Communication pubs.acs.org/IC
A Highly Stable Nanotubular MOF Rotator for Selective Adsorption of Benzene and Separation of Xylene Isomers Wei Huang,† Jun Jiang,† Dayu Wu,*,† Jun Xu,† Bing Xue,† and Alexander M. Kirillov‡ †
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis & Green Manufacturing Collaborative Innovation Center, School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, China ‡ Centro de Quimica Estrutural, Complexo I, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal S Supporting Information *
organic analogues and reversible adsorption and separation of C6−C8 arenes.18,19 Hence, we report the solvothermal synthesis, structural characterization, and topological features, as well as adsorption properties of [Zn(μ4-L)]n (Zn-MOF), which adopts a rare ptr topology and exhibits a high thermal stability.20,21 The crystal structure of Zn-MOF comprises four-coordinate Zn1 atoms with a tetrahedral {ZnO4} environment and deprotonated tetradentate μ4-L dicarboxylate ligands. These act as μ4-spacers with two μ2-η1:η1-COO groups being connected to four different Zn1 centers with the Zn−O bond lengths of ca. 1.92 Å, thus generating helical [Zn2(μ-COO)2]n 1D motifs (Figure 1a,b). These motifs are further arranged into a tubular
ABSTRACT: A remarkably stable tubular 3D Zn-MOF with hexagonal channels and a rare ptr topology was prepared under solvothermal conditions for liquid and vapor phase adsorption and separation of the C6−8 aromatic compounds. The material showed preferential affinity for benzene and can effectively separate benzene from its organic analogues under ambient conditions in both vapor and liquid phases. Furthermore, it exhibited preferable uptake of p-xylene over other C8 xylenes.
In recent years, metal−organic framework materials are attracting great attention due to their crucial applications in ion exchange, sensing, catalysis, and gas storage.1−4 In particular, self-assembled polycarboxylate-based MOFs often possess intricate nanosize channels that not only determine their topological features but also open up potential applications in selective adsorption and separation processes.5−8 Versatility in available pore and network topologies is much greater for MOFs in comparison with inorganic zeolites, thus offering opportunities to screen such materials for advanced applications. On the other hand, the three isomers of xylene, o-xylene (ox), m-xylene (mx), and p-xylene (px), together with ethylbenzene (eb) constitute the so-called C8 aromatic compounds, which are usually obtained as a mixture of isomers.9 Among them, p-xylene is the most valued component given its use in the large-scale manufacture of terephthalic acid, which is the basis for the polyester (PET) industry.10 The separation of px from mx, ox, and eb constitutes one of the most challenging issues due to their very close boiling points. Most industrial separations currently use the selective adsorption on zeolites.11,12 A very recent study reported the separation of xylene isomers by enclathration using the host−guest chemistry.13 Lusi and Barbour utilized the Werner host [Ni(NCS)2(p-phenylpyridine)4] to selectively enclathrate ox over mx and px from an equimolar ternary mixture and mx over px from a binary mixture of xylene vapors.14 Recently, Denayer’s group reported another emerging class of metal−organic framework materials to exhibit the vapor and liquid phase adsorption and separation of the C8 alkylaromatic compounds.15 However, nanotubular MOF materials with the enhanced selectivity toward a certain xylene isomer are still very limited and thus should be developed further.16,17 Just recently, Dong’s group developed nanoporous MOF showing an interesting adsorption selectivity toward benzene among its © XXXX American Chemical Society
Figure 1. (a) A MOF hexagonal nanotube in which the disordered phenyl ring resides. (b) Structural fragment view of a helical 1D strand of zinc centers via μ2-η1:η1-COO groups of μ4-L. (c) Structure of tubular 3D metal−organic framework with the disordered phenyl groups omitted for clarity. Zn, purple; C, cyan; O, red.
infinite 3D metal−organic framework (Figure 1c), which reveals the formation of hexagonal nanotubular channels when viewing along the c axis.23 The dimensions of the channels running down the c axis are approximately 14.8 Å (face to face) × 18.7 Å (corner to corner). After contracting the μ4-L moieties to their centroids, the resulting underlying net (Supporting Information, Figure S2) is assembled from the 4-connected Zn1 and μ4-L nodes.24−26 This binodal 4,4-connected net can be topologically described by the Received: July 15, 2015
A
DOI: 10.1021/acs.inorgchem.5b01581 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry point symbol of (43.62.8), revealing a very rare ptr topology according to the classification by the Reticular Chemistry Structure Source.24,25 Thermogravimetric analysis (TGA) confirmed the negligible loss of the solvent molecules on heating Zn-MOF up to 420 °C in air (Figure 2a). High thermal stability of Zn-MOF was also
Figure 3. 1H NMR spectra recorded using the CDCl3 extract from ZnMOF that was subjected to prior adsorption of the liquid phase containing benzene, the equimolar mixture of o, m, p-xylene isomers, and each single component, respectively.
Figure 2. (a) TGA trace and (b) variable temperature PXRD plots of Zn-MOF.
investigated by powder X-ray diffraction (PXRD) analyses (Figure 2b). Zn-MOF can abruptly adsorb 3.58 cm3 (STP)/g of CO2 from 6.3 × 10−6 to 0.01 atm, and then it starts to adsorb CO2 gradually and reaches 29.2 cm3 (STP)/g (5.73 wt %) at 0.98 atm (Supporting Information, Figure S3). Furthermore, the desorption isotherm does not coincide with the adsorption one. In fact, Zn-MOF shows almost no desorption until the pressure inferior to 0.73 atm and then starts to gently desorb CO2, resulting in a significantly large adsorption hysteresis.28,29 The adsorbed CO2 is not completely released even at near zero atmosphere pressure, indicating a strong interaction between CO2 and the Zn-MOF framework at 196 K. The above stepwise adsorption behavior and large adsorption hysteresis demonstrate that the nanotubes in Zn-MOF are dynamic. Given nanosize dimensions of the tubular channels in ZnMOF, it can be suitable for adsorption of aromatic organic compounds. To prove this possibility, we immersed the crystals of Zn-MOF in benzene, toluene, ethylbenzene, and o-, m-, and pxylene, respectively. The 1H NMR spectrum was first directly performed on the as-synthesized Zn(II)-MOF and gave no obvious signals corresponding to any guest molecules. The guestincorporated solid samples were then treated with d-CHCl3 to extract the adsorbed aromatic guest molecules on the basis of solvent stability (Supporting Information, Figures S4,S5). The corresponding 1H NMR spectra indicate that Zn-MOF is able to reversibly uptake benzene and m- and p-xylene.30 At room temperature, the adsorption amount was quantatively determined on the basis of 1H NMR spectra as 1.08 mg/g, 1.79 mg/g, and 1.04 mg/g for benzene, p-xylene, and m-xylene, respectively. (Supporting Information, Figures S6−S9). It is observed that the adsorption amount was enhanced as 4.14 mg/g for p-xylene at 40 °C. No signals were detected for toluene, ethylbenzene, or oxylene even after immersing Zn-MOF in them even for a week, indicating these molecules could not steadily stay in the pores (Figure 3, ESI, Figures S10,S11) Furthermore, adsorption competition was undertaken by immersing Zn-MOF in binary components, i.e., benzene/toluene and benzene/xylene in liquid phases, and the subsequent 1H NMR data clearly indicated ZnMOF can effectively separate benzene from toluene or xylene (Supporting Information, Figures S12−S15). It is known that the separation of a certain isomer from the xylene mixture based on distillation is very difficult due to quite similar boiling points. To check whether Zn-MOF is selective for
the uptake of a specific xylene isomer, a competition experiment was designed. Zn-MOF was immersed in a ternary system consisting of equimolar amounts of o-, m-, and p-xylene, and kept undisturbed at room temperature for a week. The guest-loaded solid sample was then extracted by d-CHCl3, which is subject to 1 H NMR detection, clearly evidencing that p-xylene is adsorbed by Zn-MOF as the predominant species. Such a separation performance in the liquid phase by Zn-MOF is further supported by the gas chromatography (GC) experiments. The result showed that p-xylene is dominant in the extract obtained from the liquid phase adsorption (Supporting Information, Figure S16), thus reflecting a possible guest size- and shape-controlled adsorption selectivity of Zn-MOF. Furthermore, the uploaded guest molecules can be easily removed by extracting with CHCl3 to regenerate the guest-free framework, which can be used for adsorption for the next recycle (Supporting Information, Figure S17). Notably, Zn-MOF maintains its structure during the adsorption−desorption process, which is consistent with the corresponding XRPD patterns (Supporting Information, Figure S18). To check whether there is a possibility that vapor phase xylene isomers can be selectively adsorbed on Zn-MOF, a standard static vapor phase adsorption experiment with the saturated concentrations of xylenes revealed (Supporting Information, Figure S19). A preferable adsorption of px over mx and ox in the high-pressure region, agreeing with the 1H NMR observation in the liquid phase. To experimentally obtain the dynamic parameters related with the free rotation of the anchored phenyl ring in Zn-MOF, we performed variable-temperature single-crystal X-ray diffraction studies from room temperature to 120 K. With the decreasing temperature, the dihedral angle between two disordered phenyl rings continues to decline (Figure 4). Interestingly, X-ray diffraction analysis revealed that the rotation disorder is maintained even after lowering the temperature to 120 K and the dependence of the relative rotational angle (α) vs temperature obeys the Arrhenius law, α(T) = α0 exp(−E/kBT). The best fit gives the limit dihedral angle, α0 = 103.5(7)°, and the rotational barrier, E/kB = 20.4(8) K (0.17 kJ/mol). This rotational barrier is much inferior to those of ∼8.2 and ∼20.2 kJ/mol calculated for pyrimidine and phenyl rings in UTSA-76a B
DOI: 10.1021/acs.inorgchem.5b01581 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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Figure 4. Variation of the dihedral angle between two disordered phenyl rings as the central rings of the Zn-MOF are rotated around the symmetry axis of the molecule; data obtained from variable temperature X-ray diffraction analysis.
and NOTT-101a MOFs, respectively,31 thus indicating that phenyl rings in Zn-MOF are significantly more dynamic. In conclusion, we have prepared and structurally characterized a novel thermodynamically stable nanotubular Zn-MOF derived from biphenyl-3,5-dicarboxylic acid with notable structural, topological, and adsorption features. Hence, this rather simple carboxylic acid can be applied as a useful but still very poorly explored building block for the design of tubular metal−organic frameworks with uncommon topologies.32 In fact, the described herein Zn-MOF adopts a very rare ptr topology, thus contributing to the topological classification of MOFs. It is noteworthy that this tubular MOF material is very stable and can retain its single crystallinity up to ca. 400 °C. Furthermore, it exhibits a preferable affinity for p-xylene allowing its selective uptake and separation among other xylene isomers and C8 alkyl aromatic compounds. The results deserve further exploration, namely toward engineering new functional metal−organic materials with enhanced adsorption characteristics.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01581. Additional experimental details and spectra (PDF) Crystallographic information file (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the financial support by PAPD and NSFC programs (21371010 & 21471023). REFERENCES
(1) Liu, T.; Li, D. Q.; Wang, S. Y.; Hu, Y. Z.; Dong, X. W.; Liu, X. Y.; Che, C. M. Chem. Commun. 2014, 50, 13261−13264. C
DOI: 10.1021/acs.inorgchem.5b01581 Inorg. Chem. XXXX, XXX, XXX−XXX
