Ammoxidation mechanism

Matsuura [209] has proposed an ammoxidation mechanism similar to that for the formation of acrolein [see Sect. 2.2.2(d)(i)]. Ammonia is assumed to adsorb dissociatively on B-sites, followed by reaction with A-site oxygen, finally leading to the replacement of the A-site O2- by a NH2- species. The A-site is thus altered from a site which introduces oxygen into a site that introduces nitrogen in the allyl complex. 

The ammoxidation mechanism and the role of Bi and Mo sites in the complex multistep reaction taking place on the catalyst surface have been the subject of numerous studies. The currently most accepted mechanism, proposed by Grasselli et al., is illustrated in Figure 22. 

Figure 22 Schematic representation of the ammoxidation mechanism on Bi. Mo catalysts.

Figure 2.26a. Proposed propane ammoxidation mechanism over Mo-V-Nb—Te—Oa, catalysts.

SCHEME 8.1 Propane oxidation and ammoxidation mechanism and active site (From Vedrine, J.C., Top. Catal. 2002, 21, 97-106. With permission.)... 

Fig. 2. Mechanism of selective ammoxidation and oxidation of propylene over bismuth molybdate catalysts. (31).

The surface transformations of propylene, allyl alcohol and acrylic acid in the presence or absence of NHs over V-antimonate catalysts were studied by IR spectroscopy. The results show the existence of various possible pathways of surface transformation in the mechanism of propane ammoxidation, depending on the reaction condition and the surface coverage with chemisorbed NH3. A surface reaction network is proposed and used to explain the catalytic behavior observed in flow reactor conditions. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

The purpose of the present paper is to offer a contribute to the understanding of the mechanisms of these reactions by using an IR spectroscopic method and well-characterized "monolayer" type vanadia-titania (anatase) as the catalyst. We will focus our paper in particular on the following subjects i) the nature of the activation step of the methyl-aromatic hydrocarbon ii) the mechanism of formation of maleic anhydride as a by-product of o-xylene synthesis iii) the main routes of formation of carbon oxides upon methyl-aromatic oxidation and ammoxidation iv) the nature of the first N-containing intermediates in the ammoxidation routes. 

So, our data and the known organic chemistry suggest, according to the literature (8,12), that ammoxidation and oxidation mechanisms are interplayed. We can consequently propose the following mechanism for the surface reactions relative to ammoxidation of toluene ... 

Supported vanadium oxides represent one of the technologically most important class of solid catalysts. These catalysts are useful for partial oxidation of various hydrocarbons 0), ammoxidation of alkyl substituted N-heteroaromatic compounds (2) and most recently for NO reduction (3) For a catalyst to be a successful one in industry, it should exhibit high activity with maximum selectivity, thermal and mechanical stability and long life etc. For getting some of these functionalities, the active component has to be dispersed uniformly on a support material. 

It was argued that the mechanism of ammoxidation over this catalyst was different from other metal oxides, because propylene was not observed to be an... 

Intensive studies on the mechanism of oxidation and ammoxidation have been carried out.900 939957-960 All selective oxidation and ammoxidation catalysts are... 

In spite of the accumulated mechanistic investigations, it still seems difficult to explain why multicomponent bismuth molybdate catalysts show much better performances in both the oxidation and the ammoxidation of propylene and isobutylene. The catalytic activity has been increased almost 100 times compared to the simple binary oxide catalysts to result in the lowering of the reaction temperatures 60 80°C. The selectivities to the partially oxidized products have been also improved remarkably, corresponding to the improvements of the catalyst composition and reaction conditions. The reaction mechanism shown in Figs. 1 and 2 have been partly examined on the multicomponent bismuth molybdate catalysts. However, there has been no evidence to suggest different mechanisms on the multicomponent bismuth molybdate catalysts. 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

Methyl side chains of aromatic hydrocarbons can be selectively ammox-idized to nitrile groups. The process is very similar to the ammoxidation of propene and the same catalysts are found to be effective. Identical mechanisms have been proposed, and will not be discussed here. 

The kinetics of the ammoxidation of xylenes over a vanadium catalyst and mixed vanadium catalysts were studied. The reaction rate data obtained were correlated with the parallel consecutive reaction scheme by the rate equations based upon the Langmuir-Hinshelwood mechanism where the adsorption of xylenes was strong. The reaction rates of each path are remarkably affected by the kind of xylene and catalyst. The results of the physical measurement of catalysts indicated that the activity and the selectivity of reaction were affected by the nature and the distribution of metal ions and oxygen ion on catalyst surface. 

A somewhat similar mechanism involving Movl-imino species (Mo=NH) resulting from the reaction of ammonia with Mo—O bonds has been suggested for the industrially important ammoxidation of propene to acrylonitrile (equation 109).308... 

The ammoxidation of propene to acrylonitrile described by the global equation (11.1) actually involves a very complex reaction mechanism. More generally, the reaction of ammoxidation refers to the interaction of ammonia with a hydrocarbon... 

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