The pros and cons of oxidative dehydrogenation of propane are outlined and a new catalytic system based on metal-doped cerianite catalysts is introduced. These novel materials catalyze the selective combustion of hydrogen from a mixture of hydrogen, propane, and propene at 550 degrees C. This gives three key advantages: energy is supplied directly where needed, product separation is made easier, and the dehydrogenation equilibrium is shifted to the desired products. A set of eighteen doped cerianites was synthesized in parallel, characterized, and screened for activity, selectivity, and stability in a cyclic redox system. The best results were obtained with Ce(0.89)Cr(0.02)Fe(0.09)O(2), Ce(0.98)Sn(0.02)O(2), and Ce(0.96)Cu(0.02)Zn(0.02)O(2), which gave 98 %, 91 %, and 98 % selectivity, respectively. Ce(0.89)Cr(0.02)Fe(0.09)O(2) also shows excellent stability in over 120 cycles (66 h on stream at 550 degrees C). Importantly, these doped cerias are monophasic crystalline materials. The dopants are incorporated as solid solutions throughout the fluorite lattice. This means that these catalysts are very stable (they do not sinter during reduction) as opposed to traditional supported metal oxides. The results show that both activity and selectivity towards hydrogen combustion can be tuned (increased or decreased) by selecting the appropriate dopant. Furthermore, the trends in selectivity differ from those measured on supported oxides of the same elements, which indicates that these novel materials indeed contain unique active sites. The factors governing selectivity towards hydrogen oxidation and the nature of the active site are discussed.

Ceria-based mixed oxides, in which about 10 mol % of the cerium is replaced by another metal, catalyze the selective combustion of hydrogen from a mixture of hydrogen, propane, and propene at 550 degrees C. This makes them attractive catalysts for the oxidative dehydrogenation of propane. Hydrogen combustion shifts the equilibrium to the products side, supplies energy for the endothermic dehydrogenation, and simplifies product separation. The type of metal added has an important effect on the catalytic properties. To gain insight into the process, a set consisting of six mixed oxides was synthesized and the catalytic properties and redox behavior were tested. The mixed oxides generally release more oxygen than plain ceria. Mixed oxides containing Bi, Cu, Fe, Pd or Ca release between 1.6 and 2.0 mg of oxygen per 100 mg sample (compared to only 1.2 mg for plain ceria). This result is important for reactions in which the catalyst acts as an oxygen reservoir, such as selective hydrogen combustion. The temperature at which oxygen is released is generally lower for the mixed oxides, and varies from 110 degrees C (for Cu-CeO2) to 550 degrees C (for Ca-CeO2), which enables catalytic applications over a wide temperature range. The reduction rate at 550 degrees C is related to the reduction onset of the catalysts. Those catalysts with a relatively low reduction temperature, such as Cu-, Mn-, Bi-, and Pb-CeO2, show a high reduction rate, whereas those with a high reduction temperature, such as Ca-CeO2, Fe-CeO2, and plain ceria, reduce at a slower rate. The latter catalysts also have a low selectivity towards hydrogen combustion. The influence of the catalyst composition and crystallite size on the activity and selectivity is discussed.

Perovskite-type oxides, ABO(3), can be successfully applied as solid "oxygen reservoirs" in redox reactions such as selective hydrogen combustion. This reaction is part of a novel process for propane oxidative dehydrogenation, wherein the lattice oxygen of the perovskite is used to combust hydrogen selectively from the dehydrogenation mixture at 550 degrees C. This gives three key advantages: it shifts the dehydrogenation equilibrium to the side of the desired products, heat is generated, thus aiding the endothermic dehydrogenation, and it simplifies product separation (H(2)O vs H(2)). Furthermore, the process is safer since it uses the catalysts' lattice oxygen instead of gaseous O(2). We screened fourteen perovskites for activity, selectivity and stability in selective hydrogen combustion. The catalytic properties depend strongly on the composition. Changing the B atom in a series of LaBO(3) perovskites shows that Mn and Co give a higher selectivity than Fe and Cr. Replacing some of the La atoms with Sr or Ca also affects the catalytic properties. Doping with Sr increases the selectivity of the LaFeO(3) perovskite, but yields a catalyst with low selectivity in the case of LaCrO(3). Conversely, doping LaCrO(3) with Ca increases the selectivity. The best results are achieved with Sr-doped LaMnO(3), with selectivities of up to 93 % and activities of around 150 mumol O m(-2). This catalyst, La(0.9)Sr(0.1)MnO(3), shows excellent stability, even after 125 redox cycles at 550 degrees C (70 h on stream). Notably, the activity per unit surface area of the perovskite catalysts is higher than that of doped cerias, the current benchmark of solid oxygen reservoirs.