Characterization of CO2 and mixed methane/CO2 hydrates intercalated in smectites by means of atomistic calculations.

The recent increase in anthropogenic CO2 gas released to the atmosphere and its contribution to global warming make necessary to investigate new ways of CO2 storage. Injecting CO2 into subsurface CH4 hydrate reservoirs would displace some of the CH4 in the hydrate crystal lattice, converting simple CH4 hydrates into either simple CO2 hydrates or mixed CH4CO2 hydrates. Molecular simulations were performed to determine the structure and behavior of CO2 and mixed hydrate complexes in the interlayer of Na-rich montmorillonite and beidellite smectite. Molecular Dynamics (MD) simulations used NPT ensembles in a 4×4×1 supercell comprised of montmorillonite or beidellite with CO2 or mixed CH4/CO2 hydrate complexes in the interlayer. The smectite 2:1 layer surface helps provide a stabilizing influence on the formation of gas hydrate complexes. The type of smectite affects the stability of the smectite-hydrate complexes, where high charge located on the tetrahedral layer of the smectites disfavor the formation of hydrate complexes.

Crystal structure, stability and spectroscopic properties of methane and CO2 hydrates.

Methane hydrates are highly present in sea-floors and in other planets and their moons. Hence, these compounds are of great interest for environment, global climate change, energy resources, and Cosmochemistry. The knowledge of stability and physical-chemical properties of methane hydrate crystal structure is important for evaluating some new green becoming technologies such as, strategies to produce natural gas from marine methane hydrates and simultaneously store CO2 as hydrates. However, some aspects related with their stability, spectroscopic and other chemical-physical properties of both hydrates are not well understood yet. The structure and stability of crystal structure of methane and CO2 hydrates have been investigated by means of calculations with empirical interatomic potentials and quantum-mechanical methods based on Hartree-Fock and Density Functional Theory (DFT) approximations. Molecular Dynamic simulations have been also performed exploring different configurations reproducing the experimental crystallographic properties. Spectroscopic properties have also been studied. Frequency shifts of the main vibration modes were observed upon the formation of these hydrates, confirming that vibration stretching peaks of C-H at 2915cm(-1) and 2905cm(-1) are due to methane in small and large cages, respectively. Similar effect is observed in the CO2 clathrates. The guest-host binding energy in these clathrates calculated with different methods are compared and discussed in terms of adequacy of empirical potentials and DFT methods for describing the interactions between gas guest and the host water cage, proving an exothermic nature of methane and CO2 hydrates formation process.