Thermal induced structural modifications of the high silica zeolite Gottardiite: an in-situ XRPD study

The thermally induced structural modifications of the natural zeolite gottardiite ((Na0.78K0.24Mg3.51Ca4.11Sr0.06Ba0.05)(Fe0.14Al17.47Si118.70O272) · 92.33 H2O, a = 13.631(1), b = 25.047(3) and c = 22.586(3) Å, V = 7711(1) Å3, s.g. Cmca, framework type NES) were studied in the temperature range 308 - 1203 K in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation. The unit cell parameters were refined in the whole temperature range, while structural refinements were performed approximately every 100 K, at 308 K, 371 K, 475 K, 580 K, 674 K, 779 K, 873 K, and 978 K. No signs of phase transitions were observed in the whole investigated T-range and up to the highest temperature the material does not undergo amorphization. Gottardiite shows a high thermal stability, as a consequence of the high Si content, and an extremely rigid structure with a substantial absence of cell volume change. The framework deformation induced by dehydration involves only a slight distortion of the 10-ring and 12-ring channels running in the a and b directions. From room temperature to 580 K gottardiite looses water quite slowly and regularly. The main water loss is observed in the temperature range 580 - 779 K. Above this temperature gottardiite again dehydrates slowly, and at 978 K about 13 water molecules remain.