Vapor-Phase Adsorption and Separation of Ethylbenzene and Styrene

Oct 18, 2012 - Separation of styrene (ST) from ethylbenzene (EB) remains an industrially relevant challenge in the production of polystyrene. Adsorpti...
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Vapor-Phase Adsorption and Separation of Ethylbenzene and Styrene on the Metal−Organic Frameworks MIL-47 and MIL-53(Al) Tom Remy,† Lina Ma,†,§ Michael Maes,‡ Dirk E. De Vos,‡ Gino V. Baron,† and Joeri F. M. Denayer*,† †

Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Arenbergpark 23, 3001 Leuven, Belgium

‡

S Supporting Information *

ABSTRACT: Separation of styrene (ST) from ethylbenzene (EB) remains an industrially relevant challenge in the production of polystyrene. Adsorptive separation with metal−organic frameworks (MOFs) is a potential alternative for the conventional vacuum distillation process. Adsorption and separation of ST and EB on the MOFs MIL-47 and MIL-53(Al) were studied under vapor-phase conditions. ST and EB show traditional type I isotherms on MIL-47. Contrarily, ST adsorption isotherms show steep steps on MIL-53(Al) as a result of the breathing of the flexible MOF upon increased adsorbate pressure. The separation potential of both MOFs was investigated by performing vapor-phase breakthrough experiments at total hydrocarbon partial pressures between 1.14 and 16.4 mbar and temperatures between 35 and 90 °C. ST is preferentially adsorbed on both MOFs. Although the MOFs are isostructural, the evolution of selectivity with temperature and pressure is different for both materials due to the different interaction and separation mechanisms. isomers. Jin et al. reported a novel three-dimensional flexible open microporous MOF (JUC-77) with two-dimensional rhombus shaped channels.9 JUC-77 selectively adsorbed pxylene (pX) from the other isomers by acting as a molecular sieve. Vermoortele et al. explored the pX selectivity of the MOFs MIL-125(Ti)-NH2, MIL-125(Ti), and CAU-1(Al)NH2.10 Moreira et al. confirmed the para-selective behavior of MIL-125(Ti) at low feed xylene concentrations.11 Recently, Kulprathipanja et al. patented “an adsorptive process for separation of C8 aromatic hydrocarbons”.12 The selected MOFs are MOF-5, MIL-101(Cr), and MIL-53(Al). Gu et al. compared MOF-5 ([Zn4O(terephthalate)3]n·mDMF) and MOF-monoclinic {[Zn3(terephthalate)3(H2O)3(DMF)4]n} for the adsorption and separation of xylene isomers and EB.13 EB eluted first on MOF-5 at 250 °C, while almost no separation of the xylene isomers was observed. MOF-monoclinic selectively adsorbed pX with respect to the other isomers at 120 °C. Yang et al. reported MIL-101(Cr) as the stationary phase in highperformance liquid chromatography (HPLC).14 MIL-101(Cr) is built up from a hybrid supertetrahedral building unit formed by terephthalate ligands and trimeric chromium octahedral clusters. It was demonstrated that MIL-101(Cr) is a promising stationary phase for HPLC separation, allowing selective separation of oX from the other isomers.14,15 Alaerts et al. compared the MOFs MIL-47 and MIL-53(Al) for the liquid phase separation of xylenes and EB.16,17 MIL-47 (VIVO{O2C− C6H4−CO2}) and MIL-53(Al) [AlIII(OH){O2CC6H4−CO2}] are built from infinite chains of metal octahedra that are interconnected by terephthalate linkers. This results in two three-dimensional frameworks with one-dimensional lozenge-

1. INTRODUCTION Styrene (ST) is one of the most important basic chemicals as a monomer of polystyrene. It is mainly used for polymerization into synthetic rubbers, thermoplastics, or resins. The conventional process for the production of ST involves the catalytic dehydrogenation of ethylbenzene (EB) into ST in the presence of steam.1 The dehydrogenation of EB accounts for about 60− 80% conversion to ST, leaving unreacted EB in the product stream.2 Separation of both closely boiling components typically occurs via vacuum or extractive distillation, which is not straightforward on an industrial scale.3 Therefore, the separation of EB/ST mixtures is not only expensive but also very energy demanding when using a distillation train. Hence, new methods for the production and separation of ST from EB need to be developed. For components with similar boiling points, separation via adsorption could be energetically more efficient than conventional distillation techniques,4 given that suitable nonreactive adsorbents are available as the vinyl group of ST is sensitive to unwanted side reactions. Metal−organic frameworks (MOFs) are a particular class of interesting adsorbents, offering potential opportunities in diverse specific applications.5,6 MOFs are built up from metal ions connected by organic linkers. Many researchers have already studied adsorption and separation of aromatic compounds on MOFs. Several MOFs have shown potential for the separation of xylene isomers and/or EB. Nicolau et al. investigated MOF-1 [Zn(BDC)(Dabco)0.5 (BDC = 1,4benzenedicarboxylate, Dabco = 1,4-diazabicyclo[2.2.2]octane)] by performing single and multicomponent fixed bed experiments at different hydrocarbon pressures and temperatures.7 MOF-1 showed efficient separation of o-xylene (oX) from its C8 alkyl aromatic isomers. Bárcia et al. studied UiO-66, a rigid Zr-based MOF built up from hexamers of eight-coordinated ZrO6(OH)2 polyhedra and 1,4-benzenedicarboxylate linkers.8 It was observed that adsorption of oX is favored over other © 2012 American Chemical Society

Received: Revised: Accepted: Published: 14824

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Both MOFs preferentially adsorbed ST over EB.30 However, during liquid-phase separation experiments the capacity and selectivity of the adsorbent for a given adsorbate are dependent on the solvent.29,31 Moreover, the adsorbent’s pores are almost always fully filled during liquid phase adsorption experiments. Therefore, it is worthy to investigate the adsorption and separation of EB and ST in the vapor phase in order to correctly study the separation (and separation mechanism) as a function of loading. Furthermore, it is industrially relevant for potential pressure swing separations of the crude styrene or recovery of styrene in vaporous waste streams. First, single-component vapor-phase adsorption isotherms were measured on MIL-47 and MIL-53(Al). Afterward, the separation potential of both MOFs was assessed via breakthrough experiments. In addition, the influence of important process parameters such as pressure and temperature was investigated.

shaped pore channels having a diameter of approximately 1 nm in their open form.18 MIL-47 is a quite rigid, hydrophobic structure. On the other hand, MIL-53(Al) is hydrophilic and flexible, meaning that it can contract and expand upon interaction with guest molecules.19−23 With respect to the separation of C8-alkylaromatics under liquid-phase conditions, MIL-47 has been reported to selectively adsorb pX over both m-xylene (mX) and EB with average separation factors of 3.1 and 9.7, respectively.16 Since these aromatic components have similar enthalpies, the separation has to originate from differences in adsorption entropies.24,25 Indeed, the preference over mX is related to the pairwise packing of certain xylene molecules in the MIL-47 pores. pX and oX pack pairwise, allowing strong interactions between the almost perfectly parallelly aligned aromatic rings. A similar pairwise interaction can be observed for mX, but due to the specific metasubstitution of the methyl groups, a perfect parallel alignment is not possible and a tilt between the mX molecules is observed. This also causes a pair of mX molecules to be slightly bulkier compared to the densely packed pX or oX molecules, explaining the lower preference for mX. EB tends to interact with the host rather than another EB molecule by means of an out-of-plane rotation of the ethyl group. Finsy et al. have performed a series of separations of binary and quaternary mixtures in vapor phase conditions with MIL-47.25 The binary breakthrough experiments showed that EB always eluted first. The following trend of selectivities under mixture conditions was found: oX ∼ pX > mX > EB. Moreover, the quaternary breakthrough experiments revealed that the trend of mixture elution at 70 °C is in the same order as the binary results. At 110 °C, EB elutes first from the mixture followed by mX, pX, and oX. Furthermore, Castillo et al. used grand canonical Monte Carlo (GCMC) simulations to provide molecular insight on the separation mechanism for xylene isomers on MIL-47.26 It was found that the order of preferential adsorption was oX > pX > mX. The adsorption selectivity increased with pressure, and these simulation results showed an excellent agreement with experiments. According to the simulations, the selectivity is due to differences in packing of the xylene isomers. The strongest adsorption on MIL-53(Al) has been observed for oX.17 MIL-53 is also an ortho-selective material but here the selectivity results from a more pronounced interaction with the host. Due to the ortho-substitution of the methyl groups, oX is able to interact with two adjacent carboxylate groups in the obtuse corners of the pores, whereas the other xylene isomers cannot interact in a similar way. Vapor-phase adsorption of C8 alkyl aromatics compounds (oX, pX, mX, and EB) and separation of EB from oX have been studied by Finsy et al. on MIL-53(Al).27 Pronounced framework breathing was observed during the adsorption of these adsorbates. At low pressures, below the so-called “gate-opening” pressure, MIL53(Al) showed no preference for any aromatic. At pressures high enough to induce pore opening, oX was selectively adsorbed. Remy et al. modeled the different observed experimental breakthrough profiles and the increase in selectivity starting from the gate-opening pressure by using a pressure-dependent saturation capacity for oX.28 Moreira et al. studied the influence of the eluents isooctane, n-hexane, and nheptane on the behavior of MIL-53(Al) for the adsorption and separation of xylene isomers.29 Their results confirmed the selective adsorption of oX from the other isomers. Maes et al. studied liquid-phase separation of EB from ST on MIL-47 and MIL-53(Al) at 25 °C using n-heptane as a solvent.

2. EXPERIMENTAL SECTION 2.1. Materials. MIL-47 was synthesized by dissolving 1.22 g of VCl3 and 0.32 g of terephthalic acid in 14 mL of H2O.18 This mixture was loaded into a Teflon-lined steel autoclave and placed in an oven at 200 °C during 96 h. The crystallites were washed with water after cooling, and the sample was activated by calcination under air during 21.5 h at 300 °C. The micropore volume was 0.36 mL/g, as determined from nitrogen porosimetry at 77 K. Orthorhombic crystals with diameters ranging from 0.25 to 2.0 μm were observed by scanning electron microscopy. For MIL-53(Al), a solution of 1.88 g of Al(NO3)3·9H2O and 0.41 g of terephthalic acid in 3.62 mL of water was loaded into a Teflon-lined steel autoclave. After being placed in an oven at 220 °C during 72 h, the white crystallites were washed after cooling the sample. The powder was calcined at 330 °C for 72 h in order to remove uncoordinated terephthalic acid molecules.19 The micropore volume, as calculated using the Dubinin−Raduskevitch method from the nitrogen adsorption isotherm at 77 K, was 0.495 mL/g. Crystals with diameters in the range 2−10 μm were observed by scanning electron microscopy. 2.2. Vapor-Phase Adsorption Isotherms. Vapor-phase adsorption isotherms were measured using the gravimetric technique. A reservoir, filled with the liquid adsorbate (EB or ST), is held at constant temperature through Peltier elements. Helium (He) bubbling through the container entrains the organic vapor. This He−organic vapor stream continuously flows over the sample positioned in a sample holder connected to the microbalance. About 5−10 mg of the adsorbent powder was placed in a quartz sample holder and positioned in the microbalance system (VTI Corp.). MIL-47 was activated by heating to 200 °C at a heating rate of 1 °C/min under a He flow of 350 N mL/min. For MIL-53(Al), the same procedure was followed, except that the maximum activation temperature was 190 °C. The maximum activation temperature was kept until equilibrium was reached, with a maximum equilibration time of 3 h. After cooling, adsorption isotherms of EB and ST were determined at 35, 50, and 90 °C by weighing the adsorbate uptake at different partial pressures of the adsorbate. The adsorbate partial pressure was altered by changing the temperature of the reservoir and/or by diluting the saturated flow. Vapor pressures of both EB and ST at equilibrium were calculated with the Wagner equation.32 14825

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Figure 1. Vapor-phase adsorption isotherms of EB and ST on MIL-47 and MIL-53(Al) at different temperatures expressed in wt %: (a) EB on MIL47, (b) ST on MIL-47, (c) EB on MIL-53(Al), and (d) ST on MIL-53(Al).

2.3. Preparation of Pellets. In order to avoid large pressure drops over the column, aggregates of adsorbent crystals, called pellets, with sufficient pressure and thermal stability were prepared. MIL-47 pellets were produced by compressing the MIL-47 powder into a solid disk upon application of a pressure of 500 bar. The resulting disks were crushed and sieved to obtain the desired fraction (500−630 μm). In contrast to MIL-47, a polyvinyl alcohol (PVA) binder had to be used to obtain stable pellets of MIL-53(Al).33 A 15 wt % PVA/water solution was prepared by adding 35.3 g of PVA (PVA 10−98, Fluka) granules together with 200 mL of deionized and demineralized water in a three-necked flask fitted with a thermometer, a reflux condenser, and an impeller stirrer. The solution was heated to 90 °C and vigorously stirred to ensure complete dissolution. A reflux condenser avoided the loss of water. A 0.5 g portion of this 15 wt % PVA/water solution was added to 0.5 g of MIL-53(Al) powder, homogenized, and heated up to 190 °C overnight. The resulting MIL-53(Al) pellet was crushed and sieved into the desired fraction of 500−630 μm. The obtained material is denoted as MIL-53(Al,PVA). A detailed description of the method is given elsewhere.33 It has been shown that the presence of the binder does not affect the pore size or completely block the pores of the MOF. 2.4. Binary Breakthrough Experiments. Vapor phase breakthrough experiments were performed to determine the dynamic separation potential of MIL-47 and MIL-53 using the experimental setup shown in Figure S1 Supporting Information. Breakthrough experiments were performed at varying temperatures and pressures using columns with lengths of 10,

15, and 30 cm and an internal diameter of 0.21 cm, packed with pellets (500−630 μm) of MIL-47 or MIL-53(Al,PVA). The column packed with MIL-47 pellets was in situ activated with 20 N mL/min He at 225 °C overnight using a heating rate of 1 °C/min. For MIL-53(Al,PVA), the activation was performed at 190 °C under the same He flow and heating rate. Two mass flow controllers (Bronkhorst) control the flow of He to the two evaporators. The evaporators are filled with liquid adsorbate (EB and ST). In order to control the vapor pressure, the evaporators are maintained at the corresponding temperature using a separate temperature control apparatus from Julabo (P32). The He flow is used as an inert carrier gas and takes an amount of adsorbate with it, governed by the vapor−liquid equilibrium of the adsorbate. To obtain very low vapor pressure, the saturated flow is diluted with additional He just after the evaporators using a third mass flow controller. The total hydrocarbon pressure ranged from 1.16 to 16.4 mbar. Using a multiposition valve either the mixture of He, EB, and ST or a pure He flow (for activation) can be sent through the packed column. The column is placed in an oven to maintain the desired operating temperature during the measurements and to perform the thermal regeneration of the column. The eluent stream is analyzed using an online gas chromatograph (HP-6890) equipped with an HP-5 column (15 m × 0.53 mm × 1.5 μm film thickness) containing 5% PHME Siloxane. The GC measurements are automated using an automatic injection valve with a volume of 0.500 mL. The resulting curves in the Figures 3, 5, 7, and 8 show the dimensionless concentration (C/C0) of both adsorbates as a function of the dimensionless time (t/tb). The dimensionless concentration is obtained by 14826

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Figure 2. Vapor-phase adsorption isotherms of EB and ST on MIL-47 and MIL-53(Al) at different temperatures expressed as the number of adsorbed molecules per unit cell (molec/UC): (a) EB on MIL-47, (b) ST on MIL-47, (c) EB on MIL-53(Al), and (d) ST on MIL-53(Al).

experimental temperature and pressure windows, with maximum capacities decreasing with increasing temperature (see Figure 1a,b). The maximum EB capacities on MIL-47 at 35, 50, and 90 °C are 37, 34, and 29 wt %, respectively. The corresponding experimental ST capacities on MIL-47 are 46, 44, and 40 wt %. Adsorbed amounts of both adsorbates, especially ST, do not vary largely in the temperature window under investigation. The change in ST capacity from 35 to 90 °C is only 6 wt %. To further emphasize this observation, we have added a ST isotherm at 65 °C in Figure 1b. Obtained EB capacities are in line with formerly reported EB capacities on MIL-47 by Finsy et al.25 They found EB capacities of 33, 26, and 22 wt % at 70, 90, and 150 °C, respectively. ST capacities are always higher than the corresponding EB capacities. Maes et al. reported higher liquid uptake capacities for ST with respect to EB in single-component batch experiments on MIL-47 at room temperature.30 They reported maximum liquid phase capacities for EB and ST on MIL-47 of 16 and 21 wt %, respectively. These differences in experimental capacities probably arise from the fact that Maes et al. used n-heptane as a solvent in their liquid-phase batch experiments. The alkane solvent is able to fill part of the pore volume, as evidenced by former vapor phase experiments with n-octane by Finsy et al.25 At 70 °C, n-octane reached a maximum adsorption capacity of 23 wt %, corresponding to one molecule per channel segment. Therefore, the difference of about 20 wt % in maximum capacities for EB and ST between the liquid- and vapor-phase adsorption experiments is most probably related to the coadsorption of the alkane solvent in the liquid-phase experiments.

dividing the actual concentration by the inlet concentration. The dimensionless time is the experimental time divided by the mean breakthrough time tb of the component that elutes first (and which is thus least adsorbed). The mean breakthrough time tbi of component i is defined as34 t bi =

∫0

∞⎛

x ⎞ ⎜⎜1 − i , t ⎟⎟ dt xi ,0 ⎠ ⎝

where xi,t is the mole fraction of component i at time t and xi,0 is the initial mole fraction of component i. The adsorbed amounts qi were calculated by integration of the experimental breakthrough curves. Since in the breakthrough experiment the partial pressure of the adsorbing components varies during the experiment as a result of the ongoing adsorption process, only an apparent or average separation factor can be obtained. The average separation factor or selectivity α, for a binary mixture of ST and EB, is defined as

() = () q p

αST/EB

q p

ST

EB

3. RESULTS AND DISCUSSION 3.1. Vapor-Phase Adsorption Isotherms. Figure 1 shows the adsorption isotherms of EB and ST on MIL-47 and MIL53(Al) at 35, 50, and 90 °C, respectively. Both adsorbates exhibit steep type I isotherms on MIL-47 under the chosen 14827

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phase with the open pore structure on the other hand.17,27,37 Maes et al. localized adsorbed ST and EB molecules in MIL53(Al) samples via Rietveld refinements of XRD patterns.30 Adsorption of EB and ST has a significantly different effect on the MIL-53(Al) framework. Due to its out-of-plane rotation, the alkyl group of EB interacts with the oxygen atoms of the terephthalate linkers and the Al-bridging hydroxyl group. As a result, the octahedral environment of the Al ions is distorted. ST does not cause significant distortion to the Al environment since all O−Al−O angles in the octahedron remain very close to 90°.30 The adsorption of EB thus results in an additional distortion of the framework with respect to adsorption of ST. Therefore, adsorption of EB is coupled with an extra energy cost with respect to adsorption of ST. Consequently, the transition from the single file adsorption in the contracted form to the double file adsorption in the open pore form is easier upon adsorption of ST. Therefore, ST shows steep steps in its adsorption isotherms, whereas EB has a much broader transition region. The maximum capacities of EB and ST on MIL-53(Al) are comparable: 49 wt % for EB and ST at 35 °C. At higher temperatures, full capacity was not reached, as partial hydrocarbon pressures were experimentally limited. Again, the large differences in maximum capacity between the liquid-phase uptake experiments by Maes et al. and our vapor-phase adsorption experiments are probably due to coadsorption of the alkane solvent in the liquid-phase experiments. MIL-53(Al) has a maximum capacity of 17 wt % n-octane at 110 °C (see Supporting Information, Figure S2). Maes et al. reported maximum capacities for EB and ST on MIL-53(Al) of 15 and 24 wt %, respectively. Another remarkable difference between the liquid- and vapor-phase experiments on MIL-53(Al) is the disappearance of the step in the ST isotherm during adsorption from the liquid phase. This is also due to the presence of an excess of solvent during the liquid phase adsorption experiments. Even at low ST concentrations, the alkane solvent fills the unoccupied pore volume and renders a contraction of the adsorbent pores impossible. A similar observation was already pointed out for the different xylenes isomers: the vapor-phase adsorption isotherms showed distinct steps, whereas the liquidphase isotherm did not.27 3.2. Separation of EB/ST Mixtures on MIL-47. Figure 3 shows the breakthrough profiles of an equimolar EB/ST mixture at 70 °C on MIL-47 at different total hydrocarbon pressures (PHC). All breakthrough profiles show an initial phase during which both components of the feed are fully adsorbed. After a certain time, pure EB starts to elute from the column. ST always elutes later than EB from the MIL-47 column; it is preferentially adsorbed with respect to EB. When ST starts to elute from the column, the concentration of both components at the outlet evolves to the feed concentration level, indicating that the column is saturated. 3.2.1. Effect of Vapor Pressure. MIL-47 is clearly able to separate EB from ST at 70 °C for all the total hydrocarbon pressures under investigation. This separation of EB from ST, even at low pressures, might seem unexpected given the comparable adsorbed amounts of both components in singlecomponent vapor-phase experiments (see Figure 1a,b). Since EB and ST have similar adsorption enthalpies (ΔHappEB = −10.1 kJ/mol, ΔHappST = −9.0 kJ/mol),30 the separation must rely on an entropic effect, i.e., a different stacking mode of the adsorbates. Rietveld refinements of ST-loaded MIL-47 demonstrated a pairwise stacking of ST molecules within the

For completeness, Figure 2 shows the same isotherms as in Figure 1 but with the adsorbed amounts now expressed in a number of adsorbed molecules per unit cell. The maximum loading for EB and ST at 35 °C is 37 and 46 wt %. This corresponds to 3.2 EB molecules and 4.0 ST molecules per unit cell. Maes et al. identified a maximum of 10 adsorption sites per tripled unit cell, or 3.33 molecules per unit cell, for a mixture of EB/ST.30 In pure-component vapor-phase adsorption, both adsorbates thus occupy all the available adsorption sites for C8 aromatics having a side chain containing two carbon atoms. In the liquid phase, the highest observed ST capacity corresponds to about eight adsorbed molecules per tripled unit cell according to Maes et al.30 Therefore, complete saturation of the MIL-47 pores with ST in only reached at high ST vapor pressures. The adsorption mechanism on MIL-53(Al) is completely different. ST adsorption isotherms always show a distinct steep step (see Figure 1d) starting from a certain gate-opening pressure. This gate-opening pressure increases with increasing temperature (see Figure 1d). The steps in the isotherms of ST are related to the so-called breathing phenomenon, the flexibility of the framework induced by the adsorbate.35−37 ST thus causes structural changes in the MOF structure leading to a stepwise increase in the adsorbed amounts at higher pressures. Finsy et al. studied the structural changes in MIL53(Al) at different xylene and EB loadings via Rietveld refinements.27 Within the first plateau region the pores of the adsorbent are strongly contracted and the framework structure closely resembles the formerly reported MIL-53lt(Al) structure, which has the space group Cc (a = 19.46 Å, b = 7.66 Å, c = 9.60 Å). They labeled their structure as MIL-53iX (Al) (for intermediate xylene loading), which was well-described with the space group Pnma (a = 18.51 Å, b = 6.64 Å, c = 9.60 Å). All xylenes had comparable adsorption capacities of about 20 wt % in the first plateau. This corresponds to about 1.5 molecules per unit cell or 0.75 molecules per channel segment. The comparable capacities within the first plateau region have been explained by the molecular packing mechanism in the contracted pores of the MIL-53iX (Al) framework. The adsorbates reside in the MOF pores as a single file of molecules, adsorbed along the length of the pores. Since the pore diameter of the MIL-53iX (Al) form is quite small (6.6 × 9.6 Å), only one molecule can be adsorbed in the cross section of the pore, regardless of the position of the side chains. Therefore, ST molecules reside as a single file of molecules within the contracted MIL-53 pores at low adsorbate pressures. At higher pressures, the structure reopens to accommodate up to 3.9 ST molecules per unit cell (49 wt %) at 35 °C. The situation is more complicated for EB (see Figure 1c). A first plateau region is observed for low adsorbate pressures at 50 and 90 °C. Here the EB capacity is about 20 wt % (about 1 EB molecule per unit cell). At higher EB pressures, a broad transition region is observed. This is in line with the formerly reported EB isotherm on MIL-53(Al) at 70 °C by Finsy et al.27 Their EB isotherm did also not show a steep step but rather a kink, similar to the one observed in the EB isotherm at 90 °C (see Figure 1c). The broader transition region for EB with respected to ST is related to the differences in packing of both adsorbates. The required energy to induce the structural rearrangement from a contracted to an open pore form is related to the difference in affinity between the adsorbed phase with the closed pore structure on one hand and the affinity between the adsorbed 14828

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interacts with the ligand oxygen atoms, no π−π interactions prevail, giving rise to a lower uptake of EB in comparison with ST. The evolution of the selectivity as a function of the total hydrocarbon pressure on MIL-47 at 70 °C is shown in Figure 4.

Figure 4. Selectivity as a function of the total hydrocarbon vapor pressure for the separation of ST and EB on MIL-47 and MIL53(Al,PVA) at 70 °C.

At the lowest total pressure (1.4 mbar), the selectivity is 2.5. In this situation, about one EB molecule and two ST molecules are adsorbed per unit cell, resulting in a selectivity of about 2 at this pressure (see Figure 4). At higher pressures, the adsorbed amount of ST increases while the adsorbed amount of EB remains comparable, since EB cannot be stacked pairwise inside the adsorbent pores. Consequently, the selectivity increases with increasing pressure until the pores are fully loaded. The maximum selectivity is 3.8, which is in excellent agreement with previous results from Maes et al.30 Upon breakthrough of the least adsorbed component, the outlet concentration of this component typically temporarily exceeds the inlet concentration. This phenomenon, where the most strongly adsorbed component displaces part of the preadsorbed and more weakly adsorbed component, is called roll-up.4 Although expected, the roll-up phenomenon only prevails at the highest hydrocarbon pressure under investigation (Figure 3). Maes et al. showed that ST cannot compete for every adsorption site on which EB is being preadsorbed.30 Therefore, ST is unable to displace the whole amount of preadsorbed EB, yielding less pronounced roll-ups of EB at lower pressures upon separation of EB/ST on MIL-47. 3.2.2. Effect of Temperature. Figure 5 shows the breakthrough profiles of EB and ST from an equimolar mixture at 35, 50, 70, and 90 °C on MIL-47 at a total hydrocarbon pressure of 5.2 mbar. Obviously, EB and ST can be separated at all studied temperatures. Increasing the temperature seems to have only a small effect on the relative positions of the dimensionless breakthrough curves, which is in line with the small variations in adsorbate capacity observed during isotherm measurements (see Figure 1a,b). Therefore, the separation factor does not change much with temperature (see Figure 6). This reflects the fact that the EB/ST separation on MIL-47 is mainly based on entropic effects. Maes et al. already reported the negligible influence of temperature on the separation factor during the pulse chromatographic separation (extremely low coverage) of

Figure 3. Dimensionless breakthrough curves for the separation of equimolar binary mixtures of EB and ST at 70 °C on MIL-47 at different total hydrocarbon pressures (PHC).

pores of MIL-47 allowing strong π−π interactions between adsorbed ST molecules.30 On the other hand, EB is not able to assume a pairwise configuration.16 Since the alkyl chain 14829

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Figure 6. Selectivity as a function of temperature for the separation of ST and EB on MIL-47 and MIL-53(Al,PVA) at a total hydrocarbon pressure of 5.2 mbar.

3.3. Separation of EB/ST Mixtures with MIL-53(Al,PVA). Figure 7 shows the breakthrough profiles of three equimolar EB/ST mixtures at 70 °C on MIL-53(Al,PVA) at different total hydrocarbon pressures. As for MIL-47, ST is preferentially adsorbed with respect to EB on MIL-53(Al,PVA), especially at high total hydrocarbon pressures. When ST starts to elute from the column, the concentration of both components at the outlet evolves to the feed concentration level, indicating that the column is saturated. 3.3.1. Effect of Vapor Pressure. Contrarily to MIL-47, almost no separation prevails at the lowest total hydrocarbon pressure under investigation (1.16 mbar). Both components elute at approximately the same time, resulting in a separation factor of 1.6. At this pressure, the first isotherm plateau is reached (see Figure 1c,d). Therefore, no clear separation can be expected. The separation factor is thus significantly lower on MIL-53(Al) compared to MIL-47 in the low-pressure regime (see Figure 4). The higher selectivity on MIL-47 at low hydrocarbon pressures is a result of the better packing of ST (vide supra). Since isotherms on MIL-47 are very steep for both adsorbates (see Figure 1a,b), the packing effects already influence the separation at low pressures. Higher hydrocarbon pressures give rise to different elution times for EB and ST. Therefore, the separation factor increases with total hydrocarbon pressures as for MIL-47. The maximum separation factor is 3.5 (see Figure 4). Maes et al. obtained a separation factor of 4.1 under liquid-phase conditions for an equimolar EB/ST mixture at room temperature.30 At 5.2 and 13.7 mbar total hydrocarbon pressure, the adsorbent adopts the open pore form and is able to accommodate extra ST molecules within its pores, leading to the transition from a single file to a double file adsorption for ST. The preferential adsorption of ST in the open pore form thus leads to an improved separation potential at higher hydrocarbon pressures. Contrarily to the separation on MIL-47, the elution profile of EB shows a visible roll-up for all the experimental total hydrocarbon pressures above the gate-opening pressure (see also Figure 7). In this case, ST can compete for every adsorption site of MIL-53 on which EB is being preadsorbed. 3.3.2. Effect of Temperature. Figure 8 shows the breakthrough profiles of EB and ST from an equimolar mixture at 35,

Figure 5. Dimensionless breakthrough curves for the separation of equimolar binary mixtures of EB and ST at a total hydrocarbon pressure of 5.2 mbar on MIL-47 at different temperatures.

EB/ST mixtures in n-heptane on MIL-47 in the range of 25−50 °C.30 Here, we demonstrate that for a pure EB/ST stream the separation entirely depends on entropic effects once one more than one ST molecule is adsorbed per unit cell. 14830

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Figure 7. Dimensionless breakthrough curves for the separation of equimolar binary mixtures of EB and ST at 70 °C on MIL-53(Al,PVA) at different total hydrocarbon pressures.

Figure 8. Dimensionless breakthrough curves for the separation of equimolar binary mixtures of EB and ST at a total hydrocarbon pressure of 5.2 mbar on MIL-53(Al,PVA) at different temperatures.

70, and 90 °C on MIL-53(Al,PVA) at a total hydrocarbon pressure of 5.2 mbar. Contrarily to MIL-47, the temperature has a profound effect on the separation potential of the adsorbent. At 35 °C, EB and ST are well-separated with a separation factor of 3.8 (see Figure 6). Increasing the temperature decreases the differences in breakthrough time between EB and ST. Almost no separation prevails anymore at 90 °C. Hence, the separation factor continuously decreases with temperature (see Figure 6). This reflects the enthalpic nature of the EB/ST separation on MIL-53. Maes et al. also found a clear influence of temperature on separation factor on MIL-53 in pulse chromatographic separation experiments at 25−65 °C at very low EB/ST mixture concentration and thus intrapore concentration of EB and ST.30 At 25 °C, EB and ST were baseline separated with a separation factor of 3.9, while

peaks started to overlap at temperatures above 50 °C. The effect can also be rationalized on the basis of the isotherms of ST in Figure 1d. At 35 °C and a ST pressure of 2.6 mbar, the MIL-53(Al) material is clearly in the open pore form. However at 90 °C, the ST pressure of 2.6 mbar is probably not high enough to reopen the pores of the MIL-53(Al,PVA) adsorbent. This seems even more logic if one keeps in mind that EB does hardly adsorb in the open form at 90 °C (Figure 1d) and the fact that the binder shifts the gate-opening to higher adsorbate pressures.33 Although isostructural, MIL-53(Al) with metal-bridging hydroxyl groups and MIL-47 with metal-bridging oxygen atoms show entirely different adsorption and separation mechanisms for EB/ST mixtures. Both materials show high 14831

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110 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

separation factors at the higher total hydrocarbon pressures that were investigated (see Figure 4), which is most relevant in the production of ST. For an industrial process other important factors such as the influence of impurities and stability come also into play. A typical crude ST stream contains small fractions of oX and toluene. A material that preferentially adsorbs these components is thus desired. MIL-53 has a stronger ortho-selectivity with respect to alkylaromatics than MIL-47.17 Moreover, MIL53 strongly adsorbs toluene in a mixture of ST, EB, oX, and toluene.30 The calcination of as-synthesized MIL-47 is a delicate procedure for which the time, bed thickness, and oven geometry need to be optimized.38 MIL-47 is actually a metastable phase since an unavoidable oxidation to VO2 and V2O5 will take place when exposed to air at elevated temperatures (573 K) or prolonged exposure to air at room temperature. This is not the case for MIL-53. As a final comment, it is also important to consider the presence of water in the industrial process. Most MOFs degrade upon prolonged exposure to water. MIL-47 is a hydrophobic structure due to the structure of its pore walls.18 MIL-53 undergoes large breathing in the presence of water, leading to a contraction of the pores. The transformation is entirely reversible and the adsorbed water can easily be removed by heating.19 An in depth study on the stability of both MOFs in the presence of hot water vapor has not yet been carried out. At this moment, MIL53 seems the most suited MOF for the adsorptive separation of EB from ST. MIL-53 is a more stable material than MIL-47, with a large preference for the impurities in the crude styrene stream.

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Corresponding Author

*E-mail: [email protected]. Tel.: 02 629 17 98. Fax: 02 629 32 48. Author Contributions §

These authors contributed equally. The manuscript was written through contributions of all the authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T. Remy acknowledges FWO-Vlaanderen for financial support. J. Denayer and D. De Vos are grateful to FWO-Vlaanderen for financial support (G.0453.09 N).

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REFERENCES

(1) Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley and Sons: New York, 2008. (2) de Morais Batista, A. H.; de Sousa, F. F.; Honorato, S. B.; Ayala, A. P.; Filho, J. M.; de Sousa, F. W.; Pinheiro, A. N.; de Araujo, J. C. S.; Nascimento, R. F.; Valentini, A.; Oliveira, A. C. Ethylbenzene to Chemicals: Catalytic Conversion of Ethylbenzene Into Styrene Over Metal-Containing MCM-41. J. Mol. Catal. A: Chem. 2010, 315, 86. (3) Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; John Wiley and Sons: New York, 2006. (4) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers, Inc.: New York, 1994. (5) Rowsell, J. L. C; Yaghi, O. M. Metal−Organic Frameworks: A New Class of Porous Materials. Microporous Mesoporous Mater. 2004, 73, 3. (6) Rosseinsky, M. J. Recent Developments in Metal−Organic Framework Chemistry: Design, Discovery, Permanent Porosity and Flexibility. Microporous Mesoporous Mater. 2004, 73, 15. (7) Nicolau, M. P. M.; Bárcia, P. S.; Gallegos, J. M.; Silva, J. A. C.; Rodrigues, A. E.; Chen, B. Single- and Multicomponent Vapor-Phase Adsorption of Xylene Isomers and Ethylbenzene in a Microporous Metal−Organic Framework. J. Phys. Chem. C 2009, 113, 13173. (8) Bárcia, P. S.; Guimarães, D.; Mendes, P. A. P.; Silva, J. A. C.; Guillerm, V.; Chevreau, H.; Serre, C.; Rodrigues, A. E. Reverse Shape Selectivity in the Adsorption of Hexane and Xylene Isomers in MOF UiO-66. Microporous Mesoporous Mater. 2011, 139, 67. (9) Jin, Z.; Zhao, H. Y.; Zhao, X. J.; Fang, Q. R.; Long, J. R.; Zhu, G. S. A Novel Microporous MOF with the Capability of Selective Adsorption of Xylenes. Chem. Commun. 2010, 46, 8612. (10) Vermoortele, F.; Maes, M.; Moghadam, P. Z.; Lennox, M. J.; Ragon, F.; Boulhout, M.; Biswas, S.; Laurier, K. G.; Beurroies, I.; Denoyel, R.; Roeffaers, M.; Stock, N.; Düren, T.; Serre, C.; De Vos, D. E. p-Xylene-Selective Metal-Organic Frameworks: A Case of Topology-Directed Selectivity. J. Am. Chem. Soc. 2011, 133, 18526. (11) Moreira, M. A.; Santos, J. C.; Ferreira, A. F. P.; Loureiro, J. M.; Rodrigues, A. E. Reverse Shape Selectivity in the Liquid-Phase Adsorption of Xylene Isomers in Zirconium Terephthalate MOF UiO66. Ind. Eng. Chem. Res. 2011, 50, 7688. (12) Kulprathipanja, S.; Willis, R. R.; Benin, A.; Low, J. J. Adsorptive process for separation of C8 aromatic hydrocarbons. U.S. Patent 0004491, 2012. (13) Gu, Z. Y.; Jiang, D. Q.; Wang, H. F.; Cui, X. Y.; Yan, X. P. Adsorption and Separation of Xylene Isomers and Ethylbenzene on Two Zn−Terephthalate Metal−Organic Frameworks. J. Phys. Chem. C 2010, 114, 311.

4. CONCLUSIONS In this work, it has been demonstrated that the metal−organic frameworks MIL-47 and MIL-53(Al) allow the vapor phase separation of EB/ST mixtures, with separation factors up to 4.3. Although the pure-component vapor-phase adsorption isotherms show similar adsorbed amounts for ST and EB on MIL47, dynamic experiments clearly demonstrated the separation potential of this material in the vapor phase. The separation factor increases with increasing pressure due to preferential pairwise stacking of ST at high coverage. The negligible influence of temperature on the separation confirmed the entropic nature of the separation on MIL-47. Both adsorbates show a stepwise behavior in their purecomponent vapor-phase isotherms on the flexible MOF MIL53(Al). The step is much steeper for ST since it causes much less distortion to the structure compared to EB. The selectivity strongly increases as soon as the ST pressure is high enough to trigger a structural transition of the adsorbent framework from the closed to the open pore form. Enthalpic effects govern the dynamic separation on MIL-53(Al), since changes in temperature had a profound influence on the separation factor. For industrial applications, MIL-53 seems to be better suited than MIL-47. MIL-53 has a higher stability and a higher selectivity for the important impurities in the crude styrene stream. Further research also needs to assess the stability of these MOFs upon short- and long-term water exposure.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Schematic overview of the setup for high-coverage vapor-phase breakthrough measurements on MIL-47 and MIL-53(Al), and vapor-phase adsorption isotherm of n-octane on MIL-53(Al) at 14832

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Phase Adsorption of Alkane/Alkene Mixtures on NaY. Microporous Mesoporous Mater. 2005, 82, 191. (32) Poling, B. E.; Prausnitz, J. M.; O’Connel, J. P. The Properties of Gases and Liquids; McGraw-Hill Book Co.: New York, 2000. (33) Finsy, V.; Ma, L.; Alaerts, L.; De Vos, D. E.; Baron, G. V.; Denayer, J. F. M. Separation of CO2/CH4 Mixtures with the MIL53(Al) Metal−Organic Framework. Microporous Mesoporous Mater. 2009, 120, 221. (34) Malek, A.; Farooq, S. J. Determination of Equilibrium Isotherms Using Dynamic Column Breakthrough and Constant Flow Equilibrium Desorption. Chem. Eng. Data 1996, 41, 25. (35) Kitagawa, S.; Kitaura, R.; Noro, S. Functional Porous Coordination Polymers. Angew. Chem., Int. Ed. 2004, 43, 2334. (36) Uemura, K.; Matsuda, R.; Kitagawa, S. Flexible Microporous Coordination Polymers. J. Solid State Chem. 2005, 178, 2420. (37) Coudert, F.-X.; Jeffroy, M.; Fuchs, A. H.; Boutin, A.; MellotDraznieks, C. Thermodynamics of Guest-Induced Structural Transitions in Hybrid Organic−Inorganic Frameworks. J. Am. Chem. Soc. 2008, 130, 14294. (38) Alaerts, L.; Maes, M.; Jacobs, P. A.; Denayer, J. F. M.; De Vos, D. E. Activation of the Metal−Organic Framework MIL-47 for Selective Adsorption of Xylenes and Other Difunctionalized Aromatics. Phys. Chem. Chem. Phys. 2008, 10, 2979.

(14) Yang, C. X.; Yan, X. P. Metal−Organic Framework MIL101(Cr) for High-Performance Liquid Chromatographic Separation of Substituted Aromatics. Anal. Chem. 2011, 83, 7144. (15) Gu, Z.-Y.; Yan, X.-P. Metal−Organic Framework MIL-101 for High-Resolution Gas-Chromatographic Separation of Xylene Isomers and Ethylbenzene. Angew. Chem. 2010, 122, 1519. (16) Alaerts, L.; Kirschhock, C. E. A.; Maes, M.; van der Veen, M. A.; Finsy, V.; Depla, A.; Martens, J. A.; Baron, G. V.; Jacobs, P. A.; Denayer, J. F .M.; De Vos, D. E. Selective Adsorption and Separation of Xylene Isomers and Ethylbenzene with the Microporous Vanadium(IV) Terephthalate MIL-47. Angew. Chem., Int. Ed. 2007, 46, 4293. (17) Alaerts, L.; Maes, M.; Giebeler, L.; Jacobs, P. A.; Martens, J. A.; Denayer, J. F. M.; Kirschhock, C. E. A.; De Vos, D. E. Selective Adsorption and Separation of Ortho-Substituted Alkylaromatics with the Microporous Aluminum Terephthalate MIL-53. J. Am. Chem. Soc. 2008, 130, 14170. (18) Barthelet, K.; Marrot, J.; Riou, D.; Férey, G. A Breathing Hybrid Organic−Inorganic Solid with Very Large Pores and High Magnetic Characteristics. Angew. Chem., Int. Ed. 2002, 41, 281. (19) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Férey., G. A Rationale for the Large Breathing of the Porous Aluminum Terephthalate (MIL-53) Upon Hydration. Chem.Eur. J. 2004, 10, 1373. (20) Llewellyn, L. P.; Bourrelly, S.; Serre, C.; Filinchuk, Y.; Férey, G. How Hydration Drastically Improves Adsorption Selectivity for CO2 over CH4 in the Flexible Chromium Terephthalate MIL-53. Angew. Chem., Int. Ed. 2006, 45, 7751. (21) Liu, Y.; Her, J.-H.; Dailly, A.; Cuesta, A. J. R.; Neumann, D. A.; Brown, C. M. Reversible Structural Transition in MIL-53 with Large Temperature Hysteresis. J. Am. Chem. Soc. 2008, 130, 11813. (22) Trung, T. K.; Trens, P.; Tanchoux, N.; Bourrelly, S.; Llewellyn, P. L.; Loera-Serna, S.; Serre, C.; Loiseau, T.; Fajula, F.; Férey, G. Hydrocarbon Adsorption in the Flexible Metal Organic Frameworks MIL-53(Al, Cr). J. Am. Chem. Soc. 2008, 130, 16926. (23) De Malsche, W.; Van der Perre, S.; Silverans, S.; Maes, M.; De Vos, D. E.; Lynen, F.; Denayer, J. F. M. Unusual Pressure− Temperature Dependency in the Capillary Liquid Chromatographic Separation of C8 Alkylaromatics on the MIL-53(Al) Metal−Organic Framework. Microporous Mesoporous Mater. 2012, 162, 1. (24) Duerinck, T.; Couck, S.; Vermoortele, F.; De Vos, D. E.; Baron, G. V.; Denayer, J. F. M. Pulse Gas Chromatographic Study of Adsorption of Substituted Aromatics and Heterocyclic Molecules on MIL-47 at Zero Coverage. Langmuir 2012, 28, 13883. (25) Finsy, V.; Verelst, H.; Alaerts, L.; De Vos, D. E.; Jacobs, P. A.; Baron, G. V.; Denayer, J. F. M. Pore Filling Dependent Selectivity Effects in the Vapor Phase Separation of Xylene Isomers on the Metal−Organic Framework MIL-47. J. Am. Chem. Soc. 2008, 130, 7110. (26) Castillo, J. M.; Vlugt, T. J. H.; Calero, S. Molecular Simulation Study on the Separation of Xylene Isomers in MIL-47 Metal−Organic Frameworks. J. Phys. Chem. C 2009, 113, 20869. (27) Finsy, V.; Kirschhock, C. E. A.; Vedts, G.; Maes, M.; Alaerts, L.; De Vos, D. E.; Baron, G. V.; Denayer, J. F. M. Framework Breathing Induced Xylene Separation with the Metal−Organic Framework MIL53. Chem.Eur. J. 2009, 15, 7724. (28) Remy, T.; Baron, G. V.; Denayer, J. F. M. Modeling the Effect of Structural Changes during Dynamic Separation Processes on MOFs. Langmuir 2011, 27, 13064. (29) Moreira, M. A.; Santos, J. C.; Ferreira, A. F. P.; Loureiro, J. M.; Rodrigues, A. E. Influence of the Eluent in the MIL-53(Al) Selectivity for Xylene Isomers Separation. Ind. Eng. Chem. Res. 2011, 50, 7688. (30) Maes, M.; Vermoortele, F.; Alaerts, L.; Couck, S.; Kirschhock, C. E. A.; Denayer, J. F. M.; De Vos, D. E. Separation of Styrene and Ethylbenzene on Metal−Organic Frameworks: Analogous Structures with Different Adsorption Mechanisms. J. Am. Chem. Soc. 2010, 132, 15277. (31) Daems, I.; Leflaive, P.; Méthivier, A.; Denayer, J. F. M.; Baron, G. V. A Study of Packing Induced Selectivity Effects in the Liquid 14833

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