Ammoxidation reaction conditions

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. 

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. 

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... 

For more than a century, stoichiometric methods were presumed in the preparation of benzonitriles in laboratory and industry. These particularly include the Rosenmund-von Braun reaction of aryl halides, the diazotization of anilines and subsequent Sandmeyer reaction, and the ammoxidation. Because of (over)stoichiometric amounts of metal waste, lack of functional group tolerance, and harsh reaction conditions, these methods do not meet the criteria of modern sustainable synthesis. 

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. 

Typical Reaction Conditions for the Oxidation and Ammoxidation of Propylene on the Simple and Multicomponent Bismuth Molybdate Catalyst°... 

An important dopant for rutile-type mixed oxides is Nb oxide [46a]. Banares et al. [49] found that when used as the support for V/Sb/O, Nb205 formed new phases by reaction with V and Sb under catalytic reaction conditions these phases, of unclear nature, affected the catalytic performance in propane ammoxidation. When instead Nb was added as a promoter for the alumina-supported V/Sb/O system, the interaction between the active components led to an improvement of catalytic performance with respect to the undoped V/Sb/O. Nb also forms rutile-type mixed... 

The ammoxidation of toluenes substituted with electron-donating groups, for example hydroxy- and alkoxy-substituted toluenes is rather less selective. However, under carefully chosen conditions (choice of the catalyst, feed composition, reaction conditions) adequate yields of nitriles can be achieved. Stabihty of the catalyst performances is typically an issue. 

Aromatic imides are another type of product which can be synthesized by catalytic ammoxidation. o-Xylene is converted over vanadium-titanium oxide catalysts to tolunitrile and then, depending on catalyst composition and reaction conditions, phthalimide or phthalonitrile can be selectively synthesized (Scheme 20.3) [94]. 

Figure 6. Ammoxidation of propane over V/Sb/0 catalyst. Reaction conditions temperature 430°C, residence time 2s, propane 25 mol.%, ammonia 10 mol.%, oxygen 20 mol.%.

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. 

Depending on the sequential method mentioned above, in our work of developing catalysts for propane ammoxidation to acrylonitrile, we also focused efforts on the catalytic active system of V-Sb-Al mixed oxides. The work was divided into three parts, i.e., optimizations of the composition of promoters and supporters, the conq)osition of main conq)onents as well as reaction conditions. 

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]. 

Toluene can be readily ammoxidized to benzonitrile, usually over supported vanadium oxide and vanadium phosphate catalysts [e. g. 9,57]. Besides catalyst choice, catalytic performance mainly depends on the reaction conditions. Excess ammonia, as mentioned above, significantly increases nitrile selectivity by blocking sites responsible for consecutive oxidation ammonia also frequently reduces catalyst activity [1]. Water vapor in the reactant stream can also improve selectivity by blocking sites for total oxidation [38] or providing Brdnsted sites for the activation of ammonia [51]. 

For further extension of the scope of the ammoxidation reaction new catalytic systems (based on detailed mechanistic knowledge) must be developed to establish more selective reactions, e. g. at lower temperature or under shape-selective conditions. For extension of application, effective protection of side-chains in the educts are also necessary. Finally, the search for more selective catalysts... 

This work shows the acquired experience in the preparation at pilot-scale of a novel propane ammoxidation catalyst based on a partially nitrided V-Al mixed oxide obtained hy co-precipitation. A systematic investigation of the different parameters controlling the preparation of the catalyst via a co-precipitation route at different scales was carried out. At lab-scale (50 to 100 g), the preparation parameters optimized were precipitation pH, V/Al atomic ratio, V concentration in solution and nitridation conditions, while at pilot-scale (1 kg), the optimized parameters were precipitation and ageing time, solution/solid ratio during the washing step, drying and calcination conditions, and extrusion parameters. Our results show that the optimum preparation conditions for the VAION system are pH = 5.5, V/Al atomic ratio = 0.25, concentration of V species in solution = 30.10 M. This catalyst shows the highest selectivity and yield in acrylonitrile. The samples prepared at different scales show the same activity profile in the propane ammoxidation reaction. 

The activity for the propane ammoxidation reaction was determined using a fixed bed catalytic reactor under the following experimental conditions ... 

The low pifa values for alcohols ( 16 for methanol), compared to about 30 and 44 for alkenes and alkanes, respectively, along with the acid-base character of metal oxides, allows this step to be easily facilitated by a number of (amm)oxidation catalysts. These characteristics of the reaction allow selective ammoxidation of alcohols to be conducted under milder reaction conditions than ammoxidation of alkenes and alkanes, generally at temperatures below 400°C. This promotes selectivity for the ammoxidation of the alcohol to the corresponding nitrile product by lessening the oxidation activity of the catalyst for complete oxidation to COg. 

Analysis of the solid-state chemistry of the V-Sb-0 catalyst identified the important role cation vacancies play in catalyzing the propane ammoxidation reaction (129). Reduction of the catalyst with ammonia indicated that the catalyt-ically active site is associated with the cation vacancy in the rutile structure. This was based on the results of in situ UV Raman spectroscopy along wdth computational modeling to interpret the vibrational spectra. Specifically, the two-coordinated oxygen at the cation vacation is removed from the structure during the reduction treatment and is replenished when the catalyst is reoxidized wdth 02- The study further showed that the stability of the catalyst vmder ammoxidation conditions is enhanced by the introduction of titanium into the structure as a solid solution. Titanium is incorporated into the rutile structure of V—Sb—O as a solid solution of the following composition ... 

In the presence of oxygen at 500°C and under pulse reaction conditions, se-lectivities as high as 80% to 90% are reported for a GaiSbsNii sPiOx catalyst. However, since propane conversions are quite low in these experiments (4-5%), the reported selectivities to acrylonitrile may be exaggerated because of an inability to analyze by-products other than CO2, which are known to form during propane ammoxidation and which would be produced in extremely low yields in these experiments. 

In an effort to probe the nature of the initial interaction between propane and the surface of the gallium antimonate-based catalyst, the rates of propane and isobutane ammoxidation were compared (134). In all instances, the propane conversion was greater than the conversion of isobutane vmder the same reaction conditions. Likewise, the rate of acrylonitrile formation from propane was greater than the rate of methaciylonitrile formation from isobutane. Assuming that the rate-determining step is abstraction of hydrogen from the hydrocarbon, the data suggest that abstraction does not occur at a secondary or tertiary carbon, but rather at a primary carbon site and that the first... 

The direct ammoxidation of propane into acrylonitrile by its reaction with oxygen and ammonia is an alternative route to the conventional propylene ammoxidation catalytic reaction to acrylonitrile. The use of propane instead of propylene makes the reaction conditions more demanding because of the necessity of activating the propane C-H bond which is much stronger than the propylene methyl C-H bond. Different catalytic systems have been investigated for the ammoxidation of propane into acrylonitrile with vanadium-based catalysts proving particularly efficient, particularly the Sb-V-0 mixed oxide catalyst. ... 

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. 

Among various vanadia-based catalysts, the vanadium phosphorus oxides (VPOs) have been proved to be excellent catalysts for selective O- and N-insertion reactions of aliphatic and methylaromatics, in particular for the oxidation of -butane to maleic anhydride and the ammoxidation of methylaromatics and heteroaromatics to their corresponding aldehydes and nitriles [12,44,59-65], Various VPO precursors of different structure and vanadium valence state were studied in recent years with an aim to elucidate their reaction behavior and to improve their catalytic performance in the earlier mentioned processes. Regarding the nature of active phase in these catalysts and the way its structure influences the catalytic activity and selectivity of these catalysts, comprehensive investigations were made by direct observation of the catalysts under reaction conditions with the help of various spectroscopic in situ methods. More recently, a comprehensive picture of structure-reactivity relationship for the industrially important ammoxidation of toluene to benzonitrile was demonstrated experimentally for VPO catalysts [5] ... 

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