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A study of Krypton encapsulation and adsorption in zeolite by means of neutron activation analysis
00x-7oxx
82 121345-05803.00 0 Purg;unon Press Ltd
A Study of Krypton Encapsulation and Adsorption in Zeolite by Means of Neutron Activation Analysis YASUO SHINGO ‘Tokai
ITO,’
MATSUOKA,3 Division.
TAKEMI HIRONE
TAKANO,’
HIRONAO
NAKAMURA3
KOJIMA.’
and TAKAAKI
TAMURA3
Research
Center for Nuclear Science and Technology. University of Tokyo. Tokai-mura, Ibaraki-ken. 319-11. Japan. *Faculty of Engineering. Yamanashi University. Takeda. Kofu. 400. Japan and JNuclear Engineering Research Laboratory, Faculty of Engineering. University of Tokyo. Tokai-mura. Ibaraki-ken. 319-11, Japan
Encapsulation in zeolite is one of the promising methods of immobilization of radioactive “5Kr in waste management of nuclear fuel cycles. During our test studies of krypton encapsulation into zeolite and leakage from it. we have applied neutron activation analysis to determine the amounts of zeolite and krypton. This method has proved to be particularly preferable to the gravimetrical method when absorbed water can obscure the result of weight measurements. It has been shown that krypton can be encapsulated in both I- and P-cages. but that the krypton in the r-cage can be released easily at the expense of water absorption, Kr in the p-cage seems to be relatively stable. An explanation is given for this different behavior of krypton in z- and b-cages. It is also shown that there is another site for krypton. unstable and small in number. which we attribute to surface adsorption sites.
Introduction FISSION of uranium and plutonium produces radioactive rare gases such as 133Xe, 133mXe and 85Kr. They have so far been released to the environment, but, with the gradual increase in the electric power generation capacity. it is becoming necessary to recover and store the long-lived “Kr (ri = 10.8~) for a sufficiently long period. The simplest way may be to contain it in a pressurized cylinder. But this method has the potential for leakage and release of “5Kr from failed high pressure cylinders, and the necessity of heat removal also requires an expensive storage facility. Various options for safer long-term storage are being sought, among which is the encapsulation of Kr in zeolite or in porous glass. Loading and leakage of Kr in zeolites or in porous glass have so far been studied in the U.S.A.“) and West Ciermany.‘2) and their results demonstrate the feasibility of this method. In the present paper we describe our results on Kr encapsulation in zeolite studied by means of neutron activation analysis and mass-spectroscopy. The quantitative activation analysis of Kr and zeolite has been used for the first time for this kind of experiment and has proved to be powerful when the usual gravimetrical method is not applicable due to the presence of absorbed water. The importance of this method must especially be appreciated for the present case, because, as will be
made clear in this report, the presence of absorbed water affects Kr leakage significantly. The mass-spectroscopic method has been used to monitor the Kr gas leakage during heating of the sample at a constant rate. Molecular Sieve 3A (MS-3A) has mainly been used in the present experiment.
The Structure of MS3A
and
Encapsulation The basic structures of A-type zeolitesC4’ are SiO, and AlO, groups in tetrahedral structure with either Si or Al at the center of the tetrahedron. The Al/Si ratio is 1.0. The SiO, and AlO groups are joined together to form an Archimedian truncated octahedron as shown in Fig. l(A). This is called a sodalite unit. Within the sodalite unit is a void space which is called a p-cage. Its maximum i.d. is 6.6 A. To get into the P-cage, a molecule must pass through an opening 2.2 A wide. The sodalite units are joined together along the cubic faces. and the resulting arrangement creates a larger cavity which is surrounded by eight sodalite units. This cavity is called an r-cage. and its maximum dia. is 11 A. To get into the s-cage, a molecule must pass through the aperture at the eightmembered rings. Near this aperture there is a site. called a-site. where a singly charged cation can be
I346
A
ADerture
FIG. I, (A)--The Archimedian truncated octahedron or the sodalitc unit and (B) the simple cubic arra) of truncated octahedron in the zeolite type-A from Ref. 3. When K’ occupies the r-site near the X-membered ring. the aperture is about 3 A as indicated.
accommodated. When Na+ ion occupies this site, the diameter of the aperture is almost 4 A. and this is commercially named Molecular Sieve 4A (MS-4A). When K’ ion occupies it? the diameter is decreased to 3 .& and it is named MS-3A. The diameter of the Kr atom is 3.2 A. and therefore it can get into the z-cage of MS-4A. but not into the r-cage of MS-3A or the P-cage of MS-4A and MS-3A at normal temperature. At a higher temperature it becomes possible for the Kr atom to overcome the activation barrier to enter the cavities. Once Kr atoms are put into the cavities. they cannot come out at room temperature. This effect is called encapsulation.
Experimental MS4A was purchased from Gasukuro Kogyo Co. It was dried overnight in an oven at 200 C before the encapsulation process. Kr could be encapsulated l&20 wt?,, of MS-3A depending on the conditions. but the sample studied here had typically 17 wt”. of Kr. This value was determined by weight measurement both before and after the encapsulation process and by neutron activation analysis. The amount of Kr measured by the two different methods agreed well. The activation analysis made it possible to determine simultaneously the absolute amounts of MS-3A and the encapsulated Kr. To do this we first of all determined the content of Na or K in a carefully dried MS-3A by the activation analysis. Once the value is determined. Na or K can be used as an internal standard and MS-3A can be measured without being disturbed by the presence of absorbed water. The quantitative analysis of Na and K was carried out by measuring the y-ray intensities from “Na and “K which are formed by 23Na(n,y) and “‘K(n,;) reactions. respectively. The activation analysis of Kr was carried out by measuring the intensity of 149 keV y-rays from 85mKr formed by 84K(n.y) reaction. The standard reference sample for Kr was made by col-
lecting a measured volume of Kr gas under a measured pressure into a quartz tube at a liquid nitrogen temperature. In order to insure complete collection of the gas, charcoal was put into the quartz tube. The activation analysis was carried out using research reactors JRR-2 and JRR-4 of Japan Atomic Energy Research Institute (J.A.E.R.I.). The thermal neutron flux was typically 7 x 1012!cm2 s. The sample. approximately 50mg. was sealed in a quartz tube and was irradiated together with the standard samples for Na, K and Kr for 5 min. and. after about 3 h cooling, the :-ray spectra were measured using a standard ;‘-ray spectrometer consisting of a Ge(Li) solid-state detector and a multi-channel analyzer. The mass-spectroscopic method was used to monitor the rate of Kr gas leakage during heat treatment of the Kr-containing reolite. By locking the peak detector of the spectrometer at n/i, = 84, only the Kr gas which had leaked out could be followed. while the overall gas release of Kr and H,O could bc monitored by the total ion monitor.
Results and Discussion Dctcrnlinution of‘ Nu cd
K in MS-4A
A typical y-ray spectrum for Kr-containing MS-3A is shown in Fig. 2. The y-ray peaks form ‘“Na. “‘K and ““‘Kr are all well resolved. and it is easy to determine their amounts simultaneously. First of all. the contents of Na and K were determined by the activation analysis of MS-3A. which was carefully oven-dried at 200 C. Their contents slightly differed in different lots from the manufacturer. but for the samples from the same lot the values were constant within experimental error. For MS-3A used in the present experiments. the amounts of Na and K were determined to he 3.93 + 0.05 and 8.5 & 0.1 wt”,,. respectively.
Kr encapsulation
2
’
r
65mKr
I
149
MS-3A
and adsorption
I347
in zeolite
r
24Na 1368
encapsulating in quartz
Kr ampoule I 800
I 400
Channel
I
I
1200
1600
number
FIG. 2. The y-ray spectrum of MS-3A encapsulating 17 wt% Kr. The y-rays from Na, K and Kr are clearly resolved from this spectrum. The number at each peak is the y-ray energy in keV.
Once the content of Na or K in the oven-dried is determined, then this value can be used as a conversion factor to calculate MS-3A weight from a measured amount of Na or K. This is preferable to the usual gravimetric method. because it does not involve uncertainties brought forth by absorbed water. The importance of this must especially be emphasized, because, as will be made clear, the absorbed water enhances Kr leakage significantly, and determination of the amounts of MS-3A and Kr by MS-3A
the
gravimetric
method
is essentially
made
imposs-
ible. Determinution
of the amount
of encapsulated
Kr
The encapsulation of Kr in MS-3A was carried out at 3OO‘C, lOOOatm, for 5 h. Simultaneous activation analysis of Na and Kr was done for this sample, and the amount of the encapsulated Kr was determined to be 140 &- 2 mg/g MS-3A. This value is slightly smaller than that obtained gravimetrically in a water-vapor free atmosphere, 168 mg/g MS-3A. This difference is due neither to errors of activation analysis nor to those of gravimetrical measurement. Since about 20 days had elapsed from the day of the encapsulation to the day of the activation analysis, it is most probable that some fraction of Kr had leaked out during the period. As will be shown later, Kr atoms adsorbed on surface active sites and those in the a-cages may be responsible for such leakage. Krypton
leakage
more marked. In order to see how fast Kr atoms leak out of MS3A, we kept the sample in open air for a definite time and then sealed it in a quartz tube for activation analysis. Thus the amount of Kr in MS-3A was measured as a function of the time of exposure to air. The result is shown by curve B in Fig. 3. The Kr leaks out very quickly at first, but the leakage rate levels off at about 5 wt%. It seems that there are at least two different Kr leakage mechanisms. Curve A in Fig. 3 is the weight increase during the time when the oven-dried MS-3A was allowed to stand in an open air. The weight increase is due to absorption of water vapor. It is noteworthy that curves A and B are well correlated with each other; when water absorption levels off, the Kr leakage slows down, too. It appears as if Kr leakage takes place spontaneously with the water absorption. In this connection it will be worthwhile to note that the water and cations in the cavities of open zeolites can well be compared to a concentrated aqueous salt solution.(5) For example, the heat of sorption of water in
in an open air
In the early days of this study,
MS-3A encapsulating Kr was kept in a desiccator over silica gel; by the activation analysis, we noticed that the Kr content had decreased significantly. When the sample was .. . allowed to stand in the open air, the decrease was
FIG. 3. (A&The increase in the weight of ove.r-dried MS-3A during the time when it is allowed to stand in open air. Curve (B) is the decrease in the Kr content in MS-3A during the time when MS-3A encapsulating 17 wt% Kr was . allowed to stand in the same condition as (A).
1348 TAFJL~ l(a). Leakage
Sample number
of krypton from MS-3A/Kr(r.P) vacuum Elapsed days in vacuum
Kr content (mg:g MS-3A)
0 5 9 14
136.2 140.1 141.0 140.8
1 2 3 4
TAeLf:
l(b). Leakage
Sample number
of krypton from open air
Kr content (mg. g MS-3A)
0 5 8 13
82.7 57.8 70.6 47.5
NaX zeolite is comparable water
and
MS-3A,IKr(x.P)
Elapsed days in air
1’ 2 3’ 4’
of liquid
in a
to the heat
of electrolyte
in
of vaporization
solutions,
Further-
more. the electrical conductivity of hydrated NaX and NaA zeolites is also comparable to electrolyte solutions. Low-angle X-ray scattering in them also indicates cation disorder. All these facts support a view that at least some of the cations are hydrated and are mobile. This effect of cation hydration may be related to the rapid Kr release. When K+ ions at the %-sites are hydrated, the aperture at the eight-membered rings will become larger due to displacement of the cations. Its size could even become as large as 5 A when the cations are displaced a great enough distance. It then becomes very easy for Kr atoms to pass through the enlarged aperture and leak out of the cavities. Moreover. when water is absorbed. there is the heat of sorption which. given to Kr atoms in the same cavity. can stimulate the leakage. About 5 wt”,, of Kr can remain in zeolite for a very long time (Fig. 3). This fraction of Kr probably corresponds to Kr atoms in the /I-cage. Because the size of the [i-cage is small. the water and ions in it are not expected to resemble an aqueous solutions!” They are rather expected to be bound directly to framework of oxygen atoms. Thus Kr atoms in the j-cage can hardly come out of it. Even if the cation near the aperture of the /j-cage were to be displaced due to solvation. large
the
enough
sire
of the
to allow
rapid
aperture leakage
will
not
to 2 weeks, Kr can be encapsulated stably in MS-3A in a dry atmosphere. Because the accuracy of the Kr determination by activation analysis is no better than l’?;,. however, the present technique does not have the sensitivity to detect small amounts of leakage. A long-term Kr leakage from MS-3A in a dry atmosphere was also estimated using a mass-spectrometer and by heating the sample at a constant rate. The result was analyzed using Fick’s law of diffusion, and an activation energy for the diffusion of Kr was estimated. The results. which are to be published elsewhere.(h) predicted that there will be practically no long-term Kr leakage at room temperature but that at 150’C about 60”/(, of Kr will be released within IOyr.
Using the fact that Kr atoms in the x-cage are quickly released when water is absorbed by MS-3A. we have been able to prepare MS-3A which contain Kr only in the P-cage. We will write MS-3A/Kr(x.P) for the sample which has Kr in both r- and P-cages. and MS-3AjKr(P) for the sample which has Kr only in P-cage. During our experiments using a mass-spectrometer we noticed that a very small quantity of Kr was released before a substantial amount starts to be released. An example is shown in Fig. 4(A). where MS-3A/Kr(x,P) was heated at a rate 166Cjmin. A small peak near 1lO’C precedes the broad big component of Kr release. Followings are the facts concerning this small Kr release peak: (1) This peak could be observed for both MS-3A: Kr(x./j) and MS-3A,!Kr(P).
0
become
of Kr.
Thus the result in Fig. 3 indicates that Kr can be encapsulated in both T- and [I-cages and that Kr in the r-cage is very easily released when water is absorbed. A question then arises: whether Kr will not be released if water is not absorbed at all. In order to examine this point. MS-3A encapsulating 14wt”,, of Kr was allowed to stand in a vacuum for a certain number of days. and then the Kr content was measured by activation analysis. 1 he result is shown in Table 1. Clearly. for a short-term preservation of up
0
200
I 400
I 600
Temp,
I 800
I 1000
“C
FIG. 4. Kr gas leakage during heating up (A) MS-3A cncapsulating 17 wt”,, Kr and (B) MS-3A which was simply contacted to 1 atm Kr gas. The Kr release peak at lower temperature is due to Kr adsorbed on surface active sites (see the text). Note that its intensity is approximately the same for (A) and (B). but is very small compared to the main component of Kr release at hlghcr temperatures.
Kr twapsulotion
and adsorption
(2) This peak appeared only when the samples were allowed to stand for a certain period: more than a week when MS-3A/Kr(T.P) or MS-3A!‘Kr(P) was kept in a dry atmosphere, and less than several days when it was kept in open air. (3) This peak could be produced many times if the sample is kept for a sufficiently long time before it is heated once more. (4) Oven-dried MS-3A was brought into contact with 1 atm of Kr gas and then the system was evacuated. MS-3A should not encapsulate Kr. but when it was heated subsequently. the Kr release peak at 1lo’C was observed as is shown in Fig. 4(B). These facts indicate that the small peak corresponds to Kr desorption from surface active sites. Because the activation energy for the desorption is expected to be small. a few kcalimol, the adsorbed Kr atoms can be released at a lower temperature than that at which Kr leakage from the encapsulated states takes place. Points (2) and (3) indicate that Kr atoms encapsulated in r- and p-cages can eventually come out and then be adsorbed on the surface adsorption sites, the rate of the leakage depending on the temperature. An experiment similar to that described in point (4) was also carried out for MS-4A. Oven-dried MS-4A. after being contacted to 1 atm of Kr gas. was heated. and the Kr release was monitored using the peak detector of the mass-spectrometer. The Kr release peak at 1lO’C was observed as expected, and its intensity was much larger than that observed for MS-3A. Because Kr can enter x-cages of MS-4A at normal temperature and pressure, this result clearly indicates that the adsorption sites are found not only on the surface of crystallites of zeolites but also inside x-cages.
Conclusion The quantitative
non-destructive
analysis
of zeolite
I349
in rmlire
and Kr by neutron activation analysis has been shown to be a powerful method. especially when the existence of absorbed water makes it impossible to apply gravimetrical measurements. About 50 mg or less is enough to do the experiment. which is also an advantage compared to the gravimetrical way. Further studies of Kr encapsulation using this method are under way for other kinds of zeolites processed under various encapsulation conditions. The present results have shown that there are at least three sites for Kr in MS-3A, the stability of Kr increasing in the order of surface adsorption sites, x-cage and p-cage. The existence of absorbed water has been shown to markedly enhance Kr leakage from the x-cage. The explanation given to this phenomena will have to be examined by further studies. but will undoubtedly afford important material for the understanding of the molecular sieve effect.
A~knowlcdgrmcwts This work was supported financially in part by the Research Project of the Research Center for Nuclear Science and Technology. University of Tokyo. The use of JRR-2 and JRR-4 has been made possible by the grant of the National Universities’ Program for the Common Use of J.A.E.R.I. Facilities.
References 1. BENEDICT R. W., CHRISTENSENA. B.. DFL DEBBIO J. A.. KELLER J. H. and KNECHT D. A. ENICO-1011, Sep-
tember 1979. 2. PFNZHORN R. D.. SCHCLST~R P.. NOPP~L H. E. and HELLWIG L. M. IAEA-SM-245/10, February 1980. 3. BRICK D. W. J. C’hrn~. E&c. 41, 678 (1964). 4. RIXD T. B. and BREAK D. W. J. Am. Chm. SM. 78, 5972 (I 956). 5. SHtRRY H. S. In Ion Exchanyc, (Ed. MARINSKY) Vol. 2. Chapter 3. (Marcel Dekkcr. New York. 1969). 6. MATX OKA c’t (11.(unpublished data).
82 121345-05803.00 0 Purg;unon Press Ltd
A Study of Krypton Encapsulation and Adsorption in Zeolite by Means of Neutron Activation Analysis YASUO SHINGO ‘Tokai
ITO,’
MATSUOKA,3 Division.
TAKEMI HIRONE
TAKANO,’
HIRONAO
NAKAMURA3
KOJIMA.’
and TAKAAKI
TAMURA3
Research
Center for Nuclear Science and Technology. University of Tokyo. Tokai-mura, Ibaraki-ken. 319-11. Japan. *Faculty of Engineering. Yamanashi University. Takeda. Kofu. 400. Japan and JNuclear Engineering Research Laboratory, Faculty of Engineering. University of Tokyo. Tokai-mura. Ibaraki-ken. 319-11, Japan
Encapsulation in zeolite is one of the promising methods of immobilization of radioactive “5Kr in waste management of nuclear fuel cycles. During our test studies of krypton encapsulation into zeolite and leakage from it. we have applied neutron activation analysis to determine the amounts of zeolite and krypton. This method has proved to be particularly preferable to the gravimetrical method when absorbed water can obscure the result of weight measurements. It has been shown that krypton can be encapsulated in both I- and P-cages. but that the krypton in the r-cage can be released easily at the expense of water absorption, Kr in the p-cage seems to be relatively stable. An explanation is given for this different behavior of krypton in z- and b-cages. It is also shown that there is another site for krypton. unstable and small in number. which we attribute to surface adsorption sites.
Introduction FISSION of uranium and plutonium produces radioactive rare gases such as 133Xe, 133mXe and 85Kr. They have so far been released to the environment, but, with the gradual increase in the electric power generation capacity. it is becoming necessary to recover and store the long-lived “Kr (ri = 10.8~) for a sufficiently long period. The simplest way may be to contain it in a pressurized cylinder. But this method has the potential for leakage and release of “5Kr from failed high pressure cylinders, and the necessity of heat removal also requires an expensive storage facility. Various options for safer long-term storage are being sought, among which is the encapsulation of Kr in zeolite or in porous glass. Loading and leakage of Kr in zeolites or in porous glass have so far been studied in the U.S.A.“) and West Ciermany.‘2) and their results demonstrate the feasibility of this method. In the present paper we describe our results on Kr encapsulation in zeolite studied by means of neutron activation analysis and mass-spectroscopy. The quantitative activation analysis of Kr and zeolite has been used for the first time for this kind of experiment and has proved to be powerful when the usual gravimetrical method is not applicable due to the presence of absorbed water. The importance of this method must especially be appreciated for the present case, because, as will be
made clear in this report, the presence of absorbed water affects Kr leakage significantly. The mass-spectroscopic method has been used to monitor the Kr gas leakage during heating of the sample at a constant rate. Molecular Sieve 3A (MS-3A) has mainly been used in the present experiment.
The Structure of MS3A
and
Encapsulation The basic structures of A-type zeolitesC4’ are SiO, and AlO, groups in tetrahedral structure with either Si or Al at the center of the tetrahedron. The Al/Si ratio is 1.0. The SiO, and AlO groups are joined together to form an Archimedian truncated octahedron as shown in Fig. l(A). This is called a sodalite unit. Within the sodalite unit is a void space which is called a p-cage. Its maximum i.d. is 6.6 A. To get into the P-cage, a molecule must pass through an opening 2.2 A wide. The sodalite units are joined together along the cubic faces. and the resulting arrangement creates a larger cavity which is surrounded by eight sodalite units. This cavity is called an r-cage. and its maximum dia. is 11 A. To get into the s-cage, a molecule must pass through the aperture at the eightmembered rings. Near this aperture there is a site. called a-site. where a singly charged cation can be
I346
A
ADerture
FIG. I, (A)--The Archimedian truncated octahedron or the sodalitc unit and (B) the simple cubic arra) of truncated octahedron in the zeolite type-A from Ref. 3. When K’ occupies the r-site near the X-membered ring. the aperture is about 3 A as indicated.
accommodated. When Na+ ion occupies this site, the diameter of the aperture is almost 4 A. and this is commercially named Molecular Sieve 4A (MS-4A). When K’ ion occupies it? the diameter is decreased to 3 .& and it is named MS-3A. The diameter of the Kr atom is 3.2 A. and therefore it can get into the z-cage of MS-4A. but not into the r-cage of MS-3A or the P-cage of MS-4A and MS-3A at normal temperature. At a higher temperature it becomes possible for the Kr atom to overcome the activation barrier to enter the cavities. Once Kr atoms are put into the cavities. they cannot come out at room temperature. This effect is called encapsulation.
Experimental MS4A was purchased from Gasukuro Kogyo Co. It was dried overnight in an oven at 200 C before the encapsulation process. Kr could be encapsulated l&20 wt?,, of MS-3A depending on the conditions. but the sample studied here had typically 17 wt”. of Kr. This value was determined by weight measurement both before and after the encapsulation process and by neutron activation analysis. The amount of Kr measured by the two different methods agreed well. The activation analysis made it possible to determine simultaneously the absolute amounts of MS-3A and the encapsulated Kr. To do this we first of all determined the content of Na or K in a carefully dried MS-3A by the activation analysis. Once the value is determined. Na or K can be used as an internal standard and MS-3A can be measured without being disturbed by the presence of absorbed water. The quantitative analysis of Na and K was carried out by measuring the y-ray intensities from “Na and “K which are formed by 23Na(n,y) and “‘K(n,;) reactions. respectively. The activation analysis of Kr was carried out by measuring the intensity of 149 keV y-rays from 85mKr formed by 84K(n.y) reaction. The standard reference sample for Kr was made by col-
lecting a measured volume of Kr gas under a measured pressure into a quartz tube at a liquid nitrogen temperature. In order to insure complete collection of the gas, charcoal was put into the quartz tube. The activation analysis was carried out using research reactors JRR-2 and JRR-4 of Japan Atomic Energy Research Institute (J.A.E.R.I.). The thermal neutron flux was typically 7 x 1012!cm2 s. The sample. approximately 50mg. was sealed in a quartz tube and was irradiated together with the standard samples for Na, K and Kr for 5 min. and. after about 3 h cooling, the :-ray spectra were measured using a standard ;‘-ray spectrometer consisting of a Ge(Li) solid-state detector and a multi-channel analyzer. The mass-spectroscopic method was used to monitor the rate of Kr gas leakage during heat treatment of the Kr-containing reolite. By locking the peak detector of the spectrometer at n/i, = 84, only the Kr gas which had leaked out could be followed. while the overall gas release of Kr and H,O could bc monitored by the total ion monitor.
Results and Discussion Dctcrnlinution of‘ Nu cd
K in MS-4A
A typical y-ray spectrum for Kr-containing MS-3A is shown in Fig. 2. The y-ray peaks form ‘“Na. “‘K and ““‘Kr are all well resolved. and it is easy to determine their amounts simultaneously. First of all. the contents of Na and K were determined by the activation analysis of MS-3A. which was carefully oven-dried at 200 C. Their contents slightly differed in different lots from the manufacturer. but for the samples from the same lot the values were constant within experimental error. For MS-3A used in the present experiments. the amounts of Na and K were determined to he 3.93 + 0.05 and 8.5 & 0.1 wt”,,. respectively.
Kr encapsulation
2
’
r
65mKr
I
149
MS-3A
and adsorption
I347
in zeolite
r
24Na 1368
encapsulating in quartz
Kr ampoule I 800
I 400
Channel
I
I
1200
1600
number
FIG. 2. The y-ray spectrum of MS-3A encapsulating 17 wt% Kr. The y-rays from Na, K and Kr are clearly resolved from this spectrum. The number at each peak is the y-ray energy in keV.
Once the content of Na or K in the oven-dried is determined, then this value can be used as a conversion factor to calculate MS-3A weight from a measured amount of Na or K. This is preferable to the usual gravimetric method. because it does not involve uncertainties brought forth by absorbed water. The importance of this must especially be emphasized, because, as will be made clear, the absorbed water enhances Kr leakage significantly, and determination of the amounts of MS-3A and Kr by MS-3A
the
gravimetric
method
is essentially
made
imposs-
ible. Determinution
of the amount
of encapsulated
Kr
The encapsulation of Kr in MS-3A was carried out at 3OO‘C, lOOOatm, for 5 h. Simultaneous activation analysis of Na and Kr was done for this sample, and the amount of the encapsulated Kr was determined to be 140 &- 2 mg/g MS-3A. This value is slightly smaller than that obtained gravimetrically in a water-vapor free atmosphere, 168 mg/g MS-3A. This difference is due neither to errors of activation analysis nor to those of gravimetrical measurement. Since about 20 days had elapsed from the day of the encapsulation to the day of the activation analysis, it is most probable that some fraction of Kr had leaked out during the period. As will be shown later, Kr atoms adsorbed on surface active sites and those in the a-cages may be responsible for such leakage. Krypton
leakage
more marked. In order to see how fast Kr atoms leak out of MS3A, we kept the sample in open air for a definite time and then sealed it in a quartz tube for activation analysis. Thus the amount of Kr in MS-3A was measured as a function of the time of exposure to air. The result is shown by curve B in Fig. 3. The Kr leaks out very quickly at first, but the leakage rate levels off at about 5 wt%. It seems that there are at least two different Kr leakage mechanisms. Curve A in Fig. 3 is the weight increase during the time when the oven-dried MS-3A was allowed to stand in an open air. The weight increase is due to absorption of water vapor. It is noteworthy that curves A and B are well correlated with each other; when water absorption levels off, the Kr leakage slows down, too. It appears as if Kr leakage takes place spontaneously with the water absorption. In this connection it will be worthwhile to note that the water and cations in the cavities of open zeolites can well be compared to a concentrated aqueous salt solution.(5) For example, the heat of sorption of water in
in an open air
In the early days of this study,
MS-3A encapsulating Kr was kept in a desiccator over silica gel; by the activation analysis, we noticed that the Kr content had decreased significantly. When the sample was .. . allowed to stand in the open air, the decrease was
FIG. 3. (A&The increase in the weight of ove.r-dried MS-3A during the time when it is allowed to stand in open air. Curve (B) is the decrease in the Kr content in MS-3A during the time when MS-3A encapsulating 17 wt% Kr was . allowed to stand in the same condition as (A).
1348 TAFJL~ l(a). Leakage
Sample number
of krypton from MS-3A/Kr(r.P) vacuum Elapsed days in vacuum
Kr content (mg:g MS-3A)
0 5 9 14
136.2 140.1 141.0 140.8
1 2 3 4
TAeLf:
l(b). Leakage
Sample number
of krypton from open air
Kr content (mg. g MS-3A)
0 5 8 13
82.7 57.8 70.6 47.5
NaX zeolite is comparable water
and
MS-3A,IKr(x.P)
Elapsed days in air
1’ 2 3’ 4’
of liquid
in a
to the heat
of electrolyte
in
of vaporization
solutions,
Further-
more. the electrical conductivity of hydrated NaX and NaA zeolites is also comparable to electrolyte solutions. Low-angle X-ray scattering in them also indicates cation disorder. All these facts support a view that at least some of the cations are hydrated and are mobile. This effect of cation hydration may be related to the rapid Kr release. When K+ ions at the %-sites are hydrated, the aperture at the eight-membered rings will become larger due to displacement of the cations. Its size could even become as large as 5 A when the cations are displaced a great enough distance. It then becomes very easy for Kr atoms to pass through the enlarged aperture and leak out of the cavities. Moreover. when water is absorbed. there is the heat of sorption which. given to Kr atoms in the same cavity. can stimulate the leakage. About 5 wt”,, of Kr can remain in zeolite for a very long time (Fig. 3). This fraction of Kr probably corresponds to Kr atoms in the /I-cage. Because the size of the [i-cage is small. the water and ions in it are not expected to resemble an aqueous solutions!” They are rather expected to be bound directly to framework of oxygen atoms. Thus Kr atoms in the j-cage can hardly come out of it. Even if the cation near the aperture of the /j-cage were to be displaced due to solvation. large
the
enough
sire
of the
to allow
rapid
aperture leakage
will
not
to 2 weeks, Kr can be encapsulated stably in MS-3A in a dry atmosphere. Because the accuracy of the Kr determination by activation analysis is no better than l’?;,. however, the present technique does not have the sensitivity to detect small amounts of leakage. A long-term Kr leakage from MS-3A in a dry atmosphere was also estimated using a mass-spectrometer and by heating the sample at a constant rate. The result was analyzed using Fick’s law of diffusion, and an activation energy for the diffusion of Kr was estimated. The results. which are to be published elsewhere.(h) predicted that there will be practically no long-term Kr leakage at room temperature but that at 150’C about 60”/(, of Kr will be released within IOyr.
Using the fact that Kr atoms in the x-cage are quickly released when water is absorbed by MS-3A. we have been able to prepare MS-3A which contain Kr only in the P-cage. We will write MS-3A/Kr(x.P) for the sample which has Kr in both r- and P-cages. and MS-3AjKr(P) for the sample which has Kr only in P-cage. During our experiments using a mass-spectrometer we noticed that a very small quantity of Kr was released before a substantial amount starts to be released. An example is shown in Fig. 4(A). where MS-3A/Kr(x,P) was heated at a rate 166Cjmin. A small peak near 1lO’C precedes the broad big component of Kr release. Followings are the facts concerning this small Kr release peak: (1) This peak could be observed for both MS-3A: Kr(x./j) and MS-3A,!Kr(P).
0
become
of Kr.
Thus the result in Fig. 3 indicates that Kr can be encapsulated in both T- and [I-cages and that Kr in the r-cage is very easily released when water is absorbed. A question then arises: whether Kr will not be released if water is not absorbed at all. In order to examine this point. MS-3A encapsulating 14wt”,, of Kr was allowed to stand in a vacuum for a certain number of days. and then the Kr content was measured by activation analysis. 1 he result is shown in Table 1. Clearly. for a short-term preservation of up
0
200
I 400
I 600
Temp,
I 800
I 1000
“C
FIG. 4. Kr gas leakage during heating up (A) MS-3A cncapsulating 17 wt”,, Kr and (B) MS-3A which was simply contacted to 1 atm Kr gas. The Kr release peak at lower temperature is due to Kr adsorbed on surface active sites (see the text). Note that its intensity is approximately the same for (A) and (B). but is very small compared to the main component of Kr release at hlghcr temperatures.
Kr twapsulotion
and adsorption
(2) This peak appeared only when the samples were allowed to stand for a certain period: more than a week when MS-3A/Kr(T.P) or MS-3A!‘Kr(P) was kept in a dry atmosphere, and less than several days when it was kept in open air. (3) This peak could be produced many times if the sample is kept for a sufficiently long time before it is heated once more. (4) Oven-dried MS-3A was brought into contact with 1 atm of Kr gas and then the system was evacuated. MS-3A should not encapsulate Kr. but when it was heated subsequently. the Kr release peak at 1lo’C was observed as is shown in Fig. 4(B). These facts indicate that the small peak corresponds to Kr desorption from surface active sites. Because the activation energy for the desorption is expected to be small. a few kcalimol, the adsorbed Kr atoms can be released at a lower temperature than that at which Kr leakage from the encapsulated states takes place. Points (2) and (3) indicate that Kr atoms encapsulated in r- and p-cages can eventually come out and then be adsorbed on the surface adsorption sites, the rate of the leakage depending on the temperature. An experiment similar to that described in point (4) was also carried out for MS-4A. Oven-dried MS-4A. after being contacted to 1 atm of Kr gas. was heated. and the Kr release was monitored using the peak detector of the mass-spectrometer. The Kr release peak at 1lO’C was observed as expected, and its intensity was much larger than that observed for MS-3A. Because Kr can enter x-cages of MS-4A at normal temperature and pressure, this result clearly indicates that the adsorption sites are found not only on the surface of crystallites of zeolites but also inside x-cages.
Conclusion The quantitative
non-destructive
analysis
of zeolite
I349
in rmlire
and Kr by neutron activation analysis has been shown to be a powerful method. especially when the existence of absorbed water makes it impossible to apply gravimetrical measurements. About 50 mg or less is enough to do the experiment. which is also an advantage compared to the gravimetrical way. Further studies of Kr encapsulation using this method are under way for other kinds of zeolites processed under various encapsulation conditions. The present results have shown that there are at least three sites for Kr in MS-3A, the stability of Kr increasing in the order of surface adsorption sites, x-cage and p-cage. The existence of absorbed water has been shown to markedly enhance Kr leakage from the x-cage. The explanation given to this phenomena will have to be examined by further studies. but will undoubtedly afford important material for the understanding of the molecular sieve effect.
A~knowlcdgrmcwts This work was supported financially in part by the Research Project of the Research Center for Nuclear Science and Technology. University of Tokyo. The use of JRR-2 and JRR-4 has been made possible by the grant of the National Universities’ Program for the Common Use of J.A.E.R.I. Facilities.
References 1. BENEDICT R. W., CHRISTENSENA. B.. DFL DEBBIO J. A.. KELLER J. H. and KNECHT D. A. ENICO-1011, Sep-
tember 1979. 2. PFNZHORN R. D.. SCHCLST~R P.. NOPP~L H. E. and HELLWIG L. M. IAEA-SM-245/10, February 1980. 3. BRICK D. W. J. C’hrn~. E&c. 41, 678 (1964). 4. RIXD T. B. and BREAK D. W. J. Am. Chm. SM. 78, 5972 (I 956). 5. SHtRRY H. S. In Ion Exchanyc, (Ed. MARINSKY) Vol. 2. Chapter 3. (Marcel Dekkcr. New York. 1969). 6. MATX OKA c’t (11.(unpublished data).