6th International Conference on Gas Hydrates


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   THERMODYNAMIC AND SPECTROSCOPIC ANALYSIS OF TERT- BUTYL ALCOHOL HYDRATE: APPLICATION FOR THE METHANE GAS STORAGE AND TRANSPORTATION    Youngjune Park, Minjun Cha, Woongchul Shin, Jong-Ho Cha, and Huen Lee∗ Department of Chemical & Biomolecular Engineering Korea Advanced Institute of Science and Technology 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701 REPUBLIC OF KOREA  John A. Ripmeester∗ The Steacie Institute for Molecular Sciences National Research Council of Canada 100 Sussex Drive, Rm 111 Ottawa, Ontario, K1A OR6 CANADA   ABSTRACT Recently, clathrate hydrate has attracted much attention because of its energy gas enclathration phenomenon. Since energy gas such as methane, ethane, and hydrogen could be stored in solid hydrate form, clathrate hydrate research has been considerably focused on energy gas storage and transportation medium. Especially, methane hydrate, which is crystalline compound that are formed by physical interaction between water and relatively small sized guest molecules, can contain about as much as 180 volumes of gas at standard pressure and temperature condition. To utilize gas hydrate as energy storage and transportation medium, two important key features: storage capacity and storage condition must be considered. Herein, we report the inclusion phenomena of methane occurred on tert-butyl alcohol hydrate through thermodynamic measurement and spectroscopic analysis by using powder X-ray diffractometer, and 13C solid- state NMR. From spectroscopic analysis, we found the formation of sII type (cubic, Fd3m) clathrate hydrate by introducing methane gas into tert-butyl alcohol hydrate whereas tert-butyl alcohol hydrate alone does not form clathrate hydrate structure. Under equilibrium condition, pressure-lowering effect of methane + tert-butyl alcohol double hydrate was also observed. The present results give us several key features for better understanding of inclusion phenomena occurring in the complex hydrate systems and further developing methane or other gas storage and transportation technique.  Keywords: gas hydrate, methane, tert-butyl alcohol, clathrate, storage and transportation   ∗ Corresponding author: Phone: +82 42 869 3917 Fax +82 42 869 3910 E-mail: h_lee@kaist.ac.kr ∗ Corresponding author: Phone: +1 613 993 2011 Fax +1 613 998 7833 E-mail: John.Ripmeester@nrc-cnrc.gc.ca INTRODUCTION In general, clathrate hydrates form three distinct crystalline structures, known as structure I (sI), structure II (sII), and structure H (sH) [1]. In addition, more complex clathrate hydrate structures named as type III ~ VII and structure T Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. (sT) are possibly exist [2, 3]. In particular, a type VI clathrate hydrate of tert-butylamine (t-BuNH2), which is a true clathrate hydrate as there in no evidence of hydrogen bonding between the host water and guest amine molecules, was found. For pure t-BuNH2 hydrate, large 17-dedral (43596273) cages are occupied by t-BuNH2, while the small cages (4454) remain empty. Recently, Kim et al. observed the structure transformation to mixed t- BuNH2 hydrate (sII) by the introduction of small molecules such as methane [4]. On the other hand, simple monohydroxy alcohol compounds often exhibit distinctive features in hydrate formation, as the hydrophobic-hydrophilic balance shifts with size of the alkyl groups. For example, methanol acts as inhibitor in hydrate formation, while ethanol forms clathrate hydrates of several types at low temperature [5, 6]. In this point of view, tert-butyl alcohol (TBA) is a good candidate for the study as it is fully miscible in water and not a classical hydrate promoter. However, TBA itself does not form clathrate with water. Herein, we address the inclusion phenomena occurring on TBA hydrate with introducing gaseous methane. Those findings are expected to be useful in understanding host-guest interaction as well as on potential application to methane storage.  RESULTS AND DISCUSSION  To identify the occurrence of inclusion phenomena in the TBA + methane hydrate, pressure-temperature trace was measured. The resulting phase equilibrium pattern is shown in Figure 1. By introducing methane into 5 mol% of TBA hydrate, sudden pressure drop by decreasing temperature was observed. It is implies that exposure of methane to a TBA hydrate provokes newly formed cavities, and as a result, methane molecules are entrapped. Actually, TBA + water system exhibits complex phase behavior with its concentration. It is known that such as a dihydrate (TBA•2H2O, orthorhombic Pnma) or heptahydrate (TBA•7H2O, monoclinic, P21) can be formed in low TBA concentration region [7]. However, those structures are not true clathrate hydrate because TBA and water molecules are hydrogen bonded each other. It is also notable that TBA acted as a hydrate stabilizer, even though its guest-host network is quite dissimilar to that of other guest molecules such as THF (tetrahydrofuran) or amines. When compared to pure methane hydrate which has equilibrium pressure of 113 bar at 288 K, TBA hydrate apparently exhibits pressure- lowering effect (57 bar at 288 K).   Figure 1 Pressure-temperature phase equilibrium of TBA (5 mol%) + methane hydrate   In order to evaluate the amount of enclathrated methane in TBA hydrate, we performed the quantitative analysis through direct release measurement. As shown in Figure 2, maximum about 8.4 wt% of methane could be achieved at around 5 mol% of TBA concentration, which is close to the stoichiometric concentration of sII type (cubic, Fd3m) clathrate hydrate (5.56 mol%). For the direct comparison with pure methane hydrate, TBA hydrate could store about two-thirds of the methane. In addition, when compared to THF, as it has been shown to be one of the most powerful promoter, which approximately contains 3.7 wt% of methane (θCH4sII-S = 0.45) in THF + methane double hydrate, those TBA hydrate shows much higher storage capacity.   Figure 2 Methane gas content in TBA hydrate as a function of TBA concentration.   Figure 3 shows the powder X-ray diffraction patterns of TBA and TBA + methane double hydrate. At the low TBA concentration, TBA hydrate forms two distinctive crystalline structures, named as dihydrate (orthorhombic, Pnma, a = 12.59 Å, b = 15.25 Å, c = 6.65 Å) and heptahydrate (monoclinic, P21, a = 6.02 Å, b = 6.07 Å, c = 10.38 Å, β = 106.60°). Figure 3A reveals coexistence of those two TBA hydrate structures. However, introducing methane provokes structure transition, and finally forms typical sII type clathrate hydrate (Fd3m, a = 17.30 Å).  Figure 3. Powder X-ray diffraction patterns of (A) TBA (5 mol%) hydrate, (B) TBA (5 mol%) + methane double hydrate. Black index: dihydrate, Red index: heptahydrate, Blue index: sII clathrate hydrate.     The 13C NMR spectra as shown in Figure 4 shows more detailed information. Before introducing methane, TBA hydrate contained two apparent structures as confirmed in X-ray diffraction study. 5 mol% TBA hydrate exhibits four identical peaks, allocated to COH carbon at 70.2 and 71.0 ppm, and methyl groups at 28.6 and 30.5 ppm. The peaks of 28.6 and 71.0 ppm are originated from heptahydrate, while the peaks of 30.5 and 70.2 ppm represent dihydrate. However, after exposure methane, characteristic peak representing methane in 512 cages of sII clathrate hydrate (-4.7 ppm) was found. In addition, newly formed peaks of TBA molecules at 28.6 and 67.4 ppm imply the enclathration of TBA in 51264 cages of sII clathrate hydrate. It is also confirmed that most of dihydrate and a small portion of heptahydrate have been converted to sII double hydrate.   Figure 4 13C NMR spectra of TBA (5 mol%) hydrate (upper), and TBA (5mol%) + methane double hydrate (lower). (A) CH4 peaks, (B) peaks of methyl group in TBA, (C) peaks of COH carbon in TBA.  CONCLUSION Methane inclusion phenomenon occurring on TBA hydrate was examined. From the thermodynamic equilibria, amount of methane possibly could be stored in TBA hydrate. In particular, TBA hydrate can store methane at more moderate conditions than pure methane hydrate. Spectroscopic methods, including PXRD, 13C solid-state NMR were adopted for microscopic observation. The introduction of methane into the TBA hydrate invokes a structure transition, and finally leading to the formation of cages of a clathrate hydrate structure to store methane. In addition, we observed the inclusion of TBA molecules in typical sII type clathrate hydrate. This confirms that methane molecules introduced to a TBA + H2O system play a role as a helping gas to make sII type clathrate hydrate. Comparing with liquid guest molecules such as THF or tert- Butylamine, TBA alone does not participate in the formation of clathrate hydrate without methane molecules. For a better understanding of the natural inclusion phenomena occurring on the gaseous and liquid guest molecules including alcohol, further intensive research is required. It is also expected that further investigation of inclusion phenomena occurring on energy gas and liquid guest molecules of alcohol give a various options for storing and transportation area of energy gas including methane. The present results provide several key features for better understanding of inclusion phenomena occurring in the complex hydrate systems and further developing methane or other gas storage methods.  ACKNOWLEDGMENTS This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Science and Technology (No. R0A-2005-000-10074-0(2007)), and also partially supported by the Brain Korea 21 Project.  REFERENCES [1] Sloan, E. D. Clathrate Hydrates of Natural Gases 2nd ed. New York: Marcel Dekker, 1998. [2] Jeffery, G. A. Inclusion Compounds, Vol. 1. London: Academic Press, 1984. [3] Udachin, K. A., Ratcliffe, C. I., Ripmeester, J. A. A dense and efficent clathrate hydrate structure with unusual cages. Angew. Chem. Int. Ed. 2001; 113:1343-1345. [4] Kim, D.-Y., Lee, J.-w., Seo, Y.-T., Ripmeester, J. A., Lee, H. Structural transition and tuning of tert-butylamine hydrate. Angew. Chem. Int. Ed. 2005;117:7927-7930. [5] Potts, A. D., Davidson, D. W. Ethanol hydrate. J. Phys. Chem. 1965;69:996-1000. [6] Calvert, L. D., Srivastava, P. Acta Crystallogr. Sect A 1969;25:S131. [7] Mootz, D., Staben, D. Die hydrate von tert- butanol: Kristallstruktur von Me3COH2H2O und Me3COH7H2O Z. Naturforsch 1993;48b:1325- 1330.


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