Binderless Faujasite Beads with Hierarchical Porosity for Selective CO₂ Adsorption for Biogas Upgrading

Biomethane can be isolated from biogas through selective CO₂ adsorption. Faujasite-type zeolites are promising adsorbents for CO₂ separation due to their high CO₂ adsorption capacity. While commonly inert binder materials are used to shape zeolite powders into the desired macroscopic format for application in an adsorption column, here we report the synthesis of Faujasite beads without the use of a binder and their application as CO₂-adsorbents. Three types of binderless Faujasite beads (d = 0.4-0.8 mm) were synthesized using an anion-exchange resin hard template. All the prepared beads consisted mostly of small Faujasite crystals, as demonstrated by characterization with XRD and SEM, which are interconnected through a network of meso- and macropores (10-100 nm), yielding a hierarchically porous structure, as shown by N₂ physisorption and SEM. The zeolitic beads showed high CO₂ adsorption capacity (up to 4.3 mmol g^-1 at 1 bar and 3.7 mmol g^-1 at 0.4 bar) and CO₂/CH₄ selectivity (up to 19 at the partial pressures mimicking biogas, i.e., 0.4 bar CO₂ and 0.6 bar CH₄). Additionally, the synthesized beads have a stronger interaction with CO₂ than the commercial zeolite powder (enthalpy of adsorption -45 kJ mol^-1 compared to -37 kJ mol^-1). Therefore, they are also suitable for CO₂ adsorption from gas streams in which the CO₂ concentration is relatively low, such as flue gas.