title: 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.
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. su.yan@dlut.edu.cn.
authors.forename: Xiaoxiao
authors.initials: X
authors.lastname: Cao
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. su.yan@dlut.edu.cn.
authors.forename: Yingying
authors.initials: Y
authors.lastname: Huang
authors.affiliation: School of Electronic Science and Technology, Faculty of Electronic Information and Electrical Engineering, Dalian University of Technology, Dalian 116024, China.
authors.forename: Wenbo
authors.initials: W
authors.lastname: Li
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. su.yan@dlut.edu.cn.
authors.forename: Zhaoyang
authors.initials: Z
authors.lastname: Zheng
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. su.yan@dlut.edu.cn.
authors.forename: Xue
authors.initials: X
authors.lastname: Jiang
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. su.yan@dlut.edu.cn.
authors.forename: Yan
authors.initials: Y
authors.lastname: Su
authors.affiliation: Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. su.yan@dlut.edu.cn and Beijing Computational Science Research Center, Beijing 100089, China.
authors.forename: Jijun
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao 266071, China and Qingdao Institute of Marine Geology, Qingdao 266071, China.
authors.forename: Changling
authors.initials: C
authors.lastname: Liu