Dehydration and lithium ion-exchange of the open framework vanadium silicate VSH-16Na

Abstract

Abstract Microporous VSH-16Na of Na3V(Si4O11)·H2O composition was synthesized under mild hydrothermal conditions and studied by single-crystal X-ray diffraction. Its vanadosilicate framework contains unbranched silicate double chains along the [100] direction connected by [V3+O6] octahedra to form an open framework structure with eight-membered ring channels. The compound is triclinic with space group P 1 ‾ , however the framework topology suggests higher symmetry reduced by the arrangement of the extraframework components. After removal of the H2O molecule at 733 K, the anhydrous structure adopted monoclinic symmetry (space group C2/c) with a = 15.9201 (13) b = 8.7274 (7) c = 7.4821 (6) β = 95.909 (3)°, V = 1034.05 (14) A3. The dehydration is irreversible due to the adjustment of the Na positions upon removal of the H2O molecule. In particular, the Na2 site displays positional disorder at two sub-sites, ca. 1.7 A apart. After Li+ ion exchange, the resulting structure with unit-cell parameters a = 16.638 (5) b = 8.028 (3) c = 7.382 (2) β = 90.183 (8)°, V = 986.0 (6) A3 (space group C2/c) closely simulates orthorhombic symmetry. Although the substitution of Na+ by Li+ was accompanied by hydration, the unit-cell volume decreased by ~5% compared to the dehydrated Na-bearing framework. The Li-exchanged structure is apparently alkali-deficient; however, we infer the presence of hydronium H3O+ ions at the OWa sites to counterbalance the deficit of positive charge. Framework distortions upon Li-exchange and hydration increase the micropore volume and adjust the effective pore size.