Formation of α-Eucryptite, LiAlSiO4: An In-Situ Synchrotron X-ray Powder Diffraction Study of a High Temperature Hydrothermal Synthesis

Hydrothermal conversion of zeolite Li-A(BW), LiAlSiO4·H2O, into α-eucryptite, LiAlSiO4, occurs at temperatures above 350 °C. We report here the first in-situ study of a high-temperature hydrothermal synthesis using time-resolved powder diffraction. The following hydrothermal reactions were studied: 4 M NaNO3:LiAlSiO4 [ABW] → Li(Na)AlSiO4·H2O [ABW] → Na6Al6Si6O24·2NaNO3 [SOD]; 2 M LiCl:LiAlSiO4·H2O [ABW] → α-LiAlSiO4; 2 M LiCl:LiAlSiO4[ABW] → LiAlSiO4·H2O [ABW] → α-LiAlSiO4. The hydrothermal syntheses were performed in stainless steel capillaries. To maintain hydrothermal conditions, a hydraulic pressure of 2−300 atm was applied. Synchrotron X-ray radiation with energies of 35−40 keV was used in order to penetrate the steel capillaries. Room temperature Imaging Plate powder diffraction data were collected on a sample of zeolite Li-A(BW) contained in a steel capillary in an aqueous solution at 200 atm. pressure. With the use of a 10 min exposure, it was possible successfully to refine the crystal structure of zeolite Li-A(BW) using Rietveld refinement. Angle dispersive time-resolved in-situ powder diffraction patterns were collected using a Translating Imaging Plate (TIP) camera. Crystallization and degradation curves for the high-temperature hydrothermal conversion experiments were determined using integrated intensities of selected diffraction lines. The kinetic curves were analyzed using first-order rate expressions.