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

Reactions mediated by phase transfer catalysts (PT catalysts) can be carrie d out with either a homogeneous or a heterogeneous PT catalyst. Heterogeneo us 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 diffusio nal retardation. In this work polymer bound tributylmethylammonium chloride is compared to its homogeneous analog, benzyltributylammonium chloride, fo r different reaction systems: esterification of benzyl chloride to benzyl a cetate and alkaline hydrolysis of het;yl acetate and octyl acetate. Their p erformance is distinctly superior to that of their homogeneous analogs at l ower conversions. An unusual observation, however, is the "autocatalytic" b ehavior of the hydrolysis reactions which makes it possible: for them to pr oceed faster than their heterogeneous counterparts as more product builds u p with the progress of reaction (i.e. at higher conversions). This is becau se 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 heterogeneou s components of the reactions determined. (C) 2000 Published by Elsevier Sc ience Ltd. All rights reserved.