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Study of the Self-Assembly Process of Microporous Materials Using Molecular Modeling

Zeolites are an important class of materials in modern technology with applications in catalysis, separations, biosensing and microelectronics. There are over 200 different zeolite frameworks reported in literature, but only a handful have been used commercially. Understanding their self-assembly process would assist in the fabrication of new zeolites through the control of their pore size/shape, and surface area for advanced applications. With our research we aim to elucidate aspects of zeolite formation using molecular simulations. We have extended the lattice model of silica tetrahedra developed by Jin et al. [L. Jin, S. M. Auerbach and P. A. Monson J. Chem. Phys. 134(13), 2011: 134703] to study silica polymerization under various pH values and silica concentrations. We have investigated the transition from gels, at the iso-electric point of silica, to nanoparticles in the initial stages of the formation of silicalite-1. We focus on two systems: one with low silica concentration with composition comparable to the clear solution silicalite-1 zeolite synthesis, and a high silica concentration system that leads to gel states. In the dilute system, clusters have a core-shell structure with the core predominantly comprised of silica with some SDA+ cations, surrounded by a shell of only SDA+ cations. In the concentrated system there are larger number of smaller nanoparticles than those in dilute system. Next, we focused our attention to study the effects of two different type of structure directing agent (SDA) molecules -- quasi-spherical SDA and tetramethylammonium (TMA) -- on crystalline tetrahedral frameworks in the synthesis of microporous materials. We have implemented parallel tempering Monte Carlo algorithm to simulate the formation of ordered crystalline materials, and we have demonstrated that the presence of SDAs result in the formation of previously unobserved crystalline frameworks. In the case of quasi-spherical SDAs we have observed three-dimensional fully-connected materials as well as two-dimensional layered materials in our simulations, and observed that the interaction between SDAs and silica plays a significant role in directing the final micropore structure. For TMA we have developed two types of models based on silica-TMA interaction -- silica-nitrogen interaction vs silica-methyl interaction. In both of these models we have observed that the TMA molecule forms predominantly three dimensional materials, which suggests that the molecular structure directs the preferential formation of three dimensional frameworks as opposed to two dimensional layered materials. We have also studied the kinetics for formation of crystals using forward-flux sampling method. We have estimated the rate constant of transition from amorphous silica phase to crystalline silica phase. We have also predicted and characterized the transition state for this process. This is the first time when an enhanced sampling method is applied to understand the self-assembly process of microporous crystals. This research has been crucial in understanding the nature of silica polymerization and elucidating the role of structure directing agents in zeolite synthesis; it has taken us a step closer to answering the big question in zeolite science -- "How do all silica zeolites form ?"
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