A density functional theory study on the structure formation of Al(III) carboxylate complexes in aqueous aluminum sols

Abstract

In-depth studies of the structure of precursor sols is very important for understanding the subsequent sol to gel transition. Here, density functional theory was applied to study the reaction of Al powder with formic and acetic acid in aqueous solution to find the structure of Al(III) complex in precursor sols. The Al 3+ ion obtained by dissolving Al powder in aqueous solution of formic and acetic acid would preferentially coordinate with water molecules to form aluminum hydrates. As the reaction energy barrier of aluminum hydrates with CH 3 COO − and HCOO − was comparable, both aluminum formoacetate and other aluminum carboxylate monomers could be obtained. Most Al(III) complexes formed by other carboxylate monomers were similar to that prepared from aluminum formoacetate. However, some Al(III) monomers and Al(III) dimer with different structural characteristics might be detrimental to the formation of linear polymers. The formation of other aluminum carboxylate monomers had great influence on the structure of precursor sols.

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

“…The small peak at 2426 cm −1 was attributed to free carboxylic acid vibrations. In addition, the three sharp peaks at 1596 cm −1 , 1480 cm −1 , and 1384 cm −1 were attributed to symmetric stretching vibrations of bridging bidentate COO −1 [ 22 ]. The band at 1048 cm −1 corresponded to Al-O-C stretching vibrations.…”

“…Based on the previous studies, three kinds of tetramers with different coordination groups were formed in the Al-HCOOH-CH 3 COOH-H 2 O system [ 22 ]. The molecular structures of these three tetramers—A, B, and C—are shown in Figure 2 .…”

“…They found that a mixture of hydrolysis products was formed due to the decomposing of aluminum alkoxides via ligand disproportionation. Li et al systematically studied the chemical behavior of aluminum powder with formic acid and acetic acid in neutral and acidic aqueous solutions using Density functional theory (DFT) calculation [ 21 , 22 , 23 , 24 ]. The formation mechanisms for the hydrolysate, dimer, and tetramer by polymerization were discussed.…”

“…Our previous study found that the main chain length of the aluminum sol polymerization structure was limited by coordination groups [ 21 , 22 , 23 , 24 ]. However, hydroxyl, formic, and acetic acid have strong polarity, producing a large intermolecular force.…”

Effect of the addition of acetic acid on spinning sol and mullite fibers properties

“…There are many methods for the preparation of continuous mullite fibers, and it has become the preferred method in the industrial production of mullite fibers because of the advantages of high purity, good homogeneity, and low reaction temperature using the sol-gel method [4][5] . There are two main types of raw materials: metal alcohol salts, which are difficult to apply in industrial production because of high production costs and difficult process control [6][7] ; and inorganic aluminum salts to get aluminum chloride, aluminum nitrate or aluminum carboxylate [8][9][10] . Aluminum carboxylate solutions (ACs) are usually produced by reacting aluminum powder with carboxylic acids [11][12] , and these raw materials are not only easy to obtain, but also have no other anionic impurities, do not produce polluting gases during the heat treatment, are more economical and environmentally friendly, and are suitable for industrial production.…”