Structure of Gas Hydrates:
Hydrates are
basically of three types called Type I, Type II, and Type H. Other types of
hydrates are known and but they are rare. The crystal structures of
hydrates are three-dimensional.
Type I
- It is usually smaller molecules form Type I hydrates. Type I hydrate
structures include: (1) methane, (2) ethane, (3) carbon dioxide, and (4)
hydrogen sulfide
- Type I hydrates are made up of 8 polyhedral cages -- 6 large ones and 2
small. They are made up of 46 water molecules and thus have a theoretical
composition of 8X · 46 H2O or X · 5 3/4 H2O, where X is
the guest molecule.
Type II
- It is usually larger molecules form Type II hydrates. Type II hydrate
structures include: (1) propane and (2) isobutane, however, (3)
nitrogen, a relatively small molecule, also forms a Type II hydrate.
- Type II hydrates are made up of 24 polyhedral cages -- 8 large ones and 16
small. They are made up of 136 water molecules and thus have a theoretical
composition of 24 X · 136 H2O or X · 5 2/3 H2O. If only
the large cages are occupied, which is typical, then the theoretical
composition is 8 X · 136 H2O or X · 17 H2O.
Type H
- Type H hydrates are formed by larger molecules but only in the presence of
a smaller molecule, such as methane. Type H hydrates only form in the presence
of both the large and small molecules.
- Type H hydrates are made up of six polyhedral cages - 1 large, 3 medium
and 2 small. The large molecule occupies the large cage and the small molecule
occupies the small and medium cages. They are made up of 34 water molecules
and have a theoretical composition of X · 5 Y · 34 H2O where X is the large
molecule and Y is the small.
- Type H hydrate structures include: (1) 2-methylbutane, (2)
methylcyclopentane, (3) methylcyclohexane, and (4) cyclooctane. It bears
repeating, Type H hydrates only form if another, small molecule (such as
methane) is present.
Schematic Diagrams of different Hydrate Structures:
Type 1 Structure
Type 2 Structure
Type H Structure
