Ind. Eng. Chem. Res. 1991,30,1829-1832
1829
Selective Oxidation of II -Butane to Maleic Anhydride. 3. Modeling Studies Shyamal K. Bej and Musti S. b o * Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208 016, U.P.,India The complete modeling of the selective oxidation of n-butane to maleic anhydride (MA) has been studied in this present investigation. The nature of different sites present on the VPO catalyst has been explored with a poisoning study. Two different types of sites were found. Selective sites are responsible for the selective oxidation of n-butane t o MA. Nonselective sites caused formation of oxides of carbon from MA as well as from n-butane. Two different models have been proposed and tested with the use of the differential reactor data. The experimental data substantiate the model which assumes Vv as the selective site (for the formation of MA) and VN as the nonselective site for the formation of oxides of carbon.
1. Introduction The general scheme of the selective oxidation of n-butane to MA can be represented by a combination of series and parallel reactions as follows: n-butane
(I)
MA
oxides of carbon
The reaction takes place over vanadium-phosphorus oxide (VPO)catalysts. Several investigators (Centi et al., 1984, 1988; Cavani et al., 1985; Hodnett, 1985) have reported the presence of Vv, VW,and Vm oxidation states in the VPO catalyst after its reaction with hydrocarbon. However, much doubt still remains regarding the role of different oxidation states of vanadium in this complex oxidation reaction. Hodnett (1985) and Centi et al. (1988) had reported that selective oxidation of n-butane to MA and its consecutive reaction takes place on the Vv site. But, from doping and poisoning experiments Centi et al. (1989) have reported that two different types of sites are responsible for the conversion of n-butane to MA and ita subsequent oxidation to oxides of carbon. However, there is no firm evidence regarding the occurrence of a specific reaction on a particular site. Centi et al. (1989) postulated that Vv site is responsible for the oxidation of MA to oxides of carbon. Moreover, there is no report regarding a plausible site for the third reaction, viz., the complete oxidation of n-butane to oxides of carbon. In this present work we have tried to elucidate some of these aspects. The kinetics of conversion of n-butane to MA alone has been studied in part 2 of this series. The kinetics of the entire reaction (scheme 1above) has been studied in the present investigation after identification of the sites responsible for oxidation and combustion, respectively. The reaction has been studied over a Mo- and Ce-promoted VPO catalyst (P/V atomic ratio = 1.08; weight ratio of Mo/V = 0.06; weight ratio of Ce/V = 0.022). The catalyst had a surface area of 20 m2/g. The details of catalyst preparation were given in part 1 of this series. Two sets of experiments were conducted in the present study. One set of experiments was conducted for identifying the sites responsible for the formation of MA and oxides of carbon. These experiments constitute the runs for the “poisoningstudy”. Ammonia and SOz were used as poisons. Having identified the sites responsible for the
* To whom correspondence should be addressed.
reactions, another set of experiments (called “kinetic runs”) were conducted for modeling the kinetics of the entire reaction scheme (1).
2. Experimental Section 2.1. Experimental Setup. The setup used for kinetic experiments was exactly same as that described in part 1 of this series. However, for poisoning studies, provision was made at the top of the reactor for injecting the poisons through a rubber septum. The effluent of the reactor was allowed to pass through a three-way valve. After drying, the product gases were fed to a gas chromatograph through a gas sampling valve when required; otherwise they were vented out. 2.2. Experimental Procedure. The procedure for conducting the kinetic runs was identical with the one described in part 2 of this series. For conducting experiments for poisoning study, 0.69 g of catalyst was taken in the reactor. Air was allowed to pass through the reactor at a rate of 60 mL/min. Then the reactor was heated in a controlled manner. When the desired temperature was reached, n-butane was allowed to mix with the air. After 2 h when steady state was reached, a small quantity of (known amount of) gaseous ammonia was injeded into the reactor through a gastight syringe, and 25 min after the injection, samples were taken for analysis. After an interval of 30 min after the analysis,the same procedure was repeated with different volumes of ammonia. Then the reactor was operated for 6 h to remove all the ammonia from the reactor. A similar study was carried out with different volumes of gaseous sulfur dioxide. 2.3. Analytical Methods. For the poisoning study, no liquid sample was collectad. Since MA, COz, and CO were the only products of this reaction, amounts of n-butane, COB,and CO were measured by use of gas chromatographic analysis, the details of which were described in part 1of this series, and the amount of MA was calculated from a carbon balance. For the kinetic study, the liquid sample from the water bubbler was also analyzed for MA by using a 2-m-long Porapak-QS column and a flame ionization detector. MA was found to be the only product in the liquid sample. 3. Results and Discussion 3.1. Poisoning Study. Poisoning studies have been carried out using NH3and SO2as poisons. It was seen that the effect of poisoning remained for at least 1h. Therefore,
oaaa-5aa519112630-1a29$02.50/ o 0 1991 American Chemical Society
1830 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
model I1
Table I. Results of the Poisoning Studya Product Composition (vol 70)in the Absence of Poisoning component MA 0.50
coz+ co
1.92 0.22 97.36
n-butane air
Product Composition (vol % ) after Poisoning with Ammonia amt of NH2 injected, cm3 component 1.0 1.5 2.0 2.5 3.0 MA 0.20 0.06 0.0 0.0 0.0 COz + CO 1.64 1.44 1.36 1.32 1.34 n-butane 0.88 1.06 1.11 1.10 1.11 air b b b b b Product Composition (vol %) after Poisoning with Dioxide amt of SO, injected, cm3 comuonent 1.0 1.5 2.0 2.5 MA 0.75 0.78 0.74 0.76 C 0 2 + CO 0.80 0.40 0.20 0.00 n-butane 0.40 0.40 0.46 0.45 b h air b b
+
3
V+5
+e
V+4
-e
(3)
3.2.1. Development of Model I. The steps involved in this model are as follows: KI
02+R--+X
B
+ x 2MA + R K3
MA 3.0 0.76 0.00 0.45
+R
K4
C
+F
&
02+F-R
b
the analyses of gases were done within 1 h of injection of the poisons. The results of the poisoning study are given in Table I. The reactor was operated under conditions where interparticle and intraparticle resistances were negligible. From the data it is evident that when an excess of NH, was injected into the reactor, no MA was formed indicating that the sites poisoned by NH, were the only ones responsible for the formation of MA from n-butane. At this condition, whatever n-butane was consumed was converted to form oxides of carbon. When an excess of SO2 was injected into the reactor, MA was the only product. A t this condition, the amount of MA formed was equal to the amount of n-butane consumed. No oxides of carbon were formed. From this it can be concluded that the sites poisoned by SOz were responsible for the formation of oxides of carbon from MA as well as from n-butane directly. The results of the poisoning study are in conformity with the conclusions drawn by Centi et al. (1989). Hence, the sites participating in this complex reaction can be categorized into two classes. The selective sites are responsible for oxidizing n-butane selectively to MA while the nonselective sites or combustion sites are responsible for the formation of oxides of carbon from n-butane and MA. 3.2. Model Development. On the basis of the above inferences, we have developed two redox models for the reaction based on the principles given by Thomas and Thomas (1967). The first model assumes that the selective formation of MA from n-butane takes place on the V5 sites whereas V4sites are responsible for the formation of oxides of carbon from n-butane as well as from MA. The second model is based on the reverse concept; Le., V4 sites act as selective sites while V5sites are the nonselective ones. In terms of oxidation-reduction of the sites, the following redox cycles may be proposed for the models. model I for MA formation: v+52 v+4 -e
& ~ -e
for CO, formation:
-e
B+R-C+F
air-n-butane mixture. *Remainder.
v+4
V4EV3
Sulfur
Conditions of the experiments: temperature = 410 “C; W / F = 15.5 g/(mg-mol/s); n-butane feed concentration = 1.2 vol 90 of
for CO, formation:
+e
for MA formation:
(2)
(4)
where X, R, and F stand for V+5,V4,and V3oxidation states and B, MA, and C stand for n-butane, maleic anhydride, and oxides of carbon respectively. The rate at which n-butane is depleted to form MA on site X is given by
--dpB - KgROx
(5) dt where pBis the partial pressure of n-butane at a time t , 8x is the fraction of surface occupied by X, and m is the order with respect to n-butane. Similarly, the rate at which oxygen is consumed at the site R for its oxidation to X is given by
where po is the partial pressure of oxygen, BR is the fraction of surface occupied by R, and n is the order with respect to oxygen. If a moles of oxygen are required for the oxidation of 1 mol of n-butane to MA, then, at steady state, oxygen balance on site X gives
ffKzpfPx = KlPpa,BR
(7)
Similarly, oxygen balance on site R gives PK&h
+ Y K ~ P ~=AK~@R~ $ F= K&,(l-
OR - 8,) (8)
where 0 is the number of moles of O2 required to oxidize 1 mol of n-butane to oxides of carbon, y is the number of moles of oxygen required to oxidize 1mol of MA to oxides of carbon, and r and s are the orders of these reactions with respect to n-butane and MA, respectively for their total oxidation. We have assumed the same order n with respect to oxygen for the oxidation of F to R as for the oxidation of R to X. The total rate of depletion of n-butane is (9) -rB = K2pEOx + K&OR Values of Ox and OR are obtained from eqs 7 and 8 and substituted in eq 9. After proper rearrangements, -rB takes the following form:
-rB =
(K”
+ K3PS)
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1831 Table 11. Inlet and Outlet Concentrations in the Reactor of the Components Involved in the Rate Equations temp,
"C 420
430
440
inlet concn, vol % n-butane oxygen 0.578 13.11 0.870 13.17 1.209 13.26 1.500 13.25 1.800 13.23 2.070 13.10 2.379 13.06 0.580 13.11 0.873 13.06 1.210 13.09 1.501 13.11 1.798 13.06 2.069 13.08 2.382 13.11 0.578 12.98 0.873 12.94 1.205 13.03 1.498 12.92 1.780 12.91 2.069 12.99 2.374 12.95
outlet concn, vol n-butane oxwen 0.522 12.91 0.790 12.87 1.091 12.82 1.360 12.65 1.620 12.47 1.870 12.34 2.161 12.26 0.522 12.85 0.791 12.68 1.092 12.63 1.357 12.53 1.624 12.37 1.871 12.34 2.158 12.23 0.526 12.80 0.793 12.68 1.091 12.51 1.358 12.32 1.622 12.25 1.873 12.25 2.162 12.15
%
MA 0.052 0.078 0.104 0.128 0.148 0.164 0.180 0.050 0.074 0.102 0.130 0.142 0.158 0.172 0.048 0.070 0.096 0.116 0.124 0.150 0.164
Table 111. Average Concentrations of the Components and the Rate of nButane ConsumDtion (a-mol/(a*h)) temp, O C
420
430
440
average concn in the reactor, vol % n-butane MA oxygen C02 + CO 0.550 0.026 13.01 0.006 0.830 0.039 13.02 0.010 1.150 0.052 13.04 0.023 1.430 0.064 12.95 0.030 1.710 0.074 12.85 0.040 1.970 0.082 12.72 0.053 2.270 0.090 12.66 0.065 0.551 0.025 12.98 0.010 0.832 0.037 12.87 0.018 1.151 0.051 12.86 0.025 1.429 0.065 12.82 0.026 1.711 0.071 12.73 0.052 1.970 0.079 12.71 0.071 2.270 0.086 12.67 0.093 0.552 0.024 12.89 0.009 0.833 0.035 12.81 0.024 1.148 0.048 12.77 0.034 1.428 0.058 12.62 0.043 1.701 0.067 12.58 0.064 1.971 0.075 12.62 0.095 2.268 0.082 12.55 0.127
N# a a
a a a a a a a
a a a a a a a a a a a
a
rate n-butane consumpn 0.0047 0.0066 0.0084
0.0098 0.0111 0.0123 0.0135 0.0059 0.0081 0.0103 0.0120 0.0126 0.0149 0.0164 0.0069 0.0092 0.0114 0.0131 0.0146 0.0160 0.0175
a Remainder.
where K', = K l / a . This is the complete model for the total rate of consumption of n-butane. 3.2.2. Development of Model 11. The following steps are involved in this model. KI
02+F-R
B + R -% M A + F B+XK"-C+R MA
+X
-+ K4
Oxygen balance on site R gives
C
R
1.0'
0.0
-
-1.0
-
- Y
-2.0
1"
1.35
1.40
1.45 1.50 i / i ( ~ - I x) i o 3
1.55
1.60
Figure 1. Arrhenius plots of various rate constants.
and oxygen balance on site X gives
oK$beX
+ rK4PheX = K$&eR
(13)
where the notations are same as for model I. Equation (12) can be written as ~ K Z P B ~=R ~
l p a ~exl -- 0,)
(14)
The total rate of depletion of n-butane is -rB = K&'eR
+ K3pb8x
(15)
Substituting the values of BR and ex from eqs 13 and 14 in eq 15, we obtain the final form for the total rate of depletion of n-butane as
+ 7K@hA. 3.2.3. Identification of Adequate Model. In order
where D = PK&
to obtain the parameters of eqs 10 and 16, kinetic experiments were conducted under conditions where, besides the main selective reaction, the two side reactions producing oxides of carbon also take place in appreciable amounts. From our experiments it is observed that temperatures above 410 "C favor this situation. In part 2 of this series, it was established that m = 1 and n = 0 and the values of K1 and K 2 at 370,390, and 410 O C were also obtained. These values are extrapolated to higher temperatures (Le., 420, 430, and 440 "C)and used as initial guesses for K 1 and K 2 along with m = 1 and n = 0 in eqs 10 and 16. The latter are then linearized to obtain good initial guesses of the remaining parameters. These initial guesses are then used for obtaining the optimum values of the parameters by nonlinear regression using Marquardt's Bsolve algorithm (Kuester and Mize, 1973). The experimental conditions and the rates of n-butane consumption are given in Tables I1 and 111. The values of parameters and orders of reactions along with the residual sum of squares (RSS) are given in Tables IV and V for models I and 11, respectively. From the results it appears that, for model I, the values of the parameters follow a definite (increasing) pattern with increasing temperature, while for model 11, the values of the parameters
1832 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 Table IV. Values of Parameters and Orders of the Reaction for Model I param' 420 "C 430 "C 440 "C K1 0.0700 0.0868 0.0909 Kz 0.9987 1.2672 1.6487 K3 1.2490 1.5745 2.1247 K4 1.0032 1.3675 2.2102 K5 0.4002 0.4970 0.5563 r 1.0001 Loo00 1.oooO S 0.2500 0.2500 0.2482 RSS 0.48199 X 0.22301 X lo4 0.74034 X Units of K's are g-mol/(g.atm.h). Table V. Values of Parameters and Orders of the Reaction for Model I1 440 "C paramo 420 "C 430 "C 0.0121 0.0082 0.1567 0.4797 0.0231 0.1513 2.7261 1.8001 1.0418 0.3802 2.1878 0.0300 0.0847 0.5329 0.8400 1.0002 1.oO01 0.8797 0.1712 0.3723 0.3748 0.13116 X 0.92785 X lo4 0.65180 X lo4 Units of K's are gmol/(gatm.h). Table VI. Comparison of Activation Energies with Values Reported in the Literature values of activation energy, source kcal/mol our investigation El = 12.90 E2 = 24.65 E3 = 26.68 E4 38.81 E, = 16.22 E2 = 10.77 Centi et al. (1985) E3 = 26.28 E, = 13.71 E, = 27.71 Buchanan and Sundaresan (1986) E3 = 31.06 E4 = 32.97
are suspicious. Hence it is concluded that model I is the more appropriate one with m = 1, n = 0, r = 1, and s = 0.25. The Arrhenius plots for various rate constants of model I are shown in Figure 1. Values of K1 and K 2 at lower temperatures are taken from part 2 of this series. The values of the activation energies are given in Table VI. These are comparable with those reported by Buchanan and Sundaresan (1986). 4. Conclusions With the help of a poisoning study it is established that two different types of sites are responsible for the oxidation and combustion reactions. The formation of MA from n-butane takes place on Vbsites whereas oxides of carbon are produced on V4sites. Hence it appears that the V5 e V4redox cycle is involved for the selective oxidation reaction, whereas the V4 V+3redox cycle is responsible for the combustion reaction. This is evident from the best fitting of differential experimental rate data with the model derived from the above postulation. The calculated parameters follow Arrhenius dependencies on temperature.
The activation energies for the reactions are comparable with those reported by Buchanan and Sundaresan (1986).
Nomenclature e = electron E , = activation energy for the ith step of the reaction scheme F = further reduced state of the catalyst K , = rate constant for the ith step of the reaction scheme m = order with respect to n-butane for its oxidation to MA n = order with respect to oxygen for oxidation of reduced and further reduced state p I = partial pressure of ith component r = order with respect to n-butane for its oxidation to oxides of carbon R = reduced state of the catalyst s = order with respect to MA for its oxidation to oxides of carbon W J F = contact time X = oxidized state of the catalyst Greek Symbols
a = number of moles of oxygen required to oxidize 1 mol of
n-butane to MA /3 = number of moles of oxygen required to oxidize 1 mol of
n-butane to oxides of carbon y = number of moles of oxygen required to oxidize 1 mol of
MA to oxides of carbon fraction of surface covered by ith site
8, =
Registry NO. MA, 108-31-6; PO, 1314-56-3;V, 7440-62-2; Mo, 7439-98-7; Ce, 7440-45-1; H(CH2)4H,106-97-8.
Literature Cited Buchanan, J. S.; Sundaresan, S. Kinetics and Redox Properties of Vanadium Phosphate Catalysts for Butane Oxidation. Appl. Catal. 1986,26, 211-226. Cavani, F.; Centi, G.; Manenti, I.; Trifiro, F. Catalytic Conversion of C4 Hydrocarbons on Vanadium-Phosphorus Oxides: Factors Influencing the Selectivity of 1-Butene Oxidation. Ind. Eng. Chem. Prod. Res. Dev. 1985,24, 221-226. Centi, G.; Fornasari, G.; Trifiro, F. On the Mechanism of n-Butane Oxidation to Maleic Anhydride: Oxidation in Oxygen-Stoichiometry-Controlled Conditions. J. Catal. 1984, 89, 44-51. Centi, G.; Fornasari, F.; Trifiro, F. n-Butane Oxidation to Maleic Anhydride on Vanadium-Phosphorus Oxides: Kinetic Analysis with a Tubular Flow Stacked-Pellet Reactor. Ind. Eng. Chem. Prod. Res. Deu. 1985,24, 32-37. Centi, G.; Trifiro, F.; Ebner, J. R.; Franchetti, V. M. Mechanistic Aspects of Maleic Anhydride Synthesis from C4 Hydrocarbons over Phosphorus Vanadium Oxide. Chem. Reo. 1988,88,55-80. Centi, G.; Golinelli, G.; Trifiro, F. Nature of the Active sites of (VO)2P207in the Selective Oxidation of n-Butane. Evidence from Doping Experiments. Appl. Catal. 1989, 48, 13-24. Hodnett, B. K. Vanadium-Phosphorus Oxide Catalysts for the Selective Oxidation of C, Hydrocarbons t~ Maleic Anhydride. Catal. Reu. Sci. Eng. 1985, 27 (3), 373-424. Kuester, J. L.; Mize, J. H. Optimization Techniques with Fortran; McGraw-Hill: New York, 1973; Chapter 6. Thomas, J. M.; Thomas, W. J. Introduction t o the Principles of Heterogeneous Catalysis; Academic Press: London, 1967;Chapter 8. Received for review July 2, 1990 Revised manuscript received February 6, 1991 Accepted February 12, 1991
