Insight into the Mechanism of Water Adsorption/Desorption in Hydrophilic Viologen-Carboxylate Based PCP
journal contributionposted 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  first adsorbs two water molecules which fill the pores, leading to ·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 ·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).