Coadsorption of n-Hexane and Benzene Vapors onto the Chromium

Article pubs.acs.org/JPCC

Coadsorption of n‑Hexane and Benzene Vapors onto the Chromium Terephthalate-Based Porous Material MIL-101(Cr) An Experimental and Computational Study Philippe Trens,†,* Hichem Belarbi,† Céline Shepherd,† Philippe Gonzalez,† Naseem A. Ramsahye,† U-Hwang Lee,‡ You-Kyong Seo,‡ and Jong-San Chang‡ Institut Charles Gerhardt, UMR CNRS 5253, École Nationale Supérieure de Chimie de Montpellier, 8 rue de l’École Normale, 34296 Montpellier cedex 5, France ‡ Korean Research Institute of Chemical Technology (KRICT), Daejeon, South Korea †

S Supporting Information *

ABSTRACT: The adsorption of n-hexane−benzene mixture onto a chromium terephtalate-based porous material (MIL-101(Cr)) has been studied experimentally and theoretically. The adsorption isotherms of the single components show that MIL-101(Cr) has a better affinity for benzene than for n-hexane. This is in good agreement with the enthalpies of adsorption determined at low coverage. Values of −68 kJ·mol−1 and −61 kJ·mol−1 were found for benzene and n-hexane, respectively. These are consistent with the simulated enthalpies of adsorption and also with the benzene/n-hexane selectivities which range between 2 and 3 depending on the equilibrium pressure. The saturation plateau obtained with nhexane is 30% lower than that obtained with the adsorption of benzene onto MIL101(Cr). In the case of the mixture of n-hexane−benzene, the saturation plateau is located between those obtained after adsorption of the single components. This is an indication that the coadsorption of n-hexane and benzene does not occur at the expense of one component of the mixture. However, the kinetics of adsorption of the mixture shows that benzene is adsorbed preferentially at low coverage. This is consistent with the chromatographic separation of n-hexane−benzene mixture by MIL-101(Cr).

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benzene, π−π interactions with the aromatic ligands may also come into play. This additional interaction should increase its adsorption as compared to alkanes having the same number of carbon atoms. As mentioned above, the adsorption of n-hexane or benzene onto MIL-101(Cr) has already been studied.13 Some studies describe the coadsorption of similar compounds and a few studies focus on the coadsorption of benzene and n-hexane. For instance, Modhera et al. reported on the simultaneous n-hexane isomerization and benzene saturation over Pt/nanocrystalline zeolite beta.15 However, this study was mainly devoted to the catalytic aspects of this binary system and did not describe the competition between the two species for the surface sites. Fathizadeh et al. reported on the adsorption of aromatic compounds from alkane/aromatic mixtures by NaY zeolite.16 In this particular case, the aim was to study the adsorption of benzene from mixtures in the liquid phase. A modeling approach was also used for studying the adsorption selectivity of benzene/propene mixtures for various zeolites.17 It was found that benzene is adsorbed preferentially as compared with

INTRODUCTION The extraction or separation of volatile organic compounds (VOC) from hydrocarbons mixtures is a very active research area because the present processes in use are energy intensive.1,2 n-Hexane and benzene can be regarded as model VOCs and in order to remove them from gas or liquid streams, different strategies have been developed. These include the decontamination of air streams using bacteria,3 catalytic combustion,4 adsorption,5,6 or separation from the liquid phase using ionic liquids7 or sorption using various sorbents such as silicas,8 activated carbons9,10 or more recently, metal organic frameworks (MOFs).11 In fact, among the various possible technological applications for MOFs, their use in the separation of organic compounds appears very promising due to the adjustable chemical properties of these new sorbents. It is indeed possible to design the appropriate ligand and the metal centers for the target application.11 Recently, linear alkanes have been used to understand the sorption mechanisms in rigid or flexible MOFs, namely MIL-47(V),12 MIL-101(Cr),13 or MIL-53(Cr, Al, Fe).14 It appears that van der Waals interactions were mostly directing the sorption processes. Furthermore, it was demonstrated that the enthalpy of adsorption was proportional to the number of carbon atoms in the n-alkane considered.14 In the case of © 2012 American Chemical Society

Received: August 20, 2012 Revised: November 8, 2012 Published: November 19, 2012 25824

dx.doi.org/10.1021/jp308258k | J. Phys. Chem. C 2012, 116, 25824−25831

The Journal of Physical Chemistry C

Article

Figure 1. Structure of MIL-101(Cr). (top left) large cage, (top right) small cage, (bottom) hexagonal and pentagonal windows. Free apertures are given. Chromium octahedra, carbon atoms are in green and black, respectively. 800 mg. The average particle size of the sample obtained from a scanning electron micrograph (Figure S2 of the SI) is estimated to be 1−2 μm. MIL-101(Cr) is built up from trimers of chromium(III) oxide octahedra and dicarboxylate linkers resulting in hybrid supertetrahedron that further assemble into zeotypic mesoporous MOFs reminiscent of the MTN structure type. This solid has two sets of mesoporous cages in a 2:1 ratio of dimensions of 27 and 34 Å. Accessibility to the cages occurs through microporous pentagonal and hexagonal windows of ca. 11.7 and 16.0 Å (see Figure 1). The pore volume of MIL-101(Cr) is between 1.37 and 2.15 cm3·g−1 depending on its degree of purification.21 The influence of the two different cavities has already been studied in the past and this point proved important.14 Systems. Adsorbates. n-Hexane and benzene used as adsorbates (provided by Aldrich, purity >99.9%) were outgassed and stored over activated 3 Å molecular sieve before use. The vapor phase adsorption is made possible by vaporizing a liquid mixture of both components. This vapor mixture is then allowed to contact MIL-101(Cr). It was decided to determine the adsorption experiments at constant vapor phase composition. The n-hexane−benzene mixture phase diagram exhibits an azeotropic point, at around xn‑hexane = 0.9 as shown in Figure 2. Therefore, this composition remains unchanged as the liquid mixture is being vaporized. This is to say that it would have been very convenient to perform adsorption measurements at this molar fraction. Furthermore, it is clear that industrial considerations mostly concerns benzene as an impurity to be extracted, at least in the case of hydrocarbons mixtures. However, a clear experimental drawback is the fact that at low relative pressures, the partial pressure of benzene is very low. The benzene saturation pressure is already lower than that of n-hexane (185 and 283 Torr at 313 K, respectively). After adsorption the partial pressure of benzene is even lower. This was found to be a limitation of our measurements, even with a 1m optic path length spectrometric gas cell. Our experiments were therefore conducted with a vapor molar fraction of 0.5 (which corresponds to a liquid xn‑hexane = 0.35).This is detailed in Figure 2. The depletion of the liquid phase with n-hexane could be neglected by using a very large amount of liquid mixture (250

propene. This result is in line with our previous studies showing that dispersive interactions constitute a prominent factor for adsorption, electronic interaction being able to enhance the overall sorbate/sorbent interaction.12 Yu et al. studied the adsorption of benzene mixtures on silicalite-1 and NaX zeolites from the liquid phase.18 They showed that the adsorption of nhexane is favored in silicalite-1, due to its favorable conformation. However, benzene is preferentially adsorbed in NaX due to its higher enthalpy of adsorption and its more efficient packing in the zeolite cavities. In MIL-101(Cr), two different cavities may be accessible to sorbates (see figure 1) and it is therefore very interesting to study the competition of benzene and n-hexane for these cavities. The goal of this work is the determination of partial adsorption isotherms from an nhexane−benzene vapor mixture. We therefore focused on the measurement of transient compositions of the vapor phase during adsorption processes. This allows for the comprehension of the kinetic of adsorption of both components in competition and the determination of partial adsorption isotherms for the mixture n-hexane−benzene onto MIL101(Cr).

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EXPERIMENTAL SECTION

Systems Adsorbent. MIL-101(Cr) was synthesized according to the route already published elsewhere.19,20 In brief, MIL-101(Cr) was prepared from hydrothermal reaction of terephthalic acid (1 mmol) with Cr(NO3)3.9H2O (1 mmol), HF (0.25 mmol), and H2O (267 mmol) at 220 °C for 8 h. To remove the residual terephthalic acid present in the solid product, the as-synthesized MIL-101 was further purified by two-step processes using hot ethanol and aqueous NH4F solutions. After purification, the BET surface area of the sample obtained from a N2 physisorption isotherm (Figure S1 of the Supporting Information, SI) was 3990 m2/g. The thermal stability of MIL-101(Cr) was verified prior to the thermal treatment by thermogravimetry. The amount of sample used was usually around 25825



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equilibrium.24 Longer equilibration times gave the same sorption isotherms. Depending on the relative pressure, and therefore the sorption process, different adsorption times could be observed. The desorption branches of the sorption isotherms have not be included for the sake of clarity. These followed the traces of the adsorption branches down to p/p° = 0.05. In the case of vapor mixtures, the equilibrium or transient compositions of the vapor phase were determined before and after (or during) the adsorption process in the sample cell. This has been done by measuring the absorbance of the vapor phase in a gas cell (Bruker Optics, optic path = 1 m) at 1820 and 1380 cm−1 for δC−H (benzene) and νC−H (n-hexane). The spectra of the pure components can be seen in Figure 4. More

Figure 2. Binary phase diagram at 313 K for the mixture of n-hexane− benzene determined using the UNIFAC model and Simulis Thermodynamics (Prosim).23 mL). From this liquid phase, only 5 mL were necessary to complete the full adsorption isotherm. The composition of the liquid phase reservoir was verified after completion of each sorption isotherm. In these equimolar conditions, the partial adsorption isotherms could be determined from the very low relative pressures.

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METHODS Adsorption. The vapor adsorption/desorption experiments have been performed with a homemade apparatus already described elsewhere.22 This setup is based on manometric measurements (with two capacitative pressure gauges (0−10 Torr and 0−1000 Torr). The sample cell can be disconnected from the system to undergo a thermal treatment up to 250 °C (depending on the thermal stability of the sample) under a vacuum of 10−3 Torr. The adsorption setup presented above allows for the choice of the pressure of the adsorbate to be introduced (instead of the equilibrium pressure). Modifications of this apparatus have been made allowing for the measurement of differential enthalpies of adsorption as well as the determination of the composition of the gas phase during the whole adsorption process (Figure 3). Calorimetric measurements were performed operando using a C80 Setaram microcalorimeter at 313 K. Thermal stability of the sample was better than 0.01 K. Vapor adsorption was performed at 313 K, each sorption experiment being performed with a fresh sample of MIL101(Cr) in order to ensure the initial state of activation of the samples. A duration of 600 s at the same pressure in the sample cell was chosen as criterion for the thermodynamic

Figure 4. IR absorption spectra of benzene (top) and n-hexane (bottom) gases.

classical vapor composition can be performed using GC-MS. However, the advantage of our setup is that we did not have to take a part of the vapor phase before and after each sorption process, which changes the equilibrium state. Indeed in our case, the vapor phase was continuously analyzed without disturbing this equilibrium, allowing for the kinetics of partial adsorption to be determined. Separation. The gas phase chromatographic separation setup has been built in our laboratory. It is based on two mass flow controllers allowing for the introduction of precise amounts of liquid (a few mg per hour) into a vaporization chamber. From this chamber, the vapor mixture was introduced onto the separation column through an injection loop. This column of 30 cm long, 2 mm inner diameter was packed with ML-101(Cr) without presieving and blocked by some glass wool. The column was activated at 250 °C for 8 h before being put to the chromatographic test at 180 °C. Distinct components were tested first for calibration and determination of elution times before the mixture was separated. Partial pressures could be derived from these absorbances after calibration. The sum of the partial pressures was found to match the pressure recorded with a maximum 5% discrepancy.

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MOLECULAR SIMULATIONS The general methodology for the computational approach used in this work is available elsewhere, in our previous work, therefore only brief details of the model are given here.12,14 A model of the MIL-101(Cr) structure was built from the atomic coordinates determined by X-ray diffraction experiments.25 The hydrogen atoms were then added using the Materials Studio

Figure 3. Experimental setup for the coadsorption of n-hexane− benzene vapor at 313 K. 25826



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Figure 5. Adsorption isotherms of n-hexane (left) and benzene (right) on MIL-101(Cr) at 313 K. The integral enthalpy of adsorption has been reported for the initial stages of the adsorption isotherms.

has been calculated to be −65 kJ·mol−1. These values are in good agreement with the results obtained experimentally with an overprediction of less than 10%. Furthermore, the geometries resulting from these calculations at low loadings represent the most favorable sites for these molecules. In both cases, these sites are situated inside the tetrahedral cages. The experimental enthalpy profiles indicate a saturation of the most active sites at very low relative pressure. This is evidenced by a strong decrease of the enthalpy of adsorption at increasing relative pressures. Interestingly, this observation addresses both sorbates even though benzene exhibits stronger interaction with MIL-101(Cr). This is consistent with the result discussed above which emphasized that the tetrahedral cavities are the favored sites for both sorbates. The difference in adsorbed amounts at saturation can be explained by examining the possible conformations of the different adsorbed phases. As the loading is increased past the point where interaction with the MOF surface is no longer possible, the benzene molecules are able to form stacking configurations between themselves (Figure 6).

software,26 along with water molecules on two-thirds of the μ3 oxygen atoms. F atoms were added on the remaining third of these μ3 oxygens. The charges used for the framework atoms were taken from the work of Babarao et al.27 They were calculated using the CHELPG method applied to MIL-101(Cr) clusters optimized using DFT.28 The Lennard−Jones parameters for the framework atoms were taken from the Universal Force Field (UFF) for the inorganic sections of the framework29 and the DREIDING forcefield for the organic part.30 The n-hexane molecules were treated using the TraPPE united atom model,31,32 with the charges set to zero. The atoms on the benzene molecules were described including explicit hydrogen atoms, with the parameters taken from the TraPPE forcefield derived for aromatic molecules.32 The geometries were calculated using GCMC at low loadings in order to probe the molecule− framework interactions. The full adsorption isotherms were not simulated in this work. The enthalpies were calculated using Widom’s ghost particle method (at zero coverage). The model used has been validated in previous studies, by reproduction of adsorption isotherms and adsorption enthalpies.12,14

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RESULTS AND DISCUSSION Adsorption of Single Components. The adsorption isotherms of n-hexane or benzene on MIL-101(Cr) are shown in Figure 5. These results are consistent with adsorption isotherms obtained previously, with a rather high affinity at low relative pressures. At higher relative pressures, there are two steps that have been attributed to the pore filling of cavities of different sizes.13 The adsorbed amounts at saturation are also consistent with results already published.14 Furthermore, benzene is more adsorbed than n-hexane at saturation by about 30%. Hydrophobic MOFs were also subjected to this difference at saturation.11 The right-hand axes show the adsorption enthalpy as a function of the relative pressure. A value of −68 kJ·mol−1 was extrapolated in the case of the adsorption of benzene at zero coverage whereas n-hexane adsorption led to a value of −61 kJ·mol−1. These close values are consistent with the affinities observed at low coverage on both adsorption isotherms. The derivation of Henry’s constants led to values of 375 and 275 mg·g−1 Torr−1 for benzene and n-hexane, respectively. Computational techniques based on Configurational Bias Grand Canonical Monte Carlo simulations were used to calculate the adsorption enthalpies. At zero coverage, a value of −73 kJ·mol−1 was obtained for benzene, while that for n-hexane

Figure 6. Intermolecular sorbate interaction in the case of the adsorption of benzene in MIL-101(Cr) at 313 K.

This indicates a more efficient packing of benzene molecules within the pores. In contrast, the n-hexane molecules are randomly orientated in the pores with a higher degree of entropy. Adsorption of n-hexane-benzene Mixture. The adsorption isotherm obtained in the case of the adsorption of a 50 molar % n-hexane-benzene mixture is shown in Figure 7. This adsorption isotherm exhibits different features as compared with those obtained with the pure sorbates. The saturation 25827



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be found in the case of systems in which moderate interaction are involved. The partial adsorption isotherms are shown in Figure 8. They have been extracted from Figure 7, using the determination of the composition of the vapor phase before and after adsorption. The sorption isotherms obtained for each component in the mixture are very close with each other. An interesting difference is the affinity of the sorbates at low pressure. Benzene is adsorbed at lower pressure as compared with n-hexane, which suggests a higher affinity for the former case. Another difference concerns the filling of the large cavities of the MOF. This filling usually results in a substep followed by the saturation plateau of the sorption isotherm. In the case of the mixture, the filling of the large cavities seems to occur at lower relative pressures. Indeed, the substeps are respectively located at p/p° = 0.075 or p/p° = 0.15 instead of p/p° = 0.1 or p/p° = 0.25 in the case of n-hexane or benzene as pure components. To confirm the better affinity of benzene for MIL-101(Cr), the selectivities were simulated for n-hexane and benzene mixtures (50:50) at low pressures (1 to 20 kPa). These results support the experimental findings, in that they show that adsorption of the benzene molecule is more favored than that of the n-hexane molecules. At very low pressures (1 kPa), the calculated selectivity is 2.5, and this decreases slightly with increasing pressure, although not significantly, to 2 at a pressure of 10 kPa and 1.5 at 20 kPa. At the lower pressures (1 kPa, 5 kPa), the tetrahedral pores are preferentially occupied by the benzene molecules. n-Hexane molecules are preferentially adsorbed inside the larger cages, interacting at the pentagonal and hexagonal windows (Figure 9a,b). One can also find a low proportion of the tetrahedral pores containing n-hexane molecules. From Figure 8, it can be seen that the adsorbed amounts at saturation are proportional to those found with the single components. It can be deduced that at equilibrium, benzene and n-hexane are adsorbed as if these were pure components. As the pressure increases to the point where all of the tetrahedral pockets are occupied, the benzene molecules are adsorbed in the larger cages. They also can occupy sites at the pentagonal and hexagonal windows along with the n-hexane. While the n-hexane molecules interact mainly through their H atoms,34 the benzene molecules can arrange themselves in such a way that π−π interactions between the adsorbates and the framework are possible. However, other geometries, for example a T-shape formation proposed by quantum calculations,35 are also seen during the simulations.

Figure 7. Adsorption isotherm of the mixture 50% n-hexane−benzene on MIL-101(Cr) at 313 K (diamonds) and the corresponding integral enthalpy of adsorption at low coverage (triangles). Inset: Integral enthalpy of adsorption versus coverage defined as the adsorbed amount normalized by the adsorbed amount at saturation.

plateau is located between those obtained with the pure sorbates. The enthalpy of adsorption at zero coverage was extrapolated at ΔadsH ≈ −50 kJ·mol−1. This value is quite weak as compared to the enthalpies of adsorption of the pure components. However, this value is consistent with a decrease of both Henry’s constants observed in the case of the mixture: values of 146 mg·g−1Torr−1 and 75 mg·g−1 Torr−1 were found for benzene and n-hexane, respectively. It can be deduced that both benzene and n-hexane can still be adsorbed at low pressure from the vapor mixture. A putative reason for this strong decrease of the enthalpy of adsorption could be the competition of benzene with n-hexane for the most active sites. If n-hexane cannot readily access these favorable sites, its contribution to the overall enthalpy of adsorption could be strongly lowered. An explanation using packing entropy arguments proposed by Yu et al. or Krishna et al. can also help to shed light on this matter.18,33 This is detailed below but it is likely that simultaneous adsorption takes place with an entropic gain as compared with the situation with single components. This entropic factor favors benzene, which is able to generate πstacking interaction. The inset of Figure 7 shows the enthalpic profile versus coverage. It can be seen that the enthalpy of adsorption decreases, following a rather straight line, with an extrapolation at zero coverage of around −50 kJ/mol. This profile can usually

Figure 8. Partial adsorption isotherm of n-hexane and benzene in the vapor mixture 50% benzene-n-hexane on MIL-101(Cr) at 313 K as a function of the absolute pressure (left) or the relative pressure (right). 25828



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benzene would be more favorably adsorbed within the large cages than n-hexane. To explain the selectivity toward benzene from a molecular interaction point of view, we can invoke the concept of packing entropy developed by Yu et al. to explain the adsorption of alkane mixtures in silicalite.18 This concept has been applied to the adsorption of benzene and n-hexane mixtures in zeolite NaY, where the stacking configurations of benzene were cited as being more favorable compared to the relatively disordered distribution of the n-hexane molecules.18 The same explanation can be used in this work. The stacking interactions between the benzene and organic linkers (and between the benzene molecules themselves) would take precedence over those between n-hexane and the framework. Although n-hexane can interact efficiently with the framework atoms in the case of 1D narrow pore zeolites33 and MOFs,34 via simultaneous interactions all of its −CH2 or −CH3 groups, this is not the case in larger pores, since not all the −CH2 and −CH3 groups can simultaneously interact with the framework. Kinetic Studies. The kinetic studies have been performed during the adsorption of benzene−n-hexane mixtures. Transient partial pressures have been measured for each data point on the sorption isotherm. This allows knowing the fraction of each component in equilibrium with the adsorbed phase being formed. Indeed, the composition of the vapor phase allows deriving the transient composition of the adsorbed phase between the initial state and the equilibrium state. The initial state is characterized by the composition of the pressure, precisely 50% molar of each component. The equilibrium state depends on the sorbates in hand and the final pressure. Figure 10 reports the adsorbed n-hexane to benzene molar ratio versus time for the very first data of the sorption isotherm of the n-hexane−benzene mixture. It can be noted that at short times of adsorption, this ratio is around 0.65 - 0.75. However, the initial composition of the vapor is 50% of each component. Therefore, this ratio should be equal to unit if both components were adsorbed with identical interaction with the material. It can be deduced that at the very early stages of the sorption process benzene adsorption is favored at the expense of n-hexane. This is true at least for the first 17 data point on the sorption isotherm. This result is consistent with the higher

Figure 9. Benzene adsorbed in a tetrahedral site (top), n-hexane interacting at a pentagonal window (bottom).

We performed further calculations using a model of MIL101(Cr) with the tetrahedral pores blocked with a dummy atom. For this atom, a mass was defined, but the charges and interactions were set to zero. The consequence is to force the placement of the molecules within the large pores. This calculation yielded adsorption enthalpies of −52 kJ·mol−1 for nhexane and −66 kJ·mol−1 for benzene. This is in good agreement with the experimental value of the enthalpy of adsorption of the mixture, therefore supporting the above hypothesis. Furthermore, this simulation also indicates that

Figure 10. Results obtained at 313 K. Adsorbed n-hexane to adsorbed benzene molar ratio versus time for the very first data of the sorption isotherm of the n-hexane-benzene mixture onto MIL-101(Cr) (a), Adsorbed n-hexane to adsorbed benzene molar ratio at different times of adsorption for the first 16 data of the sorption isotherm of the n-hexane−benzene mixture onto MIL-101(Cr) (b). 25829



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saturation plateau observed for benzene as compared with nhexane. However, this ratio increases with time and reaches at most 0.85 at the thermodynamic equilibrium. In the case of the first data, the ratio increases from 0.72 to 0.87 rather quickly compared to the other data. This suggests that many surface sites are available in MIL-101(Cr) for benzene, but also for nhexane. n-Hexane is therefore adsorbed more slowly than benzene, but in appreciable amounts. The curves corresponding to the data #2−#6 exhibit a rather constant n-hexane/benzene ratio up to 2000 s followed by an increase of the ratio from ∼0.67 to 0.82. Furthermore, the shape of the curves becomes flatter. This suggests that n-hexane is adsorbed onto MIL-101(Cr) with increasing difficulty as the surface sites are more saturated from one data point to the next. However, at the equilibrium, the n-hexane/benzene ratio remains around 0.8 as in the case of data point #1. The following data gave similar curves to the data point #6. This means that despite this difficulty, when equilibrium is reached, the proportion of n-hexane being adsorbed is similar for this set of data. When looking at Figure 10b, extra observations can be made. It is indeed interesting to note that the molar n-hexane/ benzene ratio exhibits a V shape when plotting the first 16 data points. This can already be observed for short times of adsorption (500 s), but it becomes even clearer for 5000 s or 7500 s. This suggests that at very low coverage, benzene is strongly favored for the surface sites of MIL-101(Cr). From data points #6, the n-hexane/benzene ratio increases, suggesting that benzene has already been adsorbed on the most favorable sites, the adsorption of benzene and n-hexane occurring on sites less specific for benzene. It can be concluded that the difference in adsorbed amounts at saturation originates from these observations. An additional reason already evoked in the literature is the better packing of the adsorbed benzene species.18 Separation of n-Hexane−benzene Mixtures by Gas Chromatography. Separation experiments have been conducted to emphasize the kinetics as well thermodynamics differences in the n-hexane/MIL-101(Cr) and benzene/MIL101(Cr) systems. It is clear that these experiments are a rather simple test with little optimization of the separation parameters. Only the temperature of the column could be optimized. However, under our conditions, two separated peaks were obtained which indicates that there the two systems are not equivalent (Figure 11).

Figure 11. Chromatographic separation of n-hexane and benzene with MIL-101(Cr) at 180 °C.

This shows that benzene and n-hexane are adsorbed at the same time. In the mixture, n-hexane molecules can prevent the favorable π stacking of benzene to some extent, although the simulations show that the stacking is still the most favorable molecular configuration. The integral enthalpy of adsorption is 30% lower than that found for the single components. It is consistent with the simulated enthalpy of adsorption limited to the large cavities. We suggest that at low pressure, the nhexane−benzene mixture favors the large cavities of MIL101(Cr) for adsorption. The kinetics of adsorption of the mixture shows that benzene is adsorbed preferentially at low coverage at the expense of nhexane. This is consistent with the clear chromatographic separation of the n-hexane−benzene mixture. Furthermore, the low retention times obtained confirm that MIL-101(Cr) is a good candidate for the separation of benzene from linear hydrocarbons.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

N2 physisorption isotherm of MIL-101(Cr) at −196 °C. Prior to the physisorption measurement, the sample was dehydrated at 150 °C for 12 h under high vacuum (

Coadsorption of n-Hexane and Benzene Vapors onto the Chromium

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