Impact of Organic Templates on the Selective Formation of Zeolite Oligomers

Uncovering the role of free lanthanum (La3+) ions and La oligomer on the surface of La (oxy)hydroxide particles for phosphate removal

La (oxy)hydroxide-based materials have been recognized as promising adsorbents for aqueous phosphate (P) removal. However, comprehending the adsorption behavior of P onto La (oxy)hydroxide particles remains challenging, given the heterogeneous low-crystalline surface encompassing La oligomers and free La3+ ions. In this study, a hydrogen (H) bond capping method was developed to construct La (oxy)hydroxide oligomers (LHOs) to simulate the low-crystalline La on the surface of La (oxy)hydroxide particles. The P uptake capacity was compared among free La3+ ions, LHOs, and La nanoparticle (La-NP) with maximum capacities of 1967.3 ± 30.8 mg/g, 461.1 ± 53.7 mg/g and 62.5 ± 6.0 mg/g, respectively. The FT-IR, Raman, in situ-XRD and XPS deconvolution analyses revealed that the removal of P by free La3+ ions mainly involve the process of chemical precipitation to form LaPO4·0.5H2O. Conversely, the elimination of P by LHOs is primarily attributed to inner-sphere complexation and hydroxyl exchange effect between LaOOH and P. Based on this study, the free La3+ ions and La oligomers on the surface of La (oxy)hydroxide particles play a primary role in P adsorption. These results also suggest that the successively decreased adsorption capacity of La (oxy)hydroxide-based adsorbents in the continuously adsorption/desorption cycles might be due to the irreversible inactivation and recrystallization of free La3+ ions and La oligomers on the surface.

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.