Acrolein formation

The proposed Re6 cluster (8) with terminal and bridged-oxygen atoms acts as a catalytic site for selective propene oxidation under a mixture of propene, Oz and NH3. When the Re6 catalyst is treated with propene and Oz at 673 K, the cluster is transformed back to the inactive [Re04] monomers (7), reversibly. This is the reason why the catalytic activity is lost in the absence of ammonia (Table 8.5). Note that NH3, which is not involved in the reaction equation for the acrolein formation (C3H6+02->CH2=CHCH0+H20) is a prerequisite for the catalytic reaction as it produces the active cluster structure under the catalytic reaction conditions. 

Molybdenum trioxide acrolein formation, 30 152 allyl iodide reactions, 30 150 azopropene reactions, 30 150 Molybdenyl compounds, M=0 bond, 31 125 N-Monoalkylation, aniline derivatives, 38 249-252... 

The formation of propene oxide as a side product of the acrolein formation or dimerization reactions is reported by many authors. Daniel et al. [95,96] demonstrated that propene oxide is formed by surface-initiated homogeneous reactions which may involve peroxy radical intermediates. The epoxidation is increased by a large void fraction in the catalyst bed or a large postcatalytic volume. In view of these results, the findings of Centola et al. [84] are understandable, as the wall of the empty reactor may have been sufficiently active to initiate the reaction. 

The exact mechanism of lattice oxygen incorporation and second hydrogen abstraction, and the precise sequence of elementary events is still a subject of speculation. Several authors assume that two distinct active sites are involved in the acrolein formation. The first, presumably a cation, participates in the formation of the initial allyl complex, while the second, which may contain a different cation and reactive oxygen anions, is the place where further hydrogen abstraction and oxygen incorporation take place. 

The main by-products of acrolein formation are carbon monoxide and carbon dioxide, as well as minor amounts of acrylic acid and lower aldehydes and acids. Combustion takes place both consecutive and parallel to the main reaction. Acrylic acid (in free or adsorbed form) is a possible intermediate in the acrolein combustion. Including this product, the following simplified scheme applies. 

The reactions in this scheme are first order with respect to the oxidized compound. Initial selectivities [kj(ki + k2)] of 90% and more are possible at 400—500°C. The decrease in selectivity at higher conversions is mainly due to acrolein combustion (k3/ki = 0.2—0.3). The activation energy of acrolein formation is approximately equal for all bismuth molybdates (18—20 kcal mol-1). 

The activity of Mo03 supported on a high surface silica carrier was studied by Vaghi et al. [331] using pulse and flow techniques at 400— 440°C. Oxidation activity and acrolein formation appear to be zero below 10 wt. % Mo03, but increase with the Mo03 content above 10%. The... 

Sn—P—O are intermediate. Comparisons with ordinary flow experiments (Table 8) reveals that much more acrolein is formed at normal flow conditions for Bi—Mo—O and Bi—W—O, while for the other catalysts the difference is small or similar for both acrolein formation and combustion. 

The one-stage conversion of propene to acrylic acid is much more difficult than the selective production of acrolein. The process is essentially a two-step process in which acrolein is the intermediate product. Further oxidation leads to acrylic acid. In fact, contrasting catalyst properties are required for these reaction steps. The acrylic acid production demands an acidic catalyst surface, while a basic, or only weakly acidic character is preferred for the selective acrolein formation. Therefore, enhanced combustion and by-product formation are unavoidable. 

An important distinction between dimerization and acrolein formation is that the selectivity of the former is evidently connected with a partially reduced state of the catalyst. It is commonly accepted, therefore, that cations like Bi3+, Sn4+, etc. play a role, presumably by adsorbing the allyl radical intermediate. Several authors assume that this is the case for allylic oxidation in general and that the role of a second oxide component is to promote dimerization byi stabilization of the allyl radical, or to direct the oxidation to aldehyde formation via a cationic allyl complex. Seiyama et al. [285] further suggest that the acidity of the promoting oxides is an important factor in this connection, and may, in part, explain why acidic oxides like Mo03 direct the oxidation to aldehydes, while basic compounds favour dimerization. 

The strong parallel with the acrolein formation initially suggested the idea that acrolein is a reaction intermediate in the ammoxidation, and can further react with ammonia and oxygen to form acrylonitrile. Although the ammoxidation of acrolein is indeed a very rapid reaction, it is generally accepted today that a direct reaction path to acrylonitrile predominates. The differences between both theories are very small, however, when one assumes that the ammoxidation of acrolein and propene involves the same reaction intermediate. Thus the various kinetic schemes proposed in the literature can be derived from the general scheme below by omitting the reaction steps (3), (4) and/or (5) and variation of the ratio between (2) and (3). 

Regarding the mechanism, it is generally accepted that, as with acrolein formation, a symmetrical allyl complex is formed (as already implied by the above scheme) for a number of catalysts, this has indeed been proved experimentally (for instance by Dozono et al. [103]). 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

A point common to all the models is that they are based upon a redox-type mechanism, in which the reoxidation of the catalyst is not a limiting factor. Corresponding, none of them employ the model expression of Mars and van Krevelen (37). On contrast newer works by Keulks (38,39) assume, at lower reaction temperatures, a limiting effect from the reoxidation which leads to a dependence on oxygen partial pressure for the acrolein formation and to a two to three-fold higher activation energy compared with the reaction at higher temperatures. 

Only two heterogeneous catalysts were evaluated Montmorillonite K-10 and SAC-13 (a Nafion catalyst), gave FFA conversions of 71% and 74% respectively after 4hrs at 180°C at a 2wt% loading, but suffered from the same issue of acrolein formation as the homogeneous acids. 

Clearly from the preceding discussion there are three separate steps involved in acrolein formation from propene, namely, abstraction of two hydrogen atoms... 

In the presence of the M0O3 component of the catalyst the radical species formed can react with lattice oxygen leading to acrolein formation as shown in Scheme 5.4. 

Scheme 5.5. Schematic of the active site for acrolein formation from propene over bismuth molybdate...

In summary, the available experimental evidence suggests that an adsorbed form of molecular oxygen is involved in partial oxidation while lattice oxygen is required for carbon dioxide production. This proposed mechanism is directly opposed to that generally accepted for propene oxidation over mixed oxide catalysts such as bismuth molybdate. In this case, lattice oxygen is responsible for acrolein formation while adsorbed oxygen results in complete combustion. This means that the fully oxidized phase is the selective catalyst while the reaction is first order with respect to alkene. 

By changing the impregnated element in the structure of MCM-41, we observe that caesium leads to the most selective catalyst to diglycerol while lanthanum leads to the less selective one (Fig. 3). Moreover if La and Mg lead to high activity, an important acrolein formation is also noticed so that the use of such catalysts must be avoided. The caesiiun additive was selected and Fig. 4 shows that the increase amount of Cs impregnated has also a positive efiect on the activity of the solid. 

I he formation of one acrolein molecule per one reduced V ion shows that the charge compensation of V in M0O3 takes place at the expence of the V° 0 complex formation. Some deviations from this ratio can be explained by acrolein formation on other centres. Thus, the catalyst reduction by ally alcohol is described by the Equation (4). This process consists of at least of two steps the surface one (7), which proceeds with participation of the surface oxygen ion, and bulk step (8), when the charge carriers diffuse to the bulk and reduce the ions ... 

Fig.6 shows the kinetic curves for the acrolein formation rate on the rhombic V-MoOg and for the change of ions content in the... 

Figure 6. Kinetics of the change of ions number in the rhombic phase of V-M0O3 and the rate at 525 K [CaHjOH] = 5.5x10 molec./cmf of acrolein formation...

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