Oligomerization of Silicic Acids in Neutral Aqueous Solution: A First-Principles Investigation

Abstract

Crystallite aluminosilicates are inorganic microporous materials with well-defined pore-size and pore-structures, and have important industrial applications, including gas adsorption and separation, catalysis, etc. Crystallite aluminosilicates are commonly synthesized via hydrothermal processes, where the oligomerization of silicic acids is crucial. The mechanisms for the oligomerization of poly-silicic acids in neutral aqueous solution were systematically investigated by extensive first-principles-based calculations. We showed that oligomerization of poly-silicic acid molecules proceeds through the lateral attacking and simultaneously proton transfer from the approaching molecule for the formation of a 5-coordinated Si species as the transition state, resulting in the ejection of a water molecule from the formed poly-silicic acid. The barriers for this mechanism are in general more plausible than the conventional direct attacking of poly-silicic acid with reaction barriers in the range of 150-160 kJ/mol. The formation of linear or branched poly-silicic acids by intermolecular oligomerization is only slightly more plausible than the formation of cyclic poly-silicic acids via intramolecular oligomerization according to the reaction barriers (124.2-133.0 vs. 130.6-144.9 kJ/mol). The potential contributions of oligomer structures, such as the length of the linear oligomers, ring distortions and neighboring linear branches, etc., to the oligomerization were also investigated but found negligible. According to the small differences among the reaction barriers, we proposed that kinetic selectivity of the poly-silicic acids condensation would be weak in neutral aqueous solution and the formation of zeolite-like structures would be thermodynamics driven.

Keywords: first-principles; nucleation; oligomerization; ortho-silicic acid; zeolite.

Figure 1

The structures of reaction species for the dimerization of ortho-silicic acid, including reactant (left panel), transition state (middle panel) and product (right panel) for the anionic-attack mechanism (AAM) (a) and the “molecular attack” mechanism (MAM) (b) mechanisms. The Si, O and H atoms are in yellow, red, and white, respectively.

Figure 2

The structures of reaction species, including reactants (left panel), transition states(middle panel) and products (right panel) for oligomerization for the formation of ${\mathrm{T}}_4$ (a), ${\mathrm{T}}_3^1$ (b), ${\mathrm{C}}_3^1$ (c), ${\mathrm{C}}_3^2$ (d) and ${\mathrm{C}}_4^1$ (e). The Si, O and H atoms are in yellow, red and white, respectively.

Figure 3

The structures of reaction species, including reactants (left panel), transition states (middle panel) and products (right panel) for oligomerization for the formation of ${\mathrm{C}}_3$ (a), ${\mathrm{C}}_3^1$ (b), ${\mathrm{C}}_3^2$ (c) and ${\mathrm{C}}_4^1$ (d). The Si, O and H atoms are in yellow, red and white, respectively.