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Insight into the Mechanism of Water Adsorption/Desorption in Hydrophilic Viologen-Carboxylate Based PCP

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journal contribution
posted on 30.03.2017, 00:00 by Maxime Leroux, Nicolas Mercier, Jean-Pierre Bellat, Guy Weber, Igor Bezverkhyy
A water stable and highly hydrophilic porous coordination polymer based on viologen-carboxylate type ligand, the 4,4′-bipyridinium,1,1-bis­(3-carboxyphenyl) (pc2), is obtained by the solvothermal method: [Cd3(pc2) (BTC)2(H2O)2]·6H2O ([1(H2O)2]·6H2O; BTC3– = 1,3,5-carboxybenzene). Its crystal structure and the ones of two partially dehydrated phases have been determined, allowing insight into the mechanism of water adsorption/desorption of this PCP material. It is shown that the dehydrated compound [1] first adsorbs two water molecules which fill the pores, leading to [1]·2H2O. On the other hand, the partial dehydration of the as-synthesized compound leads to the intermediate phase [1(H2O)]·3H2O in which one H2O molecule is bound to Cd2+ ions of trinuclear building units, and three others are localized in the pores. The structural analysis also reveals that the pyridinium N+ Lewis sites, which interact with water molecules in [1(H2O)2]·6H2O, interact with carboxylate groups after structural reorganization in [1]·2H2O. The water adsorption isotherm clearly shows that the dehydrated compound is highly hydrophilic and adsorbs water in three steps: first and second at P/PS < 0.01 and third at P/PS ∼ 0.1 with the overall water adsorption reaching 0.13 g H2O/g. This material is also able to reversibly adsorb ammonia (up to 0.3 g/g or 17.6 mmol/g).

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