Direct Structural Evidence of Molecular Packing Effects of Xylene

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Direct Structural Evidence of Molecular Packing Effects of Xylene Isomers Adsorbed in BIF-20 Richelle Lyndon, John Bacsa, Melinda L. Jue, and Ryan P Lively Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01744 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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Crystal Growth & Design

Direct Structural Evidence of Molecular Packing Effects of Xylene Isomers Adsorbed in BIF-20 Richelle Lyndon†, John Bacsa‡, Melinda L. Jue†, and Ryan P. Lively†* †

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst

Drive NW, Atlanta, Georgia 30332, United States ‡

School of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive,

Atlanta, Georgia 30332, United States

Boron Imidazolate Framework, Metal-Organic Framework, Xylene Isomers, Hydrocarbon, Diffusion, Adsorption, Separations

ACS Paragon Plus Environment

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ABSTRACT: Improved xylene separation performance through selective sorption of paraxylene on an industrial scale could significantly reduce the energy cost associated with purification and recovery. Understanding the host-guest interactions can aid in the design of more selective sorbent materials. However, only a few—primarily computational—metalorganic framework crystal structures containing xylene isomers have been reported. Here, we report a boron imidazolate framework material that displays local framework flexibility, with high xylene uptake and para-xylene/ortho-xylene single component diffusion selectivity of 3. Single-crystal X-ray diffraction studies show the different molecular packing arrangements between the xylene isomers, revealing the presence of steric effects in the ortho-xylene adsorption. These results provide direct structural insight into flexibility-assisted xylene sorption within the framework and the location of guest binding sites, which could help contribute to the design of a flexible system for selective separation applications.


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INTRODUCTION The separation of xylene isomers; ortho-, meta-, para-xylene (oX, mX, pX); is one of the most important and challenging processes in the petrochemical industry.1 Conventional methods employ a simulated-moving-bed (SMB) process using a zeolite adsorbent material. However, this approach involves slow, energy-intensive, and multi-step processing and the xylene uptake capacities of zeolites are relatively low.2 In particular, there is much interest in isolating the para-isomer due to its commercial utility for the manufacture of a variety of products.3 Metalorganic frameworks (MOFs) have promising applications in gas storage and separation,4-5 catalysis,6 and drug delivery7 due to a large variety of possible structures and properties.3,

8-11

Some zeolitic imidazolate frameworks (ZIFs, zeolite-like MOFs), have demonstrated local framework flexibility, allowing for the passage of guest molecules much larger in size than their crystallographic pore apertures without resulting in considerable changes to the crystal dimensions.11-15 In the field of gas separations, MOF framework flexibility can be advantageous as it results in surprising molecular sieving effects without the need to tailor the pore size.11 Very few MOFs display xylene separation capabilities with para-xylene preference, especially those with flexible structures.3, 16-21 A combination of packing effects and molecular sieving effects can enhance both the sorption and diffusion selectivity.20, 22-23 However, obtaining direct structural information about MOF dynamics and host-guest behavior is challenging due to the weak guest affinity or poor sample crystallinity after guest exchange.3, 10, 24-25 Oftentimes, this must be resolved through a combination of diffraction studies and other characterization methods.20-22,

26-33

For example, the para-xylene selection mechanisms in ZIF-8 and MAF-X8

have been indirectly characterized via PXRD data and computational study, respectively.21-22 Deeper understanding of the MOF host-guest interactions at the atomic scale can aid in the pre-


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selection of building blocks to engineer next generation MOFs for targeted applications such as xylene isomer separation.34 Boron imidazolate frameworks (BIFs) are another class of zeolite-like MOFs that are based on boron-imidazolate complexes.12, 35 Like some zeolite-type MOFs, we believe that some BIFs may also exhibit local flexibility, although, no evidence of BIF flexibility and pX selectivity has been reported to date. BIF-20 (Zn2(BH(mim)3)2(obb); mim = 2-methyl imidazolate, obb = 4,4′-oxybis(benzoate)) was reported as a static MOF (i.e., not flexible) with no reports on the hydrocarbon sorption capability.36 The framework features α and β cages (12 and 16 Å diameters, respectively) with a narrow, 1D interconnected channel system (3.1 Å) (Figure 1).36 Importantly, the co-ligand obb has been reported to display some flexibility within other coordinated frameworks.36-38 The small pore opening and the potential framework flexibility of BIF-20 could be advantageous for diffusion-based molecular separations. Herein, we report the first flexible BIF material that displays uncommon diffusive selectivity for xylene isomers and have successfully resolved the xylene molecules within the BIF-20 lattice. Single-crystal X-ray diffraction (SCXRD) studies were conducted to understand the structure-property relations in a guest-host system, which reveals BIF-20’s flexible pore environment that can locally rearrange to accommodate different guest molecules with varying steric hindrance. We observe the difference in molecular packing arrangements of the xylene isomers and fractional guest binding at the adsorption sites, providing direct structural insights underpinning their sorption processes. Adsorption studies show that BIF-20 can adsorb a variety of hydrocarbon molecules much larger than its crystallographic pore size, with no sharp size cutoff. The uptake capacities of ethylene, ethane, and propane at 273 K and 1 bar are the highest ever recorded for BIF materials to date.


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Figure 1. (a, c) Structural representation of BIF-20 along the b and c axes. The α and β cages are depicted by the orange and red spheres, respectively. (b) SEM image of a large BIF-20 crystal. Scale bar: 100 µm. (d) Comparison of the critical pore dimension of BIF-20 versus the molecular size of pX and oX molecules with kinetic diameters of 5.8 Å and 6.8 Å, respectively. EXPERIMENTAL SECTION Chemicals and Materials. All chemical reagents were used as purchased without purification: zinc nitrate hexahydrate (Zn(NO3)2 6H2O, 99%, Alfa Aesar), 4,4’-oxybis(benzoic acid) (4,4’-obb, 99%, Sigma Aldrich), sodium borohydride (NaBH4, 98%, Alfa Aesar), 2methylimidazole (2-mim, 97%, Alfa Aesar), 2-amino-1-butanol (97%, Sigma Aldrich), N,Ndimethylformamide (DMF, >99.8%, Alfa Aesar), 1,3,5-triisopropylbenzene (>95%, TCI) and acetonitrile (>99.5%, Alfa Aesar). Nitrogen (UHP), carbon dioxide (99.999%), methane (UHP), and ethane (99.99%) were purchased from Airgas. Ethylene (99.99%) was purchased from


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Matheson, and propane (99.5%) and n-butane (99.5%) were purchased from Tech Air. Paraxylene (99%) and ortho-xylene (99%) were purchased from Alfa Aesar. Synthesis of Na[BH(mim)3] linker (mim = 2-methylimidazolate). The linker synthesis was adapted from previously reported procedures (Figure S1).39 In a 50 mL round bottom flask, a mixture of NaBH4 (0.108 g, 2.85 mmol) and 2-mim (0.698 g, 8.50 mmol) was heated and stirred under a nitrogen atmosphere at 200 °C for 24 h, and then cooled to room temperature. The resulting off-white solid was first soaked and washed with dry acetone (3 x ~10 mL) and then dry toluene (3 x ~10 mL). The crude product was vacuum filtered and dried at 80 °C under vacuum, yielding Na[BH(2-mim)3] in 72% (0.572 g) yield. 1H NMR (D2O, 400 MHz): δ (ppm) 2.22 (s, 9H), 6.47 (d, 3H), 6.93 (d, 3H). Synthesis of small BIF-20 crystal (~150 µm). BIF-20 crystals were synthesized using a slightly modified method from the literature (Na[BH(mim)3] was used instead of K[BH(mim)3]).36 In a 60 mL vial, a mixture of Zn(NO3)2·6H2O (1.194 g, 4.01 mmol), Na[BH(mim)3] (0.32 g, 1.15 mmol) and 4,4’-obb (0.518 g, 2.01 mmol) was dissolved in 28 mL of a 2-amino-1-butanol/acetonitrile/DMF solvent mixture (in a 3:1:3 ratio, respectively). The reaction mixture was heated to 80 °C for 4 days, and then slowly cooled to room temperature. The resulting off-white crystals were washed with distilled water (3 x ~20 mL) and ethanol (3 x ~20 mL), yielding BIF-20 in 54% yield (0.295 g). Synthesis of large BIF-20 crystal (~300 µm). Larger BIF-20 crystals were synthesized using a modified method.36 In a 20 mL vial, a mixture of Zn(NO3)2·6H2O (0.302 g, 1.02 mmol), Na[BH(mim)3] (0.082 g, 0.29 mmol) and 4,4’-obb (0.066 g, 0.26 mmol) was dissolved in 3.5 mL of a 2-amino-1-butanol/acetonitrile/DMF solvent mixture (in a 3:1:3 ratio, respectively). The


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reaction mixture was heated to 80 °C for 8 days, and then slowly cooled to room temperature. The resulting off-white crystals were washed with distilled water (3 x ~10 mL) and ethanol (3 x ~10 mL), yielding BIF-20 in 57% yield (0.0458 g). Scanning Electron Microscopy (SEM). High-resolution images of the BIF-20 crystal size and morphology were acquired using a Hitachi SU8230 Scanning Electron Microscope with accelerating voltage of 1-2 kV and emission current of 5 µA at a working distance of 7-9 mm. The samples were prepared by dispersing the material onto double-sided conductive copper tape attached to a flat aluminum sample stub. ImageJ was used to analyze the crystal size distribution. The SEM images were converted to high contrast black and white images before the diameter of each crystal in the image was calculated. 1

H NMR analysis. 1H NMR analysis was performed on a Bruker Avance III 400 NMR

spectrometer. The BIF-20 linker, Na[BH(2-mim)3], was dissolved in D2O before analysis. Gas Adsorption Measurements. Single component gas adsorption isotherms of BIF-20 were measured with an ASAP 2020 (Micromeritics). The samples were evacuated and activated at 150 °C for 12 h under vacuum. No N2 adsorption was observed at 77 K, which contradicts reported measurements, suggesting that perhaps there may be disorder in the crystal structure synthesized in this work; a possible origin of this observation is supported in the SCXRD results. Organic Vapor Sorption Measurements. Single component vapor sorption isotherms of activated BIF-20 were measured with a VTI-SA+ (TA Instruments). The samples were dried in situ at 110 °C for approximately 12 h under flowing nitrogen. Diffusion studies were conducted at a relative pressure change from 0.000 to 0.025 at 55 °C using the large (~300 µm) BIF-20 crystals. Single component vapor sorption isotherms were collected up to a relative pressure of


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0.7 at 55 °C using crushed BIF-20 crystals to minimize analysis time. The equilibrium criteria for each step was set for mass changes less than 0.001 wt % over a 25 min period. During flowing gravimetric uptake measurements, the relative pressure changes are not instantaneous step changes, and require the use of an exponential boundary condition for the Fickian solution to transient sorption in a sphere.40 For a variable surface concentration, C(t), = 1 − exp−

(1)

where the final surface concentration is given by C0, and the approach to equilibrium by the inverse time constant β. The normalized total uptake of solute within the system, Mt, is given by:

= 1 − − 1 −

"

! #$

"

! %+

'

∑2 .34

)*+,-. /⁄ 1 . . - ⁄

(2)

determined by the transport diffusion coefficient D and the crystal radius a. The normalized experimental data is fit to Equation 2 to estimate D and β for a given a. The ideal diffusion selectivity was obtained by taking the ratio of the pure component diffusion coefficients for each solvent. Kinetic Batch Adsorption. Approximately 80 mg of activated crushed BIF-20 sample was added to an equimolar mixture of pX (0.2678 g) and oX (0.2671 g) diluted in 1,3,5triisopropylbenzene (7.941 g) in a 20 mL vial at room temperature. The larger 1,3,5triisopropylbenzene molecule was chosen as the solvent to minimize its adsorption in BIF-20. A


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small amount (70 µL) of the mixture was transferred into a 2 mL vial and diluted with ethanol (280 µL) for GC analysis. For imperfect oX exclusion, where pX:oX is 3:1 (based on diffusion selectivity) and approximately 30 wt% of xylene sorption, the following equation was used to estimate the mass uptake: 56,8 × :6 = 5+6,; + 5=2σ (I)] Final R indexes [all R1 = 0.0588, wR2 = R1 = 0.1081, wR2 = R1 = 0.1236, wR2 = 0.1479 0.2974 0.3864 data] 1

The sample was activated at 150 °C under vacuum prior to measurements and/or xylene

immersion.


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Figure 4. (a, b) Close up image of the boron imidazolate linker (linker A) featuring both types of disordered groups with type 1 and type 2 mim-A highlighted in red and green, respectively, and (c, d) a structural representation of linker A with only type 1 or type 2 mim-A along the c axis, respectively. For a type 1 mim-A disordered group, the boron is coordinated to an imidazole ring with its methyl group pointing away from the cavity (Figure 4a, c). For a type 2 mim-A, the imidazole ring is flipped ~180° with respect to the orientation of a type 1 mim-A so the methyl group is protruding into the cavity (Figure 4b, d). For all the single crystal structures analyzed (synthesized using the literature method and modified method), the refined occupancies of the two disordered groups were found to be consistent, with the type 1 group as the major orientation for mim-A (exact ratios reported in the Supporting Information). This suggests static disorders in the native state (Figure 4a, c). This is unlike the previously reported BIF-20, where the


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orientation of the imidazolate linkers are similar to the type 2 disordered group in this work, but with 100% occupancy.36 It is still unclear what mechanism is responsible for the formation of disordered BIF-20. However, the calculated pore sizes of BIF-20 with only type 1 or type 2 disordered groups is not significantly different from those reported in literature, which are 2.4 Å and 2.9 Å, respectively (Figure S11 and S12).36,

50-51

The smaller pore size of type 1 mim-A

(major orientation) may contribute to the absence of cryogenic nitrogen uptake in this work (the structure is expected to have significantly reduced flexibility at cryogenic temperatures). Importantly, both structures present similar solvent-accessible voids (~44%). The higher affinity of BIF-20 towards longer chain hydrocarbons, as observed in the adsorption isotherms, could be further reinforced through π -π and CH-π interactions with the aromatic imidazole rings of the type 1 mim-A (major orientation) concentrated in the cavity (Figure 3). Molecular packing arrangements of xylene isomers in BIF-20. Close inspection of the SCXRD data reveals structural information about the xylene adsorption sites and their molecular packing arrangements within the crystal. To avoid confusion, type 2 mim-A (minor orientation) is excluded in the single crystal structures (Figures 4-7). The activated BIF-20 single crystals were immersed in either xylene isomer at room temperature prior to data collection. It is also worth noting that the amount of xylene molecules sorbed into the BIF-20 crystals may not be exactly the same due to unavoidable solvent loss to the atmosphere before and during XRD measurements. Therefore, the amount of pX and oX molecules in the single crystal structures are not directly compared in this work. Structural analyses of pX@BIF-20 showed that the pX molecules could be well resolved, whereas the oX molecules in oX@BIF-20 were more disordered in the voids, suggesting weaker oX-host interactions (Figure S9 and S10). Chemical restraints were applied for oX@BIF-20 and pX@BIF-20 due to disorder and thermal motion of


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the bound xylene molecules. However, the position and orientation of xylene molecules were corroborated by the difference electron density maps (Figure S9-10). Two pX locations were identified in the pX@BIF-20 structure: site 1 (pX1) is located in the center of one of the crystal void spaces (void 1) (Figure 5a, c, d); and site 2 (pX2) is located (void 2) (Figure 5a, c). Interestingly, the pX1 molecules formed a 3-fold disordered group with 1/3 occupancy pivoted on one of the methyl groups, pointing towards the narrow pore aperture. However, pX2 molecules formed an ordered 3-fold symmetric array, which are oriented against the walls of the cavity in the other void.

Figure 5. (a, d) View of pX and oX molecules inside BIF-20 along the b axis, respectively. (b, e) View of pX and oX molecules inside BIF-20 along the c axis, respectively. (c, f) A representation of pX and oX 3-fold symmetry packing arrangements along the c axis,


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respectively. pX1 in void 1 and pX2 in void 2 are red and magenta colored, respectively. oX in void 1, and oX2 and oX3 in void 2 are blue, cyan, and green colored, respectively. In oX@BIF-20, three oX locations can be identified: site 1 (oX1) is located in void 1; site 2 (oX2) and 3 (oX3) are located in void 2. All of the oX molecules are aligned against the walls of the cavity, leaving void space in the center of the pore channel. The oX molecules also formed a 3-fold disordered group and they refine to a fractional occupancy of 50% (Figure 8). Note that the Z’ values decrease from 1 to 0.33 (Z value decreases from 36 to 12), indicating that only half of the formula unit is present in the asymmetric unit, with the other half consisting of symmetrically equivalent atoms. This means that the BIF-20 structure becomes more ordered (i.e., exhibits higher symmetry) in the presence of pX molecules, perhaps to facilitate ordering of the pX molecules. However, this is not observed in the oX@BIF-20 structure, and the oX molecules did not improve the symmetry in a similar fashion as the pX molecules, suggesting that the linear pX isomer with the smallest cross section is more compatible and sterically favorable to the geometry of the cavity. In the presence of ortho-methyl groups, the pivoting of methyl groups observed in pX@BIF-20 (pX1) molecules cannot be achieved due to the more bulky configuration of the oX molecules (Figure 2b).52 Structural analyses revealed that both xylene isomers could pass through the narrow pore aperture, and are adsorbed in the cavities. The difference in the molecular packing arrangements is not sufficient to achieve thermodynamic selectivity (Figure 2a). However, the observed difference in steric hindrance of the xylene isomers could perhaps suggest that pX with the smaller cross-sectional area can pass through the pore channels easier and results in a faster diffusion rate. The 3-fold symmetry packing arrangement observed in both xylenes@BIF-20 structures is commensurate with the void/pore


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features of BIF-20’s 3-fold crystal symmetry, which could suggest that the guest molecules may orient themselves to achieve an optimal packing position through host-guest interactions.21, 53 As mentioned previously, the unit cell parameters showed small changes, even when xylene-loaded, suggesting that perhaps the sorption and diffusion of guest molecules involved local structural flexibility (Table 1).34 Previous studies have demonstrated the presence of guestinduced rotation of the imidazole linkers in zeolite-type MOFs (e.g., ZIF-8) to explain the local flexibility of the pore windows without causing any considerable changes to the crystal dimensions.14, 54 Comparisons of selected bond angles in the BIF-20 framework indicate local structural flexibilities in the single crystal structures (Table S2). The dynamic motion of the imidazole rings within the framework is illustrated by the change in the torsion angles when loaded with xylene molecules. Compared to the activated BIF-20 structure, the change in the torsion angles between the imidazole rings (N-B-N-C) lies in the range of 1-9° from the plane (Table S2). In particular, the torsion angle for N8-B2-N9-C31 (at linker A) increased from 20.3° in activated BIF-20, to 27.1° and 29.3° in pX@BIF-20 and oX@BIF-20 to accommodate the guest molecules, respectively (Table S2). It is evident that the imidazole rings can be tilted to varying angles. However, the calculated pore apertures of the refined BIF-20 crystal structure remains approximately 3 Å despite the state of the adsorbent (activated or guest-loaded).50-51 This is clearly insufficient for enabling the sorption of xylene molecules, with kinetic diameters of about 6 Å. Therefore, it is expected that when the xylene molecules pass through the narrow hexagonal window, the initial conformation of BIF-20 may display larger tilt angles of the imidazole planes to increase the effective pore window size by almost a factor of 2 before returning to the narrow pore state.


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Interestingly, most of the adsorbed xylene molecules are located around the center of the trivalent boron linkers, indicating that π-π interactions with the imidazole rings and perhaps weak boron-π interactions may be involved (Figure 6 and 7). Such host-guest interactions may play a role in causing slight framework contraction upon adsorption (Table 1). In pX@BIF-20, the pX1 molecules are oriented at a slight angle (Figure 6c, d), with a slightly longer linker-xylene bond distance, than that of pX2, suggesting weaker host-guest interactions for that pX population (Figure 6a, b). This could be because linker A (in void 1) adopts a more distorted tetrahedron geometry, which could influence the unique disordered packing arrangements of pX1 molecules (Table S2).

Figure 6. Adsorption sites of pX molecules around the center of the tridentate boron linkers in a BIF-20 crystal to illustrate the strength of host-guest interactions. (a, b) Bond distances between pX1 and pX2 molecules, and the imidazole rings, respectively. (c, d) Bond distances from the


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centers of the aromatic rings of pX1 and pX2, to the boron atoms of the linkers, respectively. pX1 (red) and pX2 (magenta). Bond distances are shown as green dotted lines. In contrast, the oX molecules in oX@BIF-20 near the boron linkers are tilted slightly and more off-center, which further weakens the host-guest intermolecular interactions, evidenced by the longer xylene-boron distances, of up to 0.7 Å (Figure 6c, d and 7d, e). A third oX adsorption site (oX3) occurs near the zinc coordination center, which is not observed in pX@BIF-20, perhaps due to its longer molecular length (Figure 7). The relatively weaker host-guest interactions than pX@BIF-20 support the higher disorder observed in oX@BIF-20.

Figure 7. Adsorption sites of oX molecules around the center of the tridentate boron linkers and the zinc coordination center in a BIF-20 crystal to illustrate the strength of host-guest interactions. (a, b, c) Bond distances between oX1, oX2, and oX3 molecules and the imidazole rings, respectively. (d, e) Bond distances from the centers of the aromatic rings of oX1 and oX2, to the boron atoms of the linkers, respectively. (f) Bond distance from the center of the aromatic


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ring of oX3 to the zinc coordination atom. oX1 (blue), oX2 (cyan), and oX3 (green). Bond distances are shown as green dotted lines. CONCLUSIONS In summary, we have identified a locally flexible, pX-selective BIF-20 framework. The material not only displays potential diffusion separation capabilities for xylene isomers, but high adsorption capacities towards hydrocarbon molecules twice the size of the reported crystallographic aperture. The single crystal structures of xylene-loaded BIF-20 are successfully resolved, directly revealing the location of the xylene adsorption sites within the framework, and the underlying mechanism for the kinetic selectivity of pX over oX. We find that the tritopic BH(mim)3 unit serves as the primary adsorption site for xylene isomers. The tilting of the imidazole rings, providing local structural flexibility in the framework, could facilitate the adsorption and diffusion of large guest molecules through the narrow critical pore dimension (Table S2). In the presence of steric effects, the xylene isomers, exhibit varying degrees of disordered states in their 3-fold symmetry packing arrangements, with pX being more ordered, and increase the BIF-20 lattice symmetry. It is rare to obtain high quality adsorbate-loaded single crystal structures and determine the host-guest structures, particularly the location and orientation of dynamic guest molecules due to the disorder of guest molecules even at low temperatures. These results can complement existing computational modeling work to help advance the understanding of MOF-guest interactions and sorption mechanisms. Importantly, the work provides crystallographic insights to the location of guest binding sites within a locally flexible framework, facilitating the rational design and structural optimization of new materials with improved performance in gas separations. Future work in this area includes other


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mechanistic investigations through dynamic in situ SCXRD, to further understand the sorption and diffusion of xylene isomers.55


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: SEM images of BIF-20; TGA data of BIF-20; PXRD and SCXRD data of BIF-20 and xylene loaded BIF-20; difference electron density map of pX@BIF-20 and oX@BIF-20 (PDF). Accession Codes CCDC 1590583 – 1590585 contain 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]. Author Contributions The manuscript was written through contributions of all authors. R. L. carried out synthesis, gas adsorption, NMR, PXRD, SEM and wrote the first draft of the manuscript. J. B. performed single crystal XRD measurements and structure refinement. M. L. J. carried out the organic vapor adsorption measurements and edited the manuscript. R. P. L. contributed to conception of the work and finalized the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the support from STAMI (Science and Technology of Advanced Materials and Interface, Seth Marder’s center). Single-crystal diffraction experiments were performed at the Georgia Institute of Technology SCXRD facility directed by Dr. John Bacsa. The authors would like to thank Dr. Ian Walton, Dr. Hyuk Taek Kwon, and Brian Pimentel for helpful discussions and comments on the manuscript, and Dr. David Tavakoli for assistance with PXRD experiments.


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For Table of Contents Use Only

Direct Structural Evidence of Molecular Packing Effects of Xylene Isomers Adsorbed in BIF-20 Richelle Lyndon†, John Bacsa‡, Melinda L. Jue†, and Ryan P. Lively†*

SYNOPSIS Xylene molecules in a boron imidazolate framework with potential para-xylene diffusion selectivity are structurally characterized. The local flexibility of the framework, and the differences in molecular packing arrangements between the xylene isomers, can be directly observed.


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Direct Structural Evidence of Molecular Packing Effects of Xylene

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