PREPARATION OF POTASSIUM BETA-ALUMINAS BY GEL-TO-CRYSTALLITE CONVERSION AND THEIR CHARACTERIZATION

A novel method for the preparation of phase-pure K-beta-alumina is rep orted by the gel-to-crystallite conversion method. Here, coarse gels o f hydrated alumina Al2O3 . yH(2)O (80 < y < 120) were reacted with eth anolic KOH,yielding crystalline K-inserted gibbsite or K-pseudoboehmit e. Thermal decomposition of these precursors above 500 K yielded inter mediate K-inserted boehmite. Above 1473 K, K-beta-alumina was obtained via a modified K-beta-alumina intermediate. The stability range of K- P-alumina can be extended up to 1 : 17 (K2O :Al2O3) by this method. Th e extended compositional range of stability of K-beta-alumina is expla ined on the basis of the polysynthetic lamellae arising from the colla pse of the layers adjacent to the K-O plane, as revealed by high resol ution electronmicroscopy (HREM). The increased number of AlO6 octahedr a, in comparison to AlO4 tetrahedra and the prevalence of distorted Al O6 with two or more types of second nearest neighbour interaction is i ndicated from the solid state Al-27 MAS NMR studies. To obtain K-beta( n)-alumina, Li+ or Mg2+ had to be incorporated. The mechanistic aspect s of the formation of K-beta- and K-beta(n)-alumina are described in d etail.