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.

Selective Inclusion of Secondary Amine Guests in sH Hydrate Systems

“…Consequently, one can conclude that DIPA molecules that interact with water molecules in ice (Ih) via strong hydrogen bonds become fully isolated when they are entrapped in sH-L cages. This blueshift in the vibration frequency of a NH bond induced by a complete inclusion is in good agreement with the previous report of the hydrate containing DMA and CH 4 . In addition, the peak near 3273 cm –1 can be identified to be a “free NH” vibration of DIPA molecules.…”

“…15,16 Dimethylamine (DMA) was also identified as a sII former in the presence of CH 4 helper gas. 17 As another example, both CP and THF are well-known simple sII hydrate formers. 18 Although pyrrolidine (as a secondary amine) is not a simple sII former, it was recently revealed that pyrrolidine can also form binary sII hydrate with CH 4 .…”

“…This blueshift in the vibration frequency of a NH bond induced by a complete inclusion is in good agreement with the previous report of the hydrate containing DMA and CH 4 . 17 In addition, the peak near 3273 cm −1 can be identified to be a "free NH" vibration of DIPA molecules. For DEA hydrate systems, it was already suggested that the semi-clathrate hydrate (Pbcn) with a composition of DEA•8.67H 2 O is transformed into a genuine clathrate hydrate (sH) by employing CH 4 as a helper gas.…”

“…However, these selective inclusion phenomena have not been observed in sII hydrate systems. For example, both propane and dimethyl ether form simple sII hydrates without any helper gas. , Dimethylamine (DMA) was also identified as a sII former in the presence of CH 4 helper gas . As another example, both CP and THF are well-known simple sII hydrate formers .…”

Investigation of kinetics of tetrabutylammonium chloride (TBAC) + CH4 semiclathrate hydrate formation

“…Thus this kind of ionic clathrate hydrate, known as semiclathrate hydrate (SCH), shows many special features, such as cohost inclusion and improved thermal stability, due to the ionic interaction between host and guest molecules. 6 Recently, SCH has been studied as a more promising medium as compared with ordinary clathrate hydrate for many engineering practices, such as gas separation/ storage/transportation, [7][8][9][10][11][12][13][14][15][16][17][18][19][20] seawater desalination, 21 and cold energy storage. [22][23][24][25] SCHs of TBAX (X ¼ F/Cl/Br) were rst founded by Fowler et al 26 in 1940 and structurally determined by Jeffery 27 in 1969.…”