SYNTHESIS OF A MULLITE PRECURSOR FROM ALUMINUM NITRATE AND TETRAETHOXYSILANE VIA AQUEOUS HOMOGENEOUS PRECIPITATION - AN AL-27 AND SI-29 LIQUID-STATE AND SOLID-STATE NMR SPECTROSCOPIC STUDY

A simple aqueous process is described for the preparation of aluminosi licate colloids and chemically homogeneous mullite precursor gel, Star ting from a solution of aluminum nitrate and silicic acid, aluminum is slowly hydrolyzed at 80-100 degrees C by in situ generation of ammoni a. A silica gel is rapidly made, probably by a catalytic effect of ure a, the base generator. This gel is then slowly digested by partially h ydrolyzed aluminum species which break the Si-O-Si bonds and link to t he gel by Si-O-Al bonds, Progressively a clear colloidal sol is obtain ed and the colloidal particle size continues to decrease toward alumin osilicate species where the silicon atoms are in a single environment and may be linked to three hexacoordinated aluminum atoms and a hydrox yl group, by reference to natural imogolite. When the hydrolysis of al uminum is nearly complete, these particles are cross-linked and a fina l gel precursor of mullite is obtained. This gel is chemically very ho mogeneous and crystallizes to mullite at 980 degrees C. The structural evolution, from the first gel to the ceramic, has been followed by Al -27 and Si-29 liquid- and solid-state MAS NMR spectroscopy.