Mars and van Krevelen mechanism

In many of these reactions a lattice constituent — oxygen or hydrogen — is brought into reacting molecules and the lattice defect is subsequently removed by a reaction with another reaction component, or by a reaction with another reactive centre of the same molecule. This is the so-called Mars and van Krevelen mechanism. With some oxides peroxide groups are formed on the surface (from adsorbed oxygen molecules). Radicals may form at high temperatures (1000 K). 

The type (2) reactions are frequently of a very high selectivity, which is widely exploited by industry (about 20% of all chemicals are produced by oxidation reactions). The type (2) reactions proceed by the so called Mars and Van Krevelen mechanism [117], which is schematically shown for an oxidation reaction in Fig. 4.55. 

Scheme 5.2. Illustration of Mars and van Krevelen Mechanism with incorporation of gas phase oxygen into one phase and transfer to a separate surface phase for reaction with propene [114].

In most studies it has been stated that the hydrocarbon is oxidized by lattice oxygen of the oxidized form of the catalyst, KO. Subsequently, the reaction of the reduced form, K, with 0 regenerates the initial state according to the well-known Mars and van Krevelen mechanism (32) ... 

V. Oxidation catalysts have to be considered with a dynamical view under reaction conditions. This is related to the Mars and van Krevelen mechanism which involves a redox mechanism and also to the mobility of the oxide lattice. This dynamical phenomenon results in the wetting effect observed under catalytic reaction conditions for multicomponent and supported oxide catalysts [46, 47]. It follows that for many catalysts a certain time on stream is necessary before the catalyst reaches its steady state. It is frequent that in an industrial plant a... 

The kinetics of the ODH of n-butane has been investigated for unpromoted and cesium promoted a-NiMoOa catalysts. The reaction rates of dehydrogenation products as functions of the butane and oxygen partial pressures are described by a kinetic model based on the Mars and van Krevelen mechanism. The effects of Cs on the kinetic parameters can be interpreted on the basis of recently published results concerning the properties of those catalysts. 

The kinetic studies found in literature (cf Introduction) suggest that this reaction proceeds via a Mars and van Krevelen mechanism [18] and then the following steps are involved ... 

The Mars and van Krevelen mechanism for oxidation reactions used for a selective reduction reaction - influence of surface OH-groups on the selectivity. 

In this paper the Mars and van Krevelen mechanism for selective oxidation reactions is used to describe a selective reduction reaction. It will be shown that the reduction of nitrobenzene to nitrosobenzene via this mechanism is possible over various transition metal oxides. It will also be demonstrated that surface hydroxyl groups are the most important hydrogen source for the unwanted production of aniline. The selectivity of the reaction can be influenced by the number of surface hydroxyl groups. 

This reaction (2) is believed to follow the Mars and van Krevelen mechanism... 

The results presented in this paper indicate that reduction of nitrobenzene to nitrosobenzene proceeds via mechanism 1. The first argument is based on a simple comparison of oxides only those oxides which tolerate oxygen vacancies (needed for Mars and van Krevelen mechanism) are active. Secondly, production of nitrosobenzene and aniline is accompanied always with formation of CO and COj (not shown in the figures). When using 0 -labeled y-FejOj as a catalyst for the reduction of nitrobenzene, the isotope O appears both in CO and CO2. This means that at least a part of the oxidation of nitrobenzene is performed with lattice oxygen. In this way oxygen vacancies are created and selective reduction of nitrobenzene via the Mars and van Krevelen mechanism is possible. 

It can be concluded from the results presented above that reduction of nitrobenzene to nitrosobenzene is occurring via the Mars and van Krevelen mechanism. Reduction to aniline always happens in the first stages of the reaction, but it is not sure by which mechanism this reduction proceeds. In any case, surface hydroxyl groups are the most important hydrogen source. 

The most widely accepted mechanism describing the interaction between a hydrocarbon (R), molecular oxygen, and reducible metal oxide surface is the Mars and van Krevelen mechanism [13]. In this mechanism, the cationic active site oxidizes the reactant R creating lattice oxygen Ol vacancy , and being reduced... 

Pig. 5.3 Conversion of propylene into acroleine on bismuth molybdate following the Mars and Van Krevelen mechanism (from Ref. [4])... 

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