Resolution of Isomeric Hexanes by Gas-Liquid Chromotography


Optimization strategies for the development of gas-liquid chromatographic methods. Stephen L. Morgan , Stanley N. Deming. Journal of Chromatography A ...
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while the area representing chloroform accounts for 12.73 nil.

LITERATURE CITED

(1) Idnnge, s.A., “Handbook of Chemiatry,” 9th ed., pp. 1281, 1289, 1327, Handbook Publisiicrs, Inc.,

ACKNOWLEDGMENT Sandusky, Ohio, 1956. ~l~~ authors thank ~ ~ ~ 11. d ~ ( 2 )~LeRoset1, i ~ A.k L., J . d t n . ChenL. SOC. 64,1908 (1942). Hornyak for assisting in the PreliminaT (3) Tcngberg, C. O., Johnston, F., Ind. investigation. Eng. Chem. 2 5 , 733 (1933).

(4) Trueblood, K. N., llalmberg, E. AKAL. CHEX 21, 1055 (1949).

\Y.,

( 5 ) Wolfram, M. L., Thompson, h., Galkorvski, T. T., Quinn, E. J . , Ibid.. 24, 1670 (1952).

RECEIVED for rC?VieW January 10, 1957. Accepted September 18, 1957. Work sponsored by National Science Foundntion Contract 1079.

Resolution of Isomeric Hexanes by Gus-Liquid Chromatography ALBERT ZLATKIS Department of Chemistry, University of Houston, Houston, Tex.

b Hexane isomers can b e rapidly resolved by gas-liquid chromatography using short columns. N e w column packings consisting of heterocyclic amines on Celite can b e operated a t room temperature t o effect these separations. The resolution of 2,3dimethylbutane and 2-methylpentane is achieved in 15 minutes, while all hexane isomers can b e separated in less than 30 minutes.

the use of a liquid-modified solid adsorbent. This new type of column packing, consisting of a carbon black (Pelletex) coated with 1.5% squalane, was able to separate 2,3-dimethylbutane and 2-methylpentane. The resolution of C,! and Ca saturates

about 15 minutes, while all of the hexane isomers are eluted from the column 17-ithin 25 minutes.

T

methylpentanes are incompletely resolved. The present work deals with the resolution of naphthenes and paraffins and, in particular, the separation of isomeric hexanes. Gas-liquid chromatographic analysis was accomplished by using columns of diatomaceous earth with various heterocyclic amines as the stationary liquids. At ambient temperatures 2,3-dimethylbutane and 2methylpentane can be resolved in

The tlhermal conductivity cells were thermostated a t 40’ C. and the colunins were kept a t room temperature or in an ice bath. Preparation of Columns. Glass columns used were 250 cm. long and 5 mm. in inner diameter. The amine column packings consisted of 40% amine on Celite (Chromosorb, 35 to 50 mesh, Johns-Manville Co.). In the case of the quinoline-brucine and quinolinequinine packings, the ratio of quinoline to alkaloid was 7 to 1 (3.5 grams of

resolution of saturated hydrocarbons into their individual constituents has been an increasing practical problem. I n particular, the resolution of isomeric hexanes has become important in the analysis of reaction products from studies in isomerization, hydrocracking, dehydrogenation, alkylation, and polymerization. These compounds have been analyzed by the use of infrared and mass spectrometric techniques, but as instruments of this type are often unavailable to the analyst, a new approach was needed. The rapid development of gas-liquid chromatography in the field of analytical separations has made possible the analysis of complex hydrocarbon mixtures. James (3) employed this technique to determine the Cs and CO constituents in petroleum ethers. Careful screening of a wide variety of compounds for use as the stationary liquid phase in chromatographic columns has yielded columns which give good resolution of mixtures of naphthenes and alkanes. However, none of these would separate 2,3dimethylbutane and 2-methylpentane. Bradford and his coworkers (1) effected a partial separation of these isomers on a column using dinonyl phthalate as the stationary liquid. However, mass spectrometric analysis was necessary to complete the identification. Eggertsen, Knight, and Groennings (2) resolved the hexane isomers by HE

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Figure 1. Chromatogram of 2,3-dimethylbutane (2,3-DMB) and 2-methylpentane (2-MP) -1soquinoline Quinoline-brucine Quinoline

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Figure 2. Chromatogram of nine-component mixture of alkanes and naphthenes using a quinoline-brucine column a t

25' C.

CH. Cyclohexane MCP. Methylcyclapentane 24-DMP. 2,4-Dimethylpentane CP. Cyclopentane 3-MP. 3-Methylpentane

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2-MP. 2-Methylpentane 2,3-DMB. 2,3-Dimethylbutane n-Pent. n-Pentane i-Pent. lsapentane

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Figure 3. Chromatogram of isomeric hexanes and cyclopentane using squalane and isoquinoline columns in tandem a t 25' C. n-Hex. n-Hexane CP. Cyclopentane 3-MP. 3-Methylpentane

quinoline plus 0.5 gram of alkaloid per 10 grams of Celite). Ether was used a s the solvent for putting the amines on Celite. Squalane, 3%, on Celite mas prepared using petroleum ether as the solvent (0.3 gram of squalane per 30 grams of Celite). As quinine is easier t o handle than brucine, quinoline-quinine columns are preferred over quinoline-brucine columns. Other columns containing heterocyclic mixtures such as quinoline8-quinolinol and quinoline-carbazole hare not proved as effective in separations of 2.3-dimrthdbutane and 2methylpentane. Bemuse of the relatively high volatilitv of the linuid substrates. the columns &scribed should not be operated above room temperature. Helium vas used as the carrier gas and the flov rates used w r e 4-1 ml. per minute for Figures 1 and 2, and 70 ml. uer minute for Figure 3.

2-MP. 2-Methylpentans 2,3-DMB. 2,3-Dimethylbutane 2,2-DMB. 2,2-Dimethylbutane RESULTS AND DISCUSSION

Resolution of 2,3-dimethylbutane and 2-methylpentane can be effected using a variety of heterocyclic amines. Figure 1 indicates that only partial separation occurs n-hen quinoline is used alone, but by adding brucine the analysis is greatly enhanced. Best resolution is obtained using an isoquinoline column, as shonm in the same figure. As the two isomeric hexanes are still incompletely separated a t room temperature, a study mas made of columns operating a t 0" C. -4 column using a quinoline-quinine blend as the liquid phase was operated in the ice bath. I n order to facilitate rapid elution of the hydrocarbons, the Celite used n-as of 20 to 30 mesh and a helium flow of 100 ml. per minute was maintained.

While all five hexanes are resolved under these conditions, there is little improvement over operations a t 26" C. Quinoline-brucine columns give similar results, and comparable resolution of the isomeric hexanes is achieved with an isoquinoline column a t 25" C. As the resolution of naphthenes and paraffins is of considerable importance in hydrocarbon studies, a nine-component synthetic blend of C5-C; saturates IYas subjected to gas-liquid chromatographic analysis using the quinolinebrucine column packing described above. Figure 2 shows that separation of all nine hydrocarbons was accomplished. As expected, the naphthenes are retarded in their emergence as compared with the paraffins. One of the difficulties encountered in the use of thew heterocyclic bases is that n-hexane and cyclopeiitane cannot be resolved at 25" C. Partial resolution is effected a t 0" C. using a quinolinequinine column. However, another approach ~ o u l dbe the use of a %foot column, 301, squalane on Celite, a t the front end of a n isoquinoline column. This would hold back the hexane in relation to th(. cyclopentane. Figure 3 illustrates such a separation. As other C, saturates would probably interfere in analysis for methylcyclopentane and cyclohexane, it may be necessary to use longer columns and/or different packings--e.g.. 50-foot columns of squalane on Pelletex obviate this difficulty. The advantages of using quinolinebrucine, quinoline-quinine, and isoquinoline columns for the separation of isomeric hexanrs or naphthene and paraffin mixtures in the C5-Cs range are the effective resolution and the relatively short emergence times of the hydrocarbons involved. I n particular. 2,tidimethylbutane and 2-methylpentme are capable of being resolved.

LITERATURE CITED

(1) Bradford, B. W., Harvey, D., Chalkley, D. E., J . Inst. Petroleum 41, 80

(i955).

( 2 ) Eggertrsen, F. T., Knight, H. S., Groennings, S., AXAL. CHEM. 28, 303 (1956). (3) James, A. T., Mjg. Chemist 26, 5

(1955).

RECEIVED for review August 19, 1957. Accepted October 30, 1957. Presented in part before the Division of Analytical Chemistry, Symposium on Advances in Gars Chromatography, 132nd Meeting, ACS, S e w York, N. I-., September 1957.

VOL. 30, NO. 3, MARCH 1958

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