Low-temperature cross-polarization/magic angle spinning carbon-13 NMR of solid methane hydrates: structure, cage occupancy, and hydration number

Estimation of Gas Composition and Cage Occupancies in CH₄-C₂H₆ Hydrates by CP-MAS ¹³C NMR Technique

“…Almost no appreciable decreases in the signal intensities were observed during the measurements at 163 K. The hydrate samples were introduced into a zirconia sample tube (JEOL; diameter: 6 mm, length: 22 mm) in liquid N2. CP-MAS 13 C NMR measurement of hydrocarbon hydrates requires long contact time for the quantitative analysis 6) . Therefore, the CP-MAS 13 C NMR measurements were carried out under the following conditions: contact time 10 ms, pulse delay time 50 s, number of acquisitions 40-400 (depending on the S/N ratio), and spinning rate 3.0-4.0 kHz at the magic angle.…”

“…The solidstate 13 C NMR method using the Cross Polarization and Magic Angle Spinning (CP-MAS) technique can effectively enhance the signal intensities 4),6),12),13) . In addition, such enhancement in signal intensities is possible at lower temperatures, in contrast to previous 13 C NMR measurements of the hydrate samples which were usually carried out at 193-253 K 4), 6) . In the present study, CP-MAS 13 C NMR analyses of the mixed CH4 _ C2H6 gas hydrate system were carried out at 163 K, and the cage occupancies of the guest molecules in mixed gas hydrates containing CH4 and C2H6 were estimated.…”

“…The X-ray diffraction (XRD) method has been widely used to determine gas hydrate structures 2),3),5) . 13 C NMR spectroscopy is also useful to determine hydrate structures containing hydrocarbons 2),4), 6) . The cage occupancy is defi ned as the ratio of (number of cages occupied by guest molecules)/(number of total cages), and is also important to estimate the amount of natural gas, since it depends on the conditions of the hydrate formation such as pressure and gas composition.…”

“…The present study attempted to estimate the cage occupancies of mixed gas hydrates from the CP-MAS 13 C NMR data using the statistical thermodynamic model 10) . This model has been already applied to estimation of the cage occupancy by the 13 C NMR and Raman techniques 4), 6) . The values of θS/θL in the CH4 _ C2H6 system are given by Eq.…”

Capture of carbon dioxide from flue or fuel gas mixtures by clathrate crystallization in a silica gel column

“…The hydration number is the number of water molecules per guest molecule. It can be determined experimentally by simultaneously using powder X-ray diffraction and Raman spectroscopy on solid hydrate phase Kumar et al, 2008;Sum et al, 1997;Susilo et al, 2007a;Uchida et al, 1999;Udachin et al, 2007Udachin et al, , 2001 or by solid state NMR spectroscopy (Davidson et al, 1983;Ripmeester and Ratcliffe, 1988). In the presence of THF, the conversion of water to hydrate is determined by the next equation…”