Using computational tools ranging from molecular simulations – including both Monte Carlo and molecular dynamics methods – to quantum mechanical (QM) calculations (primarily at density functional theory (DFT) level), this work focuses on addressing some of the challenges faced in molecular simulations of gas adsorption in metal–organic frameworks (MOFs). This work consists of two themes: one concerns gas adsorption in MOFs with coordinatively unsaturated metal sites (cus’s), and the other one deals with predicting and understanding the breathing behaviour of the flexible MOF MIL-53(Sc). It has been shown experimentally that incorporation of cus’s – also known as “open” metal sites or unsaturated metal centres – into MOFs significantly enhances the uptake of certain gases such as CO2 and CH4. As a result of the considerably enhanced, localized guest-molecule interactions with the cus’s, it, however, remains a challenge to predict correctly adsorption isotherms and/or mechanisms in MOFs with cus’s using grand-canonical Monte Carlo (GCMC) simulations based on generic classical force fields. To address this problem, two multi-scale modelling approaches – which combine GCMC simulations with QM calculations – have been proposed in this work. The first approach is based on the direct implementation of a fluid–framework potential energy surface, calculated by a hybrid DFT/ab initio method, in the GCMC simulations. The second approach involves parameterization of ab initio force fields for GCMC simulations of gas adsorption in MOFs with cus’s. This approach focuses on the generation of accurate ab initio reference data, selection of semiempirical model potentials, and force-field fitting through a multi-objective genetic algorithm approach. The multi-scale simulation strategy not only yields adsorption isotherms in very good agreement with experimental data but also correctly captures adsorption mechanisms, including the adsorption on the cus’s, observed experimentally but absent from GCMC simulations based on generic force fields. The second challenge that this work aims to address concerns the “breathing” phenomenon of MOFs, in which the framework structure adapts its pore opening to accommodate guest molecules, for example. The breathing effect gives rise to some exceptional properties of these MOFs and hence promising applications. However, framework flexibility often poses a challenge for computational studies of such MOFs, because suitable flexible force fields for frameworks are lacking and the effort involved in developing a new one is no less a challenge. Here, an alternative to the force-field-based approach is adopted. Ab initio molecular dynamics (AIMD) simulations – which combine classical molecular dynamics simulations with electronic-structure calculations “on the fly” – have been deployed to study structural changes of the breathing MOF MIL-53(Sc) in response to changes in temperature over the range 100–623 K and adsorption of CO2 at 0–0.9 bar at 196 K. AIMD simulations employing dispersion-corrected DFT accurately simulated the experimentally observed closure of MIL-53(Sc) upon solvent removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase with increasing temperature. AIMD simulations were also used to mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework to evolve freely in response to CO2 loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies with increasing CO2 pressure; this would not have been possible for the intermediate structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53(Sc) upon temperature variation and CO2 adsorption. Both the multi-scale simulation strategy for gas adsorption in MOFs with cus’s and the AIMD study of the stimuli-responsive breathing behaviour of MIL-53(Sc) illustrate the power and promise of combining molecular simulations with quantum mechanical calculations for the prediction and understanding of MOFs.

In this work, multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy is used to investigate a range of inorganic materials, often in combination with DFT (density functional theory) studies. Solid-state NMR is particularly suited to the study of aluminophosphates (AlPOs), as the basic components of their frameworks have NMR active isotopes (²⁷Al, ³¹P, ¹⁷O), as do many of the atoms that comprise the structure directing agent (¹³C, ¹H, ¹⁵N), and the charge-balancing anions (OH⁻, F⁻). A study of the AlPO STA-15 (St Andrews microporous solid-15) provides an introduction to using solid-state NMR spectroscopy to investigate AlPOs. More in-depth studies of AlPO STA-2 (St Andrews microporous solid-2) and MgAPO STA-2 (magnesium-substituted AlPO) examine charge-balancing mechanisms in AlPO-based materials. A range of scandium carboxylate metal-organic frameworks (MOFs), with rigid and flexible frameworks, have been characterised by multinuclear solid-state NMR spectroscopy (⁴⁵Sc, ¹³C and ¹H). The materials studied contain a variety of metal units and organic linkers. ¹³C and ¹H magic-angle spinning (MAS) NMR were used to study the organic linker molecules and ⁴⁵Sc MAS NMR was used to study the scandium environment in the MOFs Sc₂BDC₃ (BDC = 1,4-benzenedicarboxylate), MIL-53(Sc), MIL-88(Sc), MIL-100(Sc) and Sc-ABTC (ABTC = 3,3`,5,5`-azobenzenetetracarboxylate). Functionalised derivatives of Sc₂BDC₃ and MIL-53(Sc) were also studied. The ⁴⁵Sc MAS NMR spectra are found to be strongly dependant on the Sc³⁺ coordination environment. ²⁷Al and ²⁵Mg MAS NMR have been used to study Ti-bearing hibonite samples (of general formula Ca(Al, Ti, Mg)₁₂O₁₉), and results compared to a recent complementary neutron powder diffraction study, in order to investigate the substitution sites for Ti³⁺/⁴⁺ and Mg²⁺. A DFT investigation was also carried out on the aluminium end member, CaAl₁₂O₁₉, due to debate in the literature on the ²⁷Al NMR parameters for the trigonal-bipyramidal site. The substitution of Mg onto the tetrahedral site (M3) and Ti primarily onto one of the octahedral sites (M4) is supported.

The research presented in this thesis aims to assess the capacity of metal organic frameworks with open metal sites for the separation of olefin mixtures. Chapter 1 provides a background to the field, including industrial separation techniques, metal organic frameworks and their applications and the current state-of-the- art for olefin separation. Chapter 3 describes the experimental techniques used in this research. Ethylene and propylene adsorption and desorption isotherms on Ni-CPO-27 and HKUST-1 at a range of temperatures are presented and compared in Chapter 4, and used to calculate isosteric heats of adsorption at varying coverages using the virial method. These pure component isotherms are used in Chapter 5 to predict selectivities for the separation of binary mixtures using ideal adsorbed solution theory. Temperature programmed desorption is used in Chapter 5 to calculate the enthalpy of desorption using Redhead's method and the heating rate variation method, and the two results are compared. The results presented in Chapters 4 and 5 conclude that propylene/ethylene separation is possible using adsorption onto metal organic frameworks with open metal sites. A new in situ environmental gas cell for single crystal X-ray diffraction is developed in Chapter 6, and the challenges encountered during this development process are discussed. The dehydration of one framework, Co-CPO-27, is studied in detail using the environmental gas cell. A dehydrated structure of HKUST-1, obtained using the gas cell, is presented for the first time. Crystal structures for the complete dehydration-adsorption-delivery cycle for biologically active NO on Co-CPO-27 are presented in Chapter 7, showing the utility of the in situ gas cell for prolonged experiments in multiple different gaseous environments. The crystal structure of NO-loaded Co-CPO-27 improves upon the models suggested in the literature, and the treatment of the dual occupancy of the open metal sites by water and NO is discussed in depth. A crystal structure of CO-loaded Co-CPO-27 is obtained for the first time, and crystal structures of Co-CPO-27 in ethylene and propylene environments are presented.

"Porous materials have been a focus of researchers for their applications as molecular storage, molecular sensing, catalysis, asymmetric synthesis and host materials. Metal-organic frameworks (MOFs) represent a promising new class of porous crystalline solids because they exhibit large pore volumes, high surface areas, permanent porosity, high thermal stability, and feature open channels with tunable dimensions and topology. We are currently investigating the design, synthesis, and structures of a new family of MOFs derived from transition metals complexes of 4-(imidazole-1-yl)benzoic acids. Here we present our effort in continuing design and synthesis MOFs composed of 4-(imidazole-1-yl)benzoic acids to expand our knowledge about 4-(imidazole-1-yl)benzoic acid MOF family. A series of ligands are synthesized and Cu MOF-3N, 4, 5 and Cd MOF-3 were synthesized, structure determination found out metal-ligand complex follows our proposal, while Cu MOF-4,5 exhibit porous framework structure via absolute structure determination. Sorption behavior is a key focus in MOF application because the great potential applications MOF bears. Here we carry out a fundamental study about MOF texture and selectivity on MOF-5 and Cd MOF-2. Non-polar polyaromatic hydrocarbons such as naphthalene, phenanthrene, and pyrene, polar molecules such as 2-naphthol, ibuprofen were selected to test our hypothesis that sorption is influenced by the degree of tight fitting, and guest-host interaction such as van der waals and hydrogen bonding. By determining Langmuir isotherms of selected guest molecules, we are able to demonstrate our hypothesis that tighter the fit of the guest molecule and the pores, the higher the amount it would sorb. The sorption difference of non-polar and polar molecules suggest hydrogen bonding is not involved in guest sorption and the dominating force of sorption is hydrophobic interaction. Polymorphism is an interesting phenomenon that bears great value in pharmaceutical industry. Here we report the first case for MOF to serve as a heterogeneous surface that induced nucleation of indomethacin. It is also a first report of this polymorph form of indomethacin. PXRD, DSC, TGA, NMR are conducted as our initial investigation effort. This polymorph exhibits exceptionally thermal stability and low solubility, indicating an unusual tight binding between indomethacin and ethanol solvate. "

The electrochemical growth of metal-organic framework (MOF) coatings, utilising the anodic dissolution method, has been investigated as a means of preparing MOF coated electrodes for various electrochemical applications. A mechanistic understanding of the formation of the electrode coatings has been further developed. This understanding has been utilised to expand the scope of this technique; to allow for the electrochemical formation of Zn and Co zeoliticimidazolate framework (ZIF) coatings which was hitherto not believed to bepossible. Electrodes coated with Co and Zn ZIFs via this methodology were assessed for their capacitive behaviour and the Co ZIFs exhibited the highest, pure MOF areal capacitance values reported to date. This was attributed to the method of coating formation, which provides well adhered coatings of MOF particles integrated into the electrode surface providing a good electrical connection between the coating and the electrode. Incorporation of GO, via electrophoretic deposition during the coating growth, is shown to improve this capacitance still further. Thecorresponding Zn ZIFs exhibited resistances orders of magnitude higher than their Co analogues; modelling can explain this behaviour with the Co analogue of a given ZIF calculated to have a greater metal contribution to its LUMO leading to a more delocalised electronic structure. Electrodes coated with the Cu MOF HKUST-1 have enabled for the first time the use of MOFs as a template for the electrodeposition of anisotropic metal nanostructures. Such MOF encapsulated metal nanostructures are demonstrated to have applications in surface enhanced Raman spectroscopy (SERS). In addition the same MOF has been discovered to display a redox based hysteresis which allows for the rewritable storage of small amounts of electrically accessible data.

Metal-organic frameworks (MOFs) are hybrid organic-inorganic micro- or mesoporous materials that exhibit regular crystalline lattices with rigid pore structures. Chemical functionalization of the organic linkers in the structures of MOFs affords facile control over pore size and physical properties, making MOFs attractive materials for application in gas-separating membranes. A wealth of reports exist discussing the synthesis of MOF structures, however relatively few reports exist discussing MOF membranes. This disparity owes to challenges associated with fabricating films of hybrid materials, including poor substrate-film interactions, moisture sensitivity, and thermal instability. Since even nanometer scale cracks and defects can affect the performance of a membrane for gas separation, these challenges are particularly acute for MOF membranes. The focus of this work is the development of novel methods for MOF film and membrane fabrication with a view to overcoming these challenges. The MOF film production methods discussed herein include in situ synthesis using ligand-modified or metal-modified supports and rapid thermal deposition (RTD).

The primary goal of this research concerns the synthesis of organic ligands in an effort to create metal-organic porous materials for the storage of gas molecules for alternative energy applications as well as other applications such as catalysis, molecular sensing, selective gas adsorption and separation. Initially, the focus of this work was on the synthesis of metal-organic polyhedra, yet the research has to date not progressed past the synthesis of ligands and the theoretical polyhedron that may form. Further efforts to obtain polyhedra from these ligands need to be explored. Concurrently, the search for a metal-organic framework that hopefully breaks the record for methane adsorption at low pressure and standard temperature was undertaken. A framework, PCN-80, was synthesized based off a newly synthesized extended bianthracene derivative, yet was unstable to the atmosphere. Hydrogen and methane adsorption capacities have been evaluated by molecular simulations; these adsorption isotherms indicated a gravimetric hydrogen uptake of 9.59 weight percent and a volumetric uptake of methane of 78.47 g/L. Following the synthesis of PCN-80, a comparison study involving the effect of the stepwise growth of the number of aromatic rings in the ligand of a MOF was pursued; the number of aromatic rings in the ligand was varied from one to eight while still maintaining a linear, ditopic moiety. The synthesis of another bianthracene-based ligand was used to complete the series of ligands and PCN-81, a two-dimensional framework with no noticeable porosity as evident by the simulated hydrogen uptake of 0.68 weight percent, was synthesized. All of these MOFs were synthesized from zinc salts to reduce the number of variables. No clear relationship was established in terms of the number of aromatic rings present in the ligand and the hydrogen adsorption capacity. However, it was confirmed that the density and hydrogen uptake in weight percent are inversely proportional. Further work needs to be done to determine what advantages are offered by these novel frameworks containing extended bianthracene derivatives. For example, with the highly fluorescent nature of the ligands from which they are composed, both PCN-80 and PCN-81 should be studied for the potential use in the application of fluorescent materials.

Metal-organic framework (MOF) materials are a novel set of porous crystalline materials that have generated great scientific interest within the past two decades due to their attractive properties such as high porosity, surface areas and tunable pore structure. These properties have made them emerge as potential candidates suitable for a broad range of applications such as gas separations and storage, catalysis and drug delivery. Despite their fascinating properties, MOFs are often unsuitable for most industrial applications due to their instability when exposed to mechanical stress. The challenge therefore is to convert the MOFs to high strength materials capable of withstanding such stress while still maintaining their exciting properties. This thesis thus focuses on investigating the effects of different binders on a zirconium based metal-organic-framework, UiO-66, in an attempt to enhance the mechanical strength of the adsorbent samples. Three different binders, kaolinite, polyvinyl alcohol and tartaric acid, are mixed with the parent MOF material in different weight percents, pressed into solid disc pellets at different pressures and calcined at different temperatures. Properties such as changes in structure, density, porosity, surface area, radial crush strength, and the adsorption capacity with CO2 are measured and evaluated. Results gathered from this work reveal that polyvinyl alcohol is the most promising of the three binders due to the increase in the strength of pellets and the slight decrease in CO2 adsorption it offers. Recommendations for future research work aimed at driving these materials towards reaching their maximum application potentials are proposed.

Metal-organic frameworks (MOFs), a new class of porous solids comprised of metal-containing nodes linked by organic ligands, have become promising materials for gas separations. In particular, their flexible chemistry makes them attractive for CO2 capture from flue gas streams in post-combustion plants. Although numerous efforts have been exerted on the investigation of MOFs for CO2 capture, the exploration of the effects from coexisting components present in very dilute proportions in flue gases is limited because of the experimental difficulty to determine the coadsorption of CO2 with trace components. In this regard, molecular simulations show superiority. In this study, molecular simulations are used to estimate the influence of impurities: water, O2, and SO2 on post-combustion CO2 capture in MOFs. Firstly, two MOFs with coordinatively unsaturated metal sites (CUMs), HKUST-1 and Mg-MOF-74 are explored. Increase of CO2 adsorption is observed for hydrated HKUST-1; on the contrary, the opposite water adsorption behavior is observed in hydrated Mg-MOF-74, leading to decrease of CO2 adsorption. Further, water effects on CO2 capture in M-HKUST1 (M = Mg, Zn, Co, Ni) are evaluated to test whether comparing the binding energy could be a general method to evaluate water effects in MOFs with CUMs. It is found that the method works well for Zn-, Co-, and Ni-HKUST1 but partially for Mg-HKUST1. In addition, the effects of O2 and SO2 on CO2 capture in MOFs are also investigated for the first time, showing that the effects of O2 may be negligible but SO2 has negative effects in the CO2 capture process in HKUST-1 systems. Secondly, the influences of water on CO2 capture in three UiO-66 MOFs with functional groups, –NH2, –OH and –Br are explored, respectively. For UiO-66-NH2 and -OH, the presence of water lowers CO2 adsorption significantly; in contrast, water shows much smaller effects in UiO-66-Br. Moreover, the presence of SO2 decreases water adsorption but enhances CO2 uptakes slightly in both UiO-66-NH2 and -Br. Finally, the effects of impurities on CO2 capture in a MOF with suitable pore size (PCN-200) are analyzed. The adsorption of both CO2 and N2 decrease substantially even with 1% water present in the mixture. In addition, the presence of low SO2 does not show obvious effect in PCN-200. However, a lower CO2 adsorption is observed for a mixture with a high SO2 content. In collaboration with experimental groups, the performances of three new MOFs in CO2 capture are evaluated using molecular simulations. The computational results demonstrate the feasibility of precisely designing single-molecule traps (SMT) for CO2 capture. Also, a multi-functional MOF with micro-porosity, open Cu2+ sites and amine groups has also proved computationally the selective adsorption of CO2 over CH4 and N2. Last, we demonstrate that charge separation is an effective strategy for improving CO2 capture in MOFs.

Submitted by Etelvina Domingos (etelvina.domingos@ufpe.br) on 2015-03-11T17:35:11Z No. of bitstreams: 2 Etelino_Dissertação final 09102012.pdf: 9253307 bytes, checksum: 22d8f2b2d1f4d51f54320eb4f05a8303 (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) / Made available in DSpace on 2015-03-11T17:35:11Z (GMT). No. of bitstreams: 2 Etelino_Dissertação final 09102012.pdf: 9253307 bytes, checksum: 22d8f2b2d1f4d51f54320eb4f05a8303 (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Previous issue date: 2012-09 / CNPq CAPES / No presente trabalho discutimos a preparação e a caracterização de nanofibras poliméricas obtidas pelo método da eletrofiação (“electrospinning”) com o objetivo de desenvolver novos materiais com morfologias fibrosas. Para isso, foram utilizados três diferentes tipos de compósitos. O primeiro foi um compósito de nanopartículas de ZnO com polipirrol (PPi) e álcool polivinílico (PVA). O segundo compósito foi constituído por PVA e dois diferentes polímeros de coordenação, [(Ln)(DPA)(HDPA)] (onde Ln = Tb3+ ou Eu3+), também conhecidos como MOF – de (“Metal Organic Framework”). Finalmente, o terceiro foi um compósito de N-(4-nitro-2-fenoxifenil)metanesulfonamida (nimesulida) com PVA. As nanofibras obtidas foram caracterizadas por espectroscopia na região do infravermelho e do UV-Vis, espectroscopia de fluorescência, medidas elétricas de corrente e voltagem, microscopia eletrônica de varredura (MEV), microscopia de fluorescência e análise termogravimétrica (TGA). Muito embora todas as nanofibras estudadas apresentassem as mesmas características morfológicas, elas exibiram diferentes propriedades físicas e químicas. As nanofibras de PVA/ZnO/PPi apresentaram fluorescência na região visível com comprimento de onda de 526 nm e, quando expostas à luz ultravioleta, sua resistência elétrica sofre um aumento de cerca de duas ordens de grandeza (um fato atribuído a um provável aumento da zona de depleção da junção ZnO/PPi quando o material é exposto a luz). Já as nanofibras com MOFs exibiram altas intensidades de fluorescência que, de acordo com a natureza do metal presente (se Tb ou Eu), pode apresentar emissão na região visível: verde ou vermelho, respectivamente. Por fim, a caracterização espectroscópica das nanofibras com nimesulida mostraram que o fármaco se encontra incorporado na matriz polimérica, pois é possível observar sua banda de absorção característica no UV-Vis em 315 nm, como também o espectro de infravermelho das nanofibras revela a presença das bandas típicas de absorção tanto da nimesulida quanto do PVA.