Rate enhancements due to autocatalysis and heterogenization in phase transfer catalysis: a comparative study

Abstract

Reactions mediated by phase transfer catalysts (PT catalysts) can be carried out with either a homogeneous or a heterogeneous PT catalyst. Heterogeneous catalysts are usually bound to a polymer support and have the potential to be used in continuous processes. However, polymer bound catalysts do not enjoy industrial acceptance mainly because of the traditional circumstance that the reaction rates are lower in solid bound catalysts due to diffusional retardation. In this work polymer bound tributylmethylammonium chloride is compared to its homogeneous analog, benzyltributylammonium chloride, for different reaction systems: esterification of benzyl chloride to benzyl acetate and alkaline hydrolysis of hexyl acetate and octyl acetate. Their performance is distinctly superior to that of their homogeneous analogs at lower conversions. An unusual observation, however, is the "autocatalytic" behavior of the hydrolysis reactions which makes it possible for them to proceed faster than their heterogeneous counterparts as more product builds up with the progress of reaction (i.e. at higher conversions). This is because the autocatalytic product increases the solubility of the PT catalyst in the organic phase (where the reaction occurs) with consequent enhancement of the reaction rate. Kinetic analyses of all the systems have been carried out and rate constants for the noncatalytic, homogeneous, and heterogeneous components of the reactions determined.