Biporous Metal-Organic Framework with Tunable CO2/CH4 Separation Performance Facilitated by Intrinsic Flexibility.

In this work, we report the synthesis of SION-8, a novel metal-organic framework (MOF) based on Ca(II) and a tetracarboxylate ligand TBAPy4- endowed with two chemically distinct types of pores characterized by their hydrophobic and hydrophilic properties. By altering the activation conditions, we gained access to two bulk materials: the fully activated SION-8F and the partially activated SION-8P with exclusively the hydrophobic pores activated. SION-8P shows high affinity for both CO2 ( Qst = 28.4 kJ/mol) and CH4 ( Qst = 21.4 kJ/mol), while upon full activation, the difference in affinity for CO2 ( Qst = 23.4 kJ/mol) and CH4 ( Qst = 16.0 kJ/mol) is more pronounced. The intrinsic flexibility of both materials results in complex adsorption behavior and greater adsorption of gas molecules than if the materials were rigid. Their CO2/CH4 separation performance was tested in fixed-bed breakthrough experiments using binary gas mixtures of different compositions and rationalized in terms of molecular interactions. SION-8F showed a 40-160% increase (depending on the temperature and the gas mixture composition probed) of the CO2/CH4 dynamic breakthrough selectivity compared to SION-8P, demonstrating the possibility to rationally tune the separation performance of a single MOF by manipulating the stepwise activation made possible by the MOF's biporous nature

Aerogels as diverse nanomaterials

Aerogels are 3-D light-weight nanoporous materials pursued for their low thermal conductivity, low dielectric constant and high acoustic attenuation. Those exceptional macroscopic properties of aerogels are dependent on the chemical nature of nanoparticles, complex hierarchical solid skeletal framework and porosity. Also, the free space can become host for functional guests such as pharmaceuticals. In chapter I, we investigated randomly mesoporous bio-compatible polymer-crosslinked dysprosia aerogels as drug delivery vehicles and demonstrated storage and release of drugs under physiological conditions. Comparative study with ordered and randomly mesoporous silica showed high drug uptake and slower release rate for random nanostructures (silica or dysprosia) relative to ordered silica. Drug release data from dysprosia aerogels showed that drug is stored successively in three hierarchical pore sites on the skeletal framework. In chapter II, we developed flexible polyurethane-acrylate aerogels from star monomer containing urethane linkage and terminal acrylate bonds by free-radical polymerization. Lower density samples were flexible, while higher density samples were mechanically strong. Those results were dependent on the particle size and interparticle connectivity of skeletal framework, pointing to a nanoscopic origin for their flexibility, rather than to a molecular one. Further, the acrylate bonds were converted to norbornene moieties and the gelation process was brought down to room temperature by using ring opening metathesis polymerization (ROMP). In chapter III, we developed polydicyclopentadiene (pDCPD) based aerogels using two different Grubbs catalysts (GC-I and GC-II) with different catalytic activity towards ROMP. The different behavior of pDCPD aerogels was traced to a different polymer configuration at molecular level. --Abstract, page v

Nanomorphology dependent optical and mechanical properties of aerogels

Aerogels are very low density, light weight open pore materials. A hypothesis that is under intense current investigation by the scientific community states that the mechanical properties of nanostructured polymers depend on their nanomorphology. Aerogels are nanostructured ultra-lightweight nanoporous materials with skeletal frameworks that can display a wide range of nanomorphologies. Thereby aerogels comprise a suitable platform for testing not only that hypothesis but also a wide range of other properties such as light scattering for applications, for example, in thermally insulating windows. To study the mechanical properties of nanostructured matter as a function of nanomorphology, various shape-memory polyurethane aerogels were prepared with identical density, porosity, and chemical composition, but with vastly different nanostructures. Based on 5 different catalysts at 5 different concentrations each, the elastic modulus of all materials followed a well-defined trend whereas, all other factors being equal, bicontinuous structures were by several times stiffer than spheroidal nanostructures. In order to develop silica aerogels as thermal insulators for windows, one must achieve a balance of clarity, strength, and thermal insulation value. Light scattering (haze) was studied with an integrating sphere, thermal conductivity with the hot plate method and mechanical strength with uniaxial compression. Delamination of wet-gels from glass substrates during drying into aerogels was traced to the nature mass fractal of the secondary particles that allows them to merge with one another --Abstract, page v

Experimental and Theoretical Study of Effects of Varying Hydration on Elastic Properties and Microstructure of Shales and Sandstones

This thesis investigates how changing hydration affects elastic properties and microstructure of shales and sandstones. The experimental measurements show that adsorption of water causes elastic weakening and deformation of the rocks. The theoretical analysis of this data indicates driving mechanisms of these effects. One of them, a changing stiffness of compliant porosity, is responsible for drastic decrease of shear velocities in the rocks. The reported results can improve seismic and well log characterisation of reservoirs

Effect of Surface Forces on the Mechanics of Sorption-deformation in Microporous Media and Environment-assisted Crack Growth in Brittle Solids

The structurization of fluid molecules at the fluid-solid interface, known as adsorption, can create noticeable forces which are ubiquitous in material systems such as fluid-saturated porous materials and cracked solids.Adsorption alters the strain and microstructure of porous materials. However, adsorption-induced forces are routinely neglected in the continuum description of the porous media. In this thesis, a poromechanics theory is developed for deformable microporous media by thermodynamically upscaling the adsorption-induced forces from the pore-scale to the continuum scale. The capability of this model is demonstrated by quantitative comparison with experimental results using physically sound parameters. The model is further extended to incorporate detailed information of pore size distribution. A comprehensive study of the influence of pore size distribution highlights plausible causes of the distinct adsorption-deformation behaviors of various porous materials. It is found that competition between the attractive disjoining pressure and the reduction of surface energy controls the shrinkage-swelling transition of microporous media during the early stage of adsorption. On the other hand, the presence of surface-reactive species in the environment can reduce materials&rsquo; resistance to withstand fracture. It is established that the alteration of surface tension due to adsorption can qualitatively explain the reduced fracture toughness in reactive environments. However, the intricate coupling between physiochemical processes of adsorption, diffusion and fracture mechanics at the vicinity of the crack tip has never been fully resolved. A mechanistic theory is thus developed and implemented in this thesis to consider both the strong disjoining pressure between the opposing solid surfaces near the crack tip and the transport processes that control the accessibility of reactive species to the crack tip. For the first time, the classical SCG curve naturally emerges from explicit consideration of the underlying physical processes. The adoption of different fluid transport models confirms that viscous flow or surface diffusion can explain certain aspects of the transition from slow environmentally promoted crack propagation to sudden fracturing. Finally, it is shown that the complete prediction of subcritical crack propagation requires not only the knowledge of the correct fluid transport mechanisms, but also the knowledge of the interplay and transition between multiple fluid transport mechanisms.</p

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

Organossílicas com mesoporosidade para aplicação em pilhas de combustível

The objective of this thesis is to assess the potential of acid-functionalized periodic mesoporous organosilicas (Ph-PMO) as fillers for the polymer membrane in polymer electrolyte fuel cells, aiming at improved performance under low relative humidity (r.h.120 °C) operation conditions. Ph-PMOs mimic the structure of Nafion®, presenting a similar acid load distributed on pores with similar width (3 nm) and with similar distance between acid sites (0.8 nm), but on a stable, rigid structure preventing the proton confinement to the pore surface that occurs in Nafion®. This offers potentially high protonic conductivity under dry conditions, in addition to improved visco-elastic behaviour. Two protogenic groups based on sulfonic (S-Ph-PMO) and phosphonic (P-Ph-PMO) acids were used to functionalize Ph-PMOs with variable structural and microstructural features. The conductivity of both types of Ph-PMOs increases with increasing specific surface area and r.h., confirming the surface nature of the protonic transport and the key role of the hydration water on the protonic transport. The strongest acid character of S-Ph-PMOs leads to a much higher conductivity, attaining values of up to ~0.1 S∙cm-1 at 94 °C and 98% r.h. Results obtained for a series of S-Ph-PMO samples with variable acid loadings, surface area and structural order, prepared by a microwave hydrothermal reaction, show that the conductivity increases with increasing acid loading, whereas no clear correlation can be established with structural order parameters. S-Ph-PMOs were selected for the preparation of composite Nafion® membranes and their transport and visco-elastic properties evaluated. The bulk effect of the fillers is demonstrated by a 10 fold increase of the storage modulus (E’) at 140 °C of composite membranes with up to 36 vol.% S-Ph-PMO, in comparison with pure Nafion®. The effect of fillers on the bulk properties is also apparent on the swelling under saturated conditions, which is reduced by 30% with addition of 36 vol.% of fillers, indicating virtually zero swelling of the fillers. These improvements may be crucial to increase the thermo-mechanical stability of the membrane and of the electrode/electrolyte interface. The conductivity of the composite membranes is less dependent on r.h. and temperature, and can be up to 1 order of magnitude higher than for pure Nafion®, at 20% r.h. and 40 °C. Differences are smaller at high r.h., with the highest conductivity of 0.2 S∙cm-1 achieved at 94 °C and 98% r.h. However, and as opposed to the bulk effects on E’ and swelling, a conductivity maximum is observed for the membranes with 20 vol.% of fillers. The use of different fillers in a series of 20 vol.% composite membranes showed that there is a slight increase of the membrane conductivity with increasing acid load and surface area of the fillers, however no direct correlation could be drawn for the structural properties. The increased conductivity at low r.h. can be interpreted considering a reduction of the proton confinement in the rigid pores of the fillers and, as the r.h. increases, by assuming a surface effect, where the presence of the mesoporous fillers disrupts the “skin-like” structure that forms at the surface of Nafion®, releasing the internal pressure and hence facilitating the access of the hydration water to the bulk of the membrane.O objectivo desta dissertação é o de avaliar o potencial de organossílicas mesoporosas periódicas (Ph-PMO) como aditivos para membranas poliméricas de pilhas de combustível, visando um melhor desempenho a baixa humidade relativa (h.r. 120 ºC). A estrutura dos Ph-PMO mimetiza a do Nafion®, apresentando semelhante tamanho de poro (3 nm) e distância entre grupos ácido (0.8 nm), mas com uma estrutura estável e rígida que reduz o efeito de confinamento protónico à superfície do poro, que ocorre no Nafion®. Este efeito pode potencialmente traduzir-se num aumento da condutividade protónica em condições anidras, para além de melhorar o comportamento visco-elástico. Foram sintetizados Ph-PMO com grupos ácido sulfónico (S-Ph-PMO) e ácido fosfónico (P-Ph-PMO), com características estruturais e microestruturais distintas. A condutividade dos dois tipos de Ph-PMO aumenta com o aumento da área superficial específica e da h.r., confirmando o papel central da superfície e da água de hidratação no transporte protónico. A maior acidez dos S-Ph-PMO resulta em valores de condutividade superiores, da ordem de 0.1 S∙cm-1 a 94 ºC e 98% de h.r. Os resultados obtidos com uma série de S-Ph-PMO com diferentes concentrações de grupos funcionais, área superficial específica e ordem estrutural, preparados por reacção hidrotermal em micro-ondas, mostram que a condutividade aumenta com o aumento da concentração de grupos ácidos, não sendo, no entanto, correlacionável com a ordem estrutural. Foram seleccionadas várias amostras de S-Ph-PMO para preparar membranas compósitas à base de Nafion®, avaliando as suas propriedades visco-elásticas e de transporte protónico. Para a membrana com cerca de 36 vol.% de aditivos, o efeito de volume dos aditivos é evidenciado por um módulo de armazenamento (E’) até 10 vezes superior ao do Nafion® puro. O efeito dos aditivos faz-se também sentir na diminuição da dilatação das membranas por absorção de água até 30%, devido à dilatação virtualmente nula dos aditivos. O aumento do E’ e diminuição da dilatação podem vir a ser cruciais para a melhoria da estabilidade termo-mecânica das membranas e da interface eléctrodo/electrólito. A condutividade das membranas compósitas é menos dependente da h.r. e da T, podendo ser até 1 ordem de grandeza superior à condutividade do Nafion® puro a 40 ºC e 20% de h.r. As diferenças são menos significativas a alta h.r., atingindo o valor máximo de 0.2 S∙cm-1 a 94 ºC e 98 % h.r. No entanto, contrariamente ao efeito de volume no E’ e na dilatação, a condutividade máxima foi atingida numa membrana com 20 vol.% de aditivos. Verifica-se que a condutividade das membranas tende a aumentar ligeiramente com o aumento da área superficial e da concentração de grupos ácido dos aditivos, sendo o efeito da ordem estrutural muito pouco visível. O aumento da condutividade das membranas compósitas, a baixa h.r., pode ser interpretado considerando a redução do confinamento protónico e, à medida que a h.r. aumenta, assumindo um efeito de superfície, no qual a presença dos aditivos provoca a ruptura de uma estrutura rígida que se forma à superfície da membrana de Nafion®, libertando a pressão interna e facilitando o acesso da água ao interior da membrana e desse modo aumentando a condutividade.Programa Doutoral em Ciência e Engenharia de Materiai

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

Nanomechanical behaviour of the monolithic framework solids: an experimental and modelling study

Metal-organic frameworks (MOFs) have established themselves as a versatile material platform for a wide variety of applications such as gas adsorption, energy conversion and storage, luminescence and chemical sensing. Over the past two decades, scientists have designed numerous MOF systems and composites that are specifically tailored for different applications, thanks to their extraordinarily large internal surface area and high tuneability. However, the integration of MOFs into real-world sensors and devices still represents a challenge. The majority of MOFs reported to date is in fact synthesised in the form of polydisperse powders, characterised by some intrinsic limitations. The aim of this thesis is to gain understanding of the mechanical behaviour of robust monolithic sol-gel MOFs, identified as a promising candidate for the transition of this class of materials from the academia to industrial deployment. The advantages and potential applications of MOF monoliths are described in Chapter 1. An overview of nanoindentation, the most used technique for the mechanical characterisation of MOFs, is provided in Chapter 2, along with a literature review of the field of MOF mechanics. Chapter 3 summarises the synthesis protocols and the material characterisation techniques utilised throughout the thesis. In Chapters 4, 5 and 6, different aspects of the mechanical response of the prototypical MOF monoliths were systematically studied by means of nanoindentation, spectroscopy, and finite element simulations. In particular, plasticity, fracture toughness and stress-strain relationships underpinning the mechanical performance of MOF monoliths are investigated, and their connections to the nanostructure and the framework architecture are established. Finally, the reported findings are critically summarised in Chapter 7, along with a personal perspective on the future development of the field

Synthesis and applications of ceramic (silicon carbide and silicon nitride), metallic (cobalt(0)) and polymeric (polyurethane) aerogels

A new method has been demonstrated for the synthesis of monolithic ceramic and purely metallic aerogels from xerogel powder compacts, and the use of polyurethane aerogels based on cyclodextrins as efficient desiccants. I. Highly porous ( \u3e 80%) monolithic SiC and Si3N4, aerogels were prepared from compressed compacts of polyurea-crosslinked silica xerogel powders. The process is time efficient as solvent-exchange through powders is fast, and energy efficient as it bypasses drying with supercritical fluids. The final ceramic objects were chemically pure, sturdy, with compressive moduli at 37 ±7 MPa and 59 ± 7 MPa, and thermal conductivities at 0.163 ± 0.010 W m-1 K-1 and 0.070 ± 0.001 W m-1 K-1, for SiC and Si3N4, respectively. II. Monolithic metallic Co(0) aerogels, synthesized from polyurea-crosslinked cobaltia xerogel powder compacts, were porous (69% v/v) and extremely sturdy (compressive modulus at 688 ± 10 MPa). They were infiltrated with molten LiClO4, and were ignited with a hot NiCr wire. The temperature during combustion reached 1515 °C. The heat released (-55.17 ± 2.01 kcal mol-1) was near the theoretical value for the reaction: 4 Co + LiClO4 ----\u3e 4 CoO + LiCl (-58.5 kcal mol-1). III. Polyurethane (PU) aerogels are low-density hierarchical nano-structured solids with high open nanoporosity, and high surface areas. Using α- and β-cyclodextrin (CD) as polyols, an aromatic triisocyanate and dibutyltin dilaurate (DBTDL) as a catalyst we obtained hyperbranched CD-based polyurethane aerogels (α- and β-CDPU-xx). Those materials show high water uptake capacities (108% w/w with α-CDPU-2.5) and can be reused multiple times by regeneration at room temperature by changing the relative humidity of the environment --Abstract, page v