Liquid-Phase Adsorption and Separation of Xylene Isomers by the

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Liquid-Phase Adsorption and Separation of Xylene Isomers by the Flexible Porous Metal−Organic Framework MIL-53(Fe) Racha El Osta,† Abel Carlin-Sinclair,† Nathalie Guillou,† Richard I. Walton,‡ Frederik Vermoortele,§ Michael̈ Maes,§ Dirk de Vos,§ and Franck Millange†,* †

Institut Lavoisier Versailles, Université de Versailles, UMR CNRS 8180, 78035 Versailles, France Department of Chemistry, University of Warwick, Coventry, CV4 7AL, U.K. § Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium ‡

S Supporting Information *

ABSTRACT: We report a study of the use of the porous metal−organic framework material MIL-53(Fe), FeIII(OH)0.8F0.2[O2C−C6H4−CO2], for the separation of BTEX mixtures (benzene, toluene, ethylbenzene, and the three xylene isomers). Crystal structures of the three host:guest materials MIL-53(Fe)[xylene], where xylene = the ortho, meta, or para isomer of dimethylbenzene, have been solved and refined from powder X-ray diffraction. Each exhibits a fully expanded form with a variety of host:guest and guest:guest interactions responsible for stabilizing the structure. While the ortho- and meta- isomers present a similar arrangement when occluded in the MIL-53 host, the para-xylene shows a distinctly different set of interactions with the host. Upon thermal treatment, xylenes are partially lost to give crystalline phases MIL-53(Fe)[xylene]0.5, the structures of which have also been solved. The kinetics of uptake of each xylene by MIL-53(Fe)[H2O], in which the water is replaced by the organic guest, have been studied using time-resolved energy-dispersive X-ray diffraction: this shows differences in kinetics of the adsorption of the three isomers. Under chromatographic conditions in heptane at 293 K, anhydrous MIL-53(Fe) is able to separate the three xylene isomers with elution of the para-xylene before the other two isomers, and at 323 K the host is able to resolve all components of the BTEX mixture. KEYWORDS: metal−organic framework, porous, separation, molecular sieve, xylenes

1. INTRODUCTION Porous metal organic frameworks (MOFs)1−4 are presently attracting much attention for applications as adsorbents for a variety of guest molecules, in particular for molecular separation and storage of gases such as CO2, CH4, and H2 for environmental and energy reasons,5,6 but also of much larger molecules, typically encountered in the liquid phase, ranging from simple hydrocarbons7 to drug molecules.8 MOFs have brought significant novelty to the field of porous materials, which has largely been dominated by aluminosilicate zeolites, for a number of reasons, such as the relative ease of modification of their chemical and physical properties by choice of the organic linker and the metal center making up their structures,9,10 the possibility of post-synthesis functionalization of the organic components to tune guest selectivity,11,12 and the structural flexibility seen in some MOFs, which offers the possibility of fine control of guest uptake compared to conventional inorganic structures.13−15 These aspects of MOF chemistry offer a new scope in the future applications of porous materials in many technological and industrial areas.16 Recent attention has turned to the separation of molecular mixtures by MOFs. Jiang and Xu have reviewed the progress in using the materials as highly selective media for the © 2012 American Chemical Society

discrimination of complex mixtures of either gas-phase or liquid-phase species, ranging from small molecules (CO2, CH4, and H2) to a variety of hydrocarbons.7 While several industrially relevant separations have been studied, one of the more challenging ones is the separation of mixtures of BTEX (benzene, toluene, ethylbenzene, and the three xylene isomers). Benzene, toluene, ethylbenzene, and all three xylenes are obtained as a mixture in the refining of crude oil. Each of these individual compounds can be processed further into valuable products, for instance, p-xylene, the precursor for terephthalic acid, is used on a large scale in the manufacture of the polyester PET.17 In order to be processed further, the BTEX mixture has to be separated into its individual compounds. Since the boiling points of the three xylene isomers are very similar (all falling between 138 and 144 °C), separation on an industrial scale is performed using either fractional crystallization, relying on a difference in freezing points,18 or adsorption, which is based upon differences in interaction between the various adsorbate molecules and the host.19 Adsorbents include microporous Received: April 23, 2012 Revised: June 4, 2012 Published: June 5, 2012 2781

dx.doi.org/10.1021/cm301242d | Chem. Mater. 2012, 24, 2781−2791

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Table 1. Unit Cell Data and Goodness of Fit Parameters for the MIL-53(Fe)[xylene] Materials Studied Resulting from the Rietveld Refinements a, Å b, Å c, Å β, deg V, Å3 space group M20 RB RWP, RP

MIL-53(Fe)[o-xylene]

MIL-53(Fe)[m-xylene]

MIL-53(Fe)[p-xylene]

15.7996(3) 16.0408(3) 13.7969(2) 115.4007(8) 3158.6(1) C2/c 155 0.080 0.110, 0.089

15.8622(3) 15.9959(3) 13.8070(2) 115.3059(8) 3167.07(9) C2/c 121 0.082 0.113, 0.090

31.7223(4) 28.9234(4) 6.89654(4) 6327.7(1) Cmca 168 0.058 0.058, 0.047

has a fully open structure in its dehydrated state, but for dehydrated MIL-53(Fe) the structure is closed,33 and while the thermal behavior of the aluminum analogue is similar to that of the chromium material, the gallium material behaves differently with the closed form persisting to higher temperature, coexisting with the open form until collapse of the structure.51 On the basis of these observations, the use of MIL-53 materials in applications for industrially relevant separations is clearly worth further investigation, with the possibility to develop highly tunable and selective sorbents that may outperform existing materials and processes. In the first part of this paper we consider the structural chemistry of the static, equilibrium structure of the MIL-53(Fe)[xylene] materials and their formation, before in the second part turning to their dynamic behavior under conditions suitable for real applications in separation.

hosts such as zeolites, either in polycrystalline or in membrane form. One of the best known examples of adsorbents for xylenes are faujasite-type zeolites, of which the selectivity toward each isomer may be tuned by ion-exchange using barium or potassium.17 The separation of xylenes using the MOFs MIL-53(Al) (a flexible AlIII terephthalate) and MIL47(V) (a rigid VIV terephthalate) has more recently been investigated.20−24 This work has shown that the former prefers o-xylene but is unable to discriminate between p- and m-xylene, while for the latter only a separation of the p- and m-xylene was possible, both in the liquid and in the vapor phase. Recently Jin et al. described a structurally unrelated indium MOF (known as JUC-77) with 2D rhombic channels which prefers p-xylene in the vapor phase over the other two isomers,25 while Gu et al. showed that a zinc terephthalate MOF allowed gas chromatographic separation of xylenes from ethylbenzene, but this material was unable to separate the xylene isomers themselves,26 and a few other examples of MOFs for xylene uptake and separation have appeared in the recent literature.27−30 In this paper we describe a systematic study of the material MIL-53(Fe) for the complete separation of BTEX mixtures in the liquid phase using a chromatographic approach: this allows us to propose a simple process for the complete separation of all components by a recyclable solid host at close to ambient conditions. The host MIL-53(Fe) is essentially isostructural to the Al analogue and is constructed from chains of trans-linked [FeO4(OH,F)]6 octahedral units, cross-linked by terephthalate ligands to give a 3D net in which one-dimensional channels run parallel to the inorganic backbone of the structure.31−33 MIL53(Fe), as well as other MIL-53-based materials, undergoes a large and reversible structural swelling depending on the presence (or absence) of guest molecules,15 an effect which may also be brought about by temperature34 or pressure.35 Thus MIL-53 materials have been investigated as vapor phase adsorbents, and they show high capacity for a variety of small molecules including CO2, H2, H2S, and hydrocarbons, where in some cases an evolution of structure with guest concentration has been observed.36−46 Similarly, liquid-phase sorption of organics by MIL-53(Fe) has also been studied,47 and various structural forms may be isolated depending on the concentration of guest: the topology and crystallinity of the structure are always maintained but the degree of expansion of the structure depends on the balance of host:guest and guest:guest interactions. Examples include pyridine and 2,6-dimethylpyridine48 and a series of aliphatic alcohols.49 It has also been observed that the behavior of the various analogues of MIL-53 in which the framework trivalent metal ion may be Cr,31 Fe,33 Al,50 Ga,51 or Sc52 is rather different: for example, MIL-53(Cr)

2. EXPERIMENTAL SECTION 2.1. Synthesis. MIL-53(Fe)[H2O] or FeIII(OH)0.8F0.2[O2C− C6H4−CO2]·H2O used in this study was synthesized as reported in previous work.48 The advantage of the fluorinated sample over its nonfluorinated analogue is its superior crystallinity for this fundamental study. On the basis of our previous work, fluorination enhances the kinetics of guest exchange.49 MIL-53(Fe)[DMF] was first isolated as a pure-phase pale orange crystalline powder under reflux conditions (set at 423 K for 12 h) from equimolar amounts of iron(III) chloride hydrate, FeCl3·xH2O (Aldrich, 97%), 1,4-benzenedicarboxylic acid, HO2C−(C6H4)−CO2H (Alfa 97%), hydrofluoric acid, and HF (Prolabo, 40% in water) in N,N-dimethylformamide (DMF, Aldrich 99%). The light orange MIL-53(Fe)[MeOH] powder was obtained after dispersion of the as-synthesized material (which contains solvent) into methanol to remove the DMF guest molecules from the pores. Exchange with water was then performed to finally obtain the hydrated form MIL-53(Fe)[H2O]. Quantitative elemental analyses (performed using ICP-MES for Fe, Schö niger flask combustion followed by titration for fluorine and by combustion analysis for CHN, by Medac Ltd., U.K.) gave the following results: Fe, 23.7%; C, 35.6%; H, 2.68%; and F, 1.31%, which compare well with the composition calculated from the formula FeIII(OH)0.8F0.2[O2C− C6H4−CO2]·H2O: Fe, 21.9%; C, 37.6%; H, 2.68%; and F, 1.48%. The ratio of F/OH in MIL-53(Fe) cannot be controlled by varying the amount of HF. MIL-53(Fe)[xylene] samples were prepared by immersing the hydrated material in a large excess of each of the three liquid xylenes at room temperature (∼0.4 g was stirred in ∼10 cm3 of the organic liquid overnight), and the product was recovered by filtration and dried at room temperature in air for X-ray thermodiffractometry, thermogravimetry analysis, and infrared spectroscopy. 2.2. Powder X-ray Diffraction. A number of powder X-ray diffraction methods were used to characterize the solids. X-ray Thermodiffractometry. X-ray thermodiffractometry was performed under static air in an Anton Paar HTK16 high temperature 2782



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state detector. Energy calibration of the detector was performed using a glass containing a series of heavy elements with well-separated fluorescence lines, and the precise angle of the detectors was calibrated using a set of Bragg peaks measured from a solid polycrystalline sample of MIL-53(Fe)[H2O]. To study the uptake of xylenes, 400 mg of solid MIL-53(Fe)[H2O] was suspended in ∼3 mL of deionized water in a 1 cm diameter borosilicate glass tube along with a Tefloncoated magnetic stirrer bar. Data were recorded in 60 s accumulations while the liquid guest was added dropwise using an electronic syringe pump supplied by KD Scientific with the rate of addition of liquid to the suspension fixed at 10 mL/h. The glass tube was sealed to prevent evaporation of the organic compound during the experiment, placed in a stand equipped with a magnetic spinner, and agitated throughout the measurement to ensure that the solid sample remained in the beam. Raw EDXRD data were converted to counts vs energy using the program PowDLL57 into a format suitable for reading into the program XFIT58 wherein peak fitting using pseudo-Voigt functions was undertaken to determine Bragg peaks areas and positions as a function of time to extract the extent of reaction. 2.3. Thermogravimetric Analysis. Thermogravimetric experiments were performed under O2 for the three MIL-53(Fe)[xylene] samples using a TA-Instrument type 2050 analyzer apparatus in the range 293−673 K at the rate of 5 K·min−1. 2.4. HPLC Apparatus and Chromatographic Conditions. Before packing the column, 5 g of the pale orange hydrated powder (with an average particle size of 2.5 μm) was activated. First, the MIL53(Fe)[H2O] sample was immersed in propan-2-ol (iPrOH) to remove the water guest molecules from the pores for 3 days, during which the activation solvent was decanted and freshly replenished three times to give the pure MIL-53(Fe)[iPrOH]. Then, the same procedure was applied using a mixture of three solvents (heptane, toluene, and ethyl acetate). It should be noticed at this point that i PrOH was only used to obtain the MIL-53(Fe)[heptane/toluene/ ethyl acetate] slurry from MIL-53(Fe)[H2O] since heptane and H2O are immiscible. A stainless steel column of 50 mm × 4.6 mm i.d. was packed using a Jasco PU-980 intelligent pump. First a pressure of ∼250 bar was applied progressively for 1 h with heptane/toluene/ethyl acetate (1:1:1 in volume) followed by 250 bar with solely dry heptane for 1 h in order to obtain a well packed column with the MIL53(Fe)[heptane] material. This column was used for both analytical HPLC and for the breakthrough experiments using heptane as eluent phase. The HPLC measurements were performed on a Waters system equipped with a 510 pump, a 2424 UV−vis detector, and a 2707 Automatic Injector. For HPLC analyses, a 10 μL loop was used while a 50 mL loop and a Waters Fraction Collector III were employed for the breakthrough experiments. Concentrations in the collected fractions for the breakthrough experiments were also determined using the UV−vis detector. A column thermostat jet stream II was used to control the temperature of the column. The detector was set at the wavelength of 254 nm. Responses were recorded and integrated using Empower (Waters) chromatography software. On the basis of liquid phase analytical chromatographic data recorded at various temperatures, the capacity factor k is the ratio of a compound’s elution volume, or retention time, relative to the bed void time tm. The bed void time was determined as the retention time of an unretained compound, in this case 1,3,5-triisopropylbenzene, minus a correction for the volume of the lines in the system. The same (small) correction was applied to the measured retention times. The capacity factor is a measure of how much time a compound spends in the stationary phase versus the mobile phase and is defined as eq 1:

device of a Siemens D-5000 diffractometer (θ−θ mode, Co Kα radiation) equipped with a M Braun linear position sensitive detector (PSD). The stability of the three MIL-53(Fe)[xylene] samples was studied at 10 °C intervals with a temperature ramp of 0.02 °C s−1. Each powder pattern was recorded in the range 6−24° (2θ) with a 2 s/ step scan, corresponding to an approximate duration of 0.5 h. Structure Determination. Powder X-ray diffraction data were collected on ID31 at the European Synchrotron Radiation Facility from powdered samples contained in 1 mm diameter quartz capillaries. MIL-53(Fe)[xylene] samples were prepared using an excess of liquid xylene within the capillary, which was then centrifuged to concentrate the solid to make a measurement with minimal liquid scatter as background. The beamline receives X-rays from the synchrotron source (which operates with an average energy of about 6 GeV) from an undulator device. The incident X-ray wavelength was 0.799989 Å using an incident beam size of 2.0 mm (horizontal) × 1.0 mm (vertical). The sample was rapidly spun during data collection to ensure good powder averaging. Extractions of the peak positions, pattern indexing, direct space strategy used to complete the structural model, and Rietveld refinements were carried out with the TOPAS program.53 For all compounds, unit cells and possible space groups were found by the LSI-Indexing method with satisfactory figures of merit (see Table 1). In the process of simulated annealing and Rietveld refinement terephthalate ions were treated as rigid bodies as well as xylene molecules. The anisotropic line broadening effect was modeled by using a spherical harmonics series. For MIL-53(Fe)[xylene] samples, powder pattern indexing converged to two unit cells (Table 1), which are superstructures of the previously reported fully expanded MIL-53(Fe)[guest] materials, and ab initio structural investigations were undertaken. The determinations were initialized with the EXPO package.54,55 For MIL-53(Fe)[p-xylene], direct methods allowed location of iron atoms with their directly coordinated atoms, as well as several carbon atoms of the terephthalate. These atoms were used as the starting model, after determination of the ligand rigid body orientation by simulated annealing. Atoms of the MIL-53(Fe) skeleton were then fixed, in order to localize p-xylene molecules by using a direct space strategy. Attempts were undertaken in several space groups compatible with systematic extinctions of the powder pattern but only converged in Cmca (with an occupancy of 0.5 for the two guest molecules) and Cmc21 with almost the same profile factors and model indicators. The centrosymmetric space group was chosen, with a disorder of the p-xylene molecules. Regarding MIL-53(Fe)[mxylene] and MIL-53(Fe)[o-xylene], the similarity between the two unit cells led us to suppose that these two compounds were isostructural. Direct method calculations were then undertaken only for the MIL53(Fe)[m-xylene] in the C2/c space group, and these allowed location of iron atoms with their environments, as well as about half of the carbon atoms of the ligands. Attempts to determine the orientation of the ligand by simulated annealing without considering the guest molecule did not lead to a consistent result, and the m-xylene molecule has to be taken into account at this stage of the calculation. Atoms of the MIL-53(Fe) skeleton were then fixed for the two compounds, in order to localize more precisely the m-xylene or o-xylene molecule by using a direct space strategy. For the three MIL-53(Fe)[xylene]0.5 samples, the atomic arrangement was found to be similar to that found for MIL-53(Fe)[lutidine]0.5.48 The atomic coordinates of its skeleton were then used directly as a starting model in the Rietveld refinement. The xylene was localized by using a direct space approach based on simulated annealing. The Cc space group instead of C2/c has been chosen arbitrarily in order to avoid large disorder of the organic moiety inside the channel. For the three compounds, the amount of xylene has been refined and converged to values of 0.46, 0.45, and 0.43 for oxylene, m-xylene, and p-xylene, respectively. However, it should be noticed that the structures can be solved either in Cc and C2/c with similar R factors. Time-Resolved EDXRD. In situ EDXRD data were measured using Beamline F3 of the HASYLAB facility at DESY, Hamburg, Germany.56 Beamline F3 receives white-beam radiation with energy 13.5−65 keV, and the incident X-ray beam is collimated to dimensions 20 × 20 μm2. Scattered X-rays are detected using a single-element germanium solid-

ki =

tri − tm tm

(1)

in which tri and tm are the retention times of the eluting compound and the bed void time, respectively. The separation factor α is the ratio of the capacity factors of two peaks. It is a measure of the difference in interactions of two analytes with the mobile and stationary phases, and therefore the difference in retention times. Like the capacity factor, the separation factor of a column is influenced by the column packing material and the eluents used (eq 2). 2783


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ki tr − tm = i kj trj − tm

(2)

Therefore, the capacity factor is related to the changes in molar enthalpy (ΔH) and molar entropy (ΔS) through the Van't Hoff equation:

ln k = −

ΔH ΔS + RT R

(3)

with T the absolute temperature. Plotting ln k vs 1/T should result in a linear graph. ΔH can then be calculated from the slope of this curve (slope × R = −ΔH); the obtained value is interpreted as an apparent adsorption enthalpy as it comprises the total change of enthalpy caused by the adsorption of the analyte and the desorption of the eluent from the framework. For breakthrough experiments at 323 K, the adsorbed amounts q were first calculated for each compound by integration of the curves using eq 4: q=

∫0

Figure 1. Single compound adsorption isotherms for o-xylene, mxylene, and p-xylene on MIL-53(Fe) at room temperature: uptake (wt %) vs equilibrium concentration (mol·L−1).

t

u(C in − Cout) dt

(4)

isomer. Each material has a “fully expanded” structure as has previously been reported for MIL-53(Fe)[2,6-lutidine]48 and MIL-53(Fe)[ROH,H2O], where ROH is a primary alcohol.49 The ordering of the guest molecules in the three MIL53(Fe)[xylene] solids gives rise to superstructures not seen for the earlier materials, but the topology of the MOF skeleton is identical. Characteristic for the xylene-loaded materials is the lack of any possible hydrogen-bonding interaction between guest and host. Figure 2 shows the structure of MIL-53(Fe)[oxylene]. The skeleton is built up from two independent iron atoms, one hydroxyl group, and one terephthalate moiety. The guest molecules, filling the pores, are stacked in an ordered manner along the lozenge-shaped channels in such a way that they form one pair in which the two xylenes are parallel to each other. There are π−π interactions between neighboring guest molecules with a distance between centers of guest molecules inside the pairs of 3.87 Å. In this pair, the aromatic rings of the two xylene molecules are also close to parallel to those of the terephthalate linkers. The distance between centers of aromatic rings is 3.79 Å, which is indicative for π−π interactions. These pairs are then stacked in an alternating manner, such that along the c-axis, neighboring pairs of guest molecules are rotated of about 90° with respect to each other in order that the xylene has to have some π−π interaction with the terephthalate linkers of the adjacent wall. Additional host−guest interactions can be identified: the distance between one of the methyl carbons of the xylene and the terephthalate center of the adjacent wall is seen at 3.90 Å, with the methyl group pointing toward the plane of the terephthalate aromatic ring. Along the c-axis, the closest C−C contact is 3.57 Å and corresponds to the distance between two methyl carbons of xylene molecules (van der Waals interactions). The structure of MIL-53(Fe)[m-xylene] is shown in Figure 3. The arrangement of guest molecules with respect to the host framework is very similar to that of MIL-53(Fe)[o-xylene], which is also reflected in the same crystal symmetry (Table 1). In MIL-53(Fe)[m-xylene], there are still π−π interactions between neighboring guest molecules inside the pairs with a distance between aromatic rings of 3.94 Å. In this pair, the two xylene molecules are also close to parallel to the terephthalate linkers. The distance between centers of aromatic rings is 3.95 Å. Regarding additional host−guest interactions, a similar distance between methyl carbons and the terephthalate center is seen at 3.83 Å. The same closest C−C contact along the c-

−1

with u being the volumetric flow rate of the feed (L·min ) and Cin and Cout the concentrations (mol·L−1) of the adsorbate in the liquid feed and eluent, respectively. Separation factors αi,j were calculated using eq 5:

⎛q ⎞ ⎛c ⎞ j αij = ⎜⎜ i ⎟⎟ × ⎜ ⎟ q c ⎝ j⎠ ⎝ i⎠

(5) −1

with qi and qj the amounts (mol·g ) of compounds i and j adsorbed per gram of MOF and ci and cj the concentrations (mol·L−1) of compounds i and j in the external liquid phase.44 As the column is fed with an equimolar mixture of compounds i and j, the separation factor α can be written as αi,j = qi/qj. 2.5. Liquid Phase Batch Adsorption Conditions. Liquid phase batch adsorption experiments were performed at 298 K in 1.8 mL glass vials containing 0.025 g of adsorbent and a single compound solution of aromatics in heptane following a literature procedure.13,20 As an aliphatic solvent is unlikely to strongly interact with the host material,13,14,20 it can be assumed that the choice for an aliphatic solvent hardly influences the adsorption capacity or separation factor for the various alkylaromatics. Uptakes were directly calculated from GC output data. Separation factors αi,j were calculated using eq 5.

3. RESULTS AND DISCUSSION 3.1. Xylene Adsorption on MIL-53(Fe). In initial experiments, the ability of MIL-53(Fe) to adsorb xylene isomers was probed by recording single compound batch isotherms in heptane (Figure 1). All three xylene isomers are taken up significantly, with a clear preference for o-xylene, which reaches a saturation level of approximately 40 wt %, corresponding to ∼0.9 molecule per iron. In the conditions chosen, the adsorption of m-xylene and p-xylene occurs stepwise, and the isotherms reach maximal levels of approximately 25 wt % or ∼0.6 molecule per iron within the studied concentration range. This stepwise behavior could be indicative of breathing effects (vide infra). After an initial plateau of 13 wt % is reached in case of m-xylene, the structure responds further to the presence of the guest, and at concentrations larger than 0.28 M more m-xylene molecules are adsorbed. In case of p-xylene, it seems that there is an additional step at 6 wt %. Since MIL-53(Fe) is able to adsorb all three xylene isomers, the structures of the loaded materials were further studied. Table 1 shows the crystallographic parameters for the three MIL-53(Fe)[xylene] materials prepared by immersion of the host in an excess of each xylene 2784



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Figure 2. Two views of the structure of MIL-53(Fe)[o-xylene] (left) in the ab plane showing stacked o-xylene molecules within the lozenge-shaped channels and (middle) along the c axis. Final Rietveld plot (right) for MIL-53(Fe)[o-xylene] from X-ray powder diffraction data measured at room temperature. Blue points are experiment data, the red line is the fit, and the black line the difference curve. The tickmarks indicate Bragg peak positions.

Figure 3. Two views of the structure of MIL-53(Fe)[m-xylene] (left) in the ab plane showing stacked m-xylene molecules within the lozenge-shaped channels and (middle) along the c axis. Final Rietveld plot (right) for MIL-53(Fe)[m-xylene] from X-ray powder diffraction data measured at room temperature. Legend as Figure 2.

Figure 4. Two views of the structure of MIL-53(Fe)[p-xylene] (left) in the ab plane showing stacked p-xylene molecules within the lozenge-shaped channels and (middle) along the c axis. Final Rietveld plot (right) for MIL-53(Fe)[p-xylene] from X-ray powder diffraction data measured at room temperature. Legend as Figure 2.

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from room temperature to 673 K. The corresponding TGA trace allows the composition of each phase to be deduced. The first mass change (∼15.5%) is equivalent to the loss of half a molar equivalent of o-xylene, and this leads to a phase that maintains crystallinity, MIL-53(Fe)[o-xylene0.5]. A subsequent mass loss of ∼7.7% up to 480 K corresponds to the loss of a further fraction of the o-xylene to give a phase of composition MIL-53(Fe)[o-xylene0.25]. By thermodiffraction, this phase can be observed solely in the temperature range 423−453 K. After that, the MIL-53(Fe)[o-xylene0.25] gradually loses the remaining guest molecules leading to a material with an XRD pattern matching that of dehydrated MIL-53(Fe), before finally collapsing into Fe2O3 (total weight loss of ∼76.8%). The initial mass loss of half an equivalent of o-xylene is irreversible in our case (working in an open system) as shown by a second thermodiffractometry experiment in which, after heating to 363 K, the sample was repeatedly cooled and reheated to 363 K without any further change in structure (Figure 6). However, it

axis at 3.57 Å corresponds to the distance between two methyl carbons of xylene molecules (van der Waals interactions). The analysis of all the distances suggests weaker host−guest interactions for the adsorbed meta-xylene compared to orthoxylene. MIL-53(Fe)[p-xylene] presents a rather different structure, as reflected by the different crystal symmetry of the material (Table 1; Figure 4). The unit cell of this material has a halved caxis value, but a and b are doubled. The skeleton is built up from three independent iron atoms (two of them in 4a and 4b Wyckoff positions and one in 8c), two hydroxyl groups, and one terephthalate moiety. There are two disordered independent guest molecules, with an occupancy of 0.5, filling the tunnels in two different manners. As a consequence two unique sets of guest molecules in neighboring channels are formed, giving rise to four possible arrangements of guest molecules (see Figure S1, Supporting Information). The major difference between the structure of this phase compared to the two discussed above is that along the channel axis the p-xylene molecules are arranged in a zigzag manner, rather than in pairs. In the tunnels (1, 1′) and (2, 2′), the distances between centers of aromatic rings of the terephthalate and the xylene moiety are 4.13 Å and 4.01 Å, respectively (see Figure S1, Supporting Information). Moreover, it should be noticed that the aromatic rings are less parallel to each other, especially in tunnels (2, 2′). If they do exist, π−π interactions are very weak. Along the caxis, the arrangement of the p-xylene molecules in a zigzag manner results again in no overlap between aromatic rings of neighboring guest molecules (5.04 Å and 5.22 Å, respectively). The overall analysis of the distances in MIL-53(Fe)[xylene] is in fair agreement with the ability of MIL-53(Fe) to adsorb preferentially o-xylene over m-xylene, p-xylene being the least favored (see Section 3.2). Thermodiffractometry was used to study the stability of the three MIL-53(Fe)[xylene] materials and the possible evolution of the structure upon removal of the guest. Figure 5 shows the case of MIL-53(Fe)[o-xylene], for which various powder patterns can be observed upon heating the material in air

Figure 6. Thermodiffractometry of MIL-53(Fe)[o-xylene] (red lines) showing irreversibility of half of the o-xylene molecules loss, leading to MIL-53(Fe)[o-xylene0.5] (blue lines).

should be noticed that if this MIL-53(Fe)[o-xylene0.5] sample is reimmersed and stirred in an excess of o-xylene, the material will readsorb organic molecules to return to the “fully expanded” structure MIL-53(Fe)[o-xylene]. A highly crystalline sample MIL-53(Fe)[o-xylene0.5] was prepared for structure analysis (Figure 7). The structure of this phase has an intermediate cell volume of ∼1200 Å3 (see Table S1, Supporting Information), which is similar to that of the previously studied MIL-53(Fe)[2,5-lutidine0.5].48 The o-xylene molecules are now stacked at the center of the tunnels, with a distance between centers of neighboring o-xylene rings of 6.88 Å. It is obvious that there are no strong host−guest interactions in this phase: the guest appears to have primarily a space-filling role, as also noted previously in MIL-53(Fe)[2,5-lutidine0.5].48 Thermodiffractometry and TGA analysis of both MIL-53(Fe)[m-xylene] and MIL-53(Fe)[p-xylene] show similar results, and the structures of the phase containing half equivalents of the guest are rather similar (see Figures S2−S7, Supporting Information). Time-resolved EDXRD was measured during the addition of each xylene isomer as a liquid to a suspension of MIL53(Fe)[H2O] in water to observe directly the uptake of the guest with time (Figure 8). In each case the only crystalline product that is observed is the fully open form of the material with all Bragg peaks matched to the unit cell parameters shown in Table 1. In this background of an excess xylene, there is no evidence for the formation of any form of MIL-53 with

Figure 5. (top) TGA trace and (bottom) thermodiffractometry experiments of MIL-53(Fe)[o-xylene] (red lines) leading to first MIL53(Fe)[o-xylene0.5] (blue lines), then MIL-53(Fe)[o-xylene0.25] (brown lines), and finally the anhydrous MIL-53(Fe) (green lines). 2786



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Figure 7. Two views of the structure of MIL-53(Fe)[o-xylene0.5] (left) in the ab plane showing stacked o-xylene molecules within the lozenge-shaped channels and (middle) along the c axis. Final Rietveld plot (right) for MIL-53(Fe)[o-xylene0.5] from X-ray powder diffraction data measured at room temperature. Legend as Figure 2.

Figure 8. Contour map representation of EDXRD recorded during the interaction of MIL-53(Fe)[H2O] with the three xylene isomers.

phase or the fully open product, even when only a small concentration of guest is present. Although we are dealing with the replacement of water by xylenes, an important conclusion of the EDXRD results is that in the presence of an excess of liquid xylene, MIL-53(Fe), is present always as a fully open structure. Note that the competitive introduction of xylene isomers cannot be studied using the EDXRD method, since all

intermediate unit cell volume, such as MIL-53(Fe)[xylene0.5], and the replacement of water by each of the xylenes occurs in a stepwise manner with no continuous evolution of structure seen on the time-scale of the experiment: during the decay of the hydrated phase and growth of expanded xylene-containing phases, the Bragg peak positions of each remain constant. The only two phases that are detected are either the initial hydrated 2787



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exchange taking around one-half of the time, Figure 9. It should be borne in mind that here we are studying the replacement of water by xylenes from a system where we have two immiscible liquids and a solid host, so a variety of complex interactions have to be considered, including the diffusion of guests to and from the host. Nevertheless our results would suggest rather different interactions between the guest and host for p-xylene compared to the other two isomers, which is consistent with the structural similarities between the packing of guest molecules in MIL-53(Fe)[o-xylene] and MIL-53(Fe)[mxylene] as described above. 3.2. Separation of BTEX Using MIL-53(Fe). While the structural studies in pure xylenes did not provide conclusive evidence for a separation of o- and m-xylene, the single compound adsorption isotherms already indicated that o-xylene is the preferred compound in a hydrocarbon background (Figure 1). This is confirmed in competitive experiments on MIL-53(Fe) columns, for instance by pulse chromatographic injections (Figure 10). Both at 293 K and at 323 K, all three xylene isomers are well separated. Furthermore, it is shown that the other compounds of the BTEX mixture can be separated at 293 K, with the exception of ethylbenzene and benzene. The latter problem, however, is solved by increasing the temperature to 323 K, at which temperature all components are completely resolved (Figure 10; Figure S8, Supporting Information, shows intermediate temperatures). Table 2 shows the separation factors calculated from the analytical chromatography data at 293 and 323 K, respectively (Table S2, Supporting Information, for intermediate temperatures). It is clear that MIL-53(Fe) preferentially retains o-xylene over the other two isomers and other BTEX components. The order of elution is para, meta, and then ortho. This is in contrast to the related materials MIL-47(V), which has a preference for pxylene, and MIL-53(Al), which shows poor selectivity between para and meta isomers.20−22 Thus MIL-53(Fe) is a more selective adsorbent than MIL-53(Al) and has a different selectivity than MIL-47(V). The difference between the MIL53(Fe) and MIL-47(V) is not unexpected given the different chemistries of the two materials: the first is flexible, while the latter is rigid; the metals are in different oxidation states (3+ and 4+, respectively), and while MIL-53(Fe) contains hydroxyl groups as part of the framework, MIL-47(V) contains oxo groups in their place. The reason for the difference in selectivities between MIL-53(Fe) and MIL-53(Al) is less obvious, although as noted previously the two materials have a different dehydration behavior,33 which indicates a strong effect of the choice of metal on host−guest interactions. While

products have very similar powder patterns that cannot be distinguished by this technique. Figure 9 shows the results of

Figure 9. Extent of exchange curves for the uptake of xylenes by MIL53(Fe)[H2O] derived from analysis of the EDXRD patterns. Red closed squares are the hydrated phase, and open blue circles are the xylene-containing product.

integration of Bragg peak intensities to produce extent of guest exchange plots. Looking at the time scales of the structural transition, the phase transformation takes place with similar rate for o-xylene and m-xylene, requiring ∼1 h to achieve full exchange of water by the xylene under the conditions used. By contrast, p-xylene is taken up more rapidly, with the complete

Figure 10. Chromatograms of the BTEX on MIL-53(Fe) at 293 K (left) and 323 K (right). 2788



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Table 2. Separation Factor α for the BTEX Molecules at 293 and 323 K Calculated from Analytical Chromatography Data ethylbenzene

p-xylene

benzene

ethylbenzene benzene p-xylene toluene m-xylene o-xylene

-

1.00 -

ethylbenzene benzene p-xylene toluene m-xylene o-xylene

-

1.25 -

α (293 K) 3.48 3.48 α (323 K) 2.91 2.33 -

m-xylene

o-xylene

4.57 4.57 1.31 -

9.17 9.17 2.63 2.01 -

12.26 12.26 3.52 2.68 1.34 -

3.87 3.10 1.33 -

5.95 4.77 2.04 1.54 -

9.39 7.52 3.22 2.42 1.58 -

as expected based on previous experiments: o-xylene is preferred most, followed by m-xylene, p-xylene, and ethylbenzene. Furthermore, at higher concentration, in each of these cases a so-called “roll-up” effect is seen after an initial period in which both compounds are being adsorbed. Once the column becomes more and more saturated, the lesser preferred compound will elute first and the more preferred compound displaces the less preferred compound out of the pores back into the effluent stream. This causes a temporary increase of the outlet concentration of the least preferred compound. As expected, this indicates that the xylene isomers compete for the same space in the pores of the MOF. While the average separation factors (Table S3, Supporting Information) are in line with the pulse chromatographic experiments, it is interesting to highlight the case of p-xylene and m-xylene. Compared to MIL-53(Al), which hardly separates both isomers, it is remarkable that MIL-53(Fe) is able to separate m-xylene and p-xylene in breakthrough experiments (equimolar mixture of 0.05 M) with an average separation factor of ∼1.5. This separation is confirmed by room temperature competitive batch adsorption isotherms (Figure S13, Supporting Information). m-Xylene is preferred most throughout the concentration range. Its maximal uptake level is 14 wt %, whereas the uptake of p-xylene does not exceed 12.2 wt %. After initially decreasing by a small amount, the separation factor tends to remain fairly constant at approximately 1.2, making MIL-53(Fe) one of the first reported MOF structures that has a preference for the meta-isomer. This once more illustrates that small alterations in a MOFs framework, in this case the use of Fe3+ instead of Al3+, can strongly influence the adsorption and separation of organic compounds.

this work reports an important interaction between two neighboring ortho- or meta-xylene molecules in the pores of MIL-53(Fe), it has been reported that such interactions were not as dominant in the pores of MIL-53(Al).59 On the basis of analytical chromatographic experiments at different temperatures, the separation factors α (Table 2 and Table S2, Supporting Information, for intermediate temperatures) can be calculated as well as apparent adsorption enthalpies ΔHapp (Table 3). The separation factors appear to be Table 3. Apparent Adsorption Enthalpies ΔHapp of p-Xylene, o-Xylene, and m-Xylene on MIL-53(Fe) in a Heptane Background ΔHapp (kJ·mol−1) p-xylene m-xylene o-xylene

toluene

−20.4 ± 1.6 −27.0 ± 2.0 −22.7 ± 2.2

relatively independent of the temperature in the separation of o-xylene and p-xylene, which indicates that the differences in adsorption enthalpy for these compounds in a heptane-filled structure are relatively minor. In case of o-xylene vs m-xylene, it appears that the apparent adsorption enthalpy is in favor of mxylene. The fact that this order of adsorption enthalpies (m > o, p) does not reflect the actual adsorption preference (o > m > p) indicates that other effects than merely enthalpic ones must be decisive for the adsorption preference. As was already obvious from the structural studies in Section 3.1, o-xylene can be packed highly efficiently in the MIL-53(Fe) pores, and this packing seems to lead to an overall more energetically favorable situation for o-xylene than for p- or m-xylene. Unlike other MIL-53 structures, MIL-53(Fe) appears to be able to separate p-xylene and m-xylene. In this case, the preference for m-xylene over p-xylene can be readily related to the more negative apparent adsorption enthalpy for m-xylene (−27 kJ·mol−1, vs −20.4 kJ·mol−1 for p-xylene). Finally, after showing the potential of MIL-53(Fe) for separating the BTEX, breakthrough experiments were performed on MIL-53(Fe) columns in order to approach more closely the real separation of C8-alkylaromatics (Figure 11 and see Figures S9−S12, Supporting Information). By pumping a feed containing an equimolar mixture of 0.005 and 0.05 M over the column, respectively, it is clear that all four C8alkylaromatics can be well separated. The order of preference is

4. CONCLUSIONS MIL-53(Fe) is able to separate effectively BTEX mixtures, including the three isomers of xylene at close to ambient conditions under chromatographic conditions. The order of the elution of the xylene isomers is para, meta, and then ortho. It must be remembered that while the diffraction studies described in the first part of the paper (the structural resolution from powder X-ray diffraction and the time-resolved diffraction experiments) used rather different conditions (hydrated MIL53(Fe) in excess, pure xylene) to those used in chromatography (dehydrated MIL-53(Fe) with xylenes diluted in heptanes), there are some important conclusions that can be made from the two sets of experiments. In particular, the differences seen 2789



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Figure 11. Breakthrough experiments with binary 0.005 M (left) and 0.05 M (right) solutions of C8 alkylaromatic compounds in heptane at 323 K: (a) effuent concentrations C of p-xylene and o-xylene as a function of eluted volume (1 mL·min−1) and (b) effuent concentrations C of p-xylene and m-xylene as a function of eluted volume (1 mL·min−1).

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in the crystal structures of the three materials described in the first part of the paper, where the MIL-53(Fe)[o-xylene] and MIL-53(Fe)[m-xylene] present similar host:guest arrangement, in contrast to MIL-53(Fe)[p-xylene], reflects a different set of host:guest interactions for the para isomer over the other two for this particular host. Clearly the underlying processes dictating the separation of the xylene isomers are a complex balance of thermodynamic and kinetic factors. The fact that the chromatographic separation factors for o-xylene and p-xylene are relatively temperature independent compared to m-xylene, despite the fact that it is the o- and m-isomers that have similar packing arrangements in the solids, suggests that a balance of enthalpic factors is at play. The flexible nature of the MIL-53 structures makes obtaining a complete explanation of the mechanism of guest uptake challenging, and future work in understanding and predicting molecular separation properties must focus on developments in methods for the simulation of these materials, particularly since they have potential for practical application.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the ESRF for beamtime on ID31 and DESY for beamtime at HASYLAB. We are grateful to Dr. Andy Fitch of the ESRF for his assistance with running the experiments on ID31 and to Mark Feyand of the Christian-AlbrechtsUniversität zu Kiel for his assistance with use of Beamline F3. 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. D.d.V. is grateful to FWO-Vlaanderen, to IAP 6/27 (Belspo), and to KULeuven (Methusalem Grant CASAS) for support.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information files of these materials and additional figures and tables of data. This material is available free of charge via the Internet at http://pubs.acs.org. 2790



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Liquid-Phase Adsorption and Separation of Xylene Isomers by the

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