ZEOLITES AS SOLID SOLVENTS

In addition to their natural appeal as crystalline solids possessing a void intracrystalline volume consisting of channels and cages, someti mes interconnected, zeolites also have remarkable features resulting f rom the surface curvature of their internal surface. These features in volve non-covalent interactions between the framework of the zeolite a nd molecules adsorbed in their intracrystalline free space. These inte ractions extend beyond the molecular level and are thus of supramolecu lar nature. They are at the origin of confinement effects which govern their behaviour as sorbents and catalysts. By contrast to molecular s hape selective effects which result from shea range repulsions imposin g restrictions on sorbates or reactants, reaction transition states, a nd/or products, the van der Waals interactions responsible for confine ment effects are long ranged and attractive. They favour the adsorptio n 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. Beca use of such confinement effects, zeolites can be assimilated to solid solvents and several of their characteristics can be described using s olvent effects as an analogy. This review analyses recent Literature s upporting this view with specific attention to the application of zeol ites 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 me asurement and optimal use of the acidic properties of zeolites. (C) 19 98 Elsevier Science B.V. All rights reserved.