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 C2H4 hydrate, prepared from finely powdered ice and C2H4 gas, using powder X-ray diffraction (PXRD). A crystallographic structural model of C2H4 hydrate, derived using the Rietveld and direct-space methods, shows that the occupancy of the 512 cages is 38%, assuming that the 51262 cages are fully occupied. Furthermore, similar to C2H6 hydrate, the centers of guest molecules (C2H4) lie out of the centers of the equatorial planes of the 51262 cages and near the centers of the 512 cages (carbon atoms are spherically arranged around these cores). However, the 512 and 51262 cages of C2H4 hydrate are larger and smaller, respectively, than the corresponding cages of C2H6 hydrate, despite the fact that C2H4 molecules are slightly smaller than C2H6 molecules. This difference is attributed to the high occupancy of the 512 cages of C2H4 hydrate, which leads to the expansion and concomitant contraction of the 512 and 51262 cages, respectively. Evidently, the relative distortion of the 51262 and 512 cages of the three-dimensional sI hydrate is a crucial factor determining its stability. In addition, the kinetic stability of C2H4 hydrate under low-temperature conditions was investigated by temperature-dependent PXRD.