1888
2008, 112, 1888-1889 Published on Web 01/30/2008
Water Cavities of sH Clathrate Hydrate Stabilized by Molecular Hydrogen: Phase Equilibrium Measurements Ana Rita C. Duarte,† Alireza Shariati,‡ Laura J. Rovetto,†,§ and Cor J. Peters*,† Delft UniVersity of Technology, Faculty of Applied Sciences, DelftChemTech, Physical Chemistry and Molecular Thermodynamics, Julianalaan 136, 2628 BL Delft, The Netherlands, Chemical Engineering Department, Shiraz UniVersity, Shiraz, Iran, and Chemical Engineering Department, Colorado School of Mines, 1600 Illinois Street, Golden, Colorado 80401 ReceiVed: NoVember 20, 2007; In Final Form: December 17, 2007
In this experimental phase equilibrium study, we show for the first time that it is possible to stabilize structure sH of hydrogen clathrate hydrate with the help of some selected promoters. It was established that the formation pressures of these systems are significantly higher than that of structure sII of hydrogen clathrate hydrate when tetrahydrofuran (THF) is used as a promoter. Although no experimental evidence is available yet, it is estimated that the hydrogen storage capacity of structure sH can be as high as 1.4 wt % of H2, which is about 40% higher compared to the hydrogen storage capacity in structure sII.
1. Introduction Dyadin et al.1 found that hydrogen can be stored as clusters in clathrate hydrates in structure sII at extreme conditions, i.e., at a pressure of 200 MPa and at a temperature of 273 K. Structure sII has 16 small 512 cages and 8 large 51264 cages per unit cell. Each unit cell of the sII structure consists of 136 water molecules. The experiments of Mao et al.2 showed that H2 and H2O mixtures crystallize in structure sII and with an approximate H2/H2O molar ratio of 1:2, which requires double occupancy of the small cages and quadruple occupancy of the large ones by clusters of hydrogen molecules. Later, Lokshin et al.3 found that the hydrogen occupancy in the large cage of the sII structure is reversible between two and four molecules per cage depending on pressure and temperature, while the occupancy of the small cage is always constant at one hydrogen molecule. According to the findings of Lokshin et al.,3 the approximate H2/H2O molar ratio is reduced to about 1:4.25 at temperatures above 130160 K and ambient pressure, while elevated pressures even may allow H2/H2O molar ratios as high as 1:2.8. Florusse et al.4 demonstrated that if the large cage of the sII structure of the clathrate hydrate is occupied by tetrahydrofuran (THF), hydrogen clathrate hydrate can be stabilized at pressures as low as 5 MPa at 279.6 K, versus 220 MPa at 280 K for pure hydrogen; i.e., a reduction of the thermodynamic equilibrium pressure as high as 95% could be achieved. Strobel and co-workers5,6 found that there is only one hydrogen molecule in the small 512 cages of the H2/THF system at the stoichiometric concentration (1 THF:17 H2O). Although the hydrate equilibrium pressure is reduced significantly, this means that the H2/(H2O + THF) molar ratio became 1:9, which * To whom correspondence should be addressed. Fax: +31-15-2784289. E-mail:
[email protected]. † Delft University of Technology. ‡ Shiraz University. § Colorado School of Mines.
10.1021/jp7110605 CCC: $40.75
corresponds to a hydrogen storage capacity as low as about 1% by weight at moderate pressures, i.e., a reduction of the storage capacity of 67% in comparison with pure H2 hydrate. In this work, it was attempted to increase the storage capacity of the clathrate hydrate of hydrogen by synthesizing sH clathrate hydrate using different promoters. 2. Experimental Section Experiments were carried out in a so-called Cailletet apparatus for pressures up to 10 MPa. In addition, a windowed-autoclave apparatus was used to cover the pressure range between 10 and 100 MPa. Both facilities operate according to the synthetic method and allow visual observation of the phase transitions to be measured. For samples of fixed overall composition, equilibrium pressures were determined at various temperatures. The temperature measurement of the Cailletet apparatus has an accuracy of (0.01 K, and the pressure measurement was performed using a dead weight gauge with an uncertainty of (0.005 MPa. The accuracy of the pressure measurements in the autoclave apparatus is better than (0.04% of the pressure reading from 3.0 to 100.0 MPa, while the uncertainty in the measured temperatures is (0.05 K. Details of the experimental facilities and procedures can be found elsewhere.7 The H2 used for the measurements was supplied by Hoekloos and had an ultrahigh purity of 99.9990 mol %, and demineralized water was used for all systems. Methylcyclohexane (MCH) and methyl tert-butyl ether (MTBE) were purchased from Fluka, and Aldrich was the supplier of dimethylcyclohexane (DMCH). Their purities were >99.5, >99.8, and >99%, respectively. 3. Results and Discussion Structure sH of clathrate hydrates has three small 512 cages, two medium-sized 435663 cages, and only one large 51268 cage per unit cell. Each unit cell of structure sH requires 34 water © 2008 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 112, No. 7, 2008 1889
TABLE 1: Experimental Data of the System H2O + H2 + Promoter MCH
DMCH
MTBE
T/K
p/MPa
T/K
p/MPa
T/K
p/MPa
274.0 274.4 274.9 275.4 275.9
83.1 85.1 90.1 95.1 100.1
274.7 276.1 277.8 278.9 279.5
60.1 70.1 80.1 90.0 95.0
269.2 269.9 270.3 270.9 271.4 272.0 272.5
70.1 75.1 80.1 85.1 90.1 95.1 100.1
molecules. To achieve our objective, different potential promoters were examined, and it was found that 1,1-dimethylcyclohexane (DMCH) with a diameter of 8.4 Å is able to stabilize the clathrate hydrate of hydrogen at 50.0 MPa and 274.7 K. If the temperature rises up to 279.5 K, the pressure increases to 95.0 MPa. Other promoters like methyl tert-butyl ether (MTBE) and methylcyclohexane (MCH) can also stabilize hydrogen clathrate hydrate in structure sH. However, the minimum pressure at which these two clathrate hydrates are stable is as high as 70.1 MPa at 269.2 K and 83.1 MPa at 274.0 K, respectively. Table 1 summarizes the experimental data of the systems studied, and Figure 1 shows the p-T diagram of the systems H2O + H2 + promoter. The equilibrium data for the pure H2 hydrate1 and the THF + H2 hydrate4 are also included in Figure 1. We consider these promoters as sH formers due to their molecular size. Their diameter (8.4 Å for DMCH, 7.8 Å for MTBE, and 8.59 Å for MCH) is large enough to stabilize the large cages of structure sH. Spectroscopic evidence for this conclusion is provided by the companion paper by Strobel et al.8 Mao et al.2 argued double occupancy of the 512 cages of structure sII. However, the findings of Strobel and co-workers5,6 indicate single occupancy by hydrogen of the small 512 cages. Therefore, single occupancy of the small 512 cages of structure sH was also assumed. In addition, complete occupancy of the large 51268 cages by the promoter is assumed and, as the medium-sized 435663 cage of sH is slightly larger than the 512 cage, single occupancy for this cage was also assumed. This scenario leads to an estimate of about 1.4 wt % of H2 stored in structure sH, which is about a 40% higher storage capacity than the storage capacity in structure sII. It should be mentioned that this estimated maximum storage capacity could not be experimentally verified yet. On the basis of what was pointed out above, it can be concluded that the H2/(H2O + promoter) molar ratio in structure sH can increase up to 1:7, which is an improvement in the gravimetric density of the clathrate hydrates in comparison with 1:9 of structure sII of the H2/THF system.
Figure 1. p-T diagram of the system H2O + H2 + promoter.
4. Conclusions In this experimental study, we showed that it is possible to stabilize structure sH of hydrogen clathrate hydrate with the help of some selected promoters. However, the formation pressures of these systems are significantly higher than that of structure sII of hydrogen clathrate hydrate when THF is used as a promoter. Although experimental evidence could not be obtained yet, it is estimated that the hydrogen storage capacity of about 1.0 wt % for structure sII is increased up to about 1.4 wt % for structure sH. Acknowledgment. The authors gratefully acknowledge the financial support of the CW/ACTS Programme “Duurzame Waterstof - Praktische Verkenning, Project number: 053.61.501”. L.J.R. is grateful to ”Daden voor Delft” for financial support. A.S. acknowledges the permission of Shiraz University for a research trip to TU Delft. References and Notes (1) Dyadin, Y. A.; Larionov, E. G.; Manakov, A. Yu.; Zhurko, F. V.; Aladko, E. Ya.; Mikina, T. V.; Komarov, V. Yu. MendeleeV Commun. 1999, 5, 209. (2) Mao, W. L.; et al. Science 2002, 297, 2247. (3) Lokshin, K. A.; et al. Phys. ReV. Lett. 2004, 93, 125503-1. (4) Florusse, L. J.; et al. Science 2004, 306, 469. (5) Strobel, T. A.; Taylor, C. J.; Hester, K. C.; Dec, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D. J. Phys. Chem. B 2006, 110, 17121. (6) Hester, K. C.; Strobel, T. A.; Sloan, E. D.; Koh, C. A. J. Phys. Chem. B 2006, 110, 14024. (7) Shariati, A.; Peters, C. J. J. Supercrit. Fluids 2003, 25, 109. (8) Strobel, T. A.; Koh, C. A.; Sloan, E. D. J. Phys. Chem. B 2008, 112, 1885 .
