Isoprene, partial oxidation

Figure 3.39 Partial oxidation of isoprene in a fixed-bed reactor and a micro reactor. The yield of citraconic anhydride is plotted as a function of reaction temperature and catalyst composition [27].

Methanol steam reforming Catalytic combustion Partial oxidation of isoprene... 

S. Walter, E. Joarmet, M. Schiel, I. Boullet, R. Philipps, M.A. Liauw, MicroChannel reactor for the partial oxidation of isoprene, in M. Matlosz, W. Ehrfeld, J.P. Baselt (Eds.), Proceedings of the 5th International Conference on Microreaction Engineering (IMRET 5), Springer, Berlin, 2001, p. 387. 

The catalysts were tested in isoprene, benzene and naphthalene hydrogenation reactions. The catalysts prepared by oxidation of the intermetallic at 703 K were found to be more active than those oxidized at room temperature. In isoprene hydrogenation the catalyst based on mischmetal has a larger activity than the other alloys and is comparable to the catalysts prepared by other routes. In benzene hydrogenation the Ni and Ce based catalysts prepared by coprecipitation were found the most active. In naphthalene hydrogenation the fully oxidized intermetallics are four times more active than the partially oxidized ones. In addition they are independent of hydrogen pressure and selective into decaline formation. 

Local emissions from specific plants were investigated by Holzinger et al. who have identified methanol, acetaldehyde, ethanol, acetone, acetic acid, isoprene, monoterpenes, toluene, and Cio-benzenes emitted from the Mediterranean helm oak (Quercus ilex L.) [36]. After flooding, ethanol and acetaldehyde were found to increase, indicating ethanol production under anoxic condition of the root system and the subsequent transport and partial oxidation to acetaldehyde inside the green leaves. Additionally, bursts of acetaldehyde with lower ethanol emission were also found in the case of the fast light/dark changes. The rapid formation of acet-... 

By far the most reports deal with the partial reduction of ZnO, used as support, and subsequent formation of Zn-containing intermetallic compovmds. In the case of Pd/ZnO, a catalyst used for the decomposition (83), the partial oxidation (84-89) and the steam reforming of methanol (90-94), the interaction between the supported Pd particles and the partial reduced support can lead to a mixture of ZnPd, ZnsPd2, and ZnPd2. When this catalyst is used for the hydrogenation of unsaturated hydrocarbons, such as butenes, isoprene, or crotonaldehyde, only the formation of ZnPd is observed (95-97). The same holds for reductive treatments of Pd/ZnO at temperatures above 400°C (Fig. 4). The supported catalyst Pd/ZnO is not the usual case, since normally a mixture of intermetallic compovmds and the originally supported transition metal is observed. 

Measurements of these relatively minor species will not only complete the budget of NO, but will also indicate if our understanding of the hydrocarbon oxidation schemes in the atmosphere is complete. The organic nitrates that completed the NO, budget in the example in Figure 9 arose primarily from the oxidation of the naturally emitted hydrocarbon, isoprene (2-methylbutadiene). To demonstrate the oxidation mechanisms believed to be involved in the production of multifunctional organic nitrates, a partial OH oxidation sequence for isoprene is discussed. The reaction pathways described are modeled closely to those described in reference 52 for propene. The first step in this oxidation is addition of the hydroxyl radical across a double bond. Subsequent addition of 02 results in the formation of a peroxy radical. With the two double bonds present in isoprene, there are four possible isomers, as shown in reactions 2-5 ... 

The data presented have important implications in the behavior of tropospheric nonanthropogenic ozone, aerosol, and other trace constituents. Observational and experimental data have been reported by Rip-perton et al. 20) indicating the natural synthesis of ozone in the troposphere. Considering this study, the ubiquitous presence of various terpenes (21), isoprene (22), and oxides of nitrogen (20) suggest that some ozone is synthesized in the lower troposphere by the reaction NO2 + a-pinene + hv. Conversely, the destruction of ozone in the troposphere is partially ascribed to reactions with the terpenes and intermediates of the photochemical mixture. 

Two possible scenarios can be envisaged for the structure of the hybrid material (see Fig. 11). The poly(ethylene oxide) block, albeit strongly interacting and partially penetrating, forms a pure PEO layer at the interface to the hydrophobic poly(isoprene) (Fig. 11, left-hand sketch) ( three-phase system). The other possibility is the complete dissolution of the PEO chains in the aluminosilicate, which results in the two-phase system depicted in the right-hand sketch of Fig. 11. Spin-diffusion NMR experiments showed that there appears to be no dynamic heterogeneity in the poly(ethylene oxide) chains, as would be expected for a three-phase system, giving rise to the conclusion that the hydrophilic... 

During a joint venture with Naarden International, Shell International developed a new route to citronellol, but this was never commercialized. The route starts with cyclodimerization of isoprene (1) to give 1,5-dimethylcycloocta-1,5-diene (137). Olefin metathesis with isobutene in the presence of a rhenium oxide catalyst produces 2,6-dimethylhepta-1,5-diene (138). The diene is hydroformylated to citronel-lal (139), which after hydrogenation produces citronellol (85) (169). Alternatively, the 1,5-dimethylcycloocta-1,5-diene can be partially hydrogenated and the monoene (140) then pyrolyzed to 2,6-dimethylocta-l,7-diene (141), which can be converted subsequently to citronellol in several steps (170). These processes are shown in Fig. 8.28. 

This does not mean that all sesquiterpene biosynthesis is independent of the 1-DXP pathway, however. In one plant system, quite the opposite has been found to be true. Bis-abolol oxides (85 and 86), matricin (81) and chamazulene (82) from chamomile flowers have been shown to contain two isoprene units derived from the 1-DXP pathway and one derived at least partially from the mevalonate pathway (Adam et al. 1999). A proposed explanation for this labelling pattern is shown in Figure 3.30. Here, sesquiterpene biosynthesis is thought to begin, unusually, in the plastids with the formation of IPP and GPP via... 

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