Boehmite (γ-AlOOH) and gibbsite (α-Al(OH) 3 ) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. Developing the ability to understand and predict the properties and characteristics of these materials, on the basis of their natural growth or synthesis pathways, is an important 1 fundamental science enterprise with wide ranging impacts. The present study describes bulk and surface characteristics of these novel materials in comprehensive detail, using a collectively-sophisticated set of experimental capabilities, including a range of conventional laboratory solids analyses and national user facility analyses such as synchrotron X-ray absorption and scattering spectroscopies, as well as small angle neutron scattering. Their thermal stability is investigated using in situ temperature-dependent Raman spectroscopy. These pure and effectively defect-free materials are ideal for synthesis of advanced alumina products.

Combination of uranium, peroxide, and mono- (Na, K) or divalent (Mg, Ca, Sr) cations under alkaline aqueous conditions results in the rapid formation of anionic uranyl triperoxide monomers (UTs), (UO(O)), exhibiting unique Raman signatures. Electronic structure calculations were decisive for the interpretation of the spectra and assignment of unexpected signals associated with vibrations of the uranyl and peroxide ions. Assignments were verified by O isotopic labeling of the uranyl ions supporting the computational-based interpretation of the experimentally observed peaks and the assignment of a novel asymmetric vibration of the peroxide ligands,(O).

Solid UO dissolution and uranium speciation in aqueous solutions that promote formation of uranyl peroxide macroanions was examined, with a focus on the role of alkali metals. UO powders were dissolved in solutions containing XOH (X = Li, Na, K) and 30% HO. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of solutions revealed linear trends of uranium versus alkali concentration in solutions resulting from oxidative dissolution of UO, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in solution. The maximum U concentration in solution was 4.20 × 10 parts per million (ppm), which is comparable to concentrations attained by dissolving UO in boiling nitric acid, and was achieved by lithium hydroxide promoted dissolution. Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) of solutions indicate that dissolution is accompanied by the formation of various uranyl peroxide cluster species, the identity of which is alkali concentration dependent, revealing remarkably complex speciation at high concentrations of base.

Uranium concentrations as high as 2.94 × 10 parts per million (1.82 mol of U/1 kg of HO) occur in water containing nanoscale uranyl cage clusters. The anionic cage clusters, with diameters of 1.5-2.5 nm, are charge-balanced by encapsulated cations, as well as cations within their electrical double layer in solution. The concentration of uranium in these systems is impacted by the countercations (K, Li, Na), and molecular dynamics simulations have predicted their distributions in selected cases. Formation of uranyl cages prevents hydrolysis reactions that would result in formation of insoluble uranyl solids under alkaline conditions, and these spherical clusters reach concentrations that require close packing in solution.

The first neutron diffraction study of a single crystal containing uranyl peroxide nanoclusters is reported for pyrophosphate-functionalized Na44K6[(UO2)24(O2)24(P2O7)12][IO3]2·140H2O (1). Relative to earlier X-ray studies, neutron diffraction provides superior information concerning the positions of H atoms and lighter counterions. Hydrogen positions have been assigned and reveal an extensive network of H-bonds; notably, most O atoms present in the anionic cluster accept H-bonds from surrounding H2O molecules, and none of the surface-bound O atoms are protonated. The D4h symmetry of the cage is consistent with the presence of six encapsulated K cations, which appear to stabilize the lower symmetry variant of this cluster. (31)P NMR measurements demonstrate retention of this symmetry in solution, while in situ (31)P NMR studies suggest an acid-catalyzed mechanism for the assembly of 1 across a wide range of pH values.

The characterization of prenucleation species is essential to understand crystallization mechanisms across many chemical systems and often involves the use of vibrational spectroscopy. Nowhere is this more evident than in the development of "green" aluminum processing technologies, where detailed understanding of the speciation of aluminum and its polynuclear analogues in highly alkaline, low water solutions is elusive. The aluminate anion Al(OH) predominates in alkaline conditions, yet equilibrium with dimeric species, either μ-oxo AlO(OH) or di-μ-hydroxo Al(OH), can be assumed. Using ab initio molecular dynamics with full solvation and the presence of counterions, this work reconciles previous contradictory studies that had concluded only a single species under relevant solution conditions. We reveal that the two dimers are energetically separated by 2 kcal/mol in pure water but that the stability of each can be reversed by ion pairing expected in saturated salt solutions. Simulated Raman and IR spectra for each species (accounting for anharmonicity and the fluctuating solvating environment) provide the first proof that the considered species are "spectroscopic siblings", whose multiple overlapping bands prevent definitive assertions in terms of speciation when compared to the experimental spectra. These observations are likely to hold in higher order aluminate oligomers and as such present a massive challenge toward understanding the crystallization mechanisms relevant to aluminum processing.

Herein, we report a new salt of a pyrophosphate-functionalized uranyl peroxide nanocluster {UPp} (1) exhibiting O molecular symmetry both in the solid and solution. Study of the system yielding 1 across a wide range of pH by single-crystal X-ray diffraction, small-angle X-ray scattering, and a combination of traditional P and diffusion-ordered spectroscopy (DOSY) NMR affords unprecedented insight into the amphoteric chemistry of this uranyl peroxide system. Key results include formation of a rare binary {U}·{UPp} (3) system observed under alkaline conditions, and evidence of acid-promoted decomposition of {UPp} (1) followed by spatial rearrangement and condensation of {U} building blocks into the {UPp} (2) cluster. Furthermore, P DOSY NMR measurements performed on saturated solutions containing crystalline {UPp} show only trace amounts (∼2% relative abundance) of the intact form of this cluster, suggesting a complex interconversion of {UPp}, {UPp}, and {UPp} ions.

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.