Applications partial oxidation reactions

Dr. Israel Wachs of Lehigh University discusses molecular engineering on oxide catalysts. These materials are especially important for partial oxidation reactions, in which selectivity is difficult to control. This chapter focuses on vanadium-based catalysts, and the approach is applicable to other supported catalysts as well. 

To meeet the demands of high-temperature applications such as hydrocarbon steam reforming and partial oxidation reactions, nickel-based alloys are frequently taken into consideration [10]. 

Aluminas are used in various catalytic applications, a-, y-, and -aluminas are all used as support materials, the first one in applications where low surface areas are desired, as in partial oxidation reactions. The latter two, and especially y-alumina, in applications where high surface areas and high thermal and mechanical stability are required. One of the most prominent applications of y-alumina as support is the catalytic converter for pollution control, where an alumina washcoat covers a monolithic support. The washcoat is impregnated with the catalytically active noble metals. Another major application area of high-surface aluminas as support is in the petrochemical industry in hydrotreating plants. Alumina-supported catalysts with Co, Ni, and/or Mo are used for this purpose. Also, all noble metals are available as supported catalysts based on aluminas. Such catalysts are used for hydrogenation reactions or sometimes oxidation reactions. If high... 

Another partial oxidation reaction that is attracting industrial attention for the application of reactive separations is the production of synthesis gas from methane [Stoukides, 2.127]. The earlier efforts made use of solid oxide solutions as electrolytes. Stoukides and coworkers (Eng and Stoukides [2.200, 2.126], Alqahtany et al. [2.201, 2.202]), for example, using a YSZ membrane in an electrochemical membrane reactor obtained a selectivity to CO and H2 of up to 86 %. They found that a Fe anodic electrode was as active as Ni in producing synthesis gas from methane (Alqahtany et al. [2.201, 2.202]), and that electro-chemically produced O was more effective in producing CO than gaseous oxygen (no ef-... 

Partial oxidation is able to convert methane and other hydrocarbons, catalyzed or non-catalyzed, at temperatures between 1100 and 1500 °C. Despite its lower efficiency compared with steam reforming, partial oxidation provides, on the other hand, exothermicity and a greater selectivity for synthesis gas production as well as advantages for certain applications such as compactness, rapid startup or load change, lower overall cost. If steam is added to the fuel and the oxidant, it is possible to heat balance the exothermal partial oxidation reaction with the endothermal reforming reaction, meaning that no external heat source (autothermal) is required. 

Use of the Reaction to Modify the Membrane and Improve Driving Force -The use of dense oxide membranes to feed O2 to partial oxidation reactions is limited by the need for high temperatures to obtain reasonable permeation rates. The potential exists for selectivity gains and the control of heat effects. One application where dense membranes have a clear role to play is in methane activation. In particular, the partial oxidation of methane to syngas. 

The three-phase membrane reactors have been mainly investigated for applications in both hydrogenation and partial oxidation reactions. 

With the exception of the lead oxide the metal oxides used are very interesting materials with respect to catalytic applications. Tungstates and molybdates are widely used in partial oxidation reaction [6], iron oxides are the basis of many important industrial cat ysts, which are for instance, used for the dehydrogenatitm of ethyl benzene to styrene. If the surfactant template could be removed from the structures, very high surface area catalysts could be accessible. 

The application of vanadium oxide catalysts in partial oxidation reactions is widely practiced. Several studies were carried out to investigate the promotional effect of alkali compounds or basic oxides to V2O5 on the catalytic performance in different reactions [58,84-88]. Generally, the promotion of V2O5 bulk and supported catalysts with alkali compounds improves the selectivity to partial oxidation products. The doping effect of mainly potassium compounds has been explained (i) by lowering of the acid sites, and/or (ii) by changing the redox properties of the catalysts. Furthermore, the addition of potassium influences the... 

Porous membranes can be utilized for both equilibrium and selectivity-limited reactions. The former mainly involve various dehydrogenation reactions, while the latter are oxidative dehydrogenation and partial oxidation reactions. Besides, other applications related to porous MRs are illustrated and summarized in the following sections. 

The performances of various Ti-zeolite systems in partial oxidation reactions catalyzed by or organic hydroperoxides are presented in [126]. Tl-ZSM-5 is also efficient in the direct conversion of isobutene and methanol into methyl tert-butyl ether. This quest is under way with the application of new classes of mesoporous and hierarchical materials. 

The quantitative analyses of reaction products due to partial or complete oxidation can be performed by different methods. This type of determination is essential to improve electrode composition. Apart from a decrease in the Coulombic efficiency (see Section 11.2), the formation of partially oxidized products can be deleterious for the DMFC application because some of these products (e.g., formic acid) may be in liquid form and are corrosive. 

A growing number of research groups are active in the field. The activity of reforming catalysts has been improved and a number of test reactors for fuel partial oxidation, reforming, water-gas shift, and selective oxidation reactions were described however, hardly any commercial micro-channel reformers have been reported. Obviously, the developments are still inhibited by a multitude of technical problems, before coming to commercialization. Concerning reformer developments with small-scale, but not micro-channel-based reformers, the first companies have been formed in the meantime (see, e.g., ) and reformers of large capacity for non-stationary household applications are on the market. 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

Fluidized bed reactors were first employed on a large scale for the catalytic cracking of petroleum fractions, but in recent years they have been employed for an increasingly large variety of reactions, both catalytic and non-catalytic. The catalytic reactions include the partial oxidation of naphthalene to phthalic anhydride and the formation of acrylonitrile from propylene, ammonia, and air. The noncatalytic applications include the roasting of ores and Tie fluorination of uranium oxide. 

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