Synthesis and structure analysis of the layer silicate DLM-2

In an attempt to prepare a novel condensed silicate containing double 4-rings, a hydrothermal synthesis using a solution of octakis(tetramethylammonium) octasilsesquioxane hydrate as the starting material was performed. The product (DLM-2) was more thermally stable than the precursor, indicating that at least some condensation had taken place, but the solution of the structure of this polycrystalline material (∣((CH3)3N)8 (H2O)20∣[Si24O48(OH)8]) turned out to be more difficult than expected. This was primarily a result of the fact that the correct symmetry was unclear. However, the structure was finally solved from powder diffraction data collected on a textured sample using the powder charge flipping algorithm pCF in the program Superflip. This algorithm is symmetry independent, so the space group problem could be circumvented. The structure proved to be non-centrosymmetric (Ima2, a = 11.5243(1) Å, b = 8.4136(1) Å, c = 27.9075(4) Å) and consists of silicate layers hydrogen-bonded to one another via water molecules with TMA+ cations encapsulated in the voids. However, there are no double 4-rings in the structure, suggesting that softer reaction conditions are needed for them to retain their structural integrity during the synthesis. The structure is very similar to that of the zeolite precursor RUB-15.