Propene to acrolein

1 Propene to acrolein. Hildenbrand and Lintz87,88 have used solid electrolyte potentiometry to study the effect of the phase composition of a copper oxide catalyst on the selectivity and yield of acrolein during the partial oxidation of propene in the temperature range of 420-510°C. Potentiometric techniques were used to determine the catalyst oxygen activity, and hence the stable copper phase, under working conditions. Hildenbrand and Lintz used kinetic measurements to confirm that the thermodynamically stable phase had been formed (it is known that propene is totally oxidised over CuO but partially oxidised over ). 

The results of Hildenbrand and Lintz showed good quantitative agreement with previous kinetic work of Riekert and Greger.84,85,89 Reaction rate measurements were indicative of which copper phase was present this phase corresponding to the thermodynamically favoured phase. Furthermore, hysteresis observed in the reaction rate data was also observed in the oxygen activity measurements as in other SEP work on oxides.35,86 

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Transition metal oxides or their combinations with metal oxides from the lower row 5 a elements were found to be effective catalysts for the oxidation of propene to acrolein. Examples of commercially used catalysts are supported CuO (used in the Shell process) and Bi203/Mo03 (used in the Sohio process). In both processes, the reaction is carried out at temperature and pressure ranges of 300-360°C and 1-2 atmospheres. In the Sohio process, a mixture of propylene, air, and steam is introduced to the reactor. The hot effluent is quenched to cool the product mixture and to remove the gases. Acrylic acid, a by-product from the oxidation reaction, is separated in a stripping tower where the acrolein-acetaldehyde mixture enters as an overhead stream. Acrolein is then separated from acetaldehyde in a solvent extraction tower. Finally, acrolein is distilled and the solvent recycled. 

Beneficial Micro Reactor Properties for the Oxidation of Propene to Acrolein... 

The CVD catalyst exhibits good catalytic performance for the selective oxidation/ammoxida-tion of propene as shown in Table 8.5. Propene is converted selectively to acrolein (major) and acrylonitrile (minor) in the presence of NH3, whereas cracking to CxHy and complete oxidation to C02 proceeds under the propene+02 reaction conditions without NH3. The difference is obvious. HZ has no catalytic activity for the selective oxidation. A conventional impregnation Re/HZ catalyst and a physically mixed Re/HZ catalyst are not selective for the reaction (Table 8.5). Note that NH3 opened a reaction path to convert propene to acrolein. Catalysts prepared by impregnation and physical mixing methods also catalyzed the reaction but the selectivity was much lower than that for the CVD catalyst. Other zeolites are much less effective as supports for ReOx species in the selective oxidation because active Re clusters cannot be produced effectively in the pores of those zeolites, probably owing to its inappropriate pore structure and acidity. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

The oxidation of propene to acrolein has been one of the most studied selective oxidation reaction. The catalysts used are usually pure bismuth molybdates owing to the fact that these phases are present in industrial catalysts and that they exhibit rather good catalytic properties (1). However the industrial catalysts also contain bivalent cation molybdates like cobalt, iron and nickel molybdates, the presence of which improves both the activity and the selectivity of the catdysts (2,3). This improvement of performances for a mixture of phases with respect to each phase component, designated synergy effect, has recently been attributed to a support effect of the bivalent cation molybdate on the bismuth molybdate (4) or to a synergy effect due to remote control (5) or to more or less strong interaction between phases (6). However, this was proposed only in view of kinetic data obtained on a prepared supported catalyst. 

Selective oxidation of propene to acrolein was carried out in a dynamic differential microreactor containing 40 to 60 mg of catalyst as described previously (12). Reaction conditions were as follows propene/02/N2 (diluting gas) = 1/1.69/5 total flow rate 7.2 dm. h-i total pressure 10 Pa and reaction temperature 380 °C. 

The oxidation of propene to acrolein has received much attention for several reasons. Firstly, the process is of industrial importance in itself, and it is also a suitable model reaction for the even more important, but at the same time more complicated, ammoxidation. Secondly, propene oxidation is, in many aspects, representative of that of a class of olefins which possesses allylic methyl groups. Last, but not least, the allylic oxidation is a very successful example of selective catalysis, for which several effective metal oxide systems have been discovered. The subject has therefore attracted much interest from the fundamental point of view. 

The conversion of isobutene to methacrolein is closely related to the selective oxidation of propene to acrolein and demands similar catalysts. It has been verified that the same mechanism applies, involving a symmetrical allylic intermediate, viz. 

Most experiments concern the application of labelled gas phase oxygen in reaction mixtures, while only in a few studies has labelling of the solid phase been used. Catalysts that have received particular attention are the bismuth molybdates and the antimonates of U, Fe and Sn, all very selective catalysts for the oxidation of propene to acrolein and similar allylic oxidations. 

The amount of Mo5 on the surface of Mo- Ti—O and Mo—Te—O catalysts has been assessed with ESR techniques by Akimoto and Echigoya [13,15,17] and Andrushkevich et al. [27]. These workers find a strong correlation between the maximum intensity of the Mo5 signal with maximum activity in the oxidation of propene to acrolein (at 8 at. % Te) and conversion of butadiene to maleic anhydride (75 at. % Ti). 

Oxo-metal complexes also intervene as active species in the heterogeneous gas-phase oxidation of hydrocarbons over metal oxide or mixed metal oxide catalysts at high temperatures. Characteristic examples are the bismuth molybdate-catalyzed oxidation of propene to acrolein and the V205-catalyzed oxidation of benzene to maleic anhydride (equations 17 and 18).SJ... 

Anaerobic Oxidation of Propene to Acrolein in a CFBR Reactor... 

Elf Atochem (now Arkema) and Du Pont have claimed a cycle process for the oxidation of propene to acrolein [70a]. In a first transport-bed reactor (a riser, where the catalyst is transported upwards by the gas) propene is put in contact with the catalyst, a Bi/Mo/W/... 

Similarly, cobaltic and argentic ion have been generated by anodic oxidation of cobaltous and argentous ion, respectively, and used for the oxidation of methyl-substituted aromatic hydrocarbons to aldehydes 18a Electrogenerated mercuric ion can be used for the conversion of propene to acrolein 18al ... 

Bi molybdates catalyst selective oxidation - for instance propene to acrolein (mostly supported)... 

Bismuth molybdate + promoter oxides mixed molybdates propene to acrolein... 

Some catalyst activation processes are extremely important this is the case for oxides used as catalysts and supports (AI2O3, SiC>2, TiC>2, ZrC>2, silica-aluminas), and zeolites. Extremely elaborate procedures are used. This concerns bulk, not supported systems, and is dealt with in Section A.2.1. The case ofSiC>2 mixed with active phases (e.g. in oxidation) has little relevance to the subject of the present section, as it seems that SiOj does not play the role of a real support, but rather that of a diluent or spacer. An electron microscopy study coupled with microanalysis on a typical oxidation catalyst (propene to acrolein) shows that only a small fraction of the active phases is attached to silica or is situated in its immediate proximity [69]. There are not many cases... 

The selective oxidation of propene to acrolein on supported vanadia catalysts was recently investigated by combined Raman, IR, and UV-vis DR spectroscopies (Zhao and Wachs, 2006). The surface vanadia species became more reduced under these reaction conditions as compared to those of alkane ODH (vide supra) because of the greater reducing power of alkenes relative to alkanes. Consequently, the reaction rates were dependent on the O2 partial pressures, because the surface vanadia sites were... 

Figure 2 Selectivity at 30% conversion for the reactions indicated as a function ofD°H C-H(reactant) - D°HC-h or c-c (product). 1 ethylbenzene to styrene 2. 1-butene to 1, 3-butadiene 3. toluene to benzoic acid 4. acrolein to acrylic acid 5. ethane to enthylene 6. n-butane to maleic anhydride 7. benzene to phenol 8. toluene to benzaldehyde 9. propene to acrolein 10. 1-butene to 2-butanone 11. isobutene to isobutene 12. methanol to formaldehyde 13. methacrolein to methacyclin acid 14. propane to propene 15. ethanol to acetaldehyde 16. isobutene to methacrolein 17. n-butane to butene 18. benzene to maleic anhydride 19. propane to acrolein 20. methane to ethane 21. ethane to acetaldehyde, 22. isobutane to methacrylic acid 23. methane to formaldehyde 24. isobutane to methacrolein.

In another example, a complex multi-component BiMoCoFeO catalyst used in the partial oxidation of propene to acrolein was characterized by Mossbauer spectroscopy. This example has been chosen because it provides a good demonstration of the high efficiency of Mossbauer spectroscopy for the characterization of working catalysts (181,182). 

Bismuth molybdates having a Bi/Mo ratio in the range of 0.67 2.0 catalyze the selective oxidation of propene to acrolein, and the ammoxidation of propene to acrylonitrile (equations 5 and 6). Both reactions proceed through an aUyhc intermediate. Three typical active phases o -Bi2Mo30i2,... 

Figure 1 Mechanism for the selective oxidation of propene to acrolein over bismuth molybdate catalysts...

The classical selective oxidation catalysts for propene to acrolein synthesis are mixed oxide catalysts such as bismuth molybdate. Despite most of these catalysts being crystalline and exhibiting long-range order, XAS has the advantage of being element specific. It can determine oxidation states and can also directly probe very low concentrations of one metal component in a matrix. 

One-step partial oxidation of propane to acrylic acid (an essential chemical widely used for the production of esters, polyesters, amides, anilides, etc.) has been investigated so far on three types of catalysts, namely, vanadium phosphorus oxides, heteropolycompounds and, more successfully, on mixed metal oxides. The active catalysts generally consist of Mo and V elements, which are also found in catalysts used for the oxidation of propene to acrolein and that of acrolein to acrylic acid. 

Based on such studies, it has been concluded that the lateral facets are active for the oxidation of methane to formaldahyde [10] and the oxidative ammonalysis of toluene [7], while the (010) facet is active for the conversion of methanol to formalahyde [1]. Studies of the oxidation of propene to acrolein illustrate that it is not always easy to relate overall activities or selectivities to the presence of a single face [3, 5, 8, 9]. Since the overall reaction is composed of several elementary steps, it is possible that different steps occur on different facets. For example, it has been proposed that the mechanism for the oxidation of propene to acrolein begins with the activation to an allyl intermediate on a lateral facet and ends with the addition of O on a basal facet [5]. The (210) facet, which is thought to consist of terraces with (010) character and ledges with (100) character, should be able to perform both elementary steps. This explanation has been used to rationalize the observation that the (210) surface is especially active for the conversion of propene of acrolein [9]. Using similar... 

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