ISOMORPHOUS REPLACEMENT IN THE ZEOLITIC FRAMEWORKS - RECENT ADVANCES AND IMPLICATIONS

Isomorphous substitution in zeolites by silicon, aluminium, boron, gal lium, germanium, titanium, vanadium and iron is described. Since the c atalytic properties of the solid are inextricably linked to its struct ure and composition, much effort has been devoted in recent years to p reparing zeolitic materials containing heteroatoms in their framework. The consequences of substitution of heteroatoms into 'regular' crysta lline aluminosilicates are described, including methods of preparation and properties of these novel materials. The theoretical basis of rep lacement is first dealt with. The experimental progress made in charac terization of molecular sieves is exemplified by extensive use of MMR, EXAFS, neutron scattering and laser Raman spectroscopy data, in addit ion to other techniques. In particular, the short-range order of Si an d X (where X = Al or other heteroatom) located in the zeolitic framewo rk has been explored by solid-state NMR. This turned out to be a key m ethod in understanding the properties of dealuminated and recently obt ained reilluminated zeolites. Fractals, at first a purely mathematical concept, have found multiple applications in chemistry and materials science. Here we present how the fractal dimension of dealuminated and realuminated faujasites can be measured. The results are interpreted satisfactorily within the frame of XRD, FT IR and MAS NMR studies of t hose solids. Finally, the catalytic behaviour of metallosilicates, in both gas phase and Liquid phase, is discussed. It is shown that in add ition to acidic catalysts, redox-type zeolite catalysts can be success fully prepared (e.g. gallium-, titanium- and vanadosilicates). Such so lids can be used in a number of oxidation and reduction reactions. Tak ing into account the progress made in understanding isomorphous substi tution in zeolites, there is no doubt that many novel and surprising m aterials will be synthesized in the future.