The Mechanism of Catalytic Oxidation

The discussion of the mechanism of catalytic oxidation falls into two extreme cases. (1) A pure auto-oxidation can be postulated wherein the oxygen is activated by the platinum and a peroxide intermediate is formed which decomposes to yield an aldehyde and hydrogen peroxide. 

A similar mechanism was formulated for the catalytic oxidation of hydrocarbons and for the photo-sensitized oxidation of 2-propanol. (2) A pure dehydrogenation can be depicted in which the platinum cleaves the hydrogen from the substrate alcohol. 

In a second reaction, the activated hydrogen (taken up from the catalyst) is oxidized with molecular oxygen. According to Macrae, this second step proceeds through hydrogen peroxide (as an intermediate), the existence of which was demonstrated by the formation of cerium peroxide when the reaction was carried out in the presence of cerium (III) hydroxide. The hydrogen peroxide produced is rapidly decomposed by the catalyst. 

At this point, there is at hand for the catalyst no hydrogen capable of dehydrogenation, whereas it is available for alcohol and aldehyde (but not for acetic acid), and the catalyst assumes the potential of the remaining molecular oxj gen present. These observations support the dehydrogenation mechanism for catalytic oxidation. 

Rottenberg and Baertschi have carried out oxidation experiments in the presence of the isotope Ethanol was oxidized to acetic acid in two experiments. In the first, the oxygen used was labeled with and, in the second, the solvent water contained H20 . In the first case, only 5% of the was found in the acetic acid and, in the second, 80 to 90% of the 0 was removed from the water. Although these results could be explained by the dehydrogenation mechanism, they are not conclusive, since the isotope distribution can also be explained through isotope exchange of the intermediate acetaldehyde, as it is known that this exchange is very rapid. 

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It is interesting that this mechanism, which was suggested during one of the early studies of tin-antimony oxides, has received so little subsequent attention, since it is a process that could be applied to some of the mechanisms of catalytic oxidation proposed in later studies. 

Catalytic oxidative transformations of lower alkanes attract the attention as possible ways to transfer these substances into more suitable chemicals - olefins and oxygenates (alcohols, aldehydes, acids, etc.) - and to involve them into the industrial use as raw materials for chemical and petrochemical synthesis. However, the yields of desirable products reached up to date are not sufficiently high. The progress in the studies of intrinsic mechanism of catalytic partial oxidation of lower alkanes is not sustainable either. We believe that these two facts are correlated and that the analysis we performed in the present work can brighten up some important details of the mechanism of catalytic oxidation of lower alkanes. ... 

In the oxidation of CH4 and CjHg, no inhibition by the reactant is operative, and initial conversion is higher near the reactor inlet, where favourable conditions for ignition are stabilized. This relationship between dynamic behaviour and adsortion-desorption properties, finds support in the literature. Site competition between hydrocarbons and oxygen has been recognized as a key point in the mechanism of catalytic oxidation [3] in the same study, a substantially different behaviour in catalytic combustion was reported for alkanes and ethylene, and this was related to the strong adsorption properties of the latter. 

Although many attempts to examine the mechanism of catalytic oxidation have been reported, the exact mechanism even for intensely studied reactions such as CO oxidation is not completely understood. However, certain general statements about p-type oxides and V2O5 deep... 

Considerable work has been expended on elucidating the mechanism of catalytic oxidation of hindered phenols. Reactions of this kind proved to be amenable to kinetic studies and structural work on isolated intermediates. Systems involving cobalt chelate complexes as catalysts have received much attention. In this section we discuss in some detail the most important mechanistic versions proposed thus far. 

Haber J, Machej T, Janik R, Rrysciak J, Sadowska H. On the Mechanism of Catalytic Oxidation of CH2CI2 on y-Al203 and its Oscillatory Behavior. Z Phys Chem 1996 197 97-112. 

A detailed account of the mechanisms of catalytic oxidation processes has been published. The mechanisms of the catalytic oxidation of terminal oleffns to methyl ketones and of the catalytic epoxidation of olefins with hydroperoxides have also been reviewed. The latter reactions are generally thought to proceed via direct attack of the substrate upon an electrophilic oxygen of a metal peroxo species rather than prior complexa-... 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

The complexity and inhomogenicity of catalytic sites of metals and metal oxides make it difficult to interpret the mechanism of catalytic reactions on solid surfaces. Investigations that may lead to a better characterization of adsorbed species on catalytic sites could add much to our understanding of heterogeneous catalysis. 

The transition metal based catalytic species derived from hydrogen peroxide or alkyl hydroperoxides are currently regarded as the most active oxidants for the majority of inorganic and organic substrates " An understanding of the mechanism of these processes is therefore a crucial point in the chemistry of catalytic oxidations. This knowledge allows one to predict not only the nature of the products in a given process, but also the stereochemical outcome in asymmetric reactions. 

Furthermore, it is likely that the mechanism of catalytic dehydrocyclizations, studied by Steiner (83) on Cr2O3 with and without additions of foreign oxides and on M0S2, will be better understood by applying the theory of electron defects and of space-charge layers. Also, it will be fruitful to use isotopes for such studies, as has been done by Winter (86,87). 

On the other hand, the rate of catalytic oxidation of CO is proportional to the specific surface area, as shown in Fig. 58. This dependence indicates ordinary heterogeneous catalysis. The linear dependence in Fig. 58 can also be explained on the basis of the redox mechanism, as both the rate of CO conversion and the rate of 02 conversion are proportional to surface area (Fig. 53). 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

The second CTL mechanism is luminescence from the excited species produced in the course of catalytic oxidation. One of the excited species, formaldehyde (HCHO), is produced during the catalytic oxidation of ethanol, and the reaction process is depicted schematically in Fig. 5. The HCHO is finally oxidized to CO2 and H2O in an atmosphere containing oxygen. 

As CTL emission is observed in the course of catalytic oxidation, we must consider the overall reaction process in order to describe the working mechanism of the CTL-based sensor. Figure 8 shows a schematic illustration of the catalyst layer to depict the simplified overall reaction processes involving CTL emission on the CTL-based gas sensor. 

Studies of the oxidation of organic sulfides with amino acid-derived ligands in acetonitrile revealed very little difference between the mechanism of their oxidation and that of halides, except for one major exception. Despite the fact that acid conditions are still required for the catalytic cycle, hydroxide or an equivalent is not produced in the catalytic cycle, so no proton is consumed [48], As a consequence, there is no requirement for maintenance of acid levels during a catalyzed reaction. Peroxo complexes of vanadium are well known to be potent insulin-mimetic compounds [49,50], Their efficacy arises, at least in part, from an oxidative mechanism that enhances insulin receptor activity, and possibly the activity of other protein tyrosine kinases activity [51]. With peroxovanadates, this is an irreversible function. Apparently, there is no direct effect on the function of the kinase, but rather there is inhibition of protein tyrosine phosphatase activity. The phosphatase regulates kinase activity by dephosphorylating the kinase. Oxidation of an active site thiol in the phosphatase prevents this down-regulation of kinase activity. Presumably, this sulfide oxidation proceeds by the process outlined above. 

Catalytic Oxidation of Carbon Monoxide. - This reaction has been used by several authors as a simple test reaction in the field of catalytic oxidation. Hirota et al.115 conclude from tracer experiments that this follows an oxidation-reduction mechanism in which lattice oxygen is used. In the mechanism proposed, two neighbouring (V=0) groups are successively reduced by CO and are then simultaneously reoxidized. 

In conclusion, the mechanism of catechol oxidation by the model compounds is very intricate, which obviously explains often contradictory literature reports on the catalytic behavior of copper(II) complexes. However, despite being sometimes controversial, studies on model compounds offer stimulating results, which improve our knowledge of the structure-activity relationships in natural systems. There is little doubt that the combination of distinct but complementary disci-... 

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