Synthesis, structure, and stability of metal-organic frameworks

Metal-organic frameworks (MOFs) obtained much attention because of their unusual structures and properties as well as their potential applications. This dissertation research was focused on (1) the effects of synthesis conditions on the structures of MOFs, (2) the thermal stability of MOFs, (3) pressure-induced amorphization, and (4) the effect of high-valent ions on the structure of a MOF. This research demonstrated that the crystal structure of MOF-5 could be controlled by drying solvents. If the vacuum solvent is dimethylformamide (DMF), the crystal structure of MOF-5 is tetragonal. In contrast, if the DMF is displaced by CH2Cl2 before the vacuum, the obtained MOF-5 occupies a cubic structure. Furthermore, it was found that the tetragonal MOF-5 exhibited a mediate surface area (300-1000 m2/g). The surface area of tetragonal MOF-5 is also dependent on Zn(NO3)2/H2BDC (H2BDC: terephthalic acid) molar ratios used for its synthesis. The optimum ratio is 1.38, at which synthesized tetragonal MOF-5 exhibits the highest crystallinity and surface area (1297 m2/g). The thermal stability and decomposition of MOF-5 were systematically investigated. The thermal decomposition of cubic and tetragonal MOF-5s resulted in the same products: CO2, benzene, amorphous carbon, and crystal ZnO. The thermal decomposition is due to breaking carboxylic bridges between benzene rings and Zn4O clusters. Identifying structural relationships between crystalline and noncrystalline states is of fundamental interest in materials research. Currently, amorphization of solid materials at ambient temperature requires an ultra-high pressure (several GPa). However, this research demonstrated that MOF-5 and IRMOF-8 can be irreversibly amorphized at ambient temperature by employing a low compressing pressure of 3.5 MPa, which is 100 times lower than that required for amorphization of other solids. Furthermore, the pressure-induced amorphization (PIA) of MOFs is strongly dependent on the changeability of bond angles. If the geometric structure of a MOF can allow bond angles to be changed without breaking bonds, it can easily be amorphized by compression. This can explain why MOF-5 and IRMOF-8 can easily be amorphized via compression than Cu-BTC. It is generally recognized that zeolitic imidazolate frameworks (ZIFs) occupy much higher stability than other types of MOFs. The representative of ZIFs is Zn(2-methylimidazole)2 (ZIF-8) exhibiting high-decomposition temperature and high chemical resistance to various solvents. However, so far, it is still unknown whether the high stability of ZIF-8 can be challenged by ions, which is important for its modification by doping ions. In this research, we performed aqueous salt solution treatment on ZIF-8, and the results showed that anions (Cl¯ and NO3¯) in a solution exhibited no effect on the crystal structure of ZIF-8. However, the effect of cations (in a solution) on structure of ZIF-8 strongly depends on the cation valences. The univalent metal cations showed no effect on the structure of ZIF-8, whereas the bivalent or higher-valent metal cations caused the collapse of ZIF-8 crystal structure. Therefore, structure stability of ZIF-8 is considered when it is subjected to the application, in which high-valent metal cations are involved.

Soft-computing based visual control for unmanned vehicles

El objetivo principal de esta Tesis es extender la utilización del “Soft- Computing” para el control de vehículos sin piloto utilizando visión. Este trabajo va más allá de los típicos sistemas de control utilizados en entornos altamente controlados, demonstrando la fuerza y versatilidad de la lógica difusa (Fuzzy Logic) para controlar vehículos aéreos y terrestres en un abanico de applicaciones diferentes. Para esta Tesis se ha realizado un gran número de pruebas reales en las cuales los controladores difusos han manejado una plataforma visual “pan-and-tilt”, un helicoptero, un coche comercial y hasta dos tipos de quadrirotores. El uso del método de optimización “Cross-Entropy” ha sido utilizado para mejorar el comportamiento de algunos de los controladores borrosos. Todos los controladores difusos presentados en ésta Tesis han sido implementados utilizando un código desarrollado por el candidato para tal efecto, llamado MOFS (Miguel Olivares’ Fuzzy Software). Diferentes algoritmos visuales han sido utilizados para adquirir la informaci´on visual del entorno, “Cmashift”, descomposición de la homografía y detección de marcas de realidad aumentada, entre otros. Dicha información visual ha sido utilizada como entrada de los controladores difusos para comandar los vehículos en las diferentes applicaciones autonomas. El volante de un vehículo comercial ha sido controlado para realizar pruebas de conducción autónoma en condiciones de tráfico similares a las de una ciudad. El sistema ha llegado a completar con éxito pruebas de más de 6 km sin ninguna interacción humana, mediante el seguimiento de una línea pintada en el suelo. El limitado campo visual del sistema no ha sido impedimento para alcanzar velocidades de hasta 48 km/h y ser guiado autonomamente en curvas de radio reducido. Objetos estáticos y móviles han sido seguidos desde un helicoptero no tripulado, mediante el control de una plataforma visual “pan-and-tilt”. ´Éste mismo helicoptero ha sido controlado completamente para su aterrizaje autonomo, mediante el control del movimiento lateral (roll), horizontal (pitch) y de altitud. El seguimiento de objetos volantes ha sido resulto mediante el control horizontal (pitch) y de orientación (heading) de un quadrirotor. Para tareas de evitación de obstáculos se ha implementado un controlador difuso para el manejo de la orientación (heading) de un quadrirotor. En el campo de la optimización de controladores se ha aportado al estado del arte una extensión del uso del método “Cross-Entropy”. Está Tesis presenta una novedosa implementación de dicho método para la optimización de las ganancias, la posición y medida de los conjuntos de las funciones de pertenecia y el peso de las reglas para mejorar el comportamiento de un controlador difuso. Dichos procesos de optimización se han realizado utilizando “ROS” y “Matlab Simulink” para obtener mejores resultados para la evitación de colisiones con vehículos aéreos no tripulados. Ésta Tesis demuestra que los controladores implementados con lógica difusa son altamente capaces de controlador sistemas sin tener en cuenta el modelo del vehículo a controlador en entornos altamente perturbables con un sensor de bajo coste como es una cámara. El ruido presentes causado por los cambios de iluminación en la adquisición de imágenes y la alta incertidumbre en la detección visual han sido manejados satisfactoriamente por ésta técnica de de “Soft-Computing” para distintas aplicaciones tanto con vehículos aéreos como terrestres.

Stabilization of APIs and nutraceuticals through cocrystallization and molecular confinement

Cocrystallization of the molecule of interest could be a smart and dainty way to tune solubility properties of solid phases leaving the molecule chemically unchanged, hence it is widely investigated by companies and by solid state scientists. Despite of this extremely high interest towards cocrystallization no particular emphasis has been paid to using it as a means to stabilize liquid molecules. In this work we define a benchmark of relevant molecules for human health that have been combined with suitable partners according to crystal engineering methods in order to obtain cocrystals. Solubility properties in different solvents of cocrystals new solid phases have been tested and compared to the properties of the drugs. A further approach to deal with volatile compounds is molecular confinement inside molecular scaffold. Nowadays metal organic frameworks (MOFs) are studied in many fields ranging from catalysis to trapping or storage of gases, such as hydrogen, methane, CO2 thanks to their extremely high porosity. Our goal is to confine liquid guests of biological relevance inside MOF pores, monitoring via X-ray diffraction, spectroscopy and thermal analysis the stabilization of the molecule of interest inside the cavities.

Polímeros de coordenação à base de cobalto(II) e N,N'-bis(4-piridil)-1,4,5,8-naftaleno diimida como ligante e suas propriedade estruturais, espectroscópicas e fotoelétricas

Polímeros de coordenação têm atraído a atenção de pesquisadores na última década por conta de sua incrível versatilidade e virtualmente infinito número de possibilidades de combinação de ligantes orgânicos e centros metálicos. Estes compostos normalmente herdam as características magnéticas, eletrônicas e espectroscópicas de seus componentes base. Entretanto, apesar do crescente número de trabalhos na área, ainda são raros os polímeros de coordenação que apresentem condutividade elétrica. Para este fim, utilizou-se a N,N\'-bis(4-piridil)-1,4,5,8-naftaleno diimida, ou NDI-py, que pertence a uma classe de compostos rígidos, planares, quimicamente e termicamente estáveis e que já foram extensamente estudados por suas propriedades fotoeletroquímicas e semicondução do tipo n. O primeiro polímero de coordenação sintetizado, MOF-CoNDI-py-1, indicou ser um polímero linear, de estrutura 1D. O segundo, MOF-CoNDI-py-2, que conta com ácido tereftálico como ligante suporte, é um sólido cristalino com cela unitária monoclínica pertencente ao grupo espacial C2/c, determinado por difração de raios-X de monocristal. A rede apresenta um arranjo trinuclear de íons Co(II) alto spin com coordenados em uma geometria de octaedro distorcido, enquanto os ligantes NDI-py se encontram em um arranjo paralelo na estrutura, em distâncias apropriadas para transferência eletrônica. Com o auxílio de cálculo teóricos a nível de DFT, foi realizado um estudo aprofundado dos espectros eletrônicos e vibracionais, com atribuição das transições observadas, tanto para o MOF-CoNDI-py-2 quanto para o ligante NDI-py livre. A rede de coordenação absorve em toda a região do espectro eletrônico analisada, de 200 nm a 2500 nm, além de apresentar luminescência com característica do ligante. Dispositivos eletrônicos fabricados com um cristal do MOF-CoNDI-py-2 revelaram condutividades da ordem de 7,9 10-3 S cm -1, a maior já observada para um MOF. Além de elevada, a condutividade elétrica dos cristais demonstrou-se altamente anisotrópica, sendo significativamente menos condutor em algumas direções. Os perfis de corrente versus voltagem foram analisados em termos de mecanismos de condutividade, sendo melhores descritos por um mecanismo limitado pelo eletrodo to tipo Space-Charge Limited Current, concordando com a proposta de condutividade através dos planos de NDI-py na rede. A condutividade dos cristais também é fortemente dependente de luz, apresentando fotocondução quando irradiado por um laser vermelho, de 632 nm, enquanto apresenta um comportamento fotorresistivo frente a uma fonte de luz branca. Estes resultados, combinados, trazem um MOF em uma estrutura incomum e com elevada condutividade elétrica, modulada por luz, em medidas diretas de corrente. Não existem exemplos conhecidos de MOFs na literatura com estas características.

Material demands for storage technologies in a hydrogen economy

A hydrogen economy is needed, in order to resolve current environmental and energy-related problems. For the introduction of hydrogen as an important energy vector, sophisticated materials are required. This paper provides a brief overview of the subject, with a focus on hydrogen storage technologies for mobile applications. The unique properties of hydrogen are addressed, from which its advantages and challenges can be derived. Different hydrogen storage technologies are described and evaluated, including compression, liquefaction, and metal hydrides, as well as porous materials. This latter class of materials is outlined in more detail, explaining the physisorption interaction which leads to the adsorption of hydrogen molecules and discussing the material characteristics which are required for hydrogen storage application. Finally, a short survey of different porous materials is given which are currently investigated for hydrogen storage, including zeolites, metal organic frameworks (MOFs), covalent organic frameworks (COFs), porous polymers, aerogels, boron nitride materials, and activated carbon materials.

Adsorbent density impact on gas storage capacities

In the literature, different approaches, terminologies, concepts and equations are used for calculating gas storage capacities. Very often, these approaches are not well defined, used and/or determined, giving rise to significant misconceptions. Even more, some of these approaches, very much associated with the type of adsorbent material used (e.g., porous carbons or new materials such as COFs and MOFs), impede a suitable comparison of their performances for gas storage applications. We review and present the set of equations used to assess the total storage capacity for which, contrarily to the absolute adsorption assessment, all its experimental variables can be determined experimentally without assumptions, ensuring the comparison of different porous storage materials for practical application. These material-based total storage capacities are calculated by taking into account the excess adsorption, the bulk density (ρbulk) and the true density (ρtrue) of the adsorbent. The impact of the material densities on the results are investigated for an exemplary hydrogen isotherm obtained at room temperature and up to 20 MPa. It turns out that the total storage capacity on a volumetric basis, which increases with both, ρbulk and ρtrue, is the most appropriate tool for comparing the performance of storage materials. However, the use of the total storage capacities on a gravimetric basis cannot be recommended, because low material bulk densities could lead to unrealistically high gravimetric values.

Coordination polymers, metal–organic frameworks and the need for terminology guidelines

Coordination polymers (CPs) and metal–organic frameworks (MOFs) are among the most prolific research areas of inorganic chemistry and crystal engineering in the last 15 years, and yet it still seems that consensus is lacking about what they really are, or are not.

Beyond the H2/CO2 upper bound: one-step crystallization and separation of nano-sized ZIF-11 by centrifugation and its application in mixed matrix membranes

The synthesis of nano-sized ZIF-11 with an average size of 36 ± 6 nm is reported. This material has been named nano-zeolitic imidazolate framework-11 (nZIF-11). It has the same chemical composition and thermal stability and analogous H2 and CO2 adsorption properties to the conventional microcrystalline ZIF-11 (i.e. 1.9 ± 0.9 μm). nZIF-11 has been obtained following the centrifugation route, typically used for solid separation, as a fast new technique (pioneering for MOFs) for obtaining nanomaterials where the temperature, time and rotation speed can easily be controlled. Compared to the traditional synthesis consisting of stirring + separation, the reaction time was lowered from several hours to a few minutes when using this centrifugation synthesis technique. Employing the same reaction time (2, 5 or 10 min), micro-sized ZIF-11 was obtained using the traditional synthesis while nano-scale ZIF-11 was achieved only by using centrifugation synthesis. The small particle size obtained for nZIF-11 allowed the use of the wet MOF sample as a colloidal suspension stable in chloroform. This helped to prepare mixed matrix membranes (MMMs) by direct addition of the membrane polymer (polyimide Matrimid®) to the colloidal suspension, avoiding particle agglomeration resulting from drying. The MMMs were tested for H2/CO2 separation, improving the pure polymer membrane performance, with permeation values of 95.9 Barrer of H2 and a H2/CO2 separation selectivity of 4.4 at 35 °C. When measured at 200 °C, these values increased to 535 Barrer and 9.1.

Retos actuales para la captura y almacenamiento de CO2

Las grandes emisiones de CO2 procedentes de la combustión de combustibles fósiles están provocando un calentamiento global en nuestro planeta. Estos problemas medioambientales están obligando a los diferentes gobiernos a buscar soluciones que permitan reducir esas emisiones y mitigar sus efectos adversos. Una de las soluciones más prometedoras consiste en la captura selectiva de CO2 en efluentes industriales mediante el uso de materiales adsorbentes porosos (zeolitas, carbón activado y materiales híbridos MOFs) que combinen una elevada capacidad de adsorción y una adecuada selectividad a CO2 frente al resto de gases del proceso industrial, además de una adecuada regeneración.

Terminology of metal–organic frameworks and coordination polymers (IUPAC Recommendations 2013)

A set of terms, definitions, and recommendations is provided for use in the classification of coordination polymers, networks, and metal–organic frameworks (MOFs). A hierarchical terminology is recommended in which the most general term is coordination polymer. Coordination networks are a subset of coordination polymers and MOFs a further subset of coordination networks. One of the criteria an MOF needs to fulfill is that it contains potential voids, but no physical measurements of porosity or other properties are demanded per se. The use of topology and topology descriptors to enhance the description of crystal structures of MOFs and 3D-coordination polymers is furthermore strongly recommended.

Síntese e caracterização de materiais mesoporosos para a captura de CO2: influência do óxido de níquel

Several materials are currently under study for the CO2 capture process, like the metal oxides and mixed metal oxides, zeolites, carbonaceous materials, metal-organic frameworks (MOF's) organosilica and modified silica surfaces. In this work, evaluated the adsorption capacity of CO2 in mesoporous materials of different structures, such as MCM-48 and SBA- 15 without impregnating and impregnated with nickel in the proportions 5 %, 10 % and 20 % (m/m), known as 5Ni-MCM-48, 10Ni-MCM-48, 20Ni-MCM-48 and 5Ni-SBA-15, 10NiSBA-15, 20Ni-SBA-15. The materials were characterized by means of X-ray diffraction (XRD), thermal analysis (TG and DTG), Fourier transform infrared spectroscopy (FT-IR), N2 adsorption and desorption (BET) and scanning electron microscopy (SEM) with EDS. The adsorption process was performed varying the pressure of 100 - 4000 kPa and keeping the temperature constant and equal to 298 K. At a pressure of 100 kPa, higher concentrations of adsorption occurred for the materials 5Ni-MCM-48 (0.795 mmol g-1 ) and SBA-15 (0.914 mmol g-1 ) is not impregnated, and at a pressure of 4000 kPa for MCM-48 materials (14.89 mmol g-1) and SBA-15 (9.97 mmol g-1) not impregnated. The results showed that the adsorption capacity varies positively with the specific area, however, has a direct dependency on the type and geometry of the porous structure of channels. The data were fitted using the Langmuir and Freundlich models and were evaluated thermodynamic parameters Gibbs free energy and entropy of the adsorption system

Síntese, estrutura e propriedades de polímeros de coordenação à base de íons lantanídeos e ácidos benzenodicarboxílicos

Metal Organic Frameworks (MOFs) are hybrids materials, often crystalline, consisting of metal or metal clusters, connected by polytopic organic ligands repetitively, leading to structures, usually porous. In this work, MOFs based on lanthanide ions (La3+ and Gd3+) and dicarboxylate type of ligands (isophthalic and terephthalic acids), were synthesized by hydrothermal, solvothermal and hydro(solvo)thermal methods. The effects of the synthetic route as well as the type of heating, conventional or by microwave, on the structure and properties of MOFs were studied. The powder samples obtained were characterized by X-ray diffraction, infrared spectroscopy, thermal analysis and scanning electron microscopy. The results suggest that the addition of an organic or inorganic base is needed to promote the deprotonation of the ligand, since in the samples prepared by the hydrothermal method, without the use of a base, no formation of the metalorganic framework was observed. On the other hand, the presence of DMF as solvent or cosolvent, afforded the deprotonation of the ligand with the consequent formation of MOFs. At least two different crystalline structures were identified for the samples prepared with terephthalic acid. These samples are isostructural with those reported for phases Eu(1,3-BDC)DMF, Eu2(1,4-BDC)3 (DMF)2 and Tb(1,4-BDC)H2O. The presence of water in the reaction medium in the hydro(solvo)thermal method, provoked the growth of the structure different from that observed in the absence of water. This can be explained by the difference in the coordination mode of water and DMF to lanthanide ions. Although not identified by XRD, the samples prepared with isophthalic acid, also present metalorganic structures, which was confirmed by the presence of the characteristic displacement of the carbonyl group band in their infrared spectra, compared to the spectrum of the pure ligand. This shift was also observed in the samples prepared with terephthalic acid. Thermal analisys shows that the metal organic frameworks do not collapse occurs at a temperature below 430°C.The analysis of scanning electron microscopy suggests that the morphology of powders is highly dependent on the type of heating used, conventional or by microwave.

Development of Metal–Organic Frameworks for Catalysis : Designing Functional and Porous Crystals

Metal–organic frameworks, or MOFs, have emerged as a new class of porous materials made by linking metal and organic units. The easy preparation, structural and functional tunability, ultrahigh porosity, and enormous surface areas of MOFs have led to them becoming one of the fastest growing fields in chemistry. MOFs have potential applications in numerous areas such as clean energy, adsorption and separation processes, biomedicine, and sensing. One of the most promising areas of research with MOFs is heterogeneous catalysis. This thesis describes the design and synthesis of new, carboxylate-based MOFs for use as catalysts. These materials have been characterized using diffraction, spectroscopy, adsorption, and imaging techniques. The thesis has focused on preparing highly-stable MOFs for catalysis, using post-synthetic methods to modify the properties of these crystals, and applying a combination of characterization techniques to probe these complex materials. In the first part of this thesis, several new vanadium MOFs have been presented. The synthesis of MIL-88B(V), MIL-101(V), and MIL-47 were studied using ex situ techniques to gain insight into the synthesis–structure relationships. The properties of these materials have also been studied. In the second part, the use of MOFs as supports for metallic nanoparticles has been investigated. These materials, Pd@MIL-101–NH2(Cr) and Pd@MIL-88B–NH2(Cr), were used as catalysts for Suzuki–Miyaura and oxidation reactions, respectively. The effect of the base on the catalytic activity, crystallinity, porosity, and palladium distribution of Pd@MIL-101–NH2(Cr) was studied. In the final part, the introduction of transition-metal complexes into MOFs through different synthesis routes has been described. A ruthenium complex was grafted onto an aluminium MOF, MOF-253, and an iridium metallolinker was introduced into a zirconium MOF, UiO-68–2CH3. These materials were used as catalysts for alcohol oxidation and allylic alcohol isomerization, respectively.

Catalytic Processes Mediated by Metal−Organic Frameworks : Reactivity and Mechanistic Studies

The present thesis describes the development of heterogeneous catalytic methodologies using metal−organic frameworks (MOFs) as porous matrices for supporting transition metal catalysts. A wide spectrum of chemical reactions is covered. Following the introductory section (Chapter 1), the results are divided between one descriptive part (Chapter 2) and four experimental parts (Chapters 3–6). Chapter 2 provides a detailed account of MOFs and their role in heterogeneous catalysis. Specific synthesis methods and characterization techniques that may be unfamiliar to organic chemists are illustrated based on examples from this work. Pd-catalyzed heterogeneous C−C coupling and C−H functionalization reactions are studied in Chapter 3, with focus on their practical utility. A vast functional group tolerance is reported, allowing access to substrates of relevance for the pharmaceutical industry. Issues concerning the recyclability of MOF-supported catalysts, leaching and operation under continuous flow are discussed in detail. The following chapter explores puzzling questions regarding the nature of the catalytically active species and the pathways of deactivation for Pd@MOF catalysts. These questions are addressed through detailed mechanistic investigations which include in situ XRD and XAS data acquisition. For this purpose a custom reaction cell is also described in Chapter 4. The scope of Pd@MOF-catalyzed reactions is expanded in Chapter 5. A strategy for boosting the thermal and chemical robustness of MOF crystals is presented. Pd@MOF catalysts are coated with a protecting SiO2 layer, which improves their mechanical properties without impeding diffusion. The resulting nanocomposite is better suited to withstand the harsh conditions of aerobic oxidation reactions. In this chapter, the influence of the nanoparticles’ geometry over the catalyst’s selectivity is also investigated. While Chapters 3–5 dealt with Pd-catalyzed processes, Chapter 6 introduces hybrid materials based on first-row transition metals. Their reactivity is explored towards light-driven water splitting. The heterogenization process leads to stabilized active sites, facilitating the spectroscopic probing of intermediates in the catalytic cycle.

Adsorption Calorimetry for Energy Conversion Technologies: Applications in Organic Photovoltaics, Catalysis, and Atomic Layer Deposition

Thesis (Ph.D.)--University of Washington, 2016-07