11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

The ion-exchange reaction occurs at the interfacial zone to form QY, then conducting theintrinsic reaction:M þ Y ðorgÞþQ þ X ðorgÞ $Q þ Y ðorgÞþM þ X ðorgÞQ þ Y ðorgÞþRX ðorgÞ !RY ðorgÞþQ þ X ðorgÞð147Þð148ÞThe regenerated Q þ X ðorgÞ continues to catalyze the formation of Q þ Y ðorgÞ, andM + X (org) is always in equilibrium with MX(s):M þ X ðorgÞ $MX ðsÞð149ÞThe rate of equation is derived asd½RXŠdt¼ k org ½RXŠ½Q þ Y Š¼ k org K e½M þ Y Š½M þ X Š ½RXŠ½Qþ X ŠK e ¼ ½Qþ Y Š½M þ X Š½Q þ X Š½M þ Y Šð150Þð151ÞIf the value f ¼½M þ Y Š=½M þ X Š is approximately a constant, the rate of the equation canbe expressed in a pseudo-first-order form:d½RXŠ¼ kdt app ½RXŠk app ¼ k org K e f ½Q þ X Šð152Þð153ÞDuring the course of reaction, when the value of f is gradually diminished, the initialreaction rate and deactivation rate should be applied. With or without adding waterinfluences the mass transfer rate from the interface into the organic phase.D. Kinetic Modeling of Heterogeneous SolubilizationNaik and Doraiswamy [206] developed a mathematical model for the case of heterogeneoussolubilization that involves the steps of ion exchange in the solid phase, interphasetransport of the catalyst and the intermediate, and the organic reaction. In this model, ionexchange occurring within the solid phase is assumed due to the possible deposition of theproduct salt MX on the solid surface retarding formation of the catalytic intermediate.The controlling step can either be the liquid-phase transfer steps, the diffusion within thereactive solid, the adsorption–desorption steps, the surface ion-exchange reaction, or theliquid organic reaction. This treatment is similar to that in gas–solid catalytic reaction.The controlling step may shift to another step continuously with time. The reaction oforganic RX with solid MY in the presence of PT catalyst QX is considered, and a poroussolid wherein the ion-exchange reaction takes place throughout the whole pellet, ratherthan at a sharp interface due to the liquid penetrating into it is assumed. The governingequations are derived as follows [206]:RX ðorgÞþMY ðsÞ ! QX RY ðorgÞ MX ðsÞ ð154ÞWithin the solid phase:Copyright © 2003 by Taylor & Francis Group, LLC