Separation of Xylene Isomers through Selective Inclusion: 1D → 2D

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Separation of Xylene Isomers through Selective Inclusion: 1D → 2D, 1D → 3D, and 2D → 3D Assembled Coordination Polymers via β‑Sheets Karabi Nath and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: The separation of xylene isomers has been achieved through sequential and selective crystallization of coordination polymers of Cu(II) with flexible bis(pyridylcarboxamide) ligand (L). The order of preference for inclusion induced crystallization was shown to be o-xylene > m-xylene > p-xylene. Although all three xylenes included CPs having distinct differences in terms of their crystal structures, they all have exhibited a tendency to assemble via β-sheet hydrogen bonds to form 3D architectures containing channels. The preferential inclusion of isomers of xylenes was confirmed by single crystal X-ray, 1H NMR, and GC. The bulk purity of xylenes was also confirmed by XRPD patterns.

demands of guest molecules.34,35 However, very few flexible coordination frameworks have ever been applied to carry out separations of small molecules.36−39 The incorporation of the flexible and hydrogen bonding components into the organic struts provides diversity in terms of topology and control of the overall geometry of the three-dimensional structure of CPs.40−45 It was shown by us earlier how β-sheet hydrogen bonds can be used to enhance dimensionality of the CPs via various assembly modes.46−49 The amenability of these CPbased materials in terms of overall architecture, using diversified assembling of CPs through hydrogen bonds, between the framework and within the framework, using the flexibility of the component, was expected to result in dynamic CPs with competitive guest inclusion, which in turn paves the way for separation of small organic molecules.50,51 Here we would like to present our study on separation of xylenes using CPs of ligand L52 (Scheme 1) which contains hydrogen bonding functionalities as well as flexibility in the form of CH2 groups. Interestingly, the CPs of L with Cu(II) were found to have an ability to include all three xylenes through a common mode of self-assembly via β-sheet hydrogen bonds. We note here that all three xylene isomers have shown an ability to template coordination networks with distinct crystal structures but with preferential order of crystallization. The reaction of L (29.6 mg, 0.1 mmol) with Cu(ClO4)2 (18.5 mg, 0.05 mmol) in MeOH (4 mL) resulted in the formation of single crystals of complex {[Cu(L)2(H2O)]· (ClO4)2}n, 1. The crystal structure analysis of 1 reveals that it

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xploration of coordination polymers (CPs) for gas storage,1−3 separation,4−7 adsorption,8−12 and catalysis13−15 applications has attracted immense interest over the decades, owing to their crystalline nature, flexibility, and amenability for fine-tuning in terms of size, shape, and nature of the cavities. Several recent reports focus upon the utility of CPs/MOFs in the separation of small molecules or isomeric mixtures.16−20 Generally, crystallization and adsorptive based separation techniques are preferred over the traditional industrial methods due to their simple, cost-effective, and greener routes. In this context, separation of xylene isomers holds one of the most challenging aspects in industry due to their very similar boiling points. The purity of these isomers is very important as they are the main ingredients for some industrial preparations and also for organic synthesis. For example, p-xylene is utilized in the polyester industry for the preparation of poly(ethylene terephthalate) derivatives, while oxylene leads to the formation of phthalic anhydride, used as plasticizers, and finally m-xylene produces derivatives of isophthalic acid, utilized in PET resin blends. Thereby, separation of these isomeric mixtures occupies a great industrial incentive. Either fractional crystallization, relying on a difference in their freezing points, or adsorption based on selective host−guest interactions21−24 meets the necessities of industrial separation. Recently, several rigid MOFs and hydrogen-bonded supramolecular assemblies have successfully administered the separations of xylene isomers, essentially by utilizing the molecular sieving phenomena and selective inclusion criteria, respectively.25−33 Flexible CPs are considered to be better adsorbents than rigid and robust analogues given their adaptive nature and selective fine-tuning of the cavities/channels depending on the © XXXX American Chemical Society

Received: September 2, 2016

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DOI: 10.1021/acs.cgd.6b01311 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Scheme 1. Molecular Structure of L

Figure 2. Illustrations for the crystal structure of 2: (a) 2D layer of (4,4)-topology with rhomboidal cavities containing o-xylene guest molecules (pink, space-filling mode) and ClO4− ion (green, cylinder mode). (b) Offset packing of the 2D layers in both a- and b-axes. Alternate layers are shown with different colors. ClO4− ion and columns of o-xylene guest molecules present in four equal channels. (c) Representation of β-sheet hydrogen bonding interactions between the 2D layers.

Figure 1. Illustrations for the crystal structure of 1: (a) 1D chain with twisted Cu2L2 macrocycles. (b) Two adjacent 1D chains interdigitate into one another in an alternate fashion generating a loop which subsequently generates a 2D layered structure. (c) Representation of β-sheet hydrogen bonding interactions between two such interdigitated 1D chains, shown in the box of panel (b).

independent L units, two Cu(II) ions, two coordinated water molecules, and four anions along with solvent molecules (H2O/ MeOH). Solvent and anions could not be located (44.5% of the crystal volume) due to heavy disorder.

crystallizes in P21/c space group (Table S1), and the asymmetric unit is composed of four crystallographically B

DOI: 10.1021/acs.cgd.6b01311 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. Illustrations for the crystal structure of 3: (a) 1D chains with rectangular cavities. (b) 3D packing of 1D chains along b-axis and column of m-xylene guest molecules (pink, cylinder mode) present in the larger channel A and ClO4− ion (green, cylinder mode) present in the smaller cavity B. (c) Representation of β-sheet hydrogen bonding interactions between the adjacent 1D chain, shown in the box of panel (b).

Figure 4. Illustrations for the crystal structure of 4: (a) 2D layer of (4,4)-topology with rhomboidal cavities containing p-xylene guest molecules (pink, space-filled mode) and ClO4− ion (green, cylinder mode). (b) Offset packing of the 2D layers in c-axis. Alternate layers are shown in different colors. ClO4− ion and columns of p-xylene guest molecules present in A and B channels, respectively. (c) Representation of β-sheet hydrogen bonding interactions between the 2D layers, shown in the box of panel (b).

Both Cu(II) ions adopt a square pyramidal geometry where the equatorial sites are occupied by four ligand molecules, and the axial position is occupied by the water molecule. The geometries of the ligands are found to be convergent in terms of the placement of the N atoms of the pyridine groups. This type of coordination and geometry of the ligand resulted in the formation of 1D chains containing M2L2 macrocycles. The 1D chains are nonplanar, as the adjacent macrocycles are perpendicular to each other and these chains are assembled to a 2D layer via β-sheet hydrogen bonding (tetramers of L) which occurs through the interdigitation of two M 2 L 2 macrocycles.

The 2D layers have cavities of dimensions 19.3 × 11.3 Å2 which pack on each other such that there are continuous channels (Figure 1). The presence of continuous channels in CP 1 prompted us to study the adsorptive based separation via guest encapsulations. Accordingly, CP 1 was immersed into ternary and binary equimolar mixtures of xylene isomers. However, the process was rendered ineffective as the apo-host C

DOI: 10.1021/acs.cgd.6b01311 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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two structures: (1) The Cu2L2 macrocycles in 3 are planar, which makes larger cavities with higher Cu(II)···Cu(II) distances (16.1 v 11.5 A). (2) The ligands are perpendicular to the plane of Cu2L2 macrocycle in 3 which makes more room available for guest molecules, whereas they are into the cavities of Cu2L2 in 1. (3) In 1 the macrocycles interdigitate which occupies the channels, whereas in 3 they interact with each other in side-on manner. Interestingly, both structures contain β-sheet hydrogen bonds but they differ in assembling the networks. In 1, discrete sheets assemble 1D → 2D. whereas in 3, there are infinite sheets which result in 1D → 3D assembly. The 3D network contains two types of channels (A and B): A are huge in size and occupied by m-xylene molecules which are in disorder, while B are smaller and contains exclusively ClO4¯ ions that are ordered (Figure 3). The presence of only m-xylene in 3 was further confirmed by GC (Figure S10) and 1H NMR analysis (Figure S11). The p-xylene included crystals of {[Cu(L)2(H2O)2]·(H2O)· (ClO4)2·(p-xylene)4}n, 4, were obtained only in the absence of other two xylenes indicating its lowest priority for inclusion. The complex 4 contains similar network features as that of 2 with differences in assembling which resulted in a space group Ccca (2, Pnnm) (Table S1). In complex 2, the L and o-xylene exhibit disorder, whereas in 4, the units of L and p-xylene are in order and also the overall cell volume is found to be larger in 4 than that of 2. Interestingly, the packing of these 2D layers resembles that of 1D chains in 3 with m-xylene (Figure 4). The major difference between 2 and 4 is that the layers are offset on both axes (xand y-axis) in 2 creating four equal channels which are occupied by anions and o-xylenes. Complexes 2−4 were subjected to thermogravimetric analysis to analyze guest release and verify thermal stability (Figure 5). TGA data were recorded with a PerkinElmer instrument, Pyris Diamond TG/DTA at a heating rate of 5 °C/ min. Complex 2 was found to exhibit gradual weight loss of 22.7% up to 320 °C which is attributed to o-xylene and coordinated H2O, whereas stepwise weight loss was observed for complexes 3 and 4 up to 312 °C. Interestingly, the first step of weight loss corresponds to the lattice water molecules that occur up to 175 °C: 2.9% (calc. 2.73%) in 3 and 4.92% (calc. 4.04%) in 4. The second weight loss of 31% and 30.58% in 3 and 4, respectively, is in agreement with the loss of m-xylene (calc. 32.2%) in 3 and p-xylene in 4 (calc. 31.8%). Crystallization experiments performed in the presence of isomeric xylene guest molecules emphasizes the role of “sequential inclusion phenomenon” as a tool for separation. It has been observed from the above studies that, whenever competitive crystallizations were conducted in the presence of an isomeric mixture solution of xylenes, with o-xylene as one of the components, 2 was found to crystallize out, irrespective of the other two isomers in the mixture. The selective inclusion of o-xylene was observed even when the reaction was conducted with less o-xylene (1 mL) and higher amounts of the other two isomers (up to 4 mL each). Similarly, from the binary mixture of m-xylene and p-xylene, the inclusion of m-xylene, i.e., formation of 3, was exclusively observed from the solution that contains m- and p-xylenes in 1:2 ratio. The in situ formed crystal lattice cavity revealed its lowest priority for p-xylene, which was obtained only in a separate crystallization experiment performed in the absence of both oxylene and m-xylene. The same priority sequence for isomeric xylene molecules has also been reported in earlier work by

Figure 5. Thermogravimetric analysis of 2, 3, and 4.

of CP 1 failed to adsorb any of the isomers of xylene, and therefore, separation of the isomers was tried via a competitive crystallization method. In this process, the ternary or binary equimolar mixture of xylene isomers was added to the methanolic solution of L and Cu(ClO4)2 and left to crystallize through slow evaporation. Interestingly, depending on the presence of mixture of isomers, it was found that the CPs of L and Cu(ClO4)2 can include all three isomers in an exclusive manner. For example, the presence of the ternary mixture resulted in crystals of CP 2 with inclusion of o-xylene. In a similar manner, m-xylene included crystals of CP were obtained from the binary mixture of mxylene and p-xylene. These results indicate the competitive inclusion of xylene isomers by the CPs of L and Cu(II) which exemplifies both the dynamic as well as the selective nature of the ligand in its CPs which can be effectively used for their separation. The reaction of methanolic solution (4 mL) of L (29.6 mg, 0.1 mmol) and Cu(ClO4)2 (18.5 mg, 0.05 mmol) in the presence of ternary equimolar mixtures (1 mL each) of xylene isomers resulted in the formation of complex {[Cu(L)2(H2O)2]·(ClO4)2·(o-xylene)2}n, 2. Single crystal X-ray diffraction studies revealed that complex 2 has a 2D network with [4,4] topology, unlike 1 which contains 1D network. The presence of only o-xylene in the resultant bulk material was confirmed by GC (Figure S8) and 1H NMR analysis (Figure S9), and PXRD also confirmed the phase purity of the material (Figure S2). In both 1 and 2, the metal is 4-coordinated with respect to ligand; however, the geometry of the ligand resulted in the differences in the network dimensionality. In 2, the ligand is divergent, which makes it to form 2D, whereas as described above it exhibits a convergent geometry in 1 to form 1D. Accordingly, within the network, the complex 2 has much bigger cavities than those in 1 and the 2D networks pack on each other in an offset fashion to form a 3D network through βsheet hydrogen bonds. The 3D network contains continuous channels in which the columns of o-xylene molecules reside (Figure 2). The above reaction was repeated using the binary mixture of the remaining two xylenes which resulted in the inclusion of only m-xylene in single crystals of the complex, {[Cu(L)2(H2O)2]·(ClO4)2·(m-xylene)4}n, 3. The crystal structure of 3 also contains 1D coordination network similar to 1; however, there are many significant differences between the D

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Barbour et al. with Werner coordination complexes.26 Further, Nassimbeni et al. also reported a similar preference for inclusion of isomeric xylenes by polyaromatic organic hosts.7f We note that the results reported here are the first of its kind using coordination polymers or MOFs for selective crystallization based separation. However, it can also be noted that recently MOFs/CPs have been used to separate xylene isomers via pure adsorption based separation, but not on the basis of selective crystallization. Furthermore, these studies also highlight the importance of β-sheet hydrogen bonding in assembly of 1D and 2D coordination networks, which creates an opportunity for the guest molecules to fine-tune the overall network according to their size and shape requirements. We note here that formation of a discrete tetrameric aggregate via β-sheet hydrogen bonds is also the first of its kind.

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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.cgd.6b01311. Experimental details, IR spectra, XRPD patterns, and other experimental data and calculation (PDF) Accession Codes

CCDC 1481292−1481295 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-3222282252. Tel: +91-3222-283346. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge DST (SERB), New Delhi, India for financial support, DST-FIST for the single crystal X-ray diffractometer. KN thanks IIT-KGP for research fellowship. We also thank Prof. M. Bhattacharjee, Department of Chemistry, IIT Kharagpur, India, for GC measurements.

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