Molecular Modeling of the Synthesis of Zeolites and Related Nanoporous Materials
Zeolites are microporous inorganic crystals with a framework that mainly contains silicon, oxygen and aluminum. The chemical composition is comparable to quartz, but the crystal structure is fundamentally different. Zeolites contain regular cavities (channels and cages) with a maximum diameter of 1.2 nanometer, large enough to contain small organic molecules. The walls of the cavities represent a huge internal surface (of the order of 1000 m2/g). Due to the well-defined shape of these channels and cages, only the molecules with a compatible structure can enter the internal cavities of a zeolite. This principle is called shape selectivity and it enables the separation of chemically similar but structurally different molecules. A Si/Al substitution in the framework introduces a catalytically active site on the walls of the zeolite cavities. During the past 60 years, all these specific properties have driven the development of new zeolites and the implementation of zeolites in industrial applications. The world-wide production of zeolites amounts to 4.2 million tons per annum, with applications in diverse markets such as petrochemistry, agriculture, animal husbandry, pharmacy, and so on. The most important applications in the western world include catalytic cracking in the petrochemical industry, ion exchange (softening and purification of water), and the separation and extraction of gases and solvents. A recent development in this field is the search for micro- and mesoporous materials. The IUPAC defines a microchannel as a pore with a diameter ranging from 0.25 nm to 2 nm, while a mesopore has a diameter between 2 nm and 50 nm. The limited diffusion rate of guest molecules in the micropores of a conventional zeolite is a well-known obstacle for the transport of reagents and reaction products. Another issue is the formation of side-products that reduce the diffusion rates and block the access to catalytically active sites. The regeneration of a zeolite catalyst, in which side products are burned, is an expensive and impractical procedure. One can avoid or at least reduce these limitations by introducing mesopores in the zeolite that have a diameter of the order of 1 to 50 nanometer. These mesopores act as highways for the transport of guest molecules. The Center for Surface Chemistry and Catalysis (COK) of the KULeuven has developed a new route for the synthesis of such micro- and mesoporous zeolites with well-controlled sizes of both types of pores. This revolutionary concept was the onset for the strategic basic research (SBO) project on biporous materials (BIPOM's). This PhD is mainly carried out in the context of the SBO-BIPOM project. A detailed description of the molecular mechanisms that give rise to the nucleation and growth of zeolite crystals is not yet available. Such insights are however of great interest for the tailor-made synthesis of zeolite catalysts in industrial applications. One of the major questions is the exact role of organic template molecules. Templates direct the synthesis process in a selective way to the formation of zeolites whose channels and cavities are structurally complementary to the shape of the template molecule. Another controversy is the exact structure of the precursors that precede the formation of zeolite crystals. A few (incompatible) models are postulated in the literature. Although several fragments of the synthesis mechanism have been unraveled through an impressive history of experimental studies, the remaining unknowns are not easily disentangled. Molecular modeling is a research field that plays a complementary role in this context. With the aid of computer simulations, one gains insights into molecular interactions that are not easily accessible to the experiment due to the small scale. There are two large categories of models to describe molecular interactions on a theoretical basis. In first instance there are the quantum-mechanical methods that solve the electronic many-body problem (approximately) to obtain the potential energy of a molecular system. An important subcategory are the ab initio methods, which rely only on the elementary quantum-mechanical postulates and do not depend on any empirical input. The second large category consists of the molecular mechanics methods that approximate molecular interactions with mainly empirical models. These models are computationally very efficient, but the downside is that molecular mechanics methods have only a limited accuracy and can not (or at least not correctly) describe important chemical processes such as the formation and the breaking of chemical bonds. Methods from both categories are applied to obtain microscopic properties of molecular systems such as optimal geometries, vibrational modes, reaction mechanisms, and so on. Only the quantum mechanical methods are reliable for the description of chemical reactions. Statistical mechanics provides the theoretical foundations to translate these microscopic data into relevant macroscopic observables. Molecular dynamics is a very versatile technique to apply the laws of statistical physics. By integrating the equations of motion of the molecular system, one runs through all the relevant microscopic states at a given temperature and pressure. Such simulations take into account the complete molecular environment such as a solvent or a zeolite framework. By using the proper statistical techniques, one can derive macroscopic parameters as averages over the ensemble of microscopic states. This thesis covers all the aspects of molecular modeling that we have addressed to gain more insights into the synthesis of zeolites. The larger part of the research deliverables in this PhD were only possible through the development of new software. The most visible example is ZEOBUILDER, a computer program to construct atomic models of biporous zeolites. The geometric models made with ZEOBUILDER are an essential starting point for molecular simulations. Another important aspect of this work is the development of molecular mechanics models that allow efficient simulations on zeolite precursors. In particular, we proposed new methods to obtain reliable force-field parameters. These technological and methodological tools are applied in theoretical studies on distinct intermediate steps of the synthesis of zeolites. Infrared spectra of zeolite precursors and zeolite nanocrystals have been derived from molecular dynamics simulations. This investigation confirms the experimental observation that a shift of the so-called MFI-fingerprint in the infrared spectrum can be associated with the formation of nanoscopic zeolite crystals. The tetrapropylammonium (TPA) template is known for the synthesis of MFI-structured zeolites such as ZSM-5 and Silicalite-I. We have studied the interactions between TPA and zeolite precursors proposed by the COK, using a broad scala of modeling techniques. The principal conclusion is that the preferential position of TPA with respect to the precursors corresponds to the place where the crossing of two channels will form in a later phase of the synthesis. This is in agreement with NMR measurements on macroscopic MFI-structured zeolite crystals from which the TPA has not been removed yet. Without the presence of the TPA, the investigated precursors would collapse, which is a confirmation of the function of the template molecule: providing a structural support for the initial zeolitic species. The new software and theoretical models in this thesis are the cornerstone for the modeling of zeolite synthesis processes. The development of polarizable force fields must be continued and their implementation in molecular dynamics software is indispensable to gain additional insights in the molecular mechanisms that lead to the formation of zeolites.
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