New aqueous mullite precursor synthesis. Structural study by 27Al and 29Si NMR spectroscopy

Abstract

A mullite precursor gel is prepared by the slow and homogeneous generation of ammonia inside an aqueous solution of aluminium nitrate and silicic acid, urea being used as the base generator. At 80–100 °C a silica gel is rapidly formed, by a catalytic reaction, and then this gel is slowly digested by the partially hydrolysed aluminium atoms which link to silica by SiOAl bonds. The structural evolution has been studied by 27Al and 29Si NMR spectroscopy. The first aluminium atoms are incorporated in tetrahedral symmetry and the following ones remain hexacoordinated. The system evolves to soluble aluminosilicate colloids with a local structure close to that of imogolite or allophane: each silicon atom being linked to three AlO 6 hexahedra and a hydroxyl group. By the completion of the hydrolysis the colloids are cross linked to a gel precursor of mullite. With thermal treatment the imogolite-like orthosilicate units arrangement is rapidly lost, but the xerogel remains chemically homogeneous with a random distribution of Al and Si atoms in the lattice, the Al atoms being four-, five- and six-fold coordinated. The xerogel crystallises at 980 °C into an alumina-rich mullite and amorphous silica, and the 3Al 2O 3· 2SiO 2 stoichiometry is reached at 1300 °C by reincorporating the silica in the lattice.