Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 2
here appear to be dominated by either of the two metals. Silver is less often considered and shows
its potential only occasionally but often with respectable results; e.g. in the aerobic oxidation of
alcohols, similar activities compared to Au reference catalysts were obtained [71, 86].
2.4.1 Catalysis by gold
In selected cases, impressive results were reported with Au catalysts. High selectivities with Au
catalysts were obtained, i.e. especially overoxidation products like carboxylic acids and esters in
alcohol oxidation were formed in low amounts and often gold catalysts provide excellent reusability.
A noteworthy feature of gold is also its capability to oxidize polyols [5] which to the best of the
author’s knowledge has not yet been achieved over Cu and Ag catalysts in the liquid phase. Similar to
alcohol oxidation, amine oxidation is dominated by gold catalysts though – especially in contrast to
alcohols – still rather high reaction temperatures and low TOFs were obtained for benzylic amines
leaving substantial potential for this rather young field of research – also for Cu and Ag catalysts.
Gold plays an outstanding role for epoxidation reactions where employing oxygen is difficult: few
examples of gold catalysts were reported being intriguing from an academic point of view but
exhibiting only low selectivities in styrene epoxidation [129]. Cyclohexene epoxidation was
conducted with great success in two studies from the group of Hutchings [120, 121] while other
groups mainly reported the formation of allylic oxidation products [122]. The reasons for this
difference are poorly understood and more fundamental studies are required to exploit the potential
of the aerobic epoxidation over gold.
Styrene epoxidations with TBHP were also intensively investigated by the group of
Choudhary [134‐138, 240] giving solid but not overly impressive results in terms of activity and
selectivity. It should also be added that styrene oligomerization was not taken into account and
therefore these studies need to be taken with care. Au catalysts are also effective in hydroquinone
and silane oxidation where Au acts as a universal catalyst in contrast to silver. In these oxidation
reactions, benzyl alcohol, cyclohexene, styrene etc. are used as model substrates demonstrating the
general applicability of the investigated catalysts for the synthesis of potentially more complex fine
chemicals. The oxidation of a number of hydrocarbons, on the contrary, is of immediate industrial
importance for the production of oxygenated commodity chemicals. Due to its high price, the use of
gold is only justifiable when outstanding selectivities and activities are obtained – considering
especially that hydrocarbon functionalization is a domain of inexpensive Co catalysts [241]. Partly
implied by the small amount of reports, gold features only low efficacy for a number of relevant
reactions, i.e. benzene hydroxylation with H2O2, side‐chain oxidation of alkyl aromatic compounds
(Au/SiO2 was found to be catalytically inactive [155]) and potentially also cyclohexane oxidation. In
the latter case, TOFs exceeding 100,000 h ‐1 (blind test resulting in 0 % conversion at 150 °C!) [164‐
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