Styrene formation

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

They observed the complete deactivation of the rhodium catalyst whether immobilized or not in the presence of free amines. When no amine was present, styrene formation was not observed. After 17 h of a reaction in which both catalysts were immobilized, the yield of the product, ethylbenzene, was 52%, again demonstrating the principle of enabling two otherwise incompatible catalysts to work concomitantly in order to achieve process intensification. 

Vinyl octanoate was obtained from TCI (Tokyo, Japan). All other chemicals with the exception of the zeolite beta are available from Sigma Aldrich. The synthesis of a particularly active modification of low-alumina zeolite beta has been described by us. Commercial material, available as samples from, for example, Zeolyst or Siidchemie can be used, but because of excessive acidity may result in up to 15 % of styrene formation. 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

New results of styrene formation over iron oxide single-crystal model catalysts were reported.326 In ultra-high-vacuum experiments with Fe304(lll) and a-Fe203(0001) films combined with batch reaction studies only Fe203 showed catalytic activity. The activity increased with increasing surface defect... 

In this regard, the above-suggested mechanism of ethylbenzene conjugated dehydrogenation with hydrogen peroxide presents a satisfactory explanation of styrene formation. Here the central place belongs to a new elementary reaction ... 

Oxidative amination of styrenes. Formation of p-aminostyrenes from a mixture of... 

As a result, a high inlet catalyst temperature is required. However, high temperature also increases the rates of nonselective thermal reactions and dealkylation reactions, which form benzene and toluene by-products. In particular, as temperature is increased, the rate of benzene formation increases significantly relative to the rate of styrene formation. This means there is an effective upper limit to the inlet temperature if high styrene selectivity is a required criterion. Reaction temperature is generally adjusted by changing either the steam temperature or the steam-to-oil ratio. 

Figure 12. Comparison of measured net rates of styrene formation with predicted formation rates for near-sooting. Conditions same as in Figure 2.

Styrene is presumably also formed in the environment from natural precursors, such as cinnamic acid. With pure cultures, the decarlxn lation of cinnamic acid has been demonstrated with numerous microorganisms, and recently a PenicUlium species was shown to synthesize styrene as a secondary metabolite [3]. An obvious pathway for styrene formation would be the transformation of phenylalanine to cinnamic acid, followed by decarboxylation to styrene. 

A process patent and subsequent publication by Fujita and cowoikers, disclosed an improved, large-scale synthesis of fingohmod. In particular, the problem of competing styrene formation that plagued the original synthesis was addressed. In this approach, a Friedel-Crafts acylation of phenylethyl bromide (8) with octanoyl chloride yielded ketone 9 (Scheme 2). Treatment of ketone 9 with sodium ethoxide affords the expected styrene product (10) however, in this case, styrene 10 can function as a Michael-type acceptor to generate the desired amino malonate product 11 in 55% yield (2 steps). Next, hydrogenolysis of the ketone with palladium on carbon in ethanol provided... 

The effects of hydrogen removal on the yield and selectivity for styrene formation will be discussed. 

The reported observations that (i) different, mainly oxide based compounds, which tend to coke deposition, are active for ODH of EB to ST, and that (ii) a characteristic induction period of several hours, during which the coke deposition occius, correlates with an increase of the catalytic activity [2], may indicate that the carbon deposited on the catalyst surface plays an important role in the styrene formation. It was also reported that amorphous carbon activated by oxidation is an active catalyst for the ODH of EB to ST [3]. On the other hand, great interest was paid to carbon nanotubes during last decade due to their stability at high temperatiu es, severe environments, and the possibility to modify them by metal introduction [4-6]. It may be assumed that pure carbon nanotubes, or nanotubes filled with Fe, could be active and stable catalysts for the ODH of EB to ST. In order to test their catalytic activity and to develop a deeper understanding of the relation between carbon structure and its catalytic activity, different types of coibons were used for the ODH of EB to ST. 

X = Br while the normal process is observed with X = Cl. The styrene formation has been explained by the mechanism of Scheme 12 and termed... 

Addition of Ce or Mo to Fe —Cr —K catalysts improves the selectivity. A relation has been reported between the activation energy for styrene formation and the electronegativity of the transition elements added, as shown in Fig. 4.31. The acid and base amounts measured correlated well with the electronegativity. According to this relation, Ce is the most effective for promoting dehydrogenation. It was suggested that Mo adjusts the activity at a moderate level to suppress the undesired formation of benzene and toluene. 

Fig. 4.31 Relation between the activation energies for styrene formation and the electronegativity of various transition metal oxides.

Besides, the chain-end reactions, initiation reactions can occasionally take place Depropagation - which leads to the monomer (styrene) formation. 

Poly(methyl methaciylate-co-styrene) 260-340 Methyl methacrylate, styrene and oligomers of styrene formation of oligomers of styrene strongly inhibited by presence of methyl methacrylate units 137... 

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