CATALYTIC SELECTIVE OXIDATION Oxygenation reactions catalyzed

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

Because the petrochemical industry is based on hydrocarbons, especially alkenes, the selective oxidation of hydrocarbons to produce organic oxygenates occupies about 20% of total sales of current chemical industries. This is the second largest market after polymerization, which occupies about a 45% share. Selectively oxidized products, such as epoxides, ketones, aldehydes, alcohols and acids, are widely used to produce plastics, detergents, paints, cosmetics, and so on. Since it was found that supported Au catalysts can effectively catalyze gas-phase propylene epoxidation [121], the catalytic performance of Au catalysts in various selective oxidation reactions has been investigated extensively. In this section we focus mainly on the gas-phase selective oxidation of organic compounds. 

Among the several types of homogeneously catalyzed reactions, oxidation is perhaps the most relevant and applicable to chemical industry. The well-known Wacker oxidation of ethylene to ethylene oxide is the classic example, although this is not a true catalytic process since the palladium (II) ion becomes reduced to metallic palladium unless an oxygen carrier is present. Related to this is the commercial reaction of ethylene and acetic acid to form vinyl acetate, although the mechanism of this reaction does not seem to have yet been discussed publicly. Attempts to achieve selective oxidation of olefins or hydrocarbons heterogeneously do not seem very successful. 

The changes in parameters Speh, C (S C), and w° and w observed in the reactions catalyzed by Fe(III)(acac)3 in the presence of R4NBr and H20 additives, and also obtained kinetics of catalytic ethylbenzene oxidation are evidently caused with the formation of catalytic active complexes of (Fe(II)(acae)2)x(R4NBr)y(H20)n and products of their transformation in the course of ethylbenzene oxidation. [11,13], The decrease in intermediate products of the (Fe(II)(acac)2)x(R4NBr)y(H20)n transformation to the end products as a consequence of coordination of H20 molecules with iron complexes. As result, the decrease in steady-state concentrations of selective catalysts Fe(II)x(acac)y(0Ac)z(L2)n(H20)m, took place. 

Various oxidations with [bis(acyloxy)iodo]arenes are also effectively catalyzed by transition metal salts and complexes [726]. (Diacetoxyiodo)benzene is occasionally used instead of iodosylbenzene as the terminal oxidant in biomimetic oxygenations catalyzed by metalloporphyrins and other transition metal complexes [727-729]. Primary and secondary alcohols can be selectively oxidized to the corresponding carbonyl compounds by PhI(OAc)2 in the presence of transition metal catalysts, such as RuCls [730-732], Ru(Pybox)(Pydic) complex [733], polymer-micelle incarcerated ruthenium catalysts [734], chiral-Mn(salen)-complexes [735,736], Mn(TPP)CN/Im catalytic system [737] and (salen)Cr(III) complexes [738]. The epox-idation of alkenes, such as stilbenes, indene and 1-methylcyclohexene, using (diacetoxyiodo)benzene in the presence of chiral binaphthyl ruthenium(III) catalysts (5 mol%) has also been reported however, the enantioselectivity of this reaction was low (4% ee) [739]. 

Initial catalytic tests indicated that LSCM catalyzes the oxidation of hydrocarbon fuels, although it also has some activity toward dry reforming [65]. Compared to the conversion of CH4 in a blank reactor, LSCM enhanced the reaction rate of CH4, and increased the selectivity toward total oxidation. Even under relatively oxygen-lean conditions (CH4 02 gas mixtures of 4 1), LSCM promoted the total oxidation of CH4 to CO2, up to 800°C. At 850°C, the CO2/CO selectivity was still 98.6% in favor of CO2. No significant steam reforming activity was measured. 

The selective oxygenation of methane and light alkanes (Ci-Cj) by the Fe(II)/H202 Fenton system was performed in the three-phase catalytic membrane reactor, enabling simultaneous reaction and product separation. Frusteri et al. reported that Nafion-based catalytic membranes catalyze the selective oxidation of... 

The selective oxidation of NH3 to NO is catalyzed by Pt/Rh. Called Ostwald reaction, it runs at high temperatures and is an important step in the conversion of nitrogen to nitrates via ammonia. Another catalytic oxidation process in inorganic chemistry is the oxidation of SO2 to SO3 to produce sulfuric acid. Platinum or vanadium oxide are the preferred catalysts. The catalyst has to be able to dissociate oxygen, and bulk sulfate formation, deactivating the catalyst, has to be suppressed. 

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