Speciation of hydroxyl-Al polymers formed through simultaneous hydrolysis of aluminum salts and urea

Urea hydrolysis has always been used to prepare alumina gels and little attention has been paid to the reactive polymeric Al species that is formed before alumina sol–gels occur. Based on the hydrolysis process of aluminum in urea solution at 90 °C, speciation and transformation of the reactive hydroxyl-Al polymers obtained by urea hydrolysis was investigated with Ferron assay, solution-state and solid-state 27Al NMR spectroscopy. Unlike the traditional viewpoint, Keggin-Al13 can form in solution without localized high alkalinity. The reaction of oligomers with Al(OH)4− resulted from the dissolving of colloidal Al hydroxides is accountable for the formation of Keggin-Al13. Alp1, the defected structure of Al13, was considered as the transient species in the transformation from Al13 to crystalline Al hydroxides. Besides Al13, some other reactive polymers with hexameric ring structure also exist. The two categories of Al species have different transformation models (i.e., forced hydrolysis and spontaneous hydrolysis). The forced hydrolysis model should be the main transformation pattern for Al species under the condition of strong base addition into Al solution. Sulphate ions inhibit the formation of high polymeric Al species and affect the crystal structure of the final Al precipitate.