Zeolites as solid solvents

In addition to their natural appeal as crystalline solids possessing a void intracrystalline volume consisting of channels and cages, sometimes interconnected, zeolites also have remarkable features resulting from the surface curvature of their internal surface. These features involve non-covalent interactions between the framework of the zeolite and molecules adsorbed in their intracrystalline free space. These interactions extend beyond the molecular level and are thus of supramolecular nature. They are at the origin of confinement effects which govern their behaviour as sorbents and catalysts. By contrast to molecular shape selective effects which result from shea range repulsions imposing restrictions on sorbates or reactants, reaction transition states, and/or products, the van der Waals interactions responsible for confinement effects are long ranged and attractive. They favour the adsorption of certain molecules vs. others, sometimes at specific locations in the intracrystalline volume, stabilise reaction transition states, and may eventually enhance the probability of bimolecular reactions. Because of such confinement effects, zeolites can be assimilated to solid solvents and several of their characteristics can be described using solvent effects as an analogy. This review analyses recent Literature supporting this view with specific attention to the application of zeolites in selective adsorption and separation and their use as catalysts for fine chemicals synthesis. Some other consequences of confinement effects will also be discussed, in particular those relating to the measurement and optimal use of the acidic properties of zeolites. (C) 1998 Elsevier Science B.V. All rights reserved.