Intermediates propane ammoxidation

Ammonia also reacts with the acrolein intermediate, via the formation of an imine or possibly oxime intermediate which transforms faster to the acrylonitrile than to the acrylamide intermediate. This pathway of reaction occurs at lower temperatures in comparison to that involving an acrylate intermediate, but its relative importance depends on the competitive reaction of the acrolein intermediate with the ammonia species and with catalyst lattice oxygens. NH3 coordinated on Lewis sites also inhibits the activation of propane differently from that absorbed on Brsurface reaction network in propane ammoxidation. 

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. 

Catalyst Chemistry. Just as in the case of selective ammoxidation of propylene to acrylonitrile, the most effective catalysts for propane ammoxidation are based on the oxides of antimony and molybdenum. In fact, in many instances, modifications in the compositions of highly effective catalysts for propylene ammoxidation result in some of the best reported catalysts for propane ammoxidation. This similarity in catalyst types is easy to rationalize, since, as is discussed in a later section, propylene is the key hydrocarbon intermediate in the mechanism of acrylonitrile formation from propane. It can be further generalized that if... 

Kinetic analysis of propane ammoxidation with V-Sb-0 catalysts has shown that the reaction proceeds through propylene as the key intermediate (130). The first step in the propane ammoxidation reaction is the oxidative dehydrogenation of propane to propylene. Essentially, all the products of the reaction derive from conversion of the propylene intermediate. The kinetic results also suggest that there is a lesser direct reaction pathway from propane to acrylonitrile. 

This preparation procedure also creates solid-state phases that are key to the performance of the Mo-V-Nb-Te-0 catalyst for propane ammoxidation. High activity and selectivity result when the x-ray powder diffraction pattern shows the presence of specific diffraction lines attributed to two separate phases denoted as Ml and M2 by Mitsubishi Chemical Corp. The diffraction lines assigned to these two phases are given in Table 7 (146). The coexistence of these two phases is viewed as key to the successful functioning of the catalyst. Specifically, the Ml phase is purportedly responsible for the oxidative dehydrogenation of propane to propylene, the key intermediate in the reaction network. This reaction sequence, in which the first step is the formation of a propylene intermediate, is the same as noted previously with other propane ammoxidation catalysts, most notably with the V-Sb-0 catalyst (see above). The M2 phase of the Mo-V-Nb-Te-0 catalyst is reportedly the center for the selective ammoxidation of the propylene intermediate to acrylonitrile. As the first-formed intermediate, propylene is apparently the source of all the observed reaction products. Although a detailed kinetic analysis has not been presented, a cursory report, published in Japan, summarized the kinetic experiments for the conversion of propane and propylene over a... 

An in-depth analysis of the solid-state chemistry of the Mo-V-Te-Nb-0 propane ammoxidation catalyst system reveals the details of the two primary phases designated as Ml and M2 (150,151). Correlations of catalytic activity and phase composition for this catalyst system establish the specific functions of the two catalytically active phases (152,153). Specifically, the Ml phase is the phase primarily responsible for propane activation and conversion to acrylonitrile via intermediate, adsorbed propylene. The M2 phase is essentially inactive for propane activation but is capable for conversion desorbed propylene intermediate to acrylonitrile. 

Fundamental studies of the mechanism of propane ammoxidation over Mo-V-Nb-0 based catalysts show that the mechanism is the same as earlier studies foimd for other propane ammoxidation catalysts, notably those based on V-Sb-O (see above). Isotope labeling studies using C-labeled propane show that the reaction proceeds through propylene as the only intermediate (156). There is no dimerization or skeletal rearrangement of C3 moities rather intermediate propylene is converted directly to acrylonitrile. Also pulse reaction studies show that the reaction occurs by a redox process wherein lattice oxygens are used in the... 

In the present chapter, we report about an inveshgahon of the catalyhc performance of rahle-type V/Sb and Sn/V/Sb/Nb mixed oxides in the gas-phase ammoxidation of n-hexane. These catalysts were chosen because they exhibit intrinsic mulhfunctional properties in fact, they possess sites able to perform both the oxidahve dehydrogenahon of the alkane to yield unsaturated hydrocarbons, and the allylic ammoxidahon of the intermediate olefins to the unsaturated lutriles. These steps are those leading to the formahon of acrylonitrile in propane ammoxidahon. The SnW/Sb/(Nb)/0 system is one of those giving the best performance in propane ammoxidahon under hydrocarbon-rich condihons (8,9). 

The synthesis of intermediates and monomers from alkanes by means of oxidative processes, in part replacing alkenes and aromatics as the traditional building blocks for the chemical industry [2]. Besides the well-known oxidation of n-butane to maleic anhydride, examples of processes implemented at the industrial level are (i) the direct oxidation of ethane to acetic acid, developed by Sabic (ii) the ammoxidation of propane to acrylonitrile, developed by INEOS (former BP) and by Mitsubishi, and recently announced by Asahi to soon become commercial (iii) the partial oxidation of methane to syngas (a demonstration unit is being built by ENI). Many other reactions are currently being investigated, for example, (i) the... 

More than a decade after the publication of the MoVNb catalyst system, scientists at Mitsubishi Chemical reported that modifying this family of mixed metal oxides with Te produced a catalyst for the amoxidation of propane to acrylonitrile [4] and the oxidation of propane to acrylic acid [5], Modification of the Union Carbide catalyst system with Te was probably not a random choice as it is a known propylene activator [5 b] and the molybdate phase TeMoO oxidizes propylene into acrolein and ammoxidizes propylene to acrylonitrile [6], a key intermediate in the commercial production of acrylic acid using Mo-based oxides. Significant efforts to optimize this and related mixed metal oxides continues for the production of both acrylic acid and acrylonitrile, with the main participants being Asahi, Rohm Hass, BASF, and BP. 

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... 

Acetonitrile can be produced by catalytic ammoxidation of ethane and propane over Nb-Sb mixed oxides supported on alumina, with selechvities to acetonitrile of about 50-55% at alkane conversions of around 30% [133]. In both cases, CO forms in approximately a 1 1 molar ratio with acetonitrile, owing to a parallel reaction from a common intermediate. When feeding n-butane, the selectivity to acetonitrile halves. Bondareva and coworkers [134] also studied ethane ammoxidation over similar types of catalyst (V/Mo/Nb/O). 

Although less apparent, oxydehydrogenation also plays a role in the work done by BP Amoco, Asahi, and others to extend ammoxidation to the direct conversion of propane to acrylonitrile. It is believed that the ammoxidation of propane proceeds through a transient propylene intermediate from which acrylonitrile is derived through a conventional ammoxidation pathway. Similarly, the conversion of n-butane to maleic anhydride could also be regarded as a form of oxydehydrogenation, except that in this case it leads to the formation of a new compound that incorporates oxygen in the molecule. 

Sohio Issued several patents claiming catalysts based on vanadium, antimony and some promoters which are able to ammoxidize propane with a completely heterogeneous mechanism (66). These catalysts can be considered intrinsically multifunctional since both dehydrogenation and nitrogen insertion functions are present (67,68). The main problem with this type of catalyst is the low rate of the subsequent ammoxidation of intermediate propylene. Indeed, propylene is always present as a by-product. 

The mechanism for acrylonitrile formation via intermediate propylene is demonstrated by the results given in Figure 6, which compares the oxidation and the ammoxidation of propane over a V/Sb/0 mixed oxide catalyst (69). The conversion of propane and the selectivity to the main products are reported (acrolein was formed only in traces). 

Ammoxidation refers to the formation of nitriles by oxidation of hydrocarbons with oxygen in the presence of ammonia (Figure 1) [1]. Ammoxidation is best conducted with olefins, or with aromatic or heteroaromatic compounds, containing a readily abstractable H atom (allylic or benzylic intermediates are formed), although the ammoxidation of alkanes (e. g. propane to acrylonitrile [e. g. 2-4] or ethane to acetonitrile [e. g. 5]) is also possible. An exceptional example is the ammoxidation of methane to hydrogen cyanide by the Andrussov reaction [6]. 

Loss of a second hydrogen from the methyl position then occurs readily to produce the propylene intermediate. The propylene may either desorb from the surface and subsequently readsorb on a propylene ammoxidation site, or it may remain on the surface and migrate to a propylene ammoxidation site. In either case, the kinetic analysis reveals that the rate of conversion of the propylene intermediate is much higher than propane conversion to propylene or any other step in the reaction network. The net effect is a virtually seamless transformation of propane to acrylonitrile. 

The catalyst operates by a redox process in which lattice oxygens, up to 70 layers deep in the Ml phase, palatinate in the activation of propane, conversion to intermediate propylene, and ammoxidation of propylene to acrylonitrile. 

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