Synthesis and characterization of trivalent metal containing mesoporous silicas obtained by a neutral templating route

A series of trivalent metal cation containing mesoporous silicas have been synthesized using a primary alkylamine, e.g., hexadecylamine as organic templating surfactant. Using this synthesis route, it was possible to prepare materials where trivalent cations were tetrahedrally coordinated, with Si/Me ratios as low as 10 without observing the presence of octahedral species. These materials possessed regular mesopores of about 37 Angstrom diameter and very high surface areas. In contrast to pure silica materials obtained following a similar synthesis route, these solids were not thermally stable, and calcination in air resulted in a partial collapse of the mesopores and in a partial extraction of trivalent cations. However, templating molecules could be easily removed from the mesopores by a solvent extraction. Removal of the template by this way preserved not only the mesopore framework but also the coordination of the cations. Solvent-extracted solids were thermally stable and could be calcined in air at high temperature without observing changes in the nature and coordination of the cations. The present synthesis route seemed thus to be particularly interesting to prepare template-free mesoporous silicas containing relatively high amounts of tetrahedrally coordinated trivalent cations.