Ammoxidation propene

A commercial iron-promoted catalyst (Sn/Sb/Fe = 1/4/0.25) was studied by Germain et al. [92,93,135,137]. Iron is reported to improve the ammoxidation qualities of the catalyst although it has no effect on the oxidation [93], The kinetics, determined in a flow reactor at 445°C and with a feed ratio C3H6/NH3/air = 1/1.2/10, are essentially similar for this catalyst and bismuth molybdate. The initial selectivity is 80% and the maximum yield is 65% (at 445°C). The initial selectivity markedly depends on the temperature (e.g. 91% at 415°C and 72% at 507°C). The effect of water is hardly significant for this catalyst the acrylonitrile formation is slightly inhibited, while some more acrolein is formed. Presumably, water and ammonia compete in the interaction with the catalyst, which is much less reactive with respect to ammonia than bismuth molybdate. The acrolein ammoxidation is very rapid (about six times the propene ammoxidation rate) and selective (86%). A comparison of the Sn—Sb—Fe—O catalyst with bismuth molybdate is presented in Table 14. 

The selectivity of the acrylonitrile formation with respect to ammonia is very low (<10%) for the molybdenum-based catalysts (mainly due to N2 formation) but very high (100%) for the Sn—Sb—(Fe) catalyst. This is in agreement with the results of the separate oxidation of ammonia, which only in the case of Sn—Sb—(Fe) demands a temperature above that of the propene ammoxidation. 

Experiments were also done to compare the propene ammoxidation... 

Compared with propene, the oxidation of isobutene is more rapid but less selective, yet selectivities of over 75% appear feasible. Combustion is the main side reaction. One would expect that some considerable attention would be shown in the literature to isobutene oxidation as a route to the industrially important methacrylic acid, but this is not the case. Nor is it with the production of methacrylonitrile, analogous to the propene ammoxidation. Only in the patent literature is a high activity noticeable. 

Several antimony-based oxide combinations are very good catalysts for the dehydrogenation of butene to butadiene. Attention has particularly been paid to the three binary oxide combinations that are well known for their propene ammoxidation qualities Sn—Sb—O, Fe—Sb—O and U—Sb—O. 

Figure 20.3 Schematic flow sheet of the SOHIO process of propene ammoxidation to acrylonitrile. Adapted from [11].

The considerations reported above suggest that the mechanism of reaction might not be the same as the well known allylic insertion of a nucleophilic NH " species onto the activated hydrocarbon, to generate the precursor of the cyano group. Indeed, none of the elements included in catalyst formulation is able to produce the M=NH species (which is generated by Mo and Sb in propene ammoxidation catalysts). 

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. 

Catalytic vapor-phase ammoxidation on mixed oxides is an important class of industrial processes. Propene ammoxidation to acrylonitrile is a well established process for the synthesis of this widely used monomer and intermediate. Over the 40 years since its commercial introduction, the yield to acrylonitrile has nearly doubled to over 80% with the fourth generation of catalysts. This is due to the intensive research effort and understanding of the several factors underpinning catalytic activity. Commercial catalysts contain over 20 elements, the presence of all of which is necessary to optimize the catalytic behavior. 

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