Template Synthesis and Characterization of Layered Al− and Mg−Silsesquioxanes

A series of new layered inorganic−organic nanocomposites, with organic functionalities directly bonded to an inorganic framework via the Si−C bond, were prepared by a template sol−gel synthesis. These layered Al− and Mg−silsesquioxanes were precipitated at room temperature by addition of aqueous base to an alcohol solution containing a mixture of AlCl 3 or MgCl 2 and a trialkoxysilane with a n -dodecyl, n -octyl, n -pentyl, 3-methacryloxypropyl, isobutyl, or phenyl functionality. The Si/Al and Si/Mg ratios of the reaction mixtures were 2:1 and 4:3, respectively, and were chosen to match the composition of clay mineral pyrophyllite (layered aluminosilicate) and talc (layered magnesiosilicate). X-ray diffraction and electron microscopy show that the products are crystalline with a layered structure. A comparison of basal spacings and length of the organic functionalities is consistent with a ∼10 Å inorganic layer and a bilayer arrangement of R groups for Al−silsesquioxanes and an interpenetrating arrangement of R groups for Mg−silsesquioxanes. FTIR and 13 C NMR spectra indicate that in the products the Si−C bond is intact and silanes are fully hydrolyzed, while the organic functionalities filling the interlayer space are in a solid-like environment. 29 Si NMR spectra reveal that silanes are not fully condensed and that the degree of condensation is higher in Mg−silsesquioxanes than Al−silsesquioxanes. Broad in-plane diffraction peaks and a variety of Al coordination states observed by 27 Al NMR are best explained by the defects in a clay-like inorganic framework caused by trifunctional Si. The most crystalline products were formed from long-chain alkyl−trialkoxysilanes with Al, while trialkoxysilanes with shorter R groups gave less well-organized structures. This dependence on chain length suggests that the formation of the layered structure is due to self-assembly of the hydrolyzed trialkoxysilanes into lamellar micelles. The micelles act as a template for the formation of a clay-like inorganic framework by condensation between the silanols and aqueous metal species attracted by the negatively charged surfactant layers. Although the thermal stability of R group limits high-temperature applications, these materials may find use as sorbents, environmental barriers, polymer fillers, catalytic supports, or chemical sensors.