Structure H (sH) Clathrate Hydrates in Methane–Halogenic Large

Article Cite This: J. Phys. Chem. C 2019, 123, 17170−17175

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Structure H Clathrate Hydrates in Methane−Halogenic Large Molecule Substance−Water Systems Yusuke Jin,*,† Masato Kida,†,‡ and Jiro Nagao† †

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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 ‡ School of Earth, Energy and Environmental Engineering, Faculty of Engineering Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan S Supporting Information *

ABSTRACT: We investigated crystallographic variations in structure H (sH) hydrates hosting CH4 and halogenic large-molecule guest substances [halogenic large-molecule guest substance (LMGS)]. The three halogenic LMGSs, namely, chlorocyclohexane (ClCH), bromocyclohexane (BrCH), and iodocyclohexane (ICH), share a common molecular structure (X-cyclohexane). The lattice constants along a and c axes of sH hydrates hosting X-cyclohexane increased with increasing molecular size: ClCH < BrCH < ICH. The c lattice constant was especially dependent on the molecular size, possibly because X-cyclohexanes align along the longitudinal direction of the 51268 cage, which coincides with the c-axis. Raman spectroscopy revealed that LMGSs altered the surroundings of CH4 molecules in the 512 and 435663 cages. As the crystal size increases, CH4 molecules encounter more attractive (less repulsive) guest−host interactions. The wavenumber shifts of the C− H vibrations of CH4 in the 512 and 435663 cages increased with temperature and were slightly greater in the 435663 cages than in the 512 cages. Different thermal responses between the 512 and 435663 cages may be caused by anisotropic lattice expansion of the sH hydrates. Finally, the phase stabilities of the sH (CH4 and X-cyclohexane) hydrates were evaluated by an isochoric method. The pressure region of equilibrium pressure−temperature conditions was lower in the sH (CH4 and X-cyclohexane) hydrates than in the pure CH4 hydrate system. Moreover, the temperature region of the equilibrium pressure−temperature conditions increased in the order ICH < ClCH < BrCH. The dissociation enthalpies of the sH (CH4 + ClCH) and sH (CH4 + ICH) hydrates were estimated as 380 kJ/mol−1.

1. INTRODUCTION Clathrate hydrate (hydrate) is a crystalline compound that traps its guest molecules in cages formed by the hydrogenbonded H2O framework.1 Hydrate crystals can be classified into three primary structures: structure I (sI), structure II (sII), and structure H (sH), which are changed by the guest molecules. More specifically, the structures of hydrates enclosing the guest molecules are combinations of five cage structures: dodecahedral (5 12 ), irregular dodecahedral (4 3 5 6 6 3 ), tetrakaidekahedral (5 12 6 2 ), hexakaidecahedral (51264), and icosahedral (51268).2 The sH hydrate consists of 512, 435663, and 51268 cages and exhibits a hexagonal crystal structure of space group P6/mmm. The sH hydrate can store large guest molecules (large-molecule guest substances, LMGSs) in its largest 51268 cages, along with small help guest molecules2,3 such as CH4, N2, Kr, and Xe.2,4−7 Almost all sH hydrates with help guests and LMGSs, hereafter called sH (help guest + LMGS) hydrates, form under lower-pressure and higher-temperature conditions than the phase equilibrium pressure−temperature (pT) condition of hydrates with help guests alone.8−11 The mild equilibrium pT conditions of sH © 2019 American Chemical Society

hydrates are expected to be appropriate for gas storage, transportation, and separation.12,13 Relationships between help guests/LMGS and the crystallographic properties of sH hydrates (such as lattice constants, guest−host interactions, and phase stabilities) have been extensively reported in the literature.5,14−25 For example, as the LMGS volume increases, the unit cell volume of the sH hydrate increases,18 whereas the lattice constants along the c and a axes decrease and increase, respectively.18,25 Conversely, Murayama et al. suggested that as the size of the helper guests increases, the c lattice constant increases remarkably more rapidly than the a lattice constant.16 Which guest parameters determine the crystallographic properties in sH hydrates remains insufficiently understood. The present paper investigates crystallographic properties variation in sH hydrates hosting CH4 (help guest) and LMGSs with common molecular structures. LMGSs are halogenic Received: May 17, 2019 Revised: June 20, 2019 Published: June 21, 2019 17170

DOI: 10.1021/acs.jpcc.9b04691 J. Phys. Chem. C 2019, 123, 17170−17175

Article

The Journal of Physical Chemistry C molecules with the molecular structure X−C6H11 (hereafter, referred to as halogenic cyclohexane or X−CH), chlorocyclohexane (ClCH), bromocyclohexane (BrCH), and iodocyclohexane (ICH). The crystal lattices were determined from powder X-ray diffraction (PXRD) measurements, and the guest−host interactions between CH4 and the cages after adding halogenic cyclohexanes were analyzed by Raman spectroscopy. We also report the phase stabilities in the new CH4−ClCH−water and CH4−ICH−water systems and compare the phase equilibrium pressure−temperature (pT) conditions in CH4−halogenic cyclohexane (ClCH, ICH, and BrCH)−water systems.

Techno Co.) with a stirring fin. The system temperature was measured by using thermistors (D642-10 and SZL-64, Technol Seven Co. Ltd., Japan) with a measurement uncertainty of ±0.02 K. The system pressure was measured by two pressure transducers: a PHS-B-10MP (Kyowa Co., Japan) at pressures ≤10 MPa and a PHS-200KA-P (Kyowa Co., Japan) at pressures above 10 MPa. Both transducers were combined with a signal conditioner (CDV-900A; Kyowa Co., Japan). The pressure uncertainties were ±0.025 MPa in the ≤10 MPa range and ±0.062 MPa in the >10 MPa range. The experimental setups, procedures, and reliability are detailed in previous reports.5,20

2. EXPERIMENTS 2.1. Materials. Chlorocyclohexane (ClCH) (>99% purity, purchased from Aldrich-Sigma Co., Inc.), ICH (>98.0% purity, purchased from Tokyo Chemical Industry Co., Ltd., Japan), bromocyclohexane (BrCH) (>98.0% purity purchased from Aldrich-Sigma Co., Inc.), and CH4 gas (>99.99% purity, purchased from Sumitomo Seika Chemicals Co., Japan) were used. Water was purified by ultrafiltration, reverse osmosis, deionization, and distillation. All materials were used without further purification. 2.2. Crystallographic Analysis. Samples for crystallographic analysis were prepared using a high-pressure vessel (TAF-SR-50, Taiatsu Techno Co., Japan). The ice + halogenic LMGS (0.5 mL) mixtures were stored in the high-pressure vessel at the stoichiometric ratios of ice/LMGS in the sH hydrates.2 After air elimination by a vacuum pump, the vessel was flushed three times with CH4 at 1 MPa. Then, the highpressure vessel was pressurized at 263.15 K by CH4 applied at 2.5 MPa. The temperature of the pressurized mixtures was ramped up to 273.05 K and maintained at that temperature for 120 h. During this time, the system pressure decreased as the hydrates were formed. The hydrate crystalline samples were then obtained by quenching the vessel at liquid-N2 temperature. PXRD patterns of crystal samples were collected using an Xray diffractometer (SmartLab; Rigaku Corp., Japan) with a Cu Kα radiation source and a one-dimensional detector (D/teX Ultra). The X-ray source voltage and current were 45 kV and 200 mA, respectively, and 2θ was measured in the range 5−42° with a scan step of 0.01° and a scan speed of 4.0°. During PXRD measurements, sample temperature was controlled by using a low-temperature controlled chamber (TTK 450, Anton Paar GmbH). The Raman spectra were collected using a Raman spectrometer (LabRAM HR-800, Horiba Ltd., Japan) with a 532-nm laser line (Torus 532, Laser Quantum), a thermoelectrically cooled charge-coupled device detector (2048 × 512 pixels) and grating with 2400 grooves/mm. Spectral measurements were performed by using a cooling stage (HFS600E-P, Linkam Scientific Instruments). The Raman shifts were calibrated by neon emission lines in the measurement wavenumber range. The estimated uncertainty in the calibrated Raman shift was ±0.06 cm−1 with a confidence level of approximately 95%, considering the mechanical reproducibility of our spectrometer (see Figure S1 in the Supporting Information). 2.3. Equilibrium Pressure−temperature Measurements. The phase-equilibrium pT data of the CH4−halogenic LMGS (ClCH and ICH)−water systems were collected by an isochoric method using a pressure vessel (TVS-N2, Taiatsu

3. RESULTS AND DISCUSSION 3.1. Crystal Characterization. Figure 1 shows the PXRD profiles of crystals sampled in the new CH4−halogenic LMGS

Figure 1. PXRD profiles of the samples obtained in the (a) CH4− ClCH−water, (b) CH4−ICH−water, and (c) CH4−BrCH−water systems. Black lines: observed data. Red lines: sH hydrate profiles. Blue line: ice Ih profiles. All PXRD profiles were collected at 83 K.

(ClCH and ICH)−water systems. The PXRD patterns in both systems resemble that of sH (CH4 + BrCH) hydrate profile in the CH4−BrCH−water system.20 Using the PXRD analysis software PDXL (Rigaku Corp.), the lattice parameters at 83 K were estimated as a = 1.21494(12) nm and c = 1.00163(9) nm in the sH (CH4 + ClCH) hydrate and as a = 1.2166(4) nm and c = 1.0056(3) nm in the sH (CH4 + ICH) hydrate. Meanwhile, the lattice parameters of the sH (CH4 + BrCH) hydrate at 83 K are a = 1.21613(10) nm and c = 1.00352(8) nm. The PXRD analysis results at 83 K are listed in Table 1. The lattice constants along the a- and c-axes of the sH hydrates hosting the halogenic LMGS were 1.215−1.217 and 1.001−1.006 nm, respectively. The molecular volumes of ClCH, ICH, and BrCH, obtained by minimizing the structural energies of the molecules in MOPAC2016,26 increased in the order ClCH (0.1156 nm3) < BrCH (0.1200 nm3) < ICH (0.1257 nm3). The lattice constants along the a- and c-axes increased with increasing volume of the halogenic LMGS (see also Table 1). A previous study reported a similar increase in the lattice constants of sH hydrates with increasing volume of LGMS.26 MCH orients along the longitudinal direction of the 51268 cage (namely, the c-axis direction.26,27). ClCH, ICH, and BrCH in the 51268 cages would also preferentially align in the c17171

DOI: 10.1021/acs.jpcc.9b04691 J. Phys. Chem. C 2019, 123, 17170−17175

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The Journal of Physical Chemistry C

Table 1. Lattice Parameters and Raman Peak Positions of the Samples Obtained in the CH4−Halogenic LMGS−Water Systemsa lattice parameter/nm (83 K) unit volume/nm3 (83 K) Raman peak/cm−1 (83 K) cage occupancy ratio, θID/θD

a c 435663 512

ClCH

ICH

BrCH

1.21494(12) 1.00163(9) 1.2804(2) 2909.11 ± 0.06 2910.97 ± 0.06 1.11 ± 0.14

1.2166(4) 1.0056(3) 1.2890(7) 2908.58 ± 0.10 2910.35 ± 0.11 1.00 ± 0.10

1.21613(10) 1.00352(8) 1.28533(18) 2909.01 ± 0.04 2910.83 ± 0.06 0.88 ± 0.14

a

Errors in the Raman peak and cage occupancy ratio were uncertainty with a confidence level of approximately 95%.

and the LMGS affect the anisotropic lattice expansion of an sH hydrate structure. Molecular length of ClCH (0.8574 nm) is longer than MCH (0.8373 nm). Nevertheless, there was a slight difference of the c-axis between sH (CH4 + MCH) and sH (CH4 + ClCH) hydrates. In sH hydrate having LMGSs with ether/hydroxy groups, the c-axis tends to be shorter than similar LMGSs without ether/hydroxy groups.18 Recently, halogen bonding (−X···O−Y) between halogen (Cl2/Br2) and water molecules of the cage structure were computationally confirmed by ab initio calculations.29,30 The short c-axis in sH (CH4 + ClCH) may be originated by a guest−host interaction between the halogen atom in LMGS and water framework like halogen bonding. Figure 3 shows the C−H vibration regions of CH4 in the three sH (CH4 + X-LMGS) hydrates. The Raman spectra of all

axis direction. The length of the halogenic LMGS molecules increased in the order ClCH (0.8574 nm) < BrCH (0.8864 nm) < ICH (0.9127 nm). Consequently, the length of the caxis remarkably changed after caging the LMGSs (ranging from 1.001 after caging ClCH to 1.006 nm after caging ICH). The thermal lattice expansions of the sH hydrates hosting CH4 and halogenic cyclohexanes (ClCH, ICH, and BrCH) were observed by PXRD measurements as the temperature increased from 83 to 183 K. The lattice expansions during temperature ramping are plotted in Figure 2. The lattice

Figure 2. Lattice expansion behavior of the sH (CH4 + Xcyclohexane) hydrates during temperature ramping. Red squares: sH (CH4 + ClCH) hydrate. Green squares: sH (CH4 + BrCH) hydrate. Blue squares: sH (CH4 + ICH) hydrate. Open squares: sH (CH4 + MCH) hydrate.15

constants linearly increased with temperature, as reported for other clathrate hydrates.15,28 The lattice expansions of the sH hydrates were anisotropic along the a- and c-axes.19 Such anisotropic behavior originates from the size of the help guest.16 The thermal expansions of the sH (CH4 + X-LMGSs) hydrates also demonstrated anisotropic behavior. In particular, the a-axis/c-axis ratios slightly increased with temperature, indicating a higher expansion tendency of the a-axis than the caxis. Furthermore, the temperature dependence of the changes in the a-axis/c-axis ratios depended on the LMGS size: the larger the LMGS, the greater was the increase in a-axis/c-axis with temperature. Therefore, the sizes of both the help guest

Figure 3. Raman spectra of the C−H stretching vibration region: (a) sH (CH4 + ClCH) hydrate, (b) sH (CH4 + ICH) hydrate, and (c) sH (CH4 + BrCH) hydrate. Red and blue lines are the C−H stretching vibrations of the CH4 molecules trapped in 435663 and 512 cages, respectively. All spectra were measured at 83 K.

sH hydrates exhibit a strong peak near 2910 cm−1, which is attributable to the CH4 molecules in the 512 (D) and 435663 (ID) cages. The strong C−H vibration peak in the spectra of the new sH hydrates hosting ClCH and ICH can be decomposed into two peaks, as in previous studies.20,21 From the peak fitting results obtained by the commercial multiple17172

DOI: 10.1021/acs.jpcc.9b04691 J. Phys. Chem. C 2019, 123, 17170−17175

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cyclohexanes. The Raman shift values were averaged over three measurements. All Raman peaks shifted toward higher wavenumbers as the temperature increased, indicating enhanced thermal vibration. The lattices of all three sH hydrates were thermally expanded (see Figure 2). In the sI hydrate hosting carbon monoxide (CO) molecules, the C−O vibration of CO shifted toward lower wavenumbers as the lattice constants increased during temperature ramping.34 This trend likely results from changes in the guest−host interaction according to the “loose cage-tight cage” model.33 The N−N vibrations in the sII and sH hydrates also showed thermal effects (see Figure S2 of Supporting Information). In nonpolar molecules (such as N2 and CH4) in cages, thermal change may affect not the guest−host interaction, but the molecular vibrations. For all three hydrates, the C−H vibrations of CH4 in the Mcages were slightly more sensitive to environmental temperature than those in the D-cages. The Raman shifts changed by approximately 0.01 cm−1/K in the ID-cage and by 0.007 cm−1/ K in the S-cage. A similar trend appeared in the N−N vibrations of sH hydrate (Figure S2). The lattices of the sH hydrates expanded anisotropically with temperature, as discussed above. The thermal vibration differences in the Dand ID-cages may originate from anisotropic lattice expansion behaviors. 3.2. Phase Equilibrium pT Conditions. The phase equilibrium data in CH4−BrCH−water have been reported in the literature,20 but the phase equilibrium pT data in the CH4−ClCH−water and CH4−ICH−water systems remain to be determined in this study (Table 2). Figure 5 shows the four-

peak-fitting program PeakFit v4.12, the C−H vibrations of the ID and D cages occurred at 2909.11 and 2910.97 cm−1, respectively, in the sH (CH4 + ClCH) hydrate, and at 2908.58 and 2910.35 cm−1, respectively, in the sH (CH4 + ICH) hydrate. The ID and D cages in the sH (CH4 + BrCH) hydrate peaked at 2909.01 and 2910.83 cm−1, respectively. The Raman shift values were averaged over three measurements. Occupancy ratios θID/θD can be estimated from relative peak area ratio 3AID/2AD. On average, the occupancy ratios θID/θD of the sH (CH4 + ClCH), sH (CH4 + ICH), and sH (CH4 + BrCH) hydrates were estimated as 1.11 ± 0.1, 1.00 ± 0.1, and 0.88 ± 0.1, respectively. θID/θD values of the two new sH hydrates agreed with those of several sH hydrates hosting CH4 in previous reports (0.7−1.3).20,21,24,26,31,32 The peak positions and θID/θD ratios of the two new sH hydrates and of other sH (CH4 + BrCH) hydrates are summarized in Table 1. The Raman peak positions of a guest molecule are determined by the “loose cage-tight cage” model.33 Because the D-cage (0.394 nm in the averaged cage radius) is tighter than the ID-cage (0.404 nm),4 CH4 molecules experience a more repulsive (less attractive) guest−host interaction in a Dcage than in an ID-cage. Consequently, the C−H vibration peak of CH4 shows a higher Raman shift in a D-cage than in an ID-cage. The C−H vibration in the ID- and D-cage increased in the order of ClCH < BrCH < ICH. Note that the crystal unit volumes increased in the same order, as revealed in the PXRD results. C−H vibrations in the spectra of D- and IDcages were approximately 0.6 cm−1 higher for the smallest ClCH than for the largest ICH. Hydrate crystals with large lattice constants enlarge the cage size. Clearly, the guest−host interaction is related with the lattice size. Fuseya et al.15 demonstrated a shift toward higher wavenumbers in the C−H peak of CH4 in sI hydrate and the composite one C−H peak in sH hydrates during temperature ramping. Meanwhile, the peaks in the spectra of sH hydrates cannot be spectrally resolved. Figure 4 shows the temperature dependence of the C−H vibrations of CH4 in the D- and ID-cages of the sH hydrates hosting halogenic

Table 2. Hydrate−Liquid Water−Liquid LMGS−CH4-Rich Vapor (H−L1−L2−V) Four-Phase Equilibrium Pressure− Temperature Conditions in CH4−Halogenic LMGS−Water Systems CH4−ClCH−water a

b

CH4−ICH−water a

T /K

p /MPa

T /K

pb/MPa

276.78 278.94 281.83 283.19 284.98 287.18 288.78

2.288 3.002 4.216 5.020 6.172 8.123 9.843

276.71 279.06 281.26 282.93 284.48 286.69 288.54

2.390 3.183 4.121 5.084 6.191 8.094 10.277

a

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

phase L1−L2−H−V equilibrium pT conditions in the CH4− X−CH (ClCH, BrCH, and ICH)−water systems. The phase equilibrium curves in the CH4−ClCH−water and CH4−ICH− water systems appear at milder (lower pressure or higher temperature) pT conditions than in the pure CH4−water system. Similar shifts from the pure system have been observed in other sH hydrate systems. The conditions of the phase equilibria in CH4−X−CH−water systems were in the order ICH < ClCH < BrCH. The dissociation enthalpies (ΔHs) were estimated using the Clausius−Clapeyron relationship. At 280 K and ∼3 MPa (compressibility factor of CH4 z = 0.9367), the ΔH values of sH (CH4 + ClCH), sH (CH4 + BrCH), and sH (CH4 + ICH)

Figure 4. Temperature dependence of the C−H vibration peaks in the examined hydrates: sH (CH4 + ClCH) (red), sH (CH4 + BrCH) (green), and sH (CH4 + ICH) (blue). Filled squares and circles are the results of CH4 molecules trapped in 435663 and 512 cages, respectively. Error bars indicate uncertainty with a confidence level of approximately 95%. 17173

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system. The equilibrium pT conditions shifted toward higher temperatures in the order ICH < ClCH < BrCH. The dissociation enthalpies of the sH (CH4 + ClCH) and sH (CH4 + ICH) hydrates were approximated as 380 kJ/mol−1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04691.

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Figure 5. Four-phase L1−L2−H−V equilibrium pressure−temperature conditions in the CH4−halogenic LMGS (ClCH, BrCH, and ICH)−water systems. Red, green, and blue filled circles refer to the CH4−ClCH−water, CH4−BrCH−water,20 and CH4−ICH−water systems, respectively. Open squares refer to the CH4−MCH−water system35,36 and the black filled circles represent the three-phase I− H−V and L−H−V equilibrium pressure−temperature conditions in the CH4−water system.9,36−38 Uncertainties in the temperature and pressure measurements were estimated as ±0.02 K, and ±0.025 MPa in the ≤10 MPa range and ±0.062 MPa in the >10 MPa range, respectively.

Uncertainty in the calibrated Raman shifts and an example of the temperature dependence of N−N vibrations in the sH hydrate hosting N2 and methylcyclohexane (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-11-857-8526. Fax: +8111-857-8417. ORCID

Yusuke Jin: 0000-0002-6256-7278 Masato Kida: 0000-0003-0075-9781 Notes

The authors declare no competing financial interest.

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hydrates in each system were estimated as approximately 378, 366, and 383 kJ/mol−1 (per mole XCH), respectively. Similar ΔH was reported for sH (CH4 + MCH) hydrate (377 kJ/ mol−1) in an earlier study.9

ACKNOWLEDGMENTS The authors thank Dr. M. Oshima and Dr. J. Yoneda of AIST and Bs. G. Fuseya of Kitami Institute of Technology for valuable discussion. The authors especially express gratitude to Dr. H. Haneda of AIST for experimental support.

4. CONCLUSIONS We synthesized sH hydrates enclosing methane (CH4) and new LMGSs and assessed their crystallographic properties by PXRD and Raman spectroscopy techniques. The new LMGSs (chlorocyclohexane (ClCH) and ICH) possess a common molecular structure (X-cyclohexane). At 83 K, the lattice parameters were determined as a = 1.21494(12) nm and c = 1.00163(9) nm in sH (CH4 + ClCH), and as a = 1.2166(4) nm and c = 1.0056(3) nm in sH (CH4 + ICH). Meanwhile, the lattice parameters of the already-reported sH (CH4 + BrCH) hydrate are a = 1.21613(10) nm and c = 1.00352(8) nm. The crystal size increases with increasing size of the LMGS. Because X-cyclohexanes are positioned along the longitudinal direction in the 51268 cages aligned along the c-axis, the lattice constant along the c-axis strongly depends on the molecular size of cyclohexanes. Thermal lattice expansions in all sH (CH4 + halogenic cyclohexane) hydrates are anisotropic, with higher temperature response along the a-axis than along the c-axis. The C−H vibrations of CH4 enclosed in the 512 and 435663 cages shifted toward lower wavenumbers with increasing crystal size of halogenic cyclohexanes. Furthermore, the C−H vibrations shifted toward higher wavenumbers as the temperature ramped up, and the lattice constants were thermally extended. Thermal vibrations can explain the shift to higher wavenumbers in the C−H vibrations of the caged CH4. The thermal component of the C−H vibrations was slightly stronger in the 435663 cages than in the 512 cages. Different thermal responses in the 512 and 435663 cages may be caused by the anisotropic lattice expansion of the sH hydrates. The four-phase L1−L2−H−V equilibrium pT conditions in the CH4−ClCH−water and CH4−ICH−water systems were also determined and compared with that of the CH4−BrCH−water

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REFERENCES

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DOI: 10.1021/acs.jpcc.9b04691 J. Phys. Chem. C 2019, 123, 17170−17175

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DOI: 10.1021/acs.jpcc.9b04691 J. Phys. Chem. C 2019, 123, 17170−17175