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Ind. Eng. Chem. Prod. Res. Dev.
1985,2 4 , 62-68
Kinetics of the Partial Oxidation of Isobutene over Silica-Supported Molybdenum-Uranium Oxide Catalyst Vlcente CortQsCorberbn, Avellno Corma, and Gojko Kremenld' Institute de Catiilisis y Petroleoqdmica, C.S.I.C., Serrano, 119, 28006 Madrid. Spain
The kinetcs of the formation of the main primary products (Le., methacrolein, acetone, CO, and COP)obtained during the oxidation of isobutene over a silica-supported molybdenum-uranium oxide catalyst have been studied over the temperature range of 320 to 380 OC in an isothermal flow reactor operated differentially. The data were fitted to a number of models based on Langmuir-Hinshelwood, Rideal-Eley, Mars-van Krevelen, and steady state of adsorption mechanisms. The preferred models for the formation of methacrolein and acetone are based on a Langmuir-Hinshelwood mechanism in which the rate-controlling step is the surface reaction between adsorbed isobutene and nondissociated oxygen. An enhanced oxygen adsorption has to be considered for acetone formation in order to explain experimental results. The best model for both CO and COPformations is consistent with a redox mechanism in which the catalyst reoxidation rate is second order in oxygen and catalyst reduction is first order in isobutene.
Introduction The catalytic oxidation of isobutene over metal oxide catalysts has received considerably less attention in the literature than the closely related oxidation of propene. The available information is concerted mainly with the catalytic properties of determined catalytic systems and mechanistic and catalyst development studies (Hucknall, 1974; Van der Wiele and van der Berg, 1978; Cullis and Hucknall, 1982). Comparativelylittle work on the reaction kinetics has been reported, and perhaps the reason is that the kinetics seems to be rather complex, as evidenced by the very different isobutene and oxygen dependencies reported and rate equations found applicable. Mann and KO (1973) oxidized isobutene over a copper-promoted bismuth molybdate catalyst in the range 300-560 "C. The rate of formation of methacrolein was satisfactorily correlated by a Langmuir-Hinshelwood mechanism which assumes the rate-controlling step to be the surface reaction between adsorbed isobutene and oxygen species. Ray and Chanda (1976) studied the oxidation of isobutene over bismuth molybdate in the range 350-500 OC and found that methacrolein and carbon oxides formation rates were independent of oxygen partial pressure and first order in isobutene when the 0xygen:isobutene ratio was higher than 61. They found that methacrolein and carbon oxides were formed by parallel routes. Cartlidge et al. (1977) analyzed the kinetics of isobutene oxidation over bismuth molybdate by means of kinetic models. The model for methacrolein formation, found applicable only below 370 OC, contained no hydrocarbon term and the best model for carbon oxides formation contained no oxygen term and assumed that all of these carbon oxides were formed by degradation of methacrolein. Vinogradova et al. (1977) studied the oxidation of isobutene in the range 320-400 "C, over a molybdenum-cobalt oxide catalyst modified with Bi and Fe. They found that methacrolein, methacrylic acid, and carbon oxides are formed according to a parallel-consecutive scheme and that the total oxidation was first order in oxygen; meanwhile, selective oxidation is first order in oxygen for oxygen:isobutene ratios below 3 and zero order for higher oxygen: isobutene ratios. Schuhl et al. (1980) oxidized isobutene over U Sb301,, in the range 300-400 "C and found that methacrolein formation is independent of oxygen and first order in isobutene while C 0 2 formation is zero order in isobutene and first order in oxygen. Zhiznevskii et al. (1978) cor0196-4321185111224-0062$01.50/0
Table I. Characteristics of t h e Mo03-U03/Si02 Catalyst particle size: 0.42-0.59 mm BET surface area: 78 m2 g-l pore volume: 0.653 cm3 g-l mean pore radius: 120 A crystalline phases: orthorhombic MOO,, monoclinic U02Mo04 equivalent oxides area9 22.1 m2 g-' a
Determined by oxygen chemisorption.
related the rate of formation of methacrolein, methacrylic and acetic acids, and carbon oxides in the oxidation of isobutene over a Te-V-Mo-0 (Te:V:Mo = 4:1.2:4.8) catalyst, with rather complex empirical expressions in which methacrolein formation was independent of hydrocarbon and dependent on oxygen and water, C 0 2 was dependent on oxygen and isobutene and inhibited by water, and CO was dependent on oxygen and isobutene, having all the rate expressions fractional orders in each reactant. These observed oxygen and isobutene dependencies are not easily related to the reaction mechanism, since as we have just seen, different kinetics are observed even in catalysts in which the reaction mechanism is the same. For this reason, it seems convenient to analyze the rate data by means of kinetic models. In a previous paper (CortBs Corberin et al., 1984) we found that the reaction network of isobutene oxidation over silica-supported molybdenum-uranium oxide catalysts is complex, the main primary products being methacrolein, acetone, CO, and COz. In the present paper we have studied the kinetics of the formation of these products and the data have been analyzed by means of kinetic models.
Experimental Section The catalyst used was a Si0,-supported molybdenum uranium mixed oxide (20.8 wt% of active phase) with an atomic ratio Mo:U = 8:l. It was prepared by a double impregnation method, which has been described elsewhere (KremeniE et al., 1982), and the main characteristics are indicated in Table I. The kinetic experiments were carried out in an isothermal stainless steel (i.d. 13 mm) flow reactor, with a coaxial thermocouple for measuring the temperature inside the catalytic bed. Reactants and products were analyzed "on line" by gas chromatography. The apparatus and procedure were described in detail in a previous paper 0 1985
American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 63
Table 11. Initial Rates of Formation of Primary Products temp, 'C 380 380 380 380 380 380 380 380 380
Ph, atm 0.0245 0.098 0.196 0.294 0.490 0.196 0.196 0.196 0.196
~ 3 atm , 0.294 0.294 0.294 0.294 0.294 0.049 0.098 0.196 0.392
350 350 350 350 350 350 350 320 320 320 320 320 320
0.0245 0.098 0.196 0.294 0.196 0.196 0.196 0.0245 0.098 0.196 0.294 0.196 0.196
0.294 0.294 0.294 0.294 0.098 0.196 0.392 0.294 0.294 0.294 0.294 0.196 0.392
CH@(CH3)CHO 32.5 67.0 70.5 91.7 96.0 20.8 32.9 60.0 81.1
(Corti%Corberln et al., 1984). All experiments were carried out at conditions in which diffusion was not a significant effect. Contact times were varied by changing the catalyst weight and/or the molar flow of isobutene. Catalyst samples (0.05-0.5 g) were diluted with carborundum bits in order to achieve a constant volume of the catalytic bed. Experiments were rejected if the errors in mass, carbon, or oxygen balances were greater than 5 % or temperature differences along the catalytic bed were more than 3 OC.
Results Kinetic Study. The kinetics for the formation of the primary products has been studied in a differential reactor by the initial rate technique. We have examined the influence of the principal operation variables, Le., contact time, temperature, and partial pressure of reactants. To study the influence of the composition of the reacting mixture we have used the usual technique of varying the concentration of one reactant while keeping constant the concentrations of the rest. All kinetic measurements were made within the following ranges: partial pressure, isobutene 0.025-0.50 atm and oxygen 0.05-0.40 atm, temperature, 320-380 OC, and atmospheric pressure. Water partial pressure was kept constant at 0.20 atm. Proper weights of catalyst samples and isobutene molar flows were chosen in order to maintain operation in a differential regime, i.e., xT I 10%. In these experimental conditions the influence of the reaction products can be neglected. The primary reaction products are methacrolein, acetone, biacetyl, methallyl alcohol, CO, and COP (Corti% Corberh et al., 1984). At initial conditions we found that yields of biacetyl and methallyl alcohol were very low, inferior, or very close to the detection limit of our analytical method, and their kinetics have not been studied. We have centered our study in the four main products, i.e., methacrolein, acetone, CO, and COP,which represent at least 95% of the total products of the reaction. Initial formation rates of each primary product have been calculated by linear regression of the typey = ax, of the yields (y) as a function of the contact time ( x ) . The results are shown in Table 11. In all cases, the initial rate increases when the partial pressure of isobutene or oxygen increases.
10.3 13.2 13.9 14.6 7.17 11.7 16.2 4.53 5.26 5.44 5.55 4.50 5.97
r; X lo4 (mol of i formed/h , e - of cat.) CH3COCH3 coz 9.44 115 37.5 319 46.0 608 62.3 708 101 752 4.27 176 8.69 389 568 22.5 512 65.1 3.97 8.00 9.61 12.0 2.81 6.09 15.5 2.19 3.40 3.88 4.28 2.41 6.16
49.2 174 353 448 144 278 359 19.3 65.2 142 159 116 150
co 28.4 89.6 156 262 310 56.8 130 164 167 12.8 48.8 99.2 117 37.7 80.0 110 5.24 19.5 41.6 44.4 32.2 46.0
Kinetic Analysis. The kinetic analysis of the experimental data was made by kinetic models based on different possible mechanisms. Only models in which the reaction rate is dependent of both isobutene and oxygen partial pressures have been considered. Four different mechanisms have been proposed in the literature for the catalytic oxidation of olefins: those of Langmuir-Hinshelwood, Rideal-Eley (Mann and KO, 1973), Mars-van Krevelen, or redox (Krenzke and Keulks, 1980),and steady state of adsorption (Muller et al., 1976). According to the literature, the rate-controlling step in the catalytic oxidation of olefins can be the adsorption of reactants or the surface reaction between the adsorbed species. In the study of the oxygen adsorption on our catalyst, it has been found that, in the temperature interval 320-380 "C, the rate of oxygen chemisorption is very high in such a way that the process can be considered instantaneous (Salazar, 1983) and consequently the oxygen adsorption can be neglected as a possible rate-controlling step. Assuming this, the models based on the Langmuir-Hinshelwood mechanism (Hinshelwood, 1940) that have been tested are the following. Models 1to 3 suppose competitive adsorption of oxygen and isobutene on one type of center. The rate-controlling step is the adsorption of isobutene in models 1A and 1B and the surface reaction in models 2A, 2B, and 3. Besides, models lB, 2B, and 3 consider that oxygen adsorption is dissociative, and model 3 considers the possibility of the two dissociated oxygen species to be involved in the rate-controlling step. Models 4A and 4B assume that adsorption of reactants is not competitive on two types of centers, and surface reaction is the rate-controlling step being the oxygen adsorption nondissociative in model 4A and dissociative in model 1B. Models based on the Rideal-Eley mechanism (Bond, 1962) suppose alternatively that the adsorbed reactant is isobutene (model 5) or oxygen (model 6). As the real kinetics of the reduction of our catalyst by isobutene, and its reoxidation by oxygen, are not known, the models based on the Mars-van Krevelen mechanism (Mars and van Krevelen, 1954) that we have tested suppose that the catalyst reduction in first order in olefin and that catalyst reoxidation can be first (model 7A) or one-half order (model 7B) in oxygen.
04
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table 111. Rate Equations Derived from the Tested Models model mechanismn rate-controlling step lA, lBb L.H. (1type of center) adsorption of isobutene 2A,2Bb L.H. (1type of center) surface reaction L.H. (1type of center) surface reaction involving two dissociated 0 atoms 3 4A, 4Bh L.H. (2 types of center) surface reaction surface reaction between adsorbed isobutene and 5 R.E. oxygen in gas phase 6 R.E. surface reaction between adsorbed oxygen and isobutene in gas phase 7A, 7B' M.v.K. equilibrium between catalyst reduction and its reoxidation 8A, 8Bb S.S.A. equilibrium between catalyst reduction and oxygen adsorption power law rate expression 9
rate eauation
"L.H. = Langmuir-Hinshelwood; R.E. = Rideal-Eley; M.v.K. = Mars-van Krevelen; S.S.A. = steady state of adsorption. bModels "A" assume nondissociative adsorption of oxygen (n = 1);models "B", dissociative adsorption (n = 'i2).'Catalyst reoxidation is first order in model 7A (n = 1) and 1/2 order in model 7B (n = I/*).
Table IV. Influence of Isobutene Partial Pressure methacrolein formation temp, "C 380
350
320
temp, "C 380
350
320
a
type of model lA, lB, 6 2A, 2B 3 4A, 4B, 5, 7A, 7B, 8A, 8B 9
Ahn 0.0254 9.13 3.60 5.12 0.36
lA, lB, 6 2A, 2B 3 4A, 4B, 5, 7A, 7B, 8A, 8B 9
0.0063 34.1 10.8 7.31 0.14
A, lB, 6 2A, 2B 3 4A, 4B, 5, 7A, 7B, 8A, 8R 9
0.0025 57.1 15.6 10.5 0.08
type of model lA, lB, 6 2A, 2B 3 4A, 4B, 5, 7A, 7B, 8A, 8B 9
Ah" 0.0717 1.89 0.833 8.22 0.83
4
F
2.92 2.07 99.4 -4.31
C.C. 0.752 0.976 0.966 0.993 0.973
1.399 0.279 0.333 0.148 0.296
1 61 42
54
0.0215 4.52 1.66 227 0.76
4.67 2.88 674 -6.34
0.284 0.987 0.978 0.997 0.986
4.233 0.229 0.297 0.118 0.234
3 74 43 288 71
0.0047 29.0 8.53 43.1 0.44
7.510 0.223 0.296 0.089 0.261
3 79 44 500 57
0.0018 57.2 146 57.1 0.27
ha
0.156 7.00 0.988 3.79 0.978 1780 0.998 -7.38 0.983 CO formation C.C. Bha 0.953 3.00 0.907 2.09 0.905 18.4 0.999 -2.78 0.995
Aha
0.340 0.543 0.549 0.065 0.125
7.77 3.96 769 -6.19
0.575 0.976 0.970 0.997 0.994
4
F
0.297 0.619 0.625 0.094 0.205
53 10 10 566 117
0.945 0.305 0.341 0.114 0.152
41 32 305 170
1188 318
0.195 2.22 1.17 1.93 0.67
0.747 1.930 10.3 0.985 0.245 4.82 0.977 0.302 2260 0.996 0.128 -7.40 0.994 0.151 C 0 2 formation Bhn C.C. 0.749 0.741 0.222 1.43 0.985 0.221 1.28 0.985 8.70 0.080 0.998 -1.91 0.248 0.981
_ _ _ . -
J.
acetone formation C.C. Bhn 0.964 5.12 0.877 2.97 0.875 24.3 0.997 -4.07 0.987
F 39 14 14
Ah"
*
1
2 65 42 243 173
F 5 99 100 776 78
lA, IB, 6 2A, 2B 3 4A, 4B, 5, 7A, 7B, 8A, 8B 9
0.436 2.11 0.839 18.8 0.92
4.26 2.63 13.3 -3.22
0.973 0.844 0.845 0.999 0.996
0.264 0.758 0.756 0.029 0.125
54 5 5 4698 259
0.162 1.07 0.532 4.85 0.91
2.21 1.70 5.70 -1.94
0.987 0.919 0.920 0.999 0.999
0.182 0.557 0.556 0.030 0.077
118 11 11 4396 678
lA, lB, 6 2A, 2B 3 4A, 4B, 5, 7A, TB, 8A, 8B 9
0.017 4.13 1.40 45.7 0.90
6.60 3.52 41.2 -4.17
0.947 0.789 0.788 0.999 0.992
0.372 0.870 0.871 0.046 0.179
26 3 3 1927 123
0.060 2.25 0.952 12.3 0.89
3.52 2.32 18.6 -2.95
0.962 0.832 0.832 0.999 0.994
0.315 0.784 0.784 0.056 0.149
37 5 5 1285 177
Constant whose units depend on the kinetic equation involved.
Models based on the steady-state adsorption mechanism (Sheltad et al., 1960; Juusola et al., 1970), which can be considered as a variation of the redox mechanism, consider the cases where oxygen adsorption is dissociative (model 8B) or not (model 8A). The power law rate, which is widely used for reactor design purposes, does not give information about the mechanism of the reaction, but we have included it in our testing as model 9 for comparison purposes. The rate expression derived from these models are summarized in Table 111. The experimental data hwe been fitted to the above rate expressions conveniently linearized as a function of isobutene (Ph) or oxygen (po)partial pressures. Simultane-
ously, several statistical tests were used to estimate the goodness of the fit and to discriminate between models. The tests used were: the correlation coefficients (c.c.), the F of Fisher (Fisher, 1955), and the $ of Exner (1966). Influence of the Isobutene Partial Pressure. The influence of ph was studied by fitting the experimental rates a t constant po and pw into the linearized rate expressions as a function of p,,. The results indicate that for the formation of each one of the primary products, only the models whose linearlized rate expression is of the type l / r = Ah(l/Ph) + Bh (1) (i.e., models 4A, 4B, 5 , 7A, 7B, 8A, and 8B) produce a statistically significant fit (Table IV).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24,
Table V. Influence of Oxygen Partial Pressure (I) methacrolein formation temp, "C type of model Ao" BOa C.C. $ 380 4A, 7A, 8A 20.3 73.9 0.996 0.114 4B, 7B, 8B 125 -93.6 0.995 0.128 5 0.024 0.898 0.492 9 0.67 -4.12 0.994 0.143 350
320
4A, 7A, 8A 4B, 7B, 8B 5 9
101 498 0.0047 0.59
361 -192
4A, 7A, 8A 4B, 7B, 8B 5 9
216 337 0.0017 0.41
1120 320
-5.86
-7.03
No. 1, 1985 65
acetone formation
F
'40"
384 302 17 244
124 765 0.015 1.34
174 -1190
BO
-3.82
0.999 0.993 0.776 0.994
0.062 0.164 0.728 0.156
1035 147 5 162
377 1830 0.0036 1.20
-280 -2360
0.999 0.996 0.904 0.995
0.067 09145 1.651 0.168
665 142 1 105
1000 3880 0.0015 1.36
-985 -4660
F
AO" 2.22
BOb
-5.41
-6.11
C.C.
+
F
0.999 0.994 0.975 0.996
0.050 0.141 0.251 0.114
2034 250 76 381
0.999 0.999 0.975 0.997
0.034 0.066 0.255 0.115
3507 338 58 298
0.998 0.995 0.973 0.999
0.097 0.175 0.282 0.053
316 97 36 1052
Constant whose units depend on the kinetic equation involved.
Table VI. Influence of Oxygen Partial Pressure (11) CO formation temp, "C type of model Aoa BO" C.C. 380 4A, 7A, 8A 6.48 33.5 0.958 4B, 7B, 8B 38.3 -15.2 0.915 5 0.055 0.670 9 0.48 03.50 0.886 10 0.285 5.54 0.994 350
320
4A, 7A, 8A 4B, 7B, 8B 5 9 10
23.4 112 0.032 0.79 1.78
20.9 -104
4A, 7A, 8A 4B, 7B, 8B 5 9 10
33.3 129 0.0132 0.48 4.27
-3.68 79.4 130 8.34 -4.92 190
C 0 2 formation
+ 0.371 0.521 1.346 0.600 0.135
33 15 1 11 269
13.2 0.739 1.02 0.097
8.71 -8.13
0.991 0.975 0.896 0.976 0,999
0.194 0.313 0.514 0.306 0.012
104 39 12 41 30137
5.69 27.4 0.109 0.69 0.433
10.1 -20.4
0.998 0.994 0.342 0.993 0.999
0.112 0.190 1.294 0.207 0.028
238 82 1 69 3973
7.89 30.4 0.044 0.38 1.02
45.4 16.6
-0.86 16.3
-2.57 24.3
-3.82 59.4
C.C.
#
F
0.968 0.929 0.390 0.650 0.996
0.324 0.479 1.030 0.981 0.119
44 19 1 2 352
0.990 0.974 0.791 0.968 0.999
0.200 0.319 0.707 0.354 0.053
98 37 5 30 1405
0.989 0.981 0.830 0.975 0.998
0.261 0.338 1.917 0.381 0.122
43 25 1 20 201
Constant whose units depend on the kinetic equation involved.
Table VII. Methacrolein Formation temp, "C Kh, atm-' 380 19.4 (6.5) . , 350 92 (19) 320 170 (24) "In mol of CjH,O/(h g of cat.).
KO,atm-'
k
X lo4" 194 (33) 137 i22j 95 (12)
3.6 (1.9) 3.6 (0.8) 5.2 (1.1)
k,
X
493 (66) 99.i (8.5) 46.3 (5.8)
k,, (= kadJ x 194 (80) 28.9 (4.3) 9.34 (0.94)
mol of C4H60/(atmh g of cat.).
Influence of the Oxygen Partial Pressure. The influence of p o was studied by fitting the experimental rates at constant P h and pwto the different rate expressions. The results are shown in Tables V and VI. From Table V it can be deduced that the most probable models for mechacrolein formation are models 4A, 7A, and 8A. The same models produce the best fit for acetone, but in this case, negative values are found for one constant. None of the previously detailed models, even the power law rate model, produces an acceptable fit of the experimental data for the formation of CO and COz (Table VI). For this reason, and looking to the curves of rate vs. oxygen partial pressure, we have tried a semiempirical model, 10, with a linear form of the rate expression as a function of oxygen partial pressure l / r = Ao(l/Po2) + Bo
(2)
and whose linear form as function of the isobutene partial pressure is that corresponding to models 4A, 7A, and 8A, i.e., eq 1. The fit of initial rate data for formation of Cc) and of COz to this new model is significant at the three temper-
atures, and therefore this will be the preferred one. Theoretical justification of this model will be seen in the Discussion.
Discussion (a) Methacrolein Formation. The most probable models for methacrolein formation are models 4A, 7A, and 8A, whose linearized expressions are equivalent although they are based on three different mechanisms. The calculated values of the constants corresponding to these models are shown in Table VII, with their 95% confidence intervals (in parentheses). The three models are acceptable from the chemical point of view, because the variations of the constants with temperature agree with the theoretical forecasts. In order to discriminate among them, the rate values calculated according to each one have been plotted vs. experimentalvalues in Figure 1. Values calculated with model 4A fit quite well to the line of slope unity with a random distribution around it; meanwhile, those predicted by models 7A and 8A are considerably biased above that line. Then, the most probable model is model 4A, which assumes a Langmuir-Hinshelwood mechanism in which rate
66
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
ts c2
:
- 4 m
0 E
MODELS
1 2 EXPERIMENTAL r,.103
4
:: c
7A 6 A
6
8
LA
(mol c 4 d 5 o /Q cot
1
0
I
Figure 1. Calculated vs. experimental initial rates of methacrolein formation.
is controlled by the surface reaction between isobutene and nondissociated oxygen, adsorbed not competitively on two types of centers. This model is the same found by Mann and co-workers for the formation of methacrolein by catalytic oxidation of isobutene on bismuth molybdate (Mann and KO, 1973) and halogen-modified copper oxide (Mann et al., 1972). The apparent activation energy calculated for this reaction is 30.0 f 3.0 kcal/mol, and the heat of adsorption of isobutene and oxygen are 21.7 f 9.6 and 3.2 f 8.2 kcal/mol, respectively. Due to the large confidence interval for the calculated heat of adsorption as a consequence of the high imprecision associated with the adsorption constants, we will give these values as an indication of their order of magnitude and they will not be considered further. (b) Acetone Formation. As has been said before, the fit of experimental rate data to models 4A, 7A, and 8A is the most significant, but it gives a negative value for one of the adsorption constants in the linear form of the rate expression as a function of oxygen partial pressure. According to Hougen and Watson’s criteria, the kinetic models whose fit leads to significant negative values of any constant determined from experimental data must be rejected. Evidently the models based on the redox or steady-state adsorption mechanisms are inconsistent with these negative values of the constants. Nevertheless, in the application of mechanisms based on Langmuir isotherm such a proposition must be carefully considered since, as Weller (1975) points out, there are cases in which the negative value of one constant may have a real meaning. The available work about adsorption from a potentially reacting mixture indicates that enhanced adsorption of at least one component (sometimes both) seems to be the general pattern (Weller, 1956; Gupta et al., 1972). The forced fitting of enhanced adsortpion data into a Langmuir form guarantees a negative value for at least one of the constant in the studied range of conditions. In the partial oxidation of olefins it is not infrequent to obtain a negative value for one of the adsorption constants. Muller et al. (1976), studying the catlytic oxidation of isobutene on a NiO-A120, catalyst, find this result when they analyze the rate of formation of methacrolein and acetone for isobutene partial pressures higher than 0.25 atm. On the other hand, enhanced adsorption phenomena
have been observed with oxygen-olefin mixtures. Gerei et al. (1973),studying the adsorption of a propene-oxygen mixtures on CuO and CuOz, find enhanced adsorption of oxygen and decreased adsorption of isobutene; at all temperature and for all the mixtures, adsorption was higher than the additive on both oxides. Although we have no data on the adsorption from isobutene-oxygen mixtures on our catalyst, experiments in course have shown that preadsorption of isobutene enhance the oxygen adsorption on its surface (Tascdn et al., 1984). Besides this, the apparent order in oxygen (A,, for model 9) is consistently higher than unity (Table V), which seems to support the hypothesis of enhanced oxygen adsorption. Then, if enhanced adsorption occurs, the question is why negative values for KOare only found for acetone formation. Although we have found that methacrolein and acetone formation involve the same type of mechanism, yet one leads to C-C bond cleavage and the other to allylic oxidation. The difference in the characteristics of selective and complete oxidation (the degradation process) lies in the reactivity of 0 species responsible for their conversion (Kubokawa and Ono, 1978). Then, the difference between the mechanism of formation of methacrolein and acetone (that is a degradation product) lies in the nature of the 0 species involved in both processes. As Bieliinski and Haber (1979) pointed out, the direction of the oxidation of the adsorbed olefin species depends on the electronic nature of the 0 species involves: electrophilic 0 species would lead to degradation products (as acetone in this case) and finally to total oxidation; meanwhile, nucleophilic 0 species would lead to selective oxidation, methacrolein in this case. Acetone formation from isobutene is a reaction equivalent to the formation of acetaldehyde from propene. Portefaix et al. (1980), based on labeled propene experiments, demonstrate that the C-C bond cleavage to give acetaldehyde does occur at the C=C double bond, by simultaneous interaction with two 0 atoms, simultaneously producing formaldehyde which is readily oxidized to COz at reaction conditions. In isobutene oxidation acetone may be formed in a similar way, its formation implying the cleavage of a C-C bond and the formation of a C1 product. We have not found CHI or HCHO at measurable quantities, and we believe that the C1 radical gives C02as final product. But the ratio between CO, and acetone formation rate is considerably higher than unity, which implies that C02must be formed also by successive scissions and oxidations of the adsorbed isobutene molecule. As we saw above, the direction of the oxidation depends on the nature of the involved oxygen (Biellnski and Haber, 1979; Kubokawa and Ono, 19781, so we can assume that the same type of oxygen is involved in both paths to C02formation. Then, the simultaneity of both reactions products a higher number of vacant sites for oxygen, per unit of time, than the acetone formation reaction by its own, which originates the enhanced adsorption. The effect of this phenomenon is not evident in COz formation because it is possibly masked by the other route of formation which is in the majority. In conclusion, the most probable model for acetone is model 4A, which assumes a Langmuir-Hinshelwood mechanism in which rate is controlled by surface reaction between isobutene and nondissociated oxygen, adsorbed noncompetitively on two types of centers, being oxygen adsorption enhanced. To avoid the negative values of KOand at the same time to maintain a Langmuir-type expression, a correction factor
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table VIII. Acetone Formation temp, OC 380 350 320
Kh, atm-'
KO,atm-'
2.5 (2.3) 17.8 (6.2) 40 (11)
1.42 (0.27) 2.09 (0.03) 1.84 (0.16)
k x lo4" 172 (51) 16.1 (4.4) 6.07 (0.75)
67
P 2.98 (0.36) 2.58 (0.17) 2.81 (0.22)
"In mol of C3H60/(h g of cat.).
Table IX. C O and COz Formation temp,
Kh, atm-'
O C
KO,atm-'
k,
X
lo2"
kerb
CO Formation 380 350 320
4.5 (2.2) 1.2 (0.9) 1.5 (1.8)
380 350 320
2.24 (0.31) 0.71 (0.89) 0.90 (1.40)
169 (49) 56 (8.2) 58 (17)
51.8 (4.8) 20.6 (8.5) 8.15 (0.63)
10.3 (1.4) 2.31 (0.17) 0.98 (0.22)
12.1 (9.0) 5.32 (0.22) 2.19 (0.14)
35.1 (5.5) 5.61 (0.09) 2.34 (0.12)
COz Formation
"In mol of CO (or C02)/(atmh g of cat.).
19.4 (6.1) 4.45 (0.16) 4.45 (0.32)
mol of CO (or C02)/(atm2 h g of cat.).
(1- /3), could be introduced in the term depending on po, leading to the expression
The introduced adimensional empirical constant /3 indicates the influence of the enhancement of the adsorption. Then, when it does not take place, /3 will be zero, and when it does, the bigger the influence, the higher /3 will be. The values of K h , Ho, k, and /3 have been calculated from the fitting constants and are shown in Table VI11 with their 95% confidence level interval (in parentheses). (c) C 0 2 and CO Formations. The most probable model for both C 0 2 and CO formation was model 15, whose linear forms were eq 1 and 2. These linear forms can arise from one of these rate expressions
r=k
KOp02
KhPh
1+ K h P h 1
+ &Po2
(4)
or
r=
k r p h kox.P2 krph
+ lZo&O2
(5)
The first expression would correspond to a LangmuirHinshelwood mechanism with two types of centers in which two oxygen molecules are adsorbed on the same center. The second one would correspond to a redox mechanism in which the catalyst reoxidation kinetics is second order in oxygen. The different constants, calculated according to both expressions, are shown in Table IX. The adsorption of two molecules on the same center seems to be a very improbable process and besides the values of the equilibrium constants KO and Kh do not decrease monotonically when temperature increases as is expected. Then, the Langmuir-Hinshelwood type model can be rejected, and only the redox mechanism is consistent with model 15. The second order in oxygen could be explained if we assume that the active center has two oxidated states. Total oxidation of isobutene would leave the active center in its most reduced state, and, if the transition from this state to the intermediate state and from the later to the most oxidated were both first order in oxygen, the kinetics of the transition most reduced most oxidated would be second order in oxygen.
+
The apparent activation energy for the formation of C 0 2 is 23.6 f 2.0 kcal/mol and that for the formation of CO is 21.9 f 0.6 kcal/mol. The total parallelism found between the kinetics of the initial formation of CO and that of C 0 2on our catalyst seems to indicate that both products are formed from a common intermediate, as has been found in propene oxidation over bismuth molydbate by Kobayashi and Futaya (1979). (d) Power Law Rate Models. From the linearization of the power law rate expression (model 9) In r = In k
+ a In p o + b In P h
(6)
it follows that the fitting constants a and b are the apparent reaction orders in isobutene and oxygen, respectively. It can be seen from Tables IV to VI that the fit to model 9 is statistically significant for both linearizations only for the case of acetone formation. This means that in our case, this type of expression is not adequate to describe the kinetics of the rest of these reactions (particularly the formation of CO and C 0 2 ) .
Conclusions The oxidation of isobutene on silica-supported molybdenum-uranium oxide catalysts is a rather complex process in which initially three parallel routes are taking part: selective oxidation to mechacrolein, partial degradation to acetone and a C1product, and total oxidation to CO and C 0 2 . Each one of these reactions follows a different mechanism, which indicates that the type of active center must be different for each route. The best model for methacrolein and acetone formations is based on a Langmuir-Hinshelwood mechahism which assumes the rate-controlling step to be the surface reaction between adsorbed isobutene and nondissociated oxygen. A correction term must be introduced, in the case of acetone, to account for the enhanced oxygen adsorption. The best model for CO and C 0 2 formations is consistent with a redox mechanism which assumes that catalyst reduction is first order in isobutene and that catalyst reoxidation is second order in oxygen. Nomenclature Ah(O)= slope of the linearized rate expression as a function
of isobutene (oxygen) parcial pressure. B,(,, = independent term constant of the linearlized rate expression as a function of the isobutene (oxygen) partial pressure.
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 ,
68
F = Fisher's function
K i= adsorption equilibrium constant for component i, atm-' k = kinetic constant kads = kinetic constant of the oxygen adsorption k,, = kinetic constant of the reoxidation of the catalyst k , = kinetic constant of the reduction of the catalyst pi = partial pressure of component i, atm rr = initial rate of formation of product i , mol of i formed/h g of cat.) W / F = contact time, g of cat./mol C4= h-' xT = total conversion and mol C,= transformed, h-' Subscripts a = acetone h = hydrocarbon (for isobutene) i = reactant or product i m = methacrolein 0 = oxygen w = water Registry No. CH2=C(CH3)CH0,78-85-3; CH3COCH3,6764-1; isobutene, 115-11-7;molybdenum oxide, 11098-99-0;uranium oxide,
11113-93-2.
Literature Cited Bielinski, A.; Haber, J. Catal. Rev.-Sci. Eng. 1979, 19, 1. Bond, G. C. "Catalysis by Metals"; Academic Press: New York, (1962), p 128. Cartlidge, J.; McGrath, L.: Wilson, S. H. Trans. Inst. Chem. Eng. 1977, 5 5 , 164. Cortds Corberin, V.; Corma, A.; Kremenld, G. Ind. Eng Chem Prod. Res. Dev. 1884, 23, 546. Cullis, C. F.; Hucknall, D. J. I n "Catalysis"; Bond, G. C.; Webb, G., Ed.; Specialist Periodical Reports; The Chemical Soclety: London, 1982: Vol. 5, Chapter 7. Exner, 0. Collect. Czech. Chem. Commun. 1988, 31, 3222.
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Fisher, T. "Statistical Methods for Chemists"; Wiley: New York, 1955. Gerai, S. V.; Rozhkova. E. V.; Gorokhovatskii, Ya. B. J . Catal. 1973, 28. 241. Gupta, R. B.; Viswanathan, B., Sastri, M. V. C. J . Catal. 1972. 26, 212. Hinshelwood, C. N. "The Kinetics of Chemical Change": Oxford University Press: New York, 1940; p 207. Hucknall, D. L. "Selectlve Oxidation of Hydrocarbons"; Academic Press: London, 1974. Juusola. J. A.; Mann, R. F.; Downie, J. J . Catal. 1970, 7 7 , 106. Kobayashi, M., Futaya, R. J . Catal. 1979, 56, 73. KremeniE, G.; Fierro, J. L. G.; Cortds Corberin, V. Actas ' 8 Simp. Iberoam. de CaGlisis, 1982; Vol. 1, p 346. Krenzke, L. D.; Keulks, G. W. J . Catal. 1980, 6 4 , 295. Kubokawa, Y.; Ono, T. Bull. Chem. Soc. Jpn. 1978, 57(12), 3435. Mann, R. S.; KO, D. W. J . Catal. 1973, 30,276. Mann. R. S.; Yao, K. C.; Dosi, M. K. J . Appl. Chem. Biotechnol. 1972, 22. 915. Mars, P.; van Krevelen, D. W. Chem. Eng., Sci. Suppl. 1954, 3 , 41. Muller, A.; Julllet, F.; Telchner, J. Bull. SOC. Chim. Fr. 1976, 9 - 7 0 , 1356. Portefaix, J. L.; Figueras, F.; Forissler, M. J . Catal. W80, 6 3 , 307. Ray, S. K.; Chanda, M. Ind. Eng. Chem. Prod. Res. Dev. 1978, 75,234. Sahzar, E. Ph.D. Thesis, Univ. de Bilbao, Spain 1983. Schuhl, Y.; Delobel, R.; Baussart, H. C . R . Acad. Sci. Paris. Ser. C 1980, 290, 5. Sheltad, K. A.: Downie, J.; Graydon, W. F. Can. J . Chem. Eng. 1960, 38, 102. Tasc6n, J. M. D.; Cortds Corberin, V.; KremeniE, G.; Gonzilez Tejuca, L. 1984; to be published. Van der Wiele, K.; Van der Berg, P. J. "Comprehensive Chemical Kinetics", Bramsford, C. H., Tipper, C. F. H., Ed.; Elsevier: Amsterdam, 1978: Vol. 20, Chapter 2. Vinogradova. 0. M.; Vytnov. G. F.; Margoiis, L. Ya. Kinef. Katal. 1977, 18, 1595. Weller, S. W. AIChE J . 1958, 2,59. Weller, S.W. Adv. Chem. Ser. 1975, No. 148, 26. Zhiznevskii, V. M.; Kabubowskaya, L. F.; Tolopko, D. K . Zh. Fiz. Khim. 1978. 52. 1058.
Received for review March 8, 1984 Accepted November 16, 1984
Effect of Nitrogen Compounds on Cracking Catalysts Chla-Mln Fu and Arnold M. Schaffer' Phllllps Petroleum Company, Phillips Research Center, Bartlesviiie, Oklahoma 74004
The present study was undertaken to determine how catalytic cracking is affected by the presence of various nitrogen compounds. The effects on conversion and selectivity of about 30 different nitrogen compounds were determined. There is good agreement between the molecule's gas-phase proton affinity and its poisoning effect on cracking catalysts. Moreover, proton affinity accounts for the strong dependence of poisoning on the nitrogen compound's molecular structure including such factors as the type of nitrogen heterocyclic, size of the molecule, and presence of alkyl substituents. For example, conversions for 0.5 wt % nitrogen added to a gas oil from pyridine, pyrrole, quinoline, indole, acridine, and carbazole are 5 1.4, 54.1, 39.2, 49.6, 34.7, and 51.7 vol % , respectively. Results have also been obtained demonstrating that conversion and selectivity are also sensitive to the nitrogen concentration, the cracking temperature, the type of feed, and the properties of the cracking catalyst.
Introduction The decreased availability of sweet light crudes has caused significant changes in refinery operation (Murphy et al., 1979; Green and Broderick, 1981). For example, fluid catalytic crackers (FCC's) must process feedstocks that are heavier, more aromatic, and higher in metals, sulfur, and nitrogen. There have been several recent studies reviewing the effects of feedstock changes on FCC operation (Tolen, 1981;Ritter et al., 1981;Magee et al., 1979);however, these reports do not deal specifically with the effects of nitrogen compounds. This is somewhat surprising since it has long been recognized that basic nitrogen compounds can poison acid cracking catalysts (Mills et al., 1950; Voge et al., 1951; Viland, 1957). Furthermore, as indicated by Table I, nitrogen poisoning will become even more important if heavy oil, shale oil, tar sands liquids, and direct coal liquids 0196-432118511224-0068$01.50/0
Table I. Nitrogen Content of Various Feedstocks N content. feedstock
wt %
E a s t Texas gas o i l A r a b l i g h t 650 O F + West Texas topped crude N o r t h Slope 650 O F + direct coal liquids Gulf o f Suez 650 O F + t a r sands l i q u i d s off-shore California Monagas 650 O F + San Joaquin 650 O F + (California) Paraho shale o i l
0.07 0.10 0.18 0.21
0.3 0.37 0.4 0.9 0.91 1.02 2.1
become available as refinery feedstocks (Hochman, 1982; deRosset et al., 1979). The present study was thus undertaken to determine how the crackability of a typical 0 1985 American Chemical
Society
