CO2 adsorption in structured Na13X zeolite-geopolymer composites: Performance comparison under dynamic flow of cold-sintered pellets vs. foamed monoliths

Adsorption on solid sorbents is a promising and versatile technology for CO 2 capture from flue gases. In this framework, zeolite–geopolymer composites represent a novel class of materials, combining good mechanical stability with tuneable composition and form. Since adsorbent porosity strongly affects gas permeability and adsorption efficiency, this study investigates the dynamic CO 2 adsorption performance of Na13X zeolite–geopolymer composites prepared using different fabrication routes and characterized by different zeolite content. A foamed geopolymer monolith was compared with pellets containing 90 wt.% Na13X, manufactured via the Cold Sintering Process (CSP). The materials were characterized in terms of density, pore structure, surface area, and microstructure, highlighting clear differences in hierarchical porosity and permeability. The foamed monolith exhibits a highly pore network with a total porosity of 78% and ultra-macropores that provide high hydraulic permeability (1.8 · 10 -11 m 2 ). Static pure CO 2 adsorption tests showed that CSP pellets achieve larger equilibrium capacities due to their higher zeolite content. However, dynamic breakthrough experiments with dry CO 2 /N 2 mixtures revealed severe diffusion limitations in CSP pellets, resulting in an approximately three-fold reduction in adsorption capacity. In contrast, the foamed monolith largely preserved its adsorption performance under dynamic conditions, reaching capacities comparable to equilibrium at longer residence times. These results demonstrate that the multimodal pore size distribution of foamed geopolymer composites enhances gas transport to active sites, making them promising materials for continuous CO 2 separation processes where mass-transfer kinetics are critical.