Exploration of the Long-Chain N-Alkanes Adsorption and Diffusion in the MOF-Type MIL-47 (V) Material by Combining Experimental and Molecular Simulation Tools

Abstract

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

Diffusion of Branched and Linear C6-Alkanes in the MIL-47(V) Metal–Organic Framework.

“…This has been obtained by defining a vector joining two CH x units corresponding to the longest CH x -CH x distance. The alkane is then considered to be parallel to the channel if this vector forms an angle lower than 45° with the channel axis, this threshold value providing a good qualitative trend as previously reported [50].…”

Diffusion of long chain n-alkanes in the metal–organic framework MIL-47(V): A combination of neutron scattering experiments and molecular dynamics simulations

“…The 3D framework type MIL-47(V) built up from infinite chains of corner sharing V 4+ O 6 octahedra interconnected by 1,4-benzenedicarboxylate groups, thus generating 1D-channels with a free diameter of 11 Â 10 Å [36], has been selected for its structural and chemical features (absence of both defects in the pore walls and structural changes upon inclusion of guest molecules) that allow a systematic measurement of D s for all the investigated n-alkanes. Beyond a fundamental interest, the knowledge gained from this study will be complementary to the equilibrium adsorption measurements published elsewhere to help the interpretation of the promising adsorption properties of this MOF-type material for alkanes [15,17,18] or other complex molecules such as xylenes [37,38].…”

“…Apart from the light hydrocarbons, it was also evidenced that such a class of hybrid MOFs shows interesting topological and chemical features that favor either large adsorption uptakes for longer chain hydrocarbons in the vapor and liquid phases or high selectivities for the separation of aromatics (xylenes) and aliphatics (hexane) isomers [14], both characteristics being also of upmost industrial interests in petrochemistry. These later conclusions were mainly deduced from equilibrium adsorption studies conducted both experimentally and computationally [15][16][17][18], while only a few investigations have focused on the dynamics of the hydrocarbons within the porosity of these MOFs that is expected to significantly impact the adsorption/separation processes. Indeed, the exploration of the dynamics has been mainly concentrated on the diffusion of short and intermediate length linear alkanes in a wide series of MOFs.…”

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

“…Experimental methods that are frequently used to measure molecular diffusion in modern MOF materials involve quasi elastic neutron scattering (QENS) (Deroche et al 2011;Yang et al 2011), pulsed field gradient (PFG) NMR (Stallmach et al 2006;Wehring et al 2010), infrared microscopy and interference microscopy (Bux et al 2010). Transport processes in the MOFs are also often studied by molecular dynamic (MD) computer simulations (Liu et al 2011;Keskin et al 2009;Sant et al 2010;Zheng et al 2012;Pantatosaki et al 2010Pantatosaki et al , 2012.…”

“…Additionally, if combined and validated with experimental diffusion studies, MD simulations may provide unique insight into the transport mechanisms at the pore level scale. Recent examples for such joint experimental and computational investigations include diffusion studies of n-alkanes in MIL-47 (Deroche et al 2011) and MOF-5 (Ford et al 2012), benzene in MOF-5, methane in CuBTC (Wehring et al 2010) and methane and carbon dioxide in UiO-66 (Yang et al 2011).…”