Structural transformation and tuning behavior induced by the propylamine concentration in hydrogen clathrate hydrates

Abstract

The structures and the guest-host distributions of iso-propylamine (i-PA) and n-propylamine (n-PA) hydrates with hydrogen as a secondary guest were identified by powder X-ray diffraction and Raman spectroscopic analysis. The structure of 11.1 mol% i-PA + H2 hydrate was identified to be hexagonal (space group P63/mmc) with a few unindexed diffraction peaks, while 5.6 mol% i-PA + H2 hydrate had a cubic structure (space group Fd3¯m). Similarly, the structure of 13.3 mol% n-PA + H2 hydrate was found to be monoclinic (space group P2(1)/n), while 5.6 mol% n-PA + H2 hydrate had a cubic structure (space group Fd3¯m). The 'tuning' phenomenon, multiple occupancy of hydrogen in the large cage at the pressure and temperature regions outside of pure hydrogen hydrate stability, was observed in the i-PA + H2 hydrate only when the amine concentration was lower than the stoichiometric value of structure II hydrate. The three-phase (H-L(w)-V) equilibria for alkylamine + H2 + water mixtures were also measured to investigate their thermodynamic stability.

Thermodynamic inhibition of 4-methylmorpholine while forming sH hydrate with methane

“…9 shows the Raman spectra of 2.9 mol% 4-mMPLþ CH 4 hydrate and 2.9 mol%, 4-mMPLþH 2 O mixture in the regions of 700-1100 cm À 1 and 2650-3050 cm À 1 . Recent studies observe conformational changes of guest molecules within the hydrate cages (Tezuka et al, 2013;Takeya et al, 2014;Park et al, 2015). 4-mMPL is a highly miscible compound in water, which gives rise to dissimilarity between the conformation state of 4-mMPL in the frozen solution and that in large cages of the sH hydrate.…”

Structural Characterization of Pyrrolidine–Including Structure II Clathrate Hydrates

“…Amines are widely used for CO 2 capture as an absorbent, and they are also LGs for sII and sH hydrates. − Due to the lower volatility and vapor pressure of amines compared to that of ethers, the uses of these former LGs in hydrate formation can be simplified. Pyrrolidine (PL) is a cyclic amine with a vapor pressure of 6.5 kPa at 298 K, which has a similar molecular size and structure (and is isoelectronic) to the cyclic ether and alkane THF and cyclopentane.…”