Binderless zeolite LTA beads with hierarchical porosity for selective CO₂ adsorption in biogas upgrading

In the context of CO2 removal from biogas, a series of binderless zeolite LTA adsorbents with a macroscopic bead format (0.5–1.0 mm) and with hierarchical porosity (i.e. with the zeolitic micropores being accessible through meso- and macropores mainly in the 10–100 nm range) was synthesized with a variety of Si/Al ratios (1.2–3.9) using Amberlite IRA-900 anion-exchange resin beads as a hard template. The CO2 and CH4 adsorption capacity of the beads in Na-form with different Si/Al ratios were measured, reaching higher CO2/CH4 selectivity and similar, yet slightly higher CO2 adsorption compared to commercial zeolite LTA pellets containing a binder. Subsequently, one the zeolitic beads was subjected to different degrees of ion-exchange (0–96%) with KCl and then tested in the adsorption of CO2 and CH4. The best performance among all the ion-exchanged beads was achieved with Na58K42-LTA beads, which gave very high CO2/CH4 selectivity (1540). Although essentially no CH4 was adsorbed on these beads, the CO2 adsorption capacity was still substantial (1.9 mmol g−1 at 0.4 bar CO2, i.e. the partial pressure of CO2 in biogas).