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Structures and Sorption Properties of Ionic Crystals of Macrocation-Dawson-Type Polyoxometalates with Different Charges

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posted on 22.03.2005, 00:00 by Sayaka Uchida, Ryosuke Kawamoto, Takeo Akatsuka, Shiro Hikichi, Noritaka Mizuno
The complexation of Dawson-type polyoxometalates of [α-P2W18O62]6-, [α2-P2W17V1O62]7-, and [α-P2W15V3O62]9- with the macrocation ([Cr3O(OOCH)6(H2O)3]+) forms ionic crystals of (NH4)4[Cr3O(OOCH)6(H2O)3]2[α-P2W18O62]·15H2O (1a), (NH4)5[Cr3O(OOCH)6(H2O)3]22-P2W17V1O62]·15H2O (2a), and (NH4)7[Cr3O(OOCH)6(H2O)3]2[α-P2W15V3O62]·15H2O (3a), respectively. The compounds 1a3a show the honeycomb packing and the symmetry of the constituent ions reflect on the crystal structures. The lengths of the a axes of 1a3a are almost the same, while those of the c axes decrease in the order of 1a2a > 3a, with the increase in the anion charges of the polyoxometalates. The water of crystallization in 1a3a is desorbed by the evacuation at 373 K to form the respective guest-free phases 1b3b. The compounds 1b3b are crystalline, and the lengths of the a axes of 1b3b are almost the same, while those of the c axes decrease in the order of 1b2b > 3b. The compounds 1b3b possess voids running perpendicular to the c axis. The sizes of the voids decrease in the order of 1b > 2b > 3b and are comparable to those of water and methanol. The water sorption profile of 1b as a function of time is reproduced by the linear driving force mass transfer model with a single rate constant k1, while that of 3b is reproduced by the two components k1 and k2. The component k1 is attributed to the diffusion of water molecules in the voids, and the component k2 is attributed to the large increase in the length of the c axis with the water sorption. The compounds 1b and 2b sorb ethanol, while the amount of sorption for 2b is smaller than that for 1b, and 3b completely excludes ethanol. Thus, the crystal structures, water sorption kinetics, and alcohol sorption properties of 1b3b systematically change with the anion charges.