COMPARATIVE-STUDY OF VARIOUS SOL-GEL PREPARATIONS OF CORDIERITE USING AL-27 AND SI-29 LIQUID-STATE AND SOLID-STATE NMR-SPECTROSCOPY

The influence of molecular precursors in the sol-gel synthesis of cordierite powders has been investigated. Several preparations have been studied by Si-29 and Al-27 liquid NMR to detect possible condensation reactions between precursors. The structure of the xerogels as well as their crystallization process into cordierite was characterized by Si-29 and Al-27 MAS NMR. A xerogel obtained from tetraethoxysilane, aluminum, and magnesium nitrates first crystallizes into MgAl2O4 spinel and then cordierite phases. It can be described as a silica gel network, with magnesium atoms and aluminum atoms in octahedral environment, dispersed in it. Preparations from silicon and aluminum alkoxides and magnesium acetate lead directly to mu- and then alpha-cordierite at 1000-degrees-C. In these gels, formation of Si-O-Al bridges have been evidenced and seems to be the key parameter to form directly cordierite at low temperature, without crystallizing spinel. Indeed, such bonds in the dried gel are related to the presence of aluminum atoms in tetrahedral environment, such as in cordierite phases. On the contrary, if no co-condensation reactions have occurred between the two precursors, the aluminum atoms stay in octahedral symmetry, and spinel formation is favored.