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Biporous Metal–Organic Framework with Tunable CO₂/CH₄ Separation Performance Facilitated by Intrinsic Flexibility
- Gładysiak, Andrzej, Deeg, Kathryn S., Dovgaliuk, Iurii, Chidambaram, Arunraj, Ordiz, Kaili, Boyd, Peter G., Moosavi, Seyed Mohamad, Ongari, Daniele, Navarro, Jorge A. R., Smit, Berend, Stylianou, Kyriakos C.
- ACS applied materials & interfaces 2018 v.10 no.42 pp. 36144-36156
- adsorption, calcium, carbon dioxide, coordination polymers, hydrophilicity, hydrophobicity, ligands, methane, temperature
- In this work, we report the synthesis of SION-8, a novel metal–organic framework (MOF) based on Ca(II) and a tetracarboxylate ligand TBAPy⁴– endowed with two chemically distinct types of pores characterized by their hydrophobic and hydrophilic properties. By altering the activation conditions, we gained access to two bulk materials: the fully activated SION-8F and the partially activated SION-8P with exclusively the hydrophobic pores activated. SION-8P shows high affinity for both CO₂ (Qₛₜ = 28.4 kJ/mol) and CH₄ (Qₛₜ = 21.4 kJ/mol), while upon full activation, the difference in affinity for CO₂ (Qₛₜ = 23.4 kJ/mol) and CH₄ (Qₛₜ = 16.0 kJ/mol) is more pronounced. The intrinsic flexibility of both materials results in complex adsorption behavior and greater adsorption of gas molecules than if the materials were rigid. Their CO₂/CH₄ separation performance was tested in fixed-bed breakthrough experiments using binary gas mixtures of different compositions and rationalized in terms of molecular interactions. SION-8F showed a 40–160% increase (depending on the temperature and the gas mixture composition probed) of the CO₂/CH₄ dynamic breakthrough selectivity compared to SION-8P, demonstrating the possibility to rationally tune the separation performance of a single MOF by manipulating the stepwise activation made possible by the MOF’s biporous nature.