Humidity-Responsive Polymorphism in CALF-20: A Resilient MOF Physisorbent for CO₂ Capture

CALF-20 is a zinc-triazole-oxalate-based metal-organic framework (MOF) that exhibits selective CO₂ physisorption, which makes it attractive for potential applications in CO₂ capture. However, to implement such applications, the CALF-20 structure must endure extended exposure to environmental conditions that may lead to changes in structure, chemistry, and, hence, performance. Here we find that the originally reported CALF-20 phase (now denoted α-CALF-20) undergoes a structure transformation following exposure to humid environments to generate a new polymorph, β-CALF-20, identified here through combined synchrotron powder X-ray diffraction and pair distribution function (PDF) analysis. This α-to-β transformation is fully reversible with α-CALF-20 being regenerated by treating β-CALF-20 under dry air or vacuum. The rapid reversion of β- to α-CALF-20 under conditions relevant to measuring the gas adsorption isotherm required the CO₂ adsorption properties for β-CALF-20 to be evaluated computationally. Experimental evaluation of the adsorption behavior of β-CALF-20 is not practical. These analyses suggest that β-CALF-20 has a higher CO₂ heat of adsorption than α-CALF-20, which may be advantageous to CO₂ sorption selectivity at low partial pressures.