Study of Oils by Infrared Spectroscopy

I. N. Zueva, N. M. Smolyaninova, G. N. Ivanov

(Presented by the scientific-methodological seminar of the CTF)

The method of infrared spectroscopy has become widely used in the study of oils and petroleum fractions.

In the present work, results are presented on the use of infrared spectroscopy to determine the type of Tomsk oils and the nature of the oxygen-containing compounds present in them.

The method for determining the type of oils from their infrared spectra—in accordance with the generally accepted chemical classification, by the predominant content of aromatic, methane, and naphthenic hydrocarbons [1]—is based on the direct relationship between the contents of aromatic and methane structures in the oil and the integral intensities of the corresponding absorption bands [2].

The measure of the quantitative content of aromatic structures is the area of the band at \(1610\ \text{cm}^{-1}\) \((S_A)\), corresponding to vibrations of the \(-C=C-\) bonds of the aromatic ring; the measure of the content of paraffinic structures is the area of the band at \(725\ \text{cm}^{-1}\) \((S_n)\), corresponding to vibrations of the \(-C-C-\) bonds of the skeleton of long polymethylene chains.

The ratio of the band areas, reflecting the ratio of aromatic and methane structures in the oil, is taken as the index of aromaticity—\(A = \frac{S_A}{S_n}\). Having determined the value of this coefficient for an oil of unknown composition, its type is established in accordance with the values of \(A\) determined by E. A. Glebovskaya [3, 4] for oils of different chemical composition.

This method makes it possible to assess rapidly the type of oil when several grams of substance are available and without resorting to laborious and lengthy chemical analysis.

The presence of oxygen-containing compounds in oil is detected from the absorption bands at \(1700\ \text{cm}^{-1}\) (the \(C=O\) group of petroleum acids, aromatic esters, and ketones) and \(1745\ \text{cm}^{-1}\) (the \(C=O\) group of aliphatic esters).

The presence of hydroxyl groups \(-OH\) in the spectrum of an oil can be determined from the absorption band at \(3200\ \text{cm}^{-1}\), corresponding to the stretching vibrations of \(-O-H\).

For the study, five oil samples were taken from the Sovetskoe field (wells Nos. 18, 27, 41, 50, 80), and one oil sample each from the Poludennoe (well 221), Katylginskoe (well 91), and Luginetskoe (well 155, from the oil rim) fields.

The oil spectra were recorded on an “IKS-14A” double-beam automatic spectrometer in the regions 1820–1540 cm\(^{-1}\) and 820–670 cm\(^{-1}\) (NaCl prism). KBr-glass cuvettes were used for recording the spectra. The thickness of the absorbing layer was set by a lead-foil spacer (\(l = 70\mu\); measured by the interference method).

To determine the aromaticity coefficient \(A\), the absorption bands of aromatic and paraffinic structures were isolated by the method of K. B. Yatsimirskii and T. V. Mal’kova [6].

Table 1

Results of determining the type of oils

The experimental results presented in Table 1 show that, according to the classification proposed by E. A. Gleboeskaya, the majority of the oils studied belong to the naphthenic type, and only two samples—Sovetskaya, well 27, and Katylyginskaya—are identified as methane-naphthenic.

Figure 1 shows the infrared absorption spectrum of Sovetskaya oil, well 18.

It should be noted that there is good agreement between the aromaticity coefficient determined by the spectral method and that calculated on the basis of data on the hydrocarbon composition of the Sovetskaya oil sample, well 18 [5]. Thus, the value of coefficient \(A\) calculated by us for

Fig. 1. IR spectrum of TN-21 oil.

is in the range \(0.6\text{–}0.75\), while that determined by spectral analysis (Table) is \(0.71 \pm 0.07\).

Consequently, the spectral method makes it possible to determine, with sufficient accuracy, the type of an oil of unknown chemical composition.

As for determining the type of oxygen-containing compounds, in the spectra of the oils studied no absorption bands at \(1700\ \text{cm}^{-1}\) and \(1745\ \text{cm}^{-1}\) are observed (Fig. 1), but a small shoulder at \(1705\ \text{cm}^{-1}\) is noticeable on the \(1610\ \text{cm}^{-1}\) absorption band of aromatic structures. It can be attributed to absorption by the \(C{=}O\) group. To confirm this assumption, spectra were recorded of the oil with additions of one and five percent propanoic acid. A sample of one such spectrum is given in Fig. 2.

Fig. 2. IR spectrum of a mixture of propanoic acid with TN-21
(\(—\) 5% propanoic acid in the mixture; \(— — —\) 1% propanoic acid in the mixture).

In the spectrum of the oil with propanoic acid, a rather intense absorption band at \(1705\ \text{cm}^{-1}\) is observed, corresponding to the stretching vibrations of the \(C{=}O\) group of propanoic acid. Consequently, the shoulder on the absorption band of aromatic structures may be assigned to absorption by \(C{=}O\) groups present in the oil.

Comparison of the absorption spectra of pure oils and of spectra of mixtures with propanoic acid in the region \(2000\text{–}3500\ \text{cm}^{-1}\) made it possible only to suggest (from the low-intensity band at \(3200\ \text{cm}^{-1}\)) the presence of acidic hydroxyl in the samples studied.

From comparison of the intensities of the absorption bands of the \(C{=}O\) group it follows that the content of oxygen-containing compounds in the oils that include these groups is apparently less than 1%.

Conclusions

1. By the method of infrared spectroscopy, the type of eight samples of Tomsk oils has been established.

2. The presence of \(C{=}O\) and \(OH\) groups, which are components of oxygen-containing compounds in oils, has been detected.

3. It has been shown that the IR-spectroscopy method makes it possible to determine the type of oil quickly and with sufficient accuracy, without resorting to chemical analysis.

References

1. S. S. Nametkin. Collected Works, vol. III, Publishing House of the USSR Academy of Sciences, Moscow, 1955.

2. Manual for Petroleum Analysis. Nedra, Leningrad, 1966.

3. E. A. Glebovskaya. Application of infrared absorption spectra in studies on petroleum geochemistry. Geol. Collection, 2, Leningrad, Gostoptekhizdat, 1956. (Proceedings of VNIGRI, new series, issue 95).

4. E. A. Glebovskaya. Determination of certain characteristics of petroleum composition from infrared absorption spectra. Geochemistry, collection 8. Leningrad, Gostoptekhizdat, 1963. (Proceedings of VNIGRI, issue 212).

5. Study of the physicochemical properties of oil from the Sovetsko-Sosninskoe field. Report on topic No. 1/66. Tomsk, 1967 (TPI archive).

6. Spectroscopic Methods in the Chemistry of Complex Compounds. Edited by Corresponding Member of the USSR Academy of Sciences V. M. Vdovenko. Pp. 102–116. Khimiya, Moscow–Leningrad, 1964.