A computational study of silicate oligomerization reactions

Abstract

The classical syntheses of zeolitic materials are based on silicates or alkoxysilanes in alkaline solutions, often with organic additives that act as structure-directing agents (SDA). The SDA are thought to be mainly responsible for the formation of a specific zeolite structures. However, this thesis shows that the silicate-template interactions also are important to be initial condensation reaction take place in the prenucleation stage. At this point it is also still under discussion whether growth proceeds by a monomer addition sequence or the assembly of preformed building blocks. In this thesis, the essential chemical properties of the silica condensation process such as the role of water, hydrogen bonding, counter ion effect, interaction with template have been addressed. The mechanism of the silica condensation reaction of small oligomer from dimer to pentamer in gas phase was studied in chapter 2. In this chapter, the reaction mechanism of silica condensation was investigated using DFT calculations for various structures of silicate oligomers. There are two different reaction paths for the condensation reaction: one reaction path proceeds via neutral species and the other occurs via the anionic species. An anionic mechanism that occurs in two steps. The first step is the formation of the SiO-Si linkage bond between two reactants. The second one is the removal of the water group from the intermediate to form a product. It infers that the water removal is the most difficult step in reaction pathway. Based on the calculated activation barriers of silicate formation, the anionic pathway is kinetically preferred over the neutral one. Thus, the polymerization of silicate species mainly concerns the anionic species. The ring closure reactions occur with high barriers because of loss of intramolecular hydrogen bonds. The decrease of the overall activation barrier for the formation of higher linear and branched molecules is ascribed to more favorable hydrogen bonding effects for these cases. The importance of inter- and intramolecular hydrogen bonding to the relative activation barriers of SiO-Si bond formation was found. In chapter 3, it was argued that it is essential to include explicitly water molecules in silica condensation and sol-gel chemistry computational studies. The rate limiting step is not the water removal step as in the gas phase simulation in chapter 2. The overall barrier is mostly depends on the first barrier of the SiO-Si bond formation. The activation barrier of the water removal step depends on the mechanism of water assisted internal or external proton transfer, independent from the size of oligomer. The kinetic and thermodynamic trends of formation of higher oligomers from dimer silica are found to be quite different between explicit solvent simulations and gas phase simulation. The gas phase model in chapter 2 proposes that the linear and branched higher silica structures are more favorable than the 3-ring and 4-ring. In contrast, this study in solution observed that 3-ring formation is more favorable than the formation of higher branched and ring silica oligomers. As a consequence, the 3-ring silica structure will be a dominant species during the first stage of pre-nucleation process in pure silica condensation. This is in good agreement with experimental studies of the early stage of zeolite synthesis. Water molecules are also essential to assist proton transfer and form stabilization hydrogen bond. The counter ion (Li+ and NH4+) effect to the silica condensation reaction was investigated in chapter 4. It was also shown that the activation barrier of the water removal step depends only on the water assisted, which is independent of the size of oligomer. The overall barrier mostly depends on the initial barrier of the SiO-Si bond formation. The position of cation has a strong effect on the barrier height of this step. When close to the reactive center of the dimerization reaction, Li+ does not change strongly the activation barrier, meanwhile NH4+ increase significantly the activation barrier. The relative rates of formation of the higher oligomers from dimer silica are found to be quite different between Li+ and NH4+ case. The presence of Li+ favors the linear and branched higher silica structures over the 3-ring structures. In contrast, with NH4+ in solution it has been observed that 3-ring formation is more favorable than the formation of higher branched silica oligomers. The positive charge of counter ion decreases most significantly the rates of SiO-Si bond formation in the dimer. For the first step of the dimerization reaction, the hydrogen bonding effects are more important than the electrostatic interaction. In consecutive oligomerization reaction steps, the cation has a weak effect on activation barrier. The importance of organic templates is well known for zeolite synthesis. Chapter 5 presents the catalytic role of template in the formation of the initial silicate structure. Formation of dimer, trimer and three-ring silicate oligomers has been studied in the presence of organic cation tetrapropylammonium TPA+. The interaction between TPA+ and anionic silica consist of an electrostatic part and a weak VdW part. TPA+ stabilises the transition state for Si-O bond formation. The ring closure reactions occur with comparable barriers to that for linear growth despite the unfavorable thermodynamics for ring closure reaction due to the loss of intramolecular hydrogen bonds. Chapter 6 reports an extensive investigation on the effect of the template on the relative stabilities of higher oligomers than could be considered in chapter 5. Various silicate structures from dimer to double 4-ring, related to initial stage of zeolite synthesis, have been studied in the presence of organic cations such as tetramethylammonium TMA+ and tetrapropylammonium TPA+. The results show that organic template, especially TPA+, stabilizes silicate oligomers. As we found before, the TPA+ favors the formation of linear small oligomer such as dimer and trimer. Condensation reactions in the presence of TPA+ gain more energy than with TMA+ or the alkali counter ion (e.g Na+). This is due to the favorable interaction between product and TPA+. It mainly relates to the difference in the Van-der-Waals interaction energies between template and oligomers. For both templates, the 4-ring fragment tends to be more stable than the 3-ring fragment. The unique figure of TMA+ is the higher stabilization of the 4-ring structures compared to the other oligomers. For TPA+ these differences are much less. This observation is consistent with experimental results. This thesis contributes to the understanding of the prenanoparticle oligomerization process that occurs initially in zeolite synthesis. Several factors control the chemical reactivity of silica condensation in solution at this early stage (e.g. hydrogen bonding, water rearrangement, electrostatic effect, Van-der-Waals interaction). The internal and external hydrogen bonding is crucially related the geometry of small silicate oligomers. The electrostatic interaction between that alkaline counter ion and reactive region of condensation reaction increase the activation barrier of silica condensation reaction. It has been shown that the organic template enhances the OSi-O bonding of small oligomer as well as the stabilities of higher oligomers. The Van-der- Waals interaction between the template and silicate structure is more important when the size of oligomer is larger. This implies that the organic templates not only act as the generally accepted role of structure directing agents to the nanoparticle formation but also have an important role in the earlier stage of oligomerization.