Mechanistic investigations of Al(OH)3 oligomerization mechanisms

Experimental and Molecular Dynamics Simulation Study on Sol-Gel Conversion Process of Aluminum Carboxylate System

Due to the lack of relevant in situ characterization techniques, the investigation of aluminum sol–gel progress is lacking. In this study, combined with molecular dynamics simulation and conventional experimental methods, the microstructures, rheological properties, and gelation process of the carboxylic aluminum sol system were studied. The experimental results showed that, with the increase in solid content, the microstructure of the colloid developed from a loose and porous framework to a homogeneous and compact structure. The viscosity of aluminum sol decreased significantly with the increase in temperature, and a temperature above 318 k was more conducive to improving the fluidity. The simulation results show that the increase in free volume and the connectivity of pores in colloidal framework structure were the key factors to improve fluidity. In addition, free water molecules had a higher migration rate, which could assist the rotation and rearrangement of macromolecular chains and also played an essential role in improving fluidity. The Molecular dynamics simulation (MD) results were consistent with experimental results and broaden the scope of experimental research, providing necessary theoretical guidance for enhancing the spinning properties of aluminum sol.

From Chain- to Graphene-like Hydroxyl-terminated (ZnO)n Clusters with n≤6 Obtained via Zinc Dimethoxide Hydrolysis and Condensation: Ab initio Structural, Electronic, Vibrational and Optical Properties Calculations

Recent reports are focusing on the structural evolution from the atomic-scale and also at the expenses of alkyl zinc alkoxide precursors towards (ZnO)_n clusters and nanostructures with different interesting motifs, but still not much is known about their electronic properties. In this manuscript, we present a theoretical study using DFT and TD-DFT methodologies on the hydrolysis and condensation of zinc dimethoxide precursor in its monomeric, dimeric and trimeric forms towards thermodynamically stable hydroxyl-terminated (ZnO)_n clusters with novel chain- and graphene-like fashions. For all cases, distinct vibrational and optical spectra features were assigned evidencing a global monotonic decrease in the opto-electronic gap with increasing oligomerization and cyclization stages. In addition, the electron-affinity of all clusters was also observed to be enhanced with increasing oligomerization and cyclization stages and the electronic charge localization in -e charged clusters was observed to be strongly related to the presence of zinc-oxo subunits and other particular structural features. Our calculations also indicate that the stabilization through hydroxyl termination of both chain- and graphene-like ZnO clusters not only could be a promising driving force to obtain larger atomic-scale 1D and 2D nanostructures but also envisage interesting properties, particularly as electronic acceptor materials for energy applications.

Mechanisms of Al3+ Dimerization in Alkaline Solutions

The molecular speciation of aluminum (Al³⁺) in alkaline solutions is fundamental to its precipitation chemistry within a number of industrial applications that include ore refinement and industrial processing of Al wastes. Under these conditions, Al³⁺ is predominantly Al(OH)₄^-, while at high [Al³⁺] dimeric species are also known to form. To date, the mechanism of dimer formation remains unclear and is likely influenced by complex ion···ion interactions. In the present work, we investigate a suite of potential dimerization pathways and the role of ion pairing on energetics using static DFT calculations and DFT and density functional tight binding molecular dynamics. Specific cation effects imparted by the background electrolyte cations Na⁺, Li⁺, and K⁺ have been examined. Our simulations predict that, when the Al species are ion-paired with either cation, the formation of the oxo-bridged Al₂O(OH)₆^2- is favored with respect to the dihydroxo-bridged Al₂(OH)₈^2-, in agreement with previous spectroscopic work. The formation of both dimers first proceeds by bridging of two monomeric units via one hydroxo ligand, leading to a labile Al₂(OH)₈^2- isomer. The effect of contact ion pairing of Li⁺ and K⁺ on the dimerization energetics is distinctly more favorable than that of Na⁺, which may have an effect on further oligomerization.

DFT Studies on the Al-Speciation and Its Structure in Aqueous Aluminum Sol Formed by Aluminum Formoacetate

The investigation of the Al-speciation and its structure in aluminum sol at the molecular level plays an important role in knowing the subsequent lager polymer gels. In this work, to study the Al-speciation and its structure in aluminum sol prepared from aluminum formoacetate at the molecule level, density functional theory studies on the chemical behaviors of aluminum formoacetate under neutral and acidic conditions were systematically conducted at the B3LYP/6-311G** level. Due to the different thermodynamic dominance of hydration, hydrolysis, and polymerization, the structure of Al-speciation formed in neutral aqueous solution obviously differed from that formed in acidic solution. The products obtained from aluminum formoacetate in neutral aqueous solution contained more carboxyl groups acted as bridging bidentate ligand but less coordinated water molecules. The addition of H⁺ ion had a great impact on the structure of Al-speciation in aluminum sol prepared by aluminum formoacetate. This work would provide guidance for the understanding of the precursor gels of materials.