Can Guest Occupancy in Binary Clathrate Hydrates Be Tuned through Control of the Growth Temperature?

Abstract

Clathrate hydrates are nonstoichiometric compounds comprised of a hydrogen-bonded water network that forms polyhedral cages that can be occupied by small guest molecules. Clathrates are candidate materials for storage and transportation of methane and H2. Promoter molecules, such as THF, reduce the pressure or temperature needed to form clathrates of these gases, resulting in the formation of binary clathrates with the promoter molecule hosted in the large cages of the hydrate. In this work, we study the growth and occupancy of binary clathrates as a function of supercooling of the solution using molecular dynamics simulations with the mW water model and small and large guest molecules of sizes similar to those of H2 and THF, respectively, but that are both highly soluble in water and produce single hydrates with identical melting point. The large guest molecules only fit into the 51264 cages, while the small guest molecules can fit into both types of cages. We find that the large guest act as a kinetic promoter for growth, increasing the rate of uptake of small guests into the clathrate. Our results also indicate that the growth of binary clathrates is limited by the arrangement of guest molecules in the large 51264 cages at the clathrate/solution interface. The occupancy of large cages of binary clathrates can be tuned by varying the growth temperature. The simulations indicate that with increasing supercooling there is an increase in the percentage of 51264 cages occupied by the small guest molecules at the expense of the large guest molecules, while the occupancy of 512 cages remains relatively constant. The results of this work show that the composition of clathrates grown at high driving force does not necessarily reflect the composition of the most stable phase.