Local structure and distortions of mixed methane-carbon dioxide hydrates

BR Cladek and SM Everett and MT McDonnell and MG Tucker and DJ Keffer and CJ Rawn, COMMUNICATIONS CHEMISTRY, 4, 6 (2021).

DOI: 10.1038/s42004-020-00441-7

A vast source of methane is found in gas hydrate deposits, which form naturally dispersed throughout ocean sediments and arctic permafrost. Methane may be obtained from hydrates by exchange with hydrocarbon byproduct carbon dioxide. It is imperative for the development of safe methane extraction and carbon dioxide sequestration to understand how methane and carbon dioxide co-occupy the same hydrate structure. Pair distribution functions (PDFs) provide atomic-scale structural insight into intermolecular interactions in methane and carbon dioxide hydrates. We present experimental neutron PDFs of methane, carbon dioxide and mixed methane-carbon dioxide hydrates at 10 K analyzed with complementing classical molecular dynamics simulations and Reverse Monte Carlo fitting. Mixed hydrate, which forms during the exchange process, is more locally disordered than methane or carbon dioxide hydrates. The behavior of mixed gas species cannot be interpolated from properties of pure compounds, and PDF measurements provide important understanding of how the guest composition impacts overall order in the hydrate structure. Natural gas can be collected from hydrate deposits by exchange with CO2, but the ensuing mixed hydrates and the path to a complete exchange are not fully understood. Here, computational analyses of neutron pair distribution functions of CH4, CO2 and mixed CH4-CO2 hydrates at 10 K reveal that the behavior of mixed gas species cannot be interpolated from properties of pure compounds.

Return to Publications page