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 degrees 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 Al-27 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. Al-13 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 Al-13 content of the sols increased with pH and the maximum fraction of Al-13 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 Al-13 polycations. Hence, a new synthetic method for the preparation of Al-13 ions containing sols was developed. Aging studies of the sols showed, that the Al-13 polycations were more stable in solutions with higher pH-value. Al-13 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 degrees to 80 degrees C caused formation of Al-13 and also of Al-30 polycations.