27Al NMR Study of Al-Speciation in Aqueous Alumina-Sols

A new method of preparing concentrated alumo-sols, the hydrolysis of Al-tri-sec-butylate in acidic aqueous media at 85°C, was studied in detail by varying the H₂O/Al- and NO₃ ⁻/Al-ratios in wide ranges. The components of the sols were characterized by ²⁷Al NMR spectroscopy. The pH-value depends on both the chosen H₂O/Al- and NO₃ ⁻/Al-ratio and on the aging time of the sols and reflects the composition of the sols. Al₁₃ polycations were detected in sols with a pH-value between 3.0 and 3.7. As a new result its presence was shown by NMR below 3.4. The Al₁₃ content of the sols increased with pH and the maximum fraction of Al₁₃ polycations was detected in the sol with the highest pH (3.7). Nearly 65% of the entire aluminium content of this solution is bound in the Al₁₃ polycations. Hence, a new synthetic method for the preparation of Al₁₃ ions containing sols was developed. Aging studies of the sols showed, that the Al₁₃ polycations were more stable in solutions with higher pH-value. Al₁₃ polycations were detected after an aging time of four months only in sols with a pH-value of 3.7. Tempering the aged sols at 40° to 80°C caused formation of Al₁₃ and also of Al₃₀ polycations.