PNNL

Abstract

Aluminum (Al) transformations between solid and liquid states in the Na2O:Al2O3:H2O system often involve changes in coordination and polymerization, with 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-situ 27Al magic angle spinning, nuclear magnetic resonance spectroscopy (27Al MAS NMR) 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 (MQ) MAS NMR. Novel metastable aluminate species were identified during the transformation. Dissolution of AlOOH in molten NaOH·H2O entails a transition from octahedral Al 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 stable at 70°C, which during cooling subsequentially crystallizes to form monomeric octahedral Al in the NSA structure. These transformations and associated intermediates provide insight into the molecular scale mechanisms of Al coordination change, which in this case appear to be mediated by an amorphous precursor containing oligomerized tetrahedral Al. The findings may ultimately help improve models designed to predict aluminate phase stability and reaction pathways that are pertinent to both environmental and industrial processes.