Accessibility of Lithium Cations in VSH-2 Zeotype: Structural Effects and Formation of Protonated Water Clusters

The accessibility of lithium cations in microporous vanadosilicate VSH-2Cs of composition Cs₂(VO)(Si₆O₁₄) ⋅ 3H₂O was investigated by Single Crystal X-ray Diffraction, Attenuated Total Reflection Fourier Transformed Infrared Spectroscopy and Density Functional Theory calculations. The topological symmetry of VSH-2Cs is described in space group Cmca. After Li-ion exchange, the structure of VSH-2Li adopted monoclinic symmetry (space group C2/c) with a=17.011(2) Å, b=8.8533(11) Å, c=12.4934(16) Å, β=91.677(4)°, V=1880.7(4) ų. The strong interactions between Li ions and oxygen-framework atoms drive the main deformation mechanism, which is based on cooperative rotation of SiO₄ and VO₅ units around their oxygen atoms that behave as hinges. Exchange of Cs⁺ by Li⁺ is incomplete and accompanied by the formation of protonated species to counterbalance the electrostatic charge. The incorporation of protons is mediated by the presence of water dimers in the structural channels. H₂O molecules in VSH-2Li account not only as “space-fillers” after the removal of large Cs ions but also mediate proton transfer to compensate the negative charge of the host vanadosilicate framework.