Distinctive Three-Step Hysteretic Sorption of Ethane with In Situ

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Distinctive Three-Step Hysteretic Sorption of Ethane with In-situ Crystallographic Visualization of the Pore Forms in a Soft Porous Crystal Prem Lama, and Leonard J. Barbour J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10352 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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Journal of the American Chemical Society

Distinctive Three-Step Hysteretic Sorption of Ethane with Insitu Crystallographic Visualization of the Pore Forms in a Soft Porous Crystal Prem Lama and Leonard J. Barbour* Department of Chemistry and Polymer Science, University of Stellenbosch, Stellenbosch 7600, South Africa

Supporting Information Placeholder ABSTRACT: A soft porous Zn(II)-MOF (1) displays distinctive three-step hysteresis breathing behavior under ethane gas pressure at ambient temperatures. In-situ single-crystal X-ray diffraction analysis was carried out at 298 K using an environmental gas cell in order to elucidate the different porous forms of the breathing framework under ethane gas. The three different phases were further characterized by pressure-gradient differential scanning calorimetry (P-DSC) and variable pressure powder X-ray diffraction analysis (VP-PXRD).

INTRODUCTION Over the past two decades a large number of porous metal organic frameworks1 (MOFs) have been reported to show potential for catalysis as well as the storage and separation of gases.2 Numerous MOFs display impressive gas uptake capacities owing to their large specific surface areas and pore volumes.3 Owing to the wide variety of potential building blocks, MOFs have a distinct advantage over conventional porous materials such as activated carbons and zeolites.4 For example, pore size and shape can be tuned by means of judicious selection of metal ions and organic linkers. In this regard, a variety of flexible/dynamic MOFs, also known as soft porous crystals5c (SPCs), have been synthesized.5 Structurally flexible MOFs can undergo ʻʻbreathingʼʼ6 in the presence of different solvents or gases to assume new pressuredependent porous forms. During activation these dynamic MOFs typically lose their single-crystal nature, but in some cases it is still possible to obtain the crystal structure of an activated material by means of single-crystal X-ray diffraction (SCD).6d In rare cases, in-situ SCD analysis can also be carried out for crystals exposed to controlled gas environments with a view to visualizing the response of the framework to pressure.7,8 However, such analyses involve overcoming significant technical challenges. Breathing MOFs generally exhibit hysteretic sorption/desorption or stepwise gas uptake/loss isotherms due to expansion or contraction of the pores under guest-specific conditions of pressure and temperature. The most dramatic examples reported to date are the porous MIL-53 compounds with M = Al, Cr and Fe. MIL-53 with M = Al or Cr9 exists in two different forms: narrow pore (NP) and large pore (LP), depending on the identity and amount of gas adsorbed. In contrast, the iron(III) version of MIL-53 undergoes more complicated breathing9b and transitions between different forms such as very narrow pore (VNP), intermediate pore (IP), NP and LP states. Several other gate-opening phenomena have been reported for MOFs,10 and in

some cases the structures of the breathing forms have also been elucidated.5,6d However, crystal structures of breathing forms in the presence of linear chain n-alkanes are rare, and have generally been determined either theoretically or using synchrotron X-ray analysis.11 Since n-alkanes such as methane (C1), ethane (C2), propane (C3) and butane (C4) do not form coordination bonds with metals, their interactions with MOFs are purely non covalent. These n-alkanes are the main constituents of natural gas and they are mainly used as combustible fuels. The selective separation of n-alkanes using porous MOFs has become an area of great commercial interest12 and in this context we now report our exploration of the recently reported6d dynamic, hydrothermally stable Zn(II)-MOF [Zn3(tp)2(1,2,3-tz)2]n (1, tp = terephthalate and 1,2,3tz = 1,2,3-triazolate, Table S1).

EXPERIMENTAL SECTION Synthesis. All chemicals and solvents were purchased from Aldrich and used without further purification. The complex {[Zn3(tp)2(1,2,3-tz)2]•x(dmf)}n, 1dmf was synthesized following a previously reported procedure.6c 1dmf was heated at 160 oC under dynamic vacuum (3.1 x 10-2 mbar) for 6 hours to yield the fully desolvated form 1o. The desolvated form is stable in air and does not appear to be moisture sensitive. Powder X-ray diffraction (PXRD). Experiments were carried out on a PANalytical X’Pert PRO instrument with BraggBrentano geometry. Intensity data were recorded using an X’Celerator detector and 2θ scans in the range of 5-40° were performed with a step size of 0.02° at the scan speed of 0.02 (°/s). During the experiment the powdered sample was exposed to CuKα radiation (λ = 1.5418 Å). The activated sample 1 was sealed within a glass capillary (environmental gas cell) and the capillary spinner configuration (with focusing mirror) of the instrument was used since this setup allows for very accurate temperature control using a short-nozzle Oxford Cryosystems Cryostream 700 Plus cryostat. Different gaseous n-alkanes (methane, ethane propane and butane) were used to pressurize the sample and its variable pressure PXRD patterns were measured at a constant temperature of 298 K. Pressure-gradient differential scanning calorimetry (PDSC). Experiments were carried out using a Setaram MicroDSC7 Evo instrument. Heat flow was recorded at 298 K in the pressure range 1 to 98 bar for methane and 1 to 30 bar for ethane. A freshly activated powdered sample of 1 was used for each experiment. For a given sample weight, the energies involved for all events (sorption and structural transformation) were calculated by

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integrating the peak area (heat flow in mW versus time in seconds) using Originpro version 8.0 software. Environmental gas cell experiments. An environmental gas cell (developed in-house) was used to determine the crystal structures of the various phases at 298 K under controlled pressures by means of single-crystal X-ray diffraction. First, the structure of 1o was determined under vacuum after activating a crystal of 1dmf. The structure of 1mI was determined after pressurizing 1o at 20 bar of methane for 24 hours. The structure of 1eI was determined after rapidly pressurizing a crystal of 1o to 6.3 bar and allowing it to equilibrate for one day. The structure of 1eII was determined after pressurizing 1eI rapidly to 12 bar and equilibrating for one day. However, the crystal lost its single crystal integrity. In a second attempt, the procedure was modified. A fresh crystal was activated, rapidly pressurized to 6.3 bar and left for one day to yield 1eI. The pressure was then increased incrementally by 0.1 bar from 6.3 bar to 6.8 bar. The crystal was allowed to equilibrate for 3 hours at each pressure. After reaching 6.8 bar (the approximate onset pressure for transformation from 1eI to 1eII), the crystal was equilibrated for 6 hours. Then the crystal was further pressurized at 0.2 bar increments and allowed to equilibrate for 30 minutes at each pressure until 10 bar was reached. The crystal was further allowed to equilibrate at 10 bar for 12 hours and the diffraction data were collected at 298 K. Gravimetric sorption analysis. Gravimetric gas sorption analysis was carried out using a Hiden Isochema Intelligent Gravimetric Analyser (IGA-002). The uptake of gas by the sample was measured with increasing pressure (up to 20 bar for methane and ethane, 8 bar for propane and 2 bar for butane) and constant temperature at 298 K. Each data points was recorded after allowing the sample to equilibrate for one hour. The effects of buoyancy due to gas pressure were corrected for automatically. The sample (114 mg) was reactivated in-situ at 50 oC prior to each sorption measurement. A Grant refrigerated recirculation bath was used to thermostat the sample chamber at a constant temperature of 298 K. RESULTS AND DISCUSSION Crystals of the solvated MOF {[Zn3(tp)2(1,2,3-tz)2]·x(dmf)}n (1dmf, dmf = N,N'-dimethyformamide) were prepared according to a previously reported procedure.6c It is important to note that form 1dmf6c,6d crystallizes in the triclinic space group Pī. The crystals were heated at 160 oC under dynamic vacuum (3.1 ×10-2 mbar) for 6 hours to yield the fully desolvated form 1o, which adopts the monoclinic space group P21/c (Table S2).6d The guest-accessible space in 1o (determined using a probe of radius 1.4 Å) is 15.8% (Table S3). With the exception of dmf, the asymmetric unit of 1o has the same composition as that of 1dmf: one tp, one 1,2,3-tz and two Zn(II) ions (one with half and the other with full site occupancy). One of the zinc centers (Zn1) is octahedrally coordinated to four oxygen atoms from four different tp ligands and to two nitrogen atoms from two different 1,2,3-tz units. Zn2 is in a tetrahedral coordination environment, being bound to two oxygen and two nitrogen atoms, all belonging to different tp and 1,2,3-tz ligands, respectively.6d Complete desolvation results in significant distortion of the host framework relative to that of the assynthesized solvate; this change is particularly evident when comparing the dimensions of the rhomboid pore formed by the Zn clusters and tp units (Figure S1).6d The dihedral angles θ1 and θ2 between the planes passing through the Zn1 centers within the rhombic grid are 38.72(1)° and 141.28(1)° in 1o, as compared to corresponding values of 59.10(1)° and 120.90(1)° in 1dmf. Desolvation causes the tp aromatic rings to rotate such that moderate π···π interactions are formed within each macrocyclic ring. The shortest contact between two atoms of two different aromatic

rings (C3···C7) is 3.35(7) Å (Figure S2). During desolvation two 1,2,3-tz units also approach each other to form π···π interactions; the shortest contact distance between the rings is 3.33(1) Å for C9···C10 (Figure S2). In our previous report6d of framework 1 we described its extreme breathing capability under high CO2 pressure at room temperature. In order to explore the porosity and breathing behavior of 1 under high pressures of the alkane gases C1-C4, gravimetric sorption measurements were carried out at three different temperatures (298, 288 and 278K) using an instrument with a pressure limit of 20 bar. The complex sorption behavior of 1 with C1-C4 is further supported by variable pressure powder X-ray diffraction analysis (VP-PXRD) and pressure-gradient differential scanning calorimetric (P-DSC) experiments.

Figure 1. n-Alkane sorption isotherms for 1 at 298K. Filled and open symbols represent adsorption and desorption, respectively; methane (black), ethane (red), propane (blue) and butane (purple). At 298 K (Figure 1) the uptake of methane by 1 is negligible, although a slight inflection (which is consistent with a gateopening structural change) is observed at approximately 17 bar. While gas uptake remains negligible on decreasing the temperature to 288 and 278 K, the onset pressure of the inflection decreases to approximately 14 and 10 bar, respectively (Figure S3). In-situ single-crystal X-ray diffraction studies were carried out under controlled methane gas pressures with a view to elucidating the open-pore host framework of 1.13 Using an environmental gas cell,13 SCD data were recorded (Table S2) for a single crystal of 1o exposed to 20 bar of methane gas (1mI). Despite slight differences in unit cell parameters, the host structure of 1mI (Figures S4-S6) is essentially the same as that of 1o. Owing to the very low guest occupancy at 20 bar, the methane molecules could not be modeled. Our environmental gas cell was easily adapted for VP-PXRD using a capillary spinner. We therefore recorded variable pressure powder diffractograms for methane at 298 K (Figures S7-S8). The powder patterns remain similar to that of 1o on increasing the pressure up to 17 bar, and correspond to form 1mI at 20 bar. Increasing the pressure to 40 bar has no further effect on the patterns. This indicates that only a single phase transformation occurs at 298 K in the pressure range studied. We also determined the unit cell parameters of 1o and 1mI by indexing powder diffractograms recorded at vacuum and 20 bar, respectively, using TOPAS14 version 4.2 (Table S4, Figures S9-S10). The data are quite consistent with those obtained from SCD experiments. PDSC measurements6d,15 were carried out at 298 K for 1 (using 1o as starting material) exposed to methane up to a pressure of 98 bar

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Journal of the American Chemical Society (Figure S11). No distinct energetic event was observed below 16 bar, after which a broad exothermic event occurs, with an onset pressure of approximately 17 bar and reaching a maximum at 20 bar. This exotherm is mainly due to the heat of adsorption associated with the gas uptake during the phase-change. No further energetic events appear to occur on further increasing the pressure to 98 bar, thus implying only a single gate-opening structural change in this pressure range at 298 K. Decreasing the pressure results in an apparently single endothermic event, which implies reversibility of the transformation between 1o and 1mI. These experiments confirm our initial speculation that the inflection at 17 bar in the sorption isotherm recorded at 298 K signals a slight phase transformation of the host framework. In the case of ethane, gas uptake proceeds as three distinct steps at all three of the temperatures 298, 288 and 278 K (Figure S12), with saturation occurring after the final open pore form 1eIII is reached. At 298 K (Figure1) the first step, with an onset pressure of approximately 5 bar, signals a structural transformation from 1o to the first intermediate pore form 1eI. The latter remains stable to a pressure of about 6.4 bar, after which the second intermediate narrow-pore form 1eII begins to emerge. Form 1eII is stable to 14 bar and, upon further increasing the pressure to 20 bar, the framework transforms to the fully open wide-pore form 1eIII. At 298 K the amounts of ethane adsorbed per formula unit by the three different pore forms 1eI→1eII→1eIII were 0.36, 3.0 and 3.5 respectively. At 20 bar the total amount of ethane taken up was approximately nine times that of methane, and we attribute this to the higher heats of adsorption generally associated with ethane.16 These results indicate a relatively high selectivity by 1 for ethane over methane in the pressure range 0 to 20 bar. The ethane sorption profile of 1 is comparable to that reported for MIL-53(Fe),9 which also displays breathing behavior. However, the phase changes in 1 occur at higher pressures and are more pronounced. As expected, lowering the temperature causes the gate opening steps for all three phase changes to shift towards lower pressures. Since ethane is adsorbed in three distinct steps, we carried out in-situ SCD studies of host 1 under controlled ethane gas pressures.13 A single crystal of 1o was pressurized at 6.3 bar and allowed to equilibrate for one day to yield form 1eI. Interestingly, the crystal retains its singular nature and we were thus able to determine

all change in the framework geometry is relatively small and the space group symmetry is retained. The crystallographic a and c axes elongate slightly, with concomitant shortening of the b axis. In addition, the β angle increases from 113.798(2)° to 115.063(2)°. Together, these changes produce a slight increase in the unit cell volume (Table S2). At the same time, the aforementioned dihedral angles θ1 and θ2 also undergo relatively small changes (Table S5). Owing to low guest occupancy at 6.3 bar, the ethane molecules could not be modeled. Interestingly, the host framework of 1eI is similar to that of form 1mI (Table S2), and this result is further supported by VP-PXRD (Table S4, Figure S16). In an attempt to elucidate the structure of form 1eII, the same crystal was pressurized rapidly to 12 bar and left for one day prior to SCD analysis. However, the crystal appeared to have lost its single crystal nature. In another attempt, a fresh crystal of 1o was gradually pressurized in a stepwise fashion to 10 bar (i.e. transformation from 1eI to 1eII) and then left to equilibrate at that pressure for 12 hours. Despite lower reflection intensities relative to 1o and 1eI, it was still possible to determine the crystal structure of 1eII. The transformation of 1eI to 1eII involves a change of the space group from P21/c to Pī and the structure resembles that of the as-synthesized6c form 1dmf (Figure 3, Figures S17-21, Table S2 and Table S5). Although, ethane molecules could not be modelled crystallographically, the most probable locations of the guest molecules within the host framework can be inferred from a difference electron density map; the highest electron density peaks reside in at the middle of the channels (Figure 3, Figure S22 and VideoS1-S2). These sites are located beyond the host-guest van der Waals contact limits, implying that the host framework of 1eII does not possess strong binding sites for ethane. The presence of large voids (49.8% per unit cell, probe radius of 1.2 Å) in 1eII leads to relatively inefficient packing of the ethane molecules, which may allow sufficient space to accommodate additional guest molecules with only a slight additional rearrangement of the host framework being required. This final structural adjustment might not be viable for the bulkier propane and butane. Unfortunately, even after several attempts, we were unable to obtain the single-crystal structure of the third form 1eIII (Figure S23). We therefore determined the unit cell parameters for 1eIII by indexing a powder diffractogram recorded at 20 bar (Table S4). The changes in bond distances and angles on going from 1o to 1eII are given in Table S6.

Figure 2. Perspective view of the host framework of 1eI, shown in capped-stick representation.

Figure 3. Perspective view showing the host framework of 1eII in capped-stick representation. The ethane guest molecules were not modelled, and the difference electron density map is shown as green surfaces at the 1.3 e−Å−3 level.

the structure of its new pore form 1eI (Figure 2 and Figures S13S15). During the structural transformation from 1o to 1eI the over-

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VP-PXRD was carried out for 1 under different pressures of ethane gas at 298 K (Figure 4 and Figures S24-S25). The powder pattern was indexed and unit cell parameters were determined by Pawley refinement (Table S4, Figures S26-S30). These values match those determined by SCD analysis. Form 1eI becomes evident at approximately 5 bar (Figure S24); it is a slightly expanded

Figure 5. Pressure-gradient differential scanning calorimetry for 1 in the presence of ethane at 298 K.

Figure 4. VP-PXRD diffractograms for 1 under ethane gas pressure at 298 K (a) under vacuum, (b) 6 bar, (c) 12 bar and (d) 20 bar. version of 1o, as evidenced by a clear shift towards lower 2θ angles of reflections corresponding to Miller indices (1 1 -1), (1 3 1) and (2 1 -1) (Figure S25). Form 1eII starts (Figure S24) to appear above 6.5 bar and persists until 14 bar. On further increasing the pressure, 1eIII is formed (Figure S24), persisting until the saturation vapor pressure of ethane at 298 K. The space group symmetry is retained during the structural transformation from 1eII to 1eIII, but there is an overall increase in crystallographic volume (Table S4). P-DSC measurements6d,15 were carried out at 298 K for 1 exposed to ethane in the pressure range 1 to 30 bar (Figure 5 and Figure S31). Three distinct exothermic peaks were recorded with increasing pressure (onset pressures of approximately 5.0, 6.5 and 14 bar). Respectively, these represent the phase changes 1o→1eII→1eII→1eIII. The energy evolved (i.e. Qst of sorption combined with ∆H of the structural change) for the first (1o→1eI), second (1eI→1eII) and third (1eII→1eIII) exothermic events are −2.24, −15.5 and −23.69 kJ mol-1, respectively (Figure S31). On decreasing the pressure from 30 back to 1 bar, the energy associated with the first endothermic event is 22.98 kJ mol-1, which is comparable in magnitude to the energy released during the transformation from 1eII→1eIII. On further lowering the pressure, the energy for the second endothermic event (17.82 kJ mol-1) corresponds in magnitude to that for the combined transformations 1o→1eI→1eII. These results suggest strongly that the gas-induced phase transformations are completely reversible, as confirmed by VP-PXRD analysis (Figure S25).

Interestingly, only single-step sorption profiles were recorded for propane and butane at all three of the temperatures considered, with saturation occurring below 1 bar in all cases (Figures S32 and S33). At 298 K, the maximum uptake amounts for propane and butane per formula unit are 3.0 and 2.9 respectively (Figure 1). These values are comparable to that for ethane uptake by form 1eII and much higher than for methane in the pressure range measured. It seems self-evident that the lower uptake of propane and butane as compared to ethane is due to the bulkier dimensions of the C3 and C4 alkanes. Despite several attempts, we were unable to obtain the propane and butane gas-loaded SCD structures. However, it was possible to determine the relevant unit cell parameters by indexing VP-PXRD diffractograms (Table S7, Figures S34-S37) recorded at 298 K. For each gas, pressurizing 1o to 1 bar results in the formation of a phase similar to that of 1eII, and this phase persists until the saturation pressure is reached (Figures S38-40). CONCLUSIONS Dynamic/flexible materials that exhibit breathing are well known,10 but this phenomenon is usually influenced by solvents or common gases such as CO2 or N2.10d Although the crystal structures of some rigid frameworks containing n-alkanes have been reported,2d,2e MIL-53 is one of the rare9b examples of a flexible MOF that shows various pore forms in the presence of different gaseous n-alkanes. However the various pore forms of MIL53 have been characterized using synchrotron PXRD analysis, and not by means of SCD. We have employed SCD to determine the host structure of 1o6d, along with open pore forms 1eI and 1eII in the presence of ethane gas. Form 1 clearly undergoes a three-step hysteretic uptake of ethane gas at near-ambient temperatures, and our conclusions regarding its structural transformations are well supported by VP-PXRD and PDSC experiments. Form 1 undergoes hinge like motion6a,6d (VideoS3), with twisting of the carboxylate benzene ring (due to knee-cap motion8) and tilting of the 1,2,3-tz unit to open the pores. The breathing behavior depends on the n-alkane being adsorbed. Since the breathing behavior of 1 in the presence of ethane occurs without substantial loss of single crystal character, a plausible mechanism can easily be proposed. Evaluation of such lucid data can advance our efforts to design new breathing materials for useful applications.

ASSOCIATED CONTENT Supporting Information

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Journal of the American Chemical Society The synthetic procedure, thermal analysis, detailed crystallographic information and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the National Research Foundation and Department of Science and Technology (SARCHI Program) for support of this work. PL also thanks the Claude Leon Foundation for financial support.

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