Physical Chemistry

N. S. Enikolopyan and G. V. Korolev

On the Yields of Formaldehyde and Acetaldehyde in the High-Temperature Oxidation of Ethane

(Presented by Academician N. N. Semenov, 30 VII 1957)

Concepts leading to the existence of limiting yields of stable intermediate products (SIP) in complex chain reactions ($^1$) were tested on the example of methane oxidation ($^{1,2}$). As an object for further testing of these concepts, the next representative, in order of complexity, of the homologous series of paraffin hydrocarbons—ethane—was chosen.

Fig. 1. Effect of additions of NO$_2$ and N$_2$ on the yield of aldehydes and on the manometric kinetics; $T = 512^\circ$, pressure of the mixture C$_2$H$_6$ : 2O$_2$ 25 mm Hg; H$_2$F$_2$ vessel; dashed line—manometric kinetics; solid curves—kinetics of accumulation of CH$_2$O; dash-dotted line—kinetics of accumulation of CH$_3$CHO; $1,1',1''$—without addition; $2,2',2''$—with addition of 0.085% NO$_2$; $3,3',3''$—with addition of 0.33% NO$_2$; $4,4',4''$—mixture diluted with nitrogen 10-fold.

The behavior of the yields of two SIPs (formaldehyde and acetaldehyde) was studied under variation of the conditions of high-temperature oxidation of C$_2$H$_6$. The apparatus and method of work have been described previously ($^1$). The presence in the polarographically analyzed solution of a mixture of two aldehydes introduced no complications into the analytical procedure, since both gave clearly separated waves in 0.05 N LiOH solution.

The experiments were carried out in two quartz reaction vessels, 45 mm in diameter and 250 ml in volume, with the walls treated by: 1) washing with H$_2$F$_2$ (H$_2$F$_2$ vessel) and 2) washing with 1% K$_2$B$_4$O$_7$ solution (K$_2$B$_4$O$_7$ vessel).

The results of experiments at low pressures (25 mm Hg) for mixtures relatively rich in oxygen (C$_2$H$_6$ : 2O$_2$) are shown in Fig. 1, from which it is seen that additions of a radical initiator (NO$_2$) and dilution of the reaction mixture with an inert gas (N$_2$) greatly increase the concentration of active centers in the reaction system, as may be judged from the strong increase in the rate of pressure rise. The yields of CH$_2$O and CH$_3$CHO in this case practically do not change, i.e., they do not differ from the limiting yields ($^{1,2}$).

The same experiments were repeated at a higher pressure, 53 mm Hg (see Fig. 2). The yield of $\mathrm{CH_2O}$ still did not change when the concentration of active centers was varied over wide limits and was approximately twice as high as the yield of $\mathrm{CH_2O}$ at a pressure of 25 mm Hg. With regard to the yield of $\mathrm{CH_3CHO}$, however, we encountered an unexpected phenomenon: at the very end of the conversion, when the chain processes in the system were sharply inhibited, the rate of accumulation of $\mathrm{CH_3CHO}$ increased sharply (“jump”) (which is especially clearly seen on curve 1 in Fig. 2), and the maximum concentration of $\mathrm{CH_3CHO}$ was reached at the moment when the rate of conversion according to $\Delta p$ had already become equal to zero.

Fig. 2. Effect of additions of $\mathrm{NO_2}$ and $\mathrm{N_2}$ on the yield of aldehydes and on the manometric kinetics; $T = 512^\circ$; mixture pressure $\mathrm{C_2H_6:2O_2}$ 53 mm Hg; $\mathrm{H_2Fe}$ vessel; $1,1',1''$—without addition; $2,2',2''$—with addition of 0.085% $\mathrm{NO_2}$; $3,3',3''$—dilution of the mixture with nitrogen by a factor of 7. The remaining conventional designations are the same as in Fig. 1.

It is evident that at the moment when the rate according to $\Delta p$ in the reaction system becomes equal to zero, the chain processes in it practically cease, since in high-temperature homogeneous oxidation of hydrocarbons in the gas phase CO is necessarily formed, which must be accompanied by an increase in pressure. Consequently, the maximum concentration of $\mathrm{CH_3CHO}$ reached after the “jump” cannot be regarded as the maximum yield of IIP in a complex chain reaction, and ideas about limiting yields are not applicable to it. It follows from this that the formation of $\mathrm{CH_3CHO}$ after the reaction has stopped according to $\Delta p$ is due to some molecular reactions of the initial or intermediate substances.

Let us consider the question of the limiting yields of IIP in a complex chain reaction in which, alongside chain pathways of formation and consumption of IIP, there occurs not only molecular consumption of IIP [2], but also its molecular formation. Then the case under consideration may be represented in the form of the scheme

[
\begin{array}{ccccc}
& \searrow^{(1)} & & \nearrow^{(2)} & \
& & \mathrm{IIP} & & \
& \nearrow_{(1')} & & \searrow_{(2')} &
\end{array}
]

where the heavy arrows depict chain processes, and the light arrows—molecular processes. In this case it is necessary to distinguish the following three cases:

1) The steady-state concentration of the intermediate product (IPP) that would occur in the presence only of processes (1) and (2) (purely chain yield) is higher than the steady-state concentration that would occur in the presence only of processes (1′) and (2′) (purely molecular yield). Then any changes in the reaction conditions that promote an increase in the concentration of active centers in the reacting system (initiation, dilution of the mixture with an inert gas, etc.) will increase the yield of IPP, since with increasing concentration of active centers the role of processes (1) and (2) grows in comparison with processes (1′) and (2′), and the yield of IPP will consequently approach the purely chain yield.

2) The purely chain yield is lower than the purely molecular yield. Then an increase in the concentration of active centers in the reacting system will entail a decrease in the yield of IPP.

3) The purely chain yield practically does not differ from the purely molecular yield. Then variation of the reaction conditions affecting the ratio of the rates of the chain and molecular pathways of formation and consumption of IPP will not affect the yield of IPP.

Case 1) is practically impossible to distinguish from the case:

[
\longrightarrow \ \mathrm{IPP}
\begin{array}{c}
\nearrow\[-2mm]
\searrow
\end{array}
]

observed in uninitiated or insufficiently initiated oxidation of methane (²), and case 3) from the case: (\longrightarrow \mathrm{IPP} \longrightarrow), observed in sufficiently initiated oxidation of (\mathrm{CH_4}) (¹).

Fig. 3. Influence of the nature of the surface of the reaction vessel and of additions of (\mathrm{NO_2}) and (\mathrm{N_2}) on the yield of aldehydes and on the manometric kinetics; (T = 452^\circ); mixture pressure (2\mathrm{C_2H_6} : \mathrm{O_2}) 53 mm Hg; (1, 1′, 1″)—without addition in an (\mathrm{H_2F_2}) vessel; (2, 2′, 2″)—dilution of the mixture with nitrogen by a factor of 5 in an (\mathrm{H_2F_2}) vessel; (3, 3′, 3″)—with addition of 1% (\mathrm{NO_2}) in an (\mathrm{H_2F_2}) vessel; (4, 4′, 4″)—with addition of 1% (\mathrm{NO_2}) in a (\mathrm{K_2B_4O_7}) vessel. The remaining symbols are the same as in Fig. 1.

The observed features of the kinetic behavior of (\mathrm{CH_3CHO}) (the “jump” and the attainment of the maximum concentration at (d(\Delta p)/dt \simeq 0)) can be explained on the assumption that case 2 occurs here. Then it is obvious that, as long as the concentration of active centers in the system is sufficiently large, the concentration of (\mathrm{CH_3CHO}) in the system tends to remain near the purely chain level. With a decrease in the concentration of active centers at the end of the conversion, the role of processes (1′) and (2′) increases in comparison with processes (1) and (2), and the yield of (\mathrm{CH_3CHO}), increasing, tends toward the magnitude of the purely molecular yield. An increase in the concentration of active centers in the system should, from this point of view, always lead to a decrease in the “pre-jump” concentration of (\mathrm{CH_3CHO}). If the concentration of active cent-

of centers is sufficiently large that molecular processes can be neglected in comparison with chain processes, then the “pre-jump” yield of CH₃CHO will be the limiting yield. The results of the experiments shown in Fig. 3 indicate that dilution of the mixture with nitrogen and additions of initiator (1% NO₂) do indeed decrease the “pre-jump” yield of CH₃CHO; moreover, the yield of CH₃CHO in the initiated reaction does not change on going from the H₂F₂ vessel to the K₂B₄O₇ vessel, although the oxidation rate, as measured by Δp and serving as a measure of the concentration of active centers, decreases appreciably. This makes it possible to regard the value 0.03% CH₃CHO in the gas as the limiting yield for the given temperature, pressure, and mixture composition.

Fig. 4. Effect of the surface of the reaction vessel and of additions of NO₂ on the yield of aldehydes and on the manometric kinetics; (T = 512^\circ); pressure of the mixture (2\mathrm{C}_2\mathrm{H}_6 : \mathrm{O}_2) 53 mm Hg; (1,1',1'') — without additive in the (K_2B_4O_7) vessel, (2,2',2'') — without additive in the (H_2F_2) vessel, (3,3',3'') — with addition of 1% (NO_2) in the (K_2B_4O_7) vessel. The remaining conditions and designations are the same as in Fig. 1.

The maximum yield of CH₃CHO (“post-jump”), as is seen from Fig. 3, does not undergo systematic changes with change in the concentration of active centers in the reaction system (as was also to be expected on the basis of the proposed interpretation).

From Figs. 3 and 4 it is evident that the yield of CH₂O in the case of mixtures (2\mathrm{C}_2\mathrm{H}_6 : \mathrm{O}_2) becomes a function of the reaction conditions: conditions that lead to an increase in the concentration of active centers in the system increase the yield of CH₂O. Treatment of the reaction vessel with a (K_2B_4O_7) solution, just as in the case of oxidation of CH₄ (^{(2)}), decreases the reaction rate according to (\Delta p) in comparison with the H₂F₂ vessel. In the noninitiated oxidation of C₂H₆, the transition from the H₂F₂ vessel to the K₂B₄O₇ vessel is accompanied by a considerable decrease in the reaction rate and, simultaneously, by a strong drop in the yield of CH₂O (cf. curves (1') and (2'), Fig. 4). In the initiated oxidation of C₂H₆, the yield of CH₂O practically does not change on going from the H₂F₂ vessel to the K₂B₄O₇ vessel (curves (3') and (4'), Fig. 3), which makes it possible to regard the yield of CH₂O in this case as limiting. Thus, in the noninitiated oxidation of mixtures (2\mathrm{C}_2\mathrm{H}_6 : \mathrm{O}_2), in contrast to mixtures (\mathrm{C}_2\mathrm{H}_6 : 2\mathrm{O}_2), the rate of molecular consumption of CH₂O becomes comparable with the rate of its chain consumption, and the yield of CH₂O becomes lower than the limiting value. Under strong initiation the rate of the molecular path is immeasurably smaller than the rate of the chain path, and the yield practically does not differ from the limiting value.

Institute of Chemical Physics
Academy of Sciences of the USSR

Received
27 VII 1957

CITED LITERATURE

1. N. S. Enikolopyan, G. V. Korolev, G. P. Savushkina, ZhFKh, 31, No. 4, 865 (1957).
2. N. S. Enikolopyan, G. V. Korolev, DAN, 118, No. 5 (1958).