Liquids processing, with supercritical fluids

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

Interestingly, many hyperthermophiles also are highly salt tolerant. This is an adaptation to life involving aqueous systems that evolve with high-pressure liquid/vapor and supercritical fluid-phase separation of hydrother-mally heated seawater. Both psychrophiles and hyperthermophiles have large numbers of species that also require heavy-metal tolerance, due to the concentration of heavy metals by the thermodynamic phase-separation processes operative in both very cold and very hot aqueous systems (Breezee et al. 2004 Kaye and Baross 2002 Summit and Baross 1998). 

Below the critical temperature, a phase transition occurs when compressing a gas. The formation of a liquid phase is usually first noted by the formation of droplets on the walls of the container. At temperatures above the critical temperature, a substance can be continuously compressed without a separate liquid phase forming. Under such conditions, the substance is a gas, because it continues to fill its container. However, because densities comparable to those of the liquid can be reached by such compression, it is customary to call a substance above its critical temperature a supercritical fluid, where the term fluid (from flow) refers to either liquid or gas. Supercritical fluids, with densities comparable to liquid and high thermal energy, can be exceedingly good solvents and have found use recently in processes such as decaffeination of coffee. 

Separation processes with supercritical gases, called supercritical fluid extraction (SFE), is a group of separation processes that applies supercritical fluids (SCFs) as separating agents in the same way as other separation processes, such as liquid-liquid extraction or absorption, make use of liquid solvents. In these processes, the solvent is a supercritical component or a supercritical mixture of components [1-3]. 

Following the logic of this tree, the multiphase processes on the left-hand side belong among the operations with immobilized catalysts but on liquid supports . The topics of this book are the processes with the liquid supports water, supercritical fluids, ionic liquids, organic liquids, soluble polymers, and fluorous liquids among these, only two processes (Ruhrchemie/Rhone-Poulenc and Shell SHOP) are operative industrially so far. The more important leaves of the family tree are shaded in gray. 

Table I. lists a few possible advantages for conducting chemical synthesis in a supercritical environment. For example, supercritical fluids might provide a means to manipulate reaction environments by altering density and temperature to influence both reaction rate and selectivity. Futhermore, in a homogeneous supercritical phase, one can, in principle, eliminate interfacial transport limitations. Effectively, temperature and pressure are used to alter density in a way to influence both solvation dynamics and equilibrium solubility. With supercritical fluid solvents, there is the possibility of integrating both reaction and separation processes, which could lead to economic advantages over conventional synthetic processes carried out in liquid solvents.

---