Polyaluminum (oxy)hydroxide species have been isolated and characterized in a wide variety of topologies, ranging from the flat-Al13 to the round Keggin Al13 and its larger, derivative polycation species such as Al30 and Al32. Each nanocluster contains similar surface oxygen functional groups but differ in their number and distribution, which ultimately governs nanocluster properties such as reactivity and stability. Such nanoclusters are attractive geochemical models due to their tractable size (1 nm) and shared functional groups with many mineral surfaces, allowing for an atomistic understanding of intermolecular interactions for processes like water remediation. The ε-MAl12 Keggin nanoclusters are the most well-known and studied, where M is the tetrahedrally-coordinated heteroatom. Three experimental heteroatom analogs (M= Al(III), Ga(III), and Ge(IV)) have been identified and studied using Density Functional Theory (DFT). Their reactivity has been compared in terms of outer-sphere anion adsorption with the increasing reactivity trend of GaAl12 &lt; Al13 &lt; GeAl12, in agreement with experimental ligand exchange rate trends. A correlation was made between crystallizing agent pKa and nanocluster deprotonation tendency, providing new insight into possible synthesis and crystallization routes for new nanoclusters. Recent experimental work has yielded new nanoclusters such as Ga2.5Al28.5 and δ-GaAl12, the first heteroatom-substituted Keggin isomer, in addition to the first Ge-substituted giant nanocluster Ge4Al49. DFT calculations explained reactivity differences and helped to shed light on why certain heteroatom-substituted Keggin nanoclusters polymerize and why others do not. A DFT + Thermodynamics model was used to predict energetics of substituting Al atoms in Keggin-type structures with different elements, combining DFT calculations with experimentally-tabulated parameters such as applied potential, ion concentration, pH, and temperature. This method has been applied to study ε-MAl12 Keggin heteroatom substitution, coupling energetics with structural trends to determine possible candidates for new ε-MAl12 species, such as FeAl12.

DFT insights on Keggin-type nanocluster stabilization and reactivity

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.