Dissertation
First-principles insights on aluminum nanocluster reactivity and crystallization for environmental remediation
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2020
DOI: 10.17077/etd.005736
Abstract
In contaminated natural waters, aluminum (Al) hydrolysis reactions result in a wide variety of products, the identity of which depend heavily on reaction conditions. When Al rich soil is exposed to acid mine drainage, naturally-occurring polyaluminum (oxy)hydroxide nanoclusters form, most notably the Al13 Keggin and its derivatives. Keggin-type nanoclusters have applications in water purification due to their sorbent properties for contaminant ions such as lead, arsenic, copper, etc. These nanoclusters are attractive for computational modeling due to their tractable size (100-200 atoms) and are used as geochemical models to study interfacial reactions due to the shared structural features with well-studied minerals such as hematite and corundum. The chemical environment and synthesis conditions govern the charge, shape, and exposed functional groups of Al nanoclusters, all of which dictate cluster properties and applications. In order to obtain nanoclusters with desirable properties, a molecular-level understanding of the interplay between Al nanoclusters and the conditions required to isolate them is needed. Density functional theory (DFT) calculations provide this insight through optimized geometries and outer-sphere anion interactions; a combination of DFT and thermodynamics modeling can provide insight into ion incorporation and substitution within Al nanoclusters. The work in this dissertation considers how synthesis conditions (temperature, pH, and other species in solution) give rise to unique structural motifs that support different functional groups in three nanoclusters: [AlO4Al12(OH)24(H2O)12]7+ (ε-Al13 Keggin), [Al8(OH)14(H2O)18]10+ (Al8), [Al13(OH)24(H2O)24]15+ (Flat-Al13). Previous work has shown a relationship between nanocluster shape and surface reactivity for Keggin-type clusters, and here we explore three nanoclusters of different topologies with a variety of functional groups. Visualization of the electrostatic potentials of the three nanoclusters reveal regions of increased positive charge in Flat-Al13 and Al8 that correspond to a collection of reactive functional groups. Alternatively, the ε-Al13 Keggin exhibits less reactive functional groups that are evenly dispersed around the exterior of the cluster. It is clear that functional group identity and distribution are important factors controlling nanocluster reactivity, and both of these factors are governed by interactions with ions in solution.
The Al13 Keggin can exist in five different geometric isomers, which differ in their connectivity but exhibit the same types and distributions of exposed functional groups. To date, only the ε-, δ-, and γ-Al13 isomers have been obtained. The ε-isomer has been shown to undergo substitution at the central, tetrahedral position known as the heteroatom M. Three ε-MAl12 (M = Al3+, Ga3+, and Ge4+) analogs have been synthesized and characterized experimentally. The DFT calculations in this dissertation detail the comparison of the analogs in terms of their outer-sphere anion adsorption reactivity, observing reactivity trends in line with experimental ligand exchange and acid-base reactions. We take note of events in which anion interactions cause nanocluster deprotonation, making connections to synthesis procedures and crystallization agents.
Insights gained by the outer-sphere interactions have helped to experimentally isolate new heteroatom-substituted nanoclusters. The first was the δ-GaAl12 isomer, which displayed a similar crystal structure to δ-Al13. DFT calculations show similar reactivity of these two analogs, revealing site-specific deprotonation events that correspond to polymerization locations to form [Al30(O)8(OH)56(H2O)24]18+ (Al30) and [Ga2O8Al28.5Ga0.5(OH)58(H2O)27]19+ (Ga2.5Al28.5). Ga2.5Al28.5 is the second new nanocluster presented in this work. DFT calculations also revealed the unique site-specific deprotonation of ε-GeAl12, linking to the formation of the third new nanocluster [Ge4O16Al48(OH)108(H2O)24]20+ (Ge4Al48). It is interesting that heteroatom identity and isomerization influence polymerization capabilities, which is only revealed through outer-sphere anion interactions and is not apparent in comparing the isolated MAl12 nanoclusters.
We go on to consider the energetics of cation substitution in Keggin-type nanoclusters modeled using a combined DFT and thermodynamics approach. We first compare the substitution observed in the δ-GaAl12 and Ga2.5Al.28.5 nanoclusters to understand why Ga cations are found in certain positions of the nanocluster. We find computed energetics in line with experimental occupancies, with substantial increased stability (greater than 0.5 eV) in the experimental configurations. Having confirmed that this approach can mirror experimental findings on cation substitution, we apply it with the goal of predicting the next ε-MAl12 analog. Coupling cation substitution energetics with derived structural criteria for new analogs, we predict ε-FeAl12 to be the next heteroatom-substituted nanocluster.
Collectively, the work presented in this dissertation presents new understanding of Al Keggin nanoclusters, with regard to their formation, stability, and transformation. Keggin-type nanoclusters are present in many facets, and the goal of this work is to provide insight on how to isolate new nanoclusters. Thus far, insight from DFT calculations have helped to isolate three new nanoclusters and hopefully can continue to guide experiment.
Details
- Title: Subtitle
- First-principles insights on aluminum nanocluster reactivity and crystallization for environmental remediation
- Creators
- Jennifer L. Bjorklund
- Contributors
- Sara E Mason (Advisor)Tori Z Forbes (Advisor)Christopher M Cheatum (Committee Member)Renee S Cole (Committee Member)Leonard R MacGillivray (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Autumn 2020
- Publisher
- University of Iowa
- DOI
- 10.17077/etd.005736
- Number of pages
- xix, 151 pages
- Copyright
- Copyright 2020 Jennifer L. Bjorklund
- Language
- English
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (page 139-151).
- Public Abstract (ETD)
Polyaluminum (oxy)hydroxide nanoclusters are ubiquitous in various aspects of our daily lives, from electronic devices and clean water to antiperspirants and medicine. Both naturally-occurring and synthetically-engineered Al nanoclusters can exist in aqueous environments, and their practical applications are dictated by their shape, charge, functional groups, and composition. These features can be controlled by altering the chemical environment, namely the ion concentration, solution pH, temperature, and the interaction with other species in solution. The Al13 Keggin is one of the most well-known Al nanoclusters, with the Keggin structural motif identified within crystalline mineral structures; the Keggin has been identified as a key species in water purification coagulants. Depending on the reaction conditions, different isomers and Keggin-type derivatives can be stabilized and isolated, each with their own unique reaction capabilities. We compare the known synthesis routes for Al nanoclusters and relate surface functional groups to anion interactions, identifying a relationship between functional group distribution and nanocluster reactivity.
The Keggin has been shown to undergo substitution, where the central Al3+ cation can be replaced with Ga3+ or Ge4+. As a consequence of cation replacement, the overall structure and reactivity of the nanocluster is altered. We compare the surface reactivity toward deprotonation of three Keggin analogs and their abilities to form different isomers and polymers, concluding that central cation identity drastically impacts transformation capabilities. We make connections to experimental synthesis and characterization, providing insight on cation substitution site selectivity and the reactivity consequences of substitution. We go on to make predictions about other possible cations that might replace Al3+ by comparing energetics and features of the known Keggin nanoclusters.
- Academic Unit
- Chemistry
- Record Identifier
- 9984036085602771
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