Abstract
Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si12O3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K+, Rb+ and Cs+), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and <SiOSi> bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).
Highlights
Zeolites including both natural and synthetic minerals have been extensively studied over the years because of their structural and chemical properties that produce interest in various fields.Zeolites are associated with ordered micro porosity, high crystallinity, strong acidity, molecular sieving capability or huge specific surface area availability [1], which makes them interesting materials for adsorption [2], catalysis [3,4], and separation [5]
These materials are usually synthesized by the hydrothermal method from hydrolysis-polycondensation of silicates or aluminosilicates in alkaline media [6]
The remaining non-hydrogen atoms were located from Fourier differences and were refined with anisotropic thermal parameters
Summary
Zeolites are associated with ordered micro porosity, high crystallinity, strong acidity, molecular sieving capability or huge specific surface area availability [1], which makes them interesting materials for adsorption [2], catalysis [3,4], and separation [5] These materials are usually synthesized by the hydrothermal method from hydrolysis-polycondensation of silicates or aluminosilicates in alkaline media [6]. They have several structural characteristics in common based on three-dimensional combinations of TO4 (T = Si or Al) tetrahedra interconnected to each other by sharing their oxygen atoms.
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