Evolution of crystalline aluminates from hybrid gel-derived precursors studied by XRD and multinuclear solid state MAS NMR IV:: Calcium dialuminate, CaAl4O7 and calcium hexaluminate, CaAl12O19

X-ray amorphous calcium aluminate gels of dialuminate and hexaluminate composition were prepared, the former by two methods, and their thermal crystallisation was monitored by thermal analysis, X-ray powder diffraction and (27)Al MAS NMR. Both gel compositions lose a large amount of solvent and organic by-products below 600 degreesC, forming amorphous precursors with (27)Al NMR spectra containing resonances corresponding to tetrahedral and octahedral sites, and another site with a peak at 36-39 ppm. The dialuminate gel crystallises abruptly at 900 degreesC to the essentially phase-purr compound containing Al predominantly in two distorted tetrahedral environments. The hexaluminate gel crystallises exothermically at 900 degreesC to gamma -alumina spinel, containing only tetrahedral and octahedral Al. The magnetoplumbite structure of the hexaluminate phase appears in an exothermic reaction at 1160 degreesC, this phase-pure reaction product containing Al atoms in one tetrahedral and two distinguishable octahedral sites. The first (43)Ca MAS NMR spectra of the crystalline dialuminate and hexaluminate are also reported. (C) 2000 Elsevier Science B,V. All rights reserved.