11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

interface, (3) bulkiness of the quaternary salt, and (4) sharpness of the interface. Starksindicated that three of the more important factors affecting the amount of interfacial areainclude interfacial tension, the presence of surfactants, and the degree of stirring or agitation.The interfacial area under steady-state stirring conditions will increase with decreasinginterfacial tension. The chemical natures of the organic and the aqueous phasesdetermine the interfacial tension that exists between these two phases. The quaternarysalt present in the reaction mixture may lower interfacial tension because of its surfactantproperties. Elegant ESCA studies [132,133] suggest that if the anion is highly hydrated itwill not be tightly bound at the interface with the quaternary cation, but rather tend to bemore dispersed in solution, removed from the interface.Reuben and Sjoberg [134] indicated that all boundaries are difficult to cross: political,legal, and geographical boundaries, and also phase boundaries in chemical systems.The interfacial mechanism is the most widely accepted mechanism for PTC reactions in thepresence of a baseInterfacial tension is an important property in the process design of liquid–liquidprocesses. The decrement of interfacial tension between both phases leads to an increasedinterfacial area [135]. Because the volumetric rate of extraction was found to be dependenton the interfacial area, interfacial tension data are useful in understanding the effect ofinterfacial area on the volumetric rate of extraction and overall reaction rates for a PTcatalyzedreaction. Dutta and Patil [136] reported that the effect on the interfacial tensionof the water/toluene system has been studied in the presence of four PT catalysts, i.e.,tricaprylmethyl ammonium chloride, hexadecyltrimethyl ammonium chloride, hexadecytrimethylammonium bromide, and hexadecyltributyl phosphonium bromide. Thedecrease in interfacial tension by surfactants increases the interfacial contact area, enhancingthe volumetric rate of extraction.Juang and Liu [74,75] presented that the interfacial tensions between water/n-hexaneand water/toluene in the synthesis of ether–ester compounds by PTC could be measured.These two-phase systems contained PT catalyst, an aqueous phase reactant, and/or alkali.The interfacial data could be well described by the Gibbs adsorption equation coupledwith the Langmuir monolayer isotherm.III.LIQUID–SOLID–LIQUID PHASE TRANSFER CATALYSISLLPTC is the most widely synthesized method for solving the problem of the mutualinsolubility of nonpolar and ionic compounds [27–31]. Two compounds in immisciblephases are able to react because of the PT catalyst. However, processes using a twophasePT-catalytic reaction always encounter the separation problem of purifying thefinal product from the catalyst. Regen [137] first used a solid-phase catalyst [triphasecatalyst (TC) or polymer-support catalyst], in which a tertiary amine was immobilizedon a polymer support, in the reaction of an organic reactant and an aqueous reactant.From the industrial application point of view, the supported catalyst can be easily separatedfrom the final product and the unreacted reactants simply by filtration or centrifugation.In addition, either the plug flow reactor (PFR) or the continuous stirred tank reactor(CSTR) can be used to carry out the reaction. The most synthetic methods used for triphasecatalysis were studied by Regen and Beese [137–141] and Tomoi and coworkers [142–146].Another advantage of triphase catalysis is that it can be easily adapted to continuousprocesses [147–149]. Therefore, triphase catalysis possesses high potential in industrialscaleapplications for synthesizing organic chemicals from two immiscible reactants.Copyright © 2003 by Taylor & Francis Group, LLC