Phase diagrams for clathrate hydrates of methane, ethane, and propane from first-principles thermodynamics

Abstract

Natural gas hydrates are inclusion compounds composed of major light hydrocarbon gaseous molecules (CH₄, C₂H₆, and C₃H₈) and a water clathrate framework. Understanding the phase stability and formation conditions of natural gas hydrates is crucial for their future exploitation and applications and requires an accurate description of intermolecular interactions. Previous ab initio calculations on gas hydrates were mainly limited by the cluster models, whereas the phase diagram and equilibrium conditions of hydrate formation were usually investigated using the thermodynamic models or empirical molecular simulations. For the first time, we construct the chemical potential phase diagrams of type II clathrate hydrates encapsulated with methane/ethane/propane guest molecules using first-principles thermodynamics. We find that the partially occupied structures (136H₂O·1CH₄, 136H₂O·16CH₄, 136H₂O·20CH₄, 136H₂O·1C₂H₆, and 136H₂O·1C₃H₈) and fully occupied structures (136H₂O·24CH₄, 136H₂O·8C₂H₆, and 136H₂O·8C₃H₈) are thermodynamically favorable under given pressure–temperature (p–T) conditions. The theoretically predicted equilibrium pressures for pure CH₄, C₂H₆ and C₃H₈ hydrates at the phase transition point are consistent with the experimental data. These results provide valuable guidance for establishing the relationship between the accurate description of intermolecular noncovalent interactions and the p–T equilibrium conditions of clathrate hydrates and other molecular crystals.