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Ind. Eng. Chem. Res. 2006, 45, 8794-8800
Selective Oxidation of Methacrolein towards Methacrylic Acid on Mixed Oxide (Mo, V, W) Catalysts. Part 1. Studies on Kinetics Harald Bo1 hnke,† Johann Gaube,*,‡ and Jochen Petzoldt† Ernst-Berl-Institut fu¨r Technische Chemie und Makromolekulare Chemie, Technische UniVersita¨t Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany, and BASF AG Ludwigshafen, Carl-Bosch Strasse, 67056 Ludwigshafen, Germany
Comprehensive kinetic studies of the oxidation of methacrolein on mixed oxide (Mo, V, W) catalysts have been undertaken in order to find ways of catalyst improvement to achieve a high selectivity toward methacrylic acid. Steady-state kinetic experiments were carried out in a differential recycle reactor that behaves like an ideal continuous stirred tank reactor (CSTR). A kinetic model based on the scheme of Mars-van Krevelen was developed for a fair representation of the reaction rates of methacrolein conversion, methacrylic acid formation, byproduct formation, and selectivity toward methacrylic acid. For a better understanding of the kinetics, transient experiments were carried out in an apparatus to render possible sorption studies as well as transient kinetic experiments, which are powerful tools to study independently both the oxidation of the aldehydes on the catalyst in the absence of oxygen and the reoxidation of the catalyst. Steady-state kinetic data and transient experiments agree well. It could be clearly shown that the conversion of methacrolein is mainly determined by the reoxidation of the catalyst and that the selective oxidation of the aldehydes and the consecutive oxidation of the acids occur on different domains of the catalyst. Introduction Mixed oxides mainly composed of molybdenum, vanadium, and tungsten oxides are excellent catalysts for the partial oxidation of acrolein (ACR), showing an exceptionally high selectivity toward acrylic acid and also an excellent long-term stability.1 Unfortunately Mo-V-W-oxide catalysts are less suitable for the partial oxidation of methacrolein (MAC) because of a considerably lower selectivity to methacrylic acid (MAA).2,3 Therefore, at present, the widely used heteropoly acids (HPA) are still the state-of-the-art catalysts for the partial oxidation of methacrolein. However, the long-term stability of these catalysts is unsatisfying. The development of improved catalysts is now focused on the modification of mixed oxide catalysts. This approach is mainly based on kinetic studies of the partial oxidation of both acrolein and methacrolein and for further interpretation also of transient experiments. Steady-state kinetic experiments for the oxidation of acrolein were carried out in a differential recycle reactor that behaves like an ideal CSTR (continuous stirred tank reactor).4,5 Transient experiments were performed in an apparatus designed to render possible sorption studies as well as transient kinetic experiments.4,6 Preliminary studies have shown that the rate of MAC oxidation on mixed oxide catalysts is 2. Above R ≈ 2, the order approaches 1. The strong influence of water, which is shown by steady-state experiments, could be confirmed directly by transient experiments presented in Figure 15. Kinetic Model The model is based on the scheme of Mars-van Krevelen, applied to the reaction scheme of the conversion of methacrolein to methacrylic acid and byproducts CO, CO2, CH3COOH, and water. Furthermore, the assumption is made that the oxidations of MAC and of MAA occur on different domains of the catalyst.
Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8799
Figure 18. Role of water in the reoxidation of the catalyst from Levy and De Groot.19
The characteristic parameter of the Mars-van Krevelen model, the degree of oxidation Φox, must be regarded as a formal parameter that may have different meanings for the oxidations of MAC and MAA because of different structures of the catalyst active in these reactions. Consequently, different degrees of oxidation are introduced for the conversion of MAC and MAA, Φ0x,MAC and Φ0x,MAA. Oxidation of MAA.
r3 ) k3pMAAΦox,MAA
(1)
rO2,MAA ) kO2,MAApOn32(1 + pHm32O)Φred,MAA ) -ν3r3
(2)
Φox,MAA + Φred,MAA ) 1
(3)
is the fraction of oxidized sites, and Φred,MAC is the fraction of blocked reduced sites. Φred,MAC is the fraction of blocked reduced sites. Then the active fraction of reduced sites is given by blocked Φred,MAC ) 1 - Φox,MAC - Φred,MAC
(7)
The latter one depends on pMAC expressed by blocked ) KpMACΦred,MAC Φred,MAC
(8)
according to the equilibrium of the following set of equations
ν3, Stoichiometric Coefficient of Oxygen Consumption. Φox,MAA results from eqs 1 and 2. Oxidation of MAC. MAC can react in parallel routes toward MAA (r1) and byproducts (r2). The rate r1 can be estimated by addition of r3 to the rate of the formation of MAA since rMAA ) r1 - r3. r2 is obtained via -rMAC ) r1 + r2 for the consumption of MAC. Figure 16 shows that, in the range 1.5 < pMAC < 5 kPa, r1 is nearly equal to rMAC; hence, in this range, the parallel reaction toward byproducts is negligible. For lower pMAC, a moderate formation of byproducts via the parallel route is stated. Obviously, another parallel route toward byproducts exists for pMAC > 6 kPa; see also Figures 3 and 4. This effect is discussed in detail in ref 11. With the exception of a very small range at low pMAC, the conversion of MAC is governed by the rate of reoxidation; thus, it is impossible to find out kinetic expressions for r1 and r2 valid in a wide range of pMAC. For low pMAC, a first order with respect to MAC is set. This assumption is strongly supported by transient kinetic studies of the oxidation of MAC on the oxidized catalyst, as presented in detail in part 2.22 The same consideration is valid for the oxidation of MAA so that also a first order with respect to MAA was set.
r1 ) k1pMACΦox,MAC r2 ) k2paMACΦox,MAC
(4) a
