Dual Oxides

The resulting catalyst contains about 2S% AljOj and is amorphous, i.e.. has no distinctive x-ray diffraction pattern indicating long-range crystallinity. Acidic properties are described in Chapter 4. 

Coprecipitated NiO-AljOi samples are typical of a wide variety of dual hydrous oxide gels. They are precursors to reduced Ni/AIjO) catalysts used in hydrogenation and methanation. Although these catalysts are produced by other routes, the intimate interactions fostered by coprecipitation are believed to impart improvements in activity and stability to the nickel crystallites/ although mechanical strength is lower. It may also be possible that irreducible compounds of NiO and AljOj, facilitated by coprecipitation, play some role in forming stable promoters. 

Both nickel and aluminum precipitate hydrous oxide sols. For best combinations, care must be taken to ensure that precipitation occurs simultaneously. Variations of pH within the vessel or during addition of the alkali result in preferential precipitation of one component, yielding product nonuniformity. One laboratory approach is to use the apparatus shown in Fig. 6.I1.  

The inner flask contains a solution oftN NaOH, the outer appropriate mixtures of nickel and aluminum nitrates. Tests are made to find the correct ratio between the two solutions such that, after mixing and precipitation, the final pH is 7. Inverting and shaking the jar gives instantaneous coprecipitation. 

Nickel aluminate is a well-defined spinel that is difficult to reduce. Formation of pseudospinels are not limited, however, to stoichiometric 

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A. Single active component and suppons 1. Single oxides 2. Dual oxides AI2O31 SiOji CrjO( SiOj-Ali<) NiO-AIjOj, CuO-AljOj zeolites... 

Catechols, hydroquinones, and their methyl ethers readily afford quinones on Ce oxidation.Partial demethylative oxidation is feasible, as shown in the preparation of several intramolecular quinhydrones (eq 5) and a precursor of daunomy-cinone. Sometimes the dual oxidant system of CAN-NaBrOs is useful. In a synthesis of methoxatin (eq 6) the o-quinone moiety was generated from an aryl methyl ether. 

Oxidation of Organosulfur Compounds. Thiols are converted into disulfides using reagents such as Bis[trinitratocerium (TV)] Chromate. Chemoselective oxidation of sulfides by Ce reagents to sulfoxides is easily accomplished. Stoichiometric oxidation under phase transfer conditions and the dual oxidant protocols permit oxidation of a variety of sulfides. 

Scheme 7.118 NHC-catalyzed dual oxidative p-functionalization reactions of aliphatic aldehydes and 1,3-dicarbonyls reported by Chi.

Fengel D., Wegener G., Wood. Chemistry, Ultrastructure, Reactions, Walter de Gruyter, Berlin, 1984. pp. 56-58 dos Santos Abreu H., Maria M.A., Reis J.L., Dual oxidation ways toward lignin evolution, Floresta e Ambiente, 8, 2001,... 

Few braze materials are compatible with SOFC operating conditions. Noble metals are considered too expensive in most SOFC stack designs. Silver is less expensive, but its use in a dual (oxidizing and reducing) environment can lead to chemical instability... 

In 2012, Karthikeyan et al. [20] reported a Rhodium(III)-catalyzed oxidative C-H coupling of JV-methoxybenzamides with aryl boronic acids to afford substituted phenanthridinones. This process involves a dual oxidative C-H bond coupling of two aryl rings (Figure 4.8). 

Hydroquinones (1,4-dihydroxybenzenes) and catechols (1,2-dihydroxybenzenes) are easily oxidized by ammonium hexanitratocerate(IV) to quinones (scheme 17) (Ho et al., 1972 Ho, 1973). However, the reaction conditions are critical, since it is not easy to avoid over-oxidation and the formation of side-products. The reactivity of ammonium hexanitratocerate(IV) can be reduced by coating the salt on silica gel (Fischer and Henderson, 1985), or by using the dual oxidant system CAN-NaBrOs (Ho, 1979). Brockhaus (1968) reported on the oxidization of catechol to 6>rf/zo-benzoquinone by cerium(IV) sulfate in a chloroform/sulfuric acid biphasic... 

Inspired by the Belousov-Zhabotinskii reaction, where cerium(III) is reoxidized by bromate ions. Ho developed a dual oxidant system consisting of ammonium hexanitratocerate(IV) and sodium bromate for the oxidation of arylmethanols (benzylic alcohols) to the corresponding aldehydes (Ho, 1978). The method has also been successfully used for the oxidation of hydroquinones to quinones (scheme 59) and for the oxidation of sulfides to sulfoxides (Ho, 1979). 

In 2011, Li etal. described a Rh(III)-catalyzed coupling of JV-aryl and Al-alkyl ben-zamidines with internal alkynes that afforded less accessible 1-aminoisoquinolines 55 [30]. In the presence of ort/zo-substituted benzamidines, the reaction occurred by dual oxidative cyclization and furnished indole-isoquinoline 56. This unusual result occurred in the presence of an ortho steric effect after the formation of the isoquinoline moiety. The steric repulsion between the ortho substituent and AT-phenyl ring forced these two groups apart (Eq. (5.54)). 

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