The effect of pressure on open-framework silicates: elastic behaviour and crystal-fluid interaction

The elastic behaviour and the structural evolution of microporous materials compressed hydrostatically in a pressure-transmitting fluid are drastically affected by the potential crystal-fluid interaction, with a penetration of new molecules through the zeolitic cavities in response to applied pressure. In this manuscript, the principal mechanisms that govern the P-behaviour of zeolites with and without crystal-fluid interaction are described, on the basis of previous experimental findings and computational modelling studies. When no crystal-fluid interaction occurs, the effects of pressure are mainly accommodated by tilting of (quasi-rigid) tetrahedra around O atoms that behave as hinges. Tilting of tetrahedra is the dominant mechanism at low-mid P-regime, whereas distortion and compression of tetrahedra represent the mechanisms which usually dominate the mid-high P regime. One of the most common deformation mechanisms in zeolitic framework is the increase of channels ellipticity. The deformation mechanisms are dictated by the topological configuration of the tetrahedral framework; however, the compressibility of the cavities is controlled by the nature and bonding configuration of the ionic and molecular content, resulting in different unit-cell volume compressibility in isotypic structures. The experimental results pertaining to compression in "penetrating" fluids, and thus with crystal-fluid interaction, showed that not all the zeolites experience a P-induced intrusion of new monoatomic species or molecules from the P-transmitting fluids. For example, zeolites with well-stuffed channels at room conditions (e.g. natural zeolites) tend to hinder the penetration of new species through the zeolitic cavities. Several variables govern the sorption phenomena at high pressure, among those: the "free diameters" of the framework cavities, the chemical nature and the configuration of the extra-framework population, the partial pressure of the penetrating molecule in the fluid (if mixed with other non-penetrating molecules), the rate of P-increase, the surface/volume ratio of the crystallites under investigations and the temperature at which the experiment is conducted. An overview of the intrusion phenomena of monoatomic species (e.g. He, Ar, Kr), small (e.g. H2O, CO2) and complex molecules, along with the P-induced polymerization phenomena (e.g. C2H2, C2H4, C2H6O, C2H6O2, BNH6, electrolytic MgCl2*21H2O solution) is provided, with a discussion of potential technological and geological implications of these experimental findings

Preparation and Characterization of Clathrates in the Systems Ba – Ge, Ba – Ni – Ge, and Ba – Ni – Si: Preparation and Characterization of Clathrates in the Systems Ba – Ge, Ba – Ni – Ge, and Ba – Ni – Si

The main focus of this work is the preparation, chemical and structural characterization along with the investigation of physical properties of intermetallic clathrates. Starting from the history of clathrate research, classification of clathrate types, their structural properties and possible application areas are evaluated in chapter 2. The methodologies of sample preparation and materials characterization as well as quantum chemical calculations are discussed in chapter 3. The complete characterization of Ba8Ge433 ( is a Schottky-symbol standing for vacancies),12-14 which is a parent compound for the variety of ternary variants, is the subject of chapter 4. Ba8Ge433 is a high temperature phase,12 which was prepared for the first time as single phase bulk material in this work.15, 16 In this way, the intrinsic transport properties could be investigated without influence of grain boundary and impurity effects. The transport behavior is analyzed at low and high temperatures and referred to the former results. In addition, crystal structure and vacancy ordering in terms of the reaction conditions are discussed. Chemical bonding in Ba8Ge433 is investigated by topological analysis of the electron localizability indicator and the electron density. Chapter 5 deals with the preparation, phase analysis, crystal structure and physical properties of BaGe5, which constitutes a new clathrate type oP60.17, 18 So far, two clathrate types were known in the binary system Ba – Ge, namely the clathrate cP124 Ba6Ge25,19-21 and the clathrate-I Ba8Ge433. Originally, BaGe5 was detected by optical and scanning electron microscopy within the grains of Ba8Ge433.12 Once the preparation of phase-pure Ba8Ge433 was achieved, it became possible to make detailed investigations of its decomposition along with the formation of BaGe5. A detailed theoretical and experimental analysis on the relation between crystal structure and physical properties of BaGe5 is presented. In chapter 6, a thorough structural characterization and the physical properties of clathrates in the system Ba – Ni – Ge is presented based on the subtle relation between the crystal structure containing vacancies and the thermoelectric properties. During the investigations in this system, a large single crystal was grown by Nguyen et al. 22, 23 from the melt with the composition Ba8Ni3.5Ge42.10.4. A systematic reinvestigation of the phase relations in this system was performed and the influence of different Ni content to the crystal structure and physical properties is evaluated. The Si-based ternary clathrate with composition Ba8–δNixySi46–x–y is the subject of chapter 7. The phase relations and the homogeneity range are established. The crystal structure taking into account vacancies in the framework is discussed. Physical properties of bulk pieces are analyzed and the results are related to the sample composition. In addition, first-principles electronic structure calculations are carried out to assess variations in the electronic band structure, phase stability and chemical bonding.24 Chapter 8 reports on the intermetallic compound Ba3Si4,25, 26 which was encountered during the investigations on the Ba – Ni – Si phase diagram. The discussion covers issues related to preparation, crystal structure, phase diagram analysis, electrical and magnetic properties, NMR measurements, quantum mechanical calculations and oxidation to nanoporous silicon with gaseous HCl. Besides my contributions to the NoE CMA, I studied under the Priority Program 1178 of Deutsche Forschungsgemeinschaft “Experimental electron density as the key for understanding chemical interactions” with the project of “Charge distribution changes by external electric fields: investigations of bond selective redistributions of valence electron densities”. Chapter 9 deals with the preparation of chalcopyrites ZnSiP2 and CuAlS2 for experimental charge density analysis. Both phases show semiconducting properties and have non-centrosymmetric structures with high space group symmetry as needed to investigate the structural changes induced by external electric field. In this chapter, I describe the preparation and the crystal structure analyses of ZnSiP2 and CuAlS2 including issues related to the data collection as well as the results of NMR investigation

6,6’-Dimethoxygossypol: Molecular Structure, Crystal Polymorphism, and Solvate Formation.

6,6’-Dimethoxygossypol (DMG) is a natural product of the cotton variety Gossypium barbadense and a derivative of gossypol. Gossypol has been shown to form an abundant number of clathrates with a large variety of compounds. One of the primary reasons why gossypol can form clathrates has been because of its ability to from extensive hydrogen bonding networks due to its hydroxyl and aldehyde functional groups. Prior to this work, the only known solvate that DMG formed was with acetic acid. DMG has methoxy groups substituted at two hydroxyl positions, and consequently there is a decrease in its ability to form hydrogen bonds. Crystallization experiments were set up to see whether, like gossypol, DMG could form clathrates. The following results presented prove that DMG is capable of forming clathrates (S1 and S2) and two new polymorphs (P1 and P2) of DMG have been reported

6,6’-Dimethoxygossypol: Molecular Structure, Crystal Polymorphism, and Solvate Formation.

6,6’-Dimethoxygossypol (DMG) is a natural product of the cotton variety Gossypium barbadense and a derivative of gossypol. Gossypol has been shown to form an abundant number of clathrates with a large variety of compounds. One of the primary reasons why gossypol can form clathrates has been because of its ability to from extensive hydrogen bonding networks due to its hydroxyl and aldehyde functional groups. Prior to this work, the only known solvate that DMG formed was with acetic acid. DMG has methoxy groups substituted at two hydroxyl positions, and consequently there is a decrease in its ability to form hydrogen bonds. Crystallization experiments were set up to see whether, like gossypol, DMG could form clathrates. The following results presented prove that DMG is capable of forming clathrates (S1 and S2) and two new polymorphs (P1 and P2) of DMG have been reported

Investigation of Formation and Dissociation Mechanisms of Pure and Mixed CO2 Hydrates in the Presence of Thermodynamic and Kinetic Promoters using Molecular Dynamics Simulation

CO2 hydrates as non-flammable solid compounds would contribute to many industrial processes. Toward developing substantial applications of CO2 hydrates, molecular dynamics (MD) simulations can aid to understand their characteristics and mechanisms involved so that complete the laboratory experimental results at a macroscopic level. In this regard, understanding the promotion mechanisms of promoters on the hydrate formation and dissociation at the molecular level would assist in either establishing feasible processes or finding more efficient promoters

Determination of mixed hydrate thermodynamics for reservoir modeling

Natural gas hydrates are likely to contain more carbon than in all other fossil fuel reserves combined worldwide. Most of the natural gas hydrate deposits contain CH4 along with other hydrocarbon gases like C2H 6, C3H8 and non-hydrocarbon gases like CO 2 and H2S. Thus, if CH4 stored in natural gas hydrates can be recovered, the hydrates would potentially become a clean energy resource for the next 10,000 years. The production of CH4 from natural gas hydrate reservoirs has been predicted by reservoir simulators that implement phase equilibria data to predict various production scenarios. Therefore, it is very important to predict accurately phase equilibria of mixed hydrates. In this work an empirical correlation of dissociation pressure with respect to temperature and gas phase composition for CH4-C 2H6 mixed hydrate system is developed by fitting to available experimental data. It is a simple method with limited accuracy. Statistical thermodynamics approach developed by van der Waals and Platteeuw in 1959 provides best approximation to predict the phase equilibrium data. They assumed that there are no lattice distortions due to the guest molecules, hence constant reference parameters are used for different guest molecules. Later, Hwang et al. by his molecular dynamics found that there are lattice distortions due to the guest molecules and Holder et al. proposed that the reference chemical potential difference Dm0w and reference enthalpy difference Dh0w varies with the guest molecule. In this work, a correlation of Dm0w and Dh0w with respect to guest molecular size is developed to estimate the values of Dm0w and Dh0w . The cell potential method developed by Anderson et al. is modified for variable reference parameters. The method is validated by reproducing the phase equilibria of simple hydrates and the structural transitions that are known to occur. Three-dimensional phase equilibria and structural transitions occurring in the mixed hydrates like CH4-C2H6, CH4-N2 and N2-CO2 are predicted accurately without fitting to experimental data. The phase equilibria of CH 4-CO2 and CH4-N2-CO 2 hydrates are predicted to assess the production of CH4 from the reservoirs by replacing CH4 in the hydrate by pure CO 2 and N2+CO2 mixture which serves dual purpose of CH4 recovery and CO2 sequestration

Formation and decomposition processes of CO2 hydrates at conditions relevant to Mars

Von einem thermodynamischen Gesichtspunkt aus gibt es kein Argument gegen die Existenz von CO2 Hydraten im Marsregolith nahe der Oberfläche. Es wurde bereits postuliert, dass CO2 Hydrate in den Eisschichten des Nordens sowie den polaren Südkappen existieren könnten. Auf dieser Basis wurden Vorschläge, die Zerzetzung von CO2 Hydrat betreffend, in Verbindung mit morphologischen Eigenschaften, vorgebracht. Ein weiterer Ideenbereich behandelt den Einfluss den die Zersetung von CO2 Hydraten auf die Umwelt, hinsichtlich einer möglichen Klimaveränderung (Greenhouse-Effekt) oder der Modifizierung von Isotopenverhältnissen in der Atmosphäre, haben könnte. Heutzutage und wahrscheinlich auch in der Vergangenheit bestimm(t)en Druck und Temperatur, dass die wahrscheinlichste Bildungsreaktion für CO2 Hydrate zwischen gasartigem CO2 und Wassereis stattfindet. Beide Bestandteile sind an der Oberfläche des Mars verfügbar. Kürzlich wurde zudem entdeckt, dass auch Wasser (H2O) häufig im Marsregolith vorkommt. Jedoch führten bislang alle Diskussionen bezüglich dieser Möglichkeit zu keinem Ergebnis, da es an elementaren Kenntnissen der Bildungs- und Zerzetzungskinetik dieser besonderen Gashydrate mangelt. Um eine physikochemische Grundlage für diese Ideen zu schaffen, wurde eine Reihe von CO2 Hydrat Bildungs- und Zerzetzungsexperimenten sowohl unter Marsoberflächenbedingungen als auch unter unterirdischen Marsbedingungen mit p-V-T-Methoden und in-situ Neutronbeugung an der ILL Grenoble, durchgeführt. Die Experimente haben gezeigt, dass die Bildungsdauer der CO2 Hydrate direkt mit der zugänglichen Eiskornoberfläche, den herrschenden Temperaturen und dem CO2 Druck zusammenhängt. Unter p-T Bedingungen die an der Marsoberfläche nahe den Polen herrschen ist CO2 Hydrat thermodynamisch stabil. Trotz dieser Tatsache zeigen die Ergebnisse, dass bei sehr niedrigen Temperaturen die langsame Kinetik sowie Schwierigkeiten die Nukleation der Hydrate betreffend, jede signifikante Bildung von Klathraten verhindern. Dennoch bleibt eine gute Chance CO2 Hydrate tiefer im Regolith aufgrund vorliegender Druck-Versiegelung durch überliegende Schichten (z.B Wassereis) zu finden. Höhere Temperaturen und höherer Druck schaffen günstigere Bedingungen für die Bildung von CO2 Hydraten. Zusätzlich stellen Klimaschwankungen ein denkbares Szenario für die Hydrat-Zersetzung und mögliche Bildungszyklen dar, sofern passende Stabilitätsbedingungen geschaffen werden können. Durch die Zersetzung von Gashydraten können Gase freigesetzt werden, die im Stande sind die Isotopenverhältnisse der Atmosphäre zu verändern. Die Freisetzung größerer Gasmengen könnte eine potentielle Ursache wärmerer Klimaepisoden darstellen. Die experimentellen Zersetzungsvorgänge, in einem Temperatubereich zwischen etwa 240 und 273 Kelvin begründen einen Prozess, welcher auch als "Selbsterhaltung" bezeichnet wird. Dieser ist in der Lage, CO2 Hydrat über einen geologisch bedeutsamen Zeitraum stabil zu halten. Der Prozess der Selbsterhaltung, der besonders die Mikrostruktur von Hydraten betrifft, ist sehr kompliziert und steht in Verbindung mit Veränderungen auf der Oberfläche der sich zersetzenden Hydrate. Sehr kleine Gashydratkristalle (Durchmesser bis zu 20μm) erzeugen bei ihrer Zersetzung eine Schicht, die durch das Verheilen oder Ausheilen von Defekten im Eisgitter sowie Kornvergröberungsprozessen zu einer Ausbreitung der Gasmoleküle und somit zu einer drastischen Verlangsamung des Zersetzungsprozesses führt. Unterhalb dieses Temperaturregimes findet der "Selbsterhaltungsprozess" auch in einem sehr schmalen P-T-Bereich statt. Die Versiegelung ist hier weniger wirksam und wird durch die Mikrostruktur des Eisfilms geregelt. Die Zerstörung dieses mechanisch oder durch das Erreichen des Schmelzpunktes von Eis erlangten metastabilen Zustandes kann zu einer sehr schnellen Gasfreisetzung aufgrund der Zersetzung von Gashydraten führen. Die plötzliche Zunahme des Porendrucks im Regolith kann die Bildung von großen geomorphologischen Phenomenen, wie z.B. chaotic terrains bewirken und so unter Druck ste hende Fluide an die Oberfläche lassen

Molecular modeling of hydrate-clathrates via ab initio, cell potential, and dynamic methods

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2005.Includes bibliographical references.High level ab initio quantum mechanical calculations were used to determine the intermolecular potential energy surface between argon and water, corrected for many- body interactions, to predict monovariant and invariant phase equilibria for the argon hydrate and mixed methane-argon hydrate systems. A consistent set of reference parameters for the van der Waals and Platteeuw model, ... and ..., were developed for Structure II hydrates and are not dependent on any fitted parameters. Our previous methane-water ab initio energy surface has been recast onto a site-site potential model that predicts guest occupancy experiments with improved accuracy compared to previous studies. This methane-water potential is verified via ab initio many-body calculations and thus should be generally applicable to dense methane-water systems. New reference parameters, ... and ..., for Structure I hydrates using the van der Waals and Platteeuw model were also determined. Equilibrium predictions with an average absolute deviation of 3.4% for the mixed hydrate of argon and methane were made. These accurate predictions of the mixed hydrate system provide an independent test of the accuracy of the intermolecular potentials.(cont.) Finally, for the mixed argon-methane hydrate, conditions for structural changes from the Structure I hydrate of methane to the Structure II hydrate of argon were predicted and await experimental confirmation. We present the application of a mathematical method reported earlier' by which the van der Waals-Platteeuw statistical mechanical model with the Lennard-Jones and Devonshire approximation can be posed as an integral equation with the unknown function being the intermolecular potential between the guest molecules and the host molecules. This method allows us to solve for the potential directly for hydrates for which the Langmuir constants are computed, either from experimental data or from ab initio data. Given the assumptions made in the van der Waals-Platteeuw model with the spherical-cell approximation, there are an infinite number of solutions; however, the only solution without cusps is a unique central-well solution in which the potential is at a finite minimum at the center to the cage.(cont.) From this central-well solution, we have found the potential well depths and volumes of negative energy for sixteen single-component hydrate systems: ethane (C₂H₆), cyclopropane (C₃H₆), methane (CH₄), argon (Ar), and chlorodifluoromethane (R-22) in structure I; and ethane (C₂H₆), cyclopropane (C₃H₆), propane (C₃H₈), isobutane (C₄H₁₀), methane (CH₄), argon (Ar), trichlorofluoromethane (R-1 1), dichlorodifluoromethane (R-12), bromotrifluoromethane (R-1 3B 1), chloroform (CHC1₃), and 1,1,1,2-Tetrafluoroethane (R-134a) in structure II. This method and the calculated cell potentials were validated by predicting existing mixed hydrate phase equilibrium data without any fitting parameters and calculating mixture phase diagrams for methane, ethane, isobutane, and cyclopropane mixtures. Several structural transitions that have been determined experimentally as well as some structural transitions that have not been examined experimentally were also predicted. In the methane-cyclopropane hydrate system, a structural transition from structure I to structure II and back to structure I is predicted to occur outside of the known structure II range for the cyclopropane hydrate.(cont.) Quintuple (Lw-SI-SII-Lho-V) points have been predicted for the ethane-propane-water (277.3 K, 12.28 bar, and Xeth,waterfree = 0.676) and ethane-isobutane-water (274.7 K, 7.18 bar, and Xeth,waterfree = 0.81) systems. A two-fold mechanism for hydrate inhibition has been proposed and tested using molecular dynamic simulations for PEO, PVP, PVCap, and VIMA. This mechanism hypothesizes that (1) as potential guest molecules become coordinated by water, form nuclei, and begin to grow, nearby inhibitor molecules disrupt the organization of the forming clathrate and (2) inhibitor molecules bind to the surface of the hydrate crystal precursor and retards further growth along the bound growth plane resulting in a modified planar morphology. This mechanism is supported by the results of our molecular dynamic simulations for the four inhibitor molecules studied. PVCap and VIMA, the more effective inhibitors, shows strong interactions with the liquid water phase under hydrate forming conditions, while PVP and PEO appear relatively neutral to the surrounding water.by Brian Anderson.Ph.D

Experimental and theoretical investigation of strong acid hydrates

188 p.En la presente tesis se estudian mediante métodos experimentales (espectroscopia Raman, espectroscopia de impedancia electroquímica y difracción de rayos X) y teóricos (cálculos de primeros principios) la formación, estructura y conductividad protónica de clatratos de agua encapsulando ácidos. En particular,presentamos un estudio teórico mediante cálculos ab initio basados en la teoría del funcional de la densidad del hidrato de HPF6 en la estructura SVII y de la estabilidad de moléculas de H3PO4, H2PO2F2 yH3PO3F como impurezas sustitucionales en dicha estructura. Nuestros cálculos confirman que la estructura es más estable cuando dichos dopantes sustituyen a aniones de PF6-, al menos a bajas concentraciones. Este resultado es consistente con resultados experimentales de la literatura y confirma la importancia del papel de las impurezas en la estabilidad y las propiedades del hidrato de HPF6. Por otro lado, nuestros resultados experimentales se centran en los hidratos mixtos de tetrahidrofurano (THF) yHClO4 para distintas fracciones HClO4/THF. Para dichos sistemas hemos realizado medidas de difracción de rayos, espectroscopia e imagen Raman, y medidas de impedancia electroquímica. De esta manera hemos podido correlacionar los cambios estructurales y de conductividad observados con el número de hidratación y la fracción ácida. A partir de estos datos se han generado modelos para entender la conductividad que ponen de relieve la importancia de la estructura y organización del sistema en la escala meso/macroscópica

CO2–Hydroquinone Clathrate: Synthesis, Purification, Characterization and Crystal Structure

Organic clathrate compounds, particularly those formed between hydroquinone (HQ) and gases, are supramolecular entities recently highlighted as promising alternatives for applications such as gas storage and separation processes. This study provides new insights into CO2–HQ clathrate, which is a key structure in some of the proposed future applications of these compounds. We present a novel synthesis and purification of CO2–HQ clathrate monocrystals. Clathrate crystals obtained from a single synthesis and native HQ are characterized and compared using Raman/Fourier transform infrared/NMR spectroscopies, optical microscopy, and thermogravimetric analysis coupled to mass spectrometry. The molecular structure of the clathrate has been resolved by X-ray diffraction analysis, and detailed crystallographic information is presented for the first time