Hydrocarbon oxidation isomerization steps

In 1968 Fish [119] proposed an alkylperoxy isomerization theory that explains the temperature dependence of the ignition delays and the complex spectrum of products that can be obtained in hydrocarbon oxidation. Once formed the ROO radical can under go an internal isomerization. Cleavage of the resulting R OOH (or QOOH) radical is a chain propagating step leading to generation of a number of different products and an OH radical. Alternatively at low temperatures the addition of O2 can compete with unimolecular dissociation. 

Peroxyl radicals can undergo various reactions, e.g., hydrogen abstraction, isomerization, decay, and addition to a double bond. Chain propagation in oxidized aliphatic, alkyl-aromatic, alicyclic hydrocarbons, and olefins with weak C—H bonds near the double bond proceeds according to the following reaction as a limiting step of the chain process [2 15] ... 

Below the temperatures for the NTC regime, the peroxy radical (ROO) may be involved in a chain-branching sequence of reactions that is responsible for the positive temperature dependence. The oxidation rate varies significantly between different hydrocarbons or hydrocarbon isomers, depending on their structure. The first step is an internal isomerization,... 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

Synthetic zeolites have gained importance as industrial catalysts for cracking and isomerization processes, because of their unique pore structures, which allow the shape-selective conversion of hydrocarbons, combined with their surface acidity, which makes them active for acid-catalyzed reactions. Many attempts have been made to introduce redox-active TMI into zeolite structures to create catalytic activity for the selective oxidation and ammoxidation of hydrocarbons as well as for SCR of nitrogen oxides in effluent gases (69-71). In particular, ZSM-5 doped with Fe ions has attracted attention since the surprising discovery of Panov et al. (72) that these materials catalyze the one-step selective oxidation of benzene to phenol... 

In the catalytic oxidation of aliphatic hydrocarbons at atmospheric pressure, the oxidation reactions are probably stepwise, each successive step occurring with greater ease. The type of reaction for the succesave steps is the same, and when a carbon compound containing oxygen is oxidized further, it is the hydrogen atoms joined to the carbon atoms already in combination with oxjgen that ar attacked. Thus, the point of first attack continues to be the point for successive attacks. This has been clearly shown for the case of the oxidation of the isomeric octanes, where... 

In solution of perchlorates and tosylates in acetic acid, the primary products of electro-oxidation of aromatic hydrocarbons are shown to be benzylic acetates, i.e., substitution occurs preferentially in the side chain. A large, primary isotope effect, ku/kjy = 2.6, provided strong evidence that in this type of substitution reaction the rate-determining step involved the loss of a proton.Further refinement of the mechanism correlated isomeric product ratios to the distribution of positive charge density in the intermediate cation radical. 

Oxides of Cr, Mo, and W are usually used for catalysts as mixed oxides with other oxides such as alumina and silica which are prepared by coprecipitation, impregnation, etc. They are seldom put to practical use as simple oxides. Principal reactions catalyzed by these oxides, unlike those observed for silica-alumina or zeolites, often involve redox-type reaction steps, and during these steps reaction intermediates having covalent carbon-metal bonds are formed. Examples of those reactions are dehydrogeneration, hydrogenation and skeletal isomerization of hydrocarbons, and polymerization of olefms, as well as metathesis of olefins and hydrodesulfurization. Therefore, acid-base properties of catalysts usually play secondary roles in catalysts. 

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