MAGHEMITE AS A CATALYST FOR OXIDATION
313
Table 1. Experiments on the oxidation of benzyl alcohol
over maghemite and magnetite catalysts with fractionation.
The air flow rate is 100 mL/min and the reaction tempera-
ture is 202–203°C
the reaction vessel is decreased below 200°C. Benzoic
acid was not detected among the distillation products.
Flow experiments. An important characteristic of
the performance of a catalyst system is its turnover
number (TON), which illustrates the on-stream sta-
bility of the catalyst. To understand how the amount of
the oxidation substrate affects the catalyst activity, we
performed experiments on the oxidation of benzyl
alcohol in the flow mode.
As is seen from Table 1, the yield of benzaldehyde
is not high; however, it slightly depends on time. Here,
benzoic acid was not detected in any of the distillates.
Catalyst
Yield of benzaldehyde
Maghemite
1st portion 8 wt %
Maghemite
Maghemite
Magnetite
Magnetite
Magnetite
2nd portion 2 wt %
3rd portion 8 wt %
1st portion 8 wt %
2nd portion 4 wt %
3rd portion 4 wt %
Recycling experiments. Recycling is one of the
most important operations in industrial organic
chemistry, which makes it possible to increase the
yield of the target product. We carried out several
experiments in the recycle mode to understand
whether the yield of the target product could be
increased by this means.
For the maghemite catalyst, the yield of benzalde-
hyde is 18% in the first cycle and 21%, in the second
cycle, i.e., recycling at least does not reduce the prod-
uct yield. Benzoic acid (the o-proton at 8.1 ppm) is not
detected in the 1st cycle, and its amount in the 2nd
cycle is about 2% of benzaldehyde (Table 2).
Table 2. Recycling experiments on the oxidation of benzyl
alcohol over maghemite. The air flow rate is 100 mL/min
and the reaction temperature is 202–203°C
Cycle Yield of benzaldehyde, Yield of benzoic acid,
number
wt %
wt %
1
2
18
21
0
0.4
It was shown in the previous experiments that the
test catalysts were active in the aerobic oxidation of
benzyl alcohol. However, the question whether this
oxidation proceeds by the radical or ionic mechanism
remained unclear. Apparently, the carbanionic mech-
anism is almost impossible in the presence of proton-
donating compounds (benzyl alcohol, water); there-
fore, two possibilities remained, namely, radical and
carbocationic mechanisms. On the other hand, it is
clear that the carbocationic mechanism is also almost
impossible if there are no acid sites with any noticeable
strength on the catalyst surface. To clarify this ques-
tion, experiments with 1-phenylethanol (PhEtOH)
were performed. Like benzyl alcohol, this alcohol is
capable of forming a benzyl-type cation; however,
unlike the benzyl cation, this cation is extremely prone
to proton elimination to form styrene. This reaction
occurs in the presence of almost all somehow strong
acids. Therefore, if such sites are present on the cata-
lyst surface, noticeable amounts of styrene should be
detected among the products.
Experiments on Alcohol Oxidation over a Catalyst
in the Presence of Trioctylamine
To clarify how important is the role of surface acid
sites in the oxidation of phenylethanol, we added
100 µL of tri-n-octylamine to the reaction mixture
(maghemite was used as the catalyst). First, trioctyl-
amine possesses high basicity and can effectively inter-
act with acid sites, and second, its boiling point is very
high and it is for sure that this additive will not be dis-
tilled off during the reaction and not contaminate the
product.
It was found that the yield of acetophenone
increased fivefold and the yield of styrene decreased
twofold in this case. This clearly indicates that the sur-
face acid sites not only do not promote the oxidation
reaction, but even inhibit it. Therefore, in this case,
the oxidation also proceeds via the radical mechanism.
An experiment was also carried out. on the oxidation
of benzyl alcohol over maghemite in the presence of
The results of oxidation of 1-phenylethanol over trioctylamine In this case, the yield of benzaldehyde
our catalysts are presented in Table 3. As is seen from increases at least fourfold, thereby fully confirming the
Table 3, phenylethanol is oxidized in the presence of above conclusion.
magnetite or maghemite (Fig. 2) (in addition to the
signal of the α-proton of phenylethanol at 4.87 ppm,
CONCLUSIONS
the signal of acetophenone α-protons at 2.61 ppm is
also present). However, styrene becomes the main
component of the obtained distillate, and its amount
exceeds by an order of magnitude the amount of
formed acetophenone. This indicates that there are
As a result of the experiments done, the following
conclusions can be drawn:
(1) It has been shown that under the slurry reactor
acid sites of sufficient strength on their surface, and conditions, maghemite is active in the oxidation reac-
the oxidation can occur via both mechanisms.
tion of benzyl alcohols with atmospheric oxygen. Ali-
PETROLEUM CHEMISTRY Vol. 60 No. 3 2020