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Ind. Eng. Chem. Res. 1998, 37, 3230-3236
Kinetic Model for Methacrolein OxidationsInfluence of Cesium and Vanadium on Heteropolyacid Catalysts CsxH3-x+y[PMo12-yVyO40] Liane M. Deusser, Jochen C. Petzoldt, and Johann W. Gaube* Institut fu¨ r Chemische Technologie, Technische Universita¨ t Darmstadt, Petersenstrasse 20, 64287 Darmstadt, Germany
Hartmut Hibst BASF AG, 67056 Ludwigshafen, Germany
A kinetic model has been developed for the heterogeneously catalyzed methacrolein oxidation, the parameters of which reflect the redox and adsorptive properties of the heteropolyacid catalysts CsxH3-x+y[PMo12-yVyO40] (x ) 0, y ) 0, 1; x ) 1, y ) 0, 1, 2). The model based on the Mars-van Krevelen mechanism describes the reaction rates of methacrolein and oxygen consumption as well as the rate of methacrylic acid and byproduct formation as a function of the methacrolein and oxygen partial pressures. Substitution of Mo by V causes a strong decrease of the ratio of reaction rate constants of reoxidation and methacrolein oxidation. The substitution of Mo by V leads to a strong decrease of the rate of consecutive methacrylic acid oxidation. Salification of the free acid by cesium provides a decrease of byproduct formation via parallel and consecutive oxidation and also a reduction of the hindrance of catalyst reoxidation caused by strong adsorption of methacrolein. 1. Introduction Methacrylic acid serves as an important intermediate for the production of methyl methacrylate polymers, which are largely used in the production of plastics. Besides the conventional cyanohydrin process with a share of about 85% of the worldwide production capacity, three additional production routes have been developed. Reasons for the replacement of the cyanohydrin process are the dangerous transport of hydrogen cyanide and the expensive disposal of ammonium sulfate. In the first step of the BASF process, ethene is hydroformylated to propanal. The following condensation with formaldehyde leads to methacrolein, which is then air-oxidized to methacrylic acid (Duembgen and Hammon, 1990). Another route using isobutene as feedstock is employed by Mitsubishi Rayon and Nippon Shokubai Co. Ltd. (Kawajiri and Hironaka, 1991; Yamamoto and Ohkita, 1991). It consists of a two-stage oxidation process also via methacrolein as intermediate. The third process, not yet in commercial use, starts with the carbonylation of propene to give isobutyric acid, which is then converted to methacrylic acid via an oxidative dehydrogenation (Gruber and Schroeder, 1983). Heteropolyacid compounds are suitable catalysts for the oxidation of methacrolein as well as for the oxydehydration of isobutyric acid. In a recent publication we presented a kinetic study that dealt with the heterogeneously catalyzed vaporphase oxidation of methacrolein (Deusser et al., 1996). The aim of this study was to elucidate the role of vanadium in catalysts of the type CsxH3-x+y[PMo12-yVyO40], abbreviated CsxVy. The vanadium content has been varied for the heteropolyacid (x ) 0; y * Phone: +49-6151-163665. Fax: +49-6151-164910. Email:
[email protected].
) 0, 1) as well as for the cesium salt (x ) 1; y ) 0, 1, 2). In this work, however, the dodecamolybdophosphoric acid was not prepared in the same way as the other catalysts of this series but was taken as commercially available (Fluka). Therefore it was not yet possible to elaborate unambiguously the effect of salification of this type of heteropolyacid by cesium. The effect of cesium is still controversally discussed in the literature. While Nakamura and Ichihashi (1980) reported for CsxH3-x[PMo12O40] a decrease of catalytic activity by substitution x ) 1 and a higher activity of x ) 2 with regard to x ) 1, Komaya and Misono (1983) found a monotonic decrease of catalytic activity per surface area by substitution x ) 1-3. Furthermore, Mizuno et al. (1985) reported that the degree of reduction in the steady state of conversion is higher for cesium-containing heteropolyacids. The aim of the presented work was to complete the kinetic study of methacrolein oxidation employing the catalyst H3[PMo12O40] prepared by the same procedure as for all catalysts of the series CsxH3-x+y[PMo12-yVyO40] in order to evaluate the effect of cesium. This experimental design renders possible the development of a kinetic model for this series of catalysts so that the effects of cesium and vanadium are reflected by the parameters of this model. A general intention of this work is to show that detailed kinetic studies and the development of an appropriate kinetic model can particularly contribute to the elucidation of the effects of catalyst components. 2. Experimental Section 2.1. Preparation and Characterization of the Heteropoly Catalysts. The catalysts were prepared by combining aqueous solutions of stoichiometric amounts of phosphoric acid (76%, pure), ammonium molybdate (99%, p.a., ca. 81% MoO3, Merck), ammonium vanadate
S0888-5885(97)00799-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/26/1998
Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3231
Figure 1. Production routes for methacrylic acid. Table 1. Characterization of the Catalysts catalyst
BET surface area (m2/g)
pore volume (Hg porosimetry) (mL/g)
H3[PMo12O40] CsH2[PMo12O40] H4[PVMo11O40] CsH3[PVMo11O40] CsH4[PV2Mo10O40]
5.0 5.4 3.2 5.1 2.4
0.21 0.24 0.14 0.22 0.26
(99%, purest, ca. 77% V2O5, Riedel-de Haen), and cesium nitrate (99.9%, pure, Chemetall) at 90 °C. The obtained slurry was dried by spray drying. The dry powder was made into cylinders (5 × 5 mm) and calcinated in two steps at 250 °C for 2 h and at 380 °C for 5 h. The catalyst samples were characterized by means of X-ray diffraction (XRD) proving their homogeneous phase composition. The XRD analysis showed that the structure of these crystalline heteropoly compounds agrees with that for the ammonium salt (NH4)3[PMo12O40]‚4H2O. All samples turned out to be single phase, which means no other crystalline oxides were present. As predicted by Vegard’s law for mixed crystals, the lattice parameters were found to rise with increasing cesium content. Furthermore, argon and nitrogen sorption measurements showed that all samples have mesopores with an average diameter of about 4 nm and that the observed micropore volume is less than 10% of the total pore volume. The BET surface areas of all fresh catalysts are listed in Table 1 together with the pore volume measured by mercury penetration. The porosimetry measurements by mercury penetration showed macropores with a maximum in diameter near 1 µm. The postmortem analysis shows a decrease in the surface area and in the pore volume detected by the mesopore analysis. 2.2. Experimental Unit. A simplified flow diagram of the experimental setup is shown in Figure 2. Methacrolein (MA) and methacrylic acid (MAA) are dosed with microprecision pumps. While methacrolein is dosed into an evaporator kept at 40 °C, methacrylic acid is directly injected through a nozzle into a hot gas streamscontaining nitrogen and water vaporsto reach immediately a temperature of 150 °C in order to prevent methacrylic acid from polymerization. The gas stream (oxygen, nitrogen, water, methacrolein, and methacrylic acid) is conducted into a differential recycle reactor (Figure 3). The recycle reactor is realized by a jet loop reactor (Luft and Herbertz, 1969), which behaves like a CSTR. The reactor is heated to 300 °C and is kept under a pressure of 0.2 MPa. A small part of the gas leaving the reactor passes through a UNIVAP precision gas sampling system to the on-line gas chromatograph. n-Octane is used as the
Figure 2. Experimental unit.
Figure 3. Differential recycle reactor: (1) gas inlet and nozzle; (2) central tube; (3) catalyst bed; (4) thermocouple; (5) flange; (6) gas outlet.
internal standard. After passing a cooling trap and a gas filter, the main part of the gas stream is analyzed with respect to oxygen, carbon monoxide, and carbon dioxide content. The rates of consumption and respectively the rates
3232 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
of formation of products are calculated by
n˘ i - n˘ 0i ri ) mcat
(1)
The rate of oxygen consumption is calculated on the basis of the oxygen balance:
1 1 [ / (n˘ 0 - n˘ 0MA) + (n˘ 0MAA - n˘ MAA) mcat 2 MA 1 1 /2n˘ Acetone - n˘ HOAc - 1/2n˘ CO - n˘ CO2] + (n˘ 0 2mcat H2O n˘ H2O) (2)
rO2 )
The (negative) rate of water formation obtained from the hydrogen balance is 0 - n˘ H2O) ) [3[(n˘ MA - n˘ 0MA) + (n˘ MAA - n˘ 0MAA) + (n˘ H 2O
Figure 4. SEM micrograph of H3[PMo12O40], magnification 5000: 1. (Reduced to 72% for reproduction.)
n˘ Acetone] + 2n˘ HOAc] (3) Before experimental work with the catalysts was started, it had to be ensured that vapor-phase oxidation and reactions on the wall of the reactor are negligible. Only at temperatures above 310 °C and concentrations beyond 2.5 vol % (pMA ) 5 kPa) were significant quantities of carbon monoxide and carbon dioxide detected. Therefore, for all experiments, 300 °C was chosen as the upper limit of reaction temperature and the methacrolein concentration was restricted to 2.5 vol %. For all experiments the reactor was loaded with 4 g of the heteropoly compound. The pressure inside the reactor was fixed to 0.2 MPa and for all experiments a constant water partial pressure of 12 kPa was maintained to ensure the structural stability of the catalyst. After each run the samples were reoxidized for 10 h at a low oxygen partial pressure in the absence of water vapor. The influence of intra- and interparticle diffusion limitation has been estimated. The calculated values for the Weisz criterion (Weisz, 1975) plainly lie below the critical value of 1. For the maximum rate of methacrolein consumption (1.5 mol/(h kgcat)) a value of ≈0.2 was obtained. The macropores with a diameter of about 1 µm provide more than 90% of the pore volume. The SEM micrograph in Figure 4 shows that the heteropolyacid catalysts mainly consist of crystallites with a size of about 1 µm. The macropores represent the space between the packed crystallites, which are hardly porous themselves. The calculated surface area formed by spherical particles with a diameter of 1 µm correlates with the measured BET surface area, thus the micropores within the crystallites have a negligible share in the overall surface area. As diffusion and reaction mainly take place between these particles, an influence of Knudsen diffusion can be neglected. Because of the high gas velocity in the recycle reactor (1 m/s) and the rather slow reaction rates, interparticle diffusion is not rate-determining. The concentration of methacrolein on the catalyst surface is about 99% of the gas-phase concentration. It must be noted that all reaction rates were related to the catalyst mass instead of the catalyst surface area. This is justified because the catalyst surface area might be changed in the course of reaction and is therefore not an appropriate point of reference. The surface area
Figure 5. Reaction network of methacrolein oxidation.
could be a function of the degree of oxidation, which is determined by the oxygen partial pressure and even by the water vapor pressure. For H4[PVMo11O40], a continuous phase evolution within the course of the reaction has been reported (Ilkenhans et al., 1995). Under reaction conditions a mixture of several different structural phases was found. This structural behavior possibly causes a change of the in-situ surface areas. Furthermore, it has been reported by Berndt et al. (1997) that BET surface areas of catalysts of the type CsxH4-x[PVMo11O40] are markedly lowered except for x ) 3 when the temperature of the thermal pretreatment was changed from 200 to 350 °C. In any case, the reaction rates can be referred to the initial surface area by use of Table 1. 3. Results The kinetics of the methacrolein oxidation can be described on the basis of the Mars-van Krevelen mechanism, which expresses the separated reduction of the catalyst by the compound that is oxidized and the subsequent reoxidation of the catalyst by supplied oxygen. For low oxygen partial pressures the reoxidation is rate-determining whereas in the range of elevated oxygen partial pressures the rate of conversion is nearly independent of the oxygen pressure, indicating that the oxidation of the substrate, respectively, the reduction of the catalyst, is the rate-determining step. The reaction network of the methacrolein oxidation (Figure 5) includes the main reaction toward methacrylic acid (r1) and the parallel reaction (r2) and the consecutive reaction (r3), which both (r2 and r3) contribute to the formation of byproducts (mainly carbon monoxide and carbon dioxide and to a smaller extent acetone and acetic acid). Figure 6 shows, exemplified for the catalysts Cs0V0 and Cs1V2, that the rate of methacrolein consumption
Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3233
Figure 7. Oxidation of methacrolein; -rMA ) f(pMA). Solid lines: calculated.
Figure 6. Oxidation of methacrolein; -rMA ) f(pMAA).
was found to be independent of the methacrylic acid partial pressure. Therefore, also for the kinetic model the assumption is made that the rate of methacrolein oxidation is not affected by methacrylic acid and vice versa. Furthermore, for both reactions the assumption of specific oxidation states of the catalyst ΦMA and ΦMAA is made. This is a major difference from our former work (Deusser et al., 1996), as the whole reaction was expressed by only one oxidation state, which does not consider the mutual independency of methacrolein and methacrylic acid oxidation. The observed independency may be due to different catalytic sites. However, there is nothing known yet about surface structures of the catalyst and oxygen species that may be specific either for the oxidation of methacrolein or that of methacrylic acid. For the steady state of substrate oxidation and reoxidation of the catalyst the following set of kinetic equations has been proved as suitable for the representation of all experimental results. Oxidation of methacrolein
r1 ) k1pMAΦMA
(4)
r2 ) k2pMAΦMA
(5)
1 MA MA n1 -rO ) kO pO2 (1 - ΦMA) ) 1/2r1 - ν2r2 (6) 2 2 1 + pm MA with
1 MA n1 pO2 kO 2 1 + pm MA ΦMA ) 1 MA n1 1 kO2 pO2 + ( /2k1 - ν2k2)pMA 1 + pm MA
(7)
Oxidation of methacrylic acid
r3 ) k3pMAAΦMAA
(8)
MAA MAA n2 ) kO pO2(1 - ΦMAA) ) -ν3r3 -rO 2 2
(9)
with MAA
Φ
)
MAA n2 kO pO2 2 MAA n2 kO pO2 - ν3k3pMAA 2
(10)
Table 2. List of Experimental Runs rate
function of
constant param
rMA rMA rMAA rMAA rO2 rO2 rMA
pMA pO2 pMAA pO2 pMA pO2 pMAA (i.e., conversion)
pO2 pMA pO2 pMAA pO2 pMA pO2; pMA
The reaction rates rMA, rMAA, and rO2 are obtained from the experiments. They are calculated by the model as follows:
rMA ) -(r1 + r2)
(11)
rMAA ) r1 - r3
(12)
MA MAA rO2 ) rO + rO 2 2
(13)
The stoichiometric coefficients of the oxygen consumption (νi) were calculated from experimental data (ν1 ) -1/2):
[
MA (k1 + k2)rO 2
-ν2 )
rMA k2 -ν3 )
MA rO 2
rMAA
]
- 1/2k1
(14)
(15)
For all investigated catalysts the evaluation of the kinetic data is based on the experimental runs listed in Table 2. For instance in Figures 7 and 8 the reaction rates of methacrolein consumption are presented as functions of methacrolein and of oxygen pressure. Figure 9 shows the rates of methacrylic acid consumption as a function of the partial pressure of methacrylic acid. The evaluation of model parameters was carried out stepwise. At first rMA ) f(pMA) and rMA ) f(pO2) as well as rMAA ) f(pMAA) and rMAA ) f(pO2) were fitted by the least-squares method. In a second step, the ratio of reaction rate constants k2/k1 and k3/k1 was corrected by fitting the model to the selectivity plot SMAA ) f(pO2), which is particularly sensitive with respect to these ratios. It turned out that the correction of k3, which was obtained from the plots rMAA ) f(pMAA) and rMAA ) f(pO2),
3234 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
Figure 8. Oxidation of methacrolein; -rMA ) f(pO2). Solid lines: calculated.
Figure 9. Oxidation of methacrylic acid; -rMAA ) f(pMAA). Solid lines: calculated. Table 3. List of Kinetic Parameters catalyst [10-4
k1 mol/(h kgcat Pa)] k2 [10-5 mol/(h kgcat Pa)] -5 k3 [10 mol/(h kgcat Pa)] MA [10-4 mol/ kO 2 (Pa1-m h kgcat)] MAA [10-3 mol/ kO 2 (Pa0.5 h kgcat)] m ν2 ν3
Cs0V0 Cs1V0 Cs0V1 Cs1V1 Cs1V2 5.2 12 59 50
3.8 7 28 50
2.2 2.4 2.5 5.7
3.2 3.5 5.8 6.1
2.1 1.9 8.7 2.0
7.7
9.0
1.9
2.3
2.4
0.54 -2.2 -1.5
0.37 -3.4 -1.8
0.28 -3.4 -3.8
0.28 -3.6 -3.8
0.16 -4.6 -3.9
is marginal. This result reflects again that the rate of consecutive oxidation r3 is independent of the partial pressure of methacrolein and rMA is independent of pMAA, which has been proved directly. With the corrected parameters k2 and k3 also the dependencies rO2 ) f(pMA) and rO2 ) f(pO2) could be well represented. Also, reaction rates for experiments with additionally dosed methacrylic acid in order to simulate higher degrees of methacrolein conversion could be well represented by the model. The reaction order with respect to oxygen was set to 1 for the oxidation of methacrolein (n1) while for the oxidation of methacrylic acid the order n2 was set to 0.5. The kinetic parameters that were obtained by fitting the model to all experimental runs are presented in Table 3. The solid lines in Figures 7-9 are calculated by this model. The curve rMA ) f(pMA) for the free heteropolyacid Cs0V0, Figure 7, shows, different from the other curves, a flat course at elevated methacrolein pressure. To enforce this flat course, we had to introduce into the
Figure 10. Oxidation of methacrolein; -rO2 ) f(pO2). Experimental data normalized to a fixed pMAA.
model an increasing hindrance of rMA as pMA is raised. This may be expressed by a negative reaction order with respect to methacrolein in the rate equation of catalyst reoxidationsin the form of the term 1/(1 + pm MA)sin analogy to the kinetics of acrolein oxidation on Mo-Voxide catalysts (Stein et al., 1997). This different behavior of the heteropolyacid Cs0V0 reveals that the acidity of the catalysts certainly plays an important role in the adsorption and activation of the reactants. The acidity of the dodecamolybdophosphoric acid was found to decrease with increasing cesium content (Komaya and Misono, 1983) and increasing vanadium content (Serwicka et al., 1991). Obviously, the elevated acidity of the free heteropolyacid causes an increased strength of adsorption of methacrolein. With the help of the detailed kinetic model the reaction rate of oxygen consumption and the selectivity toward methacrylic acid can be calculated as a function of the oxygen pressure for constant partial pressures of methacrolein and of methacrylic acid so that the reaction rate and selectivity of the investigated catalysts can be compared at the same degree of conversion as shown in Figure 10 for the rate of oxygen consumption. The free heteropolyacid Cs0V0 and the cesium salt Cs1V0 show a decreasing selectivity with increasing oxygen pressure, while for all catalysts containing vanadium the selectivity toward methacrylic acid was found to be nearly independent of the oxygen pressure. The kinetic model has also served to calculate the reduction of methacrylic acid selectivity by the formation of byproducts via both routes, the parallel oxidation of methacrolein and the consecutive oxidation of methacrylic acid. These fractions of selectivity reduction were calculated for constant partial pressures of methacrolein and of methacrylic acid in order to compare all investigated catalysts, as shown in Figure 11. The cesium salt Cs1V0 exhibits a considerably higher selectivity toward methacrylic acid than Cs0V0 mainly due to a reduction of the byproduct formation by the consecutive oxidation of methacrylic acid but also due to an evident reduction of byproduct formation via parallel oxidation of methacrolein. However, the catalysts Cs1V1 and Cs0V1 show the same product distribution (Figure 11). The reaction rates rMA and rO2 of Cs1V0 are slightly higher than the rates of Cs0V0, while with regard to the rate of methacrylic acid oxidation, Cs1V0 is less active than Cs0V0 (Figures 7, 8, 10, and 9). Both effects lead to the increased methacrylic acid selectivity of the cesium salt Cs1V0 mainly due to the relatively reduced
Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3235 MA 1 Table 4. Redox Parameter kO /( /2k1 - ν2k2) in Pam and 2 m Hindrance Factor 1/(1 + pMA) at 2000 Pa in Pa-m (in Parentheses)
Cs1 Cs0
Figure 11. Reduction of selectivity to methacrylic acid by the different reaction paths of byproduct formation, parallel oxidation of methacrolein and consecutive oxidation of methacrylic acid. pMAA ) 300 Pa; pO2 ) 20 kPa; pMA ) 2.3 kPa.
parallel oxidation, Figure 11. The comparison of Cs1V1 and Cs0V1 shows also increased reaction rates rMA and rO2 of the cesium-containing catalyst. However, in contrast to the pair Cs0V0/Cs1V0, the rate rMAA of the cesium-containing catalyst Cs1V1 is also higher than rMAA of Cs0V1 so that the ratio of consecutive oxidation and main reaction r3/r1 remains nearly unchanged, as shown in Figure 11. As already discussed by Deusser et al. (1996), substitution of Mo by V results in a decrease of reaction rate rMA and rO2 and a more strongly reduced oxidation of methacrylic acid, as shown in Figures 7, 10, and 9, respectively. The selectivity pattern (Figure 11) reveals for the catalysts free from cesium a remarkable reduction of byproduct formation via both routes, the parallel oxidation of methacrolein and the consecutive oxidation of methacrylic acid. For the pair Cs1V0/Cs1V1 the reduction mainly concerns the consecutive oxidation, while the rate of the parallel oxidation was already reduced by the addition of cesium. In comparison to the monosubstituted Cs1V1, substitution of two Mo atoms leads to a slight decrease of the selectivity toward methacrylic acid due to an increase of both routes of byproduct formation. 4. Discussion As shown by eq 16 the state of oxidation of the catalyst depends on the characteristic parameter of the MA 1 /( /2k1 - ν2k2) and on Mars-van Krevelen model kO 2 the parameter m, which determines the hindrance to reoxidation by methacrolein adsorption.
[
ΦMA ) 1 +
(1/2k1 - ν2k2) MA kO 2
]
pMA 1 n1 pO 2 1 + pm MA
-1
(16)
MA 1 Based on the figures of Table 3, the ratio kO /( /2k1 2 ν2k2) and the hindrance factor 1/(1 + pm MA) at pMA ) 2000 Pa (in brackets) are compared for all investigated catalysts in Table 4. At the V0 level the redox parameter is scarcely affected by Cs salification. However, at the V1 level a significant decrease of this ratio is observed. The strong effect of methacrolein adsorption on reoxidation found
V0
V1
V2
11.7 (0.057) 9.5 (0.016)
2.1 (0.106) 3.0 (0.106)
1.0 (0.229)
for the catalyst Cs0V0 is drastically reduced by both the formation of the Cs salt and the substitution of Mo by V. This reduction of the degree of adsorption may be due to the decrease of the catalysts acidity caused by both components Cs (Komaya and Misono, 1983) and V (Serwicka et al., 1991). Also the increase of selectivity toward methacrylic acid by Cs salification and by the substitution of Mo by V as shown in Figure 11 may be due to that change of adsorption properties. Reports of other authors on the effect of Cs salification are scarce. Mizuno et al. (1985) studied separately the rate of catalyst reduction by CO and the rate of catalyst reoxidation. They found that Cs salification leads to a reduction of both reaction rates. This is partly in agreement with our results as the reaction rate constants of methacrolein and methacrylic acid oxidation are also reduced if Cs1V0 and Cs0V0 are compared. However, it must be kept in mind that the reaction conditions of these compared studies are rather different. Ai (1981), Nakamura and Ichihashi (1980), and Staroverova et al. (1986) have reported an increased rate of methacrolein conversion and also increased selectivities toward methacrylic acid by Cs salification, in agreement with our results. However, the total salification leads to a drastic reduction of the yield of methacrylic acid. There exists an optimum of catalytic activity at a certain Brønsted acidity, indicating two competing effects. On one hand, a certain acidity is necessary in order to activate methacrolein for the nucleophilic attack of a bridging oxygen atom. On the other hand, a too strong acidity as in the case of the free heteropolyacid leads to a pronounced adsorption of methacrolein, which causes a reduction of the rate of reoxidation as discussed above. In contrast to the effect of cesium, the substitution of Mo by V causes a drastic decrease of the redox paramMA 1 /( /2k1 - ν2k2). Substitution of two Mo atoms eter kO 2 leads to a further reduction of this ratio. The remarkable reduction of the reaction rate constant of reoxidation caused by the substitution of Mo by V, Table 3, is in line with TPO experiments of Akimoto et al. (1984), which have shown an increased onset temperature of reoxidation when Mo was substituted by V. The same study has shown that within the series of V-containing heteropolyacids the monovanadium compound exhibits a maximum of reducibility, also in line with the results of this work. Substitution of two Mo atoms by V leads to a further decrease of the rate constant of reoxidation, in agreement with the reoxidation experiments of Akimoto et al. The role of vanadium has been already discussed elsewhere (Deusser et al., 1996). However, it should be noticed here that several studies have been published which show the existence of vanadyl cations within the catalyst structure after the catalytic reaction or even after dehydratation only. The process of the vanadium atoms leaving the keggin unit was examined by in-situ XRD (Ilkenhans et al., 1995), by in-situ DRIFTS (Herzog et al., 1997), by V4+-ESR comparing spectra of H4[PMo11-
3236 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
VO40]‚13-14H2O and H[VO(H2O)5][PMo12O40] (Cadot et al., 1994), and by IR spectroscopy (Cadot et al., 1993). The ESR study of Cadot et al. (1993) as well as insitu UV-vis studies of Schlo¨gl (1997) show that vanadium is present in the V(IV) state even in the initial compound and to an elevated extent in the dehydrated forms of the catalyst. This result could be an explanation for the lowered catalytic activity, particularly if one keeps in mind that V5+ acts as an electron sink because of its higher electron affinity (Jansen et al., 1994). This localization of electrons may lead to a weakening of the metal-oxygen bond, which for its part causes an easier release of oxygen atoms, as discussed by Bru¨ckmann et al. (1993). This hypothesis explains both the rather high reaction rate constants for the methacrolein oxidation and the very low rate of reoxidation, which is finally the reason for the lower catalytic activity of the vanadium-containing catalysts. Acknowledgment The financial support by the Bundesministerium fu¨r Bildung und Forschung (BMBF) is gratefully acknowledged. Nomenclature Cs0V0 ) H3[PMo12O40] Cs0V1 ) H4[PVMo11O40] Cs1V0 ) CsH2[PMo12O40] Cs1V1 ) CsH3[PVMo11O40] Cs1V2 ) CsH4[PV2Mo11O40] ki ) reaction rate constant, mol/(h kgcat Pa) MA ) methacrolein MAA ) methacrylic acid m ) reaction order with respect to methacrolein in the hindrance factor, ) dimensionless mcat ) catalyst mass, kg ni ) reaction order with respect to oxygen of site i, dimensionless pi ) partial pressure of component i, Pa ri ) reaction rate of component i, mol/(h kgcat) SMAA ) selectivity toward methacrylic acid, % TPO ) temperature-programmed oxidation Greek Letters Φi ) oxidation state of site i, dimensionless νi ) stoichiometric coefficient for reaction i, dimensionless
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Received for review November 13, 1997 Revised manuscript received May 5, 1998 Accepted May 5, 1998 IE970799W
