Computational techniques applied to the study and development of nanoporous materials

Resumen

There are around two hundred zeolites reported in the international Zeolite Database. More than ten thousand MOFs have been synthesized up to date, and an increasing number of new ZIFs are bein synthesized. Despite these astonishing numbers, most of these materials have not industrial applications yet. A lot of research is needed to analyze those materials and their properties. In this study we aim to provide new research methods an tools that will allow to increase significantly the knowledge of these materials. We also aim to study the possible use of crystalline nanoporus materials to takle current industrial challenges. Furthermore our last goal would be to design hypothetical structures suitable for solving some specific problems. The scope of this thesis is therefore three-fold: - To develop new force fields and sets of charges that allow the modeling of nanoporous materials. - To study the potential uses of different types of nanoporous materials in processe of industrial interest. - To design new materials with specific properties, for technological applications. To achieve these objectives we perform molecular simulations using the methods, force fields, and models explained in the previous sections. Development of new force fields and sets of charges that allow the modeling of nanoporous materials. (Chapters 2, 3, and 4) In chapter 2 we study the propylene models available in the literature and we propose new ones. The new models are developed by fitting to experimental adsorption isotherms in zeolites. The Lennard-Jones parameters are adjusted to reproduce the VLE curve of the gas. We have also developed a specific force field able to predict diffusion and adsorption of propylene in zeolites with very narrow channels. In chapter 3 previous available models of hydrogen sulfide are discussed and three new models are proposed. We compare their accuracy reproducing VLE and vapor-pressure curves, and the liquid density. Our results show a good agreement between the models. We also compute the adsorption isotherms, heats of adsorption and Henry coefficients of hydrogen sulfide in three MOFs. In chapter 4 we developed a transferable and scalable set of point charges for ZIFs. This set can be used in previously synthesized ZIFs as well as in theoretical ZIFs. We validate the viability of the set of charges by comparing experimental Heats of adsorption with simulation data obtained using our set of charges. Study of different types of nanoporous materials for applications of enviromental and industrial interest. (Chapters 2,3,5, and 6) In chapter 2, we use the developed models and force fields to study the adsorption and diffusion properties of propane and propylene in the ITQ-12 zeolite. Our models accurately reproduce the experimental adsorption isotherms. Using TST we compute the diffusion coefficients in order to explain the differences obtained experimentally. In chapter 3, we study the adsorption of hydrogen sulfide in three MOFs with different topologies namely IRMOF-1, MIL-47 and Cu-BTC. We compute heats of adsorption, Henry coefficients, and adsorption isotherms to have a better understanding of the behavior of this air pollutant inside MOFs. In chapters 5 and 6 we perform an extesinve study of the metal organic framework Cu-BTC. We study the adsorption of greenhouse gases such as carbon dioxide and methane in the structure. We compute adsorption isotherms of the pure gases as well of equimolar mixtures. Our results match with previous simulation and experimental data. We expand the study to all feed composition by using molecular simulations and also by using the Ideal Adsorption Solution Theory (IAST). We compute diffusion coefficients by using MD and we determine the adsorption selectivity, diffusion selectivity and mixture selectivity. We also identify the preferential adsorption sites in Cu-BTC and then carry out a systematic study to analyze the preferential adsorption sites of hydrocarbons, greenhouse gases, alcohols, water, and the main components of air. We focus on the molecular mechanisms governing the adsorption process of these molecules. We perform MC simulations in the canonical ensemble varying the number of molecules. Then we analyze the distribution of the molecules inside the cage system of the framework. Finally we explore the possibility of enhancing the adsorption of certain gases by adding ionic liquids to the structure. Design of new materials with specific characteristics (chapter 7) In chapter 7 we develop the idea of tailoring Cu-BTC in order to favor the adsorption of some gases and, prevent the adsorption of other gases. Based on the findings of chapter 6 we focus on the alcohol-water separation. We compute adsorption isotherms of equimolar mixtures in gas and liquid phase as well as adsorption energies, entropies and Henry coefficients. We follow two strategies to improve the separation: Blocking cages and poisoning metal centers of the framework. We test our strategies and we propose realistic alternatives. We also found a modification of the Cu-BTC framework that could lead to an increase of its water stability.