Computational study of adsorption and diffusion in zeolites with cations

Resumen

En esta tesis se realizó un estudio computacional de las propiedades termodinámicas de gases relevantes para el medio ambiente, como el dióxido de carbono y los alcanos ligeros (desde metano a butano), en zeolitas. Las propiedades termodinámicas estudiadas fueron los procesos de adsorción y difusión en diversos tipos de zeolitas con y sin cationes. Las técnicas de simulación utilizadas fueron: - Monte Carlo en el colectivo gran-canónico (GCMC) para el estudio de la adsorción. - Dinámica Molecular para la difusión. Para poder estudiar las propiedades termodinámicas del dióxido de carbono se desarrolló un nuevo campo de fuerzas capaz de reproducir y el comportamiento del dióxido de carbono en varios tipos de zeolitas con y sin cationes. También se estudió la influencia de la flexibilidad de la estructura y la influencia de los cationes en los procesos de difusión de metano y dióxido de carbono respectivamente, así como la influencia de los cationes en la adsorción de alcanos. Además, se propuso un nuevo modelo para optimizar las interacciones entre el adsobente y la zeolita. Summary Zeolites are nanoporous materials that are of great importance in many technological fields and environmental applications1. Zeolites consist of aluminosilicates with diverse structures: channels, windows or cages of molecular dimensions. The presence of aluminium atoms in the framework induces an electrical imbalance leading to a negatively charged framework that is compensated by additional non-framework cations, such as sodium or calcium. Due to their molecular structure, zeolites can selectively adsorb the components of gaseous and/or liquid mixtures according to their molecular size; the adsorption properties often strongly depend on the presence of non-framework cations2. Zeolites are used in gas separation processes of industrial interest such as CO2 removal from natural gas3. Remarkable separations effects can be achieved by the interplay of mixture adsorption and diffusion. Zeolites play a major role in petrochemical industry where they are used as catalysts in cracking and hydro-cracking of hydrocarbons2. For the practical application of zeolites, molecular simulation techniques provide an efficient tool to understand their thermodynamic properties. A well-designed computer simulation can predict thermodynamic properties and can be a substitute for experiments. Molecular simulation can also provide measurements that are difficult or inaccessible through experimental methods or when the experiment has components that are too dangerous or too expensive. This research focused on the understanding, from a theoretical point of view, of the mechanism of adsorption and diffusion of gases in zeolites with or without non-framework cations by applying molecular simulations. We used molecular simulations techniques to study the adsorption and the diffusion processes of gases in zeolites. In particular, we calculated adsorption isotherms by Monte Carlo simulations in the grand-canonical ensemble. Diffusion coefficients have been calculated by molecular dynamics simulations. The thesis is organized as follows: In chapter 1, we first presented a state of the art on molecular simulations and on the description of zeolites in order to provide a general overview on these fields and introduce the scientific work done in this thesis. We related the importance in technological fields and environmental applications with the simulation of the zeolite structures, as they allow to selectively adsorb the gases according to their molecular size. In chapter 2, we studied the adsorption and diffusion of small hydrocarbons in Linde Type A (LTA) zeolites as a function of their calcium/sodium ratio4. The diffusion studies focused on methane whereas the adsorption simulations were performed from methane up to pentane. Our simulation results are consistent with previous experimental studies. They provide a molecular picture of the influence of the zeolite type, the amount of cations contained and their location in the structure on the adsorption and diffusion of small hydrocarbons. In chapter 3, we studied the effect of flexibility on the adsorption and diffusion of methane in four types of zeolite A: two pure silica structure (ITQ-29 and LTASi), the sodium form (LTA-4A), and the sodium/calcium form (LTA-5A)5. Simulations were performed at different temperatures and for different methane loadings. Both processes, adsorption and diffusion, are strongly determined by the cations. In this chapter, we described how the framework flexibility affects differently to the adsorption and diffusion of methane, and we discuss about when the zeolite framework should be considered rigid or flexible. Several force fields are available to describe thermodynamic properties of light gases in zeolites but most of them are only valid to all-silica structures (zeolites without non-frameworks cations)6,7. Unfortunately, many force fields are not transferable to other systems rather than those for which they were developed8-10. In chapter 4, an accurate and transferable force field was developed to reproduce the thermodynamic properties of CO2 in all-silica structures and aluminosilicates bearing sodium non-framework cations11. This force field allows calculating the adsorption isotherms in excellent agreement with experimental data, thereby providing a more accurate and reliable tool for screening zeolites with a wide range of Al/Si ratios as well as all-silica zeolites. Regarding the diffusion process of CO2 in zeolites, in chapter 5 we investigated their diffusion in three LTA-type zeolites: ITQ-29, LTASi and LTA-4A. In order to understand the diffusion behaviour of CO2 in LTA-type zeolites and the influence of the guest-host interactions, we have compared the results for two available force fields in the literature12. The observed concentration dependencies of the self- and transport diffusions are strongly affected by the choice of the force field. To understand the physical origin of the different diffusion behaviour of CO2 in LTA-type zeolites, we have used the Relevant Site Model (RSM). This model describes the concentration dependency of CO2 in these zeolites. In addition, we investigated the influence of non-framework cations in this process. The selection or design of a zeolite for a particular use requires knowledge of the interaction between the zeolite and the adsorbate. Developing force fields is still a major task, as it requires a very large number of molecular simulations. Therefore, there is a significant interest in reducing this number. We aimed at developing a method to fit the force field parameters for describing adsorption in zeolites in a computationally easier and less time consuming way. In chapter 6, we developed a method to describe the result of a molecular simulation without performing the simulation itself13. This model represents the zeolite channel as an annular pore, where oxygen atoms are uniformly distributed over the inside of the annulus. References 1 M. E. Davis. Nature 417, 813-821 (2002). 2 S. B. Wang and Y. L. Peng. Chemical Engineering Journal 156, 11-24 (2010). 3 S. U. Rege, R. T. Yang and M. A. Buzanowski. Chemical Engineering Science 55, 4827-4838 (2000). 4 A. García-Sánchez, E. García-Pérez, D. Dubbeldam, R. Krishna and S. Calero. Adsorption Science & Technology 25, 417-427 (2007). 5 A. García-Sánchez, D. Dubbeldam and S. Calero. Journal of Physical Chemistry C 114, 15068-15074 (2010). 6 Li P. and F. H. Tezel. Journal of Chemical and Engineering Data 53, 2479-2487 (2008). 7 R. J. H. Vlugt, R. Krishna and B. Smit. Journal of Physical Chemistry B 103, 1102-1118 (1999). 8 E. D. Akten, R. Siriwardane and D. S. Sholl. Energy & Fuels 17, 977-983 (2003). 9 E. Jaramillo and M. Chandross. Journal of Physical Chemistry B 108, 20155-20159 (2004). 10 L. P. Maurin G, Bell RG. Journal of Physical Chemistry B 109, 16084-16091 (2005). 11 A. García-Sánchez, C. O. Ania, J. B. Parra, D. Dubbeldam, T. J. H. Vlugt, R. Krishna and S. Calero. Journal of Physical Chemistry C 113, 8814-8820 (2009). 12 A. García-Sánchez, J. v. d. Bergh, J. M. Castillo, S. Calero, F. Kapteijn and T. J. H. Vlugt. submitted. 13 A. García-Sánchez, E. Eggink, E. S. McGarrity, S. Calero and T. J. H. Vlugt. Journal of Physical Chemistry C 115, 10187-10195 (2011).