Clathrate hydrate, type

Wiebeke, M., and Hoebbel, D. (1992) Structural Links between ZeoUte-lype and Clathrate Hydrate-type Materials Synthesis and Crystal Structure of... 

Strong bases N(CH3)4 -1-OH Clathrate hydrates (type VII, several others) Cations in cages, OH in water lattice 5,30... 

Fig. 11. Clathrate hydrates (a) basic structural component (H4QO2Q pentagonal dodecahedron) (b) type I host stmcture (two face-sharing 14-hedra are...

Classifying particles, in filtration, 11 326 Class I hybrids, 13 536, 543, 544 Class II hybrids, 13 536, 543 Clastogenesis, 25 206 Clathrate hydrates, 14 170—171 Clathrate receptor chemistry, 16 797 Clathrates, 12 374 14 159, 170-182 formation of, 10 633-635 26 869 Hofmann- and Werner-type, 14 171-172 phenol-type, 14 180 tri-o-thymotide, 14 179 Claus catalysts... 

Illustrative examples of substances which can behave as porous hosts in one of the above ways are also given. For instance, water readily forms open ice lattices which incorporate guests in clathrate hydrates of types I and II (see later text). Ordinary ice also possesses considerable porosity so that, as shown in Table I, He and Ne can readily diffuse through it. Ice below 0°C is zeolite-like in that it has a permanent, somewhat porous structure which (unlike the open-ice frameworks of the clathrate hydrates) does not require guest molecules for stabilization. 

Hedron type III 2 6-rings 12 5-rings 24 5.3 X 6.4 (hydrates only) clathrate hydrate (81), silicide and germa-nide of type I (59) and melanophlogite (60)... 

The importance of carefully considering anomalies when studying the behavior of solute-solvent interactions has been stressed. For aqueous solutions, many anomalous results presented in the literature suggest the existence of some type of discreteness in water structure. Discreteness is consistent with a view of water structure providing distinct sites such as those found in the models of water, implying a broken down ice lattice structure or clathrate hydrate cage-like structures. 

Tables 2.5a,b provide a comprehensive list of guest molecules forming simple si and sll clathrate hydrates. The type of structure formed and the measured lattice parameter, a, obtained from x-ray or neutron diffraction are listed. Unless indicated by a reference number, the cell dimension is the 0°C value given by von Stackelberg and Jahns (1954). Where no x-ray data exists, assignment of structure I or II is based on composition studies and/or the size of the guest molecule. Tables 2.5a,b also indicate the year the hydrate former was first reported, the temperature (°C) for the stable hydrate structure at 1 atm, and the temperatures (°C) and pressures (atm) of the invariant points (Qi and Q2). Both cyclopropane and trimethylene oxide can form si or sll hydrates. Much of the contents of these tables have been extracted from the excellent review article by Davidson (1973), with updated information from more recent sources (as indicated in the tables).

Mehta, A.P., A Thermodynamic Investigation of Structure-H-Type Clathrate Hydrates, Ph.D. Thesis Colorado School of Mines, Golden, CO (1996). 

Figure 7.3 Cavity types found in structures I, II and H clathrate hydrates and the numbers of each that goes into each structure unit cell along with number of water molecules and typical guests. Each vertex denotes a water molecule connecting lines represent hydrogen-bonded contacts.

Since small gas species such as O2, N2, O3, and Kr may be trapped inside the 12-hedra, all type II clathrate hydrates usually contain non-stoichiometric amounts of air unless special precautions are taken to avoid its inclusion as guest species. 

A basic building block in many clathrate hydrates is a pentagonal dodecahedron of water molecules (Fig. 7). This structure is found in gas hydrates of structure I (12 A) and structure II (17 A) types... 

Mootz D, Oellers E-J, Wiebcke M (1987) First examples of type I clathrate hydrates of strong acids polyhydrates of hexafluorophosphoric, tetrafluoroboric, and perchloric arid. J Am Chem Soc 109 1200-1202... 

Gas hydrates usually form two crystallographic stmctures - stmcture Type I and structure Type II. Rarely, a third structure may be observed (Type H). Figure 3.1 shows these three stmctures of clathrate hydrates and Table 3.1 presents their stmctural properties. 

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