The adsorption isotherms can be used to study the properties of a sorbent and to determine the binding energy between a sorbent and a gas that is adsorbed on it. This study that was carried out on a metal organic framework called "Zeolitic imidazolate framework-8" (ZIF-8) as the sorbent. ZIF -8 is known to have a flexible structure and it has shown structural transformation during gas adsorption, at different temperatures. During this study, ZIF-8 was explored using Xenon adsorption. The range of temperatures for the Xenon adsorption isotherms was between 138 K and 157.56 K. During the adsorption of Xenon on ZIF -8 the lowest two isotherms (138 K and 140.39 K) showed two steps. The lower pressure step represents adsorption of Xenon on the "as - produced" ZIF-8. The extra step reflects the structural transition ("gate opening") that occurs due to the re-orientation of the organic linkers in ZIF-8. These changes increase the diameter of the apertures in the structure, and allow more gas molecules to enter in to the ZIF -8 structure. The Xenon adsorption isotherms were also used to determine the effective surface area of ZIF -8 by employing the "point B" method. The binding energy between Xenon and ZIF -8 was found using the isosteric heat for Xenon on ZIF-8 at low coverage. The kinetics of the Xenon adsorption was also studied during this experiment.

Thesis advisor: Chia-Kuang Tsung / Thesis advisor: Eranthie Weerapana / Metal-Organic Frameworks (MOFs) are crystalline porous materials constructed of metal ions and organic linkers, and have been widely utilized in gas storage, sensing, and chromatographic separation. The combination of MOF nanoparticles with other materials will broaden the utilization of MOF materials to a great extent. Several approaches for creating composites with the MOF, Zeolitic Imidazolate Framework-8 (ZIF-8), have been developed: dye and model drug molecules were encapsulated in ZIF-8 pores for potential drug delivery; mesoporous silica monolayer was epitaxially grown on the ZIF-8 surface for structural stability enhancement and hollow structure formation; UiO-66, another MOF subclass, was hierarchically encased inside ZIF-8 for double-phase gas separation and heterogeneous catalysis. By exploring the versatile ZIF-8 platform, these nanocomposites could have great applications in fields such as heterogeneous catalysis and drug delivery. / Thesis (MS) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Mixed matrix membranes (MMMs) have gained importance because they combine the desirable properties of the polymers and the organic/inorganic filler materials and they may have a very big potential. In this study polyethersulfone (PES) was used as polymeric material, and Zeolitic Imidazolate Framework-8 (ZIF-8) was used as porous filler material, and 2-hydroxy 5-methyl aniline(HMA), was used as a third component in membrane formulation. In this study, ZIF-8 crystals were synthesized with varying particle sizes, and a novel recycling methodology was developed to improve the efficiency of ZIF-8 production. ZIF-8 nano-crystals were synthesized by a 1-hour stirring method at room temperature and characterized by X-ray diffractometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermal gravimetric analysis (TGA). In order to investigate the effect of ZIF-8 loading on the membrane performance, different types of membranes were prepared with varying amounts of ZIF-8 between 10-60% (w/w). Moreover, ternary mixed matrix membranes were synthesized consisting of different amounts of ZIF-8 between 10-30% (w/w) and HMA 1-10% (w/w). Gas transport properties of the membranes were investigated by single gas permeation experiments of H2, CO2 and CH4 at 3 bar feed pressure. In order to investigate the effect of feed pressure on the gas transport properties of the membranes, single gas experiments were conducted on 3, 6, 8, 10 and 12 bar feed pressures. Moreover, binary gas permeation experiments of CO2/CH4 pair were conducted through selected membranes at 3 bar and 12 bar feed pressures. In addition to gas permeation experiments, the morphology and thermal characteristics of the membranes were characterized by SEM, TGA and differential scanning calorimetry (DSC) analysis. The incorporation of ZIF-8 crystals into continuous PES matrix resulted in high performance gas separation membranes. The permeabilities of all studied gases increased with ZIF-8 loading while the ideal selectivities showed a slight decrease compared to neat PES membrane. Highly reproducible and repeatable results were obtained up to 30 % w/w ZIF-8 loading, while membrane formulation reproducibility was decreased for higher ZIF-8 contents (&gt / 30 w/w %). Addition of HMA improved the gas separation performances of the binary membranes significantly by decreasing permeabilities and increasing ideal selectivities. PES/ZIF-8(%20)/HMA(%7) membrane has the best separation performance for all gases among the ternary membranes. When 7 w/w % HMA was added to PES/ZIF-8(%20) membrane, H2 permeability decreased from 26.3 to 13.7 barrer, while H2/CH4 ideal selectivity increased from 61.8 to 103.7. Increasing feed pressures appreciably increased the separation performances of all membranes. While the H2 permeability is pressure independent, the CO2 and CH4 permeabilities were reduced with increasing feed pressures and the highest selectivity improvement was observed in H2/CH4 pair for all membrane compositions. For instance, when the feed pressure was increased from 3 bar to 12 bar, the percentage improvements in ideal selectivities through PES/ZIF-8(%10)/HMA(%4) membrane were calculated as 26, 69, 113 % for the H2/CO2, CO2/CH4 and H2/CH4 gas pairs / respectively. This results show that working at higher feed pressures will be more advantageous for separation of the studied gas pairs. The ideal selectivities and the separation factors were equal to each other for all membrane compositions both for 3 and 12 bar operating pressures.

Distillation and absorption columns offer significant energy demands for future development in the petrochemical and fine chemical industries. Membranes and adsorbents are attractive alternatives to these classical separation units due to lower operating cost and easy device fabrication; however, membranes possess an upper limit in separation performance that results in a trade-off between selectivity (purity) and permeability (productivity) for the target gas product, and adsorbents require the need to be water-resistant to natural gas streams in order to withstand typical gas compositions. Composite membranes, or mixed-matrix membranes, are an appealing alternative to pure polymeric membrane materials by use of a molecular sieve “filler” phase which has higher separation performance than the pure polymer. In this thesis, the structure-property-processing relationships for a new class of molecular sieves known as zeolitic imidazolate frameworks (ZIFs) are investigated for their use as the filler phase in composite membranes or as adsorbents. These materials show robust chemical and thermal stability and are a promising class of molecular sieves for acid gas (CO₂/CH₄) separations. The synthesis of mixed-linker ZIFs is first investigated. It is shown that the organic linker composition in these materials is controllable without changing the crystal structure or significantly altering the thermal decomposition properties. There are observable changes in the adsorption properties, determined by nitrogen physisorption, that depend on the overall linker composition. The results suggest the proposed synthesis route facilitates a tunable process to control either the adsorption or diffusion properties depending on the linker composition. The structure-property-processing relationship for a specific ZIF, ZIF-8, is then investigated to determine the proper processing conditions necessary for fabricating defect-free composite membranes. The effect of ultrasonication shows an unexpected coarsening of ZIF-8 nanoparticles that grow with increased sonication time, but the structural integrity is shown to be maintained after sonication by using X-ray diffraction, Pair Distribution Function analysis, and nitrogen physisorption. The permeation properties of composite membranes revealed that intense ultrasonication is necessary to fabricate defect-free membranes for CO₂/CH₄ gas separations. Finally, the separation properties of mixed-linker ZIFs is investigated by using adsorption studies of CO₂ and CH₄ and using composite membranes with differing linker compositions. Adsorption properties of mixed-linker ZIFs reveal that these materials possess tunable surface properties, and a selectivity enhancement of six fold over ZIF-8 is observed with mixed-linker ZIFs without changing the crystal structure. Gas permeation studies of composite membranes reveal that the separation properties of mixed-linker ZIFs are different from their parent frameworks. By proper selection of mixed-linker ZIFs, there is an overall improvement of separation properties in the composite membranes when compared to ZIF-8.

This master’s thesis was conducted at the Department of Nanotechnology and Functional Materials at Ångström Laboratory as part of an on-going project to develop hybrid nanocomposites from Cladophora cellulose and a sub-type of metal-organic frameworks; zeolitic imidazolate frameworks (ZIFs). By utilizing a state-of-the-art interfacial synthesis approach, in-situ growth of ZIF particles on the cellulose could be achieved. TEMPO-mediated oxidation was diligently used to achieve cellulose nanofibers with carboxylate groups on their surfaces. These were ion-exchanged to promote growth of ZIF particles in a nanocellulose solution and lastly, metal ions and organic linkers which the ZIFs are composed of were added to the surface functionalized and ion-exchanged nanocellulose solution to promote ZIF growth. By vacuum filtration, mechanical pressing and furnace drying; freestanding nanopapers were obtained. A core-shell morphology between the nanocellulose and ZIF crystals was desired and by adjusting the metal ion concentration, a change in morphologies was expected. The nanocomposites were investigated with several relevant analytical tools to confirm presence, attachment and in-situ growth of ZIF crystal particles upon the surface of the fine nanocellulose fibers. Both the CNF@ZIF-67 and CNF@ZIF-8 nanocomposites were successfully prepared as nanopapers with superior surface areas and thermal properties compared to pure TEMPO-oxidized cellulose nanopapers. The CNF@ZIFs showcased hierarchical porosities, stemming from the micro- and mesoporous ZIFs and nanocellulose, respectively. Also, it was demonstrated that CNF@ZIF-8 selectively adsorbed CO2 over N2. Partial formation of core-shell structure could be obtained, although a relationship between increased metal ions and ZIF particle morphology could not wholly be observed.

Tesis por compendio / [ES] El desarrollo de membranas poliméricas capaces de actuar como electrolito en pilas de combustible tipo PEMFC a temperaturas intermedias constituye uno de los principales retos para conseguir la generación eficiente de energía por medio de estos dispositivos. Actualmente, las membranas basadas en polímeros perfluorosulfonados como el Nafion® son las más extendidas en pilas de combustible, ya que presentan una buena conductividad protónica además de ser estables mecánica y químicamente. Sin embargo, este tipo de membranas no son capaces de ofrecer buenos rendimientos por encima de 80 ¿C. En este sentido, el objetivo fundamental de esta tesis ha sido la síntesis y caracterización de membranas poliméricas que permitan su potencial utilización en el rango de temperaturas intermedias, por encima de 100 ¿C, donde la cinética de los electrodos y el transporte de protones a través de la membrana mejoran considerablemente, aumentando con ello el rendimiento de la celda. La investigación llevada a cabo se ha centrado en dos tipos de polímeros: poli(eter-eter-cetona) sulfonada (SPEEK) y polibencimidazol (PBI). Las membranas basadas en SPEEK ofrecen una elevada conductividad protónica y una buena estabilidad tanto mecánica como química. Si bien, estas propiedades dependen drásticamente de su grado de sulfonación. Así, altos grados de sulfonación resultan en muy buenas conductividades protónicas, aunque por el contrario, originan un excesivo hinchamiento de las membranas provocando un empeoramiento de sus propiedades mecánicas. Además, cuando la temperatura supera los 80 ¿C su conductividad disminuye debido a la deshidratación de la membrana. La estrategia seguida en este caso para mantener las propiedades mecánicas y la conductividad de las membranas basadas en SPEEK a temperaturas intermedias ha consistido en utilizar un polímero con un índice de intercambio catiónico no excesivamente alto (1.75 meq g-1), el cual ha sido dopado con dos tipos de compuestos organometálicos diferentes de tipo ZIF (Zeolitic Imidazolate Framework ) y con una mezcla de ambos. Este tipo de compuestos constituyen una subclase de los conocidos como Metal Organic Framework (MOF), los cuales adoptan una estructura tipo zeolita donde la parte orgánica está constituida por un anillo de imidazol y el nodo inorgánico es un metal. En este caso Zinc para el Z8 y Cobalto para el Z67. Las membranas compuestas SPEEK-ZIF mejoraron claramente las prestaciones de las de SPEEK puro a temperaturas intermedias. En base a los resultados anteriores, se seleccionaron las membranas dopadas con ZIF-67 para su evaluación en monocelda donde ofrecieron valores superiores a los de las membranas de SPEEK puro sin la adición de cargas y a los obtenidos con membranas de Nafion®117 a temperaturas superiores a 100 °C. En el caso de las membranas en base PBI, estas han sido capaces de ofrecer valores elevados de conductividad a altas temperaturas cuando eran dopadas con ácido fosfórico. Sin embargo, la pérdida del ácido por parte de la membrana (leaching) con el tiempo de operación y la degradación que este ácido provoca en los componentes de la celda, hacen que sea necesaria la utilización de otros agentes dopantes no volátiles y menos agresivos capaces de aportar al polímero la conductividad de la que carece. Los líquidos iónicos son sales fundidas a temperatura ambiente que poseen presiones de vapor despreciables y ofrecen buenas conductividades a temperaturas elevadas. En esta tesis, se prepararon por el método de casting, membranas de PBI conteniendo el líquido iónico 1-butil-3-metil imidazolio bis(trifluorometil sulfonil) imida (BMIM-NTf2) en diferentes porcentajes. Estas membranas alcanzaron a partir de un 10 % wt. de líquido iónico un valor de conductividad del orden de 10-2 S cm-1 a 160 ¿C. Señalando su potencial como electrolito polimérico basado en PBI libre de ácido fosfórico. / [CA] El desenvolupament de membranes polimèriques vàlides per a actuar com a electròlit en piles de combustible tipus PEMFC a temperatures intermèdies. Constitueix un dels reptes principals per aconseguir la generació eficient d'energia amb aquests dispositius. Actualment, les membranes basades en polímers perfluorosulfonats com el Nafion® són les més utilitzades en piles de combustible, ja que presenten una bona conductivitat protònica a més de tindre una bona estabilitat química i mecànica. Tot i això, aquest tipus de membranes no oferixen bons rendiments a temperaturas superiors a 80 ¿C. En aquest sentit, l'objectiu fonamental d'aquesta tesi ha segut la síntesi i caracterització de membranes polimèriques amb les característiques adequades per poder treballar a temperatures intermèdies, per damunt de 100 ¿C. En aquestes condicions la cinètica dels elèctrodes i el transport de protons a través de la membrana milloren considerablement augmentant amb això el rendiment de la cel·la. La investigació duta a terme s'ha centrat en dos tipus de polímers: poli(eter-eter-cetona) sulfonada (SPEEK) i polibencimidazol (PBI). Les membranes basades en SPEEK ofereixen una elevada conductivitat protònica i una bona estabilitat tant mecànica com a química. No obstant això, aquestes caracteristiques depenen dràsticament del seu grau de sulfonació. Així, alts graus de sulfonació resulten en molt bones conductivitats protòniques. Encara que per contra, originen un excessiu unflament de les membranes en aigua calenta provocant un empitjorament de les seves propietats mecàniques. A més, quan la temperatura supera els 80 ¿C la seva conductivitat baixa a causa de la deshidratació de la membrana. L'estratègia seguida en aquest cas per mantenir les propietats mecàniques i la conductivitat de les membranes basades en SPEEK a temperatures intermèdies. Ha segut partir d'un polímer amb un índex d'intercanvi catiònic no gaire alt (1.75 meq g-1). El qual ha segut dopat amb dos tipus de compostos órganometàl.lics diferents de tipus ZIF (Zeolitic Imidazolate Framework) (ZIF) i amb una barreja de tots dos. Aquest tipus de compostos constitueixen una subclasse dels coneguts com a Metal Organic Framework (MOF). Els quals adopten una estructura tipus zeolita on la part orgànica està constituïda per un anell d'imidazol i el node inorgànic és el metall. En aquest cas Zinc per al Z8 i Cobalt per al Z67. Les membranes compostes SPEEK-ZIF van millorar clarament les prestacions de les de SPEEK pur a temperatures intermèdies. En base als resultats anteriors es van seleccionar les membranes dopades amb ZIF-67 per a la seva avaluació en monocel·la on van oferir valors superiors als de les membranes de SPEEK pur sense l'addició de càrregues i als obtinguts amb membranes de Nafion®117 en les mateixes condicions a temperatures superiors a 100 °C. En el cas de les membranes en base PBI, aquestes han oferit valors elevats de conductivitat a altes temperatures, quan han segut dopades amb àcid fosfòric. Tot i això, la pèrdua de l'àcid per part de la membrana (leaching) amb el temps d'operació i la degradació que aquest àcid provoca en els components de la cel·la, fan que siga necessària la utilització d'altres agents dopants no volàtils i menys agressius capaços d'aportar al polímer conductivitat iònica. Els líquids iònics són sals foses a temperatura ambient que tenen pressions de vapor molt febles i ofereixen bones conductivitats a temperatures elevades. En aquesta tesi es van preparar pel mètode de càsting, membranes de PBI contenint el líquid iònic 1-butil-3-metil imidazoli bis(trifluorometil sulfonil) imida (BMIM-NTf2) en diferents percentatges. Aquestes membranes van assolir a partir d'un 10% wt. de líquid iònic un valor de conductivitat de l'ordre de 10-2 S cm-1 a 160 ¿C. Assenyalant el seu potencial com a electròlit polimèric basat en PBI lliure d'àcid fosfòric. / [EN] The development of polymeric membranes capable of acting as an electrolyte in a proton exchange membrane fuel cells (PEMFC) at intermediate temperatures. It constitutes one of the main challenges to achieve efficient energy generation through these kinds of devices. Currently, membranes based on perfluorosulfonated polymers such as Nafion® are the most widespread in fuel cells, since they have good proton conductivity as well as being mechanically and chemically stable. However, these types of membranes are not capable of offering good performance above 80 ¿C. In this sense, the main objective of this thesis has been the synthesis and characterization of polymeric membranes with potential use in the range of intermediate temperatures, above 100 ¿C, where the kinetics of the electrodes and the transport of protons through of the membrane and the performance of the cell are greatly improved. The research carried out has focused on two types of polymers: sulfonated poly(ether-ether-ketone) (SPEEK) and polybenzimidazole (PBI). SPEEK-based membranes offer high proton conductivity and good mechanical and chemical stability. However, their properties depend dramatically on its degree of sulfonation. Thus, high degrees of sulfonation result in excellent proton conductivities. On the other hand, the large amount of sulphonic groups in the membrane cause an excessive swelling in hot water, leading to a worsening of their mechanical properties, even reaching its dissolution. Furthermore, as also happens with perfluorosulfonated membranes, when the temperature is increased above 80 ¿C their proton conductivity decreases due to membrane dehydration. Focused on maintain the mechanical properties and conductivity of SPEEK-based membranes at intermediate temperatures. Membranes have been prepared from a polymer with a not excessively high cation exchange rate (1.75 meq g-1). Which has been doped with two different ZIF-type (Zeolitic Imidazolate Framework) organometallic compounds and with a mixture of both. This type of compounds constitutes a subclass of those known as Metal Organic Framework (MOF). Which adopt a zeolite-type structure where the organic part is made up of an imidazole ring and the inorganic node is the metal. In this case Zinc for Z8 and Cobalt for Z67. SPEEK-ZIF. Composite membranes clearly improved the performance of pure SPEEK membranes at intermediate tempe-ratures. Based on the previous results, the membranes doped with ZIF-67 were selected for their evaluation in a single fuel cell, where they offered higher values than those of the pure SPEEK membranes without the addition of fillers and those obtained with membranes of Nafion®117 under the same conditions at temperatures above 100 °C. PBI based membranes, have been capable of offering high conductivity values at high temperatures, when they have been doped with phosphoric acid. However, the loss of acid by the membrane (leaching) with the operation time and the degradation that this acid causes in the cell components. In this sense, it is necessary to explore the use of other non-volatile and less aggressive doping agents capable of providing ionic conductivity to the polymer. Ionic liquids are molten salts at room temperature that have negligible vapor pressures and offer good conductivities at elevated temperatures. In this thesis, PBI membranes containing the ionic liquid 1-butyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl)imide (BMIM-NTf2) at different percentages were prepared by the casting method. These membranes containing 10 wt.% of ionic liquid reached a conductivity value in the range of 10-2 S cm-1 at 160 ¿C. Noting its potential as a phosphoric acid-free PBI-based polymeric electrolyte. / This work was sponsored by the Ministerio de Economia y Competitividad (MINECO) under the project ENE/2015-69203-R. The authors acknowledge the Electron Microscopy Service from Universitat Politècnica de València for the use of instruments and staff assistance. / Barjola Ruiz, A. (2023). Membranas poliméricas de intercambio iónico con aplicación en pilas de combustible de temperatura intermedia [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/193081 / Compendio