Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
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2.3 Selective liquid‐phase oxidation reactions<br />
Contrary to homodispersed Cu in MCM‐41, CuxO particles supported on activated carbon exhibited<br />
only low efficacy in benzene hydroxylation [183] lower than that of iron and vanadium oxide particles<br />
(entry 10) supporting the assumption that CuxO particles are catalytically active, however, not the<br />
most efficient catalysts for aromatics hydroxylation. CuO incorporated in a polymer membrane was<br />
furthermore used in a more application‐oriented study [192]. Dubey et al. investigated in several<br />
reports the catalytic activity of Cu‐containing ternary hydrotalcites in the hydroxylation of phenol<br />
[58, 184, 185] affording both catechol and hydroquinone. The reaction studied with CuCoAl (Table 2‐<br />
6, entry 11) and CuNiAl hydrotalcites (entry 12) gave the best results and required H2O2. TBHP was no<br />
suitable oxidant. These hydrotalcites were also capable of oxidizing benzene to phenol with nearly<br />
100 % selectivity when pyridine was used as a solvent though catalyzing the reaction rather slowly<br />
(entry 13) [186]. The combination of Cu with Ru in MCM‐41 was catalytically active in neat benzene<br />
with H2O2 at 50 °C [143]. Phenol was the only product observed and TOFs were outstandingly high<br />
(above 600 h ‐1 , based on the combined amount of Ru and Cu).<br />
The group of Tsuruya investigated the use of molecular oxygen as oxidizing agent which<br />
required the use of ascorbic acid as a sacrificial reductant in aqueous acidic solvents (entries 14‐17)<br />
[28, 29, 43, 50, 187]. Various Cu catalysts were tested as catalysts, including Cu supported on Al2O3,<br />
SiO2 (also MCM‐41) and TiO2 as well as ion exchanged zeolites. Conversions were rather low at 30 °C<br />
(TOF < 1 h ‐1 ). Higher reaction temperatures had a negative effect on the overall conversion which<br />
was ascribed partly to ascorbic acid decomposition. The formation of H2O2 was evidenced during the<br />
reaction. Thus, it is reasonable to assume H2O2 to act as the primary hydroxylating agent. It was<br />
found that both framework incorporated Cu(II) species were catalytically active as well as leached<br />
Cu(II). Leached species gradually deactivated to Cu2O making truly heterogeneous catalysts the most<br />
effective catalysts. This also explains why heterogeneous Cu‐zeolite catalysts are better suited for<br />
this type of reaction [28] than different homogeneous Cu catalysts. CuO/Al2O3 was additionally more<br />
stable against leaching when prepared via co‐precipitation rather than impregnation [187]. In a more<br />
recent publication V‐Cu catalysts on various supports were successfully applied with ascorbic acid/O2<br />
in an acetic medium (entry 18a) [188]. The potential of this dyad was shown by more drastic reaction<br />
conditions, i.e. 80 °C as a reaction temperature and oxygen partial pressures of 7 bar. Probably due<br />
to decomposition, H2O2 was no suitable oxidant. Dissolved Cu was not observed in the reaction<br />
mixture (though insoluble Cu2O may form from leached species, vide supra), while V leaching was an<br />
issue. This approach resulted in more active catalysts with a TOF of 2 h ‐1 (based on the amount of<br />
both V and Cu). To the best of the author’s knowledge, this study is also the only example where Ag<br />
was found to be active (though also in combination with V, entry 18b). Indeed, the literature<br />
provides only few examples of heterogeneous silver or gold catalysts active for this reaction although<br />
both metals proved numerous times to be capable of activating hydrogen peroxide. A recent study<br />
47