Phase-transfer catalysis: a new rigorous mechanistic model for liquid-liquid systems

Abstract

A general kinetic model for phase-transfer catalyzed reactions involving two liquid phases and a homogeneous catalyst has been developed. The proposed new model introduces the separation of the contributions of the phase-transfer catalysis (PTC)-enhanced reaction and the non-PTC reaction toward the overall conversion. The common approach in the past was that the non-PTC reaction was either ignored or incorporated in the PTC-enhanced reaction. Even more significantly, the model also incorporates terms for explaining the variability of catalyst phase distribution with changes in electrolyte composition in the aqueous phase, which subsequently affects the amount of inorganic nucleophile that reacts in the organic phase. The industrially important reaction, synthesis of benzaldehyde from benzyl chloride, has been used to validate the model. Three reaction steps, i.e. esterification, hydrolysis, and oxidation, as well as combinations thereof, were considered. It was found that the model was able to fit the experimental data well. Model verification was done, not by parameter estimation by regression, but by determining them from separate sets of experiments. This lends greater credibility to multiparameter models (like that in the present case). Further analysis showed that the parameter values of the individual reactions can be used to classify the reactions, based on an approach proposed earlier by Starks et al. (Phase Transfer Catalysis, Chapman and Hall, New York, USA, 1994) and extended by Satrio et al. (Chem. Eng. Sci. 55 (2000) 5013).