(sH) Clathrate Hydrates with Rare Gas (Krypton and Xenon)

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Crystal Phase Boundaries of Structure‑H (sH) Clathrate Hydrates with Rare Gas (Krypton and Xenon) and Bromide 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: Phase equilibrium pressure−temperature (pT) boundaries of structure-H clathrate hydrates (sH hydrates) with rare gas (Kr and Xe)-bromide large molecule guest substances (LMGSs: bromocyclohexane, BrCH and bromocyclopentane, BrCP) were measured. The phase boundaries for the sH hydrates in the Kr−LMGS−water systems shifted to lower pressures than those for the pure Kr hydrate in the temperature range of (273.2 to 279.3) K. In this study, sH hydrate formation was not confirmed in the Xe−BrCP−water system, but sH hydrates were found in the Xe−BrCH−water system. At temperatures below 277 K, equilibrium conditions were observed at lower pressures for the Xe−BrCH−water system than for the pure Xe hydrate. However, the equilibrium pT curve for the Xe− BrCH−water system crossed over the equilibrium pT curve for the Xe hydrate at around 277 K. Intersections between the equilibrium pT curves for the Xe hydrates and the sH hydrates (Xe + LMGS) have also been found in Xe−methylcyclohexane−water systems. Using the Kr−and Xe−bromide LMGS−water systems showed that the sH hydrate phase stabilities are strongly related to the encaptured LMGS.

1. INTRODUCTION Gas clathrate hydrates (gas hydrates) are ice-like crystalline compounds, in which guest molecules (mainly gases) are stored in a hydrogen-bonded H2O framework cage.1,2 Gas hydrates primarily form three different crystal structures: structure-I (sI), structure-II (sII), and structure-H (sH). Crystal hydrate structures comprise combinations of a pentagonal dodecahedron (512) cage and one or two kinds of tetrakaidecahedron (51262), hexakaidecahedron (51264), irregular dodecahedron (435663), or icosahedron (51268) cages. Therefore, the crystal structure is generally determined by the size of the guest molecules stored in the cages.2 Among the three hydrate structures, sH hydrates with three 512 (S-cage), two 435663 (Mcage), and the largest 51268 cage (L-cage) in the structural unit can enclathrate large molecules, and these large molecules cannot be stored in the cages of sI and sII hydrates. Polar molecules (e.g., tert-butyl methyl ether; TBME) and nonpolar liquid hydrocarbons (e.g., neohexane; NH) are known to be large molecule guest substances (LMGSs) that can be encaged in the L-cage of sH hydrates.2 When guest gases and LMGSs coexist, sH hydrates can be formed by enclosing not only LMGSs but also small guest molecules that can fit into S- and M-cages. There have been many studies of the properties of different sH hydrates since they were first reported by Ripmeester et al.3 in 1987. The sH hydrates with LMGSs can store guest molecules in the S- and M-cages at lower pressures and higher temperatures than the equilibrium conditions for pure sI and sII hydrates that enclose small guest molecules allow.4−13 Methane (CH4) molecules can also be stored in clathrate hydrates, in which case the CH4−water system forms an sI hydrate. The © 2014 American Chemical Society

pure CH4 hydrates indicate equilibrium pressures higher than 3 MPa at 275 K.2 However, CH4 molecules can be encaged in sH hydrate cages in a CH4−TBME−water system up to 2 MPa at 275 K.4 The sH hydrate system of (CH4 + NH) remains stable at approximately 1.4 MPa.6,7 Tetrahydrofuran (THF) hydrate can also store CH4 in 512 cages of the sII hydrate framework at quite low pressures, the equilibrium pressure in 0.48 mol % THF solution being ca. 0.68 MPa at 279 K.14 However, the gas capacity per mole of water molecules is higher in an sH hydrate than in an sII THF hydrate. Therefore, sH hydrate systems are expected to be able to be used as gas storage or separation media. Recently, Jin et al.12 reported two new LMGSs, bromocyclohexane (BrCH) and bromocyclopentane (BrCP). Furthermore, the thermodynamic stability of sH hydrates with argon atoms was also reported in 2014.13 Thus, an understanding of a range of sH hydrates may allow us to develop suitable applications using clathrate hydrates. In this study, we report the phase boundaries of sH hydrates with the LGMSs BrCH and BrCP and the gases Kr and Xe with the aim of allowing gas hydrates to be adapted for gas storage and transfer applications.

2. EXPERIMENTAL SECTION 2.1. Measurement of Phase Equilibrium Conditions. Water−LMGS−hydrate−vapor (L1−L2−H−V) four-phase equilibrium pressure−temperature (pT) conditions for sH Received: March 7, 2014 Accepted: March 31, 2014 Published: April 7, 2014 1704

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The L1−L2−H−V four-phase equilibrium pT conditions in the Xe−bromide LMGS−water systems were measured after the pressure had been released and once the system pressure was constant and lower than the phase equilibrium pressure of the Xe hydrates. 2.2. Crystallographic Analysis. The crystallographic analysis of the hydrate samples was performed using a powder X-ray diffraction (PXRD) method, using an X-ray diffractometer (SmartLab; Rigaku Co. Japan) with a Cu Kα radiation source and a high-speed one-dimensional detector, D/teX Ultra. The X-ray source had a voltage of 40 kV and a current of 200 mA, and 2θ was measured in the range of (20 to 35)° with a scan step of 0.01° and a scan speed of (4.0 to 10.0) deg· min−1. A powdered sample was placed in a quartz glass capillary cell (2.0 mm diameter, 0.01 mm thick, and 20 mm length). The sample temperature was maintained at ca. 153 K using dry and cold nitrogen to avoid the hydrate sample dissociating. Peak indexing and refinement of the unit cell dimensions from the diffraction peak data that were acquired were performed using PXRD analysis software (PDXL, Rigaku Co., Japan). The samples were prepared for crystallographic analysis using a high pressure vessel (TAF-SR-50; Taiatsu Techno Co., Japan), which housed a polytetrafluoroethylene cylinder. Details of the procedure used have been described in a previous paper.10 The hydrate samples were synthesized using ca. 1.5 g of ice particles ( 18 MΩ·cm) was produced by deionization and distillation. All materials were used without further purification.

hydrates in rare gas (Kr and Xe)−LMGS (BrCH and BrCP)− water systems were collected using a pressure vessel equipped with an optical window and a magnetic stirrer. Here, the number of degree of freedom (F) for a system is obtained from the Gibbs phase rule,

F=C−P+2 where C and P are the number of components and the number of phases in equilibrium state, respectively. A L1−L2−H−V system indicates F = 1, because C and P are three components (water, LMGS, and gas) and four phases (water, LMGS, hydrate, and gas), respectively. Therefore, equilibrium pressure condition for the L1−L2−H−V systems in our study is given by only a system temperature without composition of components. The measured temperature (T) was reproducible to within ± 0.02 K. Considering the uncertainty in sample temperature control during the measurements, the expanded uncertainty of the collected T was estimated to be ± 0.1 K, with a confidence level of ca. 95 %. The uncertainty in the pressure measurements was ± 0.005 MPa, with a confidence level of ca. 95 %. The measurement system has been described in a previous study.15 To measure the L1−L2−H−V four-phase equilibrium pT conditions, 60 cm3 of water and 15 cm3 of LMGS were poured into the pressure vessel. Air was removed from the vessel using a vacuum pump, and then the vessel was pressurized to ca. 1.0 MPa with Kr or 0.3 MPa with Xe. After the vessel had been pressurized, the gases (including the remaining air) were removed from the vessel using the vacuum pump. To eliminate air from the vessel, the vacuum and pressurization procedure was repeated two times for the Kr−LMGS−water system and ten times for the Xe−LMGS−water system. The vessel was then pressurized to a specified pressure and chilled to ca. 273.3 K to allow the hydrates to form. The initial pressure values were 1.1 MPa for the Kr−LMGS−water system and 0.35 MPa for the Xe−LMGS−water system. Crystallization was confirmed to have occurred by inspecting the system through the optical window, and then the system temperature was increased in 1.0 K steps for the Kr−LMGS−water system and 0.5 K steps for the Xe−LMGS−water system. A pressure increase caused by hydrate dissociation, to maintain equilibrium in the system, was observed when the system pressure was lower than the L1−L2− H−V four-phase equilibrium pressure at each temperature increment step. However, the system pressure remained almost constant when the system pressure reached the phase equilibrium pressure. The temperature was maintained for (12 to 24) h after the pressure increase during the phase equilibrium pT measurements. The phase equilibrium pT conditions for the system were recorded when the system pressure became almost constant, and the presence of hydrate crystals in the vessel was visually confirmed at a constant temperature. Examples of equilibrium pT determination and the agreements in the data acquired using this procedure are shown in Figures S1 and S2 in the Supporting Information, respectively. The pressure in the Xe−LMGS−water systems remained higher than the equilibrium pressure for Xe hydrates because the initial pressure was higher than the phase equilibrium pressure for the pure Xe hydrate (ca. 0.16 MPa at 273.3 K). Therefore, before the L1−L2−H−V four-phase equilibrium conditions for the Xe−LMGS−water systems were measured the system pressure was decreased to below the phase equilibrium pT conditions, to dissociate the pure Xe hydrate.

3. RESULTS AND DISCUSSION 3.1. Kr−Bromide LMGS−Water Systems. Figure 1 shows the PXRD profiles of crystal samples obtained from the Kr− BrCH−water and Kr−BrCP−water systems. In addition to the PXRD profiles of target samples, the profiles of the sH hydrate with (Kr + MCH) obtained from the Kr−MCH−water system and the sII Kr hydrate obtained from the Kr−water system are shown in Figure 1, for structural comparisons. The peaks for the sH hydrates with (Kr + MCH) were in the 2θ range of (22 to 32)° as shown in Figure 1. The diffraction peak positions for the PXRD profiles shown in Figure 1 are listed in Table 1. The diffraction peak positions were obtained when the unit cell dimensions of the crystal samples were estimated. The peak positions of the sH hydrates with (CH4 + BrCH), which have bromide molecules as LMGSs, are also listed in Table 1. The diffraction peaks of the samples from both the Kr−BrCH− water and the Kr−BrCP−water systems were almost at the positions assigned to the sH hydrate crystals (sH (Kr + MCH) and sH (CH4 + BrCH) hydrates). This explains why both Kr− bromide LMGS−water systems could form sH hydrate crystals. We indexed the sH hydrate phase peaks, then determined the unit cell dimensions of the sH hydrates from the two Kr− 1705

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pressure and higher temperature (i.e., milder) regions than the pure Kr hydrate, similar to those found for other sH hydrate systems.4−8,11−13 The L1−L2−H−V equilibrium pT conditions were milder for the Kr−BrCH−water system than for the Kr− BrCP−water system. For example, at 276.15 K, the equilibrium pressure for the Kr−BrCH−water and Kr−BrCP−water systems were ca. (0.76 and 0.84) MPa, respectively. As shown in Figure 2, the phase equilibrium conditions for the five Kr−LMGS−water systems increased in terms of the pressure at a particular temperature in the order NH < MCH < BrCH < MCP ∼ BrCP, in temperature range of (273 to 279) K. For the sH hydrate containing CH4, the phase equilibrium conditions increased in terms of the pressure at a particular temperature in the order NH < MCH < BrCH < MCP < BrCP.5−9,11,12 The sH hydrate systems with the same small guest molecules in S- and M-cages were comparable with the systems with different LGMSs in L-cages, and the small cages containing Kr appeared to affect the phase stability of the sH hydrate little.12 Assuming that the S-and M-cages were completely occupied by Kr molecules, the hydrate dissociation enthalpy ΔH could be estimated, using the Clausius− Clapeyron equation, to be ca. (385.0 and 362.4) kJ·mol−1 (i.e., per mole of Kr molecules) for the Kr−BrCH−water and Kr−BrCP−water systems, respectively. The ΔH values for the sH hydrates with (Kr + MCH) and (Kr + MCP) were approximately (385.5 and 367.5) kJ·mol−1, respectively, in the temperature range of (273 to 279) K.9,10 The ΔH values for the sH hydrates with bromide LMGSs were lower than the ΔH values for the sH hydrates with hydrocarbon LMGSs with the Br groups in BrCH and BrCP replaced with CH3 groups. This decreasing ΔH tendency could also be seen in the sH hydrates with CH4, the ΔH for the sH hydrates with BrCH and BrCP being (376.9 and 374.8) kJ·mol−1, respectively, and the ΔH for the sH hydrates with MCH and MCP being (377.3 and 385.0) kJ·mol−1, respectively.12 Few effects on the crystal structure dimensions of the sH hydrates (CH4 + bromide LMGS) have been reported;12 however, the decrease in ΔH due to the replacement of CH3 with Br could have been caused by the Br atom acting as an electron acceptor in halogen bonding (−Br··· O−Y) or as an electron donor in hydrogen bonding (−Br···H− Y) between the Br and H2O molecules in the hydrate framework.18 3.2. Xe−Bromide LMGS−Water Systems. Figure 3 shows the PXRD profiles for the Xe−BrCH−water system and the pure sI hydrate containing Xe. Xe has a high X-ray mass

Figure 1. PXRD profiles of the crystal samples obtained from the Kr− bromide LMGS−water systems, sH, and sII hydrates. (a) From the Kr−BrCH−water system, (b) from the Kr−BrCP−water system, (c) from the Kr−MCH−water system (sH hydrate), and (d) from the Kr−water system (sII hydrate). The peaks marked with plus (+) and asterisks (*) are diffractions originating from ice Ih and LMGS, respectively. All PXRD patterns were collected at 153 K.

bromide LMGS−water systems, taking into consideration that the two sH samples had hexagonal symmetry with the space group P6/mmm. The dimensions of the unit cells of the sH hydrates with (Kr + BrCP) and (Kr + BrCH) were estimated to be a = 1.2161(3) nm and c = 1.0016(2) nm (unit volume = 1.2828(5) nm3) and a = 1.21902(15) nm and c = 1.00279(14) nm (unit volume = 1.2905(3) nm3), respectively. As a comparison, the dimensions of the unit cells of the sH hydrate with (Kr + MCH) were a = 1.2191(2) nm and c = 1.00296(19) nm (unit volume = 1.2905(4) nm3). The estimated unit cell dimensions are summarized in Table 2, which also contains the unit cell dimensions of the sH hydrate with (CH4 + LMGS), from the literature.12 The L1−L2−H−V four-phase equilibrium pT conditions for the Kr−bromide LMGS−water systems are listed in Table 3. Figure 2 shows plots of the equilibrium pT conditions determined in this study and other equilibrium data for Kr hydrates and sH hydrates with Kr as small guest molecules.9,10,16,17 The two sH hydrates from the Kr−bromide LMGS−water systems had equilibrium curves in the lower

Table 1. Diffraction Peaks of the sH Hydrates (Kr + LMGS), (Xe + LMGS), and (CH4 + LMGS)12 2θ/deg

a

Miller index of sH hydrate (hkl)

CH4 + BrCH

(201) (211) (202) (300) (301) (003) (103) (212) (220) (113) (203)

22.240(9)

25.257(8) 26.829(7)

12

a

Kr + MCH

Kr + BrCPa

24.475(6) 25.302(15) 26.850(5)

24.558(12) 25.363(9) 26.965(11)

Kr + BrCHa

Xe + BrCHa

23.979(13) 24.514(5) 25.293(15) 26.836(5) 26.854(5)

27.91(3) 28.532(4) 29.282(8) 30.485(4) 31.591(10)

28.592(3) 29.294(4) 30.498(3) 31.676(3)

28.0679 28.652(4) 29.22(4) 30.56(2) 31.712(15)

28.595(3) 29.306(4) 30.514(3) 31.739(8)

28.524(14) 29.310(4) 30.521(10) 31.539(6)

Diffraction peak positions were obtained when estimating the unit cell dimensions of the crystal samples that were obtained. 1706

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Table 2. Crystallographic Information for the sH Hydrates (Kr + LMGS), (Xe + LMGS), and (CH4 + LMGS)12 small guest LMGS unit cell a/nm c/nm volume/nm3

Kr

Xe

CH4

BrCP

BrCH

MCH

BrCH

BrCP12

1.2161(3) 1.0016(2) 1.2828(5)

1.21902(15) 1.00279(14) 1.2905(3)

1.2191(2) 1.00269(19) 1.2905(4)

1.2201(3) 1.0104(2) 1.3027(5)

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

BrCH12 1.2201(6) 1.0060(5) 1.297(1)

MCH12 1.221(2) 1.0062(19) 1.299(4)

Table 3. L1−L2−H−V Four-Phase Equilibrium Pressure and Temperature Conditions for the Kr−BrCH−Water and Kr− BrCP−Water Systems Kr−BrCH−water a

b

Kr−BrCP−water a

T /K

p /MPa

T /K

pb/MPa

273.25 274.15 275.15 276.15 277.15

0.534 0.604 0.688 0.768 0.864

273.35 274.25 275.15 276.15 277.05 278.15

0.605 0.671 0.748 0.842 0.934 1.045

Figure 3. PXRD profiles for the crystal samples obtained from the Xe−BrCH−water system and the sI hydrate. (a) From the Xe− BrCH−water system and (b) from the Xe−water system (sI hydrate). The peaks marked with plus (+) and asterisks (*) are diffractions originating from ice Ih and BrCH, respectively. The peak marked with closed circle (●) is diffraction originating from ice Ih and sI hydrate. The PXRD patterns were collected at 153 K.

a

Expanded uncertainties in dissociation temperatures were estimated to be ± 0.1 K, with a confidence level of approximately 95 %. b Uncertainties in pressure measurements were estimated to be ± 0.005 MPa, with a confidence level of approximately 95 %.

Table 4. Equilibrium Pressure and Temperature Conditions for the Xe−BrCH−Water System

Figure 2. Equilibrium pressure−temperature conditions for the Kr− bromide LMGS−water and Kr−LMGS−water systems. ●, Kr− BrCH−water system (this study); ■, Kr−BrCP−water system (this study); ○ , Kr−MCH−water system (Jin et al.); 10 □ , Kr− methylcyclopentane (MCP)−water system (Jin et al.);10 ◊, Kr− NH−water system (Ohmura et al.).9 The solid line indicates the phase equilibrium conditions for the Kr hydrate.16,17

Ta/K

pb/MPa

273.65 274.25 274.75 275.10 275.55 276.05 276.55 277.05 277.55 278.05 278.70 279.25

0.141 0.153 0.164 0.173 0.187 0.200 0.213 0.227 0.241 0.257 0.276 0.295

a

Expanded uncertainties in dissociation temperatures were estimated to be ± 0.1 K, with a confidence level of approximately 95 %. b Uncertainties in pressure measurements were estimated to be ± 0.005 MPa, with a confidence level of approximately 95 %.

attenuation coefficient;19 thus, the signal-to-noise ratios for the PXRD profiles obtained were very low. Nevertheless, the PXRD profiles for the Xe−BrCH−water system exhibited unique peaks, without being identified as being sI, in the 2θ range of (26 to 32)°, as shown in Figure 3. The index peaks for the Xe−BrCH−water system samples are listed in Table 1. Comparing the PXRD peaks for this system with the peaks for the other sH hydrates showed that the Xe−BrCH−water system could form sH hydrate crystals. The estimated unit cell dimensions for the sH hydrate with (Xe + BrCH) were a = 1.2201(3) nm and c = 1.0104(2) nm (unit volume = 1.3027(5) nm3), and these are listed in Table 2. The equilibrium pT conditions for the Xe−BrCH−water system are listed in Table 4 and plotted in Figure 4. Other equilibrium data for the Xe hydrate8,15 and the sH hydrate with Xe6,8 are also plotted in Figure 4. It was found that the equilibrium curve for the Xe−BrCH−water system crossed that

for the pure Xe hydrate system at ca. 277 K, as shown in Figure 4. Moreover, the slope of the equilibrium pT curve appeared to change slightly at around 277 K. Similar slope changes have been reported previously. The points where the slopes changed were different for the different LMGSs. The slopes changed at (281.5 and 283.2) K for the Xe−MCH−water and Xe−NH− water systems, respectively.6,8 The change in the slope for the sH hydrate with (Xe + MCH) may be caused by the phase transition from sH to sI hydrate, which has been reported in the literature.8 It is possible that the Xe−BrCH−water system mostly forms an sI Xe hydrate at above 277.05 K. Unfortunately, the formation of sH hydrates with (Xe + BrCP) was not observed in this study. The PXRD profiles for the crystals obtained from the Xe−BrCP−water system showed the diffraction pattern for the hexagonal ice (Ih). Shimada et 1707

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data presented in the literature, we believe that this change shows a transition from sH (Xe + BrCH) to sI (Xe) hydrate. Using the Kr− and Xe−bromide LMGS−water systems allowed the phase stability order for the sH hydrates to be strongly related to the encaptured LMGSs. Knowledge of gas solubility and mutual solubility would be an important factor to understand the variation in pT conditions of gas hydrate systems. Considerations of gas and liquid composition in each phase would be necessary to discuss phase equilibrium conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. Equilibrium pressure−temperature conditions for the Xe− bromide LMGS−water and Xe−LMGS−water systems. ●, Xe− BrCH−water system (this study); ○, Xe−MCH−water system (Shimada et al.);8 □, Xe−NH−water system (Makogon et al.).6 The solid line indicates the phase equilibrium conditions for the pure Xe hydrate.8,15

Examples of equilibrium pressure−temperature (pT) determination, proof of our experimental setups via the data agreements between our procedure and literature, and various equilibrium pT curves of sH hydrates with (CH4 + LMGSs). This material is available free of charge via the Internet at http://pubs.acs.org.

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al.8 reported that cis-1,3-dimethylcyclohexane (1,3-DMCH) cannot play a role as an LMGS in a Xe−water system, but the sH hydrates can form in a CH4−1,3-DMCH−water system.19 In the Shimada’s study, the L1−L2−H−V equilibrium pT curve for the sH hydrate with (CH4 + 1,3-DMCH) was close to the curve for the pure CH4 hydrate (Figure S3). For example, the equilibrium pressures at 276 K were ca. 3.1, 2.2, 2.1, 1.9, 1.8, and 1.6 MPa for the systems with 1,3-DMCH, BrCP, MCP, BrCH, MCH, and NH, respectively, and 3.4 MPa for the CH4 hydrate.6,7,11,12,18,20 Therefore, the equilibrium pT curve for the sH hydrates with (Xe + 1,3-DMCH) was considered to be very close to that for the pure Xe hydrates. Consequently, no sH hydrate would be found to form in the Xe−1,3-DMCH−water system.8 Considering the phase equilibrium pT conditions for the sH hydrates with (CH4/Kr + bromide LMGS),12 the L1− L2−H−V equilibrium pT conditions for the sH hydrate with (Xe + BrCP) were expected to be in a higher p and lower T region than those for the sH hydrates with (Xe + MCH) or (Xe + BrCH). Unfortunately, because the equilibrium pT conditions for the sH hydrates with (Xe + MCH) and (Xe + BrCH) were extremely close to the conditions for the Xe hydrates, no sH hydrate may be expected to form in the Xe− BrCP−water system using our experimental setup and procedures.

AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected] (Y.J.). Tel. no.: +81-11-8578526. Fax no: +81-11-857-8417. *E-mail address: [email protected] (J.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Y. Konno of AIST for valuable discussions. They also thank Ms. J. Hayashi and Ms. M. Watanabe of AIST for experimental support.

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REFERENCES

(1) Franks, F. Water: A Comprehensive Treatise, Vol. 2; Plenum Press: London, 1973. (2) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gasses, 3rd ed.; CRC Press: New York, 2007. (3) Ripmeester, J. A.; Tse, J. S.; Ratcliffe, C. I.; Powell, B. M. A New Clathrate Hydrate Structure. Nature 1987, 325, 135−136. (4) Hütz, U.; Englezos, P. Measurement of Structure H Hydrate Phase Equilibrium and the Effect of Electrolytes. Fluid Phase Equilib. 1996, 117, 178−185. (5) Ohmura, R.; Uchida, T.; Takeya, S.; Nagao, J.; Minagawa, H.; Ebinuma, T.; Narita, H. Clathrate Hydrate Formation in (Methane + Water + Methylcyclohexanone) Systems: The First Phase Equilibrium Data. J. Chem. Thermodyn. 2003, 35, 2045−2054. (6) Makogon, T. Y.; Mehta, A. P.; Sloan, E. D. Structure H and Structure I Hydrate Equilibrium Data for 2,2-Dimethylbutane with Methane and Xenon. J. Chem. Eng. Data 1996, 41, 315−318. (7) Ohmura, R.; Uchida, T.; Takeya, S.; Nagao, J.; Minagawa, H.; Ebinuma, T.; Narita, H. Phase Equilibrium for Structure-H Hydrates Formed with Methane and Either Pinacolone (3,3-Dimethyl-2butanone) or Pinacolyl Alcohol (3,3-Dimethyl-2-butanol). J. Chem. Eng. Data 2003, 48, 1337−1340. (8) Shimada, N.; Sugahara, K.; Sugahara, T.; Ohgaki, K. Phase Transition from Structure-H to Structure-I in the Methylcyclohexane + Xenon Hydrate System. Fluid Phase Equilib. 2003, 205, 17−23. (9) Ohmura, R.; Takeya, S.; Maekawa, T.; Uchida, T. Phase Equilibrium for Structure-H Hydrate Formed with Krypton and 2,2Dimethylbutane. J. Chem. Eng. Data 2006, 51, 161−163. (10) Jin, Y.; Kida, M.; Nagao, J. Microscopic Equilibrium Determination for Structure-H (sH) Clathrate Hydrates at the Liquid-Liquid Interface: Krypton-Liquid Hydrocarbon-Water System. J. Chem. Eng. Data 2012, 57, 2614−2618.

4. CONCLUSIONS The phase equilibrium pressure−temperature (pT) conditions for sH hydrates with Kr and Xe and with BrCH and BrCP as LMGSs were studied in the temperature range of (273.2 to 279.3) K. Three systems (the Kr−BrCH−water, Kr−BrCP− water, and Xe−BrCH−water systems) formed sH hydrates, but the Xe−BrCP−water system did not. The L1−L2−H−V fourphase equilibrium pT conditions for the three systems were determined using an isochoric method using a high-pressure vessel with an optical window and a stirrer. The phase stabilities of the sH hydrates with Kr increased, in terms of pressure, in the order NH < MCH < BrCH < BrCP ∼ MCP in the experimental temperature range. The L1−L2−H−V four-phase equilibrium pT condition pressures for the Xe−BrCH−water system were lower than the equilibrium pressures for the pure Xe hydrate below 277 K. Nevertheless, the phase equilibrium pT curve for the Xe−BrCH−water system became the reverse of that for the Xe hydrate at around 277 K. Moreover, the slope of the equilibrium pT curve changed slightly at 277.05 K. From 1708

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dx.doi.org/10.1021/je500216u | J. Chem. Eng. Data 2014, 59, 1704−1709