CONDENSED-PHASE BEHAVIOR OF ALUMINUM CHLORIDE-ZIRCONIUM CHLORIDE
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Condensed-Phase Behavior of the Aluminum Chloride-Zirconium Chloride System192
by A. J. Shor, William T. Smith, Jr., and M. A. Bredig Reactor Chemistry and Chemistry Divisions, Oak Ridge National Laboratory, Oak Ridoe, Tennessee, and the Department of Chemistry, University of Tennessee, Knoxville, Tennessee (Received October $8,1965)
According to thermal, differential thermal, and X-ray analysis, Al2Cls and ZrCll form a simple eutectic system, without solid intermediary compounds but possibly with some limited solid solution. The composition of the eutectic at 165” is approximately 75 mole % AlzC16. In disagreement with earlier literature data, visual observation above the liquidus showed the presence of only one liquid phase. Semiquantitative interpretation, with the use of the heats of fusion of the components, of the liquidus in terms of activities suggests that these largely molecular chlorides interact in the liquid to form mixtures of essentially un-ionized complex molecules ZrClr. (AlC13)n,n = 1, 2, 3, and 4, with higher values of n possible at high aluminum chloride concentration.
Except for a cursory effort by Korshunov, Reznik, and M o r o ~ o v few , ~ data are given in the literature on the solid-liquid phase behavior of the aluminum chloride-zirconium chloride system. Information on such salt systems is especially significant as a result of increased interest in reprocessing of nuclear fuel elements containing aluminum or zirconium by methods dependent on the high relative volatility of their chloride salts. Experimental difficulties arising from high vapor pressures developed at moderate temperatures has limited the study of these systems in the past. This paper reports the solid-liquid phase diagram for the aluminum chloride-zirconium chloride system. Activities of ZrClr and AlzCls in the melts are estimated and implications in relation to ZrClrA1C13 complexing in liquid solution are discussed. Discrepancies in the phase diagram between this study and the results of previous investigators are ascribed principally to the disturbing effects of the extensive supercooling observed throughout the range of composition.
Experimental Section Chemicals. Aluminum chloride was prepared from the elements, using high purity (99.99%) aluminum metal and carefully dried chlorine gas. Zirconium chloride (U. S. Industrial Chemicals Co., Reactor Grade) was purified by repeated sublimations. Melting points and freezing points obtained by thermal analy-
sis of the pure materials were 192 f 1 and 437 f 1” compared to published values of 192.6 and 438” for aluminum chloride and zirconium chloride, respect i ~ e l y . ~The ? ~ purity of the salts was also confirmed by chemical and spectrochemical analyses. The salt mixtures were prepared in a helium atmosphere drybox where the moisture level was maintained at or below 30 ppm by volume. Apparatus and Procedures. Phase reaction temperatures were determined principally by differential thermal analysis. These observations were supplemented and extended by thermal analysis and visual observations of phase changes. Interaction with atmospheric moisture and oxygen and loss of salt through volatilization were prevented by sealing the mixtures in evacuated quartz tubes or nickel capsules. (1) Research sponsored by the U. S. Atomic EnergV Commission under contract with the Union Carbide Corporation. (2) In part a revision of a thesis by A. J. S., submitted in partial fulfillment of the requirements for the degree of Master of Science, University of Tennessee, Dec 19M. (3) B. G. Korshunov, A. M. Reznik, and I. S. Morozov, T r . Moak.
Inat. Tonkoi Khim. Tekhol., ’7, 127 (1958). (4) R. E. Kirk and D. F. Othmer, “Encyclopedia of Chemicd Technology,” Vol. I, The Interscience Encyclopedia, Inc., New York, N. Y., 1952. ( 5 ) “Gmelins Handbuch der Anorganischen Chemie,” 8th ed, Zirconium System No. 42, Verlag Chemie, Gmbh., Weinheim, 1958, p 285.
Volume 70, Number 6
Mav 1966
1512
The capsules were provided with thermocouple wells. Calibrated stainless steel sheathed chromel-alumel thermocouples were assembled in a diff erential-temperature network using A1203as a reference substance. A Rubicon Type B high-precision potentiometer was employed for instrumental calibration and in direct thermal analysis measurements. High purity lead obtained from the National Bureau of Standards as well as reagent grade LiCl and NaCl was used for temperature standardization and to aid in interpretation of differential thermal analysis curves. Visual observations were made by immersing quartz capsules in a well-stirred bath of LiNOs-NaNO3-KNOt eutectic mixture which was heated and cooled a t a rate of about l”/min through the temperature range oi interest. Measurements and Accuracy. Mainly heating curves were used for the estimation of both eutectic and liquidus transition temperatures. Since very marked supercooling was observed, particularly for the eutectic and the aluminum chloride liquidus transitions, heating curves were exclusively used for the aluminum-rich region. Segregation of the components on freezing led to spurious points on the subsequent heating curves. These extraneous points were eliminated by rapid cooling of the melts before the heating runs. The observations of liquidus and eutectic temperatures are considered reliable within f3”.
Results and Discussion Both melting and freezing point observations for a series of aluminum chloride-zirconium chloride mixtures are listed in Table I, and the phase diagram derived therefrom is shown in Figure l. Data are plotted as temperature in degrees Centigrade us. mole per cent AlzC16, aluminum chloride being known6-’ to exist as the dimer both in the liquid and gaseous state. In disagreement with the phase diagram reported by Korshunov, et u E . , ~ and with a very similar diagram reported more recently for HfC14-A1C13 by Morozov, Tverskov, and Kurapova,8 visual observation failed to give any evidence for the existence of equilibria between two liquid phases. The data from thermal analysis also are representative of a simple phase diagram, showing a eutectic at 165” and 75 mole % Al&16. In order to judge the reliability of visual observation for finding two liquids in equilibrium with each other, it is to be pointed out that, in the earlier work on aluminum halide systems with various second component^,^-^^ visual observation did furnish support for the existence of a miscibility gap derived from thermal datla. X-Ray diffraction measurements both a t room and The Journal of Physical Chemistry
A. J. SHOR, W. T. SMITH, JR.,AND M. A. BREDIG
450 I
90
80
I
I
-
MOLE PERCENT ZrCI,
70
60
50
40
30
I
I
I
I
I
20
40
I
\ ZrCI,
MOLE PERCENT Al2CI6
-
A12CI6
Figure 1. Binary condensed-phase diagram for the zirconium tetrachloride-aluminum chloride syst,em: 0, behavior on heating; m, behavior on cooling; and A, data of Korshunov, et aL3
elevated temperatures gave no evidence of the formation of solid intermediary compounds, such as reported for other systems involving aluminum ~ h l o r i d e . ~ - ~ ~ However, especially for the aluminum-rich mixtures, formation of solid solutions was not excluded and may be suggested by the thermal data discussed below. The most probable form of such solutions, a simple random substitution, to a limited extent (Figure l), of 3Zr4+ 1 vacancy for 4A13+ in the (ionic12) lattice of solid aluminum chloride, AIC&(c),would have hardly been detectable by X-rays. As mentioned above, the significance of the thermal analysis data for the aluminum chloride liquidus is greatly affected by the fact that, because of the large
+
(6) W. HUckel, “Structural Chemistry of Inorganic Compounds,” Elsevier Publishing Co., Amsterdam, 1950,p 1680. (7) R. L. Harris, R. E. Wood, and H. L. Ritter, J. Am. Chem. SOC., 73, 3151 (1951). (8) I. 9. Morozov, V. A. Tverskov, and G. I. Kurapova, Russ. J . Inorg. Chem., 9 (9), 1184 (1964). The diagram showing a large miscibility gap must be wrong if for no other reason than that the melting point depression of AlaCle by HfClr shown is 70 times larger than is compatible with the known heat of fusion of AlaCls. (9) J. Kendall, E. D. Crittenden, and H. K. Miller, J. Am. Chem. SOC.,45, 963 (1923). The existence of compounds with equivalent ratios other than 1 : 1, proposed by these authors, seems poorly supported. In particular, the “A*B,” compounds are in disagreement with the phase rule. (10) I. N. Belyaev, Rusa. Chem. Rev., 29, 428 (1960). See remarks in the preceding footnote. (11) H. Houtgraaf and A. M. DeRoos, Rec. Trau. Chim., 12, 963 (1953). (12) W. Blitz and A. Voigt, 2.Anorg. Allgem. Chem., 126,39 (1923).
CONDENSED-PHASE BEHAVIOR OF ALUMINUM CHLORIDE-ZIRCONIUM CHLORIDE
supercooling, hcating data had to be used with their inherent faults, which are related to the slow establishment of equilibrium. The thermal effects which were entered in the diagram of Figure 1 as being due to the AlzC16liquidus might seem to lend themselves to the alternate interpretation as a monotectic horizontal, i.e., two immiscible liquids in equilibrium with solid aluminum chloride as reported for various other systems involving AlzCls.9-11 However, the recorder tracings from both the thermal and the differential thermal analysis runs (heating), especially around 90 mole % AlzC16,i e . , in the middle of the range between pure AlzC16and the eutectic, did not show the simultaneous presence of two invariant temperature points, namely, a monotectic and a eutectic. That this was not due to poor resolution is proven by the fact that in heating runs Ivith samples containing less than 90 mole % A12C16 that had been cooled slowly, two well-resolved peaks did appear. As the peak at the higher temperature was not present in the heating runs with previously rapidly cooled samples, it could, of course, not be interpreted as a monotectic reaction. Instead, it was attributed to the melting of more or less pure solid aluminum chloride formed as a result of segregation on slow freezing. For compositions higher in AlzCle than 90 mole yo, the temperatures at which melting made itself felt in both the differential and direct thermal analysis runs (heating) were increasingly higher than the eutectic temperature (Figure 1). This is taken to reflect one or the other of two possible situations: one, in the absence of solid solution, the amount of eutectic melt became gradually smaller with higher A12C16 concentration, and its thermal effect was increasingly overshadowed by the liquidus effect; or, alternately, and perhaps more plausibly, the rising temperatures may represent the aluminum chloride solidus, L e . , the zirconium chloride solution in solid aluminum chloride as mentioned above. The AlzC16liquidus data given represent the temperatures at which the difference between the sample and the reference thermocouple, that first arose on heating through the eutectic (or the solidus) temperature, began to diminish. This peak in the differential temperature was then interpreted as the end of the melting process, namely, the crossing of the liquidus. The finding of complete miscibility in the liquid state of this system by both visual observation and thermal analysis is further supported by the considerations that follow. The existence of liquid-miscibility gaps has been reported for a number of binary systems involving aluminum chloride or bromide, particularly at high aluminum halide concentration^.^^^^ These cases are all characterized by the second component
1513
being a typical salt, an electrolyte such as an alkali metal halide, MX, which forms a stable solid complex salt M(AlX4) with the aluminum halide. The apparent largely nonionic, molecular character of ZrC14, somewhat similar to AlzC16 in its relatively high volatility (sub1 pt 331°, as compared with the boiling point of 1400" for KC1) is consistent with the failure to observe coexistence of two liquids. The molecular nature of ZrC14 prevents the formation of a saltlike solid compound, e.g., Zr(A1C1S4, that would correspond to the existing solid salt KA1C14. The great similarity of the Zr4+ and A13+ ions, both with noble gas electron shells, in the ratio of charge to size., i.e., in polarizing power, produces a near balance in their competition for chloride ions. With the very dissimilar pair K+-A13+ this balance is very heavily in favor of A13+, enabling complex salt formation. Also, if indeed a salt-like solid, e.g., of the composition Zr(A1C14)4,would form, it would be expected to be stable enough thermally to be observed in equilibrium with liquid. No such solid compound was found, yet in the cases in which aluminum chloride did form a liquid phase in equilibrium with a liquid phase rich in the other component (e.g., KC19 and even nitrosyl chloride"), solid salt-like intermediate compounds such as K(AlC14) and NO(AlC14) did make their appearance. Their solubility, as well as that of liquids of similar composition, in the nonpolar liquid AlzC16 was always very small. The interpretation of the liquidus data with the use of the heats of fusion AHm (assumed to be temperature independent), namely, 17,000 and 9000 cal mole-' for AlzCle13and ZrC14,14at the melting points To = 466 and 709"K, respectively, in terms of the activities of the two components suggests considerable interaction in the liquid phase. These activities, a, were computed from the liquidus temperatures, T , at various compositions by means of the approximate equation
The experimental activities obtained in this manner are compared in Figure 2 with activities calculated for various solution models. The limiting slope, near a = 1, for a(ZrC14) is twice unity, Le., corresponds to the monomeric dissolution, without ionization, of ZrCll and AlCl, in the nonpolar solvent ZrC14. Because of the experimental difficulties discussed above, the ~~~~~
(13) National Bureau of Standards Circular 500, U. S. Government Printing Office, Waahington, D. C., 1952,p 731. (14) A. A. Palko, A. D. Ryon, and D. W. Kuhn,J. Phys. Chem., 62, 319 (1958).
Volume 70, Number 6 May 1966
A. J. SHOR,W. T. SMITH, JR.,AND ,If. A. BREDIG
1514
-
MOLE PERCENT ZrCI4
Table I: Liquidus and Eutectic Temperatures and Zirconium Tetrachloride Activity in the Zirconium Tetrachloride-Aluminum Chloride System
Compn, mole % AlCls AlpCls
ZrC1,
MOLE PERCENT AI2C16
--
Al2CI6
Figure 2. Comparison of experimental activities of zirconium tetrachloride and aluminum chloride with those of various models (A12C16 as 2AlC13, or as ZrC14.(A1C13),,n = 1, 2, 3, and 4, or even higher). The dotted line for a(A12Cl8) corresponds to the dotted AlzC16 liquidus of Figure 1.
0 10 20 25 33 50 60 67 75 80 86 90 93 95 96 97 98 99 100 a
limiting slope for u(AlzC16) is not established. However, with increasing ZrCL concentration, a very strong negative deviation from Raoult's law for ideal mixing of Al2C16and ZrCl4 occurs in a(Al&&) such that at the eutectic, 75 mole % A12Cls, as derived mainly from the intersection of the ZrC14 liquidus with the eutectic horizontal, this activity assumes a rather low value (a = 0.3, y = 0.4; limited solid solubility would lower these figures slightly). On the other hand, the activity of ZrCL, on addition of A12C16all the way up to 45 mole 75 Al2Cl6, decreases at what appears to be the rate expected for ideal mixing of monomeric AlC1, with the ZrC14 (dotted line, Figure 2). Above 45 mole %, it decreases much more rapidly and above 70 mole % appears to approach zero activity asymptotically. We shall see that the simple interpretation of a(ZrC14) below 45 mole % A12C16in terms of the monomeric species cannot be correct. Instead, we wish to suggest an interpretation in terms of the formation, in the liquid phase, of a series of complexes between the two chlorides, namely, ZrCl4.nA1Cl8,n = 1, 2, 3, and 4, with the--possibly terminal-formation, near 67 mole % Al&le, of the compound ZrC14.4A1C13,or Zr(A1CW4. A slightly modified formula, Zr(CIA1CIJ4, might be used to express the plausible assumption of a highly symmetrical molecule in which both aluminum and zirconium occupy the centers of altogether five interconnected chlorine tetrahedra in such a way The Journal of Physical Chemistry
0 5.3 11.1 14.3 19.8 33.3 42.8 50.4 60.0 66.7 75.4 81.8 87.0 90.5 92.3 94.2 96.1 98.0 100.0
-Transition On heating Eut Liq
. . . 437 167 425 159 419 160 . . . 163 399 162 369 166 350 168 320 164 . . , 167 . . . 163 . . . 170
...
170 172 171 178 183 185
177 188
...
...
188 189 189 193
temp, "C-On cooling Liq Eut
435 420 417
,
..
150
144 139 396 144 366 143 . . . 154 317 150 262 135 234 130 125 120 125 127 132 135 160 162 183 . . .
...
Activ-
ities,"
.4ctivity coefficient,
a(ZrCl4)
r(ZrC4)
1.00 0.89 0.84
...
1.00 0.94 0.94 ...
0.70
0.87
0.51 0.42 0.28 0.13 0.08 0.02
0.77 0.73 0.56 0.33 0.24 0.08
...
.. ...
...
...
...
... ... ...
... 0
See text for method of calculation.
that each of the four filc14 tetrahedra which surround the zirconium, again in tetrahedral arrangement, shares one of its corners with the central, ZrC14, tetrahedron. However, the suspicion seems justified that with the first additions of ZrCl4 to A12C16even larger complexes, with additional AlCh (n = 5 , 6, 7, and 8) might form. Such larger clusters may well be r e sponsible for the large increase in supercooling (Figure 1). This probably is also associated with an increased have been reportedg). viscosity. (Salts hlz+(L41~C1~),The activity of ZrCi4does not follow the curve for n = 4, nor, for that matter, any one of those for n = 1, 2, or 3, but crosses them. This might be expected, as the competition between the many ZrC14 molecules for the few AlCll groups available at low ~412C16concentration must lead to the more or less simultaneous formation of the various lower complexes, ZrCl(4--n) (ClAlCla),, beginning with a preponderance of n = 1 and continuing with rising n toward the formation, near the composition of 67 mole yoAlzC16,of Zr(C1A1CLJ4. The apparent agreement, then, at concentrations up to 45 mole % A12C16,mentioned above, of the experimental a(ZrC14) with the curve calculated for an ideal solution of independent monomeric AlC13 molecules in ZrCll (dotted line in Figure 2) must be considered
GAS-LIQUIDPARTITION CHROMATOGRAPHY OF PERDEUTERIOETHANE
purely (if strangely) coincidental: if A1C18 and ZrC14 were the species present, the experimental activity for aluminum chloride would have had to follow the line marked “a(Al~Cl6)for AlzC16as 2AlC13” all across the diagram. Actually this activity, mainly by the position of the eutectic, is shown to be much smaller at all compositions below 67 mole % A12C16. The presence of monomeric AICla is also unlikely on theoretical grounds, as with only three ligands it would represent a coordinatively most unsaturated species.
1515
Aclcnowledgments. The authors express their gratitude to Mr. R. E. Thoma of the Oak Ridge National Laboratory for the loan of the differential thermal analysis equipment and helpful discussions. Especially helpful advice was received from Professor J. E. Ricci of New York University, but the conclusions of the paper do not necessarily reflect his opinions. Mr. A. S. Dworkin of Oak Ridge National Laboratory made a valuable contribution to the experimental work.
Gas-Liquid Partition Chromatography of Perdeuterioethane. Isotope Effects on Vaporization from Solution1
by W. Alexander Van Hook and James T. Phillips Chemistry Department, University of Tennessee, KnomiUe, Tennessee 87916
(Received November 1 , 1966)
Gas-liquid chromatographic separation factors for the system CZ&-CZ& have been obtained over the temperature range 0 to -130” on a variety of liquid partition columns. The isotope effects were pronounced functions of the temperature. The logarithm of the separat’ion factors varied by more than a factor of 3 when the substrate was changed from highly polar (such as CHaCHO) to nonpolar (such as 2,3,4-trimethylpentane) liquids. These results and some earlier results of Liberti, Cartoni, and Bruner on the system C6H6C6D6 ( J . Chromatog., 12, 8, 1963) are interpreted with the aid of the statistical theory of isotope effects in condensed systems and consistency with the theory is demonstrated.
The gas chromatographic fractionation of compounds isotopically substituted with deuterium (and/or tritium) has received considerable attention. The literature has been summarized by Van Hook and Kelly2* and Bentley, Saha, and Sweeley.2b The technique is of interest both as a method of practical analysis of isotopic mixtures and for the investigation of isotope effects in solution and the intermolecular forces which give rise to them. RIost of the effort to date has been devoted to development of techniques and analytical methods, and very few results have been discussed in the context of the theory of isotopic separations. In the present work we wish to present some chromato-
graphic data for the system C Z H ~ - C ~onD a~ number of liquid phases and point out its consistency with the statistical theory of isotope effects in condensed system^.^ We shall then consider some results of Liberti, Cartoni, and Bruner4 on the system C+&(1).Presented at the 150th National Meeting, American Chemical Society, Atlantic City, N. J., Sept 1965. ( 2 ) (a) W. A. Van Hook and M. E. Kelly, Anat. Chem., 37, 508 (1965); (b) R. Bentley, N. C. Saha, and C. C. Sweeley, ibid., 37, 1118 (1965). (3) J. Bigeleiaen, J. Chem. Phya., 34, 1485 (1961). (4) A. Liberti, G. P. Cartoni, and F. Bruner, J. Chromatog., 12, 8 (1963).
Volume 70, Number 6 May 1966
