Insights into the synergistic effects of ZnO/MgO and Zn/Mg-modified porous zeolite as core-satellite materials for tuning water vapor sorption properties
Cyrille Ghislain Fotsop, Alexandra Lieb, Franziska Scheffler
Abstract
Humidity control is a major issue in industrial processes, the present work focuses on the synthesis of the core-satellite materials: Zeo-X-Mg, Zeo-X-Zn, Zeo-X-ZnO and Zeo-X-MgO, followed by a comparative study of their water vapor adsorption properties. The adsorption mechanism was elucidated using isothermal modeling with Langmuir, Freundlich, Sips and Guggenheim-Anderson-De Boer (GAB) models. The obtained materials were synthesized by an ex situ ion-exchange assisted hydrothermal method and characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), solid-state magic angle spinning (MAS)-nuclear magnetic resonance (NMR), field emission scanning electron microscopy (FE-SEM)/energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA)-differential scanning calorimetry (DSC), N₂ sorption and Fourier transform infrared spectroscopy (FTIR) techniques. XRD analysis revealed additional phases in the Zeo-X-ZnO and Zeo-X-MgO samples due to the presence of ZnO, Zn(OH)₂, MgO, and Mg(OH)₂ on the Zeo-X-Na sample's surface. SEM/EDX analysis revealed uniform octahedral particles corresponding to the six-membered rings (D6R) of sodalite cages and a homogeneous distribution of Mg and Zn elements. The water vapor adsorption capacities were 26.9, 25.1, 21.5, 18.1, and 14.1 mmol/g for Zeo-X-ZnO, Zeo-X-MgO, Zeo-X-Zn, Zeo-X-Mg, and Zeo-X-Na samples, respectively. Despite their low porosities, Zeo-X-ZnO and Zeo-X-MgO showed higher adsorption capacities comparable to samples exchanged with Mg²⁺, Zn²⁺ and pure zeolite. For Zeo-X-MgO and Zeo-X-ZnO samples, water molecule adsorption occurs preferentially on the surface of the zeolite structure, due to the formation of Zn(OH)₂ and Mg(OH)₂ complexes during to the hydration. The GAB model showed a higher R² value with low values of the error functions, indicating adsorption on infinite monolayers and multilayers at variable pressure. The Freundlich model showed that chemisorption occurred at the adsorbent surface during adsorption. The obtained isosteric heat of adsorption were 53.22, 63.03, 80.96, 65.23, and 73.15 kJ/mol for Zeo-X-Na, Zeo-X-Zn, Zeo-X-ZnO, Zeo-X-Mg, and Zeo-X-MgO, respectively. These results point to an innovative approach to industrial processes involving water vapor.