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of oxygen, supported the preferential oxidation of the methyl
group, with the primary hydroperoxide produced being readily
converted into products such as p-isopropylbenzyl alcohol, CA,
and IBA. For example, in the homogeneous catalytic Co-based
oxidation of p-cymene, the methyl group reacts approximately
a lesser extent, TOA, whereas the TCHP content was not affect-
ed very much. This may suggest that the formation of the hy-
droperoxide is fast under these conditions whereas the consec-
utive reaction pathway leading to TOA via COL and MAP is
slower, requiring longer reaction times. At 1408C (Table 1, en-
tries 9 and 10), at complete conversion of p-cymene, no residu-
al TCHP, COL, or MAP was detected; this result explains the
measured TOA and TA content. A similar behavior was found
for the pathway to TA via IBA. At 1408C, the system reached
a steady performance in 24 h. After that, no change in the
product distribution was detected (Figure 2). This was due to
19 times faster than the isopropyl group as a consequence of
an electron-transfer mechanism involving a radical cation as an
[
12]
intermediate. In terms of selectivity, the reactivity of the iso-
propyl group in p-cymene is represented by half the molar
amount of p-acetylbenzoic acid formed, whereas the reactivity
of the methyl group is represented by the sum of IBA and half
the amount of p-acetylbenzoic acid produced. Under these
conditions, the addition of chloride ions changed the reactivity
and reaction mechanism, whereas the oxidation of the isopro-
pyl group relative to the methyl group occurred in a ratio of
3
.2 to 1, which typically corresponds to a free-radical oxidation
[
25,29]
of p-cymene.
On the basis of these reports, one may
assume that the ratio of 4.7 obtained in this study may ac-
count for a free-radical mechanism.
The selectivity of the reaction was influenced strongly by
the reaction conditions (Table 1). The increase in O pressure
2
from 8 to 35 bar was conducive to a more advanced oxidation
of the isopropyl group to TOA and MAP, to the detriment of
the yield of COL (Table 1, entries 4–6), which is a common in-
termediate for TOA and MAP formation. However, this did not
affect the ratio between the products with oxidized methyl
and isopropyl groups. The increase did not influence the selec-
tivity to TA either.
Figure 2. Product distribution as a function of time for the selective oxida-
tion of p-cymene at 1408C; reaction conditions: p-cymene (6 mmol), catalyst
(50 mg), 20 bar O .
2
The increase in O pressure from 8 to 20 bar (Table 1, en-
2
tries 5 and 6) led to an increased conversion of p-cymene, with
the fact that the amount of O was less than the stoichiometric
2
a further increase in the O pressure to 35 bar promoting the
requirement; indeed, the stoichiometric O /p-cymene molar
2
2
conversion of intermediates and not the conversion of p-
ratio for the oxidation to TA (with the coproduction of two
CO molecules and four H O molecules) is 6, but a feed molar
cymene (Table 1, entry 4). Overall, the reaction was clearly limit-
2
2
ed by the amount of O loaded, which was substoichiometric.
ratio close to the stoichiometric one was achieved only for ex-
2
The influence of temperature was studied at three different
values (80, 100, and 1408C), and the respective results are de-
picted in Table 1, entries 6, 7, and 9. In general, the p-cymene
conversion decreased with the decrease in reaction tempera-
ture. However, more byproducts formed at higher tempera-
tures.
periments performed at 35 bar O pressure (at such high pres-
2
sure, however, temperatures higher than 1008C led to unsafe
operation, see entries 16 and 17). Moreover, it has to be taken
into account that the contact between oxygen and the reac-
tion mixture worsened as the reaction proceeded as the two
main products, TOA and TA, were solid under our reaction con-
ditions.
At 808C, the selectivity to COL (53%) was fairly high, and
only 14% of MAP was obtained (Table 1, entry 7). Further in-
creases in temperature enhanced the oxidation, leading to the
formation of both TOA and TA. A sharp increase in the selectiv-
ity to TOA (from 5 to 75%) was recorded as the temperature
increased from 100 to 1408C. Under the same conditions, the
selectivity to TA changed from 5 to 12% (Table 1, entries 6 and
If the reaction was performed at 1408C in the presence of
a higher amount of oxygen (initial O /p-cymene molar ratio=
2
8), the selectivity to TA increased to 51% (Table 1 entry 18).
However, longer reaction times did not change the product
distribution even under these conditions (Table 1, entry 19).
No conversion of p-cymene was detected in the absence of
catalysts (Table 1, entries 1 and 2). In addition to the clear rela-
tionship between the conversion and the catalyst loading, the
experiments performed in this study demonstrated an influ-
ence on the distribution of products (Figure 3). Low loadings
led only to TCHP and COL because of the lower conversion
achieved, whereas COL, MAP, and IBA were also produced at
larger loadings. However, an optimal substrate/catalyst ratio
has also been evidenced. Higher catalyst amounts had a small
effect on the selectivity to products but with lower conversion;
this may be attributed to a chain-termination effect that may
9). A prolonged reaction time (24 h) led to a 38% selectivity in
TA for a complete p-cymene conversion (Table 1, entry 10). This
result seems particularly important as in the presence of O TA
2
was produced in the absence of either a solvent (i.e., acetic
acid or similar) or initiators.
At constant temperature (1008C) and prolonged reaction
time (Table 1, entries 6 and 8), at which the conversion of p-
cymene reached a plateau, the changes in the distribution of
products corresponded to the transformation of the intermedi-
ates included in Scheme 1. COL was oxidized to MAP and, to
&
ChemSusChem 2016, 9, 1 – 12
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Nea?u, Florentina
