The Use of Clays as Sorbents and Catalysts

This paper attempts to show how the structural, physical and chemical properties of clay minerals relate to their laboratory, industrial and environmental uses as sorbents and catalysts. A brief review of the formulae, structures of clays and their relationship to their chemical and physical properties is given. Isomorphous substitution of layer cations generates layer charges, which are neutralised by the presence of exchangeable, compensating ions in the interlayer. Acid activation increases clay surface areas, acidity and sorption characteristics producing large volumes of acidic aluminium salt waste. Cationic clays often have high Brønsted and Lewis acidity, which enable the clays to be used as clean, often highly specific catalysts in organic and inorganic reactions or as supports for catalysts or reagents.Batch cation exchange experiments can be used in the comparison of the exchange sites available in intact and acid activated minerals, via Langmuir or Freundlich type isotherms and R_d plots. Industrially clays are used to form gels, in the removal of carotenoids and chlorophyll in the production of cooking oils and light coloured soaps, in cat litter and in carbonless copying paper. Singlet oxygen generated by these dyes in the latter case causes fading of the image, an autocatalytic effect in their own destruction. In the laboratory clays have been used extensively as acid catalysts, as selective supports in g.c. and as active supports for oxidants (e.g. clayfen and claycop) and Lewis acids (e.g. clayzic) and as regioselective catalysts for Diels-Alder reactions and for the generation of reactive intermediates such as carbenes. The thermodynamically less favoured isomer can often be selected for if the transition state of formation is less bulky.In the environment, clay minerals are highly important in soils, as they swell and retain water, act as pH buffers and as slow release agents for essential minerals and adsorb crude oil and radionuclides from spills and leaks.