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.

Understanding initial zeolite oligomerization steps with first principles calculations

“…Many investigations, both experimental and computational, have attempted to clarify how initial oligomerization steps progress using several techniques such as nuclear magnetic resonance (NMR), 15 density functional theory (DFT) calculations, [16][17][18][19][20][21] and Monte Carlo simulations. 20,22,23 Experimental studies have offered insights into the effects of reactant composition and synthesis environment on the species formed during nucleation, which has direct impact on zeolite size, topology, and functionality.…”

“…For instance, although zeolites are aluminosilicate materials, most of the computational studies focus only on oligomerization of pure silicate systems. [16][17][18][19][20][21][22][23] Additionally, cations present in the synthesis environment can assume various roles during growth. Cationic species can perform as inorganic structuredirecting agents, which serve to direct growth toward a desired topology and can determine structural characteristics in the final zeolite product.…”

“…While the contributions of pH and solvation on the nucleation steps of zeolite growth have been addressed computationally, 20 there remains a sizable gap in zeolite nucleation knowledge that has been untouched by computational studies. For instance, although zeolites are aluminosilicate materials, most of the computational studies focus only on oligomerization of pure silicate systems 16‐23 . Additionally, cations present in the synthesis environment can assume various roles during growth.…”

The zwitterion isomer of orthosilicic acid and its role in neutral pH dimerization from density functional theory.

“…This had an activation energy of 30.4 kcal/mol. The energetics of these mechanisms were corroborated by later studies with modifications (Hu et al 2013;Liu et al, 2019). However, as more studies come out, it becomes apparent that the canonical equations of orthosilicic acid speciation in water (Iler, 1979;Rimstidt and Barnes, 1980;Sjöberg et al, 1981), or…”

“…However, the explicit addition of water molecules revealed that favorable reaction paths may be starkly different from the dry, gas-phase simulations, even with implicit solvation included (Trinh et al, 2009;Heenan et al, 2020;Zhang et al, 2017) suggesting that long range interactions have a less influential role on the mechanisms than short range electrostatics and, more specifically, than hydrogen bonding. These effects are predominantly due to the ease of formation of hydrogen bonds by water molecules, which lowers energies significantly (3-8 kcal mol -1 , Huš and Urbic, 2012), and the ability of water to form proton wires where hydrogens can concertedly move around and participate in the so-called Grotthus mechanism (van Grotthus, 1806; e.g., Liu et al, 2019). Furthermore, the hydroxides of H4SiO4 themselves have been shown to participate in mobilizing hydrogen through similar mechanism (Felipe et al, 2004) in aqueous environments.…”