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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.