Thermal transformations and dissociations in polycrystalline CO<sub>2</sub> hydrates
Xinheng Li, Yongxiao Qu, Yuan Li, Xiaoyu Shi, Kaibin Xiong, Zhisen Zhang, Jianyang Wu
Abstract
Abstract CO 2 hydrates show promising application in CO 2 sequestration, as well as natural gas recovering from hydrate-bearing sediments, in which the stability of CO 2 hydrates plays a vital role in these practical applications. Here, we report the thermal dissociation and cage transformations in polycrystalline CO 2 hydrates via high-throughput molecular dynamics simulations and machine learning (ML). It is revealed that the melting points of polycrystalline CO 2 hydrates (PCO2H) are dictated by the microstructural cages, in which the 5 12 , 5 12 6 2 and 4 1 5 10 6 3 cages predominate. Upon heating, PCO2H shows reduction trend in the number of clathrate cages, while accompanied by large-scale cage reformations via 28 types of reversible/irreversible cage transformations. The cage transformations are achieved via mechanisms of removing, inserting and rotating water molecules, in which water molecules in clathrate cages substantially exchange. Cage transformations involve 5 12 , 5 12 6 2 , 4 1 5 10 6 3 , and 4 1 5 10 6 2 are pronouncedly frequent, acting as pivotal intermediate pathway in the thermal dissociation of PCO2H. The study provides a clear roadmap on the thermally-induced cage transformations and their mechanisms, and establishes ML frameworks to predict the dissociation behaviors in terms of melting points and melting dynamics.