Reaction Mechanisms of the Initial Oligomerization of Aluminophosphate

Abstract

The mechanisms of aluminophosphate oligomerization were investigated using density functional theory with the SMD solvation model. Two aluminum species, Al(OH)4(-) and Al(H2O)6(3+), and four phosphorus species, H3PO4, H2PO4(-), HPO4(2-), and PO4(3-), were considered as the monomers for polycondensation reactions. It was found that the most favorable pathway to dimerization was a Lewis acid-base reaction: the aprotic oxygen of phosphoric acid (P═O) performs a nucleophilic attack on the central aluminum atom of Al(OH)4(-). Using this mechanism as a pattern, plausible dimerization mechanisms were investigated by varying the proticity and hydration of the phosphorus and aluminum monomers, respectively. The relative reaction rates of each mechanism were estimated under different pH conditions. The chain growth of aluminophosphates to trimers, tetramers, and pentamers and the cyclization of a linear tetramer were also investigated. For oligomerization reactions beyond dimer formation, it is found that cluster growth favors the addition of the phosphoric monomers rather than aluminum monomers.

Mechanism for hydrolysis of double six‐membered ring tetraborate anion

“…This phenomenon can also be explained that the electron deficient of boron atom serves as a Lewis alkali to accept more protons from hydrogen atom. A similar explanation can also be found in the initial oligomerization of aluminophosphate system …”

“…Zhang et al reported that the 4‐ring is the most favorable product and the branched tetramer is rather unfavorable in the initial stage of silicate oligomerization . Yang et al suggested that the 4‐ring closure reaction is favorable species of aluminophosphate systems . Depla et al have found that the 4‐rings are the dominant species in the initial oligomerization of the silica sol‐gel process .…”

“…However, a series of studies concerning hydrolysis‐oligomerization reactions of silicon and alumina compounds have been reported. As for the reaction mechanism of silica condensation, the effects of factors such as pH, temperature, and counter ions on oligomerization rate of silicate were studied . Trinh et al suggested two steps of the anionic mechanism of silica‐oligomerization reaction.…”

“…As for the reaction mechanism of silica condensation, the effects of factors such as pH, temperature, and counter ions on oligomerization rate of silicate were studied. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Trinh et al [15] suggested two steps of the anionic mechanism of silica-oligomerization reaction. The first step is the formation of SiO-Si linkage bond between two reactants; the second one is the removal of the water group from the intermediate to form a product.…”

Extracting the mechanisms and kinetic models of complex reactions from atomistic simulation data

“…Despite the advances in identifying reaction species and rates from simulation data, [12][13][14][15][16][17][18][19][20][21][22][23][24] there is no established method for obtaining reaction mechanisms that can be used directly for engineering (kinetic) modeling. In a recent work by Döntgen et al, [18] a method for identifying reaction species and elementary reactions was proposed.…”