La Simulación Molecular como Herramienta para estudiar procesos de Adsorción y Difusión en Cu-BTC

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Computational techniques applied to the study and development of nanoporous materials

Programa de Doctorado en Biotecnología y Tecnología QuímicaThere are around two hundred zeolítes 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 lo 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 sorne 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 ot 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 als.o 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.Universidad Pablo de Olavide. Centro de Estudios de Postgrad

Adsorption of Carbon Dioxide in Non-Löwenstein Zeolites

We investigated the effect of the aluminum distribution in the adsorption properties of carbon dioxide in the MFI, MOR, and ITW zeolites. Because of its lack of experimental evidence and theoretical validation, Löwenstein’s rule was not generally imposed, and special attention was paid to the effect of the Al-O-Al linkages. To this end, we first generalized an existing transferable force field for CO2 adsorption in non-Löwenstein zeolites. By means of molecular simulations based on this force field, we showed that the carbon dioxide adsorption efficiency in MFI is determined by the number of Al atoms, rather than by their distribution in the framework. This was attributed to the small size of the CO2 molecules compared to the 3D wide-channel topology of the structure. Conversely, we found that the Al distribution has a higher impact on the heat of adsorption in MOR. Although structures with a very high and very low number of non-Löwenstein bonds presented significant differences, the bonds themselves do not affect the heat of adsorption directly. Instead, we found that an homogeneous distribution of the Al atoms in the sites forming the C-channel is more favorable. Finally, the small-pore distribution of the ITW zeolite led to high values of the heat of adsorption and wide error bars, which made the study feasible just for low aluminum concentrations. In that case, we report a small dependency of the heat of adsorption on the Al distribution.</p

The Boost of Toluene Capture in UiO-66 Triggered by Structural Defects or Air Humidity

This work aimed to investigate the adsorption of toluene in UiO-66 materials. Toluene is a volatile, aromatic organic molecule that is recognized as the main component of VOCs. These compounds are harmful to the environment as well as to living organisms. One of the materials that allows the capture of toluene is the UiO-66. A satisfactory representation of the calculated isotherm steep front and sorption capacity compared to the experiment was obtained by reducing the force field σ parameter by 5% and increasing ϵ by 5%. Average occupation profiles, which are projections of the positions of molecules during pressure increase, as well as RDFs, which are designed to determine the distance of the center of mass of the toluene molecule from organic linkers and metal clusters, respectively, made it possible to explain the mechanism of toluene adsorption on the UiO-66 material.</p

Defect-induced tuning of polarity-dependent adsorption in hydrophobic–hydrophilic UiO-66

Structural defects in metal–organic frameworks can be exploited to tune material properties. In the case of UiO-66 material, they may change its nature from hydrophobic to hydrophilic and therefore affect the mechanism of adsorption of polar and non-polar molecules. In this work, we focused on understanding this mechanism during adsorption of molecules with different dipole moments, using the standard volumetric adsorption measurements, IR spectroscopy, DFT + D calculations, and Monte Carlo calculations. Average occupation profiles showed that polar and nonpolar molecules change their preferences for adsorption sites. Hence, defects in the structure can be used to tune the adsorption properties of the MOF as well as to control the position of the adsorbates within the micropores of UiO-66.</p

Effect of Zeolitic Imidazolate Framework Topology on the Purification of Hydrogen from Coke Oven Gas

This work aims to shed light on the performance of zeolitic imidazolate frameworks for hydrogen purification from coke oven gases (COG). Using molecular simulation, we model COG as a mixture of six gases and study the effect of ZIF topology on the separation performance. To do this, we compare similar structures, e.g., ZIF-8 and ZIF-11, and focus on obtaining information that explains why they behave differently while being so similar. Simulation results show that the structure with the smallest pore size best separates hydrogen from carbon monoxide and nonpolar molecules. The adsorption of carbon dioxide is also strongly affected by the polarizability of the structure. However, the adsorption of the other components (methane, carbon monoxide, nitrogen, and oxygen) is strongly dependent on their pore size. We also provide molecular information on the effect of phase transition on hydrogen purification using ZIF-7 as an example, which drastically changes the pore volume of the structure when it changes phase. These findings will help to select high-performance ZIFs for adsorption- or screening-based hydrogen purification.</p

Governmental Context Determines Institutional Value: Independently Certified Performance and Failure in the Spanish Newspaper Industry

Many societies demand that independent professionals (e.g. auditors) certify the performance of firms. The value placed on such certification (i.e. the public perception of reliability/unreliability that may impact on an organization's success/failure) is not uniform, however, but contingent upon changing political contexts. This study presents and analyses data on the entire population of newspapers in Spain from 1966 to 1993, a time of peaceful transition from military dictatorship to capitalist democracy. Our results highlight the contingent nature of institutional life, demonstrating how changes in political contexts are associated with varying understandings of institutions. In particular, our findings support the prediction that, under a dictatorship, independently certified performance is not instrumental in organizational success or failure whereas, in a modern democracy, the certification process has a positive effect on the survival chances of firms.Publicad

Predictive Power of the "Trigger Tool" for the detection of adverse events in general surgery: a multicenter observational validation study

Background In spite of the global implementation of standardized surgical safety checklists and evidence-based practices, general surgery remains associated with a high residual risk of preventable perioperative complications and adverse events. This study was designed to validate the hypothesis that a new “Trigger Tool” represents a sensitive predictor of adverse events in general surgery. Methods An observational multicenter validation study was performed among 31 hospitals in Spain. The previously described “Trigger Tool” based on 40 specific triggers was applied to validate the predictive power of predicting adverse events in the perioperative care of surgical patients. A prediction model was used by means of a binary logistic regression analysis. Results The prevalence of adverse events among a total of 1,132 surgical cases included in this study was 31.53%. The “Trigger Tool” had a sensitivity and specificity of 86.27% and 79.55% respectively for predicting these adverse events. A total of 12 selected triggers of overall 40 triggers were identified for optimizing the predictive power of the “Trigger Tool”. Conclusions The “Trigger Tool” has a high predictive capacity for predicting adverse events in surgical procedures. We recommend a revision of the original 40 triggers to 12 selected triggers to optimize the predictive power of this tool, which will have to be validated in future studies

Estudio por simulación de la adsorción y difusión de gases con impacto ambiental usando Cu-BTC

[ES] Hemos llevado a cabo simulaciones moleculares para estudiar las posibles aplicaciones de la estructura metal-orgánica cristalina Cu-BTC en diversos procesos de interés ambiental: a) separación de CO2/CH4 procedentes de mezclas de biogás y gas natural; b) separación de sulfuro de hidrógeno del agua, y c) eliminación de tetracloruro de carbono del aire. Con este objetivo hemos estudiado las propiedades de adsorción y difusión de estos gases en la estructura, así como sus sitios preferentes de adsorción. De las mezclas CO2/CH4 hemos observado que la adsorción es siempre favorable al CO2 mientras el CH4 es el que presenta mayores coeficientes de difusión. Al estudiar la influencia del agua en los procesos de adsorción, concluimos que para la adsorción individual de H2S, ésta se ve drásticamente reducida por la presencia de agua, mientras que en mezclas (CCl4 en aire), la selectividad de la adsorción en presencia de agua, depende de la competición entre las moléculas adsorbidas en el sistema.Este trabajo ha sido financiado por el Ministerio Español de Ciencia e Innovación (CTQ2010-16077/BQU) y el “European Research Council” mediante una “ERC Starting Grant” (ERC-StG’11 RASPA-project). A. Martín-Calvo y J. J. Gutiérrez-Sevillano agradecen al Ministerio Español de Educación y al MEC (CTQ2007-63229) por sus becas predoctoralesPeer reviewe