Binderless SAPO-34 beads for selective CO2 adsorption

Selective adsorption of CO₂ from biogas allows isolating biomethane, which can then be used as a direct substitute for natural gas. The microporous zeotype SAPO-34 is a suitable material for CO₂ adsorption because it can achieve high working capacity at relatively mild regeneration conditions. In industrial applications, adsorbents need to be shaped into a macroscopic format (e.g. beads, pellets) in order to reduce the pressure drop over the adsorption column. Typically, an inert binder is added to the powder to achieve the desired format. In this work, novel hierarchically porous binderless SAPO-34 beads with a diameter in the range 0.7–1.2 mm were synthesised employing an ion-exchange resin as a hard template. The interior of the beads consisted mostly of small SAPO-34 crystals (< 0.3 μm) interconnected to each other and thus generating a network of meso‑ and macropores between them, as demonstrated by XRD and SEM. Around several of the beads, a crystal overgrowth was observed consisting mostly of larger SAPO-34 crystals (1–25 μm). The SAPO beads displayed good CO₂ adsorption capacity (3.0 mmol g⁻¹ at 1 bar), which was higher than that of binder-containing SAPO-34 extrudates (2.4 mmol g⁻¹ at 1 bar), but slightly lower compared to SAPO-34 in powder format (3.4 mmol g⁻¹ at 1 bar). Furthermore, the SAPO-34 beads displayed high CO₂/CH₄ selectivity (8, at partial pressures mimicking biogas, i.e. 0.4 bar CO₂ and 0.6 bar CH₄) as well as high CO₂/N₂ selectivity (33, at partial pressures mimicking flue gas, i.e. 0.15 bar CO₂ and 0.85 bar N₂). Notably, a high CO₂ working capacity of 1.8 mmol g⁻¹ was estimated based on the adsorption isotherm between 1 and 0.2 bar, and this value has the potential to be further improved by increasing the adsorption pressure to > 1 bar.