A AL-27 NUCLEAR-MAGNETIC-RESONANCE STUDY ON THE OPTIMALIZATION OF THE DEVELOPMENT OF THE AL13 POLYMER

Synthesis conditions strongly influence the yield of the tridecameric polymer Al13 ([AlO4Al12(OH)24(H2O)12]7+). An amount of 68% tridecamer was achieved by injection of alkali through a capillary tube into a 5 X 10(-2) M Al solution at a rate of 0.015 ml/s up to an OH/Al ratio of 2.2. Dropwise addition of alkali yielded significantly less tridecameric polymer. During progressive hydrolysis the monomeric Al NMR resonance moved from 0.1 to 0.9 ppm and the linewidth increased from 37 to 112 Hz. Simultaneously the resonance at 63.3 ppm due to tridecameric fourfold coordinated Al changed by 0.02 ppm. During aging the tridecamer rearranged to polymers undetectable by NMR, due to loss of the tetrahedral symmetry of the central Al, which was deduced from the decrease in intensity and the broadening of the 63.3 ppm resonance. The formation of tetrahedral Al(OH)4-, due to the inhomogeneous conditions at the point of base introduction, is essential for the synthesis of Al13. Aging over a period of 1 year caused a strong decrease in Al13 concentration, which showed that Al13 is a metastable polymer.