11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

transfer of organic or aqueous reactant between solidand liquid phase decrease. However,theapparent first-order reaction-rate constant isincreasedby decreasingtheconcentrationof organic reactant [165,181].Elemental analysis is studied by means of energy-dispersive X-ray spectrometer(EDS) methods. Ahigh Cl peak was detected due to the active site. Some chemicalcompounds (Si, Ca) added in the procedure of synthesizing the polymer resin were alsodetected. Although the pretreatment of the resin was conducted by washing with water,NaOH solution, and acetone, the salts (Si, Ca) used as reaction agents by the suspensionmethod were slightly retained in the resin. Alow Cl peak was detected due to the activesite. The peak height for the Cl atom was decreased and was increased for the Oatombetween, before, and after the reaction. This finding demonstrates that the phenoxide ionexchanged the chloride ion as counterion on the polymer-supported catalyst during thecourse of the reaction, and did not, however, occupy all the active sites in the catalyst.Hence, the result reveals that the mass transfer resistance of the ion-exchange step influencedthe concentration of anion on the active site.The volume and wet porosity of catalyst was increased about three times when thecatalyst imbibed the organic solvent and water. Different catalyst interacts differentlywith the organic phase and aqueous phase. Wu and Lee [166] indicated that the imbibedamount of organic solvent was larger than that of water because the catalyst supportwas lipophilic. The imbibed amount of water was dependent of the amount of ammoniumcation (i.e., active site). Hence, the imbibed amount of water increases withincreasing number of ring substitutions. If the structure of the resin is rigid (higherdegree of cross-linkage) or of larger particle size, the organic and aqueous phasesremains quiescent in the interior of the resin. The organic and aqueous reactants shouldnot diffuse simultaneously to the active site. The reaction occurs at a shell near thesurface of the resin. When the degree of cross-linkage of the resin is low, the structure ofthe resin is not solid. The flow rate of the organic and aqueous solutions in the interiorof the resin increases with increasing agitation rate. The number of the effective activesites in the resin is increased.WuandLee[166]indicatedthatthefree chlorideionsontheactivesite(measured byVolhard analysis) were at only 50–70% of the amount of immobilized content (measuredby element analysis). The results of the Volhard analysis method determined the freechloride ions in the bulk solution measured by the AgNO 3 titration method. Their resultsimpliedthat theactivesiteintheresincouldnotreactcompletely withtheorganic reactantin durating the triphase reaction. According to the experimental results, this reaction is atwo-zone model (or shell–core model). The reaction occurs in ashell zone, and does notoccur in acore zone. The triphasic reaction mechanism and the swollen type of resin areshowninFig.5.Thismechanismcanofferusanunderstandingofthereactionphenomenain triphase reactions.IV.SOLID–LIQUID PHASE TRANSFER CATALYSISThe function of solid–liquid phase transfer catalysis (SLPTC) is to conduct the reaction ofa solid salt and the organic reactant using a PT catalyst that is easily dissolved in theorganic phase in the absence of water. These catalysts can be tertiary amines, quaternaryammonium salts, diamines, crown ethers and cryptands, among which crown ethers, act asthe catalysts because of their specific molecular structures [183–186]. Starks et al. [183]indicated that 100% of the yield of product benzyl acetate was obtained at 258C in 2 h forCopyright © 2003 by Taylor & Francis Group, LLC