Molecular simulations of hybrid cross-linked membranes for H₂S gas separation at very high temperatures and pressure:Binary 90%/10% N₂/H₂S and CH₄/H₂S, ternary 90%/9%/1% N₂/CO₂/H₂S and CH₄/CO₂/H₂S mixtures

Molecular dynamics (MD) simulations have previously identified four hybrid inorganic-organic membranes based on POSS or OAPS silsesquioxanes hyper-cross-linked with small PMDA or 6FDA imides, which are able to maintain reasonable CO2/N2 and CO2/CH4 permselectivities at very high temperatures (300 °C and 400 °C) and pressure (60 bar). Experimentally, the polyPOSS-imides are known to degrade above 300 °C while the polyOAPS-imides can resist up to above 400 °C. In the present work, the same four model polyOAPS/POSS-imide networks are further tested for their gas separation abilities of H2S-containing mixtures. Indeed, hydrogen sulfide is a hazardous gas present in many gas feeds, and, within the context of a toxic penetrant under harsh conditions, simulations are a useful task to perform before embarking on difficult experiments. The separations of H2S with respect to N2, CH4 and CO2 by the polyOAPS/POSS-imide matrices were studied at 300 °C, 400 °C and at 60 bar, firstly with H2S as a single-gas in order to obtain its ideal permselectivities, secondly as part of binary 90%/10% N2/H2S and CH4/H2S feeds and thirdly as part of ternary 90%/9%/1% N2/CO2/H2S and CH4/CO2/H2S feeds. They were compared to separations of binary 90%/10% N2/CO2 and CH4/CO2 feeds under exactly the same conditions. At 300 °C, H2S is much more soluble in the networks than the other three penetrants. It is the only one leading to a non-negligible volume swelling at 60 bar, although this does not happen for the mixed-gas feeds due to their low H2S partial pressures. Differences are attenuated at 400 °C because of the decrease in solubilities upon heating. The linear N2 and CO2 move faster than the non-linear CH4 and H2S penetrants, but the diffusion selectivities are moderate. As such, the ideal permselectivities under harsh conditions are mainly governed by the solubility selectivities. With binary 90%/10% N2/H2S, CH4/H2S, N2/CO2 and CH4/CO2 feeds, the transport parameters of the major N2 or CH4 components remain similar to their ideal values, whereas the solubilities of the minor H2S and CO2 components increase. This leads to some of the real separation factors for H2S being different from their ideal permselectivities, and approximately twice as high as those with CO2. In the ternary 90%/9%/1% N2/CO2/H2S and CH4/CO2/H2S mixtures, replacing 1% CO2 by 1% H2S in the feeds leads to small changes but, in pratice, these are not significant enough to make a difference. Under the conditions tested, the ternary separation factors are the same than for the 90%/10% binary mixtures. In all cases, the denser polyPOSS-imides show better sieving properties than the more open polyOAPS-imides. As such, the former should preferably be used in applications up to 300 °C, i.e. in the temperature range below their degradation. However, it is also possible to use the polyOAPS-imides at higher temperatures, since they still manage maintaining separation factors between 2 and 6 for CO2 and H2S at 400 °C, which is outstanding for polymer-based membranes at such high temperatures.</p

Comparison of Eight Classical Lennard-Jones-Based H₂ Molecular Models in the Gas Phase at Temperatures and Pressures Relevant to Hydrogen On-Board Storage Tanks

This work compares eight classical H2 molecular models in the gas phase taken from the existing literature. All models are based on Lennard-Jones (LJ) 12-6 terms for the van der Waals interactions and hence easier to transfer to multiphase molecular simulations than more sophisticated potentials. The H2 potentials tested include one-site, two-site, three-site, and five-site models, with the sites being either the H atoms, the center-of-mass of the H2 molecule, or massless sites. For the multisite models, high-frequency H-H stretching modes can lead to poor equipartition of the kinetic energy, and the timestep for molecular dynamics (MD) simulations should be reduced to maintain a stable numerical integration of the equations of motion. As such, only those models with rigid bonds are considered. In the present case, 600 MD simulations of H2 gas were carried out over a large range of temperatures (−50 to +90 °C) and at densities corresponding to a pressure range of 50 to 2000 bar, which include the operating conditions of on-board storage tanks in hydrogen-fueled vehicles. Most of the models under study were found to reproduce reasonably well the experimental pVT phase diagram as well as the solubility. Discrepancies only became significant at the highest densities tested, and these could be used to rank the different models. All model diffusion coefficients were essentially indistinguishable from experimental results, and as such, kinetically dominated dynamic properties could not be used as a criterion for the choice of model. Among the eight models tested, two of them, i.e., the two-site model of Yang and Zhong and the one-site model derived from Buch performed very well over the range of conditions tested. They represent a good compromise between realism, simplicity, and computational efficiency.</p

Molecular modelling of high-performance polymer membranes and nanocomposites

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Polymers at the molecular level

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O2/N2 and CO2/CH4 separations in glassy membranes studied at the molecular level

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Modélisation moléculaire de la perméabilité du CO2 dans les membranes polymères

Les études concernant la séparation du dioxyde de carbone ont pris une importance majeure au cours du 21ème siècle. Dans le cas de matrices polymères, il est important d'examiner les relations entre les propriétés de perméation du CO2 et les caractéristiques structurales et dynamiques des polymères. Nous avons sélectionné trois polyimides fluorés (6FDA-6FpDA, 6FDA-6FmDA et 6FDA-DAM) et nous avons utilisé les techniques de simulations par dynamique moléculaire (DM) pour étudier en détails leurs propriétés relatives à la perméation du CO2 à l'échelle moléculaire. Des modèles moléculaires pour les trois polyimides fluorés ont été construits en utilisant la technique d'échantillonnage hybride de type Pivot Monte Carlo (PMC)-DM et ont ensuite été simulés par DM seule. Tous les modèles sont en bonne adéquation avec les propriétés dérivées des caractérisations expérimentales telles que les densités, les volumes libres, les d-spacings, les paramètres de solubilité, les énergies et les spectres de diffraction aux rayons-X. Les structures et morphologies des volumes libres dans les matrices pures ont aussi été analysées. Une addition par palier de CO2 a été effectuée dans les matrices polymères pures et une procédure itérative a été utilisée pour calculer les isothermes modèles de sorption de CO2. De plus, le gonflement en volume induit par la sorption du CO2 et l'effet immédiat des concentrations élevées en CO2 sur les isothermes de désorption ont été également étudiés. La morphologie des espaces vides, les énergies potentielles et la formation d'agglomérats de CO2 ont été caractérisés et comparés avec les caractérisations expérimentales. La diffusion de CO2 en fonction de sa concentration à l'intérieur des matrices polymères a été analysée et des facteurs affectant la mobilité ont été identifiés.Carbon dioxide (CO2) separation studies have become a major interest for researchers in the 21st century. In the case of polymer matrices, it is important to examine the relationships between the permeation properties of CO2 and the structural and dynamical features of the polymers. We have selected three fluorinated polyimides (6FDA-6FpDA, 6FDA-6FmDA and 6FDA-DAM) and used molecular dynamics (MD) simulations to provide a detailed picture of their CO2 permeation properties at the molecular level. Atomistic models of the three fluorinated polyimides were generated using the well-established hybrid Pivot Monte Carlo (PMC)-MD single-chain sampling technique, and were subsequently simulated using MD on its own. All the models were found to compare well with experimentally-derived properties such as densities, fractional free volumes, d-spacing parameters, solubility parameters, energies and X-ray data. Structures and void-space distributions in the pure matrices were also analysed. A realistic stepwise addition of CO2 was carried out in the pure polymer matrices and an iterative procedure was used to calculate the CO2 model sorption isotherms. In addition, the volume swelling induced by CO2 sorption and the immediate effect of exposure to high concentrations of CO2 on the desorption isotherms were also studied. The void spaces, potential energies and the formation of CO2 clusters were characterized and compared with experimental characterizations. The diffusivity of CO2 as a function of CO2 concentration within the polymer matrices was analysed, and the factors affecting mobility were reported in detail.CHAMBERY -BU Bourget (730512101) / SudocSudocFranceF

An optimized fully-atomistic procedure to generate glassy polymer films for molecular dynamics simulations

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Nanosecond-time-scale reversibility of dilation induced by carbon dioxide sorption in glassy polymer membranes

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