11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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