Although natural Na-bentonite has little catalytic activity, various cation-exchanged bentonites have proved to be effective catalysts for a wide variety of organic reactions. In the presence of these catalysts alkenes can be induced to add (a) water, to yield branched-chain symmetrical ethers; (b) alcohols, to give a variety of ethers; (c) thiols, to yield thio-ethers; (d) carboxylic acids, to give esters. The number of products obtained in each reaction depends on the ease of rearrangement of the carbocation intermediates. High yields are obtained where a single carbocation intermediate is formed, A variety of elimination reactions is also catalysed by these sheet silicates. Water is eliminated from alcohols to produce ethers, ammonia is eliminated from amines to make secondary amines, and hydrogen sulphide is eliminated from thiols to give dialkyl sulphides. In most cases the ion-exchanged bentonites react as acidic heterogeneous catalysts.

The importance of a number of techniques (including 1H and 13C NMR, XRD and IR) in exploring the important catalytic properties of synthetic and natural clays is described. A clear distinction is observed between proven catalytically-active clays (e.g. Al-exchanged) and those which are generally less effective (e.g. Na-exchanged). 13C NMR spectroscopy is used to identify directly products formed within the interlayer regions, and temperature-controlled powder XRD serves as a useful tool for identifying whether or not intercalation occurs under variously defined conditions. High-pressure XRD is used to verify the formation of different products during reaction.

Neutron scattering studies have indicated that the non-coordinated water at smectite surfaces has a similar mobility to that of bulk water, but that the water coordinated to the cations is immobile on the time scale of the neutron measurements. Thus hydrophylic polymers can readily displace the non-coordinated water and bind to the silicate surface, and to the exchangeable cations through a water-bridge mechanism. Poly(ethylene oxide) molecules with molecular weights up to 4000 appear to be bound to Na-montmorillonite in flattened conformations at the clay surface. Poly(vinyl alcohol) is extensively bound by Na-montmorillonite and by Na-Laponite (a synthetic hectorite-like clay); as binding progresses fewer molecule segments can contact the surface and so at the higher levels of adsorption extensive loops of polymer extend away from the silicate surface. Some polyanions provide good protection for smectites against flocculation with salt. The abilities of such polymers to protect the clays is dependent both on the extents of the charges and on the solution conformations which these polymers can assume.

The exchangable cations present on certain montmorillonites can be replaced by long-chain quarternary ammonium cations, and the resulting clays have the important property of being able to swell and disperse in a variety of organic solvents. Organophilic clays have a wide range of industrial applications and are, for example, important constituents of oil-well drilling fluids, paints, plastics and greases; their role in these is discussed. Organophilic clays have been extensively studied for about 50 years, but neither the structure nor the mechanisms by which they swell and disperse in organic solvents are clearly understood. An attempt is made to interpret the industrial applicatons in terms of published ideas concerning the structure and swelling properties of organophilic clays.

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.

Ion-exchanged montmorillonites behave as solid acid catalysts and are effective and selective catalysts for the hydration of ethylene. The reaction proceeds predominantly in the interlamellar environment of the clay catalyst. Ethylene conversions were found to be markedly dependent on the choice of exchangeable cation, with Al being the best of those examined.

Wyoming bentonite, exchanged with cations of high charge density, is an efficient and selective catalyst for the production of ethyl acetate, in a single step, from ethylene and acetic acid. The reaction occurs in the interlamellar space of the clay where it is proposed that strong Brönsted acid sites on the clay coupled with highly polarized intercalated reactants produce a reaction which is otherwise difficult to carry out.

Some uses of clay-supported reagents in fine organic synthesis are discussed, in particular the catalysis of the phase-transfer preparation of formaldehyde acetals by quaternary ammonium montmorillonites. The preparation and some applications of clay-supported ferric nitrate, a new reagent which oxidizes alcohols into carbonyl derivatives, couples thiols into disulphides, nitrates phenols, and oxidatively couples pyrrole and benzaldehyde into tetra-phenylporphyrin, are also described

During the reaction of solutions of the leuco dyes crystal violet lactone and N-benzoyl leuco methylene blue with acid-treated montmarillonite (as occurs with carbonless copying papers), most of the offered dye is intercalated within the interlayers of the montmorillonite structure remaining after acid attack. This phenomenon was demonstrated by XRD and also by high-resolution transmission electron microscopy. During acid treatment, octahedral cations are dissolved from exposed edges of the montmorillonite lamellae and amorphous silica builds up in these areas. As a result, the BET(N2) specific surface area increases from ∼60 m2/g to ∼300 m2/g and, consequently, the theoretical reactivity towards leuco dyes. Nevertheless, only small quantities of the dye molecules are adsorbed onto the amorphous silica. The excellent colour intensity and, in particular, the resistance to fading in the light of the montmorillonite dye complexes may be explained by the protected position of the dye molecules within the interlayers of the montmorillonite.

The unique properties of imogolite are closely related to its structure, which is a tube of 23–27 Å outer diameter and ∼10 Å inner diameter, with an AIOH outer surface and SiOH inner surface. Acid dispersions contain the long, positively-charged tubes as isolated units or small bundles, which form bulky gels in alkali, and flocculate with negatively-charged colloids, polyvalent anions, and long-chain anionic detergents. Sorption properties are associated with the 10 Å intra-tube pores and with inter-tube channels of variable dimensions. Surface acidity is less than that of layer-silicate clays. The chemical and mechanical stability, biological activity, film- and fibre-forming characteristics, and conditions of synthesis are reviewed, on the basis of both new and published findings. Areas of potential application are indicated.