11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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