Layered and Hexagonal Aluminosilicate-Hexadecylamine Mesostructures: Solid State Transformation and Ionic Conductivity.
Inorganic-organic hybrids of the aluminosilicate-hexadecylamine mesostructures (AHM) have been prepared by a sol-gel reaction of tetraethyl orthosilicate (TEOS) and Al(NO(3))(3).9H(2)O mixtures in the presence of hexadecylamine (HDA) and LiNO(3). At an HDA concentration of [HDA]/([Si] + [Al]) = 0.3, both lamellar and hexagonal AHM structures were obtained, depending on the degree of Al substitution and LiNO(3) addition. Our synthetic work illustrates that the inorganic ingredient (LiNO(3)) as well as the sol-gel precursor (Al(NO(3))(3)) plays a crucial role in the selective formation of the lamellar or hexagonal AHM. Without an Al or a Li source, products obtained by the same method were all found to have exclusively the hexagonal structure. Studies using XRD, DSC, high-resolution TEM, and selected area electron diffraction (SAED) verified that the prepared lamellar AHMs undergo (i) a reversible melting of the HDA intercalates at 60 degrees C and then (ii) an irreversible structural transformation to their hexagonal analogues at higher temperature. The temperature dependence of the ionic conductivity (sigma) revealed a strong structural sensitivity. We discuss in more detail the conduction behavior of a [Si(17.1)Al(1.00)O(36.2)(Li(0.47)H(0.53))][HDA](21.0) product in its lamellar structure (exhibiting moderate conductivities of approximately 10(-)(7) S/cm below 50 degrees C), in its highly conductive HDA-melted metastable layered structure ( approximately 10(-)(4)-10(-)(3) S/cm in the region of 60-210 degrees C), and in its comparatively insulating hexagonal structure (10(-)(9)-10(-)(8) S/cm between 130 and 260 degrees C).