29Si and 27Al NMR study of zeolite formation from alkali-leached kaolinites. Influence of thermal preactivation

Abstract

The alkali treatment of kaolinites heated between 400 and 1000°C has been studied by NMR (29Si and 27Al signals), IR, and X-ray diffraction techniques. The dissolution of kaolinite samples produces Al(OH)4- and SiO44- species which participate in the zeolite germ formation. The nature of zeolites generated (hydroxysodalite, Na-A, and tetragonal Pt) varies with thermal activation of the starting silicates, but the prolonged leaching of the samples favors the transformation of initially formed phases into the most stable Pt zeolite. The synthesis of this compound (Si/Al > 1) requires the previous polymerization of dissolved silica species, and the amount of incorporated Si depends on the temperature of pretreatment of kaolinite. © 1990 American Chemical Society.