278232 Probing the Adsorptive Behaviour of MIL-53(Al) Using Light Organics (C1-C4)
Metal-organic frameworks (MOFs)1 are three-dimensional hybrid materials, formed by the coordination of metal ions with organic linkers. They combine a high pore volume, a regular and geometrical reproducible porosity, and the presence of tunable organic groups within the framework, thus allowing an easy manipulation of the pore size. Among this new class of porous materials, the flexible MIL-53(Al, Cr) solids arise as particularly interesting substances, due to their chemically "simple" structure and unique adsorption features. Structurally, MIL-53(Al) is composed of chains of dicarboxylate groups interconnected with AlO4(OH)2 octahedra as corner-sharing units. This 3D metal-organic framework contains 1D diamond-shaped channels with pores of nanometer dimensions, and possess a chemical formula of Al(OH)(O2-C-C6H4-CO2), where Al denotes the trivalent cation. It has been reported4,5 that the MIL-53 solids exhibit a lattice breathing phenomenon over a certain temperature range, upon adsorption of special molecules (e.g., H2O and CO2) that interact with the solid via hydrogen bonding. This association between the adsorbate molecules and the framework being responsible for the switching between a narrow-pore structure (np), in which the pores are slightly deformed due to hydrogen bonding, and a large-pore form (lp), characterized by a more open porosity. In the present work, the low- to high-occupancy adsorption thermodynamics of light alkanes (C1-C4) in the large-pore structure of MIL-53(Al) is predicted from grand canonical Monte Carlo (GCMC) simulations and compared with experimental gravimetric data. The adsorption experiments span a broad range of pressures (0.01-7 MPa) and temperatures (303-353 K). In our molecular simulation work, MIL-53lp(Al) is assumed to have a perfect, rigid lattice, and both fluid-fluid and solid-fluid interactions are modeled using the TraPPE-UA force field.
II. Results and Discussion
The solid sample had been previously characterized6 by standard physico-chemical techniques, namely elemental analysis (C, H, N), powder XRD (λ = 1.5418 ), TGA, FTIR, and solid state NMR (1H,13C, 27Al), indicating a chemically pure substance (C = 43.38%, H = 2.25%) with a dry structure identical to the one proposed in the literature.2 The adsorption isotherms predicted by GCMC simulation, without any reparameterization of the TraPPE-UA force field parameters, are in good agreement with the experimental measurements within the whole temperature domain (Fig.1); particularly in the low-medium pressure range where simulation results coincide almost exactly with experimental data. For the C2-C4 molecules, the amount of adsorbed fluid increases rapidly with pressure, until reaching an approximately constant plateau and thus exhibiting type-I isotherms. The exact location of that plateau depends on the molecular nature of the adsorbate, being reached earlier for the lighter molecules.
A previously unobserved anisotropic distribution of the confined CH4 molecules (Fig.2) is interpreted in terms of a symmetry annihilation in the pseudo one-dimensional nanopores; this fact arises from antiparallel alignments of the OH groups in the inorganic octahedra.6 The total potential energy of the adsorbent/adsorbate system is decoupled into fluid-fluid and solid-fluid interactions and analyzed as a function of adsorbate loading and temperature. Macroscopic thermodynamical properties, such as the Henry constant, H, and the isosteric heat of adsorption, qst, are calculated and compared to experimentally obtained values. The agreement between simulation and experimental data is generally quite satisfactory.
FIG.1 Experimental (filled symbols) and simulated (open symbols) excess adsorption isotherms for C1–C4 alkanes
in MIL--‐53lp(Al), at 303K, 323 K and 353 K; symbols: (♦/×) methane, (■/□) ethane, (▲/Δ) propane, (●/○) n-butane.
For clarity, the adsorption data of ethane, propane and butane are displaced by 1, 2 and 3 mol/kg, respectively.
FIG.2 Molecular density field (lighter colors represent larger values of r) and zero-potential hypersurface (ZPH)
for condensed C2H4 at 152.5 K and μ/kB = –1623 K inside an even-index, diamond-shaped channel of MIL-53lp(Al).
(a) Front view of the channel; the colored lines show the perimeter of the ZPH at different axial positions along the
channel: x/a = 0 (yellow), x/a = 0.25 (red), x/a = 0.375 (green), and x/a = 0.5 (white). (b) Lateral view of the channel; the
solid line represents the ZPH for y/b = 0.5, and the dashed line shows the hypersurface extended by a sphere of diameter σff/4.
Grants from Fundao para a Cincia e a Tecnologia (FCT/MCTES, Portugal) are gratefully acknowledged by F.J.A.L. Cruz (SFRH/BPD/45064/2008), A.I. Lyubchyk (SFRH/BD/45477/2008) and I.A.A.C. Esteves (PTDC/CTM/104782/2008).
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3. S. Bourrelly, P. L. Llewellyn, C. Serre, F. Millange, T. Loiseau, G. Frey, J. Am. Chem. Soc. 127, 13519 (2005).
4. G. Frey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron- Guegan, Chem. Commun. 2976 (2003).
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