Gas Chromatographic Analysis of Aromatic Hydrocarbons from C6 through C10*

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Gas Chromatographic Analysis of Aromatic Hydrocarbons from C6 through C10* Hiroshi Miyake** and Mitsuynki Mitooka** Introduction Summary: Retention data on C6 through C10 aromatic hydrocarbons were

1 Gas Chromatographic Analysis of Aromatic Hydrocarbons from C6 through C10* Hiroshi Miyake** and Mitsuynki Mitooka** Introduction Summary: Retention data on C6 through C10 aromatic hydrocarbons were determined by the use of capillary columns. The selectivities for the aromatic coin pounds were investigated with six stationary liquids. In any cases there are some unseparated groups, and it is necessary to associate results obtained with any two substrates for the analysis of complex mixtures. When squalane and polypropylene glycol columns were used, C6 through C10 aromatic mixtures were analyzed easily. Many investigators have reported on the gas chromatographic analysis of aromatic bons, they have many isomers, and it seems considerably difficult to separate these isomers completely by the ordinary methods. To separate these mixtures, it is necessary to use the column which has higher separation efficiencies. The capillary column6), along with the development of highly sensitive ionization detectors7), will provide the highest separation efficiency. Authors used this column for the separation of C6 through C10 aromatic hydrocarbons, and studied on the selectivity of various stationary liquids for the aromatic compounds. Experimental In this study, retention data on aromatic hydrocarbons were determined by the use of capillary column-flame ionization detector following the common technique of gas chromatographic Apparatus analysis. A Perkin-Elmer Model 188 Vapor Factometer equipped with a flame ionization detector was used. Columns The columns used were as follows; Perkin- Elmer "U" column (squalane, 0.254mm in * Received December 3, ** Research Center, Maruzen Oil Co. Ltd., Shimotsucho, Kaiso-gun, Wakayama-Pref., Japan. internal diameter, 91.4m in length), "R" column (polypropylene glycol, 0.254mm, 91.4m) and the columns which atuhors have prepared by coating di-n-propyl tetrachlorophthalate (DPTCP), 7.8-benzoquinoline (BQ), 2.4-dinitrochlorobenzene (DNCB), tetrachlorobenzene (TCB), in stainless steel tubing (0.254mm,45.7m) by the following technique. Coating method of substrate in capillary tubing Each of substrates was coated in capillary tubing as follows; a weighed amount of refined liquid is dissolved in ethyl ether to prepare 10-20% solution, and this solution is injected into the capillary tubing by the aid of a 1c.c. syringe. After the tubing was completely filled with the solution, solvent and excessive substrate was purged out by heating the column at appropriate temperature in a stream of helium gas, consequently, the thin substrate is deposited on the inner wall of the capillary tubing. Pure hydrocarbons Pure C6 through C10 aromatic hydrocarbons were obtained from Phillips Petroleum Co., National Bureau of Standard (NBS), and American Petroleum Institute (API). Carrier gas Helium was used as carrier gas. Results and Discussion of C6 through C10 aromatic hydro- Separation carbons Using each of six substrates under appropriate operating conditions, chromato-

2 Miyake and Mitooka: Gas Chromatographic Analysis of Aromatic Hydrocarbons from C6 through C10 Fig. 1 Chromatograms of C6 through C10 aromatic hydrocarbons grams of the C6 through C10 aromatic hydrocarbon mixtures were obtained as shown in Fig. 1. The peak number of the figure coincides with the component number in Table 1, in which the boiling points and vapor pressures of C6 through C10 aromatic Fig. 2 Relationship between retention time and vapor pressure. hydrocarbons are shown. Six substrates used in this study are dissimilar to each other in their selectivities. Table 2 shows corrected retention volumes of components. Squalane was selected as standard substrate to compare selectivities, because this substrate is non-polar and separation of components seems to be mostly influenced by van der Waals force. Since other substrates, however, are more polar, it must be considered that separation should not be influenced only by van der Waals force. Di-n-propyltetrachlorophthalate, tetrachlorobenzene or 2.4-dinitrochlorobenzene, which have highly electronegative substituents, seems to act as electron acceptor and to form the molecular complex with aromatic hydrocarbons (as electron donor). The mechanism of separation with polypropylene glycol or 7.8-benzoquinoline is obscure. Fig. 1 shows that in any cases there are some unseparated groups, for example in the case of squalane, three groups, i. e. I; iso-butylbenzene (peak number 15) and secbutylbenzene (16), II; trimethylbenzene (18) and 1-methyl-4-iso-propylbenzene (19), and 1methyl-4-ti-propylbenzene (24), III; 1.2-diethylbenzene (25) and 1.4-diethyl- Volume 5-March 1963

Miyake and Mitooka: Gas Chromatographic Analysis Table 1 Boiling points and vapor pressure of C6 through C10 aromatic hydrocarbons.* Table 2 Corrected retention volume.

3 Miyake and Mitooka: Gas Chromatographic Analysis Table 1 Boiling points and vapor pressure of C6 through C10 aromatic hydrocarbons.* Table 2 Corrected retention volume. * Obtained from API selected values of properties of hydrocarbons ** B: Benzene T: Toluene X: Xylene M: Methyl- E: Ethyl- P: Propyl- Bu: Butyl- D: Di- T: Tri- Te: Tetrabenzene (27) elute in the same time. Consequently, to analyse all components of C6 through C10 aromatic hydrocarbons, it is necessary to associate results obtained with any two individual columns. Data on the separation of meta- and paraxylene are shown in Table 3. It is clear from these data that the best condition is 7.8-benzoquinoline Exp. No. 1, where the separation factor calculated from the retention time of meta- and para-xylene, and the degree of separation obtained from the peak height of two components have the highest value. It is interesting that some substrates inverse the elution order of meta- and paraxylene, for instance in the case of squalane, the relative retention time (benzene=1.00) of para-xylene is shorter than that of metaxylene, but in di-n-propyltetra-chlorophthalate and 2.4-dinitrochlorobenzene, reversely,

4 of Aromatic Hydrocarbons from C6 through C10 Table 3 Data of meta- and para-xylene separation * Separation factor= tr(m-x)/tr(p-x) meta-xylene is shorter. Plots of the logarithm of the corrected retention times against the logarithm of vapor pressures at operating temperature for each substrates are shown in Fig. 2. In the case of squalane, the plot yields an almost straight line, but in other substrates especially in di-n-propyltetrachlorophthalate and 2.4-dinitrochlorobenzene, the line was crooked in the range of para-xylene (point number 4) and n-propylbenzene (8). These order; for example, p-xylene (4), m-xylene (5), o-xylene (6), iso-propylbenzene (7), n-propylbenzene (7), n-propylbenzene (8) (in squalane) m-xylene (5), p-xylene (4), iso-propylbenzene (7), o-xylene (6), n-propylbenzene (8) (in di-n-propyltetrachlorophthalate), iso-propylbenzene (7), m-xylene (5), p-xylene (4), n-propylbenzene (8), o- xylene (6), (in 2.4-dinitrochlorobenzene). Fig. 3 shows similar plots to Fig. 2 about Volume 5-March 1963

5 Miyake and Mitooka: Gas Chromatographic Analysis Table 4 Selectivity index Fig. 3 Relationship between retention time and vapor pressure four groups of isomers in respect to squalane in the case of di-n-propyltetrachlorophthalate. To know the approximate quantities of forces beside van der Waals force which act on the separation, the selectivity indices were calculated on the base of the values in squalane (Table 4). where VR0=corrected retention volume of component. VR00=corrected retention volume of methane. Langer, et al.5) have reported on the between solute and solvent molecules at the separation of aromatic hydrocarbons (solute) using di-n-propyltetrachlorophthalate (solvent). When Lewis acid (electron acceptor) and Lewis base (electron donor) approach in the moderate distance, the molecular complex is formed between them in the ratio of 1:1 or 1 : n8). In this study it was expected that di-npropyltetrachlorophthalate, tetrachlorobenzene and 2.4-di-nitrochlorobenzene act as Lewis acid to aromatic hydrocarbons acting as Lewis base to form molecular complex in the column, and the degree of complex formation effects on the selectivity for the components. The inversion of elution order of metaand para-xylene on di-n-propyltetrachlorophthalate and 2.4-dinitrochlorobenzene will be explained by the fact that para-xylene has the lower base ionization potential and forms more stable complex with these substrates. From the data of Table 4, it is surmised that ortho-substitued isomers form the most stable complex with these substrates among the bi-substituted components. As a rule polymethyl-substituted compounds seem to form highly stable complex. In monosubstituted compounds, as the carbon number of substituent increases, the selectivity index decreases, and this means the formation of more unstable complex. All these tendencies do not conflict with those which are surmised from the electronegativity of substituents. Quantitative analysis of C6 through C10 aroiatic hydrocarbons To analyse quantitatively all thirty six

6 of Aromatic Hydrocarbons from C6 through C10 Fig. 4 Variation of analytical factor with retention time. Table 5 Results of quantitative analysis of platformate and Udex raffinate components of C6 through C10 aromatic hydrocarbons, it is necessary to associate results obtained with two columns because of the difficulty in separating all components with one column even if the higher efficiency column might be used. Volume 5-March 1963

7 Miyake and Mitooka: Gas Chromatographic Analysis of Aromatic Hydrocarbons from C6 through C10 Considering the separability of components and the tailing of peak, it is possible to analyse all components if we select the squalane and polypropylene glycol column, which supply the unseparability of components with each others. In many papers9,10), the ratio of peak height to the amount of each component has been used as the analytical factor. But by increasing the retention time, the analytical factors obtained from peak height decrease almost linearly, and this fact means the decrease of the relative sensitivity of detection. On the other hand, the factors from peak area are almost constant with the retention time (Fig. 4). To increase the accuracy of analysis, it seems better to use the relation of molar content and peak area expressed in the product of peak height and peak width at half height or in intergration value by printing integrator. Errors of analysis of known sample in which the unseparated components with squalane column were contained using both squalane and polypropylene glycol columns, were sufficiently small and it seems easy to analyse all components within the repeat- Table 5 shows the aromatic hydrocarbon composition of platformate and Udex raffinate. Acknowledgment The authors thank Prof. W. Funasaka of Kyoto University, under whose supervision this work was performed, and for his valuable advices and discussions in this investigation. References 1) Desty, D. H., Goldup, A., Swanton, W. T., Nature, (1959). 2) Zlatkis, A., Ling, S., Kaufman, H. R., Anal. Chem., 81, 3629 (1959). 3) Tenney. H. M., Anal. Chem., 30, 2 (1958). 4) Dan, T., Miyake, H., 12th Meeting Japan Chem. Soc., (1959). Abstracts, p ) Langer, S. H., Zahn, C., Pantazoplos, G., J. Chromatog, 3, 154 (1960). 6) Golay, M. J. E., in "Gas Chromatography", D. H. Desty, ed, p. 36. Academic Press, New York, ) Lovelock, J. E., Nature, 182, 1663 (1958). 8) Mulliken, R. S., J. Phys. Chem., 56, 801 (1952). 9) Takeuchi, T., Ishii, O., J. Chem. Soc. Japan (Ind. Chem. Sect) 64, 763 (1961). 10) Hollis, O. L., Anal. Chem., 33, 352 (1961).

Gas Chromatography. Introduction

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