Application of the ATR-IR Spectroscopic Technique to the Characterization of Hydrates Formed by CO2, CO2/H2 and CO2/H2/C3H8

The spectroscopic investigation of CO2-containing clathrate hydrates is complicated because techniques such as Raman spectroscopy cannot distinguish cage populations. C-13 NMR spectroscopy also has some complications as the isotropic chemical shifts do not change for the different CO2 cage populations. It is known that CO2 molecules in the different phases relevant to hydrates give unique infrared vibrational frequencies; however, so far only thin cryogenic films prepared at low pressure have been studied with IR transmission spectroscopy. In this study, hydrates from CO2, CO2/H-2, and CO2/H-2/C3H8 mixtures were synthesized in a high-pressure attenuated total reflection (ATR) cell and in situ infrared spectroscopy was performed at -50 degrees C to distinguish the vibrational frequencies from CO2 in small and large cages in the resultant hydrate and in the other CO2-containing phases. Quantitative estimates of cage occupancies and hydration numbers are provided as based on the analysis of the IR spectra and knowledge of hydrate gas composition from gas chromatography.