NEW AQUEOUS MULLITE PRECURSOR SYNTHESIS - STRUCTURAL STUDY BY (27)A1,AND SI-29 NMR-SPECTROSCOPY

A mullite precursor gel is prepared by the slow and homogeneous genera tion of ammonia inside an aqueous solution of aluminium nitrate and si licic acid, urea being used as the base generator. At 80-100 degrees C a silica gel is vapidly formed, by a catalytic reaction, and then thi s gel is slowly digested by the partially hydrolysed aluminium atoms w hich link to silica by Si-O-Al bonds. The structural evolution has bee n studied by Al-27 and Si-29 NMR spectroscopy. The first aluminium ato ms are incorporated in tetrahedral symmetry and the following ones rem ain hexacoordinated. The system evolves to soluble aluminosilicate col loids with a local structure close to that of imogolite or allophane. each silicon atom being linked to three AlO6 hexahedra and a hydroxyl group. By the completion of the hydrolysis the colloids are cross link ed 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 S i atoms in the lattice, the Al atoms being four-, five- and six-fold c oordinated. The xerogel crystallises at 980 degrees C into an alumina- rich mullite and amorphous silica, and the 3Al(2)O(3) . 2SiO(2) stoich iometry is reached at 1300 degrees C by reincorporating the silica in the lattice.