11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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