Propylene oxide reaction pathways

In summary, the total oxidation of propylene to C02 occurred at a higher rate than partial oxidation to propylene oxide and acetone total and partial oxidations occurred in parallel pathways. The existence of the parallel reaction pathways over Rh/Al203 suggest that the selective poisoning of total oxidation sites could be a promising approach to obtain high selectivity toward PO under high propylene conversion. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

In the investigation of hydrocarbon partial oxidation reactions the study of the factors that determine selectivity has been of paramount importance. In the past thirty years considerable work relevant to this topic has been carried out. However, there is yet no unified hypothesis to address this problem. In this paper we suggest that the primary reaction pathway in redox type reactions on oxides is determined by the structure of the adsorbed intermediate. When the hydrocarbon intermediate (R) is bonded through a metal oxygen bond (M-O-R) partial oxidation products are likely, but when the intermediate is bonded through a direct metal-carbon bond (M-R) total oxidation products are favored. Results on two redox systems are presented ethane oxidation on vanadium oxide and propylene oxidation on molybdenum oxide. 

Figure 8. Reaction pathway for partial oxidation of propylene on Au catalyst.

Reaction Pathways for Propylene Partial Oxidation on Au/Ti02 99... 

When the commodity chemical propylene oxide is heated to high temperature in the gas phase in a shock tube, unimolecular rearrangement reactions occur that generate the CsHgO isomers allyl alcohol, methyl vinyl edier, propanal, and acetone (Figure 15.9). Dubnikova and Lifshitz carried out a series of calculations to determine the mechanistic pathway(s) for each isomerization, with comparison of activation parameters to those determined from Arrhenius fits to experimental rate data to validate the theoretical protocol. 

Keulks et al. (32) have also concluded from the oxidation of 14C-labeled and unlabeled acrolein that carbon dioxide is formed almost exclusively from the further oxidation of acrolein. Thus, it can be seen that the initial step in the formation of carbon dioxide and the other side products of propylene oxidation is the formation of a symmetrical 7r-allyl intermediate. This 7r-allylic intermediate is responsible for both the selective and nonse-lective oxidation of propylene, the course of the overall reaction depending on the subsequent reaction pathway of the allylic species. 

A series of additional experiments was performed with a Ag(poly) foil [rather than a Au(poly)] foil to examine in more detail the nature of the reaction products of d6-PC with metallic Li, and possible substrate effects. According to data compiled in the literature (Table 3), all of these fragments, except mle = 4, are consistent with, albeit not unique to, ethylene oxide, e.g., acetaldehyde. No features could be identified for mle = 32 and 64, indicating that propylene oxide, if produced, yields signals too small to be detected. Furthermore, no differences were found between the peak shapes and temperatures obtained for these experiments and those observed using Au(poly) hence, the reaction pathway does not seem to be affected by the nature of the substrate. Based on the behavior found for BuOLi, for which the series of high temperature peaks are found in the range... 

Studies in experimental animals have demonstrated that propylene oxide is readily absorbed and effectively metabolized. Only a minor fraction of the compound is exhaled unchanged. The main metabolic pathways are enzyme-catalyzed reactions with glutathione and water. 

This generates opportunities to use new reaction pathways not feasible in conventional macroscale reactors. For example, Sadykov et al. utilize rapid thermal quenching in conjunction with short residence times in a microreactor to suppress undesired side reactions in propylene production by the oxidative dehydrogenation of propane. 

Several reaction mechanisms were proposed. One suggested pathway for propylene oxide polymerization pictures an initial coordination of the monomer with a cationically active center ... 

The partial oxidation of propene (PP) to propylene oxide over a Ag-Re catalyst immobilized in a porous ceramic tube membrane reactor was studied by a continuous forced flow system under a differential reactor condition. The steady state rate equations for the production of propylene oxide (PO) and carbon dioxide were separately proposed by two different reaction pathways as follows. 

Kinetic analysis of propane ammoxidation with V-Sb-0 catalysts has shown that the reaction proceeds through propylene as the key intermediate (130). The first step in the propane ammoxidation reaction is the oxidative dehydrogenation of propane to propylene. Essentially, all the products of the reaction derive from conversion of the propylene intermediate. The kinetic results also suggest that there is a lesser direct reaction pathway from propane to acrylonitrile. 

A radical pathway going through the allylic hydroperoxide to give allyl alcohol and propylene oxide cannot be the only pathway involved since the maximum epoxide selectivity would be only 50% by this route. DeRuiter [501] concludes that such a pathway is a minor one since allyl alcohol was stable under reaction conditions and its incorporation into oxidation reactions led to no increase in epoxide yields. Even a route such as (299)-(306) would not appear to give selectivities as high as are currently being reported in some systems [504]. Soviet workers describe the direct liquid phase oxidation of propylene to propylene oxide in 89% selectivity at 15% conversion [504], equation (308). If indeed radical pathways are involved. 

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