Keynote article. Environmentally friendly chemistry using supported reagent catalysts: structure–property relationships for clayzic

Abstract

Supported reagents have been widely used in organic synthesis for some 25 years and their importance is likely to increase as a result of new environmental legislation and the drive towards clean technology. While many supported reagents are stoichiometric in reactions the successful development of genuinely catalytic materials greatly enhances their value especially in liquid phase, typically fine chemical syntheses. Achieving an understanding of the nature of these fascinating materials is also an important aspect of their development and is essential if their true potential is to be realised. Solid acids are the most widely studied of supported reagents and their use as more environmentally acceptable replacements for conventional Brönsted and Lewis acids is likely to become increasingly important. Clayzic is a good example of an environmentally friendly catalyst with particular value as an alternative 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 treatment of either clayzic or its base material K10 results in the loss of any montmorillonite crystallinity that remained after the acid treatment of tonsil 13 used to form K10. Thermal treatment of clayzic also results 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 adsorbate to enter more of the polar regions of the material. These polar regions can be identified as mesopores created by the acid treatment of the clay and in which the zinc ions largely reside. Spectroscopic titration of the acid sites in clayzic show these to be largely Lewis acid in character. Thus clayzic owes its remarkable Friedel–Crafts activity to the presence of high local concentrations of zinc ions in structural mesopores. Relative reaction rates for the clayzic catalysed benzylation of alkylbenzenes also reveal the importance of these highly polar mesopores. Considerable rate enhancements can be achieved by thermally activating the catalyst and this can be largely attributed to the dehydration of the catalyst enabling better partitioning of the alkylbenzenes into the mesopores. Clayzic can be considered as being a large pore molecular sieve but where the sieving of molecules is controlled more by molecular polarities/polarisibilities than by molecular shape.