In Situ Raman Spectroscopic Studies on Small-Cage Occupancy of

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In Situ Raman Spectroscopic Studies on Small-Cage Occupancy of Methane in the Simple Methane and Methane + Deuterated Tetrahydrofuran Mixed Hydrates Caroline T. Moryama,†,‡ Takeshi Sugahara,*,† Danilo Y. Yatabe Franco,†,§ and Hiroko Mimachi¶

Downloaded by ECOLE POLYTECHNIC FED LAUSANNE on August 28, 2015 | http://pubs.acs.org Publication Date (Web): August 18, 2015 | doi: 10.1021/acs.jced.5b00533

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Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan ‡ Department of Chemical Engineering, State University of Maringá, 5790 Colombo Avenue, Maringá, Paraná 87020-900, Brazil § School of Chemistry, Department of Chemical Engineering, Federal University of Rio de Janeiro, University City, Rio de Janeiro, Rio de Janeiro 21945-970, Brazil ¶ Chiba Technology Center, Research & Development Headquarters, Mitsui Engineering & Shipbuilding Co., Ltd., 1 Yawatakaigandori, Ichihara, Chiba 290-8531, Japan ABSTRACT: To investigate the small-cage occupancies of methane (CH4) in the structure-I and structure-II hydrates, under the three-phase equilibrium conditions of clathrate hydrate, aqueous solution, and fluid CH4 phases, in situ Raman spectra of the guest molecules in the simple CH4 hydrate and the CH4 + deuterated tetrahydrofuran (THF-d8) mixed hydrate with the stoichiometric THF-d8 composition were measured. The threephase equilibrium conditions of CH4 + THF-d8 mixed hydrate phase, THF-d8 aqueous solution phase, and fluid CH4 phase were measured in a temperature range of (278 to 294) K and pressure range up to 2.6 MPa. The small-cage occupancy of CH4 in the structure-II CH4 + THF-d8 hydrate increases with increasing pressure (and also temperature) up to 3 MPa. At pressures higher than 3 MPa, the small-cages of structure-II hydrate are mostly filled up with the CH4 molecules. In the simple CH4 hydrate, the small-cage occupancy is almost independent of the pressures of (5 to 60) MPa measured in the present study. The three-phase equilibrium pressures at which the small cages are filled up with the CH4 molecules would be lower than ever expected by the statistic-thermodynamic model.

1. INTRODUCTION Clathrate hydrate is a kind of inclusion compounds composed of guest molecules and hydrogen-bonded water molecules. Guest molecules are entrapped in the host water cages and one cage normally enclathrates one guest molecule. As a guest molecule, relatively small molecules such as light hydrocarbon, carbon dioxide, hydrogen, or noble gases as well as large molecules such as tetrahydrofuran (THF), cyclopentane, or methylcyclohexane are well-known.1 Several kinds of hydrate cages, for instance, 5 12 -cage (dodecahedron, which is constructed from 12 pentagons, hereafter called S-cage), 51262-cage (M-cage), and 51264-cage (L-cage) exist. The inner volume of the cage for the guest molecule is increased with the order of S, M, and L. Methane (CH4) hydrate, one of the most famous gas hydrates, constructs the cubic crystal structure-I (sI) composed of 2 S-cages and 6 M-cages in the unit cell. The CH4 molecules are able to occupy both S- and M-cages. Natural-gas hydrates (NGHs), expected as a possible naturalgas resource, are distributed in subterranean regions of the permafrost and the bottoms of sea and lake at various places in the world.1,2 The crystal structure of NGHs, where CH4 is a main guest species, depends on the guest composition, © XXXX American Chemical Society

temperature, and pressure conditions. The coexistence of other components in natural gases predominates the crystal structure of NGHs. Especially, ethane (C2H6) and propane (C3H8) coexisted with CH4 in natural gases result in the cubic crystal structure-II (sII) hydrate formation.3−6 The C2H6 and C3H8 molecules cannot occupy the S-cages at low pressure, whereas the C2H6 molecule can occupy a part of S-cages in the simple C2H6 hydrate at pressure higher than ∼20 MPa.7 The sII has 16 S-cages and 8 L-cages and the ratio of small to large cages in sII hydrates (2.0) is much larger than that in sI hydrates (1/3). To evaluate an available amount of CH4 recovered from NGH deposits, therefore, it is significant to investigate the capability of the S-cage occupancy of CH4. Until now, many researchers have reported the cage occupancy of guest molecules and the gas−water ratio (or hydration number). X-ray diffraction, Raman, NMR, and calorimetric studies have been performed to estimate the cage Special Issue: Memorial Issue in Honor of Anthony R. H. Goodwin Received: June 25, 2015 Accepted: August 13, 2015

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DOI: 10.1021/acs.jced.5b00533 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Journal of Chemical & Engineering Data

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occupancy of CH4.1,4,8−11 However, almost all reports on the cage occupancy have been studied at atmospheric pressure and extremely low temperatures after quenching the hydrate samples (so-called “ex situ”). During quenching or depressurization of the samples, there is a high possibility that the characteristics of the hydrate samples are changed, especially the cage occupancy of guest molecules. Raman spectroscopy is one of the best methods for in situ measurements of clathrate hydrates.9 Huo et al.12 reported that the hydrate is a nonstoichiometric compound based on the powder X-ray diffraction (PXRD) and Raman studies on ethylene oxide hydrate and that the cage occupancy of guest molecule is dependent on the temperature and system composition. Also, Huo et al.13 reported that the compositions of CH4 hydrates depend on the coexisting phase, either aqueous phase (waterrich condition) or fluid CH4 phase (CH4-rich condition) at 30 MPa and 275.15 K, which is very far from the stability boundary (the three-phase equilibrium curve of hydrate + aqueous solution + fluid CH4 phases) of CH4 hydrate. In the present study, to estimate the cage occupancy of the CH4 molecule, Raman spectra corresponding to the CH4 molecule in the single crystal of the simple CH4 hydrate and the CH4 + deuterium THF (THF-d8) mixed hydrate (with the stoichiometric composition of THF-d8) were in situ measured along the three-phase equilibrium curve of hydrate + aqueous solution + fluid CH4 phases. THF-d8, instead of THF as a typical sII hydrate former, was used to avoid overlapping the C−H Raman signals of CH4 with those of THF. On the threephase equilibrium curve in the CH4 + THF-d8 mixed hydrate with the stoichiometric composition of THF-d8 as well as the simple CH4 hydrate, the cage occupancy of the guest molecules is independent of the overall composition and the amount of hydrates, because the degree of freedom is 1 according to the Gibbs phase rule. In addition to the Raman spectra, the threephase equilibrium curves of CH4 + THF-d8 mixed hydrate system were measured.

the aqueous solution of THF-d8 with the mole fractions (xTHF‑d8) of 0.0558 ± 0.0001 (dissolved air was removed by CH4 pressurization and depressurization) was introduced in the cell and afterward pressurized with CH4 up to a pressure higher than a target equilibrium pressure. The contents were cooled and agitated to generate the gas hydrate. A ruby ball was enclosed in the cell to agitate the contents. Programming thermocontroller (EYELA NCB-3100) adjusted the cell temperature. After hydrate crystals were generated by continuous agitation, the system temperature was gradually increased in order to leave a few hydrate crystals. Then it was decreased step by step (0.1 K/hour) to prepare proper-sized hydrate single crystals for Raman spectroscopy. In situ Raman spectra of the hydrate single crystals were measured through the sapphire window by using a laser Raman microprobe spectrometer with a multichannel charge-coupled device (CCD) detector (JOBIN-YVON Ramanor T64000). The CCD detector was maintained at 140 K by liquid nitrogen for heat-noise reduction. The spectral resolution was approximately 0.7 cm−1. The argon ion laser beam (MELLES GRIOT 543-GS-A03, wavelength 514.5 nm, maximum power 150 mW) condensed to 2 μm in spot diameter was irradiated from the object lens to the sample through the upper sapphire window. The backscatter of the opposite direction was taken in the same lens. The Raman shift (Δν) was calibrated with the neon emission lines in the air. The typical photo of single crystals is shown in Figure 1. All Raman spectra in hydrate

2. EXPERIMENTAL SECTION 2.1. Materials. Materials used in the present study are summarized in Table 1. All of them were used without further Table 1. Information on the Chemicals Used in the Present Study chemical name methane (CH4) deuterium tetrahydrofuran (THF-d8) sodium chloride (NaCl) water

source

Figure 1. Typical photo of the hydrate single crystals of CH4 + THFd8 mixed hydrate at 296.6 K, 3.7 MPa and xTHF‑d8 = 0.0558.

mole fraction purity

Liquid Gas Sigma-Aldrich

> 0.9999 > 0.995

Wako Pure Chemical Industries Wako Pure Chemical Industries

> 0.995

phase were obtained from the hydrate single crystal that can be completely distinct from fluid CH4 phase at microscopic field, under the three-phase equilibrium state (hydrate + aqueous + fluid CH4 phases). The experimental apparatus (for phase equilibrium measurement) used in the present study is almost the same as ones used in the previous work.15,16 The pressure-proof glass cell (Taiatsu Techno, HPG-10-1) was used. The inner volume and maximum working pressure of the glass cell were 10 cm3 and 5 MPa, respectively. These materials allowed visual observation of the phase behavior in the hydrate and aqueous phases under high-pressure conditions. All parts of the high-pressure cell were immersed in a temperature-controlled bath with the thermocontroller (Taitec, CL-80R). The THF-d8 aqueous solution with xTHF‑d8 of 0.0555 ± 0.0001 or 0.0298 ± 0.0001 (dissolved air was purged with CH4 three times) was introduced in the cell and then pressurized with CH4 up to

> 0.9999

purification. The THF-d8, NaCl, and THF-d8 + NaCl aqueous solutions were prepared with the help of an electronic balance with an accuracy of 0.01 mg (A&D, BM-22). 2.2. Experimental Procedure. The experimental apparatus (for in situ Raman spectroscopy) used in the present study is almost the same as ones used in the previous work (except for using the different type of high-pressure optical cell).14 In the present study, the high-pressure optical cell with a pair of Ti-free sapphire windows (inner volume: 0.2 cm3, maximum working pressure: 400 MPa) was used. The distilled water or B




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the desired pressure. The contents were subcooled to approximately 5 K below the anticipated equilibrium temperature. Hydrate nucleation was induced by vigorously agitating the contents by an up-and-down mixing bar driven by an exterior permanent magnetic ring. The cell temperature was then slowly (with a rate of 0.1 K for 60 min) elevated to dissociate the formed hydrates. When a negligible small amount of hydrate crystals remained and the system pressure and temperature were stabilized, the resulting temperature and pressure were measured as an equilibrium condition. The procedure is essential especially in the case of xTHF‑d8 = 0.0298, because the THF-d8 composition in hydrate phase is different from that in the aqueous phase. The equilibrium temperature was measured with the thermistor thermometer (Takara, D632, reproducibility: 0.02 K). The equilibrium pressure was measured with the pressure gauge (Valcom, VPRT, maximum uncertainty: 0.01 MPa). To verify the crystal structure of the CH4 + THF-d8 mixed hydrate, PXRD pattern was measured on a Shimadzu, MAXima_X XRD-7000 diffractometer with Cu radiation (wavelength, 0.154060 nm; generation power, 40 kV, 40 mA) and a cold stage (Anton Paar TTK450). The hydrate sample for the PXRD measurement was prepared at 281.05 K from the THF-d8 aqueous solution with xTHF‑d8 = 0.0564 ± 0.0001 by the method similar to the phase equilibrium measurement. The different procedure is the repressurization with CH4 to increase the amount of sample. Note that, at the stoichiometric THF-d8 composition, the characteristics of the hydrate sample are independent of the amount of the hydrate sample as well as of the conversion to hydrates. Once the hydrate sample was depressurized and taken from the cell in the cold room controlled at 263 K, the sample was grained in the mortar immersed in liquid nitrogen. The measurement was done in the step scan mode with scan rate of 2 deg·min−1 and step size of 0.02 degrees. The PXRD pattern was collected at ambient pressure and 153 K, not in situ. Note that the equilibrium pressure of CH4 + THF-d8 mixed hydrate with the stoichiometric composition is below atmospheric pressure at 263 K as shown in the present study (Figure 2).

Figure 2. Thermodynamic stability boundaries (temperature T, pressure p, mole fraction in aqueous phase x) of CH4 + THF-d8 mixed hydrates (open circles fitted by the red curve, xTHF‑d8 = 0.0555; open squares fitted by the blue curve, xTHF‑d8 = 0.0298 in the present study) accompanied by those of CH4 + THF mixed hydrates (closed circles, xTHF = 0.0556 in ref 20; closed triangles, xTHF = 0.05 in ref 17; closed inverse triangles, xTHF = 0.06 in ref 19; closed squares, xTHF = 0.03 in ref 18).

Table 2. Three-Phase Equilibrium Conditionsa for CH4 + THF-d8 Mixed Hydrate Systemb xTHF‑d8 = 0.0555

3. RESULTS AND DISCUSSION 3.1. Thermodynamic Stability Boundaries. The stability boundaries (three-phase equilibrium curves) of the CH4 + THF-d8 mixed hydrates at xTHF‑d8 = 0.0298 and 0.0555 (on a CH4-free basis) are listed in Table 2 and shown in Figure 2. The open and closed keys correspond to the data obtained in the present study and reported by other researchers, respectively. The stability boundaries of the CH4 + THF (not deuterated) mixed hydrates have been reported.17−20 The stability boundary of the CH4 + THF mixed hydrate at xTHF = 0.0556 originates from the maximum temperature point (277.45 K, 4.9 kPa)21 of the three-phase (sII THF hydrate + THF aqueous solution + gaseous THF phases) coexisting curve in the simple sII THF hydrate system. Phase behavior of the CH4 + THF-d8 mixed hydrate system is similar to that of the CH4 + THF mixed hydrate system. The stability boundaries of the CH4 + THF-d8 mixed hydrates are located at a lower temperature (or a higher pressure) side than those of the CH4 + THF mixed hydrates with the same THF compositions. The PXRD pattern of the CH4 + THF-d8 mixed hydrate prepared at 281.05 K and xTHF‑d8 = 0.0564 is shown in Figure 3. It is clear that the CH4 + THF-d8 mixed hydrate is the sII (cubic, Fd3m).

T

p

K 277.98 278.35 278.87 279.43 279.81 280.12 280.69 281.68 282.35 282.62 283.18 283.75 284.04 284.33 284.81 285.30 285.75 286.54 287.00 287.54 288.08

xTHF‑d8 = 0.0555 T

p

MPa

K

0.06 0.07 0.10 0.13 0.15 0.17 0.21 0.28 0.33 0.35 0.40 0.46 0.48 0.51 0.57 0.62 0.68 0.78 0.85 0.93 1.02

288.77 289.60 290.30 290.77 291.27 291.73 292.08 292.52 292.81 293.09 293.51 293.82 294.12

xTHF‑d8 = 0.0298 T

p

MPa

K

MPa

1.16 1.31 1.46 1.57 1.69 1.82 1.93 2.04 2.13 2.22 2.36 2.46 2.57

279.06 279.93 280.68 281.34 281.79 282.52 283.38 284.28 284.85 285.46 287.44 288.20 288.79 289.30 289.86 290.16 290.56 291.35 291.74 292.05 292.59

0.16 0.21 0.26 0.30 0.33 0.40 0.48 0.58 0.66 0.74 1.02 1.16 1.25 1.35 1.47 1.57 1.67 1.88 1.98 2.08 2.26

a

Temperature T, pressure p, mole fraction in aqueous phase x. Uncertainties u are u(T) = 0.02 K, u(p) = 0.01 MPa, and u(x) = 0.0001. b

The obtained lattice parameter is a = (1.712 ± 0.002) nm at 153 K. 3.2. Cage Occupancy in Simple CH4 Hydrate. Typical Raman spectra of the C−H stretching vibration (symmetric ν1 and antisymmetric ν3 detected around 2915 and 3020 cm−1, C



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(the mass fraction of NaCl is 0.035) as well as distilled water was used for the hydrate preparation. The Raman spectra of the C−H symmetric stretching vibration mode of the CH4 molecule in the CH4 hydrate single crystals at different threephase equilibrium points were measured. The relation between the peak intensity ratios of 2914 cm−1 to 2903 cm−1 and the three-phase equilibrium pressures is shown in Figure 5. The

Figure 3. PXRD patterns (intensity I, diffraction angle θ) obtained from CH4 + THF-d8 mixed hydrate prepared at 281.05 K and xTHF‑d8 = 0.0564 (recorded at 153 K). The vertical bars represent the contributions from sII hydrate (red, upper) and hexagonal ice (blue, bottom). Figure 5. Peak intensity ratio (left axis; intensity I, equilibrium pressure peq ) and calculated cage occupancy (right axis; cage occupancy of CH4 in i-cage θi) by CSMGem1 in the sI simple CH4 hydrate prepared from the distilled water (open keys with red uncertainties) and NaCl aqueous solution with NaCl mass fraction of 0.035 (closed keys with blue uncertainties).

respectively) modes of the CH4 molecule in the single crystals of the sI simple CH4 hydrate, equilibrated with aqueous (saturated with CH4) and fluid CH4 (saturated with water) phases, are shown in Figure 4. In the simple CH4 hydrate

intensity ratio (I2914/I2903) obtained from Raman spectra is almost constant at 0.26 ± 0.01 in a pressure range from (5 to 60) MPa. The effect of NaCl is negligible. Figure 5 involves the cage occupancy ratio calculated by CSMGem1 based on the van der Waals and Platteeuw model.22 In a pressure range lower than 20 MPa, the trend is different from that of the Raman results. The present experimental results suggest that, on the three-phase equilibrium curve of hydrate + aqueous + fluid CH4 phases, the variation of the S-cage occupancy of CH4 in s-I simple CH4 hydrate is smaller than expected at least in a pressure range from (5 to 60) MPa under almost full occupancy of CH4 in M-cages. We speculate that the different trends of cage occupancy in the simple CH4 hydrate system would be caused mainly by the error of M-cage occupancy of CH4 calculated by CSMGem, not by the error of the threephase equilibrium curve predicted by CSMGem. The reason is that, unlike in the simple CH4 hydrate system, the trends are relatively similar in the sII CH4 + THF-d8 mixed hydrate system (next section) where all L-cages are occupied with THF-d8 molecules. In both simple CH4 hydrate and CH4 + THF-d8 mixed hydrate systems, there was no significant difference between the measured and predicted three-phase equilibrium curves. 3.3. Small-Cage Occupancy of CH4 in CH4 + THF-d8 Mixed Hydrate. Typical Raman spectra derived from the CH4 and THF-d8 molecules in the single crystals of the sII CH4 + THF-d8 mixed hydrate, THF-d8 aqueous solution (xTHF‑d8 = 0.0558, saturated with CH4), and fluid CH4 (saturated with water and THF-d8) phases, are shown in Figure 6. The peak of the C−H stretching vibrational peak of CH4 in the sII CH4 + THF-d8 mixed hydrate is single at 2914 cm−1. The CH4 molecules occupy only the S-cages and the THF-d8 molecules occupy all of the L-cages, because, at xTHF‑d8 = 0.0558, the L-

Figure 4. Typical Raman spectra (intensity I, Raman shift Δν) of CH4 molecules in simple sI CH4 hydrate (line a, black), aqueous (line b, blue), and fluid CH4 (line c, red) phases at 20.0 MPa and 291.5 K.

phase, the spectrum of ν1 splits into a doublet (2903 and 2914 cm−1), whereas a single peak was detected around 2915 and 2910 cm−1 for the fluid CH4 and the aqueous phases, respectively. The split of the Raman peak indicates that CH4 molecules occupy both S- and M-cages of sI hydrate. The larger peak detected at 2903 cm−1 corresponds to the CH4 in the Mcage, and the smaller one at 2914 cm−1 corresponds to the Scage. The very weak peak of ν3 was detected only in the fluid CH4 phase. The single crystals of CH4 hydrates were prepared at different three-phase equilibrium points. To investigate the effect of the coexistence of electrolytes, NaCl aqueous solution D




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Figure 6. Typical Raman spectra of THF-d8 (left) and CH4 (right) molecules in the sII CH4 + THF-d8 mixed hydrate (line a, black), aqueous (line b, blue), and fluid CH4 (line c, red) phases at 5.7 MPa, 299.9 K, and xTHF‑d8 = 0.0558.

Figure 7. Normalized Raman spectra of THF-d8 (left) and CH4 (right) molecules in the sII CH4 + THF-d8 mixed hydrates at different three-phase equilibrium points: (a) 0.6 MPa and 285.2 K; (b) 1.9 MPa and 291.5 K; (c) 3.7 MPa and 296.6 K; (d) 5.7 MPa and 299.9 K. All spectra were normalized by the peak of 850 cm−1 derived from the peak of THF-d8 in the hydrate phase. The peaks around 750 and 834 cm−1 are derived from sapphire window and THF-d8 in the aqueous phase, respectively.

of Figure 7, the peak derived from THF-d8 in the aqueous solution was overlapped with that in the hydrate. When the spectra were normalized, only the peak at 850 cm−1 was used after the peak deconvolution. The cage occupancy of CH4 in the CH4 + THF-d8 mixed hydrate was estimated by the intensity ratio of the peaks (2914 cm−1 for CH4) to (850 cm−1 for THF-d8). The obtained data are summarized in Figure 8, accompanied by the molar ratio (nCH4/nTHF) of CH4 to THF estimated from the model17,28 based on the van der Waals and Platteeuw model.22 The results obtained in the present study reveal that the S-cage occupancy of CH4 in the CH4 + THF-d8 mixed hydrate increases drastically at pressures up to approximately 3 MPa along the three-phase equilibrium curve of hydrate + aqueous + fluid CH4 phases. At pressures higher than 3 MPa, the variation of S-cage occupancy with pressure becomes small. The trend is similar to the estimated data,17,28,29 although the measured equilibrium pressure where the S-cage is saturated with CH4 seems to be lower than the estimated

cage occupancy of CH4 competing with THF-d8 (so-called “tuning effect”20,23,24) does not occur. The Raman peak corresponding to the C−O−C stretching vibrational mode of THF-d8 was detected at 850 cm−1, as assigned in the ref 25, whereas the peak was detected at 834 cm−1 in the THF-d8 aqueous solution phase. The shoulder peak at lower side of the C−O−C stretching vibrational mode (920 cm−1) of THF appears clearly in the THF aqueous solution phase.26,27 In the THF-d8 aqueous solution phase, the spectral differences from that in the hydrate phase clearly emerge the peak positions detected around 800 and 850 cm−1. Same as the simple CH4 hydrate system, we measured the Raman spectra in the single crystals of CH4 + THF-d8 mixed hydrates prepared at different three-phase equilibrium points. The spectra, normalized by the peak of 850 cm−1 derived from THF-d8, are shown in Figure 7. At pressures up to 3.7 MPa, the intensity (I2914) derived from CH4 increases with the pressure, whereas the intensities at 3.7 and 5.7 MPa are similar. In the left E



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +81-66850-6293. Funding

The authors (C.T.M. and D.Y.Y.F.) sincerely thank the CAPES Foundation, Ministry of Education of Brazil, for the scholarship (No. 88888.068169/2013-00 and 88888.058098/2013-00, respectively). Notes

The authors declare no competing financial interest.


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ACKNOWLEDGMENTS We acknowledge the scientific support from the “Gas-Hydrate Analyzing System (GHAS)” of Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University.

Figure 8. Peak intensity ratio in the sII CH4 + THF-d8 mixed hydrates prepared from the THF-d8 aqueous solution (open keys with red uncertainties) and THF-d8 + NaCl aqueous solution (NaCl mass fraction of 0.035 on a THF-d8-free basis, closed keys with blue uncertainties). The molar ratio (n) of CH4 to THF (right axis) in the sII CH4 + THF mixed hydrate was calculated by de Deugd et al.17

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; Taylor & Francis-CRC Press: Boca Raton, FL, 2008. (2) Kvenvolden, K. A. Methane Hydrate − A Major Reservoir of Carbon in the Shallow Geosphere. Chem. Geol. 1988, 71, 41−51. (3) Sloan, E. D.; Fleyfel, F. Hydrate Dissociation Enthalpy and Guest Size. Fluid Phase Equilib. 1992, 76, 123−140. (4) Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D. Evidence of Structure II Hydrate Formation from Methane + Ethane Mixtures. Chem. Eng. Sci. 2000, 55, 1981−1999. (5) Takeya, S.; Kamata, Y.; Uchida, T.; Nagao, J.; Ebinuma, T.; Narita, H.; Hori, A.; Hondoh, T. Coexistence of Structure I and II Hydrates Formed from a Mixture of Methane and Ethane Gases. Can. J. Phys. 2003, 81, 479−484. (6) Hashimoto, S.; Sasatani, A.; Matsui, Y.; Sugahara, T.; Ohgaki, K. Isothermal Phase Equilibria for Methane + Ethane + Water Ternary System Containing Gas Hydrates. Open Thermodyn. J. 2008, 2, 100− 105. (7) Morita, K.; Nakano, S.; Ohgaki, K. Structure and Stability of Ethane Hydrate Crystal. Fluid Phase Equilib. 2000, 169, 167−175. (8) Handa, Y. P. Compositions, Enthalpies of Dissociation, and Heat Capacities in the Range 85 to 270 K for Clathrate Hydrates of Methane, Ethane, and Propane, and Enthalpy of Dissociation of Isobutane Hydrate, as Determined by a Heat-Flow Calorimeter. J. Chem. Thermodyn. 1986, 18, 915−921. (9) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. J. Phys. Chem. B 1997, 101, 7371−7377. (10) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A. Single Crystal Diffraction Studies of Structure I, II and H Hydrates: Structure, Cage Occupancy and Composition. J. Supramol. Chem. 2002, 2, 405−408. (11) Takeya, S.; Udachin, K. A.; Moudrakovski, I. L.; Susilo, R.; Ripmeester, J. A. Direct Space Methods for Powder X-ray Diffraction for Guest-Host Materials: Applications to Cage Occupancies and Guest Distributions in Clathrate Hydrates. J. Am. Chem. Soc. 2010, 132, 524−531. (12) Huo, Z.; Jager, M. D.; Miller, K. T.; Sloan, E. D., Jr. Ethylene Oxide Hydrate Non-stoichiometry: Measurements and Implications. Chem. Eng. Sci. 2002, 57, 705−713. (13) Huo, Z.; Hester, K.; Sloan, E. D., Jr.; Miller, K. T. Methane Hydrate Nonstoichiometry and Phase Diagram. AIChE J. 2003, 49, 1300−1306. (14) Nakano, S.; Moritoki, M.; Ohgaki, K. High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the CO2 Hydrate System. J. Chem. Eng. Data 1998, 43, 807−810. (15) 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.

one.17,28 Moreover, there is no significant effect of NaCl on the S-cage occupancy of CH4.

4. CONCLUSIONS The S-cage occupancies of the CH4 molecule in the sI simple CH4 hydrate and the sII CH4 + THF-d8 mixed hydrate (prepared from the THF-d8 aqueous solution with approximately stoichiometric composition) were measured based on in situ Raman spectroscopy along the three-phase equilibrium curve of hydrate + aqueous solution + fluid CH4 phases. At the three-phase equilibrium point, the cage occupancy of the CH4 molecule is independent of the overall composition of CH4 to water. The important findings in the present study are as follows: (1) In the sI simple CH4 hydrate, the peak intensity ratio of CH4 in S-cage to M-cage is almost constant at pressures of (5 to 60) MPa, different from the calculated results by the modified van der Waals and Platteeuw model. Regarding the intensity ratio as the occupancy ratio of CH4 in S-cage to M-cage, the S-cages are fully occupied with CH4 at relatively low pressure and low temperature conditions. (2) In the sII CH4 + THF-d8 mixed hydrate (the crystal structure was confirmed by means of PXRD analysis in the present study), considering the normalized peak intensity of CH4 in S-cage as the S-cage occupancy of CH4, it increases drastically with pressure up to 3 MPa. At pressures higher than 3 MPa, however, the effect of pressure (and temperature) on the S-cage occupancy gets weak. (3) The three-phase equilibrium curves of the hydrate + THF-d8 aqueous solution + fluid CH4 phases in the sII CH4 + THF-d8 mixed hydrate lie at a temperature slightly lower (pressure slightly higher) than those in the sII CH4 + THF mixed hydrate with the same THF compositions. F




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In Situ Raman Spectroscopic Studies on Small-Cage Occupancy of

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