Kinetics of formation of zeolite Na-A [LTA] from natural kaolinites |
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Authors: | A Gualtieri P Norby G Artioli J Hanson |
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Institution: | (1) Chemistry Department, Brookhaven National laboratory, Upton 11973, NY, USA, US;(2) Dipartimento di Scienza della Terra, Università di Milano, 20133 Milano, Italy, IT |
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Abstract: | The kinetics and mechanism of hydrothermal formation of zeolite A from natural kaolinites have been studied using as starting
materials two international kaolinite standards (KGa-1 and KGa-2 from Georgia, USA) exhibiting a different degree of stacking
disorder. Precursors utilized for the synthesis were prepared by heating the kaolinites at 800 °C. Metakaolinite was also
prepared from KGa-1 by thermal activation at 600 °C. The hydrothermal syntheses were accomplished by heating the samples in
NaOH solutions at temperatures between 70 and 110 °C. The kinetic experiments were performed by time-resolved synchrotron
powder diffraction in isothermal mode using a transmission geometry and an Image Plate detector. The results of the kinetic
analysis are interpreted in the light of the structural state of the starting kaolinite, and of the temperature of activation
of the precursor material. For kaolinite activated at high temperature the nucleation and crystallization of zeolite A is
essentially independent of the defect density of the original kaolinite, and the thermal history of the precursor seems to
be the main controlling parameter. The formation process of zeolite A from metakaolinite materials obtained at lower activation
temperatures shows significantly faster reaction rates and lower apparent activation energies. This is again interpreted in
the light of the short range inhomogeneities present in metakaolinite. As the reaction proceeds metastable zeolite A transforms
into hydroxy-sodalite.
Received April 18, 1996 / Revised, accepted September 27, 1996 |
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