Insights into the hydrothermal stability of zeolite 13X

Zeolite 13X, a hydrophilic adsorbent, can be used for storing thermal energy. However, its crystallinity can degrade under hydrothermal stress in the aqueous atmosphere of an adsorption storage device. This would result in a loss of energy storage capacity. There is still no known method for predicting the long-term hydrothermal stability of zeolites under relevant adsorption storage conditions. A solution for this could be a kinetic model designed with experimental data to predict hydrothermal degradation. The objective of the present study was to examine the hydrothermal stability of zeolite 13X under relevant adsorption storage conditions and to analyze its degradation in respect to the influencing factors of water vapor pressure, temperature and period of treatment. The investigated zeolite 13X powder and zeolite 13X beads have been treated at temperatures between 200 and 350 degrees C and water vapor pressures up to 31 kPa for treatment periods of up to 312 h. We have analyzed the changes in the zeolite structure, composition and adsorption properties by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Al-27/Si-29 MAS NMR spectroscopy, molybdate method and water sorption. The results show a degradation of the crystalline zeolite 13X phase into an amorphous phase under all hydrothermal conditions, although to different degrees. Under specific hydrothermal conditions a portion of the primary crystalline phase withstands further treatment. Based on chemical decomposition mechanisms, we discuss this phenomenon. The complex stability behavior of the zeolite 13X demonstrates that a prediction beyond experimental data cannot be recommended.