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

Abstract

In this study, the phase equilibria of binary (cyclopropylamine (CPrA) + methane (CH 4 )) and (cyclopentylamine (CPeA) + CH 4 ) hydrates were investigated using a conventional isochoric pressure−temperature trace method. The equilibrium temperatures of the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates were higher than those of pure CH 4 hydrate. The guest conformation and inclusion behaviors of the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates were also examined through 13 C solid-state nuclear magnetic resonance spectroscopy, and the results confirmed the formation of structure II hydrates with new hydrate-forming agents, CPrA and CPeA, in the presence of CH 4 . The crystal structures and possible host−guest interactions of the proposed hydrate systems were investigated through Rietveld analysis using the direct space method, and the results demonstrated the formation of sII hydrates (with cubic Fd-3m) and the possibility of host−guest interactions in the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates. These observations provide an improved understanding of the unique nature of host−guest inclusion phenomena.

Phase Equilibria and Spectroscopic Investigations of Binary (But-3-en-1-ol + CH4) Clathrate Hydrate

“…In Figure b, two remarkable peaks corresponding to the entrapped CH 4 molecules in the 5 12 small cage (sII-S, δ = −4.5 ppm) and the 5 12 6 4 large cage (sII-L, δ = −8.3 ppm) of the structure II (sII) were observed. ,,,− The peak area ratio of the entrapped CH 4 molecules ( A sII‑S / A sII‑L ) was determined to be 9.14, implying the presence of but-3-en-1-ol within the large cages of the sII hydrate. In Figure c, there are no significant changes in the chemical shifts of but-3-en-1-ol for the binary (but-3-en-1-ol + CH 4 ) hydrate (black line in Figure c) and frozen (but-3-en-1-ol + H 2 O) solution ( x = 0.0556) system (red line in Figure c); therefore, we may carefully conclude that the conformational changes of the but-3-en-1-ol molecule during hydrate formation do not occur. ,,,− Four representative peaks of the but-3-en-1-ol molecule for the binary (but-3-en-1-ol + CH 4 ) hydrate (black line in Figure ) and the frozen (but-3-en-1-ol + H 2 O) solution ( x = 0.0556) system (red line in Figure ) were observed δ = 135.3, 117.0, 61.3, and 37.6 ppm in Figure . To clarify the inclusion behaviors of the but-3-en-1-ol molecule, the carbon chemical shifts of the but-3-en-1-ol molecule (8.666 Å) with its dihedral angles (θ A and θ B ) for −115° and 0° were calculated using the Material Studio Program (with the CASTEP module using the GGA-BLYP model) (Table ).…”

“…The carbon chemical shifts of the but-3-en-1-ol molecule (8.666 Å) with its dihedral angles (θ A and θ B ) for −115° and 0° were calculated as δ = 124.4, 98.46, 50.37, and 26.73 ppm, respectively. Therefore, there are no significant changes in the chemical shift differences for carbon atoms; we expect that the enclathration of the but-3-en-1-ol molecule (8.666 Å) with its dihedral angles (θ A and θ B ) for −115° and 0° could occur in the large cages of the sII hydrate. ,,,− …”

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

“…A profile analysis performed using FullProf software indicates the hydrate to be sII-type ( Fd 3̅ m ), with the lattice parameter a = 17.428 Å ( R e = 7.69%, R wp = 45.3%, and χ 2 = 34.7). The 13 C NMR spectrum of the t BuNO 2 ( x = 0.056) + CH 4 hydrate (Figure b) shows only one clear peak at −4.78 ppm, indicating CH 4 molecules entrapped in sII-S cages. − In contrast, for the t BuNO 2 ( x = 0.030) + CH 4 hydrate, two more peaks at −6.68 and −8.29 ppm are observed, which represent CH 4 molecules in the sI-L and sII-L cages, respectively. It is therefore clear that the t BuNO 2 molecules occupy sII-L cages.…”

“…Finding suitable thermodynamic promoters is important because they can significantly reduce the energy consumed during preparation, storage, and transportation of the hydrates. Interestingly, all the thermodynamic promoters studied so far are large guest molecules (LGMs) that occupy the 5 12 6 4 (sII-L) cages of sII-type hydrates or the 5 12 6 8 (sH-L) cages of sH-type hydrates. In recent years, many new thermodynamic promoters have been discovered, and the properties of CH 4 hydrates incorporating these promoters have been studied.…”

“…Interestingly, all the thermodynamic promoters studied so far are large guest molecules (LGMs) that occupy the 5 12 6 4 (sII-L) cages of sII-type hydrates or the 5 12 6 8 (sH-L) cages of sH-type hydrates. In recent years, many new thermodynamic promoters have been discovered, and the properties of CH 4 hydrates incorporating these promoters have been studied. − These promoters contain various types of functional groups such as amines, , alcohols, , ethers, and epoxides. − …”