Selective Inclusion of Secondary Amine Guests in sH Hydrate Systems

Abstract

In this study, CH4 hydrate systems containing the six compounds 2,4-dimethylpentane (24DMP), di-isopropyl ether (DIPE), di-isopropyl amine (DIPA), n-pentane, diethyl ether (DEE), and diethyl amine (DEA) were investigated through spectroscopic analyses. According to high-resolution powder diffraction, NMR, and Raman analyses, only the secondary amine molecules of DIPA and DEA are found to be selectively accommodated in sH-L cages. Both DIPA and DEA exhibit no hydrogen bond strongly linked to the host framework, according to the clear “free NH” signals detected from Raman spectra. However, no signals of entrapped 24DMP, DIPE, n-pentane, and DEE are observed, even when mixtures of the additives (e.g., 24DMP + DIPA) are introduced. The equilibrium P–T conditions for CH4 hydrates containing the six additives were also measured. Finally, several hypotheses for interpreting the selective inclusion phenomena are suggested from geometric perspectives of the host framework and the guest molecules, in terms of the van der Waals volume. It should be emphasized that the present findings and interpretations have extended the fundamentals of clathrate hydrates, especially in relation to the unique physicochemical properties and functional usage of sH hydrates.

Structural and Thermodynamic Investigations of Nitroalkane + CH4 Hydrates with Structures II and H

“…When calculating the area ratio for cPeNO 2 + CH 4 hydrate, onethird of the area of the peak at −6.69 ppm was subtracted from the area of the peak at −4.52 ppm. The obtained area ratios (1.48 and 1.81) are consistent not only with the theoretical value of 1.5 but also with several experimental values observed in other sH hydrates, such as tert-butylmethylether + CH 4 (1.5), 19 hexamethyleneimine + CH 4 (1.75), 20 and diisopropylamine + CH 4 (1.7) 21 hydrates. The equilibrium P−T conditions of the nitroalkane + CH 4 hydrates were also investigated.…”

“…In contrast, the PXRD results (Figure S1b,c) indicate that neither cPeNO 2 nor cHxNO 2 can form simple sH hydrates without CH 4 help gas. The 13 C NMR spectra (Figure c) show two representative peaks − of the CH 4 molecules in sH-S and sH-M cages at −4.5 and −4.8 ppm, respectively. The area ratios ( A S / A M ) of the two peaks were found to be 1.48 and 1.81 for cPeNO 2 + CH 4 and cHxNO 2 + CH 4 hydrates, respectively.…”

Phase Equilibria and Spectroscopic Identification of Structure II Hydrates with New Hydrate-Forming Agents (Cyclopropylamine and Cyclopentylamine)

“…However, recent studies have reported the presence of various non-canonical clathrate hydrates. Certain large molecules with hydroxyl or amine groups in their molecular structures, such as propanols, butanols, pentanols, cyclic alcohols, and some amine molecules, can form clathrate hydrates in the presence of a help gas. ,− In addition, hydroxyl or amine moieties of large molecules in the hydrate cages may have an interaction with “host” water frameworks via hydrogen bonding. Therefore, more research is still required to achieve a comprehensive understanding of the physicochemical properties of non-canonical clathrate hydrates.…”

Geometric and Hydrophilic Effects of Oxirane Compounds with a Four-Carbon Backbone on Clathrate Hydrate Formation

“…Numerous organic compounds can act as hydrate formers and are not limited to hydrocarbons. Diverse sII and sH formers contain various functional groups have been reported thus far, including hydroxyl, − ether, − ketone, ,,− amine, − nitro, , and others. − The ability of a compound to act as a hydrate former and the specific hydrate structure it forms primarily depends on its size, although other factors also influence it. For example, diisopropyl amine is an sH former, whereas 2,4-dimethylpentane and diisopropyl ether are not, despite being isoelectric and exhibiting similar structures and size.…”

“…For example, diisopropyl amine is an sH former, whereas 2,4-dimethylpentane and diisopropyl ether are not, despite being isoelectric and exhibiting similar structures and size. Similarly, diethyl amine is an sH former, whereas n -pentane and diethyl ether are not . Additionally, Lee et al proposed that when combined with 2,2-dimethylbutane (22DMB), n -pentane or n -hexane can enter the sH-L cage, despite lacking the ability to form sH hydrates.…”