Molecular dynamics simulations of CH4/CO2 hydrates nucleation in kaolinite particles

Abstract

Nucleation and growth mechanism of CH4/CO2 hydrates has attracted great attention for CH4/CO2 mixed gas separation and CO2 sequestration. In this study, the nucleation mechanism of CH4/CO2 mixtures forming hydrate with different gas concentrations in the kaolinite (Kln) particles with hydroxyl and siloxane surface contacts was investigated using molecular dynamics simulations. The simulation results showed that CH4-occupied 512 cages and CO2-occupied 51262 cages were initially formed in the bulk-like solution of Kln particles, which was facilitated by the adsorption of CH4 and CO2 molecules to form hydrate cages, regardless of the type of surface contacts on the Kln particles. The interaction between the adsorbed CH4/CO2 molecules and the siloxane surface was the main factor that facilitated hydrate nucleation by forming clathrate-like structures. By contrast, the hydroxyl surface made a relatively minor contribution to hydrate nucleation because of the strong hydrogen bonds that formed on its surface. These results suggest that the different surface contacts of Kln particles should be considered a key factor in the nucleation and growth mechanism of CH4/CO2 hydrates in marine sediments. These findings provide molecular insight into the mechanisms of CH4/CO2 mixed gas separation and hydrate-based CO2 sequestration in Kln-rich sediments.