ENVIRONMENTALLY FRIENDLY CHEMISTRY USING SUPPORTED REAGENT CATALYSTS - STRUCTURE-PROPERTY RELATIONSHIPS FOR CLAYZIC

Supported reagents have been widely used in organic synthesis for some 25 years and their importance is likely to increase as a result of ne w environmental legislation and the drive towards clean technology. Wh ile many supported reagents are stoichiometric in reactions the succes sful development of genuinely catalytic materials greatly enhances the ir value especially in liquid phase, typically fine chemical syntheses . Achieving an understanding of the nature of these fascinating materi als is also an important aspect of their development and is essential if their true potential is to be realised. Solid acids are the most wi dely studied of supported reagents and their use as more environmental ly acceptable replacements for conventional Bronsted and Lewis acids i s likely to become increasingly important. Clayzic is a good example o f an environmentally friendly catalyst with particular value as an alt ernative to the hazardous reagent aluminium chloride in Friedel-Crafts reactions. The structure and properties of this catalyst are, however , poorly understood. X-Ray diffraction studies show that thermal treat ment of either clayzic or its base material K10 results in the loss of any montmorillonite crystallinity that remained after the acid treatm ent of tonsil 13 used to form K10. Thermal treatment of clayzic also r esults in a steady increase in the surface area of the material. While this is also consistent with structural changes the increase is also likely to be partly due to the dehydration allowing the non-polar adso rbate to enter more of the polar regions of the material. These polar regions can be identified as mesopores created by the acid treatment o f the clay and in which the zinc ions largely reside. Spectroscopic ti tration of the acid sites in clayzic show these to be largely Lewis ac id in character. Thus clayzic owes its remarkable Friedel-Crafts activ ity to the presence of high local concentrations of zinc ions in struc tural mesopores. Relative reaction rates for the clayzic catalysed ben zylation of alkylbenzenes also reveal the importance of these highly p olar mesopores. Considerable rate enhancements can be achieved by ther mally activating the catalyst and this can be largely attributed to th e dehydration of the catalyst enabling better partitioning of the alky lbenzenes into the mesopores. Clayzic can be considered as being a lar ge pore molecular sieve but where the sieving of molecules is controll ed more by molecular polarities/polarisibilities than by molecular sha pe.