(sH) Clathrate Hydrates Enclosing Nitrogen and 2,2-Dimethylbutane

Article pubs.acs.org/JPCC

Structural Characterization of Structure H (sH) Clathrate Hydrates Enclosing Nitrogen and 2,2-Dimethylbutane Yusuke Jin,* Masato Kida, and Jiro Nagao Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan S Supporting Information *

ABSTRACT: In this study, we characterized structure H (sH) clathrate hydrates (hydrates) containing nitrogen (N2) and 2,2-dimethylbutane (neohexane, hereafter referred to as NH) molecules. On the basis of the powder X-ray diffraction profile, we estimated the unit cell dimensions of the sH hydrate of N2 + NH to be a = 1.22342(15) nm and c = 0.99906(17) nm at 153 K. The c axis of this hydrate was slightly shorter (i.e., 0.00584 nm) than that of CH4 + NH, whereas we observed no difference in the a axis between these two hydrates. We successfully observed a symmetric N−N stretching (N−N vibration) Raman peak with two bumps, and we determined that the N−N vibrational mode in the 512 and 435663 cages occurred at approximately 2323.8 and 2323.3 cm−1, respectively. We found the cage occupancy ratio of the 435663/512 cages (θMθS) of the sH hydrate of N2 + NH to be approximately 1.30. From a comparison of the N−N vibrational modes in the 512, 435663, 51262, and 51264 cages of the sI, sII, and sH hydrates, we determined that N2 molecules in the distorted 435663 cages experience more attractive guest−host interaction than those in spherical 51264 cages, whereas the guest/cage diameter ratio of 435663 cages is larger than that of 51264 cages. We determined the L1−L2−H−V four-phase equilibrium pressure−temperature conditions in the N2−NH−water system in the temperature range of 274.36−280.71 K. Using the Clausius−Clapeyron equation, we estimated the dissociation enthalpies of the sH hydrates of N2 + NH to be 388.4 and 395.9 kJ·mol−1 (per one molar of N2 molecules) in the experimental temperature range.

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molecules.6−11 The methane hydrates containing CH4 as guest molecules are associated with equilibrium pressures exceeding 3 MPa at 275 K, whereas the sH hydrate containing CH4 + NH remains stable at approximately 1.4 MPa.6,7 Therefore, the sH hydrate systems are expected to be used as gas storage or separation media. The phase equilibrium pT conditions of the sH hydrates differ depending on the encapsulated LMGSs. If LMGS is methylcyclohexane, the equilibrium pressure at 275 K increases to approximately 1.6 MPa8,9 compared to a hydrate with NH (1.4 MPa). Recently, it has been revealed that the shapes and sizes of LMGSs are important for determining the equilibria of the sH hydrates.12 The Raman peaks of guest molecules encaged in hydrate structures may promise to increase our understanding of guest behavior in water-hosted frameworks.13,14 Using high-resolution Raman spectroscopy, Ohno et al.15 observed symmetric C−H stretching vibrational (C−H vibration) modes of CH4 enclosed in hydrates and identified the Raman shifts of CH4 in the 512 and 435663 cages of sH hydrates. Furthermore, Jin et al.12 used high-resolution Raman measurements to determine

INTRODUCTION The structure H (sH) clathrate hydrate (hydrate) is one of the three primary crystal structures of gas hydrates. These hydrates are ice-like crystalline compounds containing guest molecules in a hydrogen-bonded H2O framework cage.1,2 The structural unit of the sH hydrates is composed of three pentagonal dodecahedral (512) cages, two irregular dodecahedral (435663) cages, and one icosahedral (51268) cage.3 The average radii of the 512, 435663, and 51268 cages are 0.394, 0.404, and 0.579 nm, respectively.4 The 512 and 435663 cages of the sH hydrates store guest molecules (mainly gas molecules). In addition, the 51268 cage can encapsulate guest molecules; however, the guest molecules stored in these cages are large-molecule guest substances (LMGSs) that cannot be stored in the 512 or 435663 cages.5 When LMGSs coexist in a gas−water system, the formed sH hydrates encapsulate LMGSs in the 51268 cage and small guest molecules in the 512 and 435663 cages. LMGSs are polar molecules (e.g., tert-butyl methyl ether) and nonpolar liquid hydrocarbons (e.g., 2,2-dimethylbutane, neohexane, NH).5 An sH hydrate can store small guest molecules in crystals at lower-pressure and higher-temperature conditions (milder conditions) than the phase equilibrium pressure− temperature (pT) conditions required for pure structure I and II (sI and sII) hydrates that encapsulate only small guest © 2015 American Chemical Society

Received: January 18, 2015 Revised: March 20, 2015 Published: April 7, 2015 9069

DOI: 10.1021/acs.jpcc.5b00529 J. Phys. Chem. C 2015, 119, 9069−9075

Article

The Journal of Physical Chemistry C that the C−H stretching vibrations of CH4 in the 512 cages of sH hydrates are changed by LMGSs, whereas the peak locations in the 435663 cages usually remained at approximately 2909.15 cm−1. Nitrogen (N2) is a linear-shaped molecule in contrast to the spherical CH4 molecule. Because of their linear shape, N2 molecules in the 51264 cages of the sII hydrates are distributed away from the cage center and positioned at preferred orientations.16 Unfortunately, Raman frequency data for each N2 molecule in the 512 and 435663 cages of sH hydrates have not yet been reported, unlike the data for CH4 in 512 and 435663 cages,12,15 whereas the stability of the 512 and 435663 cages enclosing N2 has been addressed by ab initio calculation.17 In this study, we performed crystallographic analysis using powder X-ray diffraction (PXRD) to determine whether or not a N2 + NH + water system can form an sH hydrate. Then, by Raman spectroscopy, we investigated the Raman shifts of the N−N vibrations of N2 molecules enclosed in sH hydrates. N2 molecules are one type of guest molecule that forms hydrate crystals under high-pressure conditions than the equilibrium conditions for argon (Ar) hydrates.18,19 Because pure N2 hydrates are stable over 20 MPa at 275 K, we expect a large pressure difference (Δp) in the equilibrium conditions between pure N2 and sH hydrates containing N2 molecules. We also characterized the crystal phase boundaries of sH hydrates of N2 + NH.

obtained crystals were quenched at approximately 83 K by liquid N2. We prepared powdered samples for crystallographic investigation by grinding the quenched crystals. For comparison, we synthesized sH (CH4 + NH), pure sII N2, sI (N2 + CH4), and sII (N2 + C3H8) hydrates. The sH (CH4 + NH) hydrates were synthesized from the mixtures of ice + NH at 274 K and 2.7 MPa by a method similar to that for synthesizing sH (N2 + NH) hydrates. The pure sII N2, sI (N2 + CH4), and sII (N2 + C3H8) hydrates were prepared from ice at 253 K and 20 MPa, 272 K and 10 MPa (gas composition N2/ CH4 = 0.8:0.2), and 272 K and 1 MPa (N2:C3H8 = 0.6:0.4), respectively. The crystal structures of all the synthesized samples were determined through PXRD profiling.

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MATERIALS N2 (99.995% purity), CH4 (99.99% purity), and propane (C3H8, 99.9% purity) gases were supplied by Japan Fine Products Corp., Sumitomo Seika Chemicals Co., and Takachiho Chemical Industrial Co., Ltd., respectively. NH (≥99.0% purity) was supplied by Tokyo Chemical Industrial Co. Purified water (>18 MΩ cm) was produced by deionization and distillation. All materials were used without further purification.

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RESULTS AND DISCUSSION Figure 1 shows the PXRD pattern of samples obtained in the N2−NH−water system and the PXRD patterns of hydrates

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EXPERIMENTAL SECTION Powder X-ray Diffraction. The crystal structures of samples were determined using the PXRD method. An X-ray powder diffractometer (Rint-2500; Rigaku) equipped with a Cu Kα radiation source was used to obtain the PXRD profiles of the samples. The voltage and current of the X-ray source were 40 kV and 249 mA, respectively. The powdered samples were introduced into a quartz capillary cell (2.0 mm diameter, 0.01 mm thickness, and 10 mm length), and a temperature of approximately 153 K was maintained by blowing cool dry N2 gas on the samples during the measurements. The PXRD profiles were acquired using a step of 0.02° with a counting time of 1.2 s/step over 60 data acquisitions. Raman Spectroscopy. Raman spectra were collected using a Raman spectrometer (LabRAM HR-800, Horiba Ltd., Japan) equipped with a 2400 grooves/mm grating and a thermoelectrically cooled charge-coupled device (CCD) detector (size: 2048 × 512 pixels). The laser line wavelength was 532 nm (Torus 532, Laser Quantum). This configuration enabled Raman shifts to be collected at a spectral resolution of approximately 0.2 cm−1/pixel. The Raman shifts of the samples were calibrated using Si emission lines (520.6 cm−1) and collected at approximately 84 K using a cooling stage (HFS600E-P, Linkam Scientific Instruments) through an objective lens (Olympus SLMPLN 50; numerical aperture of 0.35; laser spot diameter of 51264). For spherical CH4 molecules, the order of the C−H vibrations may simply correlate with cage size; that is, looser cages exhibited lower Raman shifts (the “loose-cage tight-cage” model).13,14 In contrast, N2 molecules in 435663 cages (2323.31 cm−1) experience more attractive (or less repulsive) guest−host interaction than those in 51264 cages (2323.50 cm−1), whereas the guest/cage diameter ratio of 435663 cages is larger than that of the 51264 cages. The ratios of the 435663 and 51264 cages are 0.804 and 0.625, respectively. The full width at half-maximum (fwhm) of the N−N vibration peak in the 51264 cages became the broadest of the cages (Table 3). N2 molecules in 51264 cages are considered to have the highest collision rate among those in other cages, although 51264 cages show the smallest guest/cage diameter ratio. Horikawa et al.16 suggested that N2 molecules enclosed in 51264 cages distribute away from the centers of the cages, whereas N2 molecules in the slightly distorted 512 cages have preferred orientations.16,24 Consequently, the distance from the water framework to N2 in a 51264 cage would appear to be shortened. Thus, the N2 in 51264 cages would tend to experience more attractive interaction with the water framework than spherical guest molecules that tend to locate at the cage center. Simultaneously, the 435663 cages are more distorted in shape than the 512 cages due to guest enclosing.17 In the 512 cage of the sII hydrate, the preferred orientation is a lattice plane (111).16 The linear N2 molecules are also considered to have preferred orientations in 435663 cages. As shown in their small fwhm values, N2 molecules in the more distorted 435663 cages might be positioned at preferred orientations in order to decrease repulsive guest−host interaction as much as possible. In fact, N2 molecules in 435663 cages (2323.3 cm−1) experience more attractive interaction than those in sH 512 cages (2323.8 cm−1), whereas the shortest N···O distance between N2 and the water framework in the 435663 cages (0.335 nm) is much shorter than that in the sH 512 cages (0.362 nm).17 Consequently, not the cage sizes but the cage shapes would be predominantly responsible for the Raman shifts due to the linear N2 molecules. N2 molecules not only in 435663 cages but also in sH 512 cages would exhibit the preferred orientations seen in N2 in sII 512 cages, considering the fwhm value of the sH 512 cages (Table 3). Thus, 512 cages would be distorted depending on their orientation against the crystal planes. The shortened c axis of the sH hydrate enclosing N2 might be explained by the N2 orientation in the 512 cages. We also measured the L1−L2−H−V four-phase equilibrium pT conditions in the N2−NH−water system in the temperature range of 274 to 281 K. The measurement method and experimental setups are described in the SI file. Table 4 lists the

wavenumber region would be observed in the case of sII N2 hydrates.23 The 51264 cages of a sII hydrate of N2 + C3H8 are assumed to be mainly occupied by C3H8 molecules. Therefore, the intensity of N2 in 512 cages is much stronger than that of N2 in 51264 cages. Consequently, we estimated the two N−N vibration modes of the sII hydrates of N2 + C3H8 to be 2323.88 and 2323.50 cm−1 in the 512 and 51264 cages, respectively. In the sI hydrate of N2 + CH4 of Figure 2c, we can clearly observe two Raman peaks at 2323.0 and 2324.2 cm−1. The Raman peaks at the higher and lower wavenumber regions would be N2 in the 512 and 51262 cages, respectively, based on the fact that the peak position of N2 in the 512 cage is in good agreement with data from the literature (2324 cm−1).2 In the sH hydrate of N2 + NH, as shown in Figure 2a, a Raman band with two bumps was clearly composed of two peaks. The band with two bumps should be assigned to the N− N vibrations of N2 in the 512 and 435663 cages of the sH hydrate. According to the “loose-cage tight-cage” model,13,14 the Raman peaks at higher and lower frequencies represent N− N vibrations of N2 in the 512 and 435663 cages, respectively. From the peak fitting results by using mixed Gaussian− Lorentzian function, the two peaks associated with the 512 and 435663 cages appear at 2323.82 and 2323.31 cm−1, respectively, in the sH hydrate of N2 + NH. We can estimate the cage occupancy ratios (θM/θS) of the 435663 to 512 cages from the relative intensity ratio of the two peaks by considering the number of each cage (three 512 and two 435663 cages) in a unit cell. In the case of the sH hydrates of CH4 + LMGSs, the θM/θS of CH4 is 0.7−1.3.12,15,20,27,28 The θM/θS of the sH hydrate of N2 + NH was found to be 1.30. We consider the cage occupancy of N2 in the 512 and 435663 cages of the sH hydrate to be similar to that of sH hydrates having CH4. Table 3 lists the N−N vibrational modes observed in the sH, sII, and sI hydrates having N2 molecules and the size properties of each cage. In this study, the frequency shifts of the Raman peaks for N2 in the three hydrate crystal system decreased in the following order: 512 cages in sI (2324.17 cm−1) > 512 cages in sII (2323.88 cm−1) ≈ 512 cages in sH (2323.82 cm−1) > 51264 cages in sII (2323.50 cm−1) > 435663 cages in sH (2323.31 cm−1) > 51262 cages in sI (2322.98 cm−1). In the CH4 molecule, the C−H vibration Raman shift decreased in the following order: 512 cages in sI > 512 cages in sII > 512 cages in sH > 435663 cages in sH > 51264 cages in sII > 51262 cages in sI.15 The cage radii increased in the following order: 512 cages in sII (average: 0.391 nm) < 512 cages in sH (0.394 nm) < 512 cages in sI (0.395 nm) < 435663 cages in sH (0.404 nm) < 51262 cages in sI (0.433 nm) < 51264 cages in sII (0.473 nm).2 A comparison of the N2 and CH4 molecules shows that their Raman shift orders are reversed in the 51264 and 435663 cages. The C−H vibrational mode shows that CH4 in the 435663 cages (2910.1 cm−1) experiences more repulsive (or less attractive) 9072

DOI: 10.1021/acs.jpcc.5b00529 J. Phys. Chem. C 2015, 119, 9069−9075

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The Journal of Physical Chemistry C determined L1−L2−H−V four-phase equilibrium pT conditions in the N2−NH−water system. Figure 3 shows the L1−L2−H−V

Table 4. L1−L2−H−V Four-Phase Equilibrium Pressure, Temperature Conditions, and Dissociation Enthalpies for the N2−NH−Water System Ta/K

pb/MPa

compressibility factor, z30

ΔH/kJ·mol−1

274.36 275.35 276.44 276.88 277.41 278.34 279.53 280.16 280.71

6.04 6.92 7.88 8.39 8.96 9.96 11.58 12.45 13.25

0.9839 0.9842 0.9854 0.9862 0.9874 0.9900 0.9955 0.9992 1.0029

388.4 388.5 389.0 389.3 389.8 390.8 393.0 394.4 395.9

a

Uncertainties in temperature measurements were estimated to be ±0.02 K with a confidence level of approximately 95%. bUncertainty of the pressure measurements was ±0.025 MPa for pressures ≤10 MPa and ±0.062 MPa for pressures >10 MPa.

Figure 3. Equilibrium pressure−temperature conditions for the N2− NH−water (sH hydrate) and N2−water (sII hydrate) systems. Red closed circles: N2−NH−water system (this study), black closed circles: N2−water system (Yasuda et al.29), open circles: N2−water system (van Cleeff and Diepen18), and black closed squares: N2−water system (Jhaveri and Robinson19).

hydrates containing NH decrease in the following order Ar > N2 ≈ Xe > Kr > CH4 in the experimental temperature range of this study. Although the sH hydrates containing Ar exhibited the largest ΔH, Ar molecules constitute only approximately 0.93% of the atmosphere. In contrast, N2 molecules constitute approximately 78.1% of the atmosphere. Thus, the sH hydrates containing N2 may be more useful as thermal storage media than sH hydrates containing other small guest molecules.

four-phase equilibrium data for the system and plots of the L− H−V and I−H−V three-phase equilibrium conditions of N2 hydrates.18,19,29 This figure indicates a large Δp of approximately 14.5 MPa at 276 K, whereas Δp = 9 MPa between the Ar−water and Ar−NH−water systems.30 This pressure drop, caused by adding NH as LMGS, is the largest among the NHcontaining sH hydrates reported in the literature.6,7,10,11,29 Recently, Murayama et al.30 reported that Ar, which is encapsulated in sII hydrates at high-pressure conditions (>11 MPa at 275 K),31 can also be stored in the sH hydrates in NHcoexisting systems at milder pT conditions.30 From the literature, the Δp between the pure (sI or sII) and sH hydrates having small guest molecules is higher for guest molecules exhibiting higher-pressure and lower-temperature equilibrium conditions. For example, Δp between the Ar and sH hydrates of Ar + NH is approximately 9 MPa at 276 K,30 whereas Δp between the CH4 and sH hydrates of CH4 + NH is approximately 2 MPa.10 The dissociation enthalpy (ΔH) of hydrates can be estimated with the Clausius−Clapeyron equation:

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CONCLUSIONS We synthesized crystalline hydrate samples in a N2−NH−water system and characterized the sH hydrates using PXRD and Raman spectroscopic methods. The PXRD profile of the synthesized crystals revealed patterns consistent with the sH hydrate of CH4 + NH with the space group P6/mmm. The unit cell dimensions of the sH hydrate obtained in the N2−NH− water system were estimated to be a = 1.22342(15) nm and c = 0.99906(17) nm at 153 K. The c axis of the sH hydrate of N2 + NH was slightly shorter (0.00584 nm) than that of CH4 + NH, whereas no difference in the a axis was observed when N2 versus CH4 was enclosed. We succeeded in observing the symmetric N−N stretching vibrational (N−N vibration) peak with two bumps in the Raman spectrum. The N−N vibrational mode in the 512 and 435663 cages occurred at approximately 2323.8 and 2323.3 cm−1, respectively, according to the “loosecage tight-cage” model. The 435663 to 512 (θM/θS) was estimated to be 1.30 from the two N−N vibration peaks. The N−N vibrational mode in the 435663 cage peaked at a lower frequency (2323.3 cm−1) than that in the 51264 cage of the sII N2 hydrate (2323.5 cm−1). The 435663 cages (radius: 0.404 nm) were found to be much smaller than the 51264 cages (0.473 nm). The fwhm of the N−N vibration peak in the 51264 cages became the broadest among the cages of the sI, sII, and sH hydrate systems. In contrast, the fwhm of the 435663 cage shows a small value, although the cavity of the 435663 cage is as small as that of the 512 cage. This implies that the corrosion rate of the N2−water framework of 435663 cages is lower than that of 51264 cages. N2 molecules in 51264 cages distribute from the center of the cages to near the water framework. Because the 435663 cage is distorted in shape, the N2 molecules in the 435663 cages might be positioned at preferred orientations to decrease repulsive guest−host interaction as much as possible. Consequently, not the cage sizes but the cage shapes would be predominantly responsible for the Raman shifts due to the

d ln p ΔH =− d(1/T ) zR

where p, T, z, and R are the pressure, temperature, compressibility factor, and gas constant, respectively. In this study, we estimated the z values at each equilibrium condition using the Lee−Kesler method.32 Assuming that the 512 and 435663 cages in the sH hydrate crystals were completely occupied by N2, we estimated the ΔH of the sH hydrates containing N2 + NH. Table 4 lists the estimated z and ΔH values. The ΔH values of the sH hydrates having N2 + NH were estimated to be approximately 388.4 and 395.9 kJ·mol−1 (per one mole of N2 molecules) in the experimental temperature range of 274.36 to 280.71 K. The ΔH values are dependent on LMGSs in the sH hydrate systems having the same small guest molecules.33 In a temperature range similar to that of this study, the ΔH values of other sH hydrates having NH were estimated to be 425.5−432.8 kJ·mol−1 for sH hydrates containing Ar + NH, 381.7−387.5 kJ·mol−1 for Kr + NH, 370.2−382.0 kJ·mol−1 for CH4 + NH, and 389.2−392.5 kJ·mol−1 for Xe + NH.6,7,10,11,30 Thus, the values of ΔH for sH 9073

DOI: 10.1021/acs.jpcc.5b00529 J. Phys. Chem. C 2015, 119, 9069−9075

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The Journal of Physical Chemistry C linear N2 molecules. We also determined the L1−L2−H−V four-phase equilibrium pT conditions in the N2−NH−water systems were in the temperature range of 274.36−280.71 K. The L1−L2−H−V of sH hydrates having N2 + NH exhibited lower-pressure and higher-temperature (milder) shifts than the N2 hydrates. The equilibrium pressure conditions of the sH hydrate of N2 + NH and the N2 hydrate were approximately 7.5 and 22 MPa, respectively, at 276 K. Using the Clausius− Clapeyron equation, we estimated ΔH of the sH hydrates having N2 + NH to be 388.4 and 395.9 kJ·mol−1 (per one molar of N2 molecules). In the experimental temperature range of this study, the ΔH of the sH hydrate systems containing NH decreased in the following order: Ar−NH−water (approximately 430 kJ·mol−1) > N2−NH−water (approximately 392 kJ·mol−1) ≈ Xe−NH−water (approximately 390 kJ·mol−1) > Kr−NH−water (approximately 384 kJ·mol−1) > CH4−NH− water (376 kJ·mol−1).

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ASSOCIATED CONTENT

S Supporting Information *

PXRD profiles of mixed gas hydrates used in the Raman study and the method for determining the phase equilibrium conditions for a N2−NH−water system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* (Y. J.) E-mail: [email protected]. Fax: +81-11-857-8417. Tel.: +81-11-857-8526. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Y. Konno, M. Oshima, and H. Morita of AIST for their experimental support and valuable discussions.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b00529 J. Phys. Chem. C 2015, 119, 9069−9075

Article

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DOI: 10.1021/acs.jpcc.5b00529 J. Phys. Chem. C 2015, 119, 9069−9075