STUDY OF THE COMPOSITION OF BTX FRACTIONS BY GAS–LIQUID CHROMATOGRAPHY

N. M. SMOLYANINOVA, L. I. ALEKHIN

(Presented by the scientific-methodological seminar of the Chemical-Technology Faculty)

The need for a chromatographic study of the composition of BTX fractions was due to the absence of a relationship between the quality of the feedstock (characterized by the bromine number and by the fractional composition according to Spilker) and its behavior in the process of formaldehyde debenzenization [1].

In the present work, carried out at the Institute of Organic Chemistry, Siberian Branch of the USSR Academy of Sciences, the possibility is assessed of using modified cetyltrimethylammonium bentonite as a stationary phase for the analysis of crude benzene fractions; the composition of benzene–toluene–xylene fractions is determined quantitatively.

The objects of the study were BTX fractions from the Kuznetsk Metallurgical Combine, the quality indices of which are summarized in Table 1. The studies were carried out on a Czech “Khrom-1” chromatograph equipped with a flame-ionization detector, according to a method developed at NIOKh, Siberian Branch of the USSR Academy of Sciences [2]. The products were analyzed on a column 540 cm long (internal diameter 6 mm), packed with a mixture of cetyltrimethylammonium bentonite and dinonyl phthalate deposited on INZ-600 (fraction 0.28–0.40 mm). The Inza brick was preliminarily treated with acid, alkali, and water and calcined at 700°C for 9 hours. The efficiency of the column for m-xylene, determined analytically [3], was 2700 theoretical plates.

Table 1

Characteristics of BTX fractions (fractions collected during operation of the cyclopentadiene unit)

Identification of compounds was carried out by the relative retention volumes of model hydrocarbons, whose degree of purity was 98–99.9%. The relative retention volumes of individual substances are given in Table 2.

Table 2

Relative retention volumes of components. Relative retention volume of benzene \(V_R^{\mathrm{rel}} = 1.00\)

Methylcyclopentane and methylcyclohexane were identified by the presence of peaks on the chromatogram of “pure for nitration” grade benzene and from literature data [4–6].

The presence of unsaturated hydrocarbons in the raw BTX fractions was established by comparing the chromatograms of the raw fractions before and after removal of unsaturated compounds from them with concentrated sulfuric acid (fraction purification temperature 20°C). Because standards of low-boiling olefins were lacking, the latter were identified by the order of appearance of the components and from literature data [7, 8]. A typical chromatogram of a raw BTX fraction is shown in Fig. 1.

Quantitative analysis of the raw and purified products was carried out by the internal-standard method. Nonane served as the internal standard.

The content of the principal components (benzene, toluene) was calculated by the formula:

\[ C_{\mathrm{b}} = \frac{ f_{\mathrm{b}}\cdot \dfrac{h_{\mathrm{b}}}{h_{\mathrm{n}}}\cdot C_{\mathrm{n}}\cdot 100\% }{ C_{\mathrm{n}}\left( f_{\mathrm{b}}\cdot \dfrac{h_{\mathrm{b}}}{h_{\mathrm{n}}} + f_{\mathrm{t}}\cdot \dfrac{h_{\mathrm{t}}}{h_{\mathrm{n}}} \right) } \left( 1-\frac{\sum_i C_i}{100\%} \right), \]

where \(h_{\mathrm{b}}, h_{\mathrm{t}}\), and \(h_{\mathrm{n}}\) are, respectively, the peak heights of benzene, toluene, and nonane;
\(C_{\mathrm{b}}\) and \(C_{\mathrm{n}}\) are the concentrations of benzene and nonane in the fraction (%);
\(\sum_i C_i\) is the total content of the remaining components in the fraction (%);
\(f_{\mathrm{b}}\) and \(f_{\mathrm{t}}\) are the correction coefficients for benzene and toluene.

The relative error of the benzene analysis does not exceed 3%, and that of toluene 15%; for styrene, mesitylene, ethylbenzene, and the xylenes this value reaches 7%. The relative error in the determination of thiophene and of components eluting before benzene varies within the range 8–25.0%.

In the quantitative determination of substances for which standards were absent, the assumption was made that their correction coefficients have a rectilinear power dependence on relative...

retention volumes. The reference points were the correction coefficients of normal pentane, hexane, heptane, and cyclohexane.

Fig. 1. Chromatogram of the crude BTX fraction of the Kuznetsk Metallurgical Combine

Data on the composition of crude and sulfuric-acid-purified BTX fractions are presented in Table 3.

Table 3

Component composition of BTX fractions
(Analysis was carried out for the detected components)

Discussion of the Results

Comparison of the literature data with the relative retention volumes of substances on modified dinonyl phthalate cetyltrimethylammonium bentonite (Table 2) shows that the stationary phase used by us, like modified bentone-34 and bentone-245 [9–12], gives good separation of ethylbenzene and the xylene isomers (Fig. 1). The elution order of aromatic compounds and paraffins on the stationary phase investigated is analogous to the sequence of emergence of these substances on 1-bromonaphthalene, 7,8-benzoxynoline [5], benzylbiphenyl, and phenanthrene [13].

The cetyltrimethylammonium bentonite used exhibits high sensitivity to the structural features of organic compounds, to the presence in the carbon skeleton of molecules of rings, branches, and multiple bonds. This can be illustrated by a number of examples. Thus, the relative retention volume of 2,2,4-trimethylpentane (Table 2) is 1.5 times greater than the relative retention volume of n-heptane, although the difference in the boiling points of these hydrocarbons is negligibly small (0.8°C). In the same way one may compare n-heptane with methylcyclohexane, n-hexane with methylcyclopentane, cyclohexane with cyclohexene, and paraffins with iso- and cycloolefins (Fig. 1). The high sensitivity of bentonite can be successfully used for separating hydrocarbon isomers, although for this purpose it will probably be necessary to use, alongside the phase described, columns with nonpolar substances that separate organic compounds according to boiling points.

The component composition of the BTX fractions, as follows from Table 3, varies within fairly wide limits, which can probably be explained by the instability of the operation of the separating column, as well as by the operation of the plant for the production of dicyclopentadiene.

From comparison of the fractional-composition indices of the hydrocarbons studied (Table 1) with the chromatographic data (Table 3), it follows that the volume of distillate up to 100°C characterizes the concentration of benzene in the feedstock, overestimating the absolute value by 3–6%; the amount of the BTX fraction boiling up to 125°C determines, with an absolute error of 1–2%, the total concentration of benzene and toluene in the solution.

Comparing the compositions of the crude and purified BTX fractions, one may conclude that, under the conditions of sulfuric-acid purification, no noticeable course of sulfonation and alkylation reactions of benzene, toluene, and xylenes is observed.

In conclusion, the authors express their gratitude to the staff members of the Institute of Organic Chemistry, Siberian Branch of the Academy of Sciences of the USSR, V. A. Livanov, A. G. Khmel’nitskii, and V. N. Kobrina, for the assistance rendered in carrying out this study.

LITERATURE

1. M. I. Alekhin, Conference of Young Scientists and Specialists of the City of Novosibirsk. Section of Organic Chemistry (abstracts of reports). Novosibirsk, 1966.

2. V. N. Kobrina, A. G. Khmel’nitskii. Conference of Young Scientists and Specialists of the City of Novosibirsk. Section of Organic Chemistry (abstracts of reports). Novosibirsk, 1966.

3. M. Shinglyar. Gas Chromatography in Practice. Khimiya, Moscow, 55, 1964.

4. V. M. Nabivach, V. I. Dal’. Proceedings of DKHTI, issue 16, part 1, 149, 1962.

5. V. M. Nabivach. Gas Chromatography, issue 1, Moscow, 107, 1964.

6. L. I. Marinich. Coke and Chemistry, No. 3, 41, 1965.

7. L. I. Marinich. Coke and Chemistry, No. 3, 41, 1964.

8. S. A. Gader, M. G. Krishna, and S. H. Zaheer. Brennstoff-Chemie, 46, No. 6, 168, 1965.

9. M. A. Hughes, D. White, A. L. Roberts. Nature, 184, No. 4, 701, 1797, 1959.

10. M. S. Vigdergauz, O. G. Chabarova. Neftekhimiya, 4, No. 6, 932, 1964.

11. M. S. Vigdergauz, O. G. Chabarova. Neftekhimiya, 5, No. 1, 160, 1965.

12. D. W. Grant. Ind. Chem., 40, 126, 1964; Ind. Chem., 40, 240, 1964.

13. B. St. H. Langer and I. H. Purnell. J. Phys. Chem., 67, No. 2, 263, 1963.