Structure H (sH) Clathrate Hydrate with New Large Molecule Guest

Oct 11, 2013 - Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), ...
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Structure H (sH) Clathrate Hydrate with New Large Molecule Guest Substances Yusuke Jin,* Masato Kida, and Jiro Nagao* Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan S Supporting Information *

ABSTRACT: This study characterized new structure H (sH) clathrate hydrates with bromide large-molecule guest substances (LMGSs) bromocyclopentane (BrCP) and bromocyclohexane (BrCH), using powder X-ray diffraction (PXRD) and Raman spectroscopy. The lattice parameters of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined from their PXRD profiles. On the basis of their Raman spectra, the M-cage to S-cage occupancy ratio (435663 and 512 cages, respectively), θM/θS, was estimated to be approximately 1.3, and the Raman shift of the symmetric C−H vibrational modes of CH4 in S- and M-cages was 2911.1 and 2909.1 cm−1, respectively. The phase-equilibrium conditions of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined by an isochoric method. A comparison between the equilibria of sH hydrates with BrCP and BrCH and those with other typical nonpolar and polar LMGSs (methylcyclopentane, MCP; methylcyclohexane, MCH; neohexane, NH; and tert-butyl methyl ether, TBME) at the same temperature revealed that the equilibrium pressure increased in the order NH < MCH < BrCH < TBME ∼ MCP < BrCP. The phase stabilities of sH hydrates can be determined by not only molecular geometry but also their polar properties, which affect guest−host interactions.

1. INTRODUCTION Because of their ability to store gases, gas clathrate hydrates (gas hydrates) are attractive as potentially new gas-storage media. Gas hydrates are crystalline compounds formed from water and guest gas molecules and can store gases such as hydrogen (H2), methane (CH4), ethane (C2H6), and propane (C3H8).1,2 The size of the stored gas molecule helps determine the particular crystal structure of a gas hydrate, which generally falls into one of the following three types: structure I (sI), structure II (sII), or structure H (sH). Gas molecules are stored in cavities that are formed by a framework of water molecules. The framework usually consists of two or three cages from among five principal types: pentagonal dodecahedral (512), tetrakaidecahedral (51262), hexakaidecahedral (51264), irregular dodecahedral (435663), and icosahedral (51268) cages.1,2 sH hydrates consist of three 512 cages (hereafter S-cages), two 435663 cages (M-cages), and one large 51268 cage (L-cage).3 Therefore, because of the presence of the L-cage, sH hydrates can store large-molecule guest substances (LMGSs), which cannot be stored in the cages of sI and sII hydrates. LMGSs that are stored in sH hydrates coexist with help-gas molecules such as H2, CH4, C2H6, Xe, or Kr.1,2,4 In general, gas hydrates form in gas−water systems under high-pressure or lowtemperature conditions.1,2 However, sH hydrates can also form at lower pressures and higher temperatures (i.e., milder conditions) than gas hydrates that enclose only a help gas; consequently, sH hydrates are attractive gas-storage and separation media.5,6 © 2013 American Chemical Society

LMGSs enclosed in sH hydrates can be polar molecules, such as tert-butyl methyl ether (TBME) and 3-methyl-1-butanol (isoamyl alcohol), or liquid hydrocarbons (LHCs), such as methylcyclohexane (MCH), methylcyclopentane (MCP), and neohexane (NH).2,7 New LMGSs are being explored for applications that involve sH hydrates.8,9 When TBME is encaged in L-cages, the c lattice parameter of the sH hydrate is shorter than that of the sH hydrate enclosing LHCs.10 Recently, Kida et al.11 demonstrated that guest−host interactions, as indicated by the ratio of the guest diameter to the cavity diameter, affect the 13C chemical shifts and peak sharpness of CH4 in cages. Because TBME, (CH3)3C−O−CH3, contains an ether group, it interacts strongly with water molecules in solutions;12 therefore, the ether group of encaged TBME is believed to affect the lattice parameter of the hydrate. Indeed, TBME in 1H and 13C NMR relaxation times showed restriction of motion in L-cages, comparing with NH having no ether group, and then molecular dynamics simulation reveled guest−host hydrogen bonds between oxygen, O, of the ether function of TBME and hydrogen, H, of H2O molecules of H2O frameworks.13 Pinacolone (3,3-dimethyl-2-butanone), one of ketone LMGS, also shows the guest−host hydrogen bonds between O of carbonyl group of pinacolone and H of host H2O molecules in sH hydrates.14 Dynamic properties of the guest− Received: April 7, 2013 Revised: October 7, 2013 Published: October 11, 2013 23469

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lines (520.6 cm−1). Details were described in a previous paper.17 The Raman spectra of the samples were collected at the temperature of liquid N2. 2.4. Equilibrium Measurements. Equilibrium pressure− temperature (pT) data for clathrate hydrates in the gas−(BrCP or BrCH)−water systems were collected using an SUS316 stainless-steel high-pressure vessel (TVS-N2, Taiatsu Techno Co., Japan) equipped with a stirring fin. The inner wall of the vessel was coated with polytetrafluoroethylene, and the inner volume was 80 mL. The sample temperature was maintained by a circulating thermostatted bath (Haake Phoenix C41P; Thermo Fisher Scientific Inc., U.S.A.) and was measured using a thermocouple (type T, CHINO Co.). For precise temperature measurements, the thermocouple was connected to a cold-junction compensation device (ZEROCON, ZC-114, Coper Electronics Co.), which uses an ice−water slurry to establish a reference temperature of 273.15 K. The temperature measurements were reproducible within ±0.02 K. Given the uncertainty in controlling the sample temperature, the expanded uncertainty of the measured equilibrium temperatures was estimated to be ±0.1 K with a confidence level of approximately 95%. The pressure was measured using a pressure transducer (AP-14S, Keyence Co., Japan). The uncertainty in the pressure measurements was ±0.05 MPa with a confidence level of approximately 95%. Water (20 g) and either BrCP (10 g) or BrCH (10 g) were poured into the vessel. After removing air by using a vacuum pump, the vessel was flushed twice with CH4 (1 MPa). The vessel was then pressurized with CH4 to a desired pressure at room temperature. The sample was chilled to induce the formation of hydrate crystals. After a pressure decrease was observed during the decrease in temperature, the temperature was increased in increments of 0.1 to 0.5 K. When the system pressure became substantially lower than the initial pressure, we increased the system temperature in increments of 0.5 K, and as the pressure approached the initial pressure, increased the system temperature in increments of 0.1 K. At each temperature step, the sample temperature was maintained for 8−12 h until the system pressure remained constant. When hydrate crystals are formed, the pressure at each step increases owing to hydrate dissociations until it reaches the equilibrium state for the experimental system. After hydrates completely dissociate, the system pressure increases with a gentle slope against the temperature increment. Consequently, the hydrate phase equilibrium pT condition was determined from the change in slope during the pressure increment. (Figure S1 of the Supporting Information). Before measuring the phase equilibrium pT conditions in the CH4−bromide LMGS−water systems, we tested the reliability of our apparatus and procedure by measuring the equilibrium pT conditions in a CH4−water system. Our measured data agreed well with corresponding data in the literature (Table S1 and Figure S2 of the Supporting Information). 2.5. Materials. BrCP and BrCH (purity ≥ 98.0%) were obtained from Sigma-Aldrich Co., and MCH (≥99.0%) was obtained from Tokyo Chemical Industrial Co., Japan. Research grade (99.9% purity) CH4 gas was supplied by Sumitomo Seika Chemicals Co., Japan. Water was purified by ultrafiltration, reverse osmosis, deionization, and distillation (>18 MΩ·cm). All materials were used without further purification.

host hydrogen bonds was unique. For example, a lifetime of the guest−host hydrogen bonds in sH hydrates enclosing pinacolone increases as temperature increases, whereas the lifetime in sH hydrates enclosing TBME decreases.13,14 Needless to say, the guest−host hydrogen bonds can be seen in sH hydrates.14−16 The interactions related by guest molecules would lead to variation of gas hydrate natures. Bromocyclopentane (cyclopentyl bromide, hereafter, BrCP) and bromocyclohexane (cyclohexyl bromide, hereafter, BrCH) are substances in which the CH3 groups in MCP and MCH are replaced with Br. Bromine groups undergo halogen bonding with electron donors, such as the oxygen of H2O. Bromide LMGSs may interact with cage-forming water molecules. As such, the properties of an sH hydrate may change in the presence of bromide LMGSs. In this study, we investigated the possibility of using bromide substances to form new sH hydrates. The lattice parameters and cage occupancies of the new hydrate crystals were studied using powder X-ray diffraction (PXRD) and Raman spectroscopy, respectively. In addition, equilibrium pressure−temperature relationships for the new sH hydrates were measured.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation for Crystallographic Analysis. The samples were prepared using an SUS316 stainless-steel high-pressure vessel (TAF-SR50, Taiatsu Techno Co., Japan), with its inner surface coated with polytetrafluoroethylene (inner volume: 50 mL). A stoichiometric ratio of LMGS and ice (2.0 g, diameter < 212 μm) was loaded into the highpressure vessel in a cold room at approximately 263 K. The vessel was immersed in a circulating thermostatted bath (Haake Phoenix C41P; Thermo Fisher Scientific, U.S.A.) at 253 K. After air was eliminated from the vessel using a vacuum pump, the vessel was flushed twice with CH4 at 1.0 MPa. The vessel was subsequently pressurized with CH4 at 2.7 MPa, which was lower than the equilibrium pressure of the pure gas hydrate at 274 K.2 After maintaining the sample temperature at 253 K for 6 h, it was increased to 274 K. When the pressure decreased while the sample was being maintained at 274 K, we again pressurized the vessel with CH4 to 2.7 MPa and maintained this pressure for 72 h. We finally quenched the vessel in liquid nitrogen and retrieved the synthesized sample crystals. The synthesized samples were ground into powders for the PXRD and Raman shift measurements. 2.2. Powder X-ray Diffraction. An X-ray powder diffractometer (Rint-2500; Rigaku) equipped with a Cu Kα radiation source was used to obtain 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 were maintained at temperatures less than approximately 153 K under a stream of cooled, dry nitrogen (N2) gas during the PXRD measurements. The PXRD profiles were acquired using a step of 0.02°, with a counting time of 1.2 s/step over 10−40 data acquisitions. 2.3. 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 CCD detector (size: 2048 × 512 pixels). The wavelength of the laser line was 532 nm (Torus 532, Laser Quantum). The configuration allowed Raman shifts to be collected with a spectral resolution of ∼0.2 cm−1/pixel. The Raman shifts of samples were calibrated using Si emission 23470

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3. RESULTS AND DISCUSSION 3.1. Characterization of Crystal Structures. Figure 1 shows the molecular structures of BrCP and BrCH. These

Figure 1. Molecular structures of (a) bromocyclopentane and (b) bromocyclohexane: red, bromide; green, carbon; white, hydrogen. These molecular geometries were obtained from structural energy minimizations of the molecules using MOPAC2012.20 Images were drawn with Winmostar.18,19

structures were obtained by geometrical optimization using the Winmostar program,18,19 which uses MOPAC201220 to perform structural energy minimization on a molecule. For guest molecules trapped in the largest cages (51262) of sI and (51264) sII hydrates, the maximum intramolecular lengths (hereafter, maximum lengths) are less than approximately 0.6 and 0.7 nm, respectively.2 However, for LMGSs trapped in the 51268 cage of sH hydrates, the maximum lengths are approximately 0.7−0.9 nm.2 The maximum lengths in the BrCP and BrCH molecules between the atoms, which are indicated by the dashed circles in Figure 1, are approximately 0.77 and 0.79 nm, respectively, based on molar van der Waals radii. Other properties of BrCP and BrCH are summarized in Table 1, along with the properties of other LMGSs. The ovality indices listed in Table 1 represent the ratio between the molecular surface area and the minimum surface area. The minimum surface area is the surface area of a sphere with a volume equal to the molecule’s estimated volume. Therefore, the ovality index ranges from 1 to greater than 1. An ovality index of 1 means that the molecule is spherical, based on the van der Waals radii. An ovality index greater than 1 indicates that the molecule is oval. Consequently, an increase in the ovality index indicates a more oval shape. As indicated by the ovality index, the molecular shapes of BrCP and BrCH are similar to those of the other LMGSs listed in Table 1. Figure 2 shows PXRD profiles of crystals of CH4−BrCP− water and CH4−BrCH−water systems. PXRD patterns of the sH (guest molecules: CH4 and MCH) and sI (CH4) hydrates are also displayed in Figure 2. Table 2 lists the diffraction peaks observed for the sH hydrate (CH4 + MCH) and crystal samples obtained from the CH4−BrCP−water and CH4−BrCH−water systems. To allow a comparison of the diffraction peaks with

Figure 2. PXRD patterns of hydrate crystals measured at 153 K: (a) sH hydrate (guest molecules: CH4 and MCH); (b) hydrate formed in the CH4−BrCP−water system; (c) hydrate formed in the CH4− BrCH−water system; and (d) sI hydrate (CH4). Peaks marked with asterisks (*) and crosses (+) indicate peaks that originated from hexagonal ice (Ih) and LMGSs, respectively.

Table 2. Diffraction Peaks of sH Hydrates 2θ (deg) Miller index of sH hydrate (hkl)

CH4 + adamantanea

CH4 + MCHb

CH4 + BrCPb

CH4 + BrCHb

(201) (300) (301) (103) (212) (220) (113) (203)

22.2412 25.2718 26.8056 27.9277 28.5334 29.2736 30.4526 31.5975

22.220(3) 25.260(12) 26.822(4) 27.921(11) 28.537(2) 29.272(5) 30.465(3) 31.597(6)

22.215(12) 25.20(2) 26.773(15) 27.94(3) 28.484(7) 29.215(13) 30.463(8) 31.560(16)

22.240(9) 25.257(8) 26.829(7) 27.91(3) 28.532(4) 29.282(8) 30.485(4) 31.591(10)

a Diffraction peak positions under Cu Kα irradiation (λ = 0.15406 nm) were estimated using data from the X-ray crystallographic data file for structure determinations.21 bDiffraction peak positions were obtained during estimation of the unit cell dimensions of the obtained crystal samples.

literature data, diffraction peaks of the sH hydrate (CH4 + admantane)21 are also listed in Table 2. Eight unique peaks at approximately 2θ = 22.2, 25.3, 26.8, 27.9, 28.5, 29.3, 30.5, and 31.6° in the 2θ range observed in the PXRD pattern of the sH hydrate (CH4 + MCH; Figure 2a) are consistent with the

Table 1. Properties of Bromide LMGSs (BrCP and BrCH), LHC-LMGSs (MCP, MCH, and NH), and a Polar LMGS (TBME) molecular mass (M) longest lengtha (nm) volumea (nm3) surface areaa (nm2) ovalitya,b

BrCP

BrCH

MCP

MCH

NH

TBME

149.03 0.773 0.1042 1.378 1.29

163.06 0.787 0.1204 1.555 1.32

84.16 0.773 0.1023 1.376 1.30

98.19 0.843 0.1185 1.564 1.34

86.18 0.803 0.1121 1.519 1.35

88.15 0.773 0.1048 1.430 1.33

a

Each value was estimated using the Winmostar program,18,19 which performs structural energy minimization on a molecule using MOPAC201220 by considering the molar van der Waals radii. bOvality, O, was obtained using the equation O = A/(4 × π × ((3 × V)/(4 × π))2/3), where A and V indicate molecular surface area and molecular volume, respectively. 23471

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Table 3. Crystallographic Information for sH Crystal Samples with CH4 in S- and M-Cages large molecule guest substance unit cell (nm) volume (nm3) temperature (K)

a c

BrCP

BrCH

MCH

MCH22

MCH10

TBME10

1.2223(3) 1.0076(3) 1.3036(6) 153

1.2201(6) 1.0060(5) 1.297(1) 153

1.221(2) 1.0062(19) 1.299(4) 153

1.2217(1) 1.0053(1) 1.2994(1) 173

1.2203(7) 1.0056(4) 1.297(1) 163

1.22375(6) 1.00282(6) 1.3006(1) 163

than molecules of the ether groups at the PXRD measurement temperature. Figure 3 shows the Raman spectra for the symmetric C−H vibrations of CH4 encaged in sH hydrates and in a CH4 hydrate

literature data. Other diffraction peaks, which are marked with asterisks and crosses in Figure 2a, originated from hexagonal ice (Ih) and MCH molecules that were not converted into a hydrate phase. The eight unique peaks also appeared in the PXRD patterns of both CH4−BrCP−water and CH4−BrCH−water systems (Figure 2b,c and Table 2). However, three peaks in the pattern of the sI hydrate crystal (2θ = 22.7, 24.2, and 25.8°),21 which were attributed to the (222), (320), and (321) reflections, respectively, were not observed in the patterns of the CH4− BrCP−water and CH4−BrCH−water systems. Therefore, the PXRD patterns indicate that the CH4−BrCP−water and CH4− BrCH−water systems can form sH hydrates; that is, BrCP and BrCH can be used to form LMGSs in sH hydrates. After peak indexing of the sH hydrate phase, refinements of unit-cell dimensions of the samples obtained from the CH4−BrCP− water and CH4−BrCH−water systems were performed using the PXRD analysis software PDXL (Rigaku Corp.); we assumed that the samples obtained from the CH4−BrCP− water and CH4−BrCH−water systems exhibited hexagonal symmetry with space group P6/mmm. The obtained unit-cell dimensions are summarized in Table 3. In addition, the unitcell dimensions of sH hydrates (CH4 + LMGS) reported in the literature are also listed. The dimensions of the unit cells for sH hydrates in the CH4−BrCP−water and CH4−BrCH−water systems were estimated to be a = 1.2223(3) nm and c = 1.0076(3) nm in sH hydrates (CH4 + BrCP) and a = 1.2201(6) nm and c = 1.0060(5) nm in sH hydrates (CH4 + BrCH). In comparison, on the basis of the PXRD pattern shown in Figure 2a, the parameters of sH hydrates (CH4 + MCH) were a = 1.221(2) nm and c = 1.0062(19) nm. As detailed in Table 3, the lattice parameters exhibit similar values, irrespective of the LMGS. However, lattice parameter c of the sH hydrate with TBME (1.00282 nm) is substantially smaller than those of sH hydrates with other LMGSs (1.0053−1.0076 nm). TBME contains an ether group, which can engage in intermolecular hydrogen bonding between O of TBME and H of host H2O molecules,13 and the lattice parameter is affected by this functional group. BrCH and BrCP contain Br atoms. Bromine can act as an electron acceptor in halogen bonding (−Br···O−Y) or as a donor in hydrogen bonding (−Br···H−Y). Cl2 and Br2 of dihalogen molecules form sI and structure III (sIII), respectively. Here, sIII hydrate was suggested by Allen and Jeffry23 and its crystal system is tetragonal (space group P42/ mnm). As similar as hydrates having guest molecules of ether, ketone, carbonyl, and amine groups,13−16 sI and sIII hydrates having Cl2 and Br2, respectively, show guest−host interactions. In that case of dihalogen molecules, the interaction is the halogen bonding (−X···O−Y) between a halogen atom (Cl or Br) and O of host H2O molecules.24 Nevertheless, because the lattice parameters of BrCH and MCH differ slightly, the Br of BrCP and BrCH may interact less with the H2O frameworks

Figure 3. Raman spectra showing the symmetric C−H vibration modes of encaged CH4 molecules: (a) sH hydrate in the CH4−BrCP− water system; (b) sH hydrate in the CH4−BrCH−water system; (c) sH hydrate in the CH4−MCH−water system, and (d) sI hydrate in the CH4−water system. Raman shifts were measured at the temperature of liquid nitrogen.

(sI). The Raman peaks near 2910 cm−1 are attributed to the C−H vibrations of CH4 encaged in S- and M-cages.9,25,26 Therefore, sH hydrates in the CH4−BrCP−water and CH4− BrCH−water systems appear to have potential for use in gas storage. The strong Raman signal near 2910 cm−1 is a combination of two peaks of each C−H vibration in S- and M-cages. The lower- and higher-frequency peaks near the Raman peak at 2910 cm−1 represent C−H vibrations in M- and S-cages, respectively, according to the “loose cage−tight cage” model.27 The spectra showing the C−H vibrations in CH4 were decomposed into separate peaks using the commercial multiple-peak-fitting program PeakFit v4.12. Figure 4 shows the results of this decomposition. Cage occupancy ratios, θM/ θS, were obtained from the decomposed Raman peak intensities, IS and IM. Because the cross sections of CH4 molecules encaged in S- and M-cages are almost the same, the θM/θS ratios can be estimated from the relative-peakintensity ratio 3IM/2IS. The θM/θS ratio for BrCP, BrCH, and MCH is 1.28, 1.33, and 1.33, respectively (Table 4). However, the θM/θS ratio determined from X-ray diffraction patterns, NMR spectra, and other Raman spectra ranged from 0.7 to 1.2.9,10,22−26 The cage occupancy can be determined by the formation conditions.9,12,25 High-pressure conditions resulted in θS values higher than those determined under low-pressure 23472

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NMR chemical shifts of CH4 encaged in 512, 51262, and 51264 cages differs. Only the chemical shift of CH4 encaged in 512 cages of sI and sII was changed by a change in temperature, whereas no temperature dependency of the chemical shifts of CH4 molecules encaged in 51262 and 51264 cages of sI and sII was observed. Here the guest/cavity diameter ratio of a 512 cage is greater than that of 51262 and 51264 cages. The chemical shift change in only the 512 cage may have been observed because guest−host interactions between the CH4 and H2O framework in a 512 cage would be strongest among the three investigated cages. Therefore, CH4 molecules in 512 cages are believed to be more sensitive to their environment than those in other cages. The variation in the Raman shift of CH4 in the S-cages, as listed in Table 4, might result from the use of different LMGSs. 3.2. Phase Stability of sH Hydrates with Bromide LMGSs. Phase stability of hydrates is a critical issue if they are to be used as gas-storage and heat-exchange media. We measured pT data for hydrate−liquid water−liquid bromide LMGS (BrCP or BrCH)−CH4-rich gas (H−L1−L2−V) in fourphase equilibrium. Table 5 lists the H−L1−L2−V four-phase Table 5. Hydrate−Liquid Water−Liquid Bromide LMGS (BrCP or BrCH)−CH4-Rich Gas (H−L1−L2−V) Four-Phase Equilibrium pT Conditions for CH4−BrCP−Water and CH4−BrCH−Water Systems CH4−BrCP−water system

Figure 4. Spectral decomposition of symmetric C−H vibration modes: (a) sH hydrate in the CH4−BrCP−water system, (b) sH hydrate in the CH4−BrCH−water system, and (c) sH hydrate in the CH4− MCH−water system.

CH4−BrCH−water system

Ta (K)

pb (MPa)

Ta (K)

pb (MPa)

275.45 277.05 278.35 279.35 280.15

2.09 2.53 2.93 3.33 3.68

273.95 275.45 276.35 277.25 278.35 279.85 281.05

1.49 1.80 2.02 2.26 2.57 3.09 3.54

Uncertainties in dissociation temperatures were estimated to be ±0.1 K, with a confidence level of approximately 95%. bUncertainties in pressure measurements were estimated to be ±0.05 MPa, with a confidence level of approximately 95%.

a

Table 4. Raman Peaks of the C−H Vibration Mode of CH4 Enclosed in S- and M-Cages Raman shift (cm−1) system

S-cage

M-cage

cage occupancy ratio θM/θS

CH4 + BrCP CH4 + BrCH CH4 + MCH

2911.24 2911.10 2911.06

2909.17 2909.15 2909.14

1.28 1.33 1.33

equilibrium data obtained for the CH4−BrCP−water and CH4−BrCH−water systems. Figure 5 shows the measured H− L1−L2−V equilibrium conditions for the CH4−BrCP−water and CH4−BrCH−water systems, and compares these results with literature data for CH4−water, CH4−MCP−water, CH4− MCH−water, CH4−NH−water, and CH4−TBME−water systems.2,28−32 The phase equilibria of sH hydrates in the CH4−BrCP−water and CH4−BrCH−water systems shifted to lower pressures and higher temperatures (i.e., milder conditions) relative to those of pure CH4 hydrates. A comparison of the two Br systems revealed that the CH4− BrCH−water system exhibited milder pT conditions than the CH4−BrCP−water system. For example, for the CH4−BrCH− water system, the pT conditions at 277 K and 2.2 MPa are stable for sH hydrates. On the basis of several studies on phase stability of sH hydrates, LMGSs in the L-cage are considered to be a dominant factor for determining stability. In gas (CH4 and Kr)−LHCs (MCP, MCH, and NH)−water systems, the phase-equilibrium conditions of sH hydrates showed mild pT conditions in the order NH, MCH, and MCP in both sH hydrate systems.4,28−31 As shown in Figure 5, the equilibrium pT conditions for the CH4−BrCP−water system are very similar to those for the

conditions.12,25 In addition, a high LMGS ratio tends to produce a higher θM value than does a low LMGS ratio.9 In our study, the initial pressure was 2.7 MPa, which is lower than the pure CH4 hydrate equilibrium pressure at 274 K. Because a molecular dynamics study has shown that LMGSs minimally affect cage occupancy,12 our low-pressure formation conditions would lead to a lower θS and, therefore, to a θM/θS ratio higher than that reported in the literature. Peak centers are also listed in Table 4. The peak centers for CH4 in S-cages are distributed between 2911.06 and 2911.24 cm−1, whereas those for CH4 in M-cages are located at approximately 2909.15 cm−1. The variation in peak centers of CH4 in S-cages may result from changes in the guest−host interactions due to the different LMGSs. If this hypothesis is true, then peak centers could be affected by guest−host interactions. Nevertheless, no variation in peak centers of CH4 in M-cages was observed, as shown in Table 4. Kida et al.11 demonstrated that the temperature dependency of the 13C 23473

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changed to oxygen. The difference in the equilibrium pressures of TBME and NH is Δp > 0.5 MPa, as shown in Figure 5, whereas TBME has geometric properties similar to those of NH. Guest−host interactions in sH hydrates with TMBE would be much stronger, namely, severer distortion of H2O framework, than those in an sH hydrate with nonpolar NH.13 The difference in the molecular properties of TBME and NH is considered to affect the equilibrium conditions. However, as mentioned in the discussion of unit-cell dimensions, guest− host interactions in sH hydrates with BrCP and BrCH are considered to be almost same as those in sH hydrates with MCP and MCH. Therefore, the difference in the equilibrium conditions between an LMGS and a bromide LMGS (MCP and BrCP, or MCH and BrCH) would be small. Thus, not only the molecular shape but also the polarity of an LMGS may be the keys to determining the phase stability of sH hydrates.

Figure 5. H−L1−L2−V four-phase equilibrium pressure−temperature conditions of clathrate hydrates in the CH4−BrCP−water and CH4− BrCH−water systems: ● CH4−BrCP−water system (present study); ■ CH4−BrCH−water system (present study); ○ CH4−MCP−water system;28 □ CH4−MCH−water system;29,30 + CH4−NH−water system;31 and ◇ CH4−TBME−water system.32 The solid line indicates the phase-equilibrium conditions for CH4 hydrate predicted using CSMGem.2 The dotted and dashed lines are guides for the eyes.

4. CONCLUSIONS In this study, we synthesized new structure H (sH) clathrate hydrates with bromide chemical substances bromocyclopentane (BrCP) and bromocyclohexane (BrCH) as large-molecule guest substances (LMGSs). The maximum distances between BrCP and BrCH molecules were approximately 0.77 and 0.79 nm, respectively, based on the van der Waals radii. PXRD patterns revealed the formation of sH hydrates in both CH4− BrCP−water and CH4−BrCH−water systems. Through refinement of PXRD data, the unit-cell dimensions of sH hydrates in the two systems were estimated to be a = 1.2223(3) nm and c = 1.0076(3) nm in the sH hydrate (CH4 and BrCP) and a = 1.2201(6) nm and c = 1.0060(5) nm in the sH hydrate (CH4 and BrCH). The Raman shifts of the symmetric C−H vibration modes of CH4 in S- and M-cages (512 and 435663 cages) were 2911.1 and 2909.1 cm−1, respectively. The M-cage to S-cage occupancy ratio, θM/θS, was estimated to be approximately 1.3 on the basis of the relative Raman peak intensities. Phaseequilibrium conditions of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were measured via an isochoric method. Compared with other typical LMGSs, that is, methylcyclopentane (MCP), methylcyclohexane (MCH), neohexane (NH), and tert-butyl methyl ether (TBME), the order of increasing equilibrium pressures at the same temperature was NH < MCH < BrCH < TBME ∼ MCP ≤ BrCP. A comparison of the phase-equilibrium conditions of LGMSs with similar geometric properties (BrCP and MCP, BrCH, and MCH, and TBME and NH) showed that not only molecular shape, but also the polar properties of an LMGS may be the keys to determining the phase stability of sH hydrates.

CH4−MCP−water system. Furthermore, the conditions for the sH hydrate (CH4 + BrCH) system is also close to those for the sH hydrate (CH4 + MCH) and sH hydrate (CH4 + TBME) systems. As shown in Figure 5, the order of mild pT conditions (i.e., the order of decreasing phase stability) is NH > MCH > BrCH > TBME ∼ MCP ≥ BrCP. In gas−water systems such as sI and sII hydrates, gas solubility in the water phase is considered to affect phase stability.2 Gas solubility in the LGMS phase would strongly influence the manner of growth of sH hydrates.33 Nevertheless, the relationship between phase stability and gas solubility in LGMSs is not well understood. For example, in gas−LGMS− water systems, the solubility of CH4 in MCH is higher than that in NH,34 whereas sH hydrates that contain NH show milder pT phase stability than sH hydrates that contain MCH. The properties of LMGSs enclosed in the L-cage should affect the phase stability via guest−host interactions. With respect to molecules enclosed in the L-cage, Ripmeester and Ratcliffe state that “the guest molecule is required to have the correct shape for efficient space filling of the structure H large cage.”7 Because the L-cage is not only the largest but also the most prolate cage,2 the shape of the LMGS is considered to dominate the stability of L-cage frameworks. On the basis of the geometric properties of sH and non-sH formers, the LMGSs have molecular volumes that range from 0.095 to 0.16 nm3 and ovalities that range from 1.29 to 1.37 (Figure S3 in Supporting Information). The volumes and ovality indices of non-sH formers7 range from 0.07 to 0.15 nm3 and from 1.25 to 1.42, respectively. Here, n-pentane and n-hexane, which were reported as non-sH formers,7 can be encaged in the L-cage under anomalous conditions that these non-sH formers are mixed with sH former NH.35 Nevertheless, small molecules that are more or less oblate are believed to be not encaged in Lcages as a single LMGS. The equilibrium pressure values of BrCP and BrCH are similar to those of MCP and MCH, respectively (Δp < 0.2 MPa), under the same temperature conditions, whereas the bromide and methyl cycloalkanes exhibit similar molecular shapes, as listed in Table 1. Here TBME is a molecule in which the third carbon group of NH (2,2-dimethyl butane) has been

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ASSOCIATED CONTENT

S Supporting Information *

Proofs of our experimental setup for equilibrium pressure− temperature conditions and geometric properties of sHforming and non-sH-forming molecules. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-11-857-8526. Fax: +81-11-857-8417. E-mail: u-jin@ aist.go.jp (Y.J.); [email protected] (J.N.). Notes

The authors declare no competing financial interest. 23474

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ACKNOWLEDGMENTS We would like to thank Drs. Y. Konno of AIST and H. Ohno of the Kitami Institute of Technology (KIT) for valuable discussions and suggestions on sample preparation.

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