Synthesis and Crystal Structures of Two Crystalline Silicic Acids: Hydrated H-Apophyllite, H16Si16O40 • 8–10 H2O and H-Carletonite, H32Si64O144

Abstract

Hydrated H-Apophyllite (HH-Apo) and H-carletonite (H-Car) were synthesized at 0 °C by leaching an apophyllite and a carletonite single crystal in a large surplus of 1.2 molar hydrochloric acid. The XRD powder patterns of HH-Apo and H-Car were indexed with space group symmetries of P4/ncc and I4/mcm and lattice parameters of a = 8.4872(2) Å, c = 16.8684(8) Å and a = 13.8972(3) Å, c = 20.4677(21) Å, respectively. The crystal structures were solved based on model building of the structures of the precursors and a physico-chemical characterization. Rietveld structure refinements confirmed the structure models. HH-Apo and H-Car are among the very few crystalline silicic acids whose structures have been determined and confirmed based on a structure refinement. The structure of HH-Apo contains thin silicate monolayers that can be regarded as constructed by rings of interconnected [SiO3OH] tetrahedra which form a puckered silicate layer. A sheet of water molecules is intercalated between the silicate layers. There are no direct hydrogen bonds between the silanol groups, but there are hydrogen bonds of different strengths between the terminal O atoms of the silicate layers and the intercalated water molecules. The 1H MAS NMR spectrum presents a strong signal at 4.9 ppm related to the aforementioned bonds and interactions between the water molecules, as well as a small signal at 22.5 ppm corresponding to an extremely strong hydrogen bond with d(O...O) ≈ 2.2 Å. The structure of H-Car is free of structural water and consists exclusively of microporous silicate double-layers with 4-connected [SiO4] and 3-connected [SiO3OH] tetrahedra in a ratio of 1:1 and a thickness of 9.2 Å. Neighboring layers are connected to each other by medium–strong hydrogen bonds with O...O distances of 2.56 Å. The structure of HH-Apo decays within several hours while H-Car is stable. A topotactic condensation reaction applied to H-Car forms an irregularly condensed silicate which still contains the layers in a distorted form as building blocks.