Добірка наукової літератури з теми "FAU and MFI structural type zeolites"

The interzeolite conversion of faujasite (FAU-type) zeolites to chabazite (CHA-type) zeolite in the presence of N,N,N-trimethyladamantammonium and N,N,N-dimethylethylcyclohexylammonium cations was investigated over a large compositional range by carefully controlling the reaction mixture compositions. Highly crystalline CHA zeolites were also obtained by the transformation of several zeolite types including EMT, LTL, LEV, RTH, and MFI frameworks. The formation of CHA zeolite from FAU zeolite precursors was substantially faster than that from zeolite L with a similar composition. High-silica CHA zeolites were also produced successfully using a mixture of TMAda with a number of less expensive organic structure-directing agents. The CHA zeolite materials have been synthesized with high crystallinity and with a Si/Al ratio ranging from 5 to 140. Our data support the importance of structural similarity between the zeolite precursors, nucleation/crystallization processes, and the zeolite product in the interzeolite conversion compared to conventional amorphous aluminosilicate gels. Our synthetic methods could be used to prepare other 8-membered ring zeolites such as AEI and AFX frameworks, potential candidates for selective catalytic reduction of NOx, light olefin production, and CO2 abatement.

The formation of coke as a result of propene transformation at 623 K on zeolites results from a product shape selectivity mechanism of which the products are polyaromatic molecules, such as pyrene on MFI, anthracene on MOR, pyrene and coronene on FAU. Zeolite regeneration can be achieved by using non-thermal plasma (NTP), with decreased energy consumption, employing a fixed bed dielectric barrier reactor. The efficiency of this alternative regeneration process depends on the coke toxicity. On MFI and FAU (featuring three-dimensional 10 and 12 ring channel systems, respectively) coking occurs by poisoning the Brønsted acid sites; on MOR, (presenting a one-dimensional 12 ring channel system) pore blocking takes place, leading to higher coke toxicity. A complete coke removal is achieved on MFI and FAU zeolites using NTP within 3 h, while for MOR coke, removal proceeds slower and is incomplete after 3 h on stream. Hence, the efficiency of regeneration is impacted by the accessibility of active oxygenated species generated under plasma (e.g., O*, O2+) to coke molecules.

A screening of Pd/zeolite-based catalysts in the PrOx-CO reaction in H₂-rich streams was performed using zeolites with different cations (H⁺, Na⁺ and Cs⁺) prepared by ion exchange and framework type (MFI and FAU).

FAU-type(NaY) zeolites nanocrystals have been synthesized through crystallization of gel in mesoporous system of carbon nanotubes(CNTS) with a internal diameter of 20~30 nm. Investigation by using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscope (TEM) shows that the nanocrystals possess the typical nanosized zeolites structural characteristics which is different from those of microsized zeolites.

The adsorption of light hydrocarbons (C2–C5 olefins and paraffins, toluene) on HZSM-5, silicalite, and HY was studied for application in treatment of exhaust streams of the petrochemical industry and of vehicles under cold start conditions. At this aim the trapping capability was evaluated on hydrated zeolites by breakthrough curves at low hydrocarbon partial pressure (0-1 kPa), in the temperature range 298–523 K and at space velocity of 30000 h−1. The basic adsorption properties of materials were also verified for three selected hydrocarbons (ethylene, isobutene, and toluene) by equilibrium isotherms on dehydrated zeolites at 298 K. The role of physicochemical characteristics of adsorbent materials was discussed in relation with their trapping capability of different types of hydrocarbons.

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