Amm oxidation

Hydrocarbon-rich conditions imply that oxygen is the limiting reactant, due to the high oxygen-to-hydrocarbon stoichiometric ratio in n-hexane ammoxidation. Therefore, the conversion of the hydrocarbon is low this should favour, in principle, the selectivity to products of partial (amm)oxidation instead of that to combustion products. 

Table 11 shows that water primarily inhibits the combustion of propene, and thus increases the (initial) selectivity. A comparison of rate coefficients of oxidation and ammoxidation is given in Table 12, and includes the separately studied (amm)oxidation of acrolein. 

Villa et al. [340] have shown that the bismuth tungstates are comparable with bismuth molybdates with respect to dehydrogenation catalysis, although activities and selectivities are somewhat lower. Although the phase structures are different, interesting catalysts are formed in a similar composition range Bi/W = 2/3 to 2/1. (Note that, in case of propene (amm)oxidation, tungstates are definitely inferior to molybdates.)... 

The selectivity of the ammoxidation of molecules like toluene and xylene is much higher than that of the oxidation of these compounds to aldehydes. The selectivity difference is more pronounced here than in case of propene. The initial selectivities of the propene oxidation and ammoxidation are practically the same, and the selectivity difference is mainly due to the high stability of acrylonitrile compared with acrolein. For aromatic (amm)oxidation, however, the initial selectivities also differ. Apparently, ammonia interacts with the catalyst in such a way that the activity for oxidation of the aromatic nucleus is reduced. 

The participation of lattice oxygen is inherent to the redox mechanism, which is operative in many of the oxidation processes that are catalyzed by metal oxides. Reviewing the processess described in Sect. 2, participation of lattice oxygen appears to be the case for the majority of them, namely for the allylic (amm)oxidation of olefins, for the (amm)oxidation of aromatic hydrocarbons and for the oxidation of methanol, ammonia and sulphur dioxide. 

Therefore, the catalyst possesses different types of active sites one site that can activate the paraffin and oxide hydrogenate it to the olefin, and one that (amm) oxidizes the adsorbed olefin intermediate. 

Andersson developed a semi-empirical model for the charge distribution around the (V=0) bonds in V205, V6013, and V02.73 The surfaces of the lower oxides were treated, upon the basis of ESCA results discussed on p. 107, as being in an oxidized state, which is proposed to be the case under the usual conditions in (amm) oxidation reactions. The main result is that 02-in the form of (V=0) groups is responsible for the catalytic oxidation of hydrocarbons. 

Grasselli, R. K., Selectivity issues in (amm)oxidation catalysis, Catal. Today, 99, 23-31, 2005... 

Figure 11.7 Structure of the amorphous microporous mixed (AMMs) oxides discrete catalytic library L6.

Turning now to the reaction side, it is generally accepted that the rate-hmiting step of propane selective (amm)oxidation is activation of the first C—H bond. Vanadium 5-i- was proposed by several authors as being responsible for this elementary step [148, 204-206], However, XPS does not seem to support this proposal, since vanadium was observed in its formal 4+ oxidation state on the surface of Ml (Tables 6.4 and 6.5). The deficiency of the surface in vanadium and the minority of its fraction (if any) relative to V" cast serious doubts that vanadium... 

AH catalysts claimed are multi-functional systems. Indeed, the formation of acrylonitrile from propane occurs mainly via the intermediate formation of propene, which is then transformed to acrylonitrile via the allylic intermediate. It follows that the catalyst possesses different kinds of active site one site that is able to activate the paraffin and oxidehydrogenates it to the olefin, and one site that (amm)oxidizes the adsorbed olefinic intermediate. This second step must be very rapid to limit, as much as possible, the desorption of the olefin. In order to develop an effective cooperation between the two sites, it is necessary to have systems in which they are in close proximity. The muIti-functionaHty is achieved either through the combination of two different compounds (phase-cooperation), or through the presence of different elements inside a single crystaUine structure. In antimonate-based systems, the cooperation between the metal antimonate (having the rutile crystalline structure), responsible for propane oxidative dehydrogenation to propene and propene activation, and antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over... 

KAGAN MOLANDER Samanum reagent 196 KAISER JOHNSON - MIDDLETON Dinitnis cychzation 197 KAKIS Rearrangement 198 KALUZA Isothiocyanate synthesis 199 KAMETANI Amme oxidation 199 KEINAN Silane reagent 200 Kellogg 25 Kendall 246... 

Three catalytic systems active and selective in the (amm)oxidation of saturated organic substrates have been discussed, and the properties which lead to their superior catalytic performance have been examined in relationship to the mechanism of paraffin activation and transformation to the desired products. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Influence of Antimony Content in the Iron Antimony Oxide Catalyst and Reaction Conditions on the (Amm)Oxidation of Propene and Propane... 

The influence of antimony content and of ammonia partial pressure on the selectivity of the (amm)oxidation of propene and propane with FeSb04 can be understood in terms of the degree of reduction of the catalyst surface at steady state conditions. The higher the degree of reduction which can be caused either by a low antimony content or by a high ammonia partial pressure the higher the selectivity for the combustion/degradation products. 

Figure 1 shows the conversion of propene in the ammoxidation and the selectivity to the partial (amm)oxidation products, acrylonitrile plus acrolein, as a function of time on stream for the base iron antimony oxide catalyst (Sb/Fe =1) and for the impregnated catalyst with 0.27 g Sb added per g FeSb04 (method A), which corresponds to a coverage with antimony of 5... 

In the propane ammoxidation a lower selectivity for acrolein plus acrylonitrile is observed. The formation of partial (amm)oxidation products from propane requires more elemental steps than their formation from propene. All these intermediates can undergo a side reaction with electrophilic oxygen species yielding degradation products. 

Influence of ammonia partial pressure on the (amm)oxidation of propene... 

The influence of ammonia on the partial (amm)oxidation of propene was studied over the iron antimony oxide catalyst (Sb/Fe = 2) at 375 °C (see Figure 5). The yield of the partial (amm)oxidation products acrylonitrile plus acrolein decreased with increasing ammonia partial pressure. The yield of the combustion products CO and CO2 first decreased and then increased with increasing ammonia partial pressure. The opposing trends for the yield of both product groups resulted in a complex behaviour of the conversion of propene as a function of the partial pressure of ammonia. The rate of formation of the partial (amm)oxidation products can be easily modelled as a surface reaction ocupying one or two active sites, and ammonia occupying one of the sites. 

The complex dependency of the yield of combustion products on the ammonia partial pressure indicates that various factors are influencing the rate of formation of this product class. If ammonia is only inhibiting the adsorption of propene, then the yield of combustion products is expected to follow the same trend as the yield of the partial (amm)oxidation products. Ammonia is not only consumed for the formation of acrylonitrile, but can also reduce the surface under the formation of e g. N2. This will change the degree of reduction of the surface, and hence the composition of the pool of oxygen species. If ammonia cannot... 

Figure 5 Influence of ammonia partial pressure on the (amm)oxidation of propene over an iron antimony oxide catalyst (Sb/Fe = 2) at 375 °C...

The selectivity for the partial (amm)oxidation products first increased and then decreased with increasing ammonia partial pressure. A very high selectivity (close to 100 C-%) was obtained at a molar ammonia to propene ratio of less than one. The selectivity for the combustion products showed the opposite trend. At high ammonia partial pressures a slight increase in the selectivity for the degradation products was observed, which are thought to be formed similarly to the combustion products. 

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