COMPOSITION OF HYDROCARBONS IN PRODUCTS OF SYNTHESIS FROM CARBON MONOXIDE AND WATER VAPOR

A. V. KRAVTSOV, I. V. GONCHAROV, A. K. GOLOVKO

(Presented by the scientific-methodological seminar of the organic departments of the Chemical-Technological Faculty)

The prospects for syntheses based on carbon monoxide depend to a large extent on the selectivity of the process with respect to particular components. For this purpose it is absolutely necessary to determine the component composition of the substances formed. Conclusions drawn on the basis of analysis of functional groups provided incomplete information. Most researchers held the opinion that the hydrocarbon portion is represented by n-paraffins and n-olefins of $\alpha$-structure, while paraffins and olefins of iso-structure are not formed at all [1].

In recent years, owing to the rapid development of gas–liquid chromatography, the analysis of multicomponent organic mixtures has become more accessible. To simplify the analyses, we developed a method for separating the initial mixture into groups of compounds, using the results of works [2, 3].

To determine the average molecular weight and fractional composition of the product formed, analysis was carried out without preliminary isolation of alcohols and acids on a laboratory chromatograph of the LKhM-7A type with a thermal-conductivity detector. The carrier gas was helium. The chromatographic column, steel, 2 m long and with an internal diameter of 4 mm, operated in a linear temperature-programming mode from 50 to 220°C at a rate of 9°C per minute. As the stationary phase, silicone elastomer E-301 was used, deposited on Chromaton-N in an amount of 5% of the weight of the support.

The components of the mixture analyzed elute in the following order: isoolefins, isoparaffins, n-olefins, n-paraffins. Relatively clear separation is achieved only between compounds with different numbers of atoms in the molecule.

The mixture formed after synthesis is represented by paraffinic and olefinic hydrocarbons with the number of carbon atoms in the molecule from 5 to 20. Along with n-paraffins and $\alpha$-olefins, compounds of iso-structure are present, the amount of which is considerably smaller than that of compounds of normal structure.

To establish a more detailed composition of the liquid product, the hydrocarbon fraction boiling up to 130°C was subjected to chromatographic analysis on a capillary column. The analysis was performed on a “Khrom-2” chromatograph with a flame-ionization detector. A copper column 90 m long and with an internal diameter of 0.25 mm was used, which operated in the isothermal mode at

Table 1

Identification of hydrocarbons of the liquid product of synthesis from carbon monoxide and water vapor, fraction C₅—C₈

Continuation of Table 1

80°C. Squalane, deposited on the walls of the capillary from a 10% solution in hexane, was used as the liquid phase.

Identification of the components was carried out using standards (n-paraffins, hexene-1, heptene-1), and also by comparing the retention characteristics obtained with data available in the literature [4, 5], where Kovats retention indices are given for certain paraffinic and olefinic hydrocarbons. To verify the correctness of the identification, olefins were selectively removed from the overall mixture by treatment with sulfuric acid and phosphoric anhydride, and the remaining paraffin–naphthene fraction was again subjected to chromatographic separation. The composition of the C₅—C₈ fraction is presented in Table 1.

In this rather narrow fraction, 83 components were found, of which 11 were unidentified. The majority of the ve-

...substances belong to the class of alkenes—55 components. Methanonaphthenic hydrocarbons account for 28 components, of which 5 compounds are naphthenes. No aromatic compounds were detected.

In addition to α-olefins, olefins with the double bond located within the carbon chain were found in the mixture studied. The isomeric composition of the paraffins and olefins formed is represented mainly by mono- and dimethyl-containing compounds. A small amount of ethyl-substituted paraffins and olefins is also present.

After chemical separation, the alcohols and acids were analyzed on an LKhM-7A chromatograph with a thermal-conductivity detector. “PEGA” (polyethylene glycol adipate), deposited in an amount of 5 wt.% on the polymeric solid support Polysorb-1, was used as the stationary phase. The carrier gas was helium. The steel column, 1 m long and with an internal diameter of 4 mm, operated in a linear temperature-programming mode from 70 to 270°C.

Table 2

Component composition of alcohols and acids isolated from the liquid product

From our results, presented in Table 2, it is evident that alcohols and acids of composition C₁–C₁₂ are present in the mixture studied. The presence of small amounts of alcohols and acids of iso-structure was noted.

The results of the analysis of the distribution of the components according to the number of carbon atoms in the molecule made it possible to reveal one interesting regularity. The ratio between paraffinic and olefinic hydrocarbons is in direct dependence on the length of the carbon chain and, as the latter increases, shifts in favor of the paraffins.

The quantitative ratios between the components formed depend substantially on the synthesis conditions and the type of catalyst, but in any case the amount of compounds of normal structure is always greater than that of iso-structure compounds; the amount of olefins with the double bond at the terminal carbon atom is always greater than the amount of olefins with an internal position of the double bond. In the composition of acids and alcohols there are fewer discrepancies with the literature data [6]. Both in the alcohols and in the acids, the lower compounds constitute the greater part. With increasing carbon-chain length, their amount decreases.

LITERATURE

1. I. F. Bogdanov. Trudy IGI AN SSSR, No. 18, p. 34, 1962.
2. M. N. Manakov, Yu. N. Bogoslovskii, I. I. Lebedev, F. Khasan, V. V. Elmanovich, V. R. Zaretskaya. Neftekhimiya, 1969, 9, p. 253.
3. S. M. Loktev. In: Higher Fatty Alcohols, Moscow, “Khimiya,” 1970, p. 166.
4. V. Harris, G. Habgood. Gas Chromatography with Temperature Programming. Moscow, “Mir,” 1968.
5. I. I. Gudova, S. I. Leont’eva, A. K. Fedosova. In the collection: Advances in Gas Chromatography, Issue 2, Kazan, 1970, p. 141.
6. H. Kölbel, J. Huschel, H. Hammer. Liebig Ann., 32, 8, 1960.