W. HAUBACH, C. KNOBLER, A. KATORSKI, AND D. WHITE
1398
The Low-Temperature Chromatographic Separation of the Isotopic Hydrogens at 27 and 55°K'
by W. J. Haubach, C. M. Knobler, A. Katorski, and David White Cryogenic Laboratory, Department of Chemistry, The Ohw State University, Columbus, Ohw (Received October $0,1966)
The chromatographic separation of the molecular species, Hz, Dz, and HT, as well as the nuclear spin species, ortho-parahydrogen and deuterium, have been investigated at 27 and 55OK. At these low temperatures, it is found that the retention times on y-alumina are strongly dependent on the volume of the sample introduced into the column. In the limit of low surface coverage, it is found that the separation factors deduced from the retention times are in fair agreement with the theory of Katorski, White, and Lassettre.2 Furthermore, rn predicted by the theory, the retention time for the heteronuclear HT is smaller than that of the homonuclear D2 of the same mass.
Introduction A theory for the separation of the isotopic hydrogens by preferential adsorption a t low temperatures has recently been developed by Katorski, White, and Lassettre.2 This theory is based on an assumed interaction between the diatomic molecule and the adsorbent, the latter being regarded as a homogeneous plane surface. The centers of interaction are a t the component atoms of the molecule and the total interaction is given by the sum of the atom-centered potentials, f(zt), where zt is the perpendicular distance of the ith atom from the surface. If z is the perpendicular distance from the center of mass of the molecule to the surface (Figure 1) and 8 the angle between z and the internuclear axis, the potential is given by V ( Z ,COS e)
=
+ f(z2) = f(z + bl e) + f(z - bz
f(zl)
COS
COS
e) (1)
where bl and b2 are measured from the center of mass to the atom centers as shown in Figure 1. A Morse potential was chosen for f ( z ) f(z) = D/2(e-2"2- 2e-"')
(2) where D and "d' are the parametric constants. It is clear from eq 1 and 2 that the rotational coordinate, 8, is coupled to the center of mass vibrational coordinate, z, in a manner dependent upon the position of the center of mass. Although f(zt) and bl bz = b, the
+
The Journal of PhysicaZ Chemhtry
internuclear distance is, to a good approximation, the same for the isotopic hydrogens, the response of the heteronuclears (HD, DT, HT) to the surface field is quite different from that of the homonuclears (H2, Dz, Tz). An interesting prediction of the theory is that a homonuclear diatomic molecule is more strongly bound to the surface than a heteronuclear isotopic molecule of the same mass. Such an effect has been observed in the case of liquid vapor pres~ures.~ The technique of gas chromatography may be used to measure relative retention times of isotopic pairs on a given adsorbent providing separation factors4 which can be used to test the theory.2 There have been several investigations of the chromatographic separaThe extensive work tion of the isotopic (1) This work was supported in part by the Division of Research, Chemistry Branch, Atomic Energy Commission, Washington, D. C., and Mound Laboratory, Miamisburg, Ohio. (2) (a) A. Katorski and D . White, J. Chem. Phys., 40,3183 (1964); (b) D. White and E. N. Lassettre, ibid., 32, 72 (1960). (3) H. Friedmann, Advan. Chem. Phys., 32, 72 (1960). (4) V. J. Coates, H. J. Noebels, and I. S. Fagerson, "Gas Chromatography," Academic Press Inc., New York, N. Y., 1958, p 315. (5) H.A. Smith, et d., Progress Report, Atomic Energy Commission Contract AT-(40-1)-1069, Sept 25, 1958; Sept 25, 1959; Sept 25, 1960; July 16, 1961; July 15, 1962; July 15, 1963; July 15, 1964; and two doctoral dissertations, P. P. Hunt, 1960, and E. H.Carter, Jr., 1961. (6) W. R. Moore and H. R. Ward, J. Phys. Chem., 64, 832 (1960). (7) 9. Furuyama and T. Kwan, ibid., 65. 190 (1961). (8) P. L. Gant and K. Yang, Science, 129, 1548 (1959).
LOW-TEMPERATURE CHROMATOGRAPHIC SEPARATION OF ISOTOPIC HYDROGENS
I
Figure 1.
i
of Smith and collaborators6 has been primarily directed to the development of an analytical separation method with columns operating in the vicinity of the boiling point of liquid nitrogen. Except for the investigation by Mohnke and SafTert,I1there is little information concerning the temperature dependence of the chromatographic separation factors. Even in this case, the work is limited to the liquid nitrogen range. To apply the theory, two parameters, D and “a” [see eq 1 and 21, must be fitted for any isotopic series. Although in principle these can be established from two separation factors at a given temperature, it is of considerable importance to determine whether these parameters give the proper temperature dependence since they also fix the value of the heats of adsorption2 for the isotopic series. In this paper, chromatographic separation factors are reported at two widely separated low temperatures, 27 and 55°K. Since the separation factors, like vapor pressures, exhibit an exponential 1/T dependence on temperature, these experiments, unlike the earlier investigations, permit a more reliable determination of the temperature dependence. On the other hand it will be seen that practical considerations in the extension of chromatographic separations to such low temperatures introduce a new complication, namely, the strong dependence of retention times on sample volumes.
Experimental Section The chromatographic column used in these experiments consisted of a copper tube of 0.25-in. 0.d. and 6 in. in length filled with 1.84 g of y-alumina free of paramagnetic impurities, the same material used by Cunningham, Chapin, and JohnstonI2 in their ortho-para separations. The y-alumina was crushed and sieved to 20 mesh. Preliminary experiments a t the two widely separated temperatures, 27 and 55”K, indicated that in order to keep the retention times of both hydrogen and
1399
to deactivate the alumina adsorbent in the short column. This was achieved by preadsorbing of a monolayer of xenon on the y-alumina prior to the chromatographic experiments. The selection of of a monolayer was a compromise between the lengthy retention time on the pure y-alumina and a complete desensitization of the column with a monolayer of xenon. The column was placed in a vacuum jacket which in turn was immersed in a dewar containing either liquid neon or liquid nitrogen. During the chromatographic separation runs, the vacuum jacket was filled with gaseous helium for thermal contact between the bath and column. In order to preadsorb xenon on the yalumina in a reproducible manner, xenon was admitted to the chromatographic column at room temperature, the vacuum jacket was evacuated, and liquid nitrogen was then placed in the dewar so that about three-fourths of the vacuum jacket was covered. In this manner the column cooled very slowly by radiation over a period of hours allowing the xenon to achieve an equilibrium distribution on the adsorbent. The temperature of the column was measured by means of a calibratedlacopper-constantan thermocouple soldered midway between the ends of the column on the outer wall. The temperature of the column, as measured by this thermocouple, was maintained constant of h0.2’ by controlling the vapor pressure of the refrigerant bath. Neon gas at a flow rate of 54 cc/min was used as a carrier in all of the experiments. The neon was precooled to the column temperature by passage through a long copper coil immersed in the same refrigerant bath as the column. A thermomister model thermal conductivity cell was employed as a detector for the eluted nonradioactive hydrogens and an ionization chamber was used to detect radioactive HT.
Experimental Results and Discussion The initial separations were performed using normal hydrogen (7575 ortho, 25% para) in order to determine the magnitude of the ortho-para separation. As in the previous investigation^,^^ it was found that the retention time of orthohydrogen by the column was (9) W. A. Van Hook and P. H. Emmett, J . Phy8. Chem., 64, 673 (1960). (10) F. Botter, G . de la Perriere, and 9. Tistchenko, C.E.A. No. 1962, Centre d’Etudes Nucleaires de Saclay, 1961. (11) M. Mohnke and W. Baffert, “Gas Chromatography,” M. van Swaay, Ed., Butterworth and Co. Ltd., London, 1962, p 216. (12) C. M. Cunningham, D. Chapin, and H. L. Johnston, J . Am. Chem. SOC.,8 0 , 2382 (1968). (13) T. Rubin, H. I,. Johnston, and H. Altman, ibid., 73, 3401 (1951).
Volume 71, Number 6 AprQ 1967
1400
W. HAUBACH, C. KNOBLER, A. KATORSKI, AND D. WHITE
greater than that of parahydrogen, the ratio increasing with decreasing temperature. The relation‘ between separation factors and retention times is tl - to
= -
&-2
&-to
where SI-^ is the separation factor for species 1 and 2, and tz are the individual retention times, and to is the time for passage of a nonadsorbed marker gas through the column. Owing to the low temperatures employed in the present work, the only practical marker gas was helium. Since with the &in. column, helium almost instantaneously emerged from the column, to in these experiments was taken as zero. The separation factor is therefore given by
=
tl
Sl-2
=
(3) cannot be specified unless the sample volume is also known. This effect, as will be seen, is a t least partially attributable to surface heterogeneity and becomes marked only a t low temperatures and small sample volumes. The reason for this is that at higher temperatures, where the variations in surface energy become comparable to kT,there is a tendency for the adsorbed molecules to distribute more or less uniformly among all the different sites. This will also be the case for large sample volumes. Even under these conditions, the surface heterogeneity is reflected to some extent in the shape of the chromatogram. The location of the peak maxima should, however, be relatively insensitive to sample volume. The dependency of the retention time on the sample volume is also a result of the short column length in these experiments. The height of an equivalent theoretical plate (HETP)lbb is a function of the throughput in a chromatographic column just as in a distillation column. With a constant flow of carrier gas, the sample volume is related to the total throughput. In essence, when a large sample is swept through the column so rapidly, the separations are quite small. = to--HJtP--H,
The Journal of Phyaieal Chemistry
0.2
0.4 0.6 08 1.0 S a m p l e Volume ( c
1.2
d
1.4
1.6
s.T.P.)
Figure 2.
ut2
Figure 2 shows the effect of the adsorbed xenon on the retention times of para- and orthohydrogen a t 55°K. The abscissa of Figure 2 is the STP volume of the individual nuclear spin species, assuming no spin conversion. That there was no conversion was evident from the chromatograms as the ratio of the areas under the ortho and para peaks was 3 :1. It is clear from the experimental results on y-alumina shown in Figure 2 that ortho-para separation factors, SO-~a,16a calculated from the ratio of retention times SO-Hn
0
Y-
0
+
a l u m i n a 3.8 monolayer xenon
1.0 2.0 3.0 4.0 5.0 6.0 70 Volume Adsorbed (cm3 om-’@ S I R )
Figure 3.
In Figure 3 (upper curve) the differential heats of adsorption of parahydrogen on y-alumina as a function of surface coverage a t an average temperature of 65°K are shown. These were determined from vapor pressure experiments in the temperature range 50-800K using a calorimeter similar to that described by Johnston, Clarke, Rifkin, and Kerr.16 It can be seen from Figure 3 that the differential heat of adsorption shows a marked dependence on surface coverage, rising sharply a t low coverages like the retention times of Figure 2. This rise is related to the energy distribution of adsorption sites on the surface.l7 It has been shown by Halseyl8 that preadsorption of an inert gas tends to homogenize the surface. That (15) (a) The separation factors given in the text are always relative to parahydrogen; (b) “Physical and Chemical Methods of Separation,” McGraw-Hill Book Co., Inc., New York, N. Y., 1963, p 2. (16) (a) H. L. Johnston, J. T. Clarke, E. B. Rifkin, and E. C. Kerr, J . A m . Chem. Soc., 7 2 , 3933 (1950); (b) these experiments will be described in more detail in a subsequent paper where the thermodynamic properties of adsorbed hydrogen are presented. (17) L. E. Drain and J. A. Morrison, Trans. Faraday &c., 48, 316
.
(1952) (18) G. D. Halsey, Jr., J . Chem. Phys., 2 2 , 979 (1954).
LOW-TEMPERATURE CHROMATOGRAPHIC SEPARATION OF ISOTOPIC HYDROGENS
this is so in the case of y-alumina is shown by the lower curve in Figure 3 where the differential heats of adsorption of parahydrogen at an average temperature of 65°K are given for y-alumina on which " 8 of a monolayer of xenon was slowly preadsorbed. The fraction of the surface covered by xenon was calculated from the volume introduced to the adsorbent assuming the surface area of y-alumina is that given by Cunningham and Johnston19 and the area, u, of a xenon atom adsorbed on the surface is given by the relation20 u =
3.464
x
/
10'6
nx
" .01
0.1 0.5 1.0 Somple Volume( cm3 6, S.T.P.)
.05
50 I 3
\$/a
Figure 4.
(4GNp.)
where M is the atomic weight of xenon, ps its solid density, and N Avogadro's number. The units of u are square angstroms. Further addition of xenon to the surface does not significantly diminish the dependence of the differential heat of adsorption on surface coverage, although it does markedly decrease the activity of the surface to the point where the chromatographic ortho-para separation of hydrogen becomes vanishingly small at all temperatures. The influence of 3/8 of a monolayer of xenon on the retention times of ortho-parahydrogen can be seen in Figure 2. The magnitude of the retention times for ortho- and parahydrogen at 55°K have been considerably reduced; however, their variation with sample volume, although considerable, is no longer as pronounced as in the case of bare y-alumina. The retention times for various isotopic hydrogens with the column containing y-alumina on which of a monolayer of xenon has been preadsorbed is shown in Figure 4. Because of the low surface activity of the short column, it was not possible to resolve the shoulder on the high side of the ortho-D2peak owing to para-D2 at 55°K. The retention times, t , shown in Figure 4 are best represented by the empirical relation
utvni (5) where ai and nf are constants for the ith isotope or ortho-para species and V , the sample volume. The scatter of the data from the least-squares straight line is of the order of i10%. This is due to several factors, the most important being the difficulty in reproducing the surface consisting of of a monolayer of xenon. Less important, but not negligible, are uncertainties in the sample volumes and the temperature of the column. The simultaneous retention times for the ortho-para species of hydrogen and deuterium at 27°K were measured only for y-alumina on which of a monolayer of xenon had been preadsorbed. The retention times, at this temperature, vary from a few minutes in the ti =
1401
case of parahydrogen to several hours for paradeuterium. Since there is a considerable increase in the retention times when the xenon is removed, such experiments even with this short column are no longer practical. The data at 27°K show the same dependence on sample volume as that obtained a t the higher temperature. The scatter about the least-square line representing the logarithmic form of eq 5 is, however, somewhat larger at 27"K, h15%, this being primarily a result of the increased sensitivity of the retention time to small variations in temperature. The data a t 27 and 55°K are summarized in Table I where the separation factors, Si, for the ith species relative to parahydrogen are given as a function of sample volume. SO-H2 represents the ortho-para separation factor of hydrogen and SP-OD,= SP-D,/ So-D2 is the para-ortho separation factor of deuterium. Except in the case of HT, the separation factors were obtained using the relation
s,=----- ---V ti
'i
1p-H~
UP-Ha
n , - np- H a
(6)
the constants a, and n, being obtained from a leastsquare fit of the data. For HT, the assumed dependence of the retention time on sample volume is given by the broken line in Figure 4. A comparison of the experimental results with the theoretical calculations of Katorski and White2* is shown in Table 11. The comparison has been made a t a sample volume V = 0.04 cc at STP since at this value experimental separation factors for the three isotopes are available. As mentioned earlier, the theory requires a knowledge of two parameters, D, which determines the depth of the potential of interaction of the diatomic molecule with the surface and Y, (19) C. M. Cunningham and H. L. Johnston, J . Am. Chem. Soc., 80, 2377 (1958). (20) P. H.Emmett and 9. Brunauer, ibid., 59, 1553 (1937).
Volume 71, Number 6 April 1067
W. HAUBACH, C. KNOBLER, A.
1402
KATORSKI, AND
D. WHITE
Table I: Separation Factors Relative to Parahydrogen
- --r 27OK
Sample vol., STP, cm’
0.03 0.04 0.05 0.06
0.08 0.10 0.20 0.40 0.6C 0.80 1.00 1.50
’
SO-Hz
SO- Dz
SP- Dt
4.7 4.6 4.6 4.6 4.5 4.5 4.4 4.3 4.3 4.3 4.2 4.2
43.7 38.6 35.2 32.4 28.8 25.8 19.1 14.4 12.0 10.7 9.6 8.7
... ...
SP- Dr -
-
55’K-
SO-DI
... ...
...
...
...
... ...
45.9 32.1 21.2 15.2
1.8 1.7 1.5 1.3
... ...
...
...
...
...
...
SO- Ht
SHT
SO-Dt
1.9 1.9 1.8 1.8 1.8 1.8 1.7 1.7 1.6 1.6 1.6 1.6
2.9 2.8 2.7 2.6
6.4 5.9 5.6 5.3 4.9 4.6 3.7 3.1 2.7 2.5 2.3 2.1
... ... ...
... ...
... ... ...
Table 11: Comparison of The& and Experiment (Experimental Separation Factors Are Taken a t Sample Volume V = 0.04 cc) Temp
r
2 7 O K - - - - - - - - - - - - - - . SP- D: rc 55OK SO- Da So- D, SO- H r SHT
-
D,
a
Y
kcal/mole
1.50 1.80 2.10 2.40 2.70
3.28 2.56 2.14 1.87 1.74
--EP-Hs, kcal/mole
2.40 1.64 1.18 0.91 0.70
SO- HZ Exptl Theoret
4.6 4.6 4.6 4.6 4.6
4.6 4.6 4.6 4.6 4.6
Exptl
Theoret
38.6 38.6 38.6 38.6 38.6
37.8 36.8 39.0 44.0 43.0
Exptl
2.1” 2.10 2.1” 2.1” 2.1“
Theoret
2.3 2.4 2.4 2.5 2.5
Exptl
1.9 1.9 1.9 1.9 1.9
Theoret
1.8 1.8 1.8 1.8 1.9
Exptl
2.8 2.8 2.8 2.8 2.8
Theoret
3.0 3.1 3.4 3.6 3.7
SO- DI Exptl
5.9 5.9 5.9 5.9 5.9
Theopt
4.9 5.0 5.2 5.4 5.4
Extrapolated from the data in Table I.
the shape of the potential.21 The choice of D and Y not only determines the magnitude of the separation factor at any given temperature but also determines a quantity, -Ei, which represents the heat of adsorption of the ith species. To a good approximation Ei = AHi, the differential heat of adsorption. In making the comparison between theory and experiment several values of Y and D were chosen which would best fit the ortho-para separation factor of hydrogen at 27°K) while a t the same time giving a range of -Ep-Ha values above and below the differential heat of adsorption of parahydrogen at 0.02 STP cc/g (e0.04 cc/ 1.84 g of y-alumina in the column) extrapolated from the experimental data. This is estimated to be 1.35 kcal/mole. If the theory has any merit, it should predict from a set of Y , D’s which reproduce the results for one isotope and the separation factors for all other isotopes as well as their temperature dependence, the potential of interaction being, to a good approximation, the same for all isotopes. It can be seen from
The Journal of Phyeical Chemistry
Table I1 that there is fairly good agreement between theory and experiment for the entire range of Y’s and D’s chosen for the comparison. Although for values of Y = 1.8-2.1 and D = 2.56-2.14 kcal/mole, ~ to the experimental heat of a value of - E P - ~closest adsorption is obtained, the large precision errors of the separation factors, which are of the order of f10% do not allow the unique choice. They do, however, establish the applicable range of Y’s and D’s for the isotopic hydrogens on y-alumina on which xenon has been preadsorbed. It is of interest t o note that the magnitude of the experimental separation factor for HT at 55°K is considerably less than that of ortho-D2 of the same mass, a result predicted by the theory using potential parameters deduced from the ortho-para separation factor of hydrogen at 27OK.
(21) The Y in the theory of Katorski and White- is equal to a b where “a” and “b” are defined as in the Introduction and Figure 1.
