11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

Other factors in selecting asuitable catalyst that we should consider include the cost andstructure of the catalyst, the toxicity of the catalyst and solvent, the ease of separation ofthe catalyst from the products, the energy requirement for reaction, the stability of thecatalyst in process conditions, and the ease of treatment of the waste streams, in order tolead to an efficient and economic PTC process.3. Interfacial CharacteristicsSince PTC reactions are carried out between immiscible phases, the nature of the interfaceand the physical properties of the reacting compounds at the interface become veryimportant in promoting the desired reaction rate at asatisfactory level. In aliquid–liquidsystem under agitation, one phase should be dispersed as small droplets in the secondphase in amanner such that alarge interfacial area between the two phases can beobtained. The nature of the interface includes interfacial tension, the presence of surfactants,and the degree of agitation rate [3]. These three factors determine the sharpness ofthe interface (or the thickness of interfacial film), the droplet size, and the interfacialarea available to transfer the reacting anion. The interfacial behaviors of the reactinganion include the surface equilibrium distribution of the active intermediate, the ease ofpenetration of the compounds into the other phase (the depth from the interface), andthe mass transfer rate across the interface. Adding extra salts may induce achange in theproperties of the interface. For example, by adding more inorganic salts or bases, thecatalyst is salted out of the aqueous phase and an organic solvent with low polarity, andthe interfacial film grows increasingly thick, finally becoming a separate observablephase. This situation alters the original reaction zone and the apparent reaction ratebecause the properties of the interface have been changed. Hence, the thickness of theinterfacial film (sharpness) is not only limited by the nature of the interface itself, butalso affected by the introduction of other ingredients. Figure 2shows the scheme of aconcentration gradient of the reacting compound within a dispersed organic dropletunder a slow or fast diffusion rate, which indicates that the organic reaction is conductedat the interface or in the whole droplet.4. Reaction at the Interface and in the Bulk SolutionIn PT catalysis, the reaction mechanisms that have been proposed are the Starks’ extractionmechanism and Mąkosza’s interfacial mechanism. These two mechanisms describethe zone where the organic reaction occurs or the phase where the rate-determining step islocated. However, in reality, it is realized that many PTC reactions are conducted both atthe interface and in the bulk solution, especially for a reaction controlled by the intrinsicorganic reaction [3]. The distinction between these two mechanisms is recognized as thedifference in the depth of the reaction zone penetrating the organic phase.Under the conditions of no agitation with a flat interface or slow agitation with aslow mass transfer rate, as the solubility of the transferred species in the organic phase issufficiently large, the rate of diffusion within the organic phase would not influence theobserved reaction rate significantly. Fast diffusion rates may exhibit extraction mechanismbehavior, while with a slow diffusion rate the system is suitably described by an interfacialmechanism. In other words, for the case of strong agitation with extreme low solubility oftransferred species in the organic phase, the reaction should be mainly conducted near theinterface due to the short penetration depth of the transferred species, and so is describedby the interfacial mechanism. It is noted that, in general, increasing the agitation rateincreases the degree of dispersion of one phase and produces more tiny droplets, which inCopyright © 2003 by Taylor & Francis Group, LLC