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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 I. Deroche,† S. Rives,†,^ T. Trung,† Q. Yang,†,‡ A. Ghoufi,§ N. A. Ramsahye,† P. Trens,† F. Fajula,† T. Devic,|| C. Serre,|| G. Ferey,|| H. Jobic,^ and G. Maurin*,† †
)
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, UM2, UM1, ENSCM, Universite Montpellier 2, Pl. E. Bataillon, 34095 Montpellier Cedex 05, France ‡ Beijing University of Chemical Technology, Department of Chemical Engineering, Laboratory of Computational Chemistry, Beijing 100029, Peoples Republic of China § Institut de Physique de Rennes, UMR 6251 CNRS, Universite Rennes 1, 263 avenue du General Leclerc, 35042 Rennes, France Institut Lavoisier, UMR CNRS 8180, Universite de Versailles Saint-Quentin-en-Yvelines, 45 avenue des Etats-Unis, 78035 Versailles cedex, France ^ Institut de Recherches sur la Catalyse et l’environnement de Lyon, Universite de Lyon 1, CNRS, 2. Av. A. Einstein, 69626 Villeurbanne, France
bS Supporting Information ABSTRACT: The 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.
1. INTRODUCTION The adsorption of hydrocarbons on the surface of nanoporous materials constitutes a crucial step for a large number of industrial applications including capture and purification of natural gas,1 separation of alkanes in both liquid and vapor phases for increasing the octane number of gasoline as well as several catalytic reactions2 such as the cracking of crude oil fractions,3 the isomerization of alkanes and aromatics,4 or the hydro-isomerization of linear to branched alkanes.5,6 Indeed, a deeper understanding of the adsorption and transport processes of linear hydrocarbons in selected porous adsorbents is an important issue from both a fundamental and an applied point of view. Metalorganic frameworks (MOFs) are a relatively new class of hybrid porous materials that combine inorganic nodes containing metal centers and organic linker moieties (carboxylates, phosphonates, imidazolates, etc.), and are potentially interesting r 2011 American Chemical Society
for the applications cited above because of their capacity to adsorb selectively and capture various guest species including gases or vapors.713 However, whereas the adsorption of light hydrocarbons in MOFs has been extensively studied, combining several experimental and modeling tools,1419 the adsorption of longer chain alkanes has been scarcely explored in the literature. For instance, Nicolau et al.20 have studied the adsorption of C8 alkyl-aromatics in the MOF-5 type solid, whereas Barcia et al. have investigated the separation of nhexane from a mixture of hexane isomers in the Zn2(1,4bdc)2(dabco) solid, where the terms bdc and dabco refer to the 1,4-benzenedicarboxylate and 1,4 diazabicyclo2,2,2 octane groups, respectively.21 Some of us have combined several Received: April 28, 2011 Revised: June 7, 2011 Published: June 14, 2011 13868
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Figure 1. View of the MIL-47(V) structure along the chain (z axis) highlighting the 1D pores system.
experimental methods to explore the adsorption of n-alkanes in both the flexible MIL-53 (Cr,Al)22 and rigid MIL-100 (Cr) and MIL-101 (Cr) MOFs.23 More recently, Luebbers et al. have studied the specific interactions between the ZIF-8 material and a large number of volatile organic compounds, including the n-alkanes (C5C10),24 whereas the separation of C5 hydrocarbons in the liquid phase was reported by Maes et al. in the Cu-BTC structure.25 Only a few modeling studies on the adsorption of long chain n-alkanes in MOFs have been reported so far. Zhang et al.26 have explored the separation performances of the IRMOF-1 and IRMOF-6 systems with respect to the C4C6 alkane isomer mixtures. This was followed by a study by Dubbelbam et al.,27 who simulated a multicomponent mixture in the C5C7 range in MOF-1. More recently, Finsy et al. conducted a joint experimental and theoretical study on the low-coverage adsorption properties of the MIL-47(V) solid. 28 Whereas these authors reported the Henry constant and the adsorption enthalpy for linear and branched alkanes including C5C8, benzene, cyclohexane, and benzene, the adsorption isotherms were not investigated. Furthermore, in applications involving hydrocarbon gas adsorption/separation, information on the dynamics of the molecules in the porous solids is also of great importance. However, again, although a number of studies have been published for short linear alkanes,16,2931 only a few computational and experimental investigations are available on the diffusion of long chain alkanes within the pores of MOFs.26,32,33 In light of this lack of available literature on the diffusion/ adsorption properties of long chain hydrocarbons in MOFs, much effort is still required to obtain a complete picture of these processes. The aims of this study were (i) to determine the adsorption properties of n-alkanes from n-pentane (C5) to
n-nonane (C9) in the rigid MOF type MIL-47 (V) material by a combination of experimental gravimetry measurements and grand canonical Monte Carlo (CMC) simulations and (ii) to explore the microscopic diffusion mechanism of these n-alkanes within the pores of the material by means of molecular dynamics (MD) simulations. The MIL-47(V) material is built up from chains of corner-sharing V4+O6 octahedra, interconnected by linear dicarboxylate ligands that lead to 1D diamond-shaped pores of free diameter close to 8.5 Å (Figure 1). This solid can be considered as a model system because it has been previously shown that its structure presents a rather homogeneous pore surface (no acidic or basic sites) and does not undergo any significant structural changes upon adsorption of various vapors including hydrocarbons.34 The conclusions drawn from this study will be complementary to previous investigations conducted on this solid for probing the adsorption/diffusion of short linear alkanes16,29 and xylenes.28,35,36 In this study, gravimetry measurements were first performed at ambient temperature to determine the adsorption performance of the MIL-47(V) solid for each long chain n-alkane. These experimental explorations were completed by configuration bias grand CMC simulations based on two different adsorbate/ adsorbent forcefields to determine the energetic of the interactions and to explore the microscopic adsorption mechanisms, including a careful exploration of the preferential conformations for each confined hydrocarbon using the approach reported by Bates et al.37 Finally, the most accurate forcefield for reproducing the thermodynamics of the system was then transferred to the MD simulations to probe the dynamics of the n-alkanes. The selfdiffusivity for each n-alkane was thus calculated, and the diffusion mechanisms on the microscopic scale were elucidated and compared with those we previously evidenced for n-butane in the same solid29 13869
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Figure 2. Comparison between the experimental adsorption isotherms for n-pentane (0), n-hexane (O), n-heptane (4), n-octane (]), and n-nonane (r) and the calculated ones using CB-GCMC simulations. The experimental data are represented by empty symbols, wheres the simulations are described by filled symbols colored blue and red for models 1 (a) and 2 (b), respectively.
accuracy better than 0.5%. The mass uptake of the vapor flow ensured the stability of the absolute pressure in the balance with a constancy better than 0.5%. The mass uptake accuracy was better than 1 μg with a sensitivity of 0.1 μg. The typical mass sample was ∼25 mg. For each data of the adsorption isotherms, the thermodynamical equilibrium was assumed for a variation of weight dm/dt (in mg/s) lower than 0.001 for 20 min. Some data took >4 h for completion.
3. COMPUTATIONAL METHODOLOGY Figure 3. Comparison of the experimental28 adsorption enthalpy (b) with those obtained by CB-GCMC simulations using either model 1 (0) or model 2 (O) as a function of the carbon number of the n-alkane chain. All simulated values are reported for a loading of 1 molecule/u.c.
2. EXPERIMENTAL SECTION: GRAVIMETRY MEASUREMENTS MIL-47 or VIVO(O2CC6H4CO2) was synthesized and activated using the procedure reported in ref 38. The adsorption uptakes were measured at 303 K using a gravimetry apparatus at pressures up to the saturation vapor pressure of each adsorbate using a commercial gravimetry adsorption device (DVS vacuum, SMS). The vapors of n-alkanes were continuously introduced in the gravimetry device by applying a depression on a bottle containing the liquid n-alkane using a molecular pump. A constant pressure was obtained by controlling the vapor flow using a butterfly valve on the outlet of the pump and a mass flow controller on the outlet of the bottle of liquid adsorbate. (See Figure S1 of the Supporting Information.) Various pressures could be obtained by adjusting the opening of this valve, thus allowing for the determination of the full adsorption isotherm, from very low absolute pressure, up to the saturation pressure of the different adsorbates at 303 K. The balance was therefore under the vapor pressure of the adsorbate without the requirement of a gas carrier. The n-alkanes were obtained from Aldrich (purity >99.5%) and stored on 3 Å activated molecular sieve to trap the residual water moieties from the organic liquid phase. Prior to each experiment, MIL-47 (V) was outgassed at 150 C for 8 h under 105 Torr. Each adsorption isotherm was obtained using a fresh sample. n-Alkane vapor flow was set to 2 mL min1. The stability of the vapor flow ensured the stability of the absolute pressure in the balance with
3.1. Microscopic Model for the Alkanes Molecules. The n-alkane molecules were modeled using a flexible united atom (UA) model, where each CHx unit was treated as a single interaction site and the CHxCHx bonds were maintained rigid at the distance of 1.54 Å. This microscopic model has been successfully employed to simulate the adsorption behavior of various hydrocarbons in zeolites39 and MOFs.27 The intramolecular interactions were calculated using the sum of the bending term between three successive interaction sites (represented by a harmonic expression) and the sum of the dihedral torsion contributions between four successive interaction sites expressed by a cosine potential. The nonbonded intermolecular interactions between the n-alkane molecules were modeled via a repulsion-dispersion Lennard-Jones (LJ) term, and the cross LJ parameters were calculated by using Lorentz Berthelot combination rules. The values for the intramolecular terms as well as the LJ parameters were taken from the transferable potential for phase equilibria (TraPPE) forcefield,40 which was previously fitted to reproduce the liquid/vapor coexistence curves and the critical properties of various n-alkanes. 3.2. Microscopic Model for the MIL-47(V). The crystal structure of the MIL-47(V) material was built using the atomic coordinates previously derived from X-ray diffraction data.38 The framework was maintained rigid during the simulations for each n-alkane because in situ X-ray diffraction experiments have shown that the structure does not undergo any significant modification upon adsorption of such hydrocarbons.34 3.3. Interatomic Potentials. Two distinct sets of force field parameters have been subsequently applied. In a first step, all atoms of the crystalline lattice were described by the generic universal force field (UFF)41 (model 1). A further step consisted of describing only the inorganic node of the MOF material (corresponding to the metal center with the surrounding oxygen atoms) 13870
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Figure 4. Typical illustration of the preferential adsorption sites for n-pentane (a,c) and n-nonane (b,d), respectively, at low (1 molecule/u.c.) (a,b) and high (2 and 3 molecules/u.c. for C9 and C5, respectively) (c,d) loadings, extracted from the 2D probability density plots calculated through the xy plane from the CBMC simulations. The light-blue regions correspond to areas of high probability of locating n-alkane molecules, whereas the white parts are low-probability regions.
by the UFF forcefield, whereas the organic part was treated using the Dreiding forcefield.42 This was labeled as model 2. The adsorbate/MIL-framework interactions were then represented via solely Lennard-Jones (LJ) potentials. Indeed, the LJ parameters corresponding to the interactions between the CHx center and each atom of both MIL frameworks were obtained by using the LorentzBerthelot mixing rules. 3.4. Configurational Bias Grand Canonical Monte Carlo (CB-GCMC) Simulations. The absolute adsorption isotherms were computed at 303 K for each hydrocarbon under consideration by configuration-bias grand canonical Monte Carlo (CBGCMC) simulations. These simulations consisted of evaluating the average number of adsorbate molecules whose chemical potential equals those of the bulk phase for given conditions of pressure and temperature. The use of the CB-GCMC method allows an efficient sampling of the hydrocarbon conformation during the simulations.43 This efficiency comes from the insertion of one segment at a time of the hydrocarbon being transferred. A number of trial directions are selected at each growth step, and the choice of the direction along which to proceed is made based on the Boltzmann factor calculated from the interaction energies experienced by the growing segments. The chemical potentials were calculated by the Widom test particle insertion method from the NpT ensemble Monte Carlo simulation.44 During the simulations, the structure of the MIL-47(V) was treated as rigid, as mentioned above, and the periodic conditions were applied. The simulation box consisted of 16 (2 2 4) unit cells, allowing a cutoff of 12 Å to be used for the short-range interactions In each simulation, 2.5 106 Monte Carlo steps were typically performed. The Monte Carlo moves included translation, rotation, insertion of a new molecule, deletion of a
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Figure 5. Distribution of the dihedral angles over all bonds for (a) each n-alkane at low loading (1 molecule/u.c.) and for (b) n-pentane and n-nonane at both low (1 molecule/u.c.) and high loading (2 and 3 molecules/u.c. for n-nonane and n-pentane, respectively). 0 corresponds to a trans conformation.
randomly selected existing molecule, partial and complete regrowth of the n-alkane, and reptation. Finally, the differential adsorption enthalpy at low coverage was calculated through a revisited Widom test particle method reported by Vlugt et al.45 3.5. Molecular Dynamics Simulations. MD simulations were performed using the DL_POLY program in the NVT ensemble and the NoseHoover46 thermostat. The MIL-47(V) framework was maintained fixed during the simulations. In contrast, as mentioned in section 3.1, the n-alkane was treated as fully flexible because this parameter can significantly influence the diffusivity behavior in porous solids.47 For each n-alkane, the starting configurations were obtained by preliminary CMC simulations. The MD simulations were then performed at 303 K using a simulation box built using 16 unit cells, each containing 1 and 2 n-alkane molecules per unit cell. Each simulation was performed using 1 107 steps (i.e., 10 ns) with a time step of 1 fs, preceded by 1 000 000 steps of equilibration. The electrostatic interactions were calculated using the Ewald summation method, whereas a cutoff of 12 Å was used for the short-range interactions, consistent with the CB-GCMC calculations. To calculate the self-diffusion coefficients of the molecules, the mean square displacements (MSDs) for all n-alkanes were evaluated by means of the following classical equation * DS ðcÞ ¼ lim
t sf ∞
1 6t
2 + 1 N rj ðtÞ rj ðoÞ N j¼1
∑
In this equation, denotes an ensemble average, r(t) are the positions of the center of mass of the tagged guest molecule, whereas N corresponds to the number of hydrocarbon molecules (e.g., 1 molecule/u.c. for each hydrocarbon). To ensure that a 13871
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Figure 7. Proportions of n-alkanes molecules lying parallel along the direction of the tunnel according to the criteria defined in the text: Full square symbols and solid lines are the data obtained at low loading for each n-alkane (1 molecule/u.c.), and open circle symbols correspond to the high loading (2 and 3 molecules/u.c. for n-nonane and n-pentane, respectively).
Table 1. Self-Diffusion Coefficients for Each n-Alkane Simulated at 303 K for a Loading of 1 and 2 Molecules/u.c. n-alkane
Figure 6. Distribution of the angles formed between the end-to-end vector and the z axis over all conformations for (a) each n-alkane at low loading (1 molecule/u.c.) and for (b) n-pentane and n-nonane at both low (1 molecule/u.c.) and high loading (2 and 3 molecules/u.c. for n-nonane and n-pentane, respectively).
sufficient number of data points was used during the calculation, we employed multiple time origins and five different MD trajectories, as described elsewhere.48 The orientationally averaged self-diffusivities (Ds = D1D/3) were then evaluated by fitting the MSDs plots as a function of the time over period of 200500 ps, assuming Einstein’s relation.47 Complementary MD simulations were subsequently performed at higher hydrocarbon concentration to explore the influence of the loading on the diffusion mechanism.
4. RESULTS AND DISCUSSION The experimental adsorption isotherms for the C5 to C9 molecules are shown in Figure 2. The isotherms are of type I, consistent with the rigid nature of the microporous MIL-47(V) material.34 The saturation plateaus are well-defined with adsorbed amounts of 3.9, 3.45, 2.9, 2.5, and 1.9 molecules per unit cell obtained for n-pentane, n-hexane, n-heptane, n-octane, and n-nonane, respectively, suggesting that the adsorption process occurs mainly within the porosity. Indeed, the adsorbed quantity decreases by a quasi-constant amount per CH2 group on the adsorbate (∼0.45 molecules/u.c.) from n-pentane to n-octane, although this trend significantly changes when one goes from n-octane to n-nonane (0.60 molecules/u.c.). Some of us have already reported such a different behavior for n-nonane in comparison with shorter n-alkanes in the flexible MIL-53 (Cr, Al) solids for both the saturation capacity and the adsorption kinetics.22,34 Further insight into the adsorption mechanisms can be obtained from the molecular simulations. Figure 2 shows a comparison of
Ds (108 m2 s1) 1
Ds (108 m2 s1) 2
molecule/u.c.
molecule/u.c.
n-C5
0.42
0.058
n-C6 n-C7
0.38 0.34
0.018 0.011
n-C8
0.32
0.011
n-C9
0.29
0.004
the experimental adsorption isotherms measured for all of the n-alkanes considered, with those calculated by CB-GCMC simulations based on the two different forcefields labeled as models 1 and 2, as described above. Regarding model 1, a good agreement is obtained between the calculated and experimental saturation adsorption amounts for n-pentane, n-heptane, and n-octane. However, the simulations considerably overestimate the saturation amount for the n-hexane and n-nonane. If one now considers model 2, the calculated saturation capacities are in good agreement with the experimental values for all n-alkanes, except n-nonane, as already pointed out using model 1. In addition, the shape of the experimental isotherm is better reproduced at low pressure. The differential adsorption enthalpies calculated at low coverage using the CB-GCMC simulations based on models 1 and 2 have been compared with the experimental values previously reported by Finsy et al.35 using gas chromatography. Figure 3 reports our calculated values as a function of the number of carbon atoms on the alkyl chains. At a first glance, one observes a similar linear evolution for both experimental and simulated data. Elongating the n-alkane chain by a single CH2 group induces an additional dispersive interaction between the alkyl chain and the MOF surface, accompanied by an increase in the differential adsorption enthalpy by a constant increment. This latter value can be extracted from a linear fit of the reported curves. One obtains an increment of 7.50 kJ mol1 from the experimental data, whereas analysis of the CB-GCMC results lead to value of 8.00 and 7.45 kJ mol1 for models 1 and 2, respectively. Indeed, it appears that the CB-GCMC simulations with model 1 overestimate the strength of the n-alkane/MIL-47 interactions, whereas 13872
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Figure 8. 2D density probability maps extracted by MD simulations at 303 K through the yz plane of four different tunnels of MIL47(V) for n-nonane at (a) low loading (1 molecule/u.c.) and (b) higher loading (2 molecules/u.c.). Black regions correspond to higher probability of containing the n-alkanes.
the use of model 2 gives a very good agreement with the experimental data, which is consistent with a better description of the experimental adsorption isotherms in the initial stage of loading. This result is explained by the fact that the UFF force field compared with Dreiding overestimates the interactions that occur between the alkane molecules and the hydrogen atom of the organic linker. Because model 2 leads to a better description of the experimental adsorption isotherms over the whole range of pressure, the results obtained using this model were subjected to further analysis to obtain some microscopic insight into the adsorption mechanism and the preferential conformations of the n-alkanes at low and high loadings. CBMC simulations performed in the NVT ensemble using at least 1 107 MC steps per simulation allowed us to extract the probability density of each n-alkane to provide an average picture of the preferential location of the adsorbate molecules within the porosity. The resulting 2D density plots obtained for n-pentane (Figure 4a) and n-nonane (Figure 4b) at low loading provide a typical illustration of the molecular distribution. These densities are shown on the xy plane (the z axis lies along the length of the tunnel) corresponding to the orthogonal cross-section of the pore. As one can see, the n-alkanes are mainly located at the center of the pores in a diamond-shaped corridor. At higher loading, close to saturation (Figure 4c,d), the preferential location of the n-alkane molecules significantly deviates from those observed at low loading, with the probability density at the center of the corridor decreasing in both cases. This effect is much more pronounced for the n-nonane molecules. A further step consisted of analyzing both the preferential . orientations and the conformations of the confined n-alkanes by adopting the methodology previously proposed by Bates et al.37 Figure 5a shows the distributions of the dihedral angles calculated over all bonds in the n-alkanes adsorbed in MIL-47 (V) for a loading of 1 molecule/u.c. The dihedral angle of 0 corresponds to a trans conformation, whereas the angles from 90 to 150 (90 to 150) are related to a gauche +() conformation. This plot shows a small increase in the proportion of the trans conformation with increasing alkyl chain length of the adsorbate, which means that the n-pentane molecules are slightly more distorted than the other n-alkanes. This behavior is different from
that usually observed when the molecules are in the “free gas phase”, where increasing the n-alkane chain leads to a preferential gauche conformation.49,50 However, it is consistent with what has been previously reported for the same n-alkanes adsorbed in the Mordenite and silicalite-type zeolites, where the confinement effect forces the adsorbed n-alkane molecules to a more linear conformation than their counterparts in the gas phase, with this trend increasing with the hydrocarbon chain length.37,51 Figure 5b compares the extreme cases of n-pentane and n-nonane at higher loading close to saturation. One observes that whereas for the shortest n-alkane of the series the molecules tend to be significantly more distorted than in the initial stage of adsorption, the situation is the inverse for n-nonane, wherein the proportion of the trans conformations remains very similar. Figure 6a reports the angle formed between the end-to-end vector of each n-alkane and the z axis (corresponding to the direction along the tunnel), obtained for a loading of 1 molecule/u.c. The distribution of the angles is rather broad for the shorter C5 and C6 molecules and becomes narrower and centered around increasingly smaller angle values as one moves toward C9. Such a trend clearly shows that the n-alkanes become increasingly aligned along the z axis when the chain length increases. This behavior is illustrated in Figure 7 ,which shows the proportion of the chains for each n-alkane that form an angle below 45 with the z axis. This value of 45 for the angle is generally used as a threshold to obtain a qualitative trend on the molecular alignment along the pore. One can thus observe that whereas the n-nonane and n-octane align solely along the direction of the tunnel, the proportion significantly decreases for the shorter n-alkanes with only 50% for the n-pentane. This trend is mainly governed by a steric constraint as the difference between the cross section perpendicular to the direction of the tunnel of ∼10.70 Å (taking into account the σ value of 3.118 Å for the oxygen atoms), and the chain length drastically increases from C5 to C9 (from 9.31 to 14.45 Å), which results in molecules that increasingly adsorb with end-to-end vector exclusively parallel to the channel. At higher loadings, as can be seen in Figure 6b, the profile for n-nonane significantly differs, with the presence of two well-defined distributions of angles around 3 and 24.These distributions emphasize two predominant orientations of this molecule, with its global proportion parallel to the z axis being only slightly affected 13873
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Figure 9. Distribution of the molecules within the pore of MIL-47 (V) recorded during the MD runs at 303 K for (a) each n-alkane at low loading (1 molecule/u.c.) and for (b) n-pentane and n-nonane at low (1 molecule/u.c.) and high loadings (3 and 2 molecules/u.c., respectively). The origin of the distances corresponds to the middle of the pore.
(Figure 7) compared with the initial loading. This latter observation thus suggests an efficient packing of the n-nonane molecules parallel to the z axis even at high loading. One might assume that such an optimal arrangement of the molecules within the porosity is not reached in the experimental conditions used for collecting the equilibrium adsorption isotherms, which would explain the lower experimental saturation capacity compared with the simulations. In contrast, the shape of the curve obtained for n-pentane, as shown in Figure 6b, remains similar. However the orientations of these molecules deviate to a greater degree from the z axis, as summarized in Figure 7. The next step consisted of probing the diffusivity of all of these n-alkanes at 303 K by means of MD simulations based on model 2. Table 1 reports the so-obtained orientationally averaged selfdiffusivities Ds values for a loading of one molecule per unit cell. One observes that Ds decreases when the chain length increases, with the values remaining lower than those we previously reported for shorter n-alkanes (methane to n-butane) in the MIL-47(V),16,29 where the values ranged between 0.7 and 1.4 108 m2 s1. This trend is also consistent with what has been previously obtained in different zeolites.5153 Furthermore, at a higher hydrocarbon concentration, that is, two molecules per unit cell, one observes that there is a significant decrease in the self-diffusivity when one goes from n-octane to n-nonane. Such a jump in the diffusion might be at the origin of the discrepancy observed for the longest alkane between the experimental and simulated saturation capacities in the adsorption isotherms.
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To determine the microscopic diffusion mechanism, 2D probability density maps through the (xz) plane of the tunnels were extracted from the MD trajectories for each n-alkane. As a typical illustration, the profile obtained for n-nonane (Figure 8a) at low loading shows that the motions are mainly restricted along the direction of the tunnel, supporting a 1D type diffusion mechanism. Such a behavior, which remains true regardless of the alkyl chain length of the adsorbate, is consistent with our previous findings for the shorter linear n-alkanes (C1 to C4) in the MIL47(V) material.16,29 The so-obtained profiles also suggest that the n-alkanes molecules follow a pathway along the channel that corresponds to a set of successive positions each separated from one another by a small energy barrier. Furthermore, one can observe from Figure 9 that the motions of all of the n-alkanes mainly occur around the middle of the pore with the highest probability of finding the molecules at a distance from the center of the pore slightly varying from n-pentane to n-nonane (1.60 and 1.53 Å, respectively). At high loading, the n-nonane molecules still diffuse along the direction of the tunnel, as shown in Figure 8b. However, one can envisage two types of displacements: (i) motions over regions of the pore that are closer to the walls and (ii) jump sequences between positions near the pore walls and in the center of the pore. The observation of the first kind of motions is confirmed by Figure 9b, which shows that the highest probability of finding the n-nonane is significantly shifted at a higher distance from the center of the pore (2.10 vs 1.53 Å) compared with the low loading state. In addition, there is a lower half width at halfmaximum (hwhm) in this particular case (0.93 vs 1.26 Å), which means that the diffusion corridor is narrower for this molecule. Regarding the n-pentane, the same conclusions can be drawn because there is a significant shift of the distribution from 1.59 to 1.97 Å, although the diffusion corridor remains almost unchanged, as evidenced by very similar hwhm values (1.41 vs 1.45 Å). Finally, an inspection of the MD trajectories allows one to confirm the absence of a single-file diffusion for all n-alkanes regardless of the loading range explored similar to other species such as CO254 and H2.48
5. CONCLUSIONS A joint experimental-computational approach has been employed to study the adsorption and diffusion properties of longchain linear n-alkanes from n-pentane to n-nonane in the rigid MIL-47 material. The adsorption isotherms as well as the differential enthalpies of adsorption at zero coverage were simulated by means of configurational bias Monte Carlo simulations using two different forcefields for describing the n-alkane/MIL framework interactions. On the basis of a comparison with experimental data obtained by gravimetry measurements, the combined UFFDreiding forcefield, labeled as model 2, proved to be the most adapted for describing the n-alkane-MIL-47(V) system, whereas model 1 fails to reproduce the saturation capacity of n-nonane. Model 2 was used to explore the microscopic adsorption mechanism for all n-alkanes, including their preferential locations and orientations within the porosity of the solid as well as their most probable conformations. Our results show that at low loading, all of the n-alkanes are broadly distributed in the channel within a diamond-shaped corridor, and this corridor section is very similar regardless of the n-alkane. These molecules are seen to be increasingly aligned along the direction of the tunnel as the alkyl chain length of the adsorbate increases. In addition, the n-alkanes 13874
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’ ASSOCIATED CONTENT
bS
Supporting Information. Controlling the vapor flow using a butterfly valve on the outlet of the pump and a mass flow controller on the outlet of the bottle of liquid adsorbate. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ ACKNOWLEDGMENT The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 228862 and from the French ANR (ANR-07-BLAN-0284-02-SAHFS). ’ REFERENCES (1) Ackley, M. W.; Rege, S. U.; Saxena, H. Microporous Mesoporous Mater. 2003, 61, 25. (2) Zecchina, A.; Otero Arean, C. Chem. Soc. Rev. 1996, 25, 187. (3) Altwasser, S.; Welker, C.; Traa, Y.; Weitkamp, J. Microporous Mesoporous Mater. 2005, 83, 345. (4) Tromp, M.; van Bokhoven, J. A.; Oostenbrink, M. T. G.; Bitter, J. H.; de Jong, K. P.; Koningsberger, D. C. J. Catal. 2000, 190, 209. (5) Denayer, J. F. M.; Martens, J. A.; Jacobs, P. A.; Thybaut, J. W.; Marin, G. B.; Baron, G. V. Appl. Catal., A 2003, 246, 17. (6) Martens, G. G.; Marin, G. B.; Martens, J. A.; Jacobs, P. A.; Baron, G. V. J. Catal. 2000, 195, 253. (7) Zhao, X.; Xiao, B.; Fletcher, A. J.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306, 1012. (8) Li, J. R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev. 2009, 38, 1477. (9) Hamon, L.; Llewellyn, P. L.; Devic, T.; Ghoufi, A.; Clet, G.; Guillerm, V.; Pirngruber, G. D.; Maurin, G.; Serre, C.; Driver, G.; van Beek, W.; Jolimaitre, E.; Vimont, A.; Daturi, M.; Ferey, G. J. Am. Chem. Soc. 2009, 131, 17490. (10) D’Alessandro, D. M.; Smit, B.; Long, J. F. Angew. Chem., Int. Ed. 2010, 49, 6058. (11) Keskin, S.; Van Heest, T. M.; Sholl, D. S. ChemSusChem 2010, 3, 879. (12) Ferey, G.; Serre, C.; Devic, T.; Maurin, G.; Jobic, H.; Llewellyn, P. L.; De Weireld, G.; Vimont, A.; Daturi, M.; Chang, J. S. Chem. Soc. Rev. 2011, 40, 550. (13) Klein, N.; Henschel, A.; Kaskel, S. Microporous Mesoporous Mater. 2010, 129, 238. (14) Llewellyn, P. L.; Maurin, G.; Loera-Serna, S.; Rosenbach, N.; Devic, T.; Serre, C.; Bourrelly, S.; Horcajada, P.; Filinchuk, Y.; Ferey, G. J. Am. Chem. Soc. 2008, 130, 12808. (15) Llewellyn, P. L.; Horcajada, P.; Maurin, G.; Devic, T.; Rosenbach, N.; Bourrelly, S.; Serre, C.; Vincent, D.; Loera-Serna, S.; Filinchuk, Y.; Ferey, G. J. Am. Chem. Soc. 2009, 131, 13002.
ARTICLE
(16) Rosenbach, N.; Ghoufi, A.; Deroche, I.; Llewellyn, P. L.; Bourrely, S.; Serre, C.; Ferey, G.; Maurin, G. Phys. Chem. Chem. Phys. 2010, 12, 6428. (17) Keskin, S. J. Phys. Chem. C 2010, 114, 13047. (18) Guo, H.; Shi, F.; Ma, Z.; Lui, X. J. Phys. Chem. C 2010, 114, 12158. (19) Dubbeldam, D.; Frost, H.; Walton, K. S.; Snurr, R. Q. Fluid Phase Equilib. 2007, 261, 152. (20) Nicolau, M. P. M.; Garcia, P. S.; Gallego, J. M.; Silva, J. A. C.; Rodrigues, A. E.; Chen, B. J. Phys. Chem. C 2009, 113, 13173. (21) Barcia, P. S.; Zapata, F.; Silva, J. A. C.; Rodrigues, A. E.; Chen, B. L. J. Phys. Chem. B 2007, 111, 6101. (22) Trung, T. K.; Trens, P.; Tanchoux, N.; Bourrelly, S.; Llewellyn, P. L.; Loerna-Serna, S.; Serre, C.; Loiseau, T.; Fajula, F.; Ferey, G. J. Am. Chem. Soc. 2008, 130, 16926. (23) Trung, T. K.; Ramsahye, N.; Trens, P.; Tanchoux, N.; Serre, C.; Fajula, F.; Ferey, G. Microporous Mesoporous Mater. 2010, 134, 134. (24) Luebbers, M. T.; Wu, T.; Shen, L.; Masel, R. I. Langmuir 2010, 26, 11319. (25) Maes, M.; Alaerts, L.; Vermooltele, F.; Ameloot, R.; Couck, S.; Finsy, V.; Denayer, J. F. M.; De Vos, D. E. J. Am. Chem. Soc. 2010, 132, 2284. (26) Zhang, L.; Wang, Q.; Wu, T.; Liu, Y. C. Chem.—Eur. J. 2007, 13, 6387. (27) Dubbeldam, D.; Galvin, C. J.; Walton, K. S.; Ellis, D. E.; Snurr, R. Q. J. Am. Chem. Soc. 2008, 130, 10884. (28) Finsy, V.; Calero, S.; Garcia-Perez, E.; Merkling, P. J.; Vedts, G.; De Vos, D. E.; Baron, G. V.; Denayer, J. F. M. Phys. Chem. Chem. Phys. 2009, 11, 3225. (29) Jobic, H.; Rosenbach, N.; Ghoufi, A.; Kolokolov, D.; Yot, P. G.; Devic, T.; Serre, C.; Ferey, G.; Maurin, G. Chem.—Eur. J. 2010, 16, 10337. (30) Getzschmann, J.; Senkovska, I.; Wallacher, D.; Tovar, M.; Fairen-Jimenez, D.; van Baten, J. M.; Krishna, R.; Kaskel, S. Microporous Mesoporous Mater. 2010, 136, 50. (31) Bux, H.; Chmelik, C.; Krishna, R.; Caro, J. J. Membr. Sci. 2011, 369, 284. (32) Sun, X.; Wick, C. D.; Thallapally, P. K.; McGrail, B. P.; Dang, L. X. Chem. Phys. Lett. 2011, 501, 455. (33) Chmelik, C.; K€arger, J.; Wiebcke, M.; Caro, J.; van Baten, J. M.; Krishna, R. Microporous Mesoporous Mater. 2009, 117, 22. (34) Trung, T. K.; Deroche, I.; Rivera, A.; Yang, Q.; Yot, P.; Ramsahye, N.; Vinot, S.; Devic, T.; Horcajada, P.; Serre, C.; Maurin, G.; Trens, P. Microporous Mesoporous Mater. 2011, 140, 114. (35) Finsy, V.; Verelst, H.; Alaerts, L. J. Am. Chem. Soc. 2008, 130, 7110. (36) Castillo, J. M.; Vlugt, T. J. H.; Calero, S. J. Phys. Chem. C 2009, 113, 20869. (37) Bates, S. P.; van Well, W. J. M.; van Santen, R. A.; Smit, B. J. Phys. Chem. 1996, 100, 17573. (38) Barthelet, K.; Marrot, J.; Riou, D.; Ferey, G. Angew. Chem., Int. Ed. 2002, 41, 281. (39) Pascual, P.; Ungerer, P.; Tavitian, B.; Pernot, P.; Boutin, A. Phys. Chem. Chem. Phys. 2003, 5, 3684. (40) Chen, B.; Siepmann, J. I. J. Phys. Chem. B 1999, 103, 5370. (41) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddart, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (42) Mayo, S. L.; Olafson, B. D.; Goddart, W. A., III. J. Phys. Chem. 1990, 94, 8897. (43) Siepmann, J. I.; Frenkel, D. Mol. Phys. 1992, 75, 59. (44) Widom, B. J. Chem. Phys. 1963, 39, 2802. (45) Vlugt, T. J. H.; Garcia-Perez, E.; Dubbeldam, D.; Ban, S.; Calero, S. J. Chem. Theory Comput. 2008, 4, 1107. (46) Frenkel, D.; Smit, B. Understanding Molecular Simulation; Academic Press: New York, 1996. (47) Jobic, H.; Theodorou, D. N. Microporous Mesoporous Mater. 2007, 102, 21. (48) Salles, F.; Jobic, H.; Maurin, G.; Kozza, M. M.; Llewellyn, P. L.; Devic, T.; Serre, C.; Ferey, G. Phys. Rev. Lett. 2008, 24, 245901. 13875
dx.doi.org/10.1021/jp2039527 |J. Phys. Chem. C 2011, 115, 13868–13876
The Journal of Physical Chemistry C
ARTICLE
(49) Bartell, L. S.; Kohl, D. A. J. Chem. Phys. 1958, 35, 3097. (50) Compton, D. A. C.; Montero, S.; Murphy, W. F. J. Phys. Chem. 1980, 84, 3687. (51) Jobic, H.; Farago, B. J. Chem. Phys. 2008, 129, 171102. (52) Beerdesen, E.; Dubbeldam, D.; Smit, B. J. Phys. Chem. 2006, 110, 22754. (53) Garcia-Sanchez, A.; Dubbeldam, D.; Calero, S. J. Phys. Chem. C 2010, 114, 15068. (54) Salles, F.; Jobic, H.; Devic, T.; Llewellyn, P. L.; Serre, C.; Ferey, G.; Maurin, G. ACS Nano 2010, 4, 143.
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dx.doi.org/10.1021/jp2039527 |J. Phys. Chem. C 2011, 115, 13868–13876
