Resumen
Zeolites are one of the most widely used microporous materials in chemical processes due to their exceptional performances in catalysis, adsorption and ion-exchange applications. In spite of their wide range of chemical compositions, most of the industrially important zeolites are aluminosilicates (or silicates), whose synthesis process is the main topic of this thesis. With the same general chemical composition, aluminosilicate zeolites are polymorph materials, which makes it extremely difficult to predict which phase will be obtained from a given synthesis gel composition. The effect of synthesis variables such as the type of organic molecule (also called organic structure-directing agent, OSDA), Si/Al ratio, presence of defects or fluoride, will be thoroughly investigated in this thesis.
With the help of SciFinder, an analysis of ca. 70000 publications was made to generate a database of OSDAs for the synthesis of zeolites. A list of descriptors associated with each OSDA was utilized to extract a subset of 7 OSDAs that give different zeolites under aluminosilicate and silicate gel composition, pointing to a specific role of aluminum for the zeolite phase obtained. Since the role of aluminum in the synthesis process remains less understood, the analysis of this subset of experimental synthesis conditions, combined with geometry optimizations of realistic extended models containing accurate chemical composition similar to experimental results, allowed us to make predictions of which zeolite phase, among a subset of competing phases, is the most stable when the synthesis gel contains either silicate or aluminosilicate composition. Computational predictions are in agreement with the experimental results in almost all of the cases.
An extension to the synthesis energy approach was carried out in a still wider chemical space, including the presence of fluoride within the experimentally introduced 'excess fluoride' approach. The literature analysis leads now to the narrow selection of 4 OSDAs giving PST-21 (PWO), PST-22 (PWW), and ERS-7 (ESV) zeolites whose outcome as aluminosilicates has been predicted among a list of other competing zeolites such as FER, RUT, AST, RTH, ITW, STW, FER and MTW. Also, for silica composition, the outcome of ITW and AST has also been predicted among a list of zeolite phase competitors.
The role of defects in the synthesis has also been approached using a generalized extension of the synthesis energy, able to predict the thermodynamic stability of defects as one more of the 'anions' (together with aluminum and fluoride) compensating the positive charge of the OSDA. In the zeolite synthesis experiments using various isomers of trimethylpyridinium cations as OSDA under fluoride media, the presence of silanol/siloxy defects, usually not present in such conditions, was detected. Using computational approach, the formation of these defects instead of F- encapsulation was justified.
Finally, the influence of defects was explored in the diffusivity of benzene in silicalite (pure-silica ZSM-5, MFI), a well-known zeolite for its shape-selective adsorption properties. Using accurate experimental characterization of connectivity defects, a realistic model of defective ZSM-5 was generated. The self-diffusivities of benzene in both the newly generated defective ZSM-5 model and the defect-free zeolite were calculated through Molecular Dynamics simulations at various loadings and temperatures. Although the self-diffusivities had an order of magnitude discrepancy with the experimental values, the calculated heats of adsorption values were in good agreement with the experimental data. Based on the difference in diffusivities in both models, it was possible to elucidate the effect of structural defects on the diffusion of benzene in silicalite channels. The results support the counterintuitive conclusion that the presence of defects contributes to an enhanced diffusivity of benzene in silicalite.
