Thermal transformations and dissociations in polycrystalline CO2hydrates.

Abstract

CO₂hydrates show promising application in CO₂sequestration, as well as natural gas recovering from hydrate-bearing sediments, in which the stability of CO₂hydrates plays a vital role in these practical applications. Here, we report the thermal dissociation and cage transformations in polycrystalline CO₂hydrates via high-throughput molecular dynamics simulations and machine learning (ML). It is revealed that the melting points of polycrystalline CO₂hydrates (PCO2H) are dictated by the microstructural cages, in which the 5¹², 5¹²6²and 4¹5¹⁰6³cages 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 5¹², 5¹²6², 4¹5¹⁰6³, and 4¹5¹⁰6²are 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.