Density functional theory and thermodynamics analysis of MAl Keggin substitution reactions: Insights into ion incorporation and experimental confirmation.

Polyaluminum cations, such as the MAl Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al, Ga, and Ge have been experimentally studied. The identity of the heteroatom M has been shown to influence the structural and electronic properties of the nanocluster and the kinetics of ligand exchange reactions. To date, only three ε-analogs have been identified, and there is a need for a predictive model to guide experiment to the discovery of new MAl species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical ε-MAl nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new ε-MAl species, such as FeAl . Based upon these results, we evaluated the Al/Zn/Cr system and determined that substitution of Cr is unfavorable in the heteroatom site but is preferred for Zn, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr-substituted δ-Keggin species where Cr substitution occurs only in the octahedral positions. The isolated structures Na[AlOAlCr(OH)(HO)](2,6-NDS)(HO) (δ-CrAl-1) and Na[AlOAlCr(OH)(HO)](2,7-NDS)(HO) (δ-CrAl-2) are the first pieces of evidence of mixed Al/Cr Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-CrAl-2 structure also exhibits a unique placement of the bound Na cation, which may indicate that Cr substitution can alter the surface reactivity of Keggin-type species.