Microporous Mg−Si−O and Al−Si−O Materials Derived from Metal Silsesquioxanes

Abstract

Microporous amorphous metallosilicates designated as Mg−Si−O and Al−Si−O, with a very narrow pore size distribution around 6 Å diameter, typical surface areas ranging from 350 to 650 m2/g, and loadings of well-dispersed metal oxide (up to about 10 wt % metal) result from the controlled calcination of magnesium and aluminum silsesquioxane complexes [(c-C5H9)7Si7O12]2Mg4Cl2·2THF, 1, {[(c-C5H9)7Si7O12]Al}n, 2, and [(c-C5H9)7Si7O11(OSiMe3)]Al[(c-C5H9)7Si7O10(OSiMe3)(OH)], 3. Moreover, textural properties such as surface area and pore volume can be easily adjusted by varying the calcination conditions, while the pore size distribution remains practically unchanged. XPS, EDX, solid-state MAS 29Si and 27Al NMR, and HRTEM measurements suggest that the metal is present in M−Si−O materials mainly as isolated metal ions incorporated in the amorphous silica framework and also as small crystalline metal oxide particles of a few nanometers, which are well dispersed throughout the silica. SEM was employed as well to evaluate particle size and morphology. IR spectroscopy of adsorbed acetonitrile showed that both Al−Si−O materials are strong Lewis acids. Mg−Si−O and Al−Si−O were briefly tested as catalysts in 1-butanol dehydration. Mg−Si−O gave both dehydrogenation and dehydration, while on Al−Si−O only dehydration and cracking reactions occurred.