Figure 4 Optimization of different zinc compounds and ROS
diffused from the surface of the CNTs. (OH simply represents ROS).
To prove this view, an experiment has been designed to add zinc acetate
(Zn(CH3COO)2) into the cumene oxidation
reaction catalyzed by CNTs. The result shows that the conversion of
cumene with CNTs and Zn(CH3COO)2 was
37.6%, which was 4% lower than that of only with CNTs. But, compared
with zinc halide, the inhibition effect of zinc acetate is very weak
because Zn2+ is already saturated with
oxygen-containing CH3COO- groups. This
further elucidates the relationship between the coordination of
Zn2+ with oxygen-containing groups and the inhibition
of Zn2+ on cumene oxidation. Zn2+combines with ROS in the system, making it hard to extract H atom from
cumene.
The reaction (1) is the dissociation of hydrogen radical (H· )
and the production of R·. The production of R· is directly related to
the generation of RO· and ROO·. However, the bonding energy of
H· with tertiary carbon is about 3.8 eV (Table S3). Besides
that, even if the initial reaction of cumene successfully occurs. The
radicals generated from the initial reaction will quickly combine to
cumene because of its instability. This is the reason why it is
difficult for cumene to oxidize without catalyst. The energy barrier of
the initial reaction on different catalysts is shown in Figure 5 and
Table S4. ROS diffused from the surface of the CNTs effectively reduce
the initial reaction energy barrier to 0.55 eV, which can account for
the conversion of cumene by CNTs. On this basis, after adding
Cu2+, the energy barrier is lower as 0.18 eV which can
explain the promotion of
Cu2+.[14] The situation with
adding Zn2+ is completely the opposite, the energy
barrier goes up to 1.90 eV with adding Zn2+. It is
reasonable to deduce that the inhibition effect of
Zn2+ ions on cumene oxidation is due to the strong
interaction of Zn2+ between ROS in the system, which
leads to the increase of the energy barrier of the initial reaction.
Figure 5 Potential energy surface of the initial reaction of
cumene with different metal ion.
According to the above theoretical calculations, Zn2+can strongly coordinate ROS due to its positive charge and sufficient
space. According to the bonding energy between Zn2+and ROS, it is difficult to break out ROS and Zn2+.
ROS is the key substance for the oxidation of cumene catalyzed by carbon
nanotubes, so the initial reaction of cumene cannot be carried out.
Moreover, it is more difficult to extract H atom from cumene to generate
oxygen-containing free radicals (RO· and ROO·). This indicates that the
interaction characteristic between the catalyst and reactive oxygen
should be considered for the rational design of catalysts. In addition
to Zn2+, the role of other metal ions in the cumene
oxidation may also be worth considering, including the coordination
ability to ROS and the inducted chain growth process.
Conclusions
In this work, the inhibition effect of Zn2+ on the
catalytic oxidation of cumene was found by experiments, and the reason
was studied by DFT. There are two inhibitory effects of
Zn2+ on cumene oxidation. Firstly,
Zn2+ is able to strongly coordinate ROS activated by
CNTs, inhibiting this critical chain-initiation process of cumene
oxidation. After adding Zn2+, the energy barrier of
initial reaction increases to 1.90 eV, which is nearly 4 times higher
than that of the ROS promoted-process. Secondly, the interaction of
radicals (RO·, ROO·) and Zn2+ leads to those radicals
out of chain propagation reaction and annihilate. This work provides a
better understanding of carbon catalyzed cumene radical reactions. At
the same time, it is noteworthy that the influence of metal ion impurity
on the oxidation activity of cumene in industrial application.
Conflicts of interest
There are no conflicts to declare.