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.