Issue 24, 2025

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

Abstract

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
03 Mar 2025
Accepted
09 May 2025
First published
20 May 2025
This article is Open Access
Creative Commons BY-NC license

J. Mater. Chem. A, 2025,13, 18892-18899

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

D. A. Kang, N. Suphavilai, A. M. O. Mohamed, I. G. Economou, M. Shetty, J. Kim and H. Jeong, J. Mater. Chem. A, 2025, 13, 18892 DOI: 10.1039/D5TA01729G

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