Estimation of the Solubility Dependence of Aluminate Salts of Alkali Metals on Ion Radii of Alkali Metals by LDF Molecular Orbital Calculations

Abstract

The addition of a lithium salt forms an insoluble lithium aluminate salt film on an aluminum surface, effectively preventing aluminum corrosion. Other aluminate salts of alkali metals lack this behavior, being easily dissolved into water. In this study, the reason for the difference in behavior is investigated by applying molecular orbital calculation to clusters of ions and molecules. The difference in solubility among aluminate salts of alkali metals is attributed to the difference in binding energy between the anion cluster and the alkali metal ion cluster with the first layer of water molecules surrounding the alkali metal ion. Calculations show that this difference of binding energy arises from the difference in the sum of the cohesive energy, the variation of the coordinate bond energy between an alkali metal ion and water molecules surrounding it, and the interaction energy between those water molecules and an anion, in a salt molecule cluster. These three factors, in turn, derive from the difference in ion radius of the alkali metals. The binding energy in lithium aluminate is about 25 kJ/mol larger than that in sodium aluminate and about 45 kJ/mol larger than in potassium aluminate. The binding energy of antimonate salts of alkali metals is also calculated as a second example in which the value of the ion radii causes the insolubility, to verify the explanation. Then, the binding energy in the sodium antimonate is about 5 kJ/mol larger than in the lithium antimonate, and about 15 kJ/mol larger than that in the potassium antimonate. The ratio of the lithium ion radius to the aluminum ion radius is 1.4, nearly as much as that of the sodium ion radius to the antimony ion radius. This value of ion radii ratio made the binding energy largest of the aluminate or antimonate salts.

Al2O3-supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil

“…This should be attributed to the fact that NaAlO 2 and KAlO 2 can dissolve in water to give a very strong basic solution, while other alkali aluminates are less or non-soluble oxides [24]. Indeed, using commercial NaAlO 2 as the catalyst revealed the ME content as high as 92% under the same reaction conditions.…”

Theoretical Investigation of the Nature of Aluminum-Containing Species Present in Alkaline Solution

“…Hence, it is restricted to species which are either neutral or negatively charged, as it is highly unlikely that any cationic forms of aluminum will be present. There are already a large number of studies of the Al(OH) 4 - aluminate ion and its protonated form, especially in the context of modeling Brønsted acid sites in zeolites. − However, far less work has been performed on the other possible aluminum-containing anions that may exist under basic conditions. Hence, ab initio quantum mechanical results for a range of monomeric and dimeric species with aluminum coordination numbers varying from two to six are presented here.…”

Unraveling Gibbsite Transformation Pathways into LiAl-LDH in Concentrated Lithium Hydroxide

“…The deshielding of the T d resonance in the LiOH system during the course of the reaction may relate to (i) the decrease in solution concentrations of Li + and OH – following formation of LiAl-LDH and/or (ii) an evolving ensemble of Li + ···Al­(OH) 4 – ion–ion distances as aluminate concentrations change over time. The Li + ···Al­(OH) 4 – ion distance may be particularly important to the formation of LiAl-LDH because ion-pairing distorts the Al­(OH) 4 – tetrahedron . Ion pairing between Li + and OH – results in an increased delocalization of protons in water molecules solvating the ion pair .…”

“…We instead attribute the trend in chemical shift to strong shielding of the Al 3+ nucleus that is dependent on LiOH concentration, 28 − ion distance may be particularly important to the formation of LiAl-LDH because ion-pairing distorts the Al(OH) 4 − tetrahedron. 42 Ion pairing between Li + and OH − results in an increased delocalization of protons in water molecules solvating the ion pair. 43 The delocalization is attenuated in deuterated solutions.…”