In Situ Neutron Low-Temperature Pair Distribution Function (PDF) Analysis and Molecular Dynamics Simulations of CH4 and CO2 Hydrates

Natural gas hydrates (NGH) form on the ocean floor and in sub-surface permafrost deposits in high-pressure, low temperature environments.Research on these deposits is driven by their potential as an energy source.Naturally occurring CH4 hydrates primarily crystallize in the sI clathrate structure.The lattice is composed of hydrogen bonded water cages (the host), each of which occlude one gas molecule (the guest).Though the sI framework can host other molecules, this research focuses on CH4-CO2 hydrates to support current explorations in which CH4 may be harvested from hydrate deposits via exchange with CO2.CO2 replacement in the hydrate structure is energetically preferred, facilitating CO2 byproduct sequestration while providing CH4 as a fuel source.Equilibrium models predict that a mixed hydrate solid solution in which CO2 replaces some CH4 is stable at higher temperature and lower pressure compared to pure CH4 hydrate, and thermodynamic stability increases as the CO2 fraction increases.The impact of varying the type of guest molecules and mixed guest systems is a relevant topic to explore due to concerns about the stability of NGH under changing environmental conditions.A detailed understanding of the guest-host interactions in gas hydrates is necessary for the advancement of emerging technologies and processes which will utilize NGH deposits.The gas hydrate crystal has a high degree of disorder at all temperatures due to the motion of the occluded gas molecules and their interactions with the H2O lattice.Molecular dynamics (MD) simulations show that this disorder is not described by long-range crystallographic models.In situ neutron total scattering experiments and pair distribution function (PDF) analysis are used to characterize the short-range order in CH4-CO2 hydrates.Extraction of detailed information from PDF data of complex systems requires methods such as MD simulations combined with Reverse Monte Carlo (RMC) fitting.MD models of CH4-CO2 hydrates demonstrate the benefit of neutron PDF experiments by providing simulated PDFs, visualization of molecular motion, and analysis of thermodynamic interaction energies throughout the CH4-CO2 guest composition while providing large-box structural models for PDF data analysis.Variable temperature neutron PDF data of CH4, CO2, and mixed CH4-CO2 hydrate were collected on the NOMAD beamline at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory.RMCProfile was used to fit the data to large-box models produced with MD simulations.PDF experiments performed in situ provide structural characterization, while the variable temperature measurements lead to inferred dynamics.This analysis shows that when CH4 and CO2 co-occupy the hydrate, the host is more strongly distorted than in either pure CH4 or pure CO2 hydrates, but this becomes less defined with increasing temperature.The presence of CO2 in mixed hydrate increases the stability range and creates a barrier for CH4 to completely leave the structure.