Distortion of the Host Water Cages of Structure I Gas Hydrates: Structural Analysis of C₂H₄ Hydrate by Powder X-ray Diffraction

Abstract

This study investigated the crystal structure and composition of C 2 H 4 hydrate, prepared from finely powdered ice and C 2 H 4 gas, using powder X-ray diffraction (PXRD). A crystallographic structural model of C 2 H 4 hydrate, derived using the Rietveld and direct-space methods, shows that the occupancy of the 5 12 cages is 38%, assuming that the 5 12 6 2 cages are fully occupied. Furthermore, similar to C 2 H 6 hydrate, the centers of guest molecules (C 2 H 4 ) lie out of the centers of the equatorial planes of the 5 12 6 2 cages and near the centers of the 5 12 cages (carbon atoms are spherically arranged around these cores). However, the 5 12 and 5 12 6 2 cages of C 2 H 4 hydrate are larger and smaller, respectively, than the corresponding cages of C 2 H 6 hydrate, despite the fact that C 2 H 4 molecules are slightly smaller than C 2 H 6 molecules. This difference is attributed to the high occupancy of the 5 12 cages of C 2 H 4 hydrate, which leads to the expansion and concomitant contraction of the 5 12 and 5 12 6 2 cages, respectively. Evidently, the relative distortion of the 5 12 6 2 and 5 12 cages of the three-dimensional sI hydrate is a crucial factor determining its stability. In addition, the kinetic stability of C 2 H 4 hydrate under low-temperature conditions was investigated by temperature-dependent PXRD.

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

“…Cai, et al [134] studied sH hydrate of 2,2-dimethylbutane-methane hydrate with PXRD and calculated the cage occupancy. Takeya, et al [135] analyzed the sI C 2 H 4 hydrate with PXRD and calculated the cage occupancy, which is consistent with the Raman results.…”

Dissociation Kinetics and Metastability of Carbon Monoxide Hydrates Correlating with Phase Change in Water Ice

“…The dissociation of the CO hydrate was suppressed again when the phase transition to ice Ih was completed at ∼230 K. At temperatures above ∼230 K, the diffraction pattern exhibited pure ice Ih. In previous studies, ice transition due to gas hydrate dissociation from the mixture of ice Isd and Ih to ice Ih was observed in a similar temperature range as in the current study. ,,, After the ice transitions to the Ih phase above ∼240 K, the CO hydrate was completely dissociated. In a pure-water system, ice Isd forms between 130 and 220 K, and the ice Isd phase transforms into ice Ih up to 240 K .…”

“…In previous studies, ice transition due to gas hydrate dissociation from the mixture of ice Isd and Ih to ice Ih was observed in a similar temperature range as in the current study. 26,45,47,48 After the ice transitions to the Ih phase above ∼240 K, the CO hydrate was completely dissociated. In a pure-water system, ice Isd forms between 130 and 220 K, and the ice Isd phase transforms into ice Ih up to 240 K. 46 Although there was a slight difference in the phase transition temperature, the results of this study and those of a pure-water system show that the trend of the phase transition of ice was consistent.…”

Effect of Pressure on the Hydration Number of Argon Hydrate

“…To confirm the purity of argon hydrate, the powder X-ray diffraction (PXRD) measurements were conducted using an X-ray diffractometer (model Ultima-III, Rigaku) with parallel-beam optics and a low-temperature chamber. The methods and experimental conditions are described in previous work. , …”