Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

2.4 Strengths and opportunities of coinage metals as oxidation catalysts
166] need to be seen alongside with the fact that unpassivated autoclave steel can afford similar
conversions [156]. Furthermore at low reaction temperatures, only low activities of Au were
observed and additionally a radical initiator was necessary, the reaction clearly following a radical
autoxidation mechanism [159]. Further studies with accurate product determination are necessary to
clarify this point.
2.4.2 Catalysis by copper
In contrast to gold, the use of copper catalyst for the synthesis of commodity chemicals appears
more reasonable from an economic point of view and thus many Cu catalysts exist for benzene
hydroxylation with H2O2. In many cases, isolated Cu species appear to be the species resulting in the
highest catalytic activity for this reaction. EPR investigations on copper complexes suggest that this is
due to the specific interaction of ionic copper with peroxides generating oxygen centered radical
species [168‐171]. TOFs obtained for hydroxylation differ greatly being in the one‐digit area and up
to around 100 h ‐1 . Strong solvent effects were observed in many cases which might be connected to
an unfavorable side‐oxidation of the ACN solvent [180]. Interesting from an academic point of view is
the possibility to use oxygen in combination with ascorbic acid [28, 29, 50, 187]. Hydroxyl radicals are
suggested to be the hydroxylating species indirectly generated from reaction of Cu(I) with molecular
oxygen. Though reaction rates were very low compared to other examples, it is remarkable that Cu
mediates benzene hydroxylation even at room temperature. Indeed in combination with V, quite
impressive conversions were obtained with this approach [188]. There are also Cu catalysts which
were investigated for cyclohexane oxidation under aerobic conditions and especially Cu‐ZSM‐5 [27]
showed good activity. Still, these studies should be extended to industrial conditions and compared
to Au catalysts and dissolved Co. This catalyst might also be suitable for the aerobic oxidation of alkyl
aromatic compounds and vice versa. Especially CuFe/Al2O3 gave promising results for toluene
oxidation though under harsh reaction conditions [150]. Most other Cu catalysts for the oxidation of
cyclohexane and alkylaromatic compounds used H2O2 and TBHP, respectively. Due to the ring
hydroxylation activity of Cu this can be problematic (as described previously) but also high cost for
the oxidant for the production of commodity chemicals – given that these reaction can be carried out
with oxygen – make the use of H2O2 and TBHP unattractive. Additionally, the employment of
peroxides limits the maximal applicable temperature due to unselective decomposition above ~ 80 °C
making breakage of rather inert C‐H bonds difficult. It is therefore not surprising that TOFs are low
with peroxides. Still, the ability of these Cu catalysts to interact with TBHP or H2O2 might make them
suitable candidates for epoxidation reactions. Though mostly catalysts with CuO particles were used,
isolated Cu centers (as used for many other oxidation reactions) also appear to be suitable catalysts
for epoxidation reactions as they appear in MOFs [42, 127]. CuO/Ga2O3 [48] was also found to be an
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