Selective oxidation promoted iron oxides

Selective partial oxidation of hydrocarbons poses considerable challenges to contemporary research. While by no means all, most catalytic oxidations are based on transition-metal oxides as active intermediates, and the oxidative dehydrogenation of ethylbenzene to styrene over potassium-promoted iron oxides at a scale of about 20 Mt/year may serve as an example [1]. Despite this... 

Iron oxide is an important component in catalysts used in a number of industrially important processes. Table I shows some notable examples which include iron molybdate catalysts in selective oxidation of methanol to formaldehyde, ferrite catalysts in selective oxidative dehyrogenation of butene to butadiene and of ethylbenzene to styrene, iron antimony oxide in ammoxidation of propene to acrylonitrile, and iron chromium oxide in the high temperature water-gas shift reaction. In some other reactions, iron oxide is added as a promoter to improve the performance of the catalyst. 

A mixed oxide of ruthenium, copper, iron and alumnium has been developed as a catalyst for the synthesis of aldehydes and ketones from alcohols.258 Oxidation of chiral secondary 1,2-diols with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone under ultrasound wave promotion leads to the selective oxidation of benzylic or allylic hydroxyl group. The configuration of the adjacent chiral centre is retained.259 The kinetics of oxidation of ethylbenzene in the presence of acetic anhydride have been studied.260... 

Figure 4.2 shows the SIMS spectrum of a promoted iron-antimony oxide catalyst used in selective oxidation reactions. Note the simultaneous occurrence of single ions (Si+, Fe+, Cu+, etc.) and molecular ions (SiO+, SiOH+, FeO+, SbO+, SbOSi+). Also clearly visible are the isotope patterns of copper (two isotopes at 63 and 65 amu), molybdenum (seven isotopes between 92 and 100 amu), and antimony (121 and 123 amu). Isotopic ratios play an important role in the identification of peaks, because all peak intensities must agree with natural abundances. Figure 4.2 also illustrates the differences in SIMS yields of the different elements although iron and antimony are present in comparable quantities in the catalyst, the iron intensity in the spectrum is about 25 times as high as that of antimony ... 

Almost all other studies on elements other than iron are performed in absorption mode. For example, Bussiere and co-workers used the Mbssbauer effect to study the state of tin in supported Pt-Sn [37] and Ir-Sn [38] reforming catalysts, and of tin and antimony in mixed Sb-Sn oxides for the selective oxidation of propylene [39]. Also of note are Millet s investigations using the 125Te isotope to characterize the state of the tellurium promoter in multicomponent ammoxidation catalysts [40],... 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Description EB is dehydrogenated to styrene over potassium promoted iron-oxide catalyst in the presence of steam. The endothermic reaction is done under vacuum conditions and high temperature. At 1.0 weight ratio of steam to EB feed and a moderate EB conversion, reaction selectivity to styrene is over 97%. Byproducts, benzene and toluene, are recovered via distillation with the benzene fraction being recycled to the EB unit. 

The best yield reported in the literature is 13.3%, with selectivity of about 60% obtained with silica-supported K-promoted iron oxide catalysts modified by amines [43c]. The same catalyst is inactive in propene oxidation with air. However, the use of ammonia/air mixtures leads to a considerably enhanced conversion with respect to air only, with 60% selectivity for the epoxide. This observation suggests a mechanism whereby ammonia is first oxidized to nitrous oxide, which subsequently produces the active oxygen species for epoxidation. 

We also performed Mossbauer spectra measurements over various metal-promoted iron oxide catalysts [13]. We introduced selected transition/inner transition metal ions (M = Cr, Mn, Co, Ni, Cu, Zn and Ce) into the iron oxide spinel lattice that were screened for effectiveness for the WGSR. Fe Mossbauer spectra of pristine Fe203 sample exhibits a six-line Zeeman spec-tmm as a result of the complete stractural and magnetic transformation. 

The sensitivity and selectivity of the polymeric porphyrin depends not only on the potential of NO to oxidize but also on the fast process of electrochemical NO oxidation, which generates the high current. In addition, surface effects, axial ligation to the central metal in the porphyrin, nature of the central metal (iron > nickel > cobalt zinc, copper), gas permeability through sensor layer(s), and fast removal of NO" " by Nafion are all important in promoting fast and selective oxidation... 

The additive elements used to enhance the performance of the Fe-Sb-0 catalyst either enter the iron antimonate rutile phase to form a solid solution (49,50) or they form separate rutile phases (44). The promoter elements that produce the best performing iron antimonate-based ammoxidation catalysts are copper, molybdenum, tungsten, vanadium, and tellurium. Copper serves as a structural stabilizer for the antimonate phase by forming a rutile-related solid solution (23). Molybdenum, tungsten, and vanadium promote the redox properties of iron antimonate catalysts (51). They provide redox stability and prevent reductive deactivation of the catalyst, especially under conditions of low oxygen partial pressure (see above). The tellurium additive produces a marked enhancement of the selectivity of iron antimonate catalyst. How the tellurium additive functions to increase selectivity is not clear, but the presumption is that it must directly modify the active site. In fact, it is likely that it can actually serve as the site of selective oxidation because in its two prevalent oxidation states Te + and Te +, tellurium possesses the requirements for the selective (amm)oxidation site, a-hydrogen abstraction, and 0/N insertion (see below). 

Thus, DFT calculations complemented by statistical thermodynamic analysis showed that a realistic Fe/ZSM-5 catalyst should contain a small fraction of isolated Fe + species at specific positions inside the zeolite chaimels, while the predominant part of iron is present in the form of oxygenated cationic iron complexes. The questions about which of these different extraframework complexes is actually responsible for the specific catalytic properties of Fe/ZSM-5 and what the role of other species were still open. They were addressed to a large extent only when the mechanisms of different catalytic reactions over different potential intrazeolite iron sites were thoroughly investigated by DFT calculations [43,46]. The influence of the nature and structural properties of Fe sites on two important catalytic processes promoted by Fe/ ZSM-5, namely, the selective oxidation of benzene to phenol and the direct catalj4ic N O decomposition, was investigated [43,46]. 

The Fur protein from E. coli was isolated in one step due to its high affinity for metal-chelate columns loaded with zinc. In DNase footprinting experiments, the Fur protein was shown to bind DNA in the promoter region of several iron-regulated genes. The consensus sequence, called the Fur box, is GATAATGATAATCATT ATC. In vitro binding is dependent on the divalent cations Co2+ Mn2+ /s Cd2+ Cu2+ at 150 iM, while Fe2+ seemed to be less active at this concentration, probably due to oxidation to Fe3+ (De Lorenzo et al., 1987). The unspecificity for divalent metals observed in vitro shows that the cells have to select the ions transported carefully and have to balance their active concentrations. In addition, it is a caveat for the experimenter to test a hypothesis on metal-ion specificity not only in vitro, but also in vivo. 

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