Spectroscopic Identification of the Mixed Hydrogen and Carbon Dioxide Clathrate Hydrate

Abstract

In this contribution, we first found the novel clathrate hydrate containing two gaseous guests of hydrogen and carbon dioxide by spectroscopic analysis. X-ray powder diffraction and NMR spectroscopy were used to identify structure and guest distribution of the mixed H2 + CO2 hydrate. X-ray diffraction result confirmed that the unit cell parameter was 11.8602 +/- 0.0010 A, and the formed hydrate was identified as structure I hydrate. 1H magic angle spinning (MAS) NMR and 13C cross-polarization (CP) NMR spectroscopy were used to examine the distribution of hydrogen and carbon dioxide molecules in the cages of structure I, respectively. These NMR spectra showed that carbon dioxide molecules occupied both small 512 cages and large 51262 cages, and hydrogen molecules only were occluded in small 512 cages of structure I. The new finding of the mixed hydrogen hydrate is expected to contribute toward the development of hydrogen production technology and, particularly, inclusion chemistry.

Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates

“…However, a powder pattern having an anisotropic chemical shift of Ϫ55.8 ppm was observed, and this spectral shape reflects the anisotropic motion of CO 2 molecules in asymmetric sI-L. The anisotropic chemical shift is defined as ⌬ ϭ ␦ iso Ϫ ␦ zz , (15,16) where ␦ iso (ϭ (2␦ xx ϩ ␦ zz )͞3) is the isotropic chemical shift, and ␦ xx (ϭ 99.6 ppm) and ␦ zz (ϭ 183.3 ppm) are the xx and zz components of the chemical shift tensor in Fig. 1a, respectively.…”

“…For CO 2 entrapped in hydrate cages, an anisotropic chemical shift is induced by asymmetry in the immediate environment of molecules and is sensitively affected by the guest distribution in hydrate cages (14). As the sI-S produces pseudospherical symmetry, which causes molecular motions to be isotropic, only a sharp peak at an isotropic chemical shift of Ϸ123 ppm is observed for CO 2 (15,16). For hydrate samples prepared after replacement, no clear isotropic line appeared and thus CO 2 molecules were not observed in sI-S, as can be seen in Fig.…”

13C NMR Studies of Hydrocarbon Guests in Synthetic Structure H Gas Hydrates: Experiment and Computation

“…In case of NMR methods, the first measurement of cage occupancies was that using 129 Xe NMR to study xenon-containing sI and sII hydrates. 10 The first application to hydrocarbon hydrates was that of Ripmeester and Ratcliffe 11 who reported cage occupancies from 13 C NMR and its relation to the hydrate structure for CH 4 in sI and sII hydrates. Since then, the technique has been adopted in many research projects and has become one of the most powerful qualitative and quantitative methods for hydrate characterization.…”

“…Since then, the technique has been adopted in many research projects and has become one of the most powerful qualitative and quantitative methods for hydrate characterization. 12,13 When changing the nuclear probe used in the NMR measurements, such characterization can be expanded to a variety of guest species.…”

“…The bulk-cage chemical shift difference likely also has contributions from cage size effects, as described above. Thus it is valuable to have a systematic study of 13 C chemical shifts of more complex sH guest molecules, to note chemical shift differences between the encaged guests and the bulk and to try and explain the differences in terms of the two effects noted above. In this work, the 13 C NMR spectra of the hydrate samples obtained are compared with those of the hydrocarbons in the bulk.…”