Surface Transformation of Methane-Ethane sI and sII Clathrate Hydrates

The reaction of methane, ethane, and water to form structure I (sI) and structure II (sII) clathrate hydrates at 268 K was studied using solid-state C-13 NMR The reaction can be described in terms of four stages: stage 1, the simultaneous formation of methane-ethane sI and sII clathrate hydrates; stage 2, cessation Of sI hydrate formation and enhanced sII hydrate formation; stage 3, decomposition of sI hydrate and continued enhanced sII hydrate formation; stage 4, approach to a final steady state of the system. The rates of formation and decomposition of the various methane and ethane sites obtained from analysis of the time-resolved C-13 MAS NMR intensity data provide strong evidence that the sI <-> sII transformation occurs at surfaces and is dominated by two simultaneous reactions; methane-ethane sI hydrate decomposes to gas phase methane, gas phase ethane, and water while forming methane-ethane sII hydrate from gas phase methane, gas phase ethane, and water. The results of a 2D exchange C-13 MAS NMR experiment are consistent with these concurrent methane-ethane sI hydrate decomposition and methane-ethane sII hydrate formation surface reactions. The 2D exchange C-13 MAS NMR spectrum also provides conclusive evidence that methane-ethane sI and sII hydrates are not intimately mixed.