MEMORY EFFECTS IN GEL SOLID TRANSFORMATIONS - COORDINATELY UNSATURATED AL SITES IN NANOSIZED ALUMINAS

Precursors of aluminas rich in pentacoordinated aluminum are generated by a new sol-gel route. The critical step of the process is a limited hydrolysis of aluminum tri.sec.butoxide in non-aqueous media. The gel-solid transformation is investigated by Al-27 high-resolution MAS NMR and nitrogen adsorption. The structural and textural evolutions of the gel by thermal activation up to 700-degrees-C are self-determined. As the octahedral condensation in the gel increases, the Al(V) concentration in the solid increases: this observation evidences a structural memory. The percentage of small pores (d < 6 nm) in the transition alumina is correlated to the chemical shift of the Al(VI) line in the gel. Thus, a textural memory effect is also observed. A general mechanism for the Al(V) formation is proposed, and the metastability of this coordinately unsaturated aluminum site is demonstrated.