Hydrogen isotopic fractionation of methane at the formation of synthetic mixed-gas hydrate composed of methane and propane

Abstract

Hydrogen isotope fractionation for methane occurs between gas and hydrate phases at the formation of methane hydrate; D of hydrate-bound methane is about 5 lower than that of residual methane. We suppose that the relation between the sizes of guest molecules and cage size of crystallographic structure affects the isotopic fractionation. Methane is enclathrated in both large and small cages of the structure I gas hydrate, therefore the isotopic difference is D in mainly determined by molecules enclathrated in the large cages. On the other hand, in the case of methane and propane mixed-gas hydrate, methane prefers to be enclathrated in small cages, because propane is enclathrated only in the large cages of the structure II gas hydrate. In this study, we checked the effect of cage size on the hydrogen isotopic fractionation for methane by comparing pure methane hydrate and the mixed-gas hydrate composed of methane and propane. Synthetic gas hydrates were formed from fine ice powder and guest gases in a pressure cell. We retrieved residual and hydrate-bound gases and measured D of methane by a CF-IRMS. The hydrogen isotopic differences between hydrate-bound and residual methane in the case of mixed-gas hydrate was smaller than those of pure methane hydrate reported by previous works, indicating that isotopic difference in methane D for small cages is smaller than those for large cages.