CO2 Adsorption Investigation on an Innovative Nanocomposite Material with Hierarchical Porosity

A NaX nanozeolite-geopolymermonolith, with hierarchical porosity, has been produced by a one-pot hydrothermal synthesis using metakaolin as alluminosilicate source and a sodium silicate solution as activator. Its final composition, reported in terms of oxides, is 1.3–Na2O–3.0SiO2–1Al2O3–12H2O. Its microstructural and chemical features and CO2 adsorption performance have been investigated. The microstructure of the composite is characterized by NaX zeolite nanocrystals glued by the geopolymeric binder to form a complex three-dimensional network of pores. Overall porosity resulted ∼23.5%, whereas compressive strength is 16±0.7 MPa. Monolith showed BET surface area of 350 m2/g, a micropore surface area of 280 m2/g and a mesopore volume, due to the geopolymeric binder, of 0.09 cm3/g. Its CO2 adsorption capacity has been measured at the temperatures of 7, 25 and 42 C up to 15 bar using an optimized Sievert-type (volumetric) apparatus. All the adsorption data were evaluated by Toth/Langmuir isotherm model and commercial pure NaX zeolite was used as reference. CO2 adsorption isotherms show a maximum uptake value around 21 wt% at (∼7 C) that decrease to 18 wt% at high temperature (∼42 C) passing through 19 wt% at room temperature (∼25 C). The homogeneity grade of the surface, as obtained using Toth analysis performed on the adsorption isotherm, is close to t 0.40, lower than the 0.61 obtained for pure commercial NaX zeolite, as a consequence of the binder formation. Monolith exhibits a notably higher K values and quicker saturation with respect to reference that can be ascribed to the presence of mesoporosity that provides an easier and faster transport of CO2 in the NaX nanozeolite framework. The produced composite is a potential solid adsorbent candidate in industrial process.