An Efficient Reactive Force Field without Explicit Coordination Dependence for Studying Caustic Aluminum Chemistry.

Reactive force fields (RFFs) are an expedient approach to sample chemical reaction paths in complex systems, relative to density functional theory. However, there is continued need to improve efficiencies, specifically in systems that have slow transverse degrees of freedom, as in highly viscous and superconcentrated solutions. Here, we present an RFF that is differentiated from current models (e.g., ReaxFF) by omitting explicit dependence on the atom coordination and employing a small parameter set based on Lennard-Jones, Gaussian, and Stillinger-Weber potentials. The model was parametrized from AIMD simulation data and is used to model aluminate reactivity in sodium hydroxide solutions with extensive validation against experimental radial distribution functions, computed free energy profiles for oligomerization, and formation energies. The model enables simulation of early stage Al(OH)3 nucleation which has significant relevance to industrial processing of aluminum and has a computational cost that is reduced by 1 order of magnitude relative to ReaxFF.

Disordered interfaces of alkaline aluminate salt hydrates provide glimpses of Al3+ coordination changes.

HYPOTHESIS: The precipitation and dissolution of aluminum-bearing mineral phases in aqueous systems often proceed via changes in both aluminum coordination number and connectivity, complicating molecular-scale interpretation of the transformation mechanism. Here, the thermally induced transformation of crystalline sodium aluminum salt hydrate, a phase comprised of monomeric octahedrally coordinated aluminate which is of relevance to industrial aluminum processing, has been studied. Because intermediate aluminum coordination states during melting have not previously been detected, it is hypothesized that the transition to lower coordinated aluminum ions occurs within ahighly disordered quasi-two-dimensional phase at the solid-solution interface. EXPERIMENTS AND SIMULATIONS: In situ X-ray diffraction (XRD), Raman and27Al nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na9[Al(OH)6]2·3(OH)·6H2O). A mechanistic interpretation was developed based on complementary classical molecular dynamics (CMD) simulations including enhanced sampling. A reactive forcefield was developed to bridge speciation in the solution and in the solid phase. FINDINGS: In contrast to classical dissolution, aluminum coordination change proceeds through a dynamically stabilized ensemble of intermediate states in a disordered layer at the solid-solution interface. In both melting and dissolution of NSA, octahedral, monomeric aluminum transition through an intermediate of pentahedral coordination. The intermediate dehydroxylates to form tetrahedral aluminate (Al(OH)4-) in the liquid phase. This coordination change is concomitant with a breaking of the ionic aluminate-sodium ionlinkages. The solution phase Al(OH)4- ions subsequently polymerize into polynuclear aluminate ions. However, there are some differences between bulk melting and interfacial dissolution, with the onset of the surface-controlled process occurring at a lower temperature (∼30 °C) and the coordination change taking place more gradually as a function of temperature. This work to determine the local structure and dynamics of aluminum in the disordered layer provides a new basis to understand mechanisms controlling aluminum phase transformations in highly alkaline solutions.

Concentration dependent interfacial chemistry of the NaOH(aq): gibbsite interface.

Caustic conditions are often employed for dissolution of a wide variety of minerals, where ion sorption, surface diffusion, and interfacial organization impact surface reactivity. In the case of gibbsite, γ-Al(OH)3, the chemistry at the NaOH(aq) interface is deeply intertwined with industrial processing of aluminum, including metal production and the disposition of Al-containing wastes. To date, little is known about the structure, speciation, and dynamic behavior of gibbsite interfaces (and that of many other minerals) with NaOH(aq)-particularly as a function of ionic strength. Yet concentration-dependent interfacial organization and dynamics are a critical starting point to develop a fundamental understanding of the factors that influence dissolution. This work reports equilibrium molecular dynamics simulations of the γ-Al(OH)3:NaOH(aq) interface, revealing the sorption behavior and speciation of ions from 0.5-10 M [NaOH]. As inner-sphere complexes, Na+ primarily coordinates to the side of the gibbsite hexagonal cavities, while OH- accepts hydrogen-bonding from the surface-OH groups. The mobility of inner-sphere Na+ and OH- ions is significantly reduced due to a strong surface affinity in comparison to previous reports of NaCl, CaCl2, or BaCl2 electrolytes. At high [NaOH], contact ion pairing that is observed in the bulk solution is partially disrupted upon sorption to the gibbsite surface by the individual ion-surface interactions. The molecular-scale changes to surface speciation and competition between ion-surface vs. ion-ion interactions influence surface characterization of gibbsite and potential dissolution processes, providing a valuable baseline for starting conditions needed within future reactive molecular simulations.

pH dependent reactivity of boehmite surfaces from first principles molecular dynamics.

pH dependent interfacial chemistry depends upon the distribution and respective pKa values of different surface active sites. This is highly relevant to the chemistry of nanoparticle morphologies that expose faces with varying surface termination. Recent synthetic advances for nanoparticles of various minerals, including AlO(OH) (boehmite), present an excellent opportunity to compare and contrast predicted surface pKa on low Miller index planes so as to reinterpret reported interfacial properties (i.e., point of zero charge - PZC) and reactivity. This work employs ab initio molecular dynamics and empirical models to predict site-specific pKa values of accurate (benchmarked) surface models of boehmite. Using the different surface site populations, the PZC is determined and the influence this has upon reported interfacial chemistry is described.

Theory-Guided Inelastic Neutron Scattering of Crystalline Alkaline Aluminate Salts Bearing Principal Motifs of Solution-State Species.

Aluminate salts precipitated from caustic alkaline solutions exhibit a correlation between the anionic speciation and the identity of the alkali cation in the precipitate, with the aluminate ions occurring either in monomeric (Al(OH)4-) or dimeric (Al2O(OH)62-) forms. The origin of this correlation is poorly understood as are the roles that oligomeric aluminate species play in determining the solution structure, prenucleation clusters, and precipitation pathways. Characterization of aluminate solution speciation with vibrational spectroscopy results in spectra that are difficult to interpret because the ions access a diverse and dynamic configurational space. To investigate the Al(OH)4- and Al2O(OH)62- anions within a well-defined crystal lattice, inelastic neutron scattering (INS) and Raman spectroscopic data were collected and simulated by density functional theory for K2[Al2O(OH)6], Rb2[Al2O(OH)6], and Cs[Al(OH) 4]·2H2O. These structures capture archetypal solution aluminate species: the first two salts contain dimeric Al2O(OH)62- anions, while the third contains the monomeric Al(OH)4- anion. Comparisons were made to the INS and Raman spectra of sodium aluminate solutions frozen in a glassy state. In contrast to solution systems, the crystal lattice of the salts results in well-defined vibrations and associated resolved bands in the INS spectra. The use of a theory-guided analysis of the INS of this solid alkaline aluminate series revealed that differences were related to the nature of the hydrogen-bonding network and showed that INS is a sensitive probe of the degree of completeness and strength of the bond network in hydrogen-bonded materials. Results suggest that the ionic size may explain cation-specific differences in crystallization pathways in alkaline aluminate salts.

Mechanisms of Al3+ Dimerization in Alkaline Solutions.

The molecular speciation of aluminum (Al3+) in alkaline solutions is fundamental to its precipitation chemistry within a number of industrial applications that include ore refinement and industrial processing of Al wastes. Under these conditions, Al3+ is predominantly Al(OH)4-, while at high [Al3+] dimeric species are also known to form. To date, the mechanism of dimer formation remains unclear and is likely influenced by complex ion···ion interactions. In the present work, we investigate a suite of potential dimerization pathways and the role of ion pairing on energetics using static DFT calculations and DFT and density functional tight binding molecular dynamics. Specific cation effects imparted by the background electrolyte cations Na+, Li+, and K+ have been examined. Our simulations predict that, when the Al species are ion-paired with either cation, the formation of the oxo-bridged Al2O(OH)62- is favored with respect to the dihydroxo-bridged Al2(OH)82-, in agreement with previous spectroscopic work. The formation of both dimers first proceeds by bridging of two monomeric units via one hydroxo ligand, leading to a labile Al2(OH)82- isomer. The effect of contact ion pairing of Li+ and K+ on the dimerization energetics is distinctly more favorable than that of Na+, which may have an effect on further oligomerization.

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

Tetrahedrally coordinated aluminate Al(OH)4- and dialuminate Al2O(OH)62- anions are considered to be major species in aluminum-rich alkaline solutions. However, their relative abundance remains difficult to spectroscopically quantify due to local structure similarities and poorly understood effects arising from extent of polymerization and counter-cations. To help unravel these relationships here we report detailed characterization of three solid-phase analogues as structurally and compositionally well-defined reference materials. We successfully synthesized a cesium salt of the aluminate monomer, CsAl(OH)4·2H2O, for comparison to potassium and rubidium salts of the aluminate dimer, K2Al2O(OH)6, and Rb2Al2O(OH)6, respectively. Single crystal and powder X-ray diffraction methods clearly reveal the structure and purity of these materials for which a combination of 27Al MAS-NMR, Al K-edge X-ray absorption and Raman/IR spectroscopies was then used to fingerprint the two major tetrahedrally coordinated Al species. The resulting insights into the effect of Al-O-Al bridge formation between aluminate tetrahedra on spectroscopic features may also be generalized to the many materials that are based on this motif.

Ab Initio Molecular Dynamics Reveal Spectroscopic Siblings and Ion Pairing as New Challenges for Elucidating Prenucleation Aluminum Speciation.

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)4- predominates in alkaline conditions, yet equilibrium with dimeric species, either µ-oxo Al2O(OH)62- or di-µ-hydroxo Al2(OH)82-, 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.