STUDY OF THE COMPOSITION OF AROMATIC HYDROCARBONS IN MIDDLE FRACTIONS OF OIL FROM THE SAMOTLOR FIELD

N. M. SMOL’YANINOVA, G. V. SHUMAKOVA, A. A. NOVOVA

(Presented by the Scientific-Methodological Seminar of the Organic Departments of the Faculty of Chemical Technology)

With the development of new methods of oil refining and the increasing requirements for the quality of diesel and jet fuels and lubricating oils, and also in connection with the importance of understanding the genesis of oil, there arises a need to broaden scientific knowledge about the chemical composition and physicochemical properties of the middle and heavy fractions of oils.

The composition of these fractions is extremely complex and is difficult to study, despite the use of the most modern research methods.

The present work is devoted to the study of the composition of aromatic hydrocarbons of the 1st group (benzene derivatives) that are part of the kerosene–gas-oil fraction (200–300°C) of Samotlor oil, which is characterized by the following indices: yield from oil—18.4%; $n_D^{20}$—1.4680; molecular weight—194; sulfur content—0.37%; amount of methane–naphthene hydrocarbons—72.5%, aromatic hydrocarbons of the 1st group—13%, and of the 2nd, 3rd, and 4th groups—4.0, 8.5, and 2.0%, respectively.

The object of study was obtained by elution-displacement chromatography of the initial cut on silica gel of the ASK grade. The boundary between methane–naphthene and aromatic hydrocarbons was established from the value of the refractive index ($n_D^{20}$) and by the formalite reaction. Hydrocarbons with refractive indices ($n_D^{20}$) from 1.49 to 1.53 were assigned to monocyclic aromatics [1].

To facilitate further study and to increase its accuracy, the initial fraction, before chromatographic separation, was purified of heteroorganic and oxygen-containing compounds. Organo-sulfur compounds were removed by oxidation with hydrogen peroxide in glacial acetic acid, followed by removal of the oxidation products by adsorption on silica gel [2]. Nitrogen bases were extracted with a 10% aqueous solution of sulfosalicylic acid [3]. Naphthenic acids were removed by treating the fraction with a 5% solution of sodium bicarbonate saturated with carbon dioxide [4].

The mixture of monocyclic hydrocarbons isolated in this way ($n_D^{20}=1.4940$—1.5380) was subjected to precise rectification under vacuum on a column with an efficiency of 45 theoretical plates at a residual pressure of 7 mm Hg. In all, 26 narrow fractions with temperatures

Table

Characteristics of narrow fractions of monocyclic aromatic hydrocarbons of kerosene from Samotlor oil

Continuation of the table

boiling from 193 to 260°C and refractive indices from 1,4865 to 1,5140.

Ultraviolet spectra of the narrow fractions were recorded in the region 240—300 nm on a “Specord” spectrophotometer. The spectra were interpreted on the basis of literature data and atlases of spectra of individual aromatic hydrocarbons [5—8].

Infrared spectra of the indicated fractions were obtained on a “UR-20” instrument in the frequency interval 1300—600 cm\(^{-1}\). The spectra were interpreted likewise on the basis of literature data and using tables of characteristic absorption frequencies [9—11].

The table presents the principal results of the study.

On the basis of interpretation of the absorption spectra in the near ultraviolet region it was established that tetraalyl-substituted benzenes are present in almost all fractions, as indicated by a shoulder in the region 246—278 nm [12, 13]. In the first cuts, evidently, 1,2,3,5- and 1,2,4,5-tetramethylbenzenes predominate. Beginning with the 6th fraction (204—208°C) and higher, 1,2,3,4-tetramethylbenzene is present, as well as tetrasubstituted benzenes with longer chains. The maximum amount of tetrasubstituted benzenes, with predominance in their composition of the 1,2,4,5-isomer, is observed in the 13th fraction, as indicated by a pronounced maximum at a wavelength of 276—277 nm.

The second structural subgroup, present in significant amounts in almost all fractions, consists of 1,3-disubstituted alkylbenzenes, as evidenced by a pronounced maximum at a wavelength of 272 nm, despite the relatively low absorption intensity of such hydrocarbons [13].

The maximum at \(\lambda = 267—268\) nm is apparently associated with the superposition of the corresponding maxima of tetra- and 1,3-substituted benzenes.

In some fractions (11, 13, 16, and others), 1,2,4- as well as para-substituted alkylbenzenes were detected (\(\lambda = 274;\ 265—266\) nm).

In the first cuts, the presence of monoalkyl-substituted benzenes is noticeable, which is manifested in a certain elevation of the short-wavelength wing of their spectra [14].

In many fractions (8—13, 16, 17, 19, and others), there is no minimum in the region 270—272 nm. Evidently, it is masked by absorption maxima of tetra- and 1,2,3-substituted alkylbenzenes. And in fractions with boiling temperatures above 230°C, penta- and even, possibly, hexa-substituted benzenes may absorb appreciably.

IR-spectroscopy data showed that in all spectra of narrow aromatic fractions there is a band with a maximum at a frequency of 745 ± 10 cm\(^{-1}\), characteristic of monosubstituted benzenes. The upper part of the frequency interval of this band may overlap with the interval typical for bands of ortho-substituted compounds. However, monosubstituted compounds strongly

also absorb near \(690\ \mathrm{cm}^{-1}\), where ortho-substituted compounds practically do not absorb [9]. The absence of a band in the \(710\ \mathrm{cm}^{-1}\) region serves as evidence for the absence of monosubstituted compounds. From the data given in the table it can be seen that, in the spectra of all fractions, the indicated absorption band is present (usually \(705\ \mathrm{cm}^{-1}\)), which definitely indicates the presence in their composition of monosubstituted alkylbenzenes.

An absorption maximum at \(815\text{--}825\ \mathrm{cm}^{-1}\), of medium intensity in fractions from the 1st to the 7th and very intense in the subsequent distillates, indicates the presence of \(p\)-substituted compounds.

Intense absorption bands at 785 and \(770\ \mathrm{cm}^{-1}\), of medium or weak intensity at \(800\ \mathrm{cm}^{-1}\), and also a shoulder in the frequency range \(725\text{--}680\ \mathrm{cm}^{-1}\), are characteristic of 1,3- and 1,2,3-substituted benzenes.

The presence was also detected of 1,3,5- (maxima 890, 815, \(820\text{--}825\ \mathrm{cm}^{-1}\)), 1,2,4- (frequency intervals \(815\text{--}825\ \mathrm{cm}^{-1}\))-substituted compounds, as well as tetra- and pentasubstituted compounds (bands in the intervals \(900\text{--}860\) and \(890\ \mathrm{cm}^{-1}\)).

Thus, the results of spectral analysis of aromatic hydrocarbons in the IR region made it possible to confirm and refine the data obtained in interpreting the UV spectra; for example, from absorption spectra in the ultraviolet region it is difficult to judge the presence of 1,2,3- and ortho-substituted compounds [5], whereas the IR spectra unambiguously confirm the presence of one of them (1,2,3-substituted).

As a result, it may be stated that, in the composition of the monocyclic aromatic hydrocarbons of the middle fractions of Samotlor oil, the predominant components are di-, tri-, and tetrasubstituted alkylbenzenes of the \(C_9\text{--}C_{11}\) series. All narrow aromatic fractions of the kerosene studied are close in their structural-group composition, and their individual components differ in chain length and in degree of branching.

Conclusions

1. The composition of monoaromatic hydrocarbons of the \(200\text{--}300^\circ\mathrm{C}\) fraction of Samotlor oil has been studied by liquid chromatography, precise rectification, and UV and IR spectroscopy.

2. From the absorption spectra in the near-UV region and in the IR region, in the frequency interval \(1300\text{--}600\ \mathrm{cm}^{-1}\), it was established that the aromatics of the 1st group include mono-, di- (\(p\)- and \(m\)-isomers), tri- (1,2,3-, 1,2,4- and 1,3,5-), tetra- (possibly all three substitution types), and also pentasubstituted benzenes.

3. It has been shown that the narrow 2–4-degree distillates obtained from the aromatic fraction with \(n_D^{20}\ 1.49\text{--}1.53\) are close in their structural-group composition, differing only in the chain length and degree of branching of the hydrocarbons contained in them.

References

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