Shaping techniques of porous alkali bonded ceramics for adsorption applications

Alkali bonded ceramics (ABC) are synthetic and amorphous alkali aluminosilicates, also known as geopolymers, with properties varying among those of ceramics, cements, zeolites or refractories. The microstructure is intrinsically mesoporous and several forming techniques can be exploited to modify the porosity from the micro- to the ultra-macro scale, creating hierarchical pore systems. Ice-templating allows, for example, to obtain ABC with mainly unidirectional anisotropic macro-pores, where the ice crystals act as pore network template, forming unique lamellar morphology. Microporous fillers, as zeolite, improve the porosity functionalizing the material for CO2 sorption applications. Adsorbents in form of beads and spheres have lately received attention thanks to the good mobility, high packing density, ease of separation and reuse after regeneration. For this reason, several "spherification" processes are applied to obtain millimeter-size porous ABC beads, different in term of porosity and adsorption properties. Indeed an injection-solidification method in different media, i.e. polyethylene glycol or liquid nitrogen, is exploited to produce different ABC beads. In addition, ABC beads are obtained by ionotropic gelation. The process parameters are deeply investigated and the most performing samples are characterized in terms of morphology, macro- and microstructure, composition, porosity distribution and specific surface area together with the adsorption properties towards gaseous CO2 or dyes.