Surface Transformation of Methane–Ethane sI and sII Clathrate Hydrates

Abstract

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 13C 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 13C 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 13C MAS NMR exper...