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Ammoxidation reaction intermediates

MAA and MMA may also be prepared via the ammoxidation of isobutylene to give meth acrylonitrile as the key intermediate. A mixture of isobutjiene, ammonia, and air are passed over a complex mixed metal oxide catalyst at elevated temperatures to give a 70—80% yield of methacrylonitrile. Suitable catalysts often include mixtures of molybdenum, bismuth, iron, and antimony, in addition to a noble metal (131—133). The meth acrylonitrile formed may then be hydrolyzed to methacrjiamide by treatment with one equivalent of sulfuric acid. The methacrjiamide can be esterified to MMA or hydrolyzed to MAA under conditions similar to those employed in the ACH process. The relatively modest yields obtainable in the ammoxidation reaction and the generation of a considerable acid waste stream combine to make this process economically less desirable than the ACH or C-4 oxidation to methacrolein processes. [Pg.253]

The strong parallel with the acrolein formation initially suggested the idea that acrolein is a reaction intermediate in the ammoxidation, and can further react with ammonia and oxygen to form acrylonitrile. Although the ammoxidation of acrolein is indeed a very rapid reaction, it is generally accepted today that a direct reaction path to acrylonitrile predominates. The differences between both theories are very small, however, when one assumes that the ammoxidation of acrolein and propene involves the same reaction intermediate. Thus the various kinetic schemes proposed in the literature can be derived from the general scheme below by omitting the reaction steps (3), (4) and/or (5) and variation of the ratio between (2) and (3). [Pg.165]

Benzylamine- [e. g. 45] or benzaldehyde-like [e. g. 46] species have been proposed as reaction intermediates. Ammonia, which must be inserted, is chemisorbed and activated by the catalyst [47-49] and does not act directly from the gas-phase. Recent investigations using transient experiments with % support this idea [50], As indicated for vanadium phosphate catalysts, NH4+ ions can act as potential insertion species in the ammoxidation cycle, suitable chemisorption sites (OH groups) will be formed during the catalytic reaction [51]. Otherwise, the formation of imido species (=NH) is often discussed as active site for the N-insertion of catalysts, containing, e. g., molybdenum instead of vanadium [1,52-54],... [Pg.530]

In effect, the methyl group and the functionalized carbon center are converted to the nitrile functional group. However, as shown by isotope labeling experiments, ammoxidation of a hydrocarbon having an allylic hydrogen produces a symmetric intermediate wherein the nitrile functional group can be incorporated into either end of the allylic intermediate (see below). Thus, ammoxidation reactions can be divided into three major classes. The first is ammoxidation of a methyl group, effectively, to a nitrile functionality. [Pg.241]

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. [Pg.285]

The oxidation and ammoxidation of propylene (H2CHC = CH3) to acrolein (H2CHC = CHO) and acrylonitrile (H2CHC = CN) respectively, constitute major chemical intermediates. Conversion of propylene to acrolein is conducted in an oxidising environment, while conversion of propylene to acrylonitrile requires both molecular O2 and NH3. The industrial catalyst employed for these oxi-dation/ammoxidation reactions is a bulk Bi-Mo-0 mixed oxide. The alpha-Bi2 (M0O4) 3 phase is one of the most active phases and its Raman spectrum is shown in Fig. 17.7. An operando Raman-GC spectroscopy investigation of propylene... [Pg.429]

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. [Pg.216]

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. [Pg.271]

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... [Pg.273]

Niwa, M., Ando, H., and Murakami, H. Reaction mechanism of ammoxidation of toluene II. Identification of reaction intermediate adsorbed on V OjAl Oj by infrared spectroscopy. J Cala/ 49, 92-96 (1977). [Pg.285]

Vinyl chloride is an important monomer for polyvinyl chloride (PVC). The main route for obtaining this monomer, however, is via ethylene (Chapter 7). A new approach to utilize ethane as an inexpensive chemical intermediate is to ammoxidize it to acetonitrile. The reaction takes place in presence of a cobalt-B-zeolite. [Pg.171]

Ammoxidation of propylene is considered under oxidation reactions because it is thought that a common allylic intermediate is formed in both the oxidation and ammoxidation of propylene to acrolein and to acrylonitrile, respectively. [Pg.215]

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. [Pg.285]

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. [Pg.169]

The IR study performed in static controlled atmospheres in the IR cell allowed us to identify a number of adsorbed intermediate and secondary products, together with the main reaction products in oxidation and ammoxidation of toluene and the three xylene isomers. Surface reactions schemes are proposed that account for most of the mechanistic features of the heterogeneously-catalyzed industrial reactions. Our data support the following conclusions ... [Pg.181]

IsophLhalonitrile (1,3-dicyanoben/ene, 1PN), is a white solid which mells at 161°C and sublimes at 265°C. It is slightly soluble in water but readily dissolves in diinethylfonnamide, jV-inethylpyiToliclinoiie and hot aromatic solvents. IPN undergoes the reactions expected of an aromatic nitrile. It is prepared by vapor-phase ammoxidation of metci- ylcnc. Its principal use is as an intermediate to amines, As a reagent, TPN can be used to convert aromatic acids to nitriles in near quantitative yields. [Pg.1081]

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... [Pg.289]

The general aim of C-H transformation is to introduce groups with a higher complexity to hydrocarbon structures. Industrial processes therefore usually involve transformation of C-H groups starting from simple molecules. The reactions employed are selective oxidation, substitution (radical, electrophilic), nitration, ammoxidation, and sulfonation. The functionalized molecules are then further converted to more valuable products and intermediates by different reaction pathways. The latter often comprise further steps of C-H-activation. [Pg.14]

Bismuth molybdates having a Bi/Mo ratio in the range of 0.67 2.0 catalyze the selective oxidation of propene to acrolein, and the ammoxidation of propene to acrylonitrile (equations 5 and 6). Both reactions proceed through an aUyhc intermediate. Three typical active phases o -Bi2Mo30i2,... [Pg.3387]

In conclusion, research and industrial interest in the ammoxidation of methylpyridine and methylpyrazine is still active. This process could offer a greener alternative to current processes in the production of intermediates for a variety of specialty chemicals, especially in rapidly developing locations such as China where a growing interest in the reaction has been noted [115, 116]. [Pg.796]

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). [Pg.808]

The ammoxidation of cyclohexanone to cyclohexanone oxime is catalyzed by TS-1 with 98.2% selectivity to cyclohexanone oxime at 99.9 % conversion [177]. Selective oxidation of the nitrogen of ammonia by hydrogen peroxide is a key step of this reaction. The mechanism is still vividly debated and three possible routes are shown in Scheme 20. Recent evidence [163] seems to support a route via intermediate formation of hydroxylamine [mechanism B]. The high selectivity on the other hand supports the postulate that the reaction proceeds via a concerted reaction step that involves the titanium peroxo species, ammonia and cyclohexanone (mechanism C) [177],... [Pg.388]

The Sohio process is considered one of the most successful applications of FCB. Problems in industrial application of the reaction arose from the strong exothermicity of propylene ammoxidation and from the intermediate production of acrylonitrile in the consecutive reactions (V9). It is particularly noticeable that the catalyst gives high selectivity, and the reactor design aims at better fluidization and higher contact efficiency than in the FCC process. [Pg.428]


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