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SOME STRUCTURAL STUDIES OF CLATHRATE HYDRATES
D. Davidson, S. Gough, Y. Handa, C. Ratcliffe, J. Ripmeester, J. Tse
To cite this version:
D. Davidson, S. Gough, Y. Handa, C. Ratcliffe, J. Ripmeester, et al.. SOME STRUCTURAL STUD- IES OF CLATHRATE HYDRATES. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-537-C1-542.
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JOURNAL DE PHYSIQUE
Colloque C1, supplement au n o 3, Tome 48, mars 1987
SOME STRUCTURAL STUDIES OF CLATHRATE HYDRATES
D.W. DAVIDSON'~) , S.R. GOUGH, Y.P. HANDA, C.I. RATCLIFFE, J.A. RIPMEESTER and J.S. TSE
National Research Council of ~ a n a d a ' ~ ) , Division of Chemistry, Ottawa, Ontario, KIA OR9, Canada
R e s u m e Nous avons prkpar6 les hydrates d'oxyghe. d'azote, de monoxyde de carbone et d'air dans leur domaine de stabilitk B haute pression. Except6 pour l'hydrate de monoxyde de carbone, dont la structure n'est pas cependant confirme'e, les diagrammes de diffrac- tion X et de neutron montrent que leur structure est du type Stackelberg 11. Les clathrates hydrates de mkthylcyclohexane, pinacolore et t-butyl-methylether avec soit H2S soit Xe comme gaz support ont Ct6 identifiks B partir des sectres 2 H de RMN des molCcules (h6tes) deutkrCes ou partiellement deutCr6es et a partir des Bcrantages chimiques du lZ9Xe comme Ctant distincts des hydrates de type 11. A partir des diagrammes des diffraction des rayons X et de neutrons il apparait que les nouveaux clathrates hydrates sont isostructuraux avec le clathrasil dodecasil-IH.
A b s t r a c t We have prepared the hydrates of oxygen, nitrogen, carbon monoxide and air in their regions of stability at high pressure. Except for carbon monoxide hydrate, whose structure is not yet confirmed, the X-ray and/or neutron powder diffraction patterns show their structures to be von Stackelberg's type 11. Clathrate hydrates of methylcyclohexane, pinacolone and t-butyl methyl ether with H2S or Xe as help gas, were identified from the 'H NMR spectra of deuterated or partially deuterated guest molecules and from lZ9Xe chemical shieldings as being distinct from type I1 hydrates. On the basis of neutron and X- ray powder diffraction patterns, it is suggested that the new clathrate hydrates are isastruc- tural with the clathrasil dodecasil-lH.
I n t r o d u c t i o n
It is usually accepted [I] that clathrate hydrate would adopt von Stackelberg type I structure [2] if the van der Waals diameter of the encaged guest molecule is smaller than 5.80A. For bigger guest molecules with molecular dimension between 5.80 to 7.00
A,
the al- ternative von Stackelberg type I1 structure [3] is favoured. It is also believed that molecules with van der Waals diameters greater than 7.00A
do not form stable hydrates a t all. No exceptions to these general rules have been observed among more than 100 individual species known to form clathrate hydrates. Recently we reported the surprising findings that ar- gon and krypton, the two smallest species which form clathrate hydrates, exist in the type I1 modification [4]. It has been established through structural studies, that the clathrasils("~eceased
( 2 ) ~ s s u e d as NRCC 26146
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987173
Cl-538 JOURNAL DE PHYSIQUE
(clathrates with Si02 as the host lattice), are isostructural with the ciathrate hydrates [5].
Moreover, clathrasil structures with no hydrate analogues have been prepared and identi- fied. The latter observation led us to consider the possibility that new hydrate structures may exists as well. In this paper, we report the preparation and structural characterization of the hydrates formed with oxygen, nitrogen, air, and carbon monoxide. We also present evidence of a new hexagonal hydrate structure formed with large organic molecules.
Experimental Methods
The hydrates of oxygen, nitrogen, air and carbon monoxide were prepared by reacting powdered ice with the appropriate gas in a Parr pressure vessel equipped with a pressure measuring device and stainless steel rods for grinding the reactants. The pressure vessel was then put on a rotator at 243 K for several days. The structures of the nitrogen and the air hydrate were identified with X-ray powder diffraction and that of the oxygen hydrate by neutron powder diffraction. Structural analysis on carbon monoxide hydrate has not yet been completed.
The hydrates formed with large organic molecules, methyl-cyclohexane, pinacolone, t- butylmethyl ether, adamantane, tetramethylsilane and hexachloroethane were prepared by reacting the respective guest molecules with powdered ice in an ice bath in the presence of a help gas. We found both H2S and Xe to be quite efficient in stabilizing the hydrates.
Although the presence of a stable solid hydrate above 0°C is easy to establish, its melting temperature is not easily determined as the material is often contaminated with ice. 2H and laQXe NMR and powder diffraction were employed for structural characterization. Specfi- cally, the diffraction pattern of the methyl-cyclohexane hydrate was obtained using X-ray and that of t-butylmethyl ether hydrate obtained using neutron diffraction [4,6].
Results and Discussion
The powder diffraction patterns of the hydrates of nitrogen,oxygen and air were identi- fied as to be von Stackelberg structure 11 [3]. The unit cell parameters are 17.070(1), 17.11(6) and 17.24(6)
A
for oxygen, nitrogen and air hydrates respectively. It is interesting to note that the molecular dimensions of nitrogen (rvdm = 2.10A)
and oxygen molecule (rrd,,, = 2.10A)
are only slightly smaller than that of methane ( rvdw = 2.18 A), which is found to form a type I hydrate (41. The relative stabilities of the type I and I1 structures have been discussed within the context of the solid solution model proposed previously [7]. It was shown that the type I1 structure is intrinsically more stable than type I. Type I struc- ture is preferred when the guest molecule is in the size range that favours the occupancy of the 14-hedral over the 12-hedral cages. For molecules too large to enter the 14-hedral cages, only type 11 structures can be formed.An analysis of the air hydrate showed that the composition of the enclathrated gas was 71.4 % nitrogen, 27.1 % oxygen and 1.5 % argon. The composition of the parent gas was 73.4 % nitrogen, 21.6 % oxygen and 1.0 % argon. Therefore, the oxygen and argon con- tents of the hydrate are enriched over that of the parent gas by 25 % and 50 % respectively, whereas there is a smal! depletion in nitrogen of 8 %. The enrichment in oxygen content
in the hydrate has also been o b s e ~ e d in naturally-occuring air hydrate recovered from the Greenland ice cap [8].
The existence of carbon monoxide hydrate in the nuclei of comets has been speculated on for a long time. We believe this is the first preparation of this hydrate in the laboratory.
The dissociation pressure at O°C was found to be 128 f 2 bar. A dielectric measurement on the dipole reorientation reveals the presence of encaged carbon monoxide and thus substan- tiates the formation of a stable hydrate.
The powder diffraction pattern of a hydrate obtained from the reaction of ice with methyl-cyclohexane and H2S was contaminated with ice. The Bragg's reflections due to polycrystalline ice, however, can be differentiated from the overlapping spectrum by com- paring with the diffraction pattern measured after decomposing the hydrate sample(Fig. 1).
In the X-ray powder pattern, nine peaks can be indexed assuming the crystal space group to be hexagonal. The refined cell parameters are a = 12.26(3)
A
and c = 10.17(3)A.
Re- finement of the neutron diffraction pattern of a deuteriohydrate of t-butyl methyl ether and H2S obtained at 5 K gave cell constants a = 12.15(7) and c = 10.05(6)A.
Figure 1. The upper diagram shows the X-ray powder diffraction pattern of a sample obtained by reacting ice with methylcyclohexane and the lower diagram shows the X-ray powder diffraction pattern of the frozen sample recovered from the melt.
We have also obtained independent evidence supporting the formation of a new hydrate structure. Previously, we have shown that 129Xe nuclear shielding can be used to charac- terize the size and the shape of hydrate cages
[lo].
The static 1?9Xe NMR spectrum of a double hydrate of Xe with methyl-cyclohexane is compared to those of type I and and I1 hydrates also containing Xe atoms in Fig. 2. The asymmetric peak in the same spectralCI-540 JOURNAL DE PHYSIQUE
region as that associated with the 12-hedral cage in type I and I1 hydrate can be shown to be composed of two resonances by the magic angle spinning technique. This observa- tion indicates that there are at least two kinds of cages of comparable sizes in the methyl- cyclohexane hydrate. One of the cages is very similar to the 12-hedral cage in type I and I1 hydrate. The other one is more asymmetric, as evident from the anisotropic static line- shape.
Structure 1
12-hedral Structure U
16-hedral
Figure 2. State lZ9Xe NMR spectra for hydrates mntaining Xe gas.
Gathering the results obtained from the analysis of the diffraction pattern and the in- formation on the symmetry of the cages from NMR lineshapes, a likely structure for this new hydrate is that of clathrasil dodecasil-lH Ill]. The crystal structure of dodecasil-1H is hexagonal with space group PG/mmm. The host lattice is composed of 34 Si02 units with three different types of cages. A schematic drawing of the cage types present in dodecasil- 1H is depicted in Fig. 3. There are three [512] cages, two [435663] cages and one [5126*]
cage per unit cell. The [512] cage is the same 12-hedral cage found in both the type I and I1 hydrates. The [435663] cage has similar dimensions as the 12-hedral cage. The [5126']
cage is the largest of all. Moreover, both [435663] and [5126*] are not present in type I or I1 hydrates. The 129Xe results support the conjecture that the methyl-cyclohexane molecules
are encaged in the [51268] cages, with the help gas (either H2S or Xe) filling up the smaller cages. Another observation which favours the hexagonal structure is the similarity in the ra- tios of the unit cell parameters of the proposed structure to the clathrasil analogue and of other type I and I1 hydrates to the corresponding isostructural clathrasils (see Table 1).
Figure 3. Linkage of [512], [435663] and [51268] cages in dodecasil-1H taken from ref. 11.
Table 1
Comparison o f l a t t i c e parameters ( A ) f o r C l a t h r a s i l s and Hydrates
Acknowledgements
The authors wish to thank Dr. J.R. Dahn for performing the X-ray diffraction experi- ment, to Dr. B.M. Powell for performing the neutron diffraction experiment and to Dr. M.
Desantos for providing the dielectric results of carbon monoxide hydrate before publication.
References
Ratio
1.12 1.12 1.12,l.lO
( I ) D.W. Davidson, " W a t e r - A Comprehensive Treatise". vol. 3, ed., F. Franks, Plenum Press, New York (1973).
Hydrate (L=H,O)
Space Group ,g, L
Pm3n 12.03(1) Fd 3m 17.31(1)
? 12.26(3) 10.17(3) Molecular
Formula 8G.46L 24G.136L 6G.34L
C l a t h r a s i l (L=SiO,)
Space Group R w c
Pm3n 13.436(3)
Fd3 19.402(1)
P6/mmm 13.783(4) 11.190(3)
C1-542 JOURNAL DE PHYSIQUE
(2) M. von Stackelberg and H.R. Miiller, Z. Elektrochem,, 58, (1954) 25-39.
(3) M. von Stackelberg and H.R. Miiller, J. Chem. Phys., 19, (1951) 1319-20.
(4) D.W. Davidson, Y.P. Handa, C.I. Ratcliffe, J.S. Tse and B.M. Powell, Nature, 311,
(1984) 142-3.
(5) H. Gies, Nachrichten Aus Chemie, Technik und Laboratorium, 33, (1985) 387-390.
(6) J.A. Ripmeester, J.S. Tse, C.I. Ratcliffe and B.M. Powell, Nature, in press, (1987).
(7) D.W. Davidson, S.K. Garg, S.R. Gough, Y.P. Handa, C.I. Ratcliffe, J.S. Tse and J.A.
Ripmeester, J. Inclusion Phenom.,
2,
(1985) 231-238.(8) H. Shoji and C.C. Langway Jr., Nature, 298, (1982) 548-50.
(9) "International Tables of Crystallographyn, vol. A, ed., T. Hahn, Holland (1983).
(10) J.A. Ripmeester and D.W. Davidson, J . Mol. Struct., 75, (1981) 67-72.
(11) H. Gerke and H. Gies, Z. Krist., 166, (1984) 11-22.
