Thermodynamic Stability Boundaries and Structures of Methane + Monohalogenated Cyclopentane Mixed Hydrates

Abstract

Thermodynamic stability boundaries of the mixed hydrate composed of methane and fluorocyclopentane, chlorocyclopentane, or bromocyclopentane were investigated in a temperature range of (274.17 to 306.19) K and pressures up to 9.93 MPa under the four-phase (hydrate, aqueous, monohalogenated cyclopentane-rich liquid, and gas phases) equilibrium conditions. Also, the crystal structure of the methane + chlorocyclopentane hydrate was determined by means of powder X-ray diffractometry. The each boundary is laid at a pressure lower and a temperature higher than that of the simple methane hydrate. Especially, the methane + fluorocyclopentane hydrate system has the lowest equilibrium pressure among three mixed hydrate systems investigated in the present study. The powder X-ray diffraction patterns reveal that a guest species larger than chrolocyclopentane and smaller than bromocyclopentane is at the border between structure-II and structure-H hydrate formers.

Nucleophilic Reactions of Bromocyclopentane in the Structure-H Methane + Bromocyclopentane Mixed Hydrate System at High Pressures

“…At low pressures and low temperatures, the equilibrium data of the CH 4 +c-C 5 H 9 Br mixed hydrate system have been reported. 6,8 The equilibrium data plotted by solid keys below 80 MPa would be smoothly connected to the reported data 6,8 in the low-pressure region. Raman spectra derived from the CH 4 and c-C 5 H 9 Br molecules in the hydrate phase are shown in Figure 2, which is accompanied by the Raman spectra corresponding to the intermolecular vibration between water molecules (lattice mode).…”

“…The reaction products (c-C 5 H 8 and c-C 5 H 9 OH) are also enclathrated because it is reported that the c-C 5 H 8 molecule is enclathrated in the 5 12 6 4 -cage of s-II hydrate. 17,18 Moreover, the four-phase equilibrium curves in the CH 4 +c-C 5 H 8 and CH 4 +c-C 5 H 9 OH mixed hydrate systems are laid at a pressure lower (a temperature higher) than that in the CH 4 +c-C 5 H 9 Br system, 6,8 as shown in Figure 5 (Supporting Information, Table S1). The drastic change of the Raman spectrum in Figure 2a indicates that c-C 5 H 8 and c-C 5 H 9 OH as well as a quite small amount of c-C 5 H 9 Br competitively occupy the 5 12 6 4 -cages of the s-II hydrate in this system.…”

“…1 For instance, 5 12 -cage is a dodecahedron, which is constructed from 12 pentagons. In the same manner, 4 3 5 6 6 3 -cage, 5 12 6 2 -cage, 5 12 6 4 -cage, and 5 12 6 8 cage are dodecahedron, tetrakaidekahedron, hexakaidecahedron, and icosahedron, respectively. Some of them are combined to form specific hydrate structures.…”

“…1,3 Many guest species have been reported until now. 1 As special, the guest species normally occupying 5 12 6 8 cage cannot form s-H hydrate solely except at extremely high pressures. 4,5 When the small guest species such as methane (CH 4 ) and xenon are added, large guest species (LGS) occupy the 5 12 6 8 -cages of s-H hydrates with the support of small guest species occupying the 5 12 -and 4 3 5 6 6 3 -cages.…”

Four-Phase Equilibrium Relations Including Clathrate Hydrate Phase in a Ternary System of Xenon, Benzene, and Water

“…The pattern clearly indicates that the Xe+C 6 H 6 mixed hydrate is the sII (cubic, Fd 3 m ). The obtained lattice parameter recorded at 153 K and 0.1 MPa is a = (1.732 ± 0.001) nm, which falls into the category of sII hydrates having slightly larger lattice constant, even in view of their thermal expansivities. − …”