Measurement of clathrate hydrates via Raman spectroscopy

Abstract

Raman spectra of clathrate hydrate guest molecules are presented for three known structures ( I ( sI ), II ( sII ), and H ( sH )) in the following systems: CH 4 ( sI ), CO 2 ( sI ), C 3 H 8 ( sII ), CH 4 + CO 2 ( sI ), CD 4 + C 3 H 8 ( sII ), CH 4 + N 2 ( sI ), CH 4 + THF- d 8 ( sII ), and CH 4 + C 7 D 14 ( sH ). Relative occupancy of CH 4 in the large and small cavities of sI were determined by deconvoluting the ν 1 symmetric bands, resulting in hydration numbers of 6.04 ± 0.03. The frequency of the ν 1 bands for CH 4 in structures I, II, and H differ statistically, so that Raman spectroscopy is a potential tool to identify hydrate crystal structure. Hydrate guest compositions were also measured for two vapor compositions of the CH 4 + CO 2 system, and they compared favorably with predictions. The large cavities were measured to be almost fully occupied by CH 4 and CO 2 , whereas only a small fraction of the small cavities are occupied by CH 4 . No CO 2 was found in the small cavities. Hydration numbers from 7.27 to 7.45 were calculated for the mixed hydrate.