Establishing the structure of hydrates

Researchers develop novel diffraction techniques for structure determination.

An international team of chemists has developed a novel diffraction method for elucidating the structure of gas hydrates. First discovered in the early 1900s, gas hydrates are networks of water crystals that house gas molecules and were considered to be a curiosity until huge reservoirs of methane in hydrate form were discovered at the bottom of the sea.

Previous research has revealed that the water crystals can form cages of various shapes, including dodecahedrons and tetrakaidecahedrons, in which the gas molecules are trapped. Unfortunately, determining the precise structure of different gas hydrates has been hampered by the fact that creating single hydrate crystals is a difficult process, which has prevented structural analysis using neutron or X-ray diffraction. Hydrates of natural gases have therefore primarily been investigated using neutron powder diffraction, which generates less accurate structural information.

Now chemists from the University of Duisburg-Essen, Germany, Rice University, Houston, US, and the US company Halliburton, Oklahoma, have developed a novel analytical method for determining the structure of gas hydrates using X-ray diffraction.

By co-crystallising water and the relevant natural gas in a capillary tube small aggregates of crystals - termed oligocrystalline material - were formed. The researchers then developed a data treatment technique - oligo-diffractometry - to deal with the complex diffraction patterns produced.

Using this technique, the team was able to obtain structural information about the gas hydrates of methane, acetylene and propane, as well as hydrate mixtures of propane, ethanol and methane, and adamantane and methane. For example, they discovered that methane and acetylene hydrates both consist of central dodecahedra cages surrounded by tetrakaidecahedra columns.

The team now plans to expand the oligo-diffractometry technique to analyse other solutions of gases in organic solvents.

Jon Evans