Decoding Polymorphism in Zwitterionic MOFs for Carbon Capture: Insights from SCXRD and PXRD Structure Solutions

Microporous metal−organic frameworks (MOFs) featuring zwitterionic linkers are attractive for gas separation because their charged pore surfaces can promote interactions with polarizable guest molecules. Herein, we report a cadmium-based zwitterionic MOF, M1, that crystallizes as two distinct polymorphs. The initially isolated orthorhombic phase, M1A, was characterized by single-crystal X-ray diffraction but proved difficult to reproduce, with subsequent syntheses preferentially yielding the monoclinic bulk phase M1B. The structure of M1B was determined ab initio from powder X-ray diffraction data and refined by Rietveld analysis. The different ligand conformations, metal coordination environments, and framework packing arrangements of the two phases illustrate the challenges posed by polymorphism during MOF synthesis and scale-up. Their formation behavior, together with the lower calculated energy of M1B, is consistent with M1A being a metastable, kinetically accessible polymorph and M1B being the more stable form under the investigated conditions, although a direct phase transformation was not observed. Activated M1B adsorbs CO2 with capacities of 2.438, 1.607, and 1.340 mmol g−1 at approximately 1 bar and 273, 298, and 313 K, respectively. Dynamic breakthrough experiments using 15:85 and 50:50 CO2/N2 mixtures demonstrate preferential CO2 retention at 298 and 313 K under both dry and humid conditions, although competitive water adsorption reduces the CO2 capacity. These findings establish the adsorption behavior of the structurally characterized bulk polymorph and highlight the importance of accurate phase identification for developing reliable structure−property relationships in MOFs.