Oxidation of Propene to Acrolein

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 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/... 

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... 

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...

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... 

Molybdenum(VI)-oxo complexes intervene as reactive species in the selective allylic oxidation of propene to acrolein in the gas phase over bismuth molybdate catalysts at high temperatures (>300 In industrial processes, selectivities in acrolein reaching 90% can be obtained... 

The oxidation of propene to acrolein has been applied in industry since 1958, when Shell introduced a gas-phase oxidation based on a Cu20/SiC/l2 catalyst system. This process made acrolein a commodity product. A more efficient technology, still state-of-the-art, was subsequently developed by Standard Oil of Ohio (from 1957 onward), using bismuth molybdate and bismuth phosphatecatalysts... 

The allylic oxidation of propene to acrolein over a CU2O catalyst was first reported in 1948 by workers from the Shell Development Company [108]. 

The reaction pattern includes the formation of PO, its consecutive isomerization to propanal, acetone and ally alcohol on acidic sites and combustion [43aj. Propanal and acrolein are also primary products. The formation of lower alkanes, alkenes, acetaldehyde and methanol results from cracking and oxidative C—C bond cleavage of propene and products. Additional side-reactions may occur in the gas phase, including radical-type oxidation of propene to acrolein, hexadiene and other byproducts. Alkyl dioxanes and alkyl dioxolanes may form via dimerization reactions of PO on acidic catalysts. Indeed, major by-products are heavy compounds that... 

Various catalysts based on molybdates have been used both for selective oxidation of propene to acrolein... 

The allyl radical, CsHs, is an intermediate in the selective oxidation of propene to acrolein with oxygen, catalysed by bismuth molybdate and iron antimonate [69,91]... 

H. Redlingshofer, O. Krocher, W. Bock, K. Huthmacher, G. Emig, Catalytic wall reactor as a tool for isothermal investigations in the heterogeneously catalyzed oxidation of propene to acrolein, Ind. Eng. Chem. Res. 41 (2002) 1445. 

Alkene oxidation over transition metal exchanged zeolites has been of recent interest. Yu and Kevan have studied the partial oxidation of propene to acrolein over Cu2+ and Cu2+/alkali-alkaline earth exchanged zeolites.33 In both... 

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