312
I n d . E n g . C h e m . R e s . 1991, 30, 312-320
Veraa, M. J.; Bell, A. T. Effect of Alkali Metal Catalysts on Gasification of Coal Char. Fuel 1978,57, 194-200. Walker, P. L., Jr.; Shelef, M.; Anderson, R. A. Catalysis of Carbon Gasification. In Chemistry and Physics of Carbon; Walker, P. L., J r . , Ed.; Marcel Dekker: New York, 1968; Vol. 4, pp 287-380. Yuh, S. J . ; Wolf, E. E. K2C03-Catalyzed Steam Gasification of Supercritical Extracted Char. Fuel 1983, 62, 738-741.
Yuh, S. J.; Wolf, E. E. Kinetic and FT-IRStudies of the SodiumCatalyzed Steam Gasification of Coal Chars. Fuel 1984, 63, 1604-1609.
Received f o r review January 29, 1990 Revised manuscript received August 14, 1990 Accepted August 28, 1990
Kinetics and Mechanisms in the Ammoxidation of Toluene over a TiO,(B)-Supported Vanadium Oxide Monolayer Catalyst. 1. Selective Reactions Mehri Sanati and Arne Andersson* Department of Chemical Technology, Chemical Center, University of L u n d , P.O.Box 124, S-221 00 L u n d , Sweden
A kinetic investigation of the ammoxidation of toluene was carried out over a TiO,(B)-supported vanadium oxide catalyst with a loading corresponding t o a theoretical monolayer. The partial pressures of reactants toluene, oxygen, and ammonia were varied, and rates were measured for the formations of benzaldehyde and benzonitrile. By analysis of the rate dependencies on partial pressures of reactants, rate expressions completely describing the data were derived. These show that benzaldehyde is formed from two routes in which the. active ensemble accommodates one and two toluene molecules, respectively. T h e latter route is most facile. For the formation of benzonitrile, there are also two routes. In the route that is major at low partial pressures of ammonia, one ammonia molecule is adsorbed. With increase in the partial pressure of ammonia, a second route involving adsorption of two ammonia molecules at the active ensemble becomes increasingly important. Oxidation of compounds having a methyl group in the a-position relative to a double bond or an aromatic ring can be carried out in presence of ammonia, so-called ammoxidation, to produce nitriles (Smiley, 1981). The commercially most important ammoxidation process is the one developed by Sohio for production of acrylonitrile from propylene (Grasselli and Burrington, 1981). However, alkylaromatics can also be converted to nitriles by use of a similar technique; e.g., phthalonitrile, isophthalonitrile, nicotinonitrile, and benzonitrile can be produced from o-xylene, m-xylene, 3-picoline, and toluene, respectively (Sze and Gelbein, 1976). In comparison with acrylonitrile, which is used for fibers and elastomers, the market for aromatic nitriles is limited. Phthalonitrile and isophthalonitrile can be used as intermediates in the manufacture of copper phthalocyanine pigments and agricultural chemicals, respectively (Sze and Gelbein, 1976). Hydrolysis can be used to convert nicotinonitrile into nicotinamide, vitamin B5, and niacin, provitamin (Beschke et al., 1977; Paustian et al., 1981). Of the aromatic nitriles, benzonitrile is the most important. It is used in the synthesis of benzoguanamine, which is a derivative of melamine and is used in protective coatings and molding resins. Other applications of benzonitrile are as a jet-fuel additive and as a drying additive for acrylic fibers (Smiley, 1981). Ti0,-supported vanadium oxide catalysts are active and selective in both oxidation and ammoxidation of alkylaromatic compounds (Bond and Konig, 1982; Wachs et al., 1985; Cavani et al., 1987; Cavalli et al., 1987a; Jonson et al., 1988). The anatase modification is generally considered to be a superior support, in comparison with the rutile phase (van Hengstum et al., 1983; Gasior et al., 1984, 1987). It has, however, been reported that, for the ammoxidation of toluene after activation in a reactant stream, the performances of anatase- and rutile-supported vanadium oxide catalysts were quite similar and so also were the distributions between various types of vanadium species
(Cavani et al., 1988). Recently, TiO,(B), which is isotypic with VO,(B) (Marchand et al., 1980), has been used as a support for vanadium oxide (Papachryssanthou et al., 1987; Sanati and Andersson, 1990a). The active phase-support interaction existing in this system was found to enhance the catalytic performance compared to bulk Vz05when used in toluene ammoxidation (Sanati and Andersson, 1990a). Even though numerous scientific studies of V-Ti-0 catalysts have been reported during the past decade, only a few have dealt with reaction kinetic aspects of hydrocarbon oxidation (Bond and Konig, 1982; Il’inich and Ivanov, 1983; Skrzypek et al., 1984; Raevskaya and Pyatnitskii, 1984). For the ammoxidation of toluene, a kinetic investigation has been reported using a coprecipitated catalyst with TiOz in the anatase form (Cavalli et ,al., 1987b). The rate expressions derived were not valid at low partial pressures of ammonia and oxygen. In a comprehensive kinetic study of the same reaction over bulk Vz05, it was demonstrated that it is necessary to evaluate in detail all dependencies on pressures; otherwise no relevant mechanistic information is obtained (Otamiri and Andersson 1988a,b). No kinetic study has hitherto been reported on hydrocarbon (amm)oxidation over TiO,(B)supported vanadium oxide catalysts. Therefore, the present investigation on the kinetics of the ammoxidation of toluene over such a catalyst was undertaken. A vanadium loading corresponding to a theoretical monolayer was used. Following procedures described by Schmid and Sapunov (1982), a strict mathematical analysis of experimental rate dependencies on reactant pressures was performed, in order to obtain expressions describing the data completely. The corresponding mechanism was then derived. Part 1 of this work concerns the formations of benzaldehyde and benzonitrile, while the kinetics and mechanism of combustion reactions are dealt with in part 2 (Sanati and Andersson, 1990b).
0888-5885/91/2630-0312$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 313
Experimental Section Preparation of Catalyst. The support precursor, K2Ti409,was prepared by calcining an appropriate mixture of TiOp anatase and KN03 at 1000 "C for 48 h. The product was ground and then hydrolyzed for 3 days at room temperature with a 0.45 N solution of HN03. To each millimole of KzTi4O9100 mL of the acid solution was added. After filtering and washing with water, the solid material was dried at 40 "C for 24 h and further calcined a t 500 "C for 3 h. The resulting product, according to X-ray diffraction (Powder DiffractionFile, 1985), was pure TiO,(B). Chemical analysis showed that the residual potassium content was 0.4 wt 5% KzO. The catalyst sample was prepared according to a grafting technique. Powdered Ti02(B),3 g with a specific surface area of 13.4 m2/g, was placed in a two-necked flask through which pure nitrogen was passed. A solution containing 93 mg of VOCl, (Riedel de Haen 99%) in 50 mL of toluene was added. The solvent was slowly distilled off at 70 "C while stirring. Under nitrogen, the solid was washed with solvent, filtered, and dried at 40 "C for 15 h. Eventually, the sample was calcined in moistened air, 2.7 vol 5% H20, at 400 "C for 3 h. The specific surface area of the catalyst was 10.5 m2/g. From atomic absorption measurement the vanadium content was determined to be 11.6 pmol of V/m2 of support, which corresponds to 88% of a theoretical monolayer, calculated considering the structure of V206 (Sanati and Andersson, 1990a). Activity Measurements. Kinetic experiments were performed at atmospheric pressure using a differential plug-flow reactor made of Pyrex glass. A fraction of catalyst particles with diameters in the range 0.150-0.425 mm was used. In order to maintain a constant reactor temperature, the catalyst sample was diluted with washed and glowed SO2. All flows were regulated by Hi-Tec mass flow controllers. The reactant pressures were varied in the ranges as follows: oxygen (Po),2.85-28.5 kPa; toluene (PT), 0.38-1.28 kPa; and ammonia ( P A ) 0-5.71 , kPa. Nitrogen was used as an inert gas. Measurements were made at three temperatures, namely, 340, 370, and 400 "C. The formation of products benzonitrile, benzaldehyde, COP,and CO was analyzed on a Varian Vista 6000 gas chromatograph. A detailed description of the reactor and analysis arrangement has recently been published (Andersson and Hansen, 1988). The influence of heat- and mass-transfer gradients were checked both experimentally and by the use of standard criteria (Satterfield, 1980; Mears, 1971). Intraparticle and interparticle mass- and heat-transfer gradients, as well as reactor gradients, were found to have a negligible influence on rate measurements.
2
I
4
6
(kPd
P NH3
Figure 1. Rate for formation of benzonitrile as a function of partial 400 O C . PO = 11.4 pressure of ammonia a t ( 0 )340, ( 0 )370,and (0) kPa and PT = 0.77 kPa.
Y
LO
1 It
2
.
A
N '
1
1
2 P
3
(kPa)
NH3
Figure 2. Variation of rate for formation of benzaldehyde with partial pressure of ammonia a t ( 0 )370 and (0) 400 O C . Po = 11.4 kPa and PT = 0.77 kPa.
Results Influence of Partial Pressure of Ammonia. The variation of the rate for formation of benzonitrile (rCN)with the partial pressure of ammonia is given in Figure 1 for three temperatures. A t low pressures, the rate rapidly increases with increase in pressure. After passing through a maximum, the rate slowly decreases with further increase in pressure. Analysis of the rate dependency by plotting the function PA/rCN = f(pA)and its corresponding linearized form clearly showed that the rate expression is of the form
The rate for formation of benzaldehyde (rCHO)as a function of the partial pressure of ammonia is shown in Figure 2. At low pressures, a small increase in pressure causes a steep decrease of the rate for aldehyde formation. Analysis of the ratio of the rates for formation of nitrile and aldehyde showed that it can be linearized by plotting rCN/(rCHOPA2) = f ( l / P A ) .A comparison of values obtained for slope and intercept with values calculated for c1 and c2, respectively, for nitrile formation admitted the conclusion that the rate of aldehyde formation can be expressed as
rCN = (c1pA + c2pA2)/(1+ c3pA + C4p.4')
rCHO = c g / ( l + c 3 p A + c4pA2)
(1)
where ci,i = 1, ...,4, are functions of the partial pressures of oxygen and toluene.
(2)
where c5 is a function of the partial pressures of oxygen and toluene. In (1)and (2), c3 and c4 are identical.
314 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 h
4 2b
30
P (kPa) 02
Figure 5. Rate dependency on partial pressure of oxygen for for400 O C . PA = 0.14 kPa mation of benzaldehyde at).( 370 and (0) and PT = 0.77 kPa. 51
10
20
3'0
p0 (kPa)
Figure 3. Effect of partial pressure of oxygen on rate for formation 400 O C . PA = 2.85 kPa of benzonitrile a t ( 0 )340, ( 0 )370, and (0) and PT = 0.77 kPa.
0.5
To, (kPa)
i
1.5
Figure 6. Rate for formation of benzonitrile versus partial pressure of toluene at Po = 11.4 kPa. ( 0 )340 "C, PA = 2.85 kPa; ( 0 )370 O C , PA = 0.14 kPa; (A)370 O C , PA = 2.85 kPa; (0) 400 O C , PA = 0.14 kPa.
where ci,i = 6, ..., 8, might further depend upon the partial pressure of toluene. A dependency on the partial pressure of ammonia was observed for c6 and cg, which was not the case for c7. Therefore, combination of (1)and (2) with (3) and (4),respectively, gives 10
20
30
p0 (kPa)
Figure 4. Rate for formation of benzonitrile versus partial pressure of oxygen a t ( 0 )370 and (0) 400 "C. PA = 0.14 kPa and PT = 0.77 kPa.
Influence of Partial Pressure of Oxygen. The dependency of the rate for formation of benzonitrile on the partial pressure of oxygen at a high and a t a low partial pressure of ammonia is shown in Figures 3 and 4, respectively. Figure 5 shows the dependency for aldehyde formation a t low partial pressure of ammonia. For both the formation of nitrile and aldehyde, the rate increases with increase in partial pressure of oxygen and tends to a constant value at high pressures. It was found that the dependencies could be linearized by using the function l l r = f(l/Po). Analysis of slope and intercept showed that the rate expressions are of the form
rCN
=
c$8A
+ cl$8A2
+ C l l P A + c12PA2 + C 1 3 P 8 A + c 1 4 p 8 A 2 + c15P0 (5)
rCHO = c16p0
(6)
+ C l l P A + c12PA2 + c l S p 8 A + c 1 4 p 8 A 2 + cl$O where ci,i = 9, ..., 16, are possibly functions of the partial
pressure of toluene. Influence of Partial Pressure of Toluene. Figure 6 shows the rate for nitrile formation versus the partial pressure of toluene at three temperatures under low and high partial pressure of ammonia. The rate for aldehyde formation as a function of the partial pressure of toluene at a low partial pressure of ammonia is given in Figure 7. If the dependencies on toluene for the nitrile and aldehyde formations are compared, it is seen that the dependency for aldehyde formation is much stronger. The dependency in Figure 6 might in principle be expressed in the form rcN = aPT/(l + bPT). However, if the ratio rCHO/rCN is con-
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 315 Table I. Effective Constants and Apparent Activation Energies units mol/(m2.min.kPa) x lo5
13=
kao ~~(TKT'
.Eapp,kcal/mol 21.0
2.17
3.13
11.2
0.703
0.918
1.54
11.1
dimensionless x IO2
3.05
4.12
5.05
6.7
kPa-I
1.91
1.92
2.00
0.5
9.00
4.00
3.10
-9.6
kPa-'
2.46
1.50
0.941
-13.2
dimensionless
6.00
7.00
9.00
5.6
mol/(m2.min) x 10'
0.102
0.397
1.59
36.3
0.840
3.30
kPa
X
10
X 10
mol/(m2.min.kPa)
X
10'
14.1
38.4
2c
3
.E E
400 2.16
1.37
kPa-,
A
temperature, "C 370 1.03
340 0.468
2
.
N'
E
dE,
1c
g
Yo E
7
1
i
0.5
0
PToL(kPa)
Figure 7. Influence of partial pressure of toluene on rate for for400 OC. PO = 11.4kPa mation of benzaldehyde at (@) 370 and (0) and PA = 0.14 kPa.
1.50
1.BO
1.55
IT
sidered, it is found that this ratio is of first order with respect to the partial pressure of toluene. Furthermore, considering ( 5 ) and (61,it is obvious that nitrile and aldehyde are competitively formed using the same type of site. The last two facts admit the conclusion that the dependencies on partial pressure of toluene can be expressed as
1
x 103(~-')
Figure 8. Arrhenius plots of effective constants: (V)1,; 13; (v) 1,; (@I 1,; (A)16; (0) 17; ( 0 )18; (0) 1,; (m) 11@
(E) 1,;
(A)
for the formation of selective products that fit to all the dependencies shown in Figures 1-7 are as follows:
(7) + c18PT + c1pT2) rCHO = ( C Z P T + cZlpT2)/(1 + c18PT + c 1 p T 2 ) (8) where ci,i = 17, ..., 21, might be functions of the partial rCN
= Cl'IpT/(1
pressures of oxygen and ammonia. The dependencies expressed by (7) and (8) have to be combined with those of (5) and (6), respectively, in such a way that final rate expressions are obtained that are mechanistically significant. Since c16 in (6) is not a function of the partial pressure of ammonia, it follows that the denominator of the final rate expressions should not contain factors indicating formation of aldehyde a t sites also accommodating ammonia. Thus, the rate expressions
16-
+ 11-PPAT2 + 13) pT (10)
+ -1, PT
PT
Calculation of Effective Constants. Linearization methods (Schmid and Sapunov, 1982) were used in order
316 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991
1S31
Figure 9. Reaction mechanism for selective oxidation and ammoxidation of toluene.
to calculate the effective constants li, i = 1, ..., 10, of (9) and (10). The methods used were briefly described above as a part of the derivation of rate expressions. The values of the constants obtained are given in Table I. Activation energy values are also included. Arrhenius plots are shown in Figure 8. The values listed in Table I were used to draw the curves in Figures 1-7. It is clearly seen that (9) and (10) perfectly express the types of dependencies on pressure observed and that calculated rates fit excellently to experimental data. Mechanistic Interpretationof Rate Expressions. In this section the mechanistic meaning of derived rate expressions is described. This description is of course rather general, since no detailed structural information about reaction intermediates can be obtained from a rate expression. In the following section a rather detailed mechanism is proposed and discussed considering information available in literature. One implication of (9) and (10) is that the same type of site is involved in the formations of benzonitrile and benzaldehyde. For both formations, there is a common intermediate of adsorbed toluene, which is formed in a relatively fast step. When no ammonia is available, aldehyde is formed from two routes in which the active site accommodates one and two toluene molecules, respectively. In both routes, some type of surface reaction constitutes a slow and rate-determining step, which is followed by desorption of aldehyde and formation of a reduced surface site. The reoxidation step comprises a fast adsorption of molecular oxygen followed by a slow dissociation, which probably is not rate-limiting under the conditions used in the present investigation, i.e., low conversion and an excess of oxygen. When ammonia is available, benzonitrile can be formed from two routes. One route involves reaction a t a site where one ammonia molecule has been adsorbed. This route is predominant at low partial pressures of ammonia. A t high pressures, another route comprising reaction at the same site with two adsorbed ammonia species becomes important. The adsorption of ammonia is in both routes a fast step, whereas a surface reaction step constitutes a rate-determining step. After desorption of nitrile, molecular oxygen is adsorbed at oxygen vacancies. The latter
process can be considered to be in equilibrium, but the dissociation to monoatomic oxygen species is a slow step.
Discussion Figure 9 proposes a rather detailed reaction scheme that obeys the general features of the mechanisms as concluded from consideration of the final rate expressions. Additional mechanistic details have been included, considering various results published in the literature. The derivation of the rate expressions from the mechanism in Figure 9 is given in the Appendix. The relationships between the effective rate constants I;, i = 1, ..., 10, and the constants used in Figure 9 are given in Table I. The active site is considered to be an ensemble of neighboring vanadyl species with some vanadium ions exposed. The size of the ensemble has to be large enough to accommodate all necessary mechanistic steps. As an active site, it is preferable to use an ensemble instead of separate vanadium species, since the former option makes it easier to account for electronic and structural effects caused by the adsorption of a specific reactant. Vanadyl species are considered active because, upon comparing the performances of bulk vanadium oxides in the ammoxidation of 3-methylpyridine (Andersson et al., 1986), it was shown that all of the oxides found selective for nitrile formation had infrared bands in the V=O region. In a FTIR study of Ti02(B)-supportedvanadium oxide catalysts (Sanati and Andersson, 1990a), it was observed that all active structures formed at various loadings have V=O groups. Furthermore, the monolayer was concluded to be composed of interconnected tetrahedrally coordinated V4+ species having three bridging oxygens and one terminal double bonded oxygen. It was observed, Figure 1, that the rate of formation of nitrile passed through a maximum when the partial pressure of ammonia was varied. This might be due to (i) a dual-site mechanism where ammonia and another reactant are competitively adsorbed, (ii) product inhibition, or (iii) substrate inhibition. The first alternative is not the case since the constant c2 in (1)was not zero; Le., the rate does not go to zero a t high partial pressures of ammonia. As the rates in this investigation were measured a t low conversions, product inhibition seems unlikely. This is
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 317 supported by the fact that in the ammoxidation of 3- and 4-methylpyridine over V-Mo-O/A120, (Prasad and Kar, 1976) and V-Cr-O/A1203 (Das and Kar, 19791, respectively, it was in both cases observed that total rate and selectivity for nitrile formation were independent of the partial pressure of product. Thus, substrate inhibition is a possible explanation to the observed maximum, which is further supported by the finding that the maximum is shifted toward higher partial pressures when the temperature is increased. Consideration of (9) admits the conclusion that there are two routes for nitrile formation originating from the same type of surface site. In one of the routes the active site accommodates one ammonia species, while in the other route there are two ammonia species involved. From Figure 1 it follows that the second route is slower than the first one. This seems to be a general feature observed in toluene ammoxidation over vanadium oxide catalysts (Sanati and Andersson, 1990a; Cavalli et al., 1987b; Otamiri and Andersson, 1988a). Also, in studies of the ammoxidation of propylene over molybdates and antimonates, it has been concluded that the active site at high partial pressures of ammonia can activate two ammonia molecules (Burrington et al., 1983, 1984). In the system studied, the adsorption of oxygen, toluene, and ammonia have to be considered. It is generally accepted that selective (amm)oxidation occurs according to a Mars-van Krevelen type of mechanism (Mars and Krevelen, 1954) in which lattice oxygen (02-) is consumed in the reaction. The reduced site formed is then reoxidized by gaseous oxygen either directly or indirectly via a diffusion process (Haber, 1983). The participation of lattice oxygen is confirmed by, e.g., that in the ammoxidation of 3-methylpyridine over vanadium oxide, products were observed to be formed for several hours even in the absence of gaseous oxygen (Baiker and Zollinger, 1984). Experiments with alternating injections of toluene and ammonia pulses over a V,O,/Al,O, catalyst showed that benzonitrile was formed at the ammonia pulse rather than at the toluene pulse (Murakami et al., 1977). The same behavior was found when pulses of the xylene isomers and ammonia were alternatingly injected over the same type of catalyst (Niwa et al., 1981). A similar finding was also observed in a detailed transient response and temperature programmed reaction study of the ammoxidation of 3methylpyridine over Vz05(Andersson, 1986). The studies referred to clearly indicate that the first step comprises an activated adsorption of toluene, producing a species appropriate for ammonia adsorption. Such an activating step has been included in the mechanism, So SI,as an abstraction of a hydrogen atom from the methyl group, resulting in the formation of an adsorbed benzyl species and a hydroxyl group, to which ammonia can be adsorbed. This step has been represented as a concerted reaction; however, it probably occurs as two consecutive steps. In the first step, toluene is adsorbed to the ring at a vanadium ion, and in the second step, a hydrogen atom is abstracted. The benzyl intermediate formed, SI,is common for ammoxidation and oxidation routes, which is supported by the rapid decrease of the rate for aldehyde formation when the partial pressure of ammonia is increased (Figure 2). A question of great interest is whether the first hydrogen abstraction from the methyl group is a rate-limiting step, which is usually assumed to be the case in (a")oxidation reactions of alkylaromatic compounds over vanadium oxide catalysts (Prasad and Kar, 1976; Das and Kar, 1979; Grasselli, 1987). This widespread opinion originates from comparisons with results valid for the (amm)oxidation of
-
propylene over molybdates and antimonates (Grasselli and Burrington, 1981; Burrington et al., 1983, 1984) and the oxidation of substituted allylbenzenes over molybdate catalysts (Burrington et al., 1981). In the present study, consideration of rates for formation of nitrile and aldehyde at low partial pressures of ammonia, Figures 1 and 2, admits the conclusion that FCHO ( P A = 0) p-methoxytoluene > pchlorotoluene (Busca et al., 1987), in spite of the fact that quantum-chemical calculations have shown that neither the charge of the methyl group nor the methyl C-H bond length is affected by substitution in the para position (Grzybowska et al., 1987). In the mechanism for aldehyde formation, the formation of benzyloxy species, S13and S16,has been assumed to be rate limiting. With the use of FTIR spectroscopy it has not been possible to identify such species (Busca et al., 1987; van Hengstum et al., 1986),which indeed shows that they are formed in a slow step followed by a rapid desorption in the form of aldehyde. If benzyloxy species are formed as proposed in a reaction between an adsorbed benzyl radical and a double-bonded oxygen species, this reaction should be facilitated by delocalization of electrons from the V=O bond toward the oxygen. Such a delocalization can occur as a result of charge transfer from adsorbed toluene species to the surface, and it will consequently increase with the number of adsorbed species. Thus, the rate of the rate-limiting step will be higher when two toluene species are adsorbed at neighboring sites of the active ensemble compared to when there is only one species adsorbed. This is exactly what has been observed, which becomes evident from a comparison of the values of constants l9 and llo in (10) (Table I).
318 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991
On vanadium oxides, it has been established that ammonia can be both coordinatively bonded to a vanadium ion, NH,(ad), and adsorbed a t a hydroxyl group as NH4+(ad) (Busca et al., 1987; Belokopytov et al., 1979; Takagi-Kawai et al., 1980). In a FTIR study of toluene ammoxidation over a vanadium-titanium monolayer catalyst (Busca et al., 1987), it was found that when toluene was adsorbed at room temperature on an ammonia-covered surface, the bands due to NH3(ad) decreased while those of NH4+(ad) increased. Furthermore, when an ammonia-covered surface was reacted with toluene at 470 K, the bands from NH,(ad) disappeared and that of NH4+(ad) decreased, together with the appearance of bands assigned to adsorbed benzylamine. These results can be interpreted in accordance with the mechanism of the main route for nitrile formation in Figure 9. The site for toluene adsorption is a cation while ammonia reacting to nitrile is adsorbed at the hydroxyl group, formed as a result of toluene activation. From such an arrangement it follows that ammonia is favorably positioned to react with the benzyl radical, producing an adsorbed benzylamine intermediate, Sz S3. This step probably constitutes a rate-limiting step. A slow step before the nitrile desorption cannot occur at a later stage, because neither benzylamine nor imine was formed in detectable amounts under normal operating conditions. Formation of intermediate S3 is in agreement with the observation that benzylamine and benzaldehyde are formed in toluene ammoxidation using low residence times and low pressures of oxygen and ammonia (Cavalli et al., 1987a). No kinetic evidence was found for formation of nitrile from intermediate S12having two hydroxyl groups. Possibly, these hydroxyl groups are much less acidic and less prone to adsorb ammonia in comparison with the hydroxyl group in intermediate S1. This is due to the fact that the charge transferred from the surface toward a hydroxyl group increases with the number of toluene species that are flatly coordinated at neighboring cations. However, as concluded, a second route for nitrile formation exists involving two ammonia species. It has been demonstrated that, in addition to ammonia species adsorbed as a result of activation of alkylaromatic reactant, more strongly bonded ammonia species active for nitrile formation can also exist (Anderson, 1986). In ammoxidation mechanisms imido species, =NH, are usually considered to be active (Grasselli and Burrington, 1981; Otamiri and Andersson, 1988a; Prasad and Kar, 1976; Das and Kar, 1979; Burrington et al., 1983, 1984). However, it is a fact that on vanadium oxides their formation has never been spectroscopically observed (Busca et al., 1987). Due to that the adsorption of the second ammonia results in a partial deactivation, it seems unreasonable to explain the highpressure route by the formation of two NH,+(ad) species. Figure 9 suggests that the second ammonia is adsorbed in the form of a hydroxylamino species. The rate-limiting step is given as an abstraction of two hydrogen atoms from amine, which is bonded to the surface via the nitrogen atom. The mechanism reasonably accounts for the desorption of the nonreacting ammonia molecule, but the steps suggested are rather speculative in comparison with those given for the other routes. It is also possible that the second ammonia molecule can react to give water and an imido species. If so, this step has to be slow because the rate for nitrile formation should not depend on the partial pressure of water (Prasad and Kar, 1976; Das and Kar, 1979). The present work has clearly demonstrated how a kinetic investigation, combined with information obtained
from other sources, e.g., adsorption and reaction studies using infrared and pulse techniques, can significantly contribute to the understanding of reaction mechanisms. A kinetic study is an important tool that can be used to conclude whether a mechanism deduced on the basis of experiments performed under constrained conditions is actually operating under process conditions.
Aoknowledgment This work was carried out as a part of the national research program on catalysis, which is sponsored by the National Swedish Board for Technical Development (STU) and the National Energy Administration (STEV).
Appendix Expressions for the concentrations of intermediates So, ...,s16 can be derived by using expressions for equilibrium constants and the relations given below:
kl[s21= k,[s61
-
k3[s81
= h4[S11I
= k6[S151 ~ ~ [ S I+Zk7[S11 I
From the fact that the measurements were made at initial conditions, it follows
[SlZl = KTKT’pT2[sOl [SI,] = (k5KTIPT + k7)KTpT[sOI [SI,] =
(k5KT’PT
/(k6K80)
+ k7)KTPT[SOI/k6
Rates for formation of benzonitrile and benzaldehyde can be expressed in terms of rate-determining steps: rCN
= kl[S21 + k3[S81
~ C H O=
b[Sil
k5[Siz]
The expressions for [S,], [S,], [s,],and [Slz] can be inserted. [So] can be expressed by its dependence on the concentration of total number of sites, denoted ST, and partial pressures of reactants. To obtain simpler expressions, the following approximation was made based on the experimental results: After some rearrangements, the rate expressions that correspond to the general forms given as (9) and (10) can be derived. The relations between the effective rate con-
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stants of these equations and the constants used in Figure
9 are given in Table I assuming for 1,: (1 + KT‘PT +
+
+
320
Ind. Eng. Chem. Res. 1991, 30, 320-326
van Hengstum, A. J.; Pranger, J.; van Hengstum-Nijhuis, S. M.; van
Ommen, J. G.; Gellings, P. J. Infrared Study of the Selective Oxidation of Toluene and o-Xylene on Vanadium Oxide/TiOz. J . Cutal. 1986, l f f l ,323-330. Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. C. The Interaction of Vanadium Pentoxide with Titania (Anatase): Part I.
Effect on o-Xylene Oxidation to Phthalic Anhydride. Appl. Cut a l . 1985, 15, 339-352.
Received for review January 25, 1990 Revised manuscript received July 6 , 1990 Accepted July 29, 1990
Kinetics and Mechanisms in the Ammoxidation of Toluene over a Ti02(B)-Supported Vanadium Oxide Monolayer Catalyst. 2. Combustion Reactions Mehri Sanati and Arne Andersson* Department of Chemical Technology, Chemical Center, University of Lund, P.O. Box 124, S-221 00 L u n d , Sweden
To obtain knowledge about combustion mechanisms operating in the ammoxidation of toluene, the formation of carbon oxides was studied over a Ti02(B)-supported vanadium oxide catalyst with monolayer loading. The partial pressures of reactants were varied and kinetic rate expressions, completely describing experimental data, were derived. A comparison of the expressions obtained with those for the formation of nitrile and aldehyde shows that carbon oxides are formed at sites that are not involved in the mechanism of partial oxidation. Furthermore, C 0 2 and CO are formed in routes having a common type of active ensemble. All of the adsorption steps are fast and in equilibrium. For the formation of C 0 2 and CO, the rate-limiting step in both cases comprises a chemical transformation of which the details are unknown. The introduction of ammonia leads to a strong decrease of the rates for formation of carbon oxides.
For selective oxidation and ammoxidation of alkylaromatic compounds, the preferred catalysts consist of vanadium oxide supported on TiOz (Sanati and Andersson, 1990a). Several studies have been devoted to their characterization in order to determine the structure of the catalytically active and selective surface phase (Wachs et al., 1985; Kozlowski et al., 1983; Bond et al., 1986; Busca et al., 1986; Cavani et al., 1987; Haber et al., 1986; Bond and Flamerz, 1989). However, in this context, not much information has been obtained specifically on the source of formation of carbon oxides. In a study of the oxidation of o-xylene, Wachs et al. (1985) concluded that exposed titania sites lead to combustion of partial oxidation products. Later, they reported that carbon oxides can also be directly formed from o-xylene a t vanadium oxide monolayer species (Saleh and Wachs, 1987). Mechanistic studies of oxidation and ammoxidation of toluene on VTi-0 catalysts using infrared spectroscopy (van Hengstum et al., 1986; Busca et al., 1987; Miyata et al., 1988) have been concerned mainly with the routes for formation of aldehyde and nitrile, respectively. Recently, a dynamic approach to selectivity in heterogeneous partial oxidation was published (Cavani et al., 1988). According to this approach, which is a further development of the concept of site isolation (Callahan and Grasselli, 1963), the selectivity for the useful product is at maximum at a well-defined degree of surface oxidation. For the formations of nitrile and COP,the existence of a common adsorbed precursor or intermediate has been concluded in some kinetic studies of the ammoxidation of alkylaromatics over catalysts containing vanadium (Prasad and Kar, 1976; Das and Kar, 1979; Cavalli et al., 198713). In a comprehensive kinetic study of the ammoxidation of toluene over V,05, however, it was concluded that there is no common intermediate for the nitrile and combustion routes (Otamiri and Andersson, 1988). Also, in a temperature programmed reaction study of 3-picoline amm-
oxidation over reduced vanadium oxides, the stability against degradation of both nitrile and its preceding intermediate complex was demonstrated (Andersson et al., 1984). Knowledge about combustion mechanisms is of vital importance in deciding possible actions for improvement of catalyst performance (Otamiri and Andersson, 1988). In this article we present a detailed kinetic investigation of the formation of carbon oxides in the ammoxidation of toluene over a TiOz(B)-supported vanadium oxide monolayer catalyst. The kinetics and mechanisms for selective reactions are described in the preceding article (Sanati and Andersson, 1990a).
Experimental Section Experimental details, which are completely described elsewhere (Sanati and Andersson, 1990a), are summarized here. The support, obtained by acidic hydrolysis of K2Ti,Og (Sanati and Andersson, 1990b), was impregnated with a toluene solution of VOC1, and was further calcined in air at 500 "C. According to atomic absorption analysis, the vanadium content was 11.6 pmol of V/m2 of support. Kinetic experiments were carried out at atmospheric pressure using a differential plug flow reactor. Measurements were made at three temperatures, namely, 340, 370, and 400 "C. The reactant pressures were varied in the ranges as follows: oxygen (Po),2.85-28.5 kPa; toluene (PT), 0.38-1.28 kPa; and ammonia (PA),0-5.71 kPa. It was verified that the rate measurements were not controlled by mass- and heat-transfer gradients. Results Influence of Partial Pressure of Oxygen. The variation of the rate for formation of COP(rcoJ with the partial pressure of oxygen at a high and a t a low partial pressure of ammonia is given in Figures 1 and 2, respectively. The corresponding rate dependence for the for-
0888-5885/91/ 2630-0320$02.50/0 0 1991 American Chemical Society
