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TABLE 9.1 Selectivity to Major Product of a Few Commercial Catalytic Oxidation Reactions

Oxidation Process

Major Product

Selectivity (%)

Butane oxidation

Malefic anhydride

60

Propylene oxidation

Acrolein or acrylic Acid

75

Propylene ammoxidation

Acrylonitrile

80

Ethylene oxidation

Ethylene oxide

88



more expensive than the oxygenated product. Reoxidation of the reduced catalyst in the regenerator should occur at a temperature similar to that of the oxidation step to minimize the need to heat or cool the catalyst solids (energy minimization). Finally, since the reaction is essentially stoichiometric between catalyst and organic, a large amount of solid must be circulated around the reaction system. The catalyst must therefore be very resistant to attrition and must maintain structural integrity through many redox cycles. An excellent, early example of a two-step, anaerobic oxidation is the Lummus process for ammoxidation of o-xylene to o-phthalonitrile (dinitriles, DNs).2 A simplified schematic is shown in Figure 9.2. A higher selectivity is claimed for the two-step process relative to the single-stage, aerobic oxidation.

Another large development effort was carried out by ARCO Chemical during the 1970s to convert methane to ethylene.3 The reaction occurs at a very high temperature of 850-900°C. Patent and literature references, which illustrate the use of a Li0.5B0.5MnMg2.8Ox/SiO2 catalyst, show that at a conversion of 22%, the selectivity to two-carbon compounds was about 60% under both aerobic and anaerobic conditions. However, the yield of CO2 was reduced from 34% to 22% when the reaction was conducted in a cyclic mode, by carrying out the oxidation under anaerobic conditions. A major development effort was terminated when the price of oil decreased.

Scientists and engineers at Monsanto studied the oxidative dimerization of toluene to stilbene,4 as part of a new styrene process ( Figure 9.3). In the first step, using a K0.43BiOx catalyst at 575°C, the anaerobic process showed higher conversion (46 vs. 38%) and higher selectivity to stilbene (81.3 vs. 72.7%).

Emig has recently studied the oxidative dimerization of isobutylene to 2,5-dimethylhexadiene (DMH).5 Under aerobic conditions, a conversion of 24% was obtained with a selectivity of 38% to DMH giving a single-pass yield of 9.1%. Under anaerobic oxidation, the conversion dropped to 11%, the yield remained constant at 9.9%, and the selectivity to DMH increased from 38 to 90%. This is a remarkable example of CO2 reduction using a two-step process.

A recent patent has described the oxidation of propylene to acrolein and acrylic acid using a multicomponent metal oxide catalyst in a circulating solids reactor (CSR).6 Under anaerobic conditions



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FIGURE 9.1 Mars-van Krevelen oxidation of butane to maleic anhydride



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