Energy & Fuels 1998, 12, 197-200
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The Diverse Nature of Dodecahedral Cages in Clathrate Hydrates As Revealed by 129Xe and 13C NMR Spectroscopy: CO2 as a Small-Cage Guest† John A. Ripmeester* and Christopher I. Ratcliffe Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6 Received September 9, 1997. Revised Manuscript Received January 3, 1998
In this contribution it is shown that 129Xe NMR chemical shift parameters accurately reflect the relative size and geometry of the small dodecahedral cages in different hydrate structures. The relative sizes and geometric relationships were explored with CO2 as a probe molecule. Whereas CO2 is a relatively poor guest for the D cage in structure I hydrate, it appears to be a very efficient guest for the somewhat larger and asymmetric D cage in structure II hydrate. On the basis of an approximate CO2 distribution over the cages in structure I hydrate a lower limit on the hydration number of CO2 hydrate is determined to be 7.0. On the basis of Xe chemical shift parameters for structure H hydrate, it was predicted that CO2 should also be a reasonable small-cage guest for this structure. Experimental results are presented that support this prediction and show that CO2 indeed can serve as a helpgas for structure H hydrate.
Introduction The accurate prediction of conditions for hydrate formation depends strongly on the availability of good structural information.1 For clathrate hydrates, a complete description of structure involves not only the unit cell parameters and average atomic positions but also the cage occupancies. In order to provide this kind of information, it is necessary to use techniques such as diffraction, which is sensitive to long-range order, in combination with techniques such as NMR that are sensitive to local order.2 Previously, we have demonstrated the use of NMR methods for the determination of relative cage occupancies for structure I and II hydrates.3 One factor that is relatively unappreciated is the fact that although the small 512 (D) cage is common to all three structures, I, II, and H, the symmetry and size of these small cages is different,4 and hence their behavior toward guest molecules should also be quite different. This is very directly evident from the chemical shift parameters of xenon trapped in the small cages which suggest that the structure II and H small cages are significantly larger and less symmetric than the structure I small cage. In terms of guest-cage interactions, this probably is not too critical for spherical or pseudospherical guests. However, one may expect significant differences for guests that are asymmetric and that fit †
Published as NRCC no. 40858. (1) Davidson, D. W. In Water. A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2. Sloan, E. D., Jr. Hydrates of Natural Gas; Marcel Dekker: New York, 1991. (2) Ripmeester, J. A. J. Am. Chem. Soc. 1982, 104, 289. Davidson, D. W.; Handa, Y. P.; Ripmeester, J. A. J. Phys. Chem. 1986, 90, 6549. (3) Collins, M.; Ratcliffe, C. I.; Ripmeester, J. A. J. Phys. Chem. 1990, 94, 157. (4) Ripmeester, J. A.; Ratcliffe, C. I.; Tse, J. S. J. Chem. Soc., Faraday Trans. 1 1988, 3731.
quite tightly into the 512 cage. We may expect CO2 to be one such guest molecule. Some years ago, CO2 hydrate was determined to be a structure I hydrate with a lattice cell constant of 12.07 Å.5 The composition also has been reported a number of times over the years,6,7 with hydration numbers reported that generally lie between 6 and 8. Recently, interest in CO2 hydrate has intensified8 because of discussions about the disposal of industrial waste CO2 in the deep oceans as a means of reducing greenhouse gas concentrations. In this contribution we examine the role of CO2 as a small-cage guest in the three hydrate lattices of structures I, II, and H after consulting the appropriate 129Xe chemical shifts for guidance. Experimental Section The hydrate of 13CO2 was prepared as described previously.9 Double hydrates of xenon or 13CO2 were made by sealing into 10 mm Pyrex tubes measured quantities of powdered ice, along with the appropriate guests for the large cages (propane for structure II, neohexane or tert-butyl methyl ether for structure H) and either Xe or 13CO2, as appropriate. Hydrate formation takes place during conditioning of the samples over periods ranging from ∼1 day to several weeks. NMR spectra were measured on a Bruker MSL 200 spectrometer equipped with a double-tuned solenoid probe suitable for cross-polarization (5) von Stackelberg, M.; Jahns, W. Z. Elektrochem. 1954, 58, 162. (6) Wroblewski, M. S. Compt. Rend. 1882, XCIV, 1355. Villard, P. Ann. Chim. Phys. 1887, 11, 1355. (7) Bozzo, A. T.; Chen, H.-S.; Kass, J. R.; Barduhn, A. J. Desalination 1975, 16, 303. (8) Herzog, H.; Golomb, D.; Zemba, S. Environ. Prog. 1991, 10, 64. Saji, A.; Yoshida, H.; Sakai, M.; Tanii, T.; Kamata, T.; Kitamura, H. Energy Convers. Mgmt. 1992, 33, 643. Nishikawa, N.; Morishita, M.; Uchiyama, M.; Yamaguchi, F.; Ohtsubo, K.; Hiraoka, R. Energy Convers. Mgmt. 1992, 33, 651.
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Ripmeester and Ratcliffe
Table 1. Hydrate Cages, Cage Sizes, and structure I I II II H H H
cage type 512(D) 51262(T) 512(D) 51264(H) 512(D) 435663(D′) 51268(E)
sym
radius/Åa
m3 4 h 2m 3m 4 h 3m mmm 6 h 2m 6/mm
2.50 2.93 2.50 3.28 2.45 2.40 3.8
129Xe
and
13CO 2
δXeisob (ppm)
∆Xe c (ppm)
242 152 231 80 231 212.4
0 21 16 0 13.7 31.4
Chemical Shift Data ηXe d
0.701
δCiso (ppm)
∆C c (ppm)
128 128.2 128.0 128 128.5 128.5
0 -63.1 -53.0 0 +50 -82.1
ηC d
0.405
a Estimates for the cages roughly approximated as spheres from X-ray diffraction data. b Isotropic chemical shift. c ∆ ) δ iso - δzz chemical shift anisotropy. d Asymmetry parameter η ) (δyy - δxx)/(δiso - δzz); the δii are the chemical shift tensor elements; the 129Xe shifts are referenced to xenon gas at zero pressure, the 13C shifts to tetramethylsilane. The error estimates on both δiso and ∆Xe are (1 ppm, (2 ppm on ∆C, (0.04 on ηXe and (0.09 on ηC.
and dipolar decoupling.2,3 The spectra reported in this work were all recorded at temperatures between ∼250 and 270 K. Because of efficient water relaxation, rotating frame relaxation times are short and the cross-polarization times must be relatively short (0.5-1.5 ms). More detailed descriptions of the NMR experiments have been given previously.9,10 Temperature variation and control were achieved with a cold gasflow system and a temperature controller. Spectral simulations were carried out with the Bruker Xedplot package. It fits broadened (Lorentzian for powder patterns, LorentzianGaussian for the isotropic line components) spectral components to the experimental data using a nonlinear least-squares fitting procedure. The spectral parameters in Table 1 were derived from the best fits shown in the figures; error estimates are given in the table footnote.
Results and Discussion One of the first applications of 129Xe NMR spectroscopy was the chemical shift resolution of the distinct sites in structure I hydrate.2 It was also noted that the chemical shifts for the 512 cages in structures I and II were quite different. According to the empirical chemical shift-cage size relationship, the D cage in structure II is larger than that in structure I. Since relatively little is known about potential help gases for structure H, it was thought that the Xe NMR parameters could give some guidance for the prediction of the suitability of CO2 as such a help gas for the two small cages. 129Xe chemical shifts for different hydrate cages are summarized in Table 1 along with the point symmetry of the cages. The previously unreported anisotropic 129Xe chemical shift parameters for structure H were derived from a fit to the experimental powder patterns (Figure 1). We also note in passing that although the chemical shifts are dependent primarily on the structure type, there also is a small effect of the presence of other guests in the hydrate lattice. Whether this is a direct effect of the other guests on the xenon shielding or sensitivity to small differences in the lattice parameter remains unknown. For instance, the isotropic chemical shift of xenon in the structure II D cage can vary as much as 5 ppm. The principles by which the 13C shifts of CO2 are sensitive to cage shape are quite different than for the129Xe shifts of the spherical xenon atom. As explained in an early 1H and 13C NMR study on CO2 hydrate published about 10 years ago, the spectral shapes are determined by motional averaging of the chemical shift tensor. For CO2, the chemical shift tensor (9) Ratcliffe, C. I.; Ripmeester, J. A. J. Phys. Chem. 1986, 90, 1259. (10) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1988, 97, 337.
Figure 1. 129Xe NMR spectrum of a double hydrate of tertbutyl methyl ether and xenon at 273 K. The cross-polarization time was 0.8 ms. The individual cage contributions were obtained by a fitting procedure, as explained in the Experimental Section. The signal at ∼150 ppm arises from a small amount of structure I hydrate.
is axially symmetric; if we define ∆ ) δiso - δzz, where δiso is the isotropic chemical shift and δzz is the zz component of the chemical shift tensor, then the observed ∆ value averaged by molecular motion is given by ∆ave ) 0.5〈3 cos2 θ - 1〉∆stat. Here ∆stat is the value for the static molecule and θ is the angle between the rotation axis and the principal component of the shielding tensor (for CO2 this is the long molecular axis), and the angular brackets indicate an average over the motion of the guest. For CO2 in the structure I large cage, the powder pattern was used to model the molecular motion of the guest, giving either a simple model with a single θ value or a more complex model with an allowed range of θ values. In general, the NMR spectrum of CO2 in hydrate cages is a powder pattern that reflects the guest dynamic state. Since the 512 cage in structure I has pseudo-spherical symmetry so that the guest motions are isotropic, only a sharp line at the isotropic chemical shift should be observed. Because of the relative sizes of CO2 and the 512 cage, it was assumed that the small cage was essentially unoccupied. Unfortunately, excess liquid and gaseous CO2 in the sample also give rise to contributions at the isotropic shift, so that in this earlier work it was not possible to come to a conclusion about the presence of CO2 in the small cage. We note that structure I has the only pseudo-spherical
Nature of Dodecahedral Cages in Clathrate Hydrates
Energy & Fuels, Vol. 12, No. 2, 1998 199
Figure 2. 13C NMR spectra for 13CO2 structure I hydrate (top), and a structure II double hydrate of 13CO2 and propane. The spectra were recorded with cross polarization (cross-polarization times of 0.7 ms) in order to eliminate spectral contributions from liquid and gas. The spectra were recorded between 258 and 268 K. The isotropic line at 18 ppm arises from the propane carbons.
D cage. From the chemical shift data, the other D cages are not only less symmetric, as indicated by nonzero chemical shift anisotropies, but also somewhat larger, as seen from the smaller 129Xe shifts. This is likely to be of some importance for all small-cage guests, since the fact that the occupancy of the small cages in structure I decreases with increasing guest size suggests that it is the repulsive interactions that limit the cage occupancy. Also, especially for nonspherical guests, small departures from spherical symmetry are likely to be important. CO2 is a good test molecule because of its aspherical shape and a van der Waals’ dimension that closely matches the diameter of the small cages. In order to see if CO2 indeed occupies the small cage in structure I hydrate, contributions from the liquid and gas must be eliminated by using the 1H-13C cross polarization from the lattice protons of the cage to the encaged guest 13C (Figure 2 (top)). Indeed, a contribution from small-cage CO2 is visible in the spectrum. Since the relative intensities are a function of the crosspolarization dynamics, and therefore dependent on experimental parameters, they cannot be used directly to estimate the relative numbers of guests in each cage. This is illustrated in Figure 3 where the spectra shown were recorded with different cross-polarization times. At long cross-polarization times the intensity ratio is expected to approach its true value. Therefore, the occupancy ratio derived from the spectra in Figure 3 may be used to determine a lower limit on the hydration number of CO2 hydrate. The ratio of θS/θL ∼ 0. 32 gives a hydration number n ) 7.0 assuming that the ∆µ value2b of 1297 J m-1 applies to the CO2 hydrate lattice and, therefore, that the relationship determined earlier between θS/θL and n is indeed valid.3 This lower limit value actually is in good agreement with the hydration number of 7.3 derived by Bozzo et al. from thermodynamic measurements.7 Figure 2 also shows the 13C NMR spectrum of a double hydrate of propane and 13CO2. The CO2 portion of the spectrum is surprisingly similar to the spectrum observed for structure I, although there are some clear differences (see also Table 1). Of course, the assignments are very different, as the broad powder pattern
Figure 3. 13C NMR spectra of 13CO2 hydrate (268 K) illustrating the dependence of the relative signal intensities from 13CO2 in the large and small cages on the cross-polarization dynamics. Cross-polarization times: top, 1.5 ms; bottom, 0.7 ms.
Figure 4. 13C NMR spectra for a structure H hydrate of 13CO2 and neohexane. The individual cage contributions were obtained by a fitting procedure, as explained in the text. There is a small quantity of structure I 13CO2 hydrate present in the sample, as indicated.
must be associated with 13CO2 molecules in the small asymmetric D cage. The very weak central line indicates that there is very little CO2 in the pseudo-spherical H cage, although the cross-polarization dynamics make this spectral contribution appear to be smaller than it actually is. The two propane 13C resonance lines (note that the propane is not enriched in 13C) are not resolved and occur as a single peak at ∼18 ppm. The spectra shown in Figure 2 convincingly portray the importance of cage symmetry, a factor that clearly is much more important than cage size in determining the geometry of the guest dynamics. The xenon chemical shifts also are shown to be a sensitive indicator of cage size: from the 129Xe isotropic shifts, the asymmetric D cage in str II is slightly larger than the D cage in structure I and
200 Energy & Fuels, Vol. 12, No. 2, 1998
the CO2 experiments show that the former is a far better site for CO2 than the latter. On the basis of the above observations, what would one expect for the structure H small cages? From the 129Xe spectra for structure H hydrate in Figure 1 (see also Table 1), the D and D′ cages should be as large as the structure II D cage and also quite asymmetric. Therefore it should be possible for CO2 to act as small cage guest for structure H. A sample of hydrate made with neohexane and CO2 guests gave the 13C spectrum shown in Figure 4. For this sample the spectra were recorded without cross polarization, as the signals from liquid and gaseous CO2 do not need to be differentiated from CO2 trapped in spherical cages. Indeed, contributions can be found from CO2 in both D and D′ cages, along with a small contribution from a residual quantity of structure I hydrate. The NMR line shapes again reflect the dynamics of the guest molecules and have characteristic axial (D′) and nonaxial (D) chemical shift patterns. The entries for the chemical shift parameters
Ripmeester and Ratcliffe
in Table 1 were obtained from the spectral simulations shown in the figure. We can conclude that CO2 is indeed suitable as a small cage guest in structure H hydrate. Conclusions By using the Xe NMR spectrum observed for xenon trapped in the small cages in structures I, II and H, it was predicted that the small cages in structures II and H should be good sites for CO2. This was confirmed by using 13C NMR spectroscopy to examine a number of CO2-containing hydrates. CO2 is now also confirmed as a possible help gas molecule for the structure H hydrate. The fact that all of the small cages (D and D′ in structures I, II, and H) have different shapes and sizes (especially as defined by 129Xe NMR) suggests that the Langmuir constants that define the affinity of small guests for these cages should also be different. EF970171Y
