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Abstract: The present work describes the adsorption and separation of xylene isomers by ZIF-8. Although the formal pore diameter of ZIF-8 is much smaller than the molecular diameter of the xylene isomers, ZIF-8 is able to separate the isomers by molecular sieving. A structural study indicates that the diffusion of the xylenes into the pore structure of ZIF-8 happens via a transitory deformation of the pore aperture which is based on a tilt of the imidazolate linkers, followed by a return to the initial conformation. The rate of adsorption depends on the size of the isomer, i.e. it decreases from para- to meta- and to ortho-xylene. The separation of the xylene isomers is good in the gas phase. In liquid phase breakthrough experiments, the quality of the separation is deteriorated. Moreover, as expected for a separation based on molecular sieving, para-xylene cannot be well separated from ethyl-benzene. [Copyright &y& Elsevier]

The employment of metal-organic frameworks in powder form is undesirable from an industrial perspective due to process and safety issues. This work is devoted to evaluating the impact of compression on the textural and structural properties of CPO-27(Ni). For this purpose, CPO-27(Ni) was synthesized under hydrosolvothermal conditions and characterized. Then, the resulting powder was compressed into binderless pellets using variable compression forces ranging from 5–90 kN (37–678 MPa) and characterized by means of nitrogen adsorption/desorption, thermogravimetric analysis and powder X-ray diffraction to evaluate textural, thermal and structural changes. Both textural and structural properties decreased with increasing compression force. Thermal stability was impacted in pellets compressed at forces over 70 kN. CPO-27(Ni) pelletized at 5, 8 and 10 kN, and retained more than 94% of its initial textural properties, while a loss of about one-third of the textural property was observed for the two most compressed samples (70 and 90 kN) compared to the starting powder. [ABSTRACT FROM AUTHOR]

The influence of sodium cations content in zeolite Beta crystals of different sizes (between 200 nm and 2 μm) on the adsorption of pollutants (dioxin and polychlorobenzene) is investigated. In this study, protons which balance the negative charge of the zeolitic framework are partially replaced by sodium cations. Chemical, structural and textural analyses performed on the exchanged crystals indicate that the exchange rates reached 75-80 % and that the structure of zeolites is preserved. Adsorptions of pollutants in zeolite Beta samples were successfully carried out using 1,2-dichlorobenzene (gas phase) and 2,3-DiChloroDibenzo- p-Dioxin: 2,3-DCDD (liquid phase) but reveal different behaviors. Indeed, all samples adsorb the same amount (around 7 molecules) of 1,2-dichlorobenzene per unit-cell of zeolite, whereas the dioxin adsorption amounts increase with the amount of sodium cations. Compared to their protonic forms, the dioxin adsorption capacities were multiplied by a factor of 1.3 for the sodium exchanged small crystals (size between 200 nm and 1 μm) and by a factor of 4 for the sodium exchanged microcrystals (size between 1 and 2 μm) for an equilibrium concentration of 2,3-DCDD of 0.5 mmol/L in both cases. [ABSTRACT FROM AUTHOR]

Adsorptive separation of propylene (C 3 H 6) and propane (C 3 H 8) may represent an energy-efficient alternative to conventional cryogenic distillation. In this perspective, a set of zeolitic adsorbents of LTA type structure (pure-silica zeolite (Si-LTA) and cation- (Na+, 33% of Li+, 50% of Mg2+ and 50% of Ca2+) containing zeolites) were prepared, then characterized using various techniques such as PXRD, gas adsorption measurement, TGA, XRF, SEM and EDX mapping. Thanks to an in-house manometric dosing setup coupled with a Tian-Calvet type microcalorimeter, the pure gas adsorption isotherms and their corresponding differential enthalpies of adsorption were measured at 303 K and for pressures up to 5 bar. To fit those adsorption data, the dual-site Langmuir was selected as the best fit model, and by using the Ideal Adsorbed Solution Theory (IAST), thermodynamic selectivities were determined. The mass transfer constants were also estimated by fitting, separately, the Linear Driving Force (LDF) and isothermal micropore diffusion models to adsorption kinetic curves, thus allowing kinetic type selectivities to be calculated. The combination of those selectivities reveals that the thermodynamic separation of C 3 H 6 from C 3 H 8 is highly favorable on CaNa-LTA, followed by its MgNa-LTA counterpart with IAST selectivities around 15 and 5, respectively. On the other hand, the monovalent cationic zeolites (i.e., Na- and LiNa-LTA) show a predominance of steric effects. Also, except for Si-LTA which shows a moderate kinetic separation, the studied materials are potential candidates, in the following order CaNa- > MgNa- > LiNa- > Na-LTA, for the separation of C 3 H 6 from C 3 H 8 by adsorption-based technologies. [Display omitted] • Adsorption enthalpies of C3 on LTA zeolites have been measured by microcalorimetry. • LTA zeolite with Li+ and Na+ are able to kinetically separate C3 mixture. • LTA with Ca2+ and Mg2+ are promising candidates for thermodynamic separation. • Ca2+ cations effectively improve the IAST selectivity 3 times with respect to Mg2+. [ABSTRACT FROM AUTHOR]

Water adsorption in the large pore ( lp_empty ) form of Ga-MIL-53 was studied by TGA, DSC and in situ XRD and FTIR at 298 K. The large pore form can be stabilized at room temperature after activation under vacuum at 553 K. The isotherm of water adsorption in this large pore form (pore dimensions: 1.67 × 1.33 nm) is very similar to that measured on the narrow pore ( np_empty ) form (pore dimensions: 1.97 × 0.76 nm). Such a similarity is rather unusual given that the pore sizes of these two phases are very different. In order to understand the origin of this effect in situ XRD and FTIR measurements were particularly helpful. It was found that the adsorption of even small amount of water (0.05 mol per Ga atom at 0.2 hPa) in the large pore form of Ga-MIL-53 transforms ca. 50% of the solid into a narrow pore int phase, which is assumed to be present as a shell around the lp_empty core. Additional water molecules adsorbed at higher pressures do not interact with the parent lp_empty phase but with the narrow pore int phase. The phase transformations were confirmed by FTIR revealing significant band displacements in the corresponding pressure ranges. Such easy pore shrinking which occurs at very low water pressure (<0.2 hPa) can have undesirable consequences in working conditions, as for example in separation adsorption processes, because the large pore structure of Ga-MIL-53 can be preserved only under anhydrous conditions. [ABSTRACT FROM AUTHOR]