1468
COMMUNICATIONS TO THE EDITOR
TaC152 and for FeC13, respectively. Schafer and Bayer3 record the melting point of FeC4 as 307.5" and point out that the apparent melting point of FeC&when measured in a CLdeficient atmosphere becomes depressed by contamination with FeC12. Their dat,a indicate that 7 mole yo FeCh in FeCh lowers the apparent melting point to 303", the FeC13m.p. reported by Moroxov. FeCL if present should depress the observed ferric chloride liquidus temperatures, and Morozov's values consistently lie below the liquidus calculated using d In X F ~ ~ C I ~ / dT-l = -AHr/R with AHr = 20.6 kcal./mole and represented by the dotted curves in the figures. 1 0 The NbC15-Fe2C16 system, presented in Fig. 2, shows a eutectic a t 9.5 mole yo FezCls and 191". Fig. 2.-The The NbC15has :m.p. 205".* During cooling of com(2) H. Schiifer and C. Pietruck. Z. anorg. Chem., 261, 174 (1951), report Tach m.p. 216.5'; NbCIs m.p. 201.7'. J. B. Ainscough, R. Holt and F. Trowse, J . Chem. Soc., 1034 (1957), report TaCls n1.p. 216.9'. NbCls n1.p. 203.4'. ( 3 ) 13. Schzifer and L. Bayer, Z . a n o ~ g .Chen., 271, 338 (1963).
20
Vol. 65
40
MOLE
60
%
80
I
100
FezCI,j.
system NbCla-FepCla: Morozov.
0, this work;
0,
positions containing > 0.1 a weak evolution of heat, which occurred reproducibly and which was independent of rate of cooling, was observed at 193" before crystallization of the eutectic a t 191".
COMMUNICATIONS TO THE EDITOR DETECTION OF STRUCTURAL DIFFERENCES IN POLYMERS IN A DENSITY GBADIENT ESTABLISHED BY ULTIRACENTRIFUGATION Sir: W7ehave developed a method of separating polymers based on differences in partial specific volume. We have applied the method to separating highly branched material from a previously described copolymer' and Lo the separation of atactic polystyrene from stereoregular polystyrene. It is seen easily that in the vicinity of a branch point of a polymer molecule, the "density" of the molecule will be slightly greater than in the linear portion of the molecule. Likewise, if the polymer consists of stereo regular sequences the volume that the molecule occupies in solution mill vary with the amount of :stereoregularity. We have adapted the density gradient method first introduced by Meselson, Stahl and Vinograd2 to synthetic polymers in the analytical ultracentrifuge. Essentially, a system of two solvents is used to set up a density gradient; one solvent is much more dense and has a higher molecular weight than the other solvent. The concentration of the more dense solvent and the speed of ultracentrifugation are chosen so 1,hat in the vicinity of the middle of the cell the qliaiitity (1 - 6 p ) equals zero. Here d is the partial specific volume of the polymer, and p is the density of the solution. If differences in partial specific volume of the various components of the bulk polymer do exist, then each component will collect in its own region of (1 - V p ) = 0. If (1) L. H. I'eebles. Jr.. J . A m . Chem. Soc., 80, 5603 (1958). (2) hl. Meselson, IF. 1%'. Stahl and J. Vinograd, Proceed. Nut. Acad. Scz., 43, 581 (1957).
care is taken to use very small density gradients within the cell, quite small differences in 8 can be measured. A 1 E.A. solution of sample 6,' dissolved in dimethylflormamide containing 135.6 g./l. of bromoform was spun in the ultracentrifuge for 150 hours at 33,450 rpm. During this time the material clearly separated into three discrete bands, each band being located at a different position in the region of the center of the cell. I n another experiment, 85 hours at 19,180 rpm., the branched material clearly separated into two distinct bands with a partial specific volume difference of 6.1 X lo-* ml./g. This difference is too small to be detected by standard means. Under these conditions, the linear polymer did not sediment. In order to see whether atactic polystyrene could be separated from stereoregular polystyrene, a mixture of 380 g./L of bromoform in benzene containing 0.32 g./l. of atactic polystyrene of 5 X lo6 molecular weight and 0.32 g./l. of stereoregular polystyrene of 20 X lo5 molecular weight was spun for 56 hours at 33,460 rpm. The latter material is largely stereoregular since only 2% of it is soluble in hot methyl ethyl ketone. The different molecular weights were deliberately chosen so that after separation into the component parts, the fractions could be identified since the breadth of the band depends in part upon the molecular weight. The molecular weights of these polymers are sufficiently high so that the molecular weight dependence of the partial specific volume is negligible. Again, the two materials separated into distinct bands. Comparison of these results with samples run under identical conditions except that the individual polymers were used instead of a mixture showed that the individual polymers collected a t the identical density value. The atactic polymer, however, apparently contained a small
COMMUNICATIONS TO THE EDITOR
August, 1961
1469
chemisorption measureinen ts by Spenadel and B~udart.~ McHenry and co-workers'a also observed the stability of chloroplatinic acid on alumina and postulated the existence of specific platinum(1V)alumina complexes, defined as that fraction of the platinum soluble in aqueous hydrofluoric acid 01 acetylacetone. They reported4bthat the complexes were only slowly reduced by hydrogen a t 20 atmospheres pressure and 500'. Correlat,ion of soluble platinum content with activity tests led to the conclusion that a platinum(1V)-aluminachloride complex was a particularly active species for dehydrocyclization of paraffins. The existence of these divergent views has prompted us to make an additional study of reforming catalyst's. Various supported platinum catalysts were analyzed for soluble platinum using 24% hydrofluoric acid as the reagent. The catalysts were prepared by impregnation with chloroplatinic acid and contained from 0.5% to 1.5% platinum. Reduction in hydrogen for two hours a t 482' preceded the dissolution. The solution was filtered, the residue was mashed with 6 F HC1, and the platinum content of the filtrate was determined spectrophotometrically as the colored complex with stannous ~ h l o r i d e . ~Any residue was dissolved in aqua regia for determination of insoluble platinum. Combined analyses consistently gave material e H E M S T R A S D I~ESE 4RCH CENTER, INC. ROLFBUCHDAHL BOX731 HERBERT A. E m s balances for total platinum in excess of 95%. DURHAM, Nowm CAROLINA LEIGHTON H. PEEBLES, JR. Air was excluded from some catalysts prior to and during HF dissolution. The variables considered RECEIVED JUNE 14, 1061 were: (1) the ratio of chloride to platinum, (2) (3) W. R. Krigbnlll~l,11. IC. Carpenter Snd S. Newrnan, J. Phys. the type of catalyst support, and (3) the effect of Chem., 62, 1586 (1933). exposure to oxygen. The results of the analyses are summarized in Table I.
amount of stereoregular polymer which collected in the stereoregular density region. The difference in partial specific volume between these samples is 0.0283 ml./g. This can be compared with the result of Krigbaum, Carpenter, and Newman3 who found a difference of 0.004 ml./g. The discrepancy is unimportant a t this time. The important point is that a finite difference does exist and that polymers can be separated on the basis of this difference. Further, differences in partial specific volume can now be used to determine whether the bulk structural differences are due to intra or inter molecular structural differences. Examination of these structural differences must include the effect of polydispersity and concentration since they will affect the breadth of the curves; however, the general phenomenon of separation 011 the basis of partial specific volume is not eflected by polydispersity and by concentration. The details of this and other work will be published later. We wish to express our appreciation to Mr. J. 0. Threlkeld for his assistance, to the Chemstrand Research Center, Inc., for permission to publish this communication, and to Mr. Q. 0. Trcmentozzi, of the Plastics Division, Rionsanto Chemical Company, Springfield, Massachusetts, for the samples of polystyrene and their characterization.
TIK
smn; OI? t u m N u R I IN i
TABLE I OXYGEN EXPOSURE O N
i ~ i ~ i i ~ i ~ ~
CliTALYSTS
T OF
Sir : The cheniical state of platinum in pclroleum rcforming catalysts is an important but controversial subject. Mills, Weller, and Cornelius' found that platinum on alumina impregnated with chloroplatinic acid remained in the quadrivalent state w e n after calcining in air at 565'. We have noted this stabilization by alumina but find that decomposition occurs a t much lower temperatures on silica or silica-alumina supports. Visible darkening and high dehydrogenation activity of platinumdumina catalysts after reduction in hydrogen a t low temperatures led us to believe that supported chloroplatinic acid was readily reduced to metal. This view was supported by carbon monoxide (.hemisorption results.2 Chemisorption became appreciable after two hours in hydrogen at only 150'; it rose rapidly to reach a constant level over the range from about 250 to 550O. Mills, et uZ.,l reached the same conclusion from hydrogen consumption measurements, finding complete reduction to the metal in ten minutes at 245'. A similar picture of the catalyst was derived from hydrogen (1) G. A. Mills, S. Weller, and E. B Cornelius. Second International Congrees on Catalysis, Paris, 1960, VoI. 11, paper 113. (2) T. R. Hughes, R. J. Houston, and R. P. Sieg, Preprints Pet. Ihv., ACS, April, 1959.
S O 1 . U B i . E I'I,Arl'ISChI % of Total l't
Cl/I't
Ilq1t.
ator11
SuIlllurt
ratio
soluble
n?-
l.'roe
02-
Exposed
n-Alumina _- 81 .Davison silica gcsl 0 GR Davison silica gel 0 0.2 -7-Alumina 0 2 93 ?-Alumina 1.6 2 92 5.2 0 n-Alumina (i2 0 s7 Davison gel n1uiliin:i 5.9 0 9:i Filtrol 90 alrunina 6.2 3 9 q-AluminaQ 5 The 02-treated catalyst from Example 6 W:LS fluslictl with HZ for 1 hour at 28" prior to this analysis. 1 2 3 4 5 G 7 8
-
Experiments 4, 5 , and 6 show no effect of chloride level on soluble platinum values for eit'her oxygen-free or oxygen-exposed conditions. Esperiments 1 and 2 show that even with such diversc supports as a-alumina and silica, soluble platinum was obtained under oxygen-exposed conditions. The most significant feature is that essentially no soluble platinum mas obtained for reduced catalysts (3) L. Spenadel and M. Boudart, J . Phys. Chem., 64, 204 (1960). (4) K. W. hlcHenry, R. J. Bertolacini, H. M. Brennan, J. I,. Wilson, and €I. S. Seelig, (a) presented a t 138th meeting of Am. Clieiii. Soc., New York, September 1960; (b) Second International Congress on Catalysis, Paris, 1960, Vol. 11, paper 117. ( 5 ) G. H. Ayres and A . S. Meyer, Jr., Anal. Chem., 23, 299 ( 1 9 2 1 ~ .
