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Abstract: A zeolite beta–silicalite-1 composite was prepared by growing polycrystalline silicalite-1 shells on single micron-sized zeolite beta crystals. The obtained core–shell material was employed for the separation of hexane isomers. Gravimetric uptake measurements revealed that the silicalite-1 shell acted as a membrane that controlled the transport to zeolite beta core. The positive impact of the silicalite-1 membrane on the separation of mono- and di-branched hexane isomers was confirmed by breakthrough experiments. The core–shell composite material allowed more selective separation of mono- and di-branched isomers than either of its components alone and had a significantly higher adsorption capacity than silicalite-1. [Copyright &y& Elsevier]

In silico screening of CO2 adsorbents is a very powerful method for preselecting the most promising porous solids for experimental studies. Because of increased computational power, it is now possible to investigate a large number of adsorbents in a fairly short time. However, it remains difficult to rationalize structure-performance correlations because the complexity of such porous materials cannot be reduced to a few simple descriptors. In this paper, we present a different approach that was applied to the design of an optimal adsorbent for CO2 separation from gas mixtures. A CO2-CH4 mixture was used as the feed. We constructed a simplified model of the porous material by adopting a spherical pore geometry. From the dispersion-repulsion point of view, the spherical pore was modeled by homogeneously distributed Lennard-Jones sites. Besides, a charge distribution was introduced to mimic the electrostatic behavior. The selected charge distribution is constituted of negative charges homogeneously distributed over the surface of the pore. The total negative charge is counterbalanced by eight positive discrete charges placed on the corners of the cube inscribed in the sphere. This model can be characterized by two main descriptors: the pore size and the cation charge. For this model, the adsorbate-adsorbent interaction potential due to electrostatic and dispersion-repulsion interactions was determined. Then, the Henry constants of CO2 and CH4 are calculated from statistical thermodynamics and substituted in the Ruthven statistical model, which allows calculation of the binary adsorption isotherms. Finally, the adsorbed quantities are used to estimate the performance indexes of the pressure swing adsorption (PSA) process, such as the working capacity and separation factor. We show how these two parameters depend on the pore size and cation charge. For a medium-pressure PSA process, the optimal adsorbent has a large pore volume and is highly charged, which provides high working capacity and selectivity. The high cationic charge allows one to efficiently fill the large pore volume and assures high CO2 selectivity. [ABSTRACT FROM AUTHOR]

Abstract: Mesoporous materials were synthesized from solutions containing ZSM-5 precursors in the presence of a micellar template (CTACl). The effect of different parameters like aluminum content, aging conditions and hydrothermal treatment conditions on the properties of the final materials was investigated by dynamic light scattering (DLS), N2 physisorption, XRD, 27Al MAS-NMR, TEM and IR. In addition two catalytic test reactions (cracking of isopropylbenzene and 1,3,5-triisopropylbenzene) and acidity measurements via pyridine adsorption were carried out. We observed a transition of the initially formed mesoporous structure containing protozeolitic seeds to a zeolitic phase during hydrothermal treatment. The rate of zeolitization strongly depended on the conditions of the aging period and on the composition of the synthesis solution: the more protozeolitic seeds were formed during the aging procedure the faster the zeolitization. Catalytic test reactions and acidity measurements showed that the XRD amorphous materials with protozeolitic seeds in the mesoporous structure are more active and more acidic than mesoporous materials prepared without ZSM-5 precursors, but far less active and acidic than zeolites. A combination of high activity and accessibility of most active sites was found for samples containing small zeolite crystals. [Copyright &y& Elsevier]

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 objective of this work was to study the adsorption and separation of the most important families of hydrocarbon compounds on metal-organic frameworks (MOFs), in comparison with zeolites. For this purpose, we have selected four probe molecules, each of them representing one of these families, i.e., o- and p-xylene as aromatics, 1-octene as an alkene, and n-octane as an alkane. The separation of these four molecules was studied by binary breakthrough experiments. To represent the large diversity of MOF structures, the experiments were carried out with (i) two MOFs with coordinatively unsaturated metal sites (CUS), i.e., Cu-btc (HKUST-1) and CPO-27-Ni, (ii) a MOF with an anionic framework and extraframework cations, i.e. RHO-ZMOF, and (iii) two rather apolar zeolitic imidazolate framework (ZIF) materials with different pore sizes, i.e. ZIF-8 and ZIF-76. Zeolite NaY and zeolite β were used as polar and apolar reference adsorbents, respectively. The results can be briefly summarized as follows: ZIFs (not carrying any polar functional groups) behave like apolar adsorbents and exhibit very interesting and unexpected molecular sieving properties. CUS-MOFs behave like polar adsorbents but show the specificity of preferring alkenes over aromatics. This feature is rationalized thanks to DFT+D calculations. MOFs with extraframework cations behave like polar (cationic) zeolites. [ABSTRACT FROM AUTHOR]

Abstract: ZIF-76, Zn(Im) x (5-ClbIm) y (Im=imidazolate, 5-ClbIm=5-chlorobenzimidazolate), is, among the known ZIF structures, one of the few materials that combine a relatively large pore aperture with a large pore volume. Moreover, it is thermally stable up to 400°C and can be stored under ambient atmosphere without undergoing a measurable degradation. ZIF-76, therefore, appears to be an interesting material for applications in adsorption and separation. The published synthesis protocol leads to very low yields and is, thus, not applicable on a larger scale . In this work, we have succeeded in preparing ZIF-76 in suitable yields by changing the solvent and adding NaOH to the synthesis mixture. The addition of the base increases the yield and leads to smaller crystal sizes, by favoring nucleation over crystal growth. By using the same synthesis strategy, four new isomorphs of ZIF-76 were obtained, with different substituents on the benzimidazole ligand (Me, Br and NO2) and different metal sources (Co and Zn). All the samples were tested in the adsorption and separation of CO2/CH4 in order to highlight the influence of the linker and of the metal center on the adsorption/separation properties. [Copyright &y& Elsevier]

The separation of paraffin isomers is a very importanttopic inthe petrochemical industry. Zeolite 5A is industrially used to sievealkane isomers, but its pore size does not allow the separation ofmonobranched and dibranched alkanes by a kinetic mechanism. In thispublication, we compare three ZIF materials in the separation of C6-paraffinisomers: ZIF-8, ZIF-76, and a new material called IM-22. The performanceof the materials is evaluated by a breakthrough curve of binary mixturesof n-hexane, 3-methylpentane, and 2,2-dimethylbutane.We show that ZIF-8 is a very attractive alternative to zeolite 5Abecause it exhibits a high (kinetic) selectivity for the adsorptionof linear alkanes and at the same time a high adsorption capacity.The new material IM-22, a ZIF with CHAtopology, seemsto be particularly suited for the separation of mono- and dibranchedparaffin isomers. [ABSTRACT FROM AUTHOR]

The present study attempts to understand the use of the flexible porous chromium terephthalate Cr(OH)(O2C--C6H4--CO2) denoted MIL-53(Cr) (MIL = Material from Institut Lavoisier) for the separation of mixtures of CO2 and CH4 at ambient temperature. The coadsorption of CO2 and CH4 was studied by a variety of different techniques. In situ synchrotron X-ray Powder Diffraction allowed study of the breathing of the solid upon adsorption of the gas mixtures and simultaneously measured Raman spectra yielded an estimation of the adsorbed quantities of CO2 and CH4, as well as a quantification of the fraction of the narrow pore (NP) and the large pore (LP) form of MIL-53. Quantitative coadsorption data were then measured by gravimetry and by breakthrough curves. In addition, computer simulation was performed to calculate the composition of the adsorbed phase in comparison with experimental equilibrium isotherms and breakthrough results. The body of results shows that the coadsorption of CO2 and CH4 leads to a similar breathing of MIL-53(Cr) as with pure CO2. The breathing is mainly controlled by the partial pressure of CO2, but increasing the CH4 content progressively decreases the transformation of LP to NP. CH4 seems to be excluded from the NP form, which is filled exclusively by CO2 molecules. The consequences in terms of CO2/CH4 selectivity and the possible use of MIL-53(Cr) in a PSA process are discussed. [ABSTRACT FROM AUTHOR]