11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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