| The importance of solubilization kinetics in solid-liquid phase transfer catalyzed reaction systems is studied using a rotating disk reactor. Solubilization experiments are interpreted using the theory of Levich for rotating disk flow. A stagnant film model is used to analyze the results for solubilization with homogeneous chemical reaction.;The esterification reaction between solid potassium acetate (KOAc) and benzyl chloride in chlorobenzene is studied with either a crown ether, 18-crown-6, or a quaternary ammonium salt, Aliquat 336, as phase transfer catalyst. Solubilization of KOAc in the presence of 18-crown-6 is essentially instantaneous, the overall dissolution rate being limited by bulk diffusion. Furthermore, pre-complexed crown ether retains its solubilizing power by precipitation of unreactive KCl to the bulk phase. In contrast, with Aliquat 336 the solubilization reaction at the solid surface is rate determining. In addition, an unreactive layer of chloride ion is deposited on the KOAC surface as a result of solubilization. Kinetic mechanisms consistent with these results are proposed for both types of catalysts.;For solubilization with simultaneous homogeneous chemical reaction, results reveal that the overall kinetics of the solid-liquid reaction system depend on the combined effects of the solubilization reaction, homogeneous reaction, and mass transport. In particular, it is found that the quat-catalyzed reaction exhibits pseudo-first-order kinetics, not as a consequence of an equilibrium solubilization reaction in the absence of mass transport resistances, but because of the slow solubilization reaction. In light of these findings, it is clear that any kinetic study of solid-liquid phase transfer catalysis must account for the possibilities of finite solubilization reaction rate and surface deactivation when quaternary ammonium salts are used. |
