Solid-scCO2 Systems

Most reactions that have been investigated using PTC in supercritical fluids have been solid-SCF systems, not liquid-SCF. The first published example of PTC in an SCF is the displacement reaction of benzyl chloride 1 with potassium bromide in supercritical carbon dioxide (SCCO2) with 5 mol % acetone, in the presence of tetraheptylammonium bromide (THAB) [19-20] (Scheme 4.10-1) to yield benzyl bromide 2. The effects on reaction rate of traditional PTC parameters, such as agitation, catalyst type, temperature, pressure, and catalyst concentration were investigated. The experimental technique is described below. PTC appeared to occur between an SCF phase and a solid salt phase, and in the absence of a catalyst the reaction did not occur. With an excess of inorganic salt, the reaction was shown to follow pseudo-first order kinetics. 

The use of SCCO2 either alone [135-137] or in multi-phase systems [138-140] offers interesting process alternatives to other methods of catalyst immobilization such as heterogenization on solid supports [126-128], aqueous... 

In addition to this, related areas such as liquid CO2 and C02-expanded solvents should not be overlooked. Many additives and complex modifiers are being used to facilitate reactions in SCCO2 and perhaps the use of a small amount of organic solvent (perhaps from a bio-feedstock) could be justified in order to reduce the cost of a process and therefore lead to its uptake by industry. In addition to this, continued research into biphasic systems C02-water, C02-ionic liquids, CO2-PEG/surfactants and CC -solids (including heterogeneous catalysts) is needed to deliver pure products and reduced cost to future end-users of this technology. 

Gunnlaugsdottir et al. (151) studied the phase behavior of the reaction system cod liver oil + ethanol + immobilized lipase in SCCO2 using a sapphire view cell. They concluded that at 9-24 MPa and 313 K, the reaction system was comprised of three phases solid (enzyme)-liquid-vapor throughout the reaction (151). 

The water activity of pre-equilibrated substrates and enzymes may change when they are put in contact with the SCF for example, SCCO2 may take up water from the enzyme. Furthermore, if water is consumed or produced in the reaction, the water activity of the reaction system will change. Putting salt hydrates in contact with the reaction mixture should buffer the water activity. Suitable pairs of solid salt hydrates may be used to control the water activity more accurately. Examples of salt hydrates and saturated solutions are given in Table 4.9-5. 

A continuously stirred tank reactor (CSTR) would probably be the reactor vessel of choice for most liquid-scC02 systems, to ensure that efficient mixing of the phases occurs. Apart from this, the setup is likely to be similar to that employed for solid-C02 applications with regard to product isolation. A typical reactor setup for laboratory-scale exploratory explorations is depicted in Figure 6. Examples of continuous-flow systems using IL-SCCO2 mixtures using apparatus of this type... 

When the catalyst is either solid or liquid and insoluble in SCCO2, simplification of catalyst separation is expected. The merits of the procedure compared with liq-uid/solid or liquid/liquid biphasic systems are the relatively high mass transfer ability, no need for solvent removal and less catalyst leaching owing to the relatively low solubilizing power of SCCO2. 

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