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Separating Saturated Alkylaromatics from Their Unsaturated Analogues Using Metal-Organic Frameworks† Michael Maes,† Frederik Vermoortele,† Luc Alaerts,† Joeri F. M. Denayer,‡ and Dirk E. De Vos*,† Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit LeuVen, Arenbergpark 23, B-3001 LeuVen, Belgium, and Department of Chemical Engineering, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium ReceiVed: June 18, 2010; ReVised Manuscript ReceiVed: July 27, 2010
This work studies the liquid phase separation of styrene and ethylbenzene on the metal-organic framework [Cu3(BTC)2] (BTC ) 1,3,5-benzenetricarboxylate) as well as the analogous separation of vinyltoluenes from ethyltoluenes. Batch and column experiments have been performed to demonstrate the capacity of the material to separate styrene and ethylbenzene. Adsorption capacities of around 20 wt % are measured, while separation factors reach values as high as 5.5. As is demonstrated by UV-vis absorption measurements, the adsorption mechanism is based on specific interactions between the free Cu2+ ligation sites of the framework and the π-electrons of the aromatic compounds. For the separation of the vinyltoluenes and ethyltoluenes, similar capacities and separation factors have been obtained as for the separation of styrene and ethylbenzene, which suggests an analogous adsorption mechanism. 1. Introduction As ever more potential applications of metal-organic frameworks (MOFs) are described in the literature, liquid phase adsorption and separation are rapidly gaining interest. In the chemical industry, an important fraction of the unit operations and related costs is associated with the separation and purification of product streams, mainly due to the high energy demand of these unit operations. Increasing the efficiency of these processes may dramatically reduce costs and emissions. When boiling points of the molecules are similar, separation by conventional distillation becomes energetically unfavorable and unpractical. Distillation under reduced pressure and cryogenic distillation are potential but expensive solutions.1,2 An alternative separation technique could be adsorption. This implies that one compound is retained more efficiently on a bed of porous material than the other compound. Kinetic effects like shape selectivity as well as enthalpic or even entropic factors can be the driving forces.3 In liquid phase adsorption processes elevated temperatures are not strictly necessary because compounds must not be vaporized; hence side reactions are less likely to take place, at least if there are no active sites on the material that catalyze these reactions. The separation of styrene and ethylbenzene is of great importance for industry as styrene is the most important aromatic monomer produced in industry with 24.6 million tons/year.4 At elevated temperatures the reactivity of the vinyl group results in easy polymerization and copolymerization into several cheap thermoplastics, styrene-butadiene rubbers, and so forth.4 Styrene is produced by dehydrogenation of ethylbenzene, and both compounds are present in the outlet stream as conversion is incomplete, even when oxygen is added to push the equilibrium toward styrene. Ethylbenzene should be removed to obtain high †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. E-mail: dirk.devos@ biw.kuleuven.be. Tel: +32 16 321639. Fax: +32 16 321998. † Katholieke Universiteit Leuven. ‡ Vrije Universiteit Brussel.
quality polystyrene. Separation is difficult as boiling points of both compounds are close to 140 °C and styrene tends to polymerize at temperatures above 90 °C.5 To perform such separations, a number of industrial processes using vacuum distillation and extractive distillation have been described.4,6-8 On a smaller scale, pervaporation through polyurethane or crosslinked poly(hexamethylenesebacate) membranes has also been proposed.9,10 A related issue is the separation of vinyltoluenes. Although produced on a smaller scale (25 000 tons/year),5 they have important applications as paint additives and various polymers with different properties compared to those of styrene-based polymers. Like styrene, the C9 vinyltoluene analogues are produced by dehydrogenating ethyltoluenes and similar techniques as for the separation of styrene and ethylbenzene are used to separate vinyltoluenes from the respective ethyltoluenes.5,11,12 Several liquid phase adsorption studies have been performed on MOFs. MOFs have a minimal dead volume that results in high uptake capacities. Examples are the removal of sulfur compounds from a fuel feed, but also the separation of xylene, ethyltoluene, and cymene isomers of a steam cracker’s C5-cut and of cis- and trans-olefins.13-18 Recently the potential of MOFs for the separation of styrene and ethylbenzene was briefly mentioned in literature.19 This prompts us to report on our results regarding the separation of styrene and ethylbenzene on [Cu3(BTC)2] including real-time separations and a study of the adsorption mechanism. In addition to the styrene/ethylbenzene separation, the analogous separation of C9-vinyltoluenes from the respective ethyltoluenes on [Cu3(BTC)2] will also be discussed. 2. Experimental Section The MOFs used in this study were synthesized according to literature. [Cu3(BTC)2] was synthesized electrochemically. The electrolyte was prepared using 5.3 g of 1,3,5-benzenetricarboxylic acid (H3BTC) and 1 g of methyltributylammonium
10.1021/jp105637u 2011 American Chemical Society Published on Web 08/18/2010
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methyl sulfate that were dissolved in 50 g of ethanol by stirring at 55 °C.20 Two Cu-electrodes were immersed in this solution and connected to a power source (0.2 A, 17 V). The reaction took place for three hours whereafter the obtained blue crystals were washed three times with ethanol and dried overnight at 383 K to provide activated [Cu3(BTC)2]. The obtained crystals have a specific BET-surface of 1150 ( 30 m2 g-1 and a pore volume of 0.45 ( 0.02 cm3 g-1 as was determined by N2-physisorption tests. Liquid phase batch adsorption experiments were carried out at 298 K in 1.8 mL glass vials using binary solutions of alkylaromatics in dry heptane as a solvent following a literature procedure.15,18 Typically, 1.8 mL of an identical solution is injected into an empty reference vial without adsorbent, and into a vial containing 0.025 g of the adsorbent predried at 110 °C. After 2 h of stirring at room temperature, equilibrium is reached, and the supernatant of both vials is directly and automatically injected in a GC equipped with a CP-Sil5 column and an FID detector. Reproducibility of GC peak areas is within 2%. By comparing peak areas measured for the reference vial and for the vial with the adsorbent, the uptakes per mass of MOF are readily calculated, as well as the residual concentration of the adsorbate in the supernatant. Separation factors Ri,j were calculated using formula (1)
() ()
qi cj Rij ) × qj ci
(1)
with qi and qj as the amount (mol g-1) of compounds i and j adsorbed per grams of MOF, and ci and cj are the concentration (mol L-1) of compounds i and j present in the external liquid phase.3 Columns were handmade by loading approximately 0.5 g of MOF adsorbent into a stainless steel tube (L ) 5 cm, D ) 0.45 cm) under nitrogen atmosphere. In our experience, it is much easier to prepare adsorption columns starting from electrochemically prepared [Cu3(BTC)2], rather than from hydrothermally synthesized [Cu3(BTC)2]. This can be explained by the granulometry of the materials; an electrochemically prepared sample has crystals of homogeneous size of approximately 1 µm, while hydrothermally synthesized crystals have dimensions between 2 and 50 µm. Breakthrough experiments were performed following a literature procedure.15 From these breakthrough experiments, average selectivities were calculated using eq 1. First, for each compound, the adsorbed amounts q were calculated by integration of the curves using eq 2
q)
∫0t u × (Cin - Cout)dt
(2)
with u being the volumetric flow rate of the feed (L min-1) and Cin and Cout the concentration (mol L-1) of the adsorbate in the liquid feed and eluent, respectively. As the column is fed with an equimolar mixture, the average separation factor R can be written as R ) qstyrene/qethylbenzene. Regeneration of the column is performed by flushing the column with typically 150 mL of pure solvent at the same temperature and pressure as during adsorption. DRS measurements have been performed on a Varian Cary 5 UV-vis-NIR spectrophotometer with integrating sphere accessory between 4000 and 50 000 cm-1. [Cu3(BTC)2] samples were prepared by injecting 10 mL of a predried pentane solution of a compound into a sealed vial containing 0.1 g of activated
Figure 1. Breakthrough experiments with a binary 0.047 M solution of ethylbenzene (EB) and styrene (St) in heptane on a 5 cm column filled with [Cu3 (BTC) 2] at 298 K.
[Cu3(BTC)2]. Pentane has the advantage of allowing a quicker evaporation than higher aliphatic compounds. The concentration of dry styrene or dry ethylbenzene added is approximately 0.30 M, corresponding to an amount offered that is slightly lower than that needed to achieve saturation of the structure with either molecule. This facilitates the evaporation of remaining styrene or ethylbenzene in the solution after adsorption. The suspension is allowed to stir for two hours in order to reach the adsorption equilibrium. Next, excess solvent is evaporated under a He flow and the powder is transferred under inert N2 atmosphere to a UV-vis sample holder. The sample holder is sealed under inert atmosphere to avoid rehydration of the sample. In the case of the water loaded sample, [Cu3(BTC)2] is exposed to moisturized air (relative humidity 60%) for half an hour until the change of color from navy blue to pale blue is complete. The powder is then loaded into the sample holder. 3. Results and Discussion 3.1. Separation of Styrene from Ethylbenzene on [Cu3(BTC)2]. Ahmad et al. demonstrated the existence of interactions between [Cu3(BTC)2] and styrene or ethylbenzene.19 By performing a breakthrough experiment using a column of electrochemically synthesized [Cu3(BTC)2], it was evaluated if these differences allow separation of both compounds (Figure 1). It is clear that both molecules are adsorbed on the column as initially only pure solvent is eluting. Ethylbenzene and styrene are separated with an average separation factor of approximately 2. A roll-up-effect is observed, indicating that the less preferred ethylbenzene is displaced by the more strongly adsorbing styrene. This results in a temporarily elevated outlet concentration, which drops toward the inlet concentration once the column is saturated and both compounds are eluting.15 The interaction between the free Cu2+ ligation sites and styrene or ethylbenzene was further studied using UV-vis spectroscopy (Figure 2). Spectra have been measured on a dried adsorbate-free sample and samples loaded with water, dry heptane, dry styrene, and dry ethylbenzene. Some spectra of dehydrated and hydrated [Cu3(BTC)2] have been reported before.21 Particularly the frequency of the Cu2+ d-d transitions is indicative for the coordinative state of the Cu2+ centers. In
Separating Saturated Alkylaromatics from Unsaturated Analogues
Figure 2. Diffuse reflectance UV-vis spectra (expressed as F(R) according to the Kubelka-Munk theory) of thermally activated [Cu3(BTC)2] with different adsorbed species: unloaded, loaded with H2O, heptane, ethylbenzene (eb), and styrene (styr).
Figure 3. Single compound adsorption isotherms of ethylbenzene (EB) and styrene (St) dissolved in heptane on [Cu3 (BTC) 2]: uptake (wt %) as a function of equilibrium concentration at 298 K.
dry [Cu3(BTC)2], this band appears as a broad feature between 16 000 and 19 000 cm-1 with a maximum at 17 300 cm-1. An almost identical band is measured for a heptane-loaded sample. This indicates that the aliphatic heptane is not interacting with the open metal sites due to the lack of a functional group. The free Cu2+ ligation sites in [Cu3(BTC)2] are known to have a high affinity for water,22 which is confirmed by the shift of the d-d band to 14 000 cm-1. The intermediate positions of the main d-d bands for styrene or ethylbenzene loaded [Cu3(BTC)2] (∼16 000 cm-1) suggest that both compounds are interacting with the free CuII ligation sites by means of their π-electrons.18 Despite the separation observed in breakthrough experiments, single compound adsorption isotherms show almost coinciding plots at low coverage for both styrene and ethylbenzene, and a similar saturation level (Figure 3). Styrene reaches its saturation level at 21 wt %, while ethylbenzene does so at 18 wt %. These maximal uptakes correspond to approximately 21 molecules of styrene per unit cell and 17 molecules of ethylbenzene per unit cell. These values are clearly lower than the maximal uptake of p-toluidine on [Cu3(BTC)2], which in our previous study amounted to 48 molecules per unit cell.23 One unit cell of
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Figure 4. Competitive adsorption on [Cu3(BTC)2] in batch mode: uptake (wt %) from an equimolar mixture of ethylbenzene (EB) and styrene (St) in heptane as a function of total equilibrium bulk phase concentration. Separation factors R calculated from the data points are given on the right axis.
[Cu3(BTC)2] with formula [Cu48(BTC)32] consists of 4 A-type and 4-B-type cages of similar dimensions, having a diameter of approximately 1 nm, which both are accessible for the larger aromatics.24 As the free Cu2+ ligation sites are only available in the B-type cages, and as the electronic spectra evidence interaction of styrene or ethylbenzene with these sites, it is likely that the major part of the ethylbenzene and styrene adsorbates is located in these B-type cages. Considering the results of Figure 3, this would mean that ca. 5.25 molecules of styrene or ca. 4.25 molecules of ethylbenzene could be present per B-type cage. These values seem realistic considering the dimensions of this cage (1.0-1.2 nm) and the number of free Cu2+ ligation sites available per B-cage, which amounts to 12. By contrast, the high uptakes previously recorded for p-toluidine suggest that this molecule resides in both the A- and B-type cages with on the average 6 adsorbate molecules per cage. In competitive batch adsorption experiments the preference for styrene is again clear (Figure 4). The isotherm recorded for styrene is similar to the one measured in the single compound mode, indicating that styrene is as efficiently adsorbed as in the case when no ethylbenzene is present (Figure 4). By contrast, ethylbenzene uptake does not surpass 4 wt % in these competitive experiments. The separation factor remains constant at a value of approximately 5.5. One would expect a decreasing separation factor at high loading if all available coordination sites were saturated. However, in this case the number of available free Cu2+ ligation sites in the B-cage is larger than the number of aromatic molecules that can maximally adsorbed within one B-cage, explaining the contant separation factor over the studied concentration range (Figure 4). The affinity of the free Cu2+ ligation sites in [Cu3(BTC)2] for π-electron systems has previously been observed in the selective adsorption of alkenes.18,25,26 As a preliminary experiment to understand the role of the different functional groups in the π-complexation mechanism that is operative here, the adsorption of vinylcyclohexane was investigated. When offered alone, up to 19 molecules of vinylcyclohexane were adsorbed per unit cell, which is comparable to the amount of styrene that fits in the cages. The strong preference for ethylbenzene over vinylcyclohexane in competitive experiments (Table 1) indicates that for the adsorption of styrene the vinyl group is likely not the decisive element determining its preference. Additionally,
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TABLE 1: Competitive Uptake (wt %) on [Cu3(BTC)2] As a Function of Initial Concentration of Each Compound from an Equimolar Mixture of Ethylbenzene (EB) and Vinylcyclohexane (Vch) and Calculated Separation Factors at 298 K (See Experimental Section) initial concentration (M)
uptake wt % EB
uptake wt % VCH
REB/VCH
0.01 0.03 0.08 0.14
5.9 9.8 11.8 13.6
2.8 3.7 2.8 2.7
5.5 3.5 5.9 8.5
this proves that the aromatic π-system is not particularly sterically hindered in comparison with a vinyl side chain for interaction with the Cu2+ site. In a simple rationalization, one could say that the selectivity order of unsaturated compounds on [Cu3(BTC)2] is mainly governed by the total number of π-electrons present in the adsorbate: eight for styrene, six for ethylbenzene, and only two for vinylcyclohexane, which corresponds to the order of preference obtained for these molecules. 3.2. Separation of Vinyltoluenes from Ethyltoluenes on [Cu3(BTC)2]. In a second part of this work, [Cu3(BTC)2] is further assessed for its potential in the analogous separation of C9-ethyltoluenes and the respective vinyltoluenes. In Figure 5, the results of competitive batch experiments are presented for
TABLE 2: Saturation Levels Based on Single Compound Adsorption Isotherms of mVT, pVT, mET and pET on [Cu3(BTC)2] at 298 K saturation level (wt %) pET mET pVT mVT
10 14 28 26
m-vinyltoluene (mVT) versus m-ethyltoluene (mET) and pvinyltoluene (pVT) versus p-ethyltoluene (pET). [Cu3(BTC)2] shows a preferential uptake of vinyltoluenes (VTs) between 20 and 28 wt %, while only up to 7 wt % is reached for the ethyltoluenes (ETs). The separation factors between unsaturated and saturated analogues neither clearly increase nor decrease with increasing concentration. The separation factor between pVT versus pET with a value of ca. 6.5 is much higher than that between mVT and mET with a value of 2.6. The similarity of the curves in Figure 5 with those obtained for ethylbenzene versus styrene (Figure 4) is striking. Therefore it seems plausible that the underlying selectivity determining mechanism is the same, which means that selectivities are governed by the interactions between the free Cu2+ ligation sites and the πelectrons. The difference between the saturation values of the adsorption isotherms of the individual compounds is however more pronounced with for both vinyltoluenes uptake values equivalent to 5.3-5.6 molecules per B-type cage, which are comparable values those obtained for styrene (Table 2). The lower affinity for ETs compared to ethylbenzene might be attributed to sterical hindrance. Although we do not have a detailed view on the localization of the compounds inside the pores, it is clear that the ethyltoluenes interact with the free Cu2+ ligation sites via their aromatic π-system and increasing substitution of this ring could cause steric constraints. Conclusion [Cu3(BTC)2] is capable of separating the industrially important mixture of ethylbenzene and styrene with a separation factor as high as 5.5. The adsorption mechanism is based on specific interactions between the free Cu2+ ligation sites in the framework’s B-type cages and the π-electrons of both ethylbenzene and styrene. The preference seems to be determined by the number of π-electrons in the adsorbate as has been suggested by additional experiments with vinylcyclohexane. Furthermore, similar results were obtained for the analogous separation of vintyltoluenes from ethyltoluenes. These separations nicely illustrates the potential of [Cu3(BTC)2] in the field of adsorptive separation. Acknowledgment. This work has been performed in the frame of the IAP 6/27 Functional Supramolecular Systems of the Belgian Federal Government. The authors wish to thank FWO-Vlaanderen (Research Foundation Flanders) for funding this research. D.D.V. and J.D. thank FWO for support under Project G.0453.09. We are grateful to K. U. Leuven for support under the Methusalem Grant CASAS. References and Notes
Figure 5. Competitive adsorption on [Cu3(BTC)2] in batch mode: uptake (wt %) from an equimolar mixture of (a) mET versus mVT and (b) pET versus pVT in heptane as a function of the total equilibrium bulk phase concentration. Selectivites R calculated from the data points are given on the right axis.
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