Control of Polymorphisms and Functions in All-Inorganic Ionic Crystals Based on Polyaluminum Hydroxide and Polyoxometalates

Two kinds of polymorphs of all-inorganic ionic crystals were obtained by the combination of oppositely charged molecular ions (macroions) with T_d symmetry, Keggin-type polyaluminum hydroxide [ε-Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃) and polyoxometalates (POMs) [α-CoW₁₂O₄₀]^6– (POM6), [α-1,2,3-SiV^V₃W₉O₄₀]^7– (POM7), or [α-1,2,3-SiV^IVV^V₂W₉O₄₀]^8– (POM8) in aqueous solutions. The needle-type crystals were grown by dissolution-recrystallization of the precipitates, which formed immediately by combining Al₁₃ and POMs. On the other hand, the plate-type crystals were grown by adding inorganic salts (e.g., NaCl) into the aqueous solution containing the needle-type crystals: Dissolution of the needle-type crystals occurred followed by precipitation of the plate-type crystals, which is the more insoluble and stable polymorph with larger numbers of hydrogen bonds between the constituent ions. The plate-type crystals possess one-dimensional channels with large apertures (ca. 12 Å × 6 Å) and showed high activities as heterogeneous solid acid catalysts in pinacol rearrangement.