Cao Xiaoxiao, Huang Yingying, Li Wenbo, Zheng Zhaoyang, Jiang Xue, Su Yan, Zhao Jijun, Liu Changling
Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China.
School of Electronic Science and Technology, Faculty of Electronic Information and Electrical Engineering, Dalian University of Technology, Dalian 116024, China.
Phys Chem Chem Phys. 2016 Jan 28;18(4):3272-9. doi: 10.1039/c5cp06570d.
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.
天然气水合物是由主要的轻质烃类气体分子（CH4、C2H6和C3H8）与水笼形骨架组成的包合物。了解天然气水合物的相稳定性和形成条件对于其未来的开发和应用至关重要，并且需要准确描述分子间相互作用。先前对气体水合物的从头算计算主要受限于团簇模型，而水合物形成的相图和平衡条件通常使用热力学模型或经验分子模拟来研究。我们首次使用第一性原理热力学构建了封装有甲烷/乙烷/丙烷客体分子的II型笼形水合物的化学势相图。我们发现，在给定的压力-温度（p-T）条件下，部分占据结构（136H2O·1CH4、136H2O·16CH4、136H2O·20CH4、136H2O·1C2H6和136H2O·1C3H8）和完全占据结构（136H2O·24CH4、136H2O·8C2H6和136H2O·8C3H8）在热力学上是有利的。理论预测的纯CH4、C2H6和C3H8水合物在相变点的平衡压力与实验数据一致。这些结果为建立分子间非共价相互作用的准确描述与笼形水合物及其他分子晶体的p-T平衡条件之间的关系提供了有价值的指导。
