Equilibrium adsorption behaviour of a 3D-printed zeolite–geopolymer composite with high faujasitic content

Abstract

In this work, a NaX zeolite/Na-activated geopolymer composite material having a 70 wt% zeolitic content was used for fabricating 3D-printed monoliths by means of Direct Ink Writing. The produced material was characterized employing X-ray diffraction, scanning electron microscopy and gas adsorption porosimetry, confirming that the NaX zeolite substantially retained its structure and properties after being mixed with the geopolymer matrix and undergoing the 3D printing process. In particular, the textural properties, specifically the specific surface area (242 m² g⁻¹) and total pore volume (0.100 cm³ g⁻¹), exceed those already reported for comparable composites. CO₂ and water vapour adsorption isotherms of the samples were gravimetrically evaluated at 298.15, 318.15 and 338.15 K. CO₂ adsorption results (e.g., about 3.2 mol kg⁻¹ at 100 kPa and 298.15 K) suggested that, inside the composite, the NaX zeolite is the only active adsorbent for this adsorbate. On the other hand, water vapour adsorption results (e.g., about 10.1 mol kg⁻¹ at saturation and 298.15 K) showed that the geopolymer matrix was a significantly active adsorbent, working alongside the embedded NaX powders. The experimental data regarding both CO₂ and water vapour adsorption were well fitted by the semi-empirical Toth model, thus allowing determination of the respective isosteric heats of adsorption, with values significantly lower than those of pristine NaX powders. In the case of CO₂ adsorption, this is a positive finding in the light of possibly implementing this material in adsorption-based CO₂ capture technologies. Conversely, the same result for water vapour adsorption renders the material unsuitable for use in thermal energy storage devices.