11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

(a) Pseudo-Steady-State LLPTC model.The reaction relationship is given as1k app¼V K QX A þ V kQ ið31Þwhere denotes the reactivity of the phosphazene reaction. The plot of 1=k app versus , inwhich the data were measured at the initial time of different experimental runs, allows oneto obtain the mass transfer coefficient, K QX A, and the intrinsic reaction rate constant k,from the slope and intercept of the straight line.(b) Extrapolation Method. If mass transfer resistance influences the reaction, the concentrationof the active catalyst QY cannot remain constant during the course of thereaction. Decreasing the concentration of organic reactant RX increases the apparentfirst-order reaction-rate constant. When the concentration of organic reactant decreases,both the reaction rate and the effect of mass transfer decrease. If the organic reactantconcentration extrapolates to zero ð½RXŠ !0Þ, the effect of mass transfer can beneglected. The intrinsic reaction-rate constant, k, is easily evaluated.(c) Half-Reaction in the Organic Phase. The organic reactant reacted with an intermediatecatalyst, tetra-n-butyl ammonium phenolate, in a homogeneous organic phase.The intrinsic reaction-rate constant was calculated from Eq. (12).Another LLPTC is usually performed in an agitated system, in which the organicphase is mostly dispersed. Several efforts have been made in developing the theory for atwo-liquid phase with chemical reactions. For an organic phase being the dispersed phase,several phenomena take place: (1) formation of a single droplet in the continuous phase bystirring, (2) free rise or fall of a droplet through the continuous phase, and (3) coalescenceof a droplet at the end of the free-rise period. During the extraction of a catalytic intermediate,mass transfer from the bulk aqueous phase to the organic droplet surface influencesthe rate of PT reaction. Yang [71,72] studied the general analysis of the dynamics ofa PT-catalytic reaction in a dispersed system of liquid–liquid phases, considering theirreversible and reversible reactions by solving the finite difference and Runge–Kuttafourth-order methods. The rates of change of RX, RY, QX, and QY in an organic dropletare described by the instantaneous equations of diffusion and reaction with the correspondinginitial and boundary conditions as follows:@ C i@t ¼ D i @r 2 @ C iþ @r @r iR; i ¼ RX; RY; QX; and QY ð32Þr 2where i is the stoichiometric coefficient of the i component.The kinetics of inverse PT-catalytic extraction of species into the water phase wascarried out with partially water-soluble pyridines or derivatives [36,38,40,59,73], as shownin mechanism (9). These reactions can be described by a pseudo-first-order hypothesis[38,40]:k app ¼ k h þ k c ½PNOŠ ið33ÞHowever, so far, the detailed kinetics of I-LLPTC are unclear.As mentioned above, the various approaches to LLPTC modeling have been taken,and a comprehensive general model for N-LLPTC reactions is widely held. However, akinetic model for I-LLPTC and R-LLPTC reactions is yet to be developed.Copyright © 2003 by Taylor & Francis Group, LLC