Solid-State Recrystallization Pathways of Sodium Aluminate Hydroxy Hydrates

Crystallization of Al³⁺-bearing solid phases from highly alkaline Na₂O:Al₂O₃:H₂O solutions commonly necessitates an Al³⁺ coordination change from tetrahedral to octahedral, but intermediate coordination states are often difficult to isolate. Here, a similar Al³⁺ coordination change process is examined during the solid-state recrystallization of monosodium aluminate hydrate (MSA) to nonasodium bis(hexahydroxyaluminate) trihydroxide hexahydrate (NSA) at ambient temperature. While the MSA structure contains solely oxolated tetrahedral Al³⁺, the NSA structure is a molecular aluminate salt solely based upon monomeric octahedral Al³⁺. Spontaneous recrystallization of MSA and excess sodium hydroxide hydrate into NSA over 3 days of reaction time was clearly evident in X-ray diffractograms and in Raman spectra. In situ single-pulse ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and ²⁷Al multiple quantum (MQ) MAS NMR spectroscopy showed no evidence of intermediate aluminates, suggesting that transitional states, such as pentacoordinate Al³⁺, are short-lived and require spectroscopy with greater time resolution to detect. Such research is advancing upon a detailed mechanistic understanding of Al³⁺ coordination change mechanisms in these highly alkaline systems, with relevance to aluminum refining, corrosion sciences, and nuclear waste processing.