Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 2
Cu/Al2O3 and Fe/Al2O3 where almost inactive. All the catalysts in this study where synthesized by
impregnation which usually leads to formation of metal oxide particles. In the case of Cu, this can
lead to excessive leaching which was unfortunately not investigated. The synergistic effect of Fe on
Cu might result
from a stabilization of CuO particles. CuxMn3‐xO4 spinels were also investigated under the same
conditions [25]. A 1:1 ratio of Cu to Mn gave the best catalyst and the spinel was clearly favored over
amorphous mixed CuMn oxides obtained by calcination of the coprecipitate at lower temperatures.
With this catalyst, the selectivity to benzaldehyde dropped rapidly. After 3 hours reaction time 75 %
selectivity to benzoic acid was obtained at 21.3 % conversion (entry 7).
Already in the 1970s it was shown that Cu2O initiates the autoxidation of cumene (entry 8)
[151], though being highly inferior to Co3O4. The oxidation of cumene occurs readily due to the low
bond dissociation energy of the tertiary C‐H of 351.6 kJ/mol (toluene 375.0 kJ/mol) [152]. Metallic Cu
was found to be inactive as a catalyst contrary to bulk metallic silver being a good initiator for the
radical autoxidation of cumene [153]. Interestingly, the decomposition of cumene hydroperoxide
depended on the gas atmosphere and occurred only in N2 but not in O2. Ag/SiO2 was also found to be
a useful catalyst [154]. Alloying of Ag with Au supported on SiO2 via impregnation improved the
catalyst performance, Au/SiO2 being entirely inactive [155]. Ag2O supported on α‐ and γ‐Al2O3 and
zeolite 15X oxidized ethylbenzene to 1‐phenylethanol, acetophenone and ethylbenzene
hydroperoxide (entry 9) [72]. The role of silver was also to initiate a radical autoxidation but while
AiBN – also acting as an initiator – afforded mainly the peroxide, this species was decomposed on the
silver catalysts. At 70 °C, the reaction proceeded only slowly (initial TOF between 3‐8.5 h ‐1 for the
three different silver catalysts). At higher reaction temperatures (135 °C), ethylbenzene is oxidized
more rapidly in the presence of oxygen by an impregnated Ag/SiO2 catalyst [91]. Also p‐xylene and
cumene could be successfully oxidized though for p‐xylene only one methyl group was activated by
this catalyst. CeO2 and a carboxylic acid as additives increased the p‐xylene oxidation rate. On the
other hand, cumene oxidation was markedly hindered by the presence of CeO2 nanoparticles. In the
absence of a carboxylic acid, CeO2 completely suppressed the alkyl aromatic oxidation being a radical
scavenger. Toluene required higher reaction temperatures (entry 10). Significant silver leaching was
observed with the impregnated catalyst which could be circumvented by using flame‐spray pyrolysis
as a synthesis technique for Ag/SiO2‐CeO2 catalysts. This way, TOFs of 650 h ‐1 were obtained (Table 2‐
4, entry 11‐13). To the best of the author's knowledge, no studies focus on using heterogenous gold
catalysts for this reaction. However, Corma's group demonstrated the usability of gold for this
reaction by using the peroxides formed during cumene oxidation by Au/CeO2 for epoxidation of
aliphatic olefins [124].
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