Impact of Amino Acids on the Isomerization of the Aluminum Tridecamer Al₁₃

Abstract

The stability of the Keggin polycation ε-Al is monitored by Al NMR and ferron colorimetric assay upon heating aluminum aqueous solutions containing different amino acids with overall positive, negative, or no charge at pH 4.2. A focus on the effect of the amino acids on the isomerization process from ε- to δ-Al is made, compared and discussed as a function of the type of organic additive. Amino acids such as glycine and β-alanine, with only one functional group interacting relatively strongly with aluminum polycations, accelerate isomerization in a concentration-dependent manner. The effect of this class of amino acids is also found increasing with the pK of their carboxylic acid moiety, from a low impact from proline up to more than a 15-fold increased rate from the stronger binders such as glycine or β-alanine. Amino acids with relatively low C-terminal pK, but bearing additional potential binding moieties such as free alcohol (hydroxyl group) moiety of serine or the amide of glutamine, speed the isomerization comparatively and even more than glycine or β-alanine, glutamine leading to the fastest rates observed so far. With aspartic and glutamic acids, changes in aluminum speciation are faster and significant even at room temperature but rather related to the reorganization toward slow reacting complexed oligomers than to the Al isomerization process. The linear relation between the apparent rate constant of isomerization and the additive concentration points to a first-order process with respect to the additives. Most likely, the dominant process is an accelerated ε-Al dissociation, increasing the probability of δ isomer formation.

Intermediate Species in the Crystallization of Sodium Aluminate Hydroxy Hydrates

“…A key difference between Na 2 Al 2 O­(OH) 6 and K 2 Al 2 O­(OH) 6 arises in the isotropic chemical shift, which can be attributed to a combination of Na + /K + specific ion effects, slight perturbation of the Al–O–Al angle, and/or formation of a hydrated phase. The isotropic chemical shift of the Na 2 Al 2 O­(OH) 6 resonance is significantly deshielded compared to oxolated tetrahedral Al 3+ proximal to octahedral Al 3+ , such as in keggin clusters or transitional aluminas. − …”

Thermal Aging of Heteroatom-Substituted Keggin-Type Aluminum Oxo Polycation Solutions: Aggregation Behavior and Impacts on Humic Acid and Turbidity Removal

“…The peak in unaged Al 13 and GaAl 12 solution at approximately 1.5 nm aligns well with the expected hydrodynamic diameter for the known polyaluminum species Al 13 , Al 30 , GaAl 12 or Ga 2.5 Al 28.5 species in solution. 21,[33][34][35] Features at 8 nm and 15 nm for unaged GaAl 12 and GeAl 12 solution may correspond to the aggregation of smaller clusters. For the GeAl 12 solutions, the peak at 1.3 nm can be correlated to a GeAl 12 species and the feature at 2.8 nm have been reported due to the formation of the Ge 4 Al 48 species from condensation of GeAl 12.…”

Thermal aging of heteroatom substituted Keggin type aluminum oxo polycation solutions: Aggregation behavior and impacts on dissolved organic carbon and turbidity removal

“…The peak in unaged Al 13 and GaAl 12 solution at approximately 1.5 nm aligns well with the expected hydrodynamic diameter for the known polyaluminum species Al 13 , Al 30 , GaAl 12 or Ga 2.5 Al 28.5 species in solution. 21,[33][34][35] Features at 8 nm and 15 nm for unaged GaAl 12 and GeAl 12 solution may correspond to the aggregation of smaller clusters. For the GeAl 12 solutions, the peak at 1.3 nm can be correlated to a GeAl 12 species and the feature at 2.8 nm have been reported due to the formation of the Ge 4 Al 48 species from condensation of GeAl 12.…”