Structure of gas hydrates

This chapter provides a comprehensive overview of the fundamentals and applications of CO2 hydrates. Section 10.2 focuses on the microscopic perspective, looking into how gas hydrates form, the three structures of gas hydrates, and the characteristics of CO2 hydrates. From there onwards, the text focuses specifically on CO2 hydrates. The physical properties of CO2 hydrates are considered in Sect. 10.3. Section 10.4 deals with the phase equihbrium of CO2 hydrate. Experimental methods and the phase diagram are showed in this section. The last section covers the applications of CO2 hydrates, including the formation and dissociation of CO2 hydrates, ocean sequestration, the CH4 replacement in hydrates by CO2, and the use of CO2 hydrates in the refrigeration process. 

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Fig. 1. The structure of gas hydrates containing a hydrogen-bonded framework of 46 water molecules. Twenty molecules, arranged at the comers of a pentagonal dodecahedron, form a hydrogen-bonded complex about the comers of the unit cube, and another 20 form a similar complex, differently oriented, about the centre of the cube. In addition there are six hydrogen-bonded water molecules, one of which is shown in the bottom face of the cube. In the proposed structure for water additional water molecules, not forming hydrogen bonds, occupy the centres of the dodecahedra, and...

Abegg, F., Bohrmann, G., and Kuhs, W.F., subm.. Data Report Shapes and structures of gas hydrates from Hydrate Ridge imaged by CT-imaging of samples drilled during ODP Leg 204. 

Raman spectroscopy was recently used to study the structures of gas hydrates. It offers the advantage over IR analysis in that the sampling methods are significantly... 

The importance of hydrates requires the accurate knowledge of their thermodynamic and kinetic properties, mechanisms of formation and decay. Molecular simulation is a method of choice for such theoretical studies since it can explicitly capture the structure of gas hydrates and their constituents. MD is used to smdy different processes in methane hydrate, especially phase diagram of hydrates. Tung... 

It is now known that various molecules form gas hydrates, and the structures of gas hydrates have been investigated with X-ray... 

Based on the three available types of polygons mentioned above, five types of polyhedra are mainly found in the three structures of gas hydrates. As shown in... 

Fig. 10.2, they are pentagonal dodecahedron (5 " ), tetrakaidecahedron (5 6 ), hexakaidecahedron (5 6" ), irregular dodecahedron (4 5 6 ), and icosahedron (5 6 ). The formalism X" is used to describe the structure of each cavity, where X denotes the type of cage face (4—square, 5-pentagon, and 6-hexagon), and n represents the number of such kinds of face in that cavity. The three structures of gas hydrates comprise different cavities 5 and 5 6 cavities form si, 5 and 5 6 cavities form sll, and sH is formed by 5, 4 5 6, and 5 6 cavities. 

Fig. 10.3 Three structures of gas hydrates (a) si, (b) sll, and (c) sH [13]. Reprinted by permission from Macmillan Publishers Ltd NATURE COMMUNICATIONS, copyright (2011)...

Knowing the compositions and structures of gas hydrates are prerequisites for molecular-level modelling. Although gas hydrates were known in the early 1800s, the structure of these materials remained unknown until the advent of X-ray diffraction methods in the early twentieth century. Stackelberg" was the first to study the structure of gas hydrate with powder X-ray diffraction. He found that clathrate hydrates exist in two distinct cubic crystalline forms. However, the proposed structures were found to be incorrect. Remarkably, from the building of molecular models, Claussen was the first to arrive at the correct structures, which were later confirmed. Since then many hydrate structures have been studied. Under atmospheric pressure and low temperature, a majority can be classified in one of the two cubic structures termed structure I (S-I) and structure II (S-II). 

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