O. K. BOGDANOVA, Academician A. A. BALANDIN, I. P. BELOMESTNYKH

EFFECT OF THE STRUCTURE OF ALKYLAROMATIC HYDROCARBONS ON THE KINETICS OF THEIR DEHYDROGENATION

In previous works, the rate of dehydrogenation of alkylbenzenes was studied as a function of the structure of the alkyl radicals. In the present communication, the effect of replacing hydrogen in the ring by a CH\(_3\) group on the rate of dehydrogenation of an alkyl radical was investigated. For this purpose the kinetics of dehydrogenation of the following hydrocarbons was studied: m-isopropyltoluene, symmetrical isopropylxylene, and symmetrical ethylxylene. The experiments were carried out in a flow system on the same oxide catalyst, with dilution of the hydrocarbons by steam in a ratio of 1 : 2–3 by weight. The apparatus and experimental procedure were the same as before \((^{1,2})\).

The catalysts were analyzed by gas–liquid chromatography. Dinonyl sebacate deposited on fireclay brick served as the liquid phase. The contact gas was analyzed, as before, on an Orsa–Eger apparatus \((^2)\). The catalyst charge was 14.6 ml. After each experiment the catalyst was purged with a steam–air mixture and with air at the reaction temperature. The activity of the catalyst was monitored and remained constant during the experiments. In the experiments the catalyzate obtained was colorless and only at high temperatures became yellowish. According to combination-scattering spectra, the isopropyltoluene subjected to dehydrogenation consisted mainly of the m-isomer (90%); ethylxylene consisted of 1,3-dimethyl-5-ethylbenzene (85%) and 1,2-dimethyl-4-ethylbenzene (15%).

Table 1

Physical constants of the starting hydrocarbons *

* Isopropyltoluene and isopropylxylene were kindly provided to us by staff members of the Institute of Petrochemical Processes, for which we express our gratitude.

Before the experiment, the starting hydrocarbons were distilled under vacuum on a column of 30 theoretical plates. The kinetics of hydrocarbon dehydrogenation was studied in the temperature range 490–550° and at a feed rate of 800–1000 ml per 1 l of catalyst per hour. The contact gas was evolved at a constant rate, uniformly. The evolved gas was measured every 3 min. As analysis showed, the exit gas consists mainly of hydro-

gen and a small amount of carbon dioxide (from 3.5 to 9%); no unsaturated hydrocarbons were detected in the gas. A close agreement was observed between the amounts of hydrogen and vinyl compounds formed. The data obtained are given in Table 2.

Table 2

The experimental data are well described by the equation for monomolecular catalytic reactions in a flow system \((^4)\):

\[ \frac{dx}{dt} = k \frac{[A_1]}{[A_1]+Z_2[A_2]+Z_3[A_3]} . \tag{1} \]

The relative adsorption coefficients \(Z_2\) and \(Z_3\) of the reaction products entering into the equation were determined from experiments measuring the rates of dehydrogenation of the initial hydrocarbons with reaction products. Their numerical values were calculated by the formula \((^5)\):

\[ Z= \frac{\dfrac{m_0}{m}-1}{\dfrac{100}{p}-1}. \tag{2} \]

It was found that the relative adsorption coefficient of hydrogen is equal to 0.7 and does not change with temperature. Isopropenyltoluene and isopropenylxylene have close values of the relative adsorption coefficients, varying from 3.8 at 520° to 0.9 at 560°. From Fig. 1 it is evident that between \(Z_2\) and the reciprocal absolute temperature there is a logarithmic

![Fig. 1 and Fig. 2 graphs]

Fig. 1. Logarithmic relationship between the relative adsorption coefficients and the reciprocal absolute temperature: \(a\)—isopropenyltoluene, symmetrical isopropenylxylene, \(\alpha\)-methylstyrene; \(b\)—symmetrical vinylxylene

Fig. 2. Logarithmic relationship between \(k_C\) and the reciprocal temperature: \(a\)—symmetrical isopropylxylene, \(b\)—\(m\)-isopropyltoluene, \(v\)—symmetrical ethylxylene

dependence. The points lie well on straight lines. The numerical values of the relative adsorption coefficients are given in Table 3.

Table 3

Relative adsorption coefficients and thermodynamic functions of adsorption displacement

The rate constants for dehydrogenation of the hydrocarbons studied, calculated from the experimental data by equation (1), are given in Table 4. It is evident from it that the introduction of \(\mathrm{CH_3}\) groups into the benzene ring increases the rate of dehydrogenation of alkyl radicals. Thus, the rate of dehydrogenation of \(m\)-isopropyltoluene is greater than that of isopropylbenzene. Introduction of a second methyl group into the ring promotes an increase in the dehydrogenation rate, and symmetrical isopropylxylene is dehydrogenated at an even higher rate. The same conclusion can be drawn when comparing the con-

Table 4

Rate constants and activation energies

...rate constants of dehydrogenation of the ethyl radical upon replacement of hydrogen in the ring by methyl groups. Figure 2 shows the logarithmic dependence of the rate constants on the reciprocal absolute temperature. The points fall on straight lines; the Arrhenius equation is obeyed. The activation energies calculated from the rate constants are equal to 32.7 kcal/mol for isopropyltoluene, 26.3 kcal/mol for isopropylxylene, and 32.2 kcal/mol for ethylxylene. It should be noted that dehydrogenation of these hydrocarbons under the conditions studied proceeds fairly smoothly, without noticeable formation of side decomposition products. The composition of the catalysts obtained is shown in the chromatograms in Fig. 3.

Fig. 3. Chromatographic analysis of catalysts obtained in the dehydrogenation of symmetrical ethylxylene (a), m-isopropyltoluene (b), and symmetrical isopropylxylene (c).

From the values found for the relative adsorption coefficients, using known formulas, the changes in the free energy $\Delta F$, heat content $\Delta H$, and entropy $\Delta S$ of adsorption displacement were calculated (Table 3).

Received
8 VI 1962

REFERENCES CITED

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3. C. E. Egloff, Physical Constants of Hydrocarbons, N. Y., 1939.
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