Hydration mechanism in Ce-exchanged zeolites and heat release performances upon adsorption of water vapour in support of their potential use in thermochemical storage of energy under mild conditions of adsorbent regeneration and saturation

Abstract

Abstract The potential use of commercially available 13X zeolite, modified by ion-exchange with cerium compensating cations possessing high charges and high hydration energies, has been tested in view of low-temperature storage of solar energy performed under mild operating conditions of low regeneration temperatures and low pressures of water vapour during the adsorption step. Structural and textural properties, sorption behaviour towards water vapour of three selected samples containing various proportions of Ce3+ and Ce4+ compensating cations and the pristine Na+-13X zeolite were studied by a variety of experimental techniques including Wavelength Dispersive X-Ray Fluorescence, Energy Dispersive X-ray Spectroscopy, X-ray diffraction, Thermogravimetric analysis, as well as measurements of the adsorption of gaseous nitrogen at 77 K and water vapour at 313 K. Based on the structure refinement procedure applied to the experimental XRD patterns, it was demonstrated that extra-framework cerium cations were preferentially located on sites I’ and II in dry and hydrated zeolites, showing relatively little dependence on the hydration level. Monte Carlo simulations were used to determine the limit values of the amount adsorbed and differential heat of adsorption, which could be obtained experimentally if the zeolite samples were completely dried. The potential of Ce-containing zeolites as adsorbents for the thermochemical energy storage was finally determined under flow conditions by firstly dehydrating samples at 353 K or 423 K and then saturating them at 296 K with water vapour at a mole fraction of 0.03. The choice of the operating conditions was decided so as to maintain the stability of the zeolite structure while taking the risk of reduced thermal performance of zeolite adsorbents undergoing incomplete regeneration-dehydration. Under such mild conditions, the modified 13X zeolites exhibited enhanced thermal performance in comparison with that of the pristine 13X, by releasing between 700 and 1100 kJ per kg of the adsorbent during a period of 6–8 h. Through a complementary study based on calorimetry measurements and molecular simulations, the understanding of the hydration-dehydration steps in Ce-exchanged zeolites and cation displacement upon hydration has allowed to establish the best compromise for the conditions of zeolite regeneration and saturation in the case of heat long-term storage applications.