Article pubs.acs.org/JPCC
Synthesis and Characterization of sI Clathrate Hydrates Containing Hydrogen R. Gary Grim, Prasad B. Kerkar, Michele Shebowich, Melissa Arias, E. Dendy Sloan, Carolyn A. Koh,* and Amadeu K. Sum* Center for Hydrate Research, Chemical & Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: Previously, large cage occupancy of H2 has only been confirmed in the structure II (sII) hydrate. Utilizing a hydrate synthesis pathway involving pressurizing preformed structure I (sI) hydrates, we now show H2 occupancy in both the small and the large cages of sI, as evidenced by powder X-ray diffraction and Raman spectroscopic measurements. The new H2 environments were determined to be singly and doubly occupied 51262 cages occurring at 4125−4131 and 4143−4149 cm−1, respectively. This work serves as proof-of-concept that, by altering the conventional hydrate synthesis procedure to incorporate preformed hydrates, it may be possible to promote the occupancy of H2 or possibly other guests in a desired structure through a “templating” effect by simply changing the initial hydrate structure.
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INTRODUCTION Clathrate hydrates are a class of inclusion compounds that form when water and suitably sized guest molecules, normally gases, come into contact at sufficiently high pressure and low temperature. Depending on the size of available guest molecules, these ice-like solids generally form one of three common structures: structure I (sI), structure II (sII), or structure H (sH). sI is a cubic Pm3n structure comprising two pentagonal dodecahedron (512) cages and six tetrakaidecahedron (51262) cages. sII is a cubic Fd3m structure with 16 pentagonal dodecahedron (512) cages and 8 hexakaidecahedron (51264) cages. sH is characterized by a hexagonal P6/mmm crystal structure with three pentagonal dodecahedron (512) cages, two irregular dodecahedron (435663) cages, and one icosahedron (51268) cage.1 This preferred numerical nomenclature is common throughout the hydrate literature and should be read as the number of water molecules in the face of a cage raised to the number of faces for that particular cage (e.g., the 51262 cage contains 12 pentagonal faces and 2 hexagonal faces). Smaller guest molecules, such as methane and carbon dioxide, form sI, whereas larger guest molecules, such as methylcyclohexane, have been shown to occupy the largest cage of sH, in combination with a small guest to occupy the small and medium cages.1−4 In addition, very small molecules (e.g., H2, N2) form sII as they better stabilize the large number of small cages present in the unit cell of the structure.5,6 Initially, gas hydrates were viewed primarily only as a nuisance for the oil and gas industry.7 The harsh thermodynamic conditions inside subsea oil and gas lines coupled with the presence of water and light hydrocarbons provide the ideal conditions for hydrate formation and subsequent pipeline © 2012 American Chemical Society
blockages. More recently, however, gas hydrates have found the potential for many other beneficial applications. Specifically, hydrates have been studied as potential media for energy storage, natural gas transportation, and gas separation.6,8−16 With respect to energy storage in clathrates, much of the recent focus has been devoted to the storage of H2. This idea was first brought about in 1999, when Dyadin et al. demonstrated that H2 could form “classical polyhedral clathrate hydrates” at pressures of up to 15 kbar.5 The next breakthrough came in 2002, when Mao et al. demonstrated that H2 forms an sII clathrate at 220 MPa and 234 K.6 In an attempt to reduce the impractical pressure and temperature formation conditions of H2 hydrates, recent studies have shown that, by adding a second “promoter” molecule, such as tetrahydrofuran (THF), the formation conditions can be brought to “near ambient” conditions of around 5 MPa and 279.6 K.15 However, by adding promoter molecules that fully occupy the large cavities of sII, the storage capacity is typically significantly reduced from 3.8 wt % to around 1.0 wt %.17−19 As the proposed H2 storage target is currently 5.5 wt % by 2015, it is clear that neither the pure sII phase nor the binary H2 + promoter systems can achieve this goal. Therefore, H2 hydrates of structures other than sII that could potentially store H2 in amounts greater than 5.5 wt % are of interest.19 More recently, the incorporation of H2 into other hydrate structures, namely, sI and sH, has been investigated. In 2005, Kim and Lee provided both NMR and Raman spectroscopic Received: July 26, 2012 Revised: August 9, 2012 Published: August 9, 2012 18557
dx.doi.org/10.1021/jp307409s | J. Phys. Chem. C 2012, 116, 18557−18563
The Journal of Physical Chemistry C
Article
evidence of H2 occupancy in the 512 cage of sI formed from a (20/80 mol %) CO2 + H2 binary gas mixture.20 The existence of H2 in only the small cages of sI was further confirmed by Kumar et al.,12 who studied systems of (40/60 mol %) CO2 + H2 mixtures at pressures up to 8.0 MPa. However, other similar studies involving CH4 + H221 and CO2 + H211,22 systems have found that H2 cannot be incorporated in the cages of sI hydrates and act only as a diluent gas. Similar experiments with sH have shown that, although H2 can occupy the small (512) and medium (435364) cages of sH, it acts only as a help gas.2,3 Despite showing the possibility for enclathration in different structures (sI, sII, sH), what is noticeably absent in these studies and throughout the available literature is evidence of large cage occupancy of H2 for these alternative structures, a critical requirement for increasing storage capacity. Only Choi et al.23 have provided experimental data through the use of NMR to suggest the possibility of large cage occupancy of H2 in sI. The importance of large cage occupancy for H2 as an energy storage media is best highlighted in previous work by Strobel et al.19 Specifically, previously calculated storage capacities illustrate that, without large cage occupancy, the H2 storage percentages of most known structures (sI−sVII) ranges from 0 to 1.5 wt %. Conversely, if large cage filling of these structures is achieved, storage capacities can increase to a range of 3.2−7.2 wt % (e.g., sVI hydrates of tert-butylamine). Clearly, to achieve competitive storage capacities, large cage occupancy must be exploited. In this paper, we demonstrate a hydrate synthesis pathway that allows for the enclathration of H2 in what we suggest are the small and large cage environments of sI. These new environments were characterized with both Raman spectroscopy and powder X-ray diffraction (PXRD) and were determined to be singly and doubly occupied large cavities occurring at 4125−4131 and 4143−4149 cm−1, respectively. Although sI has a lower theoretical storage capacity than the extensively studied sII H2 hydrate (∼3.2 vs 3.8 wt %), this work provides evidence that suggests that, by simply changing the structure of an initial hydrate “seed”, in this case, sI, we were able to incorporate H2, a normal sII former, into the small and large cavities of sI. In the future, this technique could be applied to different structures or guests and may yield unique compositions or structures with more favorable storage capacities or other energy-related properties.
Table 1. Formation Conditions of Initial sI Structures guest
initial formation pressure (MPa)
small cage occupancy (%)a
large cage occupancy (%)a
CH4 CO2
11.0 1.7
95.7 64.8
98.8 98.2
a
Occupancy values calculated from CSMGem.25−28
min before the cell pressure was released in attempt to vent any unconverted gas. Depending on the final cell pressure, we did not always observe an audible release of pressure, indicating that some of the original sI gas may have been condensed during this step. Note that, after quenching, the internal cell temperature was measured to reach approximately 110 K, and thus, the hydrate samples were considered to be stable at atmospheric pressure. The cell was then reconnected to a separate high-pressure system and placed in a cooling bath at 258 ± 1 K. The cell was then repressurized with H2 gas up to 70 ± 2 MPa to promote the potential for H2 occupancy, yet minimize the risk for pure sII formation, as these conditions were outside of the pure sII H2 hydrate stability region.24 The hydrates of CH4 or CO2 were then given 24 h in the presence of H2. After that time, the cell was again quenched in LN2 for 10 min and the hydrate sample was then recovered for ex situ Raman and PXRD analysis. The results presented are a culmination of at least three repeats for each sI-forming gas. Raman spectroscopic measurements of the hydrate samples were performed with a JobinYvon LabRam HR spectrometer with a 532 nm excitation source and an Olympus 20× SLWD objective in conjunction with an 1800 grooves/mm grating. The Raman system was calibrated with a Ne lamp, and acquisitions were acquired over an 800 mm focal length with a 50 μm entrance slit size. Acquisition times ranged from 20 to 120 s depending on the concentration of H2 in the sample. The typical spectral resolution for this configuration was ∼0.6 cm−1. Analysis and deconvolution of Raman spectra were performed with GRAMS/AI software from Thermo Galactic. Hydrate samples were preserved during ex situ analysis with a Linkam THMS 600 cooling stage maintained at 83 K. For collection of PXRD data, a Siemens D500 diffractometer with a Co radiation source (wavelength = 0.1788965 nm) in the θ/2θ scan mode was used. Acquisitions were performed at 0.1 MPa and 77 K with a dwell time of 2 s and step size of 0.02°.29 The PXRD spectra were collected from 5° to 65° and were characterized with the programs Checkcell and PowderX.30
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EXPERIMENTAL METHOD To promote H2 occupancy in sI environments, a base sI hydrate was first formed by pressurizing ice crystals with the desired sI-forming gas (CH4 or CO2). The ice grains used in the experiments were prepared by crushing preformed hexagonal ice chips and sieving the resulting ice powder down to a final size of
