Density functional theory and thermodynamics analysis of MAl 12 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[AlO Al Cr (OH) (H O) ](2,6-NDS) (H O) (δ-Cr Al -1) and Na[AlO Al Cr (OH) (H O) ](2,7-NDS) (H O) (δ-Cr Al -2) are the first pieces of evidence of mixed Al /Cr Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-Cr Al -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.