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Kamiya. Y., Kashima, M., J . Cafal., 25, 326 (1972). de la Mare, P. B. D., Robertson, P. W., J . Chem. Soc., 279 (1943). Morimoto, T., Ogata, Y., J . Chem. SOC.B, 62, 1353 (1967). Powell, M. J. D., Comput. J., 7 , 303 (1964). Scott, E. J. Y.. J . Phys. Chem., 74, 1174 (1970). Scott, E. J. Y., Chester, A. W., J . Phys. Chern., 7 6 , 1520 (1972). Smith, C. L., Pike, R. W., Murrill, P. W., Formulation and Optimization of
Mathematical Models", International Textbook Co., Scranton, Pa., 1970. Vincow, G., "Radicallofls",p 151, E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N.Y., 1968.
Received for review January 9, 1978 Accepted April 18, 1978
The Kinetics of the Autoxidation of Aldehydes in the Presence of Cobalt(I1) and Cobalt(II1) Acetate in Acetic Acid Solution Charles F. Hendriks, Hendrlk C. A. van Beek,' and Pleter M. Heertjes Laboratory of Chemical Technology, University of Technology, Delft, The Netherlands
The mechanism of the autoxidation of several aldehydes in the presence of Co" and Co"' acetate was investigated in rapidly stirred acetic acid solutions. The kinetic parameters were determined. I n the presence of Co" acetate the autoxidation of an aldehyde is inhibited by reduction of the chain carrying acylperoxy radicals by Co", while Co"' acetate, formed by this reaction, contributes to the initiation b oxidation of the aldehyde to acyl radicals. These combined effects explain the autocatalytic conversion of Co into Co"' acetate. In the absence of Co" acetate, the corresponding peracid is formed as the first stable product of the aldehyde autoxidation. Co"' acetate catalyzes the decomposition of these peracids with formation of acylperoxy radicals, which initiate a very fast autocatalytic oxidation of the aldehyde. At high concentrations Co"' acetate also participates in chain termination by oxidizing acyl radicals.
Y'
Introduction In a previous paper (Hendriks et al., 1977) it was demonstrated that the initiation of the autoxidation of aldehydes in acetic acid solutions can be represented by reaction 1, while the propagation appeared to occur by reactions 4 and 5, which were already proposed by Bolland and Gee in 1946. We found that addition of Co" acetate to an acetic acid solution of benzaldehyde resulted in a strong inhibition of the autoxidation, analogous to the inhibition by phenols (Nemecek et al., in press). This was explained by oxidation of Co" acetate to Co"' acetate by the chain carrying benzoylperoxy radicals and perbenzoic acid (reactions 6 and 11) (Hendriks et al., in press). Co"' acetate, formed by reactions 6 and 11,contributes to the initiation by the oxidation of benzaldehyde (reaction 2). The combined effects of Co" and Co"' acetate on the autoxidation of benzaldehyde lead to a sharp increase of the oxidation rate of CO" acetate at increasing conversion. The rate of this autocatalytic reaction decreases at increasing rate of stirring of the solution. In a previous paper the course of the reaction in non- or slowly stirred solutions was discussed (Nemecek et al., 1978). In this paper experiments will be described which were carried out under such vigorous stirring conditions that higher stirring rates did not result in a further decrease of the reaction rate. The kinetics of the conversion of CO" acetate in solutions of several autoxidizing aldehydes were studied with varying initial concentrations of CO" acetate, Co"I acetate, the aldehyde, and oxygen. The influence of Co"' acetate on the autoxidation rate of the aldehydes was studied in the presence and absence of Co" acetate. The role of the decomposition of peracids (reactions 3 and 12) on the initiation of the autoxidation was investigated. From the kinetic results information was obtained concerning the chain termination reactions (reactions 7-10). 0019-7890/78/1217-0260$01 .OO/O
initiation
CO"'
+ RCHO
ColI1 + RC03H
-
k3
CO" + RCO
RCOB.
k2
[CO~~'-RCO~H] P Co"
(2)
+ RC03. (3)
propagation RCO
+ H+
+ O2
+ RCHO
-
+
RC03-
k4
RC03H
+ RCO
(4)
(5)
k5
termination
+ Co"- H+ RC03H + Co"' k6 HOO- + CO"' H+ + 02 + CO" k7 HZO RCO + Co"' RC02H + Co" + H+ k8 RC03.
+
RC03.
+ - + - + + -
2RC03.
RCO HZO
HZO
RC03H + RC02H
RC03H
RC02H + O2
(6) (7) (8)
k9 (9) kIo (10)
Co" oxidation 2Co"
+ RCOBH 2H+
2Co'I'
RCOzH + H 2 0
kll (11)
Baeyer-Villiger
RC03H
RCHO
0 1978 American Chemical Society
2RC02H
k12
(12)
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
Experimental Section Materials. Acetic acid was purified by distillation (bp 118-120 "C a t 1 atm, water content 0.1 M). Co" acetate was of A.R. quality as received. Co"' acetate was prepared by oxidation of Co" acetate by a stoichiometric amount of peracetic acid (Hendriks et al.). Perbenzoic acid was prepared by reaction of dibenzoyl peroxide (0.21 mol) with sodium methoxide (0.20 mol) in a 2:l chloroform/methanol mixture (300 mL); after extraction with water and benzene crystalline perbenzoic acid was obtained by removal of the solvent under reduced pressure (yield 8O%, purity grade 95%). Procedures. The autoxidations were carried out in a cylindrical Pyrex glass vessel (diameter 6 cm, length 15 cm), which was connected at the bottom to a Pyrex glass cell (path length 0.30 cm), placed in a Vitatron UC 200 colorimeter. The reaction vessel, which was transmittant for light with X >320 nm, was irradiated with a 100-W tungsten lamp. The solutions absorbed lo4lumen of the light transmitted. A glass stirrer was used which had two elliptical blades. The stirring rate was 2000 rpm. Reaction rates were measured by determining the extinction of Corn acetate at 652 nm, where absorption of Con acetate is negligible. Co" acetate concentrations were calculated from the difference between the total Co and the Co"' concentrations. Aldehyde concentrations were determined gas chromatographically with a Varian Aerograph 1522-1B, using a 3-m 10% SE-30 on Chromosorb W column. Peracid concentrations were determined by iodometric titration. Consumption of oxygen was measured volumetrically. The initial aldehyde concentrations were varied between and 3 M and the initial Co" and Co"' acetate concentrations between and lo-' M, and the oxygen pressure was varied between 0.2 and 1.0 atm. Reaction products were identified and determined (after conversion of carboxylic acids into the corresponding methyl esters with diazomethane) with GC/MS, using a Varianmat 111 GNOM mass spectrometer. Curve fitting of the concentration curves of reactants and products was carried out by integration of the reaction rate equation according to the Runner procedure and the method of Runge-Kutta (Smith et al., 1970). The minimum of the sum of the squares of the differences between the experimental and calculated values was determined with the aid of the Optow procedure and the method of Powell (1964). To control the accuracy, the calculations were carried out with two time-step values with a difference of a factor 2.
0 Ol0;d
Therefore the overall reaction can be written as
-
C6HbCHO + 02 + 2Co1I(CH3C02)2+ 2CH3COOH C&&OOH + 2C01"(CH3C02)3 + H2O (14)
It was found that a t longer reaction times the relative conversion of benzaldehyde and oxygen was higher than
7
_c
t 1rnl"l
Figure 1. The oxidation of 0.016 M Co" acetate with benzaldehyde (B) in acetic acid solution (25 "C, p o p = 1 atrn): 0,0, [B], = 1.40 M; .,0, [B], = 0.90 M; A,A,[B], = 0.60 M; -, calculated curves (curve fitting).
-
Results Addition of Con acetate to an aerobic acetic acid solution of an aldehyde results in the formation of the corresponding carboxylic acid, water, and Co"' acetate. No measurable amounts of peracids could be detected. For experiments carried out with benzaldehyde, the initial rates of conversion of reactants and formation of the products were measured. The following rate ratios were determined. (-d[C6H&HO] /dt):(-d[Ozl /dt): (-d[Co"] /dt):(d[C6HbCOzH] /dt):(d[Co"'] /dt) = 1:0.98:2.05:0.99:2.05 (13)
261
9
0 061
Figure 2. The oxidation of Co" acetate (A) in the presence of 1.40 M benzaldehyde in acetic acid solution (25 "C, p o p = 1 atrn): 0 , [A], = 0.01 M; X, [A], = 0.02 M; A, [A], = 0.032 M; 0,[A], = 0.046 M; 0,[A], = 0.062 M; -, calculated curves (curve fitting).
is indicated by eq 14. This is illustrated in Figure 1. This result was also obtained for the initial rate, if a mixture of Con and Corn acetate was added to an aerobic acetic acid solution of benzaldehyde. Under these conditions the stoichiometry of the overall reaction was found to be (1
+ (Y)C&jCHO+ (1
'/p~)02
+
-
2C01'(CH3C02-) + 2CH3COzH (1 + cy)C6H&OOH H 2 0 + 2Co"'(CH3C02J3 (15)
+
in which cy is a proportionality factor L 0. The value of cy increased with increasing conversion of Co" (see Figure 1). The rates of conversion of Co" acetate were determined for solutions in which the initial concentrations of Co" acetate, Co"' acetate, benzaldehyde, and oxygen were varied. Typical results are given in Figures 1, 2, and 3. The rate of conversion of Co" increased with increasing benzaldehyde and Co"' concentration (see Figures 1 and 3) and decreased with increasing Con concentration (Figure 2). The influence of the concentration of Co"' acetate on the reaction rate is complicated because at concentrations >0.01 M further addition of Co"' acetate results in a decrease of the oxidation rate of Co" acetate (Figure 3). It was already demonstrated in a previous paper (Nemecek et al.) that the rate of conversion of CO" acetate is independent of the oxygen concentration at p02> 0.2 atm. It was found, however, that if the ratio of the initial concentrations of Co" acetate and benzaldehyde was > 0.030, the initial rates of oxidation of CO" in the absence of Co"' acetate were linear with the benzaldehyde concentration and independent of the Co" concentration. The first-order rate constant k l has a value of 9 x lo-' min-' (25 OC). For the autoxidation of benzaldehyde, inhibited by phenols, the same kinetics and the same value (9 X min-') for the rate constant were found (Hendriks et al.,
262
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
Table I. The Kinetics of Aldehyde Autoxidation in Acetic Acid at 2 5 C
benzaldehyde 4-methylbenzaldehyde 4-isopropylbenzaldeh y de 4-chlorobenzaldehyde 4- bromobenzaldehyde 4-methoxybenzaldehyde 4-formylbenzaldehyde
2,4-dichlorobenzaldehyde 3,4-dichlorobenzaldehyde 2,6-dichlorobenzaldehyde 1-naphtaldehyde 2-naphtaldehy de hexanal-1 cy clohexanecarboxaldeh y de
0.9 3.2 1.1 2.0 3.8 0.6 4.2 3.6 1.9 3.5 2.3 0.7 0.9 2.5
2.1 10.1 19.2 4.5 55.1 41.3 24.5 6.3 7.4 7.2 86.2 66.5 58.6 22.4
19.0 32.4 28.4 18.4 19.2 20.1 19.2 17.6 16.4 10.7 21.5 21.0 14.1 23.1
11.4 11.4 15.0 32.2 11.0 9.7 14.1 15.3 37.7 19.0 26.1 20.2 24.0 96.7
2.1 0.6 1.2 3.0 2.3 2.5 2.7 2.9 3.0 0.9 0.9 14.1 9.0
0.15 0.03 0.02 0.06 0.04 0.03 0.16 0.07 0.14 0.05 0.04 0.05 0.04
216 72 142 304 254 202 348 379 421 309 298 268 301
10
15
6.3 4.2 1.4 1.3 3.1 2.1 2.0 5.1 3.4 4.5 2.8 3.0 2.4 2.6
3i 0
__t
t (rnin)
Figure 3. The oxidation of 0.01 M CO" acetate with 1.40 M benzaldehyde in acetic acid solution (25 "C, p o p = 1 atm) in the presence of Co"' acetate (C): 0 , [C], = 0; X, [C], = 0.01 M; A, [C], = 0.022 M; 0,[C], = 0.036 M; 0,[C], = 0.052 M; -, calculated curves (curve fitting).
tl If' __c
+
lo3
t
f
(min)
Figure 4. The oxidation of 0.1-2.0 M benzaldehyde by lo4 to lo-, M Co"' acetate in anaerobic acetic acid solution (25 "C).
1977). Similar results were obtained with other aldehydes (see Table I). In order to measure the rate of oxidation of the aldehydes by Con' acetate, this reaction was studied separately under anaerobic conditions to exclude autoxidation. Reaction products found were Co" acetate, acetic acid anhydride, and the carboxylic acid, derived from the aldehyde which was used. For the reaction between Coni acetate and benzaldehyde the relative conversions are (-d [C o"'] / dt ) : (-d [CGHBCHO] /dt ): (d [ CO"] /dt): (d [CGH~COOH] /dt) = 1:0.52:1.00:0.51 (16) The overall reaction can therefore be represented by ~CO"'(CH,CO~-)~C6HBCH0-, 2C01'(CH3C02-)2 + CGHSCOOH+ (CH3CO)zO (17) The same stoichiometry was found for other aldehydes. The reaction rates were found to be linear with the concentration of Co"' acetate and the aldehyde, as is illustrated for benzaldehyde in Figure 4. The second-order rate constants k 2 are summarized in Table I. When Co"' acetate is added to an aerobic acetic acid solution of an aldehyde a relatively very fast autoxidation occurs. Therefore, when the reaction is carried out by starting with Con acetate, the completion of the oxidation of Co" acetate coincides with a strong increase in the autoxidation rate of the aldehyde. Until the aldehyde is completely oxidized, the concentration of Co'" remains constant and the concentration of Co" is negligible. The main reaction products found were the corresponding acid and peracid. The corresponding hydrocarbon (0.01-0.02%) and phenol (0.01%) and methyl acetate (0.05-0.2%) were found as by-products.
i
0 5
1 __c
100 t (min)
200
300
400
1
500
Figure 5. The autoxidation of 0.18 M benzaldehyde in acetic acid M Co"' acetate. Experimentak 0 , at 25 "C in the presence of benzaldehyde (pOz = 1atm); 0,benzaldehyde (PO, = 0.20 atm); A, amount of oxygen converted (mol/L, p 0 2 = 1 atrn); 0,perbenzoic acid @O, = 1atm); X, benzoic acid (PO, = 1 atm);-, calculated curves (curve fitting).
The peracid concentration reaches a maximum during the catalytic autoxidation of an aldehyde. A t relatively low Coni concentration (0.01 M. Based on these facts, the oxidation of Con acetate in an aerobic acetic acid solution of an aldehyde should be explained by reactions 1,2,4-8, and 11. According to this reaction model the rate equations for the consumption of the aldehyde and Co" acetate were derived, assuming steady-state concentrations for the intermediate radicals. Curve fitting of the concentration curves of the aldehyde and Co" acetate with these rate equations gave with an accuracy of 3 7% constant values for the ratios k4/k8 and k5/k6. The difference between the calculated and the experimental concentrations is negligible, as is shown by Figures 1, 2 , and 3. When the inhibiting Con acetate is completely converted into Cornacetate, the catalytic autoxidation of the aldehyde results in the formation of peracid in the solution. Jn a following paper strong indications will be described showing that peracids are decomposed by Corn acetate into the corresponding acylperoxy radicals (Hendriks et al., in press). This reaction can contribute to the initiation of the autoxidation of the aldehyde because the acylperoxy radicals are propagating the chain reaction. This is confirmed by the increase which was found for the autoxidation rate, if a peracid was added to the solution. By comparing the rate constants of the possible initiation reactions 1, 2, and 3 (see Table I), it follows that decomposition of peracids by Co"' acetate (3) should be the most important initiation reaction already at very low peracid concentrations. This can be illustrated for the typical experiment of Figure 1 by calculating the rates of the initiation reactions 1, 2, and 3 at 2% aldehyde conversion: respectively 1.8 X 4X and 8 X M-l min-l. The autocatalytic effect of the peracid on the aldehyde oxidation was neglected by other authors (Bawn et al., 1956; Boga et al., 1973). Therefore our results indicate a more complex rate equation than is given by Bawn et al. (-d[RCHO]/dt = ~ [ C O ~ ~ I ] ~ / ~ [ R Cand H OBoga ] ~ ' ~et ) al. ] ) . decrease of the (-d[RCHO]/dt = ~ [ C O ~ ] ' / ~ [ R C H OThe reaction rate constant found by Boga et al. toward the end of the autoxidation was explained by them by assuming a decrease in catalyst activity. Based on our experiments the explanation should be that this decrease is caused by a decrease in the peracid concentration (see Figure 6). The kinetic relations given above are not valid for our experiments, as is illustrated by the nonlinearity in Figure 7. Therefore we applied a curve-fitting procedure for the concentration curves of the aldehyde and the peracid by using a reaction model consisting of initiation reactions 2 and 3, propagation reactions 4 and 5, termination reactions 8-10, the oxidation of Con acetate by peracids ( l l ) , and the Baeyer-Villiger reaction (12), assuming a steady N
264
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
state for the intermediate radical concentrations. By using the values of k z , k,, k 4 / h 8(see above) and klz (Hendriks et al., 19771, constant values with an accuracy of -3% were found for k 4 k 5 / k 9and k42/h,o(Table I). The difference between the calculated and the experimental concentrations is negligible, as is shown by Figures 5 and
follows from eq 18, which can be solved by using eq 19. With these equations an optimum catalyst concentration M was calculated. of 1.8 X d[P]/dt = (1.6 X 1.4 X lO-'[P])[B]
-
6.
d[B]/dt = (1.6
lo-,
The value of h4'/klo for the catalytic autoxidation of benzaldehyde is lower than the value determined by Ingles and Melville in 1953 (1.2 M-' min-') and Zaikov et al. in 1969 (2.0 M-' min-l) for the photooxidation of benzaldehyde in chlorobenzene. It appeared by the curvefitting procedure that at Co"' concentrations M the effect of reactions 8 and 10 on chain termination could be neglected. A t higher Co"' concentrations the effect of termination reaction 9 could be neglected. For a semiquantitative calculation of the rate of the catalytic aldehyde autoxidation, a simplified model can be used. Therefore we approximate the rate equation by assuming that the reaction is first order in the concentration of the aldehyde (see Figure 7). Pseudo-first-order rate constants were calculated by determining the minimum of the squares of the deviation cf this relation. The values of the pseudo-first-order rate constants thus obtained were found to be linear with a maximum deviation of -10% with [CO"']~~~[O,] at Co"' concentrations
