Catalysis by montmorillonites

Abstract

A preliminary set of ‘rules’ for acid-catalysis by ion-exchanged montmorillonites has been derived from the reaction of alkenes, alcohols and alicyclic acids over such catalysts. It has been shown that: 1. Cr(III) and Fe(III) are the most active interlayer cations. Although Al is also active, the exact procedures used for the ion-exchange and washing steps appear to be critical for giving catalysts of reproducible activity. 2. Below 100°C, the reactions proceed provided they involve tertiary or allylic carbocation intermediates, whereas at 150–180°C reactions involving primary and secondary carbocations are possible. 3. Reactions of carbocations with unsaturated hydrocarbons take place overwhelmingly in the interlayer region of the clay, where the hydrocarbon double bond can be effectively polarized. Reactions of carbocations with polar, oxygenated, species can take place on the surface of the clay particles as well as in the interlayer space. 4. When the acid-catalysed reactions are performed in the liquid phase and involve tertiary carbocations, the most suitable solvents are those which provide miscibility; 1,4-dioxan is especially good. However, when more acid conditions are required for the formation of primary and secondary carbocations a non-polar solvent is more efficacious.