The Role of Chloride ion in the Silicate Condensation Reaction from ab Initio Molecular Dynamics Simulations

Abstract

The comprehension of silicate oligomer formation during the initial stage of zeolite synthesis is of significant importance. In this study, we investigated the effect of chloride ions (Cl–) on silicate oligomerization using ab initio molecular dynamics simulations with explicit water molecules. The results show that the presence of Cl– increases the free energy barriers of all reactions compared to the case without the anion. The formation of the 4-ring structure has the lowest free energy barrier (73 kJ/mol), while the formation of the 3-ring structure has the highest barrier (98 kJ/mol) in the presence of Cl–. These findings suggest that Cl– suppresses the formation of 3-rings and favors the formation of larger oligomers in the process of zeolite synthesis. Our study provides important insights into the directing role of Cl– in silicate oligomerization by regulating thermodynamic and kinetic parameters. An important point to consider is the impact of the anion on aqueous reactions, particularly in altering the hydrogen bond network around reactive species. These results also provide a basis for further studies of the formations of larger silicate oligomers in solution.

Stereomutations in silicate oligomerization: the role of steric hindrance and intramolecular hydrogen bonding

“…As stated in the last paragraph, the activation barrier of Si–OH rupture for the other three cases is observed to be an almost identical value of nearly 16 kcal mol −1 . These values are similar to the activation barrier of trimer ring condensation and Si–OH rupture, which are reported to be14.58 kcal mol −1 and 15.30 kcal mol −1 in the presence of ethylamine 24 but lower than those reported in other investigations performed in presence of anions such as 17.21 kcal mol −1 and 23.42 kcal mol −1 in the presence of Cl − ions 26 and 19.36 kcal mol −1 and 21.99 kcal mol −1 in the presence of OH − ions. 25 Fig.…”

“…The activation energy of anionic-I and anionic-II condensation is determined to be 3.78 kcal mol −1 and 14.94 kcal mol −1 , respectively. These values are comparable to the activation barrier of 14.10 kcal mol −1 observed for linear trimer condensation reported in the presence of Cl − ions 26 and lower than the E a of 19.36 kcal mol −1 observed in the presence of OH − ions. 25 As stated earlier, the linear trimer silicate considered in this study has been selected in such a way that it can be an intermediate resulting from both anionic-I or anionic-II condensation; therefore, the comparison of pathways cannot be done based on anionic-I or anionic-II condensation.…”

“…The pathways without any kind of M1 or M3 alt pseudorotation suggest a contrary behavior of Si–OH rupture as the E a of 17.20 kcal mol −1 observed for Si–OH rupture in anionic-I is higher than the E a of 12.90 kcal mol −1 in the anionic-II pathway. These values are lower than the activation barriers of dimer condensation and Si–OH rupture reported in previous literature, which are 13.86 kcal mol −1 and 15.06 kcal mol −1 in the presence of ethylamine, 24 14.82 kcal mol −1 and 19.36 kcal mol −1 in the presence of Cl − ions, 26 and 16.97 kcal mol −1 and 19.84 kcal mol −1 in the presence of OH − ions. 25 It has already been established that the arrangements of hydroxyl groups in anionic-I intermediates make only M1 pseudorotation feasible and restrict M3 alt through steric hindrance or intramolecular H-bonds.…”

“…13–17 This is quite similar to the pentacoordinate formation mechanism suggested for silane reactions. 9 Silicate oligomerization has been extensively studied by the authors 13,14 along with many other researchers; 16–26 however, there is a lack of understanding regarding the role of stereomutation in the process and the forces governing stereomutations. Most of the abovementioned studies 13,14,16,17 considered the formation of silicate dimers and reported the Si–OH breaking (also known as the water removal step) as the final step.…”

Simple Molecular Model for Hydrated Silicate Ionic Liquids, a Realistic Zeolite Precursor

“…Due to the complexity of traditional zeolite growth liquids, prior studies have mostly relied on modeling isolated aspects of the synthesis liquid . Many studies have focused on modeling isolated oligomerization steps using density functional theory calculations in implicit solvent to deduce reaction networks, sometimes taking into account the influence of organic templates or cations. − Others have made valuable attempts to include explicit solvent to obtain a more realistic description of zeolite nucleation, yet these models cannot capture the complexity of sol–gel systems. − Finally, attempts have been made to describe crystal nucleation and growth at a larger scale by sacrificing atomic resolution. , …”