Two flexible cationic metal-organic frameworks with remarkable stability for CO2/CH4 separation

Cationic azole-based metal-organic frameworks (MOFs) with remarkable stability and unique pore environment have aroused great research interests. Meanwhile, flexible MOFs which can undergo pore-structure changes upon exposure to external stimuli are ideal materials for gas separation. However, introducing flexibility into the framework of cationic azole-based MOFs is rarely reported. Herein, we synthesized two stable isomorphic cationic MOFs (M-btz-as, M = Co, Ni) based on a linear azole ligand. After activated at high temperature under vacuum, M-btz-ht (M = Co, Ni) were obtained and both MOFs exhibited flexible features in which Co-btz is more flexible than Ni-btz. Different solvent-mediated activation methods were employed to explore their effects on structural flexibility and produced MOFs with different phases. Continuous phase transformation of Co-btz-e was verified by powder X-ray diffraction. In addition, these MOF phases possessed different gas separation abilities affected by their flexible frameworks, and Co-btz-ht exhibited the highest CO2/CH4 separation ability.

Automated summary

Two stable isomorphic cationic MOFs were synthesized, displaying flexible features, and the effects of different activation methods on their structural flexibility were explored. Continuous phase transformation of one MOF was verified by powder X-ray diffraction, showing higher gas separation abilities.