Catalytic partial oxidation reaction mechanism

Two reaction mechanisms for partial propane oxidation exist in the literature. One of them proposes that the reaction starts with catalytic combustion followed by reactions of a lower rate, namely steam reforming, C02 reforming and water-gas shift [54], Aartun et al. [55] investigated both reactions. The other mechanism proposes that the partial oxidation reaction occurs directly at very short residence times [56], which are easier to achieve in the micro channels. 

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... 

For other types of systems such as highly branched reaction networks for homogeneous gas-phase combustion and combined homogeneous and catalytic partial oxidation, mechanism reduction involves pruning branches and pathways of the reaction network that do not contribute significantly to the overall reaction. This pruning is done by using sensitivity analysis. See, e.g., Bui et al., "Hierarchical Reduced Models for Catalytic Combustion H Air Mixtures near Platinum Surfaces, Combustion Sci. Technol. 129(l-6) 243-275 (1997). 

Enger BC, Lodeng R, Holmen A (2008) A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanism over transition metal catalysts. Appl Catal A 346 1-27... 

The SFR with catalytically coated plates is an easy setup to investigate heterogeneously catalyzed gas-phase reactions. In situ invasive capillary techniques can be used to determine the gas-phase concentration in the one-dimensional boundary layer on top of the catalyst. The measurement species profiles can be compared with numerically predicted profiles to test surface reaction mechanisms, diffusion models but also gas-phase reaction schemes, CVD processes, and others (not discussed here). However, internal difiiision inside the catalytic disc has to be taken into account when thicker catalyst layers are used. Then, the choice of an adequate diffusion model can be crucial for a correct interpretation of the measured data. The computer code DETCHEM offers simulations with the following models to account for internal diffusion in stagnation flows on porous plates with reactions inside effectiveness factor model, ID reaction diffusion model (RD-approach), and DGM (not discussed here). While the RD-approach may even play a role in simple cases as discussed here, it is the model of choice when parallel reactions occur (e.g., catalytic partial oxidation, three-way... 

Linke, D., Wolf, D., Baems, M., et al. (2002). Catalytic Partial Oxidation of Ethane to Acetic Acid over MojVo 25Nl>o.i2Pdo.00050x- Catalyst Performance and Reaction Mechanism, J. Catal, 205, pp. 16-31. 

Nguyen, L., Loridant, S., Launay, H., efa/. (2006). Study of New Catalysts Based on Vanadium Oxide Supported on Mesoporous Silica for the Partial Oxidation of Methane to Formaldehyde Catalytic Properties and Reaction Mechanism, J. Catal., 237, pp. 38-48. 

The lack of a detailed surface reaction mechanism for propane/air combustion on platinum necessitated the use of a global-step reaction model in this work. Secondary hetero-Zhomogeneous chemistry interactions are not taken into account this way (such as the homogeneous conversion of C3Hg to CO and the subsequent conversion of the latter to CO2 on the catalytic surface) a closer-to-reahty description of the in-channel combustion processes in catalytic microreactors is thus not possible. While the impact of such a simplification in cases where total oxidation of the fuel is required is minimal, this does not hold true in catalytic partial oxidation applications. Therein, a detailed surface chemistry description would be necessary. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

In spite of the accumulated mechanistic investigations, it still seems difficult to explain why multicomponent bismuth molybdate catalysts show much better performances in both the oxidation and the ammoxidation of propylene and isobutylene. The catalytic activity has been increased almost 100 times compared to the simple binary oxide catalysts to result in the lowering of the reaction temperatures 60 80°C. The selectivities to the partially oxidized products have been also improved remarkably, corresponding to the improvements of the catalyst composition and reaction conditions. The reaction mechanism shown in Figs. 1 and 2 have been partly examined on the multicomponent bismuth molybdate catalysts. However, there has been no evidence to suggest different mechanisms on the multicomponent bismuth molybdate catalysts. 

As mentioned earlier, the multicomponent oxide catalysts currently commercialized contain bismuth, iron, and molybdenum, in addition to several other cations. Although few reports concerning multicomponent catalysts have appeared in the literature, there is agreement that iron affects several aspects of the catalyst system. Measurements on multicomponent catalysts by Wolfs et al. (109-111) showed that Fe3+ was partially reduced to Fe2+ after the catalytic reaction, indicating that Fe3+ ions are involved in the reaction mechanism. The observed Fe3+/Fe2+ redox couple was associated with the increased activity of the catalyst. 

The general redox mechanism of metal-oxide catalyzed oxidation of hydrocarbons involves two major stages in the catalytic process, reduction of the surface layers by hydrocarbons and their reoxidation by interaction with oxygen. While these two stages occur simultaneously in a reactor with the catalyst working under steady-state conditions, they can be carried out in two separate reaction zones in a reactor with catalyst circulation [37]. A hydrocarbon is fed into the first zone where a desirable intermediate product of partial oxidation is formed after interaction with the oxidized catalyst. In the second zone, gas phase oxygen reoxidizes the catalyst. Obviously, the residence time of the catalyst in the first zone should be short enough to prevent formation of an inactive reduced state of the catalyst. If only surface layers participate in the interaction with hydrocarbons, the time of catalyst reduction is approximately several seconds. 

---