Insight into the role of excess hydroxide ions in silicate condensation reactions

Abstract

Understanding the formation of silicate oligomers in the early stages of zeolite synthesis is critical. The pH and the hydroxyl ion are important in regulating the reaction rate and the...

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

“…In contrast to extensive research on the effect of cations on silicate reactions, the role of anions in this process has received relatively little attention. Recent work by Do et al 31 has demonstrated that OH – ion play a critical role in silicate condensation reactions by directly participating in these reactions. Chloride ion is another ubiquitous and crucial ion in various chemical reactions.…”

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

“…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. 13 suggests that the energy levels of the final product from two pathways are quite close to each other.…”

“…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. It is evident from Fig.…”

“…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. Instead, it has been done based on M1 or M3 alt pseudorotation.…”

“…[13][14][15][16][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][17][18][19][20][21][22][23][24][25][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.…”