Influence of H3O+ on the structure formation of oligomers in aluminium sols prepared from basic aluminium acetate: Experiments and computations

Abstract

In-depth insight into the structure of oligomers in sols plays an important role in determining the structure and properties of the final materials, such as aluminium borosilicate fibers, flakes, microspheres and composites. In this study, the influence of H3O+ on the structure formation of oligomers in sols prepared from basic aluminium acetate (BAA) was studied by Fourier-transform infrared spectroscopy (FT-IR), 27Al NMR spectroscopy and density functional theory (DFT) calculation. The results indicated that BAA consisting of tetrahedral Al species existed in dimeric form (BAA2-2). The polymerization of BAA2-2 was more thermodynamically favourable than its hydrolysis both in neutral and acidic solutions. The oligomers formed in neutral aqueous solution were beneficial to prepare different polymers with dendritic or three-dimensional structures. The oligomers formed in acidic solution (H+/BAA = 0.5) were mostly linear tetramers composed of three octahedral and one tetrahedral Al species, which were beneficial to the spinnability of sols during the preparation of ceramic fibers. The addition of H+ changed the electronic properties of Al(III) and significantly promoted the coordination ability of water and Al3+, leading to an increase in the octahedral Al species and a great change in the structure of the sol. The DFT results were consistent with the experimental results, which provided a necessary theoretical guidance to improve the properties of aluminium sols and meet the needs of the final materials.