Structural Transition Induced by CH4 Enclathration and Cage Expansion with Large Guest Molecules Occurring in Amine Hydrate Systems

Abstract

Two isomers of C2H7N, dimethylamine (DMA) and ethylamine (EA), are known to be clathrate hydrate formers by themselves. Here we introduced methane gas as a secondary guest into both dimethylamine and ethylamine clathrate hydrates and identified their structural transitions using powder X-ray diffraction (PXRD) and solid-state NMR spectroscopy. We observed the structural transitions of amine clathrate hydrates from expanded structure I (cubic Pm3n) to structure II (cubic Fd3m). In addition, from experimental results obtained through neutron powder diffraction (NPD) and PXRD, we found that neither temperature nor pressure affected the hydrate structural transition. Raman spectroscopy was used to identify the structural transition occurring in these amine clathrate hydrate systems. In addition, we measured the hydrate equilibrium conditions for amine-water-methane hydrates. The DMA and EA act as hydrate inhibitors in DMA/EA + H2O + CH4 hydrate systems compared with pure methane hydrate over our experimental pressure and temperature ranges.