Ind. Eng. C h e m . Res. 1990, 29, 713-719
713
KINETICS AND CATALYSIS Liquid-Phase Oxidation of Cyclohexanone over Cerium Oxide Catalyst Hung-Chung S h e n t and Hung-Shan Weng* Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, ROC
Catalytic oxidation of cyclohexanone in the liquid phase with glacial acetic acid as the solvent over cerium oxide was studied between 5 and 15 atm and 98 and 118 "C in a batch reactor. The products were adipic acid, glutaric acid, succinic acid, caprolactone, carbon oxides, etc. The reaction undergoes a short induction period prior to a rapid reaction regime. In both regimes, the reaction is independent of oxygen pressure when the system pressure is above 10 atm. T h e induction period is inversely proportional to both of the catalyst weight and cyclohexanone concentration. During the rapid reaction regime, the reaction rate was found to be proportional to the 0.5 power of the catalyst weight and to the 1.5 power of the cyclohexanone concentration. Reaction mechanisms and rate expressions were proposed. T h e carbon oxides produced in this study were much lower than those previously reported. T h e cerium oxide catalyst is stable during the reaction. The homogeneous liquid-phase oxidation of cyclohexanone with salts of Co, Cr, Mn, Ce, etc., as the catalyst has been studied in considerable detail (Berezin et al., 1966; Reis, 1971; Druline, 1978; Lyons, 1984; Rao and Raghunathan, 1984; Zaidi, 1988). Cyclohexanone is an intermediate in the oxidation of cyclohexane to adipic acid. The product distribution of the air oxidation of cyclohexanone is dependent on the kind of metal complex used as a catalyst (Lyons, 1984). The main reaction scheme in the liquid-phase oxidation of cyclohexanone in acetic acid can be simplified as
02
0 2
cyclohexanone a-hydroperoxide catalyst catalyst dibasic acids, caprolactone, aldehydes, carbon oxides, etc. Studies of the liquid-phase oxidation of hydrocarbons have been carried out extensively by employing transition-metal oxides (Mukherjee and Graydon, 1967; Caloyannis and Graydon, 1971; Neuberg et al., 1972,1974,1975; Varma and Graydon, 1973; Sadana and Katzer, 1974a,b; Srivastava and Srivastava, 1975). The similarity of the reaction mechanism between the homogeneous and heterogeneous reactions has been reported (Caloyannis and Graydon, 1971; Sadana and Katzer, 1974a,b; Shen and Weng, 1988). The information available on the effect of the heterogeneous catalyst on the liquid-phase oxidation of cyclohexanone is meager. The rare-earth oxides have a number of properties that make them important in catalytic applications. Some rare earths including cerium, praseodymium, and terbium form nonstoichiometric oxides (Peters and Kim, 19811, an important property possessed by many oxidation catalysts. Among the catalytically interesting rare earths forming nonstoichiometric oxides, cerium is by far the most abundant and least expensive. Recently, cerium oxide has been employed as a catalyst for the wet oxidation of poly(ethy1eneglycol), acetic acid, and ammonia (Imamura Present address: Refining & Manufacturing Research Center, Chinese Petroleum Corporation, Chia-Yi, Taiwan 60036, ROC. 0888-5885/90/2629-0713$02.50/0
and Dol, 1985; Imamura et al., 1986, 1987) and as a promoter in catalysts for automobile exhaust emission control (Yao and Yao, 1984). In this paper, we report the investigation of cyclohexanone oxidation in the liquid phase using glacial acetic acid as the solvent and cerium oxides as the catalysts. Experimental Section Catalyst Preparation. One kind of cerium oxide was purchased from Merck & Co., and the others were prepared from cerium trichloride heptahydrate (Merck & Co.) by calcination under air atmosphere. The purchased cerium oxide has a nominal purity greater than 99.9%. It was calcined at 350 "C for 3 h and designated as Ce02 (111). The calcination procedures for the other samples were as follows: CeC13.7H20was heated a t 200 "C for 1 h, was finely powdered, and then was calcined by four different procedures: for Ia, kept at 300 "C for 2 h, and increased to 350 "C, and then kept for 1 h; for Ib, kept at 300 "C for 1 h, increased to 350 "C, and then kept for 2 h; for IC,the temperature was increased to 350 "C and kept for 3 h; and for 11, calcinced at 450 "C for 3 h. The thermogram data show that a temperature around 340 "C is sufficient to transform CeC13.7H20into Ce02. Equipment and Reaction Procedure. The experimental setup and procedures have been described in detail previously (Shen and Weng, 1988). The reaction feed (total of about 125 mL), consisting of cyclohexanone (1.549-2.710 M), benzene (5 mL, as an internal standard for analysis), glacial acetic acid, and catalysts (0.1-0.4 g), was prepared according to the experimental conditions and charged into a 600-mL autoclave (Parr Co.). Experiments were conducted under a batchwise mode at constant temperature (98-118 "C) and constant pressure (5-15 atm). Analysis. The reaction samples were analyzed by two gas chromatographs. Cyclohexanone, dibasic acids, and carbon oxides were analyzed by using the same columns under the same conditions as described previously (Shen and Weng, 1988). Caprolactone was analyzed by using a fused silica WCOT capillary column containing CP-SIL 5CB. The liquid products were identified by using a G 1990 American Chemical Society
714
Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 Table 11. Comparison of Activity" concn of catalyst cyclohexanone, M none 1.936 Ia 1.936 Ib 1.936 IC 1.936 111 1.936 Ih 1.549 I1 1.549
conversion, mol 70 3.98 32.33 31.92 31.30 8.89 31.08 19.71
'Reaction pressure, 15 atm; reaction time. 3 h: reaction temperature, 108 "C; catalyst loading, 2.4 g/L.
.
e
Cyclohexarone adipic ocid
A
glutaric ocrd
c
succinic m i d caprolactm
2'4b
2.0
\
Reaction time , hrs
Figure 2. Typical concentration-time results for the oxidation of cyclohexanone at 118 "C and 15 atm. Catalyst loading: 2.4 g/L.
the possible extinction of the flame in ICP by a high content of acetic acid. The concentration of cerium ions thus determined was 0.02 ppm (which corresponded to 0.4 ppm in the spent solution). Because the detection limit of the ICP used is 0.05 ppm, we can conclude that the cerium oxide is insoluble in this system. Silvernail (1978) also reported that the cerium oxide is refractory and insoluble in acids. Reaction Kinetics. The concentration-time data for a typical run are shown in Figure 2. If the oxygen supply was shut off, the reaction pressure remained constant for a period of time that was defined as the "induction period" and then decreased rapidly (the rapid reaction regime). In this study, the reaction pressure was maintained constant after the induction period by introducing oxygen into the reactor. At the initial stage of the rapid reaction regime, the sharp decline in the oxygen pressure is probably due to the rapid reaction between the oxygen and the free radical O=R', which is produced from cyclohexanone. The existence of the induction period and the rapid reaction regime are also revealed in the curve of cyclohexanone vs time as depicted by Figure 2. In the earlier period of the rapid reaction regime, the liquid products that appear are adipic acid (major product), glutaric acid, and caprolactone. As the reaction proceeded, succinic acid and three other compounds of small amount were formed. These three compounds may be monoacids or aldehydeacid but were not identified. Effect of Oxygen Pressure. The effect of oxygen pressure on the reaction was investigated at 118 "C under oxygen partial pressures ranging from 5 to 15 atm. As
Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 715 Table 111. Effect of Reaction Pressure on Conversion and Product Distribution" reactn induction pressure, period, conversn, atm min mol % urod distrib AIGISIPIC 15 4.6 58.13 0.66/0.070/0.015/0.148/0.032 10 4.4 57.76 0.66/0.073/0.018/0.143/0.038 5 4.5 49.14 0.52/0.099/0.029/0.076/0.065 Concentration of cyclohexanone, 2.323 M; catalyst loadings, 2.4 g/L; reaction temeprature, 118 O C ; reaction time, 3 h. A, adipic acid; G, glutaric acid; S, succinic acid; P, caprolactone; C, carbon oxides.
0
1.936
C
8
0.2-
01 0
1.0
8
2.323
A
2.710
2.0
Reaction
g
10
2
3
slope
\ -
-
I
3.0
4.0
time
, hrs
5.0
Figure 4. Test for the 3/2-orderreaction a t 108 "C and 15 atm with various concentrations of cyclohexanone. Catalyst loading: 2.4 g/L.
1.0
II8.C
1.2
1
2 - 1
1.0
3.0 5.0
Concentratim of
10.0
cyclohexanone
,M
N
1
Figure 3. Effect of concentration of cyclohexanone on induction period at 15 atm and 98,108, and 118 "C. Catalyst loading: 2.4 g/L.
shown in Table 111, the reaction rate and induction period are independent of the oxygen pressure for pressures higher than 10 atm. This implies that the reaction was in the nondiffusion-controlled regime. The product distribution is also almost the same for pressures higher than 10 atm. Studies of the heterogeneous initiation of the oxidation of hydrocarbons, such as xylene, cyclohexene, tetralin, and styrene with Co203, MnOz, and Cr203 as catalysts, show that the reaction is zero order with respect to oxygen for oxygen pressures higher than 100-200 mmHg (Meyer et al., 1964; Caloyannis and Graydon, 1971). Effect of Cyclohexanone Concentration. Figure 3 shows the dependence of the induction period on cyclohexanone concentration. The induction period decreases with increasing the concentration of cyclohexanone. The slope of each line in Figure 3 is approximately equal to -1.0. This means the formation of the free radical in the induction period is first order with respect to the concentration of cyclohexanone. This is similar to the results of the oxidation of phenol catalyzed over CuO by Sadana and Katzer (1974a,b). In this study, the concentrationtime data obtained in the rapid reaction were well fitted with 3/2-order kinetics. As shown in Figure 4,four straight lines were obtained by plotting the reciprocal of the square root of the concentration vs time a t 108 "C for four different levels of concentration. The induction time was deducted when making this plot. Hence, the reaction rate of cyclohexanone can be expressed as -r = -dC/dt = k,C3I2 (1) where C is the cyclohexanone concentration and k , is the apparent reaction rate constant which is a function of temperature and catalyst loading. Similar results have been observed in the liquid-phase oxidation of p-xylene and cumene over Co203 and MnO, (Caloyannis and Graydon, 1971; Varma and Graydon, 1973) and that of cyclohexene with Mn02 (Neuberg et al., 1972). The results for 2.323 M cyclohexanone at different temperatures and
I
-se -6
0.6-
.-
6 0
0.40.2-
Ob
2.0
1.0
410
3'0
50
time , hrs
Reaction
Figure 5. Test for the 3/2-order reaction a t 15 atm and various temperatures. Catalyst loading: 2.4 g/L. 1.21
0 21 0
Weight of cafalysf
I
10
I
20
Reaction
time
,
30
,
40
, hrs
Figure 6. Test for the 3/2-orderreaction at 15 atm and 118 O C with various amounts of catalyst.
catalyst weights are shown in Figures 5 and 6. Effect of Catalyst Removal. Experiments were carried out at 108 and 118 "C in which the reaction was allowed to proceed to some extent (the conversions were 12% and 15% for 108 and 118 "C, respectively, at the time of catalyst removal); then the reaction was discontinued, and catalysts were separated by centrifuging. Figure 7 shows the concentration-time data before and after catalyst separation. It can be seen that in both cases the reaction rate decreased after catalyst removal. The ratio of oxidation rate before and after catalyst separation is about 2.0. The oxidation after catalyst separation is
716 Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990
"/I 1.
lo/
Weight o f
cotolyst ,
4
Figure 9. Test of the dependence of the reaction rate constant on the weight of catalyst a t 15 atm and 118 "C. Initial concentration of cyclohexanone: 2.323 M. The values of k , / 2 are obtained from the slopes of the straight lines in Figure 6. 0
I O
20
Reaction time
30
ihrs )
-
Figure 7 . Effect of catalyst removal a t 108 and 118 "C. ( 0 ,m) with catalyst, (0, 0)after catalyst removal.
05-
2
E , = 31 8 kcol/mole
03-
L I
,
,
T
, , ,
OK-'
Figure 10. Temperature dependence of the induction period a t 15 atm. Initial concentration of cyclohexanone: (m) 2.710 M, ( 0 )2.323, (A)1.936 M. Catalyst loading: 2.4 g/L.
The overall rate constant (k) obtained is 0.47 M-1/2g-*I2 h-' at 118 "C and 15 atm. If the reaction rate is based on the weight of the catalyst and V is the volume of the reacting mixture and kV = k', then -r'= -(V/W)(dC/dt) = k'W-1/zC-3/2
(3a)
The overall rate constant (k') will be 59 mol-'lz L3I2g-'/2 h-l. Effect of Reaction Temperature. Because the rate of free-radical formation is proportional to the concentration of cyclohexanone during the induction period, the rate equation can be expressed as d[O=R']/dt
= ki[O=RH]
(4)
By integrating from t = 0 to t = ti (ti is the length of the induction period), which corresponds to [O=R'] = 0 to [O=R'] = [O=R'Ii, and substituting in the Arrhenius equation the expression -d[O=RH]/dt = d[O=R']/dt, In [Rc] = A i exp(-Ei/RT)t
(5)
where Rc = [O=RH]o/IIO=RHlo - [O=R'liJ, [O=RHl0 is the initial concentration of cyclohexanone, and [O=R'Ii is the critical concentration of free radicals corresponding to the onset of the rapid reaction phase at t = ti. By rearrangement we can obtain In (In [Rc]/A,)
+ In ( l / t i ) = -Ei/RT
(6)
From the above equation, the plot of the reciprocal of the induction period ( l / t i ) vs 1 / T should yield a straight line. The Arrhenius plot of the reciprocal of the induction period is shown in Figure 10. Although the points seem scattered, straight lines with a slope of -1.6 X lo4 were
Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 717 Eo = z I . 3 k c o l / m f e
\
2
\
-:: 9
$!
. 1 '
Table V. Effect of the Weight of the Catalyst on Conversion and Product Distribution" catalyst wt, conversn, g mol % prod distrib A/G/S/P/C 0.1 37.96 0.62/0.070/0.010/0.173/0.034 0.2 52.61 0.63/0.062/0.012/0.167/0.036 0.3 58.13 0.66/0.070/0.015/0.148/0.032 0.4 62.03 0.67/0.074/0.021/0.146/0.034
" Concentration of cyclohexanone, 2.323 M; reaction time, 3 h; reaction temperature, 118 "C; reaction pressure, 15 atm.
#\
0.05
O i l
*.02 0.01
t
2.4 2.5
L
2.6
2.7
2.8
OK-'
T
Figure 11. Temperature dependence of reaction rate constant a t 15 atm. Initial concentration of cyclohexanone: (w) 2.710 M, ( 0 )2.323 M, (A)1.936 M. Catalyst loading: 2.4 g/L. Table IV. Effect of Concentration and Reaction Temperature on Conversion and Product Distribution' concn, temp, time, conversn, prod distrib A/G/S/P/C M OC h mol 70 0.66/0.056/0.014/0.097/0.034 2.710 118 3 63.53 108 5 46.14 0.62/0.061/0.010/0.168/0.033 98 5 26.67 0.56/0.067/- - -/0.197/0.035 2.323 118 3 58.13 0.66/0.069/0.015/0.148/0.032 108 5 49.22 0.65/0.063/0.012/0.159/0.031 98 5 24.37 0.63/0.056/- -/0.169/0.023 1.936 118 3 57.89 0.63/0.091/0.021/0.148/ 108 5 46.10 0.64/0.080/0.012/0.156/0.033 98 5 28.37 0.63/0.084/- - -/0.151/0.021
-
erogenized homogeneous cobalt catalyst (Shen and Weng, 1988). Reaction Mechanism. The homogeneous liquid-phase oxidation of cyclohexanone proceeds through a free-radical mechanism like the liquid-phase oxidation of other hydrocarbons (Berezin et al., 1966; Kamiya, 1971; Sheldon and Kochi, 1976; Druline, 1978). A heterogeneous-homogeneous reaction mechanism has been proposed in the case of hydrocarbons using transition-metal oxides (Mukherjee and Graydon, 1967; Caloyannis and Graydon, 1971; Neuberg et al., 1972, 1974; Sadana and Katzer, 1974a,b). The reaction involves initiation of free radicals on the catalyst surface and propagation of the reaction chain in homogeneous solution. Termination could occur homogeneously or heterogeneously depending on the catalyst concentration. Referring to the mechanisms proposed in the literature for the hydrocarbon oxidation and the concentration-time data obtained in this study, the possible reaction mechanism for the liquid-phase oxidation of cyclohexanone is proposed as follows: initiation
"Reaction pressure, 15 atm; catalyst loading, 2.4 g/L.
obtained, giving an activation energy of 31.8 kcal/mol. Similar observations were reported by Sadana and Katzer (1974a,b) and Will et al. (1987). The temperature dependence of the reaction during the rapid phase is shown in Figure 11. The Arrhenius plot of the rate constant yields a straight line of slope -1.07 X lo4, which corresponds to an activation energy of 21.3 kcal/mol. This value provides indirect evidence that the reaction is carried out in the kinetic control region. This value is lower than those with cobalt-type weak acid resin (28.2 kcalimol) (Shen and Weng, 1988) and cobalt acetate (26.0 kcalimol) (Kamiya, 1971) as the catalyst and acetic acid as the solvent. Product Distribution. In this reaction system, adipic acid is the main product. The byproducts include glutaric acid, succinic acid, caprolactone, and carbon oxides. Caprolactone is not the product when cobalt resin catalyst is used. This fact is worthwhile to be explored in the future. As shown in Table 111, the product distribution does not significantly depend on the system pressure as long as the oxygen pressure is greater than 10 atm. However, the cyclohexanone concentration, reaction temperature, and catalyst loading affect the product distribution significantly (Tables IV and V). The fractional yield of glutaric acid, succinic acid, and carbon oxides increased with increasing reaction temperature. As the catalyst loading increased, the fractional yield of dibasic acids increased, whereas that of caprolactone decreased. The fractional yield of carbon oxides (from 0.021 to 0.036) is rather low in comparison with ot,her homogeneous systems (fractional yield from 0.1 to 0.17) with salts of cobalt, manganese, or lead as the catalysts (Reis, 1971) and het-
+ -
+ MIv 2M"'H + 1/20,
O=RH
propagation
O2
O=R'
k10
O=RO'
(13)
O=ROH
+ O=R'
(14)
kl4
O=CHOCH(CH,),CH'
(16)
adipic acid
ki7)
k18
kisl
+
glutaric acid
-
+ O=ROO'
(12)
(15)
--
other products
(11)
O=CH(CH2)4CO'
O=CHOCH(CH2)&!H*
O=ROO'
+ M"'H
-
other products
(9)
O=ROO'
k15
kll
O=ROO'
(10)
+ MIv
O=RO'
O=CH(CH2)4CO'
k9
(8)
+ O=R' caprolactone + O=RO' O=RO' + H 2 0 + MIv
+ O=RH
termination
k0
(7)
O=ROOH
k,l
O=RO'
+ M"'H 2MIV + H,O
O=R'
-
+ O=RH O=ROO' + O=RH O=ROOH + M"'H O=ROO'
O=ROOH
k7
(17)
glutaric acid
succinic acid
k19
+ other products
+
+ other products
stable products
(18)
+ O2
(19)
Ind. Eng. Chem. Res., Vol. 29. No. 5 , 1990
718
20=R’ O=R’
km
stable products
-+
+ O=ROO’
R21
stable products
O=ROO’
+ MI1’
(noninitiating) (20)
(nonpropagating) (21)
k22
stable products
O=R’ + MI11 e23 stable products
MIv (nonpropagating) (22)
+ MIv
(noninitiating) (23)
Reactions 17 and 18 may lead to other products except dibasic acids, such as aldehydes and monoacids. Rate Expression. In the derivation of a rate expression for the consumption of cyclohexanone during oxidation, the following assumptions were made: (a) the consumption rate of cyclohexanone via reaction 14 is slow when compared with that of reactions 7, 10, and 11; (b) the involvemtn of the catalyst in the termination is negligible; (c) reactions 1’7and 18 have no effect on the consumption and formation of cyclohexanone. In addition, a t a high partial pressure of oxygen (greater than 100 mmHg), the chain termination occurs exclusively via the mutual destruction of two peroxy radicals (reaction 19). Reactions 20-23 may be neglected (Sheldon and Kochi, 19’76). The reaction rate for initiation and termination hence can be expressed as
R, = k,[M][O=RH]
(24)
R , = h,g[O=R00*12
(25)
Using the postulation of stationary radical concentrations, we get
R, = R,
(26)
When the equation for the propagation steps is introduced in the above equation, we get -d[O=RH]/dt
= (k1o
+ k,1)[0=ROO’][O=RH]
1. The induction periods of the reaction using cerium oxide and cobalt resin catalysts have the same order of magnitude and decrease as the temperature increases. Because the cerium oxide can be operated a t high temperature, the induction period by the cerium oxide catalyst can be made shorter. 2. The cerium oxide catalyst gives a slower reaction rate. However, this drawback can be compensated for by raising the reaction temperature because the cerium oxides can be used a t a higher temperature. 3. When the cobalt resin is used as the catalyst, caprolactone is not one of the products. 4. The cerium oxide catalyst results in a smaller fractional yield of carbon oxides. 5 . Because the cerium oxide does not change up to 580 “C as we can see from its TGA curve and is hardly soluble in the reacting mixture, it is more stable than the cobalt resin catalyst.
Conclusion Cerium oxide was found to be an effective heterogeneous catalyst for the liquid-phase oxidation of cyclohexanone to dibasic acids and caprolactone in the temperature range 98-118 “C. The reaction undergoes a short induction period prior to a rapid reaction phase. The reaction rate is independent of the system pressure at oxygen pressures above 10 atm. During the induction period, the reaction rate was found to be proportional to the 1st power of both the amount of catalyst and the concentration of cyclohexanone, whereas the reaction rate is of 1.5 order in cyclohexanone in the rapid reaction phase. The apparent overall activation energies for the reaction at the induction period and rapid reaction phase were found to be 31.8 and 21.3 kcal/mol, respectively. The fractional yield of carbon oxides in this study is rather low in comparison with that obtained by other homogeneous and heterogenized and homogeneous systems. The cerium oxide catalyst is far more stable than the cobalt resin catalyst. Acknowledgment
We acknowledge the financial support from the National -d[ O=RH] / d t = Science Council of ROC and the facilities provided by the (klo + k,l)(k7/k19)1’2[M]1’2[O=RH]3’z (27) Refining & Manufacturing Research Center, Chinese Petroleum C,orporation, Chia-Yi, Taiwan, ROC, during the -d[O=RH]/dt = k,(k~/kt)1’2[M]’’2[O=RH]3’2 (28) course of this work. Registry No. CeO, (III),1306-38-3; A, 124-04-9; G, 110-94-1; where k , and k , are rate constants of propagation and S, 110-15-6; P. 502-44-3; cyclohexanone, 108-94-1. termination, respectively. This rate expression reveals that the reaction is independent of oxygen pressure under the Literature Cited above assumptions. Because this rate expression has the same form as that obtained by experiment (eq 31, we may Berezin, I. V.; Denisov, E. T.; Emanuel, N. M. The Oxidation of Cyclohexane; Pergamon Press: Oxford, 1966. say that the above proposed mechanism is consistent with Caloyannis, A. G.; Graydon, W. F. Heterogeneous Catalysis in the the model. The fractional order of 0.5 can be explained Oxidation of p-Xylene in the Liquid Phase. J . Catal. 1971, 22, by assuming that the initiating reaction is of first order 287-296. with respect to the weight of the catalyst (eq 24), while the Druline, J. D. Cobalt-Catalyzed Oxidation of Isotopically Labeled termination of the chains occurs homogeneously by biCyclohexane. J . Org. Chem. 1978,43 (lo), 2069-2070. molecular recombination of peroxy radicals (reaction 19) Imamura, S.; Dol, A. Wet Oxidation of Ammonia Catalyzed by Cerium-Based Composite Oxides. Ind. Eng. Chem. Prod. Res. and thus is independent of the presence of the catalyst. D ~ u 1985, . 24, 75-80. A fractional order with respect to catalyst surface area was Imamura, S.; Nakamura, M.; Kawabata, N.; Yoshida, J. I. Wet Oxfound by other workers (Mukherjee and Graydon, 1967; idation of Poly(ethy1ene glycol) Catalyzed by Manganese-Cerium Caloyannis and Graydon, 1971; Varma and Graydon, 1973; Composite Oxide. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25. Neuberg et al., 1974, 1975) in the studies on the oxidation 34-45. of xylene, cyclohexene, and tetralin. Imamura, S.; Niroyuki, H.; Ishida, S. Preparation of Mn/Ce Composite Oxide Catalysts for the Wet Oxidation of Acetic Acid and Comparison with the Cobalt Resin Catalyst. In a their Catalytic Activities. Sekiyu Gakkaishi 1987, 30 (3), 199-202. previous paper (Shen and Weng, 19881, we reported the Kamiya, Y. The Catalytic Effect of Cobalt Salt on the Autoxidation study of the liquid-phase oxidation of cyclohexanone with of Cyclohexanone. Kogyo Kagaku Zasshi 1971, 74, 1811-1814. cobalt resin as the catalyst, which was prepared from the Lyons, J. E. Catalytic Oxidation of Hydrocarbons in the Liquid weak acid ion-exchange resin. Comparing the cerium oxide Phase. In Applied Industrial Catalysis; Leach, B. E., Ed.; Acaand the cobalt resin catalysts shows the following: demic: New York, 1984; Vol. 3, pp 131-214.
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Received for review August 3, 1989 Accepted January 8, 1990
Kinetics of Nonylphenol Polyethoxylation Catalyzed by Potassium Hydroxide E. Santacesaria,* M. Di Serio, and L. Lisi Cattedra di Chimica Industriaele dell’liniversitci di Napoli, Via Mezzocannone 4, 80134 Napoli, Italy
D. Gelosa Dipartimento di Chimica Fisica Applicata del Politecnico, Piazza Leonard0 da Vinci 32, 20133 Milano, Italy
The kinetics of nonylphenol polyethoxylation, catalyzed by potassium hydroxide, has been studied in a semibatch reactor, by performing kinetic runs a t different temperatures, catalyst concentrations, and ethylene oxide pressures. A kinetic model has been developed, which is based on the assumption that ethylene oxide ring opening is the rate-determining step in a classic SN2mechanism. Kinetic and equilibrium parameters have been determined by optimizing the fitting of experimental runs. Kinetic runs have also been performed under conditions dominated by mass transfer and have been interpreted by introducing this effect, too, in the model. T h e kinetic model and the parameters obtained have also been tested in evaluating the performance of a n industrial plant. Industrial ethoxylation of nonylphenol can be classified as a moderately slow reaction in which both the chemical and diffusional regimes are operative. Nonionic surfactants are industrially produced by base-catalyzed reactions of hydrophobic compounds containing active hydrogen, such as alkylphenols, fatty alcohols, fatty acids, mercaptan, and alkylamines with ethylene and/or propylene oxide (Schick 1967, 1987). Notwithstanding the industrial importance of these products, few papers have been published on the kinetic aspects of ethoxylation. In particular, no paper has been devoted to the kinetics of alkylphenol polyethoxylation in the presence of basic catalyst, and very few papers have been published about the same reaction with phenol (see Patat et al., 1952, 1954; Bobleter 1956; Lowe and Weibull, 1954). In those papers, a third-order kinetic law is attributed to phenol ethoxylation, in contrast with the generally accepted sN2 mechanism (see Winstein and Henderson, 1950; Ingold, 1953; Parker and Isaacs, 1959; Eliel, 1956) for the base-
* To whom
correspondence must be addressed.
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catalyzed ring-opening reaction of epoxides. Thus, the mechanism suggested for explaining third-order kinetics is controversial (Patat 1961; Patat and Wojtech, 1960; Yshii et al., 1962). On the other hand, the same reaction with fatty alcohols has been classified by other authors as a second-order reaction, in accordance with the sN2 mechanism (see Schick, 1967; Gee et al., 1959a,b). However, it must be pointed out that the kinetic data reported in the early works on phenol ethoxylation were collected only by measuring the ethylene oxide consumption, with a complicated and scarcely reliable analytical procedure (Patat et al., 1952, 1954). At present, employing the high-performance liquid chromatography technique, with a UV detector, allows us to determine the concentrations of all the oligomers formed in the ethoxylation of phenol or alkylphenol. Therefore, reaction kinetics can be followed more exactly. In the present paper, the kinetics of nonylphenol polyethoxylation, catalyzed by potassium hydroxide, has been 1990 American Chemical Society
