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J. Phys. Chem. B 2006, 110, 21788-21792
Phase Diagram and High-Pressure Boundary of Hydrate Formation in the Ethane-Water System Alexander V. Kurnosov,†,‡ Andrey G. Ogienko,*,† Sergei V. Goryainov,§ Eduard G. Larionov,† Andrey Y. Manakov,† Anna Y. Lihacheva,§ Eugeny Y. Aladko,† Fridrikh V. Zhurko,† Vladimir I. Voronin,| Ivan F. Berger,| and Aleksei I. Ancharov⊥ NikolaeV Institute of Inorganic Chemistry, SB RAS, Akad. LaVrentieVa AVenue 3, NoVosibirsk 630090, Russian Federation, BaVarian Geo-Institute, UniVersity of Bayreuth, UniVersitaetstrasse 30, 95447 Bayreuth, Germany, Institute of Geology and Mineralogy, SB RAS, Acad. Koptuga AVenue 3, NoVosibirsk 630090, Russian Federation, Institute of Metal Physics, UrB RAS, S. KoValeVskoj Str. 18, Ekaterinburg 620219, Russian Federation, Institute of Solid State Chemistry, SB RAS, Kutateladze 18, NoVosibirsk 630128, Russian Federation ReceiVed: June 13, 2006; In Final Form: August 25, 2006
Dissociation temperatures of gas hydrate formed in the ethane-water system were studied at pressures up to 1500 MPa. In situ neutron diffraction analysis and X-ray diffraction analysis in a diamond anvil cell showed that the gas hydrate formed in the ethane-water system at 340, 700, and 1840 MPa and room temperature belongs to the cubic structure I (CS-I). Raman spectra of C-C vibrations of ethane molecules in the hydrate phase, as well as the spectra of solid and liquid ethane under high-pressure conditions were studied at pressures up to 6900 MPa. Within 170-3600 MPa Raman shift of the C-C vibration mode of ethane in the hydrate phase did not show any discontinuities, which could be evidence of possible phase transformations. The upper pressure boundary of high-pressure hydrate existence was discovered at the pressure of 3600 MPa. This boundary corresponds to decomposition of the hydrate to solid ethane and ice VII. The type of phase diagram of ethane-water system was proposed in the pressure range of hydrate formation (0-3600 MPa).
Introduction Gas hydrates are inclusion compounds formed from water and gas molecules under high pressures and/or low temperatures. The host lattice of gas hydrates is a tetrahedral hydrogen-bonded network containing polyhedral cavities of one or more types. To date, six different types of gas hydrate structures have been reported.1-5 As a rule, at low pressures, proper hydrogen bond lengths and angles are crucial in determining the type of the hydrate being formed, while at high pressures, the water framework is adapted to the size and shape of the guest molecules to achieve the highest complementarity between the guest molecules and the cavities of the new structure and to maximize its overall density. In most cases, beyond a certain pressure the total volume of hydrate, constituent phases become smaller than the volume of the hydrate they form, thus resulting in the appearance of the upper pressure boundary of hydrate existence.6,7 Currently only two hydrate-forming systems lacking the upper boundary are known: water-methane8 and waterhydrogen.9 This indicates an exceptional complementarity of the guest molecules and the empty space in the water frameworks of high-pressure hydrates existing in these systems. The studies on such systems recently received a new impact from extending exploration of the inner strata of ice satellites belonging to major planets of the solar system, suggesting more * Corresponding Author. E-mail:
[email protected]. Telephone: (7383) 339 13 46. Fax: (7-383) 330 94 89. † Nikolaev Institute of Inorganic Chemistry. ‡ Bavarian Geo-Institute, University of Bayreuth. § Institute of Geology and Mineralogy. | Institute of Metal Physics. ⊥ Institute of Solid State Chemistry.
than probable existence of gas hydrates within these bodies.10 Apart from the methane-water system, among light hydrocarbons, the ethane-water one is of interest. It is known that, at normal pressure, this system yields a cubic structure I (CS-I) hydrate,1 and ethane can occupy the small cavities under pressures close to the ambient one, but the occupation factor of small cavities is below 5%, while large cavities are occupied completely.11 According to spectroscopy data, the occupation factor of small cavities significantly increases at high pressures, certainly resulting in hydrate stabilization.12 The decomposition curve of the ethane hydrate under pressures up to 479 MPa was also studied.12 In the present work, we report for the first time experimental data on the ethane hydrate decomposition curve under pressures up to 1500 MPa and some high-pressure Raman spectroscopy data for the ethane clathrate hydrate and pure ethane. Experimental Section The data for the ethane hydrate decomposition curve were obtained by differential thermal analysis under high-pressure ethane gas, the technique being described in detail in ref 13. The cubic structure I ethane hydrate was synthesized in a high-pressure piston-cylinder apparatus14 from thoroughly crushed ice and an excess of liquid ethane under pressures close to 340 (first experiment) and 700 (second experiment) MPa and room temperature. Neutron diffraction studies were performed at the research reactor IVV-2M (Ekaterinburg)15 at λ ) 1.805 Å. Powder diffraction data were analyzed using the FullProf program.16 Raman spectra of different phases in the ethane-water system under high pressure were investigated in a high-pressure
10.1021/jp0636726 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
Properties of High-Pressure Ethane Hydrate
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21789 TABLE 1: Numerical Data on l1hl2 Decomposition Curve of the CS-I Ethane Hydratea
Figure 1. Simplified P-T projection of phase diagram of the ethanewater system: h, cubic structure I ethane hydrate; l1, liquid water; l2, liquid ethane; sC2H6, solid ethane; iVI and iVII, ices VI and VII, respectively. O, our experimental data; ×, high-pressure boundary of the ethane hydrate stability determined in this work. Monovariant curves presented by the dashed lines are shown schematically. Low-pressure part of the diagram is omitted. Q1, quadruple point iVIl1hl2; Q2, quadruple point iVI iVII hl2; Q3, quadruple point iVIIhl2sC2H6.
diamond anvil cell modified for concurrent loading of water and gas. Chamber pressure was measured by the baric shift of the R1 line of ruby luminescence,17 ruby fragments being placed in the chamber together with the samples. Error in pressure determination was (30 MPa. Raman spectra were registered with a DILOR OMARS 89 spectrometer equipped with a CCD detector Princeton Instruments LN/CCD-1100Pb. Raman spectra were excited with the 514.5 nm line of an ion argon laser. We calibrated the detector frequency scale using characteristic lines of a neon discharge bulb after the recording of each spectrum. Estimated maximal possible error of the Raman band wavenumber was (0.5 cm-1. The cubic structure I ethane hydrate was synthesized in a diamond anvil cell from ice and liquid ethane under pressure close to 300 MPa. The X-ray diffraction study was performed using synchrotron radiation at the fourth beamline of the VEPP-3 storage ring (Budker Institute of Nuclear Physics, SB RAS), at λ ) 0.3675 Å.18 The Debye-Sherrer scheme was applied. A MAR345 imaging plate detector (pixel dimension 100 µm) was used to register the diffraction pattern. Integration was carried out using the FIT2D program,19 the distance from the sample to the detector was determined using the diffraction patterns of a standard substance (NaCl). Isolation of reflections, search for indexing versions, refinement of unit cell parameters, and development of model diffraction patterns were carried out with the help of programs FullProf16 and XLAT.20 Results and Discussion Our data on the decomposition curve of the ethane hydrate are presented in Figure 1 and Table 1. The comparison of our experimental points with the literature data1,12 is presented in Figure 2. With pressure increase, the decomposition curve passes a flat maximum at 68.6 °C and approximately 1300 MPa. The absence of bends, which correspond to the appearance of quadruple points in the system at the ethane hydrate decomposition curve, provides evidence that most probably the only cubic structure I hydrate exists in the whole pressure range studied (the structure characteristic of the low-pressure ethane hydrate1). For a similar system, sulfur dioxide-water (molecular volumes for ethane and sulfur dioxide are 43.3 and 33.4 Å3, respectively) studied under pressures approximately to 400 MPa,21 the hydrate decomposition curve also lacks quadruple points and passes a maximum; only one hydrate of the cubic structure I exists in the entire pressure range.
P, MPa
T, °C
P, MPa
T, °C
P, MPa
T, °C
P, MPa
T, °C
2.5* 5 7.5 10.8 16.5 20 20.6 30 35 40 45 45 50 52 60 63 65 70 101 109
14.5* 15 15.5 16.2 16.5 17.8 18 19 19.1 20.5 20.5 20.8 21.4 21.5 22.7 22.7 22.7 23.9 27.1 27.5
117 135 150 150 165 180 195 210 223 225 245 257 280 306 330 357 380 400 430 450
28.9 29.7 32.2 32.4 32.4 33.3 34.9 35.9 36.4 37 38 39.4 41.6 42.6 44 45.6 47.1 48 49.9 50.8
482 505 530 545 580 615 637 660 695 723 755 783 802 848 864 875 900 919 940 957
52.3 53.5 54.5 55.3 57 58.3 58.8 59.9 60.7 61.5 62.4 63.2 64 65 65 65 66.5 66.6 66.4 67.2
968 1000 1034 1054 1075 1094 1120 1142 1170 1200 1230 1258 1300 1325 1353 1390 1417 1449 1475 1500
67 67.4 68 68 68.1 68.1 68.5 68.1 68.5 68.5 68.6 68.6 68.6 68.6 68.4 68.4 68.4 68.4 68.1 68
a The data marked by asterisks correspond to l1hg equilibrium. l1, liquid water; l2, liquid ethane; g, gas phase; h, cubic structure I ethane hydrate.
Figure 2. The comparison of our experimental points (solid squares) with the literature data taken from ref 1 (open circles) and ref 12 (crosses).
Pressure dependence of Raman scattering peaks of C-C vibrations of ethane was studied for the hydrate phase and pure ethane under pressures varying from 200 to 6900 MPa. Some spectra are presented in Figure 3. Parameters of the equation ν(cm-1) ) ν0(cm-1) + (dν/dP)(cm-1/MPa) × P(MPa), characterizing pressure shift of ethane C-C vibration band for pure ethane and cubic structure I ethane hydrate, are presented in Table 2. At room temperature, solid ethane exists above 2700 MPa, its spectrum manifesting a doublet around 1000 cm-1 in contrast to a singlet of the liquid ethane. Room-temperature phase transformation pressure obtained in this way coincides with the value determined from volume measurements.22 The absence of singular points on the baric line shift dependence for the strongest C-C vibration band of ethane molecules in the hydrate phase within 200-3490 MPa (Figure 2) indicates that no new hydrate phases are formed. A substantial difference in the frequencies of this peak and the peak of pure ethane at low pressures also prompts that this peak originates from vibrations of ethane molecules in the hydrate phase. On the basis of data of ref 12, we attribute this peak to C-C vibrations of ethane molecules located within the large cavities of the cubic structure I; the peak corresponding to ethane molecules residing within the small cavities is much weaker and was not observable in our experimental spectra. The splitting of the peaks observed in the spectra at the pressure about 3490 MPa corresponds to
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Figure 3. Pressure shift of the C-C vibrations of pure ethane and ethane hydrate. Pure liquid ethane at 550 MPa (a); 1550 MPa (b); 2660 MPa(c). Pure solid ethane at 2750 MPa (d); 3950 MPa (e); 4550 MPa (f). Cubic structure I ethane hydrate at 340 MPa (g); 1200 MPa (h); 1750 MPa (i); 2140 MPa (j); 2980 MPa (k); 3490 MPa (l). Decomposed cubic structure I ethane hydrate at 3730 MPa (m).
TABLE 2: Pressure Shift of Ethane C-C Vibration Band for Pure Ethane and Cubic Structure I Ethane Hydrate
pure liquid ethane solid ethane, band 1 solid ethane, band 2 CS-I ethane hydrate decomposed ethane hydrate, band 1 decomposed ethane hydrate, band 2 a
pressure interval, MPa
ν0, cm-1
dν/dP, cm-1/MPa
SDa
no. of points
200-2670 2690-6900 2690-6900 220-3490 3800-6900
995.1 ( 0.7 1000.4 ( 0.8 1002.6 ( 1.0 1000.4 ( 0.4 1000.5 ( 2.3
0.0084 ( 0.0004 0.0049 ( 0.0002 0.0067 ( 0.0002 0.0062 ( 0.0002 0.0047 ( 0.0004
1.1 0.8 0.9 0.8 1.2
10 10 10 26 6
3800-6900
1001.1 ( 3.2
0.0067 ( 0.0006
1.7
6
Standard deviation.
the development of solid ethane, the only ethane-containing phase under these conditions. This can be related only to decomposition of the hydrate into solid ethane and ice VII, which is stable under these conditions. Therefore, at the pressure of approximately 3600 MPa and room temperature, a point of the three-phase equilibrium (hydrate f ice VII + solid ethane) is situated. A similar phenomenon was described in the studies7,23,24 where the occurrence of the upper pressure boundary of hydrate existence field was demonstrated. Taking into account the results of the present work, one can assume the most probable view of the P-T projection of phase diagram of the ethane-water system at high pressures (Figure 1). The phase diagram was built by application of the phase rule and some general physicochemical regularities to phase equilibria in the considered system. The extent of pressure stabilization or destabilization of hydrate phases (the slope of three-phase hydrate decomposition curves in the P-T) can be described by the Clapeyron equation, provided the composition of the phases is almost constant along the curve:
dT T∆V ) dP ∆H where ∆V is the evolution of system molar volume on hydrate formation, and ∆H is the molar enthalpy of hydrate decomposition into water (or ice) and pure guest component (mutual solubility neglected25). The large positive slope of the initial part of the ethane hydrate decomposition curve stems from the fact that one of the reactants is gaseous. In this case of hydrate formation, the
volume of the starting reagents is considerably higher than that of the hydrate phase being formed, thus resulting in large positive ∆V (the difference in the volumes of the reactants and the products) and, according to the Clapeyron equation, yielding a large positive coefficient dT/dP (hydrate decomposition enthalpy is always positive). The maximum occurring at the decomposition curve can be explained by the higher compressibility of fluid and liquid phases as compared to the solid one, therefore, the total volume of the reacting components on pressurizing decreases faster than the volume of the resulting hydrate phase, and they become equal in the maximum that, in accord with the Clapeyron equation, results in zero dT/dP derivatives. The increase in the density of hydrate packing shifts this maximum to higher pressures and temperatures. This picture is observed for the decomposition curve of the CS-I methane hydrate,26 and the trend to emergence of a maximum is found for all decomposition curves of double hydrates.27,28 The appearance in the system of dense phases of the components (ice or solid guest component) also results in development of a maximum at the decomposition curve (Xe system).13 On further rise of pressure, the derivative becomes negative, i.e., after the maximum is passed, the hydrate decomposition curve (l1hl2) goes down and inevitably crosses the melting curve of the ice VI (in the water system), thus affording the quadruple point Q1 with the following phases ranked according to decreasing water content: iVIl1hl2. Taking into account that the three-phase curves (iVIl1h) are close to the relevant melting curves iVIl in the TP projection, the Schreinemakers’ rule gives
Properties of High-Pressure Ethane Hydrate
Figure 4. Experimental X-ray powder diffraction pattern of the cubic structure I ethane hydrate at 1840 MPa.
the following order of the monovariant curves around the quadruple point Q1: l1hl2 - iVIl1h - iVIhl2 - iVIl1l2. When the pressure is further increased, the hydrate decomposition curve (iVIl1h) intersects the iVIiVII and l2sC2H6 curves of the relevant single-phase systems, yielding two new quadruple points Q2 and Q3 with the following sets of phases: iVI iVIIhl2 and iVII hCS-Il2sC2H6 (the leftmost phases are the richest with water). Around the points Q2 and Q3, the monovariant curves follow the order: iVIIhl2 - iVIiVIIh - iVIIhl2 - iVIiVIIl2 and hl2s - iVIIhl2 - iVIIl2s - iVIIhs, the iVIIhs line accounting for the experimental point of ethane hydrate decomposition. The Clapeyron equation predicts the increasing slope for the hydrate decomposition curves l1hl2 - iVIhl2 - iVIIhl2 - iVIIhs because system component densities are increased in these series due to solidification of liquid phases (yielding ice VI and solid carbon dioxide). Concerning the three-phase equilibria involving high-pressure ices (iVIhl2 and iVIIhl2), it is known that the density of ice VI (D2O) is 1.526 g/cm3 at -48 °C and 1.1 GPa, while for ice VII (D2O), it is 1.778 g/cm3 at 22 °C and 2.4 GPa.29 Lacking the data on the densities of the hydrate h and waterrich liquid l1 at these pressures, and admitting insignificant evolution of them with pressure, we suppose that the major contribution in ∆V of the hydrate decomposition reaction is made by the transition of the high-pressure ices. Hence, the larger (in absolute value) volume change should be observed for the transformation hydrate f ice VII + l1, ∆V being negative in both cases. Therefore, the ice VII curve (iVIIhl2) should go steeper than the ice VI one (iVIhl2) (Figure 1). After completion of these experiments, some uncertainty concerning the crystal structure of the ethane hydrate at high pressure still remained because, for some cases, the absence of a distinct singular point at the hydrate decomposition curve on the development of novel phases has been documented,4,30 and Raman spectroscopy experiments in diamond anvils used are possible above a minimal pressure of approximately 200 MPa. To determine the crystal structure of the high-pressure ethane hydrate, we performed a neutron and X-ray diffraction experiment. The X-ray powder diffraction pattern measured at 1840 MPa and the calculated position for reflections are presented in Figure 4. The final refinement of unit cell parameters was carried out in the cubic system (Pm3n) over seven individual reflections. The determined unit cell parameter is a ) 11.651 ( 0.001 Å and V ) 1581.4 ( 0.4 Å3 at this pressure. Powder neutron diffraction patterns measured at 340 and 700 MPa entirely matched the model of cubic structure I. The determined unit cell parameter is a ) 11.895 Å at 340 MPa; treatment of the data obtained at 700 MPa will be continued. In the present investigation, we proposed the type of phase diagram of an ethane-water system in the pressure range of
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21791
Figure 5. Pressure effect on the Raman band shift of the C-C vibration mode of ethane molecules. Filled circles, pure ethane; open squares, cubic structure I ethane hydrate. Vertical dotted lines show pressures at which solidification of ethane and decomposition of ethane hydrate occurs.
hydrate formation (0-3600 MPa). The data on the decomposition curve of the cubic structure I ethane hydrate under pressures up to 1500 MPa were obtained by the DTA method in this work; at about 1300 MPa, the decomposition curve reaches a flattened maximum (68.6 °C). It was demonstrated that no other hydrates are formed at higher pressures. The upper pressure boundary of the cubic structure I ethane hydrate existence was revealed at about 3600 MPa and room temperature; it corresponds to the monovariant curve iVIIhs. Acknowledgment. The study was supported by the Integration project of SB RAS no. 43, Russian Foundation for Basic Research (RFBR 05-05-64657 grant), the Presidium RAS program P-9-3 “Investigations of the matter at extreme conditions”, Moscow, and CRDF-BRHE (NO-008-X1 grant). References and Notes (1) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1997. (2) Jeffrey, G. A. Hydrate inclusion compounds. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vogtle, F., Eds.; Elsevier Science Ltd.: Oxford, 1996; Vol. 6, SolidState Supramolecular Chemistry: Crystal Engineering, p 757. (3) Loveday, J. S.; Nelmes, R. N.; Guthire, M. Phys. ReV. Lett. 2001, 87, 215501. (4) Manakov, A. Y.; Voronin, V. I.; Kurnosov, A. V.; Teplych, A. E.; Komarov, V. Y.; Dyadin, Y. A. J. Inclusion Phenom. 2004, 48, 11. (5) Kurnosov, A. V.; Komarov, V. Y.; Voronin, V. I.; Teplych, A. E.; Manakov, A. Y. Angew. Chem., Int. Ed. 2004, 43, 2922. (6) Dyadin, Y. A.; Bondaryuk, I. V.; Zhurko, F. V. Clathrate hydrates at high pressures. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1991; Vol. 5, Inorganic and Physical Aspects on Inclusion, p 213. (7) Manakov, A. Y.; Goryainov, S. V.; Kurnosov, A. V.; Likhacheva, A. Y.; Dyadin, Y. A.; Larionov, E. G. J. Phys. Chem. B 2003, 107, 7861. (8) Hirai, H.; Uchihara, Y.; Fujihisa, H.; Sakashita, M.; Katoh, E.; Aoki, K.; Nagashima, K.; Yamamoto, Y.; Yagi, T. J. Chem. Phys. 2001, 115, 7066. (9) Mao, W. L.; Mao, H.-K. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 708. (10) Loveday, J. S.; Nelms, R. J.; Guthrie, M.; Klug, D. D.; Tse, J. S.; Handa, Y. P. Nature 2001, 410, 661. (11) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A.; Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 1923, 2002; p 604. (12) Morita, K.; Nakano, S.; Ohgaki, K. Fluid Phase Equilib. 2000, 169, 167. (13) Dyadin, Y. A.; Larionov, E. G.; Mirinskij, D. S.; Mikina, T. V.; Aladko, E. Y.; Starostina, L. I. J. Inclusion Phenom. 1997, 28, 271. (14) Ivanov, D. F.; Litvin, B. N.; Savenko, L. S.; Smirnov, V. I.; Voronin, A. E.; Teplykh, A. E. High Pressure Res. 1995, 14, 209. (15) Aksenov, V. I. Preprint JINR, D3-94-364, Dubna, 1994.
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Kurnosov et al. (24) Sanloup, C.; Mao, H.-K.; Hemley, R. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 25. (25) Yakushev, V. S.; Istomin, V. A. In Physics and Chemistry of Ice; Maeno, N., Hondoh, T., Eds.; Hokkaido University Press: Sapporo, Japan, 1992. (26) Dyadin, Y. A.; Aladko, E. Y.; Larionov, E. G. MendeleeV Commun. 1997, 34. (27) Larionov, E. G.; Manakov, A. Y.; Zhurko, F. V.; Dyadin, Y. A. J. Struct. Chem. 2000, 41, 476. (28) Dyadin, Y. A.; Larionov, E. G.; Aladko, E. Y.; Zhurko, F. V. Dokl. Phys. Chem. 2001, 376 (4-6), 23. (29) Kuhs, W. H.; Finney, J. L.; Vettier, C.; Bliss, D. V. J Chem. Phys. 1984, 81, 3612. (30) Dyadin, Y. A.; Larionov, E. G.; Manakov, A. Y.; Zhurko, F. V.; Aladko, E. Y.; Mikina, T. V.; Komarov, V. Y. MendeleeV Commun. 1999, 209.
