11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

11Interfacial Mechanism and Kinetics ofPhase-Transfer CatalysisHUNG-MING YANG National Chung Hsing University, Taichung, Taiwan,Republic of ChinaHO-SHING WU Yuan-Ze University, Taoyuan, Taiwan, Republic of ChinaI. INTRODUCTIONA. General ConsiderationsAs the chemical reactants reside in immiscible phases, phase transfer (PT) catalysts havethe ability to carry one of the reactants as a highly active species for penetrating theinterface, into the other phase where the reaction takes place, and to give a high conversionand selectivity for the desired product under mild reaction conditions. This type ofreaction was termed ‘‘phase-transfer catalysis’’ (PTC) by Starks in 1971 [1]. Since then,numerous efforts have been devoted to the investigation of the applications, reactionmechanisms, and kinetics of PTC. Nowadays, PTC becomes an important choice inorganic synthesis and is widely applied in the manufacturing processes of specialty chemicals,such as pharmaceuticals, dyes, perfumes, additives for lubricants, pesticides, andmonomers for polymer synthesis. The global usage of PT catalysts was estimated at overone million pounds in 1996, and PTC in industrial utilization is continuously growing atan annual rate of 10–20% [2]. PTC is a very effective tool in many types of reactions, e.g.,alkylation, oxidation, reduction, addition, hydrolysis, etherification, esterification, carbene,and chiral reactions [2,3].1. Reaction Cycle of PTCThe first reaction scheme addressed by Starks in 1971 was for the reaction of aqueoussodium cyanide and organic 1-chloro-octane. In contrast with the result of no apparentreaction occurring after more than 24 h in the absence of catalyst, the cyanide displacementreaction takes place rapidly with only 1% of the quaternary ammonium salt(C 6 H 13 Þ 4 N þ Cl added, and achieving near 100% yield of 1-cyano-octane product in2–3 h [1]. The reaction scheme for the PT catalyzed cyanide displacement reaction inaqueous–organic phases is shown in the following:NaCN ðaqÞþ1-C 8 H 17 Cl ðorgÞ QCl! 1-C 8H 17 CN ðorgÞþNaCl ðaqÞ ð1ÞCopyright © 2003 by Taylor & Francis Group, LLC

ð2ÞThe PT catalyst QCl should first react with the cyanide anion to form the activeintermediate QCN, which is then transferred into the organic phase to react with theorganic reactant 1-C 8 H 17 Cl and is then regenerated back to QCl to conduct the nextcycle of reactions.2. Classification of PTC ReactionsPTC reactions can be classified into two types: soluble PTC and insoluble PTC. Each typecan be further divided into several categories. Figure 1 shows the classification of PTCreactions. Insoluble PTC consists of liquid–solid–liquid PTC (LSLPTC) and tri-liquidPTC (TLPTC), by which the catalyst can be recovered and reused, showing the greatpotential in large-scale production for industry. The catalyst used in LSLPTC is immobilizedon an organic or inorganic support, while in TLPTC it is concentrated within aviscous layer located between the organic and aqueous phases. Soluble PTC includesliquid–liquid PTC (LLPTC), solid–liquid PTC (SLPTC) and gas–liquid PTC (GLPTC).There are also nontypical PTC reactions termed inverse PTC (IPTC) and reverse PTC(RPTC), and these are different in catalyst type and transfer route, compared to normalPTC [2,3].PT catalysts commonly used are quaternary onium salts (ammonium and phosphonium),crown ethers, cryptands, and polyethylene glycols. The essential characteristics of aPT catalyst are that the catalyst must have the ability to transfer the reactive anion into theorganic phase to conduct the nucleophilic attack on the organic substrate, and effect acation–anion bonding loose enough to allow a high reaction rate in the organic phase.FIG. 1Classification of PTC reactions.Copyright © 2003 by Taylor & Francis Group, LLC

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

FIG. 2 Concentration gradient in organic droplet: (a) slow diffusion rate (or low solubility); (b) fastdiffusion rate (or high solubility).turn generates a much larger interfacial surface area for transport. Hence, the mass transferrate between the phases, the diffusion rate, and the solubility in the organic phase (orthe distribution equilibrium) incorporated with the intrinsic organic reaction play importantroles in determining whether the PT reaction is dominated by an extraction mechanismor by an interfacial mechanism.In the following sections of this chapter, the interfacial mechanism and the kineticsconcerning LLPTC, LSLPTC, SLPTC, and TLPTC will be reviewed.B. Some ApplicationsThe vast literature on PT catalysis has demonstrated in past years the very broad andeffective applications in organic synthesis [2,3]. Hundreds of articles are published per yearconcerning PTC. Hence, we do not intend to review the many uses of PTC that have beenreported, but just the typical later examples for illustration in this chapter.Copyright © 2003 by Taylor & Francis Group, LLC

1. Applications in BiologyOrsini et al. [4] synthesized biologically polyphenolic glycosides via Wittig reactions followedby glucosylation under PT conditions. These compounds include (E)-3-(-d-glucopyranosyloxy)-40 ,5-dihydroxystilbene (resveratrol 3--d-glucoside, piceid), (Z)-2 0 ,3 0 -dihydroxy-3,4,4 0 5-tetramethoxystilbene (combretastatin A-1), ;-dihydro-2 0 ,3 0 -dihydroxy-3,4,40 ,5-tetramethoxystilbene (combretastatin B-1), etc. Under PTC, the glucosylationis stereoselective and gives the best results for yields with benzyltriethylammoniumchloride and aqueous sodium hydroxide. The use of nonaqueous bases in dry solventsleads to a sluggish reaction at room temperature, probably due to the poor solubility ofthe phenolate ion in the solvents. Carrie` re et al. [5] synthesized O-, S-, Se-, and C-glycosidesby PTC. For the synthesis of O-glycosides under liquid–liquid conditions, usingdichloromethane as the organic solvent and aqueous NaOH as the base, the PT catalysttetrabutylammonium hydrogen sulfate is used to avoid the possibility of double halidedisplacement. PTC conditions are successfully applied in the synthesis of - and -naphthols to glycohydrolase substrate 7-hydroxy-4-methylcoumarin, to chromogenic substrateFat Brown B 1 , and to estrone prodrug. In the preparation of thio- and selenoglycosides,having saturated NAHCO 3 and 1 M Na 2 CO 3 as the aqueous base is sufficientwith thiols and selenols, and together with tetrabutylammonium hydrogen sulfate as thecatalyst, and ethyl acetate as the solvent instead of dichloromethane, whereby the sideproducts are produced.Albanese et al. [6] reported the synthesis of 2-substituted 3,4-dihydro-2H-1,4-benzoxazinesby ring opening of glycidols under solid–liquid PTC. They used N-(2-fluorophenyl)toluene-p-sulfonamideas the nitrogen nucleophile by incorporating the aromaticmoiety of benzoxazine as the leaving group, and performed the ring opening by stirring at90 C a heterogeneous mixture of 1,2-epoxy-3-phenoxypropane, sulfonamide, anhydrousK 2 CO 3 , the catalyst BzEt 3 NCl, and dioxane to produce a 95% yield of N-(2-fluorophenyl)-N-(2-hydroxy-propyl)toluene-p-sulfonamideafter 17 h of reaction. This method providesa straightforward and new approach to the synthesis of chiral 2-substituted 3,4-dihydro-2H-1,4-benzoxazines.Asymmetric PTC is an important method in the synthesis of -alkyl and -aminoacids. Belokon et al. [7] reported that the compound (4R,5R)-2,2-dimethyl-;; 0 ; 0 -tetraphenyl-1,3-dioxolane-4,5-dimethanol(TADDOL) was used to catalyze the C-alkylation ofC–H acids with alkyl halides to the asymmetric synthesis of -methyl-substituted -aminoacids under PTC conditions. The alkylations of the substrate C–H acids with benzylbromide or allyl bromide were conducted in dry toluene at ambient temperature withNaH or solid NaOH as base and TADDOL as a chiral promoter. The type of base isimportant in the asymmetric C-alkylation of C–H acids.Lygo et al. [8] investigated the enantioselective synthesis of bis--amino acid estersvia asymmetric PTC. Under liquid–liquid conditions, the target amino acid esters wereobtained with high enantiometric excess ( 95% ee) from the alkylation reaction of benzophenone-derivedglycineimine with an appropriate dibromide. They reported that eitherthe mono- or di-alkylated product could be obtained, depending on the reaction conditions;the monoalkylated product was obtained in good yield with excess dibromide,whereas with stoichiometric quantities of dibromide this led to the dialkylated product.By controlling the stoichiometry of the reaction, the selectivity of the desired product canbe accessed at a high level. Lygo et al. [9] also reported the asymmetric synthesis of bis-aminoacids via alkylation of a benzophenone-derived glycineimine under PTC conditions.The target bisamino acids can be produced with high yields and high levels of stereoselectivityby applying chiral quaternary ammonium salts. The core structure of the chiralCopyright © 2003 by Taylor & Francis Group, LLC

quaternary ammonium salts closely related to the cinchona alkaloid cinchonine can beused in the benzoylation of a glycineimine [10].The indan-based -amino acid derivatives can be synthesized by PTC. Kotha andBrahmachary [11] indicated that solid–liquid PTC is an attractive method that offered aneffective way of preparing optically active products by chiral PTC. They found that ethylisocyanoacetate can be easily bisalkylated in the presence of K 2 CO 3 as the base andtetrabutylammonium hydrogen sulfate as the catalyst. The advantage of isolating waterfrom the reaction medium is to avoid the formation of unwanted hydroxy compounds inthe nucleophilic substitution reaction. If liquid–liquid PTC is applied in the system withthe strong base NaOH and dichloromethane as the organic solvent, the formation ofdihydroxy or cyclic ether can be observed.2. Other ApplicationsPTC incorporated with other methods usually greatly enhances the reaction rate. Masstransfer of the catalyst or the complex between different phases is an important effect thatinfluences the reaction rate. If the mass transfer resistance cannot be neglected, animprovement in the mass transfer rate will benefit the overall reaction rate. The applicationof ultrasound to these types of reactions can be very effective. Entezari andKeshavarzi [12] presented the utilization of ultrasound to cause efficient mixing of theliquid–liquid phases for the saponification of castor oil. They used cetyltrimethylammoniumbromide (CTAB), benzyltriethylammonium chloride (BTEAC), and tetrabutylammoniumbromide (TBAB) as the catalysts in aqueous alkaline solution. The more suitablePT catalyst CTAB can accumulate more at the liquid–liquid interface and produces anemulsion with smaller droplet size; this phenomenon makes the system have a high interfacialsurface area, but the degradation of CTAB is more severe than that of BTEAC orTBAB because of more accumulation at the interface of the cavity under ultrasound.Recently, electron-transfer catalysis by viologen compounds has attracted muchattention. The compounds function as mediators of electron transfer and have beenapplied in the reduction of aldehydes, ketones, quinines, azobenzene, acrylonitrile,nitroalkenes, etc., with zinc or sodium dithionite in a monophase or a two-liquid phasesystem [13]. Noguchi et al. [13] found that a redox-active macrocyclic ionene oligomer,cyclobis(paraquat-p-phenylene), acted as an electron phase-transfer catalyst for the reductionof quinines, as compared with acyclic benzyl viologen. The enhanced activity of thiscompound is due to the inclusion of the substrate into the catalyst cavity.One of the important applications of PTC is in the field of pollution control. Anearly utilization was to apply the PTC method to recover phenolic substances from aqueousalkaline waste streams [14]. The methodology is based on the reaction of phenolicsubstances in the aqueous solution with materials such as benzoyl chloride, p-toluenesulfonylchloride, etc., dissolved in the organic solvent in the presence of PT catalysts:ð3ÞCopyright © 2003 by Taylor & Francis Group, LLC

Tundo et al. [15] reported an efficient catalytic detoxification method for toxicpolychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans(PCDFs) under mild conditions (50 C and 1 atm of hydrogen) with a supported metalcatalyst modified by the PT agent Aliquat 336. Their results show that the methodologyproved successful for hydrodechlorinating the toxic samples to yield mixtures containingconcentration of contaminants lower than the experimentally detectable limit by gas chromatography–high-resolutionmass spectrometry. This method has the potential to bepractically applied in the detoxification of PCDDs and PCDFs.PTC is also widely used in polymerization reactions. The main function of thequaternary ammonium salts is that they can transfer the diphenolate from the aqueousphase into the organic phase to react with the diacid chloride. Hodget et al. [16] presentedthe synthesis of polyesters by the reaction of dicarboxylic acid salts with bishalides ortosylates or by the self-condensation of salts of bromocarboxylic acids under liquid–liquidPTC. With benzyltrimethylammonium salts and halides in dry acetonitrile as solvent,using sodium or potassium salts, the yields of polyesters are, in degrees of polymerization(DP), in the range 17–47, and the rate of dissolution of salts is very slow and rate limiting;while in a liquid–liquid system, the DP is in the range 22–161. Liquid–liquid PTC is morefavorable in the synthesis of polyesters [16]:RCOX þ R 0 OH ! RCOOR 0 þ HXRCOO M þ þ R 0 X ! RCOOR 0 þ M þ Xð4Þð5Þwhere X ¼ -Cl, -Br, -I, -OSO 2 CH 3 , or -OSO 2 C 6 H 4 CH 3 .The applications of PTC in polymerization are gradually increasing. Tagle and coworkers[17,18] synthesized poly(amide ester)s from diphenols with the amide group in theside chain, using PT catalysts such as benzyltriethylammonium chloride, with good results.The use of anhydrous potassium carbonate as the base is to promote the organic reactionunder solid–liquid PTC. Albanese et al. [19] described some recent applications in thisarea, and the reactions of aza anions with 2-bromocarboxylic esters and expoxidesafforded protected -amino acids and -amido alcohols. Sirovski [20] described someexamples of PTC applications in organochlorine chemistry. Using a polymeric crownether the results of m-phenoxytoluene chlorination are also reported. Carboxylic acidsand picric acid act as inhibitors, while benzyl alcohol behaves as a strong promoter. Inthe absence of the promoter, the reaction is conducted either at the interface or in the thirdphase that is a border liquid film between the organic and aqueous phases.The importance of triphase catalysis in industry grows continuously. The supportsfor immobilizing the triphase catalyst are mostly of organic type, i.e., copolymers ofpolystyrene. Yadav and Naik [21] reported that clay could be used as support for thePT catalyst; benzoic anhydride was prepared from benzoyl chloride and sodium benzoateusing a clay-supported quaternary ammonium salt at 30 C. The polymer-supportedcatalysts are less active than the clay-supported catalyst for this reaction system.Desikan and Doraiswamy [22] investigated the enhanced activity of polymer-supportedPT catalysts for the esterification of benzyl chloride with aqueous sodium acetate. Theyfound that the reactivity using a triphase catalyst is higher than that using a solubleone. They hypothesized that the enhancement due to increased lipophilicity of thepolymer-supported catalyst was more than compensated by the decreased diffusionalresistance.Jayachandran and Wang [23] prepared a new PT catalyst, 2-benzilidine-N,N,N,N 0 ,N 0 ,N-hexaethylpropane-1,3-diammonium dibromide (Dq-Br), to investigateCopyright © 2003 by Taylor & Francis Group, LLC

the cycloalkylation of phenylacetonitrile (PAN) with an excess of 1,4-dibromobutaneusing aqueous sodium hydroxide as the base, and the following pseudo-first-order kineticswas observed:C 6 H 5 CH 2 CN þ BrðCH 2 Þ 4 Br Dq-Br !ð0:75 mol%Þ C 6 H 5 CðCH 2 Þ 4 CN ð6ÞHwang et al. [24] studied the Wittig reaction of benzyltriphenylphosphonium(BTPP) salts and benzaldehydes via ylide-mediated PTC. They concluded that the reactionof benzylidenetriphenyl phosphorane and the benzaldehyde in the organic phase is thedecisive step for stereoselectivity. The order of effectiveness of substituents isCF 3 > ðCl; BrÞ > MeO > F > NO 2 . Satrio and Doraiswamy [25] proposed a case studyfor the production of benzaldehyde in a possible industrial application of PTC. Thereaction between benzyl chloride and hypochlorite anion isC 6 H 5 CH 2 Cl ðorgÞþOCl ðaqÞ !C 6 H 5 CHO ðorgÞþHCl ðaqÞþCl ð7ÞThey show that the conventional route is the preferred one for a large-scale organicintermediate, and the improvements in merely one or two PTC steps can greatly enhancethe prospects of the PTC route.II.LIQUID–LIQUID PHASE TRANSFER CATALYSISA necessary condition for a reaction is to cause the collision of two reactant molecules. Itis obvious that the reaction rate of two immiscible reactants is low due to their lowsolubilities. A general method for overcoming this difficulty was to employ a protic oran aprotic solvent in order to improve their mutual solubilities. Nevertheless, thisimprovement was not very significant. The problems of two-phase reactions were notsolved until Jarrouse [26] discovered the catalyzing effect of quaternary ammonium saltin the aqueous–organic phase reaction system. PTC is an effective tool for synthesizingorganic chemicals from two immiscible reactants [27–32]. It has been extensively applied tothe synthesis of special organic chemicals by displacement, alkylation, arylation, condensation,elimination, oxidation, reduction, and polymerization. The advantage of PTC inthe synthesis of organic chemicals are fast reaction rate, high selectivity of product, moderateoperating temperature, and applicability to industrial-scale production.A. Mechanism of Liquid–Liquid Phase Transfer Catalysis (LLPTC)Quaternary salts, crown ethers, cryptands, and polyethylene glycol (PEG) are the mostcommon agents used for LLPTC. Over the last few decades, the two reaction mechanismsused to describe the phenomenon of a two-phase PTC reaction were the Starks extractionmechanism and Mąkosza interfacial mechanism.1. Starks Extraction MechanismThis reaction mechanism described by Starks [28,33] is widely accepted for a catalysttransferring between the two phases. Reactions occurring in such systems involve: (1) thereactant reacting with catalyst in the normal phase to form an intermediate catalyticreactant, (2) transfer of the intermediate catalytic reactant from its normal phase intothe reaction phase, (3) transferred intermediate catalytic reactant reacting with untransformedreactant in the reaction phase to produce the product and catalyst, and (4)Copyright © 2003 by Taylor & Francis Group, LLC

transfer of catalyst from the reaction phase to the normal phase. The reaction mechanismcan be separated in three ways based on the reaction path, and can be described asfollows.(a) Normal Liquid–Liquid Phase Transfer Catalysis (N-LLPTC). Traditionally, moreapplications of PTC have been reported in N-LLPTC. The reaction mechanism (8) ismostly applied to alkylation, esterification etherification, and simple displacement reactionsin which a nucleophilic agent is transferred to the organic phase through the solublecatalyst therein:ð8ÞMechanism (8) was first presented by Starks [33] for the reaction of 1-chloro-octane andaqueous sodium cyanide.(b) Inverse Liquid–Liquid Phase Transfer Catalysis (I-LLPTC). The organic reactantis converted, by means of a reagent (e.g., pyridine 1-oxide, PNO) partially soluble inthe organic phase, into a reactive ionic intermediate and transferred into the aqueousphase where reaction takes place to produce the desired product. The processes havebeen termed inverse phase-transfer catalysis [34–36]. The reaction mechanism can beexpressed as follows:ð9ÞThere are several examples where I-LLPTC has been used to synthesize acid anhydrides,by means of a substitution reaction, and ketones from oxidation of alcohols [37–40]. The reaction of an acid chloride (RX) with the carboylate ions (M þ R 0 ) catalyzed byPNO is to proceed through an intermediate 1-(acyloxy)pyridinium chloride formed in theorganic phase. PNO and N,N-dimethylaminopyridine (DMAP) are widely used as inversePT catalysts. The formation of hippuric acid was conducted in the presence of 4-dimethylaminopyridineas inverse PT catalyst [41].(c) Reverse Liquid–Liquid Phase Transfer Catalysis (R-LLPTC). This reactionmechanism was expressed as follows:Copyright © 2003 by Taylor & Francis Group, LLC

ð10ÞThe dehydrohalogenation reactions of alkyl halides take place in the presence ofhydroxide ion and quaternary salts to form alkenes and alkynes [42–44]. The dehydrohalogenationis promoted by hydroxide ion. In general, two reaction conditions conducted inthis system were with highly lipophilic ammonium cation and 50% aqueous sodiumhydroxide. The reaction between 4-nitrobezenediazonium chloride and N-ethylcarbazolein aqueous media was accelerated by using a water–dichloromethane system containingsodium 4-dodecylbenzenesulfonate as a transfer catalyst for the diazonium ion [34].2. Mąkosza Interfacial MechanismThis reaction mechanism described by Mąkosza and Bialecka [45,46] is the acceptedcatalyst transport between the two phases. Reactions occurring in such systems involve:(1) transfer of ionic reactant from its normal phase and catalyst from the reaction phaseinto the interfacial region, (2) the ionic reactant reacting with catalyst in the interfacialregion to form intermediate catalytic reactant, (3) the intermediate catalytic reactanttransfer into the reaction phase to react with untransformed reactant to produce theproduct and catalyst. The reaction mechanism is expressed as follows:ð11ÞUsually, the aqueous salt could be too hydrophilic to allow the quaternary salt todissolve in the organic phase, and resided exclusively in the aqueous phase; anionexchange occured at or near the interface. The mechanism is applied to carbanion reactions,carbene reactions, condensation of polymerization, and C-alkylation of activemethylene compounds such as activated benzylic nitriles, activated hydrocarbons, andactivated ketones under PTC=OH . In most cases, the reaction involves the Q þ OHcomplex because QOH is highly hydrophilic and has extremely low solubility in theorganic phase.A mechanism can also be applied when the quaternary salt is too lipophilic todissolve in the aqueous phase, and resides exclusively in the organic phase, anion exchangeoccuring at or near the interface. This parallel mechanism is called the Bra¨ ndstro¨ m–Montanari mechanism. The ion-exchange reaction existing at the interface was verifiedby Landini et al. [47] and Bra¨ ndstro¨ m [48].Copyright © 2003 by Taylor & Francis Group, LLC

A summary of characteristic kinetic criteria to distinguish between the operation ofthe extraction and interfacial mechanisms has been suggested [28,49]. The extractionmechanism is characterized by: (1) increased rates with increased lipophilicity of catalyst,(2) reaction rates that are independent of stirring speed above a certain value, (3) firstorderor fractional dependence of reaction rate on catalyst concentration, and (4) pseudofirstor second-order kinetics if the reaction in the organic phase reaction is rate controllingor zero-order kinetics if diffusion across the interface is rate controlling.The interfacial mechanism is characterized by: (1) increased rates with increasedelectrostaticity of catalyst, (2) reaction rates are dependent on agitation rate, (3) fractionalkinetic order with respect to the catalyst concentration, and (4) the value of substrateacidity pK a is in the range 16–23.B. Kinetics of a Liquid–Liquid Phase Transfer Catalysis1. Starks Extraction MechanismA typical LLPTC cycle involves a nucleophilic substitution reaction, as shown in Eq. (8).A difficult problem in the kinetics of PT-catalyzed reactions is to sort out the rate effectsdue to equilibrium anion-transfer mechanism for transfer of anions from the aqueous tothe organic phase. The reactivity of the reaction by PTC is controlled by the rate ofreaction in the organic phase, the rate of reaction in the aqueous phase, and the masstransfer steps between the organic and aqueous phases [27–29]. In general, one assumesthat the resistances of mass transfer and of chemical reaction in the aqueous phase can beneglected for a slow reaction in the organic phase by LLPTC.Although a large number of papers have been published on the synthetic applicationsof PTC in the last three decades, little mathematical analysis of the phenomenon hasbeen done, and such an analysis is especially desirable in a large-scale application. Evansand Palmer [50] considered a process of interphase mass transfer and chemical reaction.Melville and Goddard [51] and Melville and Yortsos [52] presented an analysis of masstransfer in solid–liquid PTC. Chen et al. [53] derived algebraic expressions for the interphaseflux of QY and QX. The reaction parameters were estimated from experimental datausing a two-stage method of optimal parameters. Wang and Chang [54–56] studied thekinetics of the allylation of phenoxide with allyl chloride in the presence of PEG asLLPTC. A simple mathematical model describing the liquid–liquid PT-catalyzed reactionwith the two-film theory was analyzed [57–59]. The results of the model’s prediction areconsistent with experimental data. Such mathematical analysis appears desirable andneeded in view of the widespread interest in PTC in the chemical industry in whichtwo-phase transfer and triphase catalysis are the most common industrial processes.The reactivity in phase-transfer catalysis is controlled by: (1) the reaction rate in theorganic phase, (2) the mass transfer steps between the organic and aqueous phases, and (3)the distribution equilibrium of the quaternary salts between the two phases. The distributionof quaternary salts between two phases directly affects the entire system reactivity[60–62]. On the basis of the experimental data and earlier literature [27,28,63], a generalizedapproach describing a LLPTC reaction system uses a pseudo-first-order reaction. Therate expression is written asd½RXŠ¼ kdt int ½QYŠ½RXŠ¼ k app ½RXŠð12Þð13ÞCopyright © 2003 by Taylor & Francis Group, LLC

The fixed value of k app is called the apparent first-order reaction-rate constant. The overbardenotes the species in the organic phase. The reaction rate linearly increases withincreasing QY concentration. Equation (13) is established when the QY concentration isconstant. Most observed reaction rate would follow the pseudo-first-order kinetics for anexcess amount of aqueous reactant to that of organic reactant [37]. Wu [64] indicated thata pseudo-first-order hypothesis can be used to describe the PTC experiment data, eventhough the QY concentration is not kept constant. Wang and Wu [58] developed a comprehensivemodel in a sequential phosphazene reaction. Their experimental results wereconsistent with a first-order reaction rate; the pseudo-first-order reaction-rate constantwas not linearly related to the concentration of the catalyst, because the mass transferof catalyst between the two phases influenced the reaction. Wang and Yang [57,65] andWu [63] indicated that the QY concentration is constant over time when the molar ratio ofnucleophile to catalyst is larger than unity. Therefore, in the general case, the QY concentrationcannot vary with time only when the ion-exchange rate in the aqueous phase ismore rapid than that in the organic phase [66], no mass transfer resistance of catalystbetween the two phases occurs, the molar ratio of nucleophile to catalyst is larger thanunity, and the ionic strength in the aqueous phase is high [67].The complicated nature of the LLPTC reaction system is attributed to two masstransfer steps and two reaction steps in the organic and aqueous phases. The equilibriumpartition of the catalysts between the two phases also affects the reaction rate. On the basisof the above factors and the steady-state two-film theory [60,63,64,68], a phase-planemodel to describe the dynamics of a liquid–liquid PTC reaction has been derived. Thismodel offers physically meaningful parameters that demonstrate the complicated reactivecharacter of a liquid–liquid PT-catalyzed reaction. However, when the concentration ofaqueous solution is dilute or the reactivity of aqueous reactant is weak, the onium cationhas to exist in the aqueous phase. The mathematical model cannot describe this completely.When the onium cation exists in the aqueous phase, several important phenomenainvolved in the liquid–liquid reaction need to be analyzed and discussed.ð14ÞOn the basis of Eq. (12), and mechanism (14) [64,68], the species balance equationswere solved by eliminating the time variable (phase-plane model). The relevant rate equationsaredy od ¼ y 1oy ody 1ody o¼ P 1 QYy oym 1aQY 1y 1oð15Þð16ÞCopyright © 2003 by Taylor & Francis Group, LLC

dy 1ady o¼ QYy ody 2ody o¼ QXy ody 2a¼ dy 2o y 1o y oym 1a QY 1 1 y 3a y 4a yþ 1ay 1o K d1 y 1o y 1o y 1o y oy 2o ym 2ay QX P1o y 11oy 2a 2 y 3a y 5ak d2 y 1o y o QXy oy 2oy 1om QXy 2ay 1ody 3ady o¼ 2y 3a y 5aK d2 y 1o y oþ 1y 3a y 4aK d1 y 1o y o 1y 1ay 1o y o 2y 2ay 1o y oð17Þð18Þð19Þð20ÞThe mass balances for Q i ,Y , and Xare given below:1 ¼ y 1o þ y 1a þ y 2o þ y 2a þ y 3a ð21Þy 4a ¼ P 2 y 1a y 1o þðy o 1ÞP 1 ð22Þy 5a ¼ P 3 þ 1 y 2a y 2o þð1 y o ÞP 1 ð23Þin which the dimensionless variables and parameters are defined asy o ¼ ½RXŠ o; y½RXŠ 1o ¼ V o½QYŠ oiy 3a ¼ V a½Q þ Š aQ iQ i; y 4a ¼ V a½Y Š aQ i QY ¼ K QYA=V o; Pk o Q i =V 1 ¼ V 0½RXŠ ioQ i; y 1a ¼ V a½QYŠ aQ i; y 2o ¼ V o½QXŠ o; yQ 2a ¼ V a½QXŠ a;iQ i; y 5a ¼ V a½X Š a; Q QX ¼ K QXA=V a;ik o Q i =V o; P 2 ¼ V a½MYŠ i; PQ 3 ¼ V a½MXŠ i;iQ i 1 ¼ k d1k o; 2 ¼ k d2k o; ¼ V oV a; ¼ tk oQ iV oð24Þand ½MXŠ i , ½MYŠ i , and ½RXŠ i represent the initial concentrations of reactants MX, MY,and RX, respectively. By introducing the values of the parameters into Eqs (15)–(23), thedynamic phenomena of a liquid–liquid PT-catalytic reaction was obtained.Wang and Yang [57] reported that the ion-exchange reaction-rate constant wascalculated with three differential equations as below for the dynamics of QY in boththe aqueous and organic phases in a two-phase reaction without adding the organicreactant by the numerical shooting method and correlating it with the experimental data.d C QYdtdC QYdt¼ K QY AC QY¼ K da C QY C QXC QY =m QYK QY A VV C QYC QY =m QYð25Þð26ÞdC MYdt¼ K da C QY C QX ð27ÞThe intrinsic reaction-rate constant in the organic phase is obtained by reacting QYwith RX in a homogeneous solvent and using Eq. (12). According to the literature, Wangand Yang [57] and Wu and Meng [69] have found the intrinsic reaction-rate constant fromtheir systems. The equilibrium constant and mass transfer constant of the catalyst betweentwo phases obtained are discussed in the next section.Copyright © 2003 by Taylor & Francis Group, LLC

Wu [64] characterized the transfer of Q þ X from the organic phase to the aqueousphase and of Q þ Y from the aqueous to the organic phase by definingQY ¼ y 1am QYy 1o; QX ¼ y 2oy 2a m QXð28ÞIf the PT catalysts in the two phases are in extractive equilibrium and the mass transferresistance can be neglected completely, then QY and QX are each equal to 1.The dynamics for a slow PT reaction and a mass transfer controlled instantaneousreaction were studied. Wu [63] and Wu and Meng [69] indicated that the pseudo-steadystateLLPTC model could describe the complicated nature of the LLPTC reaction. Therate equation from the report of Wu [63] is expressed asd½RXŠdtk½RXŠQ¼1 = Vm QY þ 1m þ Da ð29ÞQYDaQY m þ Da QY QX þð1 þ m QX Þ QY þ 1m þ þ QYwhere Da QY ð¼ k½RXŠ=k QY A= VÞ and Da QX ð¼ k½RXÞ=K QX A= VÞ are the Damkohlernumbers for QY and QX, respectively; ð¼ k 2 ½MXŠ=k 2 ½MYŠÞ is the reaction ratio ofthe aqueous reverse reaction to the forward reaction for ion exchange; and ð¼ k½RXŠ=k 2 ½MYŠÞ is the reaction ratio of the organic phase to the aqueous forward ionexchangereaction.Wu [63] also derived an expression for the catalyst effectiveness, which is defined asthe ratio of the actual reaction rate to that with all the catalyst present as QY, in terms ofseven physically meaningful dimensionless parameters: ¼ m QY þ 1þ Da QY Da QY þ 11þ Dam QY m QX þ 1 þ m QX þ ð30ÞQY m QYBefore evaluating Eq. [30], the parameters of kinetics, mass transfer, and thermodynamicequilibrium must be established. The aim of this work is to evaluate the equilibrium andextraction of a quaternary salt in an organic solvent/aqueous solution. The studies ondistribution equilibrium of the quaternary salts enable one to clarify the true mechanismthrough which the reactant anion is transferred.Models for LLPTC get even more complicated for special cases, e.g., reactions inboth aqueous and organic phases, systems involving a base reaction, or other complexseries–parallel multiple reactions. Wang and Wu [58] and Wu and Meng [69] studied thekinetics and mass transfer for a sequential reaction using LLPTC that involved a complexreaction with six sequential S N 2 reactions in the organic phase along with interphase masstransfer and ion exchange in the aqueous phase.Wang and Wu [70] analyzed the extraction equilibrium of the effects of catalyst,solvent, NaOH/organic substrate ratio, and temperature on the consecutive reactionbetween 2,2,2-trifluoroethanol with hexachlorocyclotriphosphazene in the presence ofaqueous NaOH. Wu and Meng [69] reported the reaction between phenol with hexachlorocyclotriphosphazene.They first obtained the intrinsic reaction-rate constant and overallmass transfer coefficient simultaneously, and reported that the mass transfer resistance ofQX from the organic to aqueous phase is larger than that of QY from the aqueous toorganic phase. The intrinsic reaction-rate constant and overall mass transfer coefficientswere obtained in three ways.Copyright © 2003 by Taylor & Francis Group, LLC

(a) Pseudo-Steady-State LLPTC model.The reaction relationship is given as1k app¼V K QX A þ V kQ ið31Þwhere denotes the reactivity of the phosphazene reaction. The plot of 1=k app versus , inwhich the data were measured at the initial time of different experimental runs, allows oneto obtain the mass transfer coefficient, K QX A, and the intrinsic reaction rate constant k,from the slope and intercept of the straight line.(b) Extrapolation Method. If mass transfer resistance influences the reaction, the concentrationof the active catalyst QY cannot remain constant during the course of thereaction. Decreasing the concentration of organic reactant RX increases the apparentfirst-order reaction-rate constant. When the concentration of organic reactant decreases,both the reaction rate and the effect of mass transfer decrease. If the organic reactantconcentration extrapolates to zero ð½RXŠ !0Þ, the effect of mass transfer can beneglected. The intrinsic reaction-rate constant, k, is easily evaluated.(c) Half-Reaction in the Organic Phase. The organic reactant reacted with an intermediatecatalyst, tetra-n-butyl ammonium phenolate, in a homogeneous organic phase.The intrinsic reaction-rate constant was calculated from Eq. (12).Another LLPTC is usually performed in an agitated system, in which the organicphase is mostly dispersed. Several efforts have been made in developing the theory for atwo-liquid phase with chemical reactions. For an organic phase being the dispersed phase,several phenomena take place: (1) formation of a single droplet in the continuous phase bystirring, (2) free rise or fall of a droplet through the continuous phase, and (3) coalescenceof a droplet at the end of the free-rise period. During the extraction of a catalytic intermediate,mass transfer from the bulk aqueous phase to the organic droplet surface influencesthe rate of PT reaction. Yang [71,72] studied the general analysis of the dynamics ofa PT-catalytic reaction in a dispersed system of liquid–liquid phases, considering theirreversible and reversible reactions by solving the finite difference and Runge–Kuttafourth-order methods. The rates of change of RX, RY, QX, and QY in an organic dropletare described by the instantaneous equations of diffusion and reaction with the correspondinginitial and boundary conditions as follows:@ C i@t ¼ D i @r 2 @ C iþ @r @r iR; i ¼ RX; RY; QX; and QY ð32Þr 2where i is the stoichiometric coefficient of the i component.The kinetics of inverse PT-catalytic extraction of species into the water phase wascarried out with partially water-soluble pyridines or derivatives [36,38,40,59,73], as shownin mechanism (9). These reactions can be described by a pseudo-first-order hypothesis[38,40]:k app ¼ k h þ k c ½PNOŠ ið33ÞHowever, so far, the detailed kinetics of I-LLPTC are unclear.As mentioned above, the various approaches to LLPTC modeling have been taken,and a comprehensive general model for N-LLPTC reactions is widely held. However, akinetic model for I-LLPTC and R-LLPTC reactions is yet to be developed.Copyright © 2003 by Taylor & Francis Group, LLC

2. Mąkosza Interfacial MechanismThe interfacial mechanism is the most widely accepted mechanism for PTC reactions in thepresence of a base. However, although there are numerous industrially important applications,very few kinetic studies or mathematical models for this mechanism are reported. Ingeneral, the mechanism is also described by a pseudo-first-order hypothesis.Juang and Liu [74,75] proposed and discussed a possible mechanism based on amixed Mąkosza and modified interfacial mechanism. The reaction rate for the etherificationof a substituted phenylacetic acid by PTC was measured using a constant interfacialarea cell, and expressed asR f ¼ k½R 0 XŠ 1=3 ½RHŠ½QXŠ½OH Š 5=21 þ k a ½QXŠ 1=2 ½OH Šþk b ½RHŠ 1=2 ½OH Šð34ÞC. Thermodynamic Equilibrium in LLPTCQuaternary salts are generally used as normal liquid–liquid PT catalysts. In general, thefunctional groups of the quaternary cation will affect the dissolution of the catalyst in theorganic phase. Further, the phase transfer of the anion will also affect the reaction rate inthe two-phase reaction. Therefore, a proper choice of PT catalyst is very important inpromoting the reaction rate. Unfortunately, a universal guideline is unavailable for selectingthe proper PT catalyst to enhance the reaction. The reactivity in PTC is controlled by:(1) the reaction rate in the organic phase, (2) the mass transfer steps between the organicand aqueous phases, and (3) the distribution equilibrium of the quaternary salts betweenthe two phases. The distribution of quaternary salts between two phases directly affects theentire system reactivity [60–62].In general, anion transfer and anion activation are the important steps involved intransferring anions from the aqueous phase to the organic phase where the reaction takesplace. Factors affecting the extraction ability of the anion from the aqueous to organicphase include cation–anion interaction energies, the ionic strength in the aqueous phase,ion-pair hydration, the lipophilicity of the catalyst, and the polarity of the organic phase.The extraction behavior and distribution coefficients of quaternary salts in various mediahave also been investigated [76–86].Bra¨ ndstro¨ m [48] indicated that the distribution of quaternary salt between two(liquid–liquid) phases exists as complicated multiequilibrium constants, which dependon the structure of the anion, cation, and solvent, as well as on pH, ionic strength, andconcentrations in the aqueous solution. Such equilibrium properties have not yet beenevaluated completely. The relationship between quaternary salt and extraction constant isan important consideration for PTC work.The distribution coefficient of quaternary cation D Q was obtained by measuring theconcentrations of quaternary cation (Q) in the organic and aqueous phases, respectively.The distribution coefficient is highly dependent on the nature and concentration of thequaternary salts:D Q ¼ ½QŠ obs½QŠ obsð35ÞThe distribution coefficient of quaternary cations between both the phases not onlyprovides information on the phases to facilitate the modeling of the two-phase transfercatalysis system, but it can also give a criterion for evaluating the suitability of the catalyst.Copyright © 2003 by Taylor & Francis Group, LLC

The order of magnitude of D Q for quaternary salts is Aliquat 336 > TBA-TBPO >TBAI > TBPB > TBAB > TBAC. The sequence of D Q for solvents is CHCl 3 >CH 2 Cl 2 > 1;2-C 2 H 2 Cl 2 > C 6 H 5 Cl. The order of influence on the extraction capabilityof quaternary salts is Br 3 C 6 H 2 O > I > Br < Cl and P þ > N þ for the anion andcentral cation, respectively. Reasons for these behaviors have been discussed in previouswork [48,76,81,85,86]. The D Q value increased on increasing the temperature.The true extraction constants of quaternary salts QX corresponding to their infinitelydilute solutions in a two-phase system were calculated using the following equation:a QXEQX T ¼a Qþ; a X½QXŠ¼½Q þ Š½X Š2ð36Þwhere a and 2 are the activity and the mean ionic activity coefficient of the quaternarysalts, respectively.The distribution constant of quaternary salt at equilibrium between two phases ism ¼ ½Qþ X Š½Q þ X Šð37ÞThe dissolved Q þ Xin the aqueous and organic phase may dissociate toQ þ X Ð Q þ þ X ð38ÞQ þ X Ð Q þ þ X ð39ÞThus, the dissociation constants K da and K da of QX in the aqueous and organic phases arewritten asK da ¼ ½Qþ Š½X Š 2 ½Q þ X ŠK do ¼ ½Qþ Š½X Š 2 ½Q þ X Šð40Þð41ÞThe dissociation constant in aprotic organic solvents can be derived from fundamentalprinciples based on Bjerrum’s theory for ion pairs. In most organic media, thedissociation constant of ion pairs is very low (of the order of around 10 5 ) [48].Bra¨ ndstro¨ m [87] reported that the ionic aggregation states of quaternary salts existingin the organic phase were of various types, i.e., dissociated ions (Q þ þ X ), ion pairs(Q þ X ), quadruples ½ðQ þ X Þ 2 Š, etc. Hence, the total concentration of quaternary salt inan organic phase can be written asC Q ¼½Q þ Šþ½QXŠþ2½Q 2 X 2 Šþð42ÞSince the organic system is in electrical neutrality,½Q þ Š¼½X ŠEquation (42) can be transformed intoð43ÞC Q ¼ E T1=2Q þ ½Q þ Š½X Š 1=2þETQX 2 ½Q þ Š½X Š þ 2E T Q 2 X 22 4 ½Q þ Š½X Š 2þ ð44Þwhere EQ T þ, ET QX, and EQ T 2 X 2are the concentration quotients represented asCopyright © 2003 by Taylor & Francis Group, LLC

E T Q ¼ ½Qþ Š½X Š2 þ½Q þ Š½X Š2EQX T ½QXŠ¼½Q þ Š½X Š2E T Q 2 X 2¼ ½Q 2X 2 Š½Q þ Š½X Š 2 ð45Þð46Þð47ÞBy using Eqs (42)–(47), the values of E T Q þ, ET QX, EQ T 2 X 2, and the distribution constantm are evaluated. Corrections for the mean activity coefficient in the organic phase weremade using the Marshall and Grunwald expression, and the values of m, K da , K do , and were calculated by a numerical iteration method. Beronius and Bra¨ ndstro¨ m [91] evenclarified the identical value of K do at ½QXŠ ¼0 within the limits of experimental errorand the conductance measurement. In view of past reports [87–92], most K da valueswere located in the range between 1 and 10; K do values were located in the range between10 1 and 10 5 . The dissociation ability of quaternary salt in the aqueous phase is greaterthan that in the organic phase.The quaternary salts QX can be completely dissociated to free ions (Q þ and X )inthe aqueous phase (, ½Q þ Š=½QXŠ > 100Þ and partially dissociated in the organic phasewhen the concentration of the quaternary salt is 0.0125 kmol/m 3 . The quaternary salts QXcan be partially dissociated to free ions in the aqueous and the organic phases when theconcentration of quaternary salt is 0:1 kmol=m 3 . The incremental rules of the dissociationdegree of the quaternary salts were obtained as follows: (1) increasing the charge-tovolumeratio of the central cation or counteranion (e.g., P þ > N þ or I > Br > Cl ),(2) increasing the electron-releasing groups on the quaternary cation (e.g., Aliquat336 > TBAC), and (3) increasing the electron-withdrawing groups on the quaternaryanion (e.g., TBA-TBPO > TBA-BPO > TBAC). Electron-releasing (or electron-withdrawing)groups apparently make the transition state more stable on the quaternarycation (or anion) while the ion-pair type of quaternary salts transferring through theinterface between two phases is a transition state. Bockries and Reddy [93] reportedthat the association constant decreased when the effective ionic radius of the ion pairwas increased.Quaternary salts in an organic phase must be determined experimentally to knowwhether the salts are dissociated or associated, and, if so, to what degree. The hydration ofthe anion plays an important role in dissociating the catalyst. Furthermore, the solvationof the anions increases the size of the ions, decreases their mobility and diffusion rate, andreduces the reactivity of the reactant. How many molecules of the coextracted water doeseach quaternary salt carry? Hence, the equation for the distribution of a tetralkylammoniumhalide into an organic phase can be written as [94,95]Q þ þ X þjH 2 O Ð Q þ þ X :jH 2 OQ þ þ X þjH 2 O Ð Q þ X :jH 2 Oð48Þð49ÞDepending on whether the species in the organic phase is dissociated as free ions [Eq. (48)]or associated as ion pairs [Eq. (49)], the corresponding equilibrium constants can bewritten asCopyright © 2003 by Taylor & Francis Group, LLC

E T Q þ ;H 2 O ¼ ½Qþ Š½X :jH 2 OŠ 2 ½Q þ Š½X Š½H 2 OŠ j 2 EQX;H T ½QX:jH2 O ¼2 OŠ½Q þ Š½X Š½H 2 OŠ j 2ð50Þð51ÞThe j value can be calculated by dividing ðH 2 OÞ by the amount of quaternary salts in theorganic phase. The water content difference in the organic phase ððH 2 OÞÞ equals thedifference between the measured water content in the solvent and that in the solution atthe same temperature.The order of magnitude of H 2 O in the organic phase for quaternary salts is Aliquat336 > TBA-TBPO > TBAI > TBPB > TBAB > TBAC. The sequence of ½H 2 OŠ for solventsis 1,2-C 2 H 4 Cl 2 > CH 2 Cl 2 > CHCl 3 > C 6 H 5 Cl. This tendency of the sequence of thecoextracted water is identical to that of the solubility of water in the organic phase of 1,2-C 2 H 4 Cl 2 ð1:3Þ > CH 2 Cl 2 ð0:81Þ > CHCl 3 ð0:08Þ > C 6 H 5 Cl ð0:05Þ at 20 C. The orders ofinfluencing extraction capability of H 2 O are Cl > Br 3 C 6 H 2 O > Br > I and N þ > P þfor the anion and central cation, respectively. The trend for water content in the organicphase varied with increasing temperature. Landini et al. [96] indicated that the solvatingcapability between quaternary salt and water could reduce the quaternary salt’s reactivityin the organic phase in a PT-catalyzed reaction. This result was confirmed by previous work[61,76]. Hence, it is significant to study the liquid–liquid PT-catalyzed reaction and toevaluate how many molecules of the coextracted water are carried by each quaternarysalt. The water content in the organic phase increased with increasing temperature. The ½H 2 OŠ value increased when the charge-to-volume ratio of the anion increased and when thepolarity of the solvent increased, but decreased as the lipophilicity of the quaternary saltincreased. These tendencies correspond to those reported by Landini and coworkers[97,98]. Kenjo and Diamond [95] reported that the average water contents in a nitrobenzene/watersystem at 23 C were 3.3, 1.8, and 1 (mol/mol quaternary salt) for Cl ,Br , andI , respectively. Starks and Owens [99] reported that the hydration numbers ofC 16 H 33 Bu 3 P þ X were 0.4, 4, and 5 for NO 3 ,Cl , and CN , respectively. The averagewater content in the organic phase ð½H 2 OŠÞ was about 1–3 mol/mol quaternary salt, exceptfor TBAC. Because the hydration numbers for different anions were different when thequaternary salt was TBA þ [(n-C 4 H 9 Þ 4 N þ Š, the results demonstrate that the water of hydrationis primarily associated with the anion, rather than with the quaternary cation.Quaternary ammonium ions are used as PT catalysts because they are least likely tointerfere in chemical reactions. According to the experimental results of Bra¨ ndstro¨ m [48],Herriott and Picker [100], and Landini et al. [97], the organophilic quaternary cationsserved as more effective PT catalysts than quaternary cations with small alkyl chains.Thus, the incremental number of C atoms surrounding the central atom (e.g., N) of aquaternary salt will increase its lipophilicity, thus raising the extraction constant.However, these researchers did not give the relationship between the extraction constantand the structure of quaternary salts. According to the literature, four relationships forquaternary cations have been reported.1. Gustavii [101] observed a linear relationship between log E QX and n, the numberof C atoms in an ammonium ion. He extracted picrates into methylene chloride usingprimary amines as well as symmetrical secondary and tertiary amines and symmetricalquaternary ammonium salts. The relationships for quaternary ammonium salts islog E Q picrate ¼ 2:0 þ 0:54n.Copyright © 2003 by Taylor & Francis Group, LLC

2. A quantitative parameter for characterizing accessibility was suggested [28]based on the strong dependence of electrostatic interaction on the distance of closestapproach between the cation and anion (which is determined by steric factors). Thisparameter, termed q, is simply the sum of the reciprocals of the length of the linearalkyl chains attached to the central nitrogen of the quaternary cation;q ¼ 1=C# 1 þ 1=C# 2 þ 1=C# 3 þ 1=C# 4 , where C# is the number of carbon atoms in eachof the four alkyl chains in the quaternary cation.3. Fukunaga et al. [102] had presented a correlation function based on hydrophile–lipophile balance (HLB) ideas to assess the efficiency of quaternary salts in the benzene–watersystem in terms of Hildebrand parameters ½Dð QX Þ¼ð QX Þ 2 =ð QX Þ 2 Š where QX , and are, respectively, the solubility parameters of the catalyst, water, and organic solvent.4. Sirovski [103] proposed that the structure–activity relationship for quaternarysalts can be described quantitatively using Hansch -hydrophobicity constants. Theseconstants are defined analogously to Hammett and Taft constants [104]: x ¼ log P x log P H , where P H is the distribution coefficient for the standard compound,and P x is the same from its derivative with the X substituent in the standard 1-octanol–water system, which has low ion selectivity in relation to halide and hydroxide ions.The former two relationships (paragraphs (1) and (2) above) were focused on to accessthe distribution of quaternary cations. The equilibrium property cannot reveal when thetotal carbon number for various quaternary salts is the same. In paragraph 3, theHildebrand parameter cannot be easily obtained for all quaternary salts. Hence, we tookthe results of paragraphs 1–3 and the concept of HLB for the surfactant to show that thedispersal efficiency of surfactant or emulsifier molecules is a function of the relative interactionsof their polar, hydrophilic ‘‘heads’’ with the aqueous phase and of their nonpolar,lipophilic ‘‘tails’’ with the hydrocarbon phase [105,106]. We developed a new model as0:475HLB ¼ qðM T M H Þ þ 9:4 M TBABð52ÞM NXin which 0.475 and 9.4 are hydrophilic group numbers of CH 2 and N, respectively, whichwere defined by Davies [107]. The equation of the HLB was developed in respect of theextraction of quaternary salts between two phases based on molecular weights of hydrophilicand lipophilic groups. A linear relationship between extraction constant and HLBwas observed for ammonium cations. An average decrease in log E QX is about 10:5 2unts per HLB value for various counteranions. The free energies of transfer for ion pairsand dissociated ions were determined and were shown to correspond to the experimentaldata in the literature.It is of interest to determine the crude free energies of phase transfer between organicand aqueous phases for the quaternaries. This is combined with the free energies oftransfer for halide ions to give the free energies for the tetrabutylammonium and tetrabutylphosphoniumions, which are not well established. Do different salts give the samevalues? Tseng [92] reports the free energy of transfer of some anions from water to variouskinds of solvents based on the distribution data for quaternary salts, and evaluates theextraction behavior of quaternary onium salts in order to understand their performance ina PT-catalyzed reaction system.An extensive and self-consistent set of data on free energies of transfer of someinorganic salts has been reported [89]. The free energy of the extraction constant, distributionconstant, and dissociation constant are expressed asG i ¼ RT lnðiÞ; i ¼ E QX ; E T QX; m; K da ; or K do ð53ÞCopyright © 2003 by Taylor & Francis Group, LLC

Gustavii [101] and Bockries and Reddy [93] indicated that the dissociation constantof quaternary salts increased when the dielectric constant of the solvents was increased.Nagata [108] reported that the logarithmic value of the association constant of a quaternarysalt was proportional to the reciprocal of the dielectric constant of the mixed solvent.According to Eq. (53), the dissociation constant decreased slightly with increasing valuesof the reciprocal of the dielectric constant. Parker et al. [109] demonstrated that the freeenergies of transfer are very useful in correlation with the solvent effects on S N 2 effects inPTC. The free energies of transfer for the quaternary salts of dissociated ions from waterto the solvent can be written asG t Q þ þX ¼ RT ln a !Qþ a X¼ RT ln E T a Qþ aQXK do ð54ÞXThe free energies of transfer for free ions from water to the solvent can be written asG t i ¼ RT ln a i; i ¼ Q þ or X ð55Þa iAbraham [88], Czapkiewicz et al. [110], and Taft et al. [111] have reported the freeenergies of transfer of (C n H 2nþ1 Þ 4 NX (n ¼ 1–3) for ion pairs and dissociated ions. The freeenergies of transfer for quaternary salts of ion pairs and dissociated ions from water tofour kinds of organic solvents were determined in these studies. The free energies oftransfer for ion pairs were less than those for dissociated ions, i.e., the transfer abilityof ion pairs was greater than that of dissociated ions. The result of the stronger cation–anion attraction in ion pairs is to reduce significantly the magnitudes of the endoergicsolvent cavity terms, as well as the exoergic anion–solvent attractive terms. The stability ofquaternary salts for ion pairs was greater than that for dissociated ions from water to theorganic phase. The result corresponds to that of Taft et al. [111]. The sequences of freeenergy of transfer for quaternary salts are of three sorts: (1) P þ > N þ , (2)TBPO < I < BPO < Br < Cl , and (3) the long chain of an alkyl group is of lowvalue (Aliquat 336 < TBAC). The stability of ion pairs in dichloromethane (or dissociatedions in chloroform) was the highest among the four kinds of solvents. These results revealthat the incremental charge localization in the anion and decrement in the cation increasesthe stability of quaternary salt in the organic phase.D. Mass Transport in LLPTCUsually, it is recognized that the rate-determining step is controlled by the chemicalreaction in the organic phase under LLPTC conditions. For a fast mass transfer rate ofcatalyst between the two phases, the influence of mass transfer on the reaction can beneglected. In the past, the reaction rate was assumed to be independent of agitation andthe surface area of the interface beyond a minimum stirring rate ( 300 rpm). However,the reaction rates can increase with increased agitation in cases where the transfer rate ofanion between both phases is slower than the organic reaction. The phenomenon of masstransfer of quaternary salt between the two phases has received little attention. The reactivityof the reaction by PTC is controlled by the rates of the organic and aqueous reactions,the partition equilibrium, and the mass transfer steps of the quaternary saltsbetween the organic and aqueous phases [27,28]. The partition equilibrium of quaternaryammonium salts was obtained in our previous work [85,86,92].Copyright © 2003 by Taylor & Francis Group, LLC

The mass transfer rates of catalysts between two phases are difficultly realized due tothe difficult identification of the active catalyst during the reaction [57,112–115]. Masstransfer coupled rapid reactions subjected to LLPTC have been studied extensively[58,63,69,115,116]. Mass transfer rates of catalysts in the reaction of 2,4,6-tribromophenoland tetra-n-butylammonium bromide in a solution of KOH were determined [57,114].Evans and Palmer [50] first consider theoretically the effect of diffusion and masstransfer in two well-mixed bulk phases of uniform composition separated by a uniformstagnant mass transfer layer at the interface. They studied the effect of the Damko¨ hlernumber, organic reaction equilibrium rate constant, reactant feed-rate ratio, flow rate ofthe organic phase, and the organic reaction reactivity on conversion. Chen et al. [53]derived algebraic expressions for the interphase flux of QY and QX. The reaction parameterswere estimated from experimental data using a two-stage method of optimal parameters.Naik and Doraiswamy [117] reported that future research should be directedtowards the use of a membrane module as a combination reactor and separator unitwith the membrane serving not merely to carry out the PT-catalyzed reaction, but alsosimultaneously and selectively to recover the organic product. Stanley and Quinn [118]reported the use of a membrane reactor for performing PT-catalytic reactions andincluded theoretical models and calculations to predict the kinetic behavior of the system.Matson [119] investigated the commercial feasibility of such membrane systems. However,the characterization of hydrodynamic phenomena in PT-catalyzed reactions has not beenattempted.Rushton et al. [120] developed a method for measuring the mass transfer coefficient.However, their method can only be used in systems with unity distribution ratio. Asai etal. [121] measured the liquid–liquid mass transfer coefficients in an agitated vessel with aflat interface. In their later work [122,123] on the alkaline hydrolysis of n-butyl acetate andoxidation of benzyl alcohol in an agitated vessel, the overall reaction rate of PTC withmass transfer at a flat interface was analyzed. The observed overall reaction rate wasconcluded to be proportional to the interfacial concentration of the actual reactant.Wang and Yang [57] investigated the dynamic behavior of PT-catalyzed reactions bydetermining the parameters accounting for mass transfer and the kinetics in a twophasesystem. The film theory was applied to interpret the behavior of PTC. The overallmass transfer coefficients of QX (or QY) from an agitated mixture of QX (or QY) werefirst calculated in known qualities of water and the organic solvent by using a simplecorrelation:"lnC QXþV C # QX1V!mQXC QX;i m QXV C QX;i V þ 1 ¼ K QX At ð56ÞThe overall mass transfer coefficient of QX was obtained by plotting the term on the lefthandside of Eq. (56) versus time. Yang et al. [124] developed a mathematical modelconcerning mass transfer in a single droplet to describe the dispersed phase system.They measured the distribution coefficient and the mass transfer coefficient of a PTcatalytic intermediate between two phases.Also, the diffusion boundary layer resistances on either side of the membrane filter inmembrane transport processes have been extensively examined [125,126]. Most of thesestudies deal with cases wherein solute diffuses across a membrane filter separating twoaqueous phases with different concentrations. However, the individual film mass transfercoefficients in both liquid phases are unavailable.Copyright © 2003 by Taylor & Francis Group, LLC

The mass transfer resistances strongly depend on the nature of the hydrodynamics inthe contacting device and the mode of operation. Many devices have been used to studytwo-phase mass transfer at or near the liquid–liquid interface. Hence, the hydrodynamiccharacteristics of ion transport through a membrane were presented to evaluate the feasibilitythat this permeation system can be calibrated as a standardized liquid–liquidsystem for studying the membrane-moderated PT-catalyzed reaction. The individualmass transfer coefficients and diffusivities for the aqueous phase, organic phase, andmembrane phase were determined and then correlated in terms of the conventional Sh–Re–Sc relationship. The transfer time of quaternary salt across the membrane and thethickness of the hydrodynamic diffusion boundary layer are calculated and then the effectof environmental flow conditions on the rate of membrane permeation can be accuratelyinterpreted [127].The mass transfer of quaternary salt from the organic phase into the aqueous phasethrough a lipophilic membrane is indicated in Fig. 3. Assume that the solute activity in thislipophilic membrane is identical to that in the bulk organic solution, then the mass fluxvalues for the individual species are described byN ¼ k o ½QXŠ ½QXŠ i ð57ÞN ¼ k a ½QXŠ i ½QXŠ ð58ÞN ¼ k m ½QXŠ i ½QXŠ ið59ÞThe asterisk denotes the species in the organic phase and the membrane phase, respectively.The distribution coefficients of quaternary salts between membrane and aqueousphase or organic phase are defined asm ¼ ½QXŠ i½QXŠ ið60Þandm ¼ ½QXŠ i½QXŠ ið61ÞFIG. 3Mass transfer of the catalyst between two phases and membrane.Copyright © 2003 by Taylor & Francis Group, LLC

According to Eqs. (57)–(61), and for organic volume V and interfacial area S, the rate ofchange of solute concentration can be expressed byV d½QXŠ¼ KS dt o m½QXŠ m½QXŠ ð62Þin whichK o ¼ m þ 1 þ m 1ð63Þk a k m k oAccording to the initial extractive concept that the content of quaternary salt is restrictedto less than 10% in the aqueous solution, the quaternary salt is completely dissociated, i.e.,[QX] approaches zero, and the magnitude of the distribution coefficient m is less than 10.By plotting ðV=SÞd½QXŠ=dt against ½QX], the overall mass transfer coefficient K o wasobtained by a least-squares regression. The regression factor is more than 0.99.If the extraction system is conducted in the absence of membrane, Eq. (63) is rewrittenasK o ¼m þ 1 1ð64Þk a k oThe values of diffusivities predicted for quaternary salts in the aqueous phase andthe organic phase are in the following descending order: TBPB > TBAB > TBAI BTBAB and RBAB > TBAI > TBPB > BTBAB; respectively. The diffusivities of quaternarysalts increased with increasing temperature. The effects of solvents on diffusivitiesare ranked in the following descending order: CH 2 Cl 2 > C 6 H 5 CH 3 > CHCl 3 > C 6 H 6 >C 6 H 5 Cl > 1; 2-C 2 H 4 Cl 2 > H 2 O. The main influencing factor may be the viscosity of solvent.The overall mass transfer coefficients were determined by Lin [127]. The values of k o ,k a ,andk m were calculated by a numerical method for four types of quaternary salts inseven kinds of solvents. Assuming that the hydrodynamic characteristics of the diffusionboundary layer in the aqueous phase and the organic phase were similar in the presence orabsence of the membrane system if the agitation rate was kept below 100 rpm, the individualmass transfer coefficient of the membrane could then be calculated by subtractingEq. (64) from Eq. (63). The individual mass transfer coefficients increased with increasingagitation rates and temperatures. The sequence of mass transfer coefficient isk a k o > k m .Kiani et al. [125] and Prasad et al. [126] reported the following equation for theintrinsic mass transfer coefficient in the membrane, k m ¼ D"= m , where " and m are theporosity and thickness of the membrane, respectively, is the tortuosity factor of themembrane defined as the actual pore length divided by the membrane thickness, and Dis the diffusivity of species in the bulk liquid phase. The average tortuosities were calculatedand found to reduce from 4.3 to 2.7 when the agitation rates increased from 90 to 600rpm. Because the individual mass transfer coefficient of a membrane is not a constant andincreases with increasing agitation rate, the tortuosity decreases slightly with increasingagitation rate according to the equation of Kinai et al. [125].If the mixing is so vigorous that the diffusion boundary layer can be eliminated, Eq.(62) can be reduced toVSd½QXŠdt k m m½QXŠ m½QXŠka ;k o !1¼ ð65ÞCopyright © 2003 by Taylor & Francis Group, LLC

The extractive effectiveness factor , defined as the effect of the diffusion boundary layeron the extraction rate of quaternary salt can be characterized in terms of the ratio of Eq.(62) to Eq (65): ¼ K o m½QXŠ m½QXŠ k m m½QXŠ m½QXŠ ð66Þ¼ðBi o þ Bi a þ 1Þ 1in which Bi o ð¼ mk m =k o Þ and Bi a ð¼ mk m =k a Þ are Biot numbers for the organic phase andaqueous phase, respectively. Equation (66) represents the mass transfer ratio of conductionrate to convection rate of the quaternary salt at the interface. According to theexperimental data of Lin [127], the values of , Bi o ,andBi a are calculated to be around0.96, 0.04, and 0.002, respectively, when the agitation rate is lower than 100 rpm. Hence, itclarified again that the membrane resistance at high agitation rates controls the masstransfer resistance of the membrane extraction.Usually, mass transfer coefficients can be correlated from the classical equation:Sh ¼ aRe b Sc cð67Þwhere Shð¼ k m d=DÞ is the Sherwood number; Re (¼ du=Þ is the Reynolds number, Sc(¼ =D) is the Schmidt number, D is the diffusivity in the bulk fluid, u is a characteristicvelocity of the fluid such as the mean fluid flow velocity, is the density, is the viscosity,and d is a characteristic dimension of the system.In Eq. (67), a is an experimental constant and c usually has a value of 1/3 [128–130].The value of b depends on the type of equipment and system, and most of the theoriespredict a one-half power on the Reynolds number [131]. The mass transfer from bulksolution to the surface of the membrane is mainly controlled by the turbulence of the fluidmotion created by stirring. The characteristic velocity is defined in terms of the stirringspeed ðu ¼ ndÞ. The values of a and b were determined from the intercept and slope of theline of Sh=Sc 1=3 against Re for the specified mass transfer coefficients of k a , k o , and K o .These parameters are different and are dependent on the system geometry and flow pattern.However, it can be concluded that the exponent value on Re varied from 0.2 to 1.0,depending on the design of the membrane permeation system.The correlating equation [67] established here can be used to evaluate the masstransfer coefficient and the thickness of the diffusion boundary layer, ð¼ d=shÞ. Thethickness of this layer calculated for an organic solvent and aqueous solution were10 3 –10 2 and 10 9 –10 7 cm, respectively, for the four types of quaternary salts studied.For a solute crossing a mass transfer resistance film, the transfer time can be approximatelyestimated by the following equation [131]:ðfilm thickness)2Transfer time ¼ð68Þdiffusion coefficientBased on the data presented here, the estimated transfer times for a solute crossing theorganic and aqueous mass transfer resistance film are about 1–10 and 10 11 –10 8 s, respectively.E. Interfacial Phenomena in LLPTCStarks [132] proposed that the transfer rate of an anion across the interface is largelygoverned by four factors: (1) interfacial area, (2) anion activity and hydration at theCopyright © 2003 by Taylor & Francis Group, LLC

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

Quaternary onium salts, crown ethers, cryptands, and polyethylene glycol have allbeen immobilized on various kinds of supports, including polymers (most commonlymethylstyrene-co-styrene resin cross-linked with divinylbenzene), alumina, silica gel,clays, and zeolites [137–156]. Because of diffusional limitations and high cost, the industrialapplications of immobilized catalysis (triphase catalysis) are not fully utilized. Thisunfortunate lack of technology for industrial scale-up of triphase catalysis is mainly due toa lack of understanding of the complex interactions between the three phases involved insuch a system. In addition to the support macrostructure, the support microenvironment isalso crucial in triphase catalysis since it determines the interactions of the aqueous and theorganic phases with the PT catalyst immobilized on the support surface [117]. However, todate, few papers have discussed the microenvironment. The effect of the internal molecularstructure of the polymer support, which plays an important role in the imbibed composition,on the reaction rate has seldom been discussed. In addition to the reactivity, for a TCin an organic and aqueous solution the volume swelling, imbibed different solvent ratio,amount of active site, and mechanical structure of the catalyst must be considered. Hence,these complex interactions in the microenvironment must be solved in order to obtain ahigh reactivity of TC.A. Characterization and Mechanism of LSLPTC1. Mechanism of LSLPTCIn general, the reaction mechanism of the fluid–solid reactions involves: (1) mass transferof reactants from the bulk solution to the surface of the catalyst pellet, (2) diffusion ofreactant to the interior of the catalyst pellet (active site) through pores, and (3) intrinsicreaction of reactant with active sites. Triphase catalysis is more complicated than traditionalheterogeneous catalysis, because it involves not merely diffusion of a single gaseousor liquid phase into the solid catalyst. Both organic reactant and aqueous reactant existwithin the pores of the polymer pellet. For step (3), a substitution reaction in the organicphase and an ion-exchange reaction in the aqueous phase occurred. Diffusion of both theaqueous and organic phases within the solid support is important and various mechanismshave been proposed for triphase catalysis. However, each mechanism can only explain asingle reaction system. Naik and Doraiswamy [117] discussed these mechanism in theirreview paper.Tundo and Venturello [155,157] proposed a mechanism for a TC system using silicagel as support to account for the active participation of the gel by adsorption of reagents.Telford et al. [158] suggested an alternation shell model that requires periodical changes inthe liquid phase filling the pores of the catalyst. Schlunt and Chau [150] from the sameresearch group tried to validate this model using a novel cyclic slurry reactor, and indicatedthat only the catalyst in a thin shell near the particle surface was utilized. Tomoi and Ford[142] and Hradil et al. [159] reported that a realistic mechanism involves the collision ofdroplets of the organic phase with solid catalyst particles dispersed in a continuous aqueousphase. Svec’s model [160] for transport of the organic reagent from the bulk phase throughwater to the catalyst particle has been developed in terms of emulsion polymerization.Because the triphase reaction involves not merely diffusion of a single phase into thesolid support, the organic reaction take places in the organic phase, and the ion-exchangereaction occurs in the aqueous phase. The catalyst support is usually lipophilic. Theorganic phase and aqueous phase fill the catalyst pores to form the continuous phaseand the disperse phase, respectively. The interaction between quaternary salts as well asCopyright © 2003 by Taylor & Francis Group, LLC

the organic phase and aqueous phase play a crucial role in promoting the triphase reactionrate. However, this information is unclear.2. Characterization of LSLPTCPoly(styrene-co-chloromethylstyrene) crosslinked with divinylbenzene, which is immobilizedwith quaternary ammonium salts, was investigated for the synthesis of the finechemicals in our previous studies [161–166]. The microenvironment of the polymer supportplayed a crucial role in enhancing the reaction rate. More information about characterizationof the polymer structure, the interaction between organic solvent, resin, andaqueous solution, and the reuse of the catalyst is required to encourage application.Wu and Lee [166] report that 24 kinds of ion-exchange resin were used to clarify thischaracter of the resin, including six kinds of commercial ion-exchange microresin, fivekinds of laboratory-produced macroresin, and 13 kinds of laboratory-produced microresin,using instrumental analysis by TGA, EA, and SEM-EDS, and the reaction method.The densities of active sites in the resin, titrated using the Volhard method for commercialanion exchangers, were higher than those for laboratory-produced resins.ð69ÞScanning electron microscopy (SEM) analyzes electrons that are scattered from thesample’s surface, and monitors the morphological observation of the polymer resin. Theelemental analysis (EA) is effected by means of energy-dispersive X-ray spectrometer(EDS) methods. The chloride density was shown to be well distributed on the resin surfaceby X-ray images of chloride. It was also demonstrated that the active sites (-NR 4 Cl) in theresin were completely dispersed. Some other chemical compounds used for synthesizingthe polymer resin were also detected. Although the pretreatment of the resin was conductedby washing with water, NaOH solution, and acetone, the salts (Al, Si, and Ca) usedas reactants in the suspension method were slightly retained in the resin.The immobilized content of tri-n-butylamine in the resin was determined by theTGA, EA, and Volhard methods. The polymer backbone formed in a one-stage processwhere the decomposing temperature range was 300 –450 C. The immobilized resin (mi4-20) was formed in a two-stage process, where the ranges of decomposing temperature forthe two stages were 160 –200 C and 350 –450 C. Although it is tempting to divide the twostages into two distinctive units, the correlation between quaternary salt content andweight loss in the first was qualitative. The weight loss in the first step is equal to theimmobilized amount of the functional group of -NðC 4 H 9 Þ 3 . The accuracy of the analyticaltechnique was within 10%. The commercial ion-exchange resins were revealed in a threestageprocess. The decomposed compound and temperature for each decomposition stepare: imbibed water ( 100 C), functional group (160 –300 C), and polymer backbone(350 –450 C). The sequence of the imbibed capability of water is: IRA-900 ð20%Þ > A-Copyright © 2003 by Taylor & Francis Group, LLC

26 > Dowex 1 2 > A-27 IRA-410 > IRA-904 > mi4-20 (4%). Most commercial ionexchangeresins are of the hydrophilic functional group type.In addition, the immobilized amount of the functional group of -NðC 4 H 9 Þ 3 in theresin was determined from the mass fraction of nitrogen by EA for C, H, and N, and fromthe chloride ion density titrated by the Volhard method. The sequence of determiningmethod for the immobilized content of tri-n-butylamine in the resin wasTGA > EA > Volhard. The analyzed result of the TGA (or EA) method was based onthe elemental weight, and it revealed the real immobilized content. However, the analyzedresult of the Volhard method determined the free chloride ion in the solution by theAgNO 3 titration method. The immobilized content of tri-n-butylamine in the resin bythe TGA (or EA) method was > 20% larger than that determined by the Volhard method.The immobilized content of tri-n-butylamine in the resin by the TGA (or EA) method wasindependent of the number of cross-linkages, and only dependent of the number of thering substitution.These experimental results demonstrate that tri-n-butylamine could be immobilizedcompletely with the active site on the resin for an immobilizion duration of 6 days.However, the immobilized content of tri-n-butylamine by the Volhard method was dependenton both the number of cross-linkages and the number of ring substitutions. Theimmobilized contents for the Volhard method are about 50–70% that for TGA (orEA). Since the analyzed results of the Volhard method determined the free chlorideions in the solution by the AgNO 3 titration method, the free chloride ion of the activesite were only measured at 50–70% of the amount of immobilized content. The trend ofthe varied content for microresin is larger than that for macroresin. This result indicatesthat the analysis by the Volhard method may be influenced by the diffusion problem, andmay be because the resin did not swell completely in the aqueous solution. On the otherhand, if the resin is used as a TC to react in an actual reaction system, and the resin couldnot swell completely to release all free chloride ions, then the reaction environment wouldbe influenced by the mass transfer of the reactant.As indicated by Ohtani et al. [32] both organic reactant and aqueous reactant existwithin the pores of the polymer pellet. The HLB of the support structure determines thedistribution of the two phases within the catalyst support [167,168]. Therefore, the distributionof the organic reactant and aqueous reactant within the pores of the polymerpellet will directly influence the reaction. The swollen capability of the resin is used toestimate the validity of the resin. The effect factor of the swollen capability of the resinincludes the cross-linkage, the number of ring substitutions (total exchange capability), theelectronic charge and diameter of the counterion, the polarity of the organic solvent, thecomposition of the functional group, the chemical bonding type between both exchangeions, and the electrolyte concentration in the aqueous solution.Wu and Lee [166] and Tang [169] reported the amount of imbibed solvent, volumeratio, and porosity of 12 kinds of ion-exchange resin for seven kinds of solvents (dichloromethane,chloroform, 1,2-dichloroethane, benzene, toluene, chlorobenzene, and water)when 1 g of the resin was placed in 25 mL of the pure solvent. The experimental results forthe commercial ion-exchange resin were as follows:1. The amounts of the imbibed solvent for the aromatic solvents (benzene,toluene, and chlorobenzene) were larger than those for halide aliphatic solvents(dichloromethane, chloroform, and 1,2-dichloroethane) since the resin was ofthe styrene type; the sequence of the imbibed amount for the aromatic solventswas benzene > toluene > chlorobenzene.Copyright © 2003 by Taylor & Francis Group, LLC

2. The imbibed amounts for water and organic solvent were around 1 g and < 1g,respectively.3. The volume ratios were mostly located between 1 and 2.4. The porosities were located between 0.5 and 0; the porosity of most ionexchangeresins was about 0.5 [170].The experimental results for the laboratory-produced resins were as follows:1. The amounts of the imbibed solvent were different, depending on the structureof the resins, and the amount for water was less than that for organic solvent.2. The amounts of imbibed solvent were in the range 0–3g.3. The volume ratios were almost all located between 1 and 3, and decreased withincreasing cross-linkage of the resin.4. The porosities were located between 0.25 and 0.75.The porosity and imbibed amount decreased for the solvents benzene, toluene, andchlorobenzene, and increased for the solvents chloroform, 1,2-dichloroethane, dichloromethane,and water, with increasing number of ring substitutions. These results indicatethat the solubility of water in chloroform, 1,2-dichloroethane, and dichloromethane isgreater than the solubility in benzene, toluene, and chlorobenzene. The imbibed amountfor aromatic solvents was larger than that for halide aliphatic solvents when the number ofring substitutions was small, and the trend was opposite when the number of the ringsubstitutions was large.Because the functional group of the laboratory-produced resin (tetrabutylammoniumchloride) is more lipophilic than that of the commercial ion-exchange resin [tetramethyl-(or ethyl-) ammonium chloride], the amount of imbibed water was larger than thatof the organic solvent for commercial resin; on the other hand, for laboratory-producedresin, the amount of water was less than that of organic solvent. The imbibed amount oforganic solvent for laboratory-produced resin was larger than that for commercial resin.Since the swollen A-27 and IRA-904 was high in the commercial resin in this study, theothers (IRA-900, A26, IRA410, Dowex IX2) were not good for swelling. Hence, they areimproperly used as PT catalysts in an organic phase/aqueous solution reaction system.Tang [169] reported the amount of the imbibed solvent for commercial resin andlaboratory-produced resin in an organic solvent and in an aqueous solution in the presenceof KOH, NaOH, KCl, and NaCl. Four kinds of salts were used to investigate the swellingphenomenon since the KOH and NaOH were usually used as reactants and the chlorideion was a byproduct in the PTC reaction. Chlorobenzene was chosen as solvent because ofits high boiling point. The imbibed amounts of chlorobenzene and water increased for thecommercial resin, and decreased for the laboratory-produced resin when the salt wasadded. The imbibed amounts of chlorobenzene and water for NaOH were less than thatfor KOH, and that for NaCl was also less than that for KCl since the diameter of the aquaion for Na is larger than that for K. The aqua interaction between metal and waterincreased to increase the swelling capability of the resin when the diameter of the aquaion increased. Also, the imbibed amounts of chlorobenzene and water for KCl were lessthan that for KOH, and that for NaCl was also less than that for NaOH since the diameterof the aqua ion for Cl is larger than that for OH. The imbibed amounts of chlorobenzeneand water for microresin was larger than that for macroresin.In general, the reaction rate increases with augmentation of the polarity of thesolvent. The apparent reaction-rate constant increased with a rise in temperature. Thesequence of the reactivity for macroresin was CH 2 Cl 2 > CHCl 3 > C 6 H 5 Cl > C 6 H 6 >Copyright © 2003 by Taylor & Francis Group, LLC

C 6 H 6 > C 6 H 5 CH 3 and the trend for microresin was similar, except for chloroform. Theincrement of the reactivity of this triphase reaction corresponds to the polarity of thesolvent. Dichloromethane has the highest reactivity among the solvents. However, theboiling point of dichloromethane is 39 C, and it is unsuitable for reaction in the highertemperature system. The activity energy varied with the structure of the resin. Mostactivity energy levels for microresins were greater than those for macroresins, except forchloroform.The four functions of a base in a liquid–solid–liquid triphase catalytic reaction werereported [165]: (1) reactant; (2) deprotonation of acidic organic compound to become thereactive form; (3) improving the reactive environment in the catalytic pellets, such asswelling volume, imbibed composition, solubility between two phases, etc; and (4) reducingthe solvation of catalyst and water to upgrade the reactivity of active catalyst in theorganic phase. Wu and Lee [166] showed the effect of base concentration for the reactivityof 4-methoxyphenylacetic acid. The apparent reaction-rate constant was maintainedalmost constant when the NaOH (or KOH) concentration was greater than 1 kmol=m 3 .The increment of the deprotonation of 4-methoxyphenylacetic acid dramatically increasedthe reactivity of the reaction when the base concentration was below 1 kmol=m 3 . When thesalt concentration was increased to change the reactivity environment, the reactivity of thereaction was slightly increased with increasing base concentration. The reactivity for KOHwas greater than that for NaOH. The result corresponded with the imbibed compositionof the resin.The advantage of using a triphase catalytic reaction is that it easily recovers thecatalyst and purifies the product and reactant. Hence, the reuse, stability, and degradationof the catalyst must always be considered. Resins with onium groups may be used forextended periods or repeated cycles only if the catalyzed reactions occur under sufficientlymild conditions to avoid degradation. The degradation of the triphase catalyst may havethree factors: high temperature, strong base, and mechanical degradation. In the pastliterature [143,147,148,166,171–173], the reactivity of the triphase reaction was slightlyinfluenced by the degradation of the catalyst (polymer-supported resin). The active sitewas seen to decrease slightly with increasing base concentration up to 9 kmol=m 3 . Thenumber of active sites remained constant up to 60 C and then decreased dramatically asthe temperature increased. The degradation of the catalyst with temperature is moresensible than that for base concentration.In addition to the diffusion resistance of reactants affected by the particle size, it isalso influenced by the characterization of the polymer pellet, i.e., the degree of crosslinkage.In principle, the cross-linkage is related to the covalent bonds between two ormore linear polymer chains. For this reason, the degree of cross-linkage of the polymerwill affect the pore size and the amount of swelling [142,161]. The structure of the polymeris compact for a higher degree of cross-linkage. The pore size of the pellet is increasedwhen a polymer with a low degree of cross-linkage is swollen in an organic solvent. Thus, alower degree of swelling for a higher cross-linking polymer in an organic solvent is adisadvantage, i.e., a large diffusion resistance is obtained for using a higher degree ofcross-linking of the polymer. Hence, the application of a highly cross-linked polymer islimited because of low reactivity in the triphase catalytic reaction.B. Kinetics and Modeling in LSLPTCThe reaction of triphase catalysis is carried out in a three-phase liquid (organic)–solid(catalyst)–liquid (aqueous) medium. In general, the reaction mechanism of the triphaseCopyright © 2003 by Taylor & Francis Group, LLC

catalysis is: (1) mass transfer of reactants from the bulk solution to the surface of thecatalyst pellet, (2) diffusion of reactants to the interior of the catalyst pellet (active sites)through pores, and (3) surface or intrinsic reaction of reactants with active sites. For step(3), the substitution reaction in the organic phase and ion-exchange reaction in the aqueousphase occurrs.r RX0¼R c V 03V cal k IRXþ V 0kM c c¼ 1 C RX0 ¼ k obs C RX0Wang and Yang [174–176] have proposed a general dynamic model for triphasecatalysis in a batch reactor. The mass transfer of reactants in the bulk aqueous and organicphases, diffusion of reactants within the pores of the solid catalyst particle, and intrinsicreactivities of the ion-exchange and organic reactions at the active sites within the solidcatalyst were investigated. Desikan and Doraiswamy [151] account for the effect of thereversibility of the ion-exchange reaction. The concentration of the catalytic active siteswithin the catalyst is given as@q QX¼ k@t 1 C Y q QX þ k 1 C X q QY þ k 2 C RX q QY ð71ÞMass balances of organic substrate and inorganic species within the catalyst are written as and" @ C RX@t" @C Y@t¼ D RXr 2¼ D Yr 2@ @ C RX@r r2 @r @ @C Y@r r2 @r s k 2C RX q QY s k 1 C Y q RXk 1 q QY C Xrespectively.In a heterogeneous catalytic reaction, the intraparticle effectiveness, c , for a firstorderreaction within a spherical catalyst at steady state is [177] c ¼ 3cothð3Þ 13 2where is the Thiele modulus:sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ R c k 2 M c c3 V cat D eAn apparent overall effectiveness factor of the catalyst is obtained by applying the pseudosteady-stateassumption to the mass balance equations within the catalyst, as" 0 ¼ 3 # app coth app 1 2 app 1 þ app coth app 1 ð76Þ=Bi mð70Þð72Þð73Þð74Þð75Þwhere app is the apparent Thiele modulus, and Bi m is the Biot number.231=2R c ðk 2 c q 0 =D RX Þ 0:5app6 1 þ D 745QYk 2D RX k 1ð77ÞCopyright © 2003 by Taylor & Francis Group, LLC

Many experimental studies on three-phase catalytic reactions indicated that the reactionrates for the organic phase and the aqueous phase follow pseudo-first-order kinetics[65,69,164,165]. The different type of the reaction expression can be written asd C RXdt¼ k appM resinV TC RX ¼ k obsC RX ¼ k 0 a V CVC RXð78ÞanddC MYdtM¼ k resinapp CV MY ¼ k obs C MY ¼ k 0 a V CTV C MYð79Þwhere k app and k app [cm 3 ðmin g resinÞ 1 ] represent the apparent reaction rate constant inthe organic phase and aqueous phase, respectively.C. Mass Transfer Problem in LSLPTCThe reactivity of a liquid–solid–liquid triphase reaction (i.e., polymer-supported catalyticreaction) is influenced by the structure of the active sites, particle size, degree of crosslinkage,degree of ring substitution, swollen volume, and spacer chain of a catalyst pellet.In the past, the characteristics of a triphase reaction, subjected to the mass transferlimitation of the reactants and ion-exchange rate in the aqueous phase, have been discussed[146,158,162,178,179]. The ion-exchange rate in the aqueous phase affects thereactivity of the triphase reaction.Past efforts have carried out this investigation macroscopically. The planar phaseboundary in a classical two-phase system cannot be described for the triphase system.Telford et al. [158] suggested an alternating shell model that requires periodical changes inthe liquid phase filling the pores of the catalyst. Schlunt and Chau [150] indicated that thereaction occurred in a thin shell near the particle surface. Tomoi and Ford [142] andHradil et al. [159] proposed that the droplet of organic (or aqueous) phase collided withthe solid catalyst. However, the mechanism and effects of the internal molecular structureof the polymer support with the reaction are seldom discussed. Although some rules werelisted in the text and clarified by the experimental results [27,28], the relationship betweenthe reaction mechanism and polymer resin in a liquid–solid–liquid triphase reaction hasnot been understood completely. Hence, this study aims to discuss the mechanism of apolymer-supported triphase reaction.Among the vast scope of PTC application [27,28], approximately 40% of PTCpatents involve the hydroxide ion and it has been estimated that approximately 60% ofcommercial PTC applications involve the hydroxide ion [28]. Many papers[61,76,96,116,164,165] have proposed that the reactivity of a reactant in an organic reactionis influenced by the base concentration. The base concentration plays a crucial role ina PT-catalyzed reaction. However, the base effect for the reactivity of reactant in a triphasereaction was rarely paid attention to.Most PTC reactions are carried out on an industrial scale in the batch mode inmixer–settler arrangements. In view of the reactor design in the liquid–solid–liquid PTcatalyzedreaction, Ragaini and coworkers [147–149] reported the use of fixed-bed reactorswith a recycling pump or with a recycling pump and an ultrasonic mixer, and emphasizedthe importance of effluent recycle concept. Schlunt and Chau [150] reported the use of acyclic slurry reactor, which allowed the immiscible reactants to contact the catalyst sites incontrolled sequential steps. However, for triphase reactions in liquid–liquid systems whereCopyright © 2003 by Taylor & Francis Group, LLC

the catalyst is asolid phase, which reactor type should be properly used in this reactionsystem is not clear.The substitution reaction of (NPCl 2 Þ 3 with phenol is asequential reaction [166,169].The reaction type is different from the common one-stage reaction. The experimentalresults can easily demonstrate the relationship between the reaction kinetic limitationand the particle diffusion limitation. In atriphase reaction, the overall kinetic cycle canbe broken up into two steps by virtue of the presence of two practically insoluble liquidphases:achemicalconversionstepinwhichtheactivecatalystsites(Resin þ withphenolateions) react with the hexachlorocyclotriphosphazene in the organic solvent, and an ionexchangestep in which the attached catalyst sites are in contact with the aqueous phase:ð80ÞThe function of base in aliquid–solid–liquid triphasic reaction has four roles asmentioned above. In previous studies, it was observed that the reactivity of organic reactantvaried with the concentration of the base concentration in aliquid–liquid PT-catalyzedreaction [96,116,180]. In the liquid–solid–liquid triphase reaction, the effect of baseonthe reactivity of reactant (or reactive environment) was rarely paid attention to the k appand k app values dramatically increased with increasing concentration of NaOH when theratio of NaOH to C 6 H 5 OHwas in the range 1–1.5. This trend corresponds to that ofliquid–liquid phase-transfer catalysis [165]. The apparent activity energies for an organicreaction decreased, and for an aqueous reaction increased when the NaOH concentrationwas increased. The reactive behavior of the organic reaction changed from reaction chemicalcontrol to diffusion control E a

FIG. 4 Yields of products and conversion of reactant as a function of reactant C 6 H 5 OH=ðNPCl 2 Þ 3consumption ratio at different NaOH concentrations: (abcd) 0.5 kmol=m 3 , (efgh) 0.9 kmol=m 3 , (ijkl)1.8 kmol=m 3 ;(*) (NPCl 2 Þ 3 ,(*)N 3 P 3 Cl 5 ðOC 6 H 5 Þ 1 ,(!)N 3 P 3 Cl 4 ðOC 6 H 5 Þ 2 ,(&)N 3 P 3 Cl 3 ðOC 6 H 5 Þ 3 ,(^) N 3 P 3 Cl 2 ðOC 6 H 5 Þ 4 ; ðÞ N 3 P 3 CLðOC 6 H 5 Þ 5 .reactant in the particle. In Fig. 4, the maximum yield of monophenolated product shifts tothe right by more than 0.2 unit, and the maximum yield of diphenolated product shifts tothe right by more than 0.1 unit. This reveals that the effect of intraparticle diffusion on theorganic reaction influences the reaction rate. This trend of shifting to the right of themaximum yield was increased with increasing concentration of NaOH.The reactivity of a triphase reaction is influenced by the structure of the active sites,particle size, degree of cross-linkage, degree of ring substitution, swollen volume, andspacer chain of a catalyst pellet. All these make the triphase reaction a complicatedone. Past efforts have carried out this investigation macroscopically. However, themechanism and effects of the internal molecular structure of the polymer support haveseldom been discussed.According to the steric effect of phenolate ion reacting with hexachlorocyclotriphosphazeneand the reports of Wu and Meng two-phase catalysis [69]; triphase catalysis[165]), the maximum yield of partially substituted phenolated product was increasedwith increasing degree of substitution reaction. Figure 4 shows that the maximum yieldof monophenolated product was larger than that of the diphenolated product, and themaximum yield of partially phenolated product decreased when the NaOH concentrationincreased (i.e., reactivity of the active site increased). This result reveals that the reactionrate of phenolate reacting with monophenolated (or diphenolated) product was greaterthan the diffusion rate of monophenolated (or diphenolated) product from active site tobulk solution and hexachlorocyclotriphosphazene from bulk solution to active site. Mostmonophenolated (or diphenolated) product reacted in situ with Resin þ OC 6 H 5 in theCopyright © 2003 by Taylor & Francis Group, LLC

neighborhood of the active site. Meanwhile, when the reactivity of Resin þ OC 6 H 5 increasedas the NaOH concentration increased, the diffusion resistance of reactants was obvious.In the present reaction, the overall reaction includes organic substitution and anaqueous ion-exchange reaction, Eq. (80). Two rate-controlling steps influence the reactionrate simultaneously. The reaction is complicated. Hence, from the literature[142,146,158,164,165,181,182], four special relationships are established between the ionexchangereaction and organic reaction with increasing concentration of organic reactant(NPCl 2 Þ 3 , according to Eqs (78) and (79), and these are listed below:1. It is assumed that the ion-exchange rate in the aqueous phase is much higherthan the substitution reaction rate in the organic phase. The effect of the ionexchangereaction could be eliminated from the reaction controlling steps.a. The intrinsic organic reaction is the rate-controlling step. Hence, the concentrationof the active-site of triphase catalyst Resin þ C 6 H 5 O remainsconstant. The value of k app is constant. The value of k app increases withhigher concentration of (NPCl 2 Þ 3 due to an increase in the consumptionrate of phenolate ion. Similar results were obtained by Wu and Tang [164].b. The organic reaction rate is limited by both reaction kinetics and particlediffusion. The values of k app increase, and the values of k app decrease withincreasing concentration of (NPCl 2 Þ 3 [181].c. The organic reaction rate is only limited by film diffusion of reactant fromthe bulk organic solution to the surface of the catalyst pellet; this is the ratecontrollingstep. The values of kapp are constant, and the values of k appdramatically increase with increasing concentration of (NPCl 2 Þ 3 .Similarresults were obtained by Tomoi and Ford [142].2. It is assumed that the organic reaction rate in the organic phase is much higherthan the substitution reaction rate in the organic phase. Controlling the reactioncould eliminate the effect of the organic reaction.a. If the film diffusion of ion from the bulk aqueous solution to the surface ofthe catalyst pellet is the rate-controlling step, the value of k app remainsconstant because the initial concentration of phenolate ion is kept constant,and the value of k app dramatically decreases [181].b. If the ion exchange rate is limited by both particle diffusion and filmdiffusion, the value of k app decreases with increasing (NPCl 2 Þ 3 concentration,and the value of k app dramatically decreases. With low-percentage ringsubstitution, the ion-exchange process is the rate-limiting step[146,153,158,182].c. If the ion-exchange rate is limited by the intrinsic ion-exchange rate, thevalue of k app remains constant with increasing (NPCl 2 Þ 3 concentration,and the value of k app dramatically decreases.3. If the organic reaction rate is limited by both reaction kinetics and particlediffusion, and the ion-exchange rate is also limited by film (or particle) diffusion,the value of k app decreases, and the value of k app also decreases [165].4. When the organic reaction rate competes with the ion-exchange rate, the valuesof k app and k app remain almost constant with increasing concentration of(NPCl 2 Þ 3 .If mass transfer resistance influences the reaction, the concentration of the activecatalyst cannot remain constant during the course of the reaction. Also, when the concentrationof organic reactant decreases, both the reaction rate and the effect of massCopyright © 2003 by Taylor & Francis Group, LLC

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

FIG. 5Mechanism of the triphasic reaction (a) and the swollen type of resin (b).the substitution reaction of potassium acetate and benzyl bromide using 18-crown-6 asthe catalyst under solid–liquid PT conditions. This phenomenon of high conversion andproduct yield using SLPTC promotes more research work in investigating this type ofreaction.The most important step in PT-catalyzed reactions is that the catalyst must have theability to transfer the reacting anion into the organic phase to react with the organicsubstrate. In an aqueous–organic two-phase system, the reacting nucleophile is locatedin the aqueous phase and is usually insoluble or slightly soluble in the organic phase underthe operating conditions. In the situation of the absence of water, the anion nucleophileshould be given by the solid salt reactant, such that the unfavorable side reaction isprobably inhibited. In addition, SLPTC can promote the weak nucleophiles, such assalts of acetate, to have much higher reactivity by eliminating the hydrolysis effect.Hence, for SLPTC, it has the advantages of easy separation of products from reactants,easy selection of organic solvents, easy recovery of catalysts, the inhibition or preventionof unfavorable side reactions, etc., and shows great potential for commercial applicationsCopyright © 2003 by Taylor & Francis Group, LLC

[187–196]. Several reactions that cannot be performed in liquid–liquid phases can becarried out efficiently in solid–liquid systems.Starks et al. [183] have addressed several questions regarding the mechanistic detailsof SLPTC, and those include what are the mechanisms of transport of anions from thesolid phase to the organic phase, the mechanisms of formation of reactive ion pairs, themechanisms of exchange of product anions located in the organic phase with reactantanions located in the solid phase, the effects of particle size on the rates of reaction, themechanistic differences between quaternary cation and crown ethers as PT catalysts, andthe mechanistic role of small quantities of water in SLPTC. Obviously, the behavior of theactive ion pairs or catalytic intermediates is important in realizing the mechanism ofSLPTC.A. Interfacial Phenomena1. The Omega PhaseFor solid–liquid PT-catalyzed reactions using crown ethers as the catalyst, the correspondingcation of the solid reactant has some limitations, e.g., a potassium salt system can onlyuse 18-crown-6 as the catalyst, while 15-crown-5 can only catalyze the reaction of asodium salt. This is because metal salts carried by crown ethers depend on their molecularstructures with the cation size just fitting into the cage of the crown ether; the activecomplex is then transported into the organic phase. Moreover, the solubility of this activecomplex is related to its lipophilicity in the organic solvent [184,185].In many solid–liquid systems using crown ethers as the catalyst, adding smallamounts of water enhances the reaction rate greatly. A trace amount of water inSLPTC obviously plays an important role. When small quantities of water are added,the solid particles are surrounded by water molecule to form a thin layer. This interfaciallayer between the solid and the organic phases is termed the omega phase, whereby thesolubility of solid reactant in the solution is enhanced to produce easily the active intermediate.Liotta et al. [186] indicated that, using 18-crown-6 as the catalyst for the solid–liquid reaction of benzyl halide and potassium cyanide, 92% of the 18-crown-6 (as asolution in toluene) and inorganic salts KCN and KCl resided in the toluene phase;however, about 1–2% of the crown ether was transferred on to the surface of the saltand coated the surface of the salt particles to form a third phase when adding smallamounts of water.When the omega phase is formed, the overall reaction rate can be described bypseudo-first-order kinetics with respect to the organic reactant. While the reaction followspseudo-zero-order kinetics as the substitution reaction is conducted in the presence ofcrown ether and in the absence of water, it is independent of the benzyl halide concentration.Crown ether directly dissociates the cation of the reacting salt. A reaction mechanismwas proposed for the esterification reaction of solid potassium 4-nitrobenzoate and benzylbromide by using crown ether [197]. The overall reaction isO 2 N C 6 H 4 COO K þ CHCl 3 ;25 Cþ C 6 H 5 CH 2 Br !O 2 N C 6 H 4 COOCH 2 C 6 H 5 þ KBr ð81ÞCopyright © 2003 by Taylor & Francis Group, LLC

The reaction steps involve [197]:1. Dissolution of solid potassium nitrobenzoate:CE org þ KNB solid Ð CE KNB org ð82Þ2. Intrinsic reaction in the organic phase:CE KNB org þ PhCH 2 Br org ! PhCH 2 Br org þ CE KBr org ð83Þ3. Release of crown ether:CE KBr org Ð CE org þ KBr solid ð84ÞWith further additions of water, the overall reaction rate does not inevitably increase, butreaches a maximum with an optimal amount of water added.2. Solubilization of Solid Salt by Quaternary Ammonium SaltsSLPTC can also be conducted by using quaternary ammonium salt as the catalyst. Thisphenomenon is somewhat different from using crown ether. Vander Zwain and Hartner[198] concluded that, for the reaction of acetate and adeninyl anions in the solid–liquidPT-catalyzed reaction using tricaprylmethylammonium chloride, showed better efficiencythan crown ether. Yadav and Sharma [199] investigated the kinetics of the reaction forbenzyl chloride and sodium acetate/benzoate by SLPTC. They found that cetyldimethylbenzylammoniumchloride was the most efficient catalyst among those studied in thetemperature range 90–139 C, and the rate of reaction in the presence of water was lessthan that in the absence of water. The solubilities of NaOAc and NaCl in toluene assolvent at 101 C are 3:85 10 5 and 3:24 10 5 gmol/mL, respectively, while being 2:1910 5 gmol=mL for the former in the presence of dimethylhexadecylbenzyl chloride. Theconcentrations of chlorides and acetates are 6:25 10 5 and 5:6 10 5 gmol=mL.Obviously, the solubilities of these two salts are affected by the reaction with the PTcatalyst. Yee et al. [200] showed that the slower reactions catalyzed by quaternary saltsin a well-mixed batch reactor were caused by the limited effectiveness of quaternary saltsin solubilizing the solid reactant.Yang and Wu [201] investigated the esterification of dipotassium phthalate withbenzyl bromide in a solid–liquid system. We found that the catalytic intermediate, formedby the solid reactant with tetrabutylammonium bromide, was the key-reacting componentin SLPTC. Yang and Wu [202] explored the kinetics of the O-allylation of sodium phenoxidewith allyl bromide in the presence of quaternary ammonium salt catalyst in a solid–liquid system. The behaviors of the catalytic intermediate tetrabutylammonium phenoxide,formed from the reaction of solid sodium phenoxide and tetrabutylammonium bromidein the solid–liquid phases, are important in conducting the etherification, andpseudo-first-order kinetics are observed.The past efforts in SLPTC show that not only can the reactions be catalyzed byquaternary ammonium salt, but the interfacial reaction of the solid reactant with thequaternary ammonium salt is also important in this type of reaction. Moreover, thebehaviors of the active intermediate are also influenced by the addition of water in conductingthe quaternary salts catalyzed reactions. A conceptual scheme describing thereaction mechanism for SLPTC was proposed by Melville and Goddard [203,204], i.e.,heterogeneous solubilization and homogeneous solubilization, by considering the solubilityof solid salts in the organic phase. For the heterogeneous solubilization mechanism,Copyright © 2003 by Taylor & Francis Group, LLC

the solid salt directly reacts with the quaternary catalyst at the solid–liquid interface toproduce the intermediate, which then transfers into the solvent and reacts with the organicsubstrate to form the product. For the homogeneous solubilization mechanism, the solidreactant can be dissolved in an organic solvent of generally higher polarity, and then reactswith the catalyst to form the intermediate. Melville and Yortsos [205] performed a theoreticalstudy regarding rapid homogeneous reactions based on a simple stagnant filmmodel in the system of SLPTC.Naik and Doraiswamy [206] reported that the homogeneous solubilization could befurther subdivided into four types, models A to D, for the following reactions:QX org þ MY s=aq Ð QY org þ MX s=aqQY org þ RX org ! QX org þ RY orgð85Þð86ÞModel A assumes that the solid dissolution and mass transfer steps are very fast comparedwith the organic reaction and that the solid particles MY and MX are present at theirequilibrium solubility levels in the organic phase. The concentrations of QY and QX in theorganic phase are both constant, i.e.,C QYo ¼ Kq 0K þ ; C QXo ¼ q 0K þ ð87ÞModel B assumes that both the ion-exchange reaction in Eq. (85) and the organic reactionin Eq. (86) are under kinetic control with the solid dissolution and mass transfer steps stillfast, and a differential equation describing the variation of QY with reaction time in theorganic phase is required. Model C assumes that MY is no longer at saturation concentrationin the organic phase, but is at some finite value. The rate of dissolution is governedby the interfacial area per unit volume of the organic phase, the dissolution rate constant,and the driving force between the saturation and the instant concentrations. Both the ionexchangeand the organic reactions take place in the bulk organic phase, and the transportof species from the solid surface to the bulk liquid is assumed to be fast; in addition, thevariation of the interfacial area according to the progress of the reaction should also beaccounted for. Model D accounts for the effect of transport of QY from the thin filmoutside the solid surface to the bulk liquid, and incorporates the rate of the organicreaction. The ion-exchange reaction is assumed to be fast and completed within the film.In order to describe the solubilization of solid reactant in the organic phase, Yangand Wu [200] performed the ion-exchange reaction of sodium phenoxide with tetrabutylammoniumbromide in a solid–liquid system. The interfacial reaction and mass transfersteps are shown as follows. The independent ion-exchange reaction isPhONa ðsÞþQBr ðorgÞ !PhOQ ðorgÞþNaBr ðsÞð88ÞThis reaction involves several steps:(a) Dissolution of PhONa. Traces of water are present in the solid reactantPhONa:3H 2 O, and the omega phase around the solid particle is formed to enhance thesolubilization of PhONa in the solution. The rate of dissolution of PhONa is very fast,leading to the solid part of PhONa readily in equilibrium with its soluble part. The concentrationof PhONa at the interface is thus kept at its saturation state.(b) Reaction of PhONa with QBr. PT catalyst QBr reacts with the soluble part ofPhONa to form PhOQ at the solid–liquid interface. The film reaction is assumed to bereversible with the equilibrium constant K 1 :Copyright © 2003 by Taylor & Francis Group, LLC

PhONa ðorgÞþQBr ðorgÞk 1!PhOQ ðorgÞþNaBr ðorgÞð89ÞK 1 ¼ C PhOQC NaBrC PhONa C QBrð90ÞIn Eq. (90) the asterisk represents the component concentration in the layer adjacent to thesurface of the solid particle.(c) Mass Transfer of PhOQ to the Bulk Organic Phase. The intermediate PhOQ transfersfrom the solid–liquid interface to the organic phase, wherein PhOQ has limitingsolubility. The equation for the rate of change is given asdC PhOQV org ¼ Kdt m A s CPhOQ C PhOQ ð91ÞThe term A s denotes the surface area of the solid particle, which gradually reduces duringthe progress of the reaction, and is derived from the mass balance of PhONa used, i.e.,with 2=3¼ A s0 1 q C PhOQ 2=3ð92ÞA s ¼ A s0N PhONa;0 N PhOQN PhONa;0q ¼N QBr;0N PhONa;0and C PhOQ ¼ C PhOQC QBr;0The mass transfer coefficient k m , which is also dependent on the particle size and theoreticallyinversely proportional to the n power of the particle size where n is in the range0.25–1.0 (from high to low Reynolds number), can be expressed as a function of saltconversion:N PhONa;0 N PhOQk m ¼ k m0N PhONa;0By combining Eqs (91)–(93), the rate of change of PhOQ in the organic phase is deduced,d C PhOQdtwhereð93Þ n=3¼ k m0 1 q C PhOQ n=3ð94Þ¼ 1 C PhOQ 1 q C PhOQ 2 nÞ=3ð95ÞC QBr;0C NaBr ¼K 1 CPhONa C QBrand ¼ k m0A s 0V orgð96ÞYang and Wu [202] showed that near-saturated concentrations of PhOQ in chlorobenzeneas solvent at different temperatures were achieved after about 20 min of operation.The difference in PhOQ concentraitons at various reaction temperatures was notsignificant in this case. This shows that the catalytic intermediate PhOQ can be formedfrom tetra-n-butylammonium salt reacted with PhONa in a solid-liquid system.Polyethylene glycols (PEGs) can also be used as the catalyst in SLPTC. Chu [207]reported the kinetics for etherification of sodium phenoxide with benzyl bromide usingquaternary ammonium salts and PEG as the catalyst in SLPTC. When PEG is used as thecatalyst, formation of the complex PEG–Na þ PhO mainly occurs at the solid–liquidinterface. The phenoxide anion carried by PEG can dissolve much more than its originalCopyright © 2003 by Taylor & Francis Group, LLC

solubility in the organic phase, thus enhancing the overall reaction. The reaction scheme isshown in Fig. 6. For the same reaction system, but using quaternary ammonium saltinsteadPEGs,theactiveintermediatebecomesPhOQproducedfromsolidNaOPhreactedwith catalyst QBr. The solubility of PhOQ varies in different kinds of solvent and leads todifferent reaction rates. The variations in the catalytic intermediate PhOQ with respect totimeforchlorobenzene, dichlorobenzene, andheptaneare shown in Fig. 7,from whichtheconcentration PhOQ in heptane is the least. However, the overall reaction rate in heptaneis still at a high level; this shows that the interfacial reaction is dominant in this case [207].B. Adsorption Effect on the Solid Surface1. Formation of the Active ComplexIn contrast with the reaction mechanism of heterogeneous and homogeneous solubilization,Yufit et al. [208] proposed a new mechanism for SLPTC that included step-by-stepformation of a cyclic ternary complex [208]. This mechanism is based on the formation oftwo pairs of binary complexes (BCs) and ternary complexes (TCs) obtained from theorganic reactant RX, the solid reactant MY, and the PT catalyst QX adsorbed on asolid salt surface as follows [208]:RX þ MY þ QX Ð TC1 Ð TC2 Ð RY þ MX þ QXð97Þð98ÞThey also analyzed the energetics of the substitutions with solid salts of different strengthof M—Y bonds and concluded that the rate-determining step was the rearrangement ofFIG. 6 Reaction scheme for benzyl bromide reacted with sodium phenoxide using PEG as thecatalyst in SLPTC.Copyright © 2003 by Taylor & Francis Group, LLC

FIG. 7 Effect of solvent on formation of PhOQ in SLPTC: benzyl bromide 0.005 mol, sodiumphenoxide 0.005 mol, TBAB 0.001 mol, solvent 50 mL, agitation 350 rpm, temperature 70 C; (*)C 6 H 5 Cl, (~) C 6 H 4 Cl 2 ,(&) n-C 7 H 16 .the TCs. From the kinetic analysis of different reaction mechanisms, the observed reactionrate constant k obs was determined by the equation:k obs ¼k½MYŠ 0 ½QXŠ 01 þ K½RXŠ 0 ½MYŠ 0 þ½QXŠ 0 ð99ÞIn Eq. (99), k is the combined rate constant and K is the equilibrium constant for thereversible reaction of TC formation.Generally, many experimental results can be described by applying pseudo-firstorderor pseudo-second-order kinetics successfully. Sometimes, however, using confinedkinetic data to elucidate exactly the reaction mechanism is indeed difficult. Hence, severalsimplified reaction mechanisms are usually employed to describe the kinetic behaviors ofthe reaction systems successfully. The technique of topochemistry is an effective methodfor achieving an approximate and quite precise interpretation of the kinetic data. Sirovskiet al. [209] discussed the applicability of the models developed for the topochemical reactionsin SLPTC. They considered that the simplest kinetic equation, called the Erofeevequation [210,211]:x ¼ 1 exp k n ð Þ ð100Þwith the rate constant k, the conversion degree x, and the parameter n depending on thegeometry of the nuclei, is more appropriate for a description of SLPTC than more complicatedones recommended in the literature. Such a kinetic equation is widely used for thedescription of topochemical processes. Sirovski et al. [209] investigated an S N 2 reaction ofbenzyl chloride with sodium acetate under SLPTC conditions:PhCH 2 Cl þ AcON Aliquat !336 PhCH 2 OAcð101ÞThey observed that the reaction rate did not follow simple kinetic laws under their operatingconditions. A possible reaction scheme was thus proposed [209]:Copyright © 2003 by Taylor & Francis Group, LLC

QCl þ AcONa ðsÞ ÐQCl:AcONa ðsÞQCl:AcONa ðsÞþPhCH 2 Cl Ð QCl:AcONa:PhCH 2 ClQCl:AcONa:PhCH 2 Cl ! QCl:NaCl:PhCH 2 OAcQCl:NaCl:PhCH 2 OAc Ð QCl:NaCl ðsÞþPhCH 2 OAcQCl:NaCl ðsÞ ÐQCl þ NaCl ðsÞð102Þð103Þð104Þð105Þð106ÞThey also found that the Erofeev equation described the observed kinetics much betterthan other simple kinetic equations. Yufit and Zinovyev [212] compared the kinetic study ofnucleophilic substitution under PTC conditions in liquid–liquid and solid–liquid systems.They observed the effect of initial exponential burst (IB) on the kinetic curve in the reactionwith solid salts for the S N 2 reaction of 2-octylmesylate with potassium halides under PTCconditions. In their study, they assumed that the active sites on which the reaction occuredwere present on the solid surface through the formation of complexes of salts, catalysts, andsubstrate [212–215]. They also concluded that the phenomenon of IB was characterized bythe first-order dependence on the initial stage of conversion and by zero-order dependenceup to high conversion. Therefore, the kinetic equation for the reaction becomes a sum oflinear and exponential terms with correlated parameters A and B½PŠ ¼At þ½XŠð1 exp BtÞ ð107Þwhere P represents the key product, [X] is the concentration of product formed by the firstorderlaw, and t is the reaction time. They also proposed a reaction mechanism includingthe adsorption on the solid surface for the solid–liquid system [212]:KBr ðsÞþQCl ð1Þ !KCl ðsÞþQBr ð1Þð108ÞQBr ð1ÞþROMs ð1Þ !QOMs ð1ÞþRBr ð1ÞQOMs ð1ÞþKBr ðsÞ ÐQOMs:KBr ðadsÞQOMs:KBr ðadsÞþROMs ð1Þ ÐQOMs:KBr:ROMs ðadsÞQOMs:KBr:ROMs ðadsÞ !RBr ð1ÞþQOMs:KOMs ðadsÞQOMs:KOMs ðadsÞþKBr ðsÞ ÐQOMs ð1ÞþKOMs:KBr ðadsÞð109Þð110Þð111Þð112Þð113ÞKOMs:KBr ðadsÞ !KOMs ðsÞþKBr ðsÞð114ÞAnother mechanism based on the concept of topochemical reaction, which meansthat the reaction rate is dependent on the characteristics or properties of the interface, hasbeen proposed by Yang and Wu [216] in investigating the esterification of linear dicarboxylateusing SLPTC for solid dipotassium sebacate (SeK 2 ) reacted with benzyl bromide(RBr). The overall reaction isKOOCC 8 H 16 COOK ðsÞþ2C 6 H 5 CH 2 Br ðorgÞ QBr!C 6 H 5 CH 2 OOCC 8 H 16 COOC H 2 C 6 H 5 ðorgÞþ2KBrðsÞð115ÞThe reaction steps involves [216]:1. Dissolution of SeK 2 from the solid surface to the organic film.SeK 2 ðsolidÞ !SeK 2 ðorgÞð116ÞCopyright © 2003 by Taylor & Francis Group, LLC

With the saturation solubility CSeK 2in the organic solvent, the dissolution rate coefficientk S is dependent on the surface property and the degree of agitation.2. Formation of a transition complex [R–Br–Q–Br] (TC) in the organic phase withequilibrium constant K 1 .RBr ðorgÞþQBr ðorgÞ ÐTC ðorgÞð117ÞFormation of the active transition complex TC from the reaction of QBr and RBr isassumed to be fast and reversible.3. The substitution reaction of TC and dissolved SeK 2 in the organic film with thethird-order rate constant k.SeK 2 ðorgÞþ2TCðorgÞ !SeR 2 ðorgÞþ2 KBr ðorgÞþ2 QBr ðorgÞð118ÞThe adsorption of QBr on the solid surface of SeK 2 plays an insignificant role in thekinetic description. The solid–liquid equilibrium of KBr between its soluble parts andsolid parts is still existed. Wu [217] reported that the kinetic data for S-shape curveswere found in this system, as shown in Fig. 8 for different amounts of potassium sebacateused. This revealed that the catalytic transition complex [R–Br–Q–Br] in the organic phasewould lead to a long induction period for the reaction of SeK 2 with TC.The concentration of TC in the organic phase and the rate of change of componentsare derived as follows [216]:C TC;org ¼ K 1 C RBr;org C QBr;orgdC SeK2 ;orgdtdC SeR2 ;orgdt¼ k S C SeK 2¼ kC SeK2 ;orgC 2 TC;orgC SeK2 ;orgkC SeK2 ;orgC 2 TC;orgð119Þð120Þð121ÞEquation (120) is simplified by neglecting the term kC SeK2 ;orgC 2 TC;org to obtain the concentrationof SeK 2 in the organic film, C SeK2 ;org ¼ C SeK 2ð1 e k St Þ, which can be furthertransformed into the equation C SeK2 ;org ¼ C SeK 2ðk s tÞ n for ease of interpreting the kineticFIG. 8 Yields of product dibenzyl sebacate for different molar ratios (r) of dipotassium sebacate tobenzyl bromide: chlorobenzene 50 mL, benzyl bromide 0.01 mol, TBAB 0.0025 mol, agitation 350rpm, temperature 80 C; r values: (*) 0.125, (^) 0.25, (&) 0.5,()1.0, (*) 2.0.Copyright © 2003 by Taylor & Francis Group, LLC

curve of the product SeK 2 . The parameters and n are determined for a definite set ofdata and have the physical meaning of characterizing the surface properties of solidreactant SeK 2 . Different sets of data in ð1 e kst ) give different sets of and n for bestfitting with a precision greater than 0.99. The apparent reaction rate constant is thendeduced from the equation:wheredYdt ¼ 2kK 2 1 C 2 QBr;orgC RBr;0 C SeK 2k n S1 þ K 1 C QBr;org 2t n ð1 YÞ 2 ¼ k app;0 C RBr;0 t n ð1 YÞ 2 ð122Þk app;0 ¼ 2k n SkC SeK 2K 2 1 C 2 QBr;org= 1 þ K 1 C QBr;org 2and Y ¼ 2C SeR2 ;org=C RBr;0It it noted that k app;0 is subjected to the effects of rate of dissolution, intrinsicreactivity, rate of formation of transition complex, catalyst amounts, the solubility ofsolid reactant in the organic phase, and the characteristics of the solid surface, and hasthe dimensions of [ðtimeÞ 1 n ðconcentrationÞ 1 ]. The resultant equation from integratingEq. (122) is similar to the conversion equation deduced from topochemistry theory. Bytaking the natural logarithm on both sides, one can obtain a rather simplified equationused to correlate the kinetic behaviors, i.e., Yln ¼ ln k app;0C RBr;0þðn þ 1Þ½ lnðtÞŠ ð123Þ1 Yn þ 1Moreover, further simultaneous generation of KBr during the reaction of SeK 2 with TCwould make the organic solution much more slushy, which in turn would reduce the filmreaction rate due to the steric hindrance when a much higher catalyst amount was used.2. Deactivation Behavior of CatalystIn SLPTC, the effect of the side-product salt on the overall reaction rate is sometimesobserved to be severe after a specific reaction time. The kinetic curve shows that thepseudo-first-order reaction initially obeyed is no longer followed at later reaction times,behaving in a diminished kinetic order, and the whole reaction is finally stopped. This sideproduct is produced from the anion-exchange reaction at the solid–liquid interface.Depending on the polarity of the solvent, this generated metal salt is usually difficult todissolve in the organic phase and has a tendency to adsorb on to the surface of the solidparticle. The adsorbed salts thus strongly influence the subsequent anion-exchange reaction,from which the active intermediate is formed. Such an effect is more significant forthe fast anion-exchange reaction or at a higher reaction temperature.Yang et al. [218] investigated the substitution reaction of sodium phenoxide withethyl 2-bromoisobutyrate in a solid–liquid PT-catalyzed system. The deactivation of catalystactivity on the apparent pseudo-first-order reaction rate was observed. Such a phenomenonresults from the salts deposited on the surface of solid particles during theprogress of a reaction. The deposition of salts decreases the rate of formation of the activeintermediate, leading to the observed reaction order change [218]. We applied the pseudofirst-orderreaction with catalytic deactivation kinetics to show that the initial reaction ratewas not influenced by the agitation rate when exceeding 350 rpm, but the deactivation rateincreased with increasing stirring speed and the amount of catalyst used. Adding a smallamount of water resulted in a reduction in the apparent reaction rate. A more lipophilicCopyright © 2003 by Taylor & Francis Group, LLC

quaternary cation solvates the solid reactant anion much more easily, and leads to a fasterinitial reaction rate. The order of reactivity for different PT catalysts is determined asTBPB > TBAB > TBAI TBAHS Aliquat 336 for the reaction.Yang et al. [219] investigated the kinetics of the etherification of ethyl 2-bromoisobutyrate(RX) with potassium 4-benzyloxyphenoxide in the presence of potassium iodidein SLPTC. In that work, we found that for various molar ratios of TBAB to RX (denotedas f ) the yield of product ArOR increased with increasing catalyst amounts up to f ¼ 0:60.Too much catalyst employed in the presence of KI results in the reduction of catalyticefficiency. This effect is due to two major reasons: first, the solubility of the catalyticintermediate in the organic solvent is limited; second, the formation of the catalytic intermediatein chlorobenzene is retarded because use of a higher amount of catalyst inducedrapid deposition of the generated potassium salts on the solid surface. Adding the extrasalt KI enhances the reactivity of PT catalyst, but the active intermediate in the organicphase is diminished when much KI is present. Small amounts of KI promote the conversionof RX into RI, which is more reactive in the organic reaction. The reaction stepsconcerning the deactivation of the catalyst are shown below [219].The overall reaction isKI;QXArOK ðsÞþRX ðorgÞ ! ArOR ðorgÞþKX ðsÞ ð124ÞThe reaction mechanism for the overall reaction is as the following steps:ArOKðsÞ ! ArOK ðorgÞKI ðsÞ ! KI ðorgÞð125Þð126ÞK 1!KI ðorgÞþQX ðorgÞ KX ðorgÞþQI ðorgÞð127ÞK 2!KI ðorgÞþRX ðorgÞ KX ðorgÞþRI ðorgÞð128ÞKXðorgÞ ! KX ðsÞð129ÞK 3!ArOK ðorgÞþQX ðorgÞ ArOQ ðorgÞþKX ðorgÞð130ÞK 4ArOK ðorgÞþQI ðorgÞ! ArOQ ðorgÞþKI ðorgÞð131ÞRX ðorgÞþArOQ ðorgÞk b! ArOR ðorgÞþQX ðorgÞð132ÞRI ðorgÞþArOQ ðorgÞk b! ArOR ðorgÞþQI ðorgÞð133ÞThe rate of change of ArOR is then expressed asdC orgArOR¼ kdt a C orgRX þ k bC org RI CorgArOQð134ÞCopyright © 2003 by Taylor & Francis Group, LLC

Taking the mass balance for the cation Q of the PT catalyst in the system givesC orgQX;0 ¼ Corg QX þ Corg QI þ Corg QrOQThe expression for the active intermediate isC orgArOQ ¼1 þ 1C org ArOKC orgQX;0C orgKXþ Corg KIK 3 K 4ð135Þ ð136ÞThe concentration of ArOQ depends on the amounts of ArOK, KX, and KI, and theinitial usage of the catalyst QX. Combining the mass balance equation for initial RX in theorganic phase with Eq. (134), a deactivation function can be introduced in the situationunder decline of catalytic efficiency, leading to the following equations:withdC orgArORdt¼ k app;0C orgRX;0C orgArORk app;0 ¼ðk a þ k b K 2 C orgKI =Corg KX ÞCorg QX;0ð137Þand1 ¼ 1 þ K 2C org ð138ÞKIC org 1 þ 1 C orgKXC org KXþ Corg KIArOK K 3 K 4In Eq. (137), k app;0 is the initial apparent reaction rate constant and is dependent on theamounts of KI, KX, and QX in the organic phase. If the rate of change of ArOR follows apseudo-first-order reaction, then would be approximately a constant. In such cases, nodeactivation effect appears. If the reaction rate behaves as a diminished first order, then decreases with the progress of the overall reaction.To evaluate the exact variation of with time is to measure the rate of deposition ofKX and the other parameters directly or to apply an empirical correlation relating to theeffect of the decrease in C orgArOKon the overall reaction. The expression of the deactivationfunction and the kinetic data would determine the form of , such as1 ¼ð1 þ k d tÞ 2 or ¼ 1ð139Þ1 þ k d tLepertit and Che [220] discussed the definitions of interfacial co-ordination chemistry(ICC) and surface organometallic chemistry (SOMC) and compared their maincharacteristics and applications. The concepts of ICC applied to catalyst preparation,adsorption, and relations with catalysis are also useful in the development of interfacialmechanisms.C. Mass Transfer EffectsSufficient kinetic information should be collected to proceed the process design for aspecific reaction system. The factors affecting the performance of SLPTC includes agitationrate, particle size of solid salt, reaction temperature, the amount of solid reactant, thekinds and amount of PT catalyst, the solubility and the dissolution rate of solid reactant inthe organic solvent, extra addition of other metal salts, the polarity, surface tension, andCopyright © 2003 by Taylor & Francis Group, LLC

viscosity of the solvent, the amount of organic reactant, and the presence or absence ofwater. The reactor employed in SLPTC is usually an agitated batch type with the solid saltsuspended in the solution. The mass transfer rate between the solid and the liquid phases isimportant in designing a SLPTC reactor.In an agitated reactor, the effect of mass transfer resistance can be reduced to aminimum by adjusting the stirring speed. The mass transfer coefficient is also a function ofthe size of suspended particles. From the point of view of reactor design, to maintain theuniformity of the desired product from batch to batch the particle size distribution of thesolid reactant should be in a rather narrow range to render the mass transfer resistanceunimportant.1. Mass Transfer CoefficientMelville and Goddard [204] used rotating disk flow to measure the mass transfer coefficientbetween the solid and liquid phases in SLPTC for the reaction of benzyl chloride andsolid potassium acetate using Aliquat 336 as the catalyst in acetonitrile as solvent. Theconcentration of quaternary ammonium acetate is expressed in the following equations:C QOAc ¼ 1 e t ð140Þ ¼ K 01C KOAc; ¼kð1 þ SK01Þ1 þ K 01ð141ÞK1 0 ¼ K 1C QCl; k ¼ A D kC KCl V ;S ¼ D QD Kð142Þwhere K 1 is the equilibrium constant for the reaction, KOAc þ QCl ! KCl þ QOAc; D i isthe diffusivity coefficient of component i; is the film thickness of mass transfer; A is thesurface area of solid potassium acetate; and V is the liquid volume. They concluded thatthe solid–liquid reaction was effected after the solid potassium acetate dissolved in thehigh-polarity solvent. Yee et al. [200] also applied rotating disk flow to carry out the masstransfer experiments for solid benzoate. The mass transfer coefficient K is obtained asK ¼ 0:6205D1=3 ! 1=2 1=6 f ðScÞf ðScÞ ¼1 þ 0:2980Sc 1=3 þ 0:01451Sc 2=3ð143Þð144Þwhere ! is the angular velocity (rad/s), is the kinematic viscosity (cm 2 =s), D is thediffusivity (cm 2 =sÞ, and SC is the Schmidt number (=DÞ:2. Pseudo-First-Order Kinetics Neglecting Mass Transfer EffectPseudo-first-order kinetics are usually observed in many solid–liquid PT-catalyzed reactionswhen the mass transfer effect is insignificant. For the reaction between the organicsubstrate RX and the nucleophile MY, the equation isRX ðorgÞþMY ðsÞþQ þ X ðorgÞ !RY ðorgÞþMX ðsÞþQ þ X ðorgÞð145ÞIn the case of some solid MY dissolved in the organic phase, the equilibrium state isachieved in a short reaction time:MY ðsÞ $M þ Y ðorgÞð146ÞCopyright © 2003 by Taylor & Francis Group, LLC

The ion-exchange reaction occurs at the interfacial zone to form QY, then conducting theintrinsic reaction:M þ Y ðorgÞþQ þ X ðorgÞ $Q þ Y ðorgÞþM þ X ðorgÞQ þ Y ðorgÞþRX ðorgÞ !RY ðorgÞþQ þ X ðorgÞð147Þð148ÞThe regenerated Q þ X ðorgÞ continues to catalyze the formation of Q þ Y ðorgÞ, andM + X (org) is always in equilibrium with MX(s):M þ X ðorgÞ $MX ðsÞð149ÞThe rate of equation is derived asd½RXŠdt¼ k org ½RXŠ½Q þ Y Š¼ k org K e½M þ Y Š½M þ X Š ½RXŠ½Qþ X ŠK e ¼ ½Qþ Y Š½M þ X Š½Q þ X Š½M þ Y Šð150Þð151ÞIf the value f ¼½M þ Y Š=½M þ X Š is approximately a constant, the rate of the equation canbe expressed in a pseudo-first-order form:d½RXŠ¼ kdt app ½RXŠk app ¼ k org K e f ½Q þ X Šð152Þð153ÞDuring the course of reaction, when the value of f is gradually diminished, the initialreaction rate and deactivation rate should be applied. With or without adding waterinfluences the mass transfer rate from the interface into the organic phase.D. Kinetic Modeling of Heterogeneous SolubilizationNaik and Doraiswamy [206] developed a mathematical model for the case of heterogeneoussolubilization that involves the steps of ion exchange in the solid phase, interphasetransport of the catalyst and the intermediate, and the organic reaction. In this model, ionexchange occurring within the solid phase is assumed due to the possible deposition of theproduct salt MX on the solid surface retarding formation of the catalytic intermediate.The controlling step can either be the liquid-phase transfer steps, the diffusion within thereactive solid, the adsorption–desorption steps, the surface ion-exchange reaction, or theliquid organic reaction. This treatment is similar to that in gas–solid catalytic reaction.The controlling step may shift to another step continuously with time. The reaction oforganic RX with solid MY in the presence of PT catalyst QX is considered, and a poroussolid wherein the ion-exchange reaction takes place throughout the whole pellet, ratherthan at a sharp interface due to the liquid penetrating into it is assumed. The governingequations are derived as follows [206]:RX ðorgÞþMY ðsÞ ! QX RY ðorgÞ MX ðsÞ ð154ÞWithin the solid phase:Copyright © 2003 by Taylor & Francis Group, LLC

@CQXS ¼ D e@t@C s QY@th i@ r 2 @CQXS =@r r 2 k@rs¼ D e @ r 2 @C s QY =@rr 2 þ k@rsIn the organic bulk solution:dC orgQX¼ kdt 2 C orgRX Corg QYdC orgRX¼dtC orgQY ¼ q 0k 2 C orgRX Corg QYhk q aC oegQXC s QXC s QXC s QXiC s QYKCQYs Kð155Þð156Þð157Þð158ÞC orgQXC s QX;a C s QY;a ð159ÞC s QX;a ¼ 3ð 10 2 C s QXdThe initial and boundary conditions are:IC: t ¼ 0; C orgRX ¼ C0 RX; C s QX ¼ 0; C s QY ¼ 0; C orgQX ¼ q 0; C orgQY ¼ 0ð160Þð161ÞBC: r ¼ 0; dCs QXdrr ¼ R; D qdC s QXdrr ¼ R; D qdC s QYdr¼ dCs QYdr¼ 0h¼ k q C orgQXh¼ k q C orgQYC s QX;RC s QY;Riið162Þð163Þð164ÞThe above equations can be rendered dimensionless in terms of Thiele’s modulus, Biotnumber for mass transfer, and nondimensional time and distance, which are defined as 2 ¼ k sR 2; BiD m ¼ k qR; ¼ D ete D q R 2 ; ¼ r Rð165ÞIn the analysis of heterogeneous solubilization, the role of the solid-phase reaction ininfluencing the overall reaction is different from that for the usual gas–solid catalyticreaction. The most important situation is that the film and internal diffusion effects withinthe solid and at the solid–liquid interface are significant.V. TRI-LIQUID PHASE TRANSFER CATALYSISNeumann and Sasson [221] investigated the isomerization of allylanisole using PEG as thecatalyst in a toluene and aqueous KOH solution. They observed that a third-liquid phasewas formed between the aqueous and the organic phases. This was the first report regardingtri-liquid PTC. In 1987, Wang and Weng [222] performed the reaction of benzylchloride and sodium bromide using tetra-n-butylammonium bromide as the PT catalystin liquid–liquid phases. They found that the overall reaction rate rapidly increased whenthe amount of catalyst used exceeded some critical value. In such reaction conditions, thePT catalyst was found to be concentrated within a viscous liquid phase that was insolubleCopyright © 2003 by Taylor & Francis Group, LLC

in both aqueous and organic phases [222]. This liquid phase enhanced the overall reactionrate as much as several fold that in two-liquid phase systems with PTC, and was called thethird-liquid phase. The third-liquid phase was found to contain little of the organic andaqueous reactants, but mainly the highly concentrated catalyst, which exhibited hydrophilicand lipophilic properties. In the bromide–chloride exchange reaction system ofWang and Weng [222], the third liquid phase was found to consist mainly of Bu 4 NBr,small amounts of toluene, water, and sodium bromide. Above about 70% of the tetrabutylammoniumbromide was forced to form a separate liquid phase. The organic andaqueous reactants readily reacted with the concentrated catalyst to yield a high reactionrate. The PTC reaction in this situation was termed as tri-liquid PTC (TLPTC).From the point of view of industrial practice, the formation of a third phaseprovides not only enhancement of the reaction rate, but also easier separation of thePT catalyst from the product stream than that in a two-liquid phase. However, in someparticular reaction systems, the catalyst could lose as much as approximately 25% ofthe initial amount used. Catalysis by TLPTC was briefly reviewed by Naik andDoraiswamy in 1998 [223]. The key results from the previous publications are discussedas follows.A. Formation of the Third Liquid PhaseTetrabutylammonium salts are found to be able to form a third liquid phase underappropriate conditions. In principle, the formation of a third catalyst phase can beobtained by using a PT catalyst having limiting solubility both in the aqueous phaseand organic phase under the interaction of other concentrated ingredients. Ido et al.[224] effected the elimination reaction of 2-bromo-octane with aqueous sodium hydroxideusing PEG as the catalyst [224]. By adding small quantities of methanol the solubilityof PEG in the organic phase was greatly reduced, leading to the formation of athird liquid phase. Mason et al. [225] investigated the elimination of phenethyl bromideto styrene using tetrabutylammonium bromide under PT-catalytic conditions. Theyfound that the rate of reaction was accelerated rapidly due to the addition of morethan the critical amount of PT catalyst, and the third phase was rich in catalyst. Whenthe PT catalyst used was replaced by the tetrapropyl- or tetrapentyl-ammonium salts,the third liquid phase was not formed, and the precipitation of excess catalyst wassimply induced.Wang and Weng [226] explored the effects of solvents and salts on the formation of athird liquid phase for the reaction between n-butyl bromide and sodium phenolate. Theyconcluded that the polarity of the solvent and the amount of NaOH are two importantfactors in influencing the formation of a third liquid phase, the distribution of catalyst, andthe reaction rate. The aqueous reactant NaOPh also exhibitis significant behavior incertain conditions. With the catalytic intermediate QOPh produced by the reaction ofNaOPh and the catalyst QBr, NaOH has the ability to extract QOPh from the organicphase or the third liquid phase into the aqueous phase. For example, when the amount ofNaOH added was 2 g, the amount of catalyst in chlorobenzene decreased to less than 10%of the original content, while the concentration of the catalyst in the aqueous phaseincreased with increasing NaOH added. In addition, when hexane was used as the solvent,adding a small amount of NaOH caused the disappearance of the third liquid phase, whichhad been formed before the addition of NaOH. This phenomenon is due to the dissolutionof QOPh in the aqueous phase.Copyright © 2003 by Taylor & Francis Group, LLC

Ido et al. (1997) reported a halogen substitution between benzyl chloride in theorganic phase and potassium bromide in the aqueous phase catalyzed under the thirdliquid phase that was formed by changing the concentration of KBr, types of PT catalysts,and the organic solvents [227]. Yadav and Reddy (1999) investigated the n-butoxylation ofp-chloronitrobenzene (PCNB) using the base NaOH under tri-liquid phase conditions.The typical composition of the third liquid phase was 55.12% of toluene, 22.52% oftetrabutylammonium bromide, 4.96% of p-chloronotrobenzene, 14.51% of water, and2.89% of n-butyl alcohol by weight. Distribution of the catalyst between the organicphase and the third liquid phase indicates about 89% of the total catalyst residing inthe third phase, and the overall reaction rate is attributed to the contribution of thereaction occurring in both the organic and the third liquid phases.Jin et al. [229] further performed the dehydrohalogenation of 2-bromo-octane withdodecane as the organic solvent and potassium hydroxide in the aqueous solution toinvestigate the synergistic effect of two PT catalysts in the situation of a third liquidphase using a combination of tetrahexylammonium bromide and PEG. They concludedthat a molecule of tetrahexylammonium bromide surrounded by many molecules of waterand some PEG 200 led to the effect of water on the catalytic activity of tetrahexylammoniumbromide becoming weaker when the amount of PEG was increased.In summary, the operating conditions influencing the formation of the third liquidphase are: (1) type and quantity of the aqueous reactant, (2) type and quantity of PTcatalyst, (3) reactant and product in the organic phase, (4) the addition of otherinorganic salts, (5) lower polarity of the organic solvent, and (6) the reaction temperature.Increasing the reaction temperature benefits the formation of the third liquidphase due to the breakage of hydrogen bonding between the PT catalyst and thewater molecule.B. Interfacial Mechanism of TLPTC1. Reaction MechanismThe typical reaction mechanism for tri-liquid PTC in a batch reactor under agitation isillustrated in the schematic diagram of Fig. 9. Three types of reaction scheme consideringthe partition of the catalyst in the different phases and the place where the inherentreaction occurred have been proposed [226,227]. For the substitution reaction of alkylhalide (RX) and aqueous reactant metal salt (MY) using quaternary ammonium salt (QX)as the catalyst, the different types of reaction are addressed as follows [226].FIG. 9Reaction scheme for benzyl bromide reacted with sodium bromide in TLPTC.Copyright © 2003 by Taylor & Francis Group, LLC

Type I.The catalyst resides in the organic phase in a significant quantity:ð166ÞFor type I, the partition of catalyst between the organic and aqueous phases is importantin determining the intrinsic reaction rate and the utilization of the catalyst in the organicphase.Type II. The catalyst resides in the aqueous phase in a significant quantity:ð167ÞFor type II, the catalyst mostly stays in the aqueous phase, and the transferred RBr fromthe organic phase to the aqueous–organic interface reacts with the catalytic intermediateQOPh that is transported from the aqueous to the interface. The intrinsic reaction ismainly conducted at the interface between aqueous and organic phases.Type III. The third liquid phase appears with the catalyst and active intermediate allresiding in this viscous phase:ð168ÞBy adding more NaOPh to the reaction system the catalyst is salted out to form the thirdliquid phase. The active intermediate is then formed at the interface of the aqueous and thethird liquid phases by the reaction of QBr and NaOPh, which is transferred from theCopyright © 2003 by Taylor & Francis Group, LLC

aqueous bulk phase. The main intrinsic reaction of QOPh and RBr is conducted in thethird liquid phase. The product ROPh is then transferred into the organic phase.In reality, not all the catalyst would exist in the third liquid phase especially for thathaving high solubility in the aqueous phase. A distribution of the catalyst between theaqueous phase and the third liquid phase is probably retained. Hence, some parts of QOPhwould be produced in the aqueous phase, resulting in the distribution of QOPh betweenthe aqueous and the third liquid phases.2. Factors Affecting Catalyst Activity in TLPTCIn TLPTC, the essential step is to form the third liquid phase by adjusting the contents ofinorganic salts and PT catalyst, and the interaction of the strong bases added. The overallreaction rates catalyzed by applying the third liquid phase are commonly enhanced tremendously,compared with the same reaction proceeding in liquid–liquid phases. Thevariables influencing the reaction rate can be summarized as follows:(a) Agitation Speed. Agitation plays an important role in a multiphase reaction system.Increasing the agitation rate increases the mass transfer rate of the componentbetween the immiscible phases and reduces the droplet size of the dispersed phase.Under agitated conditions, the mass transfer resistance at the interface between thethird liquid layer and the organic phase is affected by the droplet size. When the agitationrate increases to a critical value, the limiting step is dominated by the reactionwithin the catalyst-rich phase [224–227,230–233]. Yadav and Reddy [228] reported thatwith a speed of agitation from 650 to 1400 rpm, the rate of reaction increased withincreasing stirring, and after 1400 rpm the rate was independent of the interfacial masstransfer resistance.(b) Amount of Phase Transfer Catalyst. Different types of PT catalysts including quaternaryammonium salts and PEGs have been observed to enable the formation of thethird liquid phase, but under different conditions. Their common behaviors show thesharp discontinuity of the reaction rate before and after the formation of the thirdliquid phase. The observed reaction rate in the case of the tri-liquid phase increases linearlywith the total moles of quaternary ammonium bromide [228]. However, the quaternaryammonium salts with shorter alkyl chains show less tendency to form the thirdliquid phase, e.g., tetrapropylammonium bromide is ineffective for use as a catalyst in atri-liquid system. Mason et al. [225] indicated that, when a reaction mixture forming athree-liquid system was reconstructed by separating the middle third liquid phase, thereaction rate dropped by over half.Ido et al. [227] investigated the kinetics of a halogen exchange reaction in a threeliquidphase system and applied first-order kinetics to describe the overall reaction rate.They observed that the reaction rate constant includes the contributions of reactions in thethird-liquid phase and in the organic phase, and is a first order proportional to the totalcatalyst moles m cat . The reaction rate k inter occur at the interface between the aqueousphase and the organic phase is also important. Their results are shown as the followingequations [227]:dm A¼ kdt org x org þ k inter x aq þ k third D A x third mcat C A;org V orgmy A;org ¼exp k catV org þ D A V obs tthird V orgð169Þð170ÞCopyright © 2003 by Taylor & Francis Group, LLC

V orgk obs ¼V org þD A V thirdk org x org þk inter x aq þk third D A x thirdð171Þwhere x A denotes the mole fraction of the catalyst existing in the different phases, and K Arepresents the distribution of A(benzyl chloride) between an organic phase and athirdphase in equilibrium and is defined asD A C A;thirdC A;org¼ m A;thirdV orgm A;org V thirdð172ÞTaking the logarithm for Eq. (170), one can determine the observed rate constant k obsfrom the experimental data by plotting ½ lnðy A;org ÞŠversus time t:V orgmlnðy A;org Þ ln¼k catV org þD A V obs tð173Þthird V org(c) Reactant and Alkali Salt in the Aqueous Phase. The overall reaction rate inTLPTC usually increases with the increase in amount of strong base reactant in theaqueous phase. In contrast with abase-catalyzed elimination reaction, the third liquidphase already formed will be precipitated under the excess base to dehydrate the catalystphase. In the presence of 49% of NaOH, two liquids and one solid are observedinstead of three liquids at asomewhat lower base concentration.Ido et al. [227] found that increasing the aqueous reactantKBrincreases the reactionrateinTLPTC.Theionicstrengthintheaqueousphasealsoaffectstheeaseofformingthethird liquid phase, since adding extra salts tends to salt out ion pairs produced from theaqueous reactant with the quaternary salt. In the system of n-butyl bromide reacted withsodium phenolate [225], the water molecules form hydrogen bonds with NaOPh as well aswith QOPh, leading to the amount of tetrabutylammonium salts in the organic phase andin the third liquid phase increasing with the amount of NaOPh added, which in turnenhances the overall reaction rate.(d) Organic Solvent and the Reaction Temperature. In general, the more polar theorganic solvent the faster is the overall reaction rate in LLPTC due to the increasingsolubility of the catalytic intermediate in the organic phase, and leading to much easiertransport of ion pairs into the solvent to react with the organic substrate. In contrast,in TLPTC, the solubility of the catalytic ion pairs in the organic solvent should be lowenough to push the catalyst to form aseparate phase. Thus, asolvent with low polarityor anonpolar one is favorable. Under the same conditions of using KBr and acatalyst,the reaction rate in dodecane was observed to be much faster than in toluene [227].Increasing the reaction temperature accelerates the reaction rate [221–226,230–233].However, the catalyst existing in the third liquid phase as well as in the organic phaseshould still be maintained. Under strong base conditions in TLPTC, the catalyst and theactive intermediate have the tendency to decompose at ahigh temperature, hence, alimitingreaction temperature should be kept in maintaining the third liquid phase.C. Kinetic Modeling for TLPTCYang[234]hasdevelopedatheoreticalmodeltoinvestigatetheeffectsofmasstransferanddistribution of the catalyst within the third liquid phase and organic or aqueous phase ontheoverallreactionrate.Themodelingconsidersthedispersedorganicdropletsurroundedbyaninterfacialcatalystlayerunderagitationconditions,asshowninFig.10.Thistypeofdroplet is similar to some oil/water emulsions in the presence of surfactants. The reactantCopyright © 2003 by Taylor & Francis Group, LLC

FIG. 10Conceptual scheme for dispersed droplet and third liquid layer in TLPTC.MY in the aqueous phase undergoes a substitution reaction with the organic reactant RX toform the product RY. Under some appropriate conditions, by introducing extra inorganicsalts or reactants into the system, a separate liquid phase appears and is composed of PTcatalyst (QX), active intermediate (QY), a little water, and organic solvent. The third liquidlayer at the aqueous/organic interface exists if the solubility of QY in both the organic andaqueous phases is limited. This reaction system is shown in the following scheme:ð174ÞIn the third liquid phase system, the organic phase is considered as the dispersedphase with spherical and rigid droplets, and with a high distribution coefficient of catalystbetween aqueous and third phases. The rates of change of RX in the organic phase andQY/QX in the third phase are formulated as follows [234]:Copyright © 2003 by Taylor & Francis Group, LLC

@C orgRX@t@C catQY@t@C catQX@t¼ D RXr 2¼ D QYr 2¼ D QXr 2@r 2 @C org RX@r @r @r 2 @Ccat QY@r @r @r 2 @Ccat QX@r @rfor r r dfor r d r r cfor r D r r cð175Þð176Þð177ÞIn which r d is the radius of the organic droplet and r c is the radius of the organic dropletplus the thickness of the catalyst layer. The relationship between r c and r d isr c ¼ 1 þ V 1=3catrV d ð178ÞorgThe initial and boundary conditions areat t ¼ 0;at r ¼ 0;at r ¼ r d ;C orgRX ¼ C RX;0for 0 r r dC catQX ¼ 0; C catQY ¼ 0 for r D r r c ð179Þ@C orgRX¼ 0@r3 @CD orgRX ¼r dRX@r3 @CD catQYr dr dQY@r3 @CD catQX ¼QX@rk 2 C orgRX Ccat QY¼ k 2 C orgRX Ccat QYk 2 C orgRX Ccat QYThe distribution coefficients of QY and QX, m QY and m QX , are defined asm QY ¼ CcatðsÞ QYC aqðsÞQYAt r ¼ r c ;and m QX ¼ CcatðsÞ QXC aqðsÞQXD QY@C catQY@r@CQXcatD QX ¼@r¼ K QYC aqQYK QX ðC catQX1CQYcatm QYm QY C aqQX Þð180Þð181Þð182Þð183Þwhere K QY and K QX denote the mass transfer coefficients of QY and QX, respectively. Inthe aqueous phase, the rates of change of MY, QY, and QX aredC aqMY¼ kdt 1 C aqMY Caq QXdC aq QY¼ kdt 1 C aq3 VcatMY Caq QXK QYr c V aqdC aq QXVcatC catðsÞQXmdtQX C aqQX¼ K QX3r cV aqC aqQY1C catðsÞQYm QYk 1 C aqMY Caq QXð184Þð185Þð186ÞCopyright © 2003 by Taylor & Francis Group, LLC

The initial conditions areAt t ¼ 0;C aqMY ¼ C MY;0C aqQX ¼ C QX;0;C aqQY ¼ 0ð187ÞThe average concentration and the conservation of PT catalyst Q at any reaction time areC orgRX ¼ 3 r 3 dð rd0V aq C QX;0 ¼ V catr 2 C orgRX drQY þ C QXcat þ Vaq C aqQY þ Caq QXC catð188Þð189ÞEquations (175)–(189) constitute the system of PT-catalyzed reactions with the third liquidphase, and can be further tendered in dimensionless form and solved by finite differenceand Runge–Kutta methods.Recently, Krueger et al. [235] developed a theoretical model, based on the dispersedorganic phase, for modeling the mass transfer and interfacial reactions of the brominationof benzyl chloride in three-liquid PTC. The reaction occurring at the interface between theinner organic droplet and outer shell (or layer) of the third phase isr ¼ R: BzCl þ QBr k 1BzBr þ QCl ð190Þ!They assumed that the ion-exchange reaction occurs at the interface between the aqueousand the third liquid phase according tor ¼ R þ : Br þ Q þ Ð QBr ð191ÞQCl Ð Q þ þ ClThe governing equations in the dimensionless form for the system are@C BC¼ 1 @@ 2 2 @C BC@ @ð0

tively. When the amount of the third liquid phase has little effect on the overall reactionrate, the contribution of greater amounts of third liquid phase is to increase the masstransfer resistance. This case is similar to the situation of a very thin film of the third phasewith =R 1. They also proposed an equation [235] for the concentration of benzylchloride using the analogous analytical solution of heat transfer in terms of a mass transferproblem asC BC ¼ X1n¼1A n exp 2 n sinð n Þ n ð199Þwhere the eigenvalues n and the coefficients A n are given by1 n cotð n Þ¼N D and A n 4sinð ½ nÞ n cosð n ÞŠ2 n sinð2 n Þð200ÞVI.CONCLUSIONIn recent years, researches in PTC have achieved great progress in the development of thebasic theory and applications; the number of publications and patents in PTC has grownsteadily during the past decade. Halpern [236] estimated that the 5-year average of TBABpatents issued per year was 38 in 1985, 46 in 1990, and 57 in 1995. The PTC publicationsfor a 5-year average issued per year are 389 in 1985, 467 in 1990, and 484 in 1995. Thisinformation indicates the potential applications of PTC in industries either revamping ordeveloping new processes, including the applications of PTC in biology.The aspects of PTC publications were mostly concentrated in chemistry-based investigationsin past years. On viewing the nature of PTC, the presence of immiscible phasesand the transport of reacting species between the phases are the basic phenomena; therefore,the mass transfer resistances at the interface and within the intraphase should betaken into account in accompanying the chemical reactions for the purpose of reactordesign. The engineering analysis in various types of PTC, together with other techniquesfor enhancing the overall reactivity, has the advantage of realizing the factors influencingthe observed reaction rate, which makes the process design closer to the inherent results. Itis hoped that the review in this chapter could serve to generate more applications of PTCin the future.Copyright © 2003 by Taylor & Francis Group, LLC

SYMBOLSaactivity of quaternary salt in a solutionA interfacial area between the organic and aqueous phase ðm 2 )A s surface area of solid particles ðdm 2 )Aliquat 336 tricaprylmethylammonium chlorideCconcentration of quaternary saltC j iconcentration of component i in phase j (j ¼ org:, aq., cat.)C PhOQ dimensionless concentration of PhOQC# number of carbon atoms in each of the four alkyl chains in the quaternarycationD Q distribution coefficient of quaternary cation definedEQX T true extraction constant ðkmol=m 3 Þ 1jhydration number per quaternary salt in the organic phasek app apparent first-order reaction-rate constant (s 1 )k app;0 initial apparent reaction rate constant (1/min)k 2 forward reaction rate constant of the aqueous phase (kmol=m 3 minÞ 1k 2 reverse reaction rate constant of the aqueous phase ðkmol=m 3 minÞ 1k ireaction rate constantK iequilibrium constantK da dissociation constant of QX in the aqueous phase (kmol=m 3 )K do dissociation constant of QX in the organic phase (kmol=m 3 )K QY mass transfer coefficient of QY (m/min)K QX overall mass transfer coefficient of QX (kmol=m 3 min 1 )mdistribution coefficient of onium saltM H molecular weight of hydrophilic group in quaternary salt (e.g., N þ X )M T molecular weight of quaternary saltM TBAB molecular weight of TBABMX side product in the aqueous solutionMY aqueous reactantQ þ quaternary cationQX quaternary saltrspatial co-ordinate in radial directionr cmean radius of droplet containing organic phase and catalyst phase (m)r dmean radius of organic droplet (m)R freaction rate per unit area (mol=m 2 sÞRH reactantRX organic reactantR 0 X organic reactantttimeTabsolute temperature (K)TBAB tetra-n-butylammonium bromideTBAC tetra-n-butylammonium chlorideTBAI tetra-n-butylammonium iodideTBA-TBPO tetra-n-butylammonium 2,4,6-tribromophenoxideTBPB tetra-n-butylphosphonium bromideV volume of aqueous phase (m 3 )V aq volume of aqueous phase (L)volume of catalyst phase (L)V catCopyright © 2003 by Taylor & Francis Group, LLC

V org volume of organic phase (L)XanionXconversion of RX in the organic phaseYproduct yield[ ] molar concentration of species in brackets (kmol=m 3 )GreekG nvolume ratio (organic phase/aqueous phase)Hildebrand parametersGibbs free energymean activity coefficienteigenvaluesdimensionless spatial co-ordinate in radial directiondimensionless timeSubscripts0, i initial valueicompound iapp apparent valueobs observed valueSuperscriptsaqcatIjorgSðoverbarÞaqueous phasecatalyst phaseinterface between catalyst and organic phasesj phase (j ¼ org:, aq., cat).organic phasedroplet surface between catalyst and aqueous phasesspecies in the organic phaseCopyright © 2003 by Taylor & Francis Group, LLC

REFERENCES1. CM Starks. J Am Chem Soc 93:195–199, 1971.2. CM Starks, CL Liotta, ME Halpern. Phase-Transfer Catalysis: Fundamental Application andIndustrial Perspectives. 1st ed. New York: Chapman & Hall, 1994, pp 1–22.3. CM Starks. Modern Perspectives on the Mechanisms of Phase-Transfer Catalysis. ACSSymposium Series 659. Washington, DC: American Chemical Society, 1997, pp 10–28.4. F Orsini, F Pelizzoni, B Bellini, G Miglierini. Carbohydr Res 301:95–109, 1997.5. D Carrie` re, SJ Meunier, FD Tropper, S Cao, R Roy. J Mol Catal A: Chem 154:9–22, 2000.6. D Albanese, D Landini, M Penso. Chem Commun 2095–2096, 1999.7. YN Belokon, KA Kochetkov, TD Churkina, NS Ikonnikov, AA Chesnokov, OV Larionov,VS Parmar, R Kumar, HB Kagan. Tetrahedron: Asym 9:851–857, 1998.8. B Lygo, J Crosby, JA Peterson. Tetrahedron 40:1385–1388, 1999.9. B Lygo, J Crosby, JA Peterson. Tetrahedron 57:6447–6453, 2001.10. B Lygo, J Crosby, TR Lowdon, PG Wainwright. Tetrahedron 57:2391–2402, 2001.11. S Kotha, E Brahmachary. J Org Chem 65:1359–1365, 2000.12. MH Entezari, A Keshavarzi. Ultrasoni Sonochem 8:213–216, 2001.13. H Noguchi, H Tsutsumi, M Komiyama. Chem Commun 2455–2456, 2000.14. VK Krishnakumar, MM Sharma. Ind Eng Chem Proc Des Dev 23:410–413, 1984.15. P Tundo, A Perosa, M Selva, SS Zinovyev. Appl Catal B: Environ 32: L1–L7, 2001.16. P Hodget, R O’Dell, MSK Lee. Polymer 37:1267–1271, 1996.17. LH Tagle, FR Diaz, G Cerda, M Oyarzo, G Pen˜ afiel. Polym Bull 40:35–42, 1998.18. LH Tagle, FR Diaz, C Cares, A Brito. Polym Bull 42:627–634, 1999.19. D Albanese, D Landini, A Maia, M Penso. J Mol Catal A: Chem 150:113–131, 1999.20. FS Sirovski. Org Proc Res Dev 3:437–441, 1999.21. GD Yadav, SS Naik. Org Proc Res Dev 4:141–146, 2000.22. S Desikan, LK Doraiswamy. Chem Eng Sci 55:6119–6127, 2000.23. JP Jayachandran, ML Wang. Appl Catal A: Gen 198:127–137, 2000.24. JJ Hwang, RL Lin, RL Shieh, JJ Jwo. J Mol Catal A: Chem 142:125–139, 1999.25. JAB Satrio, LK Doraiswamy. Chem Eng J 82:43–56, 2001.26. JC Jarrouse. CR Hebd Seances Acad Sci Ser C 1424–1434, 1951.27. EV Dehmlow, SS Dehmlow. Phase Transfer Catalysis. Weinheim: Verlag Chemie, 1993.28. CM Starks, CL Liotta, ME Halpern. Phase Transfer Catalysis, Fundamental, Applicationand Industrial Perspectives. 1st ed. New York: Chapman & Hall, 1994, pp 1–17.29. WP Weber, GW Gokel. Phase Transfer Catalysis: Organic Synthesis. New York: Springer-Verlag, 1977.30. ME Halpern. Phase-Transfer Catalysis: Mechanisms and Syntheses. ACS Symposium Series659. Washington, DC: American Chemical Society, 1997.31. G. Goldberg. Phase Transfer Catalysis: Selected Problems and Applications. Netherlands:Gordon & Breach, 1992.32. N Ohtani, CA Wilkio, A Nigam, SL Regen. Macromolecules 14:516–520, 1981.33. CM Starks. J Am Chem Soc 93: 195–199, 1971.34. M Ellwood, J Griffiths, P Gregory. J Chem Soc, Chem Commun 181, 1980.35. H Iwamoto, T Sonoda, H Kobayash. Tetrahedron Lett 4703–4706, 1983.36. LJ Mathias, RA Vaidya. J Am Chem Soc 108:1093, 1986.37. ML Wang. In: Y Sasson, R Neumann, eds. Handbook of Phase Transfer Catalysis. NewYork: Chapman & Hall, 1997, pp 36–109.38. CS Kuo, JJ Jwo. J Org Chem 57:1991–1995, 1992.39. ML Wang, CC Ou, JJ Jwo. Bull Chem Soc Jpn 67:2949, 1994.40. ML Wang, CC Ou, JJ Jwo. Ind Eng Chem Res 33:2034, 1994.41. S Asai, H Nakamura, W Okada, M Yamada. Chem Eng Sci 50:943–949, 1995.42. J Dockx. Synthesis 441–456, 1973.43. A Gorgoes, A Le Coq. Tetrahedron Lett 4521, 1976.Copyright © 2003 by Taylor & Francis Group, LLC

44. Halpern, HA Zahalka, Y Sasson, M Rabinovitz. J Org Chem 50:5088, 1985.45. M Mąkosza. Pure Appl Chem 43:439–462, 1975.46. M Mąkosza, E Bialecka. Tetrahedron Lett 2:183–186, 1977.47. D Landini, A Maia, F Montanari. J Chem Soc, Chem Comm 112–113, 1977.48. A Bra¨ ndstro¨ m. Adv Phys Org Chem 15:267–330, 1977.49. M Rabinovitz, Y Cohen, M Halpern. Angew Chem, Int Ed Engl 25:960, 1980.50. KJ Evan, HJ Palmer. AIChE Symp Ser 77:104–113, 1981.51. JB Melville, JD Goddard. Chem Eng Sci 40:2207–2215, 1986.52. JB Melville, YC Yortsos, Chem Eng Sci 41:2873–2882, 1986.53. CT Chen, C Hwang, MY Yen. J Chem Eng Jpn 24:284–290, 1991.54. ML Wang, SW Chang. Ind Eng Chem Res 33:1606–1611, 1994.55. ML Wang, SW Chang. Can J Chem Eng 69:340–346, 199156. ML Wang, SW Chang. Ind Eng Chem Res 30:2378–2383, 199157. ML Wang, HM Yang. Chem Eng Sci 46:619–627, 1991.58. ML Wang, HS Wu. Chem Eng Sci 46:509–517, 1991.59. A Bhattacharya. Ind Eng Chem Res 35:645–652, 1996.60. HS Wu. J Chin Inst Chem Eng 25:183–189, 1994.61. HS Wu, SH Jou. J Chem Tech Biotechnol 64:325–330, 1995.62. HS Wu, JJ Lai. J Chin Inst Chem Eng 26:277–283, 1995.63. HS Wu. Chem Eng Sci 51:827–830, 1996.64. HS Wu. Ind Eng Chem. Res 32:1323–1327, 1993.65. ML Wang, HM Yang. Ind Eng Chem Res 30:631–635, 1991.66. ML Wang and HM Yang. Ind Eng Chem Res 29:522–526, 1990.67. CM Starks, CL Liotta. Phase Transfer Catalysis: Principles and Techniques. New York:Academic Press, 1978, p 40.68. HS Wu. J Chin Inst Chem Eng 27:185–193, 1996.69. HS Wu, SS Meng. AIChE J 43:1309–1318, 1997.70. ML Wang, HS Wu. J Org Chem 55:2344–2350, 1990.71. HM Yang. Chem Eng Commun 179:117–132, 2000.72. HM Yang. Ind Eng Chem Res 37:398–404, 1998.73. WK Fife, Y Xin. J Am Chem Soc 109:1278, 1987.74. RS Juang, SC Liu. Ind Eng Chem Res 36:5296–5301, 1997.75. RS Juang, SC Liu. Ind Eng Chem Res 37:4625–4630, 1998.76. BR Agarwal, RN Diamond. J Phys Chem 67:2785–2792, 1963.77. S Asai, H Nakamura, Y Furuichi. J Chem Eng Jpn 24:653–658, 1991.78. M Cerna, V Bizek, J Stastova, V Rov. Chem Eng Sci 48:99–103, 1993.79. EV Dehmlow, B Vehre. J Chem Res Symp 10:350–351, 1987.80. EV Dehmlow, B Vehre. New J Chem 13:117–119, 1989.81. NA Gibson, DC Weatherburn. Anal Chim Acta 58:149–157, 1972.82. NA Gibson, DC Weatherburn. Anal Chim Acta 58:159–165, 1972.83. HMNH Irvine, A D Damodaran. Anal Chim Acta 53:267–275, 1971.84. HMNH Irvine, A D Damodaran. Anal Chim Acta 53:277–285, 1971.85. HS Wu, RR Fang, SS Feng, KH Hu. J Chin Int Chem Eng 29:99–108, 1998.86. HS Wu, RR Fang, SS Feng, KH Hu. J Mol Catal A: Chem 136:135–146, 1998.87. A Bra¨ ndstro¨ m. Pure Appl Chem 54:1769–1782, 1982.88. MH Abraham. J Phys Org Chem 299–308, 1971.89. MH Abraham, AF Danil de Namor. J Chem Soc, Faraday I 72:955–962, 1976.90. AA Ansari, MR Islam. Can J Chem 66:1720–1727, 1988.91. P Beronius, A Bra¨ ndstro¨ m. Acta Chem Scand A 30:687–691, 197692. MS Tseng. Thermodynamic Properties of Quaternary Salts in an Organic Solvent andAlkaline Solution. MS thesis, Yuan-Ze University, Taoyuan, Taiwan, 2000.93. JO Bockries, KN Reddy. Modern Electrochemistry. Plenum Press, 1980, pp 261–265.94. T Kenjo, RM Diamond. J Phys Chem 76:2454–2459, 1972.Copyright © 2003 by Taylor & Francis Group, LLC

95. T. Kenjo, RM Diamond. J Inorg Nucl Chem 36:183–188, 1974.96. D Landini, A Maia, G Podda. J Org Chem 47:2264–2268, 1982.97. D Landini, A Maia, F Montanari. J Am Chem Soc 100:2796–2801, 1978.98. D Landini, A Maia, A Rampoldi. J Org Chem 54:328–332, 1989.99. CM Starks, RM Owens. J Am Chem Soc 95:3613–3617, 1973.100. AW Herriott, D Picker. J Am Chem Soc 97:2345–2349, 1975.101. K Gustavii. Acta Pharm Suec 4:233, 1967.102. K Fukunaga, H Shirai, S Ide, M Kimura. Nippon Kagaku Kaishi 1148–1153, 1980.103. FS Sirovski. Phase-Transfer and Micellar Catalysis in Two-Phase System. ACS SymposiumSeries 659. Washington, DC: American Chemical Society, 1997.104. A Leo, C Hansch, D Elkins. Chem Rev 71:525–563, 1971.105. WC Griffin. J Soc Cosmet Chem 1:311, 1949.106. WC Griffin. Am Perfum Essent Oil Rev 65:26, 1952.107. JT Davies. Proceedings of the 2nd International Congress on Surface Activity, 1957, vol. 1,p 426.108. S Nagata. Mixing Principles and Applications. New York: Halsted Press, 1975.109. AJ Parker, U Mayer, R Schmid, V Gutmann. J Org Chem 43:1843–1854, 1978.110. J Czapkiewicz, T Czapkiewicz, D Struck. Pol J Chem 52, 2203, 1978.111. RW Taft, MH Abraham, RM Doherty, MJ Kamlet. J Am Chem Soc 107:3105–3110, 1985.112. TB Lin, MY Yeh, YP Shih. Proc PAC Chem Eng Congr 3:178, 1983.113. JB Melville, JD Goddard. Ind Eng Chem Res 27:551, 1988.114. ML Wang, YM Hsieh. J Chem Eng Jpn 26:374–381, 1993.115. SS Leie, RR Bhave, MM Sharma. Chem Eng Sci 38:765–773, 1983.116. ML Wang, HS Wu. J Org Chem 55:2344–2350, 1990.117. SD Naik, LK Doraiswamy. AIChE J 612–645, 1998.118. T Stanley, J Quinn. Chem Eng Sci 42:2313–2324, 1987.119. S Matson. Advances in Catalytic Technologies Seminars, Catalystica. Mountain View, CA,USA, 1988.120. JH Rushton, S Nagata, TB Rooney. AIChE J 10:298–302, 1964.121. S Asai, J Hatanaka, Y Uekawa. J Chem Eng Jpn 16:463–469, 1983.122. S Asai, H Nakamura, Y Furuichi. AIChE J 38:397–403, 1992.123. S Asai, H Nakamura, T Sumita. AIChE J 40:2028, 1994.124. HM Yang, CK Yu, TM Chen. J Chin Inst Chem Eng 29:161–169, 1998.125. A Kiani, RR Bahave, KK Shirkar. J Membr Sci 20:125–145, 1984.126. R Prasad, A Kiani, RR Bhave, KK Sirkar. J Membr Sci 26:79–97, 1986.127. YH Lin. Mass–Transfer Behavior of Quaternary Salts in Membrane Extractor, MS thesis,Yuan-Ze University, Taoyuan, Taiwan, 2000.128. SV Save, SS Zanwar, VG Pangarkar. Chem Eng Sci 44:1591, 1989.129. N Wakao, S Kaguei. Heat and Mass Transfer in Packed Beds. New York: Gordon & Breach,1982, p 156.130. WJ McManamey, DKS Multani, JT Davies. Chem Eng Sci 30:1536, 1975.131. EL Cussler. Diffusion Mass Transfer in Fluid System. 2nd ed. New York: CambridgeUniversity Press, 1997, pp 48, 355.132. CM Starks. Modern Perspective on the Mechanisms of Phase-Transfer Catalysis. ACSSymposium Series 659. Washington, DC: American Chemical Society, 1997.133. F Moberg, O Bokman, H Bokman, OG Siegbahn. J Am Chem Soc 113:3663–3667, 1991.134. B Reuben, K Sjoberg. Chemtech 315–320, 1981.135. LL Travlarides, M Stamatoudis. In: TB Drew, GR Cokelet, JW Hooper Jr, T Vermeulen, eds.Advances in Chemical Engineering, vol. 11. New York: Academic Press, 1981, pp 119–273.136. NN Dutta, GS Patil, Can J Chem Eng 71:802–804, 1993.137. SL Regen. J Am Chem Soc 97:5956–5958, 1975.138. SL Regen. J Am Chem Soc 98:6270–6274, 1976.139. SL Regen. J Org Chem 42:875–879, 1977.Copyright © 2003 by Taylor & Francis Group, LLC

140. SL Regen. Angew Chem, Int Ed Engl 18:421–429, 1979.141. SL Regen, JJ Besse. J Am Chem Soc 101:4059–4063, 1979.142. M Tomoi, WT Ford. J Am Chem Soc 103:3821–3828, 1981.143. M Tomoi, WT Ford. J Am Chem Soc 103:3828–3829, 1981.144. M Tomoi, E Ogawa, Y Hosokawa, H Kakjuchi. J Polym Sci, Polym Chem Ed 20:3421, 1982.145. M Tomoi, E Nakamura, Y Hosokawa, H Kakjuchi. J Polym Sci, Polym Chem Ed 23:49–61,1985.146. M Tomoi, Y Hosokawa, H Kakjuchi. Makromol Chem Rapid Commun 4:227–230, 1983.147. B Ragaini, G Verzella, A Ghigone, G Colombo. In Eng Chem Proc Des Dev 25:878–885,1986.148. V Ragaini, G Colombo, P. Barzhagi. Ind Eng Chem Res 27:1382–1387, 1988.149. V Ragaini, G Colombo, P Barzhagi, E Chiellini, S D’Antone. Ind Eng Chem Res 29:924–928,1990.150. P Schlunt, PC Chau. J Catal 102:348–356, 1986.151. S Desikan, LK Doraiswany. Ind Eng Chem Res 34:3524–3537, 1995.152. WT Ford, M Tomoi. Adv Polym Sci 55:49–104, 1984.153. WT Ford, J Lee, M Tomoi. Macromolecules 15:1246–1251, 1982.154. O Arrad, Y Sasson. J Org Chem 55:2252, 1990.155. P Tundo, P Venturello. J Am Chem Soc 103:856–861, 1981.156. P Tundo, P Venturello, E Angeletti. J Am Chem Soc 104:6551–6555, 1982.157. P Tundo, P Venturello. J Am Chem Soc 101:6606–6613, 1979.158. S Telford, P Schlunt, PC Chau. Macromolecules 19:2435–2439, 1986.159. JF Hradil, C Konak, K Jurek. React Polym 9:81–89, 1988.160. F Svec. Pure Appl Chem 60:377–386, 1988.161. ML Wang, HS Wu. Ind Eng Chem Res 31:490–496, 1992.162. ML Wang, HS Wu. J Polym Sci, Polym Chem 30:1393–1399, 1992.163. ML Wang, HS Wu. Ind Eng Chem Res 31:2238–2242, 1992.164. HS Wu, JF Tang. J Mol Cata, A: Chem 145:95–105, 1999.165. HS Wu, SS Meng S. Can J Chem Eng 77:1146–1153, 1999.166. HS Wu, CS Lee. J Catal 199:217–223, 2001.167. E Ruckenstein, L Hong. J Catal 136:378, 1992.168. E Ruckenstein, JS Park. J Poly Sci C, Polym Lett 26:529, 1988.169. JF Tang. Application and Kinetics of Liquid–Solid–Liquid Triphase Reaction Ether–EsterCompound. MS thesis, Yuan-Ze University, Taoyan, Taiwan, 1998.170. DH Freeman. In: JA Marinsky, ed. Ion Exchange. vol. I. New York: Marcel Dekker, 1966,ch. 5.171. JP Idoux, R Wysocki, S Young, J Turcot, C Ohlman, R Leonard. Synth Commun 13:139,1983.172. Y Okahata, K Ariga, T Seki. J Chem Soc, Chem Commun 920, 1985.173. Y Shan, R Kang, W Li. Ind Eng Chem Res 28:1289, 1989.174. ML Wang, HM Yang. Ind Eng Chem Res 30:2384–2390, 1991.175. ML Wang, HM Yang. Ind Eng Chem Res 30:1868–1875, 1992.176. ML Wang, HM Yang. J Chin Inst Chem Eng 23:200–208, 1992.177. R Aris. The Mathematical Theory of Diffusion and Reaction in Permeable Catalysis. vol. I.London: Oxford University Press, 1975, pp 119–126.178. PF Marconi, WT Ford. J Catal 83:160–167, 1983.179. DY Cha. React Polym 5:269–279, 1987.180. HS Wu, SS Meng. Chem Eng Sci 53:4073–4084, 1998.181. ML Wang, HS Wu. Ind Eng Chem Res 29:2137–2142, 1990.182. M Tomoi, Y Hosokawa, H Kakjuchi. J Polym Sci, Polym Chem Ed 22:1243–1250, 1984.183. CM Starks, CL Liotta, ME Halpern. Phase-Transfer Catalysis: Fundamental Application andIndustrial Perspectives. 1st ed. New York: Chapman & Hall, 1994, pp 108–111.Copyright © 2003 by Taylor & Francis Group, LLC

184. Y Sasson. In: Y Sasson, R Neumann, ed. Handbook of Phase Transfer Catalysis. 1st ed.London: Blackie Academic & Professional, 1997, pp 111–134.185. A Esikova. In: Y Sasson, R Neumann, ed. Handbook of Phase Transfer Catalysis. 1st ed.London: Blackie Academic & Professional, 1997, pp 1–35.186. CL Liotta, J Berkner, J Wright, B Fair. Mechanisms and Applications of Solid–Liquid Phase-Transfer Catalysis. ACS Symposium Series 659. Washington, DC: American ChemicalSociety, 1997, pp 29–40.187. A Lo´ pez, MM Man˜ as, R Pleixats, A Roglans, J Ezquerra, C Pedregal. Tetrahedron 52:8365–8386, 1996.188. A Lo´ pez, R Pleixats. Tetrahedron: Asym 9:1967–1977, 1998.189. FS Sirovski, ER Berlin, SA Mulyashov, EA Bobrova, ZI Batrakova, DF Dankovskaya. OrgProc Res Dev 1:253–256, 1997.190. T Perrad, JC Plaquevent, JR Desmurs, D He´ brault. Org Lett 2:2959–2962, 2000.191. HM Yang, TM Chen. J Chin Inst Chem Eng 29:367–374, 1998.192. S Cohen, A Zoran, Y Sasson. Tetrahedron Lett 39:9815–9818, 1998.193. R Guilet, J Berlan, O Louisnard, J Schwartzentruber. Ultrason Sonochem 5:21–25, 1998.194. A Knochel, G Rudolph. Tetrahedron Lett 3739–3740, 1974.195. AW Herriot, D Picker. J Am Chem Soc 97:2345–2349, 1975.196. BP Czech, MJ Pugia, RA Bartsch. Tetrahedron 41:5439–5444, 1985.197. KH Wong, APW Wai. J Chem Soc, Perkin Trans II 317–321, 1983.198. MC Vander Zwan, FW Hartner. J Org Chem 43:2655–2657, 1978.199. GD Yadav, MM Sharma. Ind Eng Chem Proc Des Dev 20:385–390, 1981.200. HA Yee, HJ Palmer, SH Chen. Chem Eng Progr 83:33–39, 1987.201. HM Yang, HE Wu. Ind Eng Chem Res 37:4536–4541, 1998.202. HM Yang, CM Wu. J Mol Catal A: Chem 153:83–91, 1999.203. JB Melville, JD Goddard. Chem Eng Sci 40:2207–2215, 1985.204. JB Melville, JD Goddard. Ind Eng Chem Res 27:551–555, 1988.205. JB Melville, YC Yortsos. Chem Eng Sci 41:2873–2882, 1986.206. SD Naik, LK Doraiswamy. Chem Eng Sci 52:4533–4546, 1997.207. CS Chu. Kinetics for Synthesizing Benzyl Phenyl Ether via Solid–Liquid Phase–TransferCatalysis over Polyethylene Glycol and Quaternary Ammonium Salts. MS thesis, NationalChung Hsing University, Taichung, Taiwan, 2001.208. SS Yufit, GV Kryshtal, IA Esikova. Adsorption on Interfaces and Solvation in Phase-Transfer Catalysis. ACS Symposium Series 659. Washington, DC: American ChemicalSociety, 1997, pp 52–67.209. F Sirovski, C Reichardt, M Gorokhova, S Ruban, E Stoikova. Tetrahedron 55:6363–6374,1999.210. BV Erofeev. Dokl Akad Nauk SSSR 52:515–518, 1946.211. AY Rozovskii. Heterogeneous Chemical Reactions: Kinetics and Macrokinetics. Moscow:Nauka, 1980, pp. 180–300.212. SS Yuift, SS Zinovyev. Tetrahedron 55:6319–6328, 1999.213. SS Yufit, IA Esikova. J Phys Org Chem 4:336–340, 1991.214. IA Esikova, SS Yufit. J Phys Org Chem 4:149–157, 1991.215. IA Esikova, SS Yufit. J Phys Org Chem 4:341–345, 1991.216. HM Yang, PI Wu. Appl Catal A: Gen 209:17–26, 2001.217. PI Wu. Kinetics for Solid–Liquid and Solid–Liquid–Liquid Phase-Transfer Catalysis: I.Etherification of Halo-ester II. Esterification of Aliphatic Dicarboxylate. MS thesis,National Chung Hsing University, Taichung, Taiwan, 2000.218. HM Yang, CM Wu, HE Wu. J Chem Technol Biotechnol 75:387–393, 2000.219. HM Yang, PI Wu, CM Li. Appl Catal A: Gen 193:129–137, 2000.220. C Lepetit, M Che. J Mol Catal A: Chem 100:147–160, 1995.221. R Neumann, Y Sasson. J Org Chem 49:3448–3451, 1984.222. DH Wang, HS Weng. Chem Eng Sci 43:2019–2024, 1988.Copyright © 2003 by Taylor & Francis Group, LLC

223. SD Naik, LK Doraiswamy. AIChE J 44:612–646, 1998.224. T Ido, M Saiki, S Goto. Kagaku Kogaku Ronbunshu 15:403–410, 1989.225. D Mason, S Magdassi, Y Sasson. J Org Chem 56:7229–7232, 1991.226. DH Wang, HS Weng. Chem Eng Sci 50:3477–3486, 1995.227. T Ido, T Yamamoto, G Jin, S Goto. Chem Eng Sci 52:3511–3520, 1997.228. GD Yadav, CA Reddy. Ind Eng Chem Res 38:2245–2253, 1999.229. G Jin, T. Ido, S. Goto. J Chem Eng Jpn 32:417–423, 1999.230. HC Hsiao, HS Weng. Ind Eng Chem Res 38:2911–2918, 1999.231. HC Hsiao, SM Kao, HS Weng. Ind Eng Chem Res 39:2772–2778, 2000.232. HS Weng, CM Wang, DH Wang. Ind Eng Chem Res 36:3613–3618, 1997.233. T Ido, T Susaki, G Jin, S Goto. Appl Catal A:Gen 201:139–143, 2000.234. HM Yang. J Chin Inst Eng 21:399–408, 1998.235. JJ Krueger, MD Amiridis, HJ Ploehn. Ind Eng Chem Res 40:3158–3163, 2001.236. ME Halpern. Recent Trends in Industrial and Academic Phase-Transfer Catalysis. ACSSymposium Series 659. Washington, DC: American Chemical Society, 1997, pp 1–9.Copyright © 2003 by Taylor & Francis Group, LLC