Carbon dioxide hydrate phase equilibrium and cage occupancy calculations using ab initio intermolecular potentials.

Area of Science:

• Geochemistry
• Materials Science
• Computational Chemistry

Background:

• Gas hydrate deposits are promising for CO2 sequestration, involving methane replacement.
• Accurate characterization of CO2 cage occupancy is vital for assessing sequestration potential and methane recovery.
• Existing methods for determining guest-host interaction potentials have limitations and inconsistencies.

Purpose of the Study:

• To develop and validate a reliable method for predicting accurate cage occupancies of CO2 in gas hydrates.
• To establish accurate intermolecular potentials for CO2-H2O interactions using ab initio quantum mechanical calculations.
• To assess the CO2 sequestration potential and methane recoverability in gas hydrate systems.

Main Methods:

• Computed the potential energy surface (PES) for H2O-CO2 interactions at the MP2/aug-cc-pVTZ level, correcting for basis set superposition error (BSSE) using the half counterpoise method.
• Developed the "VAS" model by fitting Exponential-6 and Lennard-Jones 6-12 potentials to the ab initio PES, accounting for many-body interactions.
• Calculated reference parameters for structure I carbon dioxide hydrate and predicted equilibrium pressure and cage occupancies using the VAS model. Validated using molecular dynamics simulations for CO2 density and diffusion.

Main Results:

• The VAS model accurately predicted pure CO2 hydrate equilibrium pressure with <3.2% average absolute deviation from experimental data.
• Predicted small cage occupancy for CO2 hydrate ranged from 32% to 51%.
• Predicted large cage occupancy for CO2 hydrate exceeded 98%.

Conclusions:

• The VAS model, based on ab initio calculations, provides accurate intermolecular potentials for CO2-H2O systems.
• The developed model enables reliable prediction of CO2 cage occupancies in gas hydrates, crucial for sequestration assessments.
• This work advances the understanding of gas hydrate behavior for energy and environmental applications.