Добірка наукової літератури з теми "Accessibility to microporosity"

Based on silica hollow nanospheres grafted with an ionic shell, silica-based type I porous liquids remain poorly exploited, despite their huge versatility. We propose here to explore the main synthesis step of these promising materials with a thorough characterization approach to evaluate their structural and porous properties. Modifying the main synthesis parameter, the mechanism of the spheres’ formation is clarified and shows that the calcination temperature, the surfactant concentration as well as the micelle swelling agent concentration allow tuning not only the size of the nanospheres and internal cavities, but also the silica shell microporosity and, therefore, the accessibility of the internal cavities. This study highlights the key parameters of hollow silica nanospheres, which are at the basis of type I porous liquids synthesis with optimized structural and porous properties.

Adsorption isotherms of phenol, p-nitrophenol and p-hydroxybenzoic acid at three temperatures in the range 1–40° C on activated carbon have been obtained experimentally. Curve fitting to several theoretical equations shows that the Prausnitz equation gives the lowest deviations (1–3%). An inversion of the adsorption capacity of phenol with temperature has been observed, probably as a consequence of the high microporosity of the carbon and the restricted accessibility of phenol to the smaller pores. Adsorption at 40°C is also accompanied by the degradation of phenol by aerobic micro-organisms (ficomicetus-type fungi). Approximate values of the isosteric heats of adsorption of p-nitrophenol have been calculated, being in the range 5.8–6.5 kcal/mol. Differences in the behaviour of the three adsorbates are commented upon in terms of their physicochemical properties.

Silicone and pure organic binders were used to develop FAU-type zeolite coatings applied on pre-treated aluminum substrates by using a spraying method and then cured under specific conditions. The influence of the amount of binder on adhesion properties of zeolite coatings was first investigated to determine the optimum ratio between zeolite and binder. Zeolite coatings were then elaborated with a high zeolite content (between 70 and 80 wt.%) to ensure high adsorption capacities. The amount of binders involved in different zeolite coatings was sufficient to achieve interesting adhesion and cohesion properties. The accessibility of zeolite microporosity was studied by nitrogen adsorption-desorption measurements, which revealed a very small or no loss of the micropore volume for the optimized coatings. Volatile Organic Compounds (VOCs) adsorption measurements were carried out using n-hexane as probe molecule. FAU-type zeolite in powder form adsorbs 180 mg/ganhydrous zeolite, whereas the amounts of n-hexane adsorbed by zeolite coatings ranged from 131 to 175 mg/ganhydrous zeolite.

Carbon monoliths were tested as electrodes for vanadium redox batteries. The materials were synthesised by a hard-templating route, employing sucrose as carbon precursor and sodium chloride crystals as the hard template. For the preparation process, both sucrose and sodium chloride were ball-milled together and molten into a paste which was hot-pressed to achieve polycondensation of sucrose into a hard monolith. The resultant material was pyrolysed in nitrogen at 750 °C, and then washed to remove the salt by dissolving it in water. Once the porosity was opened, a second pyrolysis step at 900 °C was performed for the complete conversion of the materials into carbon. The products were next characterised in terms of textural properties and composition. Changes in porosity, obtained by varying the proportions of sucrose to sodium chloride in the initial mixture, were correlated with the electrochemical performances of the samples, and a good agreement between capacitive response and microporosity was indeed observed highlighted by an increase in the cyclic voltammetry curve area when the SBET increased. In contrast, the reversibility of vanadium redox reactions measured as a function of the difference between reduction and oxidation potentials was correlated with the accessibility of the active vanadium species to the carbon surface, i.e., was correlated with the macroporosity. The latter was a critical parameter for understanding the differences of energy and voltage efficiencies among the materials, those with larger macropore volumes having the higher efficiencies.

Due to their potential applications in catalysis, separation, gas storage, drug delivery, and biosensing, porous materials (PMs) such as porous organic polymers (POPs), polymers with intrinsic microporosity (PIMs), porous coordination polymers (PCPs), and metal–organic frameworks (MOFs), have received much attention. Various building blocks have been prepared and demonstrated various functional materials. Among the various organic building blocks, porphyrin has become one of the most important building blocks for the construction of such materials witnessed by a wide range of molecular architectures using porphyrin derivatives with various applications. Mn(III)- and Fe(III)-containing metalloporphyrins are often used to fabricate various functional molecular architectures and to mimic the extraordinary behavior of enzymes in both homogeneous and heterogeneous catalytic systems. In the case of homogeneous catalysis, Mn(III)- and Fe(III)-containing metalloporphyrins have frequent trouble with fast catalytic degradation because of µ-oxo dimer formation or ligand oxidation. To avoid the catalyst degradation, the immobilization and/or site-isolation of homogeneous catalysts using supporters such as polymers, membranes, and MOFs, are widely used strategy. In addition, they are often used to modify the surface of porous silica materials such as SBA-15, MCM-41 and MCM-48, because these porous silica materials exhibit narrow pore size distributions, high thermal stability and easy accessibility. In this presentation, we like to show the use of metalloporphyrins in various porous materials and their use as heterogeneous catalysts. References D. Y. Shin, J. H. Yoon, S. H. Kim, H. Baik, S. J. Lee,* "Immobilization of Porphyrinic Mn(III) Catalyst on a New Class of Silica Support Comprising Three-Dimensionally Interconnected Network with Two Different Sizes of Pores", Catal. Sci. Technol. 2018, 8, 6306-6310. J. Yi, H. Y. Jeong, D. Y. Shin, C. Kim, S. J. Lee,* "Mn(III)-Porphyrin Containing Heterogeneous Catalyst based on Microporous Polymeric Constituents as a New Class of Catalyst Support", ChemCatChem 2018, 10, 3974-3977. J. Yoon, H. M. Choi, S. J. Lee,* "Cu(II)Cl2 containing bispyridine-based porous organic polymer support prepared via alkyne–azide cycloaddition as a heterogeneous catalyst for oxidation of various olefins", New J. Chem. 2020, 44, 9149-9152. H. M. Choi, Y. J. Kim, E. T. Choi, S. J. Lee,* "Selective Photocatalytic Oxidative Detoxification of a Chemical Warfare Agent Simulant by Porphyrin-Containing Polymers of Intrinsic Microporocity." ACS Appl. Polym. Mater.2021, submitted .

La contamination moléculaire en orbite est l’une des problématiques majeures de l’industrie spatiale. En effet, lorsque les satellites sont en orbite, les molécules organiques contenues dans les peintures, adhésifs ou encore les colles utilisés dans la conception des satellites peuvent dégazer et ainsi former des films ou des gouttelettes en se déposant sur les surfaces sensibles comme les instruments optiques et électroniques ou encore les surfaces de contrôle thermique ce qui a pour conséquence d’endommager ces équipements. Les hydrocarbures ainsi que les plastifiants émis ont été identifiés comme étant les contaminants majeurs. Parmi les matériaux poreux testés pour l’adsorption de ces polluants organiques, les zéolithes se sont avérées être les plus efficaces de par leur capacité à piéger ces molécules présentes à de très faibles concentrations dans les conditions spatiales. La synthèse des zéolithes conduisant généralement à des poudres qui seraient elles-mêmes source de contamination particulaire, une mise en forme de ces zéolithes est donc nécessaire. Des pastilles, billes et films zéolithiques ont été élaborées lors de précédents projets mais ces mises en forme comportent leur lot de désavantages comme l’ajout d’équipements additionnels pour insérer les pastilles à la structure des satellites, les mauvaises propriétés mécaniques des billes ou encore la faible quantité de zéolithe mise en jeu dans le cas des films ainsi que de la difficulté d’application de ces derniers sur de grandes surfaces. C’est pourquoi, le développement de peintures zéolithiques a été envisagé étant donné que ces dernières présentent notamment l’avantage de pouvoir être directement appliquées sur la surface interne des satellites. L’objectif de cette thèse est donc d’élaborer des peintures zéolithiques qui adhèrent sur les éléments de surface des satellites, qui soient stables mécaniquement (chocs et vibrations subis par les satellites, gradients de température) et qui puissent piéger les polluants organiques. Des zéolithes de type structural FAU (hydrophile) et MFI (hydrophobe) ont été utilisées en combinaison avec des résines silicones comme liants dans le but d’élaborer des peintures zéolithiques pouvant répondre aux contraintes spatiales. Ces peintures zéolithiques ont montré de bonnes propriétés d’adhésion (notes de 0 au test d’adhésion selon la norme ISO 2409) ainsi qu’une bonne stabilité mécanique et thermique dans des conditions pouvant être rencontrées en orbite. Les liants utilisés n’obstruent que très peu voire pas du tout l’accessibilité à la porosité des peintures zéolithiques et de bonnes capacités d’adsorption du n-hexane ont été obtenues. Différentes quantités de pigment noir (charbon animal ou noir de carbone) ont également été ajoutées à certaines peintures zéolithiques pour développer des peintures zéolithiques carbonées dans le but d’absorber la lumière afin de répondre à un autre phénomène responsable de la contamination d’équipements optiques : la lumière parasite
The phenomenon of on-orbit molecular contamination is one of the major issues encountered by the space industry. Indeed, when satellites are placed in orbit, organic molecules contained in coatings, adhesives or glues used in the conception of satellites can degas and thus form films or droplets by depositing themselves on sensitive surfaces such as optical and electronic instruments or thermal control surfaces. This contamination leads to a drastic decrease of on-board equipment performance. Hydrocarbons as well as plasticizers have been identified as major contaminants. Among several porous materials tested for the adsorption of these organic pollutants, zeolites were found to be the most efficient due to their ability to trap organic molecules at a very low concentration in space conditions. The synthesis of zeolites generally leads to powders that would themselves be a source of particulate contamination, therefore a shaping of these zeolites appears to be necessary. Pellets, beads and zeolite films were developed in previous projects, but these processes have some disadvantages such as the addition of additional equipment to insert pellets into the structure of satellites, poor mechanical properties of beads or the small quantity of zeolite involved in the case of films and the difficulty of applying them to large surfaces. That is why, zeolite coatings were selected because they can be applied directly to the internal surface of satellites. The main goal of this project is to develop zeolite coatings that adhere to the surface elements of satellites, that are mechanically stable (shocks and vibrations undergone by satellites, temperature gradients) and that can trap organic pollutants. FAU-type (hydrophilic) and MFI-type (hydrophobic)zeolites were used in combination with silicone resins as binders in order to develop zeolite coatings that can that can fulfill spatial requirements. These zeolite coatings showed good adhesion properties (adhesion note of 0 according the ISO 2409 standard) as well as good mechanical and thermal stability under conditions encountered in orbit. Zeolite coatings porosity remain mostly accessible despite the use of a binder and good n-hexane adsorption capacities were obtained. Different quantities of black pigment (bone char or carbon black) were also added to some zeolite coatings to develop black zeolite coatings with the aim of absorbing light in order to respond to another phenomenon responsible of optial equipment contamination: stray light