Benzene ammoxidation

The mechanism of reaction included the rapid oxy-dehydrogenation of cyclohexane to cyclohexene and benzene. The latter was then transformed to successive products (see the section on benzene ammoxidation). The opening of the ring may occur either for cyclohexene or for benzene itself. 

Figure 20.12 Catalytic performance ofV/Mo/O catalyst in benzene ammoxidation [128],...

Ammoxidation of Unconventional Molecules 807 Table 20.8 Summary of results reported in [131] for benzene ammoxidation. 

Another recent patent (22) and related patent application (31) cover incorporation and use of many active metals into Si-TUD-1. Some active materials were incorporated simultaneously (e.g., NiW, NiMo, and Ga/Zn/Sn). The various catalysts have been used for many organic reactions [TUD-1 variants are shown in brackets] Alkylation of naphthalene with 1-hexadecene [Al-Si] Friedel-Crafts benzylation of benzene [Fe-Si, Ga-Si, Sn-Si and Ti-Si, see apphcation 2 above] oligomerization of 1-decene [Al-Si] selective oxidation of ethylbenzene to acetophenone [Cr-Si, Mo-Si] and selective oxidation of cyclohexanol to cyclohexanone [Mo-Si], A dehydrogenation process (32) has been described using an immobilized pincer catalyst on a TUD-1 substrate. Previously these catalysts were homogeneous, which often caused problems in separation and recycle. Several other reactions were described, including acylation, hydrogenation, and ammoxidation. 

Acrylonitrile produced industrially via propylene ammoxidation contains trace amounts of benzene. When using Pseudonocardia thermophila JCM3095 or Rhodococcus rhodochrous J-1 as microbial NHase catalyst for conversion of acrylonitrile to acrylamide, concentrations of benzene of <4 ppm produced a significant increase in the reaction rate [16]. Maintaining the concentration of HCN and oxazole at <5 ppm and <10 ppm respectively produced high-quality acrylamide suitable for polymerization. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

Many substances can be partially oxidized by oxygen if selective catalysts are used. In such a way, oxygen can be introduced in hydrocarbons such as olefins and aromatics to synthesize aldehydes (e.g. acrolein and benzaldehyde) and acids (e.g. acrylic acid, phthalic acid anhydride). A selective oxidation can also result in a dehydrogenation (butene - butadiene) or a dealkylation (toluene -> benzene). Other molecules can also be selectively attacked by oxygen. Methanol is oxidized to formaldehyde and ammonia to nitrogen oxides. Olefins and aromatics can be oxidized with oxygen together with ammonia to nitriles (ammoxidation). 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

Synthetic zeolites have gained importance as industrial catalysts for cracking and isomerization processes, because of their unique pore structures, which allow the shape-selective conversion of hydrocarbons, combined with their surface acidity, which makes them active for acid-catalyzed reactions. Many attempts have been made to introduce redox-active TMI into zeolite structures to create catalytic activity for the selective oxidation and ammoxidation of hydrocarbons as well as for SCR of nitrogen oxides in effluent gases (69-71). In particular, ZSM-5 doped with Fe ions has attracted attention since the surprising discovery of Panov et al. (72) that these materials catalyze the one-step selective oxidation of benzene to phenol... 

CoUeuille and coworkers [122] investigated catalysts for butadiene ammoxidation which are similar to those also studied in the ammoxidation of benzene (see below). Table 20.5 summarizes the results reported. The main products were fumaronitrile and maleonitrile, cro to nitrile (the unsaturated mononitrile, 1-cyano-propene, with the two trans and cis isomers) and CO with traces of acrylonitrile and furan. The residence time used was very low in this case the best performance was obtained with a typical propene ammoxidation catalyst, made of Bi/Mo/P/O under conditions of low butadiene conversion. 

Figure 20.10 Rates of adiponitrile (ADN) ( ) and benzene ( ) formation as functions of the content of Sb205 in catalysts for cyclohexane ammoxidation. Elaborated from [127a].

The ammoxidation of benzene is described in some papers and patents [124, 128, 131, 132]. Whilst benzene has been reported to remain unconverted in the presence of ammonia and oxygen with a V2O5 catalyst [124], it was efficiently transformed to nitriles with catalysts based on mixed molybdates [128]. 

Reference [132] also describes catalysts and methods for the ammoxidation of benzene to mucononitrile. The performance seems to be greatly affected by the type of reactor used and the reactor conditions. 

It is well known that the catalysts used for oxidation reactions such as those of propylene to acrolein, isobutene to methacrolein, or for ammoxidations (propylene to acrylonitrile, methyl-substituted benzenic rings to the corresponding aromatic nitriles) contain many components. This complexity in elemental composition is reflected by a complexity in phase composition. 

The first real characterization of active phases has been made for the high temperature polymorph of CoMoO (called (a) by us and later (b) by other authors) in the selective oxidation of butane to butadiene (20,21), as well as for (22) and bismuth molybdates (23,24) for oxidation, and ammoxidation of propylene. Additional examples include solid solutions such as (Mo V. )90t (with 0benzene conversion to maleic anhydride (25,26 and the solid solution up to 15% of Sb O in the SnO -Sb O system for propylene oxidation to acrolein (14,2/). 

Desulfurization of petroleum feedstock (FBR), catalytic cracking (MBR or FI BR), hydrodewaxing (FBR), steam reforming of methane or naphtha (FBR), water-gas shift (CO conversion) reaction (FBR-A), ammonia synthesis (FBR-A), methanol from synthesis gas (FBR), oxidation of sulfur dioxide (FBR-A), isomerization of xylenes (FBR-A), catalytic reforming of naphtha (FBR-A), reduction of nitrobenzene to aniline (FBR), butadiene from n-butanes (FBR-A), ethylbenzene by alkylation of benzene (FBR), dehydrogenation of ethylbenzene to styrene (FBR), methyl ethyl ketone from sec-butyl alcohol (by dehydrogenation) (FBR), formaldehyde from methanol (FBR), disproportionation of toluene (FBR-A), dehydration of ethanol (FBR-A), dimethylaniline from aniline and methanol (FBR), vinyl chloride from acetone (FBR), vinyl acetate from acetylene and acetic acid (FBR), phosgene from carbon monoxide (FBR), dichloroethane by oxichlorination of ethylene (FBR), oxidation of ethylene to ethylene oxide (FBR), oxidation of benzene to maleic anhydride (FBR), oxidation of toluene to benzaldehyde (FBR), phthalic anhydride from o-xylene (FBR), furane from butadiene (FBR), acrylonitrile by ammoxidation of propylene (FI BR)... 

Attention should be drawn It) the use of tin o.xide systems as heterogeneous catalysts. The oldest and most extensively patented systems are the mixed tin-vanadium oxide catalysts for the oxidation of aromatic compounds such as benzene, toluene, xylenes and naphthalene in the. synthesis of organic acids and acid anhydride.s. More recently mixed tin-antimony oxides have been applied to the selective oxidation and ammoxidation of propylene to acrolein, acrylic acid and acrylonitrile. 

Ammoxidation of alkylbenzenes with side-chains of different length leads to reaction at the position a to the benzene ring [38]. Reactivity increases with increasing length of the alkyl group (toluene < ethyl benzene < isopropyl benzene [58,59]), but in all cases only benzonitrile is formed. 

Red-ox type catalysts are mostly used in oxidation or related types of reactions. For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride. Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene. Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained. In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reactions. ... 

Several competing fluidized-bed processes UNIPOL and BP processes compete Partial oxidation of butane competes with fixed-bed process Supplanted by fixed-bed process Highly exothermic, well suited to fluid bed Reactor/regenerator system Raises octane content while reducing benzene Converts olefins to C5+ hydrocarbons Ammoxidation of m-xylene Means of low-temperature oxidation with favorable heat transfer So far unable to displace fixed-bed in-furnace process... 

A first group of methods starts from already made active carbons subjected to a chemical posttreatment NHj or HCN [29], amination or ammoxidation [30]. A second group starts from raw materials with a high content of nitrogen, such as vinyl pyridine-divinyl benzene copolymer [28,31]. 

More detailed discussion regarding the model performance is given by Wen and Fan (1975) and Mori and Wen (1975). The literature-reported applications of this model include a combustion study of coal with limestone injection by Horio and Wen (1975), a coal gasification study by Mori et al. (1983), a study on catalytic oxidation of benzene by Jafffes et al. (1983), a catalytic ammoxidation of propylent by Stergiou and Laguerie (1983), a silane decomposition study by Li et al. (1989), a catalytic oxidation of methane by Mleczko et al. (1992), and a study on chlorination of rutile by Zhou and Sohn (1996). 

There are several commercial gas-solid catalysed reactions in which heat transfer plays a significant, if not dominant, role in limiting the reactor productivity, lowering the process selectivity and reducing the life of the catalyst. Among these include the oxidation of ethylene, benzene, C hydrocarbons and methanol, the ammoxidation of propylene, methanol synthesis (Lurgi), the hydrochlorination of methanol and steam reforming of natural gas and naphtha. 

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