Ind. Eng. Chem. Res. 2009, 48, 9335–9337
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Characterization of the Clathrate Hydrate Formed with Methane and Propan-1-ol Keita Yasuda,*,† Satoshi Takeya,‡ Mami Sakashita,‡ Hiroshi Yamawaki,‡ and Ryo Ohmura† Department of Mechanical Engineering, Keio UniVersity, Yokohama 223-0061, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
This paper reports the identification of the crystallographic structure of the clathrate hydrate formed in a system of methane + propan-1-ol + water. A hydrate crystal sample was prepared with a 0.0556 mole fraction aqueous solution of propan-1-ol pressurized with methane and then subjected to powder X-ray diffraction and Raman spectroscopic measurements. The X-ray diffraction pattern thus obtained from the sample indicated that the crystallographic structure of the hydrate was structure II. The Raman spectra thus obtained from the same sample indicated that the methane molecules occupied not only 512 cages but also 51264 cages. cages by the CH4 molecules, we also performed a Raman spectroscopic measurement.
Introduction Clathrate hydrates are icelike crystalline compounds consisting of hydrogen-bonded water molecules forming cages that enclose so-called guest molecules which are molecules other than water. Three different crystallographic structural families are known, depending on the nature of the guest molecules; structure I, structure II, and structure H. Those space groups are Pm3n, Fd3jm, and P6/mmm, respectively.1 Many hydrophobic substances and some hydrophilic substances function as guest substances in the hydrate forming systems. Hydrocarbons, noble gases, and alcohols are the typical guest substances that form clathrate hydrates with water. Because hydrogen bonding between the water and alcohol molecules likely affects the stability of the hydrates, the hydrate formation systems including alcohols as a guest substance are interesting from a physicochemical point of view. In the systems of 2-methyl-2-propanol,2,3 propan-2-ol,4,5 or ethanol6,7 coexisting with small molecule guest gases such as methane (CH4), difluoromethane, krypton, and hydrogen sulfide, hydrate formation has been reported so far. Chapoy et al.8 recently reported the formation of clathrate hydrate in systems of CH4 + propan-1-ol + water and natural gas + propan-1-ol + water. They measured phase-equilibrium temperature-pressure conditions in the above systems and compared them to statistical-thermodynamics model predictions in which propan-1-ol was assumed to be an inhibitor for hydrate formation, a structure II hydrate former, or a structure H hydrate former. The experimental data were well represented by a model prediction based on the assumption that propan-1-ol is a structure II hydrate former, thereby strongly suggesting the formation of a structure II hydrate with CH4 or natural gas and propan-1-ol as a guest substance. However, confirmation of structure II hydrate formation with propan-1-ol by directly measuring the hydrate phase is required for a final conclusion, as noted by Chapoy et al.8 Therefore, we attempted to identify the crystallographic structure of a hydrate formed in a CH4 + propan-1ol + water system by means of a powder X-ray diffraction (PXRD) technique. To clarify the occupation of the hydrate * To whom correspondence should be addressed. E-mail: mech@ z7.keio.jp. † Keio University. ‡ National Institute of Advanced Industrial Science and Technology (AIST).
Experimental Section Fluid samples used in the experiments were liquid water (distilled from deionized), methane of 0.9999 mole fraction certified purity (Takachiho Chemical Industrial Co., Ltd., Tokyo, Japan), and propan-1-ol of 0.999 mass fraction certified purity (Sigma-Aldrich, Inc., St. Louis, MO). A liquid solution of water and propan-1-ol mixed at a molar ratio of 17:1 (i.e., 0.0556 mole fraction propan-1-ol aqueous solution) was prepared and used in all of the experiments. This molar ratio is stoichiometric for water to propan-1-ol molecules in the resulting hydrate if propan-1-ol molecules are assumed to fully occupy the 51264 cavities in a structure II hydrate. A hydrate sample was prepared in a stainless steel vessel described elsewhere.6 We prepared a hydrate sample using the procedure noted below in order to ensure a high conversion ratio from the water to hydrate. Initially, the propan-1-ol aqueous solution was pressurized up to 2.2 MPa with CH4 at 268.3 K. If we suppose propan-1-ol work as a thermodynamic inhibitor for structure I simple CH4 hydrate, the phase equilibrium pressure at the same composition of aqueous solution and temperature at the sample preparation condition is estimated to be 2.39 MPa calculated by the data reported by Yasuda and Ohmura9 and Mohammadi and Tohidi.10 For the calculation, we suppose that propan-1-ol has the same effect as methanol and ethanol. On the other hand, the phase equilibrium pressure of a CH4 + propan-1-ol hydrate forming system at the same condition is 1.23 MPa (the extrapolation data reported by Chapoy et al.8). Thus, at the sample preparation condition in the present study, the CH4 + propan-1-ol hydrate was exclusively formable. During the period of hydrate formation in the vessel, a line connecting the vessel and methane cylinder was intermittently closed. If the hydrate crystals were formed in the vessel, the pressure reduction caused by the gas consumption was observed. Almost complete conversion from water to a hydrate was suggested if no pressure reduction was observed. After it was confirmed that no further pressure reduction was observed, the vessel was subsequently immersed into liquid nitrogen to quench the hydrate sample. The hydrate sample was then removed at a temperature below 170 K. The hydrate sample was stored in a container at 77 K (liquid nitrogen temperature) and subjected to PXRD and Raman spectroscopic measurements. For the PXRD measurements, the hydrate sample was finely powdered in a nitrogen atmosphere at a temperature below 100
10.1021/ie900954w CCC: $40.75 2009 American Chemical Society Published on Web 09/28/2009
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Figure 1. X-ray diffraction profile for CH4 + propan-1-ol hydrate. The measurements were performed at 93 K. The upper tick marks represent the calculated peak positions for the structure II hydrate, and the lower marks represent hexagonal ice Ih.
K. The fine-powdered hydrate crystals were top-loaded on a specimen holder made of Cu. The PXRD measurements were done using Cu KR (λ ) 1.5406 Å) radiation by parallel beam optics (40 kV, 40 mA; Rigaku model Ultima III). The PXRD measurements were performed in the θ/θ step scan mode with a step width of 0.02° at 93 K. Determination of the unit cell parameters was done by a full-pattern fitting method using the RIETAN-2000 program.11 The Raman spectrum of the hydrate sample was measured using a cryocell at 90 K. The Raman spectrum of a pure solid propan-1-ol was also measured at the same condition. The 488 nm line of the Ar-ion laser was used for the Raman excitation. Scattered light from the sample was collected in a 180° backscattering configuration and analyzed with a single monochromator. The spectra were recorded by a charge-coupled device (CCD) detector (800 pixels × 2000 pixels) capable of covering a wavenumber span of 3000 cm-1 with a 1.2 cm-1 resolution. For calibration, Ne and Kr emission lines were employed. Because the sample preparation condition and the storing/ experimental conditions were different, there is a possibility that the hydrate sample changed during the period of storage/ experiments. However, the temperature was low enough to prevent the sample decomposition, and it was expected that the sample composition did not change during the period of storage/ experiments. Results and Discussion Figure 1 shows the PXRD pattern obtained with the hydrate sample formed in the system CH4 + propan-1-ol + water. As shown in this figure, the crystallographic structure of the CH4 + propan-1-ol hydrate was identified to be structure II with a 17.165(1) Å lattice constant at 93 K. This is almost the same as the lattice constant of the CH4 + C3H8 hydrate (the lattice constant in this system is 17.138 Å at 77 K).12 A small amount of hexagonal ice Ih coexisted with the hydrate, which is simply considered to be unreacted ice which transformed from unreacted liquid water in the vessel during the cooling procedure. Therefore, it is concluded that the addition of propan-1-ol into a CH4 + water system alters the crystallographic structure of the hydrate formed in the system, resulting in the formation of the structure II hydrate as suggested by Chapoy et al.8 Figure 2 shows the Raman spectra of the CH4 + propan-1-ol hydrate and the pure solid propan-1-ol measured at 90 K. The peak positions of the CH4 + propan-1-ol hydrate in the same region are 2875, 2901 (as a shoulder peak), 2911, 2935, 2955, and 2975 cm-1. On the other hand, the peak positions of the
Figure 2. Raman spectra for CH4 + propan-1-ol hydrate of structure II and pure solid propan-1-ol. (solid line) CH4 + propan-1-ol hydrate. (dashed line) Pure solid propan-1-ol.
pure solid propan-1-ol in this region are 2878, 2911, 2936, and 2959 cm-1. Because these peak positions may be changed due to the enclathration of propan-1-ol molecules into the hydrate cages, the contributions of propan-1-ol molecules which occupy the hydrate cages to the Raman spectrum are not clear. On the other hand, the bands observed at 2901 cm-1 which appeared as a small shoulder peak correspond to the stretching mode of CH4 encaged in 51264 cages of the structure II hydrate, and 2911 cm-1 corresponds to the stretching mode of the CH4 encaged in 512 cages of the structure II hydrate.13 Here, the peak intensities of the pure solid propan-1-ol are weak relative to the peak intensities at 2901 and 2911 cm-1 of the CH4 + propan1-ol hydrate. This indicates that the CH4 molecules occupied both 512 and 51264 cages. That is, in the CH4 + propan-1-ol hydrate, 51264 cages were occupied not only by propan-1-ol molecules but also CH4 molecules. In conclusion, it is shown that the clathrate hydrate formed with CH4 and propan-1-ol is structure II hydrate; 51264 cages are occupied by both CH4 and propan-1-ol; and 512 cages are occupied by CH4 molecules. Acknowledgment This study was supported by a Grant-in-Aid for Scientific Research (A) from the Japan Society for the Promotion of Science (Grant 19760137) and by a Grant-in-Aid for the Global Center of Excellence Program for the “Center for Education and Research of Symbiotic, Safe and Secure System Design” from the Ministry of Education, Culture, Sport, and Technology in Japan. Literature Cited (1) Ripmeester, J. A.; Ratcliffe, C. I.; Udachin, K. A. Clathrate Hydrates. In Encyclopedia of Supramolecular Chemistry; Atwood, J. A., Steed, J., Eds.; Marcel Dekker, Inc.: New York, 2004; pp 274-280. (2) Imai, S.; Miyake, K.; Ohmura, R.; Mori, Y. H. Phase Equilibrium for Clathrate Hydrates Formed with Difluoromethane or Krypton, Each Coexisting with Propan-2-ol, 2-Methyl-2-propanol, or 2-Propanone. J. Chem. Eng. Data 2007, 52, 1056–1059. (3) Ripmeester, J. A. 1H Nuclear Magnetic Resonance Study of Tunnelling in Molecules with Multiple Methyl Groups Trapped in Hydrates. Can. J. Chem. 1982, 60, 1702–1705. (4) Østergaard, K. K.; Tohidi, B.; Anderson, R.; Todd, A. C.; Danesh, A. Can 2-Propanol Form Clathrate Hydrates? Ind. Eng. Chem. Res. 2002, 41, 2064–2068. (5) Ohmura, R.; Takeya, S.; Uchida, T.; Ebinuma, T. Clathrate Hydrate Formed with Methane and 2-Propanol: Confirmation of Structure II Hydrate Formation. Ind. Eng. Chem. Res. 2004, 43, 4964–4966. (6) Yasuda, K.; Takeya, S.; Sakashita, M.; Yamawaki, H.; Ohmura, R. Binary Ethanol-Methane Clathrate Hydrate Formation in the System CH4-
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009 C2H5OH-H2O: Confirmation of Structure II Hydrate Formation. J. Phys. Chem. C 2009, 113, 12598–12601. (7) Anderson, R.; Chapoy, A.; Haghighi, H.; Tohidi, B. Binary EthanolMethane clathrate Hydrate Formation in the System CH4-C2H5OH-H2O: Phase Equilibrium and Compositional Analysis. J. Phys. Chem. C 2009, 113, 12602–12607. (8) Chapoy, A.; Anderson, R.; Haghighi, H.; Edwards, T.; Tohidi, B. Can n-Propanol Form Hydrate. Ind. Eng. Chem. Res. 2008, 47, 1689–1694. (9) Yasuda, K.; Ohmura, R. Phase Equilibrium for Clathrate Hydrates Formed with Methane, Ethane, Propane, or Carbon Dioxide at Temperatures below the Freezing Point of Water. J. Chem. Eng. Data 2008, 53, 2182– 2188. (10) Mohammadi, A. H.; Tohidi, B. A Novel Predictive Technique for Estimating the Hydrate Inhibition Effects of Single and Mixed Thermodynamic Inhibitors. Can. J. Chem. Eng. 2005, 83, 951–961.
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(11) Izumi, F.; Ikeda, T. A. A Rietveld-analysis Program RIETAN-98 and its Applications to Zeolites. Mater. Sci. Forum. 2000, 321-323, 198– 203. (12) Hester, K. C.; Huo, Z.; Ballard, A. L.; Koh, C. A.; Miller, K. T.; Sloan, E. D. Thermal Expansivity for sI and sII Clathrate Hydrates. J. Phys. Chem. B 2007, 111, 8830–8835. (13) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. J. Phys. Chem. B 1997, 101, 7371– 7377.
ReceiVed for reView June 12, 2009 ReVised manuscript receiVed September 2, 2009 Accepted September 21, 2009 IE900954W
