Znd. Eng. Chem. Res. 1991,30,617-623
617
Liquid-Phase Ozonation of Cyclohexanol Catalyzed by Cobalt(111) Acetylacetonate Jose M. Encinar and Fernando J. Beltrln* Departamento de Zngenieria Quimica y Energetica, Unioersidad de Extremadura, 06071 Badajor, Spain
JesQs M . Frades Departamento de Quimica, Uniuersidad de Castilla-La Manchu, 13400 Almaden, Spain
Liquid-phase oxidation of cyclohexanol has been studied in a semibatch reactor using an ozone-oxygen mixture as oxidant and cobalt(II1) acetylacetonate as catalyst. The influence of temperature, ozone partial pressure, and catalyst loading on cyclohexanol conversion and product distribution has been observed. The experiments carried out in the presence of catalyst yield a product distribution very different from that obtained from the noncatalytic oxidation; cyclohexanol conversion observed is higher, peroxide products are not formed, and cyclohexanohe and adipic acid yields increase in the presence of catalyst. A reaction mechanism accounting for the main oxidation products has been proposed. The initial oxidation rate of cyclohexanol and formation rates of cyclohexanone and adipic acid have been obtained from an empirical equation that fits experimental data with deviations lower than &lo%.
Introduction In the chemical industry adipic acid is obtained by means of a two-step cyclohexane oxidation process. First, cyclohexane is oxidized by air to yield a cyclohexanolcyclohexanone mixture which is further treated with nitric acid to produce adipic acid and other secondary products (glutaric, hexanoic, pentanoic acids, etc.) (Tanaka, 1974a; Berezin et al., 1966). Another way, also used in the chemical process industry, is the hydrogenation of phenol to give cyclohexanol and subsequently adipic acid via nitric acid oxidation (Winstrom, 1957). These processes consume high quantities of nitric acid, and they present some problems derived from the corrosive action of this acid. Therefore, several works have focused either on the use of oxygen as oxidant in the second step of oxidation or on carrying out the process in one step starting with cyclohexane under different experimental conditions (high pressure, with catalysts, etc.) (Reis, 1965, 1971; Tanaka, 1974b; Rao and Raghunathan, 1984,1986). In this way, both homogeneous and heterogeneous oxidation of cyclohexane have been carried out. In the fiist case acetic acid as solvent and salts of cobalt, copper, or manganese as catalyst have been used (Danley and Campbell, 1978). In the second case cobalt salts supported by different resins have been applied at moderately low pressures ranging from 5 to 15 atm (Shen and Weng, 1988). Either the oxidation of hydrocarbons, specifically, or that of organic substances, generally, can also be carried out under some advantageous conditions using ozone as oxidant agent. Ozone is a powerful oxidant that can be used to produce very valuable intermediate reaction products, such as hydroperoxides (Sotelo et al., 1985), or final reaction products of great industrial interest such as pelargonic, azelaic, or adipic acids (Oehlschlaeger, 1980). From these literature data the important role that catalysts play in different steps of the reaction mechanism through which oxidation of organic substances proceeds can be observed (Berezin et al., 1966). Among the catalysts, Cobalt(1II) acetylacetonate has been proven to be very effective in this type of processes (Tanaka, 1974b). Therefore, in this paper the results of the liquid-phase oxidation of cyclohexanol using ozone as main oxidant and *Author to whom correspondence should be addressed.
cobalt(II1) acetylacetonate as catalyst are presented with two major aims: (a) to determine the effect of several parameters (ozone partial pressure, catalyst concentration, and temperature) on the reaction rate and product distribution and (b) to show a kinetic study together with a reaction mechanism that justifies the main products obtained.
Experimental Section Materials. Reactive or analytical grade cyclohexanol, cyclohexanone, pentanoic, hexanoic, and adipic acids, and cobalt(II1) acetylacetonate were obtained from Merck. Ozone was produced from oxygen in a commercial Constrema ozonator. Experimental Setup. The experiments were carried out in the experimental setup shown in Figure 1. It consisted of an ozone generator system and a reactor. The ozone generator was a commercial ozonator, 1,which was fed with oxygen. The ozonator was provided with an electrical transformer, 3, able to modify the ozone percentage in the gas between 0 and 6% (v/v), and a rotameter, 4, which allows the gas flow rate to be varied between 20 and 200 L-h-'. The reactor, 5, was a 500-cmS glass spherical vessel provided with inlets for sampling, temperature measuring, feeding the gas mixture (ozone diluted in oxygen), and venting. The reactor was submerged in a thermostatic bath to keep the temperature constant within h0.5 O C . Analytical Methods. The unreacted cyclohexanol and remaining cyclohexanone concentrations were analyzed by gas chromatography in a stainless steel column (2-mm i.d., 3-m length). The stationary phase was of 10% 20 M Carbowax and 2% potassium hydroxide on Chromosorb WAW (80-100 mesh). Individual carboxylic acids (pentanoic, hexanoic, glutaric, and adipic acids) were also analyzed chromatographically after preparation of their methyl esthers. In this case, another stainless steel column (same dimensions as before) containing 10% FFAP on Chromosorb W-HP (100-120 mesh) was used. The hydroperoxide and ozone gas concentrations were determined iodometrically (Wagner et al., 1947; Kolthoff and Belcher, 1957). Finally, total acidity was determined by means of acid-base volumetric analysis using methanol as solvent and phenolphthalein as indicator. This was used because the acids formed were very weaks (pK,s greater than 4.8).
0888-5885/91/2630-0617$02.50/00 1991 American Chemical Society
618 Ind. Eng. Chem. Res., Vol. 30, No. 4,1991
9
2
4 Adipic Acid
Figure 1. Experimental apparatus.
1
GLutaric Acid
Pentanoic Acid
Hexanoic Acid\
A 3
6
12
9
18
15 time.h
7
/-Cyclohexanone
Figure 3. Typical distribution of acid products. Reaction temperature 100 OC; ozone partial pressure: 2.2 X 1W2atm; catalyst loading 50 mgL-'.
I
'e
o
catalyst
50mg.i'
uncatalyzd
time, h
7
Figure 2. Typical liquid reactant/product concentration profile. Reaction temperature 100 O C ; ozone partial pressure 2.2 X atm; catalyst loading 50 mg.L-'.
Procedure. The oxidation experiments were carried out at constant temperature. Once the reaction mixture (300 cm3 of cyclohexanol and the appropriate amount of catalyst) was fed to the reactor, the temperature was increased up to a given value. During this time the reaction was purged with a stream of nitrogen. Subsequently, an ozone-oxygen mixture was fed to the reaction volume and then samples were withdrawn at regular intervals. Agitation speed and gas flow rate were kept constant (300 rpm and 50 Lab-', respectively) during the experiment. Results and Discussion The influence of three variables (temperature 80,90,and 100 "C), ozone partial pressure (0.5 X to 2.65 X atm) and catalyst concentration (0, 25, 100, 200, and 400 mg-L-' Co(II1)) have been studied. Figure 2 shows as an example the distribution curves of cyclohexanol, cyclohexanone, hydroperoxide, and total acidity concentrations versus time. It is observed that cyclohexanone concentration first increases with time to reach a maximum value, then decreases for higher oxidation times (not shown), and always accounts for a 50% conversion of cyclohexanol. The major carboxylic acids formed were adipic and glutaric acids (dibasic acids); the former accounts for about 55% of the total acidity (see Figure 3). Pentanoic and hexanoic acids were also determined quantitatively though they represent only a minor fraction of total acidity. Also, other compounds, succinic and butanoic acids and the adipic acid
Cyclohexanol
65-
-4E
c
4-
3I
2-
3
6
9
12
15
18
time,h
Figure 4. Cyclohexanoland cyclohexanone concentration evolution with reaction time for catalytic and noncatalytic oxidations. Reaction temperature 100 'C; ozone partial pressure: 2.2 x atm.
hemialdehyde, were detected but not quantified. The s u m of their concentrations, plotted versus time in Figure 3 as the curve named "other acids", was calculated from the difference between total acidity and concentrations of the quantified carboxylic acids. In addition, total hydroperoxide concentration (hydrogen peroxide and l-hydroxy1-cyclohexylhydroperoxide) was also determined although, as appreciated from Figure 2, it can be considered negligible if compared to that obtained in the absence of cat-
Ind. Eng. Chem. Res., Vol. 30, No. 4,1991 619 0
catalyst: 50mg.C1
o
uncatalyzed
W g , Total Hydroperoxide
3
6
9
12
15
18
time,h Figure 5. Hydroperoxide and adipic acid concentration evolution with reaction time for catalytic and noncatalytic oxidations. Reacatm. tion temperature 100 "C; Po3 = 2.2 x
3
6
9
15
12
18
time, h
Figure 7. Effect of temperature on the formation of adipic acid. atm; [Co(III)] = 50 mpL-'. Ozone partial pressure 2.2 X Table I. Effect of Reaction Temperature on Cyclohexanol Oxidation. Initial Reaction Rates and Selectivities"
T, -rcoL, "C Msh-' 80 0.34 90 0.41 100 0.58
selectivity,* % glutaric rmp, adipic ~CONA, M.h-' Meh-' acid acid 0.27 0.024 9.70 3.60 3.40 0.35 0.031 9.14 3.60 0.44 0.042 9.50
'Experimental conditions: Po3 = 2.20 X mgL-'. For 50% cyclohexanol conversion.
0
Glutaric Acid
O - d p e n t a n o i c Acid 3
6
9
Hexanoic Acid
12 15 time,h
18
Figure 6. Typical distribution of acid products for noncatalytic oxidations. Reaction temperature 100 "C; ozone partial pressure 2.2 x atm.
alyst (Sotelo et al., 1986) (see Figure 5). Figures 4 and 5 present the evolution of product concentrations against reaction time for two experiments carried out with and without catalyst. The most significant differences were as follows: firstly, catalytic oxidation yields a negligible concentration of total hydroperoxide (maximum value was lower than 0.02 M), and secondly, concentrations of cyclohexanone and adipic acid in the catalytic experiments were about 50% and 25% higher
adipic/total acid,b equiv equiv-' 0.48 0.51 0.56
atm; [Co(III)] = 50
than those obtained from noncatalytic oxidation. Consequently conversion of cyclohexanol was also higher with catalyst. On the other hand, from Figures 3 and 6 the differences between catalytic and noncatalytic experiments regarding the acid product distribution can be observed. Adipic and glutaric acids are the major acids formed in the catalytic oxidation of cyclohexanol while in the absence of catalyst the concentration of both acids diminishes and that of pentanoic and hexanoic acids increases. These features are commented below taking into account a reaction mechanism. Influence of Operating Variables: Influence of Temperature. Figure 7 shows the variation of adipic acid concentration with time for different reaction temperatures. It can be seen that adipic acid concentration varies linearly with time except for a short initial period (induction period). It can also be observed that the greater the temperature the shorter the induction period. Table I presents the variation of the initial disappearance rate of cyclohexanol (-rcoL) and the initial formation rates of cyclohexanone (rCONA) and adipic acid (rmIp) with temperature. In the latter case, the short induction period is not considered. Adipic and glutaric acid selectivities and the ratio between adipic acid and total acid concentrations are also shown in Table I. It can also be observed that selectivity of both adipic and glutaric acids remains practically constant whatever
620 Ind. Eng. Chem. Res., Vol. 30,No. 4, 1991 Table 11. Effect of Ozone Partial Pressure on Cyclohexanol Oxidation. Initial Reaction Rates, Selectivities, and 50% Conversion Times0 1O2P0,, atm 0.50 1-00 1.45 2.20 2.65
-rcoL, M-h-' 0.18 0.26 0.49 0.51 0.55
rCONA,M.h-' 0.16 0.26 0.41 0.43 0.46
rmp, M-h-l 0.011 0.015 0.020 0.031 0.033
selectivity) % adipic acid glutaric acid 5.14 2.92 5.98 3.89 4.33 2.27 6.49 2.81 6.06 3.46
adipic/total acid) equiwequiv-1 0.46 0.50 0.44 0.48 0.46
time! h
32 16.5 11.5 10.5 9
"Experimental conditions: T = 100 O C ; [Co(III)] = 100 mgL-1. *For 50% cyclohexanol conversion.
I
[Co(111)3, q L - '
Figure 8. Influence of catalyst concentration on cyclohexanol con-
Figure 9. Influence of catalyst concentration on adipic acid/total
version.
acid concentration ratio.
the temperature, while the initial rates of cyclohexanol disappearance and product formation increase with the increasing temperature. In any case, the ratio between the concentrations of adipic acid and the rest of the acids increases with temperature probably due to the decrease of monobasic acid concentrations. Influence of Ozone Partial Pressure. From a qualitative point of view the influence of ozone partial pressure on the catalytic oxidation of cyclohexanol is similar to that presented in the preceding section in study of the temperature effect; that is, the concentration of adipic acid is a linear function of time regardless of the ozone partial pressure except for the induction period. Table 11, in a way similar to that of Table I, presents initial rates of products, acid selectivities, and ratios between adipic acid and total acid concentrations for different ozone partial pressures. The values presented correspond to reaction times needed to reach a 50% conversion of cyclohexanol. Ozone partial pressure has a positive influence on the oxidation of cyclohexanol; that is, an increase of that variable leads to an increase of both the disappearance rate of cyclohexanol and the formation rate of the main reaction products (cyclohexanone and adipic acid). On the other hand, the selectivity of adipic and glutaric acids and the ratio between adipic acid and total acid concentrations remain practically constant regardless of the ozone partial pressure applied. Obviously, the time needed to reach a 50% conversion of cyclohexanol diminishes with the in-
creasing ozone partial pressure applied. Influence of Catalyst Concentration. Figure 8 shows the variation of cyclohexanol conversion with the amount of catalyst for different reaction times. It is observed that the presence of catalyst concentrations lower than 400 mgL-' leads to cyclohexanol conversions higher than that obtained from the noncatalytic experiment. It seems that 50 mgL-' is the optimum averaged value for the catalyst concentration to get the highest cyclohexanol conversion. The presence of catalyst concentrations lower than 400 mgL-' increases the yield of cyclohexanone between about 40-50% compared to that obtained in the noncatalytic experiment. Figure 9 shows the variation of the ratio between adipic acid and total acid concentration (given in equiv-l-') with the catalyst concentration for different reaction times. The catalyst effect on this ratio is positive for concentrations of cobalt(II1) acetylacetonate lower than 400 mg.L-'. The best results are obtained with 50 mgL-' of catalyst. Finally, Table I11 presents some data about catalyst concentration influence on the variation of the initial disappearance rate of cyclohexanol, the initial formation rates of cyclohexanone and adipic acid, and the selectivity values of adipic and glutaric acids, the latter to reach a 50 % cyclohexanol conversion. The results obtained are in agreement with those shown in Figures 8 and 9. The highest formation rates, maximum selectivities, and minimum time required for a 50% cyclohexanol conversion
Ind. Eng. Chem. Res., Vol. 30, No. 4, 1991 621 Table 111. Effect of Catalyst Concentration on Cyclohexanol Oxidation. Initial Reaction Rates, Selectivities, and 60% Conversion Times" [Co(III)I, -rcoL, M-h-' 0.37 0.43 0.58 0.51 0.39 0.37
mg.L-' 0 25
50 100 200 400
selectivity? % rmp, adipic glutaric MBh-' acid acid 2.81 0.013 6.82 2.53 0.017 7.83 0.042 9.51 3.61 2.81 0.031 6.49 2.63 0.020 6.10 2.81 0.010 5.40
rCONA,
M.h-l 0.20 0.37 0.44 0.43 0.35 0.24
time! h 12 10.5 10 10.5 11.5 18
"Experimental conditions: T = 100 O C ; Po3 = 2.20 X *For a 50% cyclohexanol conversion.
atm.
are obtained with the presence of a 50 mgL-' catalyst concentration. Reaction Mechanism. It is well-known that hydrocarbon liquid-phase oxidation develops through a freeradical mechanism involving a great number of elementary reactions (Emanuel et al., 1967; Berezin et al., 1966; Hendry et al., 1976). This mechanism is largely affected by the presence of transition metal cation based catalysts. These catalysts play a double role: on one hand they increase the formation rate of free radicals and they act as radical chain terminating agents. The following radical mechanism for the oxidation of cyclohexanol with an oxygen-ozone mixture in the presence of cobalt(II1) acetylacetonate is proposed: initiation steps
- +
RH (cyclohexanol) + O3 Co(II1)
+ RH
R' + 'OH
+ O2
R' + H+
Co(I1)
(1)
(2)
propagation steps
+ 02
-+ - + + + + + + - + + + - + + -+ -+ + + + - + R'
R02'
4
RO2'
R = O (cyclohexanone)
RH
Co(II1) Co(I1)
R'
HO2'
H202
H2Oz
(3)
H02'
HzO2
(4) (5)
Co(I1)
H+
HOz'
Co(II1)
'OH
OH-
(6) (7)
termination steps RO2' ROz'
Co(I1)
Co(II1)
H+
Co(II1)
Co(I1)
H+
OH-
H20
free radicals ('OH; HO,'; R')
ROO'02
(8)
Re0
products
(9) (10) (11)
In addition to reaction 4, the following step could be considered: RO,' RH R' ROOH (44 However, at the temperatures applied in this work this reaction is negligible (Andr6 and Lemaire, 1969). Also, there exists an equilibrium between hydrogen peroxide and 1-hydroxy-1-cyclohexylhydroperoxide: R e 0 + HzOz e ROOH (12) The equilibrium constant is about 0.1 M-' for temperatures higher than 90 "C. This indicates that hydroperoxide decomposes to give hydrogen peroxide and cyclohexanone (Denisov and Kharitonov, 1964). As reaction 4a can be neglected, hydroperoxide radicals, H02', can be considered as the main propagating species of the chain
mechanism. For these radicals a first-order termination reaction is proposed (reaction 11)given their small size and the high ratio between the reactor surface and volume. As can be seen, catalysts participate in the proposed mechanism through reaction 2 as initiators, reaction 6 as propagation agents by generating hydroperoxide radicals, and, finally, reactions 8 and 9 to terminate the radical mechanism. From the sequence of reactfons 1-11, the differences observed between the catalytic and noncatalytic oxidation of cyclohexanol can be explained as follows: 1. Reactions [2] and [6] contribute to the cyclohexanol consumption. The former by generating a hydrocyclohexyl radical R', the second by giving rise to a hydroxyhydroperoxide radical so that the elimination of cyclohexanol is increased via reaction [5]. 2. The absence of hydroperoxide or peroxide compounds is in well agreement with reactions [6] and [7] or similar reactions for other hydroperoxides such as l-hydroxy-lcyclohexyl hydroperoxide or a-ketocyclohexyl hydroperoxide. 3. Three circumstances contribute to the increase of cyclohexanone concentration: firstly, the slight increase in cyclohexanol conversion; secondly, the absence of hydrogen peroxide (so there is no direct reaction of equilibrium 12); and thirdly, the occurrence of reaction 9. On the other hand, the fact that catalyst concentration has no influence on the process rate (at least for a given range of its concentration) can be due to the interchange between the species Co(I1) and Co(II1) in some reaction steps. Theoretically, there is no consumption of these species so their sum should be constant except if one of them would give rise to some molecular or nondissociating species. The results obtained when the catalyst concentration was high (especially for 400 mgL-' Co(II1)) suggest that there must be a value of the catalyst concentration above which the termination step rates are faster than the initiation rates. The distribution of acid products and other differences observed in catalytic and noncatalytic experiments (Figures 3, 6, and 9) are related to the formation of the acids. Basically, dibasic acids are formed from cyclohexanone decomposition (Pritzkow, 1954, 1955) the a-ketohydroperoxide being the first intermediate. Decomposition of this product yields cyclohexanone, hydroxycyclohexanone and adipic acid hemialdehyde; the latter eventually leads to adipic and glutaric acids and t-caprolactone. Another possible way (Denisov and Denisova, 1964) is the reaction between cyclohexanone and a-ketocyclohexyl hydroperoxide and hydroxycyclohexyl hydroperoxide radicals, although this way could be inhibited because of the scavenging effect of cobalt cations. In any case, it seems that the first way is the most probable as report by literature (Denisov et al., 1977). Monobasic acids (hexanoic and pentanoic acids) are formed mainly through radicals via pentanoic acid by means of the reaction between the adipic acid hemialdehyde and hydroxy hydroperoxides and subsequent decarboxylation while hexanoic acid is formed from the hydroxycyclohexyl alkoxide radical. These reactions cannot be developed in great extent because of the inhibiting effect of cobalt cations. These facts verify that adipic acid selectivity is higher than that of the monobasic acids. Reaction Kinetics. Reactions 1-11 can lead to the kinetic equation of cyclohexanol consumption; however, it is not possible to solve it because of the process complexity and the lack of data about rate constants of in-
622 Ind. Eng. Chem. Res., Vol. 30,No. 4, 1991
Cyclohexanol Ea: 29,3 kJ.ml-'
-1
-1
Cyclohexanone Ear271 kJ.mol-' -1
-2 L
-
C d
L C
Adipic Acid -3
-4
-L
-I
1
8
,5
-5,0
-4.5
-4,O
I
-33
I n Po3
Figure 10. Effect of temperature on the rates of cyclohexanol ox-
Figure 11. Effect of ozone partial pressure on rates of cyclohexanol
idation and cyclohexanone and adipic acid formation. Ozone partial pressure 2.2 X lo-* atm; [Co(III)] = 50 mg.L-'.
oxidation and cyclohexanone and adipic acid formation. Reaction temperature 100 "C; [Co(III)] = 100 mg.L-'.
dividual steps. The problem is even more complex if the formation rate equations of cyclohexanone and adipic acid need to be determined. Therefore, applied herein is an empirical method to determine the kinetics of the process and to account for the quantitative concentrations of the reaction products. It has been assumed that formation or disappearance rates of products are proportional to a function of temperature, ozone partial pressure, and catalyst concentration as follows: (13) I" a fAT) fAP0,) f,[CO(III)I where ti(?')is an Arrhenius-type function: fl(T ) 0:
0 Cyclohexanol
-1 I
Cy c l o h e x a n h fl:-0,43
I
I
-2
L
C
(14) and f2(P0,)and f,[Co(III)] are power-type functions of ozone partial pressure and Co(II1) concentration, respectively: e-EIRT
f,([CO(III)I) a [Co(III)lfl (16) According to proportionality 14, a plot of In r versus 1/T, for data corresponding to experiments carried out at the same ozone partial pressure and catalyst concentration, should yield a straight line, the slope being -E,/R. Figure 10 shows this kind of plot for cyclohexanol, cyclohexanone, and adipic acid. From the slopes of these lines the apparent activation energy for the cyclohexanol oxidation and cyclohexanone and adipic acid formation rates is obtained (see Figure 10). On the other hand, from proportionality 15 it is deduced that a plot of In r versus In P should yield a straight line, the slope being a. Figure 11%om this plot corresponding to experiments carried out at the same temperature and catalyst concentration for the products indicated previously. With respect to the effect of catalyst concentration (see proportionality 161, experimental results obtained at dif-
-3
-4
I
-5
4
I
5
6
Ln cco(lll)l
Figure 12. Effect of catalyst concentration on rates of cyclohexanol oxidation and cyclohexanone and adipic acid formation. Reaction temperature 100 "C; Po3 = 2.2 x atm.
ferent catalyst concentrations show that there exists an optimum value for this concentration that yields the best results. This suggests that the function f3[Co(III)]is not as simple as it has assumed to be in proportionality 16. Similar effects have been observed by Tanaka (1974a) in the catalytic oxidation of cyclohexane and Rao and Raghunathan (1984) in the catalytic oxidation of cyclohexanone. In spite of this effect, proportionality 16 has
Ind. Eng. Chem. Res;, Vol. 30, No. 4, 1991 623 Table IV. Reaction Rate Eauationao For [Co(III)] < 57 mgL-' cyclohexanol -rcoL = 20551 exp(-3530/ T)P~o.71[Co(III)]o.43 (18) cyclohexanone rCONA = 12572 exp(-3265/T)P0~~[Co(III)]~.~ (19) adipic acid rmP = 78.8 e~p(-3710/T)Po,O.~lO[Co(III)]~~~ (20) cyclohexanol
For [Co(III)] > 57 mgL-l -rcoL = 2.87 X 1@.
cyclohexanone
rCONA
adipic acid
e ~ p ( - 3 5 3 0 / T ) P ~ ~ ~ ~ [ C o(21) (III)]~~~ = 2.44 x I@exp(-3265/T)Po~~[Co(III) (22) r A D p = 3.98 x I@. e~p(-3710/T)P~~~~[Co(III) (23)
Units of reaction rates in Msh-'; ozone partial pressure in atm; catalyst concentration in mgL-'.
been applied. Figure 12 shows a plot of In r versus In [Co(III)J. Two straight lines can be observed and the critical concentration of catalyst is 57 mg.L-'. The leastsquares analysis of the lines shown in Figure 12 allows the determination of the apparent kinetic order j3 of proportionality 16 (see Table IV). Finally, taking into account proportionalities 14-16, proportionality 13 becomes r = Ae-E/RTPo;[Co(III)]@
(17)
From the experimental rate values rexp,the parameter A was calculated for every experiment. Table IV shows the averaged values of A together with the calculated parameters E J R , ct, and 6 corresponding to cyclohexanol, cyclohexanone, and adipic acid kinetics. Finally, from eqs 18-23, rate values for cyclohexanol and reaction products have been calculated a t different experimental conditions. These values compared to their corresponding experimental ones showed deviations less than or equal to 10% in 95% of the experiments.
Conclusions Oxidation of cyclohexanol to cyclohexanone and carboxylic dibasic acids, basically adipic acid, has been carried out in a semicontinuous reactor. An oxygen-ozone mixture and cobalt(II1) acetylacetonate have been used as oxidant and catalyst agents, respectively. The influence of temperature, ozone partial pressure, and catalyst concentration on cyclohexanol conversion and product distribution has been observed and qualitatively explained by a reaction mechanism. An empirical rate equation has been used for kinetic parameter determination (activation energy and reaction orders) of cyclohexanol and each reaction product. Calculated reaction rates from these equations are very close to the experimental values (deviations 5 &lo%). From the results obtained it can be concluded that the catalytic oxidation of cyclohexanol by means of ozone diluted with oxygen yields a product distribution which is different from that obtained from the noncatalytic oxidation. Higher cyclohexanol conversion, absence of peroxide products, increase of accumulated cyclohexanone, and greater adipic acid yields are the main differences observed. Both temperature and ozone partial pressure have a positive effect on cyclohexanol conversion and acid yield. Catalyst concentration is positive for cobalt(II1) acetylacetonate concentrations up to 50 mg.L-'. For higher values, the influence of catalyst on cyclohexanol disappearance rate is negative although the product distribution is the same as indicated before.
The interchange between Co(I1) and Co(III), indicated in the mechanism proposed, seems to be responsible for both the absence of peroxide products and the high selectivities achieved for the carboxylic dibasic acids. This is because the radical chain reactions for the formation of monobasic acids are blocked. Registry No. Cyclohexanol, 108-93-0;cobalt(II1) acetylacetonate, 21679-46-9;cyclohexanone, 108-94-1;adipic acid, 12444-9;glutaric acid, 110-94-1;pentanoic acid, 109-52-4;hexanoic acid, 142-62-1.
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Received for review April 9, 1990 Accepted September 28, 1990
