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J. E. MOCK1, J. E. MYERS, and E. A. TRABANT2 Department of Nuclear Engineering, Purdue University, Lafayette, Ind.
Crystallization of the Rare-Gas Clathrates The rare-gas clathrates offer the nuclear scientist and engineer a new material possessing unique properties. These compounds can be used as storage media for nuclear reactor waste gases and for handling and shipping raregas rad io isotopes
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THE VIEWPOINT of nuclear technology, interest in the rare gases has been increasing a t a rapid rate partly because of two factors: First, a large percentage of reactor fission products consists of gaseous materials, particularly xenon and krypton, which must be disposed of to prevent atmospheric pollution (7, 77). Secondly, certain rare-gas radioisotopes have already proved useful to science and industrye.g.: xenon-133 as a radiographic source and krypton-85 as a tracer element and source of radiation for gaging (78, 20). The chemical inertness of the rare gases, preventing retention in the human body, makes them particularly desirable for many applications. Many of the useful and unique properties possessed by the rare-gas nuclides have not been exploited to date, largely because these elements are gases under normal conditions of temperature and pressure. The rare gases seldom enter into true chemical combination with other elements. Fortunately, there is one interesting group of compounds in which the rare gases do participate. These clathrate compounds, discovered by Powell (9-72, 75) are of an inclusion type in which a molecule of one component is enclosed in a crystalline structure by molecules of a second component in such a way that its escape is prevented. Such rare-gas clathrate compounds were prepared by Powell upon crystallization of aqueous solutions of quinol under high pressures of argon, krypton, or xenon. I t ha5 been proposed (6) that such rare-gas compounds could be used to advantage in handling and permanent disposal of radioactive rare gases, in preparing standardized radiation sources, as radiographic and gaging sources of
radiation, in the measurement and study of nuclear properties of raregas isotopes, and as compact energy sources. This investigation was done to study possible methods of forming rare-gas clathrates which exist in the solid state under normal conditions of temperature and pressure and to investigate optimum conditions for their preparation (7). Methods investigated were: crystallization from aqueous solutions of quinol, from nonaqueous solutions, from the quinol melt, and directly from the vapor phase. All produce clathrate crystals in good yields. Under similar operating conditions, crystals with much higher gas content are obtained from nonaqueous than from aqueous solutions, and both methods give higher yields than the solventless techniques. Krypton and xenon fill more clathrate vacancies than argon under identical operating conditions. Attempts to form clathrates containing helium and neon were unsuccessful. These results tend to confirm Powell's hypothesis (13) that the helium and neon atoms are too small to be trapped in the P-quinol structure. Twenty two organic compounds were screened as potential matrix materials but only
Figure 1 . Maximum argon content is ob- g tained a t high gas 8 pressures because of increased argon con- 5 centration in the a liquid phase w
2
quinol, reported, and p-fluorophenol trapped substantial quantities of gas.
Experimental The eauiDment consisted urimarilv of six high-pressure reactors of 100-ml. capacity connected in parallel to a pressure gage and to a cylinder of rare gas. A quinol solution was prepared and added to the reactors, which were then raised to the desired operating pressure and heated in a water bath to 85' C. The reactors were then allowed to cool slowly in the bath over a 24hour period to a final temperature of 24' C. The crystals were removed, filtered, dried in a desiccator, and analyzed for average weight percentage of rare gas contained in the crystals. To determine rare-gas content, a weighed quantity of clathrate was decomposed by heating to 180' C. The gas released during the heating process was measured in a gas buret (with suitable precautions so that no moisture or quinol was carried over); the loss in weight of the crystal was determined from the difference in its weight before and after heating. From these data two independent calculations were made to determine the weight percentage of rare gas contained in the crystals. The two methods checked on the average within 1.5%, which was considered satisfactory within the framework of the method and equipment being used. T o study the effect of variation of the cooling rate on the crystallization process, quinol solutions (0.12 gram per ml. of water) were heated to 60' C. and A
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0.12
4
0.14
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0.16
0.1 8
L 2
0.20
0.24
Present address, Defense Atomic Support Agency, Department of Defense, Washington. D. C. PreLnt address, University of Buffalo, Buffalo, N. Y .
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CODE
6
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5
0 (3 K
4
4
5W 0
a
3
L 2
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'0.08
0.12
PUINOL
0.16
0.20
0.24
0.28
CONCENTRATION, GRAMS/ML.
Figure 2. Maximum argon content is obtained at low quinol concentrations because of the type of crystal which forms and low temperature at which incipient crystallization begins 6
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r - 3
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0
K
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20
30
40
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C O O L I N G TIME, HOURS
Figure 3. Maximum argon content is obtained with slow cooling rates (long cooling times)
9-
8-
7-
x W
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held at this temperature for 1 hour. Crystallization runs were made a t a pressure of 600 p.s.i.g., with time to cool the solutions from 60' to 20' C. at 0,1,2,4,6,10,24, and 48 hours. After the prescribed cooling period, the solutions were held at the final temperature for 2 hours to permit incipient crystallization to progress toward completion. Two solventleas crystallization methods were investigated. I n the high-pressure melt process, a quinol melt was subjected to a high gas pressure, then slowly cooled. A sample of approximately 5 grams of quinol was placed in a reactor, which was then sealed, purged, and pressurized with argon. The reactor was then heated in an oil bath to 220' C. and held at that temperature for 30 minutes to ensure complete fusion. The melt was then permitted to cool a t the rate of 30' C. per hour to 160' C., a t which time the cooling rate was increased as solidification oi the melt was complete. The clathrate sample was then removed and analyzed for argon content. In the sublimation process, a sample of approximately 2 grams of quinol was placed in a reactor, which was then pressurized with argon. The lower third of the reactor was immersed in an oil bath a t temperatures above 180' C. for 2 hours. After cooling, the reactor was opened. Crystals formed at the top of the reactor were analyzed. Additional experiments were conducted with helium and krypton using both processes. In screening new matrix compounds, the bolubility a t room temperature of each was measured in two solvents, such as water, ethyl alcohol, ethyl ether, or acetone. Solutions were then prepared containing sufficient solute so that upon cooling to room temperature a minimum of 0.5 gram of material would be crystallized. The solutions were placed in high-pressure reactors, pressurized to 600 p.s.i g. with argon, heated to 90" C, and cooled to room temperature over a 24-hour period. They were then removed, filtered, dried, and analyzed for argon content. The compounds screened were: paminophenol, p-phenylenediamine, hydroxyhydroquinone, fi fluorophenol, chloranilic acid, uracil (P-hydroxy3,6-dihydroxypyridazine, pyridine), tetrachlorohydroquinone, 1,5 - naphthalenediol, resorcinol, catechol, pyrogallol, maleic acid hydrazide, orcinol, oxalic acid, phloroglucinol, p,fi'-biphenol, toluhydroquinone, 4-hydroxypyridine, and chlorohydroquinone. T o determine the stability of clathrates to radiation, the Purdue University gamma radiation facility, in which the
mtoluene
5-
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d n-heptane
c n-pentane Akerosine
C'O.025 C.0.03 C=0.02§ C'0.025
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PRESSURE, RSlG
1 008
Figure 4. Oxygen-containing organic solvents yield crystals with much higher argon content than other organic solvents
INDUSTRIAL AND ENGINEERING CHEMISTRY
intensity is 3307 rads per minute, was used. Two sets of samples were prepared, one for irradiation, the other for comparative purposes. Pure quinol was used as a check to determine whether any effects should be attributed to the clathrate structure alone. Samples were placed in small glass tubes, weighed a t specified intervals. Several tubes were sealed after the samples had been inserted. This precaution precluded extensive oxidation. Results and Discussion
Aqueous Crystallization of Argon Clathrates. Three parameters were found to play a significant role-argon pressure, quinol concentration, and cooling rate. A total of 146 experimental runs were sufficient to define their effects. Argon content of the clathrate crystals increased with applied pressure (Figure 1). This trend levels off rapidly at pressures of 1000 to 1200 p.s.i.g., indicating the difficulty inherent in attempting to fill all clathrate vacancies (10.8 wt. % ' argon) by the use of high pressure. The crystallization results are estimated reproducible within 10%. Quinol concentration exerted a very marked effect on the amount of argon contained in the clathrate structure (Figure 2). This effect appears to be related to the variation in type of crystal formed as the quinol concentration is changed (from a P-quinol crystal aggregate a t low solution concentrations to a needlelike crystal at high quinol concentrations, believed to be an interstitial a-quinol crystal) and to the temperature a t which incipient crystallization begins (considering the increased gas solubility at lower temperatures). The results of varying the cooling rate (Figure 3) show that slow cooling is conducive to the formation of clathrate crystals with a high percentage of rare gas. Here again the type of crystal obtained is one of the more important factors in determining argon content in the crystals. At a cooling time of 0 to 2 hours, thin needlelike a-phase interstitial crystals were obtained with low argon content. With cooling periods from 2 to 10 hours, a mixture of needlelike crystals and large crystal aggregates was obtained, whereas longer cooling periods produced the large crystal-aggregate type of structure with high argon content. A few single crystals were obtained which had 100% of the vacancies filled. The highest argon content obtained in a uniform batch of crystals was 7.24% (67% of the vacancies filled), produced at a pressure of 1600 p.s.i.g. and a quinol concentration of 0.10 gram per ml. Nonaqueous Crystallization. Because rare gases are more soluble in many organic solvents than in water, it should be possible to duplicate the results ob-
R A R E-G A S C L AT H RATES E
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ether
A butyl acetate @ ethanol
Figure 5. Ethyl ether, butyl acetate, and ethyl alcohol are clearly superior to water as a solvent for crystallization of rare-gas clathrates
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e b u t y l acetate C = 0.16 A e t h y l ether C = 0.18 mbutyl o c e t a t e C * 0.20 b e t h y l ether C= 0.20 oethanal C= 0.50 methanal C: 0.60
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COOLING TIME,
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\, 1 QUINOL CONCENTRATION, G R A W M L OF S O L V E N T 0.10 0.12 0.14 0.16 0.18 0.20 0.50 0.55 0.60 0.65 0.70 0.75 (Ethanol) 0.16 0.18 0.20 0.22 0.24 0 . 2 6 (Ether) 0.16 0.20 0.24 0.2 8 (Butyl A c e t o t e )
(HS)
tained in aqueous crystallization of argon clathrates, but at reduced pressures. This would provide simpler, more economical operating conditions. Solvents investigated were: ethyl alcohol, acetone, ethyl ether, butyl acetate, amyl acetate, toluene, n-heptane, npentane, kerosine, and methanol. In a total of 144 crystallization runs, parametric trends were the same as had been found previously in aqueous crystallization : High pressures, low quinol concentrations, and slow cooling rates were all conducive to the formation of crystals with high rare-gas content (Figures 4,5, and 6). Several organic solvents yielded crystals with much higher argon content than had been obtained in aqueous crystallization under identical conditions of pressure and cooling rate. At 200 p.s.i.g. in dilute solutions of each solvent, a maximum of 2.870 argon was obtained in crystals from aqueous solutions compared with 7.4% argon in crystals from solutions of ethyl ether and amyl acetate. The effect of variation in cooling rate was much less pronounced in nonaqueous crystallization (Figures 3 and 6). Ethyl ether was especially insensitive to cooling rate, yielding crystals after almost instantaneous cooling with the same rare-gas content as after prolonged cooling. During this phase of the investigation numerus single crystals were obtained with the maximum possible argon content. The highest argon content obtained in a uniform batch of crystals was 10.4% (96% of the clathrate cavities filled) at a pressure of 2500 p.s.i.g. and a quinol concentration of 0.10
gram per ml. of amyl acetate. I t was discovered that quinol will not form the clathrate structure from acetone solutions because of the preferential formation of a 1 to 1 acetonequinol crystalline compound. Clathrates crystallized from methanol gave a mixed crystal containing both methanol and argon (up to 470 argon at a pressure of 600 p.s.i.g.). Solventless Crystallization Methods. I t was considered possible that solventless crystallization methods might prove to be economically advantageous in the production of rare-gas clathrates. Moreover, such methods, if simple and effective, would undoubtedly find use in the manufacture of radioactive clathrates where entrainment of radioisotopes in the solvent recovery system (if a solventcrystallization process were used) might proved to be a serious problem. Among several processes considered, two were investigated : a high-pressure melt process and a sublimation technique. Results of the former (Figure 7) showed a linear increase in argon content in the crystal structure with applied pressure. This trend can probably be explained as a solubility effect. The latter study showed that the /3quinol structure does form upon desublimation in the presence of rare gases [rather than y-quinol ( 3 ) which normally forms directly from the vapor phase] and that this method is considerably more effective in producing high gas-content crystals than is crystallization from the melt (Figure 7). Helium was found to enter the quinol structure to a very limited extent in either the melt or sublimation process-
HOURS
Figure 6. Ethyl ether is especially insensitive to variation of cooling rate, yielding crystals in zero cooling time with the same rare gas content attained under prolonged cooling periods
less than 0.005 wt. %. This gas is probably contained interstitially in the a-quinol structure. Krypton, on the other hand, fills more clathrate vacancies than does argon under identical conditions. For example a t 300 p.s.i.g., krypton fills 50% of the vacancies upon desublimation compared with 20% by argon, and in the melt process 7.7Y0 as compared with 3.9%. Variation of Matrix Materials. Quinol has proved, thus far, to be the most satisfactory matrix material for containing the rare gases. The few others discovered to date, Dianin's compound [4(p-hydroxyphenyl)-2,2,4-trimethylchroman) (2)1, phenol (76), and a-cyclodextrin (5) all suffer from one or more defects. Phenol is extremely reactive and has a low melting point, whereas Dianin's compound and acyclodextrin are unable to retain a high percentage of rare gas. Therefore, other rare gas clathrates having one or more of the following properties would be of interest : high rare-gas content, high-temperature stability, and low solubility in many common solvents.
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PRESSURE, P.S.I.A.
Figure 7. Crystallization directly from the vapor phase is more effective in producing clathrate crystals with high rare gas content than is crystallization from the melt VOL. 53,
NO. 12
DECEMBER 1961
1009
As Powell pointed out (73), it is impossible a t present to make any exact predictions as to which substances will form a clathrate structure. However, most known clathrates appear to be characterized by two factors: a capability of forming hydrogen bonds and/ or an awkward molecular shape. With these factors as a guide, 21 organic compounds were chosen for screening. I n all cases but one, the argon content of the crystals was found to be less than 0.1% by weight. I n the case of p fluorophenol, however, crystals formed from the melt under an argon pressure of 1500 p.s.i.g. contained 1% (weight) of the rare gas. These crystals were found to be stable and to release the rare gas only when the matrix material was dissolved or melted. Because of its large solubility in most common solvents, crystallizing p-fluorophenol from solution was difficult. Further investigation would undoubtedly uncover appropriate techniques for crystallizing p-fluorophenol from suitable solvents, in which case it is believed that crystals with higher rare-gas content would be obtained. If the p-fluorophenol-argon crystal has a structure similar to that of /3-quinol, then an argon content of 1% would be equivalent to filling 8.5% of the cavities; p-fluorophenol has a relatively low melting point (119O F,), which may be a serious limitation. I t appears that almost any rnodification of the quinol molecule destroys its ability to form the clathrate structure, Thus, even a minor change such as replacing a hydrogen atom on the benzene ring with a chlorine atom, a hydroxy group, or a methyl group is sufficient to preclude clathrate formation In addition, it has been found that the amino group, as in p-aminophenol and p-phenylenediamine, either because of its weaker hydrogen-bonding tendency or because of steric hindrance, fails to form an analogous structure as compared to the hydroxy group Three six-member ring systems-pyridazine, pyridine, and pyrimidine-with appropriate hydrogen bonding groups did not form the clathrate structure under the experimental conditions This may be
due to slight differences in size and configuration compared with the benzene ring or to the directive influence of nitrogen atoms in the ring structure. Krypton and Xenon Clathrates. Both xenon and krypton are considerably more soluble in most common solvents than argon. Since solubility plays such an important role in rare-gas clathrate formation, xenon and krypton would be expected to fill more clathrate vacancies than argon under identical operating conditions of pressure, quinol concentration, and cooling rate. The tabulated results (below) verify this assumption. The general parametric trends for crystallization of argon clathrates are also valid for krypton in both aqueous and nonaqueous solutions. Attempts to Form Helium and Neon Clathrates. An approximately spherical unoccupied space exists in the /3-quinol structure of 4-A. diameter (74). This imposes an upper limit on the size of atoms and molecules which can be enclosed in the clathrate structure Failures to incorporate helium and neon into the clathrate structure have led Powell to postulate that both neon and helium atoms have such a small effective size that they can escape through an opening in the cage walls (73). O n the other hand, van der Waals (79) has pointed out that the neon clathrate, if it does exist, would have a theoretical dissociation pressure of 160 atm. Attempts were made to include helium and neon in the clathrate structure by operating at low quinol concentrations, slow cooling rates, and high pressures (up to 300 p.s.i.g.). Crystals containing 0.12 wt. yo helium were obtained from ether solutions, which gave a vigorous and steady evolution of gas when dissolved in methanol. However, the specific gravity of these crystals was found to be approximately 1.36 as compared with the expected value of 1.27 for a helium-quinol clathrate. The results obtained with neon were similar in nature. Crystals containing approximately 0.3570 by weight of neon were obtained. We concluded that these crystals are interstitial crystals in which one molecule of neon (or helium) is
contained in the quinol structure for approximately 52 quinol molecules. These results tend to confirm Powell’s hypothesis. As supplementary evidence, when helium and neon crystallization experiments were made in methanol solution (where the /3-quinol structure always forms), no mixed crystals concontaining both a rare gas and methanol were formed as with argon. Stability of Clathrate Crystals. During gamma-irradiation, the quinol and clathrate crystals took on a bright brownish orange color. No weight changes (measured to 1 part in 10,000) were detected after 575 hours of irradiation (103 rads). The stability of these crystals is particularly interesting in view of the weak hydrogen bond which plays such an important role in the clathrate structure. The clathrates are very stable a t room temperature. Chleck and Ziegler ( 4 ) estimated that leakage from the clathrate structure is no greater than a few p.p.m. per day. But the clathrates are somewhat temperature sensitive, with rapid release of the caged component above 165” F. (94). Detailed analysis (8) has shown that for thicknesses of radioactive material useful in most radiation work, no difficulty arises in keeping the temperature below the decomposition point by removing heat.
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literature Cited (1) Adams, R. E., Browning, W. E., Ackley, R. D., IND. ENC. CHEM.51, 1467-70 (1959). (2) Baker, W., McOmie, J. F. W., Chem. &’ Ind. (London) 1955, p. 256. (3) Caspari, W. A., J . Chem. Soc., 1093-5 (1927). (4)’ Chlkck, D. J., Ziegler, C. A., Nucleonics 17, NO. 9, 130-3 (1959). (5) Mandelcorn, L., Chem. Revs. 59,827-39 (1959) (6) Mock, J. E., Purdue Univ.. Lafavette. . ,Ind., private communication t o Argonne National Laboratory, Oct. 14, 1958. (7) Mock, J. E., Ph.D. thesis, Purdue Univ., Lafayette, Ind., 1960. (8) Mock, J. E., Trabant, E. A., Myers, J. E., Eng. Res. Bull. No. 145, Purdue Univ. Eng. Expt. Sta., 1961. (9) Moeller, T., “Inorganic Chemistry,” pp. 223-4, 382-3, Wiley, New York, 1952. (10) Plain, D. E., Powell, H. M., J . Chem SOC. 1947, pp. 208-21. (11) Zbid.. 1948. D. 815. j12j Powell, H,’M., Ibid.,1950, p. 298-300. ,
owell, H. M., Research I, 353-7
Comparison of Clathrates
Solvent Ethyl ether
Ethyl alcohol
Butyl acetate
Hz0
Concn ., Gram/cc. 0.18 0.50 0.18 0.10
7.0 7.4 6.8 3.3
0.18 0.50 0.18 0.10
6.3 5.3 5.8 3.0
Kr =
\-’
yo Cavities Filled
Weight yo A p
Xe
A
Kr
65.2 68.8 63.5 30.5
89.5 74.8 86.5 42.3
Xe
200 p.s.i.g. 18.1 15.1 17.5 8.6
p = 175 p.s.i.g.
Ethyl ether Ethyl alcohol Butyl acetate H20
1 0 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
27.1 15.5 17.0 11.9
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58.2 49.0 53.8 27.7
96.0 54.7 60.0 42.0
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(15) Powell, H. M., Guter, M., Nature 164, 240-1 (1949). (16) Stackelberg, M., von, Rec. trau. chim. 75. 902-5 (1956). (17) ‘Steinberg, M:, Manowitz, B., IND. ENC.CHEM.51, 47-50 (1959). (18) Swift, W. E., Nucleonics 15, No. 3, 66-7 (1957). (19) Waals, J. H., van der, Trans. Faraday SOC.52, 184-93 (1956). (20) Wilson, E. J., Dibbs, H. P., Richards, S.. Eakins, J . D., Nucleonics 16. No. 4. 110-4 (1958). ‘ RECEIVED for review February 9, 1961 ACCEPTED August 15, 1961
