polystyrene based) are also produced by suspension polymerization. 

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). 

Figure 2.4 Fluid phases for biphasic chemistry -----indicates that a biphasic system may...

Keywords Carbon dioxide Polyethylene glycol Phase behavior Biphasic solvent system Supercritical fluids Phase equilibrium... 

There are many cases in which other techniques have been applied to biphasic systems in order to establish the nature of mixing. For example, fluorescence microscopy of DPPC monolayers containing 2% of a fluorescent probe have shown the coexistence of solid and fluid phases of DPPC at intermediate pressures (Weis, 1991). Similar results have been achieved with a variety of other phospholipids using the same technique (Vaz et al., 1989). The recent application of laser light scattering to this area (Street et al., unpublished data) has yet to produce any conclusive evidence, but the future for this particular technique is also promising. It also provides information about the viscoelastic properties of the monolayer and how these are affected by the inclusion of penetration enhancers. 

Since the first edition of this book a great number of articles have been published in which the different techniques to separate the catalyst from the products via two liquid phases were applied. Some general review articles in books and journals about multiphase homogeneous catalysis, catalyst recycling and fluid-fluid systems have been published [96-102]. Other review articles concentrate on aqueous organometallic chemistry and catalysis [103-108], on biphasic systems with ionic liquids [109, 110], or on fluorous solvents [111, 112] (cf. Sections 7.2, 7.3). 

Some further studies still deal with the Friedel-Crafts acylation in fluorous fluids. These fluids all have very unusual properties such as high density and high stability, low solvent strength and extremely low solubility in water and organic compounds, and, finally, nonflammability. These properties allow their easy handling and reuse. Friedel-Crafts acylation of electron-rich aromatic substrates can be very efficiently performed in a fluorous biphasic system (FBS), which represents a benign technique for phase separation, and catalyst immobilization and recycling. 

It was observed in certain experiments that even though particle synthesis was initiated in a clear one-phase microemulsion, the fluid phase became unstable during the reaction and phase separation occurred [81]. The continuing nucleation and growth of silica in the resulting biphase system resulted in a bimodal size distribution. With the aid of phase diagrams of temperature versus weight percent aqueous phase and temperature versus the ethanol/H20 mole ratio, the phase separation was traced to microemulsion destabilization via H2O depletion and ethanol release [81]. 

General aspects of fluid-fluid reactions are discussed in detail in Section 2.4 within the context of homogeneous catalytic reactions in biphasic systems. Mostly, the reaction takes place only in one phase and the reactant must be transferred from the nom-eactant phase, for example, the gas phase to the reaction phase. In consequence, the mass transfer between the different phases plays an important role on the overall kinetics and may strongly influence the... 

A third approach (Figure 6.14.9c) uses organometaDic catalysts in sohd modifications, for example, covalently bonded (see above) or immobilized in supported liquid form (see below). Here the reaction remains a sohd/supercritical fluid biphasic system during reaction and again the SCCO2 is used as the mobile phase to contact the reactant intimately with the immobilized organometallic complex. 

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

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