The Chemical Engineering
Journal, 32 (1986)
93 - 99
93
Kinetics of Oxidation of Ethylene to Vinyl Acetate Using a Homogeneous Palladium Complex Catalyst * G. S. GROVER and R. V. CHAUDHARI Chemical Engineering
Division,
National
Chemical Laboratoy,
Poona 411008
(India)
(Received January 4, 1985; in final form August 8, 1985)
ABSTRACT
A kinetic study of the oxidation of ethylene to vinyl acetate using a palladium complex catalyst is reported. The effects of the ethylene partial pressure, PdClz concentration, benzoquinone (reoxidizing agent) conten tration, sodium acetate (promoter) concentration and temperature on the rate of ethylene absorption were studied. The solubility of ethylene in acetic acid was determined at various sodium acetate concentrations and over a temperature range 30 - 50 “C. The reaction was found to be first order with respect to the catalyst (PdC12) concentration and zero order with respect to the benzoquinone concentration. The dependence of the reaction rate on the ethylene partial pressure was found to be non-linear. A rate equation is proposed on the basis of these data and the kinetic parameters were evaluated. The activation energy was found to be 14.6 kcal mol-‘. The effect of sodium acetate on the reaction rate was found to be complex, and an optimum sodium acetate concentration exists for a given set of conditions. In a certain region the rates varied sharply with sodium acetate concentration and this sensitivity increased with increase in temperature.
1. INTRODUCTION
The oxidation of ethylene in the presence of a palladium complex catalyst in acetic acid solution gives vinyl acetate as the main product [l, 21 and this reaction has been the *This is National Chemical Laboratory tion 3089. 0300-9467/86/$3.50
Communica-
basis of some commercial processes for vinyl acetate manufacture. Generally, PdC& or Pd(OAc)z, together with sodium acetate, is used as a catalyst system. Moiseev et al. [2] investigated the product distribution in the oxidation of ethylene using a [PdClz(CzH4)]2 complex catalyst with sodium acetate as a promoter. Van Helden et al. [ 31 investigated the influence of water on the product distribution and the rate of ethylene oxidation and reported that, in the presence of water, other products such as acetaldehyde, ethylidene diacetate etc. are also formed. The mechanism of ethylene oxidation using the Pd(OAc)zNaOAc catalyst system has been discussed in detail in the literature [ 4 - 81. Winstein et al. [5] studied the kinetics and mechanism of this reaction and discussed the role of NaOAc, on the basis of equilibrium data between Pd(OAc)* and NaOAc. However, there is little information available in the literature of the use of PdCl*-NaOAc as a catalyst system except in the work of Ninomiya et al. [9]. It was therefore thought desirable to investigate the kinetics of this reaction which would be applicable over a wide range of conditions. Also, no attempts to ensure that the kinetic data are obtained under conditions of negligible mass transfer effects have been made in the previous studies. In this paper, we report a detailed kinetic study using the PdCl*NaOAc catalyst system and p-benzoquinone as a reoxidizing agent. The effect of the catalyst concentration, partial pressure of ethylene, temperature, sodium acetate concentration and benzoquinone concentration on the rate of reaction was studied and a rate equation is proposed. The solubility of ethylene in acetic acid was determined at various temperatures and various sodium acetate conceptrations. For the reactor used, the gasliquid mass transfer coefficient kLa was also 0 Elsevier Sequoia/Printed in The Netherlands
94
determined using a dynamic physical absorption method.
2. EXPERIMENTAL
DETAILS
2.1. Ma teriais PdClz and sodium acetate of analytical reagent grade were used in all the experiments. Acetic acid supplied by BDH Laboratories was distilled and used. Benzoquinone obtained from BDH Laboratories was steam distilled and dried before use. Ethylene gas (generated in our own laboratory) from the cylinder was used and the purity was 99.5% as determined by gas chromatography.
3. RESULTS
2.2. Apparatus and procedure The solubility measurements and the rate studies were carried out using an apparatus consisting of a magnetically stirred vessel and a gas burette attachment. The details of the apparatus have been described elsewhere [lo]. The amount of ethylene absorbed was volumetrically recorded as a function of time under different operating conditions. The product analysis was carried out after catalyst separation using an HP 584OA gas chromatograph and a column packed with 5% Carbowax 1500 on Chromosorb W. For gas chromatography analysis, temperature programming (70 - 120 “C) with a flame ionization detector (250 “C) was used. The carrier gas (N,) flow rate was 40 cm3 min-‘.
AND DISCUSSION
The oxidation of ethylene to vinyl acetate using a palladium complex catalyst can be described by the following reaction scheme: 0
0
CzH4+ CH3COOH + 2
OH
C2H3COOCH3 + 0
(1) 0
OH
In the preliminary experiments it was found that 95% of the ethylene consumed accounted for vinyl acetate formation and the side products such as acetaldehyde and ethylidine diacetate formed were negligible. Therefore the reaction rate determination was based on the consumption rate of ethylene. As the purpose of the present study was to investigate the intrinsic kinetics of the reaction, a knowledge of the solubility of ethylene in acetic acid at various concentrations of sodium acetate is essential. Similarly, to check the absence of diffusion control, the gas-liquid mass transfer coefficient k,a is required. These properties were first determined and the results are discussed in the following sections. 3.1. Determination of the solubility and mass transfer coefficient The literature on the solubility of ethylene in acetic acid is limited and hence the relevant data were obtained experimentally using the volumetric apparatus as discussed in Section 2. Solubility experiments at various temperatures (30 - 50 “C) and various sodium acetate concentrations (0 - 1.0 mol 1-l) were carried out and the results are presented in Table 1. The reproducibility of this method was found to be within 2% - 3%. These data were correlated using the following equation: lnHe=a+
$- + s
+dlnX
95 TABLE
1
Solubility
data for the ethylene-acetic
acid system He
Sodium acetate concentration (mol 1-l)
Henry’s constant 30 “C
40°C
50 “C
0 0.101 0.203 0.406 0.609 0.813 1.016
4.066 3.991 3.689 3.292 3.058 2.839 2.635
3.307 3.215 3.102 2.753 2.495 2.272 2.137
1.824 1.713 1.628 1.424 1.325 1.146 -
The values of the constants obtained are as follows: a = -2.512
x IO2
(mol 1-l atm-‘)
a, b, c and d
x lo2
b = 1.506 X 10’ c = -2.290
x 10’
d = -1.823
X 10-l
I
Equation (2) is valid for X > 0, but the solubility values in pure acetic acid and the lowest sodium acetate concentration studied (0.1 mol 1-l) are different by only about 2% - 5%. The temperature dependence of the solubility is shown in Fig. 1, which indicates that, for this system, In He uersus l/T is not a straight line. In the presence of NaOAc the solubility is decreased as the concentration is increased. For example, when the NaOAc concentration increased from 0 to 1 mol l-i (6.66% w/w), the solubility of ethylene decreased by about 40%. The gas-liquid mass transfer coefficient hLa for magnetically stirred vessels is not well studied, and hence these data were obtained by the dynamic absorption method described by Ramachandran and Chaudhari [ 111. In this method the absorption of ethylene as a function of time was recorded in the absence of a catalyst. The results were plotted as ln( 1 - If/ IL) uersus time, and the kLa values were obtained from the slope. The FzLa values for 30 “C and 40 “C were 2.03 X 10M2 s-i and 2.54 X 1O-2 s-l respectively for a stirrer speed of 2000 rev mm’.
e3 4
3.2. Kinetic study For the kinetic study the rate of reaction was calculated from the observed volumetric uptake of ethylene versus time data. Thus, rates over a wide range of operating conditions were measured. The effect of the individual parameters is discussed below.
-3.31 3.0
I
I
3.1
32 fX
Id,
I
3.3
K-l
Fig. 1. The temperature dependence of the solubility of ethylene in acetic acid for various NaOAc concentrations: curve 1, 0.101 mol 1-l; curve 2, 0.203 mol 1-l; curve 3, 0.406 mol l-r; curve 4, 0.813 mol 1-l.
3.2.1. Effect of PdC12 concentration The rates of absorption of ethylene were studied in a PdC12 concentration range from 4.6 X lop3 to 3.2 X 10e2 mol l-l, for a sodium acetate concentration of 8.13 X 10-i mol 1-l and in a temperature range 30 - 50 “C. In all these experiments the concentration of benzoquinone used was in excess such that, during a particular run, the consumption of benzoquinone was negligible. Also, in the calculation of the absorption rates the initial period of fast absorption (required for the saturation of the liquid phase with ethylene) was excluded. A typical plot of the volumetric uptake of ethylene uersus time is shown in Fig. 2.
96
1
I
1
I
I
at a higher catalyst concentration to agglomeration of the catalyst.
I
I
i I
Of
0
I
I
I
1
20
40 TIME,1
I
I
-I
60
(MIN)
Fig. 2. A plot of the volumetric uptake of ethylene us. time for various PdClz concentrations at 40 “C with a sodium acetate concentration of 0.813 mol l-l, a benzoquinone concentration of 0.463 mol 1-l and a partial pressure of ethylene of 0.9 atm: curve 1, 4.60 x 10” mol 1-r; curve 2, 9.30 x 10e3 mol 1-r; curve 3, 2.01 X 10” mol 1-r; curve 4, 3.28 X 10e2 mol l-‘.
The effect of catalyst concentration on the rate of reaction is shown in Fig. 3 for temperatures of 30,40 and 50 “C!.It can be seen that up to a PdClz concentration of 2.0 X 1O-2 mol 1-l the reaction rate varies linearly with catalyst concentration. At higher catalyst concentrations the reaction rates were not found to increase proportionately. Such an observation is possible in the following situations: (i) when there is limited solubility of the catalyst; (ii) when the gasliquid mass transfer becomes the controlling resistance; (iii) on agglomeration of the catalytic complex. In this case the catalyst was found to be soluble in the concentration range studied. Also the reaction rates observed at higher catalyst concentrations were.found to be much less than the maximum rate of transport of ethylene from the gas phase to the liquid phase (given as h,aA*). This eliminates possibilities (i) and (ii), and the non-linear dependence of the reaction rate
may be due
3.2.2. Effect of the ethylenepartialpressure The effect of the ethylene partial pressure on the rate of absorption was studied in a pressure range 0.25 - 0.9 atm at various temperatures. The catalyst concentration and sodium acetate concentration used were 4.8 X 10e3 mol 1-i and 0.81 mol 1-l respectively. The results are shown in Fig. 4 as a plot of RA uersus ethylene partial pressure for temperatures of 30, 40 and 50 “C. As can be seen from Fig. 4, the rate of reaction is nonlinearly dependent on the partial pressure of ethylene. 3.2.3. Effect of temperature The effect of temperature on the rate of ethylene absorption was studied in the catalyst concentration range 4.6 X 1O-3 2.01 X lop2 mol 1-l. The temperature range investigated was 30 - 50 “C. The absorption rates were found to be strongly dependent on temperature and the results showed a linear variation in In RA with l/T. 3.2.4. Effect of benzoquinone concentration In a few experiments, only the benzoquinone concentration was varied whilst the other conditions were kept constant. The rate of reaction was found to be zero order with respect to the benzoquinone concentration. Here, in the entire range of benzoquinone concentrations studied (0.5 - 2.0 mol l-l), the quantity used was in excess compared with the catalyst concentration. The zero-order observation suggests that the reoxidation of Pd(0) is not a rate-controlling step in ethylene oxidation. Similar observations have also been reported earlier [ 4, 91. 3.2.5. Effect of sodium acetate concentration The effect of the sodium acetate concentration on the rate of oxidation of ethylene was studied at PdC12 concentrations of 4.6 X lop3 and 9.3 X 1O-3 mol 1-l and at various temperatures. The results are shown in Fig. 5 as a plot of RA versus sodium acetate concentration. The oxidation rate is a complex function of NaOAc concentration and passes through a maximum as the concentration of NaOAc is increased. A maximum has also been observed earlier by Belov et al. [8] and by Winstein
97
0
Fig. 3. The effect of catalyst concentration on the rate of reaction with a sodium acetate concentration mol l-l, a benzoquinone concentration of 0.463 mol 1-l and an ethylene partial pressure of 0.9 atm.
of 0.813
et al. [ 51 for the Pd(OAc)l-NaOAc system. In this work the effect of temperature was studied, which indicates that the oxidation rate is a complex function of PdClz and NaOAc concentrations and also the temperature. For example, at lower temperatures and lower PdCl* concentrations the oxidation rate appeared to be a mild function of NaOAc concentration. In contrast, at higher PdClz concentrations and higher temperatures the rate of reaction increased sharply with increase in NaOAc concentration in a certain range and showed a clear maximum. Interestingly, the range of NaOAc concentrations (0.75 - 0.9 mol 1-i) in which the maximum was observed appeared to be constant at all temperatures and PdClz concentrations. The mechanism describing the role of NaOAc in the oxidation of ethylene using the Pd(OAc)z-NaOAc system has been discussed by Winstein et al. [ 51. However, for the PdCl*-NaOAc system, no mechanistic study has been reported. The following equilibrium steps are likely to occur in this case: Pd,C&, + nNaOAc I_ AcO \ Pd Cl’
/Cl,
pd/C12-
‘Cl’
‘OAc
NazPdzC1a(OAc)z
+ 20Ac- -
2PdC12(OAc)22-
Further work on the PdC12-NaOAc system is essential to elucidate the role of NaOAc and the complex kinetics observed at different temperatures. 3.2.6. Rate expression For the purpose of kinetic studies, it was ensured that the data were in the kinetically con-
(3)
(4)
98
16I.6 -
1.2 -
!.6 -
!.46I.0 6.6-
I 0.2 PARTIAL
I
0.4 PRESSURE
I
0.6 OF
C2H,,
I
I
I
0.6
1.0 P,, ( otm
)
.4-
Fig. 4. The effect of the ethylene partial pressure on the rate of reaction with a catalyst concentration of 5 x 10” mol l-r, a sodium acetate concentration of 0.813 mol 1-r and a benzoquinone concentration of 0.463 mol 1-r.
trolled regime. The following criterion was used : o! = -<
RA
k,aA*
0.1
For each data point the above criterion was applied using the values of kLa and A* determined in this work. It was observed that for all the runs the factor o was less than 0.1, indicating that the rate of absorption is very much slower than the maximum rate of mass transfer and hence confirming the kinetic control. In order to propose a rate equation, the mechanism discussed by Ninomiya et al. [ 91 was considered. Using the principle of steady state, the following form of rate model was derived for constant sodium acetate concentration:
RA =
kCaP,He 1+ KP,He
(6)
The observed results on the variation in RA with PA and catalyst Concentration are also
O0
I 0.4
1
0.2
I
0.6
CONCENTRATION
1 2 3 4 5
I
I.0
OF
Fig. 5. The effect of the sodium tion on the rate of reaction.
Curve
I
0.8
Na OAc I mole/l
acetate
I 1.2
4
I
concentra-
PdClz concentration
Temperature
(mol 1-r)
CC)
5.00 4.60 9.30 4.80 1.02
30 40 40 50 50
x x x x x
10-s 1O-3 10-s 1O-3 10-z
consistent with eqn. (6). In order to evaluate the parameters k and K, eqn. (6) was rearranged as 1
-=
1
K
kCaP,He + hCo RA As can be seen from eqn. (7), the plot of l/RA versus l/P, will give the following quantities: 1 slope = k&He intercept = -
K
k Co
(7)
(8) (9)
99
As C,, and He are known, both k and K can be evaluated. The values of k and K obtained from the plot of l/R, versus l/P, are given in Table 2. The rates predicted using eqn. (6) agreed with the experimental data to within &5% error. The activation energy obtained from the temperature dependence of the rate constant was found to be 14.61 kcal mol-‘. Equation (6) is based on the data obtained at various temperatures and a wide range of catalyst and dissolved ethylene concentrations, unlike the earlier kinetic studies [ 1, 91 which were carried out at a single temperature. Hence the present work is likely to be more useful for design purposes. TABLE
2
Rate constants
for the oxidation
Temperature (“C)
Reaction constant k x lo2
(1 mol-’ 30 40 50
9.275 17.026 61.848
of ethylene rate
Equilibrium constant K (1 mol-*)
s-i) 47.951 40.000 87.200
sodium acetate concentration and this sensitivity was found to increase with increase in temperature.
REFERENCES I. I. Moiseev,
and J. K. Syrkin, 821. I. I. Moiseev, M. N. Vargaftik and J. K. Syrkin, Dokl. Akad. Nauk S.S.S.R., 133 (1960) 377. R. Van Helden, C. F. Kohll, D. Medema, G. Verberg and T. J. Koff, Reel. Trav. Chim. Pays-Bas, 87 (1968) 961. 4 I. I. Moiseev, A. P. Belov, V. A. Igoshin and J. K. Syrkin, Dokl. Akad. Nauk S.S.S.R., 173 (1967) Dokl.
Akad.
M. N. Vargaftik
Nauk
S.S.S.R.,
130 (1960)
863. 5 S. Winstein, J. McCaskie, Lee Hing-Bui and P. M. Henry, J. Am. Chem. Sot., 98 (1976) 6913. 6 R. N. Pandey and P. M. Henry, Can. J. Chem., 52 (1974) 1241. 7 R. N. Pandey and P. M. Henry, Can. J. Chem., 53
(1975)
1833.
8 A. P. Belov, I. I. Moiseev and N. G. Umarova, Izv. Akad. Nauk S.S.S.R., (1966) 1642. 9 R. Ninomiya, M. Sato and T. Shiba, Bull. Jpn. Pet. Inst., 7 (1965) 31. 10 S. S. Tamhankar and R. V. Chaudhari, Ind. Eng. Chem., Fundam., 18 (1979) 406. 11 P. A. Ramachandran and R. V. Chaudhari, Three Phase Catalytic Reactors, Gordon and Breach,
New York, 1983. 4. CONCLUSIONS APPENDIX
The kinetics of oxidation of ethylene using a homogeneous palladium complex catalyst were studied. It was observed that the reaction is first order with respect to catalyst concentration and zero order with respect to the reoxidation agent, benzoquinone. The reaction rate dependence on the ethylene partial pressure was found to be non-linear. A rate equation was developed and an activation energy of 14.6 kcal mol-’ was obtained. The solubility data for the ethylene-acetic acid system was obtained in the temperature range 30 - 50 “C and at various sodium acetate concentrations. From the dynamic absorption experiments, kLa values were also determined. In the presence of sodium acetate, the rate of ethylene oxidation was found to increase significantly. The effect of sodium acetate concentration on oxidation rate was studied in the temperature range 30 - 50 “C and it was found that an optimum sodium acetate concentration exists for a given set of conditions. The rates varied sharply in a certain region of
A: NOMENCLATURE
constant in eqn. (2) solubility of ethylene (mol 1-i) constant in eqn. (2) constant in eqn. (2) concentration of catalyst (mol 1-l) CO constant in eqn. (2) d He Henry’s constant (mol 1-l atm-‘) reaction rate constant (1 mol-’ s-i) k kLa gas-liquid mass transfer coefficient (s-l) equilibrium constant (1 mol-i) K partial pressure of ethylene (atm) PA rate of absorption of ethylene (mol 1-l RA s-i) temperature (K) T volume of ethylene absorbed at time t (1) V Vi! volume of ethylene absorbed at saturation point (1) concentration of sodium acetate (mol X 1-1) t-i* b c
Greek symbol ck! factor defined by eqn. (5)