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Oxidation of toluene to benzaldehyde over VSb1-xTixO4 catalyst. Kinetics studies.
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1733
Oxidation of toluene to benzaldehyde over VSbl.xTixO4 catalyst. Kinetics studies. A. Barbaro, S. Larrondo and N. Amadeo a a Labomtorio de Procesos Cataliticos. Depto.de Ing. Quimica, Facultad de Ingenieria, UBA. Pabell6n de Industrias, Ciudad Universitaria, 1428 Buenos Aires, Argentina. In this work a kinetic study of the vapor phase oxidation of toluene by air using titanium doped vanadium antimonate as catalytic system, is reported. The catalytic tests were performed in an integral fixed-bed reactor operating at atmospheric pressure. From the results obtained in the introductory kinetic tests, a kinetic scheme that considers the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides, is proposed. The corresponding kinetic parameters are estimated using non-linear regression. There is a good fitting between the regression curves and the experimental data. The oxidation of toluene takes place on a partially reduced catalyst surface. The overall rate depends on the partial pressures of the reactants. The three steps of the kinetic mechanism have similar activation energies indicating that the same intermediate is involved. In the range of the operating conditions, there is no rate-controlling step. 1. INTRODUCTION The processes of catalytic partial oxidation of aromatic hydrocarbons in vapor phase are of great importance in chemical technology[1 ]. The partial oxidation of hydrocarbon has been subject of many research works. This is because oxidation catalysis is complex and involves many parallel and consecutive reactions. There have been few comprehensive studies of the kinetics of selective oxidation reactions. In particular, for toluene oxidation, kinetic equations are usually of the power rate law type and Mars-van Krevelen redox model type [2-6]. In many of these works [4-6], low hydrocarbon pressures and high oxygen pressures were used. In consequence, the rate expressions are nearly first order in the hydrocarbon pressure and zero order in oxygen pressure. The data available in the literature about the vapor phase oxidation of toluene show that it is common to obtain selectivities to carbon oxides of 40% and higher, even at low toluene conversion levels [7-8]. This would indicate the presence of a route for direct oxidation of toluene to carbon oxide. However, there are few works [2] considering this direct oxidation of toluene by a parallel route, in the kinetic studies. In a previous work it was found that the vanadium antimonate doped with titanium, with nominal composition VSb0.8Ti0.204 [9], presents good catalytic performance in the toluene oxidation reaction. The products obtained with this catalytic system were benzaldehyde and carbon oxides. In the present work a kinetic study of vapor phase oxidation of toluene by air, using a titanium doped vanadium antimonate as catalytic system, is reported. The aim is to
1734 propose a kinetic scheme suitable to identify the main reaction steps as well as to determine a kinetic expression that provides a good fitting with the experimental data. Further studies will optimize the operation conditions in order to obtain better selectivites to benzaldehyde. 2. EXPERIMENTAL
2.1. Catalyst Preparation The catalysts were prepared by solid state reaction of mechanical mixtures of pure vanadium (V) oxide, antimony 0ID oxide and titanium (IV) oxide. The amount of each oxide in the mixtures, was the necessary for obtaining a solid catalyst with a nominal composition of VSb0.sTi0.204. The samples were put in quartz crucibles and heated in air by a conventional furnace with a temperature calcination programme, according to the method published elsewhere [9]. X-Ray powder diffraction (XRD) technique was used to check that the structure of the phase obtained was similar to that previously reported as vanadium antimonate with 20% of titanium doping [ 10].
2.2. Catalytic tests The catalytic oxidation of toluene was studied in an integral fixed-bed reactor operated at atmospheric pressure. The reactor was made of a Pyrex glass tube of 13-mm inner diameter. Toluene was fed by means of a carrier air stream flowing through a saturator. The feed toluene/air molar ratio was controlled by adjusting both the saturator temperature and the input air flow rate. The reaction temperature was measured with a sliding thermocouple placed inside the bed. Since the oxidation reactions are highly exothermal, the catalyst bed was diluted (1:10) with glass particles, of the same diameter range, in order to avoid adverse thermal effects.The composition of the feed gas and the effluent were analysed by on-line gas chromatography. The reactor was operated in steady-state conditions. The catalytic tests were performed under the following conditions: catalyst mass: 100-400 mg; total feed rate: 200-600 ml.min 1, toluene molar fraction: 0.001 to 0.01; oxygen molar fraction: 0.04 to 0.2 ; temperature range: 673-723K; particle diameter <120 ~tm. The experiments were carried out at fifteen space times, five temperatures, ten toluene partial pressures and six oxygen partial pressures.
2.3. Introductory kinetic tests. In order to insure that the kinetic experiments give meaningful results, preliminary catalytic tests were carried out at different residence times, particle diameter, total gas flow and temperatures. Some of them were performed without catalysts in order to study the contribution of the thermal oxidation. These tests showed a negligible contribution of homogeneous oxidation to total conversion and the absence of internal and external diffusion limitations, for a particle diameter below 120 l,tm and total gas flow equal or greater than 100 ml.min -1, respectively. The catalyst showed activity without the necessity of a pretreating and was stable during a typical run period of 12 hours. The main oxidation products formed were benzaldehyde, carbon monoxide and carbon dioxide. It was also detected benzoic acid in very small quantities. In Figure 1 selectivities are plotted as a function of toluene conversion at 713K. The shape of the curve representing the selectivity to benzaldehyde shows that the rate of benzaldehyde oxidation is greater than the rate of formation. The extrapolation of both curves to zero
1735 conversion of toluene lead us to the conclusion that nearly 30 to 40% of toluene would be oxidized by parallel reaction to carbon oxides. Therefore, the reaction kinetic scheme proposed is the following: 100% Y~ 0.55 mol-% T = 713 K Toluene 9 Benzaldehyde t~'''''80*,6 y~ =19.9 m o l - ~ ~
"--...
9~ 60%
OSco~ ASBA
~ 40.,6 20*,6
/
CO~ Benzoic acid was detected as product only in few experiments and in a very low concentration. Then, only the total oxidation of benzaldehyde was considered.
0%
0.0
0.1 0.2 0.3 0.4 0.5 Toluene Conversion Figure 1. Selectivities vs. Toluene conversion
3. KINETIC MODEL
1. Hydrocarbon + Oxidized Catalyst 2. Reduced Catalyst + 02 (g)
Oxidized Products + Reduced Catalyst Oxidized Catalyst
The classical redox mechanism proposed by Mars and Van Krevelen[3] was adopted in this work. The basic concepts of this mechanism may be interpreted by assuming two sucessive steps:
The rate expressions derived from this mechanism are presented in Table 1. First order in hydrocarbon and order "n" in oxygen partial pressures were assumed. Tablel. Rate expressions for each step of the kinetic mechanism proposed. Reaction step
Rate expression + (~r
rl = k l 9 PTol
.0
2. Tol + Oox ~
7 COx + 4 H20 + Or
r 2 = k 2 "PTol
"0
3.BA + Oox ~
7 COx + 3 H20 + (~r
r3 = k3 " PBA
.0
1. Tol + Oox ~
4. Oox + O2
BA + H20
=
r4=k 4.Pn .0_0 ) 02
(2Jr
Under steady state conditions, the rate of catalyst reduction equals the rate of catalyst reoxidation. Then, the surface degree of coverage by oxygen, 0, is n
k4P02 O =
(I)
k 4 P 0n2
+ (kl + fl.k2)PTol + 2.k3 PBA
where fl and 2 are stoichiometric constants that represent the number of moles of oxygen consumed by total oxidation of one mole of toluene and one mole of benzaldehyde, re~bectively. The values of fllese constants are fl = 8.11 and 2 = 7.11. When the surface is
1736 completely oxidized (O = 1) carbon oxides are mainly formed. The progressive reduction of the surface increases the formation of partial oxidized products (O <1). Theoretical calculations predicted that at O values in the 0.3-0.4 range, maximum selectivities would be achieved [11 ]. The differential equations for an ideal plug-flow reactor, with the corresponding inlet conditions are summarized in Table 2. Table2. Differential equations and inlet conditions Differential equations
dy~o,
Inlet Conditions
(k, + k 2 ) . k 4 "Yro, "Yo,
dm~,,t = - F . P .
k4 "Yo, + ( k , + f l . k 2 ) . y r o I + ./]..k 3 "YBA
dYo2 = _ F . p . ( ( k , dm~t
+ fl'k2)'Yro,
+k3"2"YsA)'k4"Y02
k4 " Yo= + (k, + fl . k~ ). Yro' + 2 - k 3 "YsA
dYBA =
F'P"
= 7.F.P.
dmcat
YO 2 = Y O 2 exp.
0
k4 "Yo, +(k~ + f l . k 2 ) ' Y r o , + 2 - k 3 "YBA
dm~a, a"c~
(kl " Yrol - k3 "YeA )" k4 " Yo,
o 0 Y]'ol = YTol exp.
(k~ "Y~ol + lc~" YB,)" k4 "Yo, k4 " Yo2 + (kl + fl " k: )" Yrot +/]"k3 " YBA
=
Y BA
0
YgO x - 0
4. RESULTS The reaction parameters of the former rate expressions were determined by minimizing the sum of the squares of the deviations between measured and calculated molar fractions of reactants and products. A specific optimization routine for multivariable non-linear regression, developed by Quiroga and Gottifredi [12] coupled with a standard routine of numerical integration of the differential equations presented in Table 2 was used. In a first step, the parameter estimation was done using the experimental data obtained at 713K. The goodness of the fitting of the regression curve and experimental data assuming n-1 in the oxygen partial pressure is shown in Figures 2,3 and 4.
'0~y~ 055mo1~ T713~ L ,00~ ~yoo~ 199 mo,~ ~ ~ - - ~~- -s-~~ / ~0~1Y~~ ~ 0T~4mo, 7 1 ~ 40~~ ' ~
O
r~
oXTo~ / ~40~1 :
0.0 0.2 0.4 0.6 0.8 1.0 1.2 Space-time W/F ( gr cat. min/ml ) Figure 2. Effect of space-time
2%
~"~
6% 10% 14% 18% Oxygen Molar Fraction
22%
Figure 3. Effect of oxygen molar fraction (W/F = 0.6 gr cat.min/ml)
1737 A significant decrease in the fitting quality for n: 0, 0.5 and 2 was observed. The surface degree of coverage by oxygen, 8, was calculated with the parameters estimated at 713K. Values within the 0.2 to 100% yO 0.8 range were found. Therefore, the oxidation 9~ 02 = n ,tool-% , 1 9 T " =9713 K takes place over a partially reduced surface 80% with the overall rate depending on oxygen ~60%
n S COx a S BA o X To l
and toluene partial pressures (Figures 3 and 4). Likewise, it was observed that 40% temperature has no significative influence on "~ -o~.._ ... selectivities to partial and total oxidized products (Figure 5). This fact suggests that all 20% o the steps of the kinetic scheme have the same 0% activation energy. The preexponential factors 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% and the activation energy (Arrhenius law, Toluene MohrFraction k~= kfl * exp(-Ea/RT)), were determined considering all the experiments performed. Figure 4. Effect of toluene molar fraction The results are presented in Table 3. It is (W/F = 0.6 grcat.min/ml) important to remark that estimated activation energy for the reoxidation reaction was close to those of the oxidation reactions. Table 3. Estimated parameters Parameter
Estimated Values ...............
k ~ mol.(min.atrn.grcat) "l
5.22E+06
k ~ mol.(min.atm.grcat) "1
3.28E+06
k ~ mol.(min.atm.grcat) "l
7.24E+07 2.02E+06
k4~ mol.(min.atm.grcat) "1 Ea kcal.mol 1
27.6
The kinetic scheme has a good fitting of the data, as can be seen in Figures 2 to 5 and in the good correlation between the experimental and the calculated toluene molar fraction showed in Figure 6. lO0~ Y~ = 0.55 mol-% y~ 2 =19.9 mol-% 80%
~ * ~ ~ ~ A 723 K SCOx n713K o 703 K -x~ o 693 K 73 K
.~ 60~ ,, ~ 40% "
20%
~176 . r "
~
0% 0%
~" 0.0% . . . . . 10% 20% 30% 40% 50% m 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% Toluene Conversion Calculated Molar Fraction Figure 5. Selectivities vs. Toluene conversion Figure 6. Parity plot for toluene molar fraction
1738 5. CONCLUSIONS The reaction network was modelled assuming the redox mechanism proposed by Mars and Van Krevelen. The approach is oversimplified with respect to the true reaction mechanism, but provides a better description of the experimental data than the empirical approaches (power law equations). The kinetic-scheme proposes three steps: the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides. The good fitting of the experimental results for the model proposed does not imply that this model represents the true reaction mechanism. Detailed mechanistic theories are out of the scope of the present work. The similar values found of apparent activation energies for each step, suggest that benzaldehyde and carbon oxides are produced from the same intermediate. Similar behaviour was obtained by Van der Wiele and Van den Berg [2] in the oxidation of toluene over bismuth molybdate catalysts. The activation energy value obtained in this work is similar to that reported by Trimm and Irshad in the oxidation of toluene over molybdenum trioxide catalyst (27.4 Kcal/mol) [5]. The overall rate was found dependent of the partial pressures of the reactants. The oxidation takes place over a partially reduced surface. The selectivity to benzaldehyde was in the 15% to 50% range and increases as the toluene molar fraction increases and oxygen molar fraction decreases. In the range of the operating conditions, there is no rate-controlling step. Further studies will be carried out in order to find the optimal conditions for benzaldehyde production. NOMENCLATURE BA : Benzaldehyde COx: Carbon oxides. Ea :activation energy. exp.: experimental F : molar feed rate. F~o : volumetric feed rate. ki: specific rate constant. ki~ Arrhenius law preexponential factor. m: catalyst mass Pi : partial pressure of component i.
P :total pressure :reaction rate of step i. T :temperature x :toluene conversion yi : molar fraction of component i. TOL : Toluene. ~ox : oxidized catalyst sites. Qr : reduced catalyst sites 0 : surface degree of coverage by oxygen W/F: space-time ri
REFERENCES 1. A.Bielanski and J. Haber, Oxygen in Catalysis, Marcel Dekker Inc., New York, 1991. 2. K. Van der Wiele and P.J. Van den Berg, J.ofCatalysis, 39 (1975) 437. 3. P. Mars and D. W. Van Krevelen, Special Supplement to Chem. Eng.Sci., 3 (1954) 41. 4.G. Gunduz and O. Akpolat, Ind. En. Chem. Res., 29 (1990) 45. 5. D.L. Trimm and M. Irshad, J.of Catalysis, 18 (1970) 142. 6. K. Papadatos and K. Shelstad, J.of Catalysis, 28 (1973) 116. 7. M. Ponzi, C. Duschatzky, A. Carrascull, E.Ponzi, Applied Catal. A: Gen., 169 (1998) 373. 8. Andersson S.L.T., J.ofCatalysis, 98 (1986) 138. 9. A. Barbaro, S. Larrondo, S. Duhalde, N. Amadeo, Applied Catal. A: Gen. (in press). 10. Berry F., Smart L., S. Duhalde, Polyhedron, 15 (1996) 651. 11. S.T. Oyama, A.N. Desikan, J.W. Hightower, ACS Symposium series, 523 (1992). 12. O.D. Quiroga and J.C. Gottifredi, Avances en la determinaci6n de parhmetros cin6ticos en sistemas de reacciones quimicas complejas, CONICET, Buenos Aires, (1982).
1733
Oxidation of toluene to benzaldehyde over VSbl.xTixO4 catalyst. Kinetics studies. A. Barbaro, S. Larrondo and N. Amadeo a a Labomtorio de Procesos Cataliticos. Depto.de Ing. Quimica, Facultad de Ingenieria, UBA. Pabell6n de Industrias, Ciudad Universitaria, 1428 Buenos Aires, Argentina. In this work a kinetic study of the vapor phase oxidation of toluene by air using titanium doped vanadium antimonate as catalytic system, is reported. The catalytic tests were performed in an integral fixed-bed reactor operating at atmospheric pressure. From the results obtained in the introductory kinetic tests, a kinetic scheme that considers the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides, is proposed. The corresponding kinetic parameters are estimated using non-linear regression. There is a good fitting between the regression curves and the experimental data. The oxidation of toluene takes place on a partially reduced catalyst surface. The overall rate depends on the partial pressures of the reactants. The three steps of the kinetic mechanism have similar activation energies indicating that the same intermediate is involved. In the range of the operating conditions, there is no rate-controlling step. 1. INTRODUCTION The processes of catalytic partial oxidation of aromatic hydrocarbons in vapor phase are of great importance in chemical technology[1 ]. The partial oxidation of hydrocarbon has been subject of many research works. This is because oxidation catalysis is complex and involves many parallel and consecutive reactions. There have been few comprehensive studies of the kinetics of selective oxidation reactions. In particular, for toluene oxidation, kinetic equations are usually of the power rate law type and Mars-van Krevelen redox model type [2-6]. In many of these works [4-6], low hydrocarbon pressures and high oxygen pressures were used. In consequence, the rate expressions are nearly first order in the hydrocarbon pressure and zero order in oxygen pressure. The data available in the literature about the vapor phase oxidation of toluene show that it is common to obtain selectivities to carbon oxides of 40% and higher, even at low toluene conversion levels [7-8]. This would indicate the presence of a route for direct oxidation of toluene to carbon oxide. However, there are few works [2] considering this direct oxidation of toluene by a parallel route, in the kinetic studies. In a previous work it was found that the vanadium antimonate doped with titanium, with nominal composition VSb0.8Ti0.204 [9], presents good catalytic performance in the toluene oxidation reaction. The products obtained with this catalytic system were benzaldehyde and carbon oxides. In the present work a kinetic study of vapor phase oxidation of toluene by air, using a titanium doped vanadium antimonate as catalytic system, is reported. The aim is to
1734 propose a kinetic scheme suitable to identify the main reaction steps as well as to determine a kinetic expression that provides a good fitting with the experimental data. Further studies will optimize the operation conditions in order to obtain better selectivites to benzaldehyde. 2. EXPERIMENTAL
2.1. Catalyst Preparation The catalysts were prepared by solid state reaction of mechanical mixtures of pure vanadium (V) oxide, antimony 0ID oxide and titanium (IV) oxide. The amount of each oxide in the mixtures, was the necessary for obtaining a solid catalyst with a nominal composition of VSb0.sTi0.204. The samples were put in quartz crucibles and heated in air by a conventional furnace with a temperature calcination programme, according to the method published elsewhere [9]. X-Ray powder diffraction (XRD) technique was used to check that the structure of the phase obtained was similar to that previously reported as vanadium antimonate with 20% of titanium doping [ 10].
2.2. Catalytic tests The catalytic oxidation of toluene was studied in an integral fixed-bed reactor operated at atmospheric pressure. The reactor was made of a Pyrex glass tube of 13-mm inner diameter. Toluene was fed by means of a carrier air stream flowing through a saturator. The feed toluene/air molar ratio was controlled by adjusting both the saturator temperature and the input air flow rate. The reaction temperature was measured with a sliding thermocouple placed inside the bed. Since the oxidation reactions are highly exothermal, the catalyst bed was diluted (1:10) with glass particles, of the same diameter range, in order to avoid adverse thermal effects.The composition of the feed gas and the effluent were analysed by on-line gas chromatography. The reactor was operated in steady-state conditions. The catalytic tests were performed under the following conditions: catalyst mass: 100-400 mg; total feed rate: 200-600 ml.min 1, toluene molar fraction: 0.001 to 0.01; oxygen molar fraction: 0.04 to 0.2 ; temperature range: 673-723K; particle diameter <120 ~tm. The experiments were carried out at fifteen space times, five temperatures, ten toluene partial pressures and six oxygen partial pressures.
2.3. Introductory kinetic tests. In order to insure that the kinetic experiments give meaningful results, preliminary catalytic tests were carried out at different residence times, particle diameter, total gas flow and temperatures. Some of them were performed without catalysts in order to study the contribution of the thermal oxidation. These tests showed a negligible contribution of homogeneous oxidation to total conversion and the absence of internal and external diffusion limitations, for a particle diameter below 120 l,tm and total gas flow equal or greater than 100 ml.min -1, respectively. The catalyst showed activity without the necessity of a pretreating and was stable during a typical run period of 12 hours. The main oxidation products formed were benzaldehyde, carbon monoxide and carbon dioxide. It was also detected benzoic acid in very small quantities. In Figure 1 selectivities are plotted as a function of toluene conversion at 713K. The shape of the curve representing the selectivity to benzaldehyde shows that the rate of benzaldehyde oxidation is greater than the rate of formation. The extrapolation of both curves to zero
1735 conversion of toluene lead us to the conclusion that nearly 30 to 40% of toluene would be oxidized by parallel reaction to carbon oxides. Therefore, the reaction kinetic scheme proposed is the following: 100% Y~ 0.55 mol-% T = 713 K Toluene 9 Benzaldehyde t~'''''80*,6 y~ =19.9 m o l - ~ ~
"--...
9~ 60%
OSco~ ASBA
~ 40.,6 20*,6
/
CO~ Benzoic acid was detected as product only in few experiments and in a very low concentration. Then, only the total oxidation of benzaldehyde was considered.
0%
0.0
0.1 0.2 0.3 0.4 0.5 Toluene Conversion Figure 1. Selectivities vs. Toluene conversion
3. KINETIC MODEL
1. Hydrocarbon + Oxidized Catalyst 2. Reduced Catalyst + 02 (g)
Oxidized Products + Reduced Catalyst Oxidized Catalyst
The classical redox mechanism proposed by Mars and Van Krevelen[3] was adopted in this work. The basic concepts of this mechanism may be interpreted by assuming two sucessive steps:
The rate expressions derived from this mechanism are presented in Table 1. First order in hydrocarbon and order "n" in oxygen partial pressures were assumed. Tablel. Rate expressions for each step of the kinetic mechanism proposed. Reaction step
Rate expression + (~r
rl = k l 9 PTol
.0
2. Tol + Oox ~
7 COx + 4 H20 + Or
r 2 = k 2 "PTol
"0
3.BA + Oox ~
7 COx + 3 H20 + (~r
r3 = k3 " PBA
.0
1. Tol + Oox ~
4. Oox + O2
BA + H20
=
r4=k 4.Pn .0_0 ) 02
(2Jr
Under steady state conditions, the rate of catalyst reduction equals the rate of catalyst reoxidation. Then, the surface degree of coverage by oxygen, 0, is n
k4P02 O =
(I)
k 4 P 0n2
+ (kl + fl.k2)PTol + 2.k3 PBA
where fl and 2 are stoichiometric constants that represent the number of moles of oxygen consumed by total oxidation of one mole of toluene and one mole of benzaldehyde, re~bectively. The values of fllese constants are fl = 8.11 and 2 = 7.11. When the surface is
1736 completely oxidized (O = 1) carbon oxides are mainly formed. The progressive reduction of the surface increases the formation of partial oxidized products (O <1). Theoretical calculations predicted that at O values in the 0.3-0.4 range, maximum selectivities would be achieved [11 ]. The differential equations for an ideal plug-flow reactor, with the corresponding inlet conditions are summarized in Table 2. Table2. Differential equations and inlet conditions Differential equations
dy~o,
Inlet Conditions
(k, + k 2 ) . k 4 "Yro, "Yo,
dm~,,t = - F . P .
k4 "Yo, + ( k , + f l . k 2 ) . y r o I + ./]..k 3 "YBA
dYo2 = _ F . p . ( ( k , dm~t
+ fl'k2)'Yro,
+k3"2"YsA)'k4"Y02
k4 " Yo= + (k, + fl . k~ ). Yro' + 2 - k 3 "YsA
dYBA =
F'P"
= 7.F.P.
dmcat
YO 2 = Y O 2 exp.
0
k4 "Yo, +(k~ + f l . k 2 ) ' Y r o , + 2 - k 3 "YBA
dm~a, a"c~
(kl " Yrol - k3 "YeA )" k4 " Yo,
o 0 Y]'ol = YTol exp.
(k~ "Y~ol + lc~" YB,)" k4 "Yo, k4 " Yo2 + (kl + fl " k: )" Yrot +/]"k3 " YBA
=
Y BA
0
YgO x - 0
4. RESULTS The reaction parameters of the former rate expressions were determined by minimizing the sum of the squares of the deviations between measured and calculated molar fractions of reactants and products. A specific optimization routine for multivariable non-linear regression, developed by Quiroga and Gottifredi [12] coupled with a standard routine of numerical integration of the differential equations presented in Table 2 was used. In a first step, the parameter estimation was done using the experimental data obtained at 713K. The goodness of the fitting of the regression curve and experimental data assuming n-1 in the oxygen partial pressure is shown in Figures 2,3 and 4.
'0~y~ 055mo1~ T713~ L ,00~ ~yoo~ 199 mo,~ ~ ~ - - ~~- -s-~~ / ~0~1Y~~ ~ 0T~4mo, 7 1 ~ 40~~ ' ~
O
r~
oXTo~ / ~40~1 :
0.0 0.2 0.4 0.6 0.8 1.0 1.2 Space-time W/F ( gr cat. min/ml ) Figure 2. Effect of space-time
2%
~"~
6% 10% 14% 18% Oxygen Molar Fraction
22%
Figure 3. Effect of oxygen molar fraction (W/F = 0.6 gr cat.min/ml)
1737 A significant decrease in the fitting quality for n: 0, 0.5 and 2 was observed. The surface degree of coverage by oxygen, 8, was calculated with the parameters estimated at 713K. Values within the 0.2 to 100% yO 0.8 range were found. Therefore, the oxidation 9~ 02 = n ,tool-% , 1 9 T " =9713 K takes place over a partially reduced surface 80% with the overall rate depending on oxygen ~60%
n S COx a S BA o X To l
and toluene partial pressures (Figures 3 and 4). Likewise, it was observed that 40% temperature has no significative influence on "~ -o~.._ ... selectivities to partial and total oxidized products (Figure 5). This fact suggests that all 20% o the steps of the kinetic scheme have the same 0% activation energy. The preexponential factors 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% and the activation energy (Arrhenius law, Toluene MohrFraction k~= kfl * exp(-Ea/RT)), were determined considering all the experiments performed. Figure 4. Effect of toluene molar fraction The results are presented in Table 3. It is (W/F = 0.6 grcat.min/ml) important to remark that estimated activation energy for the reoxidation reaction was close to those of the oxidation reactions. Table 3. Estimated parameters Parameter
Estimated Values ...............
k ~ mol.(min.atrn.grcat) "l
5.22E+06
k ~ mol.(min.atm.grcat) "1
3.28E+06
k ~ mol.(min.atm.grcat) "l
7.24E+07 2.02E+06
k4~ mol.(min.atm.grcat) "1 Ea kcal.mol 1
27.6
The kinetic scheme has a good fitting of the data, as can be seen in Figures 2 to 5 and in the good correlation between the experimental and the calculated toluene molar fraction showed in Figure 6. lO0~ Y~ = 0.55 mol-% y~ 2 =19.9 mol-% 80%
~ * ~ ~ ~ A 723 K SCOx n713K o 703 K -x~ o 693 K 73 K
.~ 60~ ,, ~ 40% "
20%
~176 . r "
~
0% 0%
~" 0.0% . . . . . 10% 20% 30% 40% 50% m 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% Toluene Conversion Calculated Molar Fraction Figure 5. Selectivities vs. Toluene conversion Figure 6. Parity plot for toluene molar fraction
1738 5. CONCLUSIONS The reaction network was modelled assuming the redox mechanism proposed by Mars and Van Krevelen. The approach is oversimplified with respect to the true reaction mechanism, but provides a better description of the experimental data than the empirical approaches (power law equations). The kinetic-scheme proposes three steps: the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides. The good fitting of the experimental results for the model proposed does not imply that this model represents the true reaction mechanism. Detailed mechanistic theories are out of the scope of the present work. The similar values found of apparent activation energies for each step, suggest that benzaldehyde and carbon oxides are produced from the same intermediate. Similar behaviour was obtained by Van der Wiele and Van den Berg [2] in the oxidation of toluene over bismuth molybdate catalysts. The activation energy value obtained in this work is similar to that reported by Trimm and Irshad in the oxidation of toluene over molybdenum trioxide catalyst (27.4 Kcal/mol) [5]. The overall rate was found dependent of the partial pressures of the reactants. The oxidation takes place over a partially reduced surface. The selectivity to benzaldehyde was in the 15% to 50% range and increases as the toluene molar fraction increases and oxygen molar fraction decreases. In the range of the operating conditions, there is no rate-controlling step. Further studies will be carried out in order to find the optimal conditions for benzaldehyde production. NOMENCLATURE BA : Benzaldehyde COx: Carbon oxides. Ea :activation energy. exp.: experimental F : molar feed rate. F~o : volumetric feed rate. ki: specific rate constant. ki~ Arrhenius law preexponential factor. m: catalyst mass Pi : partial pressure of component i.
P :total pressure :reaction rate of step i. T :temperature x :toluene conversion yi : molar fraction of component i. TOL : Toluene. ~ox : oxidized catalyst sites. Qr : reduced catalyst sites 0 : surface degree of coverage by oxygen W/F: space-time ri
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