In Situ ²⁷Al NMR Spectroscopy of Aluminate in Sodium Hydroxide Solutions above and below Saturation with Respect to Gibbsite

Abstract

Aluminum hydroxide (Al(OH), gibbsite) dissolution and precipitation processes in alkaline environments play a commanding role in aluminum refining and nuclear waste processing, yet mechanistic aspects underlying sluggish kinetics during crystallization have remained obscured due to a lack of in situ probes capable of isolating incipient ion pairs. At a molecular level Al is cycling between tetrahedral ( T ) coordination in solution to octahedral ( O) in the solid. We explored dissolution of Al(OH) that was used to produce variably saturated aluminate (Al(OH))-containing solutions under alkaline conditions (pH >13) with in situ Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy, and interrogated the results with ab initio molecular dynamics (AIMD) simulations complemented with chemical shift calculations. The collective results highlight the overall stability of the solvation structure for T Al in the Al(OH) oxyanion as a function of both temperature and Al concentration. The observed chemical shift did not change significantly even when the Al concentration in solution became supersaturated upon cooling and limited precipitation of the octahedral Al(OH) phase occurred. However, subtle changes in Al(OH) speciation correlated with the dissolution/precipitation reaction were found. AIMD-informed chemical shift calculations indicate that measurable perturbations should begin when the Al(OH)···Na distance is less than 6 Å, increasing dramatically at shorter distances, coinciding with appreciable changes to the electrostatic interaction and reorganization of the Al(OH) solvation shell. The integrated findings thus suggest that, under conditions incipient to and concurrent with gibbsite crystallization, nominally expected contact ion pairs are insignificant and instead medium-range (4-6 Å) solvent-separated Al(OH)···Na pairs predominate. Moreover, the fact that these medium-range interactions bear directly on resulting gibbsite characteristics was demonstrated by detailed microscopic and X-ray diffraction analysis and by progressive changes in the fwhm of the O resonance, as measured by in situ NMR. Sluggish gibbsite crystallization may arise from the activation energy associated with disrupting this robust medium-range ion pair interaction.

Interplay of physically different properties leading to challenges in separating lanthanide cations – an ab initio molecular dynamics and experimental study

“…16 Indeed, metal cations coordinated to one or more OH − have been known to yield reduced coordination numbers. 25,60 Another reason may be that the explicit treatment of the second hydration shell in AIMD simulations changes the first shell hydration number computed using an implicit solvent approximation in the literature. Finally, different DFT functionals can yield N hyd which are slightly different from each other and from experiments.…”

Inference of principal species in caustic aluminate solutions through solid-state spectroscopic characterization

“…33 The DFT functional consisted of a revised version of the PBE functional 34 with the empirical dispersion corrections of Grimme et al 35 Within the mixed gaussian and plane waves scheme 33 a double-zeta basis set was used with a single set of polarization functions, 36 Goedecker-Teter-Hutter pseudopotentials together with plane wave cutoffs of 600 to 900 Ry depending on the hardness of the pseudopotentials. The aluminate dimers, K 2 Al 2 O(OH) 6 (1), and Rb 2 Al 2 O(OH) 6 (2) are isostructural and crystallize in orthorhombic Aba2 (space group no. 41) unit cells.…”

“…For example, understanding how the dominant tetrahedrallycoordinated aluminate anion, Al(OH) 4 -, transforms to octahedral aluminum (Al) during the precipitation of gibbsite, α-Al(OH) 3 , is key to the efficiency of the Bayer process by which alumina, Al 2 O 3 , is extracted from bauxite ore. [1][2][3] Despite the fact that the Bayer process was invented over 120 years ago and is still used to produce nearly all of the world's Al 2 O 3 supply as an intermediate step to Al production, the mechanism and kinetics of α-Al(OH) 3 precipitation have not been comprehensively described. [4][5][6][7][8][9] Both synthetic and industrial Bayer liquors have a propensity to become supersaturated, 10,11 presenting a considerable engineering challenge as α-Al(OH) 3 precipitation suffers from slow kinetics and yields are poor even in the presence of seed crystals. 12,13 Limited mechanistic understanding of how small Al clusters of low nuclearity condense into extended sheet structures of Al octahedra in α-Al(OH) 3 , or boehmite, AlO(OH) also directly hampers the processing of ca.…”

Hierarchical phenomena in multicomponent liquids: simulation methods, analysis, chemistry

“…In another instance, 27 Al NMR sensitivity to short and mid-range to long-range counter-ion pairing effects in Al(OH) 4 was studied through MD structural sampling. 111 The shielding constant value of 27 Al was significantly affected only by short range Na + ion pairing electrostatic contributions below an ion-ion distance of 3 Å. Complementary Al Kedge X-ray absorption near edge structure (XANES) experiment and simulation of Al(OH) 4 NaOH electrolyte solutions identified a shift in the pre-edge shoulder correlated to sodium concentration. 112 In contrast to the NMR that was less sensitive to long-range interactions, the physical origin of the XANES shift was attributed to longer-range ion interactions.…”