Morphological Change in Structure H Clathrates of Methane and

ARTICLE pubs.acs.org/crystal

Morphological Change in Structure H Clathrates of Methane and Liquid Hydrocarbon at the Liquid
Liquid Interface Yusuke Jin* and Jiro Nagao* Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology, 2-17-2-1, Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan

bS Supporting Information ABSTRACT: The crystal morphology of a structure H (sH) hydrate in a methane (CH4)
liquid hydrocarbon (methylcyclohexane and neohexane)
water system in the temperature range 273.4
277.3 K was investigated in situ by optical scanning microscopy. Near the phase equilibrium condition, the sH hydrate showed a hexagonal shape with respect to its crystalline space group (P6/mmm). As the pressure increased under constant temperature, the edges of the hexagonal sH hydrate grew, and then the sH hydrate changed from a hexagonal to three-dimensional six-branched shape. We found that the branching growth of the sH hydrate appears at a specific degree of subcooling, ΔT > 4 K. This indicates that the sH hydrate with a snowflakelike crystal shape can be obtained by controlling ΔT.

1. INTRODUCTION Gas clathrate hydrates are crystalline clathrates composed of gas and water molecules under specific conditions in which gas molecules are stored in cages of hydrogen-bonded water molecules.1,2 Gas hydrates have been widely studied, from fundamental investigations to industrial applications, because of their unique structural and physical properties.2 They commonly have three crystal structures: structure I (sI), structure II (sII), and structure H (sH)2 mainly depending on the size of the guest gas molecules; for example, a methane (CH4) gas molecule is about 4.4 Å in diameter, and the CH4
water system forms an sI hydrate, whereas the propane
water system forms an sII hydrate. The space groups of the sI and sII hydrate crystals are cubic Pm3n and cubic Fd3m, respectively.2 The sH hydrates were first reported by Ripmeester et al. in 1987.3 The sH hydrates comprise one icosahedron cage (51268), three pentagonal dodecahedron cages (512), and two dodecahedron cages (435663). The space group of the sH hydrate crystal is hexagonal P6/mmm. The lattice constants on the a- and c-axes are about 12.2 and 10.1 Å, respectively.4 In the presence of small gas molecules, such as CH4, Xe, and H2S, the sH hydrates can store a large guest molecule (LGM): a liquid hydrocarbon (LHC) such as neohexane (NH), methylcyclohexane (MCH), or methylcyclopentane (MCP), and polar molecules such as tertbutyl methyl ether (TBME).2 LGMs are trapped in the large 51268 cage, and small gas molecules are trapped in both 512 and 435663 cages. The sH hydrates are formed with 34 water molecules in a unit cell. Therefore, ideally, five gas molecules other than an LGM can be stored in the unit cell of the sH hydrate crystal. The pressure and temperature conditions for sH hydrate formation are milder than those for pure gas hydrate formation. For example, at about 275.3 K, sH hydrate of MCH and CH4 is stable above 1.66 MPa,5 whereas CH4 hydrate (sI) is stable above 3.22 MPa.2 Thus, although fewer gas molecules can r 2011 American Chemical Society

be stored in an sH hydrate than in an sI hydrate, sH hydrates are of interest as storage and transportation media for natural gas. Knowledge of the appearance of a material’s crystal shape is important for understanding its crystal growth. In a gas
ice system, for example, a Xe hydrate shows anisotropic growth, and the growth of a Xe hydrate crystal on an ice surface almost entirely depends on the manner in which the water molecules diffuse.6 In the case of using an sH hydrate as a storage and transportation medium for natural gas, crystal shape is an important factor in designing the hydrate production process. Therefore, morphological changes in the hydrate crystal determine several factors for hydrate growth. Optical observation of gas hydrate growth is a classical technique and can directly provide information on the mechanism and the manner of formation and dissociation. Jin et al.7 determined the equilibrium condition of a Kr hydrate (sII) below the freezing point of water by direct observation of the morphological changes in the hydrate crystal. They clearly demonstrated that in situ visual observation of a hydrate is an effective method for characterizing its properties. Observation of the crystal morphology of sH hydrates would increase our understanding of crystal growth in gas hydrates. The sH hydrate crystals in a gas
LHC
water system form at the near interface between the LHC and water phases.8 Servio and Englezos9 observed sH hydrate growth in the CH4
NH
water system and suggested a correlation between crystal growth and the presence of CH4 in the NH phase. Many researchers have reported the visual observation of hydrate crystal growth in a gas
water system.8,10
20 However, few reports on the crystal growth of sH hydrates have been presented.9,21 The objective of this study is to understand the growth Received: March 31, 2011 Revised: May 17, 2011 Published: May 18, 2011 3149

dx.doi.org/10.1021/cg2003996 | Cryst. Growth Des. 2011, 11, 3149–3152

Crystal Growth & Design of an sH hydrate in a gas
LHC
water system and the factors that affect the crystal morphology of the sH hydrate. In this paper, we report two- to three-dimensional growth of a single crystal of sH hydrate in the CH4
MCH
water and CH4
NH
water systems under various pressure
temperature (pT) conditions and discuss the relationship between the crystal morphology of sH hydrates and the degree of subcooling.

2. EXPERIMENTAL SECTION 2.1. Materials. Water purified by ultrafiltration, reverse osmosis, deionization, and distillation was obtained from Lonza Ltd. Researchgrade (99.9% purity) methane gas used in the experiment was supplied by Sumitomo Seika Chemicals Co., Ltd. MCH used in the experiment (purity g99.0%) was supplied by Tokyo Chemical Industrial Co., Ltd. NH used in the experiment (purity g99.0%) was supplied by Aldrich Chemical Co. Inc. All materials were used without further purification. 2.2. Experimental Setup. The crystal growth of the sH hydrate in the CH4
MCH
water and CH4
NH
water systems was observed by use of an optical scanning microscope (OSM) (1HD200, Lasertec Co., Japan) equipped with a high-pressure vessel. A video camera (Handycam HDR-XR550 V, Sony Co., Japan) was attached to the OSM for real-time observations. Details of the configuration of the highpressure vessel and the observation system were reported previously.7 Distilled water (0.2 cm3) and LCH (0.2 cm3) were charged in a sample cell made of Cu with an inner diameter of 10 mm and a height of 5 mm. The sample cell containing the sample was loaded into the sample holder of the high-pressure vessel. First, the vessel was flushed two times with CH4 gas while the sample temperature was maintained at the experimental temperature, Tex. After elimination of air from the vessel, pressure was increased to the equilibrium pressure of sH hydrates at Tex by using CH4 gas. Next, to form and grow the sH hydrate crystal, the sample temperature was decreased to approximately 2.5 K lower than Tex while the pressure was maintained. After the hydrate grew to a suitable size (approximately 20 μm) for observation, the temperature was slowly increased to Tex. Then, after the sH hydrate was identified as a single crystal in a crossed-polarized-light view (Supporting Information,

Figure 1. (a) Single crystal of sH hydrate containing MCH and CH4 at 274.3 K and 2.4 MPa. (b) Growth of hydrate crystal shown in panel a after the temperature was maintained for 30 min. Scale bar indicates 100 μm.

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Figures S1 and S2), we performed observations of morphological changes in the sH hydrate by increasing the pressure while maintaining the temperature. The pressure was maintained for 30 min at each pressure condition. The sample temperature was maintained within (0.1 K by circulating liquid N2 and was measured via a thermocouple (Type K, Chino Co., Japan). The thermocouple was calibrated with a thermotracer (D641, Technol Seven Co. Ltd., Japan) and a thermistor thermometer (SXA-33, Technol Seven Co. Ltd.). The pressure was measured by pressure transducers (AP-14S, Keyence Co., Japan). The uncertainties in the temperature and pressure were estimated to be (0.2 K and (0.05 MPa, respectively, approximately at the 95% confidence level, considering the uncertainty in the measurement.

3. RESULTS AND DISCUSSION Figure 1 shows a single crystal of the sH hydrate containing MCH and CH4 grown in the CH4
MCH
water system at 274.3 K and 2.4 MPa. The crystal is considered to float in the MCH phase or near the MCH
water interface.8 The sH hydrate crystal in Figure 1 exhibits a hexagonal shape with respect to its crystalline space group. This hexagonal crystal shape is consistent with similar observations of single-crystalline sH hydrates obtained in the CH4
MCP
water system.8 In particular, the {0001} plane (basal plane) of the sH crystal in Figure 1 was perpendicular to the MCH
water interface. After 30 min at 274.3 K and 2.4 MPa (Figure 1b), the sH hydrate maintained its hexagonal shape. Figure 2 shows typical changes in the crystal growth with an increase in pressure from 2.1 to 2.2 MPa at 273.4 K. At 2.1 MPa, the sH hydrate crystal maintained its hexagonal shape. However, when the pressure was increased to 2.2 MPa, the crystalline morphology changed from hexagonal to a sixbranched shape (Figure 2b). Then after 5 min, the six-branched shape enhanced further (Figure 2c). A high growth rate is expected at the edge of the hexagon because of high thermal diffusion. The six-branched shape is reasonable in terms of latent heat transfer from the growing crystal surface. Considering this shape as shown in Figure 2b,c, the growth mechanism is expected to change with increasing pressure. Figure 3 shows the hydrate crystal in the CH4
NH
water system with an increase in pressure from 2.3 to 2.4 MPa at 275.0 K. The sH hydrate of CH4 and NH also changed from hexagonal to a six-branched shape with the increase in pressure. We performed similar observations in the CH4
MCH
water and CH4
NH
water systems and determined the pT conditions at which the crystal’s shape changes from hexagonal to branched. The morphological changes in the single crystals were observed four times at each constant temperature on increasing the pressure in steps of 0.1 MPa. Figure 4 shows the pT conditions of the

Figure 2. Sequential images of crystal change from hexagonal to branched shape at 273.4 K and 2.2 MPa in a CH4
MCH
water system (a) 5 min, (b) 10 min, and (c) 15 min after pressure was increased from 2.1 to 2.2 MPa. Scale bar indicates 100 μm. 3150

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Crystal Growth & Design

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Figure 3. Sequential images of crystal change from hexagonal to branched shape at 275.0 K and 2.4 MPa in a CH4
NH
water system (a) 5 min and (b) 30 min after pressure was increased from 2.3 to 2.4 MPa. Scale bar indicates 100 μm.

Figure 4. Graphical representation of morphological change from hexagonal to branched shape. (Blue circles) Pressure
temperature conditions for branching growth of an sH hydrate containing MCH and CH4; (red squares) pressure
temperature conditions for branching growth of an sH hydrate containing NH and CH4; (blue line) CH4
MCH
water
sH hydrate four-phase coexistence pressure
temperature conditions;5,22,23 (red line) CH4
NH
water
sH hydrate four-phase coexistence pressure
temperature conditions.23
26 Blue dashed line indicates ΔT = 4 K for sH hydrates containing MCH and CH4, and red dashed line indicated ΔT = 4 K for sH hydrates containing NH and CH4 under a given pressure.

change in crystal shape by using the phase diagram of the CH4
MCH
water
sH hydrate5,22,23 and CH4
NH
water
sH hydrate systems.23
26 Although the water solubilities in the MCH and NH phases are different,27 a change in the crystal’s shape from hexagonal to branched appeared at the degree of subcooling ΔT = 4 K in both the CH4
MCH
water and CH4
NH
water systems, as shown in Figure 4. Here ΔT is obtained by subtracting Tex from the equilibrium temperature, Teq, under a given pressure. This fact indicates that the crystal growth of the sH hydrate in a gas
LHC
water system depends on thermal diffusion that occurs at the face where crystal growth takes place. Ohmura et al.16
20 discussed the crystal morphologies of sI and sII hydrates in a gas
water system and noted that the morphology of crystal grains is only slightly reflected by the slowest-growing crystal face of each hydrate above a certain ΔT condition. For sH hydrates in a gas
LHC
water system, it has been found that the crystal morphology can be discussed in terms of ΔT. To understand what mechanism affects the growth shape of the crystal, we observed sH hydrate crystals at a constant temperature by controlling ΔT. Figure 5 shows morphological changes in the sH hydrate crystal of CH4 and MCH at 274.3 K. At this temperature, changes in the crystal’s shape appear at about 2.5 MPa (Figure 4). Each

Figure 5. Morphological changes from hexagon to snowflakelike crystal pattern of an sH hydrate at 274.3 K at various ΔT (pressure) conditions: (a) 2.4 MPa (ΔT = 3.9 K); (b) 20 min after pressure was increased from 2.4 to 2.6 MPa (ΔT = 4.8 K); (c) 20 min after pressure was decreased from 2.6 to 2.4 MPa; (d) 20 min after pressure was increased from 2.4 to 2.6 MPa; (e) 20 min after pressure was decreased from 2.6 to 2.4 MPa. Scale bar indicates 100 μm.

image was recorded after the designated pressure was maintained for 20 min. Moreover, at 2.4 MPa (ΔT = 3.9 K), the sH hydrate crystal grew while maintaining a hexagonal shape (Figure 5a), as shown in Figure 1. Figure 5b shows the result of increasing the pressure from 2.4 to 2.6 MPa (ΔT = 4.8 K). The crystalline morphology changed from hexagonal to branched shape, similar to the growth under 2.2 MPa and 273.4 K shown in Figure 2b,c. Then, after the pressure was decreased from 2.6 to 2.4 MPa, the sH hydrate showed faceted 3151

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Crystal Growth & Design growth and changed back to the hexagonal shape (Figure 5c). Figure 5d shows the crystal after the pressure was increased from 2.4 to 2.6 MPa, and Figure 5e shows the crystal after the pressure was decreased from 2.6 to 2.4 MPa. Finally, the sH hydrate crystal with a snowflakelike pattern was formed. The shape of the hydrate crystal in Figure 5e is clearly not planar but three-dimensional. At ΔT < 4 K, planar growth of the hexagonal crystal at the LHC
water interface was observed as shown in Figure 1. Smelik and King8 noted that the basal face of an sH hydrate is the slowest-growing crystal face. Therefore, it is considered that the planar growth at ΔT < 4 K is dominated by bulk growth kinetics of the sH hydrate that is reflected by the slowest basal face. On the other hand, at ΔT > 4 K, the shape changed from isotropic to anisotropic, as shown in Figure 2. Furthermore, from Figure 5, the basal face grown at ΔT > 4 K is considered to be thicker than that grown at ΔT < 4 K. This morphological instability shows that the crystal growth is dominated by the prism face and not the slowest basal face. Thus, it is believed that the collapse of bulk growth kinetics of the sH hydrate occurs at ΔT = 4 K. For an ice crystal with the same crystalline space group of an sH hydrate, Shimada and Furukawa28 suggested that the morphological instability of the ice crystal results from the difference in the growth mechanism between the basal and prism faces. Nada and Furukawa29 stated that layer-by-layer growth at the basal face and a collected molecule process at the prism face are predominant. Therefore, we consider that the growth mechanism on each crystal face of the sH hydrate at the LHC
water interface would change at ΔT = 4 K.

4. CONCLUSIONS We reported visual observation of the crystal growth of a singlecrystalline sH hydrate of CH4 and LHC (MCH and NH) in the temperature range 273.4
277.3 K. Our observations of crystal growth in the CH4
LHC
water system revealed that the crystalline morphology changed from planar hexagonal to three-dimensional six-branched shape with increasing pressure, namely, the degree of subcooling ΔT. At high ΔT, a hexagonal crystal changes to a six-branched shape, because high thermal diffusion occurs at the edge of the hexagon. Change in the shape of an sH hydrate can be classified by ΔT. The sH hydrate crystal grew two-dimensionally at ΔT < 4 K. In contrast, at around ΔT > 4 K, the growth of the prism face predominated, and the sH hydrate crystal showed three-dimensional growth. The change in the growth shape depends on the collapse of the bulk growth kinetics of the sH hydrate. ’ ASSOCIATED CONTENT

bS

Supporting Information. Two figures showing evidence for the single crystalline phase of an sH hydrate. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

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discussions. We also thank Mr. K. Matsumoto of AIST for experimental support.

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Corresponding Author

*Phone þ81-11-857-8526; fax þ81-11-857-8417; e-mail u-jin@ aist.go.jp (Y.J.) and [email protected] (J.N.).

’ ACKNOWLEDGMENT We thank Drs. H. Narita, T. Ebinuma, H. Oyama, Y. Konno, M. Kida, H. Ohno, M. Sasaki, and S. Tsuda of AIST for valuable 3152

dx.doi.org/10.1021/cg2003996 |Cryst. Growth Des. 2011, 11, 3149–3152