Contaminant adsorption on nanoscale particles: structural and theoretical characterization of Cu2+ bonding on the surface of Keggin-type polyaluminum (Al30) molecular species

Designable assembly of atomically precise Al4O4 cubane supported mesoporous heterometallic architectures

Heterometallic cluster-based framework materials are of interest in terms of both their porous structures and multi-metallic reactivity. However, such materials have not yet been extensively investigated because of difficulties in their synthesis and structural characterization. Herein, we reported the designable synthesis of atomically precise heterometallic cluster-based framework compounds and their application as catalysts in aldol reactions. By using the synergistic coordination protocol, we successfully isolated a broad range of compounds with the general formula, [Al₄M₄O₄(L)₁₂(DABCO)₂] (L = carboxylates; DABCO = 1,4-diazabicyclo[2.2.2]-octane; M²⁺ = Co²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Cd²⁺). The basic heterometallic building blocks contain unprecedented main-group γ-alumina moieties and surrounding unsaturated transition metal centers. Interestingly, the porosity and interpenetration of these frameworks can be rationally regulated through the unprecedented strategy of increment of the metal radius in addition to general introduction of sterically bulky groups on the ligand. Furthermore, these porous materials are effective catalysts for aldol reactions. This work provides a catalytic molecular model platform with accurate molecular bonding between the supporters and catalytically active metal ions.

Density functional theory and thermodynamics analysis of MAl12 Keggin substitution reactions: Insights into ion incorporation and experimental confirmation

Polyaluminum cations, such as the MAl₁₂ Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al³⁺, Ga³⁺, and Ge⁴⁺ 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 MAl₁₂ species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical ε-MAl₁₂ nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al³⁺-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new ε-MAl₁₂ species, such as FeAl₁₂ ⁷⁺. Based upon these results, we evaluated the Al³⁺/Zn²⁺/Cr³⁺ system and determined that substitution of Cr³⁺ is unfavorable in the heteroatom site but is preferred for Zn²⁺, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr³⁺-substituted δ-Keggin species where Cr³⁺ substitution occurs only in the octahedral positions. The isolated structures Na[AlO₄Al_9.6Cr_2.4(OH)₂₄(H₂O)₁₂](2,6-NDS)₄(H₂O)₂₂ (δ-Cr_nAl_13-n-1) and Na[AlO₄Al_9.5Cr_2.5(OH)₂₄(H₂O)₁₂](2,7-NDS)₄(H₂O)_18.5 (δ-Cr_nAl_13-n-2) are the first pieces of evidence of mixed Al³⁺/Cr³⁺ Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-Cr_nAl_13-n-2 structure also exhibits a unique placement of the bound Na⁺ cation, which may indicate that Cr³⁺ substitution can alter the surface reactivity of Keggin-type species.

Ga3+ Incorporation into Al13 Keggin Polyoxometalates and the Formation of δ-(GaAl12)7+ and (Ga2.5Al28.5)19+ Polycations

Keggin-type polyaluminum species (ε-Al₁₃, δ-Al₁₃, Al₂₆, Al₃₀, Al₃₂) can form upon partial hydrolysis of Al³⁺-bearing solutions and are important species for water purification and contaminant transport. While the structural features for the major Al³⁺ polyaluminum species have been delineated, much less is known regarding heteroatom substitution and resultant structures other than the previously identified ε-GaAl₁₂⁷⁺ and ε-GeAl₁₂⁸⁺ cations. Single-atom substitution within polyaluminum species can change the surface reactivity within water treatment scenarios; thus, it is important to understand heteroatom incorporation within this system. The present work describes the synthesis and characterization of two novel Ga³⁺-substituted Keggin-type polyaluminum species. Na[GaO₄Al₁₂(OH)₂₄(H₂O)₁₂](2,6-NDS)₄(H₂O)_20.5 (δ-GaAl₁₂) and [Ga₂O₈Al_28.5Ga_0.5(OH)₅₈(H₂O)₂₇(SO₄)₂](SO₄)₄Cl₇(H₂O)_8.5 (Ga_2.5Al_28.5) were crystallized from a thermally aged, partially hydrolyzed Ga³⁺/Al³⁺ solution. Structural refinement from single-crystal X-ray diffraction indicated fully occupied Ga³⁺ within tetrahedral site(s) of both isolated species. Partial substitution was observed for octahedral sites for the larger Ga_2.5Al_28.5 cluster. The chemical compositions of both clusters were confirmed by inductively coupled plasma mass spectrometry (ICP-MS). Density functional theory (DFT) calculations corroborated the structural refinement, with the energetics of Ga³⁺ substitution suggesting preferential substitution within tetrahedral sites for both species. Additional theoretical work suggests that the rotated trimer in δ-GaAl₁₂ is highly reactive, which can serve as the driving force in the formation of the Ga_2.5Al_28.5 cluster.

Humic Acid Removal from Water with PAC-Al30: Effect of Calcium and Kaolin and the Action Mechanisms

Polyaluminum chloride with a dominant species of Al30 (PAC-Al30) was prepared in laboratory and used for humic acid (HA) removal from water. The action properties and mechanisms of PAC-Al30, HA, calcium, and kaolin were tested and discussed. The results showed that the existence of calcium or kaolin contributed to the HA removal when the PAC-Al30 dosage was deficient and had no obvious effect when the amount of PAC-Al30 was sufficient. When the PAC-Al30 dosage was 0.01 and 0.02 mmol/L, the HA removal rate was increased by 66.59 and 42.20%, respectively, with a calcium concentration of 2.0 mmol/L, or increased by 53.31 and 40.92%, respectively, with the kaolin particle concentration of 150 mg/L. Calcium could compress the double electrical layers or complex with HA to neutralize a part of the surface negative charge of HA, but could not make the water system reach its isoelectric point. The mechanisms of calcium and kaolin's promoting coagulation effect were adsorption neutralization and collision aggregation respectively, but these actions were much weaker than that of PAC-Al30 with HA. The adsorption neutralization capacity of PAC-Al30 was calculated to be nearly 60 times than that of calcium, and the higher γ value of calcium modified by the Sips equation may indicate that the adsorption or neutralization sites of calcium on HA were pickier than PAC-Al30.

Using density functional theory to study shape-reactivity relationships in Keggin Al-nanoclusters

Keggin-based aluminum nanoclusters have been shown to be efficient sorbents for the removal of arsenic from water. Obtaining a molecular-level understanding of the adsorption processes associated with these molecules is of fundamental importance, and could pave the way for rational design strategies for water treatment. Due to their size and the availability of experimental crystal structures, Al nanoclusters are computationally tractable at the density functional theory (DFT) level. Here, we compare the reactivity of three aluminum polycations: [Al13O4(OH)24(H2O)12](7+) (Al13), [Al30O8(OH)56(H2O)26](18+) (Al30), and [Al32O8(OH)60(H2O)30](20+) (Al32). We use DFT calculations to determine reactivity as a function of particle topography, using sulfate and chloride as adsorption probes. Our comparative modeling of outer-sphere adsorption of Cl(-) and SO4(2-) on Al13, Al30, and A132 supports that the unique "hourglass" shape characteristic to Al30 gives rise to relatively strong adsorption in the molecular beltway, as well as a wide range of reaction energies as a function of particle topography.

Sustainable Nanotechnology: Opportunities and Challenges for Theoretical/Computational Studies

For assistance in the design of the next generation of nanomaterials that are functional and have minimal health and safety concerns, it is imperative to establish causality, rather than correlations, in how properties of nanomaterials determine biological and environmental outcomes. Due to the vast design space available and the complexity of nano/bio interfaces, theoretical and computational studies are expected to play a major role in this context. In this minireview, we highlight opportunities and pressing challenges for theoretical and computational chemistry approaches to explore the relevant physicochemical processes that span broad length and time scales. We focus discussions on a bottom-up framework that relies on the determination of correct intermolecular forces, accurate molecular dynamics, and coarse-graining procedures to systematically bridge the scales, although top-down approaches are also effective at providing insights for many problems such as the effects of nanoparticles on biological membranes.

Arsenate uptake by Al nanoclusters and other Al-based sorbents during water treatment

In many parts of the world, arsenic from geogenic and anthropogenic sources deteriorates the quality of drinking water resources. Effective methods of arsenic removal include adsorption and coagulation with iron- and aluminum-based materials, of which polyaluminum chloride is widely employed as coagulant in water treatment due to its low cost and high efficiency. We compared the arsenic uptake capacity and the arsenic bonding sites of different Al-based sorbents, including Al nanoclusters, polyaluminum chloride, polyaluminum granulate, and gibbsite. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that As(V) forms bidentate-binuclear complexes in interaction with all Al-based removal agents. The octahedral configuration of nanoclusters and the distribution of sorption sites remain the same in all types of removal agents consisting of nano-scale Al oxyhydroxide particles. The obtained distances for As(V)-O and As(V)-Al agreed with previously published data and were found to be 1.69 ± 0.02 Å and 3.17-3.21 Å, respectively. Our study suggests that As(V) binds to Al nanoclusters as strongly as to Al oxide surfaces. The As sorption capacity of Al nanoclusters was found to be very similar to that of Al clusters in a polyaluminum chloride. The most efficient Al-based sorbents for arsenic removal were Al nanoclusters, followed by polyaluminum granulate.

Isolation and characterization of the [Ga2Al18O8(OH)36(H2O)12](8+) cluster: cationic variations on the Wells-Dawson topology

The structural chemistry of Group 13 polyoxometalates lags far behind related negatively charged transition metal species and limits the development of advanced materials. A novel heterometallic cluster [Ga2Al18O8(OH)36(H2O)12](8+) (Ga2Al18) has been isolated using a supramolecular approach and structurally characterized using single-crystal X-ray diffraction. Ga2Al18 represents the Wells-Dawson structure polycations and variations in the structural topology may be related to the initial stabilization of the Keggin isomer. DFT calculations on the related ε-Keggins (GaAl12 and Al13), Ga2Al18, and theoretical Al2Al18 clusters reveal similar features of electronic structure, suggesting additional heteroatom substitution in other isostructural clusters should be possible.

Large-scale synthesis and formation mechanism study of basic aluminium sulfate microcubic crystals

Al³⁺ derived from the (100) plane of γ-AlOOH at a relatively high H⁺ concentration can form well-defined basic aluminium sulfate microcrystals through a hydrothermal process.