Oligomerization and Cyclization Processes in the Nucleation of Microporous Silicas

Abstract

The nucleation and growth of zeolites and other microporous solids continues to pose many questions. [1,2] Whilst the general features of self-assembly through condensation polymerization are understood for siliceous systems, the conditions under which both natural and synthetic zeolites form render such a complex process extremely difficult to characterize experimentally. Thus, whilst the species initially present in silicate gels can be characterized by NMR spectroscopy with some certainty, [3,4] their charge state is more difficult to probe.[5] Similarly, although scattering methods can be used to tentatively identify larger (possibly nucleation) species, [6,7] the relative stability and lifetime of the smallest clusters, significant in zeolitic structures, such as four-membered rings, remain unclear. Furthermore, in the postnucleation regime, there is much debate as to which species are responsible for crystal growth: small oligomers or larger subunits. Indeed, it has been suggested that the growth of silicalite-1 (MFI zeolite) is controlled by a unique nanocluster, [8] although there is some debate about this proposal.[9] Moreover, characterization of zeolite surfaces suggests that much smaller units are responsible for surface growth.[10]Here, we describe quantum chemical calculations that model some of the species present prior to nucleation and attempt to examine the pathways by which such key species may form and hence rationalize the assembly of zeolitic structures. In particular, we discuss the factors that drive the condensation polymerization of silicate oligomers and that favor the cyclization of such species: pH and solvent.In the past decade there have been a number of computational studies of model structures involved in the nucleation process of silicates. Structures and energies for small clusters containing up to five silicon atoms were obtained by Pereira et al. [11,12] One conclusion that can be drawn from these results is that linear silicate species are favored over cyclic structures-a situation not conducive to the formation of zeolitic structures but instead one that favors the formation of amorphous silicates.[11] These early calculations were performed using a local DFT method and considered only neutral species, although solvation effects were included using the COSMO method.[13] The roles of water and organic templates in prenucleation gels were studied by Lewis et al. [14] and Catlow et al. [15] by more approximate molecular mechan-

Effect of Counter Ions on the Silica Oligomerization Reaction

“…The author found that the condensation reaction is more favorable at high temperature than at room temperature. Free energy profiles of the silica condensation reaction have been recently presented by Mora-Fonz et al [20,28] and by Trinh et al [27,45,47] These studies revealed that the formation of the small ring fragment is thermodynamically favorable in high pH media. Especially, it is important to include explicitly the water molecules [45,47] and the counter ions in modelling the silica oligomerization.…”

“…This is more favorable and has a lower activation barrier than the neutral mechanism. [20,27,28] Catlow et al [29] have carried out MD simulations of silica precursors and a structure directing agent. They found that long range electrostatic interactions are of crucial importance.…”

“…[28] At different solution pH, the silica condensation reaction has a different mechanism and activation barrier. [20,27] It is generally accepted that the reaction occurs best in…”

Multi-level Modeling of Silica–Template Interactions During Initial Stages of Zeolite Synthesis

“…They observed that at time scales shorter than 0.5 ns fourmembered rings will be most common, while at time scales longer than 1 ns five-membered rings will dominate. Very recently, Mora-Fonz et al have shown the importance of solvent and pH to control specific oligomerization and cyclization processes in the nucleation of microporous silicas [56,57]. Wu and Deem have shown that the nucleation barrier of silica without templates at high pH (*12) lies around 50 T atoms [58].…”

The role of density functional theory in chemistry: Some historical landmarks and applications to zeolites

“…DFT has proved to be an efficient methodology for theoretical studies on these systems, as exemplified, e.g. by the recent publications of Mora-Fonz et al [81] on the formation of their cyclic structures, of Datka et al [82] reporting the activation of CaC bonds by Cu C ions in zeolites, and of Barbosa and van Santen [83] emphasizing the influence of the zeolite framework geometry on the activation of C-H and H-H chemical bonds by the [ZnOZn] C oxycation. It is also important to quote here the studies of Davidova et al [84] on the nature of the Cu C -NO bond at different types of Cu C sites in zeolite catalysts, the investigations of Lesthaeghe et al [85] describing the bifunctional acid-base properties of amine-substituted zeolites and the calculations of Baute et al [86] on carboxylate binding in copper-histidine complexes in zeolite Y.…”