Work-up procedures for product isolation

Product isolation from a surfactant-based organised reaction medium can usually not be performed by the standard work-up procedures used for reactions in conventional media. Simple distillation or extraction will result in unacceptable contamination of the product by the surfactant. This can be avoided by using procedures based on the thermodynamic properties of the microheterogeneous systems. 

Microemulsions based on non-ionic surfactants of alcohol ethoxylate type are sensitive to temperature changes and those based on ionic surfactants are sensitive to variations in the electrolyte concentration. Such variations may cause a one-phase microemulsion to form a two- or a three-phase system in which a microemulsion phase coexists with one or two excess phases. As a work-up approach the concept is particularly useful for microemulsions based on non-ionic surfactants because the transitions obtained by temperature variations are reversible. 

Whereas the surfactant will always reside in the microemulsion phase, the product is likely to partition into an excess oil phase if it is an apolar substance and into an excess water phase if it is a polar compound. The principle is illustrated in Fig. 5.14 for hydrolysis of a lipophilic ester in a Winsor I system (an oil-in-water microemulsion in equilibrium with excess oil) followed by transition into a Winsor III system [60]. The ester partitions between the excess oil phase and the oil droplets and the hydroxyl ions reside in the continuous water domain of the microemulsion. The reaction takes place at the interface. After completed reaction, acid is added to protonate the alkanoate formed and the temperature is raised so that a Winsor I to Winsor III transition occurs. The lipophilic 

In order to avoid a pronounced shift of the phase behaviour with the progress of the reaction one reactant can be fed to the reactor in a semi-batch mode. With this concept shift in phase behaviour may be compensated and the optimal state of the system is maintained during the whole reaction time. After phase separation at the most suitable temperature conventional product isolation follows. This sequence of process steps is also possible in a continuously operating process, as illustrated in Fig. 5.17 for the synthesis of 1-phenoxyoctane in a microemulsion stabilised by Triton X-100 [28], 

The product mixture that leaves the reactor (1) is cooled down so that the 2 region is formed. After phase separation unreacted 1-bromoctane is separated from the product by rectification (3). The aqueous phase is mixed with fresh bromooctane (4) and heated to generate phase inversion into the 2 region. The aqueous phase containing the by-product NaBr is released from the process (5) while the unpolar phase is fed into the reactor, where the reactant sodium phenoxide is added sequentially at different inlets of this chamber reactor. 

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