Ammoxidation hydrocarbons/mixed oxides

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). 

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. 

I 20 Catalytic Ammoxidation of Hydrocarbons on Mixed Oxides lOOi---------------- ------- --------------------... 

The term mixed oxide catalyst is commonly applied to oxides of Bi and Mo, which are used in industrial processes for partial oxidation of hydrocarbons, a prominent example being the ammoxidation of propene ... 

Light hydrocarbons consisting of oxygen or other heteroatoms are important intermediates in the chemical industry. Selective hydrocarbon oxidation of alkenes progressed dramatically with the discovery of bismuth molybdate mixed-metal-oxide catalysts because of their high selectivity and activity (>90%). These now form the basis of very important commercial multicomponent catalysts (which may contain mixed metal oxides) for the oxidation of propylene to acrolein and ammoxidation with ammonia to acrylonitrile and to propylene oxide. 

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). 

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