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Oxidation kinetics of partially reduced vanadyl pyrophosphate catalyst
Applied Catalysis A: General 223 (2002) 205–214
Oxidation kinetics of partially reduced vanadyl pyrophosphate catalyst Daxiang Wang a,b , Mark A. Barteau a,∗ a
Department of Chemical Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, USA b Conoco Inc., 1000 South Pine Street, Ponca City, OK 74602, USA Received 4 May 2001; received in revised form 17 July 2001; accepted 18 July 2001
Abstract The oxidation kinetics of partially reduced vanadyl pyrophosphate (VPO) catalyst were investigated with a novel oscillating microbalance reactor. The oxidation rate of partially reduced VPO catalyst with oxygen is close to half-order with respect to oxygen partial pressure. The activation energy for this oxidation is 70 kJ mol−1 . The kinetics obtained from the transient reaction study are applicable in the design and optimization of the circulating fluidized bed riser reactor process for the selective oxidation of butane to maleic anhydride. © 2002 Elsevier Science B.V. All rights reserved. Keywords: VPO catalyst; Kinetics; Transient reaction; Oxidation of butane; Maleic anhydride
1. Introduction Selective oxidation of butane to maleic anhydride (MA) on vanadyl pyrophosphate (VPO) catalysts has been reported to proceed through a Mars–van Krevelen mechanism [1], i.e. through the sequential reduction and reoxidation of the catalyst surface, butane is selectively converted to MA. In a newly commercialized process for the maleic anhydride production with a circulating fluidized bed riser reactor, the reduction and reoxidation steps of the VPO catalyst are spatially separated into two separate reactors [2,3]. One of the main advantages claimed for this process is the opportunity for independent optimization of operating conditions for each step [2–4]. Certainly, this
∗ Corresponding author. Tel.: +1-302-831-8905; fax: +1-302-831-8201. E-mail address: [email protected] (M.A. Barteau).
optimization requires an understanding of the reaction kinetics for the reduction and oxidation steps. Even though the kinetics for butane oxidation to MA on VPO catalysts have been thoroughly investigated under steady state reaction conditions with co-feeding of butane and oxygen [5–16], the results may not be applicable to the sequentially operated process. That is because the VPO catalyst surface structure is very sensitive to the gas phase composition [17], thus the catalyst may behave differently under transient conditions than under steady state operating conditions. Taking advantage of a newly available oscillating microbalance reactor, we can accurately study the transient kinetics by monitoring the catalyst mass change under transient reaction conditions. In a previous paper, we reported the kinetics of butane oxidation by VPO catalyst [18]. In this study, the kinetics of the oxidation of partially reduced VPO catalyst are investigated.
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2. Experimental 2.1. Catalyst A VPO catalyst with a nominal P/V ratio of unity was prepared following an organic method which has been described in detail elsewhere [18]. After activation in butane/oxygen/He (1.75/21/balance, vol.%) at 673 K for 24 h, the catalyst is composed of mainly a vanadyl pyrophosphate phase [18]. Its BET surface area is 16.9 m2 g−1 . When used under steady state reaction conditions at 616 K in a gas mixture of butane/oxygen/He = 1.75/21/balance (vol.%), at SV ≈ 1800 h−1 , it gave a 19% butane conversion with 66% selectivity to maleic anhydride, which is typical for a well crystallized VPO catalyst. 2.2. Transient kinetic study The transient oxidation kinetics of a partially reduced VPO catalyst were investigated with a novel oscillating microbalance reactor (TEOM 1500, Rupprecht & Patashnick Co. Inc., Albany, NY). The instrument set-up and its features have been described in earlier reports [18–23]. As was shown previously, when a VPO catalyst is equilibrated under steady state reaction conditions in a gas mixture with oxygen/butane molar ratios close to 12, the catalyst is in an oxidized state [18,19]. The oxidation kinetics of the catalyst can be studied by first reducing it to a specific extent. Over-reduction of VPO catalyst can result in the accumulation of V3+ on the catalyst surface [19], which may affect its oxidation kinetics. Therefore, the pre-reduction of the catalyst has to be controlled carefully. When a VPO catalyst is reduced in butane after equilibration in a gas mixture with oxygen/butane = 12, the initial reduction rate is very high, e.g. about −1 [18], which makes it very hard to 10 g g−1 cat s accurately control the reduction extent just by controlling the reduction time. Therefore, in this study we used an alternative method to achieve accurate control of the reduction extent prior to each kinetic measurement. According to the literature [17], the surface state of a VPO catalyst is sensitive to the atmosphere to which it is subjected, especially to the oxygen/butane ratio in
Fig. 1. Mass change of VPO catalyst responding to the change of gas composition from butane/oxygen/Ar/He of 1.7/2.1/1.6/balance (vol.%) to 1.7/21/1.6/balance (vol.%). Conditions: 100 mg VPO catalyst pretreated as described in the text; temperature: 674 K; total gas flowrate: 50 ml min−1 .
the feed. In turn, by controlling the gas composition, we should be able to adjust the surface state of the catalyst. Fig. 1 shows the mass change of the VPO catalyst when the reaction gas was switched from a gas mixture of butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%) in which the catalyst was equilibrated for 2 h, to a gas mixture of butane/oxygen/Ar/He (1.7/21/1.6/balance, vol.%), at 674 K. In the more oxidizing feed, the catalyst gained mass gradually and reached another steady mass value in about 5 h. The total mass change is about 200 g (100 mgcat )−1 , which corresponds to a change of about 0.04 in the average bulk vanadium valence in this vanadyl pyrophosphate sample. Compared to the total oxygen capacity of about 1000 g (100 mgcat )−1 for this catalyst [18], the mass difference of 200 g (100 mgcat )−1 in butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%) from that in butane/oxygen/Ar/He (1.7/21/1.6/balance, vol.%) represents a very modest level of reduction of the catalyst in the more reducing reactant stream. According to the above observation, the following general procedure was followed to establish a reproducible state of the “partially reduced catalyst” for the subsequent measurement of the transient oxidation
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kinetics. First, a VPO sample of 100 mg was heated up in the microbalance reactor to 674 K in a gas mixture of butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%) at 50 ml/min under atmospheric pressure. After 2 h reaction under the above conditions when the mass reading became stable, the catalyst was purged with helium (Keen Gas Grade 5, further purified with an oxygen trap) for 30 min at 674 K or cooled to the desired temperature under a helium purge. To compensate for the buoyancy effect resulting from the difference in the gas densities of the purging helium and the oxidant (a mixture of oxygen in helium), various amounts of nitrogen (Keen Gas Grade 5) were added to the purging helium. The amount of nitrogen added depended on the oxidizing gas composition which ranged from 2.1 to 21 vol.% oxygen in helium. Finally, oxidation was initiated by replacing purging gas with oxygen/He. Oxidation was carried out at several different temperatures between 617 and 674 K. The reaction order with respect to the oxygen partial pressure and the activation energy for catalyst oxidation were calculated from measurements of the catalyst mass change. 2.3. Cyclic redox operation To demonstrate the effectiveness of the transient kinetics in describing the catalyst’s behavior under cyclic redox operation, sequential redox operations were conducted on the same VPO catalyst using the microbalance reactor. In these experiments, the catalyst was first heated up in oxygen (21 vol.% in helium) to 674 K and held for 2 h, then the reaction gas was switched between oxygen (21 vol.% in helium) and butane/Ar/nitrogen/He (1.7/1.6/18/balance, vol.%) at 2 min per step. The catalyst mass was monitored during the cyclic operation. Addition of nitrogen to the reductant was used to eliminate buoyancy effects resulting from the difference in gas densities. The cyclic operations were conducted following several temperature protocols. In one protocol, after the VPO catalyst was stabilized in oxygen (21 vol.% in helium) at 674 K for 2 h, it was cooled to 653 K, and then the redox cycle was carried out at 653 K. In another protocol, after the pretreatment in 21% oxygen and a 1 h redox operation at 674 K, the catalyst was cooled down to 655, 637 and 617 K, stepwise under the cyclic operation.
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3. Results and discussions 3.1. Transient oxidation kinetics When oxygen was fed to partially reduced VPO catalyst, it resulted in a rapid mass gain of the catalyst. To identify the source of this mass gain, we conducted a purging experiment by switching the gas from oxygen/He back to nitrogen/He. The experimental results showed that purging of the oxidized catalyst did not result in any obvious change of the catalyst mass. This result suggests that reversible adsorption of oxygen on the catalyst surface did not contribute to the observed catalyst mass change. Hence, the mass change upon oxidation of partially reduced VPO catalysts results from the replacement of lattice oxygen which was consumed by the previous reduction with butane, i.e. [∗] + 21 O2 → [O]
(1)
where [∗] and [O] represent the reduced and oxidized sites on the catalyst surface, respectively. In fact, as shown previously, the active reduced and oxidized vanadium sites involved in the redox operation are V4+ and V5+ , respectively [19]. The oxidation rate for reaction (1) can be represented by the following power law rate equation: n r = kPm O2 θ
(2)
where r is the oxidation rate which can be measured as the catalyst mass change rate upon oxidation, dm/dt (g (100 mgcat )−1 s−1 ), k the reaction rate constant, PO2 the oxygen partial pressure (Torr) and θ is the lattice oxygen vacancy concentration of the catalyst. The parameters m and n are the reaction orders with respect to gas phase oxygen and catalyst oxygen vacancies, respectively. The kinetic parameters in the above expression can be determined from the results of the microbalance experiments. Fig. 2 shows the catalyst mass change upon oxidation of the VPO catalyst (equilibrated in butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%)) at several temperatures in a gas mixture of 21 vol.% oxygen in He. Noise spikes resulting from flow disturbances upon switching the gas flows have been removed, producing the gaps in the curves in Fig. 2. These curves are all concave down in shape, showing that the mass change rate is highest at the beginning of the oxidation step and decreases as the catalyst
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Fig. 2. Isothermal oxidation of partially reduced VPO catalyst in 21 vol.% oxygen in He. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; temperature as indicated.
is oxidized. This behavior indicates that the reaction order with respect to the oxygen vacancies is positive. At the beginning of each oxidation experiment, the catalyst should be in the same state owing to the consistent procedure for generating a partially reduced catalyst. That is, the initial values of θ are the same for each transient shown in Fig. 2. The initial oxidation rates at each temperature can be measured as the slope of each oxidation curve at the beginning of the oxidation step. The results are summarized in Table 1. Fig. 3 is a plot of ln(dm/dt) (g (100 mgcat )−1 s−1 ) versus 1/T (K−1 ). The solid line is a linear regression of the experimental data, which shows an activation energy of 70 kJ mol−1 .
Table 1 Effect of temperature on the oxidation rate of partially reduced VPO catalystsa Oxidation temperature (K)
Rate of oxidation, dm/dt (g (100 mgcat )−1 s−1 )
617 637 655 674
0.38 0.52 0.75 1.2
a The oxidation rates were measured from the slope of the mass change at the beginning of the oxidation. Conditions: oxidant, 21 vol.% oxygen in helium, 50 ml min−1 , sample, 100 mg VPO catalyst pretreated as described in the text.
Fig. 3. Determination of activation energy for the oxidation of partially reduced VPO catalyst.
Fig. 4 shows the catalyst mass change when oxidation was carried out at 674 K at different oxygen partial pressures from 16 to 125 Torr. The initial oxidation rates are listed in Table 2. As is shown in Fig. 5, there is a linear dependence of ln(dm/dt) (g (100 mgcat )−1 s−1 ) versus ln(PO2 ) (Torr). The empirical value for the reaction order with respect to the oxygen partial pressure thus obtained is 0.55. In summary, the oxidation rate expression can be written as follows: dm = A e8500/T PO0.55 θn 2 dt
(3)
where A is the pre-exponential factor, T the reaction temperature (K), and the other terms are same as defined for Eq. (2). As is described in reaction (1), the ultimate reaction for the oxidation of partially reduced VPO catalyst is the transfer of oxygen from the gas phase to the lattice. This reaction may involve any of a variety of adsorbed forms of oxygen such as molecular oxygen, O2 , atomic oxygen O, and charged oxygen species such as O2 − , O− and O2− , etc. [24]. On the basis of the approximately half-order dependence of the oxidation rate on the oxygen partial pressure observed in our experiments, the oxidation mechanism may be postulated as following: K1
O2 + 2[∗]↔2O∗
(4)
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Fig. 4. Isothermal oxidation of partially reduced VPO catalyst in oxygen/He. Conditions: 100 mg VPO catalyst pretreated as described in the text; temperature: 674 K; total gas flowrate: 50 ml min−1 ; oxygen partial pressure as indicated.
Table 2 Effect of oxygen partial pressure on the oxidation rate of partially reduced VPO catalystsa Oxygen partial pressure (Torr)
Rate of oxidation, dm/dt (g (100 mgcat )−1 s−1 )
16 47 70 125 152
0.37 0.58 0.96 1.17 1.21
a The oxidation rates were measured from the slope of the mass change at the beginning of the oxidation. Conditions: temperature, 674 K, gas flow rate, 50 ml min−1 ; sample, 100 mg VPO catalyst pretreated as described in the text.
slow
O∗ → [O]
(5)
This sequence involves dissociative adsorption of oxygen and the conversion of adsorbed oxygen into lattice oxygen through electron transfer from surface cations, i.e. V4+ , to the oxygen adspecies.
Fig. 5. Determination of reaction order with respect to oxygen partial pressure for the oxidation of partially reduced VPO catalyst. Conditions: 100 mg VPO catalyst pretreated as described in the text; temperature: 674 K; total gas flowrate: 50 ml min−1 ; oxygen partial pressure as indicated.
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In this scheme, an asterisk denotes a reduced surface site, species with an asterisk denote surface species, and [O] represents lattice oxygen. According to previous results [19], the reduced surface sites on the VPO catalysts are V4+ which act as adsorption sites and provide the oxygen vacancy to convert the adsorbed oxygen to lattice oxygen. Filling of oxygen vacancies near V4+ oxidizes the vanadium to V5+ , and decreases the number of adsorption sites. If one assumes that reaction (5) is rate limiting, and reaction (4) proceeds to equilibrium, the resulting rate expression is of the form: 1/2
1/2
r = k2 K1 PO2 θ
(6)
where k2 is the rate constant for reaction (5), K1 the equilibrium constant for reaction (4), and other terms are the same as in Eq. (2). Comparing Eq. (6) with Eq. (3), we see that the oxidation reaction rate has a positive first-order dependence on the oxygen vacancy concentration, i.e. n = 1 for Eq. (3).
3.2. Cyclic redox operation Fig. 6 summarizes the results for cyclic operation of the VPO catalyst at 655 and 674 K. When the oxidized catalyst was reduced in butane, a rapid mass loss was observed with the removal of lattice oxygen. This mass loss is about 420 g (100 mgcat )−1 for the first 2 min reduction at 674 K (see the curve for 674 K in Fig. 6). Reoxidation at this point only resulted in a mass gain of about 110 g (100 mgcat )−1 within 2 min. After that, the amplitude of the redox operation at 674 K stabilized at about 122 g (100 mgcat )−1 for each 2 min reduction half cycle and 107 g (100 mgcat )−1 for each 2 min oxidation half cycle. The smaller mass gain in the oxidation steps compared with the mass loss in the reduction steps resulted in the net mass loss of the catalyst after each redox cycle which is shown in Fig. 6 as a decrease in the catalyst mass with time on stream. This trend is more obvious at 674 K than that at 655 K; at the lower temperature the amplitude of the reduction half cycle is 76 g (100 mgcat )−1 while the amplitude of the oxidation half cycle is about 73 g (100 mgcat )−1 .
Fig. 6. Mass change of VPO catalyst upon cyclic redox operation. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycle time: 2 min per step; temperature as indicated.
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Fig. 7. Mass change of VPO catalyst upon cyclic redox operation. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycling time: 2 min per step; temperature as indicated.
Fig. 7 presents another set of data from the cyclic redox operation obtained following a different temperature protocol. In this experiment, after the redox amplitudes stabilized after operation at 674 K for about 1 h, the reaction temperature was decreased to 655, 637 and 617 K, stepwise. At each temperature step, the apparent mass of the catalyst decreased; these step changes were artifacts from temperature effects on the oscillating behavior of the tapered element of the microbalance reactor [23]. The amplitudes of the mass change of the reduction and oxidation half cycles were measured when stabilized at each temperature. The results are listed in Table 3. Below 550 K, there is no obvious mass change upon the switch of reaction gases, indicating that no redox reaction occurs below that temperature. Figs. 8 and 9 plot the ln(amplitude) (g (100 mgcat )−1 ), versus 1/T (K−1 ) for the oxidation and reduction half cycles (2 min duration each), respectively. Linear regression of these experimental data shows that the dependence of the mass change amplitude on the reaction temperature follows an Arrhenius relation. The apparent activation energies obtained
for the oxidation and reduction half cycles are 68 and 80 kJ mol−1 , respectively. It is the relatively larger activation energy for the reduction step compared with that for the oxidation step that results in the larger increase in the reduction amplitude than the oxidation amplitude at higher temperatures. As a result, a steeper decline of the catalyst mass with time on stream was observed for cyclic redox operation at higher temperatures (Fig. 6).
Table 3 Effect of temperature on the redox amplitudesa Redox temperature (K)
617 636 654 674
Amplitude of mass change (g (100 mgcat )−1 ) Reduction
Oxidation
34 55 76 122
32 55 73 107
a The redox cycles were operated at 2 min per step between oxygen (21 vol.%)/He and butane/Ar/N2 /He (1.7/1.6/18/balance, vol.%).
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85 kJ mol−1 for the oxidation of butane with oxidized VPO [18] and 70 kJ mol−1 for the oxidation of partially reduced VPO catalyst (Fig. 3). This agreement suggests that the kinetics data obtained from transient studies can be applied to the prediction of catalytic performance under cyclic redox operation, and potentially to the design and optimization of the circulating fluidized bed riser reactor process, as discussed below. 3.3. Applications
Fig. 8. Determination of the activation energy for the oxidation half cycles in the redox operation of VPO catalyst. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycle time: 2 min per step.
Furthermore, the apparent activation energies for the reduction and oxidation half cycles obtained from cyclic redox operation are in good agreement with those activation energies obtained from the transient kinetics studies. The corresponding values were
Fig. 9. Determination of the activation energy for the reduction half cycles in the redox operation of VPO catalyst. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycle time: 2 min per step.
The major operating variables that one can manipulate in the circulating fluidized bed riser reactor scheme include the reactor temperatures, residence times and gas compositions. Separation of the reduction and oxidation steps makes the independent optimization of each step possible in principle. The goal of such optimization is to search for a set of conditions at which butane can be continuously converted to maleic anhydride with the highest yield. The optimization of the riser reactor has been discussed elsewhere [18]. Here, we can give some guidelines in the selection of operating conditions, such as reaction temperature, residence time and oxygen concentration in the fluidized bed catalyst reoxidation reactor. 3.3.1. Reaction temperature Experimental results for cyclic redox operation show that under the conditions examined, the catalyst continuously loses mass with time on stream. This process continues over much longer times than those represented in Figs. 6 and 7. We have observed the depletion of catalyst mass continue for runs as long as 40 h involving 600 complete redox cycles — the catalyst mass never reaches a constant value about which redox cycles of constant amplitude occur for reaction conditions such as those in the experiments of Figs. 6 and 7. Such unsteady operation makes reactor design very difficult, and also in the long-term may result in catalyst deactivation due to the over-reduction of the catalyst [19]. To make the operation stable, one needs to match the reduction rate with a similar oxidation rate by improving the oxidation of the partially reduced catalyst. According to rate equation (3), the most effective way is to increase the oxidation temperature. In general, the optimum temperature for selective conversion of butane to maleic anhydride with VPO
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catalysts lies within the range of 650–700 K [25]. As an example, if the temperature in the butane oxidation (catalyst reduction) step is controlled at 674 K for the catalyst used in this study, the amplitude of the reduction half cycle is about 122 g (100 mgcat )−1 for 2 min reduction. Therefore, the oxidation amplitude needs to be increased from 107 to 122 g (100 mgcat )−1 to compensate the mass loss due to reduction. According to oxidation rate expression in Eq. (3), the oxidation temperature needs to be increased from 674 to 681 K, i.e. by 7 K. 3.3.2. Residence time Alternatively, one could increase the residence time in the reoxidation reactor to increase the amplitude of the oxidation half cycle. According to the first-order dependence of the oxidation rate on surface oxygen vacancies and the rate expression in Eq. (3), the following relation can simulate the dependence of the amplitude of mass change, A (g (100 mgcat )−1 ) in the oxidation half cycle on the duration, t, of this half cycle. A = A0 × (1 − eat )
(7)
Fig. 7 shows that at 674 K, the amplitude of the mass change in the oxidation half cycle is 107 g (100 mgcat )−1 at 2 min per oxidation half cycle. One minute into the oxidation half cycle, the mass gain is about 94 g (100 mgcat )−1 . If one parameterizes Eq. (7) using these two points, one would predict that an oxidation half cycle of about 4 min would be necessary to increase the oxidation amplitude to 122 g (100 mgcat )−1 to compensate for the mass loss in a 2 min reduction half cycle. 3.3.3. Oxygen concentration In Eq. (3), another factor affecting the oxidation rate is the oxygen partial pressure. Conceptually, the oxidation operation can also be improved by increasing the oxygen partial pressure in the regenerator. Following the 0.55 order in the oxygen partial pressure, the effect of oxygen concentration on the relative amplitude of the catalyst mass change in the oxidation half cycle can be calculated and the results are illustrated in Fig. 10. The most obvious effect of increasing oxygen partial pressure is observed at oxygen concentrations below 30 vol.%. Also, by replacing air with pure oxygen as the oxidant, the oxidation rate can be more than
Fig. 10. Simulation results on the dependence of the amplitude in the oxidation half cycles on the oxygen partial pressure. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); cycle time: 2 min per step.
doubled. However, increasing the oxygen concentration above 21% would require an oxygen production facility, which would increase capital and operating costs of the process.
4. Conclusion In conclusion, the transient kinetics for the oxidation of partially reduced VPO catalyst were accurately measured by employing a novel oscillating microbalance reactor. The oxidation rate of partially reduced VPO catalyst has a 0.55 order dependence with respect to oxygen partial pressure and an activation energy of 70 kJ mol−1 . The combination of this result with the kinetics for butane oxidation on oxidized VPO catalyst can effectively predict the catalytic performance during cyclic redox operation. These kinetic results may be useful in the design and operation of fluidized bed circulating riser reactor for the selective oxidation of butane to maleic anhydride. References [1] P. Mars, D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41. [2] R.M. Contractor, US Patent 4,668,802 issued 26 May 1987 to E.I. du Pont de Nemours and Company.
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[3] R.M. Contractor, D.I. Garnett, H.S. Horowitz, H.E. Bergna, G.S. Patience, J.T. Schwartz, G.M. Sisler, in: V. Cortes Corberan, S. Vic Bellon (Eds.), New Developments in Selective Oxidation II, Elsevier, Amsterdam, 1994, p. 233. [4] G. Emig, K. Uihlein, G.-J. Hacker, in: V. Cortes Corberan, S. Vic Bellon (Eds.), New Developments in Selective Oxidation II, Elsevier, Amsterdam, 1994, p. 243. [5] S.K. Bej, M.S. Rao, Ind. Eng. Chem. Res. 30 (1991) 1819. [6] S.K. Bej, M.S. Rao, Ind. Eng. Chem. Res. 30 (1991) 1824. [7] S.K. Bej, M.S. Rao, Ind. Eng. Chem. Res. 30 (1991) 1829. [8] S.K. Bej, M.S. Rao, Ind. Eng. Chem. Res. 31 (1991) 2075. [9] D. Suter, F. Bartroli, F. Schneider, D.W.T. Rippin, E.J. Newson, Chem. Eng. Sci. 45 (1990) 2169. [10] P. Schneider, G. Emig, H. Hofmann, Ind. Eng. Chem. Res. 22 (1987) 2236. [11] T.P. Wellauer, D.L. Cresswell, E.J. Newson, Chem. Eng. Sci. 41 (1986) 756. [12] G. Centi, F. Trifiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 55. [13] G. Centi, G. Fornasari, F. Trifiro, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 32.
[14] K. Wahlfahrt, H. Hofmann, Chem.-Ing.-Tech. 52 (1980) 811. [15] J.J. Lerou, J.F. Weiher, Paper Presented at Pittsburgh/Cleveland Catalysis Society Meeting, May 1986. [16] J.S. Buchanan, S. Sundaresan, Appl. Catal. 26 (1986) 211. [17] G. Centi, F. Trifiro, J.R. Ebner, V.M. Franchetti, Chem. Rev. 88 (1988) 55. [18] D. Wang, M.A. Barteau, J. Catal. 197 (2000) 17. [19] D. Wang, H.H. Kung, M.A. Barteau, Appl. Catal. A 201 (2000) 203. [20] J.E. Rekoske, M.A. Barteau, J. Phys. Chem. B 101 (1997) 1113. [21] S.C. Fung, C.A. Querini, K. Liu, D.C. Rumschitzki, T.C. Ho, in: B. Delmon, G.F. Froment (Eds.), Catalyst Deactivation, Vol. 88, Elsevier, Amsterdam, 1994, pp. 305–312. [22] W. Zhu, J.M. Van de Graaf, L.J.P. Van den Broeke, F. Kapteijn, J.A. Moulijn, Ind. Eng. Chem. Res. 37 (1998) 1934. [23] TEOMTM Series 1500 Pulse Mass Analyzer, Rupprecht & Patashnick Co. Inc., 1994. [24] A. Bielanski, J. Haber, Catal. Rev.-Sci. Eng. 19 (1979) 1. [25] B.K. Hodnett, Catal. Rev.-Sci. Eng. 27 (1985) 373.
Oxidation kinetics of partially reduced vanadyl pyrophosphate catalyst Daxiang Wang a,b , Mark A. Barteau a,∗ a
Department of Chemical Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, USA b Conoco Inc., 1000 South Pine Street, Ponca City, OK 74602, USA Received 4 May 2001; received in revised form 17 July 2001; accepted 18 July 2001
Abstract The oxidation kinetics of partially reduced vanadyl pyrophosphate (VPO) catalyst were investigated with a novel oscillating microbalance reactor. The oxidation rate of partially reduced VPO catalyst with oxygen is close to half-order with respect to oxygen partial pressure. The activation energy for this oxidation is 70 kJ mol−1 . The kinetics obtained from the transient reaction study are applicable in the design and optimization of the circulating fluidized bed riser reactor process for the selective oxidation of butane to maleic anhydride. © 2002 Elsevier Science B.V. All rights reserved. Keywords: VPO catalyst; Kinetics; Transient reaction; Oxidation of butane; Maleic anhydride
1. Introduction Selective oxidation of butane to maleic anhydride (MA) on vanadyl pyrophosphate (VPO) catalysts has been reported to proceed through a Mars–van Krevelen mechanism [1], i.e. through the sequential reduction and reoxidation of the catalyst surface, butane is selectively converted to MA. In a newly commercialized process for the maleic anhydride production with a circulating fluidized bed riser reactor, the reduction and reoxidation steps of the VPO catalyst are spatially separated into two separate reactors [2,3]. One of the main advantages claimed for this process is the opportunity for independent optimization of operating conditions for each step [2–4]. Certainly, this
∗ Corresponding author. Tel.: +1-302-831-8905; fax: +1-302-831-8201. E-mail address: [email protected] (M.A. Barteau).
optimization requires an understanding of the reaction kinetics for the reduction and oxidation steps. Even though the kinetics for butane oxidation to MA on VPO catalysts have been thoroughly investigated under steady state reaction conditions with co-feeding of butane and oxygen [5–16], the results may not be applicable to the sequentially operated process. That is because the VPO catalyst surface structure is very sensitive to the gas phase composition [17], thus the catalyst may behave differently under transient conditions than under steady state operating conditions. Taking advantage of a newly available oscillating microbalance reactor, we can accurately study the transient kinetics by monitoring the catalyst mass change under transient reaction conditions. In a previous paper, we reported the kinetics of butane oxidation by VPO catalyst [18]. In this study, the kinetics of the oxidation of partially reduced VPO catalyst are investigated.
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2. Experimental 2.1. Catalyst A VPO catalyst with a nominal P/V ratio of unity was prepared following an organic method which has been described in detail elsewhere [18]. After activation in butane/oxygen/He (1.75/21/balance, vol.%) at 673 K for 24 h, the catalyst is composed of mainly a vanadyl pyrophosphate phase [18]. Its BET surface area is 16.9 m2 g−1 . When used under steady state reaction conditions at 616 K in a gas mixture of butane/oxygen/He = 1.75/21/balance (vol.%), at SV ≈ 1800 h−1 , it gave a 19% butane conversion with 66% selectivity to maleic anhydride, which is typical for a well crystallized VPO catalyst. 2.2. Transient kinetic study The transient oxidation kinetics of a partially reduced VPO catalyst were investigated with a novel oscillating microbalance reactor (TEOM 1500, Rupprecht & Patashnick Co. Inc., Albany, NY). The instrument set-up and its features have been described in earlier reports [18–23]. As was shown previously, when a VPO catalyst is equilibrated under steady state reaction conditions in a gas mixture with oxygen/butane molar ratios close to 12, the catalyst is in an oxidized state [18,19]. The oxidation kinetics of the catalyst can be studied by first reducing it to a specific extent. Over-reduction of VPO catalyst can result in the accumulation of V3+ on the catalyst surface [19], which may affect its oxidation kinetics. Therefore, the pre-reduction of the catalyst has to be controlled carefully. When a VPO catalyst is reduced in butane after equilibration in a gas mixture with oxygen/butane = 12, the initial reduction rate is very high, e.g. about −1 [18], which makes it very hard to 10 g g−1 cat s accurately control the reduction extent just by controlling the reduction time. Therefore, in this study we used an alternative method to achieve accurate control of the reduction extent prior to each kinetic measurement. According to the literature [17], the surface state of a VPO catalyst is sensitive to the atmosphere to which it is subjected, especially to the oxygen/butane ratio in
Fig. 1. Mass change of VPO catalyst responding to the change of gas composition from butane/oxygen/Ar/He of 1.7/2.1/1.6/balance (vol.%) to 1.7/21/1.6/balance (vol.%). Conditions: 100 mg VPO catalyst pretreated as described in the text; temperature: 674 K; total gas flowrate: 50 ml min−1 .
the feed. In turn, by controlling the gas composition, we should be able to adjust the surface state of the catalyst. Fig. 1 shows the mass change of the VPO catalyst when the reaction gas was switched from a gas mixture of butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%) in which the catalyst was equilibrated for 2 h, to a gas mixture of butane/oxygen/Ar/He (1.7/21/1.6/balance, vol.%), at 674 K. In the more oxidizing feed, the catalyst gained mass gradually and reached another steady mass value in about 5 h. The total mass change is about 200 g (100 mgcat )−1 , which corresponds to a change of about 0.04 in the average bulk vanadium valence in this vanadyl pyrophosphate sample. Compared to the total oxygen capacity of about 1000 g (100 mgcat )−1 for this catalyst [18], the mass difference of 200 g (100 mgcat )−1 in butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%) from that in butane/oxygen/Ar/He (1.7/21/1.6/balance, vol.%) represents a very modest level of reduction of the catalyst in the more reducing reactant stream. According to the above observation, the following general procedure was followed to establish a reproducible state of the “partially reduced catalyst” for the subsequent measurement of the transient oxidation
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kinetics. First, a VPO sample of 100 mg was heated up in the microbalance reactor to 674 K in a gas mixture of butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%) at 50 ml/min under atmospheric pressure. After 2 h reaction under the above conditions when the mass reading became stable, the catalyst was purged with helium (Keen Gas Grade 5, further purified with an oxygen trap) for 30 min at 674 K or cooled to the desired temperature under a helium purge. To compensate for the buoyancy effect resulting from the difference in the gas densities of the purging helium and the oxidant (a mixture of oxygen in helium), various amounts of nitrogen (Keen Gas Grade 5) were added to the purging helium. The amount of nitrogen added depended on the oxidizing gas composition which ranged from 2.1 to 21 vol.% oxygen in helium. Finally, oxidation was initiated by replacing purging gas with oxygen/He. Oxidation was carried out at several different temperatures between 617 and 674 K. The reaction order with respect to the oxygen partial pressure and the activation energy for catalyst oxidation were calculated from measurements of the catalyst mass change. 2.3. Cyclic redox operation To demonstrate the effectiveness of the transient kinetics in describing the catalyst’s behavior under cyclic redox operation, sequential redox operations were conducted on the same VPO catalyst using the microbalance reactor. In these experiments, the catalyst was first heated up in oxygen (21 vol.% in helium) to 674 K and held for 2 h, then the reaction gas was switched between oxygen (21 vol.% in helium) and butane/Ar/nitrogen/He (1.7/1.6/18/balance, vol.%) at 2 min per step. The catalyst mass was monitored during the cyclic operation. Addition of nitrogen to the reductant was used to eliminate buoyancy effects resulting from the difference in gas densities. The cyclic operations were conducted following several temperature protocols. In one protocol, after the VPO catalyst was stabilized in oxygen (21 vol.% in helium) at 674 K for 2 h, it was cooled to 653 K, and then the redox cycle was carried out at 653 K. In another protocol, after the pretreatment in 21% oxygen and a 1 h redox operation at 674 K, the catalyst was cooled down to 655, 637 and 617 K, stepwise under the cyclic operation.
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3. Results and discussions 3.1. Transient oxidation kinetics When oxygen was fed to partially reduced VPO catalyst, it resulted in a rapid mass gain of the catalyst. To identify the source of this mass gain, we conducted a purging experiment by switching the gas from oxygen/He back to nitrogen/He. The experimental results showed that purging of the oxidized catalyst did not result in any obvious change of the catalyst mass. This result suggests that reversible adsorption of oxygen on the catalyst surface did not contribute to the observed catalyst mass change. Hence, the mass change upon oxidation of partially reduced VPO catalysts results from the replacement of lattice oxygen which was consumed by the previous reduction with butane, i.e. [∗] + 21 O2 → [O]
(1)
where [∗] and [O] represent the reduced and oxidized sites on the catalyst surface, respectively. In fact, as shown previously, the active reduced and oxidized vanadium sites involved in the redox operation are V4+ and V5+ , respectively [19]. The oxidation rate for reaction (1) can be represented by the following power law rate equation: n r = kPm O2 θ
(2)
where r is the oxidation rate which can be measured as the catalyst mass change rate upon oxidation, dm/dt (g (100 mgcat )−1 s−1 ), k the reaction rate constant, PO2 the oxygen partial pressure (Torr) and θ is the lattice oxygen vacancy concentration of the catalyst. The parameters m and n are the reaction orders with respect to gas phase oxygen and catalyst oxygen vacancies, respectively. The kinetic parameters in the above expression can be determined from the results of the microbalance experiments. Fig. 2 shows the catalyst mass change upon oxidation of the VPO catalyst (equilibrated in butane/oxygen/Ar/He (1.7/2.1/1.6/balance, vol.%)) at several temperatures in a gas mixture of 21 vol.% oxygen in He. Noise spikes resulting from flow disturbances upon switching the gas flows have been removed, producing the gaps in the curves in Fig. 2. These curves are all concave down in shape, showing that the mass change rate is highest at the beginning of the oxidation step and decreases as the catalyst
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Fig. 2. Isothermal oxidation of partially reduced VPO catalyst in 21 vol.% oxygen in He. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; temperature as indicated.
is oxidized. This behavior indicates that the reaction order with respect to the oxygen vacancies is positive. At the beginning of each oxidation experiment, the catalyst should be in the same state owing to the consistent procedure for generating a partially reduced catalyst. That is, the initial values of θ are the same for each transient shown in Fig. 2. The initial oxidation rates at each temperature can be measured as the slope of each oxidation curve at the beginning of the oxidation step. The results are summarized in Table 1. Fig. 3 is a plot of ln(dm/dt) (g (100 mgcat )−1 s−1 ) versus 1/T (K−1 ). The solid line is a linear regression of the experimental data, which shows an activation energy of 70 kJ mol−1 .
Table 1 Effect of temperature on the oxidation rate of partially reduced VPO catalystsa Oxidation temperature (K)
Rate of oxidation, dm/dt (g (100 mgcat )−1 s−1 )
617 637 655 674
0.38 0.52 0.75 1.2
a The oxidation rates were measured from the slope of the mass change at the beginning of the oxidation. Conditions: oxidant, 21 vol.% oxygen in helium, 50 ml min−1 , sample, 100 mg VPO catalyst pretreated as described in the text.
Fig. 3. Determination of activation energy for the oxidation of partially reduced VPO catalyst.
Fig. 4 shows the catalyst mass change when oxidation was carried out at 674 K at different oxygen partial pressures from 16 to 125 Torr. The initial oxidation rates are listed in Table 2. As is shown in Fig. 5, there is a linear dependence of ln(dm/dt) (g (100 mgcat )−1 s−1 ) versus ln(PO2 ) (Torr). The empirical value for the reaction order with respect to the oxygen partial pressure thus obtained is 0.55. In summary, the oxidation rate expression can be written as follows: dm = A e8500/T PO0.55 θn 2 dt
(3)
where A is the pre-exponential factor, T the reaction temperature (K), and the other terms are same as defined for Eq. (2). As is described in reaction (1), the ultimate reaction for the oxidation of partially reduced VPO catalyst is the transfer of oxygen from the gas phase to the lattice. This reaction may involve any of a variety of adsorbed forms of oxygen such as molecular oxygen, O2 , atomic oxygen O, and charged oxygen species such as O2 − , O− and O2− , etc. [24]. On the basis of the approximately half-order dependence of the oxidation rate on the oxygen partial pressure observed in our experiments, the oxidation mechanism may be postulated as following: K1
O2 + 2[∗]↔2O∗
(4)
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Fig. 4. Isothermal oxidation of partially reduced VPO catalyst in oxygen/He. Conditions: 100 mg VPO catalyst pretreated as described in the text; temperature: 674 K; total gas flowrate: 50 ml min−1 ; oxygen partial pressure as indicated.
Table 2 Effect of oxygen partial pressure on the oxidation rate of partially reduced VPO catalystsa Oxygen partial pressure (Torr)
Rate of oxidation, dm/dt (g (100 mgcat )−1 s−1 )
16 47 70 125 152
0.37 0.58 0.96 1.17 1.21
a The oxidation rates were measured from the slope of the mass change at the beginning of the oxidation. Conditions: temperature, 674 K, gas flow rate, 50 ml min−1 ; sample, 100 mg VPO catalyst pretreated as described in the text.
slow
O∗ → [O]
(5)
This sequence involves dissociative adsorption of oxygen and the conversion of adsorbed oxygen into lattice oxygen through electron transfer from surface cations, i.e. V4+ , to the oxygen adspecies.
Fig. 5. Determination of reaction order with respect to oxygen partial pressure for the oxidation of partially reduced VPO catalyst. Conditions: 100 mg VPO catalyst pretreated as described in the text; temperature: 674 K; total gas flowrate: 50 ml min−1 ; oxygen partial pressure as indicated.
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In this scheme, an asterisk denotes a reduced surface site, species with an asterisk denote surface species, and [O] represents lattice oxygen. According to previous results [19], the reduced surface sites on the VPO catalysts are V4+ which act as adsorption sites and provide the oxygen vacancy to convert the adsorbed oxygen to lattice oxygen. Filling of oxygen vacancies near V4+ oxidizes the vanadium to V5+ , and decreases the number of adsorption sites. If one assumes that reaction (5) is rate limiting, and reaction (4) proceeds to equilibrium, the resulting rate expression is of the form: 1/2
1/2
r = k2 K1 PO2 θ
(6)
where k2 is the rate constant for reaction (5), K1 the equilibrium constant for reaction (4), and other terms are the same as in Eq. (2). Comparing Eq. (6) with Eq. (3), we see that the oxidation reaction rate has a positive first-order dependence on the oxygen vacancy concentration, i.e. n = 1 for Eq. (3).
3.2. Cyclic redox operation Fig. 6 summarizes the results for cyclic operation of the VPO catalyst at 655 and 674 K. When the oxidized catalyst was reduced in butane, a rapid mass loss was observed with the removal of lattice oxygen. This mass loss is about 420 g (100 mgcat )−1 for the first 2 min reduction at 674 K (see the curve for 674 K in Fig. 6). Reoxidation at this point only resulted in a mass gain of about 110 g (100 mgcat )−1 within 2 min. After that, the amplitude of the redox operation at 674 K stabilized at about 122 g (100 mgcat )−1 for each 2 min reduction half cycle and 107 g (100 mgcat )−1 for each 2 min oxidation half cycle. The smaller mass gain in the oxidation steps compared with the mass loss in the reduction steps resulted in the net mass loss of the catalyst after each redox cycle which is shown in Fig. 6 as a decrease in the catalyst mass with time on stream. This trend is more obvious at 674 K than that at 655 K; at the lower temperature the amplitude of the reduction half cycle is 76 g (100 mgcat )−1 while the amplitude of the oxidation half cycle is about 73 g (100 mgcat )−1 .
Fig. 6. Mass change of VPO catalyst upon cyclic redox operation. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycle time: 2 min per step; temperature as indicated.
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Fig. 7. Mass change of VPO catalyst upon cyclic redox operation. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycling time: 2 min per step; temperature as indicated.
Fig. 7 presents another set of data from the cyclic redox operation obtained following a different temperature protocol. In this experiment, after the redox amplitudes stabilized after operation at 674 K for about 1 h, the reaction temperature was decreased to 655, 637 and 617 K, stepwise. At each temperature step, the apparent mass of the catalyst decreased; these step changes were artifacts from temperature effects on the oscillating behavior of the tapered element of the microbalance reactor [23]. The amplitudes of the mass change of the reduction and oxidation half cycles were measured when stabilized at each temperature. The results are listed in Table 3. Below 550 K, there is no obvious mass change upon the switch of reaction gases, indicating that no redox reaction occurs below that temperature. Figs. 8 and 9 plot the ln(amplitude) (g (100 mgcat )−1 ), versus 1/T (K−1 ) for the oxidation and reduction half cycles (2 min duration each), respectively. Linear regression of these experimental data shows that the dependence of the mass change amplitude on the reaction temperature follows an Arrhenius relation. The apparent activation energies obtained
for the oxidation and reduction half cycles are 68 and 80 kJ mol−1 , respectively. It is the relatively larger activation energy for the reduction step compared with that for the oxidation step that results in the larger increase in the reduction amplitude than the oxidation amplitude at higher temperatures. As a result, a steeper decline of the catalyst mass with time on stream was observed for cyclic redox operation at higher temperatures (Fig. 6).
Table 3 Effect of temperature on the redox amplitudesa Redox temperature (K)
617 636 654 674
Amplitude of mass change (g (100 mgcat )−1 ) Reduction
Oxidation
34 55 76 122
32 55 73 107
a The redox cycles were operated at 2 min per step between oxygen (21 vol.%)/He and butane/Ar/N2 /He (1.7/1.6/18/balance, vol.%).
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85 kJ mol−1 for the oxidation of butane with oxidized VPO [18] and 70 kJ mol−1 for the oxidation of partially reduced VPO catalyst (Fig. 3). This agreement suggests that the kinetics data obtained from transient studies can be applied to the prediction of catalytic performance under cyclic redox operation, and potentially to the design and optimization of the circulating fluidized bed riser reactor process, as discussed below. 3.3. Applications
Fig. 8. Determination of the activation energy for the oxidation half cycles in the redox operation of VPO catalyst. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycle time: 2 min per step.
Furthermore, the apparent activation energies for the reduction and oxidation half cycles obtained from cyclic redox operation are in good agreement with those activation energies obtained from the transient kinetics studies. The corresponding values were
Fig. 9. Determination of the activation energy for the reduction half cycles in the redox operation of VPO catalyst. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); oxidizing gas: oxygen (21 vol.%)/He; cycle time: 2 min per step.
The major operating variables that one can manipulate in the circulating fluidized bed riser reactor scheme include the reactor temperatures, residence times and gas compositions. Separation of the reduction and oxidation steps makes the independent optimization of each step possible in principle. The goal of such optimization is to search for a set of conditions at which butane can be continuously converted to maleic anhydride with the highest yield. The optimization of the riser reactor has been discussed elsewhere [18]. Here, we can give some guidelines in the selection of operating conditions, such as reaction temperature, residence time and oxygen concentration in the fluidized bed catalyst reoxidation reactor. 3.3.1. Reaction temperature Experimental results for cyclic redox operation show that under the conditions examined, the catalyst continuously loses mass with time on stream. This process continues over much longer times than those represented in Figs. 6 and 7. We have observed the depletion of catalyst mass continue for runs as long as 40 h involving 600 complete redox cycles — the catalyst mass never reaches a constant value about which redox cycles of constant amplitude occur for reaction conditions such as those in the experiments of Figs. 6 and 7. Such unsteady operation makes reactor design very difficult, and also in the long-term may result in catalyst deactivation due to the over-reduction of the catalyst [19]. To make the operation stable, one needs to match the reduction rate with a similar oxidation rate by improving the oxidation of the partially reduced catalyst. According to rate equation (3), the most effective way is to increase the oxidation temperature. In general, the optimum temperature for selective conversion of butane to maleic anhydride with VPO
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catalysts lies within the range of 650–700 K [25]. As an example, if the temperature in the butane oxidation (catalyst reduction) step is controlled at 674 K for the catalyst used in this study, the amplitude of the reduction half cycle is about 122 g (100 mgcat )−1 for 2 min reduction. Therefore, the oxidation amplitude needs to be increased from 107 to 122 g (100 mgcat )−1 to compensate the mass loss due to reduction. According to oxidation rate expression in Eq. (3), the oxidation temperature needs to be increased from 674 to 681 K, i.e. by 7 K. 3.3.2. Residence time Alternatively, one could increase the residence time in the reoxidation reactor to increase the amplitude of the oxidation half cycle. According to the first-order dependence of the oxidation rate on surface oxygen vacancies and the rate expression in Eq. (3), the following relation can simulate the dependence of the amplitude of mass change, A (g (100 mgcat )−1 ) in the oxidation half cycle on the duration, t, of this half cycle. A = A0 × (1 − eat )
(7)
Fig. 7 shows that at 674 K, the amplitude of the mass change in the oxidation half cycle is 107 g (100 mgcat )−1 at 2 min per oxidation half cycle. One minute into the oxidation half cycle, the mass gain is about 94 g (100 mgcat )−1 . If one parameterizes Eq. (7) using these two points, one would predict that an oxidation half cycle of about 4 min would be necessary to increase the oxidation amplitude to 122 g (100 mgcat )−1 to compensate for the mass loss in a 2 min reduction half cycle. 3.3.3. Oxygen concentration In Eq. (3), another factor affecting the oxidation rate is the oxygen partial pressure. Conceptually, the oxidation operation can also be improved by increasing the oxygen partial pressure in the regenerator. Following the 0.55 order in the oxygen partial pressure, the effect of oxygen concentration on the relative amplitude of the catalyst mass change in the oxidation half cycle can be calculated and the results are illustrated in Fig. 10. The most obvious effect of increasing oxygen partial pressure is observed at oxygen concentrations below 30 vol.%. Also, by replacing air with pure oxygen as the oxidant, the oxidation rate can be more than
Fig. 10. Simulation results on the dependence of the amplitude in the oxidation half cycles on the oxygen partial pressure. Conditions: 100 mg VPO catalyst pretreated as described in the text; total gas flowrate: 50 ml min−1 ; reducing gas: butane/Ar/nitrogen/He (1.7/1.6/18/balance); cycle time: 2 min per step.
doubled. However, increasing the oxygen concentration above 21% would require an oxygen production facility, which would increase capital and operating costs of the process.
4. Conclusion In conclusion, the transient kinetics for the oxidation of partially reduced VPO catalyst were accurately measured by employing a novel oscillating microbalance reactor. The oxidation rate of partially reduced VPO catalyst has a 0.55 order dependence with respect to oxygen partial pressure and an activation energy of 70 kJ mol−1 . The combination of this result with the kinetics for butane oxidation on oxidized VPO catalyst can effectively predict the catalytic performance during cyclic redox operation. These kinetic results may be useful in the design and operation of fluidized bed circulating riser reactor for the selective oxidation of butane to maleic anhydride. References [1] P. Mars, D.W. van Krevelen, Chem. Eng. Sci. 3 (1954) 41. [2] R.M. Contractor, US Patent 4,668,802 issued 26 May 1987 to E.I. du Pont de Nemours and Company.
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