Selective oxidation over reactions

Selective oxidation of allylic and benzylic C-H bonds over aliphatic C-H bonds has been well reported. The selectivity can be attributed to the relative weak bond strength of allylic and benzylic C-H bonds. However, unlike simple oxidation reactions, selective oxidative coupling reactions of allylic and benzylic C-H bonds have only been recently disclosed. 

Vulpescu G. D., Ruitenbeek M., van Lieshout L. L., Correia L. A., Meyer D., Pex P. P. A. C. 2004. One-step selective oxidation over a Pd-based catalytic membrane Evaluation of the oxidation of benzene to phenol as a model reaction. Catalysis Communications 5 347-351. 

Xue, Z.-Y., Schrader, G. L., 1999, Transient FTIR Studies of the Reaction Pathway for n-Butane Selective Oxidation over Vanadyl Pyrophosphate, J. Catal. 184, 87. 

Vulpescu, G., Ruitenbeek, M., van Lieshout, L., et al. (2004). One-Step Selective Oxidation over a Pd-Based Catalytic Membrane Evaluation of the Oxidation of Benzene to Phenol as a Model Reaction, Catal. Common., 5, pp. 347-351. 

Finally, the results for the partial oxidation of propane are rather frustrating, as after two decades of intense study, the yields to acrylic acid have not been enhanced. In fact, the highest yield to acryUc acid in both the open and patent literature indicates a limit to the maximum yield which can he obtained, as specifically reported by Muller. This is a consequence of the Umitations of the reaction network in consecutive steps, with propylene as the primary reaction product, as indicated in Fig. 24.6. Accordingly, mayhe we should design catalysts by considering the model of -butane selective oxidation over VPO catalysts (Fig. 24.8), in which the olefinic intermediate was not desorhed and acrylic acid was directly formed as the primary reaction product. In this sense, new crystalline strucmres could be required in which active sites for propane activation and those for propylene oxidation were near enough to directly transform the propylene intermediate into acrylic... 

In the three-step process acetone first undergoes a Uquid-phase alkah-cataly2ed condensation to form diacetone alcohol. Many alkaU metal oxides, metal hydroxides (eg, sodium, barium, potassium, magnesium, and lanthanium), and anion-exchange resins are described in the Uterature as suitable catalysts. The selectivity to diacetone alcohol is typicaUy 90—95 wt % (64). In the second step diacetone alcohol is dehydrated to mesityl oxide over an acid catalyst such as phosphoric or sulfuric acid. The reaction takes place at 95—130°C and selectivity to mesityl oxide is 80—85 wt % (64). A one-step conversion of acetone to mesityl oxide is also possible. 

In the second section, unconverted hydrogen sulfide reacts with the produced sulfur dioxide over a bauxite catalyst in the Claus reactor. Normally more than one reactor is available. In the Super-Claus process (Figure 4-3), three reactors are used. The last reactor contains a selective oxidation catalyst of high efficiency. The reaction is slightly exothermic ... 

Closed symbols in Fig. 1 show the effect of reaction temperature on ammonia oxidation over CuO by heating with a conventional electric furnace. The reaction started at about 400 K and the conversion of NH3 became 1 at temperatures higher than 500 K. Fig. 1 also indicates that selectivity to N2 was high at low temperatures but it decreased as the temperature increased. Both N2O and NO increased instead of N2, except at 623 K, at which N2O decreases. NO was detected above 583 K, and it monotonously increased by the temperature. High reaction temperature seems to tend deeper oxidation to NOx. Considering that oxidation of N2 to N2O and NO is difficult in the tested temperature range. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

This discovery was quite unexpected, since iron oxide has been never reported as an active catalyst in either partial or full oxidation. The studies of two simplest reactions, i.e. O2 isotopic exchange and N2O decomposition, revealed a dramatic change of Fe properties in the ZSM-5 matrix compared to Fe203 [4]. Fe atoms lose their ability to activate O2 but gain remarkably in their ability to activate N2O. It gives rise to a great effect of the oxidant nature in the reaction of benzene oxidation over the FeZSM-5 zeolite (Table 1). Thus, in the presence of N2O benzene conversion is 27% at 623 K, while in the presence of O2 it is only 0.3% at 773 K. And what is more, there is a perfect change of the reaction route. Instead of selective phenol formation with... 

The oxidation is carried out over layers of platinum-rhodium catalyst and the reaction conditions are selected to favour reaction 1. Yields for the oxidation step are reported to... 

The effect of Bi promotion for the selective oxidation of 1-octanol using H202 as oxidant is reported in Table 2. Since decomposition of H202 by Platinum Group Metals is rapid, H202 is fed continuously into the reactor over 2 hours. The results obtained demonstrate that the presence of Bi203 as an additive within the reaction mixture displays no significant influence on catalyst activity. However, Bi promoted Pt/C catalysts, prepared by co-precipitation of... 

The analytical phase generally involves the use of very dilute solutions and a relatively high ratio of oxidant to substrate. Solutions of a concentration of 0.01 M to 0.001 M (in periodate ion) should be employed in an excess of two to three hundred percent (of oxidant) over the expected consumption, in order to elicit a valid value for the selective oxidation. This value can best be determined by timed measurements of the oxidant consumption, followed by the construction of a rate curve as previously described. If extensive overoxidation occurs, measures should be taken to minimize it, in order that the break in the curve may be recognized, and, thence, the true consumption of oxidant. After the reaction has, as far as possible, been brought under control, the analytical determination of certain simple reaction-products (such as total acid, formaldehyde, carbon dioxide, and ammonia) often aids in revealing what the reacting structures actually were. When possible, these values should be determined at timed intervals and be plotted as a rate curve. A very useful tool in this type of investigation, particularly when applied to carbohydrates, has been the polarimeter. With such preliminary information at hand, a structure can often be proposed, or the best conditions for a synthetic operation can be outlined. 

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