11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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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
Page 1: 11Interfacial Mechanism and Kinetic
Page 5 and 6: 1. Applications in BiologyOrsini et
Page 7 and 8: Tundo et al. [15] reported an effic
Page 9 and 10: transfer of catalyst from the react
Page 11 and 12: A summary of characteristic kinetic
Page 13 and 14: dy 1ady o¼ QYy ody 2ody o¼ QXy
Page 15 and 16: (a) Pseudo-Steady-State LLPTC model
Page 17 and 18: The order of magnitude of D Q for q
Page 19 and 20: E T Q þ ;H 2 O ¼ ½Qþ Š½X :jH
Page 21 and 22: Gustavii [101] and Bockries and Red
Page 23 and 24: The mass transfer resistances stron
Page 25 and 26: The extractive effectiveness factor
Page 27 and 28: Quaternary onium salts, crown ether
Page 29 and 30: 26 > Dowex 1 2 > A-27 IRA-410 > I
Page 31 and 32: C 6 H 6 > C 6 H 5 CH 3 and the tren
Page 33 and 34: Many experimental studies on three-
Page 35 and 36: FIG. 4 Yields of products and conve
Page 37 and 38: transfer of organic or aqueous reac
Page 39 and 40: [187-196]. Several reactions that c
Page 41 and 42: the solid salt directly reacts with
Page 43 and 44: solubility in the organic phase, th
Page 45 and 46: QCl þ AcONa ðsÞ ÐQCl:AcONa ðs
Page 47 and 48: curve of the product SeK 2 . The pa
Page 49 and 50: Taking the mass balance for the cat
Page 51 and 52: The ion-exchange reaction occurs at
Page 53 and 54:
in both aqueous and organic phases
Page 55 and 56:
Type I.The catalyst resides in the
Page 57 and 58:
V orgk obs ¼V org þD A V thirdk o
Page 59 and 60:
@C orgRX@t@C catQY@t@C catQX@t¼ D
Page 61 and 62:
tively. When the amount of the thir
Page 63 and 64:
V org volume of organic phase (L)Xa
Page 65 and 66:
44. Halpern, HA Zahalka, Y Sasson,
Page 67 and 68:
140. SL Regen. Angew Chem, Int Ed E
Page 69:
223. SD Naik, LK Doraiswamy. AIChE
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11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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