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

Abstract

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.

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

“…Compared to the para‐coordinating water molecule, the formation of 6H 2 O‐HCOO‐IM2 activates the ortho‐coordinating water molecule of its formate group, resulting in this ortho‐coordinating water molecule being more easily substituted by the CH 3 COO − ion to obtain the intermediate product HCOO‐CH 3 COO‐IM3‐2. However, HCOO‐CH 3 COO‐IM3‐2 may remove some coordinating water molecules to obtain the product HCOO‐CH 3 COO‐IM4 (the protonated aluminum formoacetate, Al‐1‐H), [ 18 ] with the free energy increased by 124.1 kJ∙mol −1 . In addition, HCOO‐CH 3 COO‐IM3‐2 may also be hydrolyzed due to the presence of coordinating water molecules, and the energy barrier is 97.1 kJ∙mol −1 .…”

“…and 6H 2 O-CH 3 COO-IM5, and the hydrolysis energy barrier is 93.9 and 92.9 kJÁmol −1 , respectively. From Figure 3, the reaction energy barrier of [18] with the free energy increased by 124.1 kJÁmol −1 . In addition, HCOO-CH 3 COO-IM3-2 may also be hydrolyzed due to the presence of coordinating water molecules, and the energy barrier is 97.1 kJÁmol −1 .…”

“…Moreover, aluminum formoacetate could be spontaneously polymerized to obtain the corresponding dimer, with the free energy decreased by 139.0 kJÁmol −1 . [18] Therefore, these reaction processes are energetically favorable. The monomer products OH-2HCOO-IM4 and OH-HCOO-CH 3 COO-IM4 (aluminum formoacetate) can be obtained in these processes.…”

“…Although HCOO-CH 3 COO-IM2-2 is the most stable species in this process, and its free energy is lower than HCOO-CH 3 COO-IM6 by 63.7 kJÁmol −1 , the product HCOO-CH 3 COO-IM6 is the same as the acidic hydrolysis product of aluminum formoacetate, and it could be spontaneously bridged by two hydroxyl groups to obtain a dimer, with the free energy decreased by 90.0 kJÁmol −1 . [18] The occurrence of this dimerization can promote the hydrolysis of HCOO-CH 3 COO-IM3-2 to obtain the product HCOO-CH 3 COO-IM6.…”

“…[ 10–13 ] In comparison, basic aluminum acetate and aluminum formoacetate, as water‐soluble aluminum carboxylates, are considered to be better aluminum salts because they are economical and environmentally friendly and can produce precursor sols with suitable spinning viscosity without adding spinning aids, which have been used to prepare alumina fiber as early as the last century. [ 14–16 ] In our previous works, [ 17,18 ] the spinnability of the precursor sol prepared from basic aluminum acetate was determined by H + . Only when appropriate H + ions were added, oligomers that were favorable for the formation of linear polymers could be obtained.…”