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 alpha-CALF-20) undergoes a structure transformation following exposure to humid environments to generate a new polymorph, beta-CALF-20, identified here through combined synchrotron powder X-ray diffraction and pair distribution function (PDF) analysis. This alpha-to-beta transformation is fully reversible with alpha-CALF-20 being regenerated by treating beta-CALF-20 under dry air or vacuum. The rapid reversion of beta- to alpha-CALF-20 under conditions relevant to measuring the gas adsorption isotherm required the CO2 adsorption properties for beta-CALF-20 to be evaluated computationally. Experimental evaluation of the adsorption behavior of beta-CALF-20 is not practical. These analyses suggest that beta-CALF-20 has a higher CO2 heat of adsorption than alpha-CALF-20, which may be advantageous to CO2 sorption selectivity at low partial pressures.