Acrolein/acrylonitrile bismuth phosphomolybdate

The modern beginning of the heterogeneous catalytic oxidation of olefins to aldehydes may be taken as the discovery of the oxidation of propylene to acrolein over cuprous oxide by Hearne and Adams (5 ). This reaction has been carried to commercial operation by Shell Chemical Company. More recently, the use of bismuth phosphomolybdate has been demonstrated for the oxidation of propylene to acrolein by Veatch and co-workers (88), and, in the presence of ammonia, to acrylonitrile by Idol (89). It was also shown, by Heame and Furman (90), that diolefins could be made from C4 and higher olefins by oxidative dehydrogenation over a bismuth molybdate catalyst. From these beginnings, information on olefin oxidation has increased very rapidly, both in journal and patent literature. We shall make no attempt to review the large number of patents that have issued, but shall limit ourselves mainly to journal literature. 

Veatch, Callahan, Milberger, and Forman (88) reported that a bismuth phosphomolybdate catalyst was quite selective for oxidation of propylene to acrolein at 450°. The catalyst was compounded with SiOg, and could be used at low CgHg/Og ratio, such as 0.5. At 92 % conversion of propylene a 60% selectivity to acrolein was reported. An outstanding development with catalysts of this type is the conversion of propylene to acrylonitrile by reaction with ammonia and oxygen, as... 

Today the most cost-effective processes are those based on propylene as the starting material. There are three major variations of propylene processes, the Distillers process [21-23], the Sohio process [24], and the DuPont process [25,26]. All three processes are based on the ammonoxidation of propylene. The Distillers process is carried out in two stages. In the first, propylene is oxidized in air to form acrolein and water. These intermediate products are allowed to react in the second stage with ammonia in the presence of molybdenum oxide and air to form crude acrylonitrile. The pure monomer is recovered by a series of azeotropic distillations. The Sohio process is carried out in just one stage. Ammonoxidation of propylene takes place in air at 2-3 atmospheric pressure and 425-510°C. With catalysts, such as concentrated bismuth phosphomolybdate or other oxides of molybdenum and cobalt, the reaction takes place with over 50% yield in a reaction time of only about 15 s. In the DuPont version of this process, the ammonoxidation is brought about with nitric oxide at 500°C using silver on silica catalyst. The chemistry of acrylonitrile monomer has been reviewed by a number of authors [27-30]. 

In this method, large amounts of acetonitrile, CH3CN, occur as a by-product. Catalysis is by vanadium oxide on AI2O3, or borophosphate, titanium phosphate, or bismuth phosphomolybdate on silica acid. The mechanism is still unestablished. On the one hand, acrolein does not appear to be an intermediate product, since it is not formed in the absence of ammonia. On the other hand, acrylonitrile can also be obtained from acrolein with ammonia and oxygen, and molybdenum oxide as catalyst. In another variation, the conversion is not made with ammonia and air, but with nitric oxide ... 

A more active bismuth phosphomolybdate (Table 4.13) was prepared simply by adding an appropriate volume of phosphoric acid to the initial solution. A typical catalyst composition was claimed to be Bi9PMoi2052-55 2Si02. The same catalysts could be used to produce both acrolein and acrylonitrile. 

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