Literatura académica sobre el tema "Xerogel aerogel"

Abstract In this work ternary bioactive glasses with the molar composition 63 % SiO2, 28 % CaO, and 9 % P2O5 have been prepared via sol-gel processing route leading to xerogel or aerogel glasses, depending on the drying conditions. Two types of drying methods were used: atmospheric pressure drying (evaporative), to produce xerogels, and supercritical fluids drying, to obtain aerogels. Both dried gels were subjected to heat-treatment at three different temperatures: 400, 600 and 800 ºC in order to the removal of synthesis byproducts and structural modifications. The resulting materials were characterized by X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and differential thermal analysis (DTA), and by in vitro bioactivity tests in simulated body fluid. The influence of the drying and the sintering temperature of their structure, morphology, and bioactivity of the final products were evaluated. The results show a good bioactivity of xerogel and aerogel bioactive glass powders with the formation of an apatite layer after one day of immersion in SBF solution for aerogel bioactive glass powders and a particle size less than 10 nm. An apatite layer formed after 3 days in the case of xerogel bioactive glass powders and a particle size around 100 nm.

A simple, versatile method has been developed to fabricate the transparent superhydrophobic surface via granuliform silica aerogels. The effect of ageing on the wettability, microstructure morphology and chemical structure of the dried gels has been investigated. Silica aerogel (dried alcogel with ageing) has a 3D porous network exhibiting the high surface area and pore volume. In comparison, large aggregates of silica nanoparticles exist within the backbone of the silica xerogel (dried alcogel without ageing). Both the aerogel and xerogel exhibit analogous chemical composition with abundant of methyl groups on the surface. The rough surface due to the high porosity and low surface energy provided by the methyl groups of aerogel contribute to the superhydrophobicity. Meanwhile, glass slides coated with aerogel film is highly transparent because the roughness created by the aerogel film is limited.

Silica aerogels or xerogels are renowned dried gels with low density, high surface area, higher porosity, and better thermal stability which makes it suitable for aerospace, light weight structures, thermal insulation, and hydrophobic coatings. But brittle behaviour, low mechanical strength, and high manufacturing cost restrict its usage. Recently, the addition of various fibres like glass or carbon fiber is one of the best reinforcement methods to minimize the brittle behaviour. Supercritical drying technique usually used to develop aerogel that is expensive and difficult to produce in bulk quantities. Higher cost obstacle can be tackled by applying ambient pressure drying technique to develop xerogel. But researcher observed cracks in samples prepared through the ambient pressure drying technique is still a major shortcoming. The aim of this study is to systematically analyze the influence of silica gel fiber reinforcement on silica xerogels, encompassing morphology, mechanics, thermal behaviour, compression test, and thermogravimetric characteristics. The research used a low-cost precursor named Tetraethyl orthosilicate to synthesize low-cost composite Silica xerogel and glass and carbon fiber added to provide strength and flexibility to the overall composite. Silica gel works as binder in strengthening the xerogel network. The investigation employs scanning electron microscopy (SEM) to examine the morphology of the composites, Fourier Transform Infrared (FTIR) analysis to affirm hydrophobic characteristics, compression tests to assess mechanical strength, and thermogravimetric tests to study weight loss under different conditions. SEM results reveals that glass fibers exhibit lower adhesion to the xerogel network compared to carbon fibers. FTIR analysis confirms the hydrophobicity of the composite silica xerogel. Compression tests showed that, under a 48% strain rate, the carbon fiber composite demonstrates superior compressive stress endurance. Thermogravimetric tests revealed a 1% lower weight loss for the carbon fiber composite compared to the glass fiber composite. This work concludes that glass and carbon fiber together with silica gel particles successfully facilitated in developing flexible, less costly, hydrophobic, and crack-free silica xerogel composites by APD. These advancements have the potential to drive innovations in material science and technology across diverse industries.

This work is dedicated to the study of the effect of the synthesis conditions (drying and calcination) of sulfated zirconia on the final catalytic behavior of bifunctional composite catalysts prepared by the physical mixing of the sulfated zirconia (methanol dehydration catalyst) with Cu/ZnO/Al2O3 (CZA; methanol synthesis catalyst). The main objective was to optimize the CZA-ZrO2/SO42− composite catalyst for its use in the direct production of dimethyl ether (DME) from syngas. Sulfated zirconia aerogel (AZS) and xerogel (XZS) were prepared using the sol–gel method using different solvent evacuation conditions and calcination temperatures, while the Cu-ZnO(Al) catalyst was synthesized using the coprecipitation procedure. The effectivity of CZA-ZrO2/SO42− composite catalysts for the direct production of dimethyl ether (DME) from syngas was evaluated in a flow reactor at 250 °C and 30 bar total pressure. The characterization of the sulfated zirconia aerogels and xerogels using different techniques showed that the mesoporous aerogel (AZS0.5300) exhibited the best textural and acidic properties due to the gel drying under supercritical conditions and calcination at 300 °C. As a result, the composite catalyst CZA-AZS0.5300 exhibited seven times higher DME production than its xerogel-containing counterpart (364 vs. 52 μmolDME·min−1·gcat−1). This was attributed to its well-matched metal surface, mesoporous structure, optimal crystallite size and, most importantly, its higher acidity.

Chitosan (CS) is a natural biopolymer that shows promise as a biomaterial for bone-tissue regeneration. However, because of their limited ability to induce cell differentiation and high degradation rate, among other drawbacks associated with its use, the creation of CS-based biomaterials remains a problem in bone tissue engineering research. Here we aimed to reduce these disadvantages while retaining the benefits of potential CS biomaterial by combining it with silica to provide sufficient additional structural support for bone regeneration. In this work, CS-silica xerogel and aerogel hybrids with 8 wt.% CS content, designated SCS8X and SCS8A, respectively, were prepared by sol-gel method, either by direct solvent evaporation at the atmospheric pressure or by supercritical drying in CO2, respectively. As reported in previous studies, it was confirmed that both types of mesoporous materials exhibited large surface areas (821 m2g−1–858 m2g−1) and outstanding bioactivity, as well as osteoconductive properties. In addition to silica and chitosan, the inclusion of 10 wt.% of tricalcium phosphate (TCP), designated SCS8T10X, was also considered, which stimulates a fast bioactive response of the xerogel surface. The results here obtained also demonstrate that xerogels induced earlier cell differentiation than the aerogels with identical composition. In conclusion, our study shows that the sol-gel synthesis of CS-silica xerogels and aerogels enhances not only their bioactive response, but also osteoconduction and cell differentiation properties. Therefore, these new biomaterials should provide adequate secretion of the osteoid for a fast bone regeneration.

Low dielectric constant (Low-[Formula: see text]) films are used as inter layer dielectric (ILD) in nanoelectronic devices to reduce interconnect delay, crosstalk noise and power consumption. Tailoring capability of porous low-[Formula: see text] films attracted more attention. Present work investigates comparative study of xerogel, aerogel and porogen based porous low-[Formula: see text] films. Deposition of SiO2 and incorporation of less polar bonds in film matrix is confirmed using Fourier Transform Infra-Red Spectroscopy (FTIR). Refractive indices (RI) of xerogel, aerogel and porogen based low-[Formula: see text] films observed to be as low as 1.25, 1.19 and 1.14, respectively. Higher porosity percentage of 69.46% is observed for porogen-based films while for shrinked xerogel films, it is lowered to 45.47%. Porous structure of low-[Formula: see text] films has been validated by using Field Emission Scanning Electron Microscopy (FE-SEM). The pore diameters of porogen based annealed samples were in the range of 3.53–25.50 nm. The dielectric constant ([Formula: see text]) obtained from RI for xerogel, aerogel and porogen based films are 2.58, 2.20 and 1.88, respectively.

CuFe2O4 is an example of ferrites whose physico-chemical properties can vary greatly at the nanoscale. Here, sol-gel techniques are used to produce CuFe2O4-SiO2 nanocomposites where copper ferrite nanocrystals are grown within a porous dielectric silica matrix. Nanocomposites in the form of both xerogels and aerogels with variable loadings of copper ferrite (5 wt%, 10 wt% and 15 wt%) were synthesized. Transmission electron microscopy and X-ray diffraction investigations showed the occurrence of CuFe2O4 nanoparticles with average crystal size ranging from a few nanometers up to around 9 nm, homogeneously distributed within the porous silica matrix, after thermal treatment of the samples at 900 °C. Evidence of some impurities of CuO and α-Fe2O3 was found in the aerogel samples with 10 wt% and 15 wt% loading. DC magnetometry was used to investigate the magnetic properties of these nanocomposites, as a function of the loading of copper ferrite and of the porosity characteristics. All the nanocomposites show a blocking temperature lower than RT and soft magnetic features at low temperature. The observed magnetic parameters are interpreted taking into account the occurrence of size and interaction effects in an ensemble of superparamagnetic nanoparticles distributed in a matrix. These results highlight how aerogel and xerogel matrices give rise to nanocomposites with different magnetic features and how the spatial distribution of the nanophase in the matrices modifies the final magnetic properties with respect to the case of conventional unsupported nanoparticles.

The sol-gel method was used to prepare nickel oxide–silica and nickel–silica nanocomposite materials and the corresponding silica matrices. Different drying conditions were used to obtain aerogel and xerogel materials. The samples were characterized by thermal analysis, x-ray diffraction, N2–physisorption, transmission electron microscopy techniques, and infrared spectroscopy. Aerogel samples had a much higher surface area than the xerogel samples; moreover, different supercritical drying conditions gave rise to a different porous structure, which influenced the size and distribution of the nanoparticles in the matrix.

During this century, rapid population growth has led to an increased exploitation of oil and nuclear power to satisfy energy demands. This situation has created a growing tension between the deteriorating condition of the natural environment and the needs of society. Several research projects have sought to improve the situation through the development of innovative products and eco-friendly technologies: for example, the purification of the gaseous emissions emitted by industrial sources, or the manufacture of vehicles using sunlight as a renewable and clean energy. In addition, newly created materials are able to reduce air pollutants (organic and inorganic) through the process of photocatalysis, which consists of using solid semiconductors able to oxidize harmful substances until complete mineralization. In this context, the production of photocatalytic building materials could constitute a very interesting solution, and become an integral part of the strategy to reduce environmental pollution. Titanium dioxide (TiO2) is one of the most common photocatalytic materials. It can be used in many fields of application, including the building industry. The combination of titanium dioxide with cement makes it possible to obtain a binder that has both traditional properties, such as mechanical strength and durability, and new properties allowing the preservation of the environment and the conservation of the aesthetic value of the buildings. For instance, researchers have managed to create a material capable of accelerating the oxidation of the organic pollutants that are deposited on the external walls of the buildings. This dissertation focused on the design, synthesis and characterization of a new family of titanium dioxide 1D, 2D and 3D nano-systems, doped with lithium and some transition metals in order to decrease the band gap and to enable visible light photoactivation even in indoor conditions. The NPs samples showed an increment of the relative surface areas that was better than the increments observed in all the nanosheet samples (446 m2/g-1). The use of lithium, cobalt, cerium, and tungsten ions as dopants was evaluated through the UV-Vis absorption technique. The results showed both an increased absorption in the visible range and a decrease of band gap for the doped samples in comparison to the sample of pure TiO2. We used several methods to evaluate the photocatalytic efficiency of all the nanoparticles and nanosheets we produced. The Li-Co and Li-Ce doped nanoparticles showed very good results both in water solution and in gas phase. The photocatalytic activity of our nanosystems was comparable with the standard samples under UVb light, but the doped NPs, in particular those doped with Au@TiLi(5) and TiLi(5)Co(5), showed better results under visible light than the control samples. Our new synthesis method achieved very interesting results. It allowed us to obtain nanoparticles, nanosheets, xerogels, and aerogels covered with Li+ ions in place of the H+ ion at the surface in order to maintain them in a good dispersion in solution. In order to evaluate the toxicity of the NPs, a comparative study on human lung cells and E. coli cells was carried out in vitro. All the analyzed nanoparticles exhibited different photocatalytic activities for human cells and bacteria, in particular under irradiation. Some transition metal ions doped nanoparticles exhibited cytotoxicity even in absence of illumination, a phenomenon that was probably caused by their dopant content. The use of such systems could constitute a risk for human health; however, once inhaled, the lack of illumination within the human body could reduce such a risk, and for TiLi and Au@TiLi(5) groups the danger is virtually absent. The toxicity on E. coli cells is significantly higher in almost all cases. In the case of the TiLiCo group, in particular, there is a high contrast between the low toxicity for human cells and the high toxicity for E. coli.

Les oxydes de zirconium et d’aluminium sont des matériaux de grande stabilité thermique dans des conditions différentes et ont une surface dotée de sites acido-basiques modulables. La modification de ces oxydes par un oxo-anion, par une fonction acide et/ou métallique leur donne plus de stabilité, inhibe le frittage et développe l’acidité de leur surface et par suite ils peuvent être utilisés dans plusieurs réactions catalytiques en tant que phase active ou support.Les travaux de cette thèse portent sur la synthèse, la caractérisation et la valorisation de deux types de catalyseurs xerogels et aerogels à base d’oxydes de zirconium et d’aluminium. Nous avons fait le choix d’utiliser l’acide tellurique comme modulateur de l’acidité des supports à base de zircone et d’alumine et d’associer le nickel à l’acide tellurique pour faire des phases de tellure de nickel sur ces supports permettant la réaction d’isomérisation.Dans un premier volet, des catalyseurs xerogels et aerogels à base de zircone et d’alumine dopées par l’acide tellurique ont été synthétisés par voie « sol-gel » puis caractérisés et valorisés dans la réaction d’estérification de l’acide acétique par l’alcool benzylique comme réaction modèle pour la synthèse de biodiesel. Une étude cinétique et mécanistique ainsi que la détermination des grandeurs thermodynamiques pour la réaction d’estérification ont été réalisées en présence du catalyseur ayant les meilleures performances catalytiques.Le second volet de cette thèse porte sur la synthèse des catalyseurs xerogels et aerogels à base de zircone et d’alumine dopées par l’acide tellurique et les nitrates de nickel en une seule étape, toujours synthétisés par voie « sol-gel ». Ces catalyseurs ont été caractérisés puis testés dans la réaction d’hydrogénation isomérisation du 1-hexene comme réaction modèle pour la reformulation d’essence. Nous montrerons que la présence de nickel dans la préparation empêche la formation d’une phase Zr-Te, caractérisée sur le support seul modifié par l’acide tellurique
Zirconium and aluminum oxides are materials of great thermal stability under different conditions and have a surface with modulable acid-base sites. The modification of these oxides by an oxo-anion, by an acid and / or metallic function gives them more stability, inhibits sintering and develops the acidity of their surface and consequently they can be used in several catalytic reactions as active phase or support.The work of this thesis focuses on the synthesis, characterization and upgrading of two types of xerogels and aerogels catalysts based on zirconium and aluminum oxides. Telluric acid was chosen as a modulator of the acidity of supports based on zirconia and alumina. Nickel was combined with telluric acid to make nickel tellurium phases on these supports, allowing the isomerization reaction.In a first part, xerogels and aerogels catalysts based on zirconia and alumina doped with telluric acid were synthesized by the “sol-gel” route then characterized and tested in the esterification reaction of acetic acid by benzyl alcohol as a model reaction for the synthesis of biodiesel. A kinetic and mechanistic study as well as the determination of the thermodynamic quantities for the esterification reaction were carried out in the presence of the catalyst having the best catalytic performance.The second part of this thesis focuses on the synthesis of zirconia and alumina-based xerogels and aerogels catalysts doped with telluric acid and nickel nitrates in one-pot, always synthesized by the "sol-gel" route. These catalysts were characterized and then tested in the isomerization hydrogenation reaction of 1-hexene as a model reaction for gasoline reformulation. We will show that the presence of nickel in the preparation prevents the formation of a Zr-Te phase, characterized on the support alone modified by telluric acid

Tato práce se zabývá systematickou studií syntézy a zpracováním pokročilých tepelně stabilních aerogelů pro potenciální vysokoteplotní aplikace. V první části dizertační práce jsou podrobně popsány syntetické implikace pro přípravu aerogelů a jejich aplikace spolu s popisem depozičních metod povlaků vytvořených pomocí sol-gel procesu. Experimentální postup je rozdělen do tří částí. První z nich představuje syntetické protokoly k přípravě ZrO2, YSZ, Ln2Zr2O7 (Ln = La3+, Nd3+, Gd3+, and Dy3+) aerogelů, Ln2Zr2O7 prášků a xerogelů. Dále je popsána depoziční metoda, která byla použita pro přípravu povlaků z aerogelů na kovových substrátech. Poté jsou následně specifikovány techniky, jež byly použity pro charakterizaci. Bylo zjištěno, že množství vody a kyseliny dusičné hraje rozhodující roli v přípravě gelů vhodných pro transformaci na aerogely. Po kalcinaci při 500 °C mají ZrO2 a YSZ aerogely velký povrch, a to až do 114 m2 g-1, avšak při 1000 °C dochází k úplnému zhuštění a ztrácí se tak veškerá jejich porézní struktura. Naopak ve srovnání s ZrO2 and YSZ jsou aerogely Ln2Zr2O7 tepelně stabilnější, protože si zachovávají svou porozitu při vyšší kalcinační teplotě (1000 °C), při které dosahují hodnot > 160 m2 g-1. Experimentálně bylo dále zjištěno, že ve studovaném teplotním rozsahu ZrO2 aerogel tvoří tetragonální komplex monoklinický fázový přechod řízený velikostí krystalitů, zatímco YSZ je tvořena jedinou tetragonální fází. Fázové složení zirkoničitanů vzácných zemin je vysoce závislé na způsobu syntézy; všechny Ln2Zr2O7 materiály jsou pyrochlorické nebo fluoritové krystalické fáze. Přímým odléváním aerogelu na kovový substrát dochází k úplnému rozpraskání povlaku z důvodu smršťování, zatímco máčením kovového substrátu v suspenzi je možné vyrobit homogenní, silné a hrubé povlaky z aerogelu. Tyto povlaky neobsahují fázové změny a zůstávají vysoce porézní i po různých tepelných úpravách.

Cellulose nanofibrils are a biobased and renewable material with potential to be used in many different applications. Such applications include air filtration, absorption of liquids, and thermal insulation.  To be used for these applications the cellulose nanofibrils must form a porous and dry material. However, maintaining some degree of porosity after drying is difficult, since the fibrils are extracted in liquid and tend to collapse into a dense material upon drying. Certain methods have proven effective for making a dry porous material from cellulose nanofibrils, but these are often expensive and not suitable for large scale production. The aim of this project is to test possible methods for making a highly porous cellulose nanofibril-based material. These methods must be environmentally sustainable and suitable for large scale production. An extensive screening has been conducted with the aim of identifying methods resulting in materials with high porosity. The obtained materials have been analysed further to give a more thorough understanding of the porosity as well as other characteristics. The results indicate that cross-links in the material strengthen the structure, and that drying samples from water always results in complete collapse or very dense materials while drying samples from certain solvents other than water results in more porous materials. The analysed materials had very different porosities, some of which were relatively high. The most porous material analysed by Brunauer-Emmett-Teller gas adsorption had a surface area of 9.5 m2/g. This project gives insight into how cross-linking chemistries and treatment with different solvents and pH affect the resulting cellulose nanofibril-based material, as well as knowledge about which methods can be used to successfully produce dry porous cellulose nanofibril-based materials.

Ce travail de recherche présente la préparation et la caractérisation de matériaux monolithiques hautement poreux dérivés majoritairement de ressources naturelles. L'objectif était de préparer de nouveaux gels biosourcés jusqu'à 91%, de proposer des alternatives au séchage supercritique au CO2, et d'étudier quelques propriétés d'intérêt de tels gels après séchage, non seulement à l'état organique mais, dans certains cas, après pyrolyse pour obtenir des gels de carbone. A ces fins, le tannin et le soja ont été testés comme précurseurs, à différentes concentrations et différents pH, et trois voies de séchages ont été utilisées : supercritique, lyophilisation et séchage évaporatif. Les gels obtenus ont été caractérisés en termes de densité, porosité, distributions de tailles de pores et surfaces spécifiques, qu'ils soient sous forme organique ou carbonée selon l'application envisagée ou le type de porosité attendue. Leurs propriétés mécaniques et thermiques ont aussi été mesurées. La très large gamme de textures poreuses obtenues a permis de proposer des applications en tant qu'isolants thermiques, supports de catalyseur, ou électrodes de condensateur électrochimiques, selon les cas
This manuscript presents the preparation and the characterization of highly porous monolithic materials mainly derived from natural resources. The objectives were to: (i) develop new gels, biosourced up to the 91% level; (ii) suggest alternatives to supercritical drying in CO2, and (iii) investigate properties of interest for such gels in the organic state and, in some cases, after pyrolysis for obtaining carbon gels. For those purposes, tannin and soy flour were tested as precursors, at different concentrations and different pH, and three ways of drying were used: supercritical drying, freeze drying and evaporative drying. The obtained gels were characterized in terms of density, porosity, pore size distributions and specific surface area, whether in organic or in carbon form, depending on the intended application or expected type of porosity. Mechanical and thermal properties were also measured. The obtained broad range of porous textures allowed suggesting applications such as thermal insulators, catalyst supports or electrodes for electrochemical capacitors

Le procédé aérosol-gel est une nouvelle technique de dépôt sol-gel. Son principe repose sur la pulvérisation d'un sol par voie ultrasonore. L'aérosol ainsi généré est transporte par un gaz vecteur vers le substrat et forme, en se déposant, un film liquide. Ce film conduit après évaporation du solvant et polymérisation du dépôt, a la formation d'une couche mince xérogel homogène et transparente. Des films SiO² ont pu être obtenus a partir d'un système chimique de type tetraethoxysilane/eau/acide chlorhydrique/alcool. Apres recuit à 500°C, ces films sont purs et présentent une densité proche de celle de la silice massive. Des films parfaitement denses ont été obtenus par recuit direct à 800°C, en plaçant l'échantillon dans un four préchauffé. Des films TiO² ont été élaborés a partir d'un système chimique de type tétrabutoxyde de titane/diéthanolamine/alcool. La diéthanolamine permet une bonne stabilisation du tetrabutoxyde de titane, et empêche la formation de précipités dans la solution source. Apres recuit à 500°C, les films TiO² sont purs et présentent une porosité de l'ordre de 25%. Cette porosité peut être abaissée à moins de 10% par recuit direct. Des films SiO²-TiO² mixtes ont été élaborés à partir du mélange des systèmes chimiques précédents. L'indice de réfraction de ces films a pu être module continument entre 1,43 (0% de titane) et 2,25 (100% de titane). Aucune déviation du rapport Ti/Si entre la solution source et les films recuits n'a été observée. Cette étude montre que le procédé aérosol-gel est adaptable à un grand nombre de systèmes chimiques. De plus, sa compatibilité avec une production massive (couplage possible avec un four a défile, ou four à carrousel) lui donne d'importantes potentialités, en vue d'une utilisation a échelle industrielle.

Tese de Doutoramento em Engenharia Química apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra.
Environmental pollution is a severe issue that might cause permanent changes, such as the destruction of ecosystems and loss of biodiversity. Heavy metals are the main pollutant in European soils and groundwater. Their main source are human activities, the most relevant being mining and ore processing. Their non-biodegradability causes accumulation in the environment and the remediation of contaminated sites is difficult and might not be possible to do in situ. Thus, the preservation of natural environments is dependent on minimizing pollutant emissions, with effluent treatment being one possible approach. In this thesis the removal via sorption of copper, lead, cadmium and nickel from water is studied. These were selected due to their current relevance based on wide application, economic value, high toxicity and emissions. The focus of the work is the development of adsorbents for these metals, achieved by the synthesis of organically modified silica aerogels and xerogels, and the study of the sorption process. The goal is the development of a multipurpose solution, able to remove all pollutants, as well as producing selective copper and nickel adsorbents, due to high economic value of these metals. The adsorbents are synthesized though acid-base catalyzed sol-gel chemistry. The surface chemistry modification is achieved by using precursors containing organic moieties of interest. When appropriate, the organic groups on the precursors are modified prior to the gel’s synthesis. The effects of the drying stage, via supercritical fluids extraction with carbon dioxide or via evaporative drying, on the final material’s properties are also assessed. The adsorbents are characterized structurally, chemically and thermally. To remove all cations under study, the silica matrixes are modified with organic groups containing sulfur (thiol groups) or nitrogen (e.g., amine) electron donor atoms, resulting in several different formulations. The study of silica matrixes modified with thiol groups is presented in Chapter 4 whilst the study of nitrogen modified matrixes is in Chapter 5. The modification of the silica matrix was found to reduce the specific surface area. Formulations containing amine groups are densified when dried by evaporation, severely decreasing porosity and surface area, the latter as low as 2 m2 g−1. Comparing with pristine (non-modified) matrixes, it was revealed that the surface chemistry modification is required for this application, with the former only interacting with lead (the removal for the remainder cations is <9% with a starting concentration of 50 mg L−1). In general, the modified matrixes interact with the cations in solution, with isocyanurate and urea groups being notable exceptions (removal <10% with a starting cation concentration of 50 mg L−1, except for lead). Amine groups are the most efficient of those tested, improving even the adsorptive performance of thiol modified sorbents, even on materials of reduced porosity. The in-depth study of selected formulations revealed that the sorption takes a few hours, and the adsorbent-cation interactions are strong and not easily reversible. The aerogel containing primary and secondary amines (A_A+3A) was chosen as the best multipurpose adsorbent (Langmuir adsorption capacity of 60 mg g−1 for copper, 347 mg g−1 for lead, 83 mg g−1 for cadmium and 66 mg g−1 for nickel). Its study with binary mixtures of cations revealed that it is selective for copper up to six hours of the adsorption process (selectivity of 2.6 in relation to nickel and 49 in relation to cadmium). Thus, A_A+3A is both the multipurpose solution and the copper selective one, and the outcome of the adsorption is controlled by the operating conditions. Desorption of the cations from this material is possible with acids, but it leads to partial loss of the sportive performance, to a third of the original one. The recovery of desorbed copper can be achieved via electrodeposition. The modification of the silica matrix with certain ionophores led to the synthesis of selective aerogels. The azole modification of the silica generated a material that interacts poorly with the cations; hence a significant removal of copper was not observed. This result was attributed to the possible inaccessibility of the azole to the cations, due to the aerogel’s porous structure and steric hindrance. The nickel selective, salen based adsorbent features aerogel-like characteristics, due to a small shrinkage of 10% after evaporative drying, and has a high affinity for nickel, being its removal higher and unaffected by the presence of competing ions. Despite the isotherms being shaped like a BET curve, the existence of cation multilayers is unlikely, and the aerogel-cation interactions are defined by the hydrated radius of the cation. Its estimated selectivity reveals that this adsorbent is twice as selective for nickel than for copper and 9 times than for cadmium. Due to the high affinity with nickel, its desorption is very limited. This work allowed to demonstrate the high adsorptive performance of organically modified silica aerogels for heavy metals relevant in some industrial effluents, soil and water, when compared with other commercial or researched adsorbents. The great chemical versatility of the silica matrix also allowed to adapt its surface chemistry to obtain more general use or selective solutions, taking advantage of the high porosity and surface area of aerogels.
A poluição é um grave problema que pode conduzir a alterações irreversíveis, como a destruição de ecossistemas e perda de biodiversidade. Os metais pesados são os principais contaminantes de solos e águas subterrâneas na Europa. A sua principal fonte de emissão são atividades humanas, sendo as indústrias de extração e processamento de minérios as mais relevantes. Por serem poluentes não biodegradáveis, acumulam-se e a remediação de áreas contaminadas é difícil e nem sempre possível in situ. Desta forma, a preservação do meio ambiente passa pela minimização das emissões poluentes, sendo o tratamento de efluentes uma das estratégias para tal. Nesta tese é estudada a remoção de cobre, chumbo, cádmio e níquel de água, através do processo de adsorção. Esta seleção recai na elevada importância destes quatro metais na sociedade atual, quer seja pela sua ampla utilização e valor económico, ou pelas elevadas toxicidade e emissões. O trabalho realizado foca-se no desenvolvimento de adsorventes para estes metais, através da síntese de aerogéis e xerogéis de sílica organicamente modificados e caracterização do processo de sorção dos metais por estes. Pretende-se desenvolver adsorventes que possam funcionar como uma solução transversal, removendo todos os metais contaminantes, mas também produzir soluções seletivas para cobre e níquel, devido ao seu valor económico. Os adsorventes são sintetizados através da metodologia sol-gel, catalisada por ácidos e bases, sendo que a modificação da sua química de superfície é conseguida através de co-precursores organicamente modificados. Quando conveniente, a modificação dos grupos orgânicos dos precursores é feita a priori da síntese do adsorvente. A influência do processo de secagem, por extração com dióxido de carbono supercrítico ou por evaporação, nas propriedades do material final foi também avaliada. Os adsorventes foram alvo de caracterização física/estrutural, química e térmica. Com o objetivo de remover todos os metais em estudo, a modificação das matrizes de sílica é feita através de grupos orgânicos contendo enxofre (tiol) ou azoto (p. ex. amina) como átomos dadores de electrões, dando origem à síntese de diferentes formulações. O estudo das matrizes modificadas com grupo tiol encontra-se no Capítulo 4 enquanto que o estudo das matrizes com azoto encontra-se no Capítulo 5. A modificação da matriz de sílica conduz a uma diminuição de área de superfície específica. Quando as formulações contêm grupos amina, a sua secagem por evaporação densifica o material, levando a uma perda acentuada de porosidade e área de superfície específica, esta última podendo chegar a 2 m2 g−1. A comparação com um aerogel não modificado revelou que a modificação da sílica é necessária para esta aplicação, pois o material não modificado apenas interagiu com chumbo (remoção para os restantes catiões <9% com uma concentração inicial de 50 mg L−1). A generalidade das modificações verificou-se capaz de interagir com os metais em solução, sendo os grupos isocianurato e ureia as exceções (remoção ≤10% com uma concentração inicial de catião 50 mg L−1, exceto para chumbo). Os grupos amina são mais eficientes que os demais testados, tendo inclusivamente melhorado o desempenho sortivo de matrizes modificadas com tiol, mesmo em materiais de reduzida porosidade. O estudo mais detalhado de formulações selecionadas revelou que processo de sorção é concluído em algumas horas e as interações com os catiões são fortes e dificilmente reversíveis. Foi selecionado o aerogel contendo aminas primárias e secundárias (A_A+3A) como o melhor adsorvente transversal para os catiões em estudo (capacidade de adsorção de Langmuir de 60 mg g−1 para cobre, 347 mg g−1 para chumbo, 83 mg g−1 para cádmio e 66 mg g−1 para níquel). O seu estudo em misturas binárias permitiu concluir que até seis horas de adsorção este material revela um comportamento seletivo para cobre (2.6 em relação a níquel e até 49 a cádmio). Assim o adsorvente A_A+3A é simultaneamente uma solução transversal e seletiva para cobre, sendo o resultado da adsorção controlado pelas condições operatórias. A sua regeneração por via química com ácidos é possível, mas conduz à perda parcial de características adsortivas, para cerca de um terço da capacidade inicial. A recuperação do cobre dessorvido pode ser feita por eletrólise. A modificação da matriz de sílica com certos ionóforos conduziu ao desenvolvimento de aerogéis seletivos. A introdução de azóis na sílica resultou num material que não interage com os catiões, não se verificando uma remoção significativa de cobre. Tal facto deve-se à inacessibilidade dos átomos dadores de electrões por parte dos catiões, devido à microestrutura do material e a repulsão estérea dos grupos funcionais vizinhos. O adsorvente baseado num salen revelou propriedades estruturais semelhantes às de um aerogel mesmo após secagem evaporativa, devido a um encolhimento reduzido (10%), e tem uma acrescida afinidade para o níquel, sendo a sua capacidade remoção para este último superior e inalterada pela presença dos demais catiões. Apesar das isotérmicas apresentarem uma forma do tipo BET, a existência de multicamadas de catiões não é plausível e o tipo de interações adsorvente-adsorvato é essencialmente definido pelo raio iónico dos catiões. A seletividade estimada revela que este adsorvente é duas vezes mais seletivo para níquel do que para cobre e até 9 vezes para cádmio. Devido à forte interação com níquel a sua dessorção é muito limitada. Assim, este trabalho permitiu demonstrar a elevada capacidade adsorptiva dos aerogéis de sílica modificados organicamente para metais pesados com presença relevante em efluentes de algumas indústrias, bem como em solos e águas, quando comparados com outros adsorventes comerciais ou em investigação. A assinalável versatilidade química da rede de sílica permite ainda adaptar facilmente a química de superfície destes materiais para obter soluções de remoção mais amplas ou seletivas, tirando cumulativamente partido da elevada porosidade e área de superfície dos aerogéis.

Chapter 1: Supramolecular gels and their applications Gels are viscoelastic materials composed of a solid-like three dimensional fibrillar network that is embedded in a liquid. Supramolecular gels belong to a class of gels which are derived from low molecular weight compounds (typically < 1000). A variety of non-covalent interactions like H-bonding, π-π stacking, donor-acceptor, metal coordination, solvophobic and van der Waals interactions are involved in the formation of the self-assembled fibrous networks (SAFIN’s) in these gels. These non-covalent interactions are weak in nature and as a result, these gels can be reverted back to sol by heating and this process is reversible. These gels are further classified as hydrogels, organogels and aero/xerogels depending on the medium they encompass. Although low molecular weight gelators were known in the early part of the 20th century, it is only in the last two decades that this field has generated widespread interest among scientists. In the 90s, the investigations on these kinds of gels mainly focused on designing new gelator molecules. However, during the last decade, the research interest in this field has shifted more towards designing functional gels. Such gels Scheme 1. Various applications of functional supramolecular gels have been extensively utilized in the templated synthesis of inorganic nanomaterials, in making hybrid materials, as synthetic light harvesting systems, as sensors, in the field of biomaterials such as drug delivery, screening of enzyme inhibitors and tissue engineering and also in the field of organic optoelectronics. In this chapter a few selected examples from each of these fields are highlighted. Chapter 2: Charge transfer induced organogels from 2,3dialkoxyanthracenes and 2,4,7-trinitrofluorenone 2,3-Di-n-alkoxyanthracenes formed charge transfer (CT) interaction promoted organogels in the presence of electron acceptor 2,4,7-trinitrofluorenone (TNF). These dialkoxyanthracences (in the absence of TNF) have been reported previously to form gels in a variety of organic solvents. The gelation property was found to be dependent on the chain length and the derivatives with C6-C16 chains were found to be gelators. On the other hand derivatives with C5-C1 chains were found to be non-gelators. It was found that TNF not only modulated the gelation property of the efficient organogelators, it also transformed the weak and non-gelators into efficient gelators. This charge transfer induced gelation was observed for the derivatives with C10-C4 chains in alcoholic and hydrocarbon solvents whereas the shorter chain derivatives C3-C1 did not form gels. Several other alkoxy and dialkoxy derivatives with substituents in other positions did not show gelation in the presence of TNF. These results suggested that two structural aspects are necessary for these derivatives to form CT gels- the alkoxy chain length and the position of the alkoxy substituents. The thermal stability of all these gels was found to be maximum with a 1:1 stoichiometry of the donor and the acceptor. The common observation, the intensification of color in going from the sol to the gel phase, supported the crucial role of the charge transfer interaction behind the formation of these gels. The rheological characterization of the gels demonstrated that they Figure 1. Chemical structures of 2,3-dialkoxyanthracenes and TNF (middle) and a fluorescence confocal microscopy image (left) and a photograph (right) of DDOA-TNF gel. behaved like viscoelastic soft solids. Chapter 3: A new class of perfluorinated derivatives of bile acids: Synthesis and gelation properties A new class of bile acid based gelators was designed by connecting the side chains of the facially amphiphilic bile acid with perfluoroalkyl chains of different lengths through two different ester linkages-–O-(CO)-and –(CO)-OCH2-. All these three structural aspects i.e. the bile acid moiety, the fluoroalkyl chain length and the spacer were found to influence the gelation properties of the derivatives. Depending on them, there was a variation in terms of the nature of the solvent gelated, the CGCs, the mechanical properties of the gels, etc among the derivatives. The deoxycholic and lithocholic derivatives with the spacer –O-(CO)-formed gels in aromatic hydrocarbons and also in DMSO depending on the fluoroalkyl chain length. The mechanical properties of the gels formed in DMSO were found to be dependent on the bile acid moiety and the fluoroalkyl chain length. In general, the deoxy analogues showed higher elasticity, stiffness and yield stress values for their gels than the litho derivatives. The perfluorinated derivatives having the spacer –(CO)-OCH2-showed gelation properties in organic-aqueous media and in DMSO. Interestingly, organogelation was observed in the deoxy and lithocholic derivatives from both spacer series whereas in the literature most of the bile acid based organogelators are derived from cholic acid. (b) (c) Figure 2. (a) Perfluorinated derivatives of bile acids, (b) photographs of a few DMSO gels and (c) TEM image of a xerogel of a deoxy derivative Chapter 4: Composite aerogels and organogels from 2,3didecyloxyanthracene and bile-perfluoro derivatives Aerogels are unique materials among solids. They have extremely low densities (up to 95% of their volume is air), large pores and high inner surface area. As a result aerogels have very interesting physical properties such as extremely low thermal conductivity, low sound velocity and high optical transparency. There are only a few reports of aerogel formation by low molecular weight gelators. We have investigated the aerogel formation ability of three long 7 chain perfluoroalkyl esters (two deoxycholic and one lithocholic derivative, chart 1) in supercritical CO2. A deoxy derivative, formed aerogel in sc-CO2. When mixed with DDOA (which has been reported previously to form good aerogels in sc-CO2), the perfluoro compound formed aerogels of better quality. The mixed aerogels were characterized by the presence of very large fibers in the micron range (as observed in the aerogel formed by only the fluoro derivative) as well as fibers of smaller size observed in pure DDOA aerogel. We also investigated the behavior of the composite systems in organic solvents. It was found that in DMSO, another deoxy derivative, Figure 3. SEM images of a mixed aerogel of DDOA-DC23C13F27 (left) and a mixed organogel (DMSO) of DDOA-DC23C11F23 (right). DC23C11F23 formed gels with higher thermal stability and improved mechanical properties compared to the native gels of the perfluoro compound or DDOA. Chapter 5: Hydrogels from lanthanide(III) cholates: Tunable, multiple color luminescence from hydrogels and xerogels In this chapter, facile hydrogel formation by several lanthanide cholates is reported. When sodium cholate was added to aqueous solutions of Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III) and Yb(III) and sonicated, the mixtures formed gels within a few seconds. The gels thus obtained were transparent/translucent and thermoirreversible. Rheological measurements showed that all of them could be classified as viscoelastic soft solids. A naphthalene derivative, 2,3-dihyroxynaphthalene was found to sensitize Tb(III) emission very efficiently in its cholate gel when doped in micromolar concentrations. The importance of the gel matrix behind sensitization of Tb(III) was demonstrated by the inefficiency of the same sensitizer DHN in an SDS micellar solution. In mixed gels of Tb(III)-Eu(III) doped with DHN, a energy transfer pathway was found to occur from the sensitized Tb(III) to Eu(III). By a simple tuning of the ratio of these two lanthanide ions, multiple color emissive gels could be made.The emissive properties of the hydrogels were retained in the xerogels and the suspensions of these xerogels in n-hexane were used for making luminescent coatings on glass surface. Figure 4. Tunable, multi-color luminescent hydrogels and xerogels of lanthanide cholates

Chapter 1: Supramolecular gels and their applications Gels are viscoelastic materials composed of a solid-like three dimensional fibrillar network that is embedded in a liquid. Supramolecular gels belong to a class of gels which are derived from low molecular weight compounds (typically < 1000). A variety of non-covalent interactions like H-bonding, π-π stacking, donor-acceptor, metal coordination, solvophobic and van der Waals interactions are involved in the formation of the self-assembled fibrous networks (SAFIN’s) in these gels. These non-covalent interactions are weak in nature and as a result, these gels can be reverted back to sol by heating and this process is reversible. These gels are further classified as hydrogels, organogels and aero/xerogels depending on the medium they encompass. Although low molecular weight gelators were known in the early part of the 20th century, it is only in the last two decades that this field has generated widespread interest among scientists. In the 90s, the investigations on these kinds of gels mainly focused on designing new gelator molecules. However, during the last decade, the research interest in this field has shifted more towards designing functional gels. Such gels Scheme 1. Various applications of functional supramolecular gels have been extensively utilized in the templated synthesis of inorganic nanomaterials, in making hybrid materials, as synthetic light harvesting systems, as sensors, in the field of biomaterials such as drug delivery, screening of enzyme inhibitors and tissue engineering and also in the field of organic optoelectronics. In this chapter a few selected examples from each of these fields are highlighted. Chapter 2: Charge transfer induced organogels from 2,3dialkoxyanthracenes and 2,4,7-trinitrofluorenone 2,3-Di-n-alkoxyanthracenes formed charge transfer (CT) interaction promoted organogels in the presence of electron acceptor 2,4,7-trinitrofluorenone (TNF). These dialkoxyanthracences (in the absence of TNF) have been reported previously to form gels in a variety of organic solvents. The gelation property was found to be dependent on the chain length and the derivatives with C6-C16 chains were found to be gelators. On the other hand derivatives with C5-C1 chains were found to be non-gelators. It was found that TNF not only modulated the gelation property of the efficient organogelators, it also transformed the weak and non-gelators into efficient gelators. This charge transfer induced gelation was observed for the derivatives with C10-C4 chains in alcoholic and hydrocarbon solvents whereas the shorter chain derivatives C3-C1 did not form gels. Several other alkoxy and dialkoxy derivatives with substituents in other positions did not show gelation in the presence of TNF. These results suggested that two structural aspects are necessary for these derivatives to form CT gels- the alkoxy chain length and the position of the alkoxy substituents. The thermal stability of all these gels was found to be maximum with a 1:1 stoichiometry of the donor and the acceptor. The common observation, the intensification of color in going from the sol to the gel phase, supported the crucial role of the charge transfer interaction behind the formation of these gels. The rheological characterization of the gels demonstrated that they Figure 1. Chemical structures of 2,3-dialkoxyanthracenes and TNF (middle) and a fluorescence confocal microscopy image (left) and a photograph (right) of DDOA-TNF gel. behaved like viscoelastic soft solids. Chapter 3: A new class of perfluorinated derivatives of bile acids: Synthesis and gelation properties A new class of bile acid based gelators was designed by connecting the side chains of the facially amphiphilic bile acid with perfluoroalkyl chains of different lengths through two different ester linkages-–O-(CO)-and –(CO)-OCH2-. All these three structural aspects i.e. the bile acid moiety, the fluoroalkyl chain length and the spacer were found to influence the gelation properties of the derivatives. Depending on them, there was a variation in terms of the nature of the solvent gelated, the CGCs, the mechanical properties of the gels, etc among the derivatives. The deoxycholic and lithocholic derivatives with the spacer –O-(CO)-formed gels in aromatic hydrocarbons and also in DMSO depending on the fluoroalkyl chain length. The mechanical properties of the gels formed in DMSO were found to be dependent on the bile acid moiety and the fluoroalkyl chain length. In general, the deoxy analogues showed higher elasticity, stiffness and yield stress values for their gels than the litho derivatives. The perfluorinated derivatives having the spacer –(CO)-OCH2-showed gelation properties in organic-aqueous media and in DMSO. Interestingly, organogelation was observed in the deoxy and lithocholic derivatives from both spacer series whereas in the literature most of the bile acid based organogelators are derived from cholic acid. (b) (c) Figure 2. (a) Perfluorinated derivatives of bile acids, (b) photographs of a few DMSO gels and (c) TEM image of a xerogel of a deoxy derivative Chapter 4: Composite aerogels and organogels from 2,3didecyloxyanthracene and bile-perfluoro derivatives Aerogels are unique materials among solids. They have extremely low densities (up to 95% of their volume is air), large pores and high inner surface area. As a result aerogels have very interesting physical properties such as extremely low thermal conductivity, low sound velocity and high optical transparency. There are only a few reports of aerogel formation by low molecular weight gelators. We have investigated the aerogel formation ability of three long 7 chain perfluoroalkyl esters (two deoxycholic and one lithocholic derivative, chart 1) in supercritical CO2. A deoxy derivative, formed aerogel in sc-CO2. When mixed with DDOA (which has been reported previously to form good aerogels in sc-CO2), the perfluoro compound formed aerogels of better quality. The mixed aerogels were characterized by the presence of very large fibers in the micron range (as observed in the aerogel formed by only the fluoro derivative) as well as fibers of smaller size observed in pure DDOA aerogel. We also investigated the behavior of the composite systems in organic solvents. It was found that in DMSO, another deoxy derivative, Figure 3. SEM images of a mixed aerogel of DDOA-DC23C13F27 (left) and a mixed organogel (DMSO) of DDOA-DC23C11F23 (right). DC23C11F23 formed gels with higher thermal stability and improved mechanical properties compared to the native gels of the perfluoro compound or DDOA. Chapter 5: Hydrogels from lanthanide(III) cholates: Tunable, multiple color luminescence from hydrogels and xerogels In this chapter, facile hydrogel formation by several lanthanide cholates is reported. When sodium cholate was added to aqueous solutions of Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III) and Yb(III) and sonicated, the mixtures formed gels within a few seconds. The gels thus obtained were transparent/translucent and thermoirreversible. Rheological measurements showed that all of them could be classified as viscoelastic soft solids. A naphthalene derivative, 2,3-dihyroxynaphthalene was found to sensitize Tb(III) emission very efficiently in its cholate gel when doped in micromolar concentrations. The importance of the gel matrix behind sensitization of Tb(III) was demonstrated by the inefficiency of the same sensitizer DHN in an SDS micellar solution. In mixed gels of Tb(III)-Eu(III) doped with DHN, a energy transfer pathway was found to occur from the sensitized Tb(III) to Eu(III). By a simple tuning of the ratio of these two lanthanide ions, multiple color emissive gels could be made.The emissive properties of the hydrogels were retained in the xerogels and the suspensions of these xerogels in n-hexane were used for making luminescent coatings on glass surface. Figure 4. Tunable, multi-color luminescent hydrogels and xerogels of lanthanide cholates