Theoretical Studies on the Oligomerization of Silicate Species in Basic Solution

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

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.

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

The formation of silicate oligomers in the early stages is key to zeolite synthesis. The pH and the presence of hydroxide ions are important in regulating the reaction rate and the dominant species in solutions. This paper describes the formation of silicate species, from dimers to 4-membered rings, using ab initio molecular dynamics simulations in explicit water molecules with an excess hydroxide ion. The thermodynamic integration method was used to calculate the free energy profile of the condensation reactions. The hydroxide group's role is not only to control the pH of the environment, but also to actively participate in the condensation reaction. The results show that the most favorable reactions are linear-tetramer and 4-membered-ring formation, with overall barriers of 71 kJ mol^-1 and 73 kJ mol^-1, respectively. The formation of trimeric silicate, with the largest free-energy barrier of 102 kJ mol^-1, is the rate-limiting step under these conditions. The excess hydroxide ion aids in the stabilization of the 4-membered-ring structure over the 3-membered-ring structure. Due to a relatively high free-energy barrier, the 4-membered ring is the most difficult of the small silicate structures to dissolve in the backward reaction. This study is consistent with the experimental observation that silicate growth in zeolite synthesis is slower in a very-high-pH environment.

Understanding the mechanism and kinetics of the formation and breaking of ring structures during silica polymerization: a computational study

Ring structures are ubiquitous in porous materials and play a crucial role in the functioning of these materials. Understanding the ring formation and breaking mechanism is essential for designing and controlling the porosity, framework density, channels, and cage formation in porous materials. The current work attempts to understand the formation, breaking, and survival of rings using a computational approach. We have used the reaction ensemble Monte Carlo simulation technique and studied silica polymerization starting from monomers to inter-connected large silica clusters in dilute and concentrated silica systems. We calculated various properties of representative smaller and bigger rings at different stages of polymerization. We found that smaller rings form in the initial polymerization stages and larger ring sizes appear at later stages. The smaller rings have a larger residence time than the bigger rings in the silica system, and the residence time changes with the polymerization stage. Both smaller and bigger rings have a shorter residence time in the dilute system than the concentrated silica system. As a result, ring formation and breaking kinetics are faster in the dilute silica system, which causes reorganization within the silica cluster leading to a dense cluster. A slow reorganization of rings in the concentrated silica system is observed, due to which clusters retain their random, branched configuration and porous region within the cluster. We also investigated a series of ring formation and breaking steps to understand the formation mechanism of isolated and grouped rings in the studied silica systems. We found that rings form and break by all possible reactions during ring-formation and cluster-aggregation stages. In contrast, only one reaction is dominant in the initial and aging stages of polymerization. The concentration of silica affects the formation of isolated rings, whereas the kinetics of a grouped ring is not significantly altered. Detailed insights into the reaction dynamics of rings at various stages of polymerization would be helpful in the rational design of porous silica polymorphs.

Anion-promoted increase of the SiO2/Al2O3 ratio of zeolites

Increasing the SiO2/Al2O3 ratio (SAR) of zeolites is crucial for enhancing their acidic catalysis performance and thermal/hydrothermal stability. In this work, we reported a facile strategy to improve the SAR...