Influence of Butadiene-1,3 on Ethanal Pyrolysis at 495°C

CLAUDE RICHARD and RENE MARTIN Uniuersitk de Nancy I , Laboratoire de Chimie Radicalaire, E.R.A. n0136 du C.N.R.S.,

54037 Nancy, France

Abstract

At 495OC and a low extent of reaction, ethanal pyrolysis is slightly inhibited by the addition of small quantities of butadiene-1,3, whereas it is accelerated by more important quantities. The inhibiting effect is interpreted in terms of a free-radical chain mechanism in which the main chain carriers of ethanal pyrolysis (CH3.free radicals) reversibly add to butadiene-1,3 and yield penten-2-yl (R.) free radicals. These free radicals either react in a metathetical step:

(6) Re + CH3CHO - RH + CH3CO-

or in terminating steps. But butadiene-1,3 also gives rise to new initiation steps:

1,3-C.jHs + CH3CHO - n-C4H7. + CH3CO.

2 1,3-C& - free radicals

which account for the accelerating effect. Process (i3) seems to be more important than process (iz) in the experimental conditions, but its nature could not he identified. The results are consistent with literature data and the following value of k6:

k6 = 1012-12,000/4.57T cm?/mol sec

(4.577' in cal/mol).

Introduction

In previous publications butenes have been shown to exert an important inhibiting effect on ethanal pyrolysis [1,2], whereas ethylene was shown to exert a slight accelerating effect [2,3]. Such a difference in behavior has been ascribed to the fact that butene molecules have mobile hydrogen atoms in the allylic position and may give rise, in the gas phase, to alkenyl free radicals of comparatively low reactivity. By contrast, the ethylene molecule has no mobile hydrogen atom. Thus free radicals that carry the chains of ethanal pyrolysis can only add to ethylene and yield new free radicals of nearly the same reactivity. In order to check the validity of this inter- pretation, we though it useful to examine the effect of another compound having no mobile hydrogen atom. We chose butadiene-1,3.

International Journal of Chemical Kinetics, Vol. XII, 1021-1029 (1980) 0 1980 John Wiley & Sons, Inc. 0538-8066/80/0012-l021$01.00

1022 RICHARD AND MARTIN

Experimental

The reaction was studied in a conventional static system at 495°C and for 100-torr initial pressure of ethanal. The reaction vessel was a 250-ml Pyrex sphere enclosed in an electrically heated steel cylinder. The furnace temperature was controlled to within &0.5"C with a thermoregulator. Pyrex vessels were carefully conditioned with reagents before use. Let us recall that this wall conditioning, which is necessary to a good reproduc- ibility, is easily achieved by leaving a 100-torr mixture of butadiene-1,3- ethanal in the vessel at 495°C for 30 min and pumping it out at a pressure less than torr. An oxygen introduction, or even too long a residence time of reagents and products in the reactor, will damage or destroy the wall conditioning. Finally, greatest care was taken to work on pure and par- ticularly oxygen-free reagents. Ethanal was obtained from paraldehyde depolymerization under nitrogen pressure. Butadiene-1,3 was a product from Phillips Petroleum Co.; its molar purity was less than 99.95%. The course of reaction was followed by gas-chromatographic analyses of the products at various time intervals. Details of the mode of operation have been described elsewhere [ 1-31.

Results

Let us recall that the main primary products of pure ethanal pyrolysis at 495OC are equal amounts of CO and CHI [l-71.

In the presence of butadiene-1,3 (B) we observe the formation of a slight excess of CO over CHI, small quantities of pentenes-1 and -2, and trace amounts of methyl-3-butene-1, isoprene, and heavier products. A quan- titative tabulation of the reaction products is given in Table I. Curves for CO and CH4 formation versus time being typically S-shaped, it is difficult to get accurate values of the initial reaction rates by graphical extrapolation (see Fig. 1, for instance). By contrast, the concentration ratio (CO)/(CH4) of the products formed in the presence of B is a linear function of time (see Fig. 2). This plot yields accurate values of the ratio ( L$c~)B/( L$cH~)B of CO and CH4 formation in the presence of B. Low initial contents of B in the mixture slightly inhibit the initial formations of CO and CH4, whereas higher contents tend to increase these formations. The results at low extent of reaction are gathered in Figure 3 where the initial rate ratio ( V o ) ~ / V o in the presence and in the absence of B is plotted against the initial con- centration of (B)o. In Figure 4 the ratio of the initial rates of CO and CH4 formation in the presence of B, ( L$c~)B/( L$c& is plotted against the initial concentration of (B)o.

BUTADIENE-1,3 AND ETHANAL PYROLYSIS 1023

TABLE I. Composition (%) of reaction products of the pyrolysis of ethanal(100 torr)- butadiene-1,3 (20 torr) mixture at 495°C; extent of reaction 17%.

Reaction products Composition in X

carbon monoxide 50.5

methane 46.5

pentene-l - 0.5 pentene-2 trans - 1.5 pentene-2 cis - 1

methyl-3 butene-1 traces (detected but

not monitored)

detected but not

really searched for

isoprene I other Cs and

heavier products

Interpretation

Pure ethanal pyrolysis is known to be a long chain free-radical reaction. I t is initiated by the process

(ill CH3CHO - CHy + CHO propagated by processes (2) and (3): (2) CH3CO. - + CHr

(3) CHy + CH3CHO - CH4 + CH3CO. and terminated by (tl): (tl) 2 CHy (+ M) -+ (+ M) since the quasistationary concentration of CHy free radicals appears to be much higher than that of CH3CO- free radicals.

In the presence of B, CH3- chain carriers can only add to B in processes (4) and (5): (4, -4) CH3- + B F? CH3CH2CHCH=CH2 or R-

(5, -5) CH3- + B * CH2CH(CH3)CH=CH2 or R: R- and R: free radicals give rise to metathetical steps (6) and (7):

(6)

( 7 ) or decompose in processes (-4) (-5) or (8) and (9):

(8 ) R. - pentadiene-1,3 + H.

(9) R: - isoprene + H-

R- + CH3CHO - pentene-1 or -2 + CH3CO.

R: + CH3CBO - methyl-3 butene-1 + CH3CO-

1024

6 -

RICHARD AND MARTIN

/

0 P

2 4 6

Figure 1. Yields of methane as a function of time during the pyrolysis of pure ethanal (dashed line) or in the presence of butadiene-1,3 (19.9 torr). Po(CH3CHO) = 100 torr.

Lastly, R-, RI, and CHr may disappear by combination or disproportion- ation in chain terminating steps, and B can also give rise to free radicals in bimolecular initiation steps such as:

(i2) B + CH3CHO - free radicals (id 2 B - free radicals This scheme already allows for the formation of pentenes and trace amounts of methyl-3 butene-1 or isoprene at low extent of reaction. Since pentene yields are higher than methyl-3 butene-1, processes (5,) (-5), and (7) are probably negligible before processes (4), (-4), and (6), respectively. Pro- cesses (8) and (9) may also be disregarded as being unimportant before processes (-4) and (-5). If chains are supposed to remain long in the pres- ence of B, this additive introduces a new stoichiometry of copyrolysis [processes (2), (4), and (6))

which adds to the main stoichiometry of ethanal pyrolysis [processes (2) and (3)]:

B + CH3CHO = CO + C ~ H ~ O

CH3CHO = CO + CH4 and accounts for the slight excess of CO over CH4.

BUTADIENE-1.3 AND ETHANAL PYROLYSIS 1025

Figure 2. Concentration ratio (CO)/(CHd as a function of time during the py- rolysis of ethanal(100 torr) in the presence of butadiene-1,3 (19.9 torr).

It seems now desirable to draw some further consequences from the proposed kinetic scheme in order to compare them with our experimental results. In the following we only consider the simplified scheme:

Initiation:

6 2 ) B + CH3CHO - CH3CO. + n-C4Hy

(i3) 2 B - free radicals

Propagations:

CH3CO. -+ CO + CH3. (2) - (3)

(4, -4)

(6)

CH3. + CH3CHO --+ CH4 + CH3CO.

CH3- + B + R* R* + CHBCHO ---+ ?JiJ + CH3CO.

2 CHy (+ M) + -(+ M) Terminations:

(tl) ( tz ) CH3. + R. - /products

I (t3) 2 R - j :

With the usual assumptions-long chains, quasi-stationary state for every

1026 RICHARD AND MARTIN

t

(BI0

mole. cni3X1o6 0

Figure 3. Initial rate ratios ( V o ) ~ / V o of formation of GO and CH4 during the py- rolysis of ethanal(100 torr) in the presence and in the absence of butadiene-1,3 as a function of the initial concentration of butadiene-1,3. - theoretical plots (see text); 0, A-experimental data.

1 2

free radical-and the conventional equality k t , = 2(ktl-kt ,)1/2, we get the following relations:

with a = k4/[k-4 + k~(cH3CHO)ol. ( u ~ ) B and ui represent the initiation rates in the presence and absence of B, respectively.

Relationship (I) is quite consistent with the experimental results of Figure 4 at low (B)o values. The slight deviation that appears at high (B)o values could result from a CH4 excess due to the thermal decomposition of B, which was considered negligible in the scheme. From the tangent slope in Figure 4 at low (B)o values we get

a k6/k3 = 3.8 X 105 cm3/mol

at 768 K and for (CH3CHO)o N 2 X mol/cm3. Using this (Y k6/k3 value, we try to adapt relationship (11) to the experimental data of Figure 3. Concerning the ratio (u;)B/u; , several hypotheses are to be consid- ered.

(1) Addition of B does not give rise to any new initiation step: ( u ~ ) B / u ~

BUTADIENE-1,3 AND ETHANAL PYROLYSIS 1027

[3] 4 0

O N 0

( Blo

1 2

mole .cw3 x w 6 0

Figure 4. Initial rate ratios of co and CHI formation [ ( v & ) ) / ( p C H , ) ] B during the pyrolysis of ethanal (100 torr) in the presence of butadiene-1,3 as a function of the initial concentration of butadiene-1,3.

( Blo

1 2

mole .cw3 x w 6 0

Figure 4. Initial rate ratios of co and CHI formation [ ( v & ) ) / ( p C H , ) ] B during the pyrolysis of ethanal (100 torr) in the presence of butadiene-1,3 as a function of the initial concentration of butadiene-1,3.

= 1. Clearly, relationship (11) cannot account for the accelerating effect of B since it can be shown that k&3 < (kt3/ktl)1/2. This hypothesis must be dismissed.

(2) Addition of B gives rise to a new initiation step (i2). Then we would have (ui)B/ui = 1 + kiz(B)o/kil. Using Rosenbrock's least-squares method [S], we try to fit a theoretical curve equation:

to the experimental data with b = a(k t3 /k t l )1 /2 and c = ki2/kil. It is im- possible to get any b or c value that has a physical meaning. This hy- pothesis is also to be dismissed.

(3) B is supposed to give rise to initiation steps (i2) and (i3). Thus we have (ui)&i = 1 + d(B)o + e(B)i with d 3 kiz/ki, and e = ki,/ki,(CH~CHO)o. The least-squares method may be applied to the theoretical equation

and values of b , d, and e are obtained. Of course the results are not very

1028 RICHARD AND MARTIN

safe. Nevertheless, the experimental data are consistent with the following conclusions. If d varies in a very large range, from 0 to 5 X lo7 cm3/mol, e varies from 3.6 X 10l2 to 10l2 to 1015 cm6/moI2 and b from 2.7 X lo6 to 5 X lo7 cm3/mol, respectively. The best fit seems to be obtained for the following values:

b N 3 X lo6 cm3/mol

e N 1013 cm6/mo12

d N i05-i06 cm3/mol

The corresponding theoretical curve is given in Figure 3. Let us point out that the evaluations of a! k&3 and a (kt3/ktl)1/2, or b, seem to be reasonably precise.

Discussion

From the chemistry point of view the influence of B on ethanal pyrolysis is very similar to that of ethylene [3]. A t initial time, B gives rise to a new stoichiometry of copyrolysis:

B + CH3CHO = CO + CsHio which adds to the stoichiometry of pure ethanal pyrolysis and, thus, ac- counts for the slight excess of CO over CH4.

By contrast, the kinetic influence of B on ethanal pyrolysis at a veiy low extent of reaction is much more complex than that of C2H4 [3]. The chain carriers of ethanal pyrolysis add to B and yield allylic free radicals R. which are less reactive than the former ones. This explains the inhibiting effect that is observed at low initial B concentrations. But B also gives rise to new initiation steps which account for the accelerating effect observed a t high initial B concentrations.

The proposed scheme quantitatively describes the experimental facts and allows to get rate constant values which may be compared to literature data. First, from our values a! k&3 N 3.8 X lo5 cm3/mol at 768 K, obtained for an initial ethanal concentration (CH3CHO)o N 2 X mol/cm3, and from the following literature data (in mol, cm3, sec), with 8 = 4.57T cal/ mol:

[4-71 k 3 = 1012-8000/8

[91 k4 = 1010.9-4ioo/e

k -4 = 1013.2-37600/8 (deduced from thermochemical data around

800 K: K4 = k&-4 E 10-2.3+33*500/8 cm3/mol [9-111) we find for kg at 768 K:

(k6)768K = W7 cm3/mol sec

BUTADIENE-1.3 AND ETHANAL PYROLYSIS 1029

Supposing that the reactivity of R- free radicals on ethanal is nearly the same as that of n-C4Hy free radicals, we would have E6 ru 12 kcal/mol-[l,2] and thus compute the preexponential factor of k6. Our evaluation

Ke N- 1012-12,OOO”J’ cm3/mol sec

seems reasonable. Our best b value is also in agreement with literature data on (Y and the

equality kt,/ktzt, N 0.4. The latter equation only yields an order of magni- tude.

A discussion on d and e values is not appropriate for these values are too ill defined.

The present work only proves the occurrence of new bimolecular initia- tion processes. In order to determine the nature of these processes, it seems now useful to study the thermal reaction of pure butadiene-1,3 at very low extents of reaction.

Bibliography

[l] C. Richard, R. Martin, and M. Niclause, J. Chim. Phys., 70,1151 (1973). [2] C. Richard, doctoral dissertation, Nancy, 1972. 131 C. Richard and R. Martin, J.’ Chim. Phys. 77,353 (1980). [4] G. M. Cbme, M. Dzierzynski, R. Martin, and M. Niclause, C.R. Acad. Sci., 264C, 548,

[5] G. M. Cbme, doctoral dissertation, Nancy, 1968. [6] K. J. Laidler and M. T. H. Liu, Proc. R. SOC. London, Ser. A 297,365 (1967). [7] M. T. H. Liu and K. J. Laidler, Can. J. Chem., 46,479 (1968). [8] H. H. Rosenbrock, Computer J., 8,33 (1965). 191 J. A. Kerr and M. J. Parsonage, “Evaluated Kinetic Data on Gas Phase Addition Reac-

836 (1967); Reu. Inst. Fr. Petrole, 23,1365 (1968).

tions,” Butterworths, London, 1972. [lo] S. W. Benson, “Thermochemical Kinetics,” 2nd ed., Wiley, New York, 1976. [ll] D. R. Stull, E. F. Westrum, Jr., and G. C. Sinke, “The Chemical Thermodynamics of

Organic Compounds,” Wiley, London, 1969.

Received April 15, 1980 Accepted June 19,1980