An amorphous sodium aluminate hydrate phase mediates aluminum coordination changes in highly alkaline sodium hydroxide solutions

Abstract

Aluminum (Al) transformations between solid and liquid states in the Na₂O : Al₂O₃ : H₂O system often involve changes in coordination and polymerization, with the intermediate molecular states challenging to resolve. To detect Al transformations in situ, a solid-state mixture of sodium hydroxide monohydrate (NaOH·H₂O) and boehmite (AlOOH) was heated above the melting point of NaOH·H₂O to dissolve AlOOH and prepare nonasodium bis(hexahydroxyaluminate) trihydroxide hexahydrate (NSA). In situ²⁷Al magic angle spinning, nuclear magnetic resonance (MAS NMR) spectroscopy was used to monitor Al speciation and coordination during AlOOH dissolution into a homogenous melt, and the crystallization of NSA during cooling to room temperature, supported with ex situ X-ray diffraction, Raman spectroscopy, and ²⁷Al multiple-quantum, 3QMAS NMR spectroscopy. Novel metastable aluminate species were identified during the transformation. Dissolution of AlOOH in molten NaOH·H₂O entails a transition from octahedral Al in AlOOH to tetrahedral Al in the aluminate anion [Al(OH)₄]⁻ and mu-oxo aluminate dimer [Al₂O(OH)₆]²⁻ present in solution. These tetrahedral solution-state species then precipitate to form an intermediate, amorphous, tetrahedrally coordinated, sodium aluminate hydrate phase which is stable at 70 °C, and subsequentially crystallizes during cooling to form monomeric octahedral Al in the NSA structure. These transformations and associated intermediates provide insight into the molecular scale mechanisms of Al coordination changes, which in this case appear to be mediated by an amorphous precursor containing oligomerized, tetrahedral Al.