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Biporous Metal–Organic Framework with Tunable CO2/CH4 Separation Performance Facilitated by Intrinsic Flexibility

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journal contribution
posted on 24.09.2018, 00:00 by Andrzej Gładysiak, Kathryn S. Deeg, Iurii Dovgaliuk, Arunraj Chidambaram, Kaili Ordiz, Peter G. Boyd, Seyed Mohamad Moosavi, Daniele Ongari, Jorge A. R. Navarro, Berend Smit, Kyriakos C. Stylianou
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.