Waste-derived hierarchical zeolite/silica composites for CO₂ capture: Integrating phase composition and 3D printing for enhanced adsorption

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Waste-derived zeolite/mesoporous silica composites containing FAU, LTA, and sodalite phases in different proportions were synthesized from coal fly ash and rice husk-derived silica, providing a sustainable route for converting industrial and agricultural residues into functional CO 2 adsorbents. The composites were characterized by X-ray powder diffraction (XRD), N 2 physisorption, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) in TEM, X-ray fluorescence (XRF), Mössbauer spectroscopy, and Fourier transform infrared (FTIR) spectroscopy using NO, CO, and CO 2 as probe molecules. The results showed that varying the silica content in the synthesis mixture strongly affects the zeolite phase composition, leading to FAU/LTA-containing composites at lower silica contents and to FAU/sodalite-containing phase assemblage at higher silica content. All samples exhibited hierarchical micro/mesoporosity. It was found that the CO 2 adsorption was governed mainly by the zeolite phase composition, zeolite/ silica ratio and accessibility of extra-framework Na⁺ adsorption sites. The best performance was achieved for FA-Si-2, which exhibited both the highest CO 2 uptake under static (4.0 mmol g⁻¹) and dynamic (3.9 mmol g⁻¹) conditions. FTIR results showed that CO 2 adsorption is dominated by molecular adsorption on Na⁺-containing sites, mainly in bridged configurations, whereas carbonate-like species represent only a minor secondary population. The cycling experiments confirmed effective regeneration at 100 °C, with a capacity loss ≤ 10% over 15 adsorption–desorption cycles. A 3D-printed adsorbent based on FA-Si-1 exhibited slightly enhanced CO 2 uptake compared with the powdered material, highlighting the potential of additive manufacturing to improve accessibility and mass transfer, making the material more suitable for practical applications.