TEMPERATURE INFLUENCE ON THE AL13 COMPLEX IN PARTIALLY NEUTRALIZED ALUMINUM SOLUTIONS - A AL-27 NUCLEAR-MAGNETIC-RESONANCE STUDY

Stepwise heating to 85-degrees-C in the NMR apparatus does not notably change the monomer and tridecamer (Al13) concentrations in a 0.2M Al(NO3)3 solution neutralized with 0.2M NaOH up to an OH/Al molar ratio of 2.4. Upon heating the fourfold coordinated Al-27 NMR signal of Al13 at 63.3 ppm and the very broad sixfold coordinated Al-27 NMR signal of Al13 at 12 ppm exhibit an increasing intensity and decreasing linewidth, due to diminishing 'missing intensity' and 'quadrupole relaxation', respectively. An analogous effect for a Na2CO3 neutralized 0.2M AlCl3 solution confirmed that the 12 ppm signal must be assigned to the sixfold coordinated Al of the Al13 complex. The surface ratio of fourfold coordinated Al to sixfold coordinated Al of the Al13 complex experimentally established is smaller than the theoretical 1:12 ratio. During heating, a more intensive exchange interaction between monomer and other Al-species is proposed without any effect on the actual concentrations. High symmetry in the Al13 complex is determined at elevated preparation temperatures from the decreasing linewidth of the 63.3 ppm resonance. Above 85-degrees-C the tridecamer transforms to other species, which cannot be observed with NMR.