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Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

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Si OH<br />

TOF 130 h -1<br />

Ag/HAP<br />

CHAPTER 2<br />

H2O H2O SiH<br />

(n-Bu) 3SiH<br />

Scheme 2‐17: <strong>Oxidation</strong> of aliphatic and aromatic silanes over Ag/HAP [87] and Au/HAP [235].<br />

2.4 Strengths and opportunities of coinage metals as oxidation<br />

catalysts<br />

A great deal of the success of gold is certainly connected to the high effort which was put in this field<br />

of research. Simple synthesis methods like impregnation are hardly used and achieve poor<br />

dispersions [200]. Hence, newly developed more sophisticated synthesis strategies [24] allow tuning<br />

of the Au nanoparticles size [239]. This can also be a disadvantage: in addition to the high raw<br />

material costs of Au, more sophisticated methods are also more expensive. On the contrary, Cu<br />

combines low cost of the metal with a high efficacy of simple synthesis methods like ion‐exchange<br />

and coprecipitation. This way also mixed oxides are easily accessible. Due to the often high<br />

importance of isolated sites, optimal catalyst loadings can be very low. Silver lies somewhere<br />

between these two; though many metallic silver catalysts studied in the open literature were<br />

prepared by impregnation, more elaborate techniques usually afforded better catalysts. Potentially<br />

due to sintering, calcination times should be chosen with care for silver catalysts [71]. Calcination in<br />

air is apparently an important activation step as silver‐oxygen species are formed [73, 74]. As<br />

outlined in section 2.2, different catalyst types exist depending e.g. on the oxidation state of the<br />

metal. Table 2‐10 gives an overview over the reactivities of the different catalyst types emphasizing<br />

that – except for alcohol oxidation – the structural difference between gold and copper is big, gold<br />

being metallic and copper in the most cases ionic during the oxidation reaction. This causes a<br />

significant difference in reactivity. Both Cu and Au catalysts apparently feature areas where they are<br />

dominating. For gold, this would e.g. be the aerobic oxidation of alcohols offering high selectivity and<br />

often good reusability while the capability of Cu to generate hydroxyl radicals [170] make it a<br />

preferred catalyst material for benzene hydroxylation. Also the other oxidation reactions described<br />

56<br />

Au/HAP<br />

H 2O<br />

Si OH<br />

TOF 40 h -1<br />

(n-Bu) 3Si OH<br />

TOF 20 h -1

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