Reverse Shape Selectivity in the Liquid-Phase Adsorption of Xylene

Article pubs.acs.org/Langmuir

Reverse Shape Selectivity in the Liquid-Phase Adsorption of Xylene Isomers in Zirconium Terephthalate MOF UiO-66 Mariana A. Moreira,† Joaõ C. Santos,† Alexandre F. P. Ferreira,† José M. Loureiro,† Florence Ragon,‡ Patricia Horcajada,‡ Kyu-E. Shim,§ Young-K. Hwang,§ U.-Hwang Lee,§ Jong-S. Chang,§ Christian Serre,‡ and Alírio E. Rodrigues*,† †

LSRE - Laboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ‡ Institut Lavoisier (UMR CNRS 8180), Université de Versailles Saint-Quentin-en-Yvelines, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France § Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Taejon 305-600, Korea S Supporting Information *

ABSTRACT: Powder, agglomerates, and tablets of the microporous zirconium(IV) terephthalate metal−organic framework UiO-66 were evaluated for the selective adsorption and separation of xylene isomers in the liquid phase using n-heptane as the eluent. Pulse experiments, performed at 313 K in the presence of n-heptane, revealed the o-xylene preference of this material, which was further confirmed by binary and multicomponent breakthrough experiments in the presence of m- and p-xylene, resulting in selectivities at 313 K of 1.8 and 2.4 with regards to m-xylene and p-xylene, respectively. Additionally, because p-xylene is the less retained isomer, UiO-66 presents a selectivity pattern that is reverse of that of the xylenes' molecular dimension with respect to shape selectivity. The shaping of the material as tablets did not significantly change its selectivity toward the o-xylene isomer or toward p-xylene, which was the less retained isomer, despite a loss in capacity. Finally, the selectivity behavior of UiO-66 in the liquid n-heptane phase makes it a suitable material for o-xylene separation in the extract (heavy product) or p-xylene separation in the raffinate (light product) by simulated moving bed technology.

■

INTRODUCTION Many applications of metal−organic frameworks (MOFs) have emerged recently as a highly interesting topic in drug delivery, selective molecular sieving, and heterogeneous catalysis.1−9 Their very high porosity and versatile composition and topology, together with accessible metal sites or adaptive porosities, make MOFs very interesting candidates for separation. Xylenes constitute a family of C8 aromatics that are generally produced as a mixture containing the three xylene isomers (o-, m-, and p-xylene) and ethylbenzene. The xylene isomers differ one from another by the relative positions of the methyl groups in the benzene ring.10,11 Over time, the demand for xylenes has increased because they are used as raw materials in several industries. o-Xylene is used to produce phthalic anhydride, which is employed as a plasticizer. m-Xylene is used to produce isophthalic acid, which is gaining broader acceptance in poly(ethylene terephthalate) (PET) resin blends. p-Xylene, the isomer with the broadest commercial importance, is used as a raw material in the manufacture of PET, which in turn is used to produce fibbers, resins, films, and blown beverage bottles.10−12 In this way, their separation is an important process in the © 2012 American Chemical Society

production of pure xylenes. Adsorbents with a high selectivity toward o-xylene or p-xylene, the two isomers with the broadest commercial interest, are desired. Such materials would increase the efficiency of the existing processes. A high p-xylene-selective adsorbent would contribute to the decrease in the simulated moving bed unit size or to the decrease in the eluent consumption. A high o-xylene-selective porous material would be the core of an adsorption-based process to replace the energydemanding xylene splitter and o-xylene distillation column in an existing state-of-the-art aromatics plant. In the literature, different MOFs were presented as good materials to be used in xylene isomer separation, such as Zn(BDC),13 MOF-5, MOF monoclinic,14 MIL-47,15,16 MIL5317−20 (MIL stands for materials from Institut Lavoisier), UiO-6621(UiO stands for the University of Oslo), and, more recently, MIL-125(Ti)22,23 and MIL-125(Ti)_NH2.23 Received: January 28, 2012 Revised: March 8, 2012 Published: March 9, 2012 5715

dx.doi.org/10.1021/la3004118 | Langmuir 2012, 28, 5715−5723

Langmuir

Article

Some MOFs, such as UiO-66,21 have shown interesting features in the selective adsorption of xylene isomers in the gas phase. Some of us have recently reported21 that the adsorption of the bulkier o-xylene is favored over that of the other isomers in zirconium terephthalate UiO-66. This selective adsorption is opposite of the adsorption process usually applied in industry, which depends on the matching of the pore size and shape of the adsorbates and the adsorbent micropores.24 UiO-66, built from zirconium oxoclusters Zr6O4(OH)4 and 1,4-benzene-dicarboxylate (BDC or terephthalate),25,26 exhibits a cubic 3D structure with two types of microporous octahedral (∼11 Å) and tetrahedral cages (∼8 Å), which are accessible through microporous windows of around 5−7 Å,21,26 as presented in Figure 1. Thus, according to Bárcia et al.,21 the

Supporting Information.) For the same purpose, the Korea Research Institute of Chemical Technology (KRICT) synthesized and supplied UiO-66 in the form of tablets. Shaping of UiO-66 powder into the form of cylindrical tablets (3 mm diameter × 5 mm length) was carried out using a rotary press tabletizer. Before the shaping work was done, pretableting of the powders with subsequent grinding and sieving was done with a laboratory press in order to fit the density to more than 0.25 g/mL. The pretableted UiO-66 granule was uniformly mixed with a small amount of a graphite powder (1%) through ball milling. Then, the solid mixture was fed into the rotary press tabletizer to form a shaped body consisting of 99 wt % UiO-66 and 1 wt % graphite. Finally, the cylindrical tablets were formed by the combined pressing action of two punches and a die with holes of 3 mm diameter. After being shaped, the resulting surface area of the pelletized sample is 885 m2/g (SBET). The apparent bulk density of the tablets is 0.8 g/cm3. Gas-chromatography (GC)-grade p-xylene, o-xylene, and m-xylene (purity >99.0%, Sigma-Aldrich) were used as adsorbates. On the basis of the literature19 n-heptane (99.5% purity, Sigma-Aldrich) was used as a solvent. UiO-66 tablets were morphologically characterized at Centro de Materiais da Universidade do Porto (CEMUP) by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) operated at 15 kV (FEI Quanta 400 FEG ESEM). SEM was performed using a magnification of 60 to 1500× to observe UiO-66 tablets in detail. Helium pycnometry and mercury porosimetry analyses were performed at Laboratório de Caracterizaçaõ e Certificaçaõ de Materiais Granulares (IPN - LabGran). Pore size distribution measurements were performed using mercury porosimetry (Micromeritics AutoPore IV 9500). The N2 physisorption isotherm of the pelletized sample was collected at 77 K after evacuation at 423 K for 12 h using a volumetric sorption analyzer (Micromeritics Tristar 3000). The xylene isomers' separation using UiO-66 as an adsorbent was evaluated through breakthrough experiments using two different units: a laboratory scale unit and a pilot scale unit. The laboratory scale unit corresponds to a Shimadzu HPLC apparatus (Kyoto, Japan) comprising one model LC-20AD UFLC pump, an FRC-10A fraction collector, an oven model CTO-20A equipped with a stainless steel column, and an SPD-M20A IVDD diode array detector with a CBM20A control system. The pilot scale unit comprises two HPLC pumps, a back-pressure regulator, a fraction collector, and an oven where a stainless steel column is placed. For detailed information, see Supporting Information. The laboratory scale unit has the advantage of allowing good performance of the fixed bed experiments using a smaller amount of sample. In this way, it is suitable for the powder and pelletized material; however, the tablets had to be crushed and sieved in order to be used in the column. Fixed-bed experiments using UiO-66 tablets as provided by KRICT were conducted on the pilot scale unit. Another advantage of this unit is the possibility to test the adsorbent at higher temperature and pressure, closer to real industrial conditions. The material preparation and activation procedure will be described in subsequent subsections. Prior to any experiment, a blank experiment was conducted in both setups to determine the dead volume. The experimental results were corrected accordingly.

Figure 1. View of the UiO-66 tetrahedral and octahedral cages. The 3D optimized structure was generated with ChemBio3D Ultra from CambridgeSoft. Zirconium, oxygen, carbon, and hydrogen atoms are in dark gray, red, light gray, and white, respectively.

retention of alkanes and aromatic molecules in UiO-66 could be attributed to the rotational freedom of the adsorbed molecules inside the smaller cavity. Thus, we propose here to evaluate the potential application of the powder or shaped (as agglomerates or tablets) UiO-66 solid for xylene separation in the liquid phase by using fixed bed experiments on both laboratory and pilot scales. First, pulse experiments were performed on the laboratory unit in order to evaluate the behavior of UiO-66 at low concentrations. Then, binary breakthrough experiments were conducted on the laboratory scale to determine the adsorption isotherm at 313 K, using n-heptane as an eluent. Multicomponent breakthrough experiments were then carried out to calculate the corresponding selectivities. Finally, multicomponent breakthrough experiments were performed on the pilot scale unit at 313 K and 9 bars.

■

■

PACKED BED PREPARATION AND ACTIVATION Preparation of Packed Beds. Column A. A stainless steel column was packed with 0.45 g of UiO-66 powder (i.e., powder UiO-66, Figure 2a) as provided by ILV. The column properties are presented in Table 1. Column B. A stainless steel column was packed with the 1.97 g of shaped UiO-66. The shaped UiO-66 sample was prepared from the tablets supplied by KRICT. The tablets were crushed and sieved; the recovered fraction ranged from 300 to 500 μm (i.e., UiO-66 agglomerates, Figure 2b) and was used to pack the column. The column properties are disclosed in Table 1.

EXPERIMENTAL SECTION

Chemicals and Equipment. A UiO-66 powder sample was synthesized by Institut Lavoisier (ILV) as previously reported from a solution of 32.2 g of ZrOCl2.8H2O, 16 g of 1,4-benzenedicarboxylic acid, 500 mL of dimethylformamide (DMF), and 16 mL of HCl (37%) heated to 423 K under air for 24 h. (For more details, see the 5716

dx.doi.org/10.1021/la3004118 | Langmuir 2012, 28, 5715−5723

Langmuir

Article

were collected. Reverse breakthrough experiments were performed by switching the column feed from the xylene solution to n-heptane. The experimental protocol was repeated using the powder and the pelletized material under the same conditions. Similar experiments were performed on the pilot scale unit for further comparison using column C. Breakthrough and reverse breakthrough experiments were carried out at 313 K at a flow rate of 5.48 cm3/min. After the column was filled with n-heptane and thermal equilibrium was reached, the column feed was switched from the eluent to the xylene solution. During the experimental runs, samples were collected and analyzed according with the method described previously.

Figure 2. Photographs of the three samples: (a) powder as supplied by ILV, (b) 300−500 μm agglomerates fraction, and (c) tablets as supplied by KRICT.

Table 1. Geometry and Properties of the Columns Installed in the Experimental Units laboratory scale unit

a

Lcol Øcola madsa particle size SBETa

column A (powder)

column B (agglomerates)

column C (tablets)

12 0.46 0.45 650 ± 130 nm

11.8 0.77 1.97 300−500 μm

35.4 1.95 48.7 5 × 3 mmb

cm cm g

885

m2·g−1

1140

■

RESULTS AND DISCUSSION UiO-66 Tablet Characterization. UiO-66 tablets were characterized by SEM and EDS analysis. SEM images shown in Figure 3 reveal that UiO-66 tablets are agglomerates of around

pilot scale unit unit

Lcol − length of the column bed; Øcol − diameter of the column bed; mads − adsorbent mass; and SBET − BET surface area. bTablets with 5 mm length × 3 mm diameter a

Column C. A stainless steel column was packed with 48.7 g of shaped UiO-66. Tablets of UiO-66 (i.e., UiO-66 tablets, Figure 2c) were packed into the column as provided by KRICT. The column properties are shown in Table 1. Prior to the adsorption experiments, the material packed in the columns was heated under vacuum at 423 K for 20 h with a heating ramp rate of 0.5 K/min in order to remove the water present inside the pores. Experimental Procedure for Pulse Experiments. Pulse experiments were carried out in column B using the laboratory scale unit in order to evaluate the capability of UiO-66 for xylene isomer separation. All runs followed the same experimental protocol. Initially, the column was fed with eluent at a constant flow rate (1.0 cm3/min) at 313 K. A pulse of one of the xylene isomers was injected into the column using a loop of 20 μL. At the same time, the fraction collector program was started. All of the samples were analyzed with a Shimadzu gas chromatograph equipped with a WCOT-CP xylene fused silica capillary column of 0.53 mm diameter and 50 m length and flame ionization detector (FID). The analyses were performed using a column temperature of 308 K and an injector and detector temperature of 423 K. Helium was the carrier gas with a flow rate of 238.5 cm3/min and with a split ratio of 30. Shimadzu LCSolution software was used for graphic visualization and data acquisition. Experimental Procedure for Breakthrough Experiments. Breakthrough experiments were carried out on the laboratory scale unit to determine the adsorption equilibrium data in UiO-66 using n-heptane as the eluent. All experiments were performed at 313 K using a flow rate of 0.20 or 0.25 cm3/min, for powder (column A) or the agglomerate (column B) material, respectively. Initially, the column was fed with eluent until thermal equilibrium was reached. At this point, the column feed was switched to the xylene solutions and samples

Figure 3. Scanning electron microscopy images of UiO-66 tablets magnified (a) 45×, (b) 60×, (c) 500×, and (d) 1500×.

5 × 3 mm composed of polydistributed and faceted crystals. This observation is consistent with the technique used for shaping, a rotary press tabletizer, which usually leads to the crystal breaking. Two different zones (Z1 and Z2, Figure 3d) could be distinguished in the UiO-66 tablets. The composition of both regions was characterized by EDS, with the detection of almost no zirconium in region Z1, which might correspond to some residual graphite, and a richer zirconium content in zone Z2, supporting the presence of inorganic oxocluster Zr6O4(OH)4 from the UiO-66 phase (Supporting Information). Nitrogen sorption isotherms at 77 K for the UiO-66 tablet sample (Figure 4) and pore size distributions were determined, with an estimated BET surface area of 885 m2/g. Pulse Experiments. Pulse experiments were performed in column B, using agglomerates of UiO-66 (Table 1). Table 2 summarizes the operating conditions of the four pulse experiments. 5717

dx.doi.org/10.1021/la3004118 | Langmuir 2012, 28, 5715−5723

Langmuir

Article

Table 3. Molecular x, y, z, MIN-1, and MIN-2 Dimensions27 o-xylene m-xylene p-xylene

x

y

z

MIN-1

MIN-2

7.269 8.994 6.618

3.834 3.949 3.810

7.826 7.315 9.146

3.834 3.949 3.810

7.269 7.258 6.618

pulse (Figure 5b). One can then conclude that at low concentration (