Transient Response Study of the Ammoxidation of Propene and

Ind. Eng. Chem. Res. 1997, 36, 5207-5219

5207

Transient Response Study of the Ammoxidation of Propene and Propane on an Sb-V-Oxide Catalyst Roland Nilsson and Arne Andersson* Department of Chemical Engineering II, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden

The reaction pathways in the ammoxidation of propene and of propane over an Sb-V-oxide catalyst were studied by analyzing the modes of transient responses of reactants and products resulting from step changes from inert gas to reactant feed. The pathway from propane to acrylonitrile begins with the oxidative dehydrogenation of propane to form propene. After readsorption, the propene is then transformed into adsorbed acrolein, which eventually reacts with an NHx surface species to form acrylonitrile. The adsorption of propane is rate limiting for the propane conversion, but the desorption of water formed from the propene is slow and determines the product distribution. The response modes for the direct oxidation and ammoxidation of propene are consistent with the desorption of water being rate limiting. A comparison of the experimental responses with responses simulated under the assumption that the formation of an allylic intermediate is rate determining reveals an important difference in profiles, indicating this step to not be rate determining. The results show that substantial qualitative information concerning the reaction pathways can be obtained from the analysis of transient profile modes. Introduction Much attention today is focused on the development of catalysts for the production of acrylonitrile by the ammoxidation of propane as an alternative to producing it by the ammoxidation of propene (Roth, 1991). According to the respective patents (Guttmann et al., 1988a,b; Toft et al., 1988, 1989; Brazdil et al., 1989), one of the most auspicious catalyst systems for propane ammoxidation is the Sb-V-(W, Mo)-Al-O system. The catalytic properties of the Sb-V-O subsystem for the ammoxidation both of propane (Centi et al., 1990; Nilsson et al., 1994a,b, 1997) and of propene (Nilsson et al., 1994c) have been investigated. Agreement exists that propene is an intermediate from propane to acrylonitrile and an excess of antimony (Sb:V > 1) in the catalyst is necessary for high selectivity toward acrylonitrile to be achieved. Centi and Perathoner (1997) using infrared spectroscopy suggested that an allyl alcoholate species is an intermediate from propene to acrylonitrile on a catalyst with an Sb:V ratio being equal to 1, corresponding to the formation of SbVO4. However, kinetic data (Nilsson et al., 1994c) and TAP (temporal analysis of products) results (Buchholz and Zanthoff, 1996) reveal that rather acrolein is an intermediate to acrylonitrile on a catalyst with antimony in excess of the stoichiometric amount required for forming SbVO4. The formation of the propene intermediate from propane was reported (Nilsson et al., 1994b) to show a first-order rate dependence on the partial pressure of propane, indicating that the adsorption of propane being rate limiting. Centi et al. (1990) found acrylonitrile to be formed in two parallel pathways, one directly from propane and the other through the propene intermediate, the latter pathway dominating over a V-Sb-WAl-O catalyst. Catani et al. (1992) described the kinetics of this type of catalyst using a LangmuirHinshelwood approach, reporting the formation of the * Author to whom correspondence should be addressed. E-mail: [email protected]. Fax: (+46)-46149156. S0888-5885(97)00226-1 CCC: $14.00

propene intermediate to be rate limiting for the route to acrylonitrile. In this paper we report on an investigation of the ammoxidation of propane and of propene over an SbV-O catalyst using the transient response method. This method has been reviewed by Kobayashi and Kobayashi (1974) and by Bennett (1976). Their analyses showed that performing even some few transient experiments enables one to obtain considerable information on the reaction mechanism. In pioneer work on different model reactions, Kobayashi (1982) simulated the responses to step changes using a differential reactor. Kobayashi specified only the responses of the products, however, and not of the reactants. Nilsson and Andersson (1996), using a tanks-in-series model, simulated the responses of both the reactants and the products, studying hydrocarbon oxidation over metal oxides. Such a reaction is unique in the sense that one of the reactants, oxygen, is present at the surface initially. The reaction schemes for the oxidation and ammoxidation of hydrocarbons are usually very complex (Sanati and Andersson, 1991a,b). The fact that many different reaction paths are involved can make the interpretation of experimental results far from straightforward. In a study of propane ammoxidation over Sb-V-oxide using a step transient, Centi and Perathoner (1995) discussed their results only in terms of catalyst reconstruction without considering transient theory. The present investigation concerns the reaction pathways in the ammoxidation of propene and propane, the responses to step changes from inert gas to reactant feed being examined. The experimental results are explained in the light of model calculations. Due to both the complexity of the reaction system and to experimental limitations, it is impossible to quantitatively analyze the transients for all the byproducts. The goal, therefore, is not to estimate the kinetic parameters but rather to disclose the reaction pathway and the ratelimiting step, as well as to demonstrate the usefulness of a transient study for identifying a possible reaction mechanism. © 1997 American Chemical Society

5208 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

Experimental and Calculations Catalyst Preparation. The catalyst was prepared by adding Sb2O3 (Merck, p.a.) to a warm solution of NH4VO3 (Merck, p.a.) in water, which was then heated under reflux with stirring for 16-18 h. The bulk of the water from the resulting slurry was evaporated on a hot plate under agitation until a thick paste was formed. The paste was then dried at 110 °C for 16 h and subsequently heated at 350 °C for 5 h, after which the material was sieved, the fraction of particles in the range 150-425 µm being calcined finally at 610 °C for 3 h in a flow of air. The nominal ratio of Sb:V was 3:1. The BET surface area was 4.3 m2/g. Transient Response and Stability Experiments. Use was made of a quartz reactor with an inner diameter of 4 mm, placed in a furnace (TRANS TEMP) consisting of a heating coil and an outer quartz tube with a gold mirror to ensure a uniform temperature profile, 200 mg of the catalyst being employed. The total flow rate was 50 mL/min (STP). It was verified under stationary conditions that the influence from mass- and heat-transfer gradients were negligible. The following gases from AGA Gas were used with the composition and purity expressed in vol %: argon (99.9997%), 1.00% propane (99.5%) in argon (99.999%), 1.00% propene (99.8%) in argon (99.999%), 0.99% oxygen (99.95%) in argon (99.999%), 0.99% ammonia (99.96%) in argon (99.999%), 0.100% acrylonitrile in argon (99.999%), and 0.100% acrolein in argon (99.999%). A Pegasus quadrupole mass spectrometer from VG, rebuilt for low inlet flow, was used to analyze the transient responses. The inlet system and the tubing from the reactor to the spectrometer were heated to avoid adsorption on the walls. The m/e ratios employed, the components making a significant contribution to the signal being placed in parenthesis, were as follows: 17 (ammonia and water), 18 (water and argon), 28 (propane, carbon monoxide, carbon dioxide, and nitrogen), 29 (propane), 32 (oxygen), 38 (argon), 42 (propene and propane), 44 (propane, carbon dioxide, and nitrous oxide), 53 (acrylonitrile), and 56 (acrolein). It was not possible to separate the mass signals either from nitrogen and carbon monoxide (m/e ) 28) or from nitrous oxide and carbon dioxide (m/e ) 44). Under the reaction conditions that were used, the contribution from nitrous oxide to the total intensity of the m/e signal at 28 was found to be negligible. Likewise, acrolein contributed little to the measured intensities of the m/e signals at 28, 29, and 53. Concerning other possible products, the formation of acetonitrile was not followed, since propane and propene as well as the tail from the argon peak at m/e ) 40 contribute to the m/e ) 41 signal. Hydrogen cyanide could not be measured due to contributions from propane and propene to the m/e ) 27 signal. Moreover, no formation of nitrogen monoxide (m/e ) 30) was observed in the experiments. Prior to each experiment, the reactor was loaded with freshly prepared catalyst. This was necessary for correct interpretation of the results through their being compared with model calculations, which requires that the initial concentration of the intermediates be known. For a fresh catalyst, the concentrations of all intermediates except oxygen are zero. If a catalyst has already been used, the distribution of intermediates remaining on the surface is unknown and cannot be readily determined. The transient response studies reported here for propane ammoxidation and ammonia oxidation were

performed at 480 °C and those for propene oxidation and ammoxidation at 460 °C. However, oxidation and ammoxidation of propene at 480 °C yielded the same response features as those obtained at 460 °C, but with a higher conversion. The reaction temperatures chosen are below the onset temperature 500 °C, where lattice oxygen starts to desorb from this type of material (Nilsson et al., 1994b). This fact was also verified by following the oxygen signal (m/e ) 32) during the heating up of the catalyst in argon to the reaction temperature. Model Calculations. In order to reduce the simulation time and to simplify the discussion and focus on the partial oxidation mechanism, only propane, propene, oxygen, ammonia, acrolein, acrylonitrile, and water are taken into account in the model calculations. This limitation is also a consequence of the experimental fact that the transients of the degradation products N2 and CO, which are formed in the ammoxidation from ammonia and propane/propene, respectively, cannot be followed separately, since both the products give a signal at m/e ) 28. Likewise, both N2O and CO2 contribute to the m/e ) 44 signal. From recent simulations (Nilsson and Andersson, 1996), we knew that in using a tanksin-series model for moderate conversions the features of the responses would be the same in all the tanks. To reduce simulation time in the present case, we simulated the responses using a continuously stirred tank reactor (CSTR) model. For conversions up to about 50%, comparing calculations with the CSTR and tanks-inseries models, we verified for typical cases that the shape of the response profile is independent of the conversion level. The system can be described by the following system of ordinary differential equations, which assume a constant gas velocity through the continuously stirred tank reactor, i.e., low conversion, the inert component content being high, and/or no molar changes being caused by the reaction (for notations see Table 1):

u0 pi - pInlet i

dpi )dt

RTFB m +

x

dθk

1 )

dt

rij ∑ j)1

(1)

m

∑rkj

nk j)1

(2)

When stationary conditions are reached

f)0

(3)

where f is a vector consisting of the right-hand sides of eqs 1 and 2. In order to investigate how different rate constants influence the stationary solution, i.e., which step or steps are rate determining, we calculated the partial derivative of each p and θ with respect to each rate constant (k), using the values of p and θ that satisfy eq 3:

∂f -1 ∂f ∂y )∂k ∂y ∂k

( )

(4)

where y is the vector consisting of p:s and θ:s. Some parameter values related to the catalyst are given in Table 1. These values were selected to be of reasonable magnitude for partial oxidation catalysts. For instance, the values on the number of active sites were chosen considering the specific surface area of the catalyst together with typical values for the cation (n1)

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5209 Table 1. Parameter Values Used in the Simulations nk, number of sites available for surface species k n1, number of sites of type 1, denoted * and *′ n2, number of sites of type 2, denoted b and b′ pi, partial pressure of species i R, gas constant x, reactor length rij, rate of formation of gaseous species i in reaction j rkj, rate of formation of surface species k in reaction j T, temperature t, time total pressure u0, superficial gas velocity , void fraction of catalyst bed θk, fractional coverage with surface species k FB, bulk density of catalyst bed

5.16 × 10-5 1.32 × 10-4 82.057 0.5 753.15 1 1592 0.45 1.538

mol/g mol/g mol/g atm atm‚cm3/mol/K cm mol/g/min mol/g/min K min atm cm/min g/cm3

and oxygen (n2) concentration on metal oxide surfaces (Andersson, 1985). The exact parameter values, however, are not limiting for the present purpose to focus not on the determination of kinetic constants but on the correlation between response features and mechanistic details. Results and Discussion Stability of Acrylonitrile and Acrolein. The resistance of acrylonitrile to oxidation and of acrolein to both oxidation and ammoxidation was investigated by heating the catalyst linearly in a flow of reactants. The results are shown in Figure 1. The response of acrylonitrile decreases only slightly with an increase in temperature and is largely independent of the presence of ammonia (Figure 1a). The difference in intensity (≈10%) of the two curves is within the normal experimental variation. The curves, however, show that acrylonitrile is a stable product that almost does not oxidize to carbon oxides under the present reaction conditions. The acrolein signal decreases strongly during heating from 200 to 500 °C in an atmosphere containing oxygen (Figure 1b). The decrease is due to the degradation of acrolein to form carbon oxides (not shown in the figure). When ammonia is also present, the decrease in the acrolein signal is greater, most of the acrolein being converted then to acrylonitrile. This observation agrees with previous results (Nilsson et al., 1994c; Buchholz and Zanthoff, 1996) showing acrolein to be an intermediate in the conversion of propene to acrylonitrile over Sb-V-oxide. Moreover, the comparison of propene oxidation with propene ammoxidation showed the yield of acrolein through oxidation (20%) to be substantially less than the yield of acrylonitrile (55%) through ammoxidation (Nilsson et al., 1994c). The difference was found to be due to the easier degradation of the aldehyde, as supported by the results shown in Figure 1, comparing the stability of acrolein and of acrylonitrile during heating under oxidizing conditions. Kinetic data on propane ammoxidation over the SbV-O catalyst (Nilsson et al., 1994b) have shown that the rates of formation of carbon oxides decrease strongly with an increase in the partial pressure of ammonia, whereas the rate of acrylonitrile formation increases. Such dependence can be understood if one considers that the nitrile is a more stable product than the aldehyde. Oxidation of Propene. In Figure 2 the experimental responses of propene and oxygen are given for a step change through an empty reactor from argon to argon containing propene and oxygen. The responses of both

Figure 1. (a) The signal from acrylonitrile at the outlet of a reactor containing 200 mg of fresh Sb-V-O catalyst, heated linearly (10 °C/min) from 200 to 500 °C in a flow of either 700 ppm of acrylonitrile and 2000 ppm of oxygen in argon (solid line), or 700 ppm of acrylonitrile, 2000 ppm of oxygen, and 1000 ppm of ammonia in argon (dotted line). (b) The signal from acrolein (solid line), using the same experimental procedure but with a flow of 700 ppm of acrolein and 2000 ppm of oxygen in argon; and the signals from acrolein (dotted line) and acrylonitrile (bold solid line) in a corresponding experiment with an inlet flow of 700 ppm of acrolein, 2000 ppm of oxygen, and 1000 ppm of ammonia in argon.

propene and oxygen are very steep, reaching their final values within 1 min. No products were detected. The experimental responses of propene, oxygen, acrolein, carbon monoxide, carbon dioxide, and water obtained from a reactor filled with fresh catalyst are shown in Figure 3. Propene shows an extended increase, whereas the response of oxygen initially reaches a maximum, this being followed by a minimum and by an extended increase thereafter. The responses of acrolein, carbon monoxide, and carbon dioxide in reaching a maximum are instantaneous, this being followed


Figure 2. Experimental responses of propene and oxygen after passing through a reactor without catalyst when the inlet flow is switched from argon to argon containing 5000 ppm of propene and 5000 ppm of oxygen. Temperature: 460 °C. Sampling speed: 28 analyses per minute.

by a slow decrease. The response of water, following a delay, increases up to a maximum and decreases slowly thereafter. The results shown in Figure 3 can be explained in light of previous work (Nilsson and Andersson, 1996) in which we examined the simulated responses of different mechanisms for propene oxidation. Comparison with that work shows that the drawn out response of propene observable in Figure 3a is consistent with the adsorption onto the surface of a considerable amount of propene, i.e., with there being at least one reaction pathway in which the adsorption of propene is a rapid process. If the adsorption of propene were rate determining, the response of propene should be instantaneous, due in that case, to only a small amount of propene being adsorbed at the surface. The instantaneous rise in the oxygen response (Figure 3a) indicates that no oxygen, or only a small amount, is initially adsorbed. This behavior is reasonable in view of lattice oxygen being active in oxidation pathways on metal oxides (Grasselli and Burrington, 1981). The fact that the initial response does not reach a higher value, i.e., the inlet value, may possibly be due to the sampling speed not being fast enough. The minimum in the oxygen response implies that the adsorption and dissociation of oxygen is not rate limiting and that, prior to the rate-limiting step, there are one or more fast steps in which oxygen is consumed. Following these steps, sites suitable for oxygen adsorption are released, the adsorption of oxygen then being rapid. The instant maxima in the responses of acrolein (Figure 3b) and carbon oxides (Figure 3c), and the minimum in the oxygen response together with the fact that the response of water is drawn out in time, verify the desorption of water being a slow step. Comparison of the results in Figure 3 with our previous simulations of model mechanisms (Nilsson and Andersson, 1996) shows the experimental response features to be compatible with a model of propene oxidation comprising steps 1-3 and 5-9 in Scheme 1 and the desorption of water in step 9 being rate limiting. The mechanism comprises two sites, oxygen sites (•) and adsorption sites (*). Evidence has been presented for both being associated with vanadium centers (Nilsson et al., 1996). The mechanism in Scheme 1 for oxidation of propene to acrolein is in general agreement with the well-known mechanism presented by Grasselli and coworkers for acrolein formation over Fe-Sb-oxides and

Figure 3. Experimental responses of (a) propene and oxygen, (b) acrolein, and (c) carbon monoxide, carbon dioxide, and water, after a step change from argon to argon containing 5000 ppm of propene and 5000 ppm of oxygen. Temperature: 460 °C. Amount of catalyst: 200 mg. Sampling speed: 28 analyses per minute.

Bi-Mo-oxide catalysts (Grasselli and Burrington, 1981; Burrington et al., 1984). For the latter two catalyst systems, the abstraction of a hydrogen atom from the methyl group of propene is rate limiting. In the present case, however, neither the first hydrogen abstraction from propene (step 5 in Scheme 1) nor the insertion of oxygen (step 6 in Scheme 1) is rate limiting, since for both of these the response of acrolein should increase monotonously up to a constant value, not passing through a maximum as it does in Figure 3b. Comparison with results of the previous simulations allows us to also safely exclude both the abstraction of the second hydrogen atom (step 7 in Scheme 1) and the desorption of acrolein (step 8 in Scheme 1) as being rate determining. However, the present results are inconclusive as

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5211 Scheme 1. Mechanism for the Oxidation and Ammoxidation of Propene

to whether the abstraction of the second hydrogen atom occurs before (Adams and Jennings, 1964) or after the insertion of the oxygen atom. In Scheme 1 we subscribe to the latter view, for which there is substantial support (Grasselli and Burrington, 1981; Burrington et al., 1984). Oxidation of Ammonia. Figure 4 shows the responses of oxygen, ammonia, nitrogen, nitrous oxide, and water to a step change from argon to argon containing oxygen and ammonia. The response of oxygen (Figure 4a) reaches a maximum initially followed by an extended decrease to a nearly constant value. No ammonia appears during the first few minutes, the response gradually increasing thereafter up to a level corresponding to approximately 40% conversion. The responses of nitrogen and nitrous oxide (Figure 4b) show a somewhat extended increase before leveling out. No water was detected during the first few minutes, the response then gradually increasing. From the shape of the response of ammonia it is reasonable to conclude that the adsorption is a fast step (Nilsson and Andersson, 1996). Since the oxygen response displayed no minimum and none of the products showed a response having a clear maximum, it is reasonable to conclude that no product desorbs ahead of the ratedetermining step, which thus constitutes a surface reaction (Nilsson and Andersson, 1996). In ammonia oxidation, therefore, as opposed to propene oxidation, the desorption of water is not rate limiting. The major product of ammonia is N2 (Figure 4b). A series of dehydrogenation steps can be regarded as one mechanism for the formation of it. In TAP (temporal analysis of products) studies of propene and propane ammoxidation over an Sb-V-O catalyst, it has been concluded that short-lived NHx species are active in the formation of acrylonitrile (Buchholz and Zanthoff, 1996;

Figure 4. Experimental responses of (a) ammonia and oxygen and (b) nitrogen, nitrous oxide, and water, after a step change from argon to argon containing 2500 ppm of ammonia and 5000 ppm of oxygen. Temperature: 480 °C. Amount of catalyst: 200 mg. Sampling speed: 6 analyses per minute.

Zanthoff et al., 1996). In stopping the continuous flow of ammonia passing over the catalyst, these authors observed that the decline in acrylonitrile formation was almost identical to the decline in N2 formation (Zanthoff et al., 1996). The authors concluded that the same type of NHx species is involved in both cases. Since they obtained no evidence from DRIFT of any -NH2 or dNH species being at the surface they suggested that the NHx species is either NH3(ads) or NH4+(ads). A similar observation has been made using IR spectroscopy (Centi et al., 1996b). The fact that they were unable to detect either of the former two species may be due to dehydrogenation of the adsorbed ammonia being slow, resulting in low concentration of the intermediate species if their further transformation into N2 involves only fast steps. Since our results in Figure 4 are consistent with the dehydrogenation step being slow, we have indicated in Scheme 1 (steps 4 and 10) the formation of the dNH imido species. The assumption of such a species being formed in connection with the ammoxidation mechanism on Bi-Mo-O and Fe-Sb-O catalysts has been made previously (Burrington et al., 1984). Ammoxidation of Propene. Figure 5 shows the experimental responses produced by a step change from argon to argon containing propene, ammonia, and oxygen. In regard to the responses of the reactants as shown in Figure 5a, one can note that the oxygen response rises to a maximum instantaneously and then decreases to a minimum, after which it gradually increases to a nearly constant value. The response of


Figure 5. Experimental responses of (a) propene, ammonia, and oxygen, (b) acrylonitrile and acrolein, and (c) carbon monoxide + nitrogen, carbon dioxide + nitrous oxide, and water, after a step change from argon to argon containing 2500 ppm of propene, 2500 ppm of ammonia, and 5000 ppm of oxygen. Temperature: 460 °C. Amount of catalyst: 200 mg. Sampling speed: 6 analyses per minute.

propene increases monotonously before it levels out, and the response of ammonia is similar, although the initial delay due to adsorption there is prolonged. Of the product responses, those of acrylonitrile (Figure 5b), CO + N2 and CO2 + N2O (Figure 5c), show an overshoot. Although the response of acrolein (Figure 5b) displays similar features, the maximum is not as sharp in appearance. Almost no water desorbs initially there, but after a few minutes the response of water begins to increase to reach a flat maximum. On the basis of the results described in Figure 1b and the previous sections on the oxidation of propene and of ammonia, Scheme 1 can now be completed to include

a conceivable mechanism for propene ammoxidation over the Sb-V-O catalyst. This is achieved by including two final steps, the adsorbed acrolein reacting in step 12 with the formed imido species to give adsorbed acrylonitrile, this being eventually desorbed then in step 13. That acrolein is an intermediate from propene to acrylonitrile on Sb-V-oxides with excess antimony is confirmed by previous investigations (Nilsson et al., 1994c; Buchholtz and Zanthoff, 1996). To be able to interpret the experimental results shown in Figure 5, where our previous work (Nilsson and Andersson, 1996) was limited to the treatment of propene oxidation, we simulated for the ammoxidation mechanism in Scheme 1 the responses of the reactants and products appearing in response to a step change from inert gas to inert gas containing propene, ammonia, and oxygen. In simulating the responses, the oxygen sites were assumed to be fully covered initially and one of the steps in Scheme 1 to be rate limiting. Regarding the response behavior, the best qualitative agreement between the experimental (Figure 5) and the simulated response features was obtained for the desorption of water in step 9 being rate limiting. The simulated responses there are shown in Figure 6 for the three sets of rate constants given in Table 2 and in the figure legend. Figure 7 shows, for these same sets of rate constants under stationary conditions, partial derivatives of the partial pressures of the gaseous species with respect to each of the rate constants. Since for the first two sets of rate constants, viz. k25 ) 1 × 10-3 (Figure 7a) and k25 ) 1 × 10-4 (Figure 7b), the partial pressure of components is mainly affected by the value of k17, the desorption of water in step 9 (Scheme 1) is rate determining both for the consumption of each of the reactants and for the formation of each of the products. When k25 ) 1 × 10-5 (Figure 7c), there is also a clear influence of the k25 value on the partial derivatives, both the desorption of water (step 9) and the desorption of acrylonitrile (step 13) thus affecting the reaction rates. The theoretical responses of propene and ammonia displayed in Figure 6a,c,e show a gradual increase until the catalyst surface either is almost fully covered or is covered by an amount corresponding to the adsorption equilibrium, the latter being the case here. These results agree with the adsorption of propene and of ammonia both being fast processes (Nilsson and Andersson, 1996). For all three sets of rate constants, the response of oxygen is instantaneous (Figure 6a,c,e) since the surface is fully covered with oxygen initially. For the first two cases, Figure 6a,c, the oxygen responses have clear minima. This is due to the fast surface reactions (Scheme 1) which consume much oxygen during the first few minutes and then have to slow down as the surface becomes covered with water (H2O‚). The rates of formation of acrolein and acrylonitrile are fast during the first minutes for the same reason, yielding maxima for the responses of these two products (Figure 6b,d). The response of water also shows a maximum since water is formed in the tenth step, which is fast as long as surface oxygen is available, although it slows down when the number of oxygen species decreases because of competition between oxygen and water for the same site (•). The main difference between the first two cases, those in Figures 6a,b and 6c,d, respectively, is the height of the initial acrylonitrile response and the shift of the peak maximum toward the right as the value of k25 decreases. When the k25 value becomes less, the amount

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5213

Figure 6. Simulated responses for propene, oxygen, ammonia, acrolein, acrylonitrile, and water resulting from a step change from inert gas to inert gas containing propene, oxygen, and ammonia when (a,b) k25 ) 1 × 10-3; (c,d) k25 ) 1 × 10-4; and (e,f) k25 ) 1 × 10-5 (for notations, see Scheme 1). The values of the other rate constants are given in Table 2. The oxygen sites are initially fully covered. Table 2. Values of the Rate Constants Used To Calculate the Responses in Figures 6 and 10 for Propene and Propane Ammoxidation, Respectively rate constanta

value of rate constantb

k1, k7 k2, k8, k23 k3, k5, k9, k11, k13, k16, k19, k20 k4, k6, k10, k12, k14, k18, k22, k24 k26 k15 k17 k21

10 1 0.1 1 × 10-9 1 × 10-4 1 × 10-5 4 × 10-5

a Notations as in Scheme 1. b The units of the rate constants are in accordance with the units found in Table 1.

of adsorbed acrylonitrile increases. The duration of the period of buildup concurrently increases at the same time, leading to an extended period of increase in the

pressure of acrylonitrile, this explaining the shift in the peak maximum. However, although when the value of k25 is decreased the coverage with acrylonitrile increases, the desorption rate decreases, causing the peak height to decrease as well. The increase in the coverage with acrylonitrile leads to the acrolein coverage being lower during the first few minutes, causing the acrolein response to shift to the left and decrease in height. It is important to note, however, that the stationary solution only differs by a few percent between the two cases referred to above, since the rate-limiting step is unaffected. From this, it is apparent that useful qualitative information on the kinetics of the reaction can be obtained by studying the shapes of the responses. In the third theoretical case, both the shape of the acrylonitrile response (Figure 6f) and thus to a certain


Figure 7. The partial derivative of the partial pressure of propene, oxygen, ammonia, acrolein, acrylonitrile, and water with respect to each rate constant under stationary conditions and for the same sets of rate constants as considered in Figure 6. (a) k25 ) 1 × 10-3, (b) k25 ) 1 × 10-4, and (c) k25 ) 1 × 10-5.

extent the oxygen response as well (Figure 6e) differ very much from those in the first two cases (Figure 6ad). Although the acrylonitrile response still displays a maximum, it is very small and is shifted to the right, outside the range of the time scale in Figure 6f. The minimum for the oxygen response has practically disappeared, since almost no sites suitable for oxygen adsorption are released, their sites being occupied either by water or by acrylonitrile. Figure 7c suggests the system to have reached a transition region in which there are two steps, namely desorption of water and desorption of acrylonitrile, that each has a crucial effect on the reaction rates. Comparing in Figures 5 and 6 the shape of the experimental and the simulated responses, respectively, it is thus reasonable to conclude that the desorption of water is a slow process. The experimental response of

water, compared with the simulated responses, shows a longer initial delay and a broader maximum. This difference may be due to the selection of rate constants and/or water being readsorbed at another type of site, which is not participating in the ammoxidation mechanism. Since there is evidence that the formation of an allylic intermediate is rate determining for propene ammoxidation over other catalyst systems (Grasselli and Burrington, 1981), we simulated the responses resulting from a step change from an inert gas to an inert gas containing propene, oxygen, and ammonia when the corresponding step shown in Scheme 1 (step 5) is rate determining. The result is shown in Figure 8a,b. The values used for the rate constants are given in Table 3. The partial derivatives of the partial pressures of reactants and products with respect to each of the rate


Figure 8. Simulated responses of (a) propene, oxygen, and ammonia, (b) acrolein, acrylonitrile, and water, and (c) the partial derivatives of the partial pressure of propene, oxygen, ammonia, acrolein, acrylonitrile, and water, with respect to each rate constant under stationary conditions when the formation of an allylic intermediate is rate determining (step 5 in Scheme 1). The values of the rate constants are given in Table 3. The oxygen sites are initially fully covered. Table 3. Values of the Rate Constants Used To Calculate the Responses in Figure 8, Where the Formation of an Allylic Intermediate Is the Rate-Determining Step in Propene Ammoxidation rate constanta


k1, k7 k2 k3, k5, k8, k11, k13, k16, k17, k25 k4, k6, k10, k12, k14, k18, k22, k24 k26 k9, k21 k15 k19, k20 k23

10 0.02 0.1 1 × 10-9 5 × 10-5 0.05 0.01 1


constants under stationary conditions are shown in Figure 8c. The response features of oxygen and acrylonitrile observable in Figure 8a,b are quite different from the experimental responses shown in Figure 5, which clearly indicates that the formation of the allylic intermediate is not rate determining. Ammoxidation of Propane. The experimental responses of reactants and products resulting from a step change from argon alone to argon containing propane, oxygen, and ammonia are given in Figure 9. The response features of ammonia, acrylonitrile, acrolein, water, CO + N2, and CO2 + N2O are similar to those with use of propene instead of propane shown in Figure 5. The response of propane indicated in Figure 9a is almost instantaneous, however (cf. Figure 2), readily suggesting that a small amount of propane adsorbs on the surface, i.e., that the adsorption of

propane is rate determining. However, if such were the case, the responses of the products would be expected to be steep before leveling out (Kobayashi, 1982; Nilsson and Andersson, 1996), which obviously does not hold as can be seen in Figure 9b,c. In interpreting the experimental responses, the mechanism for propene ammoxidation found in Scheme 1 can be complemented by the mechanism shown in Scheme 2, indicating the oxidative dehydrogenation of propane to form gaseous propene. Such a mechanism for propane ammoxidation seems reasonable in view of propene being an intermediate in the formation of acrylonitrile from propane (Centi et al., 1990; Nilsson et al., 1994a,b). Figure 10a,b shows the simulated responses to a step change from an inert gas alone to an inert gas containing propane, oxygen, and ammonia, as based on the same rate constants as those used to calculate the theoretical responses shown in Figure 6c,d together with the rate constants contained in Table 4. The partial derivatives of the partial pressures of reactants and products with respect to each of the rate constants are shown in Figure 10c,d. The theoretical response of propane indicated in Figure 10a is instantaneous before leveling out. As can be clearly seen in Figure 10d, the adsorption of propane (k27) is rate determining for the consumption of propane, and thus for the rate of propene formation from propane as well if one does not take specific account of the consecutive readsorption and transformation of propene into acrolein and acrylonitrile. However, the desorption of water (k17) has a dominant influence on the consumption of oxygen and ammonia, as well as on the formation

5216 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Scheme 2. Mechanism for the Oxidative Dehydrogenation of Propane to Propene

Table 4. Values of the Rate Constants Used To Calculate the Responses for Propane Ammoxidation in Figure 10 rate constanta


k27 k28, k30, k32, k34, k36, k38, k40 k29, k31, k33, k35, k37, k39

4 × 10-3 1 × 10-9 0.1


Figure 9. Experimental responses of (a) propane, ammonia, and oxygen, (b) propene, acrylonitrile, and acrolein, and (c) carbon monoxide + nitrogen, carbon dioxide + nitrous oxide, and water, after a step change from argon to argon containing 2500 ppm of propane, 2500 ppm of ammonia, and 5000 ppm of oxygen. Temperature: 480 °C. Amount of catalyst: 200 mg. Sampling speed: 6 analyses per minute.

of propene (in terms of measured partial pressure), acrylonitrile, and water. These results thus explain the features of the experimental responses shown in Figure 9. The apparent contradiction that the adsorption of propane is rate determining for the consumption of propane but that the partial pressure of the primary product propene is mainly affected by the rate constant (k17) for water desorption can easily be understood in terms of acrylonitrile being consecutively formed from readsorbed propene. Since the desorption of water is rate determining for the formation of the nitrile, this step should also have a decisive influence on the partial pressure of propene. It is worthy to note that ∂ppropene/ ∂k17 and ∂pacrylonitrile/∂k17 differ in their sign, an increase

in the value of k17 favoring the formation of acrylonitrile and disfavoring the formation of propene and vice versa. In our previous kinetic investigation under stationary conditions, the reaction rates in propane ammoxidation were found to be almost independent of water being added to the reactant feed (Nilsson et al., 1994b). This observation can be explained, considering the present results, by the water formed in the reaction being strongly adsorbed at the surface and, therefore, an additional supply of water has little effect. The experimental response of oxygen shown in Figure 9a has the same features as the simulated one indicated in Figure 10a, namely an instantaneous maximum followed by a small decrease. Since the consumption of propane is determined by its low adsorption rate, the initial consumption of oxygen is lower than when propene is used as feedstock (cf. Figures 5 and 6). Although one might suppose that the formation of acrylonitrile from the intermediate propene would consume an appreciable amount of oxygen, resulting in a minimum in the oxygen response, this is obviously not the case since the amount of oxygen initially needed is determined by the amount of adsorbed propane, which in fact is small. The fact that the desorption of water in step 9 is rate determining for the consumption of oxygen is due to three oxygen atoms, in addition to the one needed to form propene being required to form acrylonitrile. The initial delay in the responses of ammonia and water is of shorter duration, and the overshoot in the response of acrylonitrile is broader, in the simulated profile (Figure 10) than in the experimental profile (Figure 9). Although these differences may be due to the selection of kinetic constants, they are not critical for the qualitative conclusions regarding the mechanism. As Kobayashi (1982) has pointed out, it is the mode of the response curve that is conclusive for the type of mechanism involved and not the time it takes to pass the transient state. In a recent study using deuterated propanes (Brazdil et al., 1996), a kinetic isotope effect for propane ammoxidation over a promoted Sb-V-O catalyst was


Figure 10. Simulated responses of (a) propane, oxygen, ammonia, and water and (b) propene, acrolein, and acrylonitrile, the partial derivatives of the partial pressures of (c) oxygen, ammonia, propene, acrylonitrile, and water, and (d) propane and acrolein with respect to each rate constant under stationary conditions. The values of the rate constants are given in Tables 2 and 4, and k25 ) 1 × 10-4. The oxygen sites are initially fully covered.

observed. The authors limited their rate analysis to the consumption of propane, since a rate analysis of the products was impossible due to the scrambling of hydrogen and deuterium in the formed acrylonitrile. The isotope effect suggested the rate-determining step for propane consumption to involve the abstraction of hydrogen from propane. This does not contradict our results since steps 14 and 17 in Scheme 2 can be combined. Support for the first hydrogen abstraction occurring in concert with propane adsorption is found in a kinetic study of the oxydehydrogenation of propane over Mg-V-Sb-oxide in which the heat of adsorption of propane was found to be low (Michaels et al., 1996).

In the model calculations that has been presented here, the change in the surface composition during the transient state regarding oxygen coverage has been accounted for. However, responses in general may also be affected by more severe reconstruction phenomena, e.g., transformation between phases with differing activity. Such processes are not easily accounted for in model calculations. For the catalyst system that here was used, however, we have not observed such a transformation to occur (Nilsson et al., 1997). Besides the initial transient behavior, we observed only a modest change with time of the catalytic performance. Moreover, the transient experiments were carried out using


low concentration in the feed of hydrocarbon and ammonia. Conclusions Analyses of the transient responses that result from step changes from inert gas to reactant feed were performed to obtain insight into the reaction pathways in both propene and propane ammoxidation over an SbV-oxide catalyst. The results provide the following information concerning the mechanisms: (i) If there is no ammonia in the gas phase, propene is oxidized to form acrolein and carbon oxides, the desorption of water being rate determining. Acrolein is not a stable product on the Sb-V-oxide but reacts consecutively to form carbon oxides. When ammonia is present, the adsorbed acrolein reacts with an NHx species, possibly an dNH imido species, to form acrylonitrile. Acrylonitrile is a more stable product than acrolein. This explains the observation that the selectivity to partial oxidation products is higher in ammoxidation than in oxidation (Nilsson et al., 1994c). (ii) The ammoxidation of propane to acrylonitrile occurs at an appropriate site through oxidative dehydrogenation to form propene. After readsorption at a second site, the propene reacts to give adsorbed acrolein, which after the insertion of a nitrogen species is transformed into acrylonitrile. The adsorption of propane is rate limiting for the consumption of propane during propane ammoxidation, although the desorption of water determines the propene:acrylonitrile product ratio. Our study shows that the analysis of transient responses provides useful qualitative information regarding the reaction pathways. No information is obtained here, however, regarding the detailed structure and bonding of the adsorbed species. For that purpose, spectroscopic techniques may be more useful. Centi and co-workers (Centi et al., 1996a,b; Centi and Perathoner, 1997) reported infrared spectroscopic studies recently in which they analyzed the reaction intermediates formed from propane and propene on an Sb-V-oxide, finding an allyl alcoholate adspecies to be an intermediate to acrolein. Their results also confirm the main route from propane to acrylonitrile being by way of propene. However, as we have pointed out, it is important to be aware that infrared data on the reaction intermediates is usually restricted to those intermediates that are ahead of the rate-limiting step. In the subsequent steps, the concentration of the intermediates can be low due to these steps being fast. A transient study can thus provide useful complementary information here. Use of the transient method, together with reactant and product analysis with mass spectrometry, in a small number of experiments can provide valuable qualitative information on the reaction pathways. However, a confident quantitative analysis of the rate constants from mass spectrometry data can be difficult, since several compounds can contribute to the same m/e signal and the fragmentation pattern of each compound has to be stable and analyzed. For obtaining rate expressions, conventional analysis of steady-state data from GC analysis can be more reliable. However, a few transient experiments can help substantially in reducing the number of models that need to be considered.

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Received for review March 18, 1997 Revised manuscript received August 15, 1997 Accepted August 24, 1997X IE970226J

X Abstract published in Advance ACS Abstracts, November 1, 1997.

Transient Response Study of the Ammoxidation of Propene and

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