Preparation of potassium beta aluminas by gel-to-crystallite conversion and their characterisation

Abstract

A novel method for the preparation of phase-pure K-β-alumina is reported by the gel-to-crystallite conversion method. Here, coarse gels of hydrated alumina Al₂O₃ ·yH₂O (80<y<120) were reacted with ethanolic KOH, yielding crystalline K-inserted gibbsite or K-pseudoboehmite. Thermal decomposition of these precursors above 500 K yielded intermediate K-inserted boehmite. Above 1473 K, K-β-alumina was obtained via a modified K-χ-alumina intermediate. The stability range of K-β-alumina can be extended up to 1:17 (K₂O:Al₂O₃) by this method. The extended compositional range of stability of K-β-alumina is explained on the basis of the polysynthetic lamellae arising from the collapse of the layers adjacent to the K–O plane, as revealed by high resolution electron microscopy (HREM). The increased number of AlO₆ octahedra, in comparison to AlO₄ tetrahedra and the prevalence of distorted AlO₆ with two or more types of second nearest neighbour interaction is indicated from the solid state ²⁷Al MAS NMR studies. To obtain K-β″-alumina, Li⁺ or Mg²⁺ had to be incorporated. The mechanistic aspects of the formation of K-β- and K-β″-alumina are described in detail.

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