Abstract
Polynuclear cationic aluminum hydroxide phases are known intermediates in the formation of aluminum oxides or (oxide)hydroxides upon hydrolysis of aluminum salt solutions. In the presence of sulfate anions, these aluminum polyoxocations (Al13) can form crystalline Al13 sulfates with varying chemical composition. The formation of these Al13 sulfates in aqueous solution has been poorly understood. Here, we investigate the early stage crystallization of Al13 clusters in a sulfate-containing solution, in situ and in real time. Dynamics associated with Al13 sulfate formation have been obtained for the first time, using synchrotron X-ray diffraction (XRD) of solutions suspended by acoustic levitation. Time-resolved in situ data show that the cubic phase, Na[(AlO4)Al12(OH)24(H2O)12](SO4)4·10H2O, forms after only minutes. The formation mechanism of Al13 sulfates was found to depend on the sulfate:aluminum (SO4:Al) ratio. Ex situ XRD of the product Al13 sulfates in solution shows that for SO4:Al ratio ≤1.5 two other crystalline phases form, and convert to the cubic phase upon washing and drying. In situ XRD for the same ratio shows transient formation of an intermediate during the crystallization process.
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