Another example is the catalytic oxidative dehydrogenation of propane to propene. This is not yet a commercial process because of the low yield of propene using known catalysts. Figure 10.1 summarizes the reported selectivity as a function of conversion of propane for the better catalysts. The data show that selectivities approaching 100% can be obtained only at low conversions, suggesting that the low selectivities at high conversions are due to the faster oxidation of propene to COx than of propane to propene. The former reaction is faster because allylic C–H bonds in propene are much weaker and more reactive than the C–H bonds in propane. Consequently, if catalytically active sites that are indifferent to C–H bond strengths can be constructed, such that the catalyst promotes the reaction between oxygen and propene as fast as that between oxygen and propane, then the selectivity-conversion relationship shown by the solid line in the Figure 10.1 could be obtained. Unfortunately, at present, there is insufficient information on the nature of the active sites in mixed oxide catalysts reported for this reaction to permit designing such an active site. One difficulty in attempts to elucidate the nature of active sites in mixed oxide catalysts for selective oxidation in general is the poorly crystalline state of the solid at the active site because of the facile motion of oxygen ions under reaction conditions. Development of more informative experimental methods and computational techniques will be very valuable in this area.
In addition to exothermic reactions, there are many chemical transformation processes that are endothermic. Examples include steam reforming of methane to generate hydrogen and cracking of hydrocarbon in the fluid catalytic cracking process. For these reactions, heat is required, which is often