Thermally-induced phase transformations of CALF-20: insights into its phase stability and carbon capture properties

Calgary Framework 20 (CALF-20) has garnered a great deal of attention as a promising adsorbent for CO₂ capture applications due to its high CO₂ uptake capacity and excellent stability in humid environments. Multiple studies have explored its adsorption properties and separation performance, while a few studies reveal the existence of several phases of CALF-20, including α-, β-, and τ-phases. α-Phase (i.e., referred as CALF-20) is known to transform into β- or τ-phase under humid or thermal conditions, respectively. Computational studies have shown that β-CALF-20 exhibits a higher heat of CO₂ adsorption, making it more suitable for CO₂ capture compared to α-CALF-20. However, β-CALF-20, transformed from α-CALF-20 under humid conditions, is known to be less stable, thus observed only upon moisture exposure. Here, we demonstrate that β-CALF-20 can be formed solely by thermal treatment of α-CALF-20 and this thermally-derived β-CALF-20 is significantly more stable and can be readily obtained by drying as-synthesized CALF-20 at temperatures above 100 °C. Computational analyses corroborate the experimental results, affirming that the α-to-β phase transformation can be achieved solely through thermal treatment, without the involvement of water, via a modification in the Zn coordination number from 5 to 4. More importantly, this study finds that it is challenging to determine gas adsorption properties of α-CALF-20, as degassing of α-CALF-20 at elevated temperatures under vacuum almost always leads to its transformation into β-CALF-20. The resultant β-phase demonstrates a CO₂ uptake of 2.38 mmol g⁻¹ at 0.1 bar and 30 °C, along with a CO₂/N₂ IAST selectivity of 222 for a 10 : 90 CO₂/N₂ mixture, which aligns with previous reports. Consequently, these findings imply that previously reported gas adsorption properties, including carbon (CO₂) capture capacities, may have been measured on the β-phase of CALF-20, rather than the α-phase.