Phase Diagrams for sII Clathrate Hydrates of CO2 from First-Principles Thermodynamics.

Abstract

Clathrate hydrates are crystalline solid compounds consisting of a water caged framework and guest molecules such as CH4, C2H6, and CO2. Understanding the phase equilibrium conditions of hydrates is significantly important for the industrial exploitation and experimental synthesis of hydrates. Based on the correct description of the intermolecular noncovalent interactions of clathrate hydrates with vdW-DF2, we studied the crystal structures and the chemical potential phase diagrams of sII hydrates encapsulated with CO2 molecules to provide a deep understanding of the stability mechanism of hydrates. Under the given p-T conditions, the partially occupied hydrates (136H2O·1CO2 and 136H2O·16CO2) and fully occupied hydrates (136H2O·24CO2) are thermodynamically stable, and the equilibrium temperature decreases as the relative CO2 chemical potential increases at the same pressure. We expect that the present study may provide vital information on the stability conditions of CO2 hydrates and trigger new experiments to establish an effective replacement strategy for CO2/CH4.