(665c) Low Coverage Adsorption Properties of Hydrocarbons On the Metal-Organic Framework MIL-47 Studied by Monte Carlo Simulations and Pulse Chromatography

Metal-organic frameworks (MOFs) are a relative new and emerging class of microporous materials, formed by a network of metal ion cluster held together by bridging multidendate organic ligands. Since, in most cases, the metal ions are coordinatively saturated by these ligands, the pore walls are largely organic in composition. In this way their composition differs largely from the inorganic zeolites. Whereas the potential of MOFs as catalysts, molecular sensors, luminescent and magnetic materials and drug delivey materials has already been proven, the most promising applications probably lie in the field of adsorptive storage and separations. Despite the recent interest in adsorption on MOFs little is known about the fundamental adsorption properties of these materials.

In this work, low coverage adsorption properties of linear and branched alkanes on the MIL-47 MOF were studied using pulse chromatography and Grand Canonical Monte Carlo simulations. MIL-47 is a porous terephthalate built from infinite chains of V4+O6 octahedra, held together by the dicarboxylate groups of the terephthalate linkers. In this way a 3-dimensional microporous framework with 1-dimensional diamond shaped channels with free internal diameter of about 0.85 nm is formed. Molecular modeling of MIL-47 is very recent and extremely scarce. Adsorption studies focused on carbon dioxide, using both Grand Canonical Monte Carlo and Density Functional Theory calculations [1]. Diffusion has been analyzed only for methane [2] and for hydrogen using molecular simulations [3]. Molecular simulations of linear and branched alkanes in this structure have not been reported so far.

Henry constants and low coverage adsorption enthalpies of C5?C8 linear and branched alkanes, cyclohexane and benzene were measured from 120 to 240 °C using pulse gas chromatography. An adapted force field for linear and branched alkanes in MIL-47 was used to compute the adsorption properties of those molecules [4]. A new set of charges was developed for simulations with benzene in MIL-47. The experimental adsorption enthalpy of linear alkanes increases with about 7.6 kJ/mol per additional ?CH2? group, compared to 7.8 kJ/mol in the simulations. Henry adsorption constants of iso-alkanes are slightly lower than those of the linear chains; also experimental and simulated adsorption enthalpies of the branched alkanes are slightly lower than those of the linear chains. However, the MIL-47 framework is not imposing steric constraints on the branched chains as followed from a detailed analysis of the data and a study of the molecular siting mechanisms via molecular simulation. It was found that the molecules are not preferentially adsorbed near the metal centers or the pore walls. The branched alkanes show a tendency to stay close to the center of the pore, as indicated by the condensed cloud of centers of mass for 2-methylpentane, 2,3-dimethylpentane and 2,2-dimethylpentane, whereas the centers of mass for n-hexane are much more distributed. This shows that n-hexane probes a larger area in the cross section of the pore. Also benzene adsorbs in the center of the pore. At low loading, no preferential orientation of the benzene molecules was detected in the simulations, confirming the specific interaction of this molecule with the framework.

Benzene and cyclohexane are adsorbed less strongly than n-hexane as they have less hydrogen atoms; the experimental adsorption enthalpy of benzene is considerably lower than that of n-hexane (43.4 kJ/mol) compared to 50.6 kJ/mol). Also, simulations show a weaker adsorption of benzene compared to n-hexane (44.7 k/mol compared to 55.1 kJ/mol), even though the united-atom force fields model was used for n-hexane and, thus, the hydrogen atoms were not simulated independently. The interactions between the alkane hydrogen atoms and the MIL-47 framework play a dominant role in the adsorption mechanism. Interactions between the pi-clouds of adsorbed aromatic species and the framework are less important. Therefore, it is not unreasonable to suggest that MIL-47, in contrast to most cation containing zeolites, does not exhibit a significant internal electrical force field. The additional hydrogen atoms of n-hexane compared to benzene lead to stronger interaction and thus longer retention or Henry constant. For the same reason, cyclohexane is adsorbed more strongly compared to benzene, but retained less compared to n-hexane.

The present study shows that linear and branched alkanes adsorb in an unhindered way in the pores of MIL-47. The combined simulation and experimental study revealed that, despite the significant difference in composition between the metal?organic framework and zeolites, the adsorption potential for alkanes is similar. The carbon number dependency of the low-coverage adsorption properties (Henry constants and adsorption enthalpy) correspond very well between simulations and experiments. Future work will be devoted to the comparison between experimental and simulated isotherms.

Acknowledgements

This work was performed in the frame of the IAP Functional Supramolecular Systems of the Belgian Federal Government. The authors are grateful to the Vrije Universiteit Brussel for financial support in the frame of the concerted research action ??Molecular Interactions and Transport Confined Spaces''. The authors thank the Spanish Ministerio de Educacioxn y Ciencia (MEC) (CTQ2007-63229 and CTQ2007-60910) and the Junta de Andalucýxa (P07-FQM-02595) for financial support. E. Garcýa-Perez wishes to thank the MEC for her predoctoral fellowship.

References

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[2] Rosenbach, N.; Jobic, H.; Ghoufi, A.; Salles, F.; Maurin, G.; Bourrelly, S.; Llewellyn, P. L.; Devic, T.; Serre, C.; Ferey, G., Angewandte Chemie-International Edition 2008, 47, 6611-6615.

[3] Salles, F.; Jobic, H.; Maurin, G.; Koza, M. M.; Llewellyn, P. L.; Devic, T.; Serre, C.; Ferey, G. , Physical Review Letters 2008, 100.

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