Sheet silicates: broad spectrum catalysts for organic synthesis

Abstract

Although montmorillonites have been found to have little catalytic activity in their natural Na + form, various cation-exchanged montmorillonites have proved to be effective catalysts for a wide variety of organic reactions. Alkenes can be induced to add (a) water, (b) alcohols, (c) thiols and (d) carboxylic acids to furnish ethers, thioethers and esters in the presence of these cation-exchanged sheet silicates, but amines failed to undergo addition with alkenes under similar conditions. The number of products obtained in each reaction depended upon the ease of rearrangement of the carbocation intermediates. High yields were obtained in those cases where a single carbocation intermediate was involved. The cation-exchanged sheet silicates also catalysed a variety of elimination reactions. Water was eliminated from alcohols to produce ethers, ammonia was eliminated from amines to make secondary amines and hydrogen sulphide was eliminated from thiols to give dialkyl sulphides. Cation-exchanged bentonites were also effective catalysts in the production of dioxan from ethylene oxide and in the preparation of acetals with orthoformates. Analysis of the products from the reactions suggested that the cation-exchanged montmorillonites reacted, in nearly all cases, as acidic heterogeneous catalysts.