THE CRYSTALLIZATION KINETICS OF SODALITES GROWN BY THE HYDROTHERMAL TRANSFORMATION OF KAOLINITE STUDIED BY SI-29 MAS NMR

The hydrothermal transformation of kaolinite to basic sodalite Na-8[Al SiO4](6) (OH . H2O)(2) and hydroxoborate sodalite Na-8[AlSiO4](6) [B(O H)(4)](2) has been investigated at different temperatures (353 and 473 K). In the early stage of the reactions, the crystallization kinetics was studied by X-ray powder diffraction, thermogravimetry, IR spectro scopy and Si-29 MAS NMR spectroscopy. Besides the crystallization of t he sodalites, no further intermediate phases were formed. MAS NMR of t he Si-29 nucleus has been found to be a versatile tool to follow the p rogress of reactions from the signal ratio of the initial material and the crystallization product because the differences in chemical shift s result in well-separated signals. From these measurements, the growt h rates of the sodalites could be determined quantitatively even for t he very early stages crystallization. It was found that sodium carbona te impurities in the NaOH solution used for the synthesis has an impor tant influence on the reaction kinetics. (C) 1997 Elsevier Science B.V .