Propene allylic oxidation

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

The oxyhydration of propene to acetone occurs at a much lower temperature than the allylic oxidation and demands, in principle, the presence of excess steam. The reaction is initiated by addition of a proton from the catalyst surface and the acetone formation involves oxygen originating from water. 

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 allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

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. 

A similar Te02/HBr catalyst has been used in acetic acid for the oxidation of propene at ca. 120 °C to propene oxide and propene glycol via 1,2-diacetoxypropane.361,362 However, this reaction gives lower yields because of side allylic oxidation reactions. Oxidation of toluene under similar conditions (Te02/LiBr, 160 °C) results in the formation of methylbenzyl acetate mixtures rather than benzyl acetate as observed with Se02/LiBr catalyst (equation 133).363... 

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... 

Quite a surprising reaction has recently been reported [74]. With a catalyst of palladium metal on carbon in aqueous phase, propene is oxidized with oxygen to give acrylic acid, probably via allyl alcohol in a allylic-type oxidation (for allylic oxidation see Section 3.3.14). In the presence of chloride or oxidants the normal Wacker-type reaction product acetone arises. 

The allylic oxidation of propene typifies the so-called bimetallic heterogeneous catalysis [4], a terminus technicus to emphasize cooperative effects in catalytic conversions (for multicomponent homogeneous catalysis, see Section 3.1.5). Nevertheless, the SOHIO-type oxidation is included in this book because one can imagine a number of mechanistic implications on a molecular platform, too. Studies on organometallic model compounds and reactions are available in ref. [2]. 

For the commercial production of vinyl acetate, a procedure with a heterogeneous fixed-bed catalyst is exclusively applied today. The catalysts usually consist of palladium salts, mostly the acetate, or palladium metal together with alkali acetate supported on a carrier such as alumina, silica, or carbon without any additional oxidant. This process avoids the formation of larger amounts of by-products. Thus, from ethylene vinyl acetate and from propene, allyl acetate is obtained exclusively. 

Propene oxidation to acrolein is carried out commercially over a range of bismuth molybdate catalysts to which are added 3-4 additional metal oxides to boost the activity. The final catalysts are mixtures of binary and ternary oxides and some solid solutions. One feature is the ability of lattice oxygen to transfer readily at the reaction temperature between the multiple phases that make up this catalyst and to the reacting propene. Another key feature is that the initial point of activation of the propene is one of the methyl C-H bonds with the production of a surface allyl intermediate, hence the term allylic oxidation. 

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

Allylic oxidation (i) propene to acrolein or acrylic acid and (ii) isobutene to methacrolein or methacrylic acid the synthesis of the acids may be effected in a single step, but commercially a two-step process is used due to its better selectivity. 

Duma and Honicke were the first to report the successful use of N2O in propene epoxidation. A PO yield of 5% was obtained over silica-supported iron oxide catalysts promoted with Na ions [43bj. The pore shape and diameter of the support as well as iron oxide dispersion are crucial parameters in the reaction [43b,cj. Doping vdth alkali metal may also considerably affect the Fe dispersion, and favor epoxidation over allylic oxidation [43fj. Further modification by boron can also significantly enhance the catalytic performance of the K-doped FeO /SBA-lS catalyst [43gj. 

We applied our FT-IR technique also to selective oxidation catalysts [10]. We present here our results on an FT-IR study of the interaction and oxidative conversion of n-butenes over MgFe204, i.e. an active catalyst for butene oxy-dehydrogenation. The aim of the present work was to have information on the activation of olefins over allylic oxidation catalysts and to find information on the reasons why ferrites are good catalysts for allylic oxy-dehydrogenation of butenes but they are are not good catalysts for allylic oxidation of propene. 

Scheme 3. Mechanistic alternatives for allylic oxidation of olefins, e. g. propene, at a Mo03/Bi203 catalyst site.

The mechanism of the allyl oxidation of propene is explained in terms of a reaction cycle [19]. As shown in Scheme 8-6, propene and air do not react directly with one another. Instead, the propene initially forms a n complex A witii an Mo center of the bismuth molybdate catalyst. Hydrogen abstraction by an oxo oxygen atom on bismuth leads to formation of a hydroxyl group and a 7i-aUyl complex at Mo B, whereby one... 

Seiyama and co-workers reported that the acid-base properties of catalysts controlled the reaction paths by changing the electronic state (cationic or neutral) of the reaction intermediate. They studied the allylic oxidation of propene over various metal oxides and found the relation shown in Fig. 4.34. It is seen in this figure that the selec-... 

Fig. 4.34 Correlation between the selectivity of allylic oxidation of propene over metal oxide catalysts and the electronegativity of the metal ion. ...

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