Ammoxidation reactions mechanism

The ammoxidation reaction mechanism for xylenes is similar to that for toluene. The xylene substrate is activated by hydrogen abstraction from the methyl groups with subsequent reaction with activated ammonia converting the methyl group to an amine, then to an imine, and finally to a nitrile. In the case of xylene ammoxidation, the ammoxidation of the methyl groups to cyano functionalities occurs sequentially rather than concurrently (73,76). 

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. 

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... 

The knowledge of the reaction mechanism is important for process design. Firstly, only olefins with activated methyl groups may undergo ammoxidation reactions to nitriles. Otherwise, oxidative dehydrogenation takes place preferentially. 

The ammoxidation reaction can, on the other hand, be performed continuously in fixed-bed and fluid-bed reactors, and by-products (particularly CO2) can be easily removed. The fluidized bed has some advantages in terms of heat transfer but demands are made on the mechanical durability of the catalyst and so catalyst choice is limited. The long-term stability of the catalysts is also important and so multicomponent systems are recommended [e.g. 1,12]. The separation of the nitrile formed can be achieved by condensation, centrifugation, filtration, or rectification. Sometimes the formation of hazardous by-products (HCN, CO) must be considered. 

The selectivity of a partial-oxidation catalyst can change with slight variations in its composition but is also dependent on the substrate and the reaction conditions. The design of catalysts optimized for a specific reaction should be based on a detailed knowledge and understanding of the reaction mechanism. The state of the art of catalyst development, mechanistic features, kinetics, and reaction technology in the ammoxidation of methyl aromatic compounds was summarized in 1992 by Rizayev et al. [38]. 

A redox mechanism (Mars-van Krevelen) is generally accepted for the ammoxidation reaction of methyl aromatic compounds, thus most catalysts applied contain transition metal oxides (e. g. vanadium, molybdenum) readily enabling changes in valence states. 

Scheme 8-7 Postulated reaction mechanism for the ammoxidation of propene (13)...

Surface Reaction Mechanism. The mechanism of catalytic alkene ammoxidation is invariably linked to allylic oxidation chemistry. Allylic oxidation is the selective oxidation of an alkene at the allylic carbon position. Selective allylic oxidation and ammoxidation proceed by abstraction of the hydrogen from the carbon positioned a to the carbon-carbon double bond. This produces an allylic intermediate in the rate-determining step. In the case where propylene is the hydrocarbon, the reaction is as follows ... 

Later work by Burrington and co-workers (55,56) confirmed the earlier mechanistic findings and added several important new insights into the mechanisms of both the oxidation and ammoxidation of propylene. The detailed mechanism for the ammoxidation reaction over a bismuth molybdate catalyst is shown in Figure 2. Key among the separate steps of the mechanism are as follows ... 

Investigation of the reaction mechanism of propane ammoxidation over a V-Sb-0 catalyst using deuterated propane reveals that the rate-determining step is the abstraction of hydrogen from the methylene carbon of propane (131). 

Scheme 8.2 Reaction mechanism for the ammoxidation of toluene over VPO catalysts (From Bruckner, A., Catal. Rev. 2003, 45, 97-150. With permission.)...

In general, the ammoxidation reaction of methyl aromatics and/or hetero aro-maties runs via redox mechanism as proposed by Mars and van Krevelen [12]. Most of the catalysts used so far eontain transition metal oxides with easily ehanging valence states (e.g., V, Mo, etc.). Essential steps of the reaction mechanism are (i) chemisorption of the methyl aromatic or hetero aromatic reactant on the catalyst surface followed by H-abstraetion (i.e., C-H bond disassociation) to form a benzylic intermediate, (ii) insertion of nitrogen into a surface bonded partially oxidized intermediate and (iii) desorption of the formed nitrile and iv) reoxidation of the catalyst by gas-phase oxygen. Literature survey [114, 115] revealed that the H-abstraction oeeurs via C-H bond dissociation in three different possible ways, such as (i) heterolytic with the abstraction of hydrogen atom in an anionic form followed by carbocation. 

Until now, V O, VPO, Mo, and Sb based catalysts are widely used for various ammoxidation reactions. Though the steps involved in almost all am-moxidation reactions are same irrespective of the catalyst system used, the mode of adsorption of substrate, the nature of active species involved differ from catalyst to catalyst. Although recent studies have thrown some light on the mechanism of ammoxidation reaction on different oxide catalysts, a clear understanding of various reaction steps involved is still under debate. The mechanism of ammoxidation of different substrates over different catalysts is discussed below. 

Mmakami et al. [120] and Niwa et al. [121] investigated the reaction mechanism of ammoxidation by FTIR spectroscopy using V Oj/Al Oj catalysts. They claim that the reaction proceeds via interaction of ammonium ions with surface benzoate ions. Cavalli et al. [5, 122] and Busca et al. [123] proposed benzyl amine and benzaldehyde species as reaction intermediates in the ammoxidation of toluene over V Oj/TiO catalysts. Otamiri and Andersson [124] proposed vanadium imido species (V=NH) and vanadium hydroxylami-no species (V-NH-OH) as nitrogen insertion sites in V Oj catalysts. These species in turn react with adsorbed toluene to form an amine (R-CH -NH ) and/or imine (R-CH=NH) surface intermediate, which subsequently transform into nitrile as a final product. A mechanism proposed by Sanati and Andersson [125] includes (CgHj)CH(NH2)0- and (CgHj)CH(NH2)(0-)2 species as reaction intermediates. Ramis et al. [126] reported the formation of amido... 

To design and develop novel catalysts for various oxidation and ammoxidation processes, the focal point of all research should aim at imderstanding the reaction mechanisms and deriving structure activity relationships. However, choice of active components, type of supports and promoters, method of preparation and formulation of catalyst must be related to mechanism, kinetics, and adsorption. 

Murakami, Y, Niwa, M., Hattoii, T, Osawa, S., Igushi, S., and Ando, H. Reaction mechanism of ammoxidation of toluene I. Kinetic studies of reaction steps by pulse and flow techniques. J Cala/ 49, 83-91 (1977). 

Since then Centi has also examined catalysts based on vanadiitm antimo-nates." Conversion as high as 60-80%, but only 35 0% yields, were obtained with a VSbsWO catalyst supported on alumina. Bowker confirmed Centi s conclusion that the reaction proceeds in two steps." Propane is first dehydrogenated to propylene, which is then ammoxidized to acrylonitrile by the well-established reaction mechanism. 

Another facet of surface organometallic chemistry involves modelling of the mechanisms of surface reactions on the basis of the reactivity of molecular models. For example, the reactivity of metal-imine complexes of molybdenum is considered by CHAN, who proposes elementary steps constituting the catalytic cycle of the surface-catalyzed alkene ammoxidation reaction, which is of great industrial importance. HERRMANN provides some very fine examples of molecular models of the rhenium oxide catalysts used commercially in the alkene metathesis reaction. 

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... 

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