Ammoxidation reaction

The methyl groups on xylenes can undergo ammoxidation, reaction with ammonia and oxygen (23). 

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. 

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Catalytic performances in ethylene ammoxidation as function of reaction temperature of the different catalysts are compiled in Table 2. Data were collected under stationary conditions after a transition period of one hour. All catalysts are active and selective toward acetonitrile. Wherein, Cr-Cl catalyst exhibits the higher ethylene conversion and the higher acetonitrile selectivity. Chromium with highly oxidation state (VI) seems to play a key role in the ammoxidation reaction as confirmed by TPR and DRS spectroscopy results. This idea is strongly supported by the difference between catalytic behaviour of Cr03 and Cr203 supported on ZSM-5. Nevertheless, Cr(III) oxide seems to... 

The high activity of Cr-Cl catalyst may result probably from the formation of more active species such as Cr02+ complex cation during the ammoxidation reaction. Some authors [7] assumed that Cr(V) species occurred inside the zeolite structure as complex cation such as Cr02+ coordinated to two framework oxygen atoms and suggest the following two step process as the most probable pathway of chromate formation ... 

Being one of the reactants in some of the important selective oxidation and ammoxidation reactions, NH can be a simple probe molecule to... 

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. 

Bulk Mixed-Metal Oxide Catalysts for Alkane Ammoxidation Reactions... 

The formation of VSb04 phases and their role in the ammoxidation reaction were investigated by Raman spectroscopy in combination with online GC (Guerrero-Perez and Banares, 2002). No appreciable... 

Lanthanides in oxy-anion systems In aluminosilicates In molybdates In phosphates In other systems For cracking reactions For ammoxidation reactions For hydrolysis of chlorinated aromatics... 

Depending on the preparation route of the VOHPO4 O.5H2O precursor and the gas atmosphere in which the precursor-to-catalyst transformation is performed, catalysts of various structures and compositions are obtained. However, these catalyst structures have a common property that is essential for effective catalytic performance in both selective oxidation and ammoxidation reactions, namely neighboring species that are coupled by effective spin-spin exchange inter-... 

Vanadium phosphates have been applied to a number of selective oxidation and ammoxidation reactions, although the partial oxidation of n-butane to maleic anhydride remains the most widely studied reaction for these catalysts. Even though the first patent for this reaction was filed over 40 years ago, there are stQl a large number of papers published on this system every year. 

Table 20.9 A summary of literature data on unconventional ammoxidation reactions.

The ammoxidation reaction, which is by far the largest-scale industrial allylic oxidation process, was originally discovered and developed at Sohio (2) in the early 1960s. This process, by which more than a million tons of acrylonitrile are produced annually in the United States and more than 4 million tons worldwide, revolutionized the manufacture of this important monomer, displacing the more expensive acetylene-HCN-based route (Eq. 9) ... 

The V-Ti-0 system has been extensively studied in connection with catalytic oxidation and ammoxidation reactions of aromatic hydrocarbons. Two principally different types of catalysts can be distinguished. One type of catalyst is prepared by impregnation, precipitation or mixing of the vanadium and titanium phases followed by calcination in air below the melting point of V. (1-4). The simultaneous reduction of V 0- and transformation of iiO (anatase) into rutile when heating below the V O melting point has been demonstrated to be due to topotactic reactions ( ). The formation of lower vanadium oxides can be of importance, because it has been found that reduced phases determine the activity and selectivity of catalysts (6,7). 

Towards these ends 14 selective oxidation reactions and two ammoxidation reactions have been evaluated through the use of selectivity-conversion plots, constructed fi om literature data [1]. Two examples of these plots are presented in figure 2 for ethylbenzene oxidation to styrene and methane oxidation to ethane. These selectivity-conversion plots were generated for a variety of catalysts for each reaction over a range of temperatures and space velocities. It should be stressed that the objective of this exercise was not to determine a reaction pathway or network, but simply to evaluate the best performance which has been achieved for any given reaction, hence the use of data fi om different catalysts and operating conditions. 

Figure 1. Toluene conversion during the ammoxidation reaction vs. reaction temperature on several VPO catalysts (+ - NVPO , X - NVPO, A - NVPOao, O -...

Since 1980, the applications zeolites and molecular sieves in the speciality and fine chemicals increased enormously. Zeolites are being used in the various types of reactions like cyclization, amination, rearrangement, alkylation, acylations, ammoxidation, vapour and liquid phase oxidation reactions. Zeolites and molecular sieves have also been used to encapsulate catalytically active co-ordination complexes like ship-in-bottle and as a support for photocatalytic materials and chiral ligands. Redox molecular sieves have been developed as an important class of liquid and vapour phase oxidation and ammoxidation reactions. We have discussed few typical recent examples of various types of reactions. 

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. 

Many heterogeneous catalytic systems have been developed and applied to ammoxidation reactions. Vanadium-containing oxides are preferred as supported, bulk, or multicomponent catalysts for the ammoxidation of aromatic or heteroaromatic compounds. Favored supports are titanium oxide (anatase) [18,19], zirconium oxide [20,21], tin oxide [22], or mixed supports such as titanium-tin oxide [23]. Catalytic systems used as bulk materials include vanadium-phosphorus oxides [24], crystalline vanadium phosphates [25], and vanadium oxide combined with antimony oxide [26] or molybdenum oxide [27]. Other important catalysts include multicomponent systems such as KNiCoFeBiPMoO c on silica... 

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. 

---