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 °C under
vacuum and breaking down at 350 °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.