Structure and Guest Dynamics in Binary Clathrate Hydrates of Tetrahydropyran with Carbon Dioxide/Methane

Abstract

Binary structure II (sII) clathrate hydrates of tetrahydropyran (THP) with methane and carbon dioxide are synthesized and characterized with powder X-ray diffraction (PXRD) and molecular dynamics (MD) simulations. Analysis of PXRD results shows that 83% of the small 12-sided (dodecahedral, D) sII cages are occupied by CO2 in CO2 + THP hydrate, whereas 93% of the D cages are occupied by CH4 in CH4 + THP hydrate. The effects of the tighter fit of CO2 molecules in the D cages of the binary hydrate were observed in longer lattice constants and lower cage occupancies compared to binary THP hydrates with CH4 molecule in the D cage. MD simulations of 1,4-twist-boat and chair conformers of the THP in the sII clathrate hydrate phase with the CO2 and CH4 help gases are performed at temperatures between 153 to 250 K. Simulations suggest shape and size of D cage guest molecules does not affect the conformation of H cage guest molecule, but the steric repulsions between CO2 molecule and cage’s water molecules limits i...