The gating effect in porous materials has been widely applied in highly selective gas separation and offers great potential for gas storage and sensing. Molecular simulations can improve our understanding of the gate opening/closing transitions at the atomic scale and enable the construction of quantitative models to describe the gated adsorption behaviour at the macroscale level. Emphasis is given to the temperature-regulated gating effect, where the critical admission temperature is dictated by the combined effect of the gate opening and thermodynamic factors and plays a key role in regulating guest admission. These structural variations are induced either by the host-guest interaction or by an external stimulus, such as temperature or light, and account for the gating effect at a threshold value of the stimulus. This review summarizes the mechanisms of the gating effect, which can be a result of the deformation of the framework (e.g., expansion, contraction, reorientation, and sliding of the unit cells), the vibration of the pore-keeping groups (e.g., rotation, swing, and collapse of organic linkers), and the oscillation of the pore-keeping ions (e.g. This anomalous adsorption behavior results from a gating effect, where a structural variation of the adsorbent leads to an abrupt change in the gas admission. It has long been known that some microporous adsorbents suddenly become highly accessible to guest molecules at specific conditions, e.g., above a threshold pressure or temperature. Considerable efforts have been made to regulate the pore accessibility in microporous materials for the manipulation of guest molecules' admission and release. In the past two decades, various microporous materials have been developed as useful adsorbents for gas adsorption for a wide range of industries. Les nanozéolithes de CHA et RHO ont ensuite démontrées leur efficacité pour l’adsorption sélective de CO2 du CH4. Des méthodes d’analyses utilisant la diffraction par précession des électrons en mode tomographie (PEDT) et in-situ DRX sur poudre ont été utilisées pour caractériser les zéolithes CHA et RHO après l’adsorption du CO2. Dans la seconde partie, l’analyse cristallographique des zéolithes RHO et CHA sous les formes hydratées et déshydratées est présentée. Des nanocristaux compris entre 30 et 200 nm avec un rapport Si/Al variant de 1,4 à 2,6 ont été obtenus. La première partie concerne l’élaboration d’une nouvelle voie de synthèse. La réduction de la taille des cristaux leur confère une meilleure stabilité et augmente la surface d’échange entre le matériau et les gaz. Pour cela, il a été choisi de synthétiser directement des nanocristaux de zéolithe CHA et RHO sans agent organique structurant, avec un rapport Si/Al le plus adéquat pour la séparation du CO2 du CH4. L’objectif de ce travail consiste à préparer des zéolithes à petits micropores en utilisant une voie de synthèse « verte ». The calculated energy barriers for moving the cations across the eight-membered rings are very high, which explains the experimentally observed slow kinetics of the phase transition as well as the appearance of metastable phases. A large dependence on the polarizing power of the extra-framework cations and with the loading of water has been found for the minimum aperture of the eight-membered rings that control the nanovalve effect. The distortions of the three different zeolite rings are coupled, and the six- and eight-membered rings are largely flexible. ![]() We have modeled four aluminosilicate structures, containing Li+, Na+, K+, Ca2+ cations. We have employed interatomic potentials-based simulations to obtain a detailed atomistic view of the structural distortion mechanisms of zeolite RHO, in contrast with the averaged and space group restricted information provided by diffraction studies. ![]() By varying the level of distortion of double eight-rings, it is possible to control the adsorption properties, which confer a molecular valve behavior to this material. Zeolite RHO shows unique pore deformations upon changes in hydration, cation siting, cation type, or temperature–pressure conditions. ![]() However, little is known concerning their performance in energy and environmental areas. Molecular valves are becoming popular for potential biomedical applications.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |