Article pubs.acs.org/JPCB
Pressure-Dependent Release of Guest Molecules and Structural Transitions in Hydroquinone Clathrate Byeong-Soo Kim,† Yongjae Lee,‡ and Ji-Ho Yoon*,† †
Department of Energy and Resources Engineering, Korea Maritime University, Busan 606-791, Korea Department of Earth System Sciences, Yonsei University, Seoul 120-749, Korea
‡
ABSTRACT: Structural phase transitions of N2-loaded and guest-free hydroquinone (HQ) clathrates have been investigated as a function of pressure using synchrotron X-ray diffraction and Raman spectroscopy. The N2-loaded β-form HQ clathrate reveals a structural transition to a new high pressure phase near 4 GPa and exhibits gradual N2 release from the hydrogen-bonded organic framework (HOF) cage with a further increase in pressure. Upon reducing the pressure to ambient conditions, the crystal structure reverts to the original βform HQ clathrate, indicating that the pressure-induced structural transition is fully reversible. In contrast, around 0.4 GPa, the guest-free β-form HQ clathrate undergoes an irreversible phase transition to the nonporous α-form HQ, which is retained at ambient conditions. These results suggest that HOF encaged guest molecules play an important role in the structural transitions under high pressure.
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INTRODUCTION Clathrates are crystalline inclusion compounds stabilized by physical interactions between host species and relatively small guest molecules contained in the cages of the host framework.1−4 Clathrate compounds have been recognized as potential energy storage or gas separation/sequestration media because they can store a large fraction of gas per unit volume of the solid phase. Hydroquinone (HQ), phenol, and Dianin’s compounds are widely known to form organic clathrates, and they are the basis for an extensively studied class of inclusion compounds. The β-form HQ clathrate is a structurally simple representative of this family and comprises a hydrogen-bonded organic framework (HOF) that entraps relatively volatile guest molecules.2 The HOFs have an inherent flexibility that allows them to conform their structure to guest molecules of different size and shape, such as Ar, Xe, SO2, CH4, and methanol, and can potentially adopt channel structures, which are defined by the hexagonal entrance of hydrogenbonded host HQ molecules. The flexible hexagonal entrance allows the migration of guests into the HOFs as well as their release via a dynamic pore-widening process, leading to the controlled, selective, and reversible storage of small molecules.5 An important clue to how guest−host interactions occur in inclusion compounds under high pressure comes from studies on clathrate hydrates, which indicated that guest molecules such as CH4 and H2 play an important role in these structural transitions.6−9 It is now clear that, under high pressure, such small guests encaged in the clathrate hydrates lead to changes in the nature of the hydrogen bonding network of the host molecules. On the other hand, at ambient pressure, for the βform HQ clathrate, the structural transformation to the α-form HQ occurs at 380 K, irrespective of the guest (CO2, CH4, or methanol).10−12 This indicates that the combined effects of © 2013 American Chemical Society
host−guest interactions, guest molecule size, and shape, as well as the flexibility of the host framework, may affect the structural transition of clathrate compounds. A recent study indicated that the β-form HQ clathrate containing a CH4 guest undergoes a structural transition around 5 GPa.13 The effect, however, of guest molecules on the structural integrity of the HOF has remained unclear. We report here that, under high pressure, the structural transformation of organic clathrates and, thereby, the stability of the HOF is strongly affected by the guest molecules encapsulated in the cages.
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EXPERIMENTAL METHODS Pure α-form HQ (purity 99.5%) was purchased from Riedel and used without further purification. To synthesize the N2loaded HQ clathrate, pure HQ was allowed to react with N2 gas in a high-pressure reactor. The reactor was made of 316 SUS and has an internal capacity of approximately 55 mL. To promote the reaction with N2, the powdered HQ with particle sizes less than 100 μm was used as the starting material. After loading the HQ powders, the reactor was degassed under vacuum for 24 h and then the sample was exposed to N2 gas at 10 MPa for 60 min at 80 °C and subsequently for 60 min at −10 °C. This process with a variation of temperature was repeated at least 30 times, leading to complete conversion to the N2-loaded β-form HQ clathrate. Guest-free β-form HQ clathrate was obtained by the controlled heating process to remove CO2 guests from as-synthesized CO2-loaded β-form HQ clathrate. A customized Raman spectrometer with a monochromator of 1800 grooves/mm grating and a CCD Received: May 23, 2013 Revised: June 10, 2013 Published: June 10, 2013 7621
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detector was used in this work. An Nd:YAG laser source emitting a 532 nm line was used at a power of 150 mW. Three scans were averaged to obtain each spectrum, and the signal was integrated over about 10 s for each accumulation. The calibration of the monochromator was performed regularly using the 546.09 nm line of Hg at 1122.74 cm−1. The pressuredependent structural changes of HQ samples were determined by high-pressure synchrotron X-ray diffraction using a diamond-anvil cell (DAC) at beamline 9A at the Pohang Accelerator Laboratory (PAL). A wavelength of 0.6193 Å was used, and the diffraction patterns were measured using a CCD detector to cover 1−25° in 2-theta. A Bruker DSX400 solidstate NMR spectrometer was used in this study. The powdered samples were placed in a 4 mm o.d. zirconia rotor loaded into a variable temperature probe. The solid-state 13C cross-polarization/magic angle spinning (CP/MAS) NMR spectra were recorded at a Larmor frequency of 100.6 MHz with MAS at approximately 9 kHz. A pulse length of 2 μs and a pulse repetition delay of 10 s under proton decoupling were employed with a radio frequency field strength of 50 kHz. The downfield carbon resonance peak of adamantine, which is assigned a chemical shift of 38.3 ppm at 300 K, was used as an external chemical shift reference. A symmetric DAC, equipped with two low fluorescence type-I diamond anvils (culet diameter of 400 μm, brilliant cut) and tungsten-carbide supports, was used as the high pressure vessel. The HQ samples were loaded into a sample chamber (diameter: 160 μm and thickness: 100 μm) in a preindented stainless steel gasket, along with a few small ruby chips as a pressure gauge. The pressure at the sample was measured by detecting the shift in the R1 emission of the included ruby chips. The pressure of HQ samples in the DAC was measured up to 10 GPa at room temperature. All high pressure experiments were performed without any pressure-transmitting medium, because there was no difference in XRD pattern changes when the silicon oil was used as a pressure-transmitting medium.13
Figure 1. Structure of the β-form HQ clathrate: (a) Capped-stick representation viewed along the (001) direction. Dotted lines define a unit cell. (b) Cage structure formed by 12 HQ molecules, viewed perpendicular to the c-axis. The guest space volume is represented by the yellow sphere. Hydrogen atoms are omitted for clarity.
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also show a clear difference between α-form HQ and β-form HQ clathrates and a close structural similarity of N2-loaded and guest-free β-form HQ clathrates (Figure 2b and 2c). Synchrotron XRD data of the N2-loaded HQ clathrate show significant peak broadening along with decreased peak intensities with increasing pressure (Figure 3a). The pressureinduced changes in the Raman spectra are also shown in the frequency range of 1100−1250 cm−1 and 2300−2450 cm−1 (Figure 3b and 3c). The C−O stretching vibrational modes (1160 cm−1) of the HQ host molecules and the N−N stretching vibrational mode (2322 cm−1) of the N2 guest molecule exhibit a substantial blue shift with increasing pressure. In addition, the Raman bands show similar progressive broadening and a concomitant decrease in intensity as pressure increases. These changes are characteristic for the compression of the unit cell volume including the contraction of the cage structure. Abrupt changes in the synchrotron XRD patterns and Raman spectra are observed near 4 GPa, signaling the pressure-induced structural transition. The powder diffraction data reveal a reversible pressure-induced partial amorphization under pressure. However, above 4 GPa, a new set of diffraction peaks started to grow, indicating the formation of a new high pressure phase. In particular, at 4.7 GPa, a new Raman peak appears at the high frequency side of the N−N stretching vibrational mode of N2 molecules encaged in the HQ clathrate framework. Increasing the pressure to 9.54 GPa, leads to a
RESULTS AND DISCUSSION HQ exists in three different crystalline forms: α, β, and γ.1 The rhombohedral α-form HQ is stable at ambient conditions and transforms to the β-form HQ clathrate when small guest molecules are incorporated into the ordered HQ lattice structure. The γ-form HQ forms by sublimation or rapid evaporation of HQ solutions in ether. Figure 1 shows the crystal structure of the β-form HQ clathrate as well as the cage structure. The unit cell consists of nine HQ molecules and three cages. Therefore, the molar ratio of HQ molecules to βform HQ clathrate cages is 3:1, which suggests a high guestmolecule loading capacity. In this study, a N2-loaded HQ clathrate was synthesized by the gas-phase reaction. The chemical analysis indicates that 67% of the cages of the HQ clathrate are occupied by N2 molecules, which results in a chemical formula of 0.67N2·3HQ. The guest-free HQ clathrate was prepared as well, by the gas-phase synthesis of a CO2loaded HQ clathrate, followed by controlled heating to remove the CO2 guests.5 Raman spectroscopy was used to gain information on the crystal structure of the formed HQ clathrate and the guest dynamics (Figure 2a). The two split Raman bands at 1160 cm−1 are characteristic of α-form HQ, whereas the β-form HQ clathrates feature a single peak. The Raman peak at 2322 cm−1 reveals the presence of N2 molecules encaged in the β-form HQ clathrate framework.14 The X-ray diffraction (XRD) patterns and solid-state 13C NMR results 7622
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Figure 3. (a) Pressure-dependent synchrotron XRD pattern of N2loaded HQ clathrates. Pressure-dependent Raman spectra of N2loaded HQ clathrates with increasing (b) and decreasing (c) pressure. (d) Pressure dependence of the symmetric N−N stretching mode of N2 encaged in β-form HQ and the newly observed high pressure phase. Solid lines are a guide to the eyes.
Figure 2. (a) Raman spectra, (b) X-ray diffraction patterns, and (c) solid-sate 13C NMR spectra of HQ compounds: (red) N2-loaded βform HQ clathrate; (blue) guest-free β-form HQ clathrate; (black) αform HQ.
From these results, we conjecture that the HOF of N2-loaded β-form HQ clathrate exhibits a structural transition near 4 GPa and releases its N2 guests, which subsequently form the N2 solid phase. With a further increase in pressure, the contribution of the N2 solid phase gradually increases, as shown in Figure 3b. Interestingly, at ambient temperature, the structural transition of the N2 solid β-phase to the solid δ-phase occurs at 4.67 GPa.17,18 As a result, the higher frequency Raman peaks observed at pressures over 5 GPa can be assigned to the N−N stretching vibration of the N2 solid δ-phase. This implies that the structural amorphization of the host organic clathrate might be triggered by the release of the encapsulated guest molecules and subsequent formation of its high-pressure phase.
gradual increase in intensity of the higher frequency peak, whereas the intensity of the minor peak (for entrapped N2) decreases. The higher frequency peak shows a greater blue-shift with pressure than the lower frequency peak (Figure 3d), i.e., the frequency difference between the two peaks is Δν = 11.1 cm−1 at 4.7 GPa and becomes progressively larger with increasing pressure, leading to a frequency difference of Δν = 20.3 cm−1 at 9.54 GPa. We note that the pressure dependence of the higher frequency peak is very close to that of the N−N stretching vibrational mode (ν2) of the N2 solid phase.15,16 7623
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Figure 4. Pressure-dependent synchrotron XRD patterns of (a) α-form HQ and (b) guest-free β-form HQ. Pressure-dependent Raman spectra of (c) α-form HQ and (d) guest-free β-form HQ. (e) Hydrogen bonding networks of guest-free β-form HQ and α-form HQ, viewed in perspective along the (001) and (110) directions. HQ oxygen atoms are shown as red spheres, while the connecting red lines between the oxygen atoms represent hydrogen bond interactions. White lines define a unit cell. HQ carbon and hydrogen are omitted for clarity.
forms to the α-form HQ at 0.24 GPa (Figure 4d). With further increasing pressure to 0.42 GPa (Figure 4b), the crystal structure shows a full transformation to α-form HQ. An additional important feature is that the crystal retains its α-form HQ structure when the pressure is decreased again to ambient conditions. This indicates that the pressure-induced structural transition of the guest-free β-form HQ clathrate is irreversible and that the HOF of α-form HQ is more stable and dense at ambient conditions (Figure 4e).5
The intensity of the N2 solid-phase Raman peak gradually decreases with a decrease in pressure, and a discontinuous change in the Raman frequency near 4 GPa is similarly observed (Figure 3c and 3d), possibly due to the transition from the N2 solid δ-phase to β-phase. With a further decrease in pressure to 2.37 GPa, the Raman peak of the N2 solid β-phase disappears, which is most likely due to enclathration of the N2 molecules back to the HOF cage. Here, it should be noted that, at ambient temperature, the melting of the N2 solid-state βphase to fluid-state N2 occurs at 2.4 GPa.17 Upon a decrease in pressure to ambient conditions, the XRD pattern reverts to that of the original β-form HQ clathrate (Figure 3a), indicating that the pressure-induced amorphization and release of N2 guests is fully reversible. As a result, the returned clathrate product may retain the original structure and formula of 0.67N2·3HQ. Pressure-dependent changes in the synchrotron XRD patterns and Raman spectra of guest-free β-form HQ clathrate are shown in Figure 4 and compared with α-form HQ. As shown in Figure 4a and 4c, there is no indication for a structural transition for α-form HQ with a pressure increase to 8.83 GPa. However, the guest-free β-form HQ clathrate partially trans-
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CONCLUSION In this report, we have presented here the pressure-induced structural changes of N2-loaded and guest-free HQ clathrates. Even though they exhibit the same crystalline β-form HQ structure, their phase transition behavior under high pressure is completely different. The N2-loaded β-form HQ clathrate reveals a structural transition to a new high pressure phase near 4 GPa and exhibits gradual N2 release from the HOF cage with a further increase in pressure. Upon reducing the pressure to ambient conditions, the crystal structure reverts to the original β-form HQ clathrate, indicating that the pressure-induced 7624
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quinone Clathrate Observed by Temperature-Dependent Raman Spectroscopy. J. Phys. Chem. A 2012, 116, 2435−2438. (13) Lee, Y.; Lee, J.-W.; Lee, H.-H.; Lee, D.-R.; Kao, C.-C.; Kawamura, T.; Yamamoto, Y.; Yoon, J.-H. High Pressure Investigation of α-form and CH4-loaded β-form of Hydroquinone Compounds. J. Chem. Phys. 2009, 130, 124511−124516. (14) Lee, J.-W.; Choi, K. J.; Lee, Y.; Yoon, J.-H. Spectroscopic Identification and Conversion Rate of Gaseous Guest-loaded Hydroquinone Clathrates. Chem. Phys. Lett. 2012, 528, 34−38. (15) Schiferl, D.; Buchsbaum, S.; Mills, R. L. Phase Transitions in Nitrogen Observed by Raman Spectroscopy from 0.4 to 27.4 GPa at 15 K. J. Phys. Chem. 1985, 89, 2324−2330. (16) McCluskey, M. D.; Hsu, L.; Wang, L.; Haller, E. E. Infrared Absorption of Solid Nitrogen at High Pressures. Phys. Rev. B 1996, 54, 8962−8964. (17) Vos, W. L.; Schouten, J. A. Improved Phase Diagram of Nitrogen up to 85 Kbar. J. Chem. Phys. 1989, 91, 6302−6305. (18) Belak, J.; Lesar, R.; Etters, R. D. Calculated Thermodynamic Properties and Phase Transitions of Solid N2 at Temperatures 0 ≤ T ≤300 K and Pressures 0 ≤ P ≤100 GPa. J. Chem. Phys. 1990, 92, 5430−5442.
structural transition is fully reversible. In contrast, around 0.4 GPa, the guest-free β-form HQ clathrate undergoes an irreversible phase transition to a nonporous α-form HQ, which is retained at ambient conditions. These results suggest that HOF encaged guest molecules play an important role in the structural transitions under high pressure. The reorganization of the HOF and the modification of the cage geometry point toward novel properties of the clathrate compounds under pressure.
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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 This research was supported by Basic Science Research Program (2010-0007026) and Nuclear R&D Program (M2AM06-2008-03931) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST). Experiments at PAL were supported in part by the MEST and Pohang University of Science and Technology (POSTECH).
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REFERENCES
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