Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

2.5 Conclusions and outlook
CHAPTER 2
Among the coinage metals gold catalysts are without a doubt dominating within many oxidation
reactions, especially concerning fine chemicals e.g. in alcohol or amine oxidation. Still, silver and
copper showed in this chapter that they are capable of catalyzing the same reactions: both for
alcohol oxidation and for epoxidation reactions good results were achieved. More research for better
Ag and Cu catalysts will certainly be fruitful in the future. Hydroquinone, silane and amine oxidations
(and also sulfide oxidation) are not yet excessively investigated. Good activity was obtained with gold
catalysts but Cu and Ag catalysts also showed promising results but there is still great potential for
new discoveries. When it comes to oxidation reactions where the primary role of the heterogeneous
catalyst is likely that of a radical initiator, i.e. cyclohexane, alkyl aromatics and benzene oxidation,
gold catalysts were rarely used with great success; especially the high TOFs reported in some papers
need to be evaluated with care as the product analysis is very challenging and gold simply acts as a
radical initiator. For these reactions, a higher number of Cu and Ag catalysts exist though (except for
benzene hydroxylation) Co catalysts in general give the best results. The strength of the two lighter
coinage metals as a catalyst material is – apart from the low price compared to other noble metals
used for oxidation reactions – the specific (chemo)selectivity which was reported e.g. for the
anaerobic alcohol oxidation with copper or the epoxidation of styrene with H2O2 and the silane
oxidation over silver catalysts. A prerequisite for obtaining good catalytic activity is of course a
target‐oriented synthesis of a catalyst for which some guidelines were compiled. It was the intention
of this overview to show where potential is hidden for the development of new catalyst materials
and processes. More fundamental mechanistic studies are needed in order to further understand the
catalytic processes, limitations and deactivation pathways as was briefly discussed; as an example,
one deactivation pathway investigated intensively for Cu is leaching. Moreover, examples for the use
of supercritical solvents, especially CO2, and alloying of the coinage metals with focus on Ag und Cu
has not yet been approached intensively and will offer further opportunities. There is still much
potential and therefore the coinage metals are certainly a valuable tool in oxidation catalysis and will
contribute to establishing greener and sustainable catalytic processes.
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