Flexible Structure of a Thermally Stable Hybrid Aluminosilicate Built with Only the Three-Ring Unit

Organic inorganic aluminosilicate hybrids are an attractive new class of materials that add organic functionalities to conventional properties of solid inorganic catalysts. ECS-17, a novel crystalline hybrid, was synthesized using 1,4-bis-(triethoxysilyl)-benzene as the sole silicon source. Its structure was solved by direct methods starting from high-resolution synchrotron X-ray diffraction data and is composed of inorganic layers, characterized by 10 rings, held together by phenylene rings. ECS-17 is the first aluminosilicate built from only the three-ring secondary building unit. This new material shows intriguing reversible collapsibility upon dehydration/rehydration. Mild thermal treatment under vacuum causes its crystalline structure to collapse due to facile elimination of the water molecules around the cations. Successive exposure to ambient atmospheric moisture gives back the hydrated crystalline form. ECS-17 shows remarkably high thermal stability for a hybrid, being stable up to 450 degrees C under vacuum and breaking down at 350 degrees C in air. Structural, thermal, and optical properties were examined by X-ray powder diffraction, thermogravimetric analysis, nuclear magnetic resonance, and ultraviolet-visible-near-infrared reflectance and fluorescence spectroscopies.