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Methane Clathrate Hydrates Formed within Hydrophilic and Hydrophobic Porous Media: Kinetics of Dissociation and Distortion of Host Structure Satoshi Takeya, Hiroshi Fujihisa, Yoshito Gotoh, Vladimir Istomin, Evgeny Mikhailovich Chuvilin, Hirotoshi Sakagami, and Akihiro Hachikubo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp312297h • Publication Date (Web): 08 Mar 2013 Downloaded from http://pubs.acs.org on March 11, 2013
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The Journal of Physical Chemistry
Methane Clathrate Hydrates Formed within Hydrophilic and Hydrophobic Media:
Kinetics of Dissociation and Distortion of Host Structure Satoshi Takeya,a,* Hiroshi Fujihisa,a Yoshito Gotoh,a Vladimir Istomin,b Evgeny Chuvilin,c Hirotoshi Sakagami,d and Akihiro Hachikubod a
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565,
Japan b
Gazprom VNIIGAZ LLC, Moscow Region, 142717, Russia
c
Moscow State University, Moscow 119991, Russia
d
Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan
KEYWORDS Confined space, Gas hydrate, Interfacial force, Self-preservation
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ABSTRACT The crystal structure and dissociation processes of methane (CH4) hydrates were investigated to better understand their stability in a natural environment. By using powder X-ray diffraction, we found that the unit-cell parameters of the hydrates formed with fine hydrophilic and hydrophobic beads were respectively larger and smaller by 0.02 Å than the unit-cell parameter of simple CH4 hydrates. In addition, it was found that CH4 hydrates formed with hydrophobic beads dissociated quickly above 200 K, whereas the CH4 hydrates formed with hydrophilic beads are stable up to about 273K (Hachikubo et al., Phys. Chem. Chem. Phys. 2011, 13, 17449-17452.). The interfacial forces inside the intergranular pores or void spaces of the beads affect the kinetics of dissociation of CH4 hydrate, and are important for both the macroscopic and the crystallographic structures.
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The Journal of Physical Chemistry
Introduction Gas hydrates are crystalline host-guest compounds where guest molecules are inside hydrogenbonded water cages. Natural gas hydrates, which contain methane (CH4) as a major component, exist under conditions of low temperatures and high partial pressures of the guest gases. Because natural gas hydrates exist in sea and lake bottom sediments and permafrost layers, they are considered to be a possible source of energy1–3. A comprehensive understanding of the stability of these hydrates is also important for their potential effects on global climate change when they dissociate.4-7 There have been many studies on the dissociation mechanisms of gas hydrates in the absence of sediments or porous materials; these studies are motivated by physicochemical perspectives or from gas storage applications.8-22 Although many studies have been done concerning the thermodynamic stability of gas hydrates within pore spaces,23-27 much less is known about their physical properties under natural conditions and their dissociation mechanisms at the microscopic level,28-30 except for recent simulations.31-33 Recently, we have found that the CH4 hydrates formed with hydrophilic glass beads that are less than a few microns in size remained stable up to about the melting point of ice (273 K), even though this temperature is well outside the zone of the hydrate thermodynamic stability.34 This suggests that CH4 hydrates occurring naturally within the pores of fine particles at low temperatures, such as clays, would be stable even though their thermodynamic stability within meso-pores (less than 10 nm) is lower than that of bulk CH4 hydrates because of the GibbsThomson effect. In this respect, the effects of interfacial forces inside confined spaces may affect the stability of CH4 hydrates.
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To better understand the effect by interfacial force within confined space to CH4 hydrate, we report here the crystallographic structures of CH4 hydrates within intergranular pores, i.e. voids, formed by hydrophobic and hydrophilic beads. Powder X-ray diffraction (PXRD) and Raman spectroscopy were employed to characterize the crystal structures of CH4 hydrates and their cage occupancies in several types of beads, which sizes correspond to macro-pore scale (> 50 nm). Also, temperature-dependent PXRD measurements were performed to characterize the dissociation processes of CH4 hydrates within the void spaces.
Experimental methods CH4 hydrates were formed from mixtures of water and similarly-sized and hydrophobic34 or hydrophilic non-porous beads with smooth surface. (see Fig. 1) Details of all the beads are summarized in Table 1. About 1 g of fine ice-powder (
