wileyonlinelibrary.comsuch as the mono-branched hexane isomer 3MP are usually still present and contribute to decrease the performance of the process. To overcome this drawback, there is a crucial need to search for alternative porous solids able to more selectively adsorb di-branched compounds such as the 22DMB from the mono-branched 3MP and linear nHEX.As a typical example, it was demonstrated that zeolite BETA could be used to upgrade the actual TIP processes by partially separating mono from di-branched hexane isomers in a dual layer pressure swing adsorption (PSA) bed with 5A zeolite, [ 3 ] leading to an enhancement of the RON number of the fi nal stream from 86 to 92, a value similar to the one obtained in the Hexsorb process of Axens. The achievement of higher upgrading thus calls for the design of more effi cient porous adsorbents. Metal organic frameworks (MOFs) are the latest class of crystalline porous solids. [ 6 ] These are built up from the linkage of inorganic sub-units and organic ligands, and constitute a versatile class of porous materials due to their large chemical and structural diversity, paving the way for their use in many societally relevant applications (gas storage, separation, catalysis, sensing, biomedicine, etc.) [ 7,8 ] In the fi eld of fl uid separation, it was shown for instance that MOFs could be of a great interest to separate propane from propylene, [ 9 ] xylene or alkane isomers, [ 10 ] carbon dioxide fromThe separation ability of branched alkane isomers ( n HEX, 3MP, 22DMB) of the fl exible and functionalized microporous iron(III) dicarboxylate MIL-53(Fe)-(CF 3 ) 2 solid is evaluated through a combination of breakthrough experiments (binary or ternary mixtures), adsorption isotherms, X-ray diffraction temperature analysis, quasi-elastic neutron scattering measurements and molecular dynamics simulations. A kinetically controlled molecular sieve separation between the di-branched isomer of hexane 22DMB from a mixture of paraffi ns is achieved. The reported total separation between mono-and di-branched alkanes which was neither predicted nor observed so far in any class of porous solids is spectacular and paves the way towards a potential unprecedented upgrading of the RON of gasoline.

International audienceThe adsorption properties of linear long chain alkanes (from n-pentane to n-nonane) within the rigid MOF MIL-47 (V) have been explored by combining gravimetry measurements and molecular simulations. Both experimental absolute isotherms and enthalpies of adsorption for all n-alkanes were compared with those obtained by configurational bias grand canonical Monte Carlo simulations (CB-GCMC) based on two different force fields. From a fair agreement between experimental and simulated data, a further step consisted of investigating the microscopic adsorption mechanism in play to shed some light onto the preferential orientations and conformations of all investigated n-alkanes. Whereas the trans conformation is predominantly observed for all n-alkanes, the proportion of the n-alkane conformations lying parallel to the direction of the tunnel significantly increases with the chain length, emphasizing that the confinement effect is stronger for the longer chain n-alkanes. Finally, molecular dynamics simulations allowed us to emphasize that all n-alkanes follow a pathway along the direction of the tunnel, leading to a 1D type diffusion mechanism, the motions being mainly centered around the middle of the pores at low loading, whereas they are significantly shifted toward the pore wall when the alkane concentration increases

Some bacteria move with a corkscrew motion, meeting less resistance from surrounding water. A spiral movement is also considered during RNA polymerase translocation and it has been observed for water and proteins. We have found that benzene molecules confined in the metal−organic framework MIL-47 move in a corkscrew fashion. This spectacular diffusion effect could be put into evidence by combining experimental and computation tools; the two experimental techniques, quasielastic neutron scattering and 2 H NMR, cover a wide range of time scales.

The dynamics of xylene isomers in the metal−organic framework MIL-47(V) has been investigated by combining molecular dynamics (MD) simulations and experimental tools including quasielastic neutron scattering (QENS) and deuterium nuclear magnetic resonance ( 2 H NMR).The experimental and simulated self-diffusion coefficients (D s ) values for each single component isomer are in reasonable agreement in the whole range of temperatures. More interestingly, the simulations predict a nonmonotonous evolution of D s with the temperature for all xylenes. Such an unusual trend is experimentally confirmed for p-xylene. Two distinct diffusion regimes are elucidated at the microscopic level: a low-temperature regime where the xylene molecules are close to the MIL-47(V) pore wall with a high activation energy barrier for the diffusion and a hightemperature regime where the xylene molecules are mainly located in the center of the channel associated with a lower activation energy for the diffusion. This dynamic behavior remains also true whatever the degree of pore filling. It has been further shown that the diffusivity is only slightly affected when one compares the case of xylenes in single component and in mixture. 2 H NMR experiments enlighten that packing effects and guest−guest interactions are crucial when considering the dynamics of confined xylenes in MIL-47(V).

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A combination of experimental (quasi-elastic neutron scattering and 2 H NMR) and computational (molecular dynamics) tools was used to uncover the molecular mobility of benzene trapped inside the flexible channel-type MIL-53 (Cr 3+ ) MOF. This material was shown to undergo a contraction of the structure upon benzene adsorption with the formation of a narrow pore phase with a smaller aperture. This confinement was found to strongly influence the dynamics of the guest: benzene diffuses in a region centered in the middle of the pore by a 1D-jump translational mechanism along the tunnel ruled by the presence of the μ 2 -OH groups present at the MOF pore wall. This translational diffusion is combined with a fast uniaxial rotational motion around the C 6 -axis. Any other rotational motion that involves the tumbling of the phenyl rings about the channel axis is much less probable due to a high activation energy barrier (49 kJ mol −1 ). In this way benzene can be pictured as a rotating disc that diffuses rapidly through the central part of the channel by short jumps between neighboring low energy basins located in the vicinity of the μ 2 -OH groups of the MIL-53 channels.

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The self-diffusion of hexane isomers in MIL-47(V) has been investigated by combining quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. Experimentally, the diffusivity of n-hexane is found to be larger than the one of 3-methylpentane. MD simulations bring further insight into the microscopic diffusion mechanism in play confirming that diffusion proceeds via a jump sequence, as observed by QENS.